Selected organic syntheses

This chapter presents a case history of selected syntheses of a class of natural products known as the pyrrolizidine alkaloids to bring together the aspects of organic synthesis. The pyrrolizidines are a group of alkaloids which are characterized by the presence of the basic necine skeleton. The pyrrolizidine alkaloids exhibit remarkably diverse types of biological activity and have been reported to act as antitumor agents. The chapter then describes four methods for the synthesis of the pyrrolizidine alkaloids. Each route has its own particular merits, e.g. the route to platynecine gives the highest yield, the second method for the synthesis of retronecine uses stereoselectivity to good effect, and of course, the route to (+)–heliotridine is the only enantioselective synthesis.