Probing the Effect of Acylation on Arabinofuranose Ring Conformation in Di- and Trisaccharide Fragments of Mycobacterial Arabinogalactan

A major component of the cell wall of mycobacteria is the mycolyl-arabinogalactan (mAG) complex. The arabinose and galactose residues in mAG are found solely in the furanose form, and it has been suggested that the flexibility of these five-membered rings allows for the tight packing of mycolic acids. In order to probe the "flexible scaffold hypothesis", we designed and synthesized glycolipids 3-6 and 8-11 as simple models of the terminal portion of mAG. A set of donors and acceptors were explored for preparing the key beta-(1-->2) linkage in 2-6, and the best selectivity and yield can be obtained by using the electron-rich thioglycoside donor 14 and the O-5 p-methoxybenzyl-protected acceptor 17. Both alpha-linkages in the trisaccharides 7-11 were formed in a one-pot reaction. The conformations of compounds 2-11 were studied using solution-state NMR spectroscopy, but little change was observed in the coupling constants for the ring protons between 2 and 3-6 or between 7 and 8-11. However, the rotamer populations about the C-4-C-5 bond for the beta-linked ring in disaccharide 2 did change upon acylation at O-5.