SYNTHESIS OF 6H-1,2-OXAZINES BY HETERO-DIELS-ALDER REACTIONS OF NITROSO ALKENES TOWARDS METHOXYALLENES

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 state is favoured in this [4 + 2] cycloaddition. The reaction of the sterically hindered methoxyallene 1 h with nitrosostyrene affords the cycloadduct 22 in low yield. 22 is 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 and do not provide 1,2-oxazines.