Synthesis and biological evaluation of novel benzoxazinyl-oxazolidinones as potential antibacterial agents

Article abstract of DOI:10.1016/j.bmcl.2013.05.023

A number of benzoxazinyl-oxazolidinones bearing 3-trizolylmethyl or 3-carboxamide side chain were designed and synthesized with the aim to develop antibacterial agents with improved properties. In vitro antibacterial activities of these novel compounds we

Full text of DOI:10.1016/j.bmcl.2013.05.023

Bioorganic & Medicinal Chemistry Letters 23 (2013) 3697–3699
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological evaluation of novel benzoxazinyl-
oxazolidinones as potential antibacterial agents
a
a
a
a
b
a,
⇑
Bin Guo , Houxing Fan , Qisheng Xin , Wenjing Chu , Hui Wang , Yushe Yang
a
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institute for Biological Sciences, Chinese Academy of Sciences, Shanghai, China
Department of Microbiology, China Pharmaceutical University, Nanjing, Jiangsu Province 210009, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A number of benzoxazinyl-oxazolidinones bearing 3-trizolylmethyl or 3-carboxamide side chain were
designed and synthesized with the aim to develop antibacterial agents with improved properties. In vitro
antibacterial activities of these novel compounds were evaluated against a panel of resistant and suscep-
tible Gram-positive bacteria. Most analogues bearing 3-trizolylmethyl showed good to moderate antibac-
terial activities. Compound 12a exhibited a fourfold increase in activity compared with linezolid against
all the tested strains, which was identified to be a promising antibacterial agent for further evaluation.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 5 April 2013
Revised 5 May 2013
Accepted 8 May 2013
Available online 17 May 2013
Keywords:
Benzoxazinyl-oxazolidinone
Linezolid
Antibacterial agents
Gram-positive organisms
The emergence and worldwide spread of bacterial pathogens
resistant to existing antimicrobials has become a serious threat
to the well-being of mankind.1 In particular, multidrug-resistant
Gram-positive bacteria including methicillin-resistant Staphylococ-
cus aureus (MRSA) and S. epidermidis (MRSE), vancomycin-resistant
Enterococci (VRE) and penicillin-resistant Streptococci (PRSP) are a
major concern. Oxazolidinones, as exemplified by linezolid (1,
Fig. 1) are a new class of synthetic antibacterial agents with potent
activity against a broad range of Gram-positive bacteria such as
MRSA, MRSE and VRE.4 Unfortunately, a short time after its
launch, linezolid-resistant S. aureus and Enterococcus faecium began
tion of oxazolidinones. A reverse amide analogue 3 of linezolid
have a K of 146 M versus MAO-A, a value approximately three-
fold higher than linezolid.
i
l
–3
16
In our previous efforts in the development of oxazolidinones,
we first reported a highly potent benzoxazinyl-oxazolidinone ser-
ies as typified by the candidate compound 4. Compared to linezo-
lid, compound 4 showed a 3- to 4-fold increase in efficacy in vivo
1
7
and an excellent pharmacokinetic (PK) profile. As a novel core
structure, [6,6,5] tricyclic fused benzoxazinyl-oxazolidinone is
suitable for further structural modification to develop new oxazo-
lidinones with more potent antibacterial activity, especially with
an improved safety profile with regard to MAO-A inhibition. Here-
in, we reported our efforts towards the synthesis and in vitro anti-
bacterial profile of a number of benzoxazinyl-oxazolidinones
bearing 3-trizolylmethyl or 3-carboxamide side chain, which led
to the discovery of compound 12a as a potent antibacterial agent
against Gram-positive bacteria.
–6
7
–11
to emerge.
Another concern with oxazolidinones as antibacte-
rial agents is inhibition of monoamine oxidase (MAO), especially
type A (MAO-A), which could potentially lead to severe hyperten-
sive crises as a result of ingestion of tyramine-containing food to-
gether with an oxazolidinone drug.1
2–14
Therefore, it is necessary
to develop novel oxazolidinones, which not only have improved
antibacterial activity but also show a decreased MAO-A inhibition.
A group in AstraZeneca reported that the 1,2,3-triazole is a good
replacement for the conventional acetamide functionality in oxa-
zolidinones. Analogues with 1,2,3-triazole (such as compound 2,
Fig. 1) showed good antibacterial activity with reduced inhibition
The general synthesis of these novel compounds (12a–o and
17a–d) was carried out as depicted in Schemes 1 and 2. First,
intermediate 7 was synthesized from the commercially available
materials 4-bromo-2-fluoro-1-nitrobenzene (5) and cis-2-butene-
1,4-diol (6) in seven steps according to the method previously
1
5
17,18
against MAO-A. In addition, a group from Pfizer reported another
C-5 modification which also significantly reduces the MAO inhibi-
reported by us.
m-Nitrobenzene sulfonyl chloride was used
to form an aryl sulfonate in compound 8, to provide a good leaving
group. Substitution of 8 by sodium azide gave compound 9. Then,
compound 10 was obtained through click reaction from 9 and
⇑
Corresponding author at present address: Shanghai Institute of Materia Medica,
No. 555 Zu Chongzhi Road, Zhangjiang High-tech Park, Shanghai 201203, China.
Tel./fax: +86 21 5080 6786.
2
,5-norbornadiene at 110 °C. Next, compound 10 was converted
to key intermediate boric acid esters 11 through the Miyaura reac-
tion. Finally, compound 11 readily underwent the Suzuki coupling
E-mail address: ysyang@simm.ac.cn (Y. Yang).
0
960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.05.023

3
698
B. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3697–3699
F
O
F
O
bromide-substituted pyridine fragments through the Miyaura–Su-
zuki coupling reaction. All the newly synthesized compounds were
characterized by spectroscopic data ( H NMR, C NMR, MS and
HMRS) and melting point.
The target compounds were evaluated for their in vitro antibac-
terial activity against a panel of susceptible and resistant Gram-po-
sitive organisms. The results are summarized in Table 1.
As shown in Table 1, most of the analogues bearing 3-trizolylm-
ethyl (compounds 12a–o) displayed moderate or good antibacte-
rial activity, especially compound 12a, the most potent one, with
O
O
O
O
N
N
N
O
N
N
S
N
1
13
NHAc
NH2
linezolid
1
2
F
O
O
O
O
O S
N
NC
N
NHAc
N
O
O
PF-00422602
3
4
MIC values of 0.25–0.5
l
g/mL against MSSA, MRSA, MSSE and
Figure 1. Structure of oxazolidinone compounds.
MRSE, 0.25–1 g/mL against PRSP and 1
l
l
g/mL against E. faecalis.
Analogues substituted on the pyridyl ring with heteroaromatic
rings (compounds 12b–d) only exhibited moderate antibacterial
activity, which was equivalent or inferior to that of linezolid, while
nonaromatic rings substituted analogues (compounds 12e–f)
showed more potent than linezolid.
Substitution with –CH OH or –CH(OH)CH moieties (com-
2 3
pounds 12g and h) on the pyridyl ring gave compounds with sim-
ilar activity to linezolid. Oxidation of alcohols to ketones
reaction with a series of bromide-substituted pyridine fragments
in dioxane/H O as solvent at 80 °C afforded compounds 12a–o in
moderate to good yields. The synthesis of compounds 17a–d was
outlined in Scheme 2. First, compound 7 was oxidized to acid 13,
and then the acid was converted to acyl chloride. Amide 15 was
2
gained from acyl chloride and NH
3
gas. Finally, compounds
1
7a–d was synthesized from compound 15 with a series of
O
O
O
seven steps
a
b
O
O
O
Br
NO
2
HO
OH
Br
N
Br
N
Br
N
OH
O
O
N
3
F
O
7
O
S
O
O
5
6
8
9
c
NO2
O
O
O
O
O
e
d
O
O
O
R
N
N
N
B
N
N
N
Br
N
N
N
N
N
N
N
O
O
O
1
1
2
11
0
1
1
1
1
1
1
1
1
2a: R = CN
12i: R = acetyl
12j: R = 1-(ethoxyimino)ethyl
12k: R = 1-(isopropoxyimino)ethyl
12l: R = (1H-1,2,4-triazol-1-yl)methyl
2b: R = 1-methyl-1H-tetrazol-5-yl
2c: R = 2-methyl-2H-tetrazol-5-yl
2d: R = 5-methyl-1,2,4-oxadiazol-3-yl
2e: R = 5-(hydroxymethyl)-2-oxooxazolidin-3-yl 12m: R = (2H-tetrazol-2-yl)methyl
2f: R = 6-((S)-3-methyl-2-oxooxazolidin-5-yl
2g: R = hydroxymethyl
12n: R = (1H-tetrazol-1-yl)methyl
12o: R = (1H-1,2,3-triazol-1-yl)methyl
2h: R = 1-hydroxyethyl
Scheme 1. Synthetic route of benzoxazinyl-oxazolidinones (12a–o). Reagents and conditions: (a) m-nitro benzene sulfonyl chloride, Et
3
N, CH
2
Cl
2
, 0 °C to room temp; (b)
, dioxane/H O, 80 °C.
NaN
3
, DMF, 80 °C; (c) 2,5-norbornadiene, dioxane, 110 °C; (d) bis(pinacolato)diboron, PdCl
2
(dppf)ÁCH
2
Cl
2
, KOAc, DMSO, 80 °C; (e) Pd(PPh , Cs
)
3 4
2
CO
3
2
O
O
O
a
b
O
O
O
Br
N
Br
N
Br
N
OH
OH
Cl
O
O
O
O
O
7
13
14
c
O
O
O
e
O
O
d
O
O
O
Br
N
R
N
B
N
N
NH2
NH2
NH
2
O
O
O
7
O
O
O
15
1
16
1
1
1
1
7a: R = CN
7b: R = 5-methyl-1,2,4-oxadiazol-3-y
7c: R = 2-oxooxazolidin-3-yl
7d: R = (1H-1,2,4-triazol-1-yl)methyl
Scheme 2. Synthetic route of benzoxazinyl-oxazolidinones (17a–d). Reagents and conditions: (a) TEMPO, NaClO
2 3 2 3
/NaClO, CH CN, 50 °C; (b) (COCl) , room temp, Ar; (c) NH
(
gas), THF, 0 °C, Ar; (d) bis(pinacolato)diboron, PdCl (dppf)ÁCH Cl , KOAc, DMSO, 80 °C. (e) Pd(PPh , Cs CO , dioxane/H
2
2
2
3
)
4
2
3
2
O, 80 °C.

B. Guo et al. / Bioorg. Med. Chem. Lett. 23 (2013) 3697–3699
3699
Table 1
panel of resistant and susceptible Gram-positive bacteria. Most of
analogues bearing 3-trizolylmethyl showed more potent or equiv-
alent antibacterial activity to linezolid, while all analogues with
3-carboxamide lost antibacterial activity. Compound 12a, the most
potent of all the synthesized compounds, exhibited a fourfold in-
crease in antibacterial activity compared with linezolid against
all the tested strains. This led to the promotion of 12a as a prom-
ising antibacterial agent for further evaluation.
In vitro antibacterial activity of new benzoxazinyl-oxazolidinones against Gram-
positive clinical isolates
Compounds
MIC (
l
g/mL)
MSSAa
MRSAb
MSSEc
MRSEd
PRSPe
E.ff
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
7a
7b
7c
7d
0.25–0.5 0.25–0.5 0.25–0.5 0.25–0.5 0.25–1
1
4
4
8
1
2
4
2–4
2
>32
>32
1–2
2
4
4
>32
>32
>32
>32
2–4
2–4
2
1–2
2
2–8
1–2
1
1–2
1–2
0.5
>32
>32
0.5–1
1–2
2–4
1–2
>32
>32
>32
>32
1–2
1–2
1–2
2–4
0.5–1
0.5–1
1
1–2
0.5–1
>32
>32
0.5
0.5–1
1–2
1
>32
>32
>32
>32
1–2
1–2
2–4
4
1–2
1
2
1–4
1–2
0.5–1
0.25–0.5
2–4
0.5–2
0.25–0.5
>32
>32
2
2–4
0.5–1
0.5–1
2–4
1–2
0.5
>32
>32
0.5–1
1–2
2–4
2–4
>32
>32
>32
>32
1–2
Acknowledgment
1–2
1–2
1–2
>32
>32
0.5–1
1–2
1–2
1–2
>32
>32
>32
>32
1–2
This work was supported by the National Natural Science
Foundation of China (No. 81172922).
Supplementary data
2
4
Supplementary data (experimental details and spectroscopic
characterization of compounds involved in the synthesis) associ-
ated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.bmcl.2013.05.023.
2–4
>32
>32
>32
>32
2
Linezolid
References and notes
a
b
c
d
e
f
MSSA: methicillin-sensitive S. aureus, five strains.
MRSA: methicillin-resistant S. aureus, six strains.
MSSE: methicillin-sensitive S. epidermidis, five strains.
MRSE: methicillin-resistant S. epidermidis, five strains.
PRSP: penicillin-resistant S. pneumoniae, four strains.
E.f.: E. faecalis, six strains.
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follow the trend 1,2,4-triazol > 2H-tetrazol P 1,2,3-triazol > 1H-
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To our surprise, all analogues bearing 3-carboxamide side chain
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1
6
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ture–activity relationship (SAR) from the previous report.
In conclusion, a number of benzoxazinyl-oxazolidinones bear-
ing 3-trizolylmethyl or 3-carboxamide side chain were synthesized
and evaluated for their in vitro antibacterial activities against a
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