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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

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posted on 06.08.2010, 00:00 by Mitchell H. Weston, Masood Parvez, Thomas G. Back
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.