Unusual Diastereoselection in the Synthesis of Nine-Membered Ring Lactams and Conformation-Controlled Transannular Reactions to Generate Optically Active Indolizidinones

The aza‐Claisen rearrangement of vinylpyrrolidines 1 yielded almost exclusively the trans‐3,8‐disubstituted nine‐membered ring lactams 2 (TBS=tBuMe2Si), independent of whether cis or trans isomers were used as starting materials. The conformation (which provided facial chirality) of the medium‐sized ring controlled the regio‐ and diastereoselectivities of the transannular reactions that afforded indolizidinones 3.