Enantiospecific Synthesis of the Bridged Pyrrolizidine Core of Asparagamine A: Dipolar Cycloadditions of Azomethine Ylides Derived from the Sulfonylation of Vinylogous Amides

An important precursor to the bridged pyrrolizidine core 2 of asparagamine A is vinylogous amide 1. The key reaction in the synthesis of 2 is an intramolecular 1,3‐dipolar cycloaddition of an azomethine ylide derived from the sulfonylation of 1. A variety of highly functionalized pyrrolidines, pyrrolizidines, and indolizidines can be prepared from vinylogous amides such as 1. Tf=trifluoromethanesulfonyl.