Rubipodanin B, a New Cytotoxic Cyclopeptide from Rubia podantha

Using the TLC cyclopeptide protosite detection method, a new cyclohexapeptide named rubipodanin B (1), together with 11 known Rubiaceae‐type cyclopeptides (RAs), RA‐X‐OMe (2), RA‐IV (3), RA‐XI (4), RA‐XIII‐OMe (5), rubiyunnanin C (6), RA‐I (7), RA‐III (8), RA‐V (9), RA‐VII (10), RA‐XII (11) and rubipodanin A (12), were obtained from the roots and rhizomes of Rubia podantha Diels. The structures were determined using various spectroscopic methods. Among them, 2 was firstly identified as a natural product, and 3–6 were firstly isolated from this species. Cytotoxicity and NF‐κB signaling pathway activity of 1, 2, 4, 6, 7 and 9 were evaluated. All these compounds showed cytotoxic activities against three human tumor cell lines, MDA‐MB‐231, SW620 and HepG2, with the IC50 values between 0.015 and 10.27 μm, and only 7 and 9 possessed NF‐κB inhibitory activities with the IC50 values of 2.42 and 0.046 μm, respectively, which demonstrated that 2‐alanine amino acid plays a key role to maintain the RAs bioactivity.