A new synthesis of bicyclic N,O- and N,S-enaminals by the anionic cyclization of alk-4-ynals with amino alcohols and amino thiols

A reaction of alk-4-ynals with aliphatic amino alcohols or 2-aminoethanethiol in the system DMSO-KOH gives bicyclic N,O- and N,S-enaminals: 6-methylidenehexahydro-2H-pyrrolo[2,1-b][1,3] oxazines, 5-methylidenehexahydropyrrolo[2,1-b] oxazoles, or 5-methyl-idenehexahydropyrrolo[2,1-b] thiazoles. The reaction proceeds through the formation of equilibrium mixtures of the corresponding imines and monocyclic aminals with subsequent 5-exo-dig-cyclization catalyzed by the superbasic system DMSO-KOH.