2, -Methylthiazole, irradiation

The irradiation of thiazole did not give any interesting products [69JCS(CC) 1018]. However, 2-, 4-, and 5-methylthiazole gave the corresponding isothiazoles in low yields when irradiated in trifluoroacetic acid (Scheme 31) (93JOC3407). 

The irradiation of isothiazole with a low-pressure mercury arc leads to the formation of thiazole [69JCA(CC)1018], The photochemical behavior of alkyliso-thiazoles has been studied. 3-Methylisothiazole gave 2-methylthiazole in a low yield. 4-Methylisothiazole was converted into 4-methylthiazole, and 5-methyliso-thiazole gave a mixture of 5-methylthiazole (55%) and 4-methylisothiazole (Scheme 38) (72T3141 93JOC3407). Either a ZI (72T3141) or an ICI mechanism was invoked to justify these reactions (93JOC3407). 

Hydroxy-4-methylthiazole-2(3//)thione carbamates, e.g. the cyclohexyl derivative 336, are precursors for monoalkylaminium cation radicals, which cannot be prepared from 2-thioxopyridinyloxycarbamates. The carbamate is obtained from 3-hydroxy-4-methylthiazole-2(3//)-thione and cyclohexyl isocyanate. When irradiated in the presence of malonic acid and /-butyl hydrogen sulphide it yields the cyclohexylaminium cation... 

Although methylthiazoles do not undergo phototransposition upon irradiation in a variety of neutral solvents, methylthiazolium cations (49a-c), formed by dissolving the corresponding neutral methylthiazoles in trifluoroacetic acid (TFA), do undergo (Scheme 25) phototransposition to methylisothiazolium ions (50a-c) <93JOC3407>. 

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