1.3- Oxazin-6-ones thermolysis

Flash vacuum thermolysis (FVP) at 600°C or microwave excitation of 1-substituted perhydropyrido[l,2-f][l,3]oxa-zines afforded 1-substituted 1,4,5,6-tetrahydropyridines <2005TL5451>. Perhydropyrido[l,2-f][l,3]oxazin-l-ones were hydrolyzed with 2M ethanolic KOH to 2-(2-hydroxyalkyl)piperidines <1996CJC2434, 2005EJ01378>. (+)-9- />z -6-Epipinidinol 102 was similarly obtained from 3,8-dimethylperhydropyrido[l,2-f][l,3]oxazin-l-one 101 (Equation 16) <1998T13505>. 

The flash vacuum thermolysis of 4-hydroxy-2,5-diphenyl-l,3-oxazin-6-one resulted in carboxy(phenyl)ketene and benzonitrile as the major and benzoyl isocyanate as the minor product. This was interpreted in terms of the influence of the tautomeric forms on the thermal fragmentation pathways <2007JOC1399>. 

Routes to AH-1,3-oxazines and -thiazines involve the cyclization of amides or thioamides with acidic reagents (Scheme 35) (78AHC(23)l). l,3-Oxazin-2-ones can be made by the thermolysis of carbonyl azides (262 Scheme 36) (79CC719). 

Breslow94 introduced an interesting method of forming 2-oxo compounds, which was based on his earlier work.95 Thermolysis of n-octadecyl azidoformate gives some tetrahydro-l,3-oxazin-2-one (23) [Eq. (14)]. 

As in many other areas of 1,3-oxazine chemistry, ketenes and isocyanates generated in situ are often used to prepare oxazin-6-ones. For example, 2,4-disubstituted l,3-oxazin-6-ones (135) are readily available through the thermolysis of 5-[(A-acylamino)alkylidene]-l,3-dioxane-4,6-diones (134). The latter are generated from the reactions of the appropriate ethyl acylimidate (133) and Meldrum s acid (Scheme 36) <86CPB1980>. Similarly, the bismethylthiomethylene derivative (136) reacts with benzamides in the presence of potassium hydroxide to afford the methyl-thioacylaminomethylene derivatives (137 R = MeS), which can be reacted further with ammonia to give the amino compounds (137 R = NHj). In turn, these products can be thermolyzed to afford the oxazinones (138 R = MeS) or (138 R = NH2), respectively (Scheme 37) <91SC1213>. 

Although 2,3-dihydro-l,3-oxazin-4-ones (205) are frequently obtained by other methods, one approach utilizes 1,3-dioxinones (204), which on thermolysis afford acylketenes. These undergo cycloadditions with imines in situ to provide the required heterocycles (Equation (17)) <84CPB2602>. 

As one would expect, oximes and nitroso-compounds are frequently encountered starting materials throughout the chemistry of this group of heterocycles. Further examples of their reactions have appeared. Reaction with enol ethers produces dihydro-oxazines (260), which may be converted to pyridine N-oxides upon treatment with HCl. Dihydro-oxazines are also obtained upon treatment of 7,6-unsaturated dicarbonyl compounds with nitrous acid. Thermolysis of these products leads to nitrones (261). 

Iminoketenes, 508, generated in the thermolysis of benzotriazinones or isatoic anhydride, react with aryl isocyanates to give 2-aryliminobenz-l,3-oxazin-4-ones 509 °. 

Thermolysis of furandione (310) affords the dibenzoylketen (311), which enters into (4 + 2) cycloadditions with nitriles to give the l,3-oxazin-4-ones (312). ... 

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