The Diels–Alder cycloadditions of the α-acetoxynitroso dienophile 1 in water are reported. The rapid and high yielding synthesis of structurally diverse 3,6-dihydro-1,2-oxazines complements the straightforward elaboration of aminoalcohols obtained from the α-acetoxynitroso derivative 1 in anhydrous medium. A rationale for this solvent-dependent product distribution is proposed.