Sequential [1 + 4]- and [2 + 3]-Annulation of Prop-2-ynylsulfonium Salts: Access to Hexahydropyrrolo[3,2-b]indoles.

A powerful sequential [1 + 4]- and [2 + 3]-annulation has been developed using prop-2-ynylsulfonium salts and sulfonyl-protected o-amino aromatic aldimines, affording a series of hexahydropyrrolo[3,2-b]indoles in high yields. Prop-2-ynylsulfonium salts act as C2 synthons in the reaction, and fused rings containing two five-membered azaheterocycles can be constructed in a single operation with readily accessible starting materials.