Brønsted acid catalyzed cyclization of hydroxylated enynes: a concise synthesis of five-membered heterocycles

Abstract

A mild and direct pathway for the formation of five-membered heterocyclic compounds from hydroxylated enynes has been developed. In this reaction, hydroxylated enynes were selectively transformed into five-membered heterocyclic compounds 2, with an allene moiety at the 3-position, in the presence of F(3) CSO(3) H (0.1 mol %). When R(1) , R(2) =Ph, diphenylvinyl-2,3-dihydro-1H-pyrrole (2 y) was obtained. With HSbF(6) (5 mol %) as the catalyst, polycyclic skeletons 3 and 4 with adjacent stereocenters were obtained. When R(1) =H and R(2) =styrene, 1,3-dienyl-2,5-dihydro-1H-pyrrole (6 as) was formed. This Brønsted acid catalyzed domino process involves the formation of an allene carbocation intermediate, which can be readily trapped by olefins to give various novel five-membered heterocyclic skeletons.