Synthesis of the celogentin C right-hand ring

Synthesis of the cyclic tetrapeptide comprising the right-hand ring of celogentin C, a potent inhibitor of tubulin polymerization, has been accomplished. A mild oxidative coupling reaction permitted construction of the indole-imidazole linkage via a convergent union of two fully functionalized dipeptides. A high-yielding macrolactamization and subsequent deprotection of the N-terminus furnished the target compound.