Pyrrolizidine alkaloid biosynthesis. Synthesis of 14C-labelled homospermidines and their incorporation into retronecine

Abstract

[1,9-¹⁴C]Homospermidine (12) was synthesized and fed to Senecio isatideus plants. The ¹⁴C radioactivity was located predominantly in the base [retronecine (2)] portion of the alkaloid. Degradation of the retronecine showed that 43–44% of the radioactivity was at C-9 and 2–4% was at C-(5 + 6 + 7). In a complementary experiment, [4,6-¹⁴C]homospermidine (15) was prepared and fed to S. isatideus plants. The ¹⁴C radioactivity was again located mainly in retronecine (2). Degradation of retronecine demonstrated that 1–4% of the radioactivity was at C-9 and 45–47% was at C-(5 + 6 + 7). Homospermidine is therefore incorporated without significant breakdown into retronecine. Homospermidine was also shown to be present in S. isatideus plants by an intermediate trapping experiment.