Gas-phase chemistry of acylium ions. Seven-to-five ring contraction of 1,3-dioxepane and 1,3-dioxep-5-ene

Abstract

As shown by pentaquadrupole triple-stage mass spectrometric and O-18-labeling experiments, two seven-membered cyclic acetals, 1,3-dioxepane and 1,3-dioxep-5-ene, fail to react by transacetalization with the gaseous acylium ions CH3C+=O and (CH3)(2)NC+=O. Instead, a novel and less exothermic but more kinetically favored reaction, seven-to-five ring contraction, occurs predominantly, and to great extents with the most reactive acylium ion, (CH3)(2)NC+=O. 1,3-Dioxepane yields O-acylated tetrahydrofurans; 1,3-dioxep-5-ene yields O-acylated 2,5-dihydrofurans. Copyright (C) 1999 John Wiley & Sons, Ltd.