6-Bromochromone

Chromones are also Michael acceptors, and Scheme 18 shows how 3-bromochromone reacts with 1,3-diketones in basic media. The reaction is fairly general and the yields can be as high as 90%, moreover, phenolic furans are not common and the approach provides an effective way of protecting the phenolic hydroxy group during furan ring formation.100... 

Dibromochroman-4-one yields chromone with zinc dust (58BSF329), whilst the 3,3-dibromo compound is dehydrobrominated by pyridine to 3-bromochromone (71AP543). 

Bromochromone reacts readily at room temperature with acetylacetone in chloroform in the presence of two equivalents of DBU. The product, isolated in 83% yield in the usual manner after quenching of the reaction mixture with 5% HC1, is 3-acetyl-5-(2-hydroxybenzoyl)-2-methylfuran. A variety of (3-diketones and P-keto esters react similarly to give moderate to excellent yields of highly functionalised trisubstituted furans. 

It was further discovered that 3-bromochromones readily react with arylboronic acids or their esters under the Suzuki coupling conditions to provide the corresponding isoflavone derivatives in excellent yields (Eq. (69)) [112]. 

Both 3-bromochromone and 4-bromocoumarin have been successfully used in... 

Mn02 converted this compound into the 2-aldehyde in about 30% overall yield. When 3-bromochromone (167 X = Br) is treated briefly with either a 1,3-diketone or a 3-oxo-ester in the presence of l,5-diazabicyclo[4.3.0]non-5-ene, the pyrone ring is cleaved and a trisubstituted furan (172) is formed, in high yield. 

The variants on this route are many for example, condensation of ortho-hydroxyacetophenone with the Vilsmeier reagent produces 3-formylchromone, and combination with dimethylformamide dimethyl acetal, then bromine, gives 3-bromochromones. ... 

There are, however, some cases in which the aqueous technique can lead to the formation of side-products, as e.g. in arylation of 3-bromochromone by sterically hindered mesitylboronic acid, which is complicated by protonolysis [37] when the reaction is carried out in aqueous solvent in the presence of Na2C03. This reaction can be made selective if it is run without water with TI2CO3 as a solid base ... 

Hoshino, Y. Miyaura, N. Suzuki, A. (1988) Novel synthesis of isoflavones by the palladium catalysed cross-coupling reaction of 3-bromochromones with arylboronic acids or their esters. Bull. Chem. Soc. Jpn., 61, 3008-10. 

---