Pyrrolizidine alkaloids - A carbon-13 N.M.R. study

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Abstract
A carbon-13 n.m.r. data base on 30 pyrrolizidine alkaloids (19 new) of the crotalane and senecane series and a group of alkaloids containing non-cyclic ester functions is presented. Attention is focused on the effects of macrocyclic ring size, some hydrogen bonding, substituents and in particular of stereochemistry on the 13C n.m.r. chemical shifts. Useful observations in the 13C-1H coupled spectra of these molecules are described, which may be used as an aid in the characterization of this group of alkaloids. Deuterium isotopic shifts have been used in some highly oxygenated (hydroxyl) systems to confirm assignments. A new structure for grantaline (6) is confirmed and an alkaloid previously identified as crispatine (4) is suggested to be a new diastereoisomer.