Carbohydrates with Metal Salts

Complexes of Carbohydrates with Metal Salts 1. Proof of Existence 

—Solubility Relations11 in the System n-Glucose—Sodium Chloride—Water at 24°. 

—Solubility Relations in the System Sucrose—Sodium Iodide—Water at 30°. 

Tegge12 and Lebedev and coworkers14 have extensively studied the relation between temperature and the stability of 2 D-glucose N aCl H20 in aqueous systems. The fact that this adduct crystallises better than pure D-glucose from aqueous solution is important industrially. o-Glucose may be recovered from its adduct in 77.1% yield by agitating the adduct with water at 5° for 1 hour.14 The sodium chloride dissolves, and the D-glucose remains as a crystalline solid. 

Dialysis experiments have shown that Ca2 , Mg2 , Ba2 , and Sr2 form soluble chelates in aqueous alkaline solution with D-galactose, D-glucose, D-fructose, D-arabinose, D-ribose, maltose, and lactose. The absence of any precipitation of alkaline-earth metal hydroxide when an aqueous solution containing D-fructose and an alkaline-earth metal salt is made alkaline 

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Besides stereoselective synthesis of various monosaccharides, stereoselective reaction for the preparation of glycosides is an important problem in the synthetic field of carbohydrate chemistry. However, the classical methods, which require the assistance of heavy metal salts or drastic reaction conditions, are still employed by and large in the synthesis of such compounds. Taking these disadvantages into consideration, new glycosylation reactions, which proceed under mild reaction conditions with high selectivity, have been developed and exploited. 

In relation to carbohydrate chemistry, the Koenigs-Knorr synthesis of glycosides involves the treatment of glycosyl halides with an alcohol or phenol in the presence of a heavy metal salt.267 Karrer268 discovered that reaction of silver salts of hydros acids could also be used and use of these reagents has been extended more recently. Numerous variations and improvements on the original method have now been reported, and silver oxide, silver carbonate or silver trifluoromethanesulfonate have since become the accepted standard reagents.267... 

In dilute ethanolic solutions of alkali metal salts, many simpie carbohydrates unite with the salt to give an isolable crystalline solid of either 1 1 or 2 1 ratio of carbohydrate to salt. The frequency with which these ratios are obtained argues not only for the existence of true adducts, but also for a generally preferred stoichiometry at low concentration of salt. 

TiTany commercial uses of carbohydrates depend upon reversible inter- actions of the hydroxyl groups with ions to form complexes. The processes are intricate, and analyses of the equilibria involved have been difficult or impossible (I). For example, the nature of the complexes involving polysaccharides and polyanions, such as borate, can only be hypothesized (2). Similarly the gelling of pectates with Ca(II) (3), the reactions of heptono-y-lactones with Fe(III), or the nature of Fe(III)-dextran complexes in intramuscular injectable solutions are awaiting proper description. Although the systems involving monomeric components are more amenable to study, as for the reactions of metal salts or hydroxides with monosaccharides or their simple derivatives (4,5, 6, 7)... 

The carbonates and thiocarbonates may be classified quite simply into the following types, (i) Mixed Esters. In these esters, the carbonic acid or thiocarbonic acid is esterified with both an alcohol and a carbohydrate, (ii) Acid Esters. Here, a metal salt of a half-ester of a carbohydrate has been formed, (iii) Intermolecular Esters. Here, an ester has been formed between one molecule of the acid and two molecules of carbohydrate, (iv) Cyclic or Intramolecular Esters. Here, one molecule of the acid is di-esterified with one molecule of carbohydrate. 

It ia usually met with in cylindrical sticks, hard, white, opaque, and brittle. The KHO by alcohol has a bluiali tinge and a smoother surface than the common ep. gr. 2.1 fuses at dull redness is freely soluble in H,0, forming a strongly alkaline and caustic liquid less soluble in alcohol. In air, solid or in solution, it absorbs H,0 and CO, and is converted into K,CO,. Its solutions dissolve Cl, Br, 1, S, and P. It decomposes the am-moniaic salts with liberation of NH, and the salts of many of the metals, with formation of a K salt and a metallic hydrate. It dissolves the albuminoids and, when heated, decomposes them with formation of leucin, tyroein, etc. It oxidizes the carbohydrates with formation of potassium oxalate and carbonate. 

The unique character of liquid crystals allows the sensitive change of molecular orientation with both temperature and chemical environment. Steroidal crown compounds entrapped in a mixture of cholesteryl nonaoate and cholesterol thus form a liquid crystalline phase whose helical pitch can be modified by different complexed metal salts (Fig. 3.5.4). Potassium R-mandelate, for example, induces a green assembly color, potassium S-mandelate a blue one. For the detection of carbohydrates by steroidal borates, see Figure 4.5.1 (Shinkai et al., 1991). 

The ability of carbohydrates to complex with metals was used in the determination of an Fe + salt in a strongly alkaline medium, by using sucrose as the complexing agent. ... 

From the experiments, it ean be concluded that two methods can be used for the coupling reaction of methyl 9 [4] or methyl 12-hydroxyoctadeeanoate [5] with mono-and disaccharides. The first is the well-known Koenigs-Knorr reaetion the second is a newly developed more environmentally fiiendly method using triethylamine instead of heavy-metal salts. The combination of the compounds [4,5] and carbohydrates leads to the formation of a new class of alkyl polyglycosides. 

Sihca impregnated with saturated and unsaturated hydrocarbons (squalene, paraffin oil) silicone and plant oils complexing agents (silver ions, boric acid, and borates carbohydrates unsaturated and aromatic compounds) chelating compounds [ethylene diamine tetra-acetic add (EDTA), digitonin] transition metal salt synthetic peptides 18-crown-6 and ammonium sulfate silanized sdica gel impregnated with anionic and cationic surfactants Cross-hnked, polymeric dextran gels (Sephadex)... 

Iron salts are easily accessible, inexpensive and abundant and the metal itself is non-toxic. Their use should therefore become attractive from an economic and environmental point of view in a wide variety of carbohydrate transformations, in either stoichiometric applications or as a catalyst. As stated in the introduction, this review concentrates on a few transformations promoted by ferric salts used as Lewis acids in our laboratories and does not present exhaustive work done in carbohydrate chemistry with these salts. Many more other applications have been reported. However, their uses could be far more developed for fast and selective transformations of carbohydrates to useful new molecular constructs. Besides the acidic properties of iron(iii) presented here, iron chemistry is rich and could be particularly fruitful with carbohydrates in generating new types of complexes for regioselective transformations or in carbon-carbon forming reactions based on iron-catalyzed cross-coupling reactions. The glycochemistry community should certainly expect many more useful accomplishments in the near future. 

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