Arabinan, Galactan, and Related Glycoconjugates

FIGURE 14.6 Structure of mycobacterial mycolyl arabinogalactan-peptidoglycan complex. Ara, arabinose MurNGc, iV-glycolylmuramic acid. 

Stereocontrolled formation of P-Ara/ is intrinsically problematic because of its 1,2-cis, nonaxial nature [173], In order to solve this, several innovative methods have been developed. Of particular note were approaches based on SN2-type displacement of a-triflates derived from 2,3-anhydro-modified and carboxybenzyl-substituted donors, which were developed by Todd Lowary [174, 175] and by Kwan Soo Kim [176], respectively. Alternatively, certain cyclic protections gave promising results by virtue of their abilities to bring conformational constrain to arabinofuranosyl donors. For instance, bicyclic donors such as those protected with 3,5-O-di-tert-butylsilylidene (DTBS) [177-179], 3,5-0-tetraisopropyldisiloxanylidene (TIPDS) 

FIGURE 14.7 Structures of the docosasaccharide arabinan prepared by the laboratories of Lowary and Ito. 

Ito s synthetic strategy toward the docosasaccharide arabinan 125 involves (i) the stereoselective p-arabinofuranosylation using both 3,5-O-TIPDS-protected and 

SCHEME 14.16 Synthesis of the docosasaccharide arabinan 124 by Lowary and coworkers. CAN, ceric ammonium nitrate. 

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