4-Pyrones with Grignard reagents

The reaction of 2-pyrones with Grignard reagents is complex305"310 the mechanism may be rationalized as in Scheme 14 and includes a knowledge of valence-bond isomerism of 2//-pyrans (see Section V,E). 

As shown in Scheme 15, 4-substituted 4-hydroxy-2,6-dimethyl-4//-pyrans 324 and 325 were obtained by reaction of 4-pyrone 322 with BuLi320 or substituted furyllithium reagents,321 respectively. 4,4-Disubstituted 2,6-dimethyl-4//-pyrans 326 were prepared by the reaction of a threefold excess of the corresponding Grignard reagents RMgX with 4-pyrone 322.3 1,316... 

The reaction of 3,4,5,6-tetrachloro-2-pyrone with Grignard reagents results in substitution of the chlorine atom at C6 (77CB1000). Nucleophilic... 

The best-known reaction pertaining to this type is the condensation of pyrones with Grignard reagents. One year after Gomberg and Cone153 had reported that they had observed no reaction between... 

Pyrones also add Grignard nucleophiles at the carbonyl carbon, C-4 dehydration of the inunediate tertiary alcohol product with mineral acid provides an important route to 4-mono-substituted pyrylium salts." More vigorous conditions lead to the reaction of both 2- and 4-pyrones with two mole equivalents of organometallic reagent and the formation of 2,2-disubstituted-2H- and 4,4-disubstituted-4//-pyrans, respectively." Perhaps surprisingly, hydride (lithium aluminium hydride) addition to 4,6-dimethyl-2-pyrone takes place, in contrast, at C-b." ... 

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