Sugar subunits

A large number of chiral crowns have been prepared by numerous groups. The reader is directed to the tables at the end of this chapter to obtain an overview of these structures. It would not be useful to try to recount the synthetic approaches used in the preparation of all of these compounds we have chosen rather to subdivide this mass of compounds into three principal groups. The groups are (1) Cram s chiral binaphthyl systems (2) chiral crowns based on the tartaric acid unit and (3) crowns incorporating sugar subunits. These are discussed in turn, below. 

As in the previous categories in this section, there are numerous compounds which have been prepared based on a sugar subunit. Examples may be found in Refs. 7,35,42-45, 57, 82-85, 117—121,175,176,193 and 208. Much of the work in these references has been reported by Stoddart and his coworkers, who have pioneered this field. As with the compounds prepared by Cram, the goal was to prepare a chiral receptor for ammonium ions which could be utilized in enzyme model studies. 

The first total synthesis of the nucleoside antibiotic herbicidin B was accomplished in the laboratory of A. Matsuda. The key step was a novel aldol-type C-glycosidation reaction promoted by Sml2 between a 1-phenylthio-2-ulose derivative and a 1- 3-D-xylosyladenine-5 -aldehyde derivative. During the preparation of the phenylthio sugar subunit, the Moffatt oxidation was applied to convert the primary alcohol to the corresponding aldehyde, which was immediately oxidized with PDC in DMF/MeOH to the methyl ester. The reaction conditions were completely compatible with the silyl protecting group as well as the thioacetal functionality. 

Hemicelluiose. A degradation-resistant polymer of the sugar D-xylose, with side chains of other sugars. Hemicelluiose is related structurally to cellulose by its similar linkage between the sugar subunits, but is somewhat less resistant to degradation than cellulose. 

Polysaccharide. A carbohydrate polymer composed of monomeric sugar subunits, such as glucose, mannose, and fructose commonly used as a... 

Benzothiazol-2-yl C-nucleosides were also obtained by a different mode of synthesis that involved the free radical coupling of the sugar subunit with the heterocycle residue. The 2-deoxy-D-ribofuranosyl free radical 170, formed by photolysis of the 2-thiopyridone-iV-(2,5-anhydro-3-deoxy)-D-allonoate 169, reacted with benzothiazolium camphorsulfonate to form a mixture of 2-(a and 3-2-deoxy-D-ribofuranosyl) benzothiazoles 171 (92CL1673) (Scheme 50). 

The O-protected 9-deazaadenosine derivative 329 was transformed to other pyrrolo[3,2-d]pyrimidin-7-yl C-nucleosides (333 and 334) by performing alterations in the sugar subunit (86MI11 93MI7) (Scheme 99). The prepared compounds were active as antileukemic agents, yet were much less active than the parent 9-deazaadenosine (322) (86MI11). 

Formycin and formycin B with modified sugar subunits have also been prepared using, mostly, the nucleoside-nucleoside approach. In one instance, however, 2 -deoxyformycin B (458) was prepared by regio- and stereospecific palladium-mediated C—C bond formation between the fu-ranoid glycal 454 and protected 3-iodopyrazolo[4,3-d]pyrimidine. The C-nucleoside 455 having an unsaturated sugar moiety was subsequently elaborated to 458 (92JOC4690) (Scheme 125). 

The three kinds of polymers that are prevalent in nature are polysaccharides, proteins, and nucleic acids. You have already learned about polysaccharides, which are naturally occurring polymers of sugar subunits (Section 22.18), and nucleic acids are covered in Chapter 27. We will now look at proteins and the structurally similar, but shorter, peptides. Peptides and proteins are polymers of amino acids linked together by amide bonds. The repeating units are called amino acid residues. 

Normal or A/-nucleosides (1), the building units of DNA or RNA, have Cl of their sugar subunits (2-deoxy-D-ribose or o-ribose) linked to heterocycle subunits (purine or pyrimidine bases) through carbon-nitrogen bonds. 

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