Nitrosoalkene2 generated in situ from oxime 3 underwent smooth hetero Diels-Alder reaction with enol ethers 1 to afford 1,2-oxazine derivatives 4 bearing an exocyclic C=C bond. Methoxyallene 8 and 2 provided 6 H-1,2-oxazine 10 in good overall yield. The exocyclic double bond of this type of 1,2-oxazines can be employed for addition reactions as demonstrated by dihydroxylation of 4A with potassium