Total Synthesis of Starfish Cyclic Steroid Glycosides.Part 3,Formal Total Synthesis of Luzonicosides A and D

Comprehensive Summary Luzonicosides A(1)and D(2)are the major saponins isolated from pacific starfish Echinaster Luzonicus,featuring a sterically congested 16-membered ring which is formed by bridging the C3 and C6 of a 3β,6β-dihydroxy-Δ7-ene steroid with a 1,2-trans-linked linear trisaccharide via glycosidic and ether bonds,respectively.Here,we describe a formal total synthesis of starfish cyclic steroid glycosides 1 and 2.The synthetic route is highly modular and versatile,involving construction of the 16-membered macrocycles via Au(I)-catalyzed intramolecular glycosylation,highly efficient installation of unsaturated sugar moieties with ortho-hexynylbenzoates,elaboration of glucuronic acid unit via post-oxidation,and a de novo synthesis of the ether-linked pyranose unit.