Synthesis of the mangostins

Abstract

Acylation of 5,7-bisbenzyloxy-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran (10c) with 6,8-dimethoxy-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-5-carboxylic acid (9b) in the presence of trifluoroacetic anhydride gave 5,7-dibenzyloxy-3,4-dihydro-2,2-dimethyl-2H-1- benzopyran-8-yl, 3,4-dihydro-6,8-dimethoxy-2,2-dimethyl-2H-1-benzopyran-5-yl ketone (8c), which was converted, in three steps, into dimethyl-1-isonormangostin (7). Reaction of (7) with boron trichloride resulted in the cleavage of the two terminal pyran rings to give 1,7-bis-(3-chloro-3-methylbutyl)-2,8-dihydroxy-3,6-dimethoxyxanthen-9-one (6a). Selective methylation of (6a) afforded the known 1,7-bis-(3-chloro-3-methylbutyl)-8-hydroxy-2,3,6-trimethoxyxanthen-9-one (6b). Whereas dehydrochlorination of the methoxycarbonyloxy derivative (6c) of (6b) with lithium chloride in dimethylformamide gave a mixture from which dimethylmangostin (1d) and 1,7-bis-(3-methylbut-3-enyl)-8-hydroxy-2,3,6-trimethoxyxanthen-9-one (22c) were isolated by preparative high pressure liquid chromatography, that of (6b) with potassium t-butoxide in dimethyl sulphoxide gave a mixture from which β-mangostin (1b) was obtained, by the same technique.