Free radical cyclizations in alkaloid synthesis: (+)-heliotridine and (+)-hastanecine

Treatment of phenylthiolactams 5a–f with tri-$\text{n}$-butyltin hydride and AIBN affords mixtures of reduction and cyclization products. Cyclization products partition between indolizidinones and pyrrolizidinones depending on the terminal alkyne substituent. When the terminal substituent is a trimethylsilyl group, synthetically useful yields of pyrrolizidinones are obtained. Applications of this chemistry to the synthesis of (+)-heliotridine (2) and (+)-hastanecine (3) via the key intermediates 37 and 38 are described.