Polyketo-enols and chelates. Part II. The chemistry of the xanthophanic enols
Study is made of the chemistry of the xanthophanic enols. On boiling with water pyrone decarboxylation occurs to give, in the case of dimethyl xanthophanic enol, methyl acetoacetate, methyl 2-hydroxy-5-acetylbenzoate, and 3-acetyl-8-methoxycarbonyl-5-methylcoumarin. These products are consistent with our new structure but not with Feist's earlier proposal and other alternatives; this is further emphasised when the treatment is extended to mixed-ester xanthophanic enols.
When treated with sodium methoxide a 5-substituted 6-hydroxy-4-methyl-1,3-benzenedicarboxylate is obtained from dimethyl xanthophanic enol. Similarly, magnesium methoxide with less than 2 mol. quantities gives a coumarin and a ketal, both 5-substituted 6-hydroxy-4-methyl-1,3-benzenedicarboxylates. When an excess of magnesium methoxide (e.g., 6 mol.) is employed, however, a fourth compound, a 2,4-dihydroxybenzoate are with a substituted side-chain at position 5, is obtained instead in high yield. Its formation is explained as involving Claisen cyclisation of the carbanion from the bismagnesium chelate whereas the 1,3-benzenedicarboxylate mode of cyclisation is by a Michael-aldol reaction. The principle of such control of cyclisation by metal ions may be of biogenetic and synthetic interest. The schemes proposed are consistent with marked ester experiments and the reactions of 3′,3′,5-triacetyl-3-ethoxycarbonyl-6-propenyl-α-pyrone.
Whereas diethylxanthophanic enol and p-bromophenylhydrazine give a substituted pyridone, hydrazine yields a substituted pyrazolopyridine carboxylic acid with elision of an acetoacetate fragment. A mechanism is suggested. In concentrated sulphuric acid both dimethyl and diethylxanthophanic enol give the same gold-coloured compound. Mainly on the basis of proton magnetic resonance and other spectral information this is formulated as an enolised acetylglutaconic anhydride substituted in the 3-position by an acetylpyrone unit. A mechanism of formation is outlined.