Hetero-diels-alder reaction of ethyl 2-nitrosoacrylate and cyclohexadienes, and bromine induced diene isomerization

Bicyclic 8a-methoxy-1,2-oxazine (3a) and bicyclic 7-methoxy-l,2-oxazine (3b) were prepared via Hetero-Diels-Alder reaction between 1-methoxy-1,4-cyclohexadiene (2a) and ethyl 2-nitrosoacrylate generated in situ from ethyl bromopyruvate oxime (1). The formation of 3b was ascribed to bromine, released from 1, which induced transformation of 2a to 1-methoxy-1,3-cyclohexadiene (2b). 3b could be easily transformed to oxazinone (3c) in CHCl 3 , or other organic solvents in the presence of a trace amount of acid.