REVISED STRUCTURES FOR PLUVIATILOL, METHYL PLUVIATILOL AND XANTHOXYLOL - GENERAL METHODS FOR ASSIGNMENT OF STEREOCHEMISTRY TO 2,6-DIARYL-3,7-DIOXABICYCLO[3.3.0]OCTANE LIGNANS

The isolation and characterization of methyl piperitol, (+)-epieudesmin and (+)-episesamin from the stem bark of Zanthoxylum acanthapodium was reported. The 1H and 13C NMR spectra are correlated with those of other known lignans and 13C NMR shifts can be used to assign unambiguously the nature and configuration of the aryl groups in unsymmetrically substituted 2,6-diaryl-3,7-dioxabicyclo[3.3.0]octanes. Other less satisfactory methods which were used for this purpose are discussed and in particular the use of coupling constants to assign stereochemistry in this series is strongly criticized. Revised structures for pluviatilol, methyl pluviatilol (fargesin) and xanthoxylol are proposed.