Formation of 2-acetylpyridines by the base-catalysed ring opening of dihydro-4H-furo[2,3-e]oxazines

Furo-oxazines (1), which were formed by the addition of nitrosoalkenes to 2,5-dimethylfuran or 2-methylfuran, reacted with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give 2-acetylpyridines (2). The reaction is postulated to involve an initial base-catalysed elimination to give the 6H-1,2-oxazines (3) which are then cleaved in a second base-catalysed elimination. In support of this proposal, the oxazine (3a) has been isolated and has been converted into the pyridine (2a) by reaction with DBU. A second intermediate, which has been detected spectroscropically, is formulated as the dihydropyridine (7). A deuterium labelled 6H-oxazine (6) has been prepared in order to elucidate the course of the reaction. An analogous furopyridazine (9) has been synthesized and its reaction with DBU has been investigated. Only the initial elimination reaction took place and the dihydropyridazine (10) was obtained as the reaction product.