Synthesis of the Pentasaccharide Moiety of Thornasterside A

The synthesis of the pentasaccharide moiety of thornasteroside A, the first asterosaponin isolated from starfish in 1978 has been achieved for the first time. Initially, a [3+2] convergent strategy was attempted, but the β(1→4) glycosidic linkage between galactopyranose (sugar IV) and xylopyranose (sugar II) was formed with a low stereoselectivity and in low yield. Subsequently, a [3+1+1] strategy was adopted. A galactopyranosyl donor ( 18 ) equipped with a neighboring participating Lev (levulinoyl) group at the 2‐position was first coupled with a trisaccharide acceptor to construct the β(1→4) glycosidic bond. Then the Lev group was selectively removed, and subsequent glycosylation with a perbenzoylated D ‐fucopyranosyl Schmidt donor efficiently gave the desired pentasaccharide.