Shielding of H7β in pyrrolizidine derivatives

Abstract

The conformations of 3-oxopyrrolizidine derivatives are discussed on the basis of an analysis of the nuclear magnetic resonance spectrum of 3,5-dioxopyrrolizidine and partial data for methyl 2,3- dioxopyrrolizidine-1-carboxylate and methyl 2β-hydroxy-3- oxopyrrolizidine-1a-carboxylate. In each compound, H 7β is located forward into the fold of the rings and is subject to a shielding influence which is thought to be due to the C 8-C 1 single bond.