Aldoses reactivity

The presence of an aldehyde function m their open chain forms makes aldoses reactive toward nucleophilic addition of hydrogen cyanide Addition yields a mixture of diastereo meric cyanohydrins... 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

The common 1,2-enediol provides an explanation for the formation of identical products from an aldose and the corresponding 2-ketose, and, because of the large differences in reactivity of aldoses and ketoses, its formation probably constitutes the rate-determining step. A dual pathway is proposed, as only 47, the cis form, is produced on treatment of 45 with acid,60 whereas both 47 and 48 were reported to have been isolated from the reaction of D-fructose.61... 

The reactivity of the 2,5-anhydrides of aldoses is determined by two essential structural features that do not exist in the sugars, namely, the presence of an oxolane ring and of a carbonyl group (most frequently, free) a to the ring-oxygen atom. These two characteristics make the 2,5-anhydroaldoses closer to tetrahydro-2-furaldehyde than to the aldoses, where only in exceptional cases is the carbonyl group not masked by the formation of an intramolecular, five- or six-membered, hemiacetal ring. 

In this Section, hydrolysis, acetolysis, and isomerization of acetals are considered. Selective deprotection of acetals may also be achieved through halogenation, hydrogenolysis, ozonolysis, and photolysis, but these topics, are covered in an accompanying article in this Series, on the reactivity of cyclic acetals of aldoses and aldosides,3 and will not be discussed here. 

Fortunately, the rules of chemical reactivity and conformational analysis, coupled with the laws of thermodynamics, join forces to allow us to functionalize polyhydroxy aldehydes and ketones (aldoses and alduloses) in a selective and predictable fashion. 

J. Gelas, The reactivity of cyclic acetals of aldoses and aldosides, Adv. Carbohydr. Chem. Biochem. 39 71 (1981). 

The ruthenium-catalyzed oxidation of aldoses by NBS under acidic46 and basic47 conditions have been investigated. The order of reactivity of some pentoses and hexoses has been determined for their oxidation by NBS in aqueous acidic media containing Hg(II) acetate. A mechanism for the reaction has been suggested on the basis of kinetic measurements.48... 

Kinetics of oxidation of four pentoses by bromamide-T were conducted in alkaline medium at different temperatures and the overall activation parameters have been calculated.52 Aldonic acids were the oxidation products, and a mechanism was suggested in which formation of the enediol anion of the sugar is the rate-limiting step. As aldoses may undergo epimerization in alkaline solutions, the oxidation of monosaccharides with bromamide-T was also performed in hydrochloric acid solution.53 Kinetic parameters revealed a low reactivity of ketoses relative to aldoses, and indicated that the cyclic forms of the latter are involved in the oxidations. 

K. K. Sen Gupta, A. Sanyal, P. S. Tribedi, and S. Sen Gupta, Kinetic behaviour and relative reactivities of some aldoses, amino sugars and methylated sugars towards permanganate in perchloric acid medium, J. Chem. Res. (S), (1993) 484-485. 

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