| Research Abstract |
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.
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