Phenyliodine Bis(trifluoroacetate) (PIFA) as an Excellent Promoter of 2-Deoxy-2-phthalimido-1-thioglycosides in the Presence of Triflic Acid in Glycosylation Reactions

A combination of phenyliodine bis(trifluoroacetate) (PIFA) and trifluoromethanesulfonic acid was found to be an effective activator for the glycosylation reaction, of which the glycosyl donor was p-(octyloxy)phenyl 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-1-thio-D-glucopyranoside or p-(octyloxy)phenyl 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-1-thio-D-galactopyranoside. The reaction proceeded with good yields when naturally occurring and derived alkanols, such as cholestanol, (-)-menthol, 1- and 2-adamantanols and nor-kauranol, were employed as acceptor substrates. Interestingly, the glycosylation reaction of sterically hindered terpenoid alcohols, such as (-)- and (+)-borneols and (+)-fenchyl alcohol, with p-(octyloxy)phenyl 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-1-thio-D-glucopyranoside afforded the desired -glycoside in quantitative yield. Moreover, the present reaction was useful to synthesize disaccharides of which the non-reducing end was composed of a D-glucosamine or D-galactosamine unit.