Cyclopropyl building blocks for organic synthesis, 72. Cyclobutylidenecyclopropane: new synthesis and use in 1,3-dipolar cycloadditions - a direct route to spirocyclopropane-annulated azepinone derivatives.
(cyclopropylidene)cyclobutane was prepd. in multi-gram quantities by a new three-step sequence starting from Et cyclobutanecarboxylate. The 1,3-dipolar cycloaddn. of N-(phenylmethylene)methanamine, N-(2-pyridinylmethylene)methanamine and 1,4-dioxa-7-azaspiro[4.4]non-6-ene 7-oxide onto (cyclopropylidene)cyclobutane resulted in the regioselective formation of the corresponding adducts [i.e., dispiro[(cyclobutane)isoxazolidine(cyclopropane)] derivs.], with the spirobutane moieties adjacent to the oxygen atom in the oxazolidine rings. Under flash vacuum pyrolysis conditions, these cycloadducts furthermore underwent thermal rearrangement with opening of the four-membered ring, to afford spiro[cyclopropane]azepinones. In one case, an indolizidinone was also isolated (13% yield). Mechanistically this rearrangement is interpreted in terms of a cyclobutylmethyl-to-penten-5-yl radical rearrangement.
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