Sugar vanadates: synthesis and characterisation of mannopyranoside and ribofuranoside esters incorporating VO³⁺

Abstract

The sugars, methyl 4,6-di-O-methyl-α-D-mannopyranoside (H₂m) and methyl 5-O-methyl-β-D-ribofuranoside (H₂r) have been synthesized. These react smoothly with [V^IVO(L)(H₂O)] in methanol in air affording the dark coloured vanadates [V^VO(Hm)(L)] and [V^VO(Hr)(L)] in excellent yields (L^2– = N-salicylideneglycinate). The crystal structure determination of [V^VO(Hm)(L)] revealed five-membered chelation of VO³⁺ by the alcoholic and alkoxide oxygen atoms of the monoionised carbohydrate. The two atoms lie respectively trans to the oxo oxygen and aldimine nitrogen atoms. The five V–O bonds are unequal and span the range 1.57–2.37 Å. The asymmetric unit of the complex consists of two metrically similar molecules locked in carboxylate–alcohol O · · · O hydrogen bonding generating a macrocyclic cavity. In solution each ester displays a single ⁵¹V resonance near δ –544. The ¹H NMR parameters of the alkoxidic and alcoholic CH protons and of OMe protons are consistent with the O,O-chelation mode for both the vanadates.