Generation of cationic 2-azabutadienes from N,S-acetals and their use for the regio- and diastereoselective synthesis of 1,2,3,4-tetrahydroquinolines by intermolecular [4 pi(+)+2 pi] cycloadditions

Abstract

Substituted 1,2,3,4-tetrahydroquinolines and related N-heterocycles are formed highly regio- and diastereoselectively with yields ranging from 57 to 100% by intermolecular polar [4 pi(+) + 2 pi] cycloadditions of cationic 2-azabutadienes and various dienophiles. The cationic 2-azabutadienes can be generated in situ by Lewis acid mediated heterolytic cleavage of N,S-acetals. Best results have been obtained using a new mixed Lewis acid consisting of a mixture of TiCl4 and PPh3.