Configuration aldoses

The influence, on the reaction, of different substituents in the phenyl-hydrazine molecule and the specificity due to these molecular changes have been studied. The specificity in respect to certain aldose configurations was proved, and it was also shown that D-fructose and other 2-ketoses react with such weakly basic hydrazines as (p-bromo-, (p-carboxy-, (p-carbeth-oxy- and (p-nitro-phenyl)hydrazine. Correlations between the reaction rate and ease of interconversion between cyclic and acyclic forms were also determined. Studies with tritium-labeled D-fructose showed that the... 

The simplest aldose. Configurational standard for carbohydrates. In aq. soln., undergoes equilibration with dimers and with l,3-Dihydroxy-2-propanone, D-710 via the enediol tautomer. 

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. 

As shown for the aldotetroses an aldose belongs to the d or the l series accord mg to the configuration of the chirality center farthest removed from the aldehyde func tion Individual names such as erythrose and threose specify the particular arrangement of chirality centers within the molecule relative to each other Optical activities cannot be determined directly from the d and l prefixes As if furns ouf bofh d eryfhrose and D fhreose are levorofafory buf d glyceraldehyde is dexfrorofafory... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

Configurations of the D series of aldoses containing three through six carbon atoms. 

Aldoses with at least three carbons and ketoses with at least four carbons contain chiral centers (Chapter 4). The nomenclature for such molecules must specify the configuration about each asymmetric center, and drawings of these molecules must be based on a system that clearly specifies these configurations. 

FIGURE 7.2 The structure and stereochemical relationships of D-aldoses having three to six carbons. The configuration in each case is determined by the highest numbered asymmetric carbon (shown in gray). In each row, the new asymmetric carbon is shown in red. 

Fischer projections and, 977-978 glycosides and, 989-990 l- 4 links in, 997-998 origin of name. 973 photosynthesis of, 973-974 see also Aldose, Monosaccharide vaccines from, 1004-1005 Carbon, ground-state electron configuration of, 6... 

Anomeric configuration in cyclic forms of higher aldoses 2-Carb-9. Dialdases... 

Chart I. Trivial names (with recommended three-letter abbreviations in parentheses) and structures (in the aldehydic, acyclic form) of the aldoses with three to six carbon atoms. Only the D-forms are shown the L-forms are the mirror images. The chains of chiral atoms delineated in bold face correspond to the configurational prefixes given in italics below the names... 

Note. For simple aldoses up to aldohexoses, and ketoses up to hept-2-uloses, the anomeric reference atom and the configurational atom are the same. 

Systematic names are formed from a stem name and a configurational prefix or prefixes. The stem names for the aldoses with three to ten carbon atoms are triose,... 

Note 2. Since all aldoses up to the hexoses have trivial names that are preferred, the systematic names apply only to the higher aldoses. However, the configurational prefixes are also used to name ketoses (see below) and other monosaccharides. 

Systematic names for individual dialdoses are formed from the systematic stem name for the corresponding aldose (see 2-Carb-8.2), but with the ending odialdose instead of ose , and the appropriate configurational prefix (Chart I). A choice between the two possible aldose parent names is made on the basis of 2-Carb-2.2.2. 

For 2-ketoses, configurational prefixes are given in the same way as for aldoses (see 2-Carb-8.2 and 2-Carb-8.3). 

NeuA, has broad substrate specificity for aldoses while pyruvate was found to be irreplaceable. As a notable distinction, KdoA was also active on smaller acceptors such as glyceraldehyde. Preparative applications, for example, for the synthesis of KDO (enf-6) and its homologs or analogs (16)/(17), suffer from an unfavorable equilibrium constant of 13 in direction of synthesis [34]. The stereochemical course of aldol additions generally seems to adhere to a re-face attack on the aldehyde carbonyl, which is complementary to the stereoselectivity of NeuA. On the basis of the results published so far, it may be concluded that a (31 )-configuration is necessary (but not sufficient), and that stereochemical requirements at C-2 are less stringent [71]. 

A more general access to biologically important and structurally more diverse aldose isomers makes use of ketol isomerases for the enzymatic interconversion of ketoses to aldoses. For a full realization of the concept of enzymatic stereodivergent carbohydrate synthesis, the stereochemically complementary i-rhamnose (Rhal EC 5.3.1.14) and i-fucose isomerases (Fuel EC 5.3.1.3) from E. coli have been shown to display a relaxed substrate tolerance [16,99,113,131]. Both enzymes convert sugars and their derivatives that have a common (3 J )-OH configuration, but may deviate in... 

Through extensive screening of compounds, " " " it was revealed that this enzyme accepts a very wide range of substrates. In addition to phosphorylated aldose, which are the native substrate, non-phosphorylated aldose, simple aliphatic, aromatic, heterocyclic and functionalized aldehydes, even with an increased hydropho-bicity, work as substrates. The stereochemical course has been elucidated in Fig. 18. The hydroxyl group on the 2-position of the aldehyde is very important and 2-deoxygenated aldehydes were rather weak substrates. The substrates with d-configuration at the 2-position have a stronger affinity to TKase than L-form. 

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