Boronic acids as fructose sensors. Structure determination of the complexes involved using 1 J CC coupling constants

Abstract

Complex formation between p-tolylboronic acid and D-fructose was investigated by 13C NMR spectroscopy both in neutral nonaqueous and alkaline aqueous solutions to evaluate the possibilities of boronic acid based fructose sensors. Under both conditions a mixture of complexes was observed. The structures of the complexes in solution were assigned on the basis of 1JCC coupling constants which provided the information of the binding sites for the formed cyclic boronic esters. In alkaline aqueous solution seven different complexes were observed. At a 1:1 boronic acid: fructose ratio the major complex (82%) was β-D-fructofuranose 2,3,6-tri-O-(p-tolylorthoboronate)1. The minor complex was β-D-fructofuranose 2,3-(p-tolylhydroxyboronate) present in two diastereomeric forms 2a and 2b. At higher boronic acid:fructose ratios four additional complexes were observed namely the four possible diastereomeric forms of β-D-fructopyranose 2,3:4,5-bis(p-tolylhydroxyboronate) 3a–d. Under nonaqueous conditions signals arising from 13 different compounds were observed. Five of these complexes were structurally assigned. At a 1:1 boronic acid : fructose ratio the major complex was β-D-fructofuranose 2,3(p-tolylboronate) 5 present in 46%. At a ratio of 4:1 the major species was β-D-fructopyranose 2,3:4,5-bis(p-tolylboronate) 7 present in 67%. Two additional monoesterified pyranoses and one furanose were observed. They were assigned to the β-D-fructopyranose 1,2-(p-tolylboronate) 8, the α-D-fructopyranose 1,3-(ptolylboronate) 9 and the β-D-fructofuranose 1,3-(p-tolylboronate) 6. It is concluded that results of earlier studies on binding constants bear no relation to the actual formed complexes.