3- Oxazolin-5-ones, thermolysis

A nitrile ylide intermediate has also been postulated for the thermolysis of the thiapyrans 32 (X = S) [75CR(C)37], The intermediacy of a nitrile ylide 37 and its 1,7-electrocyclization onto a nitro group to give intermediate 38 have been proposed as key steps in the thermolysis of 3-oxazolin-5-ones 36 to 1-acyloxyindazoles 39 (Scheme 14) (73CB2870). 

Steglich52 obtained the 2-azabicyclo[3.1.0]hex-2-en system (45) stereospe-cifically by thermolysis of the oxazolin-5-one 43, a method known to produce nitrile ylides. The intermediate 44 could be trapped with dimethyl acetylene-dicarboxylate. Brief photolysis (4 min) of 45 caused epimerization to 47 (Scheme 9).52 Evidently, diradicals (or zwitterions) such as 46 are formed in a secondary photochemical process, which would also appear to apply to Scheme 8. 

Hoefle and Steglich164 have recently found that thermolysis of 2,4-disubstituted 4-acyl-2-oxazoline-5-ones and 2,4-disubstituted 2-acyl-3-oxazolin-5-ones gives 2,4,5-trisubstituted oxazoles (93 and 94, respectively), with elimination of carbon dioxide. In the latter case, the substituents at C-2 and C-4 were interchanged. 

Reactions of Oxazoles. 2-Amino-4-methyloxazole (359) forms the rearranged hydro-peroxide (360) by the action of hydrogen peroxide. Thermolysis of 2- (3,3-di-methylallyloxy)-4,5-diphenyloxazole yields the oxazolin-2-one (361) with inversion of the allyl group by [ , ] sigmatropic shift irradiation, on the other... 

Thermolysis of the hydroxyimino derivative 40 of Meldrum s acid generates nitrosoketene 41, which adds to ketones to give cyclic nitrones, e.g. 42 from cyclohexanone (94CC281). Methyl (Z)-3-dimethylamlno-2-i3ocyanoacrylate reacts with acyl chlorides to give 2-acyl-4-(dimethyl-aminomethylene)oxazolin-5-ones 43 (93JHC575). 

Imidazolidines are quite common species, and may be prepared in numerous ways, for example by treatment of a guanidine amidine-thiourea with a 1,2-di-iminium species, or reaction of metallated isocyanides with nitrones, or even by thermolysis of 3-oxazolin-5-ones. The most novel preparation of imidazolidines in 1980 has surely been the work of Coyle et on the... 

Also, thermolysis of a carbonyl azide attached to an oxiranering generates the isocyanate 357, which undergoes an intramolecular cycloaddition to give a 3-oxazolin-2-one 358" . ... 

Isothiazol-3-ones (366) have been obtained in 92% yield by thermolysis of 2-ethoxy-3-cepham-lj8-oxide (365) in ethyl acetate. The latter was derived from the cephem (363), which gave the oxazoline (364) with t-butylhypochlorite. ... 

In connection with studies on the thermolysis of oxazolin-5-ones, the 2-arylthio-substituted derivative (49) has been observed to undergo thermally induced cycloelimination of carbon dioxide to give the 2H-1,3-benzothiazine (52), presumably via the ylide intermediates (50) and (51). Like 2-ethoxybenzoxazin-4-one, the sulphur analogue (53) undergoes ethanol elimination under the influence of an acid catalyst to give the unstable cyclic acylamine (54), which can be trapped as Diels-Alder adducts... 

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