EVIDENCE FOR MONODENTATE AND BISDENTATE BORONATE COMPLEXES OF GLUCOSE IN THE FURANOSE FORM - APPLICATION OF (1)J(C-C)-COUPLING-CONSTANTS AS A STRUCTURAL PROBE

Complex formation between aromatic boronic acids and D-glucose was investigated by H-1 and C-13 NMR spectroscopy both under neutral nonaqueous and alkaline aqueous conditions. The structure of the bisdentate complex between 2,2'-dimethoxydiphenylmethane-5,5'-diboronic acid and glucose was reinvestigated and reassigned. The reactions studied are shown in Scheme 1. The complexes contain in all cases the furanose form of glucose bound to two boronic acid groups where the one is bound in the (1,2) position. The binding site for the second boronic acid group is (3,5) under neutral nonaqueous conditions. In aqueous alkaline solution the second binding site varies from the mono- to the diboronic acid. The second monoboronic acid binds to the (3,5,6) position in a tris coordinating manner, and the diboronic acid binds its second boronic acid group only to the (5,6) position. Two closely related complexes (ratio 4:1) are observed in the spectra with the monoboronic acid. This is ascribed to diastereomerism from the 1,2-complexed stereocenter. (1)J(C-C) are used in the structure assignment. Exceptionally low values of 34-35 Hz for (1)J(C-C) are measured for vicinal diols when the O-C-C-O fragment becomes incorporated in a five membered ring as is the case in cyclic boronic esters with a vicinal ester bonding.