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

Abstract

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