Superacids in carbohydrate chemistry

O-Glycosylation—that is, the condensation of a sugar derivative bearing an appropriate functional group (donor) with an appropriately protected other sugar derivative with a free hydroxyl group (acceptor)—is a crucial methodology in carbohydrate and natural product chemistry.744 

Glycosyl fluorides, widely used reagents in carbohydrate and natural product chemistry,745 746 can be used to carry out stereoselective synthesis of glycosides with a catalytic amount (5 mol%) of triflic acid. The appropriately protected /3-D-glycosyl fluorides of glucose and galactose as donor molecules, when applied in dichloro- 

Wong and co-workers753 and Hashimoto and co-workers754 have demonstrated that triflic acid is an ideal promoter in chemo- and stereoselective glycosylation with phosphites [Eq. (5.285)]. 

Franck and Marzabadi755 have developed the heterocyclic donor 180 and used it in coupling with a variety of alcohol acceptors to obtain /8-glycosides in good yields with excellent stereoselectivities using equimolar amount of triflic acid [Eq. (5.286)]. 

Catalytic amount (5 mol%) of triflic acid has been shown to be effective in the intramolecular condensation-dehydration-type glycosylation to form anhydroketo-pyranoses without the necessity to introduce any leaving groups756 [Eq. (5.287)]. Interestingly, the twist-boat conformers were also detected in the transformation of the 1-phenyl- and 1-benzyl-susbtituted compounds. 

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