3.5- Dibromo-2-pyrones, Stille couplings

Stille couplings on 3,5-dibromo-2-pyrones are normally selective for C(3), but the presence of Cu(I) reverses this selectivity <2003JA14288>. 

Finally, a new effect of the addition of copper salts was noted recently. The regioselectivity of the Stille coupling of 3,5-dibromo-2-pyrone can be modified in the presence of Cul in DMF as the solvent, leading to the products shown in Scheme 5.3.5. ... 

Sdieme 27. The Stille coupling reactions of 3,5-dibromo-2-pyrone 38... 

The synthesis of pyrone derivatives attracted attention due to their synthetic potential. In an illustrative example Cho and co-workers studied the Suzuki-coupling of 3,5-dibromo-2-pyrone with arylboronic acids (8.9.). Under regular conditions the aryl group is introduced selectively into the more electron deficient 3-position, while in the presence of an equimolar amount of copper(I) iodide the coupling is diverted selectively into the 5-position (N.B. drop of the reaction temperature from 50 °C to ambient temperature negated the effect of copper and led to 3-arylation). The way copper effects the coupling is still unclear, but it was successfully used in the preparation of a range of 5-aryl-2-pyrons.17... 

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