Influence of ethylene-oxy spacer group on the activity of linezolid: Synthesis of potent antibacterials possessing a thiocarbonyl group

Article abstract of DOI:10.1016/j.bmcl.2003.08.068

The influence of an ethylene-oxy spacer element between the heterocycle and the aromatic ring in linezolid is reported. The introduction of such spacer group generated compounds with inferior antibacterial activity. However, the conversion of the acetamide group present in the linezolid analogues to either thiocarbamate or thioacetamide functionality restored the activity. The synthesis of linezolid analogues possessing the ethylene-oxy spacer group along with SAR studies with different heterocycles and preparation of some thiocarbonyl compounds possessing potent antibacterial property are presented.

Full text of DOI:10.1016/j.bmcl.2003.08.068

Bioorganic & Medicinal Chemistry Letters 13 (2003) 4169–4172
Influence of Ethylene-Oxy Spacer Group on the Activity of
Linezolid: Synthesis of Potent Antibacterials Possessinga
Thiocarbonyl Group§
N. Selvakumar,* Mohammed A. Raheem, Manoj Kumar Khera,
Trideep V. Rajale, Magadi Sitaram Kumar, Sreenivas Kandepu,
Jagattaran Das, R. Rajagopalan, Javed Iqbal and Sanjay Trehan
Anti-infectives Discovery Group, Discovery Research, Dr. Reddy’s Laboratories Ltd., Miyapur, Hyderabad 500 049, India
Received 8 May 2003; accepted 17 August 2003
Abstract—The influence of an ethylene-oxy spacer element between the heterocycle and the aromatic ring in linezolid is reported.
The introduction of such spacer group generated compounds with inferior antibacterial activity. However, the conversion of the
acetamide group present in the linezolid analogues to either thiocarbamate or thioacetamide functionality restored the activity. The
synthesis of linezolid analogues possessing the ethylene-oxy spacer group along with SAR studies with different heterocycles and
preparation of some thiocarbonyl compounds possessing potent antibacterial property are presented.
# 2003 Elsevier Ltd. All rights reserved.
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 including methicillin-resistant
Staphylococcus aureus (MRSA)3 and Staphylococcus
epidermidis (MRSE) and vancomycin resistant enter-
ococci (VRE) are of major concern.4 Oxazolidinones are
a new class of synthetic antibacterials with activity
against Gram-positive bacteria and anaerobic bac-
teria.5,6 They have been shown to selectively bind to the
50S ribosomal subunit and inhibit bacterial translation
at the initiation phase of the protein synthesis.7 Line-
zolid 1, developed by Pharmacia and Upjohn, is the first
compound commercialised world wide from the oxazo-
lidinone class of antibacterials.8 This class of com-
pounds is particularly active against Gram-positive
organisms such as MRSA, MRSE and VRE. The novel
mechanism of action combined with the biological
activity against resistant organisms aroused widespread
attention and stimulated others to explore chemistry in
the oxazolidinone class.4 In an ongoing project on anti-
infectives in our laboratory, we have explored the
introduction of an ethylene-oxy spacer group between
the heterocycle and the phenyl ring in linezolid 1 (Fig.
1). In this letter, we report our initial results on the
influence of this spacer element, as illustrated in struc-
ture 2, on the antibacterial activity of linezolid.9
The general method of introducing the spacer element
involved the aromatic nucleophilic substitution of eth-
ylene glycol with mono or difluoro nitrobenzenes. Thus,
substitution of fluorine by ethylene glycol under basic
conditions in 4-fluoronitrobenzene and 3,4-difluoroni-
trobenzene resulted in the displacement of the fluorine
atom leading to the nitro compounds 3 and 4, respec-
tively (Scheme 1). The nitro alcohol 3 and the fluoro
nitro-alcohol 4 were the starting materials for the line-
zolid analogues possessing no fluorine and fluorine
respectively. The nitro compounds 3 and 4 afforded the
benzylated compounds 5 under standard conditions.
Having obtained the nitro compounds 5, further steps
for the synthesis of 8 were carried out along the lines of
established protocol.8 Thus, compound 5 upon reduc-
tion followed by protection afforded the Cbz-protected
compound 6. Deprotonation of compound 6 followed
by treatment with (R)-glycidyl butyrate yielded the
oxazolidinone alcohol 7, which was converted into the
corresponding azide by standard procedures. The treat-
ment of the azide with thioacetic acid produced the tar-
get acetamides 8 and 9 that in turn underwent
§DRF Publication No. 361.
*Corresponding author. Tel.:+91-40-2304-5439; fax:+91-40-2304-
5438; e-mail: selvakumarn@drreddys.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.08.068

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N. Selvakumar et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4169–4172
The thiocarbamates 23 and 24 and thioacetamides 25–
27 were prepared following a route illustrated in Scheme
2. The nitro-alcohol 4 was converted to its mesylate 18,
which was treated with various heterocycles to afford
the nitro-compounds 19. Having obtained the nitro
compounds possessing different heterocycles, the rest of
the synthesis was carried out following a route discussed
in Scheme 1 to afford the azides 21. These azides were
transformed to the corresponding amines 22 by stan-
dard procedure. The above amines were converted to
the corresponding thiocarbamates 23 and 24 via the
respective isothiocyanates using standard experimental
procedures.4 Treatment of the amines with ethyl-
dithioacetate resulted in the thioacetamides 25–27 in
good yields.
Figure 1.
The analogues of linezolid possessing the spacer group
were screened for in vitro activity against a panel of
Gram-positive organisms and the results are summar-
ized in Table 1. The compounds without a fluorine atom
in the aromatic ring 13–15 exhibited no antibacterial
activity. Consequently, analogues possessing a fluorine
atom in the aromatic ring were evaluated. The closest
variant of linezolid with ethylene-oxy spacer group 2
exhibited moderate activity having MIC values ranging
4–16 mg/mL. While the activity of its thiomorpholine
analogue 16 was similar from MIC values, the piper-
idine analogue 17 exhibited nearly no activity. Thus, the
effort to introduce the ethylene-oxy spacer group gen-
erated molecules having inferior activity when com-
pared to linezolid. It is interesting that the benzyl
compound 9 containing fluorine atom in the aromatic
ring and the corresponding hydroxy compound 11
showed MIC values of 8 mg/mL against MRSA strain
ATCC 33591.
At this juncture, we turned our attention towards mod-
ifying the right hand side of the molecule in an effort to
restore the activity of the above analogues. We, along
with others, have recently established that the conver-
sion of acetamide moiety of the oxazolidinone class of
antibacterials into either thioacetamide or thiocarba-
mate results in significant improvement of activity.4
Consequently when the above compounds were con-
verted into their thiocarbamate or thioacetamide analo-
gues, there was significant improvement in the in vitro
activity. The linezolid analogue 23 having a thiocarba-
mate group exhibited a one- to three-fold better activity
compared to the acetamide analogue 2. Interestingly,
the thiomorpholine analogue 24 showed even better in
vitro results possessing excellent MIC values that are
comparable to that of linezolid. Similarly, the thioace-
tamide analogues 25 and 27 showed activities equivalent
to their thiocarbamate counterparts and compound 25
appeared to be slightly more potent with respect to the
MRSA strain. Once again, the piperidine analogue 26
had poorer antibacterial spectrum compared to its clo-
sest variants 25 and 27.
Scheme 1. Reagents and conditions: (a) ethylene glycol, K2CO3,
DMF, rt, 48 h; (b) NaH, BnBr, THF, rt, 12 h; (c) Fe, HCl, EtOH,
0 ꢀC–rt, 2 h; (d) Cbz–Cl, aq Na2CO3, acetone, 0 ꢀC, 2 h; (e) BuLi, (R)-
glycidyl butyrate, THF, À78 ꢀC to rt, 14 h; (f) MsCl, Et3N, CH2Cl2,
0 ꢀC, 4 h; (g) NaN3, DMF, 80 ꢀC, 1 h; (h) CH3COSH, rt, 14 h; (i) 10%
Pd/C, H2, THF, 14 h; (j) heterocycle, Et3N, DMF, 80 ꢀC, 12 h.
hydrogenolysis to afford the hydroxy compounds 10
and 11. The compounds 10 and 11 were converted to
their corresponding mesylates 12 by standard methods
to facilitate introducing heterocycles. The above mesyl-
ates underwent nucleophilic displacement with various
heterocycles under basic conditions yielding the target
compounds 13–17 and 2.
In conclusion, a study conducted towards under-
standing the influence of introducing ethylene-oxy
spacer group on the activity of linezolid has been
accomplished. While, the acetamide analogues resulted

N. Selvakumar et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4169–4172
Table 1. In vitro antibacterial activity (MIC, mg/mL)a of novel oxazolidinonesb
4171
Entry
Compd
R1
R2
S.a
019
S.a
213
S.a
035
E.f
034
E.f
153
E.fm
154
1
2
3
13
14
15
NHCOCH3
NHCOCH3
NHCOCH3
>32
>32
32
NDc
ND
ND
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
>32
4
5
6
9
11
2
OCH2Ph
OH
NHCOCH3
NHCOCH3
NHCOCH3
8
8
4
ND
ND
16
16
8
32
16
16
32
16
16
32
16
4
16
7
16
17
23
24
25
26
27
NHCOCH3
NHCOCH3
NHCSOCH3
NHCSOCH3
NHCSCH3
NHCSCH3
NHCSCH3
4
32
2
16
64
4
16
64
4
16
64
2
16
64
4
4
64
2
8
9
10
11
12
13
1
4
4
1
2
2
0.5
2
2
1
2
2
2
1
8
8
8
4
4
2
2
2
2
2
14
15
Linezolid
1
2
2
1
2
1
2
2
2
2
Vancomycin
>32
>32
aS.a 019=Staphylococcus aureus ATCC 33591 (methicillin-resistant); S.a 213=S. aureus ATCC 49951; S.a 035=S. aureus ATCC 29213; E.f
034=Enterococcus faecalis ATCC 29212 (vancomycin-susceptible); E.f 153=E. faecalis NCTC 12201 (vancomycin-resistant) and E.fm
154=Enterococcus faecium ATCC 12202 (vancomycin-resistant).
bThe MIC values were obtained as described previously.4
cND, not determined.

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N. Selvakumar et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4169–4172
Scheme 2. Reagents and conditions: (a) MsCl, Et3N, CH2Cl2, 0 ꢀC, 4 h; (b) heterocycle, Et3N, DMF, 80 ꢀC, 12 h; (c) 10% Pd/C, H2, THF, 14 h; (d)
Cbz–Cl, aq Na2CO3, acetone, 0 ꢀC, 2 h; (e) BuLi, (R)-glycidyl butyrate, THF, À78 ꢀC to rt, 14 h; (f) NaN3, DMF, 80 ꢀC, 1 h; (g) PPh3, H2O, THF,
rt, 36 h; (h) CSCl2, aq NaHCO3, CH2Cl2, 0 ꢀC, 0.5 h; then MeOH, reflux, 12 h; (i) ethyldithioacetate, THF, Et3N, rt, 12 h.
Mamidi, N. V. S. R.; Sarnaik, H.; Chandrasekar, C.; Rao,
in compounds possessing inferior antibacterial activity,
B. S.; Raheem, M. A.; Das, J.; Iqbal, J.; Rajagopalan, R. J.
Med. Chem. 2002, 45, 3953.
their corresponding thiocarbamate and thioacetamide
analogues produced certain potent compounds such as
24, 25 and 27. Further work to vary the length of the
spacer group in 2 along with changing the oxygen atom
5. Brickner, S. J. Curr. Pharm. Des. 1996, 2, 175.
6. For a comprehensive list of activities in this area: Phillips,
O. A. Curr. Opin. Invest. Drugs 2003, 4, 117.
in the spacer element to other heteroatoms is currently
7. Swaney, S. M.; Aoki, H.; Ganoza, M. C.; Shinabarger,
underway in our laboratory.
D. L. Antimicrob. Agents Chemother. 1998, 42, 3251, and the
references cited therein.
8. Brickner, S. J.; Hutchinson, D. K.; Barbachyn, M. R.;
Manninen, P. R.; Ulanowicz, D. A.; Garmon, S. A.; Grega,
K. C.; Hendges, S. K.; Toops, D. S.; Ford, C. W.; Zurenko,
G. E. J. Med. Chem. 1996, 39, 673.
9. For a study with a different spacer group, Leuhr, G. W.;
Gordeev, M. F.; Hackbarth, C. J.; Lopez, S.; Wu, C.; Trias, J.;
Yuan, Z.; Patel, D.V. 40th Intersci. Conf. Antimicrob. Agents
Chemother., 2000, 1831.
References and Notes
1. Service, R. F. Science 1995, 270, 724.
2. Swartz, M. N. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 2420.
3. Tomasz, A. N. Engl. J. Med. 1994, 330, 1247.
4. For a comprehensive list of references for this area: Selva-
kumar, N.; Srinivas, D.; Khera, M. K.; Kumar, M. S.;