4-Ulose intermediate

The stereochemistry of the reaction in which C-4 is oxidized to give intermediate 41 is difficult to envisage, because the hydride abstraction and addition would have to take place from opposite sides of the carbonyl group. Various solutions to this problem have included (a) a double binding-site for the substrate, which can transfer from one site to the other as the intermediate 4-ulose,146... 

Derivatives of 6-deoxy-hex-5-enopyranosides are important intermediates in the synthesis of dicarbonyl sugars, namely hexos-5-uloses. For... 

The type-specific capsular polysaccharide from Streptococcus pneumoniae type 5 contains 2-acetamido-2,6-dideoxy- -D-x>>/o-hexopyranosyl-4-ulose residues (17). Sugar nucleotides of hexos-4-uloses are important intermediates in the transformation of sugars during the biosynthesis, but this is the only known example of such a sugar as a polysaccharide component. 

For most of the sugar components, little or nothing is known about their biosynthesis. Nucleoside hexosyl-4-ulose diphosphates are, however, almost certainly key intermediates in the biosynthesis of several of these sugars, as discussed in Ref 7. The biosynthesis of the 6-deoxyheptoses is probably analogous to that of the 6-deoxyhexoses, and proceeds by way of nucleoside heptosyl-4-ulose diphosphates. 

The presence of a dichloromethylene group at the anomeric center of 82 facilitates proton abstraction at C-3 by a strong base (77), aifording the 4-deoxyglycos-3-ulose derivative 83. Reduction of the dichloromethylene group by Raney nickel gave a 1-C-methyl derivative with high stereospecificity, which opens the way to a series of 2,5-anhydro-l-deoxyalditols. Compound 83 was the key intermediate for the synthesis (78) of tosyl L-(+)-epi-muscarine (84a) and tosyl L-(+)muscarine (84b). 

Saccharinic acid formation has been studied for several years. The four-step reaction proceeds rapidly in alkaline solution because of basic catalysis, particularly in the last two steps. Initially formed is an enediol that can undergo j8-elimination of a functional group, usually a hydroxyl group. The final two steps involve tautomerization to an a,j8-dicarbonyl intermediate followed by a benzilic acid rearrangement. This sequence is shown in Scheme 6 for the formation of the a- and j8 -xylometasac-charinic acids (30) by way of 3-deoxy-D-g/ycero-pentos-2-ulose (29). 

A prior report proposed that 3-deoxyhexosuloses participate in the formation of 11 (see Scheme 7). The 3-deoxyhexosuloses were isolated from heated, acid reaction mixtures involving D-fructose and L-sorbose. It was suggested that 3-deoxyhexos-2-ulose (39) underwent reversible equilibrium with 38, its cis form, and was further dehydrated at C-3 and C-4, to form 3,4-dideoxyhex-3-enos-2-ulose (40). This intermediate cy-clized to the furaldehyde. Experiments involving treatment of both... 

Scheme 16 shows that 3-deoxy-D- ry < -hexos-2-ulose (39) can be formed by the cleavage of a diketosamine. Such an intermediate has already been implicated in the formation of 5-(hydroxymethyl)-2-furalde-hyde (see Scheme 7). Interest in 39 and its formation is directed towards its role in the production of two major groups of products observed in...

During the course of the Amadori rearrangement, the 1,2-enolamine intermediate (82) can rearrange to form 3-deoxyhexos-2-ulose (39) under aqueous conditions while releasing the amine or, more importantly, the... 

Many DOHs, such as L-daunosamine, L-epivancosamine or L-ristosamine, contain an amino group at C3, which is introduced by an aminotransferase. The substrate for this reaction is the 3-keto sugar intermediate that arises as a consequence of the action of a 2,3-dehydratase. This transaminahon reaction has been biochemically characterized in the biosynthesis of L-epivancosamine [10]. Using a coupled reaction with EvaB (2,3-dehydratase) and EvaC (aminotransferase), with pyridoxal-5-phosphate (PEP) as a coenzyme and L-glutamate as a cosubstrate, they were able to show conversion of TDP-4-keto-2,6-dideoxyglucose into thymidine-5 -diphospho-3-amino-2,3,6-trideoxy-D-threo-hexopyranos-4-ulose. 

Many DOHs, such as L-mycarose, L-epivancosamine, L-noviose or L-dauno-samine, show an L configuration. Formation of L-DOH requires the action of a 5- or 3,5-epimerase. The epimerase EvaD from the biosynthesis pathway of L-epivancosamine was shown to act as a 5-epimerase on the intermediate thymidine-5 -diphospho-3-amino-2,3,6-trideoxy-D-tIireo-hexopyranos-4-ulose [10]. On the... 

Stoicheiometric RuOyCCl was also used to oxidise several furanoses, partially acylated glycosides and l,4 3,6-dianhydrohexitols [317] pyranosides to pyrano-siduloses [313] methyl 2,3,6-tri-O-benzoyl-a-D-glucopyranoside and its C-4 epimer to the a-D-xy/o-hexapyranosid-4-ulose (Table 2.3) [317], and methyl 2,3,6-trideoxy-a-D-e 7f/tro-hexapyranoside to the -a-D-,g/yceri9-hexa-pyranosid-4-ulose, an intermediate in the synthesis of forosamine [318], It was also used to oxidise benzyl 6-deoxy-2,3-0-isopropylidene-a-L-mannopyranoside to the a-L-/yxo-hexapyranosid-4-ulose [319] and for oxidation of isolated secondary alcohol functions, e.g. in the conversion of l,6-anhydro-2,3-0-isopropylidene-P-D-man-nopyranose to the-P-D-/yxo-hexa-pyranos-4-ulose mannopyranose (Fig. 2.16, Table 2.3 [20, 320, 324]). 

As shown in Fig. 2, two mechanisms involving an intermediate oxidation may be written for the epimerization at C-4". In the first one (A), the oxidation results in an a-D-xj/lo-hexopyranosyl-4-ulose derivative (96), which is then attacked by a hydride ion from the opposite side of the carbonyl group a change in conformation of the enzyme-intermediate complex seems necessary for such a process. The mechanism depicted under (B) postulates oxidation at C-3", and the resulting hexopyranosyl-3-ulose derivatives (54 and 97) then achieve equilibrium through the common enediol intermediate (98) before undergoing reduction at C-3". Compound 98 may also be formed from the hexopyranosyl-4-ulose ester 96, and in such a manner, both of the pathways may be linked. 

The mechanism suggested243 includes successive, intermediate formation of the 4"-ulose ester 114, the carbanion 115, and the 4"-ulose 116, all of which are firmly bound to the enzyme-NADH complexes. The sequence for the 5"-labeled substrate is given in Scheme 13. 

The syntheses of both l-(3-deoxy-/ -D-g/i/cm>-pentofuranosyl-2-ulose)uracil (25c) and l-(3-deoxy-/ -D-g/ /cm>-pentofuranosyl-2-ulose)cytosine (30b) by selective elimination reactions have been reported.4,5 Thus, the reaction of sulfonyl derivatives of the cytosine nucleoside 26, and uracil nucleosides 23a, 23c, and 28, with sodium benzoate in N,N-dimethylformamide (DMF) leads to 3 -deoxy-2 -ke-tonucleosides by way of such (presumed) unsaturated intermediates as 24a, 24c, 29a, and 29b. However, in one instance, the intermediate... 

The synthesis of 7-(2,3,6-trideoxy-/ -L-gh/cero-hex-2-enopyranosyl-4-ulose)theophylline (71), which constitutes an important intermediate in the synthesis of branched-chain nucleosides (see Section VI), has been achieved42 by reaction54 55 of the 2, 3 -anhydroketonucleoside 59 with sodium iodide and sodium acetate. 

Among the unsaturated ketonucleosides may be classified the disaccharide derivative 74, which is an analog of the biologically active compound 68c. The key intermediate for the synthesis of this unsaturated ketodisaccharide nucleoside was the partially protected, disaccharide nucleoside 73, which was prepared by two separate routes,56 Treatment of 73 with the Me2SO-acetic anhydride reagent for two days at room temperature afforded 7-[2,3-di-0-benzoyl-4-0-(3-0-benzoyl-2,6-di-deoxy - / - d - glycero - hex - 2 - enopyranosyl - 4 - ulose) - 6 - deoxy - / - d -glycopyranosyl]theophylline (74), isolated crystalline.56... 

The key intermediates in the biosynthesis of 6-deoxy sugars are the nucleoside 6-deoxyhexosyl-4-ulose diphosphates (7), formed through enzymic reactions catalyzed by NDP-sugar 4,6-dehydratases (EC 4.2.1.45-47) from primary glycosyl nucleotides. These reactions were observed... 

The conversion includes at least three enzymic reactions.169-171 In the first stage, which requires pyridoxamine 5 -phosphate as a cofactor,171,172 dehydration of 7b occurs through intermediate formation of the Schiff base.173 Reduction of the resulting, unsaturated derivative with NADPH, the mechanism of which is not completely clear,174 leads to CDP-3,6-dideoxy-D-eryf/iro-hexos-4-ulose,169 and, in the third stage, further reduction of the latter at C-4 of the hexosyl group produces the derivatives of paratose or abequose the stereochemistry of the reaction is determined by the source of the enzyme.168 The tyvelose derivative is formed as a result of enzymic epimerization at C-2 of the hexosyl group in CDP-paratose.175... 

A similar mechanism seems probable for conversion of GDP-6-deoxy-D-(yxo-hexos-4-ulose (7c) into GDP-L-fucose (GDP-Fuc),40,159 although, in this case, the enzymes that participated in the process were not separated, and intermediate formation of the 6-deoxy-L-xy/o-hexosyl-4-ulose derivative (8b) was not demonstrated. 

Chapters 17 through 21 deal with carbohydrate-enzyme systems. Hehre presents some new ideas on the action of amylases. Kabat presents some new immunochemical studies on the carbohydrate moiety of certain water-soluble blood-group substances and their precursor antigens. Hassid reviews the role of sugar phosphates in the biosynthesis of complex saccharides. Pazur and co-workers present information obtained by isotopic techniques on the nature of enzyme-substrate complexes in the hydrolysis of polysaccharides. Gabriel presents a common mechanism for the production of 6-deoxyhexoses. An intermediate nucleoside-5 -(6-deoxyhexose-4-ulose pyrophosphate) is formed in each of the syntheses. 

Considering this property of 4-uloses to be applicable to the postulated intermediate TDP-D-xylo-4-hexulose formed in the enzymatic reaction, we decided to verify this reaction mechanism by the use of specifically labeled substrate. For this purpose TDP-D-glucose-4T was prepared in the following way. 

Several independent lines of experimental evidence have been given (41, 42) to substantiate the proposed reaction mechanism for UDP-galactose-4-epimerase, shown in Figure 9. Recently, Kalckar and coworkers (42) used TDP-glucose-4T as a substrate. The initial attack occurs at carbon 4 and results in conversion to the enzyme-bound-4-ulose intermediate, accompanied by formation of enzyme-NADT. The 4-ulose intermediate serves as hydrogen acceptor to restore enzyme-NAD+ and to release product. 

When this mechanism is compared with the one for TDPG-oxido reductase (Figure 2), several similarities are apparent. The axial hydrogen at carbon 4 is the initial point of attack which leads to the enzyme bound 4-ulose intermediate. Release of product from the enzyme occurs only after reoxidation of enzyme-NADH to enzyme-NAD+. A summary of available data to further substantiate our comparative study of TDPG-oxido reductase and UDP-galactose-4-epimerase is given in Table III. 

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