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
摘要:
N-Acyl-3,4-dihydro-1H-2,1-benzoxazines (3) undergo a thermal decomposition involving loss of formaldehyde in a retro-Diels-Alder reaction. The resultant N-acylazaxylylenes dagger (4) undergo a 6 pi electrocyclisation to give 2-substituted-4H-3, 1-benzoxazines (5) rather than a 4 pi electrocyclisation to give the N-acyl-1,2-dihydrobenzazetes (6). Compounds 5 have been fully characterised spectroscopically and their data is inconsistent with that reported previously by other workers for what are purported to be the same compounds. 2-Methyl-4H-3,1-benzoxazine (5b) and other 2-alkyl-substituted compounds undergo facile hydrolysis to o-aminobenzyl esters (9) which rearrange to the thermodynamically more stable o-hydroxymethylanilides (10). 2-Phenyl-4H-3,1-benzoxazine (5a) is relatively stable to hydrolysis but undergoes a novel photochemical 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 thermal rearrangement of 3a to 5a, N-benzoylazaxylylene (12), was trapped as o'-cyclohexyloxymethylbenzanilide (11b). The rearrangements in mesitylene are unimolecular with activation energies of 35, 37 and 42 kcal mol(-1)double dagger for 3a; 3c and 3d,.respectively. The extrusion and electrocyclisation reaction pathways for N-acetyl-3,4-dihydro-2, 1-benzoxazine (3b) have been modelled using AM1 molecular orbital theory which predicts both a non-synchronous transition state for the retro-Diels-Alder reaction and the preferred mode of ring closure to be the 6 pi rather than the 4 pi electrocyclisation.
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
摘要:
N-Acyl-3,4-dihydro-1H-2,1-benzoxazines (3) undergo a thermal decomposition involving loss of formaldehyde in a retro-Diels-Alder reaction. The resultant N-acylazaxylylenes dagger (4) undergo a 6 pi electrocyclisation to give 2-substituted-4H-3, 1-benzoxazines (5) rather than a 4 pi electrocyclisation to give the N-acyl-1,2-dihydrobenzazetes (6). Compounds 5 have been fully characterised spectroscopically and their data is inconsistent with that reported previously by other workers for what are purported to be the same compounds. 2-Methyl-4H-3,1-benzoxazine (5b) and other 2-alkyl-substituted compounds undergo facile hydrolysis to o-aminobenzyl esters (9) which rearrange to the thermodynamically more stable o-hydroxymethylanilides (10). 2-Phenyl-4H-3,1-benzoxazine (5a) is relatively stable to hydrolysis but undergoes a novel photochemical 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 thermal rearrangement of 3a to 5a, N-benzoylazaxylylene (12), was trapped as o'-cyclohexyloxymethylbenzanilide (11b). The rearrangements in mesitylene are unimolecular with activation energies of 35, 37 and 42 kcal mol(-1)double dagger for 3a; 3c and 3d,.respectively. The extrusion and electrocyclisation reaction pathways for N-acetyl-3,4-dihydro-2, 1-benzoxazine (3b) have been modelled using AM1 molecular orbital theory which predicts both a non-synchronous transition state for the retro-Diels-Alder reaction and the preferred mode of ring closure to be the 6 pi rather than the 4 pi electrocyclisation.
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
作者:Stephen A. Glover、Katherine M. Jones、Ian R. McNee、Colleen A. Rowbottom
DOI:10.1039/p29960001367
日期:——
N-Acyl-3,4-dihydro-1H-2,1-benzoxazines (3) undergo a thermal decomposition involving loss of formaldehyde in a retro-Diels-Alder reaction. The resultant N-acylazaxylylenes dagger (4) undergo a 6 pi electrocyclisation to give 2-substituted-4H-3, 1-benzoxazines (5) rather than a 4 pi electrocyclisation to give the N-acyl-1,2-dihydrobenzazetes (6). Compounds 5 have been fully characterised spectroscopically and their data is inconsistent with that reported previously by other workers for what are purported to be the same compounds. 2-Methyl-4H-3,1-benzoxazine (5b) and other 2-alkyl-substituted compounds undergo facile hydrolysis to o-aminobenzyl esters (9) which rearrange to the thermodynamically more stable o-hydroxymethylanilides (10). 2-Phenyl-4H-3,1-benzoxazine (5a) is relatively stable to hydrolysis but undergoes a novel photochemical 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 thermal rearrangement of 3a to 5a, N-benzoylazaxylylene (12), was trapped as o'-cyclohexyloxymethylbenzanilide (11b). The rearrangements in mesitylene are unimolecular with activation energies of 35, 37 and 42 kcal mol(-1)double dagger for 3a; 3c and 3d,.respectively. The extrusion and electrocyclisation reaction pathways for N-acetyl-3,4-dihydro-2, 1-benzoxazine (3b) have been modelled using AM1 molecular orbital theory which predicts both a non-synchronous transition state for the retro-Diels-Alder reaction and the preferred mode of ring closure to be the 6 pi rather than the 4 pi electrocyclisation.