2-Silyloxy-1,2-oxazines, a New Type of Acetals of Conjugated Nitroso Alkenes

3-Alkyl-substituted 1,2-oxazine N-oxides 2 can
be selectively transformed into 2-silyloxy-1,2-oxazines 1 upon
treatment with silylating reagents. In the solid state derivatives 1 adopt a chair conformation with the pyramidal
nitrogen atom, whereas in solution they exist as an equilibrating mixture of two conformers (ΔG⧧ 55−60 kJ/mol). A
preliminary study of the reactivity of nitrosals 1 has shown
that they react with O- and N-stabilized carbocations to yield
1,2-oxazine N-oxides with a functionalized alkyl substituent
at the 3-position. Moreover, compounds 1 can rearrange into
silyloxy-1,2-oxazines 5 and react with morpholine to produce
3-morpholinoalkyl-1,2-oxazines 7 existing in a tautameric
equilibrium with open-chain oximes 6.