Pyrolysis of isoxazoles

High-temperature gas-phase pyrolysis of isoxazole derivatives 99M127. 

On pyrolysis of isoxazoles, decarbonylation has also been observed. Carbon labeling experiments have indicated that the formation of 2,3-diphenyl-2/f-azirine (86) and 2-phenylindole from 3,5-diphenylisoxazole takes place as shown in Scheme 22 thus, 3,5-13C-labeled starting material gave 2-[2-13C]phenylindole containing 51% of the original 13C.205... 

G. I. Yranzo Flash vacuum pyrolysis of isoxazoles, pyrazoles and related compound 2004COR1071... 

High-temperature gas-phase pyrolysis of isoxazole derivatives 99MI27. Isoxazole derivatives in synthesis of prostanoids 99ZOR1749. 

It is fairly stable as an ethereal solution, but the isolated acid is explosively unstable, and sensitive to heat, shock or friction [1], In a new method of preparation of the acid or its salts, pyrolysis of 4-oximato-3-substituted-isoxazol-5(4//)-ones or their metal salts must be conducted with extreme care under high vacuum to prevent explosive decomposition [2],... 

Pyrolysis of 4-phenyl-l,2,3-benzotriazine 3-oxide (100) at 420 °C gave 3-phenylbenz-isoxazole (101), 3-phenylindazole (102) and acridin-9-one (103). However, the pyrolysis of 4-phenylnaphtho[2,3-. 

A new synthesis of isoxazoles is by successive treatment of a ketoxime with butyllithi-um, the ester of a carboxylic acid and sulfuric acid, e.g. 1 -> 2 (94S989). Hitrovinyl oximes 3 (R1, R3 = alkyl or aryl) undergo oxidative cyclization to isoxazoles 4 by the action of DDQ or iodine/potassium iodide (94JHC861). Flash-vacuum pyrolysis of the 1,3-dipolar cycloadduct 5 of acrylonitrile oxide to norbornadiene results in a retro-Diels-Alder reaction to give cyclopentadiene and 3-vinylisoxazole 6 (94CC2661). 

Oxazoles are the products of the rearrangement of isoxazoles and of C-acylazirines, these processes being intimately related (81MI41800, 76ACR37I, 77H(6)143). Photolysis of 3-acylazirines gives nitrile ylides which cyclize thermally to oxazoles (equation 131). Flash vacuum pyrolysis of 3,5-diphenylisoxazole yields as major products 2,5-diphenyloxazole, 1,2-diphenylazirine, 2-phenylindole and benzamide. 13C labelling experiments indicate that the first two compounds are formed as shown in Scheme 31. 

Formation of an isoxazole system has been reported in the case of the pyrolysis of o-azidoketones. 2-Azidobenzophenone (208) gave a good yield of 3-phenylanthranil (209)... 

Pyrolysis of the terminal ketovinyl azides 73 at temperatures below 110° led to the formation of isoxazoles 74 and nitriles 75 with the... 

In flash vacuum photolysis and pyrolysis experiments on isoxazole derivatives, IR is used to monitor the reaction and to elucidate the reaction mechanisms <84JOC2772>. Evidence is found for the formation of azirine during the transformation of isoxazole to oxazole. 

In some cases, mixtures of 2,4-disubstituted and 2,5-disubstituted oxazoles were isolated. For example, pyrolysis of 2-benzoyl-3-phenyl-isoxazol-5-one 106 gave the expected 2,4-diphenyloxazole 110 in 60% yield together with 10% of 2,5-diphen-yloxazole 111 and 15% of benzanilide (Scheme 1.30). The authors explained these products by initial formation of a singlet iminocarbene 107. This then... 

Secondary aminoacetylenes, which readily tautomerize to ketenimines, have been isolated and characterized for the first time by pyrolysis of 4-methylene-isoxazol-5(4//)-ones and trapping the products at -196 °C e.g. Scheme 115). Bis-dialkylaminocyclopropenones, prepared from tetrachlorocyclopropene and dialkylamines, pyrolize smoothly at 525 °C to furnish bis-dialkylamino-... 

The conversion of small rings to smaller ones, without loss, is not common. 3-Chloroazetidine isomerizes reversibly to 2-chloromethylaziridine (Section 5.09.2.2.5). Flash vacuum pyrolysis can convert isoxazoles to azirines (Section 5.04.4.3). More common is the isomerization of medium-sized, i.e. five- or six-membered rings, e.g. certain succinimides (Scheme 23) (81JOC27) to azetidinediones, or bicyclic 1,2-dioxetanes to bis-oxiranes (Section 5.05.4.3.2). 

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