An unexpected double cycloaddition of [1,2,4]triazolo[1,5-a]pyrimidine N-ylide with activated acetylenes and alkenes

Abstract

The reaction of 5,7-dmethyl-[1,2,4]triazolo[1,5-a]pyrimidinio-3-phenacylide (5) with activated acetylenes gave 1 : 2 adducts of ylide–R–CC–R. The structures of the products were determined as 3,3a-dihydropyrazolo[1,5-c]pyrimidine derivatives (7) by hydrolysis, ¹H and ¹³C n.m.r., and X-ray crystallography. Molecular orbital calculations (ab initio) of the model compounds were performed in order to elucidate the mechanism for the formation of the 1 : 2 adducts. The results of the calculations suggested that an intermediate, 1 : 1 adduct would be less reactive than the starting ylide. However, despite many attempts isolation of the 1 : 1 adduct was unsuccessful. The 1 : 2 adduct was obtained even from the reaction of the ylide with 0.5 equiv. of the acetylenic compound. Easy formation of the 1 : 2 adducts can be explained by an equilibrium between the 1 : 1 adducts and the starting materials.