Total Synthesis and Biological Evaluation of 19-Hydroxysarmentogenin-3 -O-α-l-rhamnoside, Trewianin, and Their Aglycons.

Cardenolides comprise an important family of natural steroids with a wide spectrum of biological activities. Although 19-hydroxysarmentogenin-3-O-α-l-rhamnoside (1a) and trewianin (1b) were structurally determined to have cardenolide structures, their biological activities have not been evaluated. The 6/6/6/5-membered ABCD-ring systems of both 1a and 1b are decorated by a β-oriented C17-butenolide, three C11,14,19-hydroxy groups, and a C3–O-l-rhamnoside moiety. On the other hand, 1a and 1b are epimeric at the C5-position. The structures of 1a and 1b were assembled from four simple fragments by applying a convergent and unified strategy. The AB-ring 10a/b and the D-ring 8/9 were tentatively tethered at the acetal moiety, and a subsequent stereoselective 6-exo radical reaction linked the two fragments. Next, an aldol reaction enabled simultaneous introduction of three new stereocenters of the C-rings of 5aa and 54. Attachment of the C17-butenolide led to aglycons 2a and 2b. l-Rhamnose was then installed int...