Carbohydrate binding to VO3+. Sugar vanadate esters incorporating L-amino acid Schiff bases as coligands

The glycosides methyl-2,6-dimethoxy-beta-D-galactopyranoside (beta-D-H(2)Me (3)GP), methyl-4,6 -dimethoxy-alpha-D-mannopyranoside (alpha-D-H2Me3MP), an d methyl-5-methoxy-beta-D-ribofuranoside (beta-D-H2Me2RF) have been synthes ized, and their reaction with VO(L-Asal)(OMe)(OHMe) in dichloromethane has afforded esters of the type VO(beta-D-HMe(3)GP)(L-Asal), VO(alpha-D-HMe3MP) (L-Asal), and VO(beta-D-HMe2RF)(L-Asal) as dark-colored solids (red in solu tion). Here, L-Asal(2-) is the deprotonated salicylaldimine of L-alanine (A = a), L-valine (A = v), and L-phenylalanine (A = p). The X-ray structures of VO(beta-D- HMe(3)GP)(L-vsal) . H2O and VO(alpha-D-HMe3MP)(L-psal) . H2O have revealed five-membered (O,O)-chelation by monoionized carbohydrates, t he undissociated hydroxyl group lying trans to the oxo oxygen atom. In the carbohydrate frame, the alkoxidic oxygen atom is axial in the former ester and equatorial in the latter. The V-O(alkoxidic) and V-O(alcoholic) distanc es are, respectively, similar to 1.80 and similar to 2.30 Angstrom. The ONO coordinating tridentate salicylaldimine ligand is folded (by similar to 35 degrees) along a C-N bond. The chiral configuration of the metal site corr esponds exclusively to the endo disposition of the V=O and the amino acid C -R (R = Me, CHMe2, CH2Ph) bonds. In VO(beta-D-HMe(3)GP)(L-vsal) H2O two est er molecules constitute the asymmetric unit and these along with the two wa ter molecules form a macrocyclic supramolecule (diameter, similar to 6.1 An gstrom) held by hydrogen bonds involving alcohol and an OMe function as wel l as water (O...O distance, 2.59-2.86 Angstrom). On the other hand, in VO(a lpha-D-HMe3MP)(L-psal) . H2O the water molecule bridges two symmetry-relate d ester molecules via alkoxide water and alcohol water hydrogen bonds formi ng an infinite chain structure (O...O lengths, 2.65 and 2.85 Angstrom). The molecular structures observed in the solid state are preserved in solution (H-1 and V-51 NMR). No isomerization is detectable either at the metal sit e or at the anomeric carbon atom, and the V-O(alkoxidic) and V-O(alcoholic) sites and the metal-carbohydrate binding remain in tact. The VO(beta-D-HMe 2RF)(L-Asal) species did not afford single crystals but NMR results are con sistent with (O,O)-chelation by the ribose fragment, the alkoxidic carbon b eing C3. Crystal data are as follows. VO(beta-D-HMe(3)GP)-(L-vsal) . H2O: c hemical formula, C21H32NO11V; crystal system, orthorhombic; space group, P2 (1)2(1)2(1); a 13.146(7) Angstrom, b = 15.142(5) Angstrom, 25.631(9) Angstr om; Z = 8. VO(alpha-D-HMe3MP)(L-psal) . H2O: chemical formula, C25H32NO11V; crystal system, monoclinic; space group, P2(1); a = 13.645(4) Angstrom, b = 7.022(2) Angstrom, c = 15.500(4) Angstrom; beta = 113.98(2)degrees; Z = 2 .