Pyrrolizidine alkaloid biosynthesis. Synthesis of 13C-labelled putrescines and their incorporation into retronecine

Abstract

The biosynthesis of the retronecine (3) portion of the pyrrolizidine alkaloid retrorsine (1) has been studied in Senecio isatideus plants using ¹³C-labelled putrescines. Samples of [1-¹³C]-(10), [1,4-¹³C₂,]-(8), and [2,3-¹³C₂]-putrescine (9) were prepared and used to obtain labelled retronecine. The labelling patterns, determined by ¹³C n.m.r. spectroscopy, demonstrated that two molecules of putrescine are incorporated to about the same extent into each molecule of retronecine. [1-amino-¹⁵N, 1-¹³C] Putrescine (11) was synthesized and fed to S. isatideus plants. The labelling pattern in the retronecine (12) produced was consistent with the formation of a later C₄–N–C₄ symmetrical intermediate in retronecine biosynthesis.