Crystal structures of four thioglycosides involving carbamimidothioate groups

Abstract

The compounds 2',3',4',6'-tetra-<i>O</i>-acetyl-β-d-gluco-pyranosyl <i>N</i>'-cyano-<i>N</i>-phenyl-carbamimido-thio-ate (C₂₂H₂₅N₃O₉S, <b>5a</b>), 2',3',4',6'-tetra-<i>O</i>-acetyl-β-d-galacto-pyranosyl <i>N</i>'-cyano-<i>N-</i>phenyl-carbamimido-thio-ate, (C₂₂H₂₅N₃O₉S, <b>5b</b>), 2',3',4',6'-tetra-<i>O</i>-acetyl-β-d-galacto-pyranosyl <i>N</i>'-cyano-<i>N</i>-methyl-carbamimido-thio-ate (C₁₇H₂₃N₃O₉S, <b>5c</b>), and 2',3',4',6'-tetra-<i>O</i>-acetyl-β-d-galacto-pyranosyl <i>N</i>'-cyano-<i>N</i>-<i>p</i>-tolyl-carbamimido-thio-ate (C₂₃H₂₇N₃O₉S, <b>5d</b>) all crystallize in <i>P</i>2₁2₁2₁ with <i>Z</i> = 4. For all four structures, the configuration across the central (formal) C=N(CN) double bond of the carbamimido-thio-ate group is <i>Z</i>. The torsion angles C5-O1-C1-S (standard sugar numbering) are all close to 180°, confirming the β position of the substituent. Compound <b>5b</b> involves an intra-molecular hydrogen bond N-H⋯O1; in <b>5c</b> this contact is the weaker branch of a three-centre inter-action, whereas in <b>5a</b> and <b>5d</b> the H⋯O distances are much longer and do not represent significant inter-actions. The C-N bond lengths at the central carbon atom of the carbamimido-thio-ate group are almost equal. All C-O-C=O torsion angles of the acetyl groups correspond to a synperiplanar geometry, but otherwise all four mol-ecules display a high degree of conformational flexibility, with many widely differing torsion angles for equivalent groups. In the crystal packing, <b>5a</b>, <b>5c</b> and <b>5d</b> form layer structures involving the classical hydrogen bond N-H⋯N_cyano and a variety of 'weak' hydrogen bonds C-H⋯O or C-H⋯S. The packing of <b>5b</b> is almost featureless and involves a large number of borderline 'weak' hydrogen bonds. In an appendix, a potted history of wavelength preferences for structure determination is presented and it is recommended that, even for small organic crystals in non-centrosymmetric space groups, the use of Mo radiation should be considered.