The role of SET in the deprotection of (Thio)ketals under photosensitization by pi-acceptors

Efficient deprotection of 2-methyl-2-tert-butyl-1,3-dithiolane and of the corresponding oxathiolane to give the ketone is obtained by photosensitization by a catalytic amount of aromatic nitriles (benzenetetncarbonitrile, TCB, and anthracenedicarbonitrile. DC4 the latter only in the presence pf biphenyl) or of heterocyclic salts (triphenylpyrylium tetrafluoborate, TPPT, and methylacridinium perchlorate. MAP), in oxygen saturated solutions. The reaction involves SET to the excited pi-acceptor and interception of the substrate radical cation either by oxygen (in the case of the heterocyclics) or by the superoside anion (in the case of the nitriles). Triplet chloranil (Chi) acts both as oxidant and as proton acceptor. and thus deprotection is accompanied by formation of a stoichiometric amount of the hydroquinone ChlH(2). The corresponding dioxolane is a weaker donor: SET occurs only with photoexcited TCB and causes C-C bond fragmentation, not deprotection. The other pi-acceptors photosensitize the deprotection of the dioxolane, but in this case they act as Lewis acids (Chi gives a trimer under this condition). This method is unsatisfactory with protected aldehydes. The different reactivity of the radical cations of thioketals and of ketals and the scope of this deprotection method are discussed. (C) 1997, Elsevier Science Ltd.