1- methyl-3-ethynylpyrazole

The composition of the mixture of products of different structures depends on the diacetylene diazomethane ratio (68LA124). With a 1 1 ratio of butadiyne and diazomethane, 3(5)-ethynylpyrazole dominates (55%). The yields of isomeric 3- and 5-ethynyl-l-methylpyrazoles are 8 and 11%, respectively. The double excess of diazomethane leads mainly to a mixture of N-methylated isomers (81%), 10% of 3(5)-ethynylpyrazole, and a small amount (3%) of bipyrazole (68LA124) (Scheme 3). 

Using chloro- or bromoenines, one can immediately obtain 4-ethynylpyrazoles (omitting the stage of pyrazolenine isolation) which is accompanied by methylation and the formation of the mixture of 3- and 5-sulfonyl-l-methyl-4-acetylenylpyra-zoles in about 1 2 ratio (total yield 37%) (Scheme 14). 

Ring-methylated 1-ethynylpyrazoles were similarly obtained as minor produets in the pyrolysis of 3,5-dimethyl- and a mixture of 3- and 5-methyl- 1-propynoylpyrazoles. Pyrolysis of the 3-methyl derivative gave only pyrazolo[l,5-a]pyridin-5-ol... 

Thus, the hydrogenation of isomeric l-ethynyl-3-methyl- and l-ethynyl-5-methylpyrazoles by hydrogen with 10% Pd/C in l-ethyl-3-methyl- and 1-ethyl-5-methylpyrazoles was used to prove the structure of iV-ethynylpyrazoles formed from pyrolysis of the corresponding iV-propynoylazoles (94AJC991) (Scheme 79). 

The effect of substituents in the ring on the mobility of a methyne proton can be followed from the series of 5-substituted l-methyl-4-ethynylpyrazoles and 1,3-dimethyl-4-ethynylpyrazoles. In both structural series, the acidity of compounds increases with a change in the character of substituents in the following order CH3 < H < CH2NH2 < Cl. 

The acidity of 4-acetylenenylpyrazoles increases by 0.3-0.7 log units on introducing CH2NH2 group into position 5 of the ring. Chlorine atoms have the greatest acidifying effect. Chlorine atom in position 5 increases the acidity of l-methyl-3-ethynylpyrazole. 

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