1,2‐Asymmetric Induction in Diastereoselective Zwitterionic Aza‐Claisen Rearrangements: Key Steps in Optically Active Alkaloid Synthesis

The zwitterionic aza‐Claisen rearrangement of optically active N‐allylpyrrolidines and α‐phenoxyacetyl fluorides proceeds with complete simple diastereoselectivity (internal asymmetric induction) and complete 1,2‐asymmetric induction to generate a new C–C bond adjacent to a chiral C‐N‐Boc functionality. The resulting γ,δ‐unsaturated amides were cyclised to give the corresponding pyrrolizidinones, which enabled the determination of the relative configuration of the stereotriads. Vinyl group degradation and a final lactam reduction gave an optically active analogue of (+)‐petasinine (a pyrrolizidine alkaloid). Furthermore, the stereotriad‐containing amides should be useful key intermediates for the total synthesis of optically active elaeocarpin (an indolizidine alkaloid).