N-Methylation of syn- or anti-configured 3,6-dihydro-2H-1,2-oxazines and subsequent treatment with triethylamine smoothly provided enantiopure α,β-unsaturated β-alkoxy-γ-amino aldehydes bearing different protected diol, triol, or tetrol side chains in good to excellent yields. The N-O bond cleavage occurs under mild conditions and involves an internal redox process. The method is also applicable to tetrahydro-2H-1,2-oxazines, which either lead to 4-amino ketose or aldose derivatives (d-sorbose or d-idose configuration). The equivalency of the generated β-alkoxyenal moiety with 1,3-dicarbonyl compounds could be demonstrated by condensation reactions with hydrazine or 2-aminoimidazole derivatives providing a series of new pyrazole or imidazo[1,2-a]pyrimidine derivatives with stereodefined and protected aminopolyol side chains.