Selective transformation of acetonides to orthoesters: an application of a photoinduced electron transfer process

Irradiation of α,β-acetonides of polyols and sugars in the presence of benzene 1,2,4,5-tetracarbonitrile causes cleavage of a methyl radical and generates an α,α-dioxy carbocation. The subsequent nucleophile addition gives different kinds of derivatives depending on the acetonide structure. Cyclic othoesters are formed when a δ-hydroxyl group is present, while in the other case, intermolecular trapping by moisture, present in the solvent (acetonitrile) or by added alchols occurs and gives orthoacids. With galactopyranose acetonide, efficient intermolecular trapping by a second molecule of sugar has been obtained. Useful synthetic intermediates such as hydroxy acetates are in turn obtained by selective hydrolysis of the methoxyethylidene derivatives.