Ketoses stability

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

The Lewis acid catalyzed C-glycosylation method is probably one of the most efficient to prepare C-glycosides of ketoses, the anomeric carbocation intermediate being in this case more stabilized by a further substituent. We applied this procedure to synthesize C-fructosides [15] (Scheme 8).The reaction of methyl... 

Studies on the rates of hydrolysis of di-O-isopropylidene derivatives of ketoses indicated that the relative order of stability of 2,2-di-methyl-l,3-dioxolane rings is in the order cis-fused to a furanoid ring > sptro-fused to a pyranoid ring > cts-fused to a pyranoid ring > sptro-fused to a furanoid ring.8 Thus, partial hydrolysis of l,2 3,4-di-0-isopropylidene-/3-D-en/fhro-2-pentulofuranose (1) to the 3,4-acetal (2)... 

Transketolase requires the cofactor thiamine pyrophosphate (TPP), which stabilizes a two-carbon car-banion in this reaction (Fig. 14—26a), just as it does in the pyruvate decarboxylase reaction (Fig. 14-13). Transaldolase uses a Lys side chain to form a Schiff base with the carbonyl group of its substrate, a ketose,... 

Because of its ready availability and stability, the transketolase from yeast has been studied more frequently than enzymes from other sources for mechanistic as well as synthetic purposes. A valuable feature is that during the first round of a ping-pong catalytic cycle the natural ketose donor component(s) can... 

Aldoses and ketoses also react with the amino groups of proteins, to give Schiff bases (2 in Scheme 11). It has been considered that D-glu-cose and related sugars may become attached in. this way to hemoglobin in vivo.76 The Schiff base formed between a reducing sugar and a protein may also be stabilized by reduction to the polyhydroxy alkylamine.78,79... 

A similar order should hold for ketoses, although the additional equatorial group (at the anomeric center) may increase the stability of individual isomers, and the diketose dianhydrides are actually always formed. 

Assuming that 1,2-enolization is rate-limiting, the stability of aldoses compared to ketoses could be due to a lower concentration of the acyclic, carbonyl form, and to differences in the basic strength of the carbonyl groups and in the acidity of the protons at C-1 and C-2. 

De Beider has reviewed the stability of cyclic acetals of the aldoses and aldosides to heat, light, bases, and oxidants and reductants. His Chapter gives many examples of the kinds of reactions that may be expected to occur when cyclic acetals of ketoses are subjected to such conditions. 

The kinetic parameters for aldose and ketose transformations in the D-glucose-D-mannose-D-fructose system at 27 C on aluminate resin and hydroxide resin have been obtained. On both resins hydroxide ion catalyses isomerization. By complexation, resin-bound aluminate stabilizes D-fructose so that the equilibrium mixture contains 72% of the ketose. Lower temperatures were found to give the highest yields of D-fructose. A similar study on maltose gave a yield of 63% maltulose at 24 C, but higher temperatures caused elimination of D-glucose at C-4. In a study of isomerization, enolization, etc. of mono-... 

We carried out an extensive survey of the C-2 epimerization of various aldoses and ketoses [44b]. Because of the variety of the possible aldose configurations, the stability of the complexes should vary accordingly. In order to understand the effects of configuration on the epimerization of aldoses, a variety of aldohexoses, aldopentoses, and 6-deoxyaldohexose were studied along with four ketoses. The stereospedfic and regiospecific rearrangement of these ketoses will be studied in detail in Sect 4. 

---