Synthesis and SAR of novel conformationally restricted oxazolidinones possessing Gram-positive and fastidious Gram-negative antibacterial activity. Part 2: Amino substitutions on heterocyclic D-ring system

Article abstract of DOI:10.1016/j.bmcl.2007.05.085

A novel series of conformationally restricted oxazolidinones was synthesized, in which the heterocyclic D ring was substituted with various amino groups. Several analogs exhibited potent activity against both Gram-positive and fastidious Gram-negative org

Full text of DOI:10.1016/j.bmcl.2007.05.085

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4699–4702
Synthesis and SAR of novel conformationally restricted
oxazolidinones possessing Gram-positive and fastidious
Gram-negative antibacterial activity. Part 2: Amino
substitutions on heterocyclic D-ring system
Allison L. Choy,* J. V. N. Vara Prasad, Frederick E. Boyer,
Michael D. Huband and Michael R. Dermyer
Pfizer Global Research and Development, 2800 Plymouth Rd., Ann Arbor, MI 48105, USA
Received 17 April 2007; revised 23 May 2007; accepted 25 May 2007
Available online 31 May 2007
Abstract—A novel series of conformationally restricted oxazolidinones was synthesized, in which the heterocyclic D ring was substi-
tuted with various amino groups. Several analogs exhibited potent activity against both Gram-positive and fastidious Gram-nega-
tive organisms. Certain amino-substituted analogs also exhibited improved aqueous solubility compared to the corresponding
un-substituted heterocyclic D-ring analogs.
Ó 2007 Elsevier Ltd. All rights reserved.
Linezolid (ZyvoxTM) 1 is the first oxazolidinone to be ap-
N
NH
O
F
proved for clinical use against serious Gram-positive
bacterial infections (Fig. 1).1 We have been interested
in developing oxazolidinone antibacterial agents with a
broader spectrum of antibacterial activity. We recently
disclosed a novel series of conformationally restricted
oxazolidinones, exemplified by the lead compound 2.2,3
Compound 2 has activity against Gram-positive
(Staphylococcus aureus MIC = 0.5 lg/mL, Streptococcus
pneumoniae MIC = 0.25 lg/mL) and the fastidious
Gram-negative respiratory tract pathogens (Haemophilus
influenzae and Moraxella cattarhalis MICs = 2 lg/
mL).2 However, the poor aqueous solubility of
compound 2 (14 lg/mL) was considered a barrier to
good oral absorption. In order to improve the aqueous
solubility of this series, the 3-position of the pyrazole
ring was substituted with hydrophilic functionality to
give amino-substituted analogs 3. The synthesis and
SAR of these amino-substituted analogs is presented
here.3
N
O
O
N
O
N O
1
2
NHAc
O
NHAc
N
RHN
DNH
C
B
N A
O
3
NHAc
Figure 1.
Scheme 1. Bromination of ketone 4 with pyridinium
tribromide and glacial acetic acid gave the bromoketone
5 in quantitative yield. Treatment of bromoketone 5
with the appropriately substituted thiosemicarbazide
gave the desired amino-substituted pyrazoles 3, albeit
in low to modest yields (5–50%).5
Thiosemicarbazides that were not commercially avail-
able were prepared according to literature methods6 as
shown in Scheme 2. The appropriately substituted
amines 6a–h were treated with triethylamine and carbon
disulfide to give the dithiocarbamates 7a–h. The dithio-
carbamates 7a–h were then heated with hydrazine mono-
hydrate to give the thiosemicarbazides 8a–h. In the case
The amino-substituted pyrazoles 3 were synthesized
from the known ketone 44 in two steps as shown in
Keywords: Oxazolidinones; Antibacterials; Pyrazoles.
*
Corresponding author. Tel.: +1 734 622 4829; fax: +1 734 622
2265; e-mail: Allison.Choy@pfizer.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.05.085

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A. L. Choy et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4699–4702
S
N
RHN
O
O
R
NH
Br
N
NHNH2
b
a
H
O
O
O
N
O
N
O
N O
4
5
3
NHAc
NHAc
NHAc
Scheme 1. Reagents and conditions: (a) pyridinium tribromide, glacial HOAc, CH2Cl2, rt, 100%; (b) abs EtOH, 88 °C, 5–50%.
54% yields by removal of the Boc-protecting groups in
3u and 3v with acetyl chloride in methanol.
S
a
b
X
X
N
SCH3
NH2
n
n
H
6a-h
7a-h
The amino-substituted oxazolidinone analogs were
tested against a panel of Gram-positive and fastidious
Gram-negative bacteria.7 Minimum inhibitory concen-
tration (MIC, in lg/mL) values were determined by
microbroth methodology.7 The Escherichia coli in vitro
transcription and translation (EC TnT) assay was per-
formed in 96-well microtiter plates using a luciferase
reporter system.8 The activities of the amino-substituted
analogs are summarized in Table 1. MIC data for
linezolid 1 and the lead pyrazole compound 2 are
provided for comparison.
S
X
N
n
NHNH2
H
8a-h
a, n = 1; X = CH3; b, n = 1; X = OCH3; c, n = 2; X
= OCH3; d, n = 1; X = NHCO2 -Bu; e, n = 2; X =
NHCO2 -Bu; f, n = 2; X = N(CH2CH3)2; g, n = 1; X
= OSi(CH3)2 -Bu; h, n = 2; X = OSi(CH3)2 -Bu
t
t
t
t
Scheme 2. Reagents and conditions: (a) i—CS2, NEt3, CH3OH, Et2O,
0 °C–rt; ii—CH3I, 0 °C–rt; (b) hydrazine monohydrate, 2-methoxy-
methanol, 80 °C.
The in vitro activity of analogs with small alkylamino
substituents at the 3-position of the pyrazole ring (3a–
e) was similar to that of the lead pyrazole compound
2 (Table 1), except that they were somewhat less active
against the Gram-positive pathogens S. aureus and
Enterococcus faecalis. The aminopyrazoles 3a–c re-
tained activity against the fastidious Gram-negative
bacteria H. influenzae and Moraxella catarrhalis
(MICs = 2–4 lg/mL). Analogs with larger alkylamino
substituents (3d–e) showed a loss of fastidious Gram-
negative activity, but maintained Gram-positive anti-
bacterial activity similar to or better than linezolid 1.
In general, analogs with ‘R’ groups containing polar
functionalities showed reduced antibacterial activity.
The substituted ethylaminopyrazole analogs 3i–m and
the substituted propylaminopyrazole analogs 3o–r
exhibited decreased antibacterial activity against both
Gram-positive and fastidious Gram-negative organ-
of 4-(2-tert-butyldimethylsilanyloxyethyl)-3-thiosemi-
carbazide 8g and 4-(3-tert-butyldimethylsilanyloxypro-
pyl)-3-thiosemicarbazide 8h, reaction with bromoketone 5
resulted in in situ cleavage of the silyl ethers and
isolation of the hydroxy analogs 3i and 3o, respectively.
The 3-aminopyrazole analog 3a (R = H) and the N-acyl
pyrazoles 3s–t were prepared from the p-methoxybenzyl
(PMB) amine 3h as shown in Scheme 3. Treatment of
the p-methoxybenzylamine 3h with triethylsilane and tri-
fluoroacetic acid (TFA) gave the free amine 3a in 87%
yield. Coupling of amine 3a with (S)-2,2-dimethyl-1,3-
dioxolane-4-carboxylic acid, followed by treatment with
1 M hydrochloric acid, gave amide 3s. Similarly, amine
3a was coupled with benzyloxyacetic acid, followed by
debenzylation with palladium on carbon to give amide
3t. The amino analogs 3k and 3q were prepared in 43–
OH
O
H
H
N
N
N
N
N
H2N
HO
NH
NH
NH
PMB
a
b
O
O
O
N
O
N
O
N O
3h
3a
3s
NHAc
NHAc
NHAc
H
N
N
NH
HO
c
O
O
O
N
3t
NHAc
O
H
N
n
H
N
n
N
N
NH
NH
t-BuO
N
H2N
d
H
O
O
N
O
N O
3u n = 1
3v n = 2
NHAc
3k n = 1
3q n = 2
NHAc
Scheme 3. Reagents and conditions: (a) Et3SiH, TFA, CH2Cl2, 87%; (b) i—N-(3-dimethylaminopropyl)-N0-ethylcarbodiimide hydrochloride,
pyridine, (S)-2,2-dimethyl-1,3-dioxolane-4-carboxylic acid, 25%; ii—1 M HCl, THF, 85%; (c) i—N-(3-dimethylaminopropyl)-N0-ethylcarbodiimide
hydrochloride, pyridine, benzyloxyacetic acid, 47%; ii—Pd/C, MeOH, 73%; (d) CH3COCl, CH3OH, 0 °C–rt, 43–54%.

A. L. Choy et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4699–4702
4701
Table 1. Escherichia coli in vitro transcription and translation (EC TnT) assay results (IC50, lM) and minimum inhibitory concentrations (MICs,
lg/mL) for compounds 1, 2, 3a–t
Compound
R
EC TnT IC50 (lM)
S.a.
S.p.
E.f.
H.i.
M.c.
1 linezolid
2
n/a
n/a
0.95
0.45
1.2
1
2
0.50
1
4
0.25
8
2
8
2
0.25
0.5
0.5
0.25
0.5
0.25
0.5
1
3a
3b
3c
3d
3e
3f
–H
–CH3
–CH2CH3
4
2
2
4
2
1
4
2
1.3
2.2
2.5
—
1
1
4
4
–CH(CH3)2
–CH2CH2CH3
–Ph
4
2
8
8
2
1
4
8
1
0.5
1
4
2
3g
3h
3i
–CH2Ph
2
2
8
4
–CH2Ph(4-OMe)
–(CH2)2OH
–(CH2)2OMe
–(CH2)2NH2
–(CH2)2(N-Morpholine)
–(CH2)2(2-Pyridyl)
–(CH2)2Ph(4-OH)
–(CH2)3OH
–(CH2)3OMe
–(CH2)3NH2
–(CH2)3NEt2
–C(@O)CH(OH)CH2OH
–C(@O)CH2OH
2.2
1.2
2.6
0.8
1.9
1.7
2.1
1.8
2.9
0.45
13
1
0.5
2
1
8
4
8
8
2
16
8
3j
2
1
2
16
1
3k
3l
32
16
8
4
16
8
4
2
8
1
16
8
3m
3n
3o
3p
3q
3r
3s
3t
0.5
0.5
4
8
2
1
4
2
8
8
16
16
>64
32
32
16
32
16
32
32
32
16
4
1
4
32
64
16
16
4
8
4
32
16
4
0.78
1.3
2
4
Strains: S.a., Staphylococcus aureus UC-76 SA-1; S.p., Streptococcus pneumoniae SV1 SP-3; E.f., Enterococcus faecalis MGH-2 EF1-1; H.i., Hae-
mophilus influenzae HI-3542; M.c., Moraxella catarrhalis BC-3531.
isms, when compared to the alkylaminopyrazoles 3a–e
and the lead pyrazole compound 2. The N-acyl pyra-
zoles 3s–t retained activity only against S. pneumoniae.
Analogs with ‘R’ groups that contained aryl functional-
ity (3g–h) maintained activity against the Gram-positive
pathogens S. aureus, S. pneumoniae, and E. faecalis, but
showed decreased activity against the fastidious Gram-
negative bacteria H. influenzae and M. catarrhalis.
However, the aniline derivative 3f and the tyramine
derivative 3n maintained Gram-negative activity com-
parable to 2.
heterocyclic ring was substituted with various hydro-
philic amino groups as an effort to improve the aqueous
solubility of these compounds. Several analogs showed
improved aqueous solubility compared to the lead pyra-
zole compound 2, while retaining Gram-positive and
fastidious Gram-negative antibacterial activity. In par-
ticular, analog 3b had excellent solubility (>70 lg/mL)
and showed antibacterial activity similar to the lead pyr-
azole compound 2. Further research results with oxazo-
lidinones with an expanded spectrum of activity will be
reported in due course.
The apparent aqueous solubility9 was determined for se-
lected oxazolidinone analogs and the results are shown
in Table 2. Although the lead pyrazole analog 2 has
poor aqueous solubility (14 lg/mL), introduction of
amino substituents at the 3-position of the pyrazole ring
resulted in significant improvements in the solubility of
these compounds. The alkylamino analog 3b, and the
ethylamino and propylamino analogs 3k and 3q, had
excellent solubility (>70 lg/mL) compared to the
un-substituted pyrazole 2. The N-acyl pyrazole 3s also
showed improved solubility (46 lg/mL) over the lead
compound 2.
References and notes
1. For recent reviews of linezolid, see: (a) Wilcox, M. H.
Expert Opin. Pharmacother. 2005, 6, 2315; (b) Stevens, D.
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L.; Dermyer, M. R.; Ding, Q.; Huband, M. D.; Jiao, W.;
Kaneko, T.; Khlebnikov, V.; Kim, J.-Y.; Lall, M. S.; Maiti,
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1
In summary, a novel series of conformationally
restricted oxazolidinones were synthesized in which the
3. For experimental details and H NMR data, see: Chupak,
L. S.; Kaneko, T.; Vara Prasad, J. V. N.; Kim, J.-Y.; Choy,
A. L.; Hagen, S. E.; Boyer, F. E. U.S. Patent 0288273 A1,
2005.
Table 2. Aqueous solubilities (lg/mL) of selected analogs
4. Vara Prasad, J. V. N.; Boyer, F. E.; Chupak, L.; Dermyer,
M.; Ding, Q.; Gavardinas, K.; Hagen, S. E.; Huband, M.
D.; Jiao, W.; Kaneko, T.; Maiti, S. N.; Melnick, M.;
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5. For examples of the preparation of pyrazole derivatives from
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Compound
R
Solubility (lg/mL)
2
n/a
–CH3
14
>70
>70
53
3b
3k
3i
–(CH2)2NH2
–(CH2)2OH
–(CH2)3NH2
–C(@O)CH(OH)CH2OH
3q
3s
>70
46

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A. L. Choy et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4699–4702
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