Facilitated synthesis of peptaibols: Alamethicin via enzymatic segment condensation

We have used a combined chemical‐enzymatic approach to facilitate the total synthesis of the 20‐residue peptaibol, alamethicin. The 1–11 segment of alamethicin, having a C‐terminal Gly, and the 12–20 segment, having an N‐terminal Leu, were prepared by well‐established chemical methods, and then coupled using papain to afford a 54% yield of alamethicin in straightforward fashion. In contrast to the reported chemical syntheses of alamethicin requiring side‐chain protection at Glu, ¹⁸ the papain‐catalyzed coupling proceeded readily and selectively using a C‐terminal segment having a free γ‐carboxyl group at this position. Several alamethicin partial sequences were obtained via enzymatic formation of the Gly ¹¹−Leu¹² bond. The high efficiency of this route is illustrated by the enzymatic assembly of the 1–17 alamethicin fragment on a 400–mg scale in 62% yield. An alternative route to alamethicin through enzymatic formation of the Ala⁶−Gln⁷ bond was less successful because of a low yield in the final coupling. © 1992 John Wiley & Sons, Inc.