Expedient Pyrrolizidine Synthesis by Propargylsilane Addition to N-Acyliminium Ions Followed by Gold-Catalyzed α-Allenyl Amide Cyclization.

A reaction sequence, involving the addition of (substituted) propargylsilanes to lactam-derived N-acyliminium ions followed by gold-catalyzed cyclization of the resulting α-allenyl amide, is applied in expedient syntheses of pyrrolizidine alkaloids heliotridine and ent-retronecine in five steps from (S)-malic acid.