Journal of the Chemical Society. Perkin transactions I p. 165 - 171 (1981)
Update date:2022-08-04
Topics:
Baker, S. Richard
Crombie Leslie
Dove, Roderick V.
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 exess (>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) with 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.
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