Transition metal complexes carbohydrate

Initially, the de novo synthesis of enantiomerically pure carbohydrates [110] and glycolipids [111] using transition metal complexes and chiral auxiliaries afforded only modest success. Ultimately, it was the use of enantiomerically pure aldehydes, such as the R and S enantiomers of 2-(phenylseleno)propionaldehyde, to convey facial selectivity upon the LACDAC reaction that enabled the synthesis of optically pure glycals. Syntheses of several complex monosaccharides such as the main sialic acid-type N-acetylneuraminic acid (Neu5Ac) and rac-3-deoxy-ma o-2-octulosonic acid (KDO) were accomplished with this technology [112, 113], The LACDAC... 

Figure 6.48 Synthesis of transition metal complexes with carbohydrate functionalised NHC ligands.

This reaction type has been applied to several other aldehydes and dienes and can be used in the synthesis of optically pure natural and non-natural carbohydrate-type systems, such as L-glucose. Hetero-Diels-Alder cycloadditions of this type can also be catalyzed by other chirally modified transition metal complexes, such as CpRu(CH2 = CH2)L3PF6 [L2 = (S)-Chi-raphos, (-)-Diop]68, (binaphthol)TiX269 70 102, vanadium103 104, or carbohydrate-modified titanium catalysts105. 

Asymmetric induction using a transition metal complex (67), use of a carbohydrate template (68), and use of the chiral lactone 49 (69) or ester 50 (70) have all given effective syntheses of stereospecifically labeled samples of glycine. A further synthesis by Santaniello etal. (71, 72) has used glutamate decarboxylase to prepare labeled samples of y-aminobutyric acid 51 (Scheme 16). On cyclization, protection, and oxidation, these gave the labeled enamides 55, which were degraded to the labeled samples of glycine 23 (71, 72). 

Emil Fischer s result involving cyanide additions to carbohydrates had demonstrated the power of diastereoselective synthesis early as the 1890s (Equation 1) [4, 34,162]. The corresponding enantioselective formation of cyanohydrins has been the subject of immense efforts. It has long been appreciated that optically active cyanohydrins are synthetically useful intermediates that can be elaborated into a number of chiral building blocks, such as hydroxy acids. In general, there are three main classes of catalysts for the preparation of chiral cyanohydrins enzymes, cyclic dipeptides, and transition metal complexes [163-166]. 

The homo- and cross-addition of alkenes catalyzed by a transition-metal provided another economical way of forming C-C bonds.155 These reactions are carried out by using nickel, palladium, or ruthenium phosphine complexes to yield vinylarenes and some can occur in aqueous media. By using carbohydrate-derived ligands, asymmetric hydrovinylations can be carried out in aqueous conditions.156... 

Carbon-carbon bond-forming reactions are of central importance in synthetic organic chemistry. Although a host of asymmetric transition metal-catalyzed (5) and organocatalyzed (6) reactions of this type are known, enzyme catalysts are often complementary and sometimes even superior. Aldolases allow the synthesis of many complex carbohydrates in one or two steps, with enantio- and diastereoselectivity... 

Figure 6.47 Synthesis of a carbohydrate functionalised imidazolium salt and its transition metal NHC complexes.

The "toxicity" of O2 is exerted primarily by its penetration to important sites, where it is converted into further, more reactive oxygen species. When a reaction between O2 and carbohydrates was observed, this was nearly exclusively the case when complex biomaterials, for instance cartilage [231] or body fluids, for instance joint (synovial) fluid [232], were used for the experiments. Such complex materials do always contain at least traces of transition metals. This was convincingly demonstrated by Carlin and Djursater [233]. These authors have shown that there is no reduction of the viscosity of a given HA solution when solely the enzyme xanthine oxidase is used as source of 02. In contrast, a marked depolymerization of HA occurs in the presence of ferritin-... 

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