Diastereoselective Zwitterionic Aza‐Claisen Rearrangement: Synthesis of Nine‐Membered Ring Lactams and Transannular Ring Contraction

The zwitterionic aza‐Claisen rearrangement of optically active 3‐pyrrolidine acryl esters and various acid chlorides to generate optically active azoninones proceeds with high simple diastereoselectivity (internal asymmetric induction) and a complete 1,3‐chirality transfer. The reaction path observed depends on the subsitution pattern of the allylic system: while the more electron‐rich alkylated allyl amine fromed predominantly von Braun type products, the α,β ‐unsaturated esters could be rearranged with high yields. The azoninones thus obtained were treated with electrophiles, inducing regio‐ and diastereoselective transannular ring contractions. The resulting indolizidinones should be useful key intermediates in alkaloid synthesis.