1 -(1 -Pyrrolidinyl)-1,3-butadienes (2, 3, 5, and 7), all having an electron-withdrawing substituent at C-3, undergo a thermal rearrangement to pyrrolizine derivatives (1, and 10-14). The rate of reaction generally increases with increasing polarity of the solvent; in aprotic solvents Lewis acids act as catalysts. The rate of reaction is also dependent on the configuration of the l-( 1-pyrrolidinyl)-1,3-butadiene (k^y¡/k(Z)-5 = 5.2). These thermal rearrangements take place in two steps. The first step comprises a concerted antarafacial [1,6] hydrogen shift that generates a 1,5-dipolar species, e.g., 15a, that may undergo stereomutation to 15b; in the second step a disrotatory electrocyclization takes place to give the corresponding pyrrolizines
The four-membered cyclic nitrones (2,3-dihydroazete 1-oxides) la-d react with dimethyl acetylenedicarboxylate by 1,3-dipolar cycloaddition to give the dihydroazeto[ 1,2-b]isoxazole derivatives 2a-d; cyclic nitrone le reacts in a similar way via the not isolated intermediate 2e to give the l-azabicyclo[5.1.0]octane derivative 4e. Upon being heated, cycloadduct 2b rearranges to the aziridine derivative 4b, the structure of which is elucidated by single-crystal X-ray analysis. Irradiation of nitrone lb yields the 5-oxa-l-azabicyclo[2.1.0]pentane derivative 5, which upon prolonged irradiation rearranges to the aziridine 6. Reactions of the nitrones la-c with methyl propicíate yield two isomeric dihydroazeto[1,2-6]isoxazoles (7a-c and 8a-c). Hydrogenation of the 1,3-dipolar adducts 2 and 7 in the presence of palladium on charcoal (5%) gives the pyridine derivatives 9 or 10.Recently we described the synthesis of four-membered cyclic nitrones (1, 2,3-dihydroazete 1-oxides) by the reac-
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.