Synthesis and antibacterial activity of 3-O-acyl-6-O-carbamoyl erythromycin A derivatives

Article abstract of DOI:10.1016/j.bmcl.2005.10.074

A series of 3-O-acyl-6-O-carbamoyl erythromycin A derivatives has been synthesized. Several functional groups were identified as the optimal C3-substituents, and the best compounds in this series possess potent in vitro antibacterial activity against eryt

Full text of DOI:10.1016/j.bmcl.2005.10.074

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1054–1059
Synthesis and antibacterial activity of
3-O-acyl-6-O-carbamoyl erythromycin A derivatives
Bin Zhu,* Brett A. Marinelli, Darren Abbanat, Barbara D. Foleno, Todd C. Henninger,
Karen Bush and Mark J. Macielag
Drug Discovery, Johnson & Johnson Pharmaceutical Research and Development, L.L.C. 1000 Route 202, PO Box 300, Raritan,
NJ 08869, USA
Received 14 September 2005; accepted 21 October 2005
Available online 11 November 2005
Abstract—A series of 3-O-acyl-6-O-carbamoyl erythromycin A derivatives has been synthesized. Several functional groups were
identified as the optimal C3-substituents, and the best compounds in this series possess potent in vitro antibacterial activity against
erythromycin-susceptible and erythromycin-resistant bacteria.
Ó 2005 Elsevier Ltd. All rights reserved.
Erythromycin A is a well-established macrolide antibiot-
ic that has been used to treat bacterial infections for
more than five decades. Erythromycin A derivatives
with improved pharmacokinetic properties, such as clar-
ithromycin and azithromycin, were introduced in the
early 1990s. These second-generation macrolides are
now widely prescribed to treat upper and lower respira-
tory tract infections.1 The emergence of bacterial resis-
tance to macrolide antibiotics in recent years, however,
has compromised the therapeutic utility of the available
macrolides.2 The most widespread mechanisms of bacte-
rial resistance in the important respiratory pathogen,
Streptococcus pneumoniae, involve the Erm(B) methyl-
transferase, which methylates a specific adenine residue
in the macrolide binding site of the bacterial ribosome,
and the Mef(A) efflux pump.3
C11,C12-cyclic carbamate and an aryl side chain into
the macrolide skeleton to achieve potent antibacterial
activity against most erythromycin-susceptible and
erythromycin-resistant pneumococci.
The C3-cladinose group was long considered to be cru-
cial for the antibacterial activity of erythromycin A.
However, recent structural studies have revealed that
the C3-substituent of macrolides and ketolides contrib-
utes little to the binding interaction with the bacterial
ribosome and appears to occupy a region of consider-
able steric bulk tolerance.7 In addition, researchers from
Taisho have shown that macrolides in which the cladi-
nose sugar is replaced by a C3-O-acyl group are active
against many bacterial respiratory pathogens, including
erythromycin-resistant strains.8 We have recently identi-
fied a series of novel ketolides in which the aryl side
The need for more potent macrolide antibiotics has
prompted renewed interest in the macrolide research
area. The recent discovery of ketolides, in which the nat-
ural C3-cladinosyl group of erythromycin A is replaced
by a keto group, represents a significant breakthrough in
the search for macrolide antibiotics with activity against
resistant pneumococci.4 The two most advanced
ketolides reported to date are telithromycin5 and cethro-
N
N
N
N
O
O
H
N
O
O
N
O
O
O
O
11
11
OH
OH
6
6
12
12
mycin6 (Fig. 1). Both compounds incorporate
a
N
N
O
O
O
O
3
3
O
O
O
O
O
O
Keywords: Macrolide; Erythromycin; Antibiotic; Antibacterial.
*
Telithromycin
Cethromycin (ABT-773)
Corresponding author. Tel.: +1 908 704 4967; fax: +1 908 203
8109; e-mail: bzhu@prdus.jnj.com
Figure 1.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.10.074

B. Zhu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1054–1059
1055
O
O
O
R1
O
R1
O
O
O
O
O
O
H
N
H
N
H
N
R1
OH
2'
O
O
O
H
N
NH
NH
R1
O
N
H
N
H
O
O
a or b,c
O
O
O
11
11
O
OH
OH
6
6
6
12
12
OAc
6
N
N
N
O
O
O
O
O
O
N
O
O
2'
3
3
3
O
O
O
3
O
O
O
O
O
O
O
O
OH
R2
R
O
6-O-Carbamoyl Ketolides
3-O-Acyl-6-O-Carbamoyl
5
6
Macrolides
Scheme 2. Reagents and conditions: (a) (R2CO)2O, Et3N, DMAP,
CH2Cl2; (b) R2CO2H, DCC, DMAP, CH2Cl2; (c) MeOH, rt, yields
range from 16% to 73% for two steps.
Figure 2.
chain is attached to the macrolide core via a C6-carba-
mate linkage.9 To broaden our investigation of this 6-
O-carbamoyl series, we synthesized 6-O-carbamoyl
macrolides with various ester and carbamate groups at
the C3-position (Fig. 2). Herein we report the chemistry
and antibacterial activity of this series of 3-O-acyl-6-O-
carbamoyl macrolides.
O
O
O
H
N
R1
OH
2'
O
O
O
H
N
N
H
R1
OAc
O
N
H
O
O
a or b,c
O
6
6
N
O
O
N
O
O
2'
3
O
R2
R3
3
O
O
O
O
OH
N
O
The synthesis of the 3-O-acyl-6-O-carbamoyl macrolide
derivatives is outlined in Schemes 1–3. Intermediate 4
was prepared by methods described previously.9 Briefly,
erythromycin A was reacted with acetic anhydride in the
presence of triethylamine and 4-dimethylaminopyridine
(DMAP) to give 11,20,400-triacetylerythromycin A. Sub-
sequent elimination of the C11-O-acetyl group using
sodium bis(trimethylsilyl)amide led to 10,11-anhydro-
5
7
Scheme 3. Reagents and conditions: (a) R2NCO, DMAP; (b) (I)
Cl3COC(O)Cl, Et3N, DMAP, CH2Cl2; (II) HNR2 R3; (c) MeOH, rt,
yields range from 5% to 72% for two steps.
erythromycin A derivative 1. Treatment of compound
1 with trichloroacetyl-isocyanate, followed by base
hydrolysis (Et3N, MeOH/H2O), generated the C6,
C12-primary carbamates.
O
O
Under the reaction conditions, the C12-primary carba-
mate underwent spontaneous intramolecular Michael
addition to form the C11,12-cyclic carbamate. The Mi-
chael addition product 2 was obtained as a mixture of
C10-methyl epimers, which could be equilibrated to
the desired C10-b-epimer 3 by treatment with potassium
tert-butoxide. The C20-hydroxyl group of 3 was repro-
tected, and the C3-cladinose sugar was then selectively
removed under acidic conditions (1 N aq HCl, EtOH)
to give 3-descladinosyl derivative 4. Compound 4 was
reacted with various aldehydes under reductive alkyl-
ation conditions (trifluoroacetic acid, triethylsilane) to
install the desired aryl side chain at the C6-position
via a carbamate linkage (Scheme 1). The resulting C3-
hydroxy compound 5 was converted to the correspond-
ing C3-ester derivative using standard acylation
conditions. Removal of the C20-hydroxy-protecting
group by methanolysis yielded the target C3-ester com-
pounds 6 (Scheme 2).
10
11
11
OH
OH
O
OH
O
HO
a,b
HO
OH
2'
OAc
12
6
12
N
N
O
O
O
O
O
O
O
O
O
O
4"
MeO
MeO
O
OH
OAc
O
Erythromycin
1
O
O
H
N
H
N
O
O
NH2
NH2
e
c,d
O
6
O
O
O
11
11
OH
2'
OH
6
12
12
N
N
O
O
O
O
3
O
O
O
O
O
O
O
O
MeO
MeO
OAc
OAc
O
3
O
2
O
O
H
N
H
N
Two methods were utilized to synthesize the C3-carba-
mate macrolides 7 (Scheme 3). In the first method, C3-
hydroxy compound 5 was reacted with isocyanates in
the presence of 4-dimethylaminopyridine (DMAP) to
form the C3-secondary carbamate. Removal of the
C20-hydroxy-protecting group gave macrolide com-
pounds 7a–c. The second method involved treatment
of compound 5 with diphosgene in the presence of tri-
ethylamine and 4-dimethylaminopyridine (DMAP),
followed by reaction with various secondary amines.
Deprotection of the C20-hydroxyl group led to the
desired C3-tertiary carbamate compounds 7d-l.
R1
OAc
O
O
N
H
NH2
OAc
f,g
h
O
O
6
O
O
6
N
N
O
O
2'
O
O
3
3
O
O
O
OH
O
OH
4
5
Scheme 1. Reagents and conditions: (a) Ac2O, Et3N, cat DMAP,
CH2Cl2; (b) NaN(TMS)2, THF, 0 °C; (c) Cl3CC(O)NCO, CH2Cl2,
0 °C; (d) Et3N, MeOH, H2O, reflux; (e) KOtBu, THF, 0 °C; (f) Ac2O,
Et3N, CH2Cl2; (g) aq HCl, EtOH rt, 35% for seven steps; (h) R1CHO,
CF3CO2H, Et3SiH, CH3CN, 65 °C, yields range from 28% to 66%.

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B. Zhu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1054–1059
Table 1. In vitro antibacterial activity of 3-O-acyl-6-O-carbamoyl macrolides 6a–i and 7a–i
Compound
R
MIC (lg/ml)
Sta. aureusa
S. pneumoniaeb
S. pneumoniae [erm(B)]c
S. pneumoniae [mef(A)]d
H. influenzaee
EryA
Telith
8
—
—
—
0.5
0.06
0.03
0.03
>16
0.06
0.06
4
8
2
4
0.25
0.12
0.25
0.25
6a
6b
6c
6d
6e
6f
1
0.03
0.03
0.06
0.06
0.06
0.03
0.03
0.03
0.03
0.12
0.12
0.12
0.03
0.06
0.06
0.06
0.03
0.06
0.5
0.25
0.12
0.12
4
0.12
0.12
0.12
0.12
0.25
0.12
0.06
0.12
0.06
0.25
0.25
0.25
0.06
0.06
0.12
0.25
0.06
0.12
16
O
0.5
0.5
1
8
O
8
O
O
16
16
4
O
0.5
0.25
0.12
0.25
0.25
1
N
N
0.12
0.12
0.25
2
O
6g
6h
6i
4
O
N
4
O
N
O
8
N
H
N
7a
7b
7c
7d
7e
7f
4
16
16
16
8
O
O
OCH3
H
N
1
2
F
H
N
1
4
O
O
N
N
0.25
0.5
0.5
1
0.25
0.12
0.25
0.25
0.5
0.25
8
O
N
N
16
8
O
O
7g
7h
7i
N
N
N
N
0.25
0.5
4
O
O
N
16
a Staphylococcus aureus OC4172 (erythromycin-susceptible).
b Streptococcus pneumoniae OC9132 (erythromycin-susceptible).
c Streptococcus pneumoniae OC4051 (erm(B)-containing, erythromycin-resistant).
d Streptococcus pneumoniae OC4438 (mef(A)-containing, erythromycin-resistant).
e Haemophilus influenzae OC4882.

B. Zhu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1054–1059
1057
O
O
HN
O
HN
O
H
N
O
H
N
N
O
N
O
O
11
O
11
N
O
OH
N
6
12
OH
6
12
N
O
O
N
O
O
3
3
O
O
O
O
O
O
R
8
6a-i & 7a-i
Figure 3.
Table 2. In vitro antibacterial activity of 3-O-acyl-6-O-carbamoyl macrolides 6j–o and 7j–l
O
O
R1
OH
H
N
O
N
H
O
11
O
12
6
N
O
O
3
O
O
O
R
6j-o & 7j-l
Compound
R
R1
—
MIC (lg/ml)
Sta. aureusa S. pneumoniaeb S. pneumoniae [erm(B)]c S. pneumoniae [mef(A)]d H. influenzaee
Telith
—
0.25
0.5
0.5
0.5
0.25
0.5
0.5
0.5
0.5
1
0.03
0.06
0.06
0.03
0.03
0.06
0.06
0.06
0.06
0.12
0.06
0.12
0.25
0.12
0.06
0.25
0.12
0.12
0.25
0.5
0.25
0.06
0.25
0.12
0.03
0.12
0.06
0.06
0.12
0.12
2
4
2
2
4
4
2
4
4
4
N
N
6j
O
N
N
N
6k
6l
O
O
N
N
N
6m
6n
6o
7j
O
O
O
N
N
N
N
N
N
N
O
O
O
N
N
7k
7l
N
N
a Staphylococcus aureus OC4172 (erythromycin-susceptible).
b Streptococcus pneumoniae OC9132 (erythromycin-susceptible).
c Streptococcus pneumoniae OC4051 (erm(B)-containing, erythromycin-resistant).
d Streptococcus pneumoniae OC4438 (mef(A)-containing, erythromycin-resistant).
e Haemophilus influenzae OC4882.
The in vitro antibacterial activity of the 3-O-acyl-6-O-
carbamoyl macrolide compounds was assessed against
both erythromycin-susceptible and erythromycin-resis-
tant bacteria, with particular emphasis on respiratory
pathogens. Data are presented in Table 1 as the minimal
inhibitory concentration (MIC)10 against a panel of five

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B. Zhu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1054–1059
selected bacterial strains, including macrolide-suscepti-
ble Staphylococcus aureus (Smith) OC4172, macrolide-
susceptible S. pneumoniae OC9132, macrolide-resistant
S. pneumoniae OC4051 with the erm(B) ribosomal meth-
ylase gene, macrolide-resistant S. pneumoniae OC4438
containing the Mef(A) efflux pump, and Haemophilus
influenzae OC4882, a Gram-negative respiratory patho-
gen. Twofold differences in the MIC value are within the
error of the method.
H. influenzae compared to the corresponding C6-O-
(E)-3-[4-(2-pyrimidinyl)phenyl]-prop-2-enyl analog 6c.
In conclusion, an efficient synthesis of a novel series of
3-O-acyl-6-O-carbamoyl macrolide compounds has
been developed. Compounds with various ester and car-
bamate groups at the C3-position were studied for their
antibacterial activity against erythromycin-susceptible
and erythromycin-resistant bacteria. Several functional
groups have been identified as the optimal substituents
at the C3-position, and the combination of these substit-
uents with a variety of C6-O-carbamoyl side chains was
examined. The best compounds of this series (6g, 6l, 6m,
and 6o) possess in vitro antibacterial activity compara-
ble to that of telithromycin. Further studies on this
series of macrolide compounds will be reported in the
future.
To readily interpret the structure–activity relationships
of the C3-position substituent, the C6-carbamoyl side
chain was initially held constant, while various C3-esters
and C3-carbamates were examined. (E)-3-[4-(2-Pyrimid-
inyl)phenyl]prop-2-enyl was chosen as the initial C6-side
chain based on the antibacterial profile of the corre-
sponding ketolide analog 8 (Fig. 3).9
Among the C3-ester derivatives, compounds with an
isopropyl or pyridylmethyl substituent possessed the
best antibacterial activity, particularly against erythro-
mycin-resistant S. pneumoniae strains and H. influenzae
(Table 1). Attempts to improve activity by shortening
or lengthening the pyridylalkyl chain, as in compounds
6e and 6i, had a detrimental effect against erm(B)-con-
taining S. pneumoniae. Compound 6g, with a 3-pyr-
idylmethyl substituent, exhibited in vitro activity
comparable to those of telithromycin and the C6-carba-
mate ketolide analog 8. In the C3-carbamate series, ana-
logs with a tertiary carbamate, such as compounds 7d–i,
had an antibacterial profile better than that of com-
pounds with a secondary carbamate (compounds 7a–
c). The detrimental effect of the secondary carbamate
functionality was particularly apparent in the activity
of 7a–c against the erm(B)-containing S. pneumoniae
strain, with MIC values ranging from 2 to 4 lg/ml.
N,N-Diethyl carbamate appeared to be the preferred
C3-carbamate substituent, in view of the activity of 7e
against the erythromycin-resistant S. pneumoniae
strains.
Acknowledgment
The authors thank Ellyn Wira for contributions to
in vitro testing.
References and notes
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In reviewing the data of Table 1, it is noteworthy that
while the C3-substituent had a dramatic effect on activ-
ity against the erm(B)-containing S. pneumoniae strain,
activity against the S. pneumoniae strain resistant to
erythromycin due to efflux was largely unaffected by
the structure of the ester or carbamate group. This result
is consistent with previous studies which indicate that
the cladinose sugar is a key molecular feature for recog-
nition by the Mef efflux pump.11
Once the optimal C3-substituents were identified, we
combined these preferred substituents with different
C6-carbamoyl side chains to search for compounds with
further improvements in antibacterial activity, especially
against H. influenzae. Table 2 provides data for some
representative macrolides (6j–o and 7j–l) containing
such combinations. Compound 6m, with a C6-O-(E)-3-
(3-quinolinyl)prop-2-enyl-carbamoyl group and a C3-
isopropyl ester, demonstrated improved activity against
erythromycin-resistant S. pneumoniae strains compared
to 6c. Compounds with less lipophilic C6-position sub-
stituents, such as 6o, showed improved activity against
7. Schlunzen, F.; Harms, J. M.; Franceschi, F.; Hansen, H.
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B. Zhu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1054–1059
1059
10. The minimal inhibitory concentrations (MICs) were
determined by the broth microdilution method according
to National Committee for Clinical Laboratory Standards
(NCCLS). Methods for Dilution Antimicrobial Susceptibil-
ity Tests for Bacteria that Grow Aerobically, 5th ed.;
Approved standard: NCCLS Document M7-A5, 2000.
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