Synthetic Access Toward Cycloastragenol Glycosides.

The first efficient synthetic approach toward four types of the cycloartane glycosides, the cycloastragenol 25-O; 3-O; 3,6-O-bis; and 3,25-O-bisglycoside, have been established, which featured the PPY-mediated, concentration-controlled acetylation and Au(I)-catalyzed Yu glycosylation. Through the synthetic investigation, the reactivity sequence of the four OHs in cycloastragenol was fixed for the first time and a detour strategy for the highly efficient removal of bulky pivaloyl protecting groups was discovered.