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Changes to the dienone region of myxopyronin B
References and notes
revealed several interesting features required for the
activity of this class of compounds (Table 1, entries
2–5 and 16). The geometrical isomer of myxopyronin
B, 1e, showed a threefold decrease in activity in the
RNAP enzymatic assay and a 10-fold decrease in
potency in the growth inhibition test.
1. Cragg, G. M.; Newman, D. J.; Snader, K. M. J. Nat. Prod.
1997, 60, 52–60.
2. (a) Kohl, W.; Irschik, H.; Reichenback, H.; Ho¨fle, G.
Liebigs Ann. Chem. 1983, 1656–1667; (b) Kohl, W.;
Irschik, H.; Reichenback, H.; Ho¨fle, G. Liebigs Ann.
Chem. 1984, 1088–1093; (c) Jansen, R.; Irschik, H.;
Reichenback, H.; Ho¨fle, G. Liebigs Ann. Chem. 1985,
822–836.
Although compound 1d, lacking the methyl d to the carb-
onyl group, showed similar activity against RNAP in
the biochemical assay and toward E. coli (Tol C), the
activity against S. aureus decreased significantly.
Removing the methyl adjacent to the carbonyl group
(1c) resulted in an almost complete loss of antibacterial
activity. Replacement of the dienone with an amide
functionality as in 33 or complete absence of the diene
moiety, as in 9, resulted in a loss of antibacterial
potency, as well as the enzymatic activity. The enecarba-
mate component of myxopyronin B was even more sen-
sitive toward changes (Table 1, entries 6–14). Extending
the carbamate with solubilizing groups (12 and 13),
replacement of the carbamate with an ester (8b) and
the incorporation of the carbamate alkene into an aro-
matic moiety (26–27) reduced the enzymatic activity
and resulted in a complete loss of potency against all
three bacterial strains. Aldehyde 18, which is a potential
degradation product of myxopyronin B, lacked potency
and was therefore dismissed as an RNAP inhibitor.
3. Irschik, H.; Gerth, K.; Ho¨fle, G.; Kohl, W.; Reichenback,
H. J. Antibiot. 1983, 36, 1651–1658.
4. (a) Campbell, E. A.; Korzheva, A.; Mustaev, A.; Mura-
kami, K.; Nair, S.; Goldfarb, A.; Darst, S. Cell 2001, 104,
901–912; (b) For other inhibitors of RNAP see: Artsi-
movitch, I.; Chu, C.; Lynch, A. S.; Landick, R. Science
2003, 302, 650–654.
5. Hu, T.; Schaus, V.; Lam, K.; Palfreyman, M. G.;
Wuonola, M.; Gustafso, G.; Panek, J. S. J. Org. Chem.
1998, 63, 2401–2406.
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N.; Dolak, L. A.; McGrath, J. P.; Lynn, J. C.; Horng,
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R. J.; Shiou, L.; Possert, P. L.; Rush, B. D.; Wilkonson,
K. F.; Howard, G. M.; Toth, L. N.; Williams, M. G.;
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P. A. J. Med. Chem. 1995, 38, 4968–4971.
7. Cook, L.; Ternai, B.; Ghosh, P. J. Med. Chem. 1987, 30,
1017–1023.
The cis analog, 10, displayed comparable IC50 value as
the parent trans compound with a small decrease in
activity toward E. coli and S. aureus. The saturated
carbamate analog 19 displayed comparable activity
against RNAP, but a significant decrease in MIC was
observed. Methylation of the pyrone skeleton (11)
resulted in fivefold decrease toward RNAP and com-
plete loss of cellular activity, indicating the importance
of the free hydroxyl group for biological activity.
8. Groutas, W. C.; Stanga, M. A.; Brubaker, M. J.; Huang,
T. L.; Moi, M. K.; Carroll, R. T. J. Med. Chem. 1985, 28,
1106–1109.
9. (a) Aldehyde 5 was prepared from methyl 2-butynoate
immediately prior to addition, in a three-step reaction
sequence: (i) n-BuLi, CuI, methyl 2-butynoate; (ii) LiAlH4;
(iii) TPAP, NMO. See: Anderson, R. J.; Corbin, V. L.;
Cotterrell, G.; Cox, G. R.; Henrick, C. A.; Schaub, F.;
Siddall, J. B. J. Am. Chem. Soc. 1975, 97, 1197–1204; (b)
Goura, K.; Nishino, T.; Koyama, T.; Seto, S. J. Am.
Chem. Soc. 1970, 92, 1197–1204.
The desmethyl analog, 1a, is the most active analog
synthesized in this series. The activity against RNAP
was improved threefold, without any increase in the cel-
lular potency. This result shows that it is possible to
remove the methyl group and hence the chirality from
this class of compounds without diminishing the biolog-
ical activity. Thus, the preparation of the next genera-
tion of myxopyronin B analogs, was designed without
the chiral center. The synthesis and biological evaluation
on the second-generation analogs will be reported
elsewhere.
10. Satisfactory spectroscopic data were obtained for all
newcompounds and all final analogs were purified by
reverse phase HPLC and characterized by 1H NMR and LC–
MS. HPLC (MeOH/H2O/AcOH = 70:30:4, flowrate =
40mL/min) tr = 2.6min (1a, minor), tr = 3.9min (1a, major).
11. 6-Ethyl-4-hydroxy-2-pyrone was prepared from 3-oxo-
pentanoic acid and meldrumÕs acid using similar condi-
tions as described for the preparation of other 2-pyrones,
see: Lokot, I. P.; Pashkovsky, F. S.; Lakhvich, F. A.
Tetrahedron 1999, 55, 4783–4792.
12. Poleschner, H.; Heydenreich, M.; Dietemr, M. Synthesis
1991, 1231–1235.
13. Hu, T. Q.; Weiler, L. Can. J. Chem. 1994, 72, 1500–1511.
14. Cumming, J. N.; Wang, D.; Park, S. B.; Shapiro, T. A.;
Posner, G. H. J. Med. Chem. 1998, 41, 952–964.
15. Gorins, G.; Kuhnert, L.; Johnson, C. R.; Marnett, L. J.
J. Med. Chem. 1996, 39, 4871–4878.
16. Sen, S. E.; Roach, S. L. J. Org. Chem. 1998, 61, 6646–
6650.
17. Molho, D. U.S. Patent 3,122,557, 1964.
18. Lee, B. H.; Clothier, M. F.; Dutton, F. E.; Conder, G. A.;
Johnson, S. S. Bioorg. Med. Chem. Lett. 1998, 8, 3317–
3320.
19. Johnson, J. C.; Shanoff, M.; Boezi, J. A.; Hansen, R. G.
Anal. Biochem. 1968, 26, 137–145. The cited assay was
adapted to 96-well plates to obtain reasonable throughput.
In conclusion, the SAR generated from this study
showed that minor structural changes to myxopyronin
B led to loss of biological activity. At the present time
natural product within this class do not represent viable
lead candidates for antibacterial therapy until a better
understanding of the exact mode of action is
determined.
Acknowledgement
We thank Dr. K. Steffy for the cytotoxicity studies.