Thermal and acid-catalysed rearrangements of 5,6-dihydro-4H-1,2-oxazines

Abstract

2-Arylpyridines have been isolated in good yields from the pyrolysis of 3-arylcyclopent[e]oxazines (1) in the melt above 200 °C; a mechanism is proposed for this reaction which involves tautomerism of the 4H- to 2H-oxazines, followed by cleavage of the N–O bond. A similar ring-opening occurs with the benzo-derivative (3). The oxazines are methylated at nitrogen by methyl fluorosulphonate and methylation facilitates the ring-opening.

Acid-catalysed rearrangement of the cyclopent[e]oxazine (1a) takes a different course, giving the spiro-isomer (9) by cleavage of the C–O bond. Similarly, the furo-oxazine (2) is cleaved in acid to give α-(2-furyl)acetophenone oxime, which is further transformed into cis-3-(3-phenylisoxazol-5-yl)prop-2-enol (10a) in trifluoroacetic acid. Acid-catalysed ring-opening of 6-morpholino-3,6-diphenyl-5,6-dihydro-4H-1,2-oxazine (16) results in the formation of the syn and anti mono-oximes of 1,4-diphenylbutane-1,4-dione.