Unusual anthocyanin reaction with acetone leading to pyranoanthocyanin formation

Anthocyanins undergo unusual facile reactions with acetone to give rise to pyranoanthocyanins, thus showing that the new class of pigments isolated from blackcurrant seeds were a result of solvent participation. This oxidative addition was extended to the successful synthesis of pyranomalvidin or 2-(3,5-dimethoxy-4-hydroxyphenyl)-8-hydroxy-5-methyl-3-O-β-d-glucopyranosyloxypyrano[4,3,2-de]-1-benzopyrylium by the reaction of the principal grape anthocyanin malvidin 3-O-glucoside with acetone. The chemical structures of pyranomalvidin, together with pyranocyanin B and pyranodelphinin B, were fully characterised by 2D NMR.