Two Rearrangement Pathways in the Geminal Acylation of 2-Methoxyoxazolidines Leading to Substituted 1,4-Oxazines

Lewis acid‐mediated geminal acylation of 2‐methoxyoxazolidines with five‐ or six‐membered acyloins followed by heterocyclization afforded 1,4‐oxazines fused to cyclopentenone or cyclohexenone rings. Overall yields ranged from 30 to 73 %. The position of the carbonyl group in the products depended on whether or not water was present during the ring‐expanding acyl migration step. The route lacking water during the acyl migration step was best conducted in one pot. The addition of water effected cleavage of a silyloxy group in the intermediate during the initial Mukaiyama aldol reaction prior to the acyl migration. The course of BF3‐mediated geminal acylation of cyclic orthoamides with five‐ and six‐membered acyloins was altered by carrying out the second rearrangement step with, or without, added water. Subsequent heterocyclization afforded isomeric cyclopenta‐ and cyclohexa[b][1,4]oxazinones in yields of 30 to 73 % over three steps.