SYNTHESIS OF METHYL 3-O-[3-O-(2,3,4-TRI-O-METHYL-ALPHA-L-RHAMNOPYRANOSYL)-ALPHA-L-RHAMNOPYRANOSYL]-ALPHA-L-RHAMNOPYRANOSIDE - THE OUTER TRISACCHARIDE UNIT OF A UNIQUE MYCOBACTERIUM-XENOPI GLYCOPEPTIDOLIPID

The combination of sugars present in Mycobacterium xenopi glycopeptidolipid (GPL X-1) has been characterised as O-(2,3,4-tri-O-methyl-alpha-L-rhamnopyranosyl)-(1-3)-O-(alpha-L-rhamnopyranosyl)-(1-3)-O-(alpha-L-rhamnopyranosyl)-(1-3)-6-deoxy-L-glucose. Two synthetic routes (A and B) towards the outer trisaccharide segment (2) have been designed. Route A involved the condensation of the aglycone (7) with 2,3,4-tri-O-acetyl-alpha-L-rhamnopyranosyl bromide (13) to afford which was sequentially deacetylated, methylated, deallylated and reacetylated to afford 12. Condensation of 12 with 16 in the presence of borontrifluoride etherate was unsuccessful. In route B, the disaccharide 21 was synthesised by the coupling reaction between 16 and 18 in the presence of borontrifluoride-etherate followed by deallylation. Alternatively, the disaccharide triacetate (22) was first prepared and routine modifications on it led to the formation of 21. Final condensation of 21 with the trichloroacetimidate (29) followed by sequential deacetylation, methylation and debenzylation afforded the trisaccharide (2).