MNDO STUDY OF THE MECHANISM OF THE ACID-CATALYZED REARRANGEMENT OF 3-ACETYL-6-ETHOXY-5,6-DIHYDRO-4H-1,2-OXAZINE.

MNDO semi-empirical calculations are reported for the gas-phase acid-catalyzed rearrangement of the 3-acetyl-6-ethoxy-5,6-dihydro-4H-1,2-oxazine. Insight into the reaction mechanism may be obtained by the theoretical study of the reaction hyper surface and intrinsic reaction coordinate analysis by semi-empirical mechanical calculations. The potential energy surface of the reaction is determined by characterizing intermediates and transition states, the reaction is found to take place by a step-wise mechanism involves protonation of the ring oxygen, subsequent C?O bond cleavage reproduces the oxime, which cyclized to the five membered nitrone on which the solvent methanol affected the protonated alcohol exchange. N?C bond cleavage (oxime exchange), followed by cyclization utilizing the carbonyl carbon produces the six membered ring, aromatization reproduces the desired pyridine N-oxide cation. The acid-catalyzed rearrangements of 1,2-oxazines was found to involve the ring C?O bond cleavage.