Polyketo-enols and chelates. Part I. The formation and constitution of xanthophanic enol and the xanthyrones

Abstract

A new structure is proposed, on the basis of spectroscopic and chemical evidence, for the yellow compound diethyl xanthophanic enol formed when ethyl sodioacetoacetate and ethyl ethoxymethyleneacetoacetate are heated together. A mechanism of formation is outlined and supported. The new information is used to prepare various compounds of the xanthyrone class and two of these, 3,3′,3′,5-tetra-acetyl-6-propenyl-α-pyrone and 3′-acetyl-3,3′,5-triethoxycarbonyl-6-propenyl-α-pyrone, enable structural rivals to be discounted.

In the synthesis of 3′,3′-diethoxycarbonyl-6-propenyl-α-pyrones, the simple predictions of the formation scheme may be overridden by the thermodynamic stability of products with an ionised acetyl acetone or acetoacetate side-chain. A mechanism for interchange between the alkoxymethylene and the β-dicarbonyl carbanion provides the necessary components.

Pyrolysis of diethylxanthophanic enol gives 3-acetyl-5-ethoxycarbonyl-6-methyl-2-pyrone which can also be formed by pyrolysis of diethyl 1,3-diacetylglutaconate.