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-alpha-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 la and lb are decorated by a beta-oriented C17-butenolide, three C11,14,19-hydroxy groups, and a C3-O-L-rhamnoside moiety. On the other hand, la and lb 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 Saa and 54. Attachment of the C17-butenolide led to aglycons 2a and 2b. L-Rhamnose was then installed into 2a and 2b to yield the targets la and lb, respectively. Finally, the growth inhibitory activity of 1a, 1b, 2a, and 2b was assessed against MCF-7 human breast carcinoma cells. The significantly higher activities of 1a and 1b in comparison to 2a and 2b demonstrated the biological importance of the monosaccharide substructure.