Ribose, cyclic acetals

Lactones are in principle reducible to hydroxyaldehydes, and when applied to lactones of sugar acids the method works well because cyclic acetal or ketal formation stabilizes the carbonyl function and prevents overreduction [3,96-100]. In particular, d-ribose may be prepared from the cathodic reduction of D-ribono-y-lactone as in Eq. (24) the optimum conditions for this reaction include careful buffering of the electrolyte [97-100]. 

The first example of methoxyl participation reported in carbohydrate chemistry was the migration454 of a methoxyl group from C-l to C-4 during an attempted benzoate displacement reaction with 2,3,5-tri-O-benzyl-4-O-p-tolylsulfonyl-D-ribose dimethyl acetal (185). Instead of the 4-O-benzoyl-L-lyxose derivative expected, the isomeric l-0-benzoyl-2,3,5-tri-0-benzyl-4-0-methyl-L-lyxose methyl hemi-acetal (187) was obtained, presumably by way of the cyclic, oxonium ion 186. Solvolysis of the related aldehydo sugar, namely, 2,3,5-tri-O-benzyl-4-O-p-tolylsulfonyl-D-ribose, readily gives 2,3,5-tri-O-benzyl-L-lyxofuranose, and the reaction probably involves participation by the free aldehyde group.455... 

With benzaldehyde, and zinc chloride as catalyst, n-ribose yields 2,3-0-benzylidene- 8-D-ribofuranose and 1,5 2,3-di-0-benzylidene-D-ribofur-anose. The 1,5-cyclic acetal system has not been found in other sugars. With zinc chloride and acetic acid as catalyst, Vis and Fletcher reported the formation, also, of l,5-anhydro-2,3-0-benzylidene-/3-D-ribofuranose. At higher temperatures, the dianhydride (21) was formed. 

Ribose, a carbohydrate with the formula shown, forms a cyclic hemiacetal, which, in principle, could contain either a four-membered, five-membered, or six-membered ring. When D-ribose is treated with methanol in the presence of an acid catalyst, two cyclic acetals, A and B, are formed, both with molecular formula C HijOj.These are separated, and each is treated with sodium periodate (Section 10.8C) followed by dilute aqueous acid. Both A and B yield the same three products in the same ratios. 

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... 

The known adduct (385) of furan and vinylene carbonate, previously used for the synthesis of some cyclitols,256,257 has been transformed into DL-ribose derivatives. After hydroxylation of 385 and subsequent formation of the isopropylidene derivative, the carbonate group was removed by treatment with barium hydroxide, and the resulting diol was cleaved by oxidation with permanganate. Dicarboxyl-ic acid 386 gave, upon treatment with acetic anhydride, cyclic anhydride 387. The reaction of 387 with azidotrimethylsilane produced... 

In general, only one of these four isomers crystallizes at a given condition (e.g., a-glucopyranose from water at high temperature or p-glucopyranose at low temperature). It is either the predominant species that crystallizes or the least soluble one in the solvent used. Crystallization then gives rise to optical mutarotation, which is caused by establishment of the equilibrium mixture in a solution from which one component is removed. In the literature crystal structures of pyranoses predominate with only a few samples of crystalline fura-noses. The furanose isomers of D-ribose and 2-deoxy-D-ribose, for example, have never been isolated and crystallized since configurational interconversion of cyclic compounds in solution tends to inhibit crystallization. For this reason, the 1-0-methyl acetals, which cannot isomerize, crystallize more readily than half-acetals. 

Volatile buffers with a pH ranging from 2.0 to 9.0 are generally utilized in PE. Typically, a formic acid/acetic acid/water mixture at pH 2.0 is used for nucleotides. Noncyclic 5 -nucleotides can be separated from cyclic 3, 5 - nucleotides, which are simultaneously present in animal, plant, and microbial tissues, by applying a nonvolatile sodium borate buffer at pH 9.3 indeed borate interacts specifically with the cis-vicinal hydroxyl groups of 5 -nucleotide ribose but not with cyclic nucleotides. 

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