Emerimicins III and IV and their ethylalanine12 epimers. Facilitated chemical-enzymatic synthesis and a qualitative evaluation of their solution structures

The peptaibol antibiotics, emerimicin III and IV (Ac-Phe 1 -MeA 2 -MeA 3 -MeA 4 -Val 5 -Gly 6 -Leu 7 -MeA 8 -MeA 9 - Hyp 10 -Gln 11 -R-EtA 12 -Hyp 13 -Xxx 14 -Phol 15 , where Xxx=Ala for emerimicin III and Xxx=MeA for emerimicin IV) and their EtA 12 epimers have been synthesized using a combined approach involving solution-phase fragment condensation with a final papain-mediated coupling of the 1-6 and 7-15 fragments