Sugar moieties

The C-30 skeleton group accounts for about 60% of the polyethers for which stmctures have been deterrnined. Most of these contain a sugar moiety, usually 2,3,6-trideoxy-4-0-methyl-D-erythto pyranose [65104-53-2]. The single spiroketal subset is ikustrated by two closely related compounds, semduramicin (4, R = H, R =) and maduramicin alpha (4, R = CH3, R = ). Maduramicin is marketed as an anticoccidial and semduramicin is under development for the same appHcation. 

Nucleosides are much more water-soluble than the free bases because of the hydrophilicity of the sugar moiety. Like glycosides (see Chapter 7), nucleosides are relatively stable in alkali. Pyrimidine nucleosides are also resistant to acid hydrolysis, but purine nucleosides are easily hydrolyzed in acid to yield the free base and pentose. 

As is well-known, nucleic acids consist of a polymeric chain of monotonously reiterating molecules of phosphoric acid and a sugar. In ribonucleic acid, the sugar component is represented by n-ribose, in deoxyribonucleic acid by D-2-deoxyribose. To this chain pyrimidine and purine derivatives are bound at the sugar moieties, these derivatives being conventionally, even if inaccurately, termed as pyrimidine and purine bases. The bases in question are uracil (in ribonucleic acids) or thymine (in deoxyribonucleic acids), cytosine, adenine, guanine, in some cases 5-methylcytosine and 5-hydroxymethylcyto-sine. In addition to these, a number of the so-called odd bases occurring in small amounts in some ribonucleic acid fractions have been isolated. 

Nucleoside N -oxides have proved useful in preventing intramolecular cyclizations during manipulation of the sugar moiety. A key step is the reductive removal of the oxide when needed. In the presence of Raney nickel, the oxide can be reduced selectively even when such easily reduced substituents as iodo are present. Azides, however, are reduced concomitantly with the oxide 105). 

The next step of our study, therefore, was to determine the structure of the side chain attached to the amino sugar moiety. 

Pressured cycloaddition of (E)-l-methoxybutadiene (18b) with l 2,3 4-di-0-isopropylidene-a-D-galactopyranos-6-ulose (73) diastereoisomerically afforded pure cycloadduct 74, that exhibits the cis arrangement of the methoxy group with respect to the sugar moiety [27] (Scheme 5.7). Similar results were obtained with the sugar aldehydes 75 and 76. 

Scheme 33 Microwave-enhanced introduction of sugar moieties...

The acyclic nucleoside phosphonates (ANPs) can be considered as nucleotide rather than nucleoside analogues, in that, besides the purine or pyrimidine base, they contain an (acyclic) sugar moiety to which a phosphonate is attached. In these nucleotide analogues (Fig. 2), the phosphoric ester grouping (= P-O---C-----) is... 

O-acetylation of the aliphatic hydroxyl groups of the ribose moiety (or other sugar moieties) to the 2, 3, 5 -tri-0-acetates ... 

In RNA, the sugar moiety to which the phosphates and purine and pyrimidine bases are attached is ribose rather than the 2 -deoxyribose of DNA. 

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