CONFIGURATIONAL AND CONFORMATIONAL-ANALYSIS OF HIGHLY OXYGENATED PYRROLIZIDINES - DEFINITIVE IDENTIFICATION OF SAME NATURALLY-OCCURRING 7A-EPI-ALEXINES

The NMR spectra of a number of naturally occurring alexines (tetrahydr oxylated pyrrolizidine alkaloids) are analyzed and the consequences of changes in the configuration on the conformation of these bicyclic sy stems discussed. Unambiguous syntheses of australine (7-epi-alexine) a nd of 7,7a-epi-alexine have now unequivocally established the structur es of two natural products isolated from Castanospermum australe which were insecure due to erroneous NMR data. Chemical shift parameters ar e unreliable as a method of comparing different samples of identical c ompounds; however,H-1-H-1 three bond coupling constants ((3)J(HH)) pro vide easy direct comparison between samples and allow assignments of b oth the relative configurations for the ring protons and the conformat ion of the pyrrolizidine framework. (C) 1998 Elsevier Science Ltd. All rights reserved.