Total synthesis of the tiacumicin B (lipiarmycin A3/fidaxomicin) aglycone.

Tiacumicin B (lipiarmycin A3, fidaxomicin) is an atypical macrolide antibiotic which is used for the treatment of Clostridium difficile infections. Tiacumicin B is also a potent inhibitor of Mycobacterium tuberculosis, but due to its limited oral bioavailability is unsuitable for systemic therapy. To provide a basis for structure-activity studies that might eventually lead to improved variants of tiacumicin B, we have developed an efficient approach to the synthesis of the tiacumicin B aglycone. The synthesis features a high-yielding intramolecular Suzuki cross-coupling reaction to effect macrocyclic ring closure. Key steps in the synthesis of the macrocyclization precursor were a highly selective, one-pot Corey-Peterson olefination and an ene-diene cross-metathesis reaction. Depending on the reaction conditions, the final deprotection delivered either the fully deprotected tiacumicin B aglycone or partially protected versions thereof.