N,N-Dimethylbenzamide andN,N-dimethylacetamide dimethyl acetals react with vicinalcis-diols to give cyclic α-(dimethylamino)benzylidene and 1-(dimethylamino)ethylidene acetals respectively. In aqueous methanol, the acetals are hydrolyzed to the original diol. At pH4–5, cleavage occurs to give benzoate and acetate esters, presumably by the sequential formation of acyloxonium and orthoacid intermediates. The acetals are thus useful as temporary protecting groups for vicinal diols and can be used for indirect, selective benzoylation and acetylation. They are compatible with the reaction conditions of acylation (acetylation, benzoylation, tosylation) and nucleophilic displacement