D-Ribose forms

Administration203 to chicks of acetic acid labeled with C14 (in the carboxyl group) gave D-glucose (from the glycogen) labeled equally at C3 and C4, but the D-ribose (from nucleic acids) had more label at C3 than at C2, indicating that in vivo D-ribose does not arise exclusively from hexose by loss of Cl. The 20-30% isotope content of D-ribose formed from D-glucose-l-C14 by Escherichia coli indicates that direct conversion is the major pathway but that a part is probably derived from transketolase action.204 206... 

This same reaction of 2-deoxy-D-ribose forms deoxyribonucleosides. 

The same nomenclature applies to the furanose ring form of fructose, except that a and p refer to the hydroxyl groups attached to C-2, the anomeric carbon atom (see Figure 11.5). Fructose forms both pyranose and furanose rings. The pyranose form predominates in fructose free in solution, and the furanose form predominates in many fructose derivatives (Figure 11.6). Pentoses such as d-ribose and 2-deoxy-d-ribose form furanose rings, as we have seen in the structure of these units in RNA and DNA. 

Oligonucleotides containing 2 -deoxy-2 -fluoro-D-arabinose or -D-ribose form stable, triple-heUcal complexes stabilized by the intermolecular 2 -OH phosphate contacts and sugar puckering these derivatives have promoted interest in relation to compounds synthesized for DNA targeting in vivo Fig. 6 [31]. 

The essential elements of Table 2.1 meet these demands. In all cases they are components of the metabolic system in cell or of important final products for example, cellulose for the upright standing of the plant. The function as constituents of such compounds is clear for C, H, and O. These three elements are together components of nearly all organic compounds in the plant [only hydrocarbons (e.g., carotins) are free of O], and therefore they build up the planfs shape. A similarly clear situation holds true for N and P, both of which are constituents of the information carriers DNA and RNA. N is a component of their purine and pyrimidine bases, while phosphoric acid esters of D-ribose or 2-deoxy-D-ribose form the backbone of their nucleotide sequences. Moreover, P plays a very important role in energy metabolism, the key compounds being nucleotide phosphates (e.g., adenosine triphosphate, ATP) (see Scheme 2.1) and the homologous molecules... 

The mechanism for this process is analogous to that of glycoside formation. Two carbohydrates, in particular, D-ribose and 2-deoxy-D-ribose, form especially important A -glycosides. 

In solution D-ribose forms an equilibrium mfitture containing 6% a-furanose, 18% P furanose, 20% a-pyranose, and 56% p-pyranose. Explain why P-pyranose forms predominates at equilibrium. 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

During the discussion of hemiacetal formation in d ribose in the preceding section you may have noticed that aldopentoses have the potential of forming a six membered cyclic hemiacetal via addition of the C 5 hydroxyl to the carbonyl group This mode of ring closure leads to a and p pyranose forms... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

FIGURE 25 5 Distribution of furanose pyranose and open chain forms of d ribose in aqueous solution as mea sured by H and NMR spectroscopy... 

FIGURE 7.8 D-Ribose and other five-carbon saccharides can form either fnranose or pyranose strnctnres. 

A stereospecific total synthesis of polyoxin C and related nucleosides is reported, in which thereacdon of l-Cphenylthioi-l-nitroalkenes v/ith nucleophiles and siibseqiientozono lysis are key reacdons Addidonof potassium trimethylsilanoate to l-Cphenylthioi-nitroalkenes derived from D-ribose followed by ozonolysis gives the cr-hydroxy thioester, which is formed v/ith excellent diastereoselecdvity fScheme 4 5 This conformadon meets the stereo-electronic requirements for andperiplanar addition of the nucleophile with the result of high 5-fS stereochemical bias in the reacdons... 

Problem 25.11 Ribose exists largely in a furanose form, produced by addition of the C4 -OH group to the Cl aldehyde. Draw D-ribose in its furanose form. 

From L-arabinose, carba-sugars of the a-D-gluco and h-altro types were prepared from D-arabinose, carba-a-L-mannopyranose from D-ribose, that of the fi-L-manno modification from D-xylose, those of the -L-gluco and a-D-altro forms and, from D-glucose, those of the P-L-allo a-L-... 

Posttranslational modification of preformed polynucleotides can generate additional bases such as pseudouridine, in which D-ribose is linked to C-5 of uracil by a carbon-to-carbon bond rather than by a P-N-glycosidic bond. The nucleotide pseudouridylic acid T arises by rearrangement of UMP of a preformed tRNA. Similarly, methylation by S-adenosylmethionine of a UMP of preformed tRNA forms TMP (thymidine monophosphate), which contains ribose rather than de-oxyribose. 

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