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 α-Man p , Me α- and β-Gal p and selected O -methyl derivatives in D 2 O was determined by 51 V, 1D and 2D 13 C NMR spectroscopy at pD 7.8. The 51 V 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 51 V 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 13 C 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 51 V 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 . 51 V 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 13 C 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 51 V signals of complexes at δ −492 (9%) and −508 (9%), showing more than one structure of oxyvanadium derivatives.