Catalytic Stereoseletive Synthesis of Glycosides

在催化量的四(五氟苯基)硼酸酯(TrB(C_6F_5)_4)存在下，2，3，5-三-O-苯基-D-呋喃核糖与醇反应，高产率、高立体选择性地合成了多种β-D-呋喃核苷。在锂盐共存下，还观察到了反向立体选择性，从2，3，3-三-O-苯基-D-核呋喃糖合成了α-D-呋喃核苷。在LiClO_4或LiNTF_2存在下，5-三-O-苄基-D-呋喃核糖和几种醇发生了反向立体选择，这可能是由于α-异构体通过形成含锂离子的配位络合物而选择性稳定的结果。在呋喃糖苷的合成中，仅从1-羟基糖开始的糖基化反应的立体选择性是令人满意的，但在吡喃糖苷的合成中并不总是如此。在SnOTf_2-Me_3SiCl催化体系和有效添加剂LiC_1O_4的联合作用下，成功地实现了几种醇类亲核试剂与2，3，4，6-四-O-苯基-D-半乳糖(1-羟基糖)的糖基化反应。几个α-D-半乳糖苷以较高的立体选择性得到了几个α-D-半乳糖苷。脱氧糖苷通常存在于蒽环类、金黄色酸、正霉素类和其他有药用价值的化合物的糖组分中。2-脱氧-α-D-糖苷是含糖天然产物中重要的生物结构单元。以3，4，6-三-O-苯基-2-脱氧-D-吡喃葡萄糖(1-羟基糖)为原料，在TrB(C_6F_5)_4催化下，高产率地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.