Heterocycles derivatives as outer-ring dienes

Treatment of 4,5-bis(bromomethyl)-3-phenylisoxazole (375) with sodium iodide and subsequently with azodicarboxylate gave Diels-Alder cycloadduct (377) (91TL4603). Intermediate 1,2-quinodime-thyde (376) may participate in the reaction as an active 1,3-diene. 

By analogy, flash vacuum pyrolysis of a p-chlorobenzoate ester (378, Ar=4-C1C6H4) (90TL1487) produced oxazole-4,5-xylylene (379), which could be trapped with S02 to give adduct (381). The unstable intermediate also gave a Diels-Alder adduct (380) with methyl acrylate (for the similar reactions see Section 3.3.3.3.3). 

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