An Unusual 23-Secotaraxerene and Other Cytotoxic Triterpenoids from Pleiocarpa pycnantha (Apocynaceae) Leaves Collected from Nigeria

摘要

Three known triterpenoids, namely ursolic acid (>1), and the 27-E- and 27-Z-p-coumaric esters of ursolic acid (compounds >2, >3), were isolated together with a new triterpene 2,3-seco-taraxer-14-en-2,3-lactone [pycanocarpine (>4)] from an ethanolic extract of Pleiocarpa pycnantha leaves. The structure of >4 was unambiguously assigned using NMR, HREIMS and X-ray crystallography. The cytotoxic activities of the compounds were evaluated against HeLa, MCF-7, KMST-6 and HT-29 cells using the WST-1 assay. Ursolic acid (>1) displayed potent cytotoxic activity against HeLa, HT-29 and MCF-7 cells with IC50 values of 10, 10 and 20 µM respectively. The new compound >4 and its hydrolysed derivative >5 were selectively cytotoxic to the breast cancer cell line, MCF-7 with IC50 values 20 and 10 µM respectively. This is the first report on isolation of a 2,3-seco-taraxerene derivative from the Apocynaceae family and cytotoxic activityof P. pycnantha constituents.