Synthesis and antibacterial activity of novel modified 5-O-desosamine ketolides

Article abstract of DOI:10.1016/j.bmcl.2012.10.064

A series of novel modified 5-O-desosamine-ketolides were synthesized. The 5-O-desosamine fragment was removed from ketolide by an efficient and mild manipulation. 4-O-substituted desosamine was introduced into the ketolide aglycon and various coupling methods were essayed for the glycosylation. Three novel ketolides were tested for in vitro antibacterial activity against a panel of susceptible and resistant pathogens. Compound 26 showed potent activity against all the methicillin-sensitivity and resistant pathogens.

Full text of DOI:10.1016/j.bmcl.2012.10.064

Bioorganic & Medicinal Chemistry Letters 22 (2012) 7402–7405
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and antibacterial activity of novel modified 5-O-desosamine ketolides
⇑
Xiaozhuo Chen , Peng Xu , Yanpeng Xu, Lu Liu, Yi Liu, Di Zhu, Pingsheng Lei
State Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Beijing Key Laboratory of Active Substances Discovery and Drugability Evaluation, Department
of Medicinal Chemistry, Institute of Materia Medica, Peking Union Medical College, Chinese Academy of Medical Sciences, Beijing 100050, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel modified 5-O-desosamine-ketolides were synthesized. The 5-O-desosamine fragment
was removed from ketolide by an efficient and mild manipulation. 4-O-substituted desosamine was
introduced into the ketolide aglycon and various coupling methods were essayed for the glycosylation.
Three novel ketolides were tested for in vitro antibacterial activity against a panel of susceptible and
resistant pathogens. Compound 26 showed potent activity against all the methicillin-sensitivity and
resistant pathogens.
Received 15 August 2012
Revised 25 September 2012
Accepted 13 October 2012
Available online 23 October 2012
Keywords:
Desosamine
Ketolide
Ó 2012 Elsevier Ltd. All rights reserved.
Glycosylation
Antibacterial activity
Antibiotic
With intensive emergence of antibacterial resistance against
antibiotics,1 great efforts have been made to find new structures
which were potent against resistant pathogens. The third genera-
tion macrolide—ketolide, telithromycin and cethromycin as typical
cases,2,3 became a main field of macrolide research (Scheme 1).
Further researches based on the ketolide focus on modifying the
different positions of 14 member macrolide,4–6 but the modifica-
tion of 5-O-desosamine has rarely been reported.7 In fact, the
5-O-desosamine plays an important role in the antibacterial mech-
anism and has been considered to be critical to the antibacterial
activity of macrolide antibiotics.8,9 According to Steitz T.A.’s
report,10 one-half to two-thirds of the interaction surface was con-
tributed by the sugar substitutes of macrolide. So macrolides with
modified saccharride fragment may assist in the design of new
antibiotics that will work against the presently known resistance
mutations.
The 5-O-desosamine residue was first removed and replaced
with a series of optimized sugars by Romero, A. et al.11,12 In their
reports, some new ketolides with modified 5-O-desosamine resi-
due exhibited potent activity against several key erythromycin-
resistant pathogens. Unfortunately, only p-tolyl-thio glycosides
were used as donors for the glycosylation in their work and the
method of glycosylation were not published in detail.
5-O-desosamine fragment. (2) Facile glycosylation at 5-OH group
of 14-membered ketolide.
The 5-OH ketolide 5 was synthesized from 11, 12-carbonate-20-
OAc clarithromycin 1 (Scheme 2). The Swern Oxidation was carried
to convert the 3-OH to the carbonate compound 2 in a yield of 72%.
The 20-OAc was removed in methanol to form 3 in a yield of 95%.
Once more Swern Oxidation to form the unstable intermediate
product 4, and then stirred it in methanol at neutral condition to
remove the desosamine. Due to the acid sensitivity, the purifica-
tion of ketolide aglycon 5 was carried out on a column of Sephadex
LH-20 by using CHCl3/CH3OH (1:1) as eluent. The pure compound 5
was obtained in a yield of 52% for two steps. The structure of 5 was
confirmed by its 1HNMR, 13CNMR and HRMS spectra.
The modified desosamine glycosyl donors were synthesized
from methyl
a-D-glucopyranoside which was converted into the
4, 6-benzylidene acetal 6 in a yield of 75% (Scheme 3). The acetal
6 was converted into the dibenzoyl intermediate 7 in a yield of
89%. Reductive cleavage of the dioxane ring with borane-tetrahy-
drofuran complex–Copper (II) trifluoromethanesulfonate yielded
exclusive the 4-benzyl ether 8 in a yield of 90%. Compound 8
was converted into the 6-bromo intermediate 9 with carbon tetra-
bromide in THF in a yield of 86% which underwent a facile radical
reaction with 2, 2-azobisisobutyronitrile and tri-n-butyltin hydride
to form the 6-deoxy compound 10 in a yield of 91%. The 2, 3-diben-
zoyl were removed and then converted into the ditosylate 12.
Treatment of 12 with sodium methoxide/methanol in methylene
chloride resulted in ready conversion into the allo 2, 3-epoxide
13 in a yield of 90%.
In this report, we are going to modify the 5-O-desosamine of
14-membered ketolide. Especially, we focus on the following
respects: (1) an efficient procedure for synthesis of modified
⇑
Corresponding author. Tel.: +86 10 63162006; fax: +86 10 6301775.
E-mail address: lei@imm.ac.cn (P. Lei).
In epoxidation reaction, a trans-isomer was favorable to get.
Since the 1HNMR, 13CNMR and HRMS were not enough to confirm
These authors contributed equally to this work.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.10.064

X. Chen et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7402–7405
7403
N
N
N
N
N
O
N
O
HO
O
HO
O
O
O
O
H
N
O
N
O
O
O
O
O
O
O
O
O
O
cethromycin
telithromycin
Scheme 1. Structures of telithromycin and cethromycin.
the configuration of compound 13, the nuclear overhauser effect
spectroscopy (NOESY) was used. In case the 2, 3-epoxide and 1-
OCH3 were opposite in direction, irradiate the signal of 1-OCH3
in 1HNMR, the correlation with 2-H would be detected. However,
no NOE effect with 2-H was detected while irradiating to com-
pound 13 in deuterated acetone solution at 3.32 ppm (the signal
for OCH3 of 13). It was confirmed that the 2-H and 1-OCH3 were
in trans-configuration.
Cleavage of the oxirane ring in 13 with dimethylamine in H2O
yielded the 3-dimethylamine gluco 14 as the target product while
the 2-dimethylamine altro was also formed with a proportion of
about 30%. The 2-OH of 14 was protected by benzoyl group to form
the intermediate 15 which was easily separated from the altro iso-
mer. Exhaustive acetolysis of methyl pyranoside 15 with a mixture
of acetic anhydride, acetic acid and sulfuric acid yielded the acetate
16, as a mixture of anomers.
acceptor 5 (Scheme 5). Under catalysis of TESOTf at ꢀ35 °C, accep-
tor 5 was glycosylated with trichloroacetimidate 22 to provide 5-
O-desosamine substituted product 24 with a yield of 30%. The
two thioglycoside donors 18 and 20 were coupled respectively
with acceptor 5 in the presence of NIS-AgOTf and 2, 6-di-tert-
butylpyridine at ꢀ78 °C to afford target product 24 with the yields
of 2.2% and 16%. In the glycosyl fluoride donor system, we choice
Cp2HfCl2: AgClO4 1: 2 as catalyst, and appropriate content of 2,
6-di-tert-butylpyridine were added. The glycosylation was pro-
ceeding mildly at 40 °C, 24 was obtained with a yield of 40%. For
2-OBz of the glycosyl donors plays an important orientating role
in stereochemistry of glycosylation reaction, b-configuration of
the product was obtained only.
The structure of product 24 was confirmed by 1H NMR, 13C NMR
1
and HRMS. The H NMR signal at d 4.54 ppm for H-10 (d, 1H,
J = 7.2 Hz) and the 13C NMR signal at d 101.1 ppm for the C-10 con-
firmed that the configuration of glycosidic bond was b-configura-
tion. The 20-OBz of 24 was removed in CH3OH to form 40-OBn
macrolide 25. Continue removing the benzyl by catalytic hydroge-
nation, 26 was obtained in a yield of 68%
For careful investigation of the glycosylation with the unstable
5-OH ketolide 5, four different donors were prepared from 16
(Scheme 4). Under catalysis of boron fluoride, compound 16 was
converted to p-methylphenyl thio b-D glucopyranoside 18 with p-
thiocresol. Unfortunately, the 4-Bn was unstable at this condition
40-O-allyl 5-O-desosamine ketolide 28 were also prepared from
40-allyl glycosyl fluoride donor 27. The benzyl group of 28 was re-
moved to form the 40-O-allyl ketolide 29. (Scheme 6).
and compound 19 became the major product which could be con-
verted to 18 again by treating with BnBr. The ethyl thio b-D gluco-
pyranoside 20 was obtained with ethanethiol under the same
condition. The acetyl of 16 was removed smoothly by hydrazine
acetate in DMF to form compound 17 in a yield of 75%. Treat com-
pound 17 with diethylaminosulfur trifluoride (DAST) in THF
yielded the glycosyl fluoride 23 in a yield of 91%. The trichloroace-
timidate 22 was obtained by the reaction of 17 and trichloroaceto-
nitrile in the presence of 1, 8-diazabicyclo [5.4.0] undec-7-ene
(DBU).
The prepared ketolides were tested against a panel of represen-
tative respiratory tract pathogens together with telithromycin,
clarithromycin and erythromycin as references (Table 1). Various
macrolide sensitive and resistant strains were included in order
to evaluate the activity of these novel ketolide derivatives.
ATCC29213, 09O077 and 09U035 are methicillin-sensitivity staph-
ylococcus aurous (MSSA). 09L075 and 09N120 are methicillin-resis-
tant staphylococcus aurous (MRSA). 09A011 and 09G291 are
methicillin-sensitivity staphylococcus epidermidis (MSSE). 09R476
and 09M124 are methicillin-resistant staphylococcus epidermidis
The trichloroacetimidate, two thioglycoside donors and glycosyl
fluoride donor were taken respectively into glycosylation with
N
N
N
O
AcO
O
RO
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
O
O
O
O
C
c
O
O
O
O
O
d
a
O
O
O
OH
O
O
O
O
O
O
O
O
O
O
O
1
2 R = Ac
3R = H
5
4
b
Scheme 2. Reagents and conditions: (a) (COCl)2, DMSO, Et3N, CH2Cl2, ꢀ70 °C, 72%; (b) MeOH, reflux, 95%; (c) (COCl)2, DMSO, Et3N, CH2Cl2, ꢀ70 °C; (d) MeOH, 50 °C, 52% for
two steps.

7404
X. Chen et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7402–7405
OH
R
Ph
O
a
c
h
f
O
O
O
O
HO
HO
BnO
RO
O
BnO
BzO
RO
OH
OR
OR
OBz
OCH3
OCH3
OCH3
OCH3
8 R = OH
9 R = Br
10 R = H
11 R = H
12 R = Ts
6 R = H
7 R = Bz
d
b
g
e
O
i
O
O
O
k
j
BnO
BnO
Me2N
BnO
Me2N
BnO
Me2N
OAc
O
OH
OBz
OBz
OCH3
OCH3
OCH3
13
14
16
15
Scheme 3. Reagents and conditions: (a) PhCH(OMe)2, TsOHꢁH2O, DMF, 55 °C, 75%; (b) BzCl, pyridine, 89%; (c) BH3ꢁTHF, Cu(OTf)2, 90%; (d) Ph3P, CBr4, THF, 86%; (e) Bu3SnH,
AIBN, toluene, 91%. (f) CH3ONa/CH3OH; (g) TsCl, Py, DMAP, 80 °C, 4d; (h) CH3ONa/CH3OH, CH2Cl2, 90%; (i) Me2NH/ H2O, 30 °C; (j) BzCl/Py, 0oC, 48% for two steps; (k) Ac2O/
AcOH/H2SO4, rt.67%.
O
O
f
O
BnO
Me2N
BnO
Me2N
BnO
Me2N
d
OAc
F
OH
OBz
OBz
OBz
23
16
17
a
b
e
O
O
BnO
Me2N
BnO
Me2N
O
BnO
Me2N
STol
SEt
OBz
OBz
20
18
OBz
O
CCl3
c
c
22
+
+
NH
O
O
HO
Me2N
HO
Me2N
STol
SEt
OBz
OBz
19
21
Scheme 4. Reagents and conditions: (a) p-thiocresol, BF3ꢁEt2O, CH2Cl2, 48 h, 60%; (b) EtSH, BF3ꢁEt2O, CH2Cl2, 48 h, 71%; (c) BnBr, 60% NaH, DMF, 74–80%; (d) H2NNH2ꢁHOAc,
DMF, 50 °C, 75%; (e) CCl3CN, DBU, CH2Cl2, 0 °C, 67%; (f) DAST, THF, ꢀ30 °C, 91%.
N
O
N
O
HO
OCH3
O
OR
O
BzO
OCH3
O
OBn
O
O
OCH3
OH
O
O
O
O
O
O
a,b,c
O
d,e
BnO
Me2N
O
O
O
+
O
O
O
O
O
OBz
O
LG
O
O
O
25 R=Bn
26 R=OH
24
5
LG= ¦Â-STol
18
20 LG= ¦Â-SEt
LG= ¦Á-O-trichloro-acetimidate
22
23 LG= F
Scheme 5. Reagents and conditions: (a) (for 18 and 20), NIS-AgOTf, 2, 6-di-tert-butylpyridine, ꢀ78 °C; (b) (for 22), TESOTf, 4 Å MS, CH2Cl2, ꢀ35 °C, 30%; (c) (for 23), Cp2HfCl2-
AgClO4, 4 Å MS, CH2Cl2, 40 °C, 40%. (d) MeOH, reflux, 77%; e. Pd/C, H2, ethyl acetate, 68%.
N
O
RO
OCH3
O
O
O
OCH3
OH
O
O
O
O
O
O
O
O
a
+
O
Me2N
F
O
OBz
O
O
O
O
5
O
27
28 R=Bz
29 R=H
b
Scheme 6. Reagents and conditions: (a) Cp2HfCl2, AgClO4, 4 Å MS, CH2Cl2, rt; (b) CH3OH, reflux.

X. Chen et al. / Bioorg. Med. Chem. Lett. 22 (2012) 7402–7405
7405
Table 1
In vitro antibacterial activities of compounds 25, 26 and 29
Pathogens
Ery.
Clar.
Teli.
25
26
29
ATCC29213
09°077
09U035
S.aureus
S.aureus
S.aureus
S.aureus
MSSA
MSSA
MSSA
MRSA
MRSA
MSSE
MSSE
MRSE
MRSE
ESSP
ESSP
ESSP
ERSP
ERSP
ESSPy
ERSPy
0.25
0.25
>32
>32
2
>32
0.125
>32
0.25
32
0.125
0.062
0.062
>32
0.125
>64
>64
0.125
>64
0.125
>64
0.125
0.25
0.031
0.031
0.031
0.125
0.25
0.016
2
32
0.125
>32
>32
0.125
>32
0.125
>32
0.25
0.25
0.062
0.062
60.031
>32
4
>256
>256
0.5
>256
0.062
>256
0.125
128
0.062
0.062
0.062
>256
>256
0.031
>256
>32
>32
>32
>32
16
>32
32
32
4
4
4
>32
>32
>32
>32
>8
>8
8
>8
2
>8
4
4
0.5
0.5
1
>8
>8
>8
>8
09L075
09N120
09A011
09G291
09R476
09M124
ATCC49619
09H065
09H071
09D016
09E427
09U070
09Q149
S.aureus
S.epidermidis
S.epidermidis
S.epidermidis
S.epidermidis
S.pneumoniae
S.pneumoniae
S.pneumoniae
S.pneumoniae
S.pneumoniae
S.pyogenes
S.pyogenes
>32
0.031
>32
>32
60.031
>32
(MRSE). ATCC49619, 09H065 and 09H071 are erythromycin-sensi-
tivity Streptococcus pneumoniae (ESSP). 09D016 and 09E427 are
erythromycin-resistant Streptococcus pneumoniae (ERSP), 09U070
is erythromycin-sensitivity Streptococcus pyogenes (ESSPy) and
09Q149 is erythromycin-resistant Streptococcus pyogenes (ERSPy).
All the strains chosen in this test were supplied by the Ministry
of Health National Antimicrobial Resistance Investigation Net
(MOHNARIN, China). The in vitro antibacterial activity was re-
ported as minimum inhibitory concentrations (MICs), which was
determined by the broth microdilution method as recommended
by the NCCLS.13
hydrogen donor to the amino acid residues of the binding pocket,
so compound 26 showed improved antibacterial activity compared
with erythromycin and clathromycin. 40-hydrophobic group, like
aryl or alkyl, could not increase any interactions between macro-
lides and ribosomal subunits and then the antibacterial activity
of 25 and 29 were not improved.
This study is helpful for better understanding of the interaction
between ketolides and bacterial ribosomes. Further studies are in
progress.
Acknowledgment
Compared with erythromycin and clathromycin, compounds 25
and 29 showed no improvement of antibacterial activity. These re-
sults indicate that an aryl or alkyl group existed at 40-O-position of
5-O-desosamine is disfavorable for the amelioration of antibacte-
rial activity. But compound 26 exhibited obviously more active
than clarithromycin for ATCC29213 (MSSA), 09L075(MRSA),
09M124(MRSE), 09H071(ESSP) and 09U070 (ESSPy) strains. It sug-
gests that a hydrophile group (hydroxyl group) at 4-position of 5-
O-desosamine is favorable to increase the antibacterial activity
against both methicillin-sensitivity and -resistant strains. Com-
pared to telithromycin, 26 showed the same potent activity against
all the methicillin-sensitivity and resistant pathogens. It kept the
same antibacterial activity compared with telithromycin against
the erythromycin-sensitivity ESSP and ESSPy strains, but not the
erythromycin-resistant ERSP and ERSPy strains.
In summary, by using a bis-Swern Oxidation and mild removing
of 5-O-desosamine under neutral condition, intermediate 5 was
obtained successfully. Four different modified desosamine glycosyl
donors were obtained with multiply steps. The glycosylation of 5-
OH ketolide core was carefully studied with various coupling
methods. Three novel 40-modified 5-O-desosamine ketolides were
synthesized.
This work was supported by the National S&T Major Special
Project on Major New Drug Innovation (Item Number:
2008ZX09401-004 and 2009ZX09301-003) and National Natural
Science Foundation of China (81072524).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.10.064.
References and notes
1. Hoban, D.; Baquero, F.; Reed, V.; Felmingham, D. Int. J. Infect Dis. 2005, 9, 262.
2. Alexis, D.; Constantin, A., et al. Bioorg. Med. Chem. Lett. 1999, 9, 3075.
3. Richard, F. C.; Zhenkun Ma, et al. Bioorg. Med. Chem. Lett. 2000, 10, 815.
4. Hong Yong; Yu Gui Gu, et al. Bioorg. Med. Chem. Lett. 2005, 15, 2653.
5. Ghilsoo Nama; Yang Soo Kim, et al. Bioorg. Med. Chem. Lett. 2010, 20, 2671.
6. DailyDrugNews.com, April 23, 2010.
7. Ly, T. Phan; Tianying Jian, et al. J. Med. Chem. 2004, 47, 2965.
8. Takashima, H. Curr. Top. Med. Chem. 2003, 3, 991.
9. Pfister, P.; Jenni, S. J. Mol. Biol. 2004, 342, 1569.
10. Steitz, T. A. et al Mol. Cell 2002, 10, 117.
11. Romero, A.; Liang, C. H. Tetrahedron Lett. 2005, 46, 1483.
12. Liang, C. H.; Yao, S. Bioorg. Med. Chem. Lett. 2005, 15, 1307.
13. National committee for clinical laboratory standards, methods for dilution
antimicrobial susceptibility tests for bacteria that grow aerobically, fifth ed.
Vol. 20, 2000, approved standard: NCCLS document M7–A5.
For the Structure-activity relationships, desosamine sugar binds
to domain V of the 23S rRNA through a hydrogen bond. Existence
of 40-OH on desosamine sugar could possible provide another