Total Synthesis of Starfish Cyclic Steroid Glycosides

Starfishes have evolved with a special type of secondary metabolites, namely starfish saponins, to ward off various predators and parasites; among them, the starfish cyclic steroid glycosides stand out structurally, featuring a unique 16-membered ring formed by bridging the steroidal C3 and C6 with a trisaccharide. The rigid cyclic scaffold and the congested and vulnerable steroid-sugar etherate linkage present an unprecedented synthetic challenge. Here we report a collective total synthesis of the major starfish cyclic steroid glycosides, namely luzonicosides A (1) and D (2) and sepositoside A (3), with an innovative approach, which entails a de novo construction of the ether-linked hexopyranosyl units, use of olefinic pyranoses as sugar precursors, and a decisive ring-closing glycosylation under the mild gold(I)-catalyzed conditions.