Polyketo-enols and chelates. Chemistry of the formation of xanthophanic enol and its glutaconate and pyran intermediates

摘要: 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 cyclic form (4) in solution, increasing solvent polarity, temperature, or catalysis pyridine, promotes n.m.r. equivalence of acetyl methyls. In case 1,1,3,3-tetra-acetylpropene, first two, then all four signals coalesce showing an equilibrating system structures involving hemi-acetal Z–E changes. 1,1,3,3-Tetrakismethoxycarbonylpropene has a normal acyclic structure (9).Glutaconate gives (11) when treated with sodium methoxide (0.5–4.0 mol) : at higher concentrations (12 aldol product dimethyl 5-hydroxytoluene-2,4-dicarboxylate (15) its half-ester are formed. With magnesium base, 1 mol less pyran (11), but compounds 2 above Claisen 5-acetyl-2,4-dihydroxybenzoate (18) (15). These differing responses to bases considered, effects chelation on reactivity discussed terms which explain why condensations favoured whilst disfavoured during attack chelate.