Studies on borate esters. Part 5. The system glucarate–borate–calcium(II) as studied by 1H, 11B, and 13C nuclear magnetic resonance spectroscopy

Abstract

A combination of ¹H, ¹¹B, and ¹³C n.m.r. spectroscopy is used to determine the identity, structure, and stability of compounds present in the aqueous glucarate–borate–calcium(II) system. Borate ester formation was found to occur preferbly at the threo-3,4-position of glucarate. Two diastereoisomeric borate diesters of glucarate are the major calcium(II) co-ordinating species. Each borate diester contains two calcium(II) co-ordinating sites consisting of two carboxylate groups, two borate ester ring oxygens, and, depending on the α-CHOH configurations, up to two α-hydroxy groups. The overall calcium(II) co-ordinating strength of both borate diester diastereoisomers of glucarate is about equal, whereas this is not the case for the arabinonate diastereoisomers.