Polyketo-enols and chelates. Product control by magnesium chelation in the rearrangement of dimethyl xanthophanic enol and other xanthyrones

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Abstract
The products formed when dimethyl xanthophanic enol (1) is treated with magnesium methoxide depend strikingly on the molar ratio of the reactants. Up to near a 1 : 1 molar ratio of methoxide to enol, the enol is recovered unchanged. At a 1 : 1 molar ratio the main product is the isophthalate (11) formed by aldol reaction. When the molar ratio is >2 : 1, the main product is the pyran (13) and formation of (11) is much diminished. At a 3 : 1 molar ratio no isophthalate (11) is formed, pyran (13) has diminished in amount, and the resorcylic ester (14), a product of Claisen condensation, is now dominant. At a >6 : 1 molar ratio of magnesium methoxide to enol the resorcylic ester is the sole product, formed in high yield. These results are explained in terms of methoxide-initiated pyran opening and the role of the resulting mono- and bis-magnesium chelated species as substrates for aldol and Claisen rearrangements. The protective action of magnesium complexing on an otherwise base-sensitive chain is also a significant factor. For comparison, the reaction of dimethyl xanthophanic enol is examined using initial sodium methoxide : enol ratios of 1–24 : 1. Only the aldol product, the isophthalate (6, R = H), is formed across the whole concentration range.The ester-interchange situation, when variously substituted xanthyrones are transformed into resorcylic esters by excess of magnesium methoxide, is studied in support of the proposed mechanisms. The xanthyrones (34)–(36), without a pyrone acetyl and thus incapable of undergoing the resorcylic ester transformation, form substituted pyrans (39)–(41). Sodium methoxide, however, causes chain degradation and cyclisation of the major fragment to dimethyl 4-hydroxyisophthalate, again demonstrating a substantial diversion of reaction pathway resulting from the complexing effect of magnesium methoxide.
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