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

The sugars, methyl 4,6-di-O-methyl-alpha-D-mannopyranoside (H(2)m) and meth yl 5-O-methyl-beta-D-ribofuranoside (H(2)r) have been synthesized. These re act smoothly with [(VO)-O-IV(L)(H2O)] in methanol in air affording the dark coloured vanadates [(VO)-O-V(Hm)(L)] and [(VO)-O-V(Hr)(L)] in excellent yi elds (L2-=N-salicylideneglycinate). The crystal structure determination of [(VO)-O-V(Hm)(L)] revealed five-membered chelation of VO3+ by the alcoholic and alkoxide oxygen atoms of the monoionised carbohydrate. The two atoms l ie respectively trans to the oxo oxygen and aldimine nitrogen atoms. The fi ve V-O bonds are unequal and span the range 1.57-2.37 Angstrom. The asymmet ric unit of the complex consists of two metrically similar molecules locked in carboxylate-alcohol O ... O hydrogen bonding generating a macrocyclic c avity. In solution each ester displays a single V-51 resonance near delta - 544. The H-1 NMR parameters of the alkoxidic and alcoholic CH protons and o f OMe protons are consistent with the O,O-chelation mode for both the vanad ates.