Synthesis and biological activity of new 5-O-sugar modified ketolide and 2-fluoro-ketolide antibiotics

Article abstract of DOI:10.1016/j.bmcl.2005.01.027

A series of new triazole-containing ketolides and 2-fluoro-ketolides in which the 5-O-desosamine was replaced by unnatural sugars were synthesized and evaluated against relevant macrolide-sensitive and macrolide-resistant respiratory pathogens. Excellent

Full text of DOI:10.1016/j.bmcl.2005.01.027

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1307–1310
Synthesis and biological activity of new 5-O-sugar modified
ketolide and 2-fluoro-ketolide antibiotics
Chang-Hsing Liang, Sulan Yao, Yu-Hung Chiu, Po Yee Leung, Nicole Robert,
Jaime Seddon, Pam Sears, Chan-Kou Hwang, Yoshi Ichikawa and Alex Romero*
Department of Chemistry and Department of Biology, Optimer Pharmaceuticals, Inc., 10110 Sorrento Valley Road, Suite C,
San Diego, CA 92121, United States
Received 22 October 2004; revised 11 January 2005; accepted 13 January 2005
Abstract—A series of new triazole-containing ketolides and 2-fluoro-ketolides in which the 5-O-desosamine was replaced by unnat-
ural sugars were synthesized and evaluated against relevant macrolide-sensitive and macrolide-resistant respiratory pathogens.
Excellent in vitro antibacterial activities were demonstrated for ketolide analogues having the 60-OBz-30-dimethylamino-glucose
and 60-OBz-40-deoxy-30-dimethylamino-glucose substituents.
Ó 2005 Elsevier Ltd. All rights reserved.
Ketolide antibiotics, such as telithromycin (1) and
cethromycin (2) (Fig. 1), are a newer generation of mac-
rolide antibiotics that show excellent activity against
specific macrolide-resistant organisms, such as inducible
macrolide, lincosamide, and streptogramin B phenotype
(MLSB-inducible) and pathogens having efflux (mef)
resistant mechanisms.1 This improved activity, over
their macrolide counterparts, has been attributed to
the 3-keto group, 11,12-cyclic carbamate, and the teth-
ered hetero-aromatic substituent. However, cross-resis-
tance due to constitutive expression of erm gene
(methylation of the 23S rRNA) still remains an unsolved
problem for the efficacy of ketolides and macrolide
antibiotics.2
The 5-O-desosamine substituent plays an important role
in this ubiquitous erm-resistance mechanism and has
been considered critical for the overall antibacterial
activity of the macrolide and ketolide class of antibiot-
ics.3 One possible approach to overcome bacterial erm-
resistance is to replace the resistance causing 5-O-desos-
amine with a optimized sugar scaffold having the poten-
tial for increased binding to the resistant bacterial
ribosome. Up until our previous report4 and now with
our current disclosure, there have been only few reports
describing the removal or modification of this sugar.1,2,5
The limited SAR investigating the role of the 5-O-deso-
amine substituent is partly due to both the lack of effi-
cient methods for the removal of this sugar and the
lack of appropriate and efficient techniques for glycosyl-
ating acid sensitive macrolide and ketolide aglycons.
N
N
N
N
HO
NMe2
HO
NMe2
O
O
O
H
N
O
O
N
O
O
O
O
O
O
O
O
O
O
O
Desosamine
Desosamine
O
O
Telithromycin (1)
Cethromycin (2)
We have recently described an efficient synthetic proce-
dure for preparing ketolide aglycon 4 and its subsequent
utilization in glycosylation and triazole-forming reac-
tions.4 The current communication will highlight the
syntheses of other triazole containing 5-O-glycosylated
ketolide analogues, including novel triazole-containing
5-O-glycosylated-2-fluoro-ketolides derived from the
Figure 1.
Keywords: Glycosylated; Ketolide; 2-Fluoro-ketolide; Macrolide
resistance.
*
Corresponding author. Tel.: +1 858 909 0736; fax: +1 858 909 0737;
e-mail: aromero@optimerpharma.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.01.027

1308
C.-H. Liang et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1307–1310
new fluoro-ketolide aglycon 5, and will present the anti-
microbial activities against relevant respiratory patho-
gens for this novel class of ketolide antibiotics.
OBz
NHFmoc
OBz
NHFmoc
OBz
BzO
TolS
BzO
TolS
BzO
TolS
OBz
O
O
O
17a
17b
17c
NHFmoc
OBz
OBz
NHFmoc
Scheme 1 describes the syntheses of the ketolide aglycon
4 (condition a) and 2-fluoro-ketolide aglycon 5 (condi-
tion b), while their subsequent glycosylation, deprotec-
tion, and triazole-forming reactions are showcased in
Scheme 2. Starting with compound 3, ketolide aglycon
4 was obtained in 54% yield after methanolysis and sub-
sequent one-step Swern oxidation–hydrolysis (Scheme 1,
condition a).4 Similarly, the new 2-fluoro-ketolide agly-
con 5 was obtained in 59% overall yield after fluorina-
tion using N-fluorobenzenesulfonimide followed by
one-step Swern oxidation–hydrolysis procedure (condi-
tion b).
BzO
TolS
BzO
TolS
OBz
O
O
17d
17e
Figure 2. Examples of p-tolyl-thio glycosides utilized in the glycosyl-
ations of ketolide aglycons.
requisite N,N-dimethylamino group in the presence of
the tethered azide group contained in the ketolide cores.
Ketolide aglycon 4 was successfully glycosylated with
appropriately protected p-tolyl-thio glycosides 17a–e6
(Fig. 2), under mild conditions (NIS/AgOTf/di-tert-but-
yl-pyridine) affording the glycosylated products 6a–e
(X = H) in yields ranging from 60% to 70% (Scheme
2).4 Similar yields were obtained for ketolides 7d and
7e derived from the glycosylation of 2-fluoro-ketolide
aglycon 5 with p-tolyl-thio glycosides 17d and 17e. The
desired b-stereoisomer was predominantly obtained
(>8:1) from each of the glycosylation reactions due to
neighboring group participation of the 2-OBz group of
the designed p-tolyl-thio glycosides. The deprotection
sequences for the newly constructed glycosylated keto-
lides were as follows (Scheme 2): In cases of where the
monosaccharide contained a protected amine (NHF-
moc), the Fmoc-protecting group was removed with
10% piperidine in DMF and reductive amination was
accomplished with NaBH(OAc)3 and formaldehyde in
THF to give the requisite N,N-dimethylamine substitu-
ent. Glycosylated products having 20- and/or 40-OBz
groups were selectively removed by heating in MeOH.
Glycosylated products having additional OBz groups
(i.e., 60-OBz or 30-OBz) could be deprotected, if desired,
by reaction with LiOH in MeOH. For each of the gly-
cosylated-deprotected ketolides, the azide group was
converted in excellent yields (>90%) to a 4-substituted-
[1,2,3]-triazole as a single regio-isomer by reaction with
the corresponding acetylenes to afford ketolides 8–13
(R1 = 2-pyridyl) and 2-fluoro-ketolides 14–16 (R1 = 3-
NH2–Ph).4,7
The p-tolyl-thio glycosides that were employed in the gly-
cosylation of the novel ketolide aglycons are shown be-
low (Fig. 2). The p-tolyl-thio glycoside of 6-deoxy-
glucose, 17a, was chosen in order to investigate the
importance of the 3-amino substituent of the naturally
occurring desosamine group, while the p-tolyl-thio glyco-
sides, 17b–e, were chosen to investigate the effect on anti-
microbial activities resulting from the addition of
hydrophobic or hydrophilic groups to 40- and/or 60-posi-
tion of the 5-O-desosamine. The p-tolyl-thio glycosides
having the 3-amino group were protected as the NHF-
moc group since the Fmoc group could be easily re-
moved and the amine could be converted to the
N3
N3
AcO
NMe2
O
O
O
O
O
O
N
N
OH
O
O
O
a (X = H)
O
O
O
or
O
O
b (X =F)
X
O
O
3
4 (X = H)
5 (X = F)
Scheme 1. Reagents and conditions: (a) (i) MeOH, rt (98%); (ii)
(COCl)2, DMSO, Et3N, CH2Cl2; (iii) Silica-gel, MeOH, N2, 65 °C
(55%); (b) (i) NaH, (PhSO2)2NF, THF (90%); (ii) MeOH, rt (98%); (iii)
(COCl)2, DMSO, Et3N, CH2Cl2; (iv) Silica-gel, MeOH, N2, 65 °C
(65%).
N
N
R1
N3
N3
N
R2
R3
R5
R2
R3
R5
O
O
O
O
O
O
O
O
O
R4
N
N
OH
R4
N
O
O
O
a
b, c, d, e
O
O
O
O
O
O
O
O
O
O
X
X
X
O
O
O
4 (X = H)
5 (X = F)
6a-6e (X = H)
7d-7e (X = F)
8-13 (X = H)
14-16 (X = F)
Scheme 2. Reagents and conditions: (a) 17a–e, NIS, AgOTf, ꢀ20 °C–rt (60–70%); (b) piperidine, DMF (80–85%); (c) NaBH(OAc)3, CH2O, THF
(80–90%); (d) (i) MeOH, rt (99%) or (ii) LiOH, MeOH (85–90%); (e) 2-pyridyl or 3-NH2–Ph acteylene, CuI (cat), toluene 80 °C (90–98%).

C.-H. Liang et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1307–1310
1309
N
N
N
N
NMe2
O
NMe2
OH
NMe2
HO
HO
HO
HO
OH
OH
OH
O
O
8
O
9
O
10
O
O
18
N
O
R =
NMe2
NMe2
NMe2
R
O
HO
HO
HO
OH
OH
OH
O
OBz
OBz
O
O
O
O
12
13
11
O
Figure 3. Ketolides 8–13, 18.
The newly prepared ketolides (Fig. 3) were tested
against erythromycin-sensitive (Ery-S) and erythromy-
cin-resistant (Ery-R) strains of S. aureus, S. pyogenes,
S. pneumoniae and H. influenzae ATCC 49247 (Table
1).8,9 The ketolide analogue 8, containing the 5-O-myc-
aminose (60-deoxy-30-dimethylamino-glucose) displayed
reduced activity against Ery-R S. pneumoniae 303
(erm) and Ery-S H. influenzae, having minimum inhibi-
tory concentrations (MIC) of 16 lg/mL for both strains
compared to 60.125 and 4 lg/mL, respectively, for its
ketolide counterpart, 18,4 having the naturally occurring
5-O-desosamine sugar (Table 1). The glycosylated keto-
lide, 9, having the 60-deoxy-glucose substituent, which
lacks the ÔsignatureÕ 30-N,N-dimethylamino group, was
significantly less active than both compounds 8 and 18
against most of the strains tested. The role of the 50-
methyl substituent was deemed important since the
activity of ketolide 10, having the 30-dimethlyamino-xy-
lose group, was less active than ketolide 8 against strains
of S. aureus, Ery-R S. pneumoniae 163 (mef), and H.
influenzae. We also prepared and tested ketolides, 11–
13, having the 60-modified groups (OH and OBz) in
order to investigate hydrophilic/hydrophobic substituent
effects of the 60-position. Ketolide 11, having the 30-
dimethylamino-glucose substituent, was significantly
less active than ketolides, 12 and 13, which contained
the hydrophobic 60-OBz-30-dimethlyamino-glucose and
60-OBz-40-deoxy-30-dimethylamino-glucose groups, res-
pectively, against strains of S. aureus, Ery-R S. pyogenes
3029 (erm), and Ery-R S. pneumoniae 163 (mef).
This trend of improved activity with increased hydro-
phobicity of the 60-position was also witnessed for the
2-fluoro-ketolides (Fig. 4). The selected 2-fluoro-keto-
lides, 14–16, having the 4-(3-aniline)-[1,2,3]-triazole
group (R1 = 3-NH2–Ph) were tested against a secondary
panel of Ery-S and Ery-R strains of S. pyogenes and S.
pneumoniae and H. influenzae ATCC 49247 (Table 2).8,9
The 2-fluoro-ketolide, 14, was highly active (60.125 lg/
mL) against S. pneumoniae ATCC 49619 and Ery-R
S. pneumoniae 163 (mef) and 303 (erm); however it was
completely inactive (>32 lg/mL) against the other
Table 1. In vitro antibacterial activity of triazole-containing 5-O-substituted ketolide derivatives 8–13, 18a
Entry
Minimum inhibitory concentration (lg/mL)9
S. aureus
S. pyogenes
S. pneumoniae
H. influenzae
ATCC 29213 96:11480
Ery-S
ATCC 19615 3029
ATCC 49619 163
303
Ery-R (mef) Ery-R (erm) Ery-S
ATCC 49247
Ery-R (MLSB-ind) Ery-S
Ery-R (erm) Ery-S
Azithromycin
1
>64
60.125
60.125
60.125
>64
16
60.125
60.125
8
>64
60.125
4
4
Telithromycin 60.125
60.125
18
8
60.125
60.125
>64
4
0.125
0.25
>64
4
60.125
60.125
8
>64
>64
nt
60.125
60.125
4
60.125
0.5
60.125
16
16
4
16
9
2
>64
>64
64
10
11
12
13
60.125
60.125
60.125
nt
nt
60.125
60.125
60.125
60.125
8
4
4
64
60.125
nt
>64
2
4
60.125
60.125
60.125
60.125
nt
60.125
60.125
64
64
0.5
a nt = not tested.
NH2
N
O
N
N
NMe2
NMe2
OH
NMe2
O
O
O
HO
HO
HO
OH
OH
N
O
R
R =
O
OBz
OBz
O
O
O
14
15
16
O
F
O
Figure 4. 2-Fluoro-ketolides 14–16.10

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C.-H. Liang et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1307–1310
Table 2. In vitro antibacterial activity of triazole containing 5-O-substituted 2-fluoro-ketolide derivatives 14–16a
Entry
Minimum inhibitory concentration (lg/mL)9
S. pneumoniae
ATCC 49619 163 303
S. pyogenes
ATCC 19615 3029 1850
Ery-R Ery-R Ery-R Ery-S
H. influenzae
3262
1721
3773
Ery-R Ery-R Ery-S
5032
ATCC 49247
Ery-S
Ery-R
Ery-R
(mef)
Ery-R
(erm)
(erm)
(erm)
(erm)
(erm)
(erm)
(erm)
Azithromycin
60.125
>64
16
>64
8
>64
32
>64
64
60.125
60.125
8
>64
>64
1
>64
0.5
4
4
Telithromycin 60.125
60.125 60.125
14
15
16
60.125
60.125
nt
>32
0.5
>32
0.25
>32
4
>32
8
60.125
60.125
60.125
60.125 60.125 >32
>32
4
32
4
60.125 60.125
60.125 60.125
8
4
60.125 0.25
0.5
8
0.25
4
a nt = not tested.
S.; Hoban, D. J. Drugs 2002, 62, 1771; (b) Berisio, R.;
Harms, J.; Schluenzen, F.; Zarivach, R.; Hansen, H.;
Fucini, P.; Yonath, A. J. Bacteriol. 2003, 185, 4276.
2. Asaka, T.; Manaka, A.; Sugiyama, H. Curr. Topics Med.
Chem. 2003, 3, 961.
3. Hansen, J.; Ippolito, J.; Ban, N.; Nissen, P.; Moore, P. B.;
Steitz, T. A. Mol. Cell 2002, 10, 117.
4. Romero, A.; Liang, C. H.; Chiu, Y. H.; Yao, S.; Duffield,
J.; Sucheck, S.; Marby, K.; Rabuka, D.; Leung, P. Y.;
Shue, Y. K.; Ichikawa, Y.; Hwang, C. K. Tetrahedron
Lett. 2005, 46, 1483.
5. (a) Omura, S. Macrolide Antibiotics Chemistry, Biology,
and Practice; Academic: London, 2002; (b) Jones, P. H.;
Powley, E. K. J. Org. Chem. 1968, 33, 665; (c) LeMahieu,
R. A.; Carson, M.; Kierstead, R. W. J. Med. Chem. 1974,
17, 953; (d) Djokic, S.; Kobrehel, G.; Lazarevski, G.;
Lopotar, N.; Tamburasev, Z. J. Chem. Soc., Perkin Trans.
1 1986, 1881.
macrolide-resistant strains, including Ery-R strains of S.
pneumoniae 3773 and 5032 (erm), and all Ery-R strains
of S. pyogenes 3029, 1850, 3262, and 1721 (erm). This
compound was also 8-fold less active than azithromycin
and telithromycin against Ery-S H. influenzae. On the
other hand, ketolides 15 and 16, containing an addi-
tional 60-OBz group such as 60-OBz-30-dimethylamino-
glucose and 60-OBz-40-deoxy-30-dimethylamino-glucose,
respectively, were as active as azithromycin and telithro-
mycin against Ery-S H. influenzae. Interestingly, keto-
lides 15 and 16 were both significantly more active
than ketolide 14 against most macrolide-resistant strains
of S. pyogenes and S. pneumoniae. Particularly notewor-
thy, ketolides 15 and 16 were also significantly more ac-
tive than telithromycin and azithromycin against Ery-R
strains of S. pyogenes 3029, 1850, 3262, and 1721 (erm).
6. The p-tolyl-thio glycosides 17a–e were prepared in-house.
The protecting group scheme (i.e., 2-OBz) was chosen in
order to direct the b-configuration at the anomeric center
resulting from the glycosylation reactions.
7. Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
B. Angew. Chem., Int. Ed. 2002, 114, 2596.
8. Clinical isolates were provided by Dr. Dwight Hardy,
University of Rochester, NY; Dr. Marilyn Roberts,
University of Washington, Seattle, Wa; and Dr. Peter
Appelbaum, Hershey Medical Center, Hershey, Pa.
9. National Committee for Clinical Laboratory Standards.
Methods for Dilution Antimicrobial Susceptibility Tests
for Bacteria that Grow Aerobically, 6th ed.; Approved
standard: NCCLS Document M7-A6, 2003.
In summary, series of new triazole containing ketolides
and 2-fluoro-ketolides in which the 5-O-desosamine
was replaced by unnatural sugars were synthesized.
Ketolides having the 60-OBz-30-dimethylamino-glucose
and 60-OBz-40-deoxy-30-dimethylamino-glucose substit-
uents displayed excellent in vitro activity against macro-
lide-sensitive and macrolide-resistant pathogens. These
compounds are promising candidates for further efficacy
evaluation. This work should also provide significant
opportunity for the discovery of novel 5-O-sugar substi-
tuted macrolide and ketolide antibiotics having potent
activity against bacteria expressing erm-mediated
resistance.
10. Representative spectroscopic data. Compound 15: 1H
NMR (400 MHz, CDCl3) d 8.09–8.06 (m, 2H), 7.82 (s,
1H), 7.60–7.51 (m, 1H), 7.49–7.32 (m, 2H), 7.25–7.15 (m,
3H), 6.65–6.60 (m, 1H), 4.95–4.90 (m, 1H), 4.75–4.52 (m,
2H), 4.48–4.32 (m, 3H), 4.13 (d, J = 8.1 Hz, 1H), 3.80–3.35
(m, 7H), 3.15–3.05 (m, 1H), 2.80–2.70 (m, 5H), 2.54 (s,
6H), 2.43 (s, 3H), 2.11–1.35 (m, 8H), 1.81 (d, J = 21.5 Hz,
3H), 1.51 (s, 3H), 1.23 (s, 3H), 1.21 (d, J = 6.9 Hz, 3H),
1.18 (d, J = 6.9 Hz, 3H), 1.04 (d, J = 7.0 Hz, 3H), 0.90 (t,
J = 7.5 Hz, 3H). 13C NMR (100 MHz, CDCl3) d 216.5,
202.5, 167.1, 166.4, 157.2, 147.9, 146.9, 133.5, 131.7, 129.9,
129.5, 128.5, 128.4, 119.7, 116.2, 114.8, 112.4, 103.9, 97.9
(d, J = 205 Hz), 82.1, 80.4, 78.6, 78.4, 75.8, 70.3, 70.1,
66.2, 63.9, 60.8, 49.7, 49.1, 44.5, 42.8, 41.7, 40.6, 39.5,
39.2, 27.6, 25.4, 24.3, 22.1, 19.7, 17.9, 15.1, 14.7, 13.6, 10.5.
MS: Calcd for C50H70FN6O13 (M+H): 981.5, found:
981.3.
Acknowledgements
This work was partially supported by SBIR research
grant 1R43 AI058395-01, awarded by the National
Institute of Allergy and Infectious Diseases, National
Institute of Health. We thank Dr. Youe-Kong Shue
for helpful scientific discussions and insights.
References and notes
1. (a) Zhanel, G. G.; Walters, M.; Noreddin, A.; Vercaigne, L.
M.; Wierzbowski, A.; Embil, J. M.; Gin, A. S.; Douthwaite,