The First Total Synthesis of the Peptaibol Hypomurocin A1 and Its Conformation Analysis: an Application of the 'Azirine/Oxazolone Method'

The first total synthesis of Hypomurocin A1 (HM A1) in solution phase is described. As members of the peptaibol family, hypomurocins are constituted by two groups of peptides: six undecapeptides (undecamers) in the HM A group and six octadecapeptides (18‐mers) in the HM B group. The synthesis presented has been successfully achieved by the ‘azirine/oxazolone method’ to introduce the two Aib‐Pro sequences included in this undecapeptaibol in one step with methyl 2,2‐dimethyl‐2H‐azirine‐3‐prolinate as the building block. The coupling reactions of the Z‐protected amino acids or peptide acids involved the use of N,N,N′,N′‐tetramethyluronium tetrafluoroborate (TBTU) and 1‐hydroxybenzotriazole (HOBt), and led to the peptides in good‐to‐very‐good yields. The peptides were purified by reverse‐phase HPLC and characterized by NMR spectroscopy (1H, 13C, COSY, TOCSY, HSQC, HMBC, ROESY), ESI‐MS, IR, elemental analysis, optical rotation, and X‐ray crystallography. An NMR analysis of HM A1 was also carried out in deuterated micelles to perform a structural comparison of the helix in solution and in membranes.