Dienone-Phenol Rearrangement of (+ )-2,-Demethoxydehydrogriseofulvin into a 4-Methylxanthone Derivative

Treatment of (+)-2‘-demethoxydehydrogriseofulvin (2b) with magnesium iodide afforded 5-chloro-2,8-dihydroxy-6-methoxy-4-methylxanthone (3a) via dienone-phenol rearrangement. The structure of 3a was determined by means of a carbon-13 nuclear magnetic resonance (₁₃C-NMR) long-range selective proton decoupling (LSPD) experiment performed on its diacetate 3b. The rearrangement of 2b was also effected with p-toluenesulfonic acid to give 5-chloro-6,8-dimethoxy-2-hydroxy-4-methylxanthone (6a). On the other hand, reaction of (— )-dehydrogriseofulvin (2d) with p-toluenesulfonic acid under more vigorous conditions resulted in racemization, no rearrangement being observed.