Attachment to the Sugar

After attachment of the building block 97 at the 5 -end, cleavage of the trifluoroacetyl group proceeded by use of aqueous ammonium hydroxide. Such 5 -aminated oligomers have been reacted with various fluorescent dyes [271]. 2 -0-aminoalkylated ribonucleotides allow postsynthetic derivatization after cleavage of the phthalimide protective group 98 [150]. 

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As shown in Figure 45.1, the bases appear in complementary pairs, A with T and G with C in this particular example, the sequence for one strand of DNA is A-T-C-G-T- while the other strand is -T-A-G-C-A-. The sequences of the bases attached to the sugar-phosphate backbone direct the production of proteins from amino acids. Along each strand, groups of three bases, called codons, correspond to individual amino acids. For example, in Figure 45.1, the triplet CGT, acting as a codon, would correspond to the amino acid serine. One codon, TAG, indicates where synthesis should begin in the DNA strand, and other codons, such as ATT, indicate where synthesis should stop. 

Fig. 8. Non-Watson-Crick base pairs occurring in double-stranded RNA where — represents the site of attachment to the sugar (a) A—U reverse-Watson-Crick (b) G—C reverse-Watson-Crick (c) A—U Hoogsteen (d) A—U reverse-Hoogsteen (e) G—U wobble and (f) G—U reverse-wobble.

Protons bound to heteroatoms in heterocyclic compounds are likely to be very mobile in solution and, where two or more heteroatoms are present in a structure, different isomers (tautomers) may be in equilibrium. As a case in point, consider the nucleotide bases (indicates the point of attachment to the sugar-phosphate backbone). 

Grouped in this Section are the C-D-pentofuranosyl-imidazoles, -pyrazolopyrimidines, and -adenines. The last two analogs are positional isomers of formycin in which the heterocyclic moiety is attached to the sugar at an unnatural position. A rationale103 for the synthesis of this type of analog is of interest it was based on the possibility of a close structural similarity between a natural adenine nucleoside and synthetic analog with respect to available hydrogenbonding sites. 

Nature provides us with many glycoside esters in which the acyl group is attached to the sugar portion an example is the compound vacciniin already mentioned. However, the largest group of natural acyl derivatives of sugars is to be found in the gallotannins and related substances. 

Figure 6.2. Molecular architecture of DNA. Each strand of DNA is composed of alternating pentose sugar (deoxyribose) and phosphate moieties linked to each other via phosphodiester linkage. The first carbon position of the sugar is attached to one of the four nitrogenous bases (A, T, G, or C). The two strands are in opposite orientation to each other with respect to a 5 or 3 phosphate group attached to the sugar moiety. Cytosine (C) pairs with guanine (G) via three hydrogen bonds, and adenine (A) pairs with thymine (T). (Reproduced from Textbook of Biochemistry with Clinical Correlations, T. M. Devlin, ed., Wiley, New York, 1982.)...

We took advantage of this observation in order to effect selective debenzylations, once an allylic appendage is attached to the sugar. For example, when the polybenzylated allyl-C-glucopyranoside 7 (10) is treated with iodine in THF at 0 °C, the bicyclic compound 8 is obtained (85 15 in favor to the 2 R isomer), which upon treatment with zinc and acetic acid undergoes a reductive elimination affording the C-glucoside 9 selectively deprotected at C-2. (11)... 

Nucleotide (Section 27.1) A nucleoside that has a phosphate group attached to the sugar. 

On catalytic hydrogenation, diphosphothymidine gave diphospho-dihydrothymidine which is readily hydrolyzed by mineral acid to dihydrothymine and a reducing diphospho-sugar. Hence both phosphoryl groups are attached to the sugar component and, since thymidine is furanose, they must be situated at positions (3) and (5). 

The manipulation of appropriately activated hydroxyl groups by nucleophilic displacement reactions is an indispensable tool for the introduction of functionalities directly attached to the sugar framework. 

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