Substituent and solvent isotope effects on the reaction of 2-alkylnaphth[1,8-de]-1,3-oxazines to 1-hydroxy-8-acylaminonaphthalenes through reversible formation of a cyclic tetrahedral addition intermediate

Abstract

The first-order rate coefficient for the addition of water to substituted 2-alkylnaphth[1,8-de]-1,3-oxazines to give the corresponding 1-hydroxy-8-acylaminonaphthalenes in aqueous solution at pH 0 to 6 goes through a maximum value with pH. Substituent effects and solvent isotope effects on the rate–pH profile are explained by a mechanism involving reversible addition of water to the protonated naphthoxazine to give a cyclic tetrahedral intermediate and hydromum ion followed by ring-opening of the intermediate to form product. In aqueous alkaline solution and in 50%(v/v) Me₂SO–H₂O containing hydroxide ion the reaction is first-order with respect to hydroxide ion and involves nucleophilic addition of hydroxide ion to the neutral naphthoxazine.