Ring , furanose

The furanose rings of the deoxyribose units of DNA are conformationally labile. All flexible forms of cyclopentane and related rings are of nearly constant strain and pseudorotations take place by a fast wave-like motion around the ring The flexibility of the furanose rings (M, Levitt, 1978) is presumably responsible for the partial unraveling of the DNA double helix in biological processes. 

Hemiacetal formation between the carbonyl group and the C 4 hydroxyl yields the five membered furanose ring form The anomenc carbon is a new chirality center its hydroxyl group can be either cis or trans to the other hydroxyl groups of the molecule... 

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... 

Conformation of D ribose suitable for furanose ring formation... 

The pyrimidine and purine bases are cis to the —CH2OH group of the furanose ring (p stereochemistry)... 

Benzoates. The selective debenzoylation of sucrose octabenzoate [2425-84-5] using isopropylamine in the absence of solvents caused deacylation in the furanose ring to give 2,3,4,6,1/3/6 -hepta- and 2,3,4,6,1/6 -hexa-O-benzoyl-sucroses in 24.1 and 25.4% after 21 and 80 hours, respectively (54). The unambiguous assignment of partially benzoylated sucrose derivatives was accompHshed by specific isotopic labeling techniques (54). Identification of any benzoylated sucrose derivative can thus be achieved by comparison of its C-nmr carbonyl carbon resonances with those of the assigned octabenzoate derivative after benzoylation with 10 atom % benzoyl—carbonyl chloride in pyridine. 

Most free pentoses, hexoses, and heptoses occur primarily in less strained pyranose rings, but the furanose ring is also quite important. The furanose ring is formed in the same way as the pyranose ring and also occurs in a and P forms. This is demonstrated with L-arabinose, which is commonly found in polysaccharides in the form of a-L-arabinofuranosyl units (see Fig. 2). 

Figure 3 The underlying tree of a furanose ring in nucleic acids. Atoms are numbered 1,. . . , 5 corresponding to the natural tree ordering. All bond lengths are fixed. Arrows illustrate five internal coordinates that determine the ring conformation.

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open-chain form is present at equilibrium. To understand their strorctures and chemical reactions, we need to be able to translate Fischer projections of car bohydrates into their cyclic hemiacetal forms. Consider first cyclic hemiacetal formation in D-erythrose. To visualize furanose ring formation more clearly, redraw the Fischer projection in a form more suited to cyclization, being careful to maintain the stereochemistry at each chirality center. 

Furanose ring formation involves this hydroxyl group -... 

Carbon-4 of o-xylose must be rotated in a counterclockwise sense to bring its hydroxyl group into the proper orientation for furanose ring formation. 

Potential hydrogen-bonding groups (—NH2 and C=0) point away from the furanose ring. 

Nucleic acids are linear polymers of nucleotides linked 3 to 5 by phosphodi-ester bridges (Figure 11.17). They are formed as 5 -nucleoside monophosphates are successively added to the 3 -OH group of the preceding nucleotide, a process that gives the polymer a directional sense. Polymers of ribonucleotides are named ribonucleic acid, or RNA. Deoxyribonucleotide polymers are called deoxyribonucleic acid, or DNA. Because C-1 and C-4 in deoxyribonucleotides are involved in furanose ring formation and because there is no 2 -OH, only... 

Stereospecificity was observed in the hydrogenation and hydroboration of the alkene (39) (38). Attack from below the furanose ring of both D-erythro-4-enosides (39 and 43) is hindered by the 2,3-O-isopropylidene ring and the products resulting from topside attack therefore predominate. 

Once again no molecular-ion peak is seen, but the M-CH3 peak is prominent at m/e 189 in Figure 7. An important peak at m/e 159 from C-4-C-5 cleavage with charge retention on C-4 establishes the presence of a furanose ring and of a 6-deoxy function. Charge retention on C-5 leads to the ion at m/e 45 (see Equations 19 and 20). 

The practice of including the conformation after the name of the parent monosaccharide should be used only with caution because not all conformations are known with certainty. In the case of furanose rings especially, conformations might differ between the crystalline and solution states. 

Historically, the trivial names diheterolevuloson 1, 11, III, and IV have been used to describe the di-D-fructose dianhydrides, which contain one or two pyra-nose rings. Similarly, the names di-D-fructose dianhydride (or difructose anhydride) 1,11, III, IV, and V have been used to describe those compounds in which two furanose rings occur. The names diheterosorhosan I and II have also been coined. Trivial names should not be used in other than a secondary manner for example, they may be listed in parentheses after the IUPAC name. 

The mechanism of HMF formation from D-fructose and sucrose was reviewed by Antal et a/.48 Several arguments were advanced for favoring a mechanism involving furanose rings and a fructose oxocarbonium ion over an open-chain [3-elimination mechanism that proceeds via an enediol intermediate to a... 

Jh-3. f 20.2 Hz in D2O), concluded that the furanose ring adopts the envelope form with C-3 endo, which differs greatly from the conformation of... 

Ab initio molecular orbital calculations (using the Gaussian 80 computer program) on the barrier to pseudorotation (for the furanose ring) of two model compounds, 2-deoxy-/ -D- /) cero-tetrofuranosylamine (781) and 2-deoxy-2-fluoro-)3-D-erythrofuranosylamine (782) were reported. Al-... 

Deoxy-3-fluoro- -D-xylofuranosyl)cytosine (821) was prepared by condensation of 2,5-di-0-benzoyl-3-deoxy-3-fluoro-D-xylofuranosyl bromide with bis(trimethylsilyl)cytosine (773). It was proposed that 1 -(3 1eoxy-3-fluoro-)S-D-arabinofuranosyl)uracil (823), prepared from l-(2,3-anhy-dro-)S-D-lyxofuranosyl)uracil (822) with anhydrous HF ( 8% in 1,4-dioxane 116 °, 41 h, 3% yield), has a twist conformation of the furanose ring(°r, acetone- 4-D20 . Jn-vf 13.5, Hz). Reexamination ... 

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