Confirmation by trapping, synthesis, and reactivity of 2,3-dehydro-N-methylmorpholine (DNMM)

The elusive 4-methyl-3,4-dihydro-2H-[1,4]oxazine (2,3-dehydro-N-methylmorpholine, DNMM, 5) was confirmed to occur as degradation product of N-methylmorpholine-N-oxide (1) by trapping with the α-tocopherol-derived ortho-quinone methide in a hetero-Diels–Alder reaction with inverse electron demand. The regioselectivity of the addition was in good agreement with DFT computational data. An authentic sample of 5 was synthesized from formmorpholide (16) via 2-methoxy-formmorpholide in 38% overall yield.