AbstractRegioselective hetero Diels‐Alder reactions of in‐situ generated nitroso alkenes 2–4 with methoxyallene derivatives1provide 4H‐1,2‐oxazines5–7with an exo‐methylene group at C‐5. These primary cycloadducts are smoothly transformed into conjugated 6H‐1,2‐oxazines11–13by base or acid catalysis. The bicylic nitroso alkene17and methoxyallene (1a) combine to give the tricyclic 1,2‐oxazine18, thus demonstrating that an exo‐transition state is favoured in this [4+2] cycloaddition. The reaction of the sterically hindered methoxyallene1hwith nitrosostyrene affords the cycloadduct22in low yield.22is very likely formed by a two‐step Diels‐Alder reaction via a zwitterion. Allene derivatives lacking a methoxy group are not sufficiently reactive towards nitroso alkenes an