Hydrogenation of carbohydrates

Derivation (1) By-product of soap manufacture (2) from propylene and chlorine to form allyl chloride, which is converted to the dichlorohydrin with hypo-chlorous acid this is then saponified to glycerol with caustic solution (3) isomerization of propylene oxide to allyl alcohol, which is reacted with peracetic acid, (the resulting glycidol is hydrolyzed to glycerol) (4) hydrogenation of carbohydrates with nickel catalyst (5) from acrolein and hydrogen peroxide. 

In this contribution, we first review the hydrogenation of carbohydrates to polyols. Section 8.4.3 will focus on the reductive amination of carbohydrates and, finally (Section 8.4.4), the facile dehydrogenation of carbohydrates under alkaline conditions will be reported. The hydrogenolysis of carbohydrates is beyond the scope of this chapter. Readers interested in this subject should consult Ref. 3 and references cited therein. 

Efficient hydrogenations of carbohydrates such as fructose, u-glucose, and D-mannose was achieved with [HRuCl(TPPTS)3] and with Ru(II)/TPPMS and Ru(II)/TPPTS catalysts prepared in situ [12, 48, 49]. 

Crystallizes from water in large colourless prisms containing 2H2O. It is poisonous, causing paralysis of the nervous system m.p. 101 C (hydrate), 189°C (anhydrous), sublimes 157°C. It occurs as the free acid in beet leaves, and as potassium hydrogen oxalate in wood sorrel and rhubarb. Commercially, oxalic acid is made from sodium methanoate. This is obtained from anhydrous NaOH with CO at 150-200°C and 7-10 atm. At lower pressure sodium oxalate formed from the sodium salt the acid is readily liberated by sulphuric acid. Oxalic acid is also obtained as a by-product in the manufacture of citric acid and by the oxidation of carbohydrates with nitric acid in presence of V2O5. 

The carbonyl group of carbohydrates can be reduced to an alcohol function Typi cal procedures include catalytic hydrogenation and sodium borohydnde reduction Lithium aluminum hydride is not suitable because it is not compatible with the solvents (water alcohols) that are required to dissolve carbohydrates The products of carbohydrate reduc tion are called alditols Because these alditols lack a carbonyl group they are of course incapable of forming cyclic hemiacetals and exist exclusively m noncyclic forms... 

Other potential synthetic methods include fermentation (qv) of certain carbohydrates (qv), oxidation of propane, hydrogenation of acetone, and hydrolysis of isopropyl acetate. The hydrogenation of by-product acetone is the only method practiced commercially. 

Most parent structures consist essentially of an assembly of rings and/or chains, the degree of hydrogenation of which is defined (usually completely saturated or containing the maximum number of non-cumulative double bonds in cyclic portions), and having no attached functional substituents (the carbohydrates are a notable exception to this). The stereochemistry at all (or most) chiral centres is defined thus such parent structures are sometimes referred to as stereoparents . Some examples are shown (77)-(83). 

All of these functions are made possible by the characteristic chemical features of carbohydrates (1) the existence of at least one and often two or more asymmetric centers, (2) the ability to exist either in linear or ring structures, (3) the capacity to form polymeric structures via glyeosidie bonds, and (4) the potential to form multiple hydrogen bonds with water or other molecules in their environment. 

Three types of carbohydrate sample have been reported to give data above mass 4000, namely, permethylated polysaccharides, permethylated glycosphingolipids, and naturally acylated forms of a mycobacterial O-methyl-D-glucose polysaccharide. All are hydrophobic, and desorption is probably facilitated by their inability to form strongly hydrogen-bonded aggregates, either with themselves or with the matrix. 

Scheme 8.5 Hydrogenation of methyl a-acetamidocinnamate with S/P ligands derived from carbohydrates.

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