Isoxazolo[2,3-d][1,4]benzoxazepine rings from 1,3-dipolar cycloaddition of [1,4]benzoxazepine N-oxides with acetylenic and olefinic dipolarophiles

Abstract

1,3-Dipolar cycloaddition of [1,4]benzoxazepine N-oxides to triple and double bonded dipolarophiles affords di-and tetrahydro-isoxazolobenzoxazepines in the first reported synthesis of this tricyclic ring system. It is observed that in addition to the acetylenic dipolarophiles the diastereofacial selectivity varied both with the nature of the C-nitrone substituent as well as on the degree of substitution of the dipolarophile. Addition to alkene substrates occurred regiospecifically furnishing "5-substituted" isoxazolidine rings. For each olefinic dipolarophile the major cycloadduct arose via an endo-addition of the dipolarophile to the β-face of the dipole. Some of the new tricycles displayed sharp signals in their pmr spectra whilst signal duplication/broadening was evident for others indicating the adducts exist in solution as pairs of conformational isomers which reach coalescence at or close to room temperature. Preliminary investigations show the isoxazolidine ring of the tricycles can be cleaved under reductive and oxidative conditions affording novel benzoxazepines. The structure of the cycloadduct 6a formed between the C-phenyl nitrone 5a and dimethyl acetylenedicarboxylate and has been solved by single crystal xray analysis.