Synthesis and antibacterial activity of 6-O-heteroarylcarbamoyl-11,12- lactoketolides

Article abstract of DOI:10.1016/j.bmcl.2005.12.097

A new series of erythromycin A derivatives, the 6-O-heteroarylcarbamoyl-11, 12-lactoketolides, with activity against macrolide-resistant streptococci, are described. Structurally, these macrolide antibiotics are characterized by a heteroaryl side chain at

Full text of DOI:10.1016/j.bmcl.2005.12.097

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1929–1933
Synthesis and antibacterial activity of
6-O-heteroarylcarbamoyl-11,12-lactoketolides
Eugene B. Grant,* Deodialsingh Guiadeen, Darren Abbanat, Barbara D. Foleno,
Karen Bush and Mark J. Macielag
Johnson & Johnson Pharmaceutical Research & Development, L.L.C., 1000 Route 202, PO Box 300, Raritan, NJ 08869, USA
Received 17 October 2005; accepted 22 December 2005
Available online 30 January 2006
Abstract—A new series of erythromycin A derivatives, the 6-O-heteroarylcarbamoyl-11,12-lactoketolides, with activity against mac-
rolide-resistant streptococci, are described. Structurally, these macrolide antibiotics are characterized by a heteroaryl side chain
attached to the macrolactone core through a carbamate linkage at the C6 position, as well as 11,12-c-lactone and 3-keto function-
alities. The synthesis and antibacterial activity of this new series of ketolides are discussed.
Ó 2006 Elsevier Ltd. All rights reserved.
The macrolide antibiotics, erythromycin A (1), clari-
thromycin, and azithromycin, have been widely pre-
scribed to treat respiratory tract infections.1 Due to
their widespread use, resistance in the important respira-
tory pathogen, Streptococcus pneumoniae, has emerged
at an alarming rate.2 The primary mechanisms of resis-
tance in S. pneumoniae are ribosomal methylation (cata-
lyzed by the erm methylase) and intracellular removal of
the macrolide by efflux (mediated by the mef-gene prod-
uct).3 As the prevalence of erythromycin resistance has
increased, the interest in discovery of new, more effective
macrolide antibiotics with activity against resistant
respiratory pathogens has intensified.
Telithromycin (2) and cethromycin (3) share certain
structural features, including an 11,12-cyclic carbamate
functionality and a 3-keto group. The cyclic carbamate
appears to play a conformational role in ribosomal
N
N
O
N
OH
HO
OH
OH
O
O
O
N
O
N
O
O
O
O
The ketolides, which include telithromycin (2)4 and
cethromycin (3),5 are the latest generation of semisyn-
thetic macrolide antibiotics, with improved activity
against erythromycin- and penicillin-resistant isolates
of S. pneumoniae. Telithromycin (2) has demonstrated
clinical efficacy against most erythromycin-susceptible
and -resistant strains of S. pneumoniae, Streptococcus
pyogenes, and Haemophilus influenzae, and is marketed
for the treatment of community-acquired upper and
lower respiratory tract infections.6 Cethromycin has
progressed to late stage clinical development.5
O
11
OH
6
12
O
N
O
O
3
OMe
O
O
O
OH
Telithromycin
Erythromycin
(1)
(2)
O
N
H
O
N
O
O
O
11
OH
6
12
N
O
3
O
O
O
Keywords: Ketolide; Antibacterail activity.
*
Cethromycin
Corresponding author. Tel.: +1 908 218 6589; e-mail:
egrant@prdus.jnj.com
(3)
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.12.097

1930
E. B. Grant et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1929–1933
O
O
binding, whereas the ketone is important in circumvent-
ing efflux resistance. Both 2 and 3 contain hetero-
arylalkyl side chains that are critical in overcoming
resistance due to ribosomal methylation. Although the
side chains are tethered to different positions of the mac-
rolactone nucleus (i.e., at the carbamate nitrogen and
O6, respectively), structural studies have shown that
they bind to similar sites within domain II of the bacte-
rial ribosome.7
9
OH
O
OH
O
HO
HO
OH
6
12
OBz
OH
a, b, c
O
N
O
N
3
O
O
O
O
O
O
O
O
OMe
O
OMe
OH
OAc
O
NH2
1
7
O
O
O
Recent structure–activity relationship (SAR) studies have
shown that a variety of synthetic modifications of the ba-
sic ketolide core structure can provide analogs with simi-
lar activity as 2 and 3 against erythromycin-susceptible
and -resistant respiratory pathogens. In particular,
researchers at Johnson & Johnson Pharmaceutical
Research & Development have reported on a novel series
of ketolides, represented by 4, in which the hetero-
arylalkyl group is attached to the macrolactone ring by
a C6-carbamate linkage.8 In addition, scientists at Roche
and Basilea have described a new series of ketolides with a
fused five-membered lactone ring in place of the cyclic
carbamate.9 Compound 5 has an antimicrobial spectrum
similar to that of 2. Wewere interested in the effect on anti-
bacterial activity of combining these two structural
modifications in a single molecule. Herein, we report the
synthesis and antibacterial activity of a new series of
macrolide antibiotics, the 6-O-carbamoyl-11,12-lacto-
ketolides represented by structure 6.
g,h,
d,e,f
12
O
6
OBz
O
N
O
O
O
O
OMe
O
O
NH2
O
O
OAc
O
NH2
O
8
O
O
O
i
O
OBz
OBz
O
O
N
O
O
N
O
OH O
O
O
O
R
1.5: 1 S:R
1.5: 1 S:R
O
9
10
O
O
NH
O
O
j,k
OH
O
O
N
O
O
O
6
1.5: 1 S:R
The synthesis of 6 started with commercially available
erythromycin A (1) (Scheme 1). Selective acylation of
the 20 hydroxyl group of 1 with benzoic anhydride was
followed by exhaustive acetylation to provide the
400,11-diacetate. Elimination of the C11-acetate under
basic conditions afforded the 10,11-anhydroerythromy-
Scheme 1. Synthesis of 6-O-carbamoyl-11,12-lactoketolides. Reagents
and conditions: (a) Bz2O, Et3N, CH2Cl2, rt, 18 h; (b) Ac2O, Et3N,
DMAP, CH2Cl2, rt, 18 h; (c) NaHMDS, THF, 0 °C, 2 h; (d) Ac2O,
Et3N, DMAP, pyridine, rt, 18 h; (e) Cl3CC(O)NCO, CH2Cl2, 0 °C, 3 h;
(f) 10% aq NaOH, rt, 2 h; (g) LDA, THF, ꢀ78 to 0 °C, 5 h; (h) HCl,
EtOH, H2O, rt, 20 h; (i) EDCI, pyr.TFA, DMSO, CH2Cl2, 0 °C, 3 h;
12% overall yield from 1; (j) RCHO, Et3SiH, TFA, CH3CN; (k)
MeOH, reflux, 12 h 14–25%.
N
N
N
H2N
cin A derivative 7. Selective acetylation of the 12-hy-
droxyl group was readily accomplished upon treatment
with acetic anhydride in pyridine containing a catalytic
amount of dimethylaminopyridine. Carbamoylation of
the C-6 hydroxy with trichloroacetylisocyanate, fol-
lowed by treatment with 10% aqueous sodium hydrox-
ide, afforded primary carbamate 8 in good yield.8
Ester 8 was treated with 5 equivalents of lithium
diisopropylamide in THF at ꢀ78 °C, and the resulting
heterogeneous mixture was allowed to warm to 0 °C
over 5 h. Following acidic workup that effected the
removal of the cladinose sugar, and careful chromatog-
raphy, alcohol 9 was isolated as a 1.5:1 inseparable
mixture of C10-epimers in 30% yield. With the core
structure prepared, alcohol 9 was converted to ketolide
10 by Pfitzner–Moffatt oxidation. Attempts to epimerize
the C10-methyl group of 9 by treatment with the
equilibrating base potassium tert-butoxide, the strong
base lithium diisopropylamide, or simply by stirring in
methanol were unsuccessful,9 with no change in the ratio
of diastereomers. At this point, the stereochemical
outcome of the intramolecular conjugate enolate addi-
tion was studied further.10 While the selective attack of
the C12-ester enolate of 8 to the C11-position must
N
N
N
O
9
O
9
S
HN
O
O
NH
O
O
O
OMe
O
O
OH
O
N
11
11
OH
N
12
6
6
12
O
O
O
O
O
O
O
O
5
4
6-Carbamate
Linker
11,12-
Lactone
Heteroaryl
O
9
HN
O
O
O
O
OH
O
N
11
12
6
O
O
O
O
6-O-Carbamoyl-11,12-lactoketolide (6)

E. B. Grant et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1929–1933
1931
occur from the Si-face of the double bond leading to the
11-(S) absolute configuration,11 the stereochemistry at
C10 could not be predicted a priori. A small amount
of the major diastereomer of 9 could be isolated by
RP-HPLC and the absolute configuration at C10 was
investigated by NMR. After assignment of the reso-
nances by COSY, HETCOR, and HMBC, NOESY
experiments revealed the unnatural (S) stereochemistry
at C10, with C2, C11, and C12 retaining the expected
R, S, and R absolute configurations, respectively.12
Additional support for this structural assignment was
provided by the NMR data for the C10-diastereomers
of compound 11,9 an intermediate in the synthesis of a
ketolide lactone analog from Roche/Basilea. In 11, the
chemical shift of the proton at C13 appeared at
5.01 ppm in the unnatural C10-(S) diastereomer and at
5.53 ppm in the natural C10-(R) diastereomer. In the
case of 9, the chemical shift of the C13-proton appeared
at 5.10 ppm, again suggesting the (S)-configuration at
C10. The heteroaryl side chain was appended to the pri-
mary carbamate of 9 (used as a 1.5:1 mixture of S:R dia-
stereomers) by treatment with an appropriately
substituted aldehyde in the presence of trifluoroacetic
acid and triethylsilane in acetonitrile at 80 °C for 12 h.
This was followed by methanolysis of the 20-benzoyl
protecting group to afford a 10–25% yield of heteroaryl-
substituted 6 as a mixture of epimers.
included in the testing panel. For S. aureus (Smith),
MIC values were determined in the absence and pres-
ence of 50% mouse serum to assess the effect of serum
proteins on antibacterial activity in the ketolides.
Activities of the ketolides against the susceptible S. aure-
us and S. pneumoniae strains were essentially indepen-
dent of the composition of the heteroaryl side chain,
with MIC values ranging from 0.25 to 1 lg/ml and
60.015 to 0.06 lg/ml, respectively, comparable to those
of erythromycin and telithromycin. MIC values of the
ketolides against the mef(A)-containing S. pneumoniae
strain were 8- to 16-fold lower than for erythromycin,
but once again the structure of the heteroaryl side chain
had little impact on the level of activity.
In contrast, the side-chain substituent had a more dra-
matic effect on activity against the erm(B)-containing
S. pneumoniae strain. Generally, lower MIC values were
observed for ketolides containing a biaryl- or a fused
heteroarylpropenyl substituent than for analogs with a
single aromatic ring in the side chain (6i) or with a ben-
zylic linker (6a and b). Quinoline analogs 6c, h, and j had
comparable activity to telithromycin (2) against this
strain.
Activity against the Gram-negative respiratory patho-
gen, H. influenzae, was somewhat more capricious, with
MIC values ranging from 2 lg/ml for the 6- and 7-quin-
olinyl analogs 6d and 6h to 16 lg/ml for pyridylphenyl
analog 6e. The ketolides with the best activity against
H. influenzae (6d, h, i, j, k, and l) had similar MIC values
as telithromycin (2).
As expected, the diastereomeric ratio observed for 9
remained constant after alkylation and deacetylation
to afford 6a–l. Generally, Attempts to separate the dia-
stereomers of 6a–l by chromatography were unsuccess-
ful; thus, these analogs were tested as a 1.5:1 mixture
of S:R isomers at C10.
An unanticipated property of this novel series of keto-
lides was the significant reduction of antibacterial activ-
ity (4- to 16-fold increase in MIC) for nearly all analogs
in the presence of mouse serum, presumably due to
binding to serum proteins. It was possible to reduce pro-
tein binding by decreasing the number of aryl rings in
the side chain (6i), but as mentioned above this came
at a cost of activity against the erm-containing S. pneu-
moniae strain.
S
O
9
O
O
OMe
O
OAc
O
N
6
13
O
In the case of the pyrimidinylphenyl analog 6m, separa-
tion of the C-10 epimers was accomplished by HPLC in
order to determine the effect of C-10 stereochemistry on
antibacterial activity (Table 2). As expected from
previous literature reports,10 the MIC values of the lac-
toketolides were significantly influenced by C-10-stereo-
chemistry, with MIC values for the (S)-isomer 2- to
32-fold higher than those for the natural (R)-isomer
(compare (10R)-6m to (10S)-6m). It is noteworthy that
the antibacterial activity of (10R)-6m was comparable
to that of the corresponding analog from the ketolide
series with the 11,12-cyclic carbamate (4),8 but with a
slight decrease in activity against the mef(A)-containing
S. pneumoniae strain.
O
O
11
In vitro antibacterial activities of ketolides 6a–l were
established against a five-membered panel of bacteria,
including erythromycin-susceptible and -resistant
strains. Minimum inhibitory concentrations (MIC) were
determined by the NCCLS-approved broth microdilu-
tion method and are reported in Table 1.13 Twofold dif-
ferences are considered to be within the error of the
method. Among the strains tested, Staphylococcus aure-
us (Smith) OC4172 and S. pneumoniae ATCC6301 are
erythromycin-susceptible, S. pneumoniae OC4051 is
erythromycin-resistant due to an erm(B)-encoded ribo-
somal methylase, and S. pneumoniae OC4421 is erythro-
mycin-resistant due to a mef(A)-encoded efflux-pump. A
representative strain (OC4882) of the important Gram-
negative respiratory pathogen H. influenzae was also
In conclusion, the SAR study of the heteroaryl-substi-
tuted lactoketolides (6) showed the importance of tether
length, the nature of the heteroaromatic substituent,
and C10-stereochemistry. As a class, the lactoketolides

1932
E. B. Grant et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1929–1933
Table 1. In vitro antibacterial activity of 6-O-heteroarylcarbamoyl-11,12-lactoketolidesa
Compound
Heteroaryl
MIC (lg/mL)
S. aureus
S. aureus
S. pneum.
S. pneum.
S. pneum.
H. inf.
(+serum)
erm(B)
mef(A)
1
—
—
0.5
0.12
0.5
4
0.06
>16
4
8
2
0.12
0.5
1
0.015
0.06
0.06
0.5
0.12
0.5
2
8
6a
6b
6c
6d
6e
6f
6g
6h
6i
N
4
0.06
2
0.5
8
N
0.5
0.5
1
2
60.015
0.03
0.12
0.12
0.25
0.25
0.5
0.25
0.25
0.5
8
N
2
2
N
8
0.03
16
8
N
0.5
0.5
0.25
1
2
0.06
0.5
N
2
NDb
0.03
0.5
8
N
4
0.12
2
0.25
0.5
2
N
1
0.03
4
N
6j
0.5
4
60.015
0.25
0.25
4
N
N
N
6k
6l
0.5
0.5
2
2
0.03
0.06
0.25
0.25
0.5
0.5
4
4
N
N
a S. aureus: Staphylococcus aureus (Smith) OC4172; S. aureus (+serum): Staphylococcus aureus (Smith) OC4172 in the presence of 50% mouse serum;
S.pneum.: Streptococcus pneumoniae ATCC6301; S. pneum. erm(B): Streptococcus pneumoniae OC4051; S. pneum. mef(A): Streptococcus pneu-
moniae OC4421; H. inf.: Haemophilus influenzae OC4882. See text for detailed description.
b Not determined.
Table 2. In vitro antibacterial activity
Compound
MIC (lg/mL)
S. pneum. S. pneum. erm(B)
S. aureus
S. aureus. (+serum)
S. pneum. mef(A)
H. inf.
1
0.5
0.12
0.5
0.06
0.015
0.03
0.03
0.12
>16
0.06
0.06
0.06
2
4
8
2
4
8
8
2
0.12
0.12
0.12
2
0.12
0.06
0.25
1
4
0.25
0.5
(10R)-6m
(10S)-6m
16
See Table 1 for detailed description of strains.

E. B. Grant et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1929–1933
1933
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N
N
N
O
O
HN
HN
O
O
O
O
O
O
O
O
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Bonnefoy, A.; Bretin, F.; Chantot, J.-F.; Dussart, A.;
OH
N
OH
N
`
Fromentin, C.; D’Ambrieres, S. G.; Lachaud, S.; Laurin,
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O
O
O
O
O
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O
O
O
O
(10R)-6m
(10S)-6m
appeared to be more highly protein bound than corre-
sponding analogs bearing an 11,12-cyclic carbamate.8
The best compounds in this series, however, possess
in vitro antibacterial activity comparable to telithromy-
cin (2). In preparing this new series of ketolides, the dual
synthetic challenges of intramolecular 1,4-addition of an
unsubstituted acetate in the presence of a base labile car-
bamate and selective acetylation of the C12-tertiary
alcohol were overcome.
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Acknowledgments
9. Hunziker, D.; Wyss, P.-C.; Angehrn, P.; Mueller, A.;
Marty, H.-P.; Halm, R.; Kellenberger, L.; Bitsch, V.;
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Johnson, P. M.; Flamm, R. K.; Henry, R. F.; Plattner, J.
J. J. Antibiot. 1996, 49, 465.
Contributions to in vitro testing by Ellyn Wira are grate-
fully acknowledged. The authors thank Amy Maden
for NMR structure determination of the C10-epimers
and Dr. Xun Li for the preparation of advanced
intermediates.
11. Baker, W. R.; Clark, J. D.; Stephens, R. L.; Kim, K. H.
J. Org. Chem. 1988, 53, 2340.
References and notes
12. A weak NOE (ꢁ5%) was observed between the protons of
the lactone ring and the C10-proton, suggesting a syn-
relationship.
13. National Committee for Clinical Laboratory Standards.
Methods for Dilution Antimicrobial Susceptibility Tests
for Bacteria that Grow Aerobically, 5th ed.; Approved
Standard: NCCLS Document M7-A5, 2000.
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