The widespread emergence of drug-resistant bacteria threatens to return humans to a pre-antibiotic era where common infections are lethal.1 Two million people in the US are infected annually with antibiotic-resistant bacteria, resulting in 23,000 deaths, at an economic cost in excess of USD 55 billion/year.2 Private investment in antibiotic development has diminished due to demanding regulatory hurdles and poor economic incentives.3 Pleuromutilins are a class of antibiotics that inhibit the growth of primarily Gram-positive bacteria4 by binding the peptidyl transferase center of the bacterial ribosome via an induced-fit mechanism.5 Most pleuromutilins tested have a low mutational frequency (10–9–10–11;6 by comparison the mutational frequency of S. aureus to rifampin is 10–7), leading to low rates of resistance development. The semisynthetic derivative retapamulin (3) was approved for use in humans in 2007, and as of 2014 clinical resistance to retapamulin (3) had not been recorded.7 Consistent with this, pleuromutilins display minimal cross-resistance with existing antibiotics.6 In addition to the clinical agent retapamulin (3), tiamulin and valnemulin (4, 5, respectively) are C14 analogs that have been in veterinary use since the late '80s. Lefamulin (6) has advanced to Phase III trials for the treatment of community-acquired bacterial pneumonia and acute bacterial skin and skin structure infections (ABSSIs).6

The C14 glycolic acid ester has served as the focal point for optimization, and thousands of C14 analogs have been prepared by semisynthesis. Evidence suggests there is substantial potential for further optimization, particularly within the hydrophobic tricyclic core. For example, epimerization of the C12 quaternary position, followed by functionalization of the transposed alkene, provides extended spectrum pleuromutilins with activity against Gram-negative and drug-resistant pathogens.8 In all, pleuromutilins inhibit the three bacterial strains identified as urgent threats by the CDC: Clostridium difficile, carbapenem-resistant Enterobacteriaceae (CRE), and drug-resistant Neisseria gonorrhoeae.

We have developed a convergent and enantioselective synthesis of pleuromutilins, including 12-epi-mutilins, that is amenable to the production of novel antibiotics.9 As illustrated below, our strategy features the convergent union of a strained lactam (7) and a conjunctive iodoether (8).

pleuro retro.jpg

We envisioned beginning our synthesis of 7 with an asymmetric conjugate addition–C-acylation reaction of cyclohexenone (9). To achieve this, we merged Feringa’s asymmetric conjugate addition technology10 with the reactivity of zincate enolates. Asymmetric 1,4-addition of dimethylzinc to cyclohexenone afforded a zinc enolate, which after conversion to the corresponding zincate enolate 10 with methyl lithium, underwent site-selectivite C-acylation.11 Diastereoselective methylation of the resulting β-ketoester 11 then formed the vicinally disubstituted β-dicarbonyl 12. 12 was converted to cyclopentenone 15 by triflation of the ketone, carbonylative vinylation, and Nazarov electrocyclization. Next, 1,4-cyanation was achieved with 3:1 stereoselectivity at C9 and inverted stereochemistry at C4. However, treatment of the unpurifed material with dilute base resulted in epimerization of the C4 stereocenter to yield the cis-hydrindanone 17. Ketone protection followed by a cascade reaction addition–cyclization then provided the strained lactam 7. This cascade likely comprises addition of the methyl lithium to the nitrile, cyclization of the resulting lithio-enamine onto the methyl ester, deprotonation of the acyl-imine, and N-acylation, as shown.

pleuro syn 1.jpg

Synthesis of the conjunctive iodoether 8 was accomplished by diastereoselective α–alkylation of a tiglic acid derivative (21), reduction, and iodo-deoxygenation. This fragment features the inverted stereochemistry at C12 to access 12-epi-mutilins.

The key fragment coupling step involved addition of the organolithium reagent derived from 8 to the strained lactam 7. Hydrolysis of the resulting lithio-enamine provided the methyl ketone 23. The ketone was converted to an alkyne via base-induced elimination of a transient vinyl triflate. Next, the para-methoxybenzylether was deprotected and oxidized to the corresponding aldehyde 25. We envisioned forging the macrocycle of the targets via reductive cyclization of the alkynyl aldehyde. The structural rigidity of the 5,6-bicyclic ring system and the presence of sp2 centers at C10 and C14 of the allylic alcohol product 26 were thought to reduce the entropic and enthalpic penalties of ring closure. After extensive experimentation with various transition metals catalysts, we found that treatment of the alkynyl aldehyde with Ni(cod)2, the N-heterocyclic carbene ligand IPr, and stoichiomectric triethylsilane resulted in smooth reductive cyclization to afford, after desilylation, the allylic alcohol 26. The allylic alcohol 26 was next subjected a high-yielding redox isomerization to afford diketone 27, the structure of which was confirmed by X-ray analysis. Reduction and deprotection provided (+)-12-epi-mutilin (28) and (±)-11,12-di-epi-mutilin (29).

Finally, site-selective acylations and deprotections enabled us to convert these mutilin-derivatives to 12-epi-pleuromutilin (30) and 11,12-di-epi-pleuromutilin (33) respectively. Additionally, the C12-quaternary center was epimerized using the diethylzinc-promoted retroallylation-allylation reaction developed by Berner12 to afford (+)-pleuromutilin (1) itself.

With a viable and convergent strategy to access the pleuromutilin skeleton, we are in position to investigate the preparation of antibiotic candidates that have modifications to the core (substituents, atomic substitution) that complement the derivatives prepared in earlier semi-synthetic approaches.

References.

  1. Martens, E.; Demain, A. L. "The antibiotic resistance crisis, with a focus on the United States." J. Antibiot. 2017, 70, 520.
  2. (a) Hampton, T. "Report reveals scope of us antibiotic resistance threat." JAMA, J. Am. Med. Assoc. 2013, 310, 1661. (b) CDC. Antibiotic Resistance Threats in the United States, 2013. Atlanta, GA: U.S. Department of Health and Human Services, CDC, 2013.
  3. Silver, L. L. "Challenges of antibacterial discovery." _Clin. Microbiol. Rev. 2011, 24, 71.
  4. Isolation and structure determination: (a) Kavanagh, F.; Hervey, A.; Robbins, W. J. "Antibiotic substances from Basidiomycetes. VIII. Pleurotus multilus (fr.) Sacc. and Pleurotus Passeckerianus Pilat." _Proc. Natl. Acad. Sci. U. S. A. 1951, 37, 570. (b) Kavanagh, F.; Hervey, A.; Robbins, W. J. "Antibiotic substances from Basidiomycetes: IX. Drosophila Subtarata. (Batsch Ex Fr.) Quel." _Proc. Natl. Acad. Sci. U. S. A. 1952, 38, 555. (c) Birch, A. J.; Holzapfel, C. W.; Rickards, R. W. "Structure and some aspects of the biosynthesis of pleuromutilin." Tetrahedron 1966, 22, 359. (d) Arigoni, D. "Some studies in the biosynthesis of terpenes and related compounds." Pure Appl. Chem. 1968, 17, 331. For a recent review, see: (e) Fazakerley, N. J.; Procter, D. J. "Synthesis and synthetic chemistry of pleuromutilin." Tetrahedron 2014, 70, 6911.
  5. Davidovich, C.; Bashan, A.; Auerbach-Nevo, T.; Yaggie, R. D.; Gontarek, R. R.; Yonath, A. "Induced-fit tightens pleuromutilins binding to ribosomes and remote interactions enable their selectivity." Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 4291.
  6. Paukner, S.; Riedl, R. "Pleuromutilins: Potent drugs for resistant bugs—mode of action and resistance." Cold Spring Harbor Perspect. Med. 2017, 7, a027110.
  7. Walsh, C. T.; Wencewicz, T. A. "Prospects for new antibiotics: A molecule-centered perspective." J. Antibiot. 2014, 67, 7.
  8. Wicha, W. W.; Paukner, S.; Strickmann, D. B.; Thirring, K.; Kollmann, H.; Heilmayer, W.; Ivezic-Schoenfeld, Z. In Efficacy of Novel Extended Spectrum Pleuromutilins Against E. coli In Vitro and In Vivo, 25th European Congress of Clinical Microbiology and Infectious Diseases, Copenhagen, Denmark, April 25-28, 2015.
  9. Murphy, S. K.; Zeng, M.; Herzon, S. B. "A modular and enantioselective synthesis of the pleuromutilin antibiotics." Science 2017, 356, 956.
  10. Feringa, B. L.; Pineschi, M.; Arnold, L. A.; Imbos, R.; de Vries, A. H. M. "Highly enantioselective catalytic conjugate addition and tandem conjugate addition–aldol reactions of organozinc reagents." Angew. Chem., Int. Ed. 1997, 36, 2620.
  11. Murphy, S. K.; Zeng, M.; Herzon, S. B. "Stereoselective multicomponent reactions using zincate nucleophiles: β-Dicarbonyl synthesis and functionalization." Org. Lett. 2016, 18, 4880.
  12. Berner, H.; Vyplel, H.; Schulz, G.; Schneider, H. "Inversion of configuration of the vinylgroup at carbon 12 by reversible retro-en-cleavage." Monatsh. Chem. 1986, 117, 1073.