Crystal structure of two brom containing aza-tetracyclic fused N-heterocycles including isoxazolidine ring compounds have been determined in single-crystal X-ray diffraction studies. The compound, C14H19BrN2O3, ( 1), and C12H15BrN2O3, ( 2), were obtained from a stereospecific[ 3 + 2] 1,3-cycloaddition of oxime based tandem nitrone generation reactions. The envelope conformation of the isoxazolidine rings are different, leading the substituents to be pseudoaxial in ( 1) and pseudoequatorial in ( 2). The stereochemistry of these fused compounds are exo- and endo-stereoisomer geometry contributed by exo and endo transition state of nitrone 1,3-dipolar cycloaddition reactions. The title compound ( 1) crystallizes in the monoclinic space group P2(1)/n with a = 12.6221( 5) Angstrom, b = 7.5917( 4) Angstrom, c = 16.2350( 9) Angstrom, beta = 112.254( 5) degrees, V = 1439.81( 12) Angstrom(3), and D-calc = 1.583 Mg/m(3) for Z = 4 and compound ( 2) crystallizes in the monoclinic space group P2(1)/c with a = 11.6906( 9) Angstrom, b = 6.4255( 7) Angstrom, c = 17.2070( 10) Angstrom, beta = 109.264( 5) degrees, V = 1220.2( 2) Angstrom(3), and D-calc = 1.716 Mg/m(3) for Z = 4. The antibacterial activity of both compounds were investigated for three Gram (+) and two Gram (-) bacteria by employing broth microdilution method and subsequently inhibitory activity against yeast-like fungi was also determined.