Ambident enophilic character

The 2-pyrones 4 and 5 with a bromine atom substituted at the C3 or C5 position, respectively, are particularly versatile 1,3-diene synthons, which can readily undergo [4-1-2] cycloadditions to give the cycloadducts 6 and 7 with high regio- and endo/exo selectivity under milder conditions than the parent 2-pyrone (Scheme 4) <91TL5295 92TL7839>. They are viable synthetic equivalents of the parent 2-pyrone as the bromine atom can be reductively removed after cycloaddition. Additionally, they exhibit ambident enophilic character and are capable of undergoing both normal and inverse electron demand cycloadditions. 

The ambident enophilic character of the resultant 3-aryl-5-bromo-2-pyrone 81 was demonstrated by the easy cycloadditions of 3-phenyl-5-bromo-2-pyrone 94 (Scheme 28) with both methyl acrylate (MA) and benzyl vinyl ether (BVE), affording the corresponding cycloadducts 95 and 96 in good yields with varying endo/exo selectivity. 

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