DOUBLE-BOND MIGRATION IN PYRANO[4,3-B]PYRYLIUM CATIONS

That hydrolysis of [1]benzopyrano[4,3-b] [1]benzopyrylium derivatives 1 includes a double-bond shift giving derivatives 5 of 3- (2-hydroxybe nzyl) [1] benzopyran-4-ones has been confirmed. Chromones like 5 are r elated to the 7H-[1]benzopyrano[3,2-c][1]benzopyrylium cations 9 into which they are transformed by strong acids. The double bond shift occu rs in the cations and is mediated not by prototropy but by hydride exc hanges via the 6H,7H-[1]benzopyrano[4.3-b][1]benzopyrans 6, thus: 1 6 half arrow pointing right over half arrow pointing left 6 + 1 and 1 + 6 half arrow pointing right over half arrow pointing left 6 + 9. The equilibria favour the cations 9 which are hydrolysed by water to the chromones 5. Initiation occurs when the cations 1 add water giving the alcohols 2 which reduce them to the catalysts 6 and are themselves ox idised to 6H,7H-[1] benzopyrano[4,3-b] [1] benzopyran-7-ones 7.