Intramolecular acyl transfer between ester and amide groups in 1-acyloxy-8-acylaminonaphthalene-3,6-disulphonates

Abstract

The reversible intramolecular acetyl transfer from 1-acetoxy-8-acetylaminonaphthalene-3,6-disulphonate to give 1-hydroxy-8-bis(acetyl)aminonaphthalene-3,6-disulphonate is promoted by hydroxide ion but catalysis by buffer bases is not observed. In the presence of 0.001 mol dm^–3 hydroxide ion in aqueous solution at 278 K the half-life for the approach to equilibrium is ca. 5 ms. Transfer of a propionyl group occurs at a similar rate. The mechanism involves rapid ionisation of the amide followed by intramolecular nucleophilic attack of the amide anion on the ester carbonyl. By comparison of the rate coefficient for this reaction with that for the intermolecular attack of hydroxide ion on the ester carbonyl, an effective molarity of ca. 1 × 10⁵ mol dm^–3 is calculated for the intramolecular process. In the reverse direction, the reaction involves intramolecular nucleophilic attack by the ionised naphthol on the imide carbonyl and by comparison with the rate of the intermolecular reaction of the imide with hydroxide ion an effective molarity of ca. 5 × 10⁵ mol dm^–3 is estimated.