An efficient method of synthesis of isoxazolidine-fused β-lactams via base-promoted cyclization–ring opening of carbamoyl-spirocyclopropane isoxazolidines.

Abstract: 1,3-Dipolar cycloadditions of dialkyl-substituted 2-benzylidenecyclopropane-1,1-dicarboxylates and a number of C-carbamoyl nitrones proceed with high efficiency and selectivity with the formation of single isomeric spiro[cyclopropane-1,4-isoxazolidine] cycloadduct. Obtained cycloadducts easily undergo cyclopropyl ring opening–cyclization to form new β-lactams fused with isoxazolidine ring in high yields. [Copyright &y& Elsevier]