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.
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