The formation of 1,3-dioxepane-4-ylium ions ( 3 ) is achieved by protonation of 4,5-dihydro-1,3-dioxepines ( 2 ). These are prepared by π-bond isomerization of 4,7-dihydro-1,3-dioxepines ( 1 ) under basic conditions. Thermal fragmentation of 3 and subsequent recyclization lead...