On the rearrangement mechanism of cyclic O,N-acetals derived from acylquinones and enamines

Abstract

A kinetic study of the acid-catalysed rearrangement of 4-acetyl-2,3-dihydro-3,3-dimethyl-5-hydroxy-2-(1-morpholino)- and 4-acetyl-2,3-dihydro-3,3-dimethyl-5-hydroxy-2-(1-piperidino)-benzo[b]furan (1a and 1b, respectively) as a function of the hydrochloric acid concentration, the reaction medium, and the dialkylamino group of the substrate is reported. The substrate (1a) shows hydrolysis and rearrangement reactions in dioxane–water (5.3 mol dm^–3 in water) mixture but (1b) does not exhibit hydrolysis reaction. Both substrates show a limiting [HCl] dependence in dioxane–water (5.3 mol dm^–3 in water) mixture but in ethanol–water (5.3 mol dm^–3 in water)(1b) shows a first order in [HCl] kinetic behaviour. In dioxane–ethanol (40% v/v ethanol) both substrates exhibit closely similar rate-constant values for the same HCl concentrations. The kinetic results are explained through a reaction scheme and discussed taking into account the basicity of the amino groups and the stabilisation of the ionic species involved in the probable rearrangement mechanism.