Synthesis of the Glycopeptidolipid of Mycobacterium avium Serovar 4:  First Example of a Fully Synthetic C-Mycoside GPL

The preparation of the glycopeptidolipid (GPL) present in the cell wall of Mycobacterium avium Serovar 4, namely 3,4-di-O-Me-α-l-Rhap-(1→1){R-C₂₁H₄₃CH(OH)CH₂CO-d-Phe-[4-O-Me-α-l-Rhap-(1→4)-2-O-Me-α-l-Fucp-(1→3)-α-l-Rhap-(1→2)-6-deoxy-α-l-Talp-(1→3)]-d-allo-Thr-d-Ala-l-Alaol} (1), is described. The synthesis was based on the disconnection of the final structure into four building blocks, an l-rhamnosyl pseudodipeptide, a 6-deoxy-l-talosyl dipeptide, a trisaccharide donor, and a 3-hydroxyalkanoic acid. The key steps are the creation of the glycosidic linkage between the trisaccharide donor, used as a pentenyl glycoside, and the 6-deoxy-l-talose unit of an appropriate d-Phe-O-(6-deoxy-l-talosyl)-d-allo-Thr derivative and the final coupling of the two glycodipeptide fragments. Pentenyl glycosides were shown to provide useful donors in several glycosylation steps. This work constitutes the first synthesis of the full structure of a so-called “polar mycoside C” GPL.