HETEROCYCLES

Review | Regular issue | Vol. 89, No. 8, 2014, pp. 1767-1800
Received, 16th March, 2014, Accepted, 2nd May, 2014, Published online, 22nd May, 2014.
DOI: 10.3987/REV-14-796

■ Asymmetric Synthesis of Cyclopropylamine Derivatives

Han Wang, Xiaokun Zhou, and Yongjun Mao*

Department of Medicinal Chemistry, Shanghai University of Engineering Science, 333 Longteng Rd., Songjiang, Shanghai, 201620, China

Abstract

Cyclopropylamines are important synthetic intermediates and their enantiopure forms are also useful as chiral resolving agents. The present review outlines the recent developments on the asymmetric synthesis of cyclopropylamine derivatives. The transformations based on carbene or carbenoid species, asymmetric catalysis, ylide formation, Simmons-Smith, Kulinkovich and Mitsunobu reaction, conjugate addition, Favorskii-type rearrangement and intramolecular cyclopropanations are elaborated.

CONTENTS
1. Introduction
2. Using Carbene and Carbenoid to Build the Cyclopropane
2.1 Diazomethane as Carbene
2.2 Diazo Esters as the Carbenoid
2.3 Using Ylides to Build the Cyclopropane
(1) Sulfur Ylide
(2) Iodonium Ylides
2.4 Building the Cyclopropane Ring by Simmons-Smith Reaction
3. Building the Cyclopropane Ring by Kulinkovich Reactions
4. Building the Cyclopropane Ring by Favorskii-type Rearrangement
5. Addition of Bromonitroalkanes to α,β-Unsaturated Aldehydes to Form Chiral Nitrocyclopropanes
6. Building the Chiral Nitrocyclopropane Ring by Mitsunobu Reaction
7. Other Asymmetric Synthesis Method of Cyclopropylamines
7.1 Using Tosylhydrazones to Build the Cyclopropylamine Ring
7.2 Wadsworth-Emmons Cyclopropanation
7.3 Intramolecular Cyclopropanation of 1-Nitropropane Derivatives
8. Conclusion


1. INTRODUCTION
Amine functionalities are widely present in a large number of drugs and drug candidates. Asymmetric synthesis of amine derivatives has been an extremely challenging task for the design of potent drugs and their precursors. The widespread occurrence of highly strained cyclopropylamines as pharmaceutical or agrochemical subunits has stimulated their studies in recent years as potential chiral resolving agents or as synthetic intermediates for further reactions.1
Cyclopropylamine (CPA) is the key intermediate for the preparation of antibacterial drugs such as ciprofloxacin (1), sparfloxacin (2), balofloxacin (3) and grepafloxacin (4) (Figure 1).2 CPA is also used in the synthesis of herbicides such as 2-N-cyclopropylamine-4,6-dichlorotriazine (5) and cyprazine (6), insecticides cyromazine (7) (Figure 1) as well as many other plant protection agents and feed additives.3 1-Aminocyclopropane carboxylic acid (ACC, 8) is present in many plants as the immediate precursor to the plant hormone ethylene.4 The diazepinone derivative Nevirapine (9) is an anti-HIV drug candidate.5

Chiral cyclopropylamine derivatives are also used in many drug molecules, such as an anticoagulant ticagrelor (10) (Figure 2) approved by FDA in 2011.6 2,5-Dimethoxyamphetamine derivatives 11 and 12 (Figure 2) are lead compounds with high affinity for 5-hydroxytryptamine 2 (5-HT2) receptor with an EC50 = 2.0–6.3 nM, the more potent stereoisomer of the cyclopropane analogs has the expected (–)-(1R,2S)- configuration.7 trans-2-Phenylcyclopropylamine (2-PCPA) (13) also exhibits lysine specific demethylase 1 (LSD1) and monoamine oxidase (MAO) inhibitory activities. Some other derivatives, such as compounds 14 and 15 (Figure 2) have inhibitory activities both on LSD1 (IC50 = 1.3–1.9 μM) and MAO-A (IC50 = 0.5–1.2 μM). In particular, the compound 16 displays better inhibition selectivity over LSD1 (IC50 = 1.9 μM) and MAO-A and B (IC50 = 230 μM).8–10 The Streptomyces sp. UCK 14 metabolite Belactosin A (17) and its derivatives, which contain the unusual 2-trans-(2-aminocyclopropyl)alanine residue, exhibit remarkable proteasome inhibitory activities.11

Similar to the synthesis of CPA, most common methods for the preparation of the chiral cyclopropylamine derivatives are based on asymmetric addition of a carbene or carbenoid species (such as diazomethane and its derivative, sulfur ylides, Simmons-Smiths reagents, etc.) to an olefin promoted by a chiral catalyst or through the introduction of a chiral auxiliary group to the olefin to control the sterochemical configuration of the product.12 Generally α,β-unsaturated carboxylic esters are used as the olefin substrate. The cyclopropylamine products are obtained from the cyclopropanecarboxylate intermediates through the traditional ester hydrolysis, Hofmann degradation or Curtius rearrangement reactions. Other methods to produce chiral cyclopropylamine derivatives include Kulinkovich reactions, intramolecular Mitsunobu reaction, Favorskii rearrangement, and so on.13

2. USING CARBENE AND CARBENOID TO BUILD THE CYCLOPROPANE
2.1 Diazomethane as Carbene
The arylated methyl acrylate derivatives were cyclopropanated with diazomethane in the presence of 0.5 mol% of palladium-(II) acetate. Higher concentrations of the catalyst often led to precipitation of palladium-(0) with accompanying termination of the reaction. Alkaline hydrolysis gave the desired trans-arylcyclopropanecarboxylic acids 23 (Scheme 1).14 These acids were transformed to the corresponding primary amines by Curtius rearrangements. The one-pot procedure with diphenyl phosphorazidate (DPPA)15 worked well in many cases. An alternative method used involved preparation of acyl chloride 24 which was subsequently treated with sodium azide to obtain acyl azides 25. These were transformed into tert-butyl carbamates via the corresponding isocyanates 26. The resulting carbamates were finally hydrolyzed with 1 M hydrochloric acid to give the desired cyclopropanamine 28. When the aryl moiety was acid sensitive, the 2-(trimethylsilyl)ethyl carbamate 29 was prepared, which was hydrolyzed with tetrabutylammonium fluoride (TBAF).16

Besides tert-butanol, the intermediate isocyanates can be trapped with benzyl alcohol in the Curtius rearrangement, as reported by Vangveravong17 and Erhardt.18 As shown in Scheme 2, indole-3- carboxaldehydes 31 were first protected as the N-p-toluenesulfonyl derivatives 32. This protection was key to the success of the synthesis, not only because it enhanced reactivity of the carboxaldehyde but also because attempts to deprotect prior to the final step led to unstable intermediates that could not be isolated and purified. The protected indolecarboxaldehydes were condensed with malonic acid to afford the trans-β-indol-3-yl-acrylic acids 33. The acids were converted to their methyl esters, and cyclopropanation with retention of trans stereochemistry was then accomplished using ethereal diazomethane in the presence of catalytic palladium diacetate.19 Hydrolysis of the cyclopropyl methyl esters provided acids 35, which were reacted under the conditions of Curtius rearrangement, trapped with benzyl alcohol and then subjected to catalytic hydrogenation to give the amines 37. N-Detosylation as the final step was accomplished using sodium amalgam under buffered conditions to afford the desired indolecyclopropylamines 38.

In order to obtain (1S,2R)-2-(thiophen-3-yl)cyclopropanamine, the (+)-enoyl sultams were prepared from L-(+)-camphorsultam (40) and the methyl acrylate derivatives 41 (Scheme 3).14 The enoyl sultams were cyclopropanated, and the products were recrystallized to provide the corresponding cyclopropanoyl derivatives (+)-43 with high diastereomeric purities. The camphorsultam moiety of the diastereoisomers was removed by a two-step procedure involving (a) treatment with titanium-(IV) isopropoxide in benzyl alcohol and (b) base promoted hydrolysis of the resulting benzyl ester, thus providing the desired (+)-44.

D-(–)-Camphorsultam (Oppolzer's Sultam, 49) was also adopted in the asymmetric synthesis of cyclopropylamine, which functions as an easily removable, inexpensive, and efficient chiral auxiliary, as reported by Vallgarda20, 21 and Vangveravong 22 (Scheme 4). Adopting the similar methods as described before, 51 were obtained in good yield, and the stereochemical purity can be easily increased by recrystallization. The cyclopropanation was quantitative at temperatures between –30 °C and 28 °C. At temperatures below –30 °C the reaction became very sluggish and above 30 °C the volatility of diazomethane caused problems. The stereoselectivity of the reaction increased with the reaction temperature, the diastereomeric excess (de %) of the product increased from about 53 to 87% as a result of an increase in reaction temperature from –30 °C to 0 °C. Thus, it appears that reaction conditions favored formation of thermodynamically controlled product with enhanced stereoselectivity.

2.2 Diazo Esters as the Carbenoid
To date developments in cyclopropanation have been restricted to diazo esters and α-substituted analogues for two reasons: (i) diazo esters are more stable (ethyl diazoacetate is commercially available) and therefore easier to handle than aryl-, alkenyl-, or alkynyldiazomethanes, which are unstable and potentially explosive,23–25 and (ii) diazo esters are much less prone to metal-catalyzed diazo dimerization than the above diazo compounds.
As shown in Scheme 5, ethyl diazoacetate reacted with olefins 55a–f in the presence of catalyst rhodium acetate to give the cyclopropanecarboxylate 56a–f.26,27 The transformations of 56a–f into 59a–f were carried out without isolation of the intermediate azides 57a–f and the products of Curtius rearrangement 58a–f. The hydrochlorides 59a–f were purified by recrystallization from chloroform or methanol before conversion into the corresponding amines 60a–f. The cyclopropane products were mixture of four isomers, for example, 1,4-diazidospiro[2.2]pentane (57f):A:B:C:D = 50:20:20:10, and Spiro[2.2]- pentane-1,4-diamine dihydrochloride (59f):A:B:C:D = 35:30:25:10.

Recently, Chanthamath28 developed a highly enantioselective cyclopropanation of vinyl carbamate derivatives with diazoesters, using the Ru(II)-Pheox complex as a catalyst, as shown in Scheme 6. The reaction proceeds smoothly under mild conditions, giving the corresponding protected cyclopropylamine products 67 in high yield, with excellent diastereoselectivity (up to 96:4) and enantioselectivity (up to 99% ee).

2.3 Using Ylides to Build the Cyclopropane
(1) Sulfur Ylide
Gooden29 reported a facile route to synthesize trans-2-arylcyclopropylamines 73a–k (Scheme 7). Two cyclopropanation methods were employed for the preparation of compounds 71a–k (Table 1). Reaction of α,β-unsaturated esters 70a–k with the Corey-Chaykovsky reagent (dimethylsulfosonium methylide) in DMSO (Method A) generally gave poor yields of the desired cyclopropanated products than the traditional diazomethane approach (Method B).30 Detosylation of the indole nitrogen in 70k was anticipated under the strongly basic conditions of Method A, thus cyclopropanation by this method was not attempted. On the other hand, use of diazomethane with palladium (II) acetate (Method B) afforded the corresponding cyclopropanated products in excellent yields.

For the two substrates (entries 9 and 10), cyclopropanation by Method B gave complex reaction mixtures whereas all other substrates reacted under these conditions yielding the cyclopropyl adduct as the sole reaction product. However, cyclopropanation by Method A provided 71i and 71j in acceptable yield as the sole reaction products without the need for further purification.
Using Oppolzer's sultam (49) as the auxillary group, Khan31 synthesized the trans- cyclopropanecarboxylic acid 79 diastereoselectively based on the sulfur ylide cyclopropanations (Scheme 8). The cyclopropanation of N-acryloyl derivative 76 with trimethylsulfoxonium iodide proceeded in DMSO:THF (1:1) to give on the average of three runs a 3:1 mixture of (1S,2S)-cyclopropane-sultam 77 and (1R,2R)-cyclopropane-sultam 78 respectively, with 80% isolated yield of the diastereomeric mixture.

Clark32 reported a kilogram-scale method to prepare trans-(1R,2S)-2-(3,4-difluorophenyl)- cyclopropylamine (86), the key intermediate of ticagrelor (9), using L-menthol as the auxiliary group (Scheme 9). The key sulfur ylide cyclopropanation step from 82 gave a 31% isolated yield and 91.8% de of 83 and 84. After the ester hydrolysis, Curtius rearrangement process, the product 86 was obtained with 91.8% de, which can be purified by resolution with R-(–)-mandelic acid in ethyl acetate.

(2) Iodonium Ylides
Moreau33 reported a method for synthesis of cyclopropane α-amino acids by a catalytic asymmetric cyclopropanation of alkenes using iodonium ylides derived from methyl nitroacetate (Scheme 10). A three-step synthesis of enantiomerically enriched products was developed using the Cu(I)-catalyzed asymmetric cyclopropanation reaction of phenyliodonium ylides with alkenes. Commercially available isopropylidene-bis(4-phenyl-2-oxazoline) (91) and AgSbF6 were used as catalyst precursors.

Several alkenes (90a–j) were employed and the results are depicted in Table 2. Substitution of the aromatic ring could be accomplished with success, as the cyclopropanation of sterically hindered 2,4,6-trimethylstyrene 90g led to the desired product in 54% yield and 93% ee (entry 7). Indene (90i) also furnished excellent yield, diastereoselectivity, and enantioselectivity of the corresponding cyclopropane (entry 9).

The cyclopropylamine ester 93a was obtained in high yield from the nitroester 92a by a simple Zn-mediated reduction. Similarly, a simple two-step process was used to decarboxylate and reduce 92a into the amine 94a. The high enantioselectivity was preserved in both cases.

2.4 Building the Cyclopropane Ring by Simmons-Smith Reaction
The Simmons-Smith reaction is an organic cheletropic reaction in which a carbenoid reacts with an alkene (or alkyne) to form a cyclopropane.34 Zhao35 reported an asymmetric synthesis of (l’S,2’R)-cyclopropyl carbocyclic nucleoside based on this reaction. Their initial attempts to directly convert β-unsaturated ester 96 to the cyclopropyl derivative, using dimethyloxosulfonium methylide, gave a low yield (~10%) without stereoselectivity. Thus, the ester 96, which was prepared by the Wittig reaction of 95, was reduced by DIBALH at –78 oC to 97 in 84% yield (Scheme 11). Treatment with Et2Zn and CH2I2 at 0 oC gave optically pure cyclopropylmethyl alcohol 98 as the major isomer, which was oxidized with NaIO4 in the presence of RuO2 to obtain acid 99. The cyclopropylamine 102 was obtained after the Curtius process.

The asymmetric Simmons-Smith cyclopropanation reaction was introduced in 199236 by employing a reaction of cinnamyl alcohol (104) with diethylzinc, diiodomethane and a chiral disulfonamide 105 in dichloromethane (Scheme 12). The hydroxyl group is a prerequisite serving as an anchor for zinc.

In another version of this reaction37 the ligand was based on the Salalen reagent 119 and an Al Lewis acid / N-Lewis base bifunctional catalyst (Table 3). The experimental results suggested that the bifunctional catalysis of the Al (salalen) complex (120) is essential for obtaining high enantioselectivities.

Long38 found that the readily available dipeptide N-Boc-L-Val-L-Pro-OMe (139) (Table 4) is an effective ligand for asymmetric cyclopropanation of unfunctionalized olefins. Using this methodology, up to 91% ee was attained. These results suggest that the development of a highly enantioselective Simmons-Smith type cyclopropanation of unfunctionalized olefins via transfer of a simple methylene group is a real possibility.

Wang39 reported an asymmetric Simmons-Smith reaction using Charette chiral dioxaborolane ligand (151) for the construction of enantiomerically enriched cyclopropane ring (Scheme 13). Substituted iodomethylzinc reagents proved ideal for the Charette asymmetric SS reaction to obtain excellent enantioselectivities (90–98% ee) and high diastereoselectivities (from 10:1 to > 50:1 dr).40

It is interesting to note that the cyclopropanation reactions of 2-substituted allylic alcohols and homoallylic alcohols gave relatively lower levels of enantioselectivities (around 80% ee). Computational studies suggest that when Charette chiral ligand 151 is employed, monomeric iodomethylzinc allyloxide is converted into an energetically more stable four-coordinated chiral zinc/ligand complex 152. The chiral complex with the zinc metal bonded to the CH2I group and linked to three oxygen atoms through coordinate linkage (from the allylic alcohol, carbonyl oxygen and the dioxaborolane ligand) can readily undergo the desired cyclopropanation.

3. BUILDING THE CYCLOPROPANE RING BY KULINKOVICH REACTIONS
The Kulinkovich reaction describes the synthesis of cyclopropanols via reaction of esters with dialkyldialkoxytitanium reagents, generated in situ from Grignard reagents bearing hydrogen in beta-position and titanium(IV) alkoxides such as titanium isopropoxide.41 Titanium catalysts employed for this purpose are ClTi(OiPr)3 or Ti(OiPr)4, ClTi(OtBu)3 or Ti(OtBu)4, whereas Grignard reagents are EtMgX, PrMgX or BuMgX. With amides instead of esters, the reaction product is an aminocyclopropane in the de Meijere variation (Scheme 14).42 The intramolecular version of the reaction is also known (Scheme 15),43 while in the Szymoniak variation the substrate is a nitrile and the reaction product a cyclopropane with a primary amine group (Scheme 16).44

de Meijere45 conducted an excellent study based on the Kulinkovich reaction by treating N,N-dialkyl- and N-alkyl-N-phosphorylalkyl-substituted carboxamides (156) with unsubstituted as well as with 2-alkyl-, 2,2-dialkyl-, and 3-alkenyl-substituted ethylmagnesium bromides (155) in the presence of stoichiometric amounts of titanium tetraisopropoxide or methyltitanium triisopropoxide to furnish substituted cyclopropylamines (157) in 20–98% yield. Depending on the substituents, the reaction afforded from practically negligible (1:1) to excellent (>25:1) diastereoselectivities (Scheme 17). With substituted ethylmagnesium bromides, two chiral centers are produced in the cyclopropylamine ring; the results are depicted in Table 5.

The N,N-dibenzylformamide 156d with n-hexylmagnesium bromide 155b (Table 5, entry 5) was chosen for further experiments to prepare 2-butyl-N,N-dibenzylcyclopropylamine 157bd (Table 6). It was found that chiral titanium ligands generated from one equivalent of Ti(OiPr)4 and one equivalent of diamine/diol such as DACH (158) and DADPE (159), or BINOL (160) did only furnish racemic products (Table 6, entries 1–3) with concomitant loss of yield (52–65% vs 87% for the variant with a chiral MeTi(OiPr)3 (Table 5, entry 19). In contrast, application of tetraphenyldioxolanedimethanol (TADDOL) 161 led to 157bd in 59% yield with moderate enantiomeric excesses of 41% for the Z and 35% for the E isomer (Table 6, entry 4). Further improvement of the enantiomeric excesses to 66% for the Z and 42% for the E isomer could be achieved by generating the chiral titanium from two instead of one equivalent of 161 (Table 6, entry 5).

These results induced an attempt to optimize the enantioselectivity by systematic modification of the TADDOL ligand (Table 7). Switching from the two methyl substituents R1 and R2 on the dioxolane ring to the more bulky groups improved the enantiomeric excess to 70–84% for the Z and 55–77% for the E isomer. When the aryl substituents on the diol moiety were changed from phenyl to 3,5-dimethylphenyl or 3,5-trifluoromethylphenyl substituents, yields as well as enantioselectivities were lowered.

The established Kulinkovich-Szymoniak procedure is simple and the reaction appears to be quite general. A wide range of nitriles and organomagnesium reagents can react to afford diversely substituted cyclopropylamines. Furthermore, bicyclic cyclopropylamines can be obtained via an intramolecular coupling from unsaturated nitriles.46 Bertus47 performed detailed studies on this and presented a new method for the preparation of primary cyclopropylamines 165 (Table 8) involving a cooperative Ti(II) and Lewis acid-mediated coupling of nitriles 163 with Grignard reagents 164. With other substituted EtMgBr Grignard reagents, 1,2-disubstituted cyclopropylamines could be obtained as shown in entries 6–9. In all cases, a moderate diastereoselectivity of about 2:1 was observed.

Pradhan48 reported a simple and stereoselective method for the preparation of (Z)-2-substituted 1-aminocyclopropane (169) and (Z)-1-(2-hydroxyethyl)[2.4]4-azaspiroheptan-5-one (171). The common key step for these reaction sequences involves the stereoselective Ti-mediated coupling of nitrile 167 or 170 and homoallylic alcohol 168 (Scheme 18). The major advantages of this method are the simplicity and high diastereoselectivity of the cyclopropanation key step and low cost as well as ready availability of the starting materials for scale-up.

4. BUILDING THE CYCLOPROPANE RING BY FAVORSKII-TYPE REARRANGEMENT
In principle, the Favorskii rearrangement is a rearrangement of cyclopropanones and α-halo ketones which leads to carboxylic acid derivatives.49 In the case of cyclic α-halo ketones, the rearrangement constitutes a ring contraction, as shown in Scheme 19.

This kind of reaction can be used for the stereoselective synthesis of cyclopropylamines, as reported by Denolf.50 N-Sulfinyl α-chloro ketimines (RS)-175, a new class of functionalized N-sulfinylimines, were synthesized via condensation of α-chloro ketones 173 with (RS)-tert-butanesulfinamide 174 in the presence of 2 equiv of Ti(OEt)4 (Scheme 20) and subsequently tested for their reactivity upon treatment with Grignard reagents. The best results were obtained when 2.2 equiv of PhMgCl was added to ketimine (RS)-175a in dichloromethane at –78 °C. Subsequent stirring for 2 h at –78 °C and 4 h at –40 °C afforded 1-phenylcyclopropylamine (RS,R)-176a after aqueous NH4Cl workup in high yield (70%) and excellent diastereoselectivity (95:5 dr). Stirring for 15 min at room temperature afforded the HCl salts of the cyclopropylamines 177 in high yield (> 90%) and purity (85–95%) (Scheme 20).

It is proposed that the cyclopropanation reaction proceeds via a Favorskii-type reaction mechanism (Scheme 21). Hence, the first equivalent of Grignard reagent acts as a base, an unprecedented reaction in this field. A proton at the α-position of the imino function of ketimines (RS)-175 is abstracted, the resulting 1-azaallylic anion 177 undergoes chloride expulsion to produce the intermediate N-(cyclopropylidene)-tert-butanesulfinamide 178, which is attacked by the second equivalent of Grignard reagent at the reactive imino function of 178 giving rise to cyclopropylamine 176 after aqueous NH4Cl workup. The highly strained Favorskiitype intermediate 178, in combination with the bulky tert-butanesulfinyl group, results in the formation of an enantioenriched cyclopropylamine 176.

5. ADDITION OF BROMONITROALKANES TO Α,Β-UNSATURATED ALDEHYDES TO FORM CHIRAL NITROCYCLOPROPANES
Vesely51 and Zhang52 reported their respective results on asymmetric conjugate addition of bromonitromethane to α,β-unsaturated aldehydes. Several chiral secondary amines (181–188) (Scheme 22) were chosen to catalyze the reaction and diphenylprolinol triethylsilyl ether (188) was identified as the best catalyst for the reaction (Table 9). Excellent enantioselectivities and good yields were achieved for a number of β-arylacroleins under MeOH/AcONa system (Table 10). Substituted 1-bromonitromethanes, such as 1-bromonitroethane and 1-phenyl-1-bromonitromethane, also provided excellent enantioselectivities and improved diastereoselectivities. The reaction is efficient for preparing highly substituted chiral nitrocyclopropanes, which can be used as the precursor of cyclopropylamines.

Accordingly, efficient shielding of the Si-face of the chiral iminium intermediate 198 by the bulky aryl groups of chiral pyrrolidine 188 leads to stereoselective Re-facial nucleophilic conjugate addition by the bromonitromethane195 (Scheme 23). Next, the generated chiral enamine intermediate 199 undergoes intramolecular 3-exo-tert nucleophilic attack to form the cyclopropane ring of 200. The intramolecular ring-closure pushes the equilibrium forward and makes this step irreversible. Hydrolysis of iminium intermediate 200 releases the catalyst and gives the corresponding 2-formylcyclopropane 196 and 197. Due to steric repulsion between the nitro-group and the catalyst of iminium complex 200 or the nitro- and 2-formyl-groups of 197, diastereoisomer 196 is formed.

6. BUILDING THE CHIRAL NITROCYCLOPROPANE RING BY MITSUNOBU REACTION
The Mitsunobu Reaction allows the conversion of primary and secondary alcohols to esters, phenyl ethers, thioethers and various other compounds.53 Triphenylphosphine and an azodicarboxylate such as diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD) are used. The alcohol undergoes an inversion of stereochemistry as depicted in Scheme 24.

Yu54 reported a method to afford α-nitrocyclopropanes by treatment of a wide variety of γ-nitroalkanols with a preformed complex of DEAD and Ph3P in good to excellent yields (Table 11). The reaction proceeds rapidly at ambient temperature under essentially neutral conditions in benzene or THF. This represents a highly efficient intramolecular variant of the Mitsunobu displacement procedure in which a nitronate anion acts as a carbon nucleophile resulting in a new carbon-carbon bond.

Khile et al.55 prepared the trans-(1R,2S)-cyclopropylamine 86 (intermediate of ticagrelor) on a hundred-gram scale based on the intramolecular Mitsunobu reaction (Scheme 25). 1-(3,4-Difluorophenyl)-3-nitropropan-1-one (206) was obtained from difluorobenzene 205 in acceptable yield, which was then converted to the (S)-benzyl alcohol 208 by treating with borane-N,N-diethylaniline and 1 mol% (S)-(–)-2-methyl-CBS-oxazaborolidine (207) through an asymmetric process with high selectivity. Then it underwent the Mitsunobu reaction to give the trans-(1R,2S)-nitrocyclopropane 209 with good yield and selectivity. The cyclopropylamine product 86 was afforded through reduction and subsequent resolution with (R)-(–)-mandelic acid in high purity and medium overall yield.

7. OTHER ASYMMETRIC SYNTHESIS METHOD OF CYCLOPROPYLAMINES
7.1 Using Tosylhydrazones to Build the Cyclopropylamine Ring
Aggarwal56 reported a user-friendly, one-pot process for catalytic cyclopropanation of alkenes from tosylhydrazones. The cyclopropanation of N-vinylphthalimide (214) provides a new route to 2-arylcyclopropylamines. This process provided a route to the less easily accessible cis isomer with high diastereoselectivity, which was exemplified in the efficient synthesis of the HIV-1 reverse transcriptase inhibitor 217 (Scheme 26). Aldehyde 212 was converted into the tosylhydrazone 213. Deprotonation with LiHMDS and treatment with 214, PTC, and rhodium acetate gave the cyclopropane 215 in 76% yield and as an 85:15 mixture in favor of the required cis isomer. It was interesting to note that even though a hindered 2,3,6-trisubstituted arylhydrazone was employed, high yield and high cis selectivity was still achieved. Following hydrazinolysis, the amine was coupled with 2-amino-5-cyanopyridine in the presence of triphosgene to give the urea 217.

7.2 Wadsworth-Emmons Cyclopropanation
Armstrong57 reported a concise synthesis of 3-(trans-2-aminocyclopropyl) alanine, a component of belactosin A, using asymmetric alkylation of a glycine enolate in the presence of chiral phase-transfer catalysts to control the configuration at C2. Reaction of protected glycidol with triethyl phosphonoacetate (Wadsworth-Emmons cyclopropanation) is used for enantiospecific preparation of an intermediate cyclopropanecarboxylate that is converted to a cyclopropylamine via Curtius rearrangement (Scheme 27).

Based on the above cyclopropanation procedure, Mitsuda58 prepared the trans-(1R,2S)-cyclopropylamine 86 (intermediate of ticagrelor) on a grams scale (Scheme 28). (2S)-2-(3,4-Difluorophenyl)oxirane (225) was synthesized from (2S)-2-chlorobenzyl alcohol 224 in basic condition. Then it was treated in accordance with the Wadsworth-Emmons cyclopropanation method to give the (1R,2R)- cyclopropanecarboxylate 226 in good yield. After the ester hydrolysis and amidation, the (1R,2R)-cyclopropanecarboxamide 227 was obtained, which was reacted with NaOH / NaOCl to give the trans-(1R,2S) product 86 through a Hofmann rearrangement process.

7.3 Intramolecular Cyclopropanation of 1-Nitropropane Derivatives
Averina59 summarized the present knowledge on the methods of synthesis and transformations of nitrocyclopropanes systematically, mainly including the methods of stereoselective cyclopropanation, which can be used as the precursors to synthesize chiral cyclopropylamines. The synthetic routes involve an intramolecular 1,3-elimination to afford the desired nitrocyclopropanes (Scheme 29).

The dibromopropanoate 231 eliminated an HBr molecule by treating with K2CO3 to give the 232. Subsequent addition by MeNO2 followed by cyclopropanation afforded the desired product 234 (Scheme 30).60

During the process research of trans-(1R,2S)-cyclopropylamine 86, Rasparini et al.61 developed an effective method based on introducing a γ-leaving group to the nitropropane, as shown in Scheme 31. Mesylate, tosylate, triflate, or just -Br can be the γ-leaving group, while the mesylate example was described in detail. There were two route to give the (1S,2R)-2-nitrocyclopropylbenzene 209. One is treating the (S)-benzyl alcohol 208 with mesyl chloride in pyridine to give 235, which was cyclized in DBU to obtain 209 with good yield and ee value. The other is using enantiomer benzyl alcohol 236 as the reactant to afford the enantiomer product 237, which has a high stereoselectivity.

Besides sulphonate and halogen, other leaving group such as -SeO2Ph can also be employed for cyclopropanation. Thus, (hex-1-en-1-ylselenonyl)benzene 238 was treated with MeNO2 to afford the product 240 (Scheme 32).62

When a rigid substrate 241 was adopted, the reaction exhibited some stereoselectivety to give the product 242 with a medium to good ee value (Scheme 33).63

Reaction of a halohydrocarbon with a nitro olefin is another method that leads to cyclopropanation. The nitroethene 243 reacted with diethyl 2-chloromalonate under basic condition to give the nitrocyclopropane 244 with good yield and high de and medium ee value (Scheme 34).64

Recently, Sakae reported a Cu-catalyzed aminoboration of 1-methylenecyclopropanes (245) with bis(pinacolato)diboron (246) and O-benzoyl-N,N-dialkylhydroxylamines (247), which provides a rapid and concise access to (borylmethyl)cyclopropylamines (248) in a highly regio- and diastereoselective manner (Scheme 35).65

8. CONCLUSION
Since optically active cyclopropylamines have been recognized as useful building blocks for biologically active compounds, several methodologies are encountered in the literature for their synthesis. The preparation of cyclopropylamines or the introduction of a cyclopropyl residue to nitrogen containing functional group has only begun to commence in true sense since the last two decades. The most common approach involves reaction of α,β-unsaturated carboxylic esters with carbene sources such as diazomethane and 1-methyl-1-nitrosourea or carbenoid species which include diazo esters, sulfur ylide, iodonium ylide or Simmons-Smith reagents. Generally the stereoselectivity of the reaction is achieved through the introduction of a chiral auxiliary (such as camphorsultam, L-menthol) into an α,β-unsaturated carboxylic ester to afford another chiral α,β-unsaturated carboxylic ester or amide. Subsequent alkaline hydrolysis generates a chiral cyclopropanecarboxylic acid which then undergoes a Curtius or Hofmann rearrangement to give the desired cyclopropylamine. Although there are many successful examples of carbene or carbenoid based synthesis of cyclopropylamines, lack of operational safety, high cost and difficulties associated to recycle the chiral auxiliary often impose a restriction to the general applicability of the process. In recent years, synthesis of chiral cyclopropylamines based on asymmetric catalysis, ylide generation and Simmons-Smith reaction have gained wide attention due to their simple operation, high stereoselectivity and goodatom economy. Whereas Simmons-Smith reactions afford satisfactory results only with prop-2-en-1-ol derivatives, the lower yield associated with sulfur ylide based reactions needs to be addressed in future.
Kulinkovich reaction has been developed in-depth by de Meijere and other scholars for the asymmetric synthesis of substituted cyclopropylamines. TADDOL (161) and its derivatives were proved to be effective catalysts, which gave medium to good stereoselectivity in many cases. (RS)-tert-Butanesulfinamide could be introduced to α-chloro ketones as the chiral auxiliary as well as the active ketimine intermediate. The chiral cyclopropylamines are obtained through a Favorskii-type rearrangement and subsequent acid hydrolysis. In addition, since the Kulinkovich reaction and Favorskii-type rearrangement introduce the amino functionality directly onto a cyclopropane ring, both of them offer better atom economy in the absence of traditional Curtius rearrangement process.
Addition of bromonitroalkanes to α,β-unsaturated aldehydes provides yet another route to nitrocyclopropane, a potent precursor to cyclopropylamine. Several chiral secondary amines were chosen to catalyze the reaction, some of which displayed excellent stereoselectivity. Besides nitro group, a formyl group can also be introduced onto the cyclopropane ring but the method has not been found useful for the synthesis of cyclopropylamines.
The intramolecular Mitsunobu reaction can be used for the synthesis of chiral cyclopropylamines for its high chemical selectivity. The main drawback of this method is the requirement of prior preparation of the optically pure substrate through asymmetric synthesis. The same problem is also encountered in Wadsworth-Emmons cyclopropanation, which employs enantiopure epoxypropane derivatives. Intramolecular cyclopropanation of 1-nitropropane derivatives also affords nitrocyclopropanes with moderate chemical selectivity. Nevertheless, further studies are still required to optimize reaction parameters for preparing asymmetric cyclopropylamines through easily accessible and inexpensive techniques.

ACKNOWLEDGEMENT
This work was supported by the Shanghai University of Engineering and Technology Scientific Research Foundation (A-0501-12-050) and the Special Scientific Foundation for Outstanding Young Teachers in Shanghai Higher Education Institutions (ZZGJD13017).

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