Abstract Regioselective hetero Diels-Alder reactions of in-situ generated nitroso alkenes 2– 4 with methoxyallene derivatives 1 provide 4H-1, 2-oxazines 5–7 with an exo-methylene group at C-5. These primary cycloadducts are smoothly transformed into conjugated 6H-1, 2- oxazines 11–13 by base or acid catalysis. The bicylic nitroso alkene 17 and methoxyallene (1a) combine to give the tricyclic 1, 2-oxazine 18, thus demonstrating that an exo-transition ...
