3.5- Dibromo-2-pyrone, synthesis

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... 

The palladium-catalyzed Suzuki-Miyaura reaction of 3,5-dibromo-2-pyronc 100 with benzo[, ]furan-2-boronic acid 101 was applied to the synthesis of 3-(benzo[ ]furan-2-yl)-5-bromo-pyrone 102 in 50% yield (Equation 92) <2004SL2197>. 

ZnBr2-catalyzed IMDA cycloaddition of the 2-pyrone 152 (Scheme 37), tethered with a chiral enol silaketal, provided (+)-153-exo in 74% yield, along with a small amount of (-)- 5 i-endo <95JOC1617>. In this case, the -geometry of the dienophile was not preserved during the cycloaddition, as in the cycloaddition of 3,5-dibromo-2-pyrone with dimethyl maleate (cf. entry 9, Table 2), implying a stepwise reaetion mechanism. The isolated exo product (+)-153 was used as the key intermediate for the asymmetric total synthesis of a 2-alkyl-vitamin D3 analog. 

The iV-phenylmaleimide derivative (518), prepared in two steps from a-pyrone and nitrosobenzene, is a dienophile and yields the cycloaddition product (519) on reaction with cyclohexa-l,3-diene." ° The sealed-tube reaction of anthracene with excess l,2-dibromo-3,3-difluorocyclopropene at 120°C gave the crystalline adduct (520), presumably by way of intermediate (521) the mechanism is not known. A synthesis of the tricyclo[5,2,2,0 ]undecane skeleton related to isoeremolactone has been reported. The Diels-Alder adduct (522) was a key intermediate in this synthesis, being converted in two standard steps into an epimeric mixture of compounds... 

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