Conjugate Additions, Aza-Cope, and Dissociative Rearrangements and Unexpected Electrocyclic Ring Closures in the Reactions of 2-(2-Pyrrolidinyl)-Substituted Heteroaromatic Systems with Acetylenic Sulfones

The reactions of 2-heteroaryl-substituted pyrrolidines with acetylenic sulfones proceeded via two pathways. The first involves conjugate addition of the pyrrolidine to the acetylenic sulfone to afford a zwitterion, followed by dissociation of the C−N bond and recombination of the resulting carbocation and vinyl anion to afford the corresponding azepine derivative. The second comprises a cascade of conjugate addition, aza-Cope rearrangement and anionic 6π electrocyclic ring-closure steps. The stability of the carbocation intermediate formed by C−N cleavage determines the dominant pathway.