2-BENZOYLBENZOIC ACID - A PHOTOLABILE MASK FOR ALCOHOLS AND THIOLS

Photolysis of 2-benzoylbenzoate esters of primary and secondary alcoho ls 1 in the presence of a hydrogen donor (2-isopropanol) or an electro n donor (primary amines) produces the corresponding alcohol in high yi eld. The fate of the benzoate is dependent on the conditions used for the photoreaction. In 2-propanol, the ketyl radical that derives from photoreduction dimerizes, to afford the benzpinacol product 3,3'-diphe nylbiphthalidyl, 5. In the presence of amines the product is 3-phenylp hthalide, 6, a benzhydrol derivative which is the result of simple red uction of the ketone followed by lactonization. While the photoproduct of the benzoate-2-propanol reaction results from anticipated free rad ical chemistry, the amine-promoted reaction appears to result from a s econd, ''dark'', electron transfer process. We conclude that 2-benzoyl benzoic acid is an effective photolabile protecting group for primary and secondary alcohols, and preliminary studies indicate that thiols c an be protected in an analogous way. Studies on the effect of benzophe none substituents and reaction solvent on the benzhydrol:benzpinacol p roduct ratio provide mechanistic insight into the process.