3-Bromo-2-pyrone, Diels-Alder reactions

Because of their low reactivity, a Diels-Alder reaction of 2-pyrones usually requires such a high temperature that the initial bicyclic lactone adducts often undergo cycloreversion [30,33] with loss of CO2. In some cases this limitation has been overcome by carrying out the reaction imder high pressure conditions. Posner and coworkers have shown [34-36] that the presence of a tolylthio group or a bromine atom at the 3- or 5-position increases the reactivity of 2-pyrones. 3-Bromo-2-pyrone (35) (Scheme 2.15), as well as its regioisomer 5-bromo (36)... 

Diels-Alder reaction of pymn-2-ones. Diels-Alder reaction of 2-pyrones, if successful, can provide unusual cyclohcxcnecarboxylic acids, but thermally promoted cycloadditions with these electron-deficient dienes usually result in decarboxylation and aromatization of the adducts as a result of the required high temperatures (6,291-292). Successful Diels-Alder reactions of 3-bromo-2-pyrone (1) with the electron-rich dioxolc... 

Bromo-2-pyrone. Based on literature precedent and also on the C NMR chemical shift data in Table 1, our expectation was that 3-bromo-2-pyrone (55) would be unexciting as a diene in Diels-Alder reactions. We were pleasantly surprised to find that pyrone 55 undergoes [4+2] cycloadditions with both electron-rich and electron-poor dienophiles under relatively mild reaction conditions. It has the additional advantage of being considerably more stable than 2-pyrone. The bicyclic lactone adducts formed typically undergo reductive debromination, therefore making 3-bromo-2-pyrone a synthetic equivalent of 2-pyrone. 

Diels-Alder cycloaddition of 5-bromo-2-pyrone with the electron-rich tert-butyldi-methylsilyl (TBS) enol ether of acetaldehyde, using superheated dichloromethane as solvent, has been investigated by Joullie and coworkers (Scheme 6.90) [188]. While the reaction in a sealed tube at 95 °C required 5 days to reach completion, the anticipated oxabicyclo[2.2.2]octenone core was obtained within 6 h by microwave irradiation at 100 °C. The endo adduct was obtained as the main product. Similar results and selectivities were also obtained with a more elaborate bis-olefin, although the desired product was obtained in diminished yield. Related cydoaddition reactions involving 2-pyrones have been discussed in Section 2.5.3 (see Scheme 2.4) [189]. 

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