2011 Volume 23 Issue 131 Pages 122-133
We describe L-O-rhamnosylation and D-O-glucosylation reactions using glycosyl donors in which the pyranose rings were restricted in a conformation that possessed more axial substituents. We call this conformation the axial-rich form. Introduction of the bulky silyl-protecting groups onto the adjacent trans-diols on the sugars restricted the conformation to the axial-rich form. The axial-rich L-rhamnosyl donor provided the b-isomer preferentially in the glycosylation reaction when trichloroacetimidate was adopted as the leaving group; however, the α/β ratio was only 1/4. This value was insufficient for practical applications. By contrast, the axial-rich glucosyl donor induced a highly β-selective reaction in which the anomeric α/β ratio was up to 2/98. In this case, the axial-rich skew-boat conformation of the pyranose ring was the key to successful stereoselective glycosylation. This method is an alternative for construction of the β-O-glucosidic linkages without relying upon participation of neighboring groups, a method that has long been employed. The new method was particularly effective for the synthesis of 2-O-glycosylated β-O-glucosides, for which direct construction using the neighboring group participation method is difficult.
