Double-bond migration in pyrano[4,3-b]pyrylium cations

Abstract

That hydrolysis of [1]benzopyrano[4,3-b][1]benzopyrylium derivatives 1 includes a double-bond shift giving derivatives 5 of 3-(2-hydroxybenzyl)[1] benzopyran-4-ones has been confirmed. Chromones like 5 are related to the 7H-[1]benzopyrano[3,2-c][1]benzopyrylium cations 9 into which they are transformed by strong acids. The double bond shift occurs in the cations and is mediated not by prototropy but by hydride exchanges via the 6H,7H-[1]benzopyrano[4,3-b][1]benzopyrans 6, thus: 1+6⇌6+1 and 1+6⇌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 oxidised to 6H,7H-[1] benzopyrano[4,3-b][1] benzopyran-7-ones 7.