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

The compounds 2',3',4',6'-tetra- O -acetyl-β-d-gluco-pyranosyl N '-cyano- N -phenyl-carbamimido-thio-ate (C 22 H 25 N 3 O 9 S, 5a ), 2',3',4',6'-tetra- O -acetyl-β-d-galacto-pyranosyl N '-cyano- N- phenyl-carbamimido-thio-ate, (C 22 H 25 N 3 O 9 S, 5b ), 2',3',4',6'-tetra- O -acetyl-β-d-galacto-pyranosyl N '-cyano- N -methyl-carbamimido-thio-ate (C 17 H 23 N 3 O 9 S, 5c ), and 2',3',4',6'-tetra- O -acetyl-β-d-galacto-pyranosyl N '-cyano- N - p -tolyl-carbamimido-thio-ate (C 23 H 27 N 3 O 9 S, 5d ) all crystallize in P 2 1 2 1 2 1 with Z = 4. For all four structures, the configuration across the central (formal) C=N(CN) double bond of the carbamimido-thio-ate group is Z . The torsion angles C5-O1-C1-S (standard sugar numbering) are all close to 180°, confirming the β position of the substituent. Compound 5b involves an intra-molecular hydrogen bond N-H⋯O1; in 5c this contact is the weaker branch of a three-centre inter-action, whereas in 5a and 5d 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, 5a , 5c and 5d 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 5b 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.