Glycosylidene Carbenes. Part 17. Glycosidation of benzyl ?-D- and ?-L-ribopyranosides. Further evidence for the effect of stereoelectronic control on the regioselectivity of glycosidation
作者:Peter Uhlmann、Andrea Vasella
DOI:10.1002/hlca.19940770426
日期:1994.6.29
which were characterized as their diacetates 14–19 and 26–31 (Scheme). Glycosidation in CH2Cl2 and in dioxane yielded mostly the 1,3-linked disaccharides 10/11 and 22/23 (α/β ca. 4:1), while glycosidation in THF leads mostly to the 1,2- and 1,4-linked regioisomers (β>α). There are small, but significant differences in the glycosidation of 4 and 5. These, the regio-, and the stereoselectivities are rationalized
描述了对映体核糖苷4和5的H键及其被二嗪1的糖苷化作用。HO–C(2)和HOC(4)(分别为4和5)形成“触发器” H键系统,HOC(3)充当OC(2)或OC(4)的H键供体。因此,单体4和5的HOC(2)和HOC(4)是最强酸性的OH基团。4和5乘1的糖苷化取决于溶剂,温度和浓度。它产生高达1%的1,2-,1,3-和1,4-连接的二糖8-13和20-25的端基异构体混合物的91%分别以双乙酸盐14-19和26-31(方案)为特征。CH 2 Cl 2和二恶烷中的糖苷化主要产生1,3连接的二糖10/11和22/23(α/β约4:1),而THF中的糖苷化主要导致1,2-和1 ,4-连接的区域异构体(β>α)。4和5的糖苷化存在微小但显着的差异。这些,区域选择性和立体选择性被合理化为立体电子控制从HOC(2)或HOC(4)到中间卡宾的H转移以及糖苷键的形成以及中间体氧碳鎓离子与THF的配位