Ketoses formation

As would be expected from this mechanism, 2,3,4,6-tetra-O-methyl-glucose gives only 2,3,4,6-tetra-O-methylmannose when treated with alkali (saturated lime solutions). The same equilibrium point is reached from tetra-O-methylmannose (80). In this instance, ketose formation is precluded because of the absence of an ionizable hydrogen atom on carbon 2... 

This colorimetric determination of ketose formation by a redox transformation has several limitations. The method is restricted to nonhydroxylated aldehyde acceptors and requires further handhng steps by addition and removal of solid reagent to ehminate residual Li-HPA. Lastly, this method, based on an endpoint determination of the reaction product, does not allow continuous measurement of enzyme kinetics. 

If the hydroxyl group on C-2 is substituted, then ketose formation is prevented since the substituent group cannot migrate to C-i, thus 2,3,4-tri-0-methylxylose (CXLVIII) 5delds only 2,3,4-tri-O-methyllyxose (CXLIX). 

Enediol formation is also rate limiting in the oxidation of aldoses and ketoses by alkaline ferricyanide , the rate expression being... 

The synthesis of cellulose by A. xylinum from various polyalcohols14 is accompanied by the formation of carbon dioxide, formic acid, nonvolatile acids, ketoses and sometimes ethanol. The much greater variety of substrates suitable for cellulose synthesis, as compared with the small number for dextran or levan, may account for the widespread natural occurrence of cellulose. 

Similarly, ketose sugars participate in polysaccharide formation by reaction of their ano-meric carbon with a hydroxyl of another monosaccharide to create a ketal linkage. The acetal and ketal bonds within polysaccharides are termed o-glycosidic linkages. 

Measurement of the acidity produced in the periodate oxidation of carbohydrates is confined mainly to the determination of formic acid. Only from unsubstituted ketoses is the formation of such acids as glyoxylic and glycolic to be expected in carbohydrate chemistry.274... 

D-erythro-Pentulose 5-phosphate (XLIV) has been formed by the action of transketolase on hydroxypyruvate (XLII) and D-glycerose 3-phosphate, the hydroxypyruvate being decarboxylated196 to active glycolaldehyde which then reacts with the triose phosphate by an acyloin reaction.28 The active glycolaldehyde is also formed from L-glycero-tetrulose, d-altro-heptulose 7-phosphate, D-fructose 6-phosphate, and D-i/ireo-pentulose 5-phosphate and it reacts with various aldehydes (acceptors) to give ketoses.198, 200 Thus, substitution of L-gfh/cero-tetrulose for hydroxypyruvate in the above experiment also resulted in formation of D-en/i/iro-pentulose... 

Figure 3 Formation of 2-amino-1-keto sugars from ketose sugar...

OZTs from aldoses and ketoses. The second and the oldest-as well as the less studied—method is based on the condensation of O-unprotected sugars with thiocyanic acid, generated in situ from potassium thiocyanate and a protic acid. The reaction involves the free anomeric position and a y- or (5-hydroxyl group able to promote intramolecular cyclization of a transient open-chain isothiocyanate, to form the thermodynamically most stable OZT. The first results obtained by Zemplen in d-gluco and D-Fru series reported the formation of OZTs fused to pyran backbones (Scheme 20).42... 

Ketoses should react under a similar scheme. Indeed they do but an important problem in the chemistry of ketoses consists on the lack of selectivity due to (1) the complexity of their tautomeric equilibria and (2) their tendency to form tertiary oxocarbenium ions under acidic conditions. Thus, mixtures of open-chain, cyclic and dehydrated products are frequently obtained.7 The discussion about OZT structures obtained from D-fructose as proposed by Zemplen, Wickstrom and more recently by Grouiller et al. continues today. In fact, the first authors claimed the fusion of OZT on a pyran form of D-fructose, while Grouiller suggested the formation of a mixture of fused OZTs with /1-pyran (major) and p-furan (minor) forms (Scheme 22).18... 

Open-chain OXT from a ketose. Recently Leconte et al. reported the formation of an open-chain oxazoline-2-thione (OXT) in low yield from D-fructose based on hydroxyketone chemistry (Scheme 30).30... 

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... 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

The initiating reaction between aldoses and amines, or amino acids, appears to involve a reversible formation of an N-substituted aldosyl-amine (75) see Scheme 14. Without an acidic catalyst, hexoses form the aldosylamine condensation-product in 80-90% yield. An acidic catalyst raises the reaction rate and yet, too much acid rapidly promotes the formation of 1-amino-l-deoxy-2-ketoses. Amino acids act in an autocat-alytic manner, and the condensation proceeds even in the absence of additional acid. A considerable number of glycosylamines have been prepared by heating the saccharides and an amine in anhydrous ethanol in the presence of an acidic catalyst. N.m.r. spectroscopy has been used to show that primary amines condense with D-ribose to give D-ribopyrano-sylamines. ... 

Reduction of a ketose yields a secondary alcohol, and reduction of an aldose yields a primary alcohol (ccilled an alditol). A possible reducing agent is hydrogenation in the presence of a catalyst, such as platinum another reducing agent is sodium borohydride (NaBH ) followed by hydrolysis. Figure 16-14 illustrates the formation of an alditol. 

Although many analyses are performed on alditol acetates (see Section VII, p. 56), in order to avoid the formation of multiple peaks, such a reduction is not practical when the mixture contains ketoses, notably fructose. Such analyses are mainly encountered with medical samples and in the examination of sugars occurring free in Nature. Furthermore, the peak-area ratios may be used as a means of identification, to check on the completeness of trimethylsilylation,67,89 and, despite the complex chromatograms obtained from trimethyl-silyl derivatives, they have the merit of being rapidly formed.89 For all of these reasons, improvements in the separation of monosaccharides as their trimethylsilyl derivatives continue to be of considerable importance. 

The importance of 2,3-enediol as compared to 1,2-enediol formation from a ketose presents a problem that has not yet been quantitatively investigated. Lemieux30 has pointed out that the large, non-... 

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... 

C. B. linn, USP 2881066 (1959) CA 53, 14522 (1959) claims incendiary fuels consisting of gels formed by the condensation of 2 moles of an aromatic hydrocarbon with a ketose and gel-formation in benzene. The condensation of d-fructose with 2 moles of toluene, in an autoclave with HF catalyst, is given as an example... 

Another class of pyrrole derivative may be obtained by the interaction of l-amino-l-deoxy-2-ketoses or 2-amino-2-deoxyaldoses with a jQ-dicarbonyl compound. Unlike the previous type (which is N-substituted), these pyrrole derivatives have a tetrahydroxybutyl group in the a- or /8-position with respect to the nitrogen atom of the ring, in addition to other groups arising from the dicarbonyl compound used in the condensation. The formation and reactions of this type of pyrrole derivative have been discussed in detail in two articles in this series48,49 they will, therefore, only be treated briefly. 1-Amino-l-deoxy-D-fructose (53) reacts with 2,4-pentanedione to give50 pyrrole derivative 54a similar pyrroles were obtained with ethyl acetoace-tate,50,51 which yields 54b. 

---