POLYKETO-ENOLS AND CHELATES - CHEMISTRY OF THE FORMATION OF XANTHOPHANIC ENOL AND ITS GLUTACONATE AND PYRAN INTERMEDIATES

The formation mechanism for xanthophanic enol (3) under 'melt' conditions is clarified by stepwise synthesis via 2,4-diacetylglutaconic ester (1) and the anion from methyl 5-acetyl-6-oxo-2-methyl-6H-pyran-3-carboxylate (11). Dimethyl 2,4-diacetylglutaconate exists mainly as the cyclic form (4) in solution, and increasing in solvent polarity, increasing temperature, or catalysis by pyridine, promotes n.m.r. equivalence of the acetyl methyls. In the case of 1,1,3,3-tetra-acetylpropene, first two, and then all four acetyl methyl signals coalesce showing an equilibrating system of cyclic structures involving hemi-acetal and Z-E changes. 1,1,3,3-Tetrakismethoxycarbonylpropene has a normal acyclic structure (9). Glutaconate (1) gives methyl 5-acetyl-6-oxo-2-methyl-6H- pyran-3-carboxylate (11) when treated with sodium methoxide (0.5-4.0 mol): at higher concentrations (12 mol) the aldol product dimethyl 5-hydroxytoluene-2,4- dicarboxylate (15) and its half-ester are formed. With magnesium methoxide as base, 1 mol or less gives the pyran (11), but the compounds with 2 mol and above are the Claisen product methyl 5-acetyl-2,4-dihydroxybenzoate (18) and the aldol product (15). These differing responses to bases are considered, and the effects of magnesium chelation on reactivity are discussed in terms which explain why Claisen condensations are favoured whilst aldol condensations are disfavoured during anion attack on the chelate.