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

Abstract

The glycosides methyl-2,6-dimethoxy-β-d-galactopyranoside (β-d-H₂Me₃GP), methyl-4,6-dimethoxy-a-D-mannopyranoside (α-D-H₂Me₃MP), and methyl-5-methoxy-β-D-ribofuranoside (β-d-H₂Me₂RF) have been synthesized, and their reaction with VO(L-Asal)(OMe)(OHMe) in dichloromethane has afforded esters of the type VO(β-D-HMe₃GP)(L-Asal), VO(a-D-HMe₃MP)(L-Asal), and VO(β-D-HMe₂RF)(L-Asal) as dark-colored solids (red in solution). 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(β-D-HMe₃GP)(L-vsal)·H₂O and VO(a-D-HMe₃MP)(L-psal)·H₂O have revealed five-membered (O,O)-chelation by monoionized carbohydrates, the 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) distances are, respectively, ~1.80 and ~2.30 Å. The ONO coordinating tridentate salicylaldimine ligand is folded (by ~35°) along a C-N bond. The chiral configuration of the metal site corresponds exclusively to the endo disposition of the V=O and the amino acid C-R (R = Me, CHMe₂, CH₂Ph) bonds. In VO(β-D-HMe₃GP)(L-vsal)·H₂O two ester molecules constitute the asymmetric unit and these along with the two water molecules form a macrocyclic supramolecule (diameter, ~6.1 Å) held by hydrogen bonds involving alcohol and an OMe function as well as water (O···O distance, 2.59-2.86 Å). On the other hand, in VO(a-D-HMe3MP)(L-psal)·H₂O the water molecule bridges two symmetry-related ester molecules via alkoxide···water and alcohol···water hydrogen bonds forming an infinite chain structure (O···O lengths, 2.65 and 2.85 Å). The molecular structures observed in the solid state are preserved in solution (¹H and ⁵¹V NMR). No isomerization is detectable either at the metal site 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(β-D-HMe₂RF)(L-Asal) species did not afford single crystals but NMR results are consistent with (O,O)-chelation by the ribose fragment, the alkoxidic carbon being C₃. Crystal data are as follows. VO(β-D-HMe₃GP)(L-vsal)·H₂O: chemical formula, C₂₁H₃₂NO₁₁V; crystal system, orthorhombic; space group, P2₁2₁2₁; a = 13.146(7) Å, b = 15.142(5) Å, c = 25.631(9) Å; Z = 8. VO(a-d-HMe₃MP)(L-psal)·H₂O: chemical formula, C₂₅H₃₂NO₁₁V; crystal system, monoclinic; space group, P2₁; a = 13.645(4) Å, b = 7.022(2) Å, c = 15.500(4) Å; β = 113.98(2)°; Z = 2.