Catalyst-Controlled Stereoselective O-Glycosylation: Pd(0) vs Pd(II)

Stereoselective construction of various O-glycosidic bonds was first achieved by different palladium sources using 3,4-O-carbonate galactal as the donor to reach yields up to 95% under mild conditions. With Pd(II) catalyst coordination of this glycal donor from the β-face directed by carbonate group, hard nucleophiles (aliphatic alcohols) gave β-glycosides and α-glycosides were obtained from soft nucleophiles (phenols). In contrast, with the Pd(0) catalyst coordinating the donor from the β-face due to steric effect, both hard and soft acceptors could only generate β-glycosides via hydrogen-bond-mediated aglycone delivery.