Solid-Phase Total Synthesis of Bacitracin A Jinho Lee and John H. Griffin* Department of Chemistry, Stanford University, Stanford, California 94305-5080 Thalia I. Nicas Infectious Disease Research, Lilly Research Laboratories, Lilly Corporate Center, Indianapolis, Indiana 46285 Received March 28, 1996 X An efficient solid-phase method for the total synthesis of bacitracin A is reported. This work was undertaken in order to provide a general means of probing the intriguing mode of action of the bacitracins and exploring their potential for use against emerging drug-resistant pathogens. The synthetic approach to bacitracin A involves three key features: (1) linkage to the solid support through the side chain of the L-asparaginyl residue at position 12 (L-Asn 12 ), (2) cyclization through amide bond formation between the R-carboxyl of L-Asn 12 and the side chain amino group of L-Lys 8 , and (3) postcyclization addition of the N-terminal thiazoline dipeptide as a single unit. To initiate the synthesis, Fmoc L-Asp(OH)-OAllyl was attached to a PAL resin. The chain of bacitracin A was elaborated in the C-to-N direction by sequential piperidine deprotection/HBTU-mediated coupling cycles with Fmoc D-Asp(OtBu)-OH, Fmoc L-His(Trt)-OH, Fmoc D-Phe-OH, Fmoc L-Ile-OH, Fmoc D-Orn(Boc)-OH, Fmoc L-Lys(Aloc)-OH, Fmoc L-Ile-OH, Fmoc D-Glu(OtBu)-OH, and Fmoc L-Leu- OH. The allyl ester and allyl carbamate protecting groups of L-Asn 12 and L-Lys 8 , respectively, were simultaneously and selectively removed by treating the peptide-resin with palladium tetrakis- (triphenylphosphine), acetic acid, and triethylamine. Cyclization was effected by PyBOP/HOBT under the pseudo high-dilution conditions afforded by attachment to the solid support. After removal of the N-terminal Fmoc group, the cyclized peptide was coupled with 2-[1′(S)-(tert-butyloxycarbo- nylamino)-2′(R)-methylbutyl]-4(R)-carboxy-∆ 2 -thiazoline (1). The synthetic peptide was deprotected and cleaved from the solid support under acidic conditions and then purified by reverse-phase HPLC. The synthetic material exhibited an ion in the FAB-MS at m/z 1422.7, consistent with the molecular weight calculated for the parent ion of bacitracin A (MH + ) C 73 H 84 N 10 O 23 Cl 2 , 1422.7 g/mol). It was also indistinguishable from authentic bacitracin A by high-field 1 H NMR and displayed antibacterial activity equal to that of the natural product, thus confirming its identity as bacitracin A. The overall yield for the solid-phase synthesis was 24%. Introduction Bacitracin A prototypifies the family of dodecapeptide lariat antibiotics produced nonribosomally by Bacillus subtilis and licheniformis. 1 These agents exhibit a novel, receptorlike mode of actionsin conjunction with a diva- lent metal ion, bacitracins bind to and sequester bacto- prenyl pyrophosphate, the lipid carrier of intermediates involved in cell wall biosynthesis. 2 Bacitracin is widely used as a component of topical antibacterial ointments and an additive in animal feeds, and it has recently been found that bacitracin eradicates intestinal colonization by vancomycin-resistant Enterococcus faecium. 3 Unfrac- tionated bacitracin is not suitable for systemic use, however, because some elements of the complex, difficult to separate naturally occurring mixture are nephrotoxic. 4 Solution-phase synthetic studies directed toward the bacitracins were carried out some time ago, 5 and a combination solid-phase/solution-phase total synthesis of the inactive, nephrotoxic component bacitracin F has been reported. 6 However, no total synthesis of a biologi- cally active bacitracin has been described. In order to probe the intriguing mode of action of the bacitracins and to further explore their potential for use against emerging drug-resistant pathogens, we undertook the development of synthetic methods for the preparation of biologically active bacitracins. In this communication, we report an efficient solid-phase method for the total synthesis of bacitracin A. Results and Discussion Advances with orthogonal protecting-group strategies, solid supports, and coupling reagents have led to the X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Discovery: (a) Johnson, B. A.; Anker, H.; Meleney, F. L. Science 1945, 102, 376-377. (b) Arriagada, A.; Savage, M. C.; Abraham, E. P.; Heatley, N. G.; Sharp, A. E. Br. J. Exp. Pathol. 1949, 30, 425-427. Structure: Ressler, C.; Kashelikar, D. V. J. Am. Chem. Soc. 1966, 88, 2025-2035. Biosynthesis: Ishihara, H.; Sasaki, T.; Shimura, K. Biochem. Biophys. Acta 1968, 166, 496-504. (2) (a) Weinberg, E. D. Antibiotic. Ann. 1958-1959, 924-929. (b) Alder, R. H.; Snoke, J. E. J. Bacteriol. 1962, 83, 1315-1317. (c) Stone, K. J.; Strominger, J. L. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 3223- 7. (d) Storm, D. R.; Strominger, J. L. J. Biol. Chem. 1973, 248, 3940- 5. (3) (a) O’Donovan, C. A.; Fan-Havard, P.; Tecson-Tumang, F. T.; Smith, S. M.; Eng, R. H. K. Diagn. Microbiol. Infect. Dis. 1994, 18, 105-109. (b) Eng, R. H. K.; Ng, K.; Smith, S. M. J. Antimicrob. Chemother. 1993, 31, 609-10. (c) Chia, J. K. S.; Nakata, M. M.; Park, S. S.; Lewis, R. P.; McKee, B. Clin. Infect. Dis. 1995, 21, 1520-1524. (4) Drapeau, G.; Petitclerc, E.; Toulouse, A.; Marceau, F. Antimicrob. Agents Chemother. 1992, 36, 955-961. (5) For leading references see: (a) Ariyoshi, Y.; Shiba, T.; Kaneko, T. Bull. Chem. Soc. Jpn. 1967, 40, 2648-2654. (b) Munekata, E.; Shiba, T.; Kaneko, T. Bull. Chem. Soc. Jpn. 1973, 46, 3835-3839. (6) Hirotsu, Y.; Nishiuchi, Y.; Shiba, T. Pept. Chem. 1978, 110, 171- 176. (7) (a) Trzeciak, A.; Bannwarth, W. Tetrahedron Lett. 1992, 32, 4557-4560. (b) Kates, S. A.; Sole ´, N. A.; Johnson, C. R.; Hudson, D.; Barany, G.; Albericio, F. Tetrahedron Lett. 1993, 34, 1549-1552. (c) Kates, S. A.; Sole ´, N. A.; Albericio, F.; Barany, G. In Peptides: Design, Synthesis, and Biological Activity; Basava, C.; Anantharamaiah, G. M., Eds.; Birkha ¨ user: New York, 1994; pp 30-59. 3983 J. Org. Chem. 1996, 61, 3983-3986 S0022-3263(96)00580-4 CCC: $12.00 © 1996 American Chemical Society