STRUCTURE OF MESEMBRANOLS AND ABSOLUTE CONFIGURATION OF MESEMBRINE AND RELATED ALKALOIDS

The relative configurations of the epimeric alcohols, (-)-mesembranol (2) and (-)-6-epimesembranol (3), have been established by spectral studies of the alcohols and their Oacetyl derivatives 4 and 5. Supporting evidence for the configurational assignments is presented from the saponification rates of 4 and 5 and rates of acetylation of the alcohols. In the latter reaction an unusually facile acetylation of 6-epimesembranol is consistent with neighboring group participation by the nitrogen to account for the rate enhancement. Analysis of the nmr spectrum of the alkaloid mesembrine indicates that it also exists predominantly with ring B in the chair conformation 1A in which the aryl substituent is quasi-axial. Interpretation of the CD spectrum of mesembrine on the basis of this conformation for ring B leads to a reassignment of the absolute configuration of mesembrine as indicated by structure 1. A discussion of the conformational features of mesembrine is presented. © 1969, American Chemical Society. All rights reserved.