Pyrrolizidine alkaloid biosynthesis. Incorporation of 14C-labelled precursors into retronecine

Abstract

The biosynthesis of the retronecine (3) portion of the pyrrolizidine alkaloid retrorsine (1) present in Senecio isatideus plants has been studied with the aid of ¹⁴C-labelled precursors. Putrescine (5), spermidine (6), and spermine (7), are the most efficient precursors of the pyrrolizidine ring. Degradations of labelled retronecine samples indicated that two molecules of each precursor are utilised, and that from these two molecules, a symmetrical intermediate of the type C₄–N–C₄ is formed and converted into retronecine.