The Synthesis of Pyrazoles

The synthesis of pyrazoles, indazoles and their derivatives generally follows classical methods, the two most important methods for practical purposes being the reaction between hydrazines and /3-difunctional compounds, and 1,3-dipolar cycloadditions (Section 4.04.3.1.2). Both procedures are well documented (64HC(20)l, 66AHC(6)327, 67HC(22)l) and thus the length of the sections in this part of the chapter reflects not only the number of publications dealing with a particular method but also its interest and novelty. 

Sucrow, studying the chemistry of the enehydrazines (83CB1520), has found several methods for the synthesis of pyrazoles. For example, he has described (79CB1712) the cyclization of monomethylhydrazones of dialkyl oxalacetates to 5-pyrazolones via an enehydrazine (Scheme 47). 

The use of diphenyl hydrazone 33 has been used in the synthesis of pyrazoles under modified conditions where the hydrazine is released in situ. Some reversal of regiochemistry is seen in the reaction with unsymmetrical dicarbonyls. With aryl hydrazine and diphenyl hydrazone, the ratio of 41 to 42 is 22 1 and 5 1, respectively. 

The synthesis of pyrazoles and isoxazoles from l-heterobut-l-en-3-ynes is performed, as a rule, in an aqueous acid medium, i.e., under conditions favoring the hydrolysis and hydration of the initial enyne. This circumstance impedes rationalization of the experimental results. Therefore, it is reasonable to consider in brief the protonation and behavior of l-heterobut-l-en-3-ynes under the conditions of acid-catalyzed hydrolysis. 

The concept of the use of dilithio reagents for the preparation of heterocyclic systems has been extended to the synthesis of 2-isoxazolin--5-ones10 by carboxylation of a dilithio oxime, followed by eyolin-zation, and the synthesis of pyrazoles from dilithiophenylhydrazones and trilithiothiohydrazones.11 12... 

Heterocycles can be employed as precursors for the synthesis of pyrazoles. Pyrazoles can be synthesized by three-membered ring substrates. For example, allyl amines 12 and pyrazoles 13 could be obtained by hydrazinolysis of 2-ketoaziridines 11 <06TL255>. Regioselective ring opening of 3-aryl-2-benzoyl-l,l-cyclopropanedicarbonitriles 14 with hydrazine provided a new process for the synthesis of 5-aryl-3-phenylpyrazoles 15 <06JHC495>. 

Szczepaikiewicz et al. [85] reported the synthesis of pyrazoles (e.g., 167) from the diketone 168 with hydrazine hydrate, shown in Scheme 43. Nigram et al. executed a similar synthetic sequence [86]. 

Pyrazoles were synthesized in the authors laboratory by Le Blanc et al. from the epoxy-ketone as already stated in Sect. 3.1.1a, Scheme 35 [80]. The synthetic strategy employed by Le Blanc et al. [80] was based upon that the strategy published by Bhat et al. [81] who also described the synthesis of pyrazoles but did not report cytotoxic evaluation on the synthesized compounds. Scheme 48 shows the synthesis of the most active compound (178). Dissolution of the epoxide (179) with a xylenes followed by treatment with p-toluenesulfonic acid and hydrazine hydrate produced the pure nitro-pyrazole 180 in good yield (60%). Catalytic hydrogenation with palladium on activated carbon allowed the amino-pyrazole (178) to be obtained in a pure form. This synthesis allowed relatively large numbers of compounds to be produced as the crude product was sufficiently pure. Yield, reaction time, and purification compared to reported approaches were improved [50, 61, and 81]. Cytotoxicity of these pyrazole analogs was disappointing. The planarity of these compounds may account for this, as CA-4, 7 is a twisted molecule. 

Since the early review on sydnones by Stewart (192) and the subsequent coverage by Potts (1), several new applications of these remarkably stable mesoionic heterocycles have been described. In particular, the synthesis of pyrazoles from sydnones has been pursued by several groups. Badachikar et al. (193) prepared several new potential antibacterials (297) from the appropriate sydnones 296, which were synthesized in the standard fashion by the cyclodehydration of the corresponding A-nitroso-A-arylglycine. 

A review on the synthesis of pyrazoles and condensed pyrazoles covering the period of 1989-1998 was written <99JHC321>. 

Quiroga et al. have described the synthesis of pyrazole 26c g and pyrimidine 27a,b TB derivatives and suggested the mechanism of their formation from isolated intermediates (02TL5617, 02JCS(P1)1588). In this case, C-alkylation served to initiate... 

Enol ethers of 1,3-diketones react in weakly acidic media as the diketones themselves.236 The synthesis of pyrazoles with various heterocyclic substituents has been liberally covered in the literature.237-259... 

Acetals of /9-keto aldehydes are hydrolyzed in acid media and hence under these conditions they give pyrazoles just as the ketoaldehydes themselves.297-300 Recently the bis-acetal of malondialdehyde has been used extensively for the synthesis of pyrazoles unsubstituted on the ring carbon atoms the reaction yields single products since the bis-acetal is symmetrical.217,271,273,301-307 Cyclization takes place in... 

Kost and Sagitullin,531 Fridel and Combes,532 Domnin,833, 534 and Kauffmann5340 For the synthesis of pyrazoles consequent on the formation of an N—N link, see Belov,45 Fowden,130 Brian et al.,300 and others.306,535,536 A number of pyrazoles have been prepared from pyrazolinones see, for example, Michaelis and Lachwitz537 or Mustafa and co-workers.53 7a... 

A further example of electrophilic substitution is the synthesis of pyrazole aldehydes by formylation with dimethylformamide.620,628... 

Only a few examples of photoinduced rearrangement reactions for the synthesis of pyrazoles are worthy of mention. For example, N-ethoxycarbonylpyrazoles 43 are formed in the irradiation, in acetone or benzene solutions (high-pressure Hg... 

Diazo compounds have also been used as precursors in the preparation of pyrazoles and indazoles. The copper-promoted cycloaddition reaction of lithium acetylides 18 with diazocarbonyl compounds 19 provided a direct and efficient approach to the synthesis of pyrazoles 20 <07AG(I)3242>. A facile, efficient, and general method for the synthesis of 1-arylated indazoles 22 and A-unsubstituted indazoles 23 by the 1,3-dipolar cycloaddition of benzynes, generated from 21, with diazomethane derivatives has been reported <07AG(I)3323>. Reaction of diazo(trimethylsilyl)methylmagnesium bromide with aldehydes or ketones gave 2-diazo-2-(trimethylsilyl)ethanols, which were applied to the synthesis of di- and trisubstituted pyrazoles via [3+2] cycloaddition reaction with ethyl propiolate or dimethyl acetylenedicarboxylate <07S3371>. 

Diazonium intermediates have also been employed in the synthesis of pyrazoles. A convenient one-pot procedure for the preparation of 3-phenyl- or 3-pyridylpyrazoles 27 from the 1,3-dipolar cycloadditions of phenylacetylene or 3-(pyridyl)acetylene with diazo compounds 26 generated in situ from aldehydes 25 has been reported <03JOC5381>. Cyclization of ortho-(arylethynyl)benzene diazonium salts 28 having substituents at the para-position of the aryl ring furnished indazoles 29 <03TL5453>. 

One unusual approach to the synthesis of pyrazoles 509 from azo compounds 510 under phase-transfer conditions for the generation of dichloro-carbene was proposed by Baldoni ef al. (89JHC241). 

In contrast to the synthesis of pyrazoles, carbene synthetic approaches to imidazoles are highly diversified. Photolytic reactions of 2//-azirines provide one of the well-known synthetic methods. When irradiated, these compounds undergo irreversible C—C bond cleavage to give nitrile ylides capable of reacting as 1,3-dipoles or as arylideneaminocarbenes both inter-... 

Cycloaddition of diazo and azido compounds with acetylenes constitutes a well-established method for the synthesis of pyrazoles and triazoles. Addition of diazomethane to ethynylphosphonates provides... 

In this section, all the reactions have in common the formation of an N—C bond as the last step in the synthesis of pyrazoles and indazoles. First, pyrazoles and then indazoles are discussed. Pyrazole itself and many other N f-pyrazoles have been obtained from A-allyl-A-nitrosamides <90H(30)789> to illustrate the reaction. Scheme 6 represents the case of pyrazole (7). The reaction of... 

As is also true for silicon and tin compounds, the high stability of boronates, particularly cyclic esters, allows them to be incorporated into and carried through as substituents in a range of reaction types, such as the synthesis of pyrazole boronates for cross couplings (see 4.2.7.4). 

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