Photochemical Reactions of Isothiazoles

Preliminary observations on the photochemical behaviour of isothiazoles have been briefly reported. The parent base is partially converted into thiazole on irradiation (7% in propylamine, 1% in ether), several unidentified products being formed (15%) side by side. 3-Phenyl- and 3,5-diphenyl-isothiazole are converted into 4-phenyl- (12%) and 2,4-diphenyl-thiazole (48%), respectively, but 5-phenylisothiazole preserves its ring system, merely producing small quantities (2.3%) of the 3-phenyl isomer. The reactions are believed to proceed by way of tricyclic sulphonium cations of the type previously postulated by Kojima and Maeda. It is recalled that the reverse photoisomerization is also on record (see syntheses, above). 

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). 

In analogy to the isoxazole-to-oxazole rearrangement, thiazoles can be obtained photochemically from irradiation of the corresponding isothiazoles. In several cases, however, irradiations have been conducted for mechanistic purposes only, and mixtures of photoisomers arising from competing reaction mechanisms are often formed [4, 6]. 

Although the photochemistry of 4-substituted isothiazoles in the presence of NH3 or TEA is dominated by reactions initiated by cleavage of the S-N bond, 5-phenyl- and 3-phenylthiazoles, (56) and (54), react under these conditions by competing photochemical pathways . 

Irradiation of 2-phenylbenzoxazole forms a head-to-tail dimer, 1,3-diazetidine 436, whereas benzoxazole itself, in the presence of oxygen, gives the dimeric product, 437. Isothiazoles yield lactams 438 on reaction with diphenylketene. Benzotriazole and its 2-alkyl derivatives undergo photochemical ( > 290 nm) [2 + 2] cycloaddition to maleinimides to give adducts 439 <20020L1487>. 

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