Synthesis of All Eight L-Glycopyranosyl Donors Using CH Activation

The synthesis of all eight rare, but biologically important L-hexoses as the according thioglycosyl donors was achieved through a procedure involving the CH activation of their corresponding 6-deoxy-L-hexoses. The key steps of the procedure were the silylation of the OH group at C4 followed by an intramolecular CH activation of the methyl group in [gamma]-position; both steps were catalyzed by iridium. The following Fleming-Tamao oxidation and acetylation gave the suitably protected L-hexoses. This is the first general method for the preparation of all eight L-hexoses as their thioglycosyl donors ready for glycosylation and the first example of an iridium-catalyzed C(sp3)H activation on sulfide-containing compounds.