3.4.5- Trisubstituted pyrazoles synthesis

Application of the Knorr pyrazole synthesis has also been demonstrated on solid support. ° To prepare trisubstituted pyrazoles, the diketone was linked to the solid support to make 57 using a linker with an amide bond. Alkylation of the diketone followed by condensation of the hydrazine with the resulting diketone gave the desired pyrazoles as mixtures of isomers. Subsequent cleavage of the amide bond linker then provided the pyrazole amides 59. ... 

The groups of Giacomelli and Taddei have developed a rapid solution-phase protocol for the synthesis of 1,4,5-trisubstituted pyrazole libraries (Scheme 6.194) [356]. The transformations involved the cyclization of a monosubstituted hydrazine with an enamino-/8-ketoester derived from a /8-ketoester and N,N-dimethylformamide dimethyl acetal (DMFDMA). The sites for molecular diversity in this approach are the substituents on the hydrazine (R3) and on the starting j3-keto ester (R1, R2). Subjecting a solution of the /8-keto ester in DMFDMA as solvent to 5 min of microwave irradiation (domestic oven) led to full and clean conversion to the corresponding enamine. After evaporation of the excess DMFDMA, ethanol was added to the crude reaction mixture followed by 1 equivalent of the hydrazine hydrochloride and 1.5 equivalents of triethylamine base. Further microwave irradiation for 8 min provided - after purification by filtration through a short silica gel column - the desired pyrazoles in >90% purity. 

Novel applications of benzotriazole methodology in organic synthesis include regiospecific preparation of 1,4,5-trisubstituted pyrazoles <2007ARK(i)9>, efficient synthesis of 1,5-disubstituted tetrazoles <2007SL1204>,... 

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>. 

Willy B, Muller TJJ (2008) Regioselective three-component synthesis of highly fluorescent 1,3,5-trisubstituted pyrazoles. Eur J Org Chem 4157 168... 

The orientation of the substituent gtoups in 1,2,4-oxadiazole substituted pyrazoles 39, formed by reaction of benzonittile oxides with an unsymmetrically substituted hydrazine, has been determined by C NMR assignments <1998JHC161>. The scope and limitations in the regioselective synthesis of 1,3,5-trisubstituted pyrazoles from / -amino enones and hydrazine derivatives were investigated by C chemical-shift prediction mles for 1,3,5-trisubstituted pyrazoles <2001H(55)331>. 

Solid-phase synthesis of substituted pyrazolones 550 from polymer-bound /3-keto esters 549 has been described (Scheme 68) <2001EJ01631>. Trisubstituted pyrazole carboxylic acids were prepared by reaction of polymer-bound arylidene- or alkylidene-/3-oxo esters with phenylhydrazines <1999S1961>. 2-(Pyrazol-l-yl)pyrimi-dine derivatives were prepared by cyclocondensation of ethyl acetoacetate and (6-methyl-4-oxo-3,4-dihydropyrimi-din-2-yl)hydrazine with aromatic aldehydes <2004RJC423>. Reactions of acylated diethyl malonates with hydrazine monohydrochloride in ethanol afforded 3,4-disubstituted-pyrazolin-5-ones <2002T3639>. Reactions of hydrazines with A -acetoacetyl derivatives of (45 )-4-benzyloxazolidin-2-one (Evans oxazolidinone) and (2R)-bornane-10,2-sultam (Oppolzer sultam) in very acidic media gave pyrazoles retaining the 3(5)-chiral moiety <1999S157>. 

Table 8 Tpx ligands derived from trisubstituted pyrazole rings and references to their synthesis.

Synthesis of l-iodo-3,4,5-trisubstituted pyrazoles 25 (R=Me, X=C1, Br, I, Me) may be carried out by the reaction of the corresponding silver salts of pyrazole with iodine (70CB1949). 

The synthesis of 3-dimethyl-2-benzoyl propenenitriles l(a-b) is the vital key intermediate for the synthesis of various nitrogen heterocycles, such as pyrazole and pyrimidine derivatives. The literature reports suggest that 1,3,4-trisubstituted pyrazole derivatives are important compounds in the preparation of 1,5-diphenylpyrazole nonnudeoside derivatives, which are used as HIV-1 normucleoside reverse transcriptose inhibitors [15]. Similarly, 4-cyano pyrazole... 

A small library of alkyl, sulfone, and carboxamide-functionalized pyrazoles and isoxazoles has been developed via a rapid sequential condensation of various R-acylketene dithioacetals with hydrazine hydrate or hydroxylamine hydrochloride, followed by oxidation of sulfide to sulfone using water as the reaction medium [3] (Scheme 8.3). An efficient and safe oxidation of sulfides to the corresponding sul-fones using sodium per borate system in aqueous medium has also been reported. The concise and two-step synthesis of trisubstituted pyrazoles and isoxazoles was investigated under a variety of reaction conditions. The newly developed methodology has the advantage of excellent yield and chemical purity with short reaction time using water as a solvent. 

Paulson, A. S. Eskildsen, J. Vedso, P. Begtrup, M. Sequential functionalization of pyrazole 1-oxides via regioselective metalation synthesis of 3,4,5-trisubstituted 1-hydroxypyra-zoles. /. Org. Chem. 2002, 67, 3904-3907. 

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