Synthesis of 8-deoxygartanin. Trapezifolixanthone and related xanthones

Abstract

Reaction of 1,3,5-trihydroxyxanthone (6) with 3-methylbut-2-enyl bromide in the presence of methanolic sodium methoxide yields a mixture of 2,4-di(3-methylbut-2-enyl)-5-(3-methylbut-2-enyloxy) derivative (7), natural 8-deoxygartanin (3) and the 2-(3-methylbut-2-eny1)-5-(3-methylbut-2-enyloxy) derivative (8). Cyclodehydrogenation of (3) and (8) with DDQ affords natural trapezifolixanthone (5) and 5-(3-methylbut-2-eny1oxy)-6-deoxyjacareubin (9) respectively.