Catalytic Stereoseletive Synthesis of Glycosides

在催化量的三苯甲基四硼酸五氟苯酯（TrB（C_6F_5）_4）、2，3，5-三-O-苄基-D-呋喃核糖与醇类反应，以高产率、高立体选择性地合成了多种β-D-呋喃核糖苷，在锂盐共存下也观察到反向立体选择性，以2，3，5-三-O-苄基-D-呋喃核糖为原料，5-三-O-苄基-D-呋喃核糖和几种醇在LiClO_4或LiNTf_2存在下的反应。这些反向立体选择性的发生可能是由于使用锂盐的α-端基异构体的选择性稳定，通过形成一个配位络合物，包括锂阳离子。糖基化的立体选择性仅从1-羟基糖在呋喃糖苷合成中是在令人满意的水平，但并不总是这样的吡喃糖苷合成。联合使用Sn（OTf）_2-Me_3SiCl催化剂体系和有效添加剂LiClO_4，成功地进行了几种醇类亲核试剂与2，3，4，6-四-O-苄基-D-半乳糖（1-羟基糖）的糖基化反应。几种α-D-吡喃半乳糖苷以高产率和高立体选择性获得。脱氧糖苷经常在蒽环类、金酸、正霉素和其他具有药用价值的化合物的糖组分中发现。2-脱氧-α-D-糖苷在含糖天然产物领域中被认为是生物学上重要的结构单元。以3，4，6-三-O-苄基-2-脱氧-D-吡喃葡萄糖（1-羟基糖）为原料，在催化量的TrB（C_6F_5）_4作用下，立体选择性地合成了几种2-脱氧-α-D-吡喃葡萄糖苷。

In the presence of a catalytic amount of trityl tetrakis(pentafluoropheny1) borate (TrB(C_6F_5)_4), 2,3,5-tri-O-benzyl-D-ribofuranose smoothly reacted with alcohols to give various beta-D-ribofuranosides in high yields with high stereoselectivitiy.Reverse stereoselection was also observed in the coexistence of lithium salts, and alpha-D-ribofuranosides were synthesized from 2,3,5-tri-O-benzy1-D-ribofuranose and several alcohols in the presence of LiClO_4 or LiNTf_2. These reverse stereoselection by using lithium salts occurred probably due to the selective stabilization of the alpha-anomer by forming a coordinated complex including lithium cation.The stereoselectivity of the glycosylation starting just from 1-hydroxy sugar in furanoside synthesis was in satisfactory level but not always so in pyranoside synthesis. The glycosylation reaction of several alcoholic nucleophiles with 2,3,4,6-tetra-O-benzy1-D-galactopyranose (1-hydroxy sugar) is successfully carried out by the combined use of Sn(OTf)_2-Me_3SiCl catalyst system andLiC1O_4, an effective additive. Several alpha-D-galactopyranosides are obtained in good yields with high stereoselectivities.Deoxyglycosides are often found in sugar components of anthracyclines, aureoic acids, orthosomycines and in other compounds of pharmaceutical interest. 2-Deoxy-alpha-D-glycosides are known as biologically important structural units in the fields of sugar-containing natural products. Several 2-deoxy-alpha-D-glucopyranosides are stereoselectively synthesized in high yields by glycosylation of various alcoholic nucleophiles with 3,4,6-tri-O-benzy1-2-deoxy-D-glucopyranose (1-hydroxy sugar)using a catalytic amount of TrB (C_6F_5)_4.