Metal complexes of carbohydrates: isolation and characterisation of cobalt(III) complexes containing N-substituted glycosylamine ligands derived from ethane-1,2-diamine and glucosamine

Abstract

Reaction between cis-or trans- [Co(en),CI,] + (en = ethane-I ,2-diamine) and D-glucosamine (2-amino-2-deoxy-~-glucopyranose) in neutral aqueous solution results in a very complicated product mixture including as a major component several isomeric complex ions in which the co-ordination sphere is formally made up of two ethane-I ,2-diamine units and one sugar unit. Separation of these complexes by ion-exchange chromatography, and subsequent spectroscopic and structural studies have shown that they are diastereomeric forms in which the co-ordination sphere actually contains one bidentate ethane-1.2-diamine chelate and a multidentate ligand which is an N-(2'-aminoethyl)amino-substituted glycosylamine derived from glucosamine, fructosamine or mannosamine. The presence of the fructosamine moiety reveals that the complex formation reaction is accompanied by the Amadori rearrangement of glucosamine, while the presence of the mannosamine moiety indicates that this rearrangement may be reversible and possibly stereospecific in particular diastereoisomers. Crystal structure determinations have definitively characterised eight of the reaction products as: 1 a complex of the 1 -N-(2'-aminoethyl)amino-substituted glucosamine in which the sugar, although chelated to the metal as part of a tridentate ligand, is in its open-chain form and in which the four nitrogen atoms derived from the original two ethane-I ,2-diamine units are essentially coplanar; 2 a complex of tridentate 1 -N-(2'-aminoethyl)amino-substituted glucosamine in which the sugar has its pyranose form and the chelate edges spanned by the original ethane-I ,2-diamine units are in the A configuration; 3 a complex of 1 -N-(2'-aminoethyl)amino-substituted glucosamine in which the sugar is in its open-chain form and the ligand is bound in a quadridentate manner, with the chelate edges spanned by the original ethane-I ,2diamine units in the A configuration; 4 a complex of 1 -N-(2'-aminoethyl)amino-substituted mannosamine in which the sugar has a furanose form and is part of a quadridentate ligand, and the chelate edges spanned by the original ethane-1.2-diamine units are in the A configuration; 5 a complex of quadridentate 1 -(2'-aminoethylamino) -1 -deoxy-fructopyranosylamine in which the chelate edges spanned by the original ethane-1.2-diamine units are in the A configuration; 6 a complex of quadridentate 1 -(2'-aminoethylamino) -1 -deoxy-fructofuranosylamine in which the chelate edges spanned by the original ethane-l,2-diamine units are in the A configuration; 7 the same complex as 6 but in the form where its co-ordinated sugar hydroxyl group is deprotonated; and 8 a complex of 1 -(2'-aminoethylamino) -1 -deoxyfructopyranosylamine in which the chelate edges spanned by the original ethane-I ,2-diamine units are in the A configuration.

Complexes of natural carbohydrates with metal cations

“…Subsequently, it was shown 330 that this affords four isomers with a D : L ratio of 7 : 3, which differ in the anomeric configuration of the ligand and configuration of the chelate unit (complexes like 43 with an amino group instead of a hydroxyl group). Finally, recently 331 it was shown that the composition of the product mixture is even more complicated; eight(!) isomers were isolated from the mixture and characterised by X-ray diffraction analysis.…”

Spin−Orbit-Induced Anomalous pH-Dependence in 1H NMR Spectra of CoIII Amine Complexes: A Diagnostic Tool for Structure Elucidation

“…Corresponding complexes with chelating polyamines have been the subject of numerous subsequent studies, serving for example as model compounds for the investigation of the kinetics and mechanism of ligand exchange reactions . Co III -amine complexes with polyalcohols have been prepared as models for the elucidation of metal binding to carbohydrates . The diamagnetic Co III center is particularly suitable for this purpose, because the different species formed in solution can readily be identified by conventional NMR methods …”

Basic achievements in the coordination chemistry of modified monosaccharides

“…9,102 The use of an alternative workup of the D-glucosamine (103a) ± [Co(en) 3 ] 3+ mixture (see Ref. Among the complexes isolated, 103 only compound 106 with a pyranose form of the carbohydrate ligand is a normal product. In the other complexes, D-glucosamine 103a is transformed into both D-mannosamine 103b by the Lobry de Bruyn ± van Ekenstein rearrangement, 104,105 and D-fructosylamine by the Amadori rearrangement 105 (complex 107).…”

“…The complexes formed are diastereomerically pure and have a Dor L-configuration at the chelate point. The study 103 is undoubtedly one of the key studies in the coordination chemistry of carbohydrates, since it demonstrates extreme flexibility of unprotected monosaccharides in complexation with metals.…”