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Synthesis of 6 H ‐1,2‐oxazines by hetero Diels‐Alder reactions of nitroso alkenes towards methoxyallenes
Author(s) -
Zimmer Reinhold,
Reißig HansUlrich
Publication year - 1991
Publication title -
liebigs annalen der chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0170-2041
DOI - 10.1002/jlac.1991199101101
Subject(s) - chemistry , oxazines , nitroso , cycloaddition , alkene , allene , zwitterion , regioselectivity , steric effects , medicinal chemistry , methylene , diels–alder reaction , catalysis , stereochemistry , organic chemistry , molecule
Regioselective hetero Diels‐Alder reactions of in‐situ generated nitroso alkenes 2–4 with methoxyallene derivatives 1 provide 4 H ‐1,2‐oxazines 5–7 with an exo‐methylene group at C‐5. These primary cycloadducts are smoothly transformed into conjugated 6 H ‐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 1h 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.