Complexes of sodium vanadate(V) with methyl α-D-mannopyranoside, methyl α- and β-D-galactopyranoside, and selected O-methyl derivatives:: a 51V and 13C NMR study

The hydroxyl group stereochemistry of complexation of sodium vanadate(V) with Me alpha-Manp, Me alpha- and beta-Galp and selected O-methyl derivatives in D2O was determined by V-51, 1D and 2D C-13 NMR spectroscopy at pD 7.8. The V-51 approach served to show the extent of complexation and the minimum number of esters formed. That of Me a-Manp gave rise mainly to a 51V signal at delta -515, identical with that of its 4,6-di-O-methyl derivative, which had only a 2,3-cis-diol exposed. The C-13 NMR spectra contained much weaker signals of the complexes, but both glycosides showed strong C-2 and C-3 alpha-shifts of +17.3 and +10.8 ppm, respectively. As expected, Me 2,3-Me-2-alpha-Manp, which contains a 4,6-diol, did not complex. Me Galp anomers and their derivatives showed more diversity in the structure of its oxyvanadium derivatives. Me alpha-Galp, with its 3,4-cis-diol, complexed to give rise to V-51 signals at delta -495 (9%) -508 (10%), and -534 (4%). These shifts and proportions were maintained with Me beta-Galp and Me Me alpha-Galp. V-51 NMR spectroscopy showed that Me 3Me-beta-Galp, with its possibly available 4,6-diol, did not complex. Similarly, Me alpha-Galp + vanadate gave a C-13 DEPT spectrum that did not contain an inverted signal at delta > 71.4, as would be expected of a C-6 resonance suffering a strong downfield alpha-shift. Me 2,6-Me-2-alpha-Galp, with a 3,4-cis-diol group, gave rise to two V-51 signals of complexes at delta -492 (9%) and -508 (9%), showing more than one structure of oxyvanadium derivatives. (C) 2003 Elsevier Ltd. All rights reserved.