title: Lewis acids as α-directing additives in glycosylations by using 2,3-O-carbonate-protected glucose and galactose thioglycoside donors based on preactivation protocol.
abstract: Catalytic or stoichiometric amounts of Lewis acids were found to be very effective α-directing additives in the stereoselective glycosylations of diverse 2,3-O-carbonate-protected glucose and galactose thioglycoside donors by preactivation protocol. The poor stereoselectivities of 4,6-di-O-acetyl-2,3-O-carbonate protected thioglycoside donors in glycosyl coupling reactions were greatly improved, and excellent α-stereoselectivities were achieved by the addition of 0.2 equiv of BF(3)·OEt(2). On the other hand, the β-selectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thioglucoside donor toward glycosylations were reversed completely to the α-selectivities by the use of 1 equiv of SnCl(4), making the stereoselectivity controllable. Furthermore, the poor stereoselectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thiogalactoside donor in glycosylations were also improved by using SnCl(4) as additive.
authors.affiliation: State Key Laboratory of Natural and Biomimetic Drugs and School of Pharmaceutical Sciences, Peking University, Xue Yuan Road No. 38, Beijing 100191, China.
authors.forename: Yiqun
authors.initials: Y
authors.lastname: Geng
authors.forename: Qi
authors.initials: Q
authors.lastname: Qin
authors.forename: Xin-Shan
authors.initials: XS
authors.lastname: Ye
mesh_terms: D002254:Carbonates / Q000737:chemistry*; D002384:Catalysis; D005690:Galactose / Q000737:chemistry*; D005947:Glucose / Q000737:chemistry*; D006031:Glycosylation; D058116:Lewis Acids / Q000737:chemistry*; D015394:Molecular Structure; D013237:Stereoisomerism; D013862:Thiogalactosides / Q000737:chemistry*; D013865:Thioglycosides / Q000737:chemistry*
chemical_list: D002254:Carbonates; D058116:Lewis Acids; D013862:Thiogalactosides; D013865:Thioglycosides; D005947:Glucose; D005690:Galactose