6H-1,3-Oxazin-6-one

Another interesting formation of 6H-1,3-oxazin-6-one derivatives by the oxidation of pyrrole derivatives was described by Sprio215 [Eq. (57)] (see also Yee et a/.216). 

H-1,3-Oxazin-6-one (68) is readily hydrolyzed to an open-chain product. It is isomerized by UV irradiation to the bicycle with fused four-membered rings (100), which on heating reverted to 68.210,211 Prolonged irradiation decomposed the substance into hydrogen cyanide, carbon dioxide, and acetylene.211... 

The imine bond of 4H- and 6H- 1,3-oxazines enters into addition reactions with quinones and alcohols. Thus, for example, the triphenyl derivative (50) forms a 1 1 adduct with 1,4-benzoquinone (69LA(723)lll) and the oxazin-6-ones (51), when heated in methanol, give 2-methoxy-2,3-dihydro derivatives (Scheme 15) (72CJC584). 

Mesoionic pyrido[2,l- >][1,3]oxazines (54) afforded 4-oxo-4//-pyrido[l,2-a]pyrimidin-l-iumolates (55) and 4//-quinolizin-4-one (56) with phenyl iso(thio)cyanates [78LA1655 79CB1585 82ZN(B)222] and dimethyl acetylenedicarboxylate (79CB1585), respectively. Reaction of 2-cyano-3-methyl-lH,6//-pyridol[l,2-a][3,l]benzoxazine-l,6-dione with ammonium acetate and hydroxylamine, hydrazines, primary aliphatic or aromatic amines, and (thio)ureas gave 5-unsubstituted and 5-substituted 2-cyano-3-methyl-l//,6H-pyrido[l,2-a]quinazoline-l,6-diones (93CCC1953). 

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