Ring transformation of 5‐acylpyrimidines into 4‐acylpyrazoles with hydrazine and phenylhydrazine in acidic medium

Abstract 5‐Benzoyl‐4‐methylpyrimidines 4a,b and 5‐acetyl‐4‐phenylpyrimidines 5a,b reacted with hydrazines in alcoholic acidic medium to give respectively 4‐acetyl‐3‐phenylpyrazoles 7, 9 and 10 and 4‐benzoyl‐3‐methylpyrazoles 6, 8 and 11. In the reaction with phenylhydrazine, 5‐benzoyl‐4‐methyl‐2‐methylthiopyrimidine (4a) led exclusively to 4‐acetyl‐1,3‐diphenylpyrazole (10) as 5‐acetyl4‐phenyl‐2‐methylthiopyrimidine (5a) led to 4‐benzoyl‐3‐methyl‐1‐phenylpyrazole (11) via the initial formation of phenylhydrazones of pyrimidines 4a and 5a. However, 5‐benzoyl‐4‐methyl‐2‐phenylpyrimidine (4b) and 5‐acetyl‐2,4‐diphenylpyrimidine (5b) reacted with phenylhydrazine to afford, each of them, a mixture of two isomeric pyrazoles. The mechanism of these ring contraction reactions is discussed.