Synthesis of a pentasaccharide fragment of Polysaccharide II of Mycobacterium tuberculosis

Stereocontrolled, stepwise synthesis of decyl glycosides of α-(1 → 2)-linked di- to pentaglucosides (1–5) is described; these constitute fragments of Polysaccharide II of Mycobacterium tuberculosis. Phenyl 3,4,6-tri-O-acetyl-2-O-benzyl-1-thio-α-d-glucopyranoside (7) was used as the single key intermediate, obtained from 1,3,4,6-tetra-O-acetyl-2-O-benzyl-β-d-glucopyranose (6) and PhSSiMe3. Halogenolysis of 7 afforded the isolated β bromide (10) and β chloride (13). Solvolysis of 10 with decanol without heavy metal salts gave decyl 3,4,6-tri-O-acetyl-2-O-benzyl-α-d-glucopyranoside (14) in a highly stereoselective reaction, in high yield. Subsequent, iterative hydrogenolytic removal of the O-benzyl group and glycosylation with the β-chloride 13 under catalysis by silver salts afforded the protected di- to penta-saccharide glycosides 16, 19, 21, and 23, which were conventionally deblocked.