Nuclear magnetic resonance spectra of pyrrolizidine alkaloids. II. The pyrrolizidine nucleus in ester and non-ester alkaloids and their derivatives

Abstract

Chemical shifts and coupling constants are tabulated for the protons of the pyrrolizidine nucleus in 40 pyrrolizidine alkaloids and derivatives. The effect of acylation of hydroxyl substituents on C7 and C9 and the very large difference in chemical shift of the H9 protons in the macrocyclic diester alkaloids is discussed. The direction of buckling of the saturated ring can be ascertained from the H5,H6 vicinal coupling constants or from the width of the H7 multiplet if H7 bears an oxygen substituent. In general, retronecine derivatives are exo-buckled whereas heliotridine derivatives consist of interconverting exo- and endo-buckled forms.