Polyketoenols and chelates. The mechanism by which glaucophanic enols are formed
Attempts to make dimethyl glaucophanic enol (4) by coupling the anion from 3-acetyl-5-methoxycarbonyl-6-methyl-2-pyrone (2)(2 mol equiv.) through a one-carbon source were unsuccessful, as were attempts to derive it by treating the anion with dimethyl xanthophanic enol (5). However, a ‘melt reaction’ between pyrone (2)(2 mol equiv.), methyl methoxymethyleneacetoacetate (1 mol equiv.), and dry sodium methoxide gave glaucyrone (4)(49%). By using methyl methoxy[14C]methyleneacetoacetate in the reaction, labelled glaucyrone (4) and xanthyrone (5) were obtained with the same specific activities. The central (C-8) position of the label in (4) was shown by magnesium methoxide-catalysed transformation into the chalcone (7) which was ozonised to (8) and (9). Extraction of the atom, which corresponds to C-8 in (4), from the latter by Dakin reaction accounted for 89% of the original radioactivity. The two residual aromatic fragments each contain 4% of the activity, which can be explained by equilibration of the label into the pyrone rings of (4). A unified mechanism for the formation of xanthyrones and glaucyrones in ‘melt reactions’ can now be advanced and factors favouring one or the other product understood. This is supported by comparison of the xanthyrone–glaucyrone-forming ability of five alkoxymethylene compounds.