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 (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.