Extractives from Guttiferae. Part XXIX. Synthesis of celebixanthone methyl ether and related 1,5,6,7-tetraoxygenated xanthones

Abstract

The total synthesis of celebixanthone methyl ether [1,5-dihydroxy-6,7-dimethoxy-8-(3-methylbut-2-enyl)xanthen-9-one](2) is described. The route involves preparation of 2-hydroxy-2′,3,4,5,6′-pentamethoxybenzophenone (8) and subsequent base catalysed cyclisation to give 1,5,6,7-tetramethoxyxanthen-9-one (4). Selective demethylation and methylation gives 1,5-dihydroxy-6,7-dimethoxyxanthen-9-one (3). Preparation of the 5-(3-methyl-2-butenyloxy)xanthen-9-one (10) followed by a Claisen rearrangement gives celebixanthone methyl ether.