Stereocontrolled Synthesis of 2-Deoxy-galactopyranosides via Isopropylidene-Protected 6- O-Silylated Donors.

The stereocontrolled synthesis of 2-deoxy-d- arabino-hexopyranosides ("galactopyranosides") using 3,4- O-isopropylidene-6- O- tert-butyldiphenylsilyl-protected glycosyl donors is reported. 2-Deoxy-thioglycoside 3e gives excellent α-selectivity, while galactal 9 leads to, in a two-step protocol, 2-deoxy-ß-glycosides in high stereoselectivity. The selectivity of both reagents is believed to arise from the combination of the isopropylidene acetal spanning O-3 and O-4 together with the sterically demanding silyl group on O-6. The utility of the method was demonstrated through the synthesis of a trisaccharide that contains both 2-deoxy α- and ß-d-galactopyranosyl residues.