Conversion of xanthyrones into glutaconic anhydride–pyrone (GP) compounds in strong acid media: selective deuteriation as between chelated and unchelated acetyls

Abstract

The structure, and mechanism of formation, of the pyrone–glutaconic anhydride (GP1) obtained by treatment of dialkylxanthophanic enols with concentrated H₂SO₄ or F₃CCO₂H, is further supported, and a new example GP2 is prepared. Experiments with deuteriated acids show that deuterium is introduced at C-5′ and the unchelated acetyl of GP1, the second acetyl being apparently protected by chelation. This is supported by benzenoid test examples, though in the naphthalene series acetyl migration also occurs through deacylation/reacylation. An explanation of the protective effect of chelation against acid catalysed enolisation of the acetyl methyl is suggested.