A new series of macrolide derivatives with 4″-O-saccharide substituents

Article abstract of DOI:10.1016/j.bmcl.2010.07.072

A series of novel derivatives of macrolide with 4″-O-mono- or disaccharides were synthesized. The corresponding glycosyl trichloroacetimidates were used as the donors in the glycosylations. The in vitro antibacterial activities of 7a-f and 13-16 against a panel of susceptible and resistant pathogens were tested. The modification of 4″-O-mono- or disaccharides may lead to the understanding of interaction of the macrolide and the bacterial ribosome.

Full text of DOI:10.1016/j.bmcl.2010.07.072

Bioorganic & Medicinal Chemistry Letters 20 (2010) 5527–5531
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
A new series of macrolide derivatives with 400-O-saccharide substituents
Peng Xu a, , Xiao-zhuo Chen a, , Lu Liu a, Zhi-ping Jin b, Ping-sheng Lei a,
*
a Key Laboratory of Bioactivity Substance and Resources Utilization of Chinese Herbal Medicine, Ministry of Education Institute of Materia Medica, Peking Union
Medica College & Chinese Academy of Medical Sciences, Beijing 100050, PR China
b Jingxin Pharmaceutical. Co., Xinchang, Zhejiang 312400, PR China
a r t i c l e i n f o
a b s t r a c t
A series of novel derivatives of macrolide with 400-O-mono- or disaccharides were synthesized. The cor-
responding glycosyl trichloroacetimidates were used as the donors in the glycosylations. The in vitro anti-
bacterial activities of 7a–f and 13–16 against a panel of susceptible and resistant pathogens were tested.
The modification of 400-O-mono- or disaccharides may lead to the understanding of interaction of the
macrolide and the bacterial ribosome.
Article history:
Received 6 May 2010
Revised 23 June 2010
Accepted 16 July 2010
Available online 21 July 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Macrolide
Synthesis
Resistant pathogen
Glycosylation
Macrolide antibiotics have been used clinically to treat respira-
tory tract infections for over 50 years, since the first agent erythro-
of antibacterial activities.5 In the case of telithromycin, 11- and 12-
positions are appropriate for introducing a cyclic carbamate sub-
structure with a N-tethered aryl–alkyl side chain. 400-Modified
macrolide derivatives have been investigated by some research
groups6–9 and compounds such as 1–3 were found to have potent
activity against susceptible and resistant bacterial strains (Fig. 1).
These observations led us to focus on the 400-position and elon-
gate the saccharide chain at this position, with considerations
based on following facts. First, the 400-OH group shows good reac-
tivity for glycosylation and modification at the 400-position can be
achieved through a few reaction steps. Second, the elongated sac-
charide fragment has several –OH groups which could be donors or
acceptors of hydrogen bond interactions. And last, recent research
has indicated that the contributions of van der Waals interactions
are more important and significant than electrostatic contributions
in the binding of macrolides to the binding pocket.10 The –OH
groups of a 400-O-saccharide could be further modified, for example,
with benzoyl or benzylidene acetal groups. These modifications
provide more opportunities to have non-bonded atom–atom inter-
actions, such as hydrophobic interactions or aromatic stacking ef-
fects between a macrolide and ribosome.
mycin
A was discovered. With the intensive emergence of
antibacterial resistance against macrolides, great efforts have been
made to find new structures potent against resistant pathogens.
The most well-known series of third generation macrolides is the
ketolides, represented by telithromycin1 and cethromycin.2 (Fig. 1)
Their structural features, including a 3-keto group, a 11,12-cyclic
carbamate, and a tethered hetero-aromatic substituent, play key
roles in the observed superior activity against erythromycin-resis-
tant bacterial strains.
It is noteworthy to mention that ketolides are not the only class
of new macrolides to overcome macrolide resistance. Many deriv-
atives of nonketolide families have been synthesized by different
research groups3 in the search for compounds with potent antibac-
terial activity.
Carbohydrates are involved in a broad variety of biological phe-
nomena and are, in particular, associated with tremendous diagnos-
tic and therapeutic potential. Macrolides with 14-membered
lactone ring have two monosaccharides,
nose, in their structures and both components, especially the
D
-desosamine and
L
-cladi-
-des-
D
osamine, are important for the binding of macrolides to ribosomes.
Using saccharide as a component to modify the macrolide core
has rarely been reported.4 A hydroxy group is necessary for intro-
duction of a saccharide fragment and it is generally accepted that
modification on 20-hydroxy group will lead to significant decrease
In this study, the synthesis of 7a–f started from commercially
available clarithromycin (Scheme 1). Selective acetylation of the
20-OH of clarithromycin left a remaining 400-OH as an ideal site at
which to attach a saccharide section. We deducted that the steric
hindrance of two cis-methyl groups made the reactivity of 11-OH
lower than that of 400-OH. Glycosylation of 5 with an appropriately
protected glycosyl trichloroacetimidate donor 4a–d11 (Fig. 2), in
the presence of triethylsilyl trifluoromethanesulfonate (TESOTf),
produced 6a–d in yields ranging from 25% to 46%. The presence of
* Corresponding author. Tel.: +86 10 63162596; fax: +86 10 63017757.
E-mail addresses: Lei@imm.ac.cn, wangdizhu007@163.com (P. Lei).
Authors contributed equally to this work.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.07.072

5528
P. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5527–5531
N
N
N
N
N
O
N
HO
HO
OCH3
O
O
O
O
H
N
O
N
O
O
O
O
O
O
O
O
O
O
O
Cethromycin
Telithromycin
N
N
HO
HO
3'
H2N
O
OH
1'
OH
O
O
O
O
HO
HO
O
O
5
O
3
O
O
O
O
O
O
O
O
1"
H
1
O
4"
N
N
O
N
O
O
H
O
O
O
O
O
2 (CP-544372)
1 (A-60565)
N
N
N
3'
N
HO
O
9
1'
O
O
N
5
11
12
O
O
3
N
O
O
1"
O
O
1
N
4"
N
O
O
H
O
N
3
Figure 1. Structures of Telithromycin, cethromycin and compounds 1–3.
the proton signal of 11-OH (about 4.0 ppm) confirmed that the gly-
cosylation had taken place at 400-position. The acetyl substitution on
2-OH of the donors ensured the b-configuration was generated at
the anomeric centre (coupling constant of J1,2 = 7.2–8.1 Hz). React
6a with K2CO3 in methanol at 50 °C for 12 h afforded products 7a
and 7b in a 1:1 ratio. The 20-OAc of 6b and 6c could be selectively re-
moved by refluxing in MeOH overnight. Products 7c and 7e were ob-
tained in high yields (85% and 88%, respectively). An attempt to
selectively deacetylate 6b failed using guanidine/EtOH. Compound
7d with 2000-OAc and 6000-OBz was obtained. Considering that the po-
lar surface area is an important factor in determining the biophar-
maceutical properties of a lead compound and that a reduced
polar surface area will possibly improve the pharmacokinetic profile
of macrolide compounds,12 one or several acyl groups remained on
compounds 7b–7e. The complete deacetylation of 6d, affording 7f,
was accomplished by a reaction in MeOH/K2CO3.
chains attached at 11,12-carbamate substructure were proved to
be potent in our previous work.13 Acylimidazole intermediate 814
was reacted with [4-(3H-imidazo[4,5-b]pyridin-3-yl)butyl]amine,
using 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU) as base, and pro-
ducing a cyclic carbamate product 9 (Scheme 2). Deprotection of
two acetyl groups of 9 was carried out by heating 9 in MeOH in
the presence of K2CO3. After selective protection of 20-OH gave
11 in excellent yield. Glycosylation of 11 with donor 4a or 4e,11
using TESOTf as a promoter, produced two macrolide derivatives
with saccharide substituted at 400-position. Removal of the acetyl
groups by heating these products in MeOH/K2CO3, respectively,
gave 13 and 14.
Followed a same manner as described above, 8 was coupled
with [4-(1H-imidazo[4,5-b]pyridin-1-yl)butyl]amine to give 10.
After protecting group manipulation on 10 yielded 12. Glycosyla-
tion of 12 with 4a or 4e followed by deacetylation, 15 and 16 were
afforded.
The synthesis of 13–16 followed a different procedure with 7a–
f. A heteroaryl–alkyl side chain was introduced in 11,12-position
before glycosylation at 400-OH. The two fused-heteroaryl side
The antibacterial activities of 7a–f and 13–16, with clarithro-
mycin and telithromycin as reference compounds, against both

P. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5527–5531
5529
N
N
O
O
HO
O
O
AcO
O
O
O
2'
1'
9
HO
HO
HO
HO
O
5
a
3
O
O
1"
O
O
O
O
1
O
4"
OH
O
OH
O
O
5
Clarithromycin
N
N
O
O
HO
O
O
AcO
O
O
O
HO
HO
HO
HO
O
b
c or d
O
O
O
O
O
O
OR1
O
OR2
O
O
O
6a-d
7a-f
R1
OAc
R2
OH
AcO
OAc
6'"
HO
OH
2'"
1'"
7a
7b
7c
7d
7e
6a
O
O
OAc
OH
OH
OH
AcO
O
OH
OAc
O
OAc
AcO
OBz
OBz
AcO
AcO
AcO
OBz
6b
O
OBz
OH
OH
O
OBz
OBz
OAc
OAc
O
AcO
OBz
6c
6d
O
OAc
O
OAc
O
OAc
OAc
OH
OH
OAc
O
OH
O
AcO
OAc
O
HO
OH
AcO
HO
7f
O
OAc
OH
Scheme 1. Reagents and conditions: (a) Ac2O, Et3N, rt, 95%; (b) 4a–4d, TESOTf, CH2Cl2, ꢀ15 °C, 25–46%; (c) K2CO3, MeOH, 40 °C; (d) guanidine, EtOH, CH2Cl2, ꢀ15 °C.
erythromycin-susceptible and erythromycin-resistant strains are
shown in Table 1. A 6-membered panel of bacteria were employed,
including methicillin-susceptible Staphylococcus aureus (MSSA)
ATCC29213, methicillin-resistant S. aureus (MRSA) BK2464, meth-
icillin-resistant Staphylococcus epidermidis (MRSE) MRSE-1, eryth-
romycin-resistant Streptococcus pneumoniae ERSP-2 (ermB and
mef encoded), and erythromycin-resistant Streptococcus pyogenes
8902 (ermB encoded) and ERSPy-1(mef encoded). The in vitro anti-
bacterial activities were reported as minimum inhibitory concen-
trations (MICs), determined by the broth microdilution method,
as recommended by the NCCLS (National Committee of Clinical
Laboratory Standard).15
Among 7a–f, regardless of a mono- or disaccharide substitution
at the 400-position and with acyl groups on the saccharide chain or

5530
P. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5527–5531
AcO
AcO
O
OC(NH)CCl3 BzO
OAc BzO
O
OC(NH)CCl3 AcO
OAc BzO
O
OC(NH)CCl3
OAc
OAc
OAc
OAc
4c
4a
4b
OAc
OAc
OAc
O
AcO
O
OC(NH)CCl3
O
OC(NH)CCl3
O
O
Ph
O
OAc
OAc
OAc
OAc
OAc
4d
4e
Figure 2. Structures of glycosyl donors 4a–e.
R1
N
N
O
O
AcO
O
AcO
O
O
O
N
O
O
O
a
O
O
N
O
O
O
O
O
N
O
O
O
OAc
O
OAc
O
O
N
N
N
R1=
10 R1=
8
9
N
N
N
R1
R1
N
N
O
O
HO
O
O
AcO
O
O
O
N
N
O
O
O
O
O
O
O
O
O
d,e
O
O
b,c
O
OR2
O
OH
O
O
N
N
N
N
N
12 R1=
11 R1=
13-16
N
R1
R2
OH
N
N
HO
OH
OH
13
14
N
O
OH
HO
HO
N
N
O
Ph
N
O
O
OH
N
N
N
OH
15
16
O
OH
O
OH
N
HO
N
N
Ph
O
O
Scheme 2. Reagents and conditions: (a) [4-(3H-imidazo[4,5-b]pyridin-3-yl)butyl]amine or [4-(1H-imidazo[4,5-b]pyridin-1-yl)butyl]amine, acetonitrile/H2O (10/1 v:v),
50 °C, 36 h; (b) K2CO3, MeOH, 40 °C; (c) Ac2O, Et3N, rt; (d) 4a or 4e, TESOTf, CH2Cl2, ꢀ15 °C; (e) K2CO3, MeOH, 40 °C.

P. Xu et al. / Bioorg. Med. Chem. Lett. 20 (2010) 5527–5531
5531
Table 1
In vitro antibacterial activities of compounds 7a–f and 13–16
Pathogens
MIC (lg/mL)
S. aureus ATCC 29213
S. aureus BK2464
S. epidermidis MRSE-1
S. pneumoniae ERSP-2
S. pyogenes 8902
S. pyogenes ERSPy-1
Clarithromycin
Telithromycin
0.25
0.125
>16
>16
>16
16
>32
>32
>16
>16
>16
>16
>16
>16
>16
>16
>16
>16
16
0.125
16
>16
16
16
16
0.125
16
16
16
8
16
16
8
32
0.25
16
>16
16
16
16
>16
16
4
2
0.25
16
16
8
8
1
>16
16
0.5
8
7a
7b
7c
7d
7e
7f
13
14
15
16
16
16
>16
>16
4
>16
4
>16
>16
4
>16
4
2
8
1
16
8
2
not, the compounds were inactive against either susceptible or
resistant strains.
Special Project on Major New Drug Innovation (2008ZX09401-
004 and 2009ZX09301-003).
Among compounds 13–16, no improvement in antibacterial
activity could be detected when an additional 11,12-carbamate
substructure with an aryl–alkyl side chain was introduced. This
is different from previous observations of ketolide series16 and
supposed here to be mainly because the conformations of the mac-
rolide core, different from those of ketolides,17 did not lead the side
chain to domain II of the 23S rRNA and, thus, no additional inter-
actions with the ribosome took place.18 Compared with compound
13, compound 14 showed at least a 4-fold more potent activity
against most pathogens. The same trend was observed when com-
paring 15–16, indicating that a 4000,6000-O-benzylidene acetal moiety
contributed to some extent to the enhancement of activity. We
speculated that the stronger activities of compounds 14 and 16
than 13 and 15 were due to hydrophobic interactions or aromatic
stacking effects between benzylidene groups with bacterial ribo-
some. The most active compound 14 exhibited 4-fold more active
against S. pyogenes ERSPy-1 than clarithromycin, 8-fold more ac-
tive against S. pneumoniae ERSP-2 and S. pyogenes 8902 than clar-
ithromycin, but still less potent than telithromycin.
In conclusion, a series of novel macrolides derivatives with
400-O-saccharide were synthesized. Derivatives with 400-O-saccha-
ride or with an incorporated 11,12-carbamate aryl–alkyl side chain
could not dramatically increase antibacterial activity compared
with the parent compound, suggesting that the bulky saccharide
structure did not fit well in the binding pocket and may have dis-
turbed the interactions of the macrolide with bacterial ribosome.
Even through, this work will enrich the structure types of de-
scribed macrolide derivatives and provide hints for future struc-
ture–activity relationship studies of macrolide antibiotics.
Supplementary data
1H NMR, 13C NMR, HRMS data of compounds 7a–7f, 13–16 and
experimental procedures of these compounds are available online
version, at doi:10.1016/j.bmcl.2010.07.072.
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This work was supported financially by the National Natural
Science Foundation of China (30572275) and National S&T Major