Direct observation by 1H NMR of 4,5-benzoxepin-2,3-oxide and its surprisingly rapid ring-opening rearrangement to 1H-2-benzopyran-1-carboxaldehyde

The first unambiguous observation of an oxepin-2,3-oxide is reported. Between 5 and 10 °C it rearranges rapidly to its isomer 1H-2-benzopyran-1-carboxaldehyde. In contrast, 2,3-oxides of monocyclic oxepins rearrange to stable, ring-opened dialdehydes or diketones.