STRUCTURAL-ANALYSIS OF 6-S-(BENZOXAZOL-2-YL)-6-DEOXY 1,2 DI-O-ISOPROPYLIDENE-6-THIO-ALPHA-D-GALACTOPYRANOSE BY MEANS OF X-RAY-DIFFRACTION, HIGH-RESOLUTION NMR-SPECTROSCOPY, AND MOLECULAR MODELING/

The conformational behaviour of a representative azaheterocycle/thiosu gar hybrid: 6-S-(benzoxazol-2-yl)-6-deoxy- di-O-isopropylidene-6-thio- alpha-D-galactopyranose 1, has been characterised by X-ray crystallogr aphy, molecular modelling and NMR studies, The crystal of 1 (C19H23NO6 S) belongs to the monoclinic space group P2(1), with a = 8.4330(4), b = 10.287(3), c = 11.417(4) Angstrom, beta = 101.66(5)degrees, V = 970. 0(5) Angstrom(3), and Z = 2, The structure has been determined by X-ra y diffraction at 299 K using an area detector. Least-squares refinemen t based on 2352 reflections yielded a final wR2 of 0.098, with R1 = 0. 040 [for 2210 reflections with F-0 > 4 sigma(F-0)], The pyranose ring adopts a conformation between that of the screw-boat, S-O(S), and the twist-boat, T-O(2) forms. Proton NMR chemical shifts have been assigne d by measurement of 1D and 2D spectra. The C-5-C-6 conformer populatio ns in CDCl3 solution have been estimated, In parallel, a conformationa l analysis using the crystal coordinates as the starting point has bee n made using the Tripes force field. The resulting potential energy su rface indicates a high flexibility about the C-5-C-6 and C-6-S-6 bonds . The agreement between experimental and theoretical data is satisfact ory and constitutes a validation of the conformational energy surface. (C) 1996 Elsevier Science Ltd.