Pyrrolizidine alkaloid analogues. Synthesis of 11-membered macrocyclic diesters of (±)-synthanecine A

Abstract

Treatment of (±)-synthanecine A (4) with various glutaric anhydride derivatives [(5a–f) and (7)] yielded the corresponding 6-monoesters of (±)-synthanecine A. Lactonisation of these monoesters was carried out by the Corey–Nicolaou method to give a range of 11 -membered macrocyclic diesters [(6a–f), (8), and (9)] of synthanecine A. The macrocyclic nature of these new pyrrolizidine alkaloid analogues was established by spectroscopic data and by comparison of their ¹H n.m.r. and mass spectra with those of macrocyclic pyrrolizidine alkaloids.