Issue 0, 1981

Polyketoenols and chelates. New types of xanthyrones lacking enolised side-chain termini: their reactions with magnesium methoxide

Abstract

Xanthyrones (4) and (8) as well as (9), (11), and (13) have been synthesised: blocking the side-chain terminus, which is normally enolised as in (5), by dicyano or cyano-ester end groups, leads to enolisation of the 3-acetylpyrone end as shown by n.m.r. data in CDCl3. The dicyanobismethoxycarbonyl representative (9) exists in the 1′-H form in CDCl3–CF3CO2H whilst in CDCl3 the tetraester (13) is a 3′-H form. Xanthyrone (11) is present mainly in the 3′-H form in CDCl3 but some enolate (12) is observed. All the xanthyrones are highly ionised in ethanol, (9) especially so. Treated with excess ([gt-or-equal]6 mol. equiv.) magnesium methoxide, xanthyrones (4) and (8) cyclise by an aldol pathway and (9) and (13) by an analogous Claisen pathway. This contrasts with dimethylxanthophanic enol (5) which undergoes a different type of Claisen cyclisation. The results accord with the view that cyclisation in the latter case involves a bischelate (20) and in the other cases monochelates, e.g. (22) and (25). Reactions of xanthyrones (4) and (8) with boiling water follow expected pathways.

Article information

Article type
Paper

J. Chem. Soc., Perkin Trans. 1, 1981, 165-171

Polyketoenols and chelates. New types of xanthyrones lacking enolised side-chain termini: their reactions with magnesium methoxide

S. R. Baker, L. Crombie and R. V. Dove, J. Chem. Soc., Perkin Trans. 1, 1981, 165 DOI: 10.1039/P19810000165

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