Hydroxy dehydrative glycosylation

Dehydrativeglycosylation of 1-hydroxy donors with Ph2S0/Tf20 in conjunction with thioglycoside acceptors opens the way for sequential double glycosylation, one-pot procedures for trisaccharide synthesis, as exemplified by the efficient one-pot synthesis of the a-Gal epitope and a hyaluronan trisaccharide [566]. This study also shows the potential of selenoglycoside as acceptors in dehydrative glycosylation (Scheme 4.112). 

Seeberger PH, Bilodeau MT, Danishefsky SJ. Synthesis of biologically important oligosaccharides and other glycoconjugates by the glycal assembly method. Aldrichim. Acta. 1997 30 75-92. Gin D. Dehydrative glycosylation with 1-hydroxy donors. J. Carb. Chem. 2002 21 33-52. 

Gin, D, Dehydrative glycosylation with 1-hydroxy donors, J. Carbohydr. Chem., 21, 645-665, 2002. 

The reaction of sugar isothiocyanates with aminosugars described above has also been studied with 1-amino (alkyl and arylamino)-l-deoxy-D-fructose 143 (Scheme 32). Thus, treatment of different 0-acylated sugar isoth-iocyanates with D-fructosamines 143 gave M-glycosyl-5-hydroxy-5-tetritolyl-imidazolidine-2-thiones 144 as pairs of diastereomers in almost quantitative yield [ 152]. The reaction times were shorter (15-20 min) for fructosamine or its M-methyl derivatives than in the cases of N-aryl derivatives (8-24 h). Acid-catalyzed dehydration of 144 with ethanolic TFA at rt led to 147 in 65-77% yield. These compounds can be considered simultaneously as M-nucleosides and acyclic C-nucleosides of imidazoline-2-thiones. 

Example an heteropoly acid act both as promoter and dehydrating agent in the glycosylation of an anomeric hydroxy sugar donor... 

---