Synthesis of osajaxanthone

Abstract

Prenylation of 1,3-dihydroxy-7-methoxyxanthone (1) with prenyl bromide and methanolic methoxide yielded a mixture of 2-C-prenyl derivative (4a), 2,4-di-C-prenyl derivative (2) and 3-O-prenyl derivative (3). 2-C-Prenyl derivative (4a) with HI gave a mixture of dihydro-osajaxanthone (8) and its angular isomer (9). The acetate of the former with excess of NBS gave a dibromo derivative which on dehydrobromination with pyridine and deacetylation with alcoholic potash formed 4'-bromo-osajaxanthone (11) in good yield. Refluxing with zinc and glacial acetic acid afforded osajaxanthone (13) identical with a natural sample.