Aldose, conversion into ketose

Transketolase catalyzes the reversible transfer of a hydroxyacetyl fragment from a ketose to an aldehyde. Because the ketose products formed by transketolase reactions are not acceptors for a consecutive transformation by the same enzyme, we have investigated the option to include a xylose (glucose) isomerase (Xyll E.C. 5.3.1.5), which has similar stereochemical specificity, for ketose to aldose equilibration (Scheme 2.2.5.13). Starting from racemic lactaldehyde 32a, the transketolase forms 5-deoxy-D-xylulose 35a, which indeed was accepted by the Xyll in situ for diastereospecific conversion into 5-deoxy-D-xylose 36a. The latter again proved to be a substrate of transketolase which completed a tandem operation to furnish 7-deoxy-sedoheptulose 37a as the sole bisadduct in 24% overall yield and in enantio- and diastereomerically pure quality [35, 36]. All four stereocenters of the resulting product are completely controlled by the enzymes during this one-pot operation. The procedure profits from the limited tolerance of the isomerase... 

In the year 1884, Emil Fi8cheri prepared the first sugar osazones and suggested their use for identification purposes. He later used the fact that D-glucose and n-mannose give the same osazone in his determination of the structure of these two monosaccharides, and he accomplished, through osazone formation, the conversion of aldoses into ketoses. Since then, osazone formation has been used in a multitude of syntheses in the carbo-... 

The conversion of aldoses into ketoses is also caused by dilute sodium hydroxide solutions. However, these reactions occur very slowly and are not observed in the time period necessary for chromatography. 

D-Xylose isomerase catalyzes the reversible conversion of D-xylose into D-xylulose, and it is also capable of converting other sugars from aldose into ketose. The enzyme requires Co +, or Mn " " for activation while zinc and calcium dications inhibit cataiysis. Crystaiiographic data indicate... 

Walker et al. " found the CHjl-AgjO-DMF method eflective for conversion of reducing carbohydrates into completely methylated glycosides without protection of the reducing group. The reaction proceeds smoothly with a variety of aldoses, ketoses, and uronic acids completely methylated products are obtained in 80-90% yield in a reaction period of 16 hrs. 

The Nef reaction12,13 is the conversion, by treatment with mineral acid, of a nitronate into an aldehyde or ketone. Early applications of this reaction to deoxynitroalditols have proved extremely useful in sugar synthesis, and have been reviewed.3 The general utility of the reaction is emphasized by several more-recent syntheses of aldoses,16-183,22,23 2-deoxyaldoses,18a,133 135 ketoses,29-34 and disacchar-... 

Using curved arrows to symbolize the flow of electrons, write a stepwise mechanism for the base-catalyzed conversion of a ketose into an aldose. 

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