Ring contraction 1.2- oxazines, 3,6-dihydro

The mass spectral behavior of 2,3-dihydro-4//-l,3-oxazin-4-ones under electrospray ionization could be characterized by a predominant RDA fragmentation pathway, producing a pair of complementary ions 18 and 19. However, the decomposition of 17 also followed another pathway, resulting in ring contraction to form oxete ions 20 by the loss of isocyanate (Scheme 3) <2004RCM1116>. 

Reaction of the 2,3-dioxo-2,3-dihydro-4//-l,4-oxazines with dimethylsul-foxonium methylide introduced an exomethylene group with concomitant ring contraction to give the respective 4,5-dioxo-2,3,4,5-tetrahydrooxazole (94CPB739) (Scheme 78A) where an endocyclic carbon atom became exo-cyclic. 

Oxazines, Thiazines, and their Fused E>erivatives Oxazines. - The action of nitrous acid on the enones (205) gives mixtures of oximes (206) and 4-oxo-5,6-dihydro-l,2-4//-oxazines (207) the former can be converted into the latter by brief heating, whereas heating for a longer period results in ring-contraction (Scheme 93). ... 

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