Crystal structures of four thio-glycosides involving carbamimido-thio-ate groups.

The compounds 2',3',4',6'-tetra--acetyl-β-d-gluco-pyranosyl '-cyano--phenyl-carbamimido-thio-ate (CHNOS, ), 2',3',4',6'-tetra--acetyl-β-d-galacto-pyranosyl '-cyano-phenyl-carbamimido-thio-ate, (CHNOS, ), 2',3',4',6'-tetra--acetyl-β-d-galacto-pyranosyl '-cyano--methyl-carbamimido-thio-ate (CHNOS, ), and 2',3',4',6'-tetra--acetyl-β-d-galacto-pyranosyl '-cyano---tolyl-carbamimido-thio-ate (CHNOS, ) all crystallize in 222 with = 4. For all four structures, the configuration across the central (formal) C=N(CN) double bond of the carbamimido-thio-ate group is . The torsion angles C5-O1-C1-S (standard sugar numbering) are all close to 180°, confirming the β position of the substituent. Compound involves an intra-molecular hydrogen bond N-H⋯O1; in this contact is the weaker branch of a three-centre inter-action, whereas in and 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, , and form layer structures involving the classical hydrogen bond N-H⋯N and a variety of 'weak' hydrogen bonds C-H⋯O or C-H⋯S. The packing of 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.