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

As shown by pentaquadrupole triple‐stage mass spectrometric and ¹⁸O‐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)₂NC⁺⁼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)₂NC⁺⁼O. 1,3‐Dioxepane yields O‐acylated tetrahydrofurans; 1,3‐dioxep‐5‐ene yields O‐acylated 2,5‐dihydrofurans.