HETEROCYCLES

Review | Regular issue | Vol. 78, No. 11, 2009, pp. 2661-2728
Received, 2nd July, 2009, Accepted, 24th August, 2009, Published online, 26th August, 2009.
DOI: 10.3987/REV-09-656

■ Metal-Catalyzed Heterocyclization: Formation of Five- and Six-Membered Oxygen Heterocycles through Carbon-Oxygen Bond Forming Reactions

Krishna C. Majumdar,* Pradip Debnath, and Brindaban Roy

Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India

Abstract

This review describes the synthesis of five- and six-membered oxygen heterocycles through carbon-oxygen bond formation by metal catalyzed heterocyclization published during 2005 to 2008.

CONTENTS
1 Introduction
2 Catalyst and general aspect of the catalysis
3 Synthesis of five-membered oxygen heterocycles
3. 1 Synthesis from alkenes
3. 2 Synthesis from alkynes
3. 2. 1 Intramolecular cyclization of alcoholic-, phenolic-, carbonyl-, ether- and epoxy-oxygen to alkyne
3.2. 2 Intermolecular cyclization of alkyne
3. 2. 3 Tandem cyclization of alcoholic oxygen to enynes
3. 3 Synthesis from allenes
3. 4 Intramolecular cyclization of haloaromatic compounds
3. 5 Synthesis from diazo compounds
3. 6 Synthesis via cyclopropane ring opening
3. 7 Synthesis of furanones
4 Synthesis of six-membered oxygen heterocycles
4. 1 Synthesis from alkenes: Cyclization of alcoholic- and phenolic-oxygen to alkenes
4. 2 Synthesis from alkynes: Cyclization of alcoholic-, benzylic-, carbonyl- and epoxy-oxygen to alkynes
4. 3 Synthesis from allenes
4. 4 Synthesis from diazo compounds
4. 5 Synthesis of pyranones
4. 6 Synthesis of coumarin derivatives
5 Miscellaneous reactions
6 Conclusion
7 Acknowledgements
8 References

1. INTRODUCTION
Various heterocyclic backbones appear in different natural products and are very much important for their therapeutic properties. They have wide applications in pharmaceutical field.1 Heterocyclic compounds are also versatile building blocks in organic synthesis by virtue of their multiple reactivity profiles. Thus from the drug discovery perspective the synthesis of these heterocycles have attracted organic chemists for active research directed towards the development of novel and effective synthetic strategies. Many of these strategies involve the formation of either carbon-carbon or carbon-heteroatom bond from the corresponding acyclic precursors. Most of the classical methods employ comparatively harsh conditions and suffer disadvantages such as the use of expensive catalyst, elevated temperature, longer reaction time, poor yield of the final product, tedious work-up etc. So these methodologies have obvious limitations. These drawbacks and limitations necessitate the search for new methods for the construction of organic molecules from simple starting materials which is an ongoing challenge for the organic chemists. The search may be for the modification or expansion of the existing one, the development of complementary methods, scale up etc. The protocol should be simple, mild, direct method to be compatible with the previous one, for the rapid access to heterocycles and should have further application in future for rapidly constructing other useful heterocycles.
Among several newly developed methodologies, the employment of transition metal catalysis in the synthesis of heterocycles have proven its efficiency and importance to the level where this is now routinely considered in strategy level planning of complex targets. The wide utility of metals in organic synthesis is evident from the huge number of name reactions where the deep influences of this versatile transition metal enable it in the formation of C-C, C-O, C-N and even C-S bond under relatively mild conditions. The catalytic requirement and excellent tolerance of functional groups avoiding the protection-deprotection chemistry has made possible the use of transition metal in the synthesis of small to large ring heterocyclic compounds. Moreover, the development of asymmetric transformation using chiral ligand is a major progress in the metal-catalyzed synthesis of heterocycles.2 In addition, the metal-catalyzed domino multiple transformation, in recent years, have been the general need both from economical and ecological ground. Recently, several reviews have appeared describing the metal catalyzed various type of reactions.3-12 However, there is no such review describing the synthesis of five- and six-membered oxygen heterocycles via the selective formation of C-O bond. In this brief review we provide an updated summary of transition metal-catalyzed approaches for the preparation of heterocycles through the formation C-O bond directly.

2. CATALYST AND GENERAL ASPECT OF THE CATALYSIS
Heteroannulation processes involving the activation of alkenes, alkynes, and allenes bearing a tethered nucleophilic substituent are among the most versatile and efficient synthetic way for constructing a wide array of heterocycles. For this purpose various transition metals3-12 such as Cu, Pd, Pt, Ru, Rh, Au, W, Mo, Ir etc have been extensively used. Only late transition metal complexes behave as soft Lewis acid. Such a property allows the metal to activate the unsaturated carbon-carbon bonds, to create carbon-carbon and carbon-heteroatom bonds under extremely mild conditions. Choice of catalyst and ligand variation is the most powerful tool in metal catalysis and key features of transition metal catalysts such as activities, selectivity and stability are dictated by the steric and electronic properties of ligands that are co-ordinated to the metal.
Generally, the reactions are initiated via the formation of a π-complex between the metal catalyst and unsaturated bond followed by the nucleophilic attack onto activated unsaturated bonds of alkynes or alkenes or allenes (Scheme 1).

Protodemetalation provides various types of five- and six-membered heterocycles. The nucleophile (X) may be the oxygen atom of alcohols, carbonyls, carboxylic acids, carbonates, carbamates and amides etc. Important heterocycles such as furans, pyrans, benzofurans, benzopyrans, furanones and pyranones have been accessed using this protocol.

3. SYNTHESIS OF FIVE-MEMBERED OXYGEN HETEROCYCLES
The furan or dihydrofuran ring widely occurs as key structure subunits in numerous natural products,13 which find a variety of application as pharmaceuticals, flavor and fragrance compounds.14 Thus, there are continuous demand for increasingly clean and efficient chemical synthesis of furans from both economic and environmental points of view. The development of synthetic tools to connect highly functionalized fragments still remains an exciting challenge for organic chemists.
Transition metal mediated synthesis is increasingly employed for the formation of biologically important molecules in a regio- and chemoselective manner.15 Transition metal-mediated addition of oxygen nucleophiles across a carbon-carbon double and/or triple bond is one of the most interesting and important reactions to the synthesis of oxygenated heterocycles.16 The intramolecular version of this reaction falls under the broad category of cycloisomerization. Cycloisomerization reactions are characterized by their complete atom economy and have been recognized as an attractive tool for the preparation of complex molecules.17

3. 1 SYNTHESIS FROM ALKENES
Pd-catalyzed synthesis of a number of five- and six-membered oxygen heterocycles has been reported via oxidative cyclization of a variety of nucleophiles such as phenol, alcohol, and carboxylic acid, onto unactivated alkene.5a,18 The use of chiral bisoxazoline ligands based on binaphthyl19 and biphenyl backbones20 in the Pd(II)-catalyzed enantioselective Wacker-type cyclization of o-ally phenols have been well documented. In a related study, C2-asymmetric bisoxazoline ligand bearing an axis-unfixed biphenyl backbone-(L-1) was utilized in highly enantioselective Pd(II) catalyzed Wacker-type cyclization of 2-allylphenols 1. The reactions were catalysed by Pd(II)-(L-1) complexes generated in situ by mixing Pd(CF3COO)2 with bisoxazolines-(L-1) (Pd/ligand 1:2) in the presence of 4 equiv of p-benzoquinone (BQ) in methanol (MeOH) to afford the corresponding chiral 2,3-dihydrobenzofurans 2 with good to excellent enantioselectivity (Scheme 2).21

Yamaguchi et al. reported22 that asymmetric cyclization of 2-(3-methylbut-2-enyl)phenol with a chiral ligand and catalytic palladium afforded both the desired product 2-isopropenyl-2,3-dihydrobenzofuran and the unwanted pyran derivative. The reaction suffered from poor conversion and poor enantioselectivity. Koning et al. showed that Trost condition for the synthesis of chiral 2-substituted-2-vinyl chromans using chiral ligand (L-2)23 and catalytic Pd(dba)2 furnished the required volatile 2-isopropenyl-2,3-dihydrobenzofuran 4 from the alcohol 3a.24 They also synthesized the opposite enantiomer, 5 by similar treatment of 3b with Pd(dba)2 in the presence of acetic acid (AcOH) and enantiomeric Trost-ligand, (L-3) (Scheme 3).

Sakurai et al. recently carried out the cycloisomerization of a variety of γ-hydroxy alkenes 6, under the air atmosphere, using a gold nanocluster, Au/PVP, as the catalyst in H2O/DMF mixture containing DBU (Scheme 4).25

A highly efficient gold-catalyzed annulation of phenol 8 with diene 9 to construct tricyclic furan derivative 10 was reported by Li and co-workers.26 The use of an excess of diene in the presence of AuCl3 (5 mol%)/AgOTf (15 mol%) led to a high yield of the desired product. However, the use of AuCl3 or AgOTf alone, or AuCl/AgOTf as the catalyst gave only a trace or no product. On the other hand, acyclic diene did not give the product under similar reaction conditions. Mechanistically, the reaction involves two gold-catalyzed processes including a Friedel-Crafts reaction to form intermediate 11 and a subsequent hydrophenoxylation of intermediate 12 giving the product 10 (Scheme 5).

3. 2 SYNTHESIS FROM ALKYNES
Similar to alkenes, the metal complexes catalyzed intramolecular cycloisomerization of precursors containing alkynes and various oxygenated nucleophiles was also exploited for the construction of a range of oxygen containing heterocycles.27

3. 2. 1 INTRAMOLECULAR CYCLIZATION OF ALCOHOLIC-, PHENOLIC-, CARBONYL-, ETHER- AND EPOXY-OXYGEN TO ALKYNE
Cycloisomerization of alkynols11,28 has been utilized as a tool to synthesize oxygen-containing heterocycles encompassing functionalized furan, pyran, benzopyran, and spiroketal skeletons. Ru29- and Pd30-catalyzed cycloisomerization of a variety of Z-enynols have also been reported.
Substituted 2,3-dihydrofurans were synthesized by the palladium-catalyzed reaction of propargylic carbonate containing a homopropargylic hydroxyl group with different phenols in moderate to excellent yields.30 The reaction was best carried out using Pd2(dba)3.CHCl3-dppf catalytic system in dioxane. The key issue is the mode of cyclization i.e., exo-dig versus endo-dig.31 Ramana et al. very recently reported32 the electronic control over the 5-exo-dig versus 6-endo-dig modes of Pd-catalyzed cycloisomerization of 3-C-alkynylfuranosyl derivatives using a set of two different alkynol substrates 15 and 18 (Scheme 6). It is interesting to note that electron-donating group attached to the aromatic ring favored 6-endo-dig while electron attracting group favored 5-exo-dig modes of cyclization.

The intramolecular asymmetric hydroxylation of alkynols for the synthesis of five- and six-membered cyclic ethers was examined by Yamamoto et al.33 Better result was observed in the presence of Pd2(dba)3.CHCl3 (10 mol%) PhCO2H (20 mol%) and (R,R)-renorphos (60 mol%) in benzene and a variety of five- and six-membered cyclic ethers were obtained with moderate to good enantioselectivities (Scheme 7). The reaction is also successful with benzo substitutent at 2,3-position of the alcohol 22.
The tandem cycloisomerization-hydroalkoxylation of various homopropargylic alcohols 24 in the presence of the dual catalyst system Ph3PAuCl/AgBF4 and a Brønsted acid p-TsOH in alcohol at room temperature gave tetrahydrofuranyl ethers 25 (Scheme 8).34 The hydroalkoxylation step occurs with various primary and secondary alcohols. Both gold(I) and gold(III) precatalysts were found active for the

cyclization and afforded the cyclic acetal. However, Ph3PAuCl alone was inactive towards the cyclization. But, gave the desired product rapidly along with the silver salts as additive, may be due to the formation of cationic gold species. Other gold catalyst such as AuCl, Au(OH)3, Au(OAc)3, AuCl3, HAuCl4 were also found to be active for the cyclization but gave lower yields of the products. Subsequently, Pale et al. synthesized35 functionalized tetrahedrofurans and pyrans 27, regio- and stereoselectively, via oxycyclization of primary 4(or 5)-yn-1-ols 26 catalyzed by a 1:1 mixture of AuCl and K2CO3 (Scheme 8). They proposed that stereoselectivity of the cycloisomerization is due to the trans-addition of the alcohol to the Au-coordinated C≡C bond.

Recently, Hammond et al. reported the Ag-catalyzed direct activation of electron deficient triple bonds by using the combined electron withdrawing effects of the fluorine atoms to modulate the electronic density of the triple bond. Substituted 3,3-difluro-4,5-dihydrofurans (unstable) was synthesized in excellent yields from gem-difluorohomopropargyl alcohols 28 by activation of the triple bond using AgNO336 (10 mol%) as Lewis acid, which on treatment with SiO2 or Pd/H2 yielded the corresponding 3-fluorinated furans 29 and 3,3-difluorotetrahydrofurans 30, respectively (Scheme 9). However, the cyclization of gem-difluorohomopropargyl alcohols 28 mediated by catalytic amounts of AuBr3 or CuI afforded 3-fluoro-furans 29 directly in poor yield.37

Genet and co-workers described38 the cycloisomerization of various homopropargylic diols, catalyzed by AuCl or AuCl3 leading to the strained dioxabicyclo[2.2.1], -[2.2.2], or -[3.2.1] ketals. The cyclization was attempted with bis-homopropargylic diols 31 in the presence of 2 mol% of AuCl or AuCl3 in MeOH at room temperature to afford the corresponding bicyclic ketals 33 regioselectively (Scheme 10). In these reactions the styrene-like olefin in the other side chain, which is equidistant from the hydroxyl groups, does not compete with the alkyne. The reaction may be initiated by the formation of the π-alkynyl complex 32, followed by two intramolecular additions of alcohols leading to the bicyclic ketals 33.

The addition of AgOTf to the Au catalyst in 1,2-dichloroethane, led to the participitation of the tethered C=C bond and traces of water to afford a triol.39 Similarly, Liu et al. have carried40 out the cyclization of 4-nonyne-1,9-diol and its monoprotected form by the Au-catalyzed cyclization conditions. Mixture of [5,5]- and [4,6]-spiroketal resulting from the 6-exo-dig and 7-endo-dig cyclizations, respectively, were obtained. The yield of the products and the ratio of the two isomers depend upon the nature of the catalyst. Intramolecular hydroxylation/cyclization of alkynyl and allenyl alcohols mediated by homogeneous lanthanide catalyst to give cyclic ethers 35 was reported41 (Scheme 11). It was observed that homoleptic lanthanide amides, La[N(SiMe3)2]3 (5 mol%) is an effective precatalyst for the hydroxylation/cyclization of alkynyl and allenyl alcohols than the other lanthanide amide complexes, Ln = Nd, Sm, Y, and Lu.

PdI2-catalyzed oxidative aminocarbonylation of the triple bond of (Z)-2-en-4-yn-1-ols 36 may initially give the corresponding 2-ynamide intermediates 38, which then undergo intramolecular conjugate addition to give 2-(5H-furan-ylidene)acetamide derivatives 39.42 The intermediates 39 may finally undergo aromatization to afford the furanacetamide derivatives 37 (Scheme 12).

Hashmi et al. reported43 that gold-catalyst is more effective in the cyclization of 2-methylpent-2-en-4-yn-1-ol to furans. Liu et al. synthesized44a a variety of highly substituted furans and 5-ylidene-2,5-dihydrofurans from (Z)-enynols containing a secondary and tertiary alcoholic group, respectively, based on gold-catalyzed cyclization. Treatment of (Z)-enynols 40 having a secondary alcoholic group with 1 mol% of AuCl3 in DCM or with cationic gold(I) complex, (PPh3)AuCl/AgOTf in THF for 3 h, afforded cycloisomerized products furans 41 in high yields (Scheme 13). However, when the same cyclization of the tertiary alcohols 42 was carried out at room temperature, stereoisomerically pure compounds (Z)-5-ylidene-2,5-dihydrofurans 43 were obtained (Scheme 13).44b

The 3,3,3-trifluoroprop-1-en-2-yl substituted furans 45 were efficiently synthesized by PdCl2(CH3CN)2 catalyzed cyclization-isomerization of 1,1,1-trifluoro-2-[tert-butyldimethylsilyloxy)methyl]-3-alkynylbut-2-en-1-ols 44 (Scheme 14).45 The catalysts like Pd(OAc)2, Pd(PPh3)4, Pd2(dba)3 and PdCl2(PPh3)2 were found to be inactive under the reaction conditions. The furan ring is assumed to form via a 5-endo-dig ring closure of the hydroxyl group to activated alkyne.

Different approachs, such as palladium-catalyzed isomerization of 1,4-alkyne diols to respective 1,4-dicarbonyl compounds was reported by Lu et al. In the presence of an acid resin, in situ ring closure of the diketones gave the corresponding furan derivatives.46 Williams et al. synthesized47 a range of 2,5-disubstituted furans 48 from 1,4-alkynediols 46 based on ruthenium catalyzed isomerization to diketones 47, followed by in situ conversion into the corresponding furans 48 by acid-catalyzed dehydration (Scheme 15). Alkyl/alkyl disubstituted alkynediols and aryl/alkyl disubstituted alkyne diols were converted into the required furan derivatives along with a small amount of unreacted diketones 47.

o-Alkynylphenols 49 underwent regioselective 5-endo cyclization with 4 mol% of Ir(III)-catalyst to give the corresponding benzofurans 50.48 The mechanism involves electrophilic activation of the alkyne towards the nucleophilic attack by binding to the Ir(III) center, followed by direct selective protonolysis of the Ir-C bond to give the desired product (Scheme 16).

Similarly, Li et al.49 and Liao et al.50 independantly carried out the palladium-catalyzed oxycyclization of 2-alkylphenols to 2-substituted benzo(b)furans and benzo(b)furans-3-carboxylic acids, respectively. Liang et al. reported51 a novel route to disubstituted benzodifurans in moderate to good yields by Pd(OAc)2-catalyzed double cyclization of the dihydroxy-bis(alkyl-substituted 1-alkynyl)benzenes. Cacchi et al. extended the alkyne cyclization chemistry to the preparation of a variety of lipophilic 2-substituted and 2,3-disubstitutedbenzo[b]furans 55 from cardanol 53 by palladium-catalyzed Sonogashira cross coupling/cyclization (Scheme 17).52

Venkataraman showed53 that 2-iodophenols coupled with phenylacetylenes underwent cyclization to give
2-phenylbenzo[b]furans in excellent yields in the presence of [Cu(phen)(PPh3)2]NO3 as catalyst, Cs2CO3 as base in refluxing toluene. Subsequently, Zhang et al. reported54 that NaAuCl4 catalyzed cyclization of (E)-3-benzylidene-1,1,1-trifluoro-5-phenylpent-4-yn-2-ol gave an inseparable mixture of products. However, Pale et al. reported55 the synthesis of 3-hydroxybenzofuran 57 by the AuCl catalyzed cyclization of 2-(1-hydroxy-3-phenylprop-2-ynyl) phenol 56 (Scheme 18).

A novel methodology for the synthesis of benzofurans and indole derivatives starting from ortho-substituted aryl diynes has been reported.56 The product of the reaction depends upon the Cu-catalyst used in the reaction. For example, treatment of compound 58 by employing copper salt, Cu(OAc)2.H2O in pyridine/MeOH at room temperature gave bis-(1H)-benzofuran 60, with the two benzofuran rings linked

to the positions 2,2’-by two triple bonds. However, when the reaction was performed with CuCl in DMF at 70 oC led to the tetrayne derivative 59 and with TBAF in THF at 50 oC, gave 2-ethylnylbenzofuran 61 (Scheme 19). However, a different selectivity of the copper salts was observed in a similar reaction for the synthesis of indole derivatives. Activation of alkynes by coordination of electrophilic transition-metal complexes has led to the development of a variety of catalytic cyclizations involving a carbon-carbon or carbon-heteroatom bond formation.57 Recently, Echevarren and co-workers58 investigated the late-transition metal-based Lewis acid catalyzed reaction for hydroxy- or alkoxy cyclization of enynes. Larock et al.59 used AuCl3 and Yamamoto et al.60 used CuBr as catalyst, respectively to the synthesis of substituted furans by sequential nucleophilic domino attacks onto a metal-complexed alkyne. Liang et al. used Bu4N[AuCl4] as catalyst for the same cyclization in ionic liquid, namely [bmim]BF4.61
In a related reactions, Oh et al.62 observed that Pt complexes were also highly effective catalysts for the hydroxy- or alkoxy cyclization of 2-(1-alkynyl)-2-alkene-1-ones. The Pt(0) complexes like Pt(PPh3)4 and Pt(C2H4)(PPh3)2 were ineffective for the transformation. However, treatment of compound 62 with 5 mol% of Pt(II) catalyst in MeOH afforded the corresponding methoxy-incorporated furan derivative 63 in high yield (Scheme 20).

Subsequently, Schmalz and Zhang have reported63 the synthesis of a furan derivative 65 from cyclopropyl alkynyl ketone 64, by Ph3PAuOTf-catalyzed ring-opening of the cyclopropane ring by nucleophilic attack of MeOH (Scheme 21).

Similarly, Liu et al. have prepared64 cyclic acetals through a two-step reaction: an Au-catalyzed cycloisomerization followed by acidic treatment of the resulting product with MeOH/HC(OMe)3. Nishizawa et al.65 utilized Hg(OTf)2 as catalyst for the cycloisomerization of alkynyl-1,3-cyclohexanedione and cyclopentanedione to the corresponding oxygen heterocycle (Scheme 22). Gosselin et al. also carried out similar cyclization of terminal propargylic ketones to furan derivatives in the presence of Hg(OTf)2.TMU complex.66

Oxidative cyclization-carbonylation of 2-alkyl-2-propargylcyclohexane-1,3-diones 68 mediated by Pd(CF3CO2)2 in the presence of ligand 2,2’-isopropylidenebis[(4R)-4-(3,4-dimethoxyphenyl)-2-oxazoline], L-4 afforded cis-fused bicyclic-β-alkoxyacrylates, hydrindanes 69 (Scheme 23).67

A number of cyclic 6-(phenyl)furo[2,3-d]pyridin-2(3H)-ones 71 were synthesized by AgNO3-catalyzed oxycyclization of 5-alkynyluracil 70 (Scheme 24). Electron-rich alkynes were cyclized more rapidly than electron-deficient alkynes.68 By applying similar methodology Eycken et al. reported69 the preparation of trisubstituted furo[2,3-b]pyrazines 73.

Zhang et al. developed70 a [PdCl2(MeCN)2]-catalyzed three-component Michael addition/cyclization/cross-coupling reaction of 2-(1-alkynyl)-2-alken-1-ones 74 with various nucleophiles and allyl chloride, to provide an efficient route to functionalized tetrasubstituted furans 76 (Scheme 25). For substituted allyl chloride, coupling occurs exclusively at the less substituted terminus of the allyl chloride. The catalyst [PdCl2(MeCN)2] acts as a Lewis acid and transition metal (dual character) in this transformation for the activation of the carbonyl and alkyne, respectively of 74. The alkynes bearing aryl groups afforded relatively higher yields than those bearing alkyl groups.

Forsyth et al. reported71 gold(I)-catalyzed bis-spiroketalization towards the synthesis of trioxadispiroketal containing A-D rings of azaspiroacid, in which one of the key steps was AuCl catalyzed oxycyclization. The enyne 79 was converted into the desired bis-spiroketal 83 using AuCl and PPTS. This process may proceed through an initial syn addition of the C-6 hydroxy group and π-coordinated alkynes-Au(I) complex across the alkynes to generate A-ring enol ether 81, followed by protodeauration in the presence of an acid, and cyclization of the resultant enol ether at C-11 would attract the C-13 methoxy oxygen atom to add to C-10 and close the B ring. Methyl group of the solvent molecule would quench the B-ring oxonium species and gave the desired azaspiroacid 83 (Scheme 26).

This method provided thermodynamically favoured establishment of both the newly formed spiroketal centers. The cyclization of 6-(1-alkoxyethyl)hex-2-ynoates 84 in the presence of platinum-olefin catalyst system gave the corresponding multisubstituted 2-(dihydrofuran-2(3H)-ylidene)acetates 85 and 86 as a mixture of two isomers (Scheme 27).72 The stereoselectivity can be controlled by switching the electronic property of the ester group. For example, the reaction of 2,2,2-trichloroethyl ester (R = CH2CCl3) in the presence of PtCl2 (10 mol%) and 1,5-hexadiene in toluene at 35 oC gave exclusively (Z)-85 in 84% yield,

derived from trans-addition of the acetal C-O bond to the alkynyl moiety, while substrates having a relatively electron-rich ester, particularly phenyl ester (R = Ph) gave E isomers, from cis addition of the acetal C-O bond.
Metal-catalyzed cycloalkoxylation of epoxy alkynes is an important route for the synthesis of substituted furans. Hashmi et al. synthesized73a number of furan derivatives from α-epoxy alkynes in the presence of AuCl3 catalyst. Recently, Liang et al. reported74 the cycloisomerization/alkoxylation of alkynyloxiranes 87, using a Au(I) or Au(III) catalysts and alcohols (Scheme 28).

The cyclization may proceed by the addition of alcohol to the intermediate 89 (path a) or by simultaneous addition of alcohol and cyclization (path b), to give the gold intermediate 91. The protodeauration followed by elimination of AcOH gave the 2,5-disubstituted furan 88 (Scheme 29).

SmI2- catalyzed reduction of alkynyloxiranes into cumullenes followed by Pd-catalyzed cyclization is an expeditious entry into the functionalized furans. Recently, an efficient Pd(II)-catalyzed heterocyclization-coupling reaction to the synthesis of polysubstituted furans having an vinylic group at C-3 position starting from buta-1,2,3-trienyl carbinols 92 and electron-deficient alkenes 93 has been reported (Scheme 30).75 The formation of either Heck- or conjugated-addition type products is in part dependent upon the choice of electron-withdrawing group on the alkene. Activating groups such as ester, amide, nitrile, and sulfone cause the reaction to follow exclusively the Heck pathway, whereas the coupling with methyl vinyl ketone 95 affords selectively either Heck- or hydroarylation-type products depending on the reaction conditions (Scheme 30).

3. 2. 2 INTERMOLECULAR CYCLIZATION OF ALKYNE
Zhou et al. reported76 Yb(OTf)3-catalyzed coupling reaction of 1,3-dicarbonyl compounds with propargylic alcohols. Selective propargylation or allenylation products were obtained depending on the nature of the propargylic alcohols. By applying this reaction as the key step, multi-substituted furocoumarin 100 was synthesized by the treatment of 4-hydroxycoumarin 98 with propargylic alcohol 99

in the presence of 5 mol% Yb(OTf)3 (Scheme 31).

Fluoropropargyl chloride reacted with carbonyl compounds to give propargylic fluorohydrins at room temperature, but when higher temperature and longer reaction time was employed, the same starting materials gave 2,5-disubstituted furans 103 (Scheme 32).77

A plausible mechanism for the above conversion suggests that fluoropropargyl chloride 101 initially reacts with zinc and aldehydes 102 to give intermediates 104, which undergo a stepwise addition-elimination or a concerted SN2’ displacement, to give the corresponding furan 103, through an allene intermediate 106. An alternative mechanism, in which a vicinal SN2 attack by intermediate 104 yield a propargyl epoxides 107, which then isomerize to the corresponding furan derivatives(Scheme 33).

Synthesis of tetrasubstituted furans via InCl3 catalyzed propargylation of 1,3-dicarbonyl compounds was developed by Xiang et al.78 The reaction of propargyl alcohols 108 with three equivalent of 1,3-diketone or ethyl acetoacetate (109) catalyzed by 10 mol% of InCl3 gave tetrasubstituted furan derivatives 110 in high yields (Scheme 34). The yields of furans are lower with ethyl acetoacetate than that with acetylacetone.

A tentative mechanism for the reaction is outlined in Scheme 35. Initially, InCl3 coordinates with the oxygen atom of the propargyl alcohol to generate a carbocation, which is subsequently trapped by the dicarbonyl compounds to form the substituted product 112. InCl3 may again coordinate with the triple bond of 112, followed by the nucleophilic attack at the activated triple bond by the lone-pair electron of the carbonyl group to generate 114. The carbon-indium bond was broken down and the desired furan product was obtained through a series of proton addition/elimination, double bond isomerization processes.

3. 2. 3 TANDEM CYCLIZATION OF ALCOHOLIC OXYGEN TO ENYNES
The catalytic metal-mediated intramolecular cyclization of precursors containing enyne and different oxygenated nucleophiles can be exploited for the construction of a wide variety of oxygen-containing heterocycles. Zhang et al. observed79 that the treatment of a 1,5-enyne 116 armed with a primary alcohol, in the presence of a catalytic amount of AuCl3 triggered the double cyclization to produce intermediate 117, which underwent subsequent protonolysis to afford the oxabicyclic products 118 (Scheme 36). According to them the reaction involved a 6-endo-dig cyclization of the enyne with a concomitant intramolecular formation of the C-O bond. However, the structure of the product depended upon both the geometry of the alkene moiety and the length of the hydroxylated tether. Enynes 119 (E-alkenes) afforded the trans-oxabicyclic products 120, whereas enynes 119 (Z-alkenes) produced the cis-oxabicyclic products 120 (Scheme 36). The tosyl amine moiety behaved similarly in the presence of the [Au(PPh3)]ClO4 catalyst.

The proposed mechanism of the AuCl3-catalyzed double cyclization of (E)-4,6,6-trimethyl-8-phenyloct-3-en-7-yn-1-ol (119, n = 1) involves a concerted process from 121A or the ring opening of cyclopropyl gold carbene 121B (Scheme 37).

Similarly, the double cyclization of 1,6-enynes armed with secondary alcohol has been reported to undergo Ph3PAuCl and AgSbF6 catalyzed formal 5-exo-trig addition of the alcohol to the double bond followed by a 5-exo-dig addition to the triple bond.80 In a related study Cossy et al. reported81 a diastereoselective Au(I)-catalyzed cycloisomerization of ene-ynamides bearing a propargylic alcohol moiety, leading to 2-azabicyclo[3.1.0]hexane framework and Toste et al. reported82 a stereoselective synthesis of substituted 5,6- and 6,6-spiroketal from 1,5-enynes bearing a hydroxyalkyl chain, catalyzed by Au-complex.

3. 3 SYNTHESIS FROM ALLENES
The transition metal-catalyzed cyclization reaction of functionalized allenes has attracted the attention of many synthetic chemists due to their unique reactivity and stereoselectivity.83 Metal catalyzed addition of heteroatom nucleophiles to allenes followed by trapping of the intermediate alkenyl metal intermediate by proton or any electrophilic species have found extensive applications in the synthesis of heterocycles.84
Electrophile-induced 5-endo-trig cyclization of α-allenyl alcohols constitute a well-known and elegant route to synthetically useful 2,5-dihydrofurans. Gold- salts have been found to be very efficient catalyst for this purpose.85 Reissig et al. synthesized 3-alkoxy-2,5-dihydrofuran 124 (Scheme 38) by AuCl catalyzed 5-endo oxycyclization of α-hydroxy allene (sec-alcohol).86 Hashmi et al. reported87 the formation of various side products, when α-hydroxy allenes were reacted with AuCl3 catalyst in MeCN. However, substrates 125 afforded 2,5-dihydrofurans 126 in high yields with Ph3PAuNTf2 in DCM (Scheme 39). On the other hand, Huang and Zhang obtained88 a substituted bicycle[4.3.0]nonene as the main product instead of dihydrofuran, from the Au(III)-catalyzed cycloisomerization of 1-cyclohexenyl-2-(methoxymethoxy)buta-2,3-dien-1-ol.

Gold(I) or gold(III)-catalyzed cycloisomerization of α-and β-hydroxyallenes bearing alkyl substituents at

the allene takes place with complete transfer of chirality from the axis to the newly formed stereogenic center in the product.89 For example, the synthesis of furan derivatives 128 with complete chirality transfer was observed with the catalytic system Ph3PAuNTf2 and AgBF4 in DCM at room temperature (Scheme 40).90
Krause et al. observed91 that gold(I) and gold(III) salts are able to epimerize aryl-substituted hydroxyallenes, thereby causing a diminished stereochemical purity of the product. However, this problem was overcome by the addition of a sigma-donar ligand such as 2,2’-bipyridine as an additive or by using a moderately coordinating solvent such as THF, which reduces the Lewis acidity of the gold catalyst (Scheme 41).

Recently, the same group also reported the synthesis of bicyclic furanomycin derivative 133 as a mixture of two diastereomers using AuCl3 catalyzed cycloisomerization of α-hydroxyallenes as the key step.92 The cycloisomerization of 131 in the presence of 1 mol% of AuCl3 in THF proceeded smoothly and gave 89% yield of the bicyclic dihydrofuran 132, from which after several steps the bicyclic furanomycin derivative 133 was obtained as a mixture of diastereomers (Scheme 42).

Subsequently, an excellent stereoselectivity without loss of optical purity and the participation of a second hydroxyl group in the β-position have also been reported by Krause et al. (Scheme 43)93

Widenhoefer et al. obtained a 1.3:1 mixture of tetrahydrofuran and dihydropyran derivatives from toluene solution of 2,2-diphenyl-4,5-hexadien-1-ol 136 containing a catalytic 1:1 mixture of [Au{P(tBu)2(o-biphenyl)}Cl] and AgOTf. Switching from AgOTf to AgOTs, to form the highly active cationic Au(I) catalyst, gave almost exo-hydroalkoxylation products, 2-vinyltetrahydrofurans 137 in excellent yields via the transfer of chirality from the allenyl moiety to the newly formed stereogenic tetrahedral C-atom. (Scheme 44)94

Recently, Zhang et al.95 demonstrated that exo-hydroalkoxylation of γ- and δ-hydroxyallenes occurred with high enantioselectivity when a cationic gold(I) catalyst generated from 1:2 mixture of [Au2(S)-ACl2] (2.5 mol%) and AgOTs. Enantioselective hydroalkoxylation of (R)-139 (94% ee) led to the isolation of (Z)-140 in 88% yield with greater than 95% ee and greater than 20:1 diastereoselectivity (Scheme 45). Subsequently, Toste et al.96 carried out the enantioselective hydroxylation of terminal disubstituted allenes in high yields and up to 99% ee, by using a mixture of Au(I) complex [AuCl)2dppm] and the chiral silver phosphate ligand L-5 (Scheme 46).

The transition metal-catalyzed cyclizative dimerization of allenes is attractive to the organic chemists for the enantioselective synthesis of furan derivatives with chirality transfer.97 The Pd-catalyzed first homodimerization reaction of 1,2-allenyl ketones has been reported by Hashmi et al. for the synthesis of 3-(3’-oxo-1’-alkenyl)substituted furan derivatives.98 The AuCl3-catalyzed reaction of 1,2-allenyl ketones and 2,3-allenols also provide substituted furan derivatives.99 Recently, a PdCl2/NaI catalyzed homodimeric coupling-cyclization of 2,3-allenols 143 has been reported by Ma et al.100 to provided an efficient route to the stereoselective synthesis of 4-(1’,3’-dien-2’-yl)-2,5-dihydrofuran derivatives 144. Optically active 2,3-allenols 145 afforded optically active furans 146 and 147 as a mixture of isomers in good yields (Scheme 47). An efficient metal-controlled regiodivergent preparation of tetrahydrofurans and tetrahydrooxepines101 was recently reported from the enantiopure γ-allenols substituted in the α-position by a protected hydroxyl moiety. The TBS protecting group was inert under experimental conditions, whereas MOM group underwent cleavage, and controls the regioselectivity of the reaction.

Reactivity of AuCl3 catalyst depends upon the presence of the MOM protecting group at the γ-allenol oxygen atom. The free γ-allenols 148 having a MOM-protected OH group at the α-position, gave 5-endo hydroalkoxylation products 149, whereas γ-allenols 148 having a TBS protected OH group at the α-position gave 5-exo-product 150 and MOM-protected γ-allenols 151 exclusively underwent a 7-endo oxycyclization. However, cyclizative coupling reaction of γ-allenols 148 with allyl halide in the presence of PdCl2 gave the seven-membered adducts 153, exclusively, via 7-endo oxycyclization (Scheme 48). Thus, it seems that a (mothoxymethyl)oxy protecting group not only masks the hydroxyl functionality, but also exerts directing influence as a controlling unit in regioselectivity reversal.

Mechanistically, it is assumed that the initially formed allene-gold complex 154 undergoes an intramolecular attack by the (methoxymethyl)oxy group via a 7-endo mode to give intermediate 155. Protonolysis of the carbon-gold bond followed by elimination of methoxymethanol may give the bicyclic compounds 150. The 5-exo oxycyclization via 156 is restricted by the steric hinderance between the (methoxymethyl)oxy group and the substituent at the quaternary stereocenter (Scheme 49). Similarly, PdCl2-catalyzed heterocyclization/cross-coupling reaction of two different α-allenols gave 2,3,4-trifunctionalized 2,5-dihydrofurans, regioselectively. The α-allenols 157 reacted smoothly with the protected α-allenols 158 to give the desired optically active dihydrofuran derivatives 159 (Scheme 50).102

Gevorgyan group, recently, reported the gold-catalyzed regiodivergent cycloisomerization of bromoallenyl ketones 160 into isomeric bromofurans 161 and 162 via 1,2-migration.103 Initially, it was thought that more oxophilic Au(III) catalyst coordinates to the carbonyl oxygen 163, which, via the halirenium intermediate 164 produces the 1,2-Br migration product, 3-bromofuran 161. Alternatively, the more π-philic Au(I)(PR’3)Cl- (R’ = H, Me)-catalyst may coordinate to the distal double bond of the allene

to form 165, from which the gold carbenoid intermediate 166 may be formed. Subsequent 1,2-hydride shift may give 2-bromofurans 162 (Scheme 51). Recently, the mechanism of Au-catalyzed cycloisomerization of bromoallenyl ketones was studied by DFT calculations.104 Both Au(I) and Au(III) catalysts activate the distal double bond of the allene to produce cyclic zwitterionic intermediates, which undergo a kinetically favoured 1,2-migration. However, in the cases of Au(PR’3)L (L = Cl, OTf) catalysts, the counterion-assisted H-shift is the major process, indicating that the regioselectivity of the Au-catalyzed 1,2-H vs 1,2- Br migration is ligand dependent.
Che et al. used gold(III) porphyrin complex, [Au(TPP)]Cl as a resonable catalyst for the cycloisomerization of allenones 167 into the corresponding furans 168 in good to excellent yields (Scheme 52).105 Mechanistically, the gold catalyst [Au(TPP)]+ reversibly binds to the C=C=C moiety which facilitates the nucleophilic attack of the carbonyl oxygen at the terminal of the allene carbon. The furyl-gold intermediate undergoes acid-catalyzed demetalation to give the furan products.

3. 4 INTRAMOLECULAR CYCLIZATION OF HALOAROMATIC COMPOUNDS
A more sustainable protocol to give 2-alkyl- or 2-aryl substituted benzo[b]furans was reported involving a copper-TMEDA complex which catalyzed the transformation of readily available ketone derivatives 169 into the corresponding benzofurans 171 (Scheme 53).106 The reaction is proceeded smoothly in water without any organic co-solvents. The TMEDA act as a ligand and it may also coordinate with the copper to facilitate the formation of enol-type intermediate through coordination to the oxygen as shown in Scheme 53. Similar reaction was also applied by Chen et al. to synthesize a variety of benzo[b]furans via CuI-catalyzed ring closure107 of 2-haloaromatic ketones. The methodology is tolerant to various functional groups, affording benzo[b]furans in 72-99% yields. However, Pd(II)-catalyst in similar reactions yielded products in lower yields.108

Recently, an efficient methodology for the indirect anti-Markovnikov hydration of unsymmetrically substituted terminal and internal alkynes to 2-haloaromatic ketones was developed based on TiCl4-catalyzed hydroamination reaction. It’s application to ortho-alkynylhaloarenes, followed by a copper iodide catalyzed O-arylation, provides flexible access to substituted benzo[b]furans.109
Willis et al. demonstrated110 the effectiveness of the combination of KOH and CuI/TMEDA for the synthesis of benzofurans 215 from aryl bromide-alkenyl triflates 214 (Scheme 54). The yield of the products was reduced on decreasing the amount of TMEDA, or lowering the reaction temperature. The reaction may proceed via the hydrolysis of the alkenyl triflates to generate enolates, which may then get converted to the benzofurans under the action of Cu(I).

3. 5 SYNTHESIS FROM DIAZO COMPOUNDS
Metal carbenes are easily obtained from diazo compounds in the presence of copper and rhodium complexes.111 Stabilized diazo compounds, particularly α-diazo ketones or esters, are suitable precursors for the metal carbenes that exhibit electrophilic properties at the carbon center, which allow them to undergo attack of the nucleophiles with eventual release of the metal. Lee et al. applied this strategy112 to synthesize a number of dihydrofurans with exo-olefin and furans from diazocarbonyl compounds and allyl halides catalyzed by Rh2(OAc)4 (1 mol%).
Fu et al. reported the stereoselective synthesis of 2,3-dihydrofurans by copper-catalyzed [4+1] cycloaddition reaction of enones and diazo compounds. Treatment of α,β-unsaturated ketones 174, and 2,6-diisopropylphenyl diazoacetate 175 in the presence of 1.0 mol% CuOTf and 1.3 mol% planar-chiral 2,2’-bipyridine (-bpy) ligand (L-5) in DCM gave the 2,3-dihydrofuran derivatives 176 (Scheme 55).113 The enantiomeric excess (ee) is highest (92%) when the enone substitutents (R and R1) are unsaturated. Similarly, Liang et al. synthesized114 highly substituted furans 179 by CuI catalyzed [4+1] cycloaddition reaction of α,β-acetylenic ketones 177 with α-diazo esters 178 in dry DCE at 90 oC (Scheme 55).

3. 6 SYNTHESIS VIA CYCLOPROPANE RING OPENING
2-Methylenecyclopropanyl ketones 180 owing to the presence of the exo-cyclic C=C bond and the strained cyclopropane ring, underwent highly selective ring-opening cycloisomerization with PdCl2(CH3CN)2 catalyst in the presence of NaI to afford substituted furan derivatives 181 (Scheme 56).115 Interestingly, the reaction of the cyclopropyl ketones in the absence of NaI afforded substituted pyrans in good yields. The formation of pyrans or furans presumably proceed through a highly regioselective cleavage of a carbon-carbon single bond in the cyclopropane ring, triggered by regioselective halometalation of the C=C bond and β-decarbopalladation, halogen anion attack on the unsubstituted carbon atom of the cyclopropane ring, or the direct oxidative addition of the distal C-C single bond of the cyclopropane ring with Pd(0).

Subsequently, Johnson et al. demonstrated Ni(0)-catalyzed rearrangement of 1-acyl-2-vinylcyclopropanes 182 to dihydrofurans 183 in high yields.116 The vinyl cyclopropyl ketones 182, having geminal electron- withdrawing substitution, on treatment with Ni(COD)2 (2 mol%) and 2,2’-bipyridyl as ligand (2.2 mol%) in CH3CN gave dihydrofurans 183 (Scheme 57). Use of triphenylphosphine as ligand, reduced the catalyst loading to 1 mol% and use of (PPh3)Ni(COD) gave incomplete conversion of the starting materials. Lewis acid Cu(OTf)2 also facilitated the complete conversion of the cyclopropane at room temperature with catalyst loading 10 mol%. The overall reaction proceeds with the retention of configuration at the vinyl bearing stereogenic center.

A new strategy for the synthesis of furo[2,3-b]quinoline derivatives 189 through SnCl4 mediated tandem ring-opening/recyclization reaction of the doubly activated cyclopropanes of readily available 1-acyl N-aryl cyclopropylcarboxamides 184 was reported by Liu et al.117 The overall transformation may involve the SnCl4.5H2O initiated opening of the cyclopropane ring of 184 followed by oxycyclization to form the dihydrofuran intermediate 186. Furoquinolines 189 may then be generated through Combes-type annulation reactions (Scheme 58).118

3. 7 SYNTHESIS OF FURANONES
Transition metal-catalyzed 5-endo heterocyclization of alkynyl alcohol through the activation of triple bond is an important method for the synthesis of 3-(2H)-furanones.119,120 Recently, Liu et al. demonstrated44b the oxidative cleavage of triple bonds in (Z)-enynols by AuCl(PPh3)/AgOTf (2 mol%) in the presence of oxygen to butenolides. Initially the intermediate of (Z)-5-ylidene-2,5-dihydrofurans is formed via 5-exo-dig oxycyclization, which is oxidized by gold-catalyst in the presence of oxygen into the corresponding butenolides in 70-97% yields. Xiao et al. carried out the reaction between 2-methylbut-3-yn-2-ol 190 and thiophenol in the presence of Pd(OAc)2 (3 mol%) to give thiolactonization product 191 as major one along with mono- and dithiocarboxylation product (Scheme 59).121

PdI2-catalyzed oxidative aminocarbonylation of the terminal alkynes is a facile route for the synthesis of five-membered oxygen and nitrogen heterocycles.122 Propargyl alcohol 192 when subjected to PdI2, carbon monoxide and oxygen in the presence of a secondary amine afforded 4-dialkylamino-5H-furan-2-ones 193 (Scheme 60) by a sequential oxidative aminocarbonylation-intramolecular conjugate addition-cyclization route.

The intramolecular addition of carboxylic acids to alkynes gives lactones. Perfect regioselectivity for the Au-catalyzed cyclization step in favor of the 5-exo isomer was observed for terminal alkynes, whereas a mixture of 5-exo and 6-endo products resulted from the cyclization of internal alkynes. For example, terminal acetylenic acids 194 (R = H) undergo selective exo-cyclization in the presence of gold(I)-catalyst, AuCl to give a number of functionalized γ-lactones 195 (R = H).123 However, the reaction of internal alkynes 194 afforded selectively (Z)-γ-lactones 195 (Scheme 61), which indicate an intramolecular addition of the carboxylic acid to the Au-alkyne intermediate, resulting from an initial activation of the triple bond by Au(I)-catalyst. Even in the presence of a styrene chain, no competition of the activated alkene with the alkyne was observed. More recently, Michelet et al. also observed124 5-exo mode of cyclization with Z-stereoselectivity resulting from anti aeration using Au2O3 catalyst.

Hg(OTf)2 tetramethylurea (TMU)-catalyzed cyclization of alkynoic acid 196 to ene-γ-lactone 197 has been reported (Scheme 62). The alkynoic acid residue has been used as the leaving group for Hg(OTf)2-catalyzed glycosylation via SN1 reaction mechanism.

Access to 2-(5H)-furanone skeleton could also be accomplished through palladium-mediated cyclization-carbonylation125 or coupling-cyclization126 reaction of propargyl esters. Palladium–catalyzed reaction of unsaturated triflates and halides with methyl-4-hydroxy-2-butenoate and tetrahydropyranyl derivatives, afforded 4-aryl- and 4-vinyl-2-(5H)-furanones through in situ vinylic substitution/anulation sequence.127
Recently, 4-substituted-2(5H)-furanones were synthesized by Arcadi et al. through a sequential regioselective rhodium-catalyzed addition/lactonization reaction of organoboron derivatives to γ-hydroxy-α,β-acetylenic esters.128 The reaction was carried out with Rh(acac)(C2H2)/dppf or [Rh(cod)OH]2/dppb

as catalytic system in dioxane/H2O (10/1) at 100 oC using following molar ratios 198:199:[Rh(acac)(C2H2)]:dppf (A) = 1:5:0.03:0.066 or [Rh(cod)OH]2:dppb (B) = 1:2.0:0.03:0.06 (Scheme 63). Rh-catalyzed reaction of alkyl 4-hydroxy-2-alkynoates bearing a tertiary propargyl alcohol group resulted in reversal of the regioselectivity compared to that of palladium-catalyzed process. However, the regioselectivity of the secondary propargylic alcohol is not affected by the bulkiness of groups close to the C-C triple bond.
The optically active γ-hydroxy-α,β-acetylenic esters 201 undergo regiospecific hydration in the presence of Zeise’s dimer, [PtCl2(C2H2)]2, to produce the optically active tetronic acid derivatives 203.129 The observed regiospecific hydration is explained by considering intermediate 202 (Scheme 64). Electron withdrawing effect of the ester group, Lewis acidity of Pt(II) center and the chelating effect of the acetylenic ester to Pt induce attack of water at β-position.

Furanones can also be obtained from alkynones having a hydroxyl group at the α-position.130 This tandem reaction consisting of heterocyclization and 1,2-migration is believed to proceed via a acyclic oxonium ion intermediate 207. For example, treatment of the alkynes 204, with the catalyst, AuCl3 or PtCl2 underwent a domino heterocyclization and subsequent 1,2-alkyl shift to give spirocyclic compounds, 3-(2H)-furanone derivatives 205 or 206, respectively (Scheme 65).131 When 5 mol% of AuCl3 was used as the catalyst, spirocyclic compounds 205 were obtained in high yields, but R was restricted to aryl substituent only. The use of Au(I) catalysts such as Ph3PAuBF4 and Ph3PAuCl mainly caused decomposition. However, PtCl2-catalyzed reaction proceeded soomthly with contraction of the ring size.

Recently, there has been a report132 of a novel electrophile-induced tandem cyclization/1,2-migration reaction of 2-alkynyl-2-siloxy carbonyl compounds 208 to fully substituted 3(2H)-furanones 209 in excellent yields containing an iodo-substituent at C-4 position, catalyzed by AuCl3 (5 mol%) in DCM at room temperature (Scheme 66). However, without AuCl3, a variety of trimethylsilyl ethers were also effectively converted into the corresponding spiro 4-iodo-3-furanones but took longer time for cyclization. The reaction of substrates with R1 = R2 = Et, failed to give furanones by method A, but by method B these cyclized giving low yields (27% and 43% only).
Gouverneur et al. devised a palladium(II)-catalyzed Wacker-Heck reaction involving the union of structurally diverse hydroxyynone and ethyl acrylate to the synthesis of 4(2H)-pyranones.133 In addition, the same author also applied the domino cyclization reaction to α-hydroxyenones. Tetrasubstituted furanones 213 were prepared under the optimized reaction condition in isolated yields ranging from 52-65% (Scheme 67). Additionally, Liu et al. have succeeded134 in achieving a number of substituted 3(2H)-furanones 216 from 2-oxo-3-butynoic esters or disubstituted-1,2-diones 215 via gold-catalyzed cyclization and nucleophilic addition sequence.

The reaction proceeded smoothly with 2 mol% AuCl3 catalyst in DCM at room temperature in the presence of nucleophiles to give the cyclized products in moderate to excellent yields (Scheme 68).

Ma et al. reported CuCl-catalyzed oxycyclization of optically active 2,3-allenoic acids or 1:1 salts of optically active 2,3-allenoic acids with chiral amine to the corresponding 2(5H)-furanones (Scheme 69).135 It was observed that the reaction of optically active 2,3-allenoic acids 217 in methanol required only catalytic amount of CuCl, whereas the reaction of the 1:1 salts of optically active 2,3-allenoic acids with chiral amine required one equiv. of CuCl and DCM as solvent to ensure high efficiency of the chirality transfer. Shin et al. utilized trans-2,3-allenoic esters for the synthesis of 2-furanones, catalyzed by AuCl3 (Scheme 70).136
The reaction between two same or differently functionalised allenes, i.e, homodimerization reaction of functionalised allenes is of considerable interest. Hashmi et al. first reported the homodimerization reaction of allenyl ketone.137 The intermolecular dimerization of 2,3-allenoic acids using PdCl2 as catalyst afforded bicyclic butenolides.138 Moreover, in the heterodimerization the reactions between 2,3-allenoic acids or 2,3-allenamides and 1,2-allenyl ketones, both allenes were cyclized to form products with two different rings.139 An interesting β-hydroxy elimination to dienyl unit was observed during the palladium

catalyzed cyclization of 2,3-allenoic acids in the presence of 2,3-allenols.140 Alcaide and co-workers also reported similar cross-coupling cyclization reactions of (R)-allenols in the presence of 2,3-allenyl carboxylates.141
Recent investigation of Ma et al. also demonstrated the intermolecular cross coupling reaction between 2,3-allenoic acids 223 and simple allenes 224 in the presenece of Pd(OAc)2, LiBr.H2O, and benzoquinone (BQ) in AcOH to give highly substituted furan-2(5H)-ones, Z-225.142 The reaction of optically active 2,3-allenoic acid 226 in the presence of allene afforded the product Z-228 in high enantiopurity (Scheme 71).

The catalytic cycle leading to furan-2(5H)-one is assumed to proceed via initial cyclic oxypalladation of 2,3-allenoic acid with Pd(II) to generate furanonyl palladium intermediate 229 which is trapped by the allene to generate a π-allylic intermediate anti-231. This is presumably nucleophilically attacked by Br- in anti-231 to yield 4-(1’-bromoalk-2’(Z)-en-2’-yl)furan-2(5H)-one derivatives, Z-225 with the regeneration of Pd(0) by the oxidant BQ (Scheme 72). The exclusive formation of the Z-isomer may be explained by face-selective coordination of the allene 224 with the palladium atom in intermediate 230 to avoid steric congestion.

4. SYNTHESIS OF SIX-MEMBERED OXYGEN HETEROCYCLES
Six-membered oxygenated heterocycles i.e. pyran ring system can be synthesized by intramolecular cyclization, cycloaddition or five-membered ring expansion. In addition to these processes, there is a set of important methodologies based on the cyclization of oxygenated precursors catalyzed by different transition metals that afford pyran rings in a highly efficient and straightforward manner.143

4. 1 SYNTHESIS FROM ALKENES: CYCLIZATION OF ALCOHOLIC- AND PHENOLIC- OXYGEN TO ALKENES
The cyclization of δ-hydroxy-cis-alkenes promoted by Hg(II) salts144 and chiral bisoxazolines as ligands affords 2-substituted tetrahydropyrans in excellent ee.155 The intramolecular oxymercuriation has been applied to the synthesis of the C22-C26 tetrahydropyran of phorboxazole B. The mercury(II)-mediated electrophilic ring opening reaction of hydroxy cyclopropylcarbinol strategy146 has been applied to the construction of the C3-C7 tetrahydropyran ring of zincophorin.147
Toste et al. reported vanadium(v)-oxo complex catalyzed highly diastereo- and enantioselective synthesis of 2,5-trans-tetrahydropyran 236 and 2,4-cis-tetrahydrofuran 237 using sequential resolution/oxidative cyclization of racemic bis- and homoallylic α-hydroxyesters (Scheme 73).148 Both steps in the reaction sequence are catalyzed by vanadium(v)-oxo complex with a readily available tridentate Schiffs base ligand L-6. The reverse selectivity has been explained by assuming the chelation of the ester carbonyl group to the vanadium catalyst during the epoxidation and formation of a transition state in which the sterically more demanding substituent occupies in a position to minimize the steric interaction. This synthetic protocol also provides an enantioselective synthesis of (-)-pantofuranoid E.

Pd-complexes catalyze the cyclization of δ-hydroxy alkenes having an allylic leaving group such as OH, OAc, CO2R, OPO(OR)2, OAr, Cl, Br etc, leading to the pyrans149 via the π-allylpalladium cations followed by intramolecular attack of the hydroxy oxygen. From a stereochemical point of view, substrate-controlled cyclization usually proceeds with the retention of configuration of the reacting centre. Coordination of the palladium to the carbon-carbon double bond occurs on the less hindered side and opposite to the leaving group. In this context, Trost et al. demonstrated that intramolecular asymmetric allylic alkylation of hydroxy alkenes can be highly enantioselective, with the nucleophilic addition of the alcohol to the π-allylic cation being the enantio-determining step of the process. Indeed, treatment of the hydroxy alkene 238 (R = H) with Pd2(dba)3.CHCl3, chiral diphosphine ligand L-7, and Et3N provided the tetrahedropyran 241. Moreover, the chiral starting material 238 (R = CH2OCOPr) with L-7 gave the tetrahydropyran trans-239 or isomer cis-240, exclusively, by switching the configuration of L-7 (Scheme 74).150 High stereochemical control has also been observed in δ-hydroxy alkenes containing other allylic leaving groups.151

Similar to Pd(0)-complexes, Pd(II) complexes also catalyze the intramolecular cyclization of hydroxy alkenes.152 By applying this strategy, Antiosteoporotic diarylheptanoids (-)-diospongins A and B were synthesized stereoselectively. The key steps in the synthesis is the stereospecific PdCl2 catalyzed oxycyclization of chiral 1,5,7-trihydroxy-2-heptenes, 242 and 245, to form cis and trans tetrahydropyran rings 243 and 246, respectively, depending upon the the configuration of the allylic stereocentre (Scheme 75). The regioselective Wacker oxidation of 243 and 246 gave (-)-diospongins A and B, respectively.153 This strategy has also been applied to the constraction of various pyran ring system,154 present in many natural products.

Besides palladium, other metal complexes such as platinum,155 tin,156 cerium,157 silver158 have been successfully used for the cyclization of δ- and γ-hydroxy alkenes, based on the activation of the olefin, followed by a 6-endo cyclization (Scheme 76).

It has been observed that the intermediate, σ-alkylpalladium(II) can be trapped by CO and the resulting acylpalladium species is easily converted into the corresponding methylester. This methodology has been successfully applied to the synthesis of leucascandrolide A and phorboxazole.159 The σ-alkylpalladium

intermediate can also be trapped through a Heck reaction.160 This possibility is illustrated by a palladium-catalyzed sequence synthesis of α-tocopherol. The reaction of phenol 254 with methyl acrylate in the presence of catalytic amounts of Pd(OCOCF3)2, the chiral ligand L-8, and BQ afforded chroman 255 (Scheme 77).
Recently, Gagne et al. disclosed161 a Pt-based catalyst, which catalyzed the highly enantioselective Prins reaction between 2-allylphenol 257 and glyoxylate esters 258 to provide the Prins product 259 (Scheme 78). Enantiomeric excess (ee) of the product depends upon the R group of the ester. However, treatment with Lewis acid gave only Prins reaction product, homoallylic alcohols.

Perumal et al. observed162 that indium trichloride and triphenyl phosphonium perchlorate (TPP) are very effective catalyst for the cyclization of o-hydroxyaldimines with 3,4-dihydro-2H-pyran and 2,3-dihydrofuran, and afforded cis-pyrano and furobenzopyran ring system, respectively. In a related reaction Yadav et al. observed that use of Sc(OTf)2 as a Lewis acid gave trans-fused pyranobenzopyrans, stereoselectively.163
Atom economical sequential C-C/C-O bond formations between phenols and dienes using the reusable catalyst Sc(OTf)3 and an ionic liquid [bmim][PF6] have been developed by Youn for the synthesis of a variety of dihydrobenzopyran and dihydrobenzofuran ring systems in good yields (Scheme 79).164 In this reaction ionic liquid plays an important role as not only an efficient additive but also an immobilizing agent for facilitating the catalysis.

Tanaka et al. recently reported the [Rh(cod)2]BF4/(R)-H8-binap complex catalyzed [2+2+2] cycloaddition of 1,6-enynes with electron-deficient ketones to give fused dihydropyrans containing two quaternary carbon centers with excellent regio-, diastereo-, and enantioselectivity.165 However, the reaction of electron-rich aryl ketones with 1,6-enynes in the presence of the same catalyst gave ortho-functionalized aryl ketones with excellent regio-and enantioselectivity.

4. 2 SYNTHESIS FROM ALKYNES: CYCLIZATION OF ALCOHOLIC-, BENZYLIC-, CARBONYL- AND EPOXY-OXYGEN TO ALKYNES
The transition metal catalyzed isomerization of 4-hydroxy terminal alkynes represent a highly valuable route to 3,4-dihydro-2H-pyrans.166 The reaction proceeds through the vinylidene intermediate that undergoes intramolecular attack by alcohol oxygen.
Tungsten has also attracted much attention. The most common tungsten complex used for the cycloisomerization of 4-alkynols is W(CO)6 which is photochemically activated in situ to generate the true catalytic species, presumably W(CO)5.167 The methodology has been successfully applied to the construction of the trisaccharide component of digitoxin.168
Stereoselective synthesis of D-desosamine diacetate ester was achieved from the glycal which is obtained by tungsten carbonyl catalyzed exclusive 6-endo-cycloisomerisation of the corresponding N-protected amino alkynol. The cyclization reaction proceeds smoothly with both diastereoisomers of the alkynol substrates 263 and 265 in the presence of 5-15 mol% of catalyst to afford the glycal cycloisomerization products 264 and 266, respectively (Scheme 80). 169

Recently, McDonald et al. reported170 a new catalyst system 268 that does not require photochemical activation. The optimal results are obtained with 25 mol% of oxacarbene 268 in the presence of Et3N (10 equiv.) by simple warming at 40 oC in THF (Scheme 81). The methodology has been successfully applied to the synthesis of altromycin disaccharide 270.

Treatment of homopropargylic ether 271 with Ph3PAuCl (5 mol%) and AgSbF6 (5 mol%) in wet DCM generated the enone intermediate which underwent cyclization to 2,6-cis-tetrahydropyran 273 in excellent yield (Scheme 82).171

5-Alkynols with a terminal triple bond undergo complete regioselective 6-exo-dig ring closure to exocyclic enol ethers in the presence of palladium, iridium,11 platinum172 or gold.173 For example, Barluenga et al. examined173 the metal-catalyzed cycloisomerization of 1-en-8-yn-4-ols 274 using alcohol as the solvent (Scheme 83). The use of AuCl3 or PtCl2 catalyst smoothly catalyzes the domino reaction, leading to the bicyclic compounds 275 with incorporation of one molecule of solvent. The transformation has a low efficiency with AuCl, and was not observed with Ph3PAuCl. Experiments carried out with deuteriated substrates and deuteriated alcohols supported the 6-exo-dig cyclization leading to an exocyclic enol 277, which is in equilibrium with the oxocarbenium 278 under the reaction conditions. Finally a Prins-type cyclization occurred by the addition of the counterion to the C=C.

The homoallylic alcohols, carbonyl compounds and nitriles underwent a tandem Prins-Ritter type cyclization in the presence of CeCl3.7H2O/AcCl at ambient temperature to produce 4-amidotetrahydropyrans in high yields with cis-selectivity. Cyclic ketones gave spirocyclic 4-amidotetrahydropyrans.174
Intramolecular cyclization of benzylic oxygen to activated C≡C of 2-(1-alkynyl)-benzyl alcohol gave isochromenes. Recent reports on the cyclization of 4- and 5-alkynol have established the synthetic potentiality of such an approach. Palladium175 and iridium48 complexes catalyzed the 6-endo-dig ring closure of 2-alkynylbenzyl alcohol with internal triple bond to yield the corresponding isochromenes in good yield (Scheme 84).

Recently, Hashmi et al. described related gold-catalyzed ring closure by C-H activation at the benzylic position of 2-(1-alkyknyl)-benzyl alcohol.176 The substrates 281 underwent expected 6-endo-dig cyclization to give the isochromene derivatives 282. When AuCl3 was used as catalyst, the initially formed 282 underwent significant decomposition and conversion was low. However, Au(I)-catalyst, [(Mes3PAu)2Cl]BF4 affords good conversion and good yields when the substituent R is sterically shielded. When the alkynyl moiety contains an additional nucleophilic group such as ester or amide function, the reaction leads to unexpected dimer products 283 along with the monomeric products 282, with both the gold(I) and gold(III) catalysts. (Scheme 85)176

Electrophilic activation of unsaturated C-C bond of 2-alkynylbenzaldehydes and related systems, followed by the nucleophilic attack of the carbonyl oxygen on the activated alkene or alkynes yielded the benzopyran derivatives.177, 178
In this context, Yamamoto et al. reported the reaction of alkenyl aldehydes with methanol in the presence of Pd(II) catalyst to give a mixture of five- and six-membered alkenyl ethers.179 However with alkynyl benzaldehyde, the cyclization in the presence of 5 mol% of Pd(OAc)2, 1 equiv. of benzoquinone (BQ) and 2 equiv. of MeOH in 1,4-dioxane gave exclusively six-membered product in moderate yield.180 It is to note that in this transformation Pd(OAc)2 played dual role both as a Lewis acid for enhancing the electrophilicity of aldehyde as well as a transition metal catalyst for enhancing the electrophilicity of the alkyne bond, for constructing the (R)-methoxycyclic alkenyl ether from the o-alkynylaryl aldehyde.
Belmont et al. have recently disclosed the synthesis of pyranoquinolines 285 from the Au-catalyzed reaction between 1-alkynyl-2-carbonyl-quinolines 284 and methanol (Scheme 86).181 No reaction or low yields were obtained using AuCl3 or Ph3PAuCl as the catalyst. The acetalization/cycloisomerization process was efficiently promoted with a 1:1 mixture of Ph3PAuCl and AgSbF6. Similar result was also obtained by using silver salt.
Subsequently, Yamamoto et al utilized CuI in DMF for similar cyclization of alkenyl ethers from acetylenic aldehydes.182 It is proposed that the resonance-stabilized oxonium ion 287 formed by the nucleophilic attack of the aldehydic oxygen to the copper coordinated alkynes, is being trapped by alcohols to give the desired products (Scheme 87). Recently, Yao and Li183 developed a highly efficient gold-catalyzed Grignard-type alkynylation of ortho-alkynylaryl aldehydes with terminal alkynes in water. In this reaction, terminal alkynes 290 reacted with ortho-alkynylaryl aldehydes 289 by the application of

5 mol% of Me3PAuCl and 20 mol% of i-Pr2NEt as base in toluene and H2O mixture to afford 1-alkynyl-1H-isochromene derivatives 291 (Scheme 88).

The reaction was found to be dually promoted by an electron-donating phosphine ligand and the presence of the ortho-alkynes. Presumably, alkynyl gold intermediate which is formed in the presence of base, may add to the aldehyde 289 to form intermediate 292. The resulting secondary alcohol 293 may then undergo an intramolecular trans-oxyauration, and protodeauration to give the isochromene 291 (Scheme 88).
It has also been observed that 4-propargyl-1,3-cyclopentanediones 295 gave exclusively bicycles 296 upon treatment with 10 mol% PtCl2 at room temperature (Scheme 89). These 6-endo-dig cyclizations are supposed to proceed under kinetic control by coordination of the catalyst to the triple bond.65a Palladium acetate catalyzed reaction of enynals 299 with dimethyl acetylenedicarboxylate 302 produces the pyran derivatives 303, fused with medium and large rings (n = 0-7). The key step of this reaction is the intramolecular oxycyclization of 299 by Pd(OAc)2, to give an electron rich olefin 300, which undergoes cycloaddition reaction with 302 in the presence of an additive and subsequent ring expansion may give

the product 303 (Scheme 90).184 However, in the absence of an additive cycloaddition product is obtained in poor yield.

A new palladium-catalyzed chemoselective cycloisomerization of cis-2,4-diene-1-als to 4-alkylidene-3,4-dihydro-2H-pyrans was reported by Liu et al.185 The reaction is carried out in the presence of PdCl2(C6H5CN)2 in toluene and is found to be very much efficient for the construction of 2H-pyranderivatives.
Shi et al. disclosed gold catalyzed ring-opening of epoxides 304, which can undergo highly regio- and diastereoselective cascade double intermolecular addition of alcohol to alkyne to give 2,6-trans-substituted morpholines 306 (Scheme 91).186 A possible mechanism involves alcohol addition to the complex 305 formed by the coordination of both the triple bond and the oxirane, followed by protodeauration, and re-coordination to promote the addition of a second molecule of alcohol.

4. 3 SYNTHESIS FROM ALLENES
Allenes containing a nucleophile can undergo intramolecular cyclizations on treatment with transition metal catalysts. In particular, gold-catalyzed cycloisomerization of β-hydroxy or γ-hydroxyallenes give access to six-membered oxygenated heterocycles. Krause and co-workers showed89b that β-hydroxyallenes and β-aminoalllene can readily be converted to the corresponding chiral dihydropyrans and tetrahydropyridine in good yields via stereoselective gold(I)-catalyzed or gold(III)-catalyst 6-endo cycloisomerizations. The increase in products by the addition of 3-hydroxypropionitrile or AgBF4, may be due to the formation of cationic gold species.

But in the case of gold(I) chloride, the addition of pyridine or 2,2’-bipyridine induced a remarkable increase of the reactivity. The chiral transfer observed for the β-hydroxyallenes 307 to the dihydropyrans 308 could be explained by the coordination of the gold catalyst to the terminal double bond of the allene 307 resulting in the formation of 309 which upon nucleophilic attack of the oxygen, is conducted into the δ-gold complex 310. Protodemetalation of the latter provides the heterocyclic product 308 and releases the gold catalyst into the catalytic cycle (Scheme 92).
A number of 2-alkenyl tetrahydropyrans 312 was prepared from 5,6-heptadien-1-ols 311 under achiral experimental conditions, catalyzed by cationic gold complex, (o-biphenyl)(t-Bu)2PAuCl (Scheme 93).94 Remarkably, gold-mediated hydroalkoxylation of 313 in the presence of the chiral diphosphine Au2{(S)-A)}Cl2 (5 mol%) afforded 314 (Scheme 94).95

4. 4 SYNTHESIS FROM DIAZO COMPOUNDS
Catalytic conversion of the diazo groups into the corresponding metal carbenes followed by intramolecular reaction with alcohol, ether, or carbonyl compounds afford a wide variety of oxygenated heterocycles.187
The use of carbonyl as oxygenated nucleophiles produces carbonyl ylide from the attack of the oxygen atom of C=O bond on the metal carbene. This carbonyl ylide undergoes 1,3-dipolar cycloaddition to give oxygenated heterocycles. By applying this methodology structurally complex natural products such as zaragozic acid C188 and polygalolide A189 have been synthesized. The intramolecular trapping of the carbonyl ylide, obtained from α-diazo ester 315 by treatment with Rh2(OAc)4 (5 mol%), by internal alkene gave the cycloadduct 317 as a single diastereoisomer, from which polygalolide A (318) was obtained after several steps (Scheme 95). Similarly, Geng et al. achieved190 the enantioselective synthesis of diastereoisomer 320 as the major product from the α-diazo ketone 319 by treatment with a chiral Rh-catalyst, Rh2[(S)-bptv]4, whereas in the presence of achiral rhodium catalyst the oxatricyclic compound 322 was obtained as the major product (Scheme 96).

The ether nucleophile also reacts with metal carbenes via an oxonium yilde intermediate to give six-membered oxygenated heterocycles.191 For example; copper-catalyzed cyclization of diazo ketones containing allyl ether has been applied to the synthesis of natural products. Treatment of O-allyldiazo ketone 323 with Cu(tfacac)2 provides the bicyclic compound 324 in excellent diastereomeric ratio. Epimerization to more stable isomer followed by reduction and further functional group manipulations afford the diazo ketone 325. This diazo compound after a second cyclization-epimerization-reduction sequence gave polyether 327 via 326 (Scheme 97).192
In a similar approach, Padwa et al. utilized ester carbonyl as a nucleophile towards the synthesis of icetexane core of komaroviquinone 330 using a rhodium(II)-catalyzed cyclization/cycloaddition sequence as the key step.193 The ylide generated by the cyclization of rhodium carbenoid intermediate onto the

proximal ester group, followed by intramolecular dipolar-cycloaddition of the carbonyl ylide dipole across the tethered π-bond afforded cycloadduct 329 (Scheme 98).

4. 5 SYNTHESIS OF PYRANONES
A novel annulation of simple o-hydroxyaldehydes with alkynes catalyzed by gold (I) catalyst gives isoflavanones. The best yield for this annulation was obtained by using 1 mol% of AuCN and 25 mol% of PBu3.194 The annulation efficiently generates isoflavanone-type structures with many possible applications in the synthesis of isoflavanone natural products. By applying this strategy, recently, Li et al. reported195 an efficient two steps procedure for the synthesis of (±)-pterocarpan and isoflavone196 natural products. To synthesize (±)-pterocarpans 334, salicylaldehydes 331 and 2-(methoxymethoxy)-1-ethynylbenzene 332 were treated with AuCN in the presence of PBu3 in toluene at 150 oC, to give the desired isoflavanone 333, followed by reduction with NaBH4 and addition of excess BF3.OEt2 afforded (±)-pterocarpans 334 (Scheme 99). The tentative mechanism for the novel annulation is similar to the previous mechanism discussed in Scheme 88.

The oxidative cyclization of β-hydroxy enones with PdCl2 gave 2,3-dihydro-4H-pyranones.197 This methodology represents a new approach to the enantioselective synthesis of pyranones based on the 6-endo ring closure of Pd(II)-complex followed by complete regioselective β-hydride elimination, without disturbing the stereochemistry of the existing stereocenter (Scheme 100).

A one pot, two components InCl3-mediated cascade reaction between β-keto esters 341 and alkynals 342 gave highly functionalized 1-oxadecalins 343 in good yields and excellent diastereoselectivity.198 The high diastereoselectivity could be rationalized by the six-membered chair-like transition state 344, which on Prins-type cyclization gave tetrahydropyran. The subsequent Conia-ene reaction leads to cis- oxadecalin ring junction selectively (Scheme 101).

When 2’-hydroxychalcones 346 was treated with 2.5 equivalent of FeCl3.6H2O in refluxing MeOH for 8 h, the O-heterocyclization occurred to give the corresponding flavones 347 (Scheme 102).199

A novel palladium-catalyzed cyclocarbonylation and thiocarbonylation reaction of allylic alcohols with thiols afford double carbonylated products, thioester-cotaining 6-membered-ring lactones using THF as solvent.200 The effect of ionic liquids on this palladium-catalyzed carbonylation of enols with thiols was also studied.201 The reaction of allylic alcohols with a variety of thiols under 500 psi of carbon monoxide in the presence of a catalytic amount of Pd(OAc)2 (2 mol%) and PPh3 (8 mol%) in ionic liquid, BMIM.PF6 or BMIM.NTf2 gave chemoselectivly monocarbonylated products 349 in variable yields (Scheme 103).201

Backvall et al. reported202 the cyclization of allene-substituted malonates leading to β,γ-unsaturated δ-lactones by AuCl3 catalyzed intramolecular nucleophilic attack of the carboxy oxygen onto the allene. Indeed, the allenes 350, on treatment with AuCl3 (5 mol%) in the presence of silver salts AgSbF6 (15 mol%) as additive in AcOH furnished β,γ-unsaturated δ-lactones 351 (Scheme 104). The other gold catalysts such as AuCl and Au(PPh3)3Cl also furnished lactones in lower yields than AuCl3.

4. 6 Synthesis of coumarin derivatives
Bahekar et al. reported the Pechmann condensation using Sm(III) as the catalyst under solvent free conditions. Sm(NO3)3.6H2O (10 mol%) effectively catalyzed the reaction of ethyl acetoacetate and resorcinol at 80 oC to give 7-hydroxy-4-methylcoumarin in 98% yield.203
Sharma et al. observed204 that an equimolecular mixture of resorcinol and β-ketoesters such as ethyl-4-chloroacetoacetate, ethyl benzoylacetate and ethyl furoacetate gave 4-substituted coumarins 354 in 90-95% yields on treatment with 10 mol% ZrCl4 at room temperature under solvent free condition via Pechmann reaction (Scheme 105). Similarly, Kirsch et al. also synthesized coumarin derivatives by using zirconyl octahydrate (1 mol%) as a Pechmann catalyst.205 BiCl3 has also been found to be an efficient catalyst in the Pechmann condensation reaction of phenols with β-keto esters leading to the formation of coumarin derivatives under solvent free conditions.206

Interestingly, Murakami et al. reported Rh(I)-catalyzed enantioselective synthesis of 3,4-dihydrocoumarin.207 When 3-(2-hydroxyphenyl)cyclobutanone 355 was treated with a catalytic amount of a rhodium(I) catalyst, obtained in situ from [Rh(OH)(cod)]2 (7 mol%) and (R)-SEGPHOS (16 mol%), in toluene at room temperature, 4-methyl-3,4-dihydrocoumarin 356 was obtained. BINAP and Tol- BINAP were also effective as the chiral ligand and gave 96% ee (Scheme 106).
The proposed mechanism for the transformation involoves of the generation of rhodium aryloxide 357 followed by its addition to the carbonyl group to give the rhodium cyclobutanoate intermediate 358. The ring opening of the cyclobutane skeleton may give intermediate coumarin derivative 359, followed by a series of β-hydride elimination and re-addition afforded coumarin 356 (Scheme 106).

Larock et al. reported208 that the palladium-catalyzed annulation of internal alkynes by o-iodophenol in the presence of CO employing 5 mol% Pd(OAc)2, 2 equiv. of pyridine, and 1 equiv. of n-Bu4NCl in DMF at 120 °C, afforded coumarin derivatives in moderate yields. The methodology was found to be very much effective for the simultaneous double annulation of 2,5-diiodo-1,4-hydroquinone.
2-Allyloxyaryl-2yn-1-ols undergo deallylation to the corresponding 2-propargylphenols, catalyzed by Pd(0). The resulting 2-propargylphenols undergo Pd(II)-catalyzed heterocyclization-alkoxycarbonylation reactions with CO and MeOH, to give 2-benzofuran-2-ylacetic esters.209 2-(1-Hydroxyprop-2-ynyl)phenols 360 bearing a terminal alkyne selectively underwent a dicarbonylation reaction with the formation of 3-[(methoxycarbonyl)methyl]coumarins 361.210 The reaction was carried out in the presence of PdI2 in conjugation with KI in MeOH at room temperature. However, a benzofuran derivative was obtained instead of coumarin in the case of propargyl phenol having an internal alkyne. The reaction may proceed through the formation of intermediate 362, followed by elimination of water and protonolysis of allylpalladium complex by HI, leading to the coumarin derivatives 361 (Scheme 107).

2-Ethynyl benzoic acid underwent metal-catalyzed cyclization to give a mixture of phthalides and isocoumarins.211 Pal et al. reported that isocoumarins could be obtained as major products when o-iodobenzoic acid was reacted with terminal alkynes in the presence of Pd/C-Et3N-CuI catalytic system.212 Jiang et al. described213 a phosphine and copper free protocol for the synthesis of phthalides (major) and isocoumarins (minor), via Pd/carbon nanotubes-catalyzed tandem coupling-cyclization between ortho-iodobenzoic acids 364 and terminal alkynes 365. Interestingly, when 5 mol% of H2O was added to DMF as the solvents, phthalides 366 were isolated as the sole product (Scheme 108).

Gold(I)-catalyzed intramolecular cyclization of γ-and δ-alkynes acids 367 gave various alkylidene lactones in high yields.214 A slight electronic effect of the R group was observed on the regioselectivity of the cyclization. Bulky substituent on the R group bearing the alkyne strongly modifies the reactivity. The exo-dig mode of cyclization predominated. The lactones 368 were obtained as major product and as a single stereoisomer Z. The electron-rich group on the aromatic ring of R decreases the exo selectivity (Scheme 109). The cycloisomerization of methyl o-alkynylbenzoate with AuCl3 in aqueous medium also furnished the isocoumarins exclusively, via 6-endo cyclization intermediate.214

Similarly, Li et al. utilized CuX2 (X = Cl, Br) as catalyst to cyclize a variety of o-(alk-1-ynyl)benzoates and (Z)-alk-2-en-4-ynoate to the corresponding 4-haloisocoumarins and 5-halo-2-pyrone, respectively, in moderate to excellent yields.215 It was observed that Cy2NH.HX could improve the rate of the reaction and the selectivity of the product.216a For example, in the presence of CuCl2 (2 equiv.), cyclization of methyl 2-(2-phenylethynyl)benzoate 370 afforded the corresponding isocoumarin 371 in 83% yield, whereas the yield was enhanced to 92% when 0.1 equiv. of Cy2NH.HCl was added to the reaction mixture (Scheme 110).

In a similar approach regiocontrolled cyclization of 2-(2-arylethynyl)heteroaryl esters 370 (Y = N) to isocoumarins in high yields was carried out through a 6-endo-dig cyclization.216 The reaction was performed in the presence of a catalytic amount of Lewis acids such as Cu(OTf)2, AuCl3 or (CF3CO2)Ag in combination with Brønsted acid, TFA under microwave irradiation. For example, the microwave irradiation of methyl nicotinate 370 (Y = N, R1 = Me) in TFA in the presence of either Cu(OTf)2 (5 mol%) or AuCl3 (5 mol%) or CF3CO2Ag (5 mol%) gave the lactone 372 in 92%, 83%, 72% yield, respectively (Scheme 110). A range of N-heterocyclic esters underwent similar cyclization to the corresponding lactones in high yields by this protocol.

5. MISCELLANEOUS REACTIONS
A novel three-component reaction gave access to new dihydrobenzofuran derivatives.217 Indeed, the domino reaction of 2-iodophenol, methyl bromomethylacrylate and phenylboronic acid in the presence of Pd(OAc)2, K2CO3 and n-Bu4NCl in DMF at 80 °C provided 3,3-disubstituted 2,3-dihydro benzofuran in moderate yield. In a similar approach, a three-component coupling reaction of aliphatic or aromatic aldehydes, homoallylic alcohols and ammonium thiocyanate by In(OTf)3 (10 mol%) gave 4-thiocyanotetrahydropyrans through heterocyclization in excellent yields with all cis-selectivity.218
The combination of 5 mol% Cu(OTf)2 and CuCl in the presence of DMAP effectively catalyzed the coupling reaction involving an alkynylsilane 373, an o-hydroxybenzaldehyde derivatives 374, and a secondary amine 375.219 The reaction proceeded via intramolecular 5-exo-dig cyclization, resulting in the direct synthesis of the corresponding benzofuran derivatives 376 in moderate to excellent yields (Scheme 111). Recently, Shi et al. reported gold(I)-catalyzed condensation of (E)- and (Z)-2-(arylmethylene)cyclopropylcarbinols 377 with terminal alkynes 378 and alcohols 379 to afford 3- oxabicyclo[3.1.0]hexane (Scheme 112).220 The mechanism of the addition reaction was confirmed by deuterium labeling and trapping of the intermediates. This reaction may proceed via intramolecular tandem hydroalkoxylation/Prins-type reaction pathway.

Helmchen et al. reported the first intermolecular gold-catalyzed addition of aldehydes and ketones to 1,6-enynes. The reaction proceeds smoothly with [AuCl(PPh3)]/AgSbF6 (5 mol%) to give the tricyclic oxygenated compounds 384 in high diastereoselectivity (Scheme 113).221

Li et al. reported222 the synthesis of 2-methyleneoxetanes 389 by O-vinylation of γ-bromohomoallylic alcohols 388 with CuI as catalyst in the presence of 1,10-phenanthroline as ligand and Cs2CO3 as base in CH3CN (intramolecular Ullmann Coupling, Scheme 114). The selectivity between 4-exo and 5-exo cyclization is completely reversed by changing the catalyst from Cu to Pd.

Jaisankar et al. synthesized tetrasubstituted furan derivatives 395 by InCl3-catalyzed reaction between but-2-ene-1,4-diones 390 and acetoacetate esters 391 using i-PrOH as solvent.223 In this reaction but-2-ene-1,4-diones act as Michael acceptors and acetoacetate esters as the nucleophiles resulting in Michael adduct 392 which under the influence of InCl3, formed the hemiacetal 393 followed by spontaneous dehydration afforded furans 395 in 78-90% yields (Scheme 115).

The insertion of aldehydes into a C-H bond of aromatic ketimines activated by rhenium complex, [ReBr(CO)3(thf)]2 (2.5 mol%), provided the isobenzofuran derivatives 398 (Scheme 116).224

Stereoselective synthesis of trans-2,6-disubstituted 3,6-dihydro-2H-pyrans 404 from δ-hydroxy-α,β-unsaturated aldehydes 399 and allyltrimethylsilane/TMSCN based on InBr3 (5 mol%)-catalyzed heterocyclization was also reported (Scheme 117).225 The Lewis acid induced tandem allylation or cyanation of δ-hydroxy-α,β-unsaturated aldehydes to produce dihydropyrans in good yields and with trans-selectivity at room temperature. Mechanistically, the reaction proceeds with activation of aldehyde

by In(III) bromide and subsequent nucleophilic attack by oxygen of OH group to give an oxonium intermediate 403 in which stereoelectronic and/or steric factors dictate the direction of the incoming new nucleophile and gave the trans products selectively.
Ihara et al. observed that the reaction of chiral propargylic carbonates proceeded in a highly enantiospecific manner to give chiral cyclic carbonates via an overall cascade chirality transfer process.226 The catalyst Au(PAr3)Cl (5 mol%, Ar = C6F5) in combination with AgSbF6 (5 mol%) gave the better results than corresponding PPh3 complex.227

6. CONCLUSION
Due to immense importance of heterocyclic compounds various classical methods were developed for their synthesis. Many of those methods are endowed with inherent limitations. The challenge to synthetic chemists is to overcome these problems and make the synthesis clean, straightforward and environment friendly. One of the several efforts is to search for suitable transition metal based catalysts. The development of catalysts has simplified the earlier harsh, tedious and time consumining reactions. Moreover, some of the unsuccessful reactions have been made successful with the use of catalysts. Use of catalysts has broaden the scope of the reaction. Moreover, the use of co-catalysts and additives in combination with the catalyst sometimes make the reaction selective and more useful. Thus the process becomes more effective and economic and the target compounds can be easily accessed. The foregoing discussion clearly demonstrates that a wide variety of transition metal based catalysts have already been developed. Though a vast number of catalysts and their useful applications have appeared in the literature, the complete listing is not possible due to lack of space. Only those recent catalysts which are typical for the synthesis of oxygen heterocycles are included in this report. The development and scope of transition metal catalysts is tremendous and the last decades have seen enormous growth and the growth is ever increasing. It seems like the transition metal catalysts hold the future of organic synthesis. Therefore, there is still much scope for development of new catalysts and application to challenging selective and target oriented synthesis. We believe this report will be useful to heterocyclic and medical chemists in particular and synthetic chemists in general.

7 ACKNOWLEDGEMENTS
We thank the CSIR (New Delhi) for financial assistance. P. Debnath is grateful to the CSIR (New Delhi) for his research fellowship.

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