Keto hexoses

Both aldoses and ketoses reduce Fehling s solution (for details, see under 4). This fact may appear surprising when it is remembered that Fehling s solution is one of the reagents for distinguishing between aldehydes and ketones (see 4). The explanation lies in the fact that a-hydroxyketones are much more readily oxidised than simple ketones, perhaps because the hydroxy ketone allows its isomerisation, in the presence of alkali, into an aldehyde. For example, fructose, a keto-hexose, might Isomerlse thus ... 

A similar series of reactions applied to L-sorbose (once a rare sugar but now easily obtained from sorbitol by bacterial oxidation16) gives rise to 2-keto-L-gulonic acid.4 The method is not confined to keto-hexoses and has been employed in the preparation from L-erythropentulose (XXXI) of the 2-keto-L-ribonic acid (XXXV) which undergoes immediate transformation to the corresponding L-erythroascorbic acid (XXXVI).16... 

This phosphotransferase [EC 2.7.1.1] catalyzes phosphorylation of D-glucose as well as many other aldo- and keto-hexoses (with the notable exception of o-galac-tose) MgATP is the active phosphoryl donor, and the overall reaction is highly favorable T eq = [ADP]... 

Among the keto-hexoses, fructose is of major natural occurrence. 

CH,OH Keto-hexose CH,OH Ene-diol CHjOH Aldo-hexose... 

Tnformation about the characteristics of keto-hexoses in solution has been - derived mainly from optical rotatory data (I, 2, 3, 4) and in recent years by application of the principles of conformational analysis (5, 6). In the current study an attempt is made to describe the conformation and composition of these sugars in solution by nuclear magnetic resonance (NMR) spectroscopy, a highly sensitive means for examining stereochemistry and for differentiating between isomeric species. 

OH group in an axial alignment which, assuming that the anomeric effect in keto-hexoses is of the same magnitude as in aldoses (5), supplements the conformational impact of the equatorial 1-carbinol group.)... 

NMR spectral information about D-psicose (D-allulose) is particularly interesting because little is known of the solution characteristics of this keto-hexose. Having not as yet been obtained in any crystalline modifica-... 

Feingold DS (1982) Aldo (and Keto) hexoses and uronic acids. In Loewus FA, Tanner W (eds) Encyclopedia of plant physiology New Series Volume 13A Carbohydrates I Intracellular Carbohydrates. Springer, Berlin Heidelberg New York, p 3... 

The reaction is effectively an aldol condensation between the enol of the keto-sugar phosphate and the electrophilic aldehyde of glyceraldehyde phosphate and the enzyme is named appropriately aldolase. The product is the keto-hexose fructose-1,6-diphosphate. 

Feingold, D.S. 1982, Aldo (and keto) hexoses and uronic acids. In Loewus, F.A. and Tanner, W. (eds.), Plant Carbohydrates I. Intracellular Carbohydrates, Vol. 13A, New Series, Encyclopedia of Plant Physiology. Springer-Verlag, Berlin, pp. 3-76. 

Genes and Proteins Involved in the Conversion of NDP-4-keto-6-Deoxyhexose to NDP-3,6-dideoxy-4-keto-Hexose... 

We may then write the constitutional formulas for the aldo-hexose and the keto-hexose as follows ... 

The keto-hexose, corresponding to the aldo-hexose given above, is oxidized as follows ... 

Naturally occurring substances are known in the two structural forms of aldo-hexoses and keto-hexoses. 

Fructose, CH2OH—CHOH—CHOH—CHOH—GO—CH2OH, Keto-hexose. 

CHoOH—CHOH—CHOH—CHOH—CO—CH2OH, Keto-hexose. 

Kiliani accomplished the purpose of his plan of research he established the aldopentose structure for arabinose and proved that glucose and galactose are aldohexoses and fructose a 2-keto-hexose. His discovery that these polyhydroxy acids lose water readily and pass to stable lactones of crystalline habit soon became of great significance. 

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