C13H24BF4NO3, orthorhombic, C222i (no. 20), a = 9.320(2) A, b = 10.6761(8) A, c = 33.786(2) A, V= 3361.6 A, Z = 8, Rgi(F) = 0.064, wR^F) = 0.185,7= 293 K. Source of material The title compound has been obtained by 1,3-dipolar cycloaddition of ethylene and the nitrile oxide obtained in situ from hydroximoyl chloride, derived from 2,3-O-cyclohexylidene-Dglyceraldehyde [1-3]. This was followed by N-methylation of the intermediate isoxazoline with trimethyloxonium tetrafluoroborate and reduction on treatment with sodium borohydride in ethanol [2]. Further methylation of the corresponding major isoxazolidine isomer (separated by MPLC, ratio of diastereoisomers = 84:16) with trimethyloxonium tetrafluoroborate [3-5] and recrystallization from ethanol afforded the isoxazolidinium salt in the form of colorless crystals (m.p. 443-444 Κ, αο = -29.4°, c = 0.81, CH2CI2). Discussion The isoxazolidine ring system shows an envelope conformation (figure, top). Three atoms (C10,CI 1,C12) of thecyclohexylidene moiety exhibit very large displacement parameters. A resolution of eventual disorder via split positions was not performed, the distances of the C-C bonds deviate less than 5 % from normal values. The fluorine atoms of the tetrafluoroborate anion have also large displacement parameters, which indicates considerable rotation of the anion. The packing diagram reveals a bilayer-type stacking of the molecules with alternating polarity along the c axis (figure, bottom). The non-polar bilayer is built up by a face-to-face orientation of the cyclohexylidene moieties; the polar bilayer is built by the hetero atoms of the isoxazolidines and the tetrafluoroborate anions. Table 1. Data collection and handling. Crystal: Wavelength: μ· Diffractometer, scan mode: 2βηωχ: N(hkl)masimd, N(hklhm^ Criterion for / 2 o(ldta), 1174 199 SHELXS-97 [6], SHELXL-97 [7] Table 2. Atomic coordinates and displacement parameters (in A). Correspondence author (e-mail: jager.ioc@po.uni-stuttgart.de) Atom Site X y ζ Um, H(1A) 8c 0.0618 0.5178 0.4513 0.146 H(1B) 8c -0.1065 0.5198 0.4543 0.146 H(2A) 8c 0.0584 0.4363 0.3923 0.120 H(2B) 8c -0.1006 0.4873 0.3909 0.120 H(3) 8c -0.1956 0.3044 0.4095 0.082 H(4) 8c -0.0728 0.1290 0.3796 0.083 H(5A) 8c -0.2387 0.1880 0.3358 0.102 H(5B) 8c -0.1904 0.3296 0.3366 0.102