Chemically and photochemically induced radical couplings of xanthyrones and glaucyrones
3′,3′-Diacetyl-3,5-bismethoxycarbonylxanthyrone (1) undergoes dehydrodimerisation leading to racemic and meso-products (3) and (6) from 1′[1′] radical coupling, when treated with manganese dioxide. Also formed is a 2,8-dioxabicyclo[3.2.1]oct-3-ene (7) apparently arising by 1′[3′] coupling followed by cyclisation: the structure is established by X-ray diffraction. A similar bicyclo-system (11) is formed from diethyl 2,4-diacetylglutaconate (10). 3,3′-Diacetyl-5,5′-bismethoxycarbonylglaucyrone (2) undergoes 7 radical coupling to give (13) either photochemically in the presence of acetone as hydrogen acceptor, or by one-electron oxidation by manganese dioxide.