Synthesis and cycloadditions of substituted monobromo-2-pyrones

Ar= Ph, 4-O2NC6H4, 2-O2NC6H4, 2-Me02CC6H4, 4-MeOCeH4, 4-FC6H4, 3-ICeH4, 4-NCC6H4. 

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

Under the reaction conditions, 3-(2-furyl)-5-bromo-2-pyrone and 3-(2-thienyl)-5-bromo- 

2-pyrone also undergo ZnBrj-catalyzed D-A cycloadditions with BVE, but not as readily as does 100. After 12 hours at ambient temperature, cycloaddition products were produced in 30 - 40% yield. Mono-coordinating Lewis acids, such as BF3 etherate, did not promote the reaction. Additionally, no cycloaddition products were observed with electron-deficient methyl acrylate under identical reaction conditions. 

The 3-amino-5-bromo-2-pyrones 106 were expected to favor normal electron demand cycloadditions, compared to the parent 3,5-dibromo-2-pyrone, because of the presence of the electron-donating amino group. Summarized in Table 6 are the results of the cycloadditios of 

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