Dioxazolidines, formation

Following a report by Biichi and Ayer > on the photolysis of nitrobenzene in cyclohexene and 2-methyl-2-butene, a thorough study on nitrobenzene cycloaddition to olefines has been undertaken by de Mayo and co-workers 3 . ). Evidence was presented for the previously postulated ) formation of 1,3,2-dioxazolidines ... 

A mixture of PhCHO, RCHO, and R1N=0 was obtained in an attempt to perform the cyclocondensation of carbonyl oxide with a-phenyl-feA-butyl nitron PBN <2002JPG3917>. The authors believe that a preferable pathway for this reaction is the formation of 1,2,4-dioxazolidine A-oxide 39 as an unstable intermediate (Scheme 1). 

The most characteristic reactions for these compounds are thermolysis and decomposition under the action of both acids and nucleophilic reagents. 1,2,4-Dioxazolidines 15 are thermally stable only below 100 °C. The thermolysis of compound 15a in solution at 130 °C afforded benzophenone and benzamide in quantitative yields. The formation of these products is consistent with a radical mechanism involving initial 0-0 homolysis (Scheme 21). The base-catalyzed decomposition of compound 15a afforded the same products. Similar results were observed for the decomposition of compounds 15b and 15c. The interaction of dioxazolidine 15a with triphenylphosphine resulted in deoxygenation of the system with the concomitant formation of benzophenone and its imine with aniline <1995J(P1)41>. 

Other intramolecular cyclizations accompanied by elimination are the preparation of the l,2,4-dioxazolin-3-ones (190) by HC02H dehydrative cyclization of a precursor percar-bamic acid (72JPR145) formation of the l,3,4-dioxazol-5-ones (191) from the hydroxamic ester (192) (78JAP78127479) and preparation of the 1,3,2-dioxazolidine (57) by base treatment of the precursor A-methoxy-A-hydroxyethoxy compound (80IZV2181). 

Finally, the DCA-sensitized photooxygenations of several substituted or unsubstituted aziridines, affording the corresponding 1,2,4-dioxazolidines, do not require the BP cosensitization since they efficiently quench the DCA fluorescence [109,181,182]. Of particular interest is the fact that the unsubstituted derivatives cis- and/or tram-2,3-diphenylaziridines 77 a, b follow a similar stereochemical course of epoxides with the exclusive formation of the m-isomer 3,5-diphenyl-l,2,4-dioxazolidine 78 ... 

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