The first quantitative determination of conformational equilibrium in quinolizidine-piperidine alkaloids

The fraction of the conformer with the boat ring C in the conformational eq uilibrium in N-methylangustifoline (7) in chloroform and benzene solutions was determined to be ca. 34% using coupling constant J(7-13 beta). In angus tifoline (5) J(7-13 beta) could not be determined directly from the spectra ; a simulation gave the result of 4.25 Hz corresponding to ca. 23% of the b oat conformer. In tetrahydrorhombifoline (6), rings B and C have a chair co nformation. Low temperature C-13 NMR measurements seem to corroborate these results qualitatively. Factors influencing conformational equilibria are d iscussed. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.