Boronic acids as fructose sensors. Structure determination of the complexes involved using 1JCC 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 ¹J_CC 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.