The first synthesis of clausenamine-A and cytotoxic activities of three biscarbazole analogues against cancer cells

Clausemine-A (Scheme 1 and Scheme 2), isolated from the stem and root bark of Clausena excavata, was synthesized using Suzuki cross-coupling and Oxidative coupling as the key step. Compound Scheme 1 and Scheme 2, and the other two structurally related biscarbazoles 1 and 2, showed potent cytotoxic activities against a variety of human cancer cell lines in vitro. Scheme 1. Scheme 2. Reagents and conditions: (i) TsCl, Et3N, CH2Cl2, rt, 84%; (ii) 48% HBr, DMSO, rt, 94%; (iii) 5% Pd(PPh3)4, 2 N Na2CO3, benzene, 3,5-dimethoxyphenyl boronic acid, ethanol, reflux, 99%; (iv) 48% HBr, DMSO, rt, 79.5%; (v) 1.2 equiv Pd(PPh3)4, Na2CO3, toluene, reflux, 97%; (vi) KOH, H2O, EtOH, reflux, 88%; (vii) (t-BuO)2, chlorobenzene, reflux, 90%.