1.3- Oxazines ring fission

Although direct oxidation is sometimes possible (54CCC282), N- or S-oxides and dioxides are normally prepared by indirect routes. 3,6-Dihydro-l,2-oxazines can be reduced catalyti-cally to their tetrahydro derivatives, but zinc and acetic acid cause ring fission to 4-aminobut-3-enols which cyclize to dihydropyrroles (54MI22700). 

Tetrahydro-l,3-oxazines are normally assumed to adopt a chair conformation in which the NH bond has an axial orientation. This view is substantiated by NMR spectroscopy and also by dipole moment analysis <73JCS(P2)325). The ring system is not stable and, for example, when the parent molecule is allowed to stand, it slowly ring opens and then forms a trimer (Scheme 33) (78AF937). In acidic media ring fission is accelerated and the open-chair imines may then hydrolyze. This property has been utilized in the synthesis of aldehydes as previously noted (see Section 2.27.2.2.4(0). 

Oxazine JV-oxides 386 obtained from cross-conjugated keto enamines 384, and 1-nitrocyclohexene 385 do not ring open to nitroalkylated enamines, but undergo ring fission and rearrangement to five-membered carbocycles - . Nucleophilic ring fission of 386 to the intermediate 387 followed by recyclization gave the hexahydroindene... 

Dihydro-2//-l,3-oxazines (122), which are available through cycloaddition reactions (see Section 6.05.6.2.2(vi)), may be reduced with sodium and 2-propanol to yield perhydro derivatives (123) without ring fission. These products may then be ring opened by treatment with hydrochloric acid to give the diastereomeric 3-aminopropanols (124) (Scheme 31) <91S387>. 

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