Total Synthesis of the Protected Aglycon of Fidaxomicin (Tiacumicin B, Lipiarmycin A3)

Fidaxomicin, also known as tiacumicin B or lipiarmycin A3, is a novel macrocyclic antibiotic that is used in hospitals for the treatment of Clostridium difficile infections. This natural product has also been shown to have excellent bactericidal activity against multidrug-resistant Mycobacterium tuberculosis. In spite of its attractive biological activity, no total synthesis has been reported to date. The enantioselective synthesis of the central 18-membered macrolactone is reported herein. The key reactions include ring-closing metathesis between a terminal olefin and a dienoate moiety for macrocyclization, a vinylogous Mukaiyama aldol reaction, and a Stille coupling reaction of sterically demanding substrates. The retrosynthesis involves three medium-sized fragments, thus leading to a flexible yet convergent synthetic route.