Synthesis and antibacterial activities of novel oxazolidinones having spiro[2,4]heptane moieties

Article abstract of DOI:10.1016/j.bmcl.2009.03.025

The synthesis of a new series of oxazolidinones having spiro[2,4]heptane moieties is described. Their in vitro antibacterial activities against both Gram-positive and Gram-negative bacteria were tested and the effect of substituents on the oxazolidinone ring was investigated. A particular compound Ih having fluoro group showed the most potent antibacterial activity.

Full text of DOI:10.1016/j.bmcl.2009.03.025

Bioorganic & Medicinal Chemistry Letters 19 (2009) 2558–2561
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and antibacterial activities of novel oxazolidinones having
spiro[2,4]heptane moieties
*
So-Young Kim, Hyeong Beom Park, Jung-Hyuck Cho, Kyung Ho Yoo, Chang-Hyun Oh
Life Sciences Research Division, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a new series of oxazolidinones having spiro[2,4]heptane moieties is described. Their
in vitro antibacterial activities against both Gram-positive and Gram-negative bacteria were tested and
the effect of substituents on the oxazolidinone ring was investigated. A particular compound Ih having
fluoro group showed the most potent antibacterial activity.
Received 5 January 2009
Revised 4 March 2009
Accepted 7 March 2009
Available online 13 March 2009
Ó 2009 Published by Elsevier Ltd.
Keywords:
Oxazolidinones
Antibacterial activity
Spiro[2,4]heptane
The emergence of bacterial resistance to the antibiotics poses a
serious concern for medical professionals during the last decade.1
In particular, multi-drug-resistant Gram-positive bacteria2 includ-
ing methicillin-resistant Staphylococcus aureus (MRSA)3 and Staph-
ylococcus epidermidis (MRSE), and vancomycin resistant enterococci
(VRE) are of major concern.4
Oxazolidinones, a new class of synthetic antibacterial agents,
exhibit activity against a large number of Gram-positive organ-
isms. Linezolid is the first oxazolidinone approved for the treat-
ment of Gram-positive bacterial infections in humans.5 Since
Linezolid, the many attractive traits of oxazolidinone series have
encouraged further work in this area, and also the literature reveals
extensive chemical programs exist.6,7 At present, most efforts are
focused on substituted phenyl oxazolidinones. Eperezolid and
AZD2563 have been extensively used as the structural precursors
for modification.8
7-Oxo-5-azaspiro[2,4]heptane 811 was prepared via seven steps
from diketene and benzylamine as shown in Scheme 1. Aceto com-
pound 2 was obtained from diketene and benzylamine, which was
then cyclized to compound 3 using 1,2-dibromoethane. Carbonyl
group of 3 was protected by ethylene glycol and subsequently bro-
minated to give 5, which was converted to spiro[2,4]heptane 6
using sodium hydride. Cyclized compound 6 was reduced with
lithium aluminum hydride to give 7. The key compound 8 was ob-
tained by hydrogenation of 7 in the presence of palladium carbon.
The syntheses of derivatives Ia–i are outlined inSchemes 2 and 3.
7-Oxo-5-azaspiro[2,4]heptane as a starting material on condensa-
tion with 3,4-difluoronitrobenzene in acetonitrile gave compound 9.
Hydrogenation of compound 9 with 10% Pd–C/H2 followed by
condensation with benzyl chloroformate afforded the protected
In this Letter, we describe the synthesis and structure–activity
relationship of oxazolidinones having spiro[2,4]heptane moieties
instead of morpholine (Fig. 1). In addition, our approach for the
improvement of antibacterial activity of oxazolidinones is also
discussed.
O
N
O
N
O
O
O
O
N
N
N
NHAc
NHAc
OH
F
F
It is revealed9,10 that a spiro[2,4]heptane substituent could en-
hance largely the activity of quinolone antibiotics especially
against both Gram-positive and Gram-negative bacteria. Based
on the facts, a positive effect of a spiro[2,4]heptane moiety on
the activity of oxazolidinone was anticipated.
Linezolid
Eperezolid
O
N
R
O
N
NHAc
F
Spiro[2,4]heptane substituted oxazolidinones
* Corresponding author. Tel.: +82 2 958 5160; fax: +82 2 9585189.
E-mail address: choh@kist.re.kr (C.-H. Oh).
Figure 1. Structures of Linezolid, Eperezolid, and target molecules.
0960-894X/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2009.03.025

S.-Y. Kim et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2558–2561
2559
O
O
O
O
ii
iii
i
N
H
N
H
Diketene
1
2
3
O
O
O
Br
O
O
iv
v
vi
HN
O
N
O
N
H
O
O
6
5
4
O
O
O
vii
N
O
HN
8
7
Scheme 1. Reagents and conditions: (i) benzylamine, CH2Cl2; (ii) 1,2-dibromoethane, K2CO3, DMF; (iii) ethylene glycol, p-toluensulfonic acid, benzene; (iv) Br2, dioxane,
ether; (v) NaH, DMF; (vi) LiAlH4, THF; (vii) Pd/C, H2, EtOH.
O
O
O
i
ii
N
O
NO2
O
O
N
NHCbz
O
F
9
HN
F
10
8
O
O
O
iii
iv
v
O
O
N
N
O
N
N
O
N
OH
OMs
F
F
12
11
O
O
O
O
vi
O
N
O
vii
N
O
O
N
N3
NHCOCH3
F
F
13
14
O
O
O
HO
viii
N
N
O
N
N
O
NHCOCH3
F
NHCOCH3
F
Ib
Ia
Scheme 2. Reagents and condition: (i) 3,4-difluoronitrobenzene, CH3CN, reflux, 67%; (ii) (1) H2, Pd/C, THF (2) benzylchloroformate, NaHCO3, acetone–H2O; (iii) n-BuLi, (R)-
glycidyl butyrate, À78 °C, 65%; (iv) MsCl, TEA, CH2Cl2, 5 °C, 79%; (v) NaN3, DMF, 75 °C, 78%; (vi) (1) H2, Pd/C, EtOAc (2) Ac2O, pyridine, 69%; (vii) p-toluenesulfonic acid
monohydrate, acetone–H2O; (viii) NaBH4, THF, À5 °C, 75%.
O
O
HO
F
vi
N
N
N
N
N
N
O
O
O
NHAc
NHAc
NHAc
F
F
Ih
Ib
i
OH
N
OCH3
N
O
O
O
O
ii
NHAc
iii
N
N
O
N
N
O
F
F
NHAc
F
Ia
Ic
iv
v
Id
NH2
N
OR
O
O
N
N
N
N
N
O
O
NHAc
NHAc
F
F
Ig
Ie = R: Et
If = R: Pr
Scheme 3. Reagents and conditions: (i) NaBH4, THF, À5 °C, 75%; (ii) NH2OHÁHCl, TEA, EtOH, 60 °C, 70%; (iii) NH2OCH3ÁHCl, TEA, MeOH, rt, 65%; (iv) Ie: C2H5Br, KOH, DMF,
40 °C, 71%; If: C3H7Br, KOH, DMF, 40 °C, 73%; (v) N2H4, TEA, EtOH, 60 °C, 55%; (vi) DAST, CH2Cl2, À75° C to rt, 53%.

2560
S.-Y. Kim et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2558–2561
O
O
MsO
O
H2N
HO
ii
i
N
N
O
N
N
N
N
O
O
NHAc
NHAc
NHAc
F
F
F
Ib
Ii
15
Scheme 4. Reagents and conditions: (i) MsCl, TEA, CH2Cl2, 5 °C, 76%; (ii) (1) NaN3, DMF, 75 °C, 78% (2) H2, Pd/C, EtOH, 69%.
Table 1
Introduction of fluoro group of compound Ih was carried out by
treatment of Ib with DAST in CH2Cl2.
Compound Ib was reacted with methane sulfonyl chloride and
subsequent treatment with sodium azide to yield azide compound.
The title compound Ii was obtained by hydrogenation of azide in
the presence of palladium carbon (Scheme 4).
In vitro antibacterial activity(MIC,
standard strains
lg/ml) of oxazolidinone derivatives against
Compound
S. a.a
C. s.b
E. f.c
E. f.d
S. p.e
S. p.f
S. a.g
H. i.h
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
3.12
12.5
12.5
6.25
12.5
25.0
25.0
3.12
6.25
3.12
3.12
12.5
25.0
12.5
12.5
12.5
12.5
3.12
6.25
3.12
6.25
12.5
25.0
12.5
25.0
25.0
25.0
1.56
6.25
1.56
1.56
6.25
12.5
6.25
12.5
12.5
25.0
0.78
6.25
1.56
0.78
3.12
12.5
3.12
12.5
12.5
25.0
1.56
6.25
0.78
3.12
6.25
6.25
6.25
12.5
12.5
12.5
1.56
6.25
1.56
1.56
6.25
6.25
6.25
12.5
12.5
25.0
0.78
6.25
1.56
1.56
6.25
12.5
6.25
12.5
25.0
25.0
0.39
6.25
1.56
The minimal inhibitory concentrations (MICs) were determined
by the agar dilution method using test agar. An overnight culture of
bacteria in tryptosoy broth was diluted to about 106 cells/mL with
the same broth and inoculated with an inoculating device onto
agar containing serial twofold dilutions of the tested compounds.
Organisms were incubated at 37 °C for 18–20 h. The MICs of a com-
pound were defined as the lowest concentration that visibly inhib-
ited growth.
In particular, compound Ih having fluoro substituted
spiro[2,4]heptane moiety was 2–3-fold superior active against
most of the targeted both Gram-positive and vancomycin-resistant
strains to Linezolid. As to the substituents on the oxazolidinone
ring, it was found that compounds Ib and Ii having hydroxy and
amino groups, respectively didn’t show good activity compared
to other compounds. With increasing bulkiness from methyl in
compound Id to ethyl or propyl in compounds Ie and If, respec-
tively, the activity was found to be decreased.
The in vitro antibacterial activities of the new oxazolidinones
(Ia–i) prepared above against both Gram-positive and Gram-nega-
tive organism such as Haemophilus influenzae are listed in Tables 1
and 2. For comparison, the MIC values of Linezolid are also listed.
Among the tested compounds, Ia and Ih12 displayed superior or
similar antibacterial activities to Linezolid against Gram-positive,
methicillin- and vancomycin-resistant strain sequence.
In case of Ie and If, any bulky substituents also led to significant
loss in antibacterial activity. This suggests that bulky substituents
are not favorable.
In summary, the introduction of 7-fluoro-5-azaspiro[2,4]hep-
tane moiety to oxazolidinone afforded potent compound with
in vitro antibacterial activity comparable or superior to Linezolid
against Gram-positive, methicillin- and vancomycin-resistant
strains.
Ii
Linezolid
a
S. a. = Staphylococcus aureus C463.
b
c
d
e
f
C. s. = Coagulase negative staphylococci.
E. f. = Enterococcus faecalis C474.
E. f. = Enterococccus faecium C803.
S. p. = Streptococcus pneumoniae C402.
S. p. = Streptococcus pyogenes ATCC8736.
S. a. = Streptococcus agalactiae ATCC2901.
H. i. = Haemophilus influenza.
g
h
compound 10. Conversion of compound 10 to oxazolidinone 11
was accomplished by use of n-butyllithium and (R)-glycidyl buty-
rate in dry THF at À78 °C. Compound 11 was reacted with meth-
anesulfonyl chloride and subsequent treatment with sodium
azide to yield azide 13. Reduction and acylation of compound 13
and subsequent deprotection using p-toluenesulfonic acid pro-
vided gave the title compound Ia.
The title compounds Ib–Ih were prepared from Ia as shown in
Scheme 3. Reduction of Ia with sodium borohydride in THF gave
the hydroxyl compound Ib. Preparation of oxime Ic and methoxy-
imino compound Id was accomplished by treatment of the car-
bonyl group with hydroxyl and methoxyl amine. The oxime Ic
was converted to the ethyloxyimino Ie and propyloxyimino If
using ethyl bromide and propyl bromide, respectively, in the pres-
ence of potassium hydroxide.
Preparation of the hydrazone Ig was accomplished by treatment
of the carbonyl group with hydrazine.
Table 2
In vitro antibacterial activity (MIC,
lg/mL) of oxazolidinone derivatives against MRSA and VRE
Compound
MRSA
VRE 1
VRE 2
VRE 3
VRE 4
VRE 5
VRE 6
VRE 7
VRE 8
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
3.12
6.25
12.5
6.25
12.5
12.5
25
0.78
6.25
1.56
1.56
6.25
12.5
6.25
12.5
25
1.56
6.25
12.5
6.25
12.5
12.5
25
1.56
6.25
1.56
3.12
3.12
12.5
6.25
12.5
12.5
12.5
1.56
12.5
1.56
1.56
6.25
6.25
3.12
6.25
12.5
25
1.56
6.25
1.56
1.56
6.25
6.25
6.25
6.25
12.5
12.5
3.12
6.25
1.56
3.12
6.25
6.25
6.25
12.5
12.5
25
0.78
6.25
1.56
3.12
6.25
6.25
6.25
6.25
12.5
12.5
0.78
6.25
1.56
1.56
3.12
12.5
6.25
6.25
12.5
25
1.56
12.5
1.56
50
1.56
12.5
1.56
Ii
Linezolid
MRSA: methicillin-resistant Staphylococcus aureus, VRE 1: vancomycin-resistant Enterococcus faecalis, VRE 2: vancomycin-resistant Enterococccus faecium, VRE 3: vancomycin
resistant enterococci 1, VRE 4: vancomycin resistant enterococci 2, VRE 5: vancomycin resistant enterococci 3, VRE 6: vancomycin resistant enterococci 4, VRE 7: vancomycin
resistant enterococci 5, VRE 8: vancomycin resistant enterococci 6.

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