REARRANGEMENT OF N-ACYL-3,4-DIHYDRO-1H-2,1-BENZOXAZINES TO 2-SUBSTITUTED-4H-3,1-BENZOXAZINES THROUGH A RETRO-DIELS-ALDER EXTRUSION OF FORMALDEHYDE

N-Acyl-3,4-dihydro-1H-2,1-benzoxazines (3) undergo a thermal decomposi tion involving loss of formaldehyde in a retro-Diels-Alder reaction. T he resultant N-acylazaxylylenes dagger (4) undergo a 6 pi electrocycli sation to give 2-substituted-4H-3, 1-benzoxazines (5) rather than a 4 pi electrocyclisation to give the N-acyl-1,2-dihydrobenzazetes (6). Co mpounds 5 have been fully characterised spectroscopically and their da ta is inconsistent with that reported previously by other workers for what are purported to be the same compounds. 2-Methyl-4H-3,1-benzoxazi ne (5b) and other 2-alkyl-substituted compounds undergo facile hydroly sis to o-aminobenzyl esters (9) which rearrange to the thermodynamical ly more stable o-hydroxymethylanilides (10). 2-Phenyl-4H-3,1-benzoxazi ne (5a) is relatively stable to hydrolysis but undergoes a novel photo chemical ring opening (> 254 nm) to give the N-benzoylazaxylylene (12) which can be trapped with alcohols giving o'-alkoxymethylbenzanilides (11). In cyclohexanol at 160 degrees C, the intermediate in the therm al rearrangement of 3a to 5a, N-benzoylazaxylylene (12), was trapped a s o'-cyclohexyloxymethylbenzanilide (11b). The rearrangements in mesit ylene are unimolecular with activation energies of 35, 37 and 42 kcal mol(-1)double dagger for 3a; 3c and 3d,.respectively. The extrusion an d electrocyclisation reaction pathways for N-acetyl-3,4-dihydro-2, 1-b enzoxazine (3b) have been modelled using AM1 molecular orbital theory which predicts both a non-synchronous transition state for the retro-D iels-Alder reaction and the preferred mode of ring closure to be the 6 pi rather than the 4 pi electrocyclisation.