Rearrangements of isoxazoles

Table 10 Rearrangement of Isoxazoles with Suitable Three-atom Side Chains...

Oxazoles are generally photostable and are, indeed, produced by light-induced rearrangements of isoxazoles (see Section 4.18.4.1.2) (b-80MI41800>. However, irradiation of 2,5-diphenyloxazole (117) in ethanol gives a mixture of 3,5-diphenylisoxazole (118), 4,5-diphenyloxazole (119), the phenanthrooxazole (120) and a trace of benzoic acid. These reactions, which proceed by two distinct paths, are rationalized as shown in Scheme 2 <77JCS(P1)239>. 

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. 

Furazans are available via the mononuclear rearrangement of isoxazoles and 1,2,4-oxadiazoles bearing oxime groups in the 3-position (81AHC(29)14l). The approach is illustrated in Scheme 5, with X=Y—Z being N=C—O or C=C—O. 

A formally similar rearrangement of isoxazol-5-ones probably proceeds by hydrolysis of the lactone linkage followed by recyclization.247... 

Sigmatropic rearrangements of cyclopropylimines <91TL7127> and cyclobutylimines <88H(27)1665> to generate fused bicyclic aziridines have been reported <84CHEC-I(7)47>. The sigmatropic rearrangement of isoxazoles to azirines will be covered in Section 1.01.9.6. Electrocyclic closure of vinyl-nitrenes to azirines will be discussed in Section 1.01.9.5. 

Although rearrangement of isoxazoles can be used to prepare oxazoles, unexpected limitations have been reported. Perez and Wunderlin investigated the FVP of various 5-alkyl-3-methyl-4-nitroisoxazoles 87 as a method for preparing... 

Pascuar developed a one-pot synthesis of 2-(alkylamino)-4-cyanooxazoles and 5-alkyl-2-(alkylamino)-4-cyanooxazoles by rearrangement of isoxazole-derived thioureas (Scheme 1.27). Reaction of an isoxazole isothiocyanate 94 with primary amines gave the thioureas 95, which were dehydrated to carbodiimides 97... 

Nishimoto and co-workers reported a theoretical smdy on the photochemical rearrangement of isoxazoles to oxazoles. In a later smdy of the thermal and photochemical transposition of oxazoles, the authors concluded that these rearrangements involve the formation of azirine intermediates. ... 

A base-induced rearrangement of isoxazoles to imidazole derivatives is a three-atom side-chain rearrangement involving a CNC sequence (Scheme 57). ° ... 

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