2-Acyl-3-arylisoxazol-5(2H)-ones give 2-alkyl(aryl)-4-aryloxazoles in good yields at 540°C under flash vacuum pyrolysis conditions, but at higher temperatures the expected oxazoles are accompanied by increasing amounts of isomeric 2,5-disubstituted oxazoles, as well as anilides and decomposition products of the 2,4-disubstituted oxazole. The rearrangement mechanisms have been studied by the use of 13C labelled substrates and p-substituted 3-arylisoxazolones. The 2,5-disubstituted oxazoles are considered to arise from 1H-azirines, and the anilides from the nitrone ketene isomer of the acylisoxazolone.

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this was unstable; other quaternisations could not be carried out. (17) n = 2 (19) 1.2 = 2 (18) Pi? = 3 (20) t a = 3Acyl derivatives of 2-thiazoline-2-thiol (I 1) may be obtained by treatment with acyl halides but it appears not to have been established unequivocally whether these are in fact S-acyl derivatives 596 (2-acylthio-2-thiazolines) (16) or N-acyl derivatives 'i-9 (3-acylthiazolidine-2-thiones), e.g. ( 21). The position of the G Oband in their i.r. spectra does not permit unequivocal distinction between SCOR and N-COR, but the U.V. spectra of these compounds are unaffected by added acid, and they do not form picrates; these properties are similar to those of the corresponding iN-alkyl compounds, e.g. ( 25), but contrast with those of the S-alkyl compounds, e.g. (13).Attempts to make a 2-thiazolin-2-yl disulphide by oxidation of 2-thiazoline-2-thiol (1 1) with hydrogen peroxide were unsuccessful. Though 5,5-dimethyl-2-thiazoline-2-thiol is said to yield the disulphide (28), in our hands 2-thiazoline-2-thiol (1 1), under similar conditions, yielded the thiazolinylthiazolidine-2-thione (29). However, the reported characterisation of compound ( 28) c D (27) R = H (29) R = H (28) R = Me (30) R = Me rests on a nitrogen analysis only, and this fits structure

While isoxazol-5(2H)-ones substituted with heterocycles at C2 but unsubstituted at C3 react with amines to give either amidines or malonamides, their reaction at low temperatures with lithium dialkylamides is a preparatively useful procedure for obtaining the amidines in most cases. Longer reaction times may lead to formation of pyrimidin-4-ones when ester groups are present at C4 of the isoxazolone.

The potential cyclization of N-heteroarylamidines to annelated pyrimidines by flash vacuum pyrolysis or by treatment with bases has been investigated. The pyrolytic process with the title compounds is complicated by competing formation and rearrangement of an imidoylketene. The major pyrolysis products, the 4-aminopyrimidin-6-ones, were best prepared from the title compounds by reaction with hindered alkoxides, while the pyrolysis byproducts, the 4-alkoxypyrimidin-6-ones, were conveniently prepared from the appropriate isoxazol-5(2H)-one by reaction with the appropriate alkoxide, followed by acidic workup.