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.