HETEROCYCLES

Review | Regular issue | Vol. 85, No. 9, 2012, pp. 2073-2109
Received, 23rd March, 2012, Accepted, 15th June, 2012, Published online, 25th June, 2012.
DOI: 10.3987/REV-12-739

■ Synthetic Methods for Phosphorus Compounds Containing Pyrazole Rings

Tarik E. Ali* and Somaia M. Abdel-Kariem

Department of Chemistry, Faculty of Education, Ain Shams University, El-Maqreezy St, Roxy, Heliopolis, Cairo 11711, Egypt

Abstract

The present review considers all the literature data on methods developed for the synthesis of phosphorus compounds containing pyrazole rings starting from their appearance up to the 2011. The described methods for the synthesis of phosphorus compounds containing pyrazole rings can be divided into three routes: a) direct phosphorylation of pyrazole rings, b) ring closure of acyclic phosphorus compounds with different reagents into phosphono-pyrazoles and c) cyclization of side functional groups to give isolated and fused phosphorus heterocyclic systems containing pyrazole rings.

CONTENTS
1. Introduction
2. Synthetic approaches
2.1. Phosphorylation of pyrazole rings
2.1.1. C−Phosphorylation of pyrazole rings
2.1.2. N−Phosphorylation of pyrazole rings
2.2. Ring closure of acyclic phosphorus compounds with electrophilic and nucleophilic reagents into phosphonopyrazoles
2.2.1. Cyclization of phosphonic dihydrazones
2.2.2. Cyclization of α-hydrazonomethylphosphonate
2.2.3. Cyclization of β-hydrazonomethylphosphonate
2.2.4. Cyclization of α-cyanomethylphosphonate
2.2.5. Cyclization of α,β-unsaturated phosphonate
2.2.6. Cyclization of α-diazomethylphosphonate
2.2.7. Cyclization of triphenylphosphonium halide salts
2.2.8. Cyclization of allenyl phosphonate
2.2.9. Cyclization of 1,3-dicarbonyl phosphonate
2.3. synthesis of isolated and fused phosphorus heterocyclic systems containing pyrazole rings
2.3.1. Five-membered rings
2.3.2. Six-membered rings
2.3.3. Seven and higher-membered rings
3. Conclusion

1. INTRODUCTION
Heteroaromatic compounds have attracted considerable attention in the design of biologically active molecules and advanced organic materials.1 Hence, a practical method for the preparation of such compounds is of great interest in synthetic organic chemistry. Pyrazole and its derivatives, a class of well known nitrogen containing heterocyclic compounds, occupy an important position in medicinal and pesticide chemistry with having a wide range of bioactivities such as antimicrobial,2 anticancer,3 anti-inflammatory,4 anti-depressant,5 anti-convulsant,6 anti-hyperglycemic,7 antipyretic,8 antibacterial,9 antifungal activities,10 CNS regulants11 and selective enzyme inhibitory activities.12 It has been found that these compounds have hypoglycemic activity, and are also known as inhibitors and deactivators of liver alcohol dehydrogenase and oxidorApplying of a similar type of substitution reaction on the pyrazole 1 via its treatment with phosphoryl chloride afforded the dichloride derivative 4 which underwent esterification with sodium ethoxide to give the corresponding ester 2 (Scheme 2).25eductases.13 It has been shown in vivo that some of the pyrazole derivatives have appreciable antihypertensive activity.14 These compounds also exhibited properties such as cannabinoid hCB1 and hCB2 receptor, inhibitors of p38 Kinase and CB1 receptor antagonists.15,16
On the other hand, it is known that phosphorus substituents regulate important biological functions such as pesticides, antichloine esterase, antiviral, antimicrobial activity, and war gases,17-20 and that molecular modifications involving the introduction of organophosphorus functionalities in simple synthons could be very interesting for the preparation of biologically active compounds.
pyrazole derivatives containing a phosphorus atom have been showed good biological activities, for example, pyraclofos and flupyrazofos which have been developed as good insecticides.21-23 For many years, the synthesis of phosphorus derivatives of pyrazoles has been a subject of interest in several laboratories. The present survey considers all the literature data on methods developed for the synthesis of phosphorus compounds containing pyrazole moieties starting from their appearance up to the 2011. the described methods for the synthesis of phosphorus compounds containing pyrazole rings can be divided into three routes: a) direct phosphorylation of pyrazole rings, b) ring closure of acyclic phosphorus compounds with electrophilic and nucleophilic reagents into phosphonopyrazoles and c) cyclization of side functional groups to give isolated and fused phosphorus heterocyclic systems containing pyrazole rings. It is hoped that this survey will demonstrate the synthetic potential of the synthesis of phosphorus containing pyrazole moieties and generate some new ideas in this area.

2. SYNTHETIC APPROACHES
2.1. DIRECT phosphorylation of pyrazole rings.
2.1.1. C-phosphorylation of pyrazole rings
It is known that pyrazoles with unsubstituted 4-positions and lacking electron-withdrawing groups in the ring can facilitate the reaction in this position. This property was used in the synthesis of a series of phosphonylated pyrazoles.24,25 Thus, heating of alkyl substituted pyrazoles 1 and POCl3 in an ampule for 12 h, followed by treatment with absolute alcohol afforded dialkyl 1,3,5-trialkylpyrazol-4-yl- phosphonates 2 in 31-44% yield (Scheme 1). The dibutyl 1-phenylpyrazol-4-yl-phosphonate 2 (R3= n-Bu) could be converted into the monobutyl ester 3 by heating with KOH in propanol, followed by acidification. Hydrolysis of the dialkyl esters 2 with acids gave syrupy phosphonic acids that could not be purified.

Applying of a similar type of substitution reaction on the pyrazole 1 via its treatment with phosphoryl chloride afforded the dichloride derivative 4 which underwent esterification with sodium ethoxide to give the corresponding ester 2 (Scheme 2).25

The synthesis of diethyl 3-cyanopyrazol-4-ylphosphonate 8 and the corresponding phosphonic acid 10 was achieved (Scheme 3).26,27 The synthetic route depends on phosphonylation of the pyrazole derivative 5 with phosphorus trichloride, followed by esterification to give the diester 7. The latter diester underwent an oxidation with hydrogen peroxide to prepare the corresponding phosphonate 8. Conversion of the phosphonate 8 to the phosphonic acid 10 occurred in two steps, via the monoalkyl ester 9 (Scheme 3). The envisaged end products were used for pest control and preparation of veterinary drugs, since they are useful for the control of arthropods and helminthes.
Treatment of 5-ethoxy-3-methyl-1-phenylpyrazole (11) with phosphorus trichloride in pyridine provided 5-ethoxy-3-methyl-1-phenylpyrazol-4-yl-dichlorophosphine (12). Chlorination of 12 with chlorine afforded the phosphorane 13. This phosphorane 13 underwent elimination of ethyl chloride to yield the phosphorus ylide 14 under strictly controlled conditions. The latter compound reacted with methanol to yield the dimethylpyrazol-4-yl-phosphonate 15 (Scheme 4).28

Reaction of 5-alkoxy-3-methyl-1-phenyl-1H-pyrazole (16) with dialkylaminophosphinyl chloride in pyridine afforded 4-dialkylaminophosphinyl pyrazoles 17. Further, it was found that the chlorination of 17 with chlorine led to stable chloro phosphorus ylides 19a-c rather than to the expected chlorophosphonium chloride 18 (Scheme 5).29

Similarly, the pyrazole 11 reacted with chloro(diphenyl)phosphine to give the phosphine 20. Contrary to the behavior of compounds 17, the phosphine 20 was chlorinated to provide a rather stable chlorophosphonium salt 21 that could not be isolated in a pure state, although its structure was supported by 31P NMR spectral data as well as by hydrolyzing it to diphenyl (3-methyl-1-phenyl-5-ethoxypyrazol-4-yl) phosphine oxide (22). When the chlorophosphonium salt 21 was maintained at 20 oC in benzene for 24 h, it decomposed to give 4-[chloro(diphenyl)-λ5-phosphanylidene]-3-methyl-1-phenyl-5H-pyrazol-5-one (23) and ethyl chloride (Scheme 6).29

The chloro phosphorus ylides 19b,c were readily hydrolyzed by atmospheric moisture or by reaction with aqueous sodium carbonate to give the corresponding phosphonates 24 (Scheme 7).29

Also, the chloro phosphorus ylide 19b reacted with hydrogen chloride to furnish tetraethyldiamino (3-methyl-5-oxo-1-phenyl-2,5-dihydropyrazol-4-yl)chlorophosphonium chloride (25A,B) as air-stable compounds (Scheme 8).29

Under the action of gaseous ammonia, the chlorine atoms in the chloro phosphorus ylides 19b,c were readily displaced by amino groups to produce the iminophosphonates 27b,c (Scheme 9).29

The ease of substitution of the chlorine atom in chloro phosphorus ylides 19a-c by a dialkylamino or an arylamino group was mainly governed by steric effects of substituents at the phosphorus atom. For example, in the case of dimethylamino groups bonded to the phosphorus atom, the reaction with diethylamine and aniline was carried out for 24 h to reach completion (Scheme 10).29

Treatment of the phosphine derivative 12 with diisopropylamine gave 4-[chloro(diisopropylamino) phosphino]pyrazole (30). Chlorination of 30 led to the stable dichlorophosphonium chloride 31, which underwent transformation into the 4-[dichloro(diisopropylamino)-λ5-phosphanylidene]-3-methyl-1-phenyl-5H-pyrazol-5-one (32), as a result of dealkylation through loss of ethyl chloride (scheme 11).30

Further, the chlorine atoms in the dichloro phosphorus ylide 32 at the electrophilic phosphorus atom could readily be substituted by various groups through interaction with O-, N-, and S-nucleophiles, giving the corresponding compounds 33-37 (scheme 12).30

The sulfur dioxide gas was bubbled through the reaction mixture of compound 13 to give 5-ethoxy-3-methyl-1-phenyl-1H-pyrazol-4-yl-phosphonic dichloride (38), which was transformed into 4-[5-ethoxy-3-methyl-1-phenyl-1H-pyrazol-4-yl)(4-morpholinyl)phosphoryl]morpholine (39), by its reaction with morpholine hydrochloride in the presence of aprotic solvent (scheme 13).30

It was found that dichlorophosphino moiety was successfully introduced into the 4-position of the pyrazole ring using N1,N1-dimethyl-N2-5-pyrazolylformamidine 40 as the model system. Thus, reaction of compound 40 with phosphorus trichloride in the presence of pyridine and triethylamine gave 4-[5-(3-methyl-1,3-diazobut-1-enyl)-3-methyl-1-phenyl]pyrazolyl-dichlorophosphine (41). The latter compound was treated with secondary amine to give bis(dialkylamino)phosphines 42 which were transformed into the corresponding phosphonium salts 43 by the action of methyl iodide and/or 4-nitrobenzyl bromide (Scheme 14).31

Treatment of the 4-lithiated bis-pyrazole 44 with chlorodiphenylphosphine in THF gave bis-(4-{diphenylphosphinyl}-1H-pyrazole)methane (45). This compound was used as a ligand with some metals as Hg, Sn….etc (scheme 15).32

The reactions of tris(dimethylamino)phosphine with the aldehyde 46 proceeded smoothly in THF to afford tris-aminophosphonium dipolar ion structure 48A,B. Nucleophilic attack of the phosphines at C-5 of the α,β-unsaturated system in the aldehyde 46 afforded the C-phosphonium betaine 47, which followed by further hydrogen migration to give the more stable ylide enol structure 48A,B (Scheme 16).33

2.1.2. N-phosphorylation of pyrazole rings
Reaction of compound 49 with pyrazole in dichloromethane in the presence of triethylamine as a base afforded 5-alkoxy-2,2-methylenediphenol-2-(1H-pyrazol-1-yl)-1,3,4,2-λ5-oxadiazaphosphole-3(2H)-carboxylic ester 50 (Scheme 17).34

Condensation of two molecules of 3-benzoyl-4-phenyl-2-pyrazoline (51) with various alkyl/aryl phosphorodichloridate in diethyl ether in the presence of triethylamine resulted in the formation of alkyl/aryl bis(3-benzoyl-4,5-dihydro-4-phenyl-1H-pyrazol-1-yl)phosphinates 52a-e (Scheme 18).35

2.2. RING CLOSURE OF ACYCLIC PHOSPHORUS COMPOUNDS with electrophilic and nucleophilic reagents INTO PHOSPHONOPYRAZOLES
2.2.1. Cyclization of phosphonic dihydrazones
A convenient procedure for the synthesis of 4-formylpyrazole derivatives is based on the Vilsmeier-Haack reactions with methyl ketone aryl hydrazones. Thus, application of Vilsmeier-Haack reaction on phosphonic dihydrazones 53a,b afforded bis-{4-formyl-3-aryl-1H-pyrazol-1-yl}phosphine oxides (54a,b) (Scheme 19).36

Similarly, application of the Vilsmeier-Haack reaction on phosphonic dihydrazones 53c,d which contain active functional groups in ortho positions that led to new fused pyrazole systems. Thus, when phosphonic dihydrazone 53c was treated with Vilsmeier reagent afforded a red crystalline product namely, bis-{4-hydroxy-2,4-dihydrochromeno[4,3-c]pyrazol-2-yl}phosphine oxide (55). The oxidation reaction of 55 with selenium dioxide in dry dioxane yielded bis-{chromeno[4,3-c]pyrazol-4-oxo-2-yl}phosphine oxide (56) (Scheme 20).36 Consequently, the effect of Vilsmeier reagent on the phosphonic dihydrazone 53d afforded bis-{chromeno[2,3-g]indazol-11-oxo-2-yl}phosphine oxide (57) in moderate yield (Scheme 20). this transformation involved monoformylation at each methyl group of 53d, followed by two steps of cyclization process.36

2.2.2. Cyclization of α-hydrazonomethylphosphonate
The reaction of phosphonyl hydrazones 58 with the Vilsmeier reagent (DMF/POCl3) in equimolar ratio afforded 1-phenyl-3-diethoxyphosphonyl pyrazoles 59. Compound 59a underwent formylation via its reaction with another mole of the Vilsmeier reagent to give the corresponding aldehyde 60 (Scheme 21).37

Dimethyl[bromo(phenylhydrazono)methyl]phosphonate (61) underwent cycloaddition reaction with different alkynes to give a mixture of 3-pyrazolylphosphonates 62a-d and 63a-d, while its reaction with alkenes afforded a mixture of 3-pyrazolinylphosphonates 64a-d and 65a-d. Also, their phosphonic acids, 1,5-diphenyl-1H-pyrazole-3-phosphonic acid (66) and 1,5-diphenyl-4,5-dihydro-1H-pyrazole-3- phosphonic acid (67) were obtained in quantitative yield via hydrolysis of the diesters 62a and 64a, respectively, with a 10 fold excess of trimethylsilyl bromide in methylene chloride (Scheme 22).38

2.2.3. Cyclization of β-hydrazonomethylphosphonate
Cyclization of β-hydrazonophosphonate 68 with triethyl orthoformate yielded 4-phosphopyrazoles 69. Furthermore, reaction of 69 with HSiCl3 led to pyrazoles 70, which underwent sulfuration by sulfur to afford 71 (Scheme 23).39

Similarly, cyclization of some N-(2,2,2-trifluoroethyl)-β-phosphonate hydrazones 72 into the corresponding 4-phosphonopyrazoles 73 was achieved via their reaction with triethyl orthoformate in xylene containing few drops of glacial acetic acid (Scheme 24).40

1-Phenyl-4-diethoxyphosphonylpyrazoles 75 were obtained by reaction of phosphonyl ethylene hydrazones 74 with Vilsmeier reagent (Scheme 25).37

Palacios et al.,41 explored the behavior of substrate 76 in the presence of LiClO4 as a catalyst. Addition of LiClO4 to [3-(dimethylhydrazono)-2-(diphenylphosphinoyl)but-1-enyl]dimethylamine (76) in the presence or even in the absence of dienophile and using nitromethane as a solvent led to the formation of 4-(diphenylphosphinoyl)-1,3-dimethyl-1H-pyrazole (78). The formation of this pyrazole 78 could be explained by intramolecular cyclization involving nucleophilic attack of the dimethylamino group at N-1 to the carbon–carbon double bond to give pyrazolidine 77 which underwent loss of trimethylamine to afford the 4-phosphorylated pyrazole 78 (scheme 26).

Treatment of β-hydrazono phosphine oxides 79 with lithium diisopropylamide (LDA) in THF followed by addition of isocyanates or isothiocyanates and aqueous work-up giving the functionalized phosphine oxides 80A,B. Compounds 80A,B were reacted with phosphoryl chloride in the presence of triethylamine led to the formation of diphenyl (5-aminopyrazol-4-yl)phosphine oxides (83) (Scheme 27).42

The phosphoranylidene hydrazones 84 could be easily converted into the corresponding 5-oxo-4-phoshoranylidene-4,5-dihydro-1H-pyrazoles 85 by treatment with a catalytic or a stoichiometric amount of sodium hydride in a mixture of tetrahydrofuran/methanol (1:1) at room temperature. Compounds 86 could also be obtained by hydrolytic cleavage of 84 (R4 =i-Pr), by magnetically stirring in methanol/water (95:5) at room temperature (Scheme 28). The hydrazone derivatives 86 have proven once again their synthetic utility as they were converted in good yields into (3-oxo-2,3-dihydro-1H-pyrazol-4-yl)phosphonamidates 87 and 88 by treatment with a catalytic amount of sodium hydride in tetrahydrofuran/methanol (1:1). The formation of 2-unsubstituted 3-oxo-pyrazole 87 took place by base-promoted hydrolytic cleavage of the aminocarbonyl group linked to the nitrogen atom at the 2-position of the pyrazole ring (Scheme 28).43

2.2.4. Cyclization of α-cyanomethylphosphonate
Treatment of phenyl hydrazone 89 with diethylphosphonoacetonitrile (90) in sodium ethoxide at room temperature with stirring afforded two different products namely, 4-cyano-2,5-diphenyl-1,2,3-diazaphosphole (93) (route a) and diethyl 5-amino-1,3-diphenylpyrazol-4-ylphosphonate (95) (route b) (Scheme 29).44

2-diazo-1,3-indandione (96) was treated with a little excess of molar amount of diethyl cyanomethylphosphonate (90) in a mixture of LiOH/H2O/CHCl3 at room temperature then heated for 30 h at the reflux temperature to give diethyl (4`-amino-1,3-dioxo-1,3-dihydrospiro[indene-2,3`-pyrazol]-5`-yl) phosphonate (98) and 4-ethoxy-1-ethyl-1,3-dioxo-1,2,3,4-tetrahydrospiro[1,2,4]diazaphosphole-5,2`-indene]-3-carbonitrile-4-oxide (100) (Scheme 30).45

Meanwhile, the Schiff base 101 was heated under reflux with diphenylphosphoryl acetonitrile for 12 h in THF in the presence of sodium hydride as a catalyst to afford 7-(4-chlorophenyl)-8-(diphenylphosphoryl)-3-methyl-4-oxo-4,8-dihydropyrazolo[5,1-c][1,2,4]triazine-8-carbonitrile (104) (scheme 31). This reaction pathway proceeds via C-nucleophilic attack by the reactive methylene of diphenylphosphoryl acetonitrile on the N–N=CH–Ar moiety to give the intermediate 102 which underwent cyclization by elimination of one molecule of H2S followed by an air oxidation process (Scheme 31).46

2.2.5. Cyclization of α,β-unsaturated phosphonate
An alternative route to dihydropyrazole phosphorus compounds 107 was provided in the addition of the nitrile imine 105 to vinylphosphorus compounds 106 (Scheme 32).47

Reaction of 2-diazo-1,3-inandione (96) with diethyl vinylphosphonate (106) was proceeded in the presence of (LiOH/H2O/CHCl3) to produce diethyl (1`-ethyl-1,3-dioxo-1,2`,3,4`-tetrahydrospiro[indene-2,5`-pyrazol]-3`-yl)phosphonate (108) (scheme 33). Compound 108 showed high antibacterial activity towards B. tumefaciens, S. aureus, K. pneumoniae, A. niger, A. flavus and P. crysogenous.45

Also, it was found that the cycloaddition of diazomethane onto dimethyl 1-(formylamino)- ethylenephosphonate (109) smoothly proceeds in ether at room temperature to furnish dimethyl 3-(formylamino)-4,5-dihydro-3H-pyrazol-3-phosphonate (110) in a quantitative yield (Scheme 34).48

Dialkyl (1-cyano-2,2-bismethylsulfanylvinyl)phosphonates (111) reacted with hydrazines to yield the 4-pyrazolylphosphonates 112 (Scheme 35).49

Similarly, cyclization reaction of dialkyl (2-cyano-2-substituted-1-methylsulfanylvinyl)phosphonates 113 with some hydrazines to construct the phosphonylpyrazoles 114 were achieved under carefully controlled reaction temperature (Scheme 36).50

1-Phenyl-5-diethoxyphosphonyl pyrazoles 117 were readily prepared by the action of phenylhydrazine on 115 in the presence of sodium metal (Scheme 37).37

Also, the phosphonic enamines 118 were reacted with methylhydrazine to provide (1-methyl-1H-pyrazol-4-yl)phosphonates 119. The hydrolysis of the phosphonates 119 could be affected either with HCl or bromotrimethylsilane to give the corresponding (1-methyl-1H-pyrazol-4-yl)phosphonic acids 120 (Scheme 38).51

The reaction of 3-phosphonocoumarins 121 with an excess of ethyl diazoacetate in benzene or chloroform at room temperature for 60 days afforded ethyl 3a-(diethoxyphosphoryl)-4-oxo-3,3a,4,9b-tetrahydrochromeno[3,4-c]pyrazole-1-carboxylates (122) in moderate yields (Scheme 39).52

On treating 1,3-indandion-2-hydrazone (123) with 1,3-dithietane 124, gave the unexpected diethyl 2-imino-3H-4-thioxo(11,12-dihydroindan-10-one)[12,11-a](5,13,1-oxadiazole)[13,1-b](1,13-pyrazole-3-yl)phosphonate (127). The formation of the product 127 could be rationalized by the generally accepted mechanism of the initial formation of the reactive intermediate 125 to give the oxadiazole 127 via the dipolar intermediate 126 (Scheme 40).53

The 1,3-dipolar cycloaddition reaction of 1,3-dipoles with alkenes was used in the synthesis of heterocyclic phosphorus derivatives in two different ways: the first in which the phosphorus substituent is present in the alkene (or alkyne) and the second in which it was attached to the 1,3-dipole. Diisopropyl ethynyl phosphonate (128) upon reaction with diazomethane was converted into the pyrazolyl phosphonate 129 in 28% yield.54 The ethynyldiphosphonate 130 was converted into the diphosphonyl pyrazole 131 in 95% yield in a similar reaction with diazomethane.55 Also, at low temperatures diphenyldiazomethane (132) was added to diethyl vinylphosphonate (106) to yield dihydropyrazolylphosphonate 133 in 73% yield (Scheme 41).56

2.2.6. Cyclization of α-diazomethylphosphonate
1,3-Dipolar cycloaddition of the anion of diethyl 1-diazo-methylphosphonate, generated in situ from diethyl 1-diazo-2-oxopropylphosphonate (134) (Bestmann-Ohira reagent), with conjugated nitroalkenes 135 provided regioisomerically pure phosphonylpyrazoles 136 in moderate to good yields. These pyrazoles were formed in one-pot via spontaneous elimination of the nitro group. However, nitropyrazoles could be synthesized by the same strategy using α-bromonitroalkenes. The methodology worked for the synthesis of phosphonylpyrazoles 138 fused to other carbo- and heterocycles as well (Scheme 42).57,58 Similarly, recently a new multicomponent reaction allowed to give the regioselctive synthesis of phosphonopyrazoles 139 by combination an aldehyde, cyano acid derivative and diethyl 1-diazo-2-oxopropylphosphonate (134) (Scheme 42).59


2.2.7. Cyclization of triphenylphosphonium halide salts
5-Alkyl/aryl substituted 2-pyrazolin-3-yltriphenylphosphonium salts 142 were prepared from vinyl triphenylphosphonium bromide (140) and substituted diazomethanes 141 in high yields (Scheme 43).60,61

1,3-Dipolar cycloaddition reaction between 2-diazo-1,3-indandione (96) and vinyl triphenyl phosphonium bromide (140) in basic medium, led to the corresponding phosphonium salt 143 (Scheme 44).62

2.2.8. Cyclization of allenyl phosphonate
Recently, the phosphorane-promoted addition of dialkyl azodicarboxylates ROCN=NCO2R [R= Et (DEAD) or i-Pr (DIAD)] was elegantly utilized to generate new pyrazole derivatives. Thus, reaction of allenes 144 with dialkyl azodicarboxylates 145 and 146 was performed in the presence of triphenylphosphine to afford the 4-phosphonopyrazoles 147-149 (scheme 45).63

2.2.9. Cyclization of 1,3-dicarbonylphosphonate
1,3-Dicarbonylphosphonate 150 was condensed with phenylhydrazine to give the pyrazoline phosphonate 151 in low yield (Scheme 46).64

2.3. SYNTHESIS OF isolated AND FUSED PHOSPHORUS HETEROCYCLIC SYSTEMS CONTAINING PYRAZOLES RINGS
2.3.1. Five-membered rings
it was found that the one-pot reaction of bis-chalcone 152 with diethyl phosphite in the presence of BF3.Et2O at 80 oC for 8 h, afforded bis-{[2-ethoxy-2-oxo-5-phenyl-2,3-dihydro-1,2-oxaphosphol-3-yl]-3-phenyl-1H-pyrazol-1-yl}phosphine oxide (154) (Scheme 46). The proposed mechanism involved an initial Michael type addition of phosphorus atom of diethyl phosphite to the activated double bond in compound 152 followed by cyclization via elimination of ethanol molecules to give 154 (Scheme 47).36

The cyclization reaction of 3-amino-5-benzylthio-4-ethoxycarbonylpyrazole (155) with phenylphosphonus dichloride and aromatic aldehydes gave 2-aryl-6-benzylthio-7-ethoxycarbonyl-3-phenyl-2,3-dihydro-1H-pyrazolo[5,1-e][1,4,2]diazaphosphole-3-oxides 156 (Scheme 48).65

2.3.2. Six-membered rings
Abdel-Ghaffar et al.36 have reported that heterocyclization of bis-thiosemicarbazone 157 with diethyl phosphite at 80 oC in the presence of BF3.Et2O for 10 h, afforded an interesting type of phosphorus heterocycle, namely bis-{3-(4`-biphenyl)-4-[2-ethoxy-6-phenylamino-2-oxo-3,4-dihydro-2H-1,4,5,2-thiadiazaphosphinin-3-yl]-1H-pyrazol-1-yl}phosphine oxide (159) (Ar= 4`-biphenyl) (Scheme 49). The formation of 159 may be occurred via addition of phosphorus atom of diethyl phosphite to CH=Nexocyclic groups to give the nonisolable intermediate 158, which underwent cyclization by nucleophilic attack of SH groups at phosphonate groups to eliminate two molecules of ethanol (Scheme 49).36

Reaction of dimethyl-4-oxo-4H-chromen-3-ylphosphonate (160) with an equimolar amount of methylhydrazine, performed on a larger scale without any solvent, gave three isolated products. The major product, 5-(2-hydroxyphenyl)-1,3-dimethyl-4-phosphonyl-substituted pyrazole 161 was isolated from the reaction mixture by addition of acetone, while the two other products were separated chromatographically from the remaining mixture. The second reaction product 162 was originated from an analogous cyclization of 161 in the lowest yield. Unexpectedly, no isomeric 3-(2-hydroxyphenyl)-1,5-dimethyl-4-phosphonyl-substituted pyrazole 163 was obtained. Instead, compound 164 was isolated in 20% yield. Probably 163, formed in the first step of the reaction, was unstable under the chromatographic conditions and underwent intramolecular cyclization giving rise to 164 (scheme 50). The reaction proceeded according to the general mechanism described for the reaction of chromone with nitrogen nucleophiles, including hydrazines. Attack of a nitrogen nucleophile at C-2 of the chromone led to the opening of the γ-pyrone ring. Similarly, reaction of 160 with 2-hydrazinopyridine in methanolic solution afforded 5-(2-hydroxybenzoyl)-3-methyl-1-(2-pyridinyl)-1H-pyrazol-4-phosphonic acid dimethyl ester (165). Compounds 161 and 165 reacted with Pt(II), Cu(II) and Pd(II) salts forming metal complexes which showed low cytotoxic activity against human acute leukemia HL-60 and NALM-6 and melanoma WM-115 cell lines. The cytotoxic effect of its palladium and platinum complexes are rather low (scheme 50).66-69

Two diastereoisomeric 1,3a-diethoxycarbonyl-4-ethoxy-4-oxo-3,9b-dihydro-4,5-benzoxaphosphorino[3,4-c]pyrazoles (167A,B) were isolated from reaction of 3-ethoxycarbonyl-1,2-benzoxaphosphorine (166) with ethyl diazoacetate. Compounds 167A,B are epimers towards phosphorus atom of the oxaphosphole ring (scheme 51).52

Also, the 1,3-dipolar cycloaddition of ethyl diazoacetate to 1,2-benzoxaphosphorin-3-phosphonates 168a,b proceeded with the formation of two epimeric methylenebisphosphonates 169A and B (scheme 52).52

The C-phosphorylation of 1,3,3-trimethyl-2-(3-methyl-1-(cyanoethyl/phenyl)-5-pyrazolyliminoethylidene)indolines 170 with phosphorus trihalide and dibromo(phenyl)phosphine proceeded simultaneously at the two nucleophilic carbon centers to result in the pyrazoloazaphosphinines 171 and 172, respectively. The cyclization was most effective when conducted in dichloromethane in the presence of triethylamine as the base (scheme 53).70

The cyclic bromophosphines 171 easily reacted with secondary amines and anilines to give unstable aminophosphines 173 identified by NMR spectra. The latter compound was transformed to stable 3-methyl-4-morpholino-1-substituted-5-(1,3,3-trimethyl-2,3-dihydro-1H-2-indolylidene)-4,5-dihydro-1H-4λ5-pyrazolo[3,4-b][1,4]azaphosphinine-4-thione 174 via reaction with sulfur element (scheme 54).70

5-Hydroxy-3-methyl-1-phenyl-1H-pyrazol-4-yl-N,N-diisopropyl phosphonamidic chloride hydrochloride (175) was obtained via partial hydrolysis of the dichloride 32. The reaction of 175 with triethylamine led to the abstraction of hydrogen chloride in the presence of aceto- or benzonitrile to afford a new type of phosphorus containing heterocyclic system namely, 1-(diisopropylamino)-3,8-dimethyl-5-phenyl-1,5-dihydro-1λ5-pyrazolo[4,3-e][1,3,4]oxazaphosphorin-1-one (177) suggesting the formation of diisopropylaminophosphorus ylide 176 as an intermediate (Scheme 55).30

Reaction of 1-aryl-5-arylcarboxamidopyrazoles 178 with phosphorus trihalide gave pyrazolo[4,3-c][1,5,2]oxazaphosphinines 180, which were formed through the acyclic O-phosphorylated intermediate 179 (scheme 56). Upon treatment with secondary aliphatic amines or anilines, cyclic bromoanhydrides 180 underwent halogen substitution yielding amides 181 and 182 respectively, which could be oxidized with elemental sulfur to give air-stable cyclic thioamides 183 and 184. Also, the amides 181 reacted with some arylazides to give the iminoamides 185 (scheme 57).71

Also, bromoanhydrides 180 (R=Br) reacted with equivalent amount of alcohol or phenol giving the corresponding cyclic esters 186, which were oxidized with elemental sulfur to give the thioesters 187 (scheme 58).71

Cyclic phosphines 180 (R=Ph) could also be transformed into air-stable derivatives of phosphorus (v). Thus, they were oxidized into thioxides 188 and 189 by elemental sulfur and iminated with arylazides to cyclic phosphaza compounds 190. Also, cyclic phosphine 180 reacted with chlorine giving the chlorophosphonium chloride 191 (scheme 59).71

Bromination of amides 181 gave the bromophosphonium bromides 192 which were hydrolyzed with water in the presence of a base giving cyclic phosphorus (v) amides 193 (scheme 60).71

Also, bromination of anilides 182 led to bromophosphaza compound 194, which was reacted with morpholine affording the iminoamide 185a. The latter compound underwent hydrolysis in aqueous pyridine to yield the 5-carboxamidopyrazole 195 (scheme 61).71

It should be noted that reaction of opening cyclic esters with an excess of alcohol was reversible. It was found during an attempt to isolate acyclic ester 196 that it was stable in methanol solution in the absence of water and oxygen, however under evaporating methanol in vacuo cyclic ester 186 was obtained (Scheme 62).71

When 1-substituted 5-benzylamino-4-pyrazolecarboxamides 197 were reacted with phenylphosphonic dichloride in dioxane, 7-alkyl-1-benzyl-2-phenyl-2-oxido-1,2,3,7-tetrahydro-4H-pyrazolo[3,4-d][1,3,2]diazaphosphinin-4-one 198 was produced (scheme 63).72

The pyrazolodiazaphosphinethione 200 was also achieved in high yield from treatment of aminocyanopyrazole 199 with Lawesson’s reagent in THF (Scheme 64).73

The phosphonium salts 43 reacted with ammonia or sodium ethoxide to give the corresponding phosphorus ylides 201, which underwent intramolecular nucleophilic substitution in situ to form pyrazolo[5,4-b]azaphosphinines 202 (Scheme 65).31,74

6-Benzylthio-5-alkyl-1-alkoxy/aryloxy-1-oxido-1,2-dihydropyrazolo[1,5-c][1,3,5,2]triazaphosphinin-3-(4H)-ones 204 were synthesized by reaction of 3-benzylthio-5-aminopyrazoles 203 with alkyl phosphorochloroiso-cyanates in THF in the presence of Et3N (scheme 66).75,76

2.3.3. Seven and higher-membered rings
Treatment of N-[1-(1-p-iodophenyl-2,5-dimethyl-3-pyrrolyl)methylidene]-N-(3-methyl-1-phenyl-1H-5-pyrazolyl)amine 205 with dibromoarylphosphines provided 6-(p-iodophenyl)-3,5,7-trimethyl-4-(substituted)-1-phenyl-4,6-dihydro-1H-pyrazolo[3,4-b]pyrrolo[3,4-e][1,4]azaphosphepine (206a,b). Further, reaction of 205 with phosphorus tribromide in pyridine in the presence of sulfur element and methanol afforded 6-(p-iodophenyl)-4-methoxy-3,5,7-trimethyl-1-phenyl-4,6-dihydro-1H-4∆5-pyrazolo[3,4-b]pyrrolo[3,4-e]azaphosphepine-4-thione (208), through the formation of azaphosphepine 207 as intermediate (scheme 67).77

5-Hydroxy-3-methyl-1-phenylpyrazolyl-4-phosphonic tetraethyldiamide (24b) was found to display unusual chemical behavior. Thus, on heating it to 230 oC under vacuum, one molecule of diethylamine was eliminated to give 3,8-dimethyl-1,6-diphenyl-4,9-dioxo-4,9-bis-(diethylamino)-4,5,9,10-tetrahydro[1,5,2,6]dioxadiphosphocino[3,4-d:7,8-d]dipyrazole (209) (scheme 68).29

The condensation of 5-azido-4-formyl-3-methyl-1-phenyl-1H-pyrazole (210) with 1,4-diaminobutane in ethanol at room temperature provided the bis(azide) 211 in moderate yield, which was treated with 1,4-bis(diphenylphosphino)butane affording the 20-membered macrocyclic bis(iminophosphorane) 212 as a crystalline solid (scheme 69).78

CONCLUSION
Phosphorus compounds containing pyrazole rings prove to be of interest for a variety of socially relevant fields: the medicinal and pharmaceutical field, the agrochemical and biochemical field, the field of metal complexation, and so forth. Because of these well-known biological properties of compounds containing pyrazole rings have attracted and are still attracting the attention of many research groups. Their efforts resulted in a vast diversity of synthetic pathways and a better understanding of the reactivities of this class of compounds. During the preparation of this review, we have been trying to sort the different synthetic pathways into general subdivisions based on the used reagents type and cyclization reactions. Some examples of biologically active compounds have already been presented in this review, and from our point of view, more research in this exciting field will result in even more active examples upon further biotesting.

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