Design and synthesis of pyrankacin: A pyranmycin class of broad-spectrum aminoglycoside antibiotic

Article abstract of DOI:10.1021/ol0529750

A novel broad-spectrum aminoglycoside antibiotic, pyrankacin, has been prepared. In addition to the synthetic innovation in dideoxygenation and regioselective Staudinger reduction, we have obtained prominent antibacterial activity against several clinical

Full text of DOI:10.1021/ol0529750

ORGANIC
LETTERS
2006
Vol. 8, No. 5
887-889
Design and Synthesis of Pyrankacin:
A Pyranmycin Class of Broad-Spectrum
Aminoglycoside Antibiotic
Ravi Rai, Hsiao-Nung Chen, Przemyslaw G. Czyryca, Jie Li, and
Cheng-Wei Tom Chang*
Department of Chemistry and Biochemistry, Utah State UniVersity,
0300 Old Main Hill, Logan, Utah 84322-0300
chang@cc.usu.edu
Received December 8, 2005
ABSTRACT
A novel broad-spectrum aminoglycoside antibiotic, pyrankacin, has been prepared. In addition to the synthetic innovation in dideoxygenation
and regioselective Staudinger reduction, we have obtained prominent antibacterial activity against several clinically important pathogens in
the course of this work.
Aminoglycosides have long been antibiotics of choice for
their impressive antibacterial activity (Figure 1).1 These
bactericidal substances exert their antibacterial effects by
interfering with the protein synthesis at the prokaryotic
ribosomal RNA level.2 Although alterations to the ribosomal
binding site3 and decreased permeability4 into the cells have
been implicated as modes of resistance toward aminogly-
cosides, their broad-spectrum activity has, in the main, been
compromised by the prevalence of aminoglycoside modifying
enzymes in resistant strains of certain pathogenic bacteria.
For some time now, our group has been working on the
synthesis of novel aminoglycosides (Figure 1), and we
recently came up with a neomycin class of aminoglycoside,
called pyranmycin,5 and identified a lead structure TC005.
We then synthesized the 3′,4′-dideoxypyranmycin analogue,
RR501, which had impressive antibacterial activity against
bacteria that harbor one modifying enzyme, APH(3′)-I.6
(1) Haddad, J.; Kotra, L. P.; Mobashery, S. In Glycochemistry Principles,
Synthesis, and Applications; Wang, P. G., Bertozzi, C. R., Eds.; Marcel
Dekker: New York, 2001, pp 307-424.
Figure 1. Structure of kanamycin and neomycin classes of
(2) Vakulenko, S. B.; Mobashery, S. Clin. Microbiol. ReV. 2003, 16,
aminoglycosides.
430-450.
(3) Eliopoulos, G. M.; Farber, B. F.; Murray, B. E.; Wennersten, C.;
Moellering, R. C. Antimicrob. Agents Chemother. 1984, 25, 398-405.
(4) Damper, P. D.; Epstein, W. Antimicrob. Agents Chemother. 1981,
Although TC005 and RR501 are both not as active as
20, 803-812.
neomycin against aminoglycoside susceptible bacteria, they
10.1021/ol0529750 CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/10/2006

have superior acid stability. In an attempt to further improve
the activity of this dideoxy analogue, we have been dedicat-
ing our synthetic efforts toward attaching the (S)-2-hydroxy-
4-aminobutyl (AHB side chain) at the N-1 position. Ami-
kacin, a kanamycin derivative with AHB at N-1, has very
impressive activity against resistant bacteria.7 Therefore, it
is expected that the pyranmycin analogue with AHB at N-1
will display similar or even improved activity against
resistant bacteria while maintaining the advantageous acid
stability.
The synthesis of pyrankacin started from the chloroben-
zoylation of 26 to yield 3, which was then subjected to a
selective Staudinger reaction to yield the N-1 Boc-protected
compound 4 (Scheme 1). Interestingly, the obtained selectiv-
Scheme 1. Synthesis of Pyrankacin
There are only a few examples of an N-1 modified
neomycin class of aminoglycosides,8 and the synthetic
strategies to such molecules are not suitable for modifying
tetraazidoneamine, the key intermediate compound we have
employed for our aminoglycoside synthesis. We have
recently developed a novel method to selectively reduce the
N-1 azido group of the 3′,4′-di-O-benzoyltetraazidoneamine,
1, by tuning the stereoelectronic environment of the azido
groups.9 Nevertheless, we still have to extend the same
methodology to dideoxyneamine 2.6 It has been shown that
an electron-deficient azido group has greater reactivity toward
the Staudinger reduction than an electron-rich one.10 The
presence of the double bond in 2 could perturb the stereo-
electronic environment of 2′-N3 and prevent the desired
selective Staudinger reduction from occurring at 1-N3. The
higher chemical shift of H-2′ in 2 (3.92) as compared to 1
(3.62) confirms our speculation (Table 1). Fortuitously, by
Table 1. Proton Chemical Shifts (ppm) on Acylated Neamine
Derivatives
compounds
H-1
H-3
H-2′
H-6′
1
2
3
4
3.28
3.30
3.75
4.20
3.43
3.38
3.61
3.65
3.62
3.92
3.38
3.48
3.57/3.41
3.45/3.27
3.40/3.24
3.48/3.30
ity was even better than when 5,6-di-O-acyl-3′,4′-di-O-
benzoyltetraazidoneamine was employed.9 Hydrolysis of the
ester protecting groups followed by selective benzoylation
at the O-6 position gave 6. Glycosylation of 6 with 75
followed by the hydrolysis of the acyl groups offered the
corresponding trisaccharide, 10. Deprotection of the Boc
group and coupling with the (S)-N-carbobenzyloxy-4-amino-
2-hydroxybutyric acid yielded 10. Global deprotection and
ion exchange provided the desired final product, which we
named pyrankacin.
using the 4-chlorobenzoyl group on the O-5 and O-6, as in
the case of 3, the needed stereoelectronic effect can still be
obtained with the N-1 (H-1) being the most reactive
(electron-deficient) one. Upon further investigation, we think
the observed selectivity of the Staudinger reaction is
governed by a combination of both steric and stereoelectronic
effects.11
Pyrankacin was assayed against various strains of bacteria,
and the minimum inhibitory concentration (MIC) was
determined using amikacin, neomycin, butirosin, gentamicin,
and kanamycin as the controls (Table 2). Aminoglycoside
susceptible Escherichia coli (ATCC 25922), Staphylococcus
aureus (ATCC 25923), and Klebsiella pneumoniae (ATCC
13883, resistant to ampicillin, susceptible to aminoglycosides)
were used as standard reference strains. E. coli (pSF815)
(5) (a) Chang, C.-W. T.; Hui, Y.; Elchert, B.; Wang, J.; Li, J.; Rai, R.
Org. Lett. 2002, 4, 4603-4606. (b)Elchert, B.; Li, J.; Wang, J.; Hui, Y.;
Rai, R.; Ptak, R.; Ward, P.; Takemoto, J. Y.; Bensaci, M.; Chang, C.-W.
T. J. Org. Chem. 2004, 69, 1513-1523.
(6) Rai, R.; Chen, H.-N.; Chang, C.-W. T.; J. Carbohydr. Chem. 2005,
24, 131-143.
(7) Kawaguchi, H.; Naito, T.; Nakagawa, S.; Fujisawa, K. J. J. Antibiot.
1972, 25, 695-698.
(8) (a) Woo, P. W. K.; Haskell, T. H. J. Antibiot. 1982, 35, 692-702.
(b) Umezawa, S.; Tsuchiya, T.; Torii, T. J. Antibiot. 1982, 35, 58-61. (c)
Takagi, Y.; Komuro, C.; Tsuchiya, T.; Umezawa, S.; Hamada, M.;
Umezawa, H. J. Antibiot. 1981, 34, 1-4.
(9) Li, J.; Chen, H.-N.; Chang, H.; Wang, J.; Chang, C.-W. T. Org. Lett.
2005, 7, 3061-3064.
(10) (a) Nyffeler, P. T.; Liang, C.-H.; Koeller, K. M.; Wong, C.-H. J.
Am. Chem. Soc. 2002, 124, 10773-10778. (b) Ariza, X.; Vrpi, F.;
Viladomat, C.; Vilarrasa, J. Tetrahedron Lett. 1998, 39, 9101-9102.
(11) We have recently completed the synthesis of several azidoneamine
analogues and studied the effect of various protecting groups on the
regioselectivity of Staudinger reduction. The obtained results suggest that
the regioselectivity is affected by both steric and stereoelectronic factors.
Li, J.; Chang, C.-W. T. Unpublished result.
888
Org. Lett., Vol. 8, No. 5, 2006

Table 2. Minimum Inhibitory Concentrations (MIC)a
entry
strains
E. colib
amikacin butirosin gentamicin neomycin ribostamycin kanamycin B pyrankacin RR501 JT005
1
2
3
4
5
6
7
8
9
1
2
2
4
8
2
4
4
1
1
1
2
8
2
2
ND
8
ND
E. coli (TG1)c
E. coli (pSF815)d
E. coli (pTZ19U-3)e
K. pneumoniaef
K. pneumoniaeg
S. aureush
1
1
1
0.25
0.5
2
8
2
2
16
32
inactive
4
4
4
4
4
1-2
2
inactivek
inactive
inactive
inactive
1
inactive
1-2
4
4
2-4
2-4
4
0.5
0.5-1
1
16
1
1
inactive
inactive
2
inactive
1
0.5
8-16
1
4
0.5-1
inactive
2
inactive
8
inactive
ND
S. aureusi
P. aeruginosaj
0.5
ND
0.5-1
inactive
0.5-1
inactive
inactive
inactive
inactive inactive
a Unit: µg/mL. ND: not determined. b Escherichia coli (ATCC 25922). c E. coli (TG1) (aminoglycoside susceptible strain). d E. coli (TG1) (pSF815
plasmid encoded for (AAC(6′)/APH(2”)). e E. coli (TG1) (pTZ19U-3 plasmid encoded for APH(3′)-I). f Klebsiella pneumoniae (ATCC 700603). g K. pneumoniae
(ATCC 13883). h Staphylococcus aureus (ATCC 33591) (MRSA). i S. aureus (ATCC 25923). j Pseudomonas aeruginosa (ATCC 27853). k Inactive is defined
as MIC g 32 µg/mL.
and E. coli (pTZ19U-3) are laboratory resistant strains using
E. coli (TG1) as the host. K. pneumoniae (ATCC 700603)12
is a clinical isolate that is resistant to ceftazidime, other
â-lactams, and several aminoglycosides (ANT(2′′)). Pseudo-
monas aeruginosa (ATCC 27853) that expresses APH(3′)-
IIb manifests modest resistance toward aminoglycosides.13
Methicillin-resistant S. aureus (ATCC 33591) (MRSA) is
the leading cause of bacterial infections and a global scourge.
Many MRSA strains contain genes encoded for APH(3′),
ANT(4′), and AAC(6′)/APH(2′′), which render the bacteria
resistant to many aminoglycosides.14
From the MIC values, pyrankacin appears to be the one
with the most prominent broad-spectrum antibacterial activity
against all the examined strains. For example, for the
clinically used gentamicin and amikacin, the former is
ineffective against bacteria with the bifunctional enzyme,
AAC(6′)/APH(2′′) and K. pneumoniae (ATCC 700603)
(entries 3 and 5) whereas the latter is less active against
MRSA (entry 7). Pyrankacin is more active than gentamicin
against E. coli (pSF815) and K. pneumoniae (ATCC 700603)
(entries 3 and 5). While being less active than gentamicin
against MRSA, pyrankacin is more active than amikacin
against the same strain. More interestingly, even pyrankacin
can be viewed as a neomycin class of aminoglycoside; it is
the only active compound against P. aeruginosa among
JT005,9 neomycin, butirosin, and ribostamycin. The attach-
ment of the AHB group at N-1 of the kanamycin class of
aminoglycoside as in the case of amikacin revives the
antibacterial activity, whereas the same modification on
butirosin and JT005 does not produce the same effect. This
result suggests that a combination of 3′,4′-dideoxygenation
and the N-1 AHB group is essential for the neomycin class
of aminoglycoside to be active against P. aeruginosa.
Molecular modeling of pyrankacin bound to the targeted
rRNA did not reveal significant conformational alteration
on ring III, and rings I and II and the AHB are almost
identical to the reference compound.15 The AHB group
encounters steric hindrance in the active sites of APH(3′)-
III and AAC(2′). Interestingly, when pyrankacin is docked
in the binding site of AAC(2′), the 6′′-CH3 of ring III that is
unique to pyrankacin is pointing toward a hydrophilic area
thus creating unfavorable interaction. This result may, in part,
explain the difference in activity against MRSA and P.
aeruginosa.
In conclusion, we have not only designed and synthesized
a novel dideoxypyranmycin with the AHB side chain but
also been able to devise a novel regioselective Staudinger
reaction that can be performed in the presence of a double
bond. Pyrankacin maintains the acid stability while displaying
an impressive broad-spectrum antibacterial activity against
several clinically important pathogens. This novel aminogly-
coside can serve as the lead for further synthetic modification,
pave the way to the development of a new generation of
antibiotics, and tilt the fight against bacterial infections. For
example, our work on pyrankacin has yielded the result that
the combined modifications of 3′,4′-dideoxygenation and
attaching the AHB group at N-1 are pivotal in reviving the
antibacterial activity against MRSA. We are currently
working on both the neomycin and kanamycin classes of
aminoglycosides bearing the combination of 3′,4′-dideoxy-
genation and the N-1 AHB group.
Acknowledgment. We acknowledge the National Insti-
tute of Health (AI053138) for financial support. We thank
Prof. Mobashery from Notre Dame University for providing
the pTZ19U-3 and pSF815 plasmids.
Supporting Information Available: Experimental pro-
1
cedures for the preparation of compounds, H, 13C, and
COSY spectra of selected compounds, the antibacterial assay,
and the molecular modeling results. This material is available
free of charge via the Internet at http://pubs.acs.org.
(12) Rasheed, J. K.; Anderson, G. J.; Yigit, H.; Queenan, A. M.;
Domenech-Sanchez, A.; Swenson, J. M.; Biddle, J. W.; Ferraro, M. J.;
Jacoby, G. A.; Tenover, F. C. Antimicrob. Agents Chemother. 2000, 44,
2382-2388.
(13) Hachiler, H.; Santanam, P.; Kayser, F. H. Antimicrob. Agents
Chemother. 1996, 40, 1254-1256.
(14) Ida, T.; Okamoto, R.; Shimauchi, C.; Okubo, T.; Kuga, A.; Inouqe,
M. J. Clin. Microbiol. 2001, 39, 3115-3121.
OL0529750
(15) Please refer to Supporting Information for more details.
Org. Lett., Vol. 8, No. 5, 2006
889