Syntheses and regiochemistry of enol addition to 9-phenyl-9H-xanthen-9-ol

Regioselective C–C bond formation of 9-phenyl-9H-xanthen-9-ol 1 with various enolizable ketones I–X in an acidic (HBr) medium, obtained by the reaction of 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) with ketone is observed. Except for ketone, 4-methyl-pentan-2-one VII in all other cases examined the attack to xanthenyl carbocation is from the thermodynamically stable enolizable side of the unsymmetrical ketones. In the case of 3-methyl-butan-2-one VIII the equilibrium is in favor of the more stable enolizable ketone, which has large steric factor, hence no reaction was observed during its addition to alcohol 1.