Ketoses separation from aldoses

Stroh914 has reviewed the influence of substituents on the reaction of phenylhydrazines with sugars and their selectivity in reaction with sugar mixtures. The so-called diphenyl-methanedimethylhydrazine [p,p/-methylenedi(Arl-methylphenylhydrazine)], for instance, is sometimes useful for separating sugar mixtures as it condenses more easily with most aldoses than with ketoses.915 1-Methyl-1-phenylhydrazine can also be used to differentiate aldoses from ketoses with aldoses it forms the hydrazone but with ketoses the osazone.916... 

Scheme II.—Separation of Aldoses from Ketoses in a Formose Mixture, ...

The aldoses in such mixtures may be oxidized by bromine, which has no action on the ketoses. The aldonic acids formed in this manner are conveniently separated from the ketoses by ion-exchange resins. 

Ketoses may be separated from contaminating aldoses by oxidation of the latter with bromine and removal of the aldonic acids with an ion-ex-change resin (2), Ion-exchange resins are also useful in the recovery of sugar acids and of some nitrogen-containing derivatives of sugars. 

The ketonic sugars are resistant to bromine oxidation, and thus may be separated from the aldoses. On more powerful oxidation, ketoses rupture at the carbonyl group, with formation of two acids. 

On the other hand if an aldonic acid is treated with alkali, partial conversion to the C-2 epimer occurs. After separation, the pure epimeric acid can be converted to the y-lactone by heating, and this on controlled reduction yields the corresponding aldose uncontaminated with ketose produced in the Lobry de Bru3m reaction (see p. 40). In this way a rare sugar such as talose may be S3mthesised from the aldonic acid of the comparatively abundant sugeur, galactose. ... 

In order to characterize them and more readily separate them from interfering accompanying substances carbonyl compounds (aldehydes, ketones) can be converted to hydrazones at the start. The reagent mainly employed is 2,4-dinitro-phenylhydrazine in acidic solution [70], This yields osazones with aldoses and ketoses. Some examples are listed in Table 15. 

Thus, the 2-nitroethanol synthesis results in the addition of two carbon atoms to an aldose sugar to produce two higher-carbon ketose sugars epimeric at carbon 3. While the synthesis is general in nature, the two ketoses produced from any one aldose are seldom as readily separable as is fortunately the case with D-mannoheptulose and D-gluco-heptulose. However, the newly developed techniques of adsorption and partition chromatography will undoubtedly be of service for the more difficult separations. 

The aldose-ketose mixtures have usually been separated by oxidiaing the aldoses with hypobromite. Occasionally, an aldose has been removed by fermentation— as in the isolation of n-tagatose from a n-tagatose-D-galactose mixture. Levene and Tipson separated the mixtures from the respective transformations of n-xylose and of L-arabinose by forming and fractionally distilling the isopropylidene derivatives. 

We have developed preparative enzymatic syntheses of several unusual hexoketoses using fructose-1,6-diphosphate aldolase (FDP-aldolase, E.C.4.1.2.13) as catalyst and dihydroxyacetone phosphate (DHAP) and an aldehyde as substrates (15). The enzyme appears to be very specific for DHAP but will accept a variety of aldehydes as acceptors. The ketose-1-phosphates prepared are converted to the phosphate free ketoses after removal of the phosphate group by acid- or phosphatase-catalyzed hydrolysis. The ketoses can be isomerized stereospecifically to aldoses catalyzed by glucose isomerase (E.C.5.3.1.5.) from Flavobacteriuum arborescens. The equilibrium mixtures of aldoses and ketoses are then separated by chromatography on Dowex 50 (Ba ) or Dowex 1 (HSO "). Figure 1 illustrates the preparation of a mixture of 6-deoxy-6-fluoro-D-fructose... 

The influence of cross-linking and bead size on the resolution of mannitol/sorbitol and arabinitol/xylitol mixtures on seven Ca -form cation exchange resins has been studied. Alditols with at least four vicinal hydroxy-groups are retained more strongly than aldoses and ketoses when applied to anion exchange resins as their complexes with hexaammonium heptamolybdate. While the free sugars can be eluted with water, the alditols require 0.1 M aq. ammonia. Oxalic, citric and a-hydroxycarboxylic acids afford stable complexes with molybdate ions, so that they need to be removed from mixtures, if the separation of aldoses and alditols is to be achieved." ... 

The individual 2-C-(hydroxymethyl)aldoses were obtained after a combined separation of the epimers via their phenylhydrazones followed by chemisorption chromatography on a cation-exchange resin in the Ba form [50]. Then, on treatment with molybdic acid at increased temperature, 2-C-(hydroxymethyl)-D-mannose (16c) rearranged to D- /Mco-hept-2-ulose (15 c) as expected. Under similar conditions, 2-C-(hydroxymethyl)-D-glucose (15 d) was transformed to d-ma o-hept-2-ulose (16d). The analysis of the reaction equilibria by NMR spectroscopy revealed the presence of small quantities of the respective starting branched-chain aldoses 15d (3.5%) and 16c (8%). The same equilibria were also obtained from the side of the model ketoses 15 c and 16 d. Thus, for example, the equilibrium of the sugars 15c and 16c at their total 2% concentration in a 0.2%... 

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