The 4,6-O-[α-(2-(2- Iodophenyl)ethylthiocarbonyl)benzylidene] Protecting Group:  Stereoselective Glycosylation, Reductive Radical Fragmentation, and Synthesis of β-d-Rhamnopyranosides and Other Deoxy Sugars

In the thioglycoside/BSP/Tf2O glycosylation method, the 4,6-O-[α-(2-(2-iodophenyl)ethylthiocarbonyl)benzylidene] group enforces β-selectivity in mannopyranosylations. Following glycosylation, treatment with Bu3SnH in toluene at reflux affords regioselective, reductive fragmentation to the 6-deoxy-β-mannosides (β-rhamnosides). Applied to glucosides, the radical fragmentation provides 6-deoxyglucosides, whereas 4-deoxygalactosides are the preferred products in the galactose series. The radical fragmentation is fully compatible with the presence of benzyl and p-methoxybenzyl ethers and with acetate esters