Catalytic Stereoseletive Synthesis of Glycosides

在一定催化量的三烷基四（五氟苯1）硼酸酯（TrB(C_6F_5)_4）存在下，2,3,5-三- o -苄基- d -核呋喃糖与醇类反应顺利，产率高，立体选择性好，生成了多种- d -核呋喃苷。在LiClO_4或LiNTf_2存在下，由2,3,5-三- o -苄基- d -核呋喃糖和几种醇合成了α - d -核呋喃苷。使用锂盐的这些反向立体选择可能是由于通过形成包括锂阳离子的配位配合物来选择性稳定α -异头物。呋喃糖苷合成中仅从1-羟基糖开始的糖基化反应的立体选择性较好，而吡喃糖苷的立体选择性则不尽如人意。采用Sn(OTf)_2-Me_3SiCl催化体系和lico_4作为有效添加剂，成功地进行了几种醇性亲核试剂与2,3,4,6-四- o -苄基- d -半乳糖醛酸糖基化反应。几种α - d -半乳糖苷以较高的立体选择性产率制得。脱氧糖苷通常存在于蒽环类药物、金葡酸、正霉素的糖组分和其他具有药学意义的化合物中。2-脱氧- α - d -糖苷在含糖天然产物领域被认为是生物学上重要的结构单元。以TrB (C_6F_5)_4为催化剂，以3,4,6-三- o -苄基1-2-脱氧-d -葡萄糖吡喃糖（1-羟基糖）为原料，立体选择性地合成了几种2-脱氧- α -d -葡萄糖吡喃苷。

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.