Studies in the xanthone series. Part 13. Structural and synthetic studies on toxyloxanthone B

Abstract

A re-consideration of the ¹H n.m.r. data for toxyloxanthone B trimethyl ether and an unambiguous total synthesis both show that toxyloxanthone B has the 1,1-dimethylpyrano[3,2-a]xanthone structure (1a) and not the 3,3dimethylpyrano[3,2-a]xanthone system (2) previously assigned. The synthesis is based on the preparation of 1,7-dihydroxy-3,5-dimethoxyxanthone (5) from cyclisation of a benzophenone precursor (3a) or (3b) and selective demethylations. A Claisen rearrangement of the 7-prop-2-ynyloxyxanthone (6) followed by cyclisation and methylation gives toxyloxanthone B trimethyl ether.