Intramolecular nucleophilic assistance in the reactions of OO′-benzylidenecatechols

Abstract

3-(2-Hydroxyphenoxy)phthalide is an intermediate in the hydrolysis of OO′-(2-carboxybenzylidene) catechol in aqueous buffers of pH 2·9–5. This reaction is general acid catalysed by carboxylic acids and also occurs spontaneously in a reaction which probably involves water as a general acid catalyst. The catalytic constants for the reactions catalysed by acetic and formic acids are similar to those of the analogously catalysed hydrolyses of OO′-(4-carboxybenzylidene) catechol, but the catalytic constant for the spontaneous reaction is ca. 30 times greater than that for spontaneous hydrolysis of the latter compound. This is attributed to nucleophilic assistance in reaction of OO′-(2-carboxybenzylidene) catechol. In the pH range 2–5, 2,3-OO′-(2-carboxybenzylidene)-2,3-dihydroxybenzoic acid is converted into 3-(3-carboxy-2-hydroxyphenoxy)phthalide. The pH-rate profile is bell-shaped but at the maximum the reaction occurs only ca. three times faster than the hydrolysis of 2,3-OO′-(4-carboxybenzylidene)-2,3-dihydroxybenzoic acid. The neighbouring carboxylate group of the aldehyde moiety of this acid does not provide appreciable anchimeric assistance in the intramolecularly catalysed opening of the dioxolan ring.