Conformations of eleven-membered dilactone rings in pyrrolizidine alkaloid analogues. X-Ray crystallographic studies of meso-2,4-dimethylglutaryl, 3,3-tetramethyleneglutaryl, and 3,3-dimethylglutaryl derivatives of synthanecine A

Abstract

The pyrrolizidine alkaloid analogues (2R,10R,12S)- and (2S,10S,12R)-6,7-O,O-(meso-2,4-dimethyl-glutaryl)synthanecine A (2), (±)-6,7-O,O-(3,3-tetramethyleneglutaryl)synthanecine A (4), and (±)-6,7-O,O-(3,3-dimethylglutaryl)synthanecine A (5) have been characterized by X-ray crystal structure analysis. The 11-membered dilactone ring in compound (2) has an unsymmetrical conformation in which both ester carbonyl groups are directed above the plane of the macrocycle. The dilactone rings in compounds (4) and (5) have conformations that approximate closely to C₂ symmetry with the ester carbonyl groups anti-parallel. N.m.r. data suggest that the conformations persist in solution.