Furanoheliangolides from Centratherum punctatum and a General Approach for Stereochemical Assignment of Flexible...

Human topoisomerase 3β (TOP3B) is a potential molecular therapeutic target for cancer and viral infections. A high-throughput differential cell viability assay using colon cancer cell lines was developed to identify natural product modulators of TOP3B-associated cancer cell viability. The assay identified an organic extract of the plant Centratherum punctatum as having cytotoxic activity. Seven new furanoheliangolides, centratherolides A-G (1-7), along with two known analogues (2,3-epoxybutyryloxy)-goyazensolanolide (8) and goyazensolide (9), were isolated. Compounds 1, 8, and 9 exhibited selective cytotoxic activities against the TOP3B-knockout (TOP3B-KO) human colon carcinoma HCT116 cells compared with the wild-type HCT116 cells (TOP3B-WT). The challenging absolute configuration determination of the flexible chiral side chains in selected analogues (1-4 and 8) was resolved by combined approaches, including synthesis of chemical standards, DFT ECD calculation, and chiral HPLC analysis. Application of this elucidation methodology to a commercial sesquiterpene lactone clarified a contradiction in the stereochemical assignments reported for centaurepensin/chlorohyssopifolin A and 17-epi-chlorohyssopifolin A.