The saccharide–hydrazide linkage: molecular and crystal structures of the semicarbazide derivatives of d-glucose, d-galactose, and d-xylose, including a ‘forbidden’ conformation of the galactose derivative Original Research Article

The X-ray crystal structures of the semicarbazide derivatives of d-glucose, d-galactose, and d-xylose are described. All are glycopyranosyl derivatives in the solid state. The glucose semicarbazide crystallizes as a dihydrate. Two different conformations of the galactose semicarbazide are found, one having the gg side-chain orientation unfavorable for monosaccharides having the 4C1-d-galacto configuration. In the two other known examples, this conformation is stabilized by an intramolecular hydrogen bond, but in the current structure, the hydrogen bonds involving the side chain are intermolecular. In these semicarbazides the NNCO torsional angle is approximately 180°, in contrast to the angles of 0.60 and 13.9° in the two solid-state conformations of 2-benzoyl-1-(α-d-xylopyranosyl)hydrazine [Ernholt, B. V.; Thomsen, I. B.; Lohse, A.; Plesner, I. W.; Jensen, K. B.; Hazell, R. G.; Liang, X.; Jakobsen, A.; Bols, M. Chem. Eur. J. 2000, 6, 278–287]. The water molecules and the carbonyl oxygen are heavily involved in H-bonding.