Synthesis of triazole-oxazolidinones via a one-pot reaction and evaluation of their antimicrobial activity

Article abstract of DOI:10.1016/j.bmcl.2008.07.087

C-5-substituted triazole-oxazolidinones were synthesized using a bromide catalyzed cycloaddition between aryl isocyanates and epibromohydrin followed by a three-component Huisgen cycloaddition. The library of compounds was screened for antibacterial activity against Mycobacterium smegmatis ATCC 14468, Bacillus subtilis ATCC 6633, and Enterococcus faecalis ATCC 29212. Notably, the 3-(4-acetyl-phenyl)-5-(1H-1,2,3-triazol-1-yl)methyl)-oxazolidin-2-one (18) showed an MIC of 1 μg/mL against M. smegmatis ATCC 14468, fourfold lower than the MIC measured for isoniazid.

Full text of DOI:10.1016/j.bmcl.2008.07.087

Bioorganic & Medicinal Chemistry Letters 18 (2008) 4868–4871
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of triazole-oxazolidinones via a one-pot reaction
and evaluation of their antimicrobial activity
*
Jeffrey A. Demaray, Jason E. Thuener, Matthew N. Dawson, Steven J. Sucheck
The University of Toledo, Department of Chemistry, 2801 W. Bancroft Street, Toledo, OH 43606, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
C-5-substituted triazole-oxazolidinones were synthesized using a bromide catalyzed cycloaddition
between aryl isocyanates and epibromohydrin followed by a three-component Huisgen cycloaddition.
The library of compounds was screened for antibacterial activity against Mycobacterium smegmatis ATCC
14468, Bacillus subtilis ATCC 6633, and Enterococcus faecalis ATCC 29212. Notably, the 3-(4-acetyl-phe-
Received 27 June 2008
Accepted 18 July 2008
Available online 24 July 2008
nyl)-5-(1H-1,2,3-triazol-1-yl)methyl)-oxazolidin-2-one (18) showed an MIC of
M. smegmatis ATCC 14468, fourfold lower than the MIC measured for isoniazid.
1 lg/mL against
Keywords:
Antimycobacterial
Antibacterial
Oxazolidinone
One-pot
Published by Elsevier Ltd.
The N-aryloxazolidin-2-one moiety is present in a number of
compounds which show diverse biological activities. Some of
these activities include monoamine oxidase (MAO) inhibition,1
GP IIb/IIIa antagonism,2 neuroleptic activity,3 and antibacterial
activity.4 Due to this medicinal importance, methods for generat-
ing libraries of these compounds have received significant
interest.5
Chemistry. One of the earliest methods for N-alkyloxazolidin-2-
ones preparation is the quaternary ammonium halide catalyzed
cycloaddition of alkyl isocyanates with monosubstituted oxiranes
reported by Speranza and Peppel.10 In that chemistry the halide
is proposed to open the oxirane to form a 1,2-alkoxy halide11
which subsequently adds to an isocyanate. The intermediate
cyclizes to form predominately 3,5-disubstituted oxazolidin-2-
ones. A variety of halides; for example, LiBr–OPPh3,4b,12 n-Bu3S-
nI–Lewis base complexes,13 and lanthanide chlorides,14 also cata-
lyze the reaction. Since alkyl isocyanates are abundant the
method allows for considerable variation of the N-aryl moiety
and was therefore chosen to prepare various N-aryl-substituted
C-5 (bromomethyl)-oxazolidinones, Scheme 1. Tetrabutylammo-
nium bromide (TBAB) was chosen as a catalyst as it is inexpensive,
commercially available, soluble in most solvents, and can be re-
moved by aqueous extraction. Phenyl isocyanate 1a, tolyl isocya-
nate 1b, or 4-acetylphenyl isocyanate 1c were then condensed
The compound class is particularly well recognized for its activ-
ity against clinically important susceptible and resistant Gram-po-
sitive bacteria such as methicillin-resistant Staphylococcus aureus,
vancomycin-resistant Enterococcus faecium, and penicillin-resis-
tant Streptococcus pneumouniae.6 N-aryloxazolidin-2-ones are also
being rediscovered for their activity against Mycobacterium sp.7
In the latter case, the increased prevalence of drug resistant
Mycobacterium tuberculosis combined with the rise of Mycobacte-
rium avium complex (MAC) infections in immuno-compromised
populations has placed renewed attention on the potential use of
oxazolidines for the treatment of mycobacterial infections.8
Recently, a new class (1H-1,2,3)-triazole C-5 substituted oxaz-
olidin-2-ones was described.9 These oxazolidinones showed
impressive antibacterial activity and were less prone to inhibit
MAOs. In this letter we describe an expedient method for the intro-
duction of 1,4-disubstituted-(1H-1,2,3)-triazoles at the C-5 posi-
tion of oxazolidinones. The resulting library of compounds were
then screened against a small panel Gram-positive reference
strains which included the strain Mycobacterium smegmatis, which
has been explored as a surrogate for pathogenic Mycobacterium.
O
TBAB
O
R
+
N
O
R N
C
O
Br
tolune or
xylenes
Br
1a: R = Ph
2a: R = Ph
1b: R =
1c: R =
p
-CH3-Ph
2b: R =
2c: R =
p
-CH3-Ph
p-COCH3-Ph
p
-COCH3-Ph
* Corresponding author. Tel.: +1 419 530 1504; fax: +1 419 530 1990.
Scheme 1. Preparation of C-5 (bromomethyl)-oxazolidinones from 1,2-epoxides
E-mail address: Steve.Sucheck@UToledo.edu (S.J. Sucheck).
and isocyanates.
0960-894X/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2008.07.087

J. A. Demaray et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4868–4871
4869
with epibromohydrin in the presence of TBAB to form oxazolidi-
nones 2a–c.
2-ones without the need to isolate (azidomethyl)-oxazolidinones
(2a–c).17 During the investigation we noted that trimethylsilyl
acetylene failed to react in the Huisgen cycloaddition using the
Cu(II)SO4 10 mol %–10 mol % ascorbic acid–1:1 tert-butanol water
conditions; however, the reaction proceeded when run in DMSO
using 10 mol % CuI as catalyst (data not shown). It has been
recently reported that the copper catalyzed Huisgen cycloaddition
proceeds faster in aqueous media when the alkyne is electron poor
while electron rich alkynes are more reactive in non-aqueous med-
ia.18 In our case, trimethylsilyl acetylene was the most electron
rich alkyne employed. Since DMSO was compatible with both the
azide displacement and all the Huisgen cycloadditions, this solvent
was chosen to explore a one-pot three-component variant of the
reaction. Thus, bromomethyl-oxazolidinones 2a–c and NaN3 were
dissolved in DMSO and heated to at 80–90 °C. Azide formation was
followed by TLC and was generally complete within 30 min. At that
time 1-decyne, 3-butyn-1-ol, phenyl acetylene, or trimethylsilyl
acetylene were added directly to the reaction followed by addition
of 10 mol % CuI as shown in Scheme 3. Reactions were maintained
at 80–90 °C and were generally complete within 1 h. The condi-
tions are essentially the same as those reported by Molander.17c
The structures and isolated yields of the various triazole 1,4-disub-
stituted-(1H-1,2,3)-triazole-oxazolidinones 4–15 are reported in
Table 2.
Hoping to avoid the high temperatures and prolonged reactions
times reported for isocyanate–epoxide cycloadditions,10 we briefly
investigated the dependence of oxazolidinone yield with respect to
reaction time, temperature, and stoichiometry using phenyl isocy-
anate 1a–c, Table 1. Overall higher yields corresponded with longer
reaction times and temperatures. Lower yields were accompanied
by the formation of putative isocyanurate side products. The
results are consistent with rapid isocyanurate formation followed
by a relatively slow reaction of the isocyanurate with epoxide lead-
ing to oxazolidinone, a conclusion supported both experimentally
and computationally.15 We noted isocyanates with electron-with-
drawing groups required higher temperatures and longer reaction
times compared with less electrophilic isocyanates also consistent
with earlier studies.13
In order to familiarize ourselves with the properties of the tria-
zole-oxazolidin-2-ones we first investigated the yield of a stepwise
displacement of bromide 2b with NaN3 to provide C-5 (azido-
methyl)-oxazolidinone 3b.4b Azide 3b was subsequently converted
to 1,4-disubstituted-triazoles 4–6 in the presence of either 1-de-
cyne, 3-butyn-1-ol, or phenyl acetylene using a Cu(I) catalyzed
1,3-dipolar Huisgen cycloaddition to afford triazoles in high yield
and regioselectivity, Scheme 2.16 For these initial studies Cu(I)
was generated in situ by the reduction of Cu(II)SO4 10 mol % to
Cu(I) with 10 mol % ascorbic acid in 1:1 tert-butanol water.
Having produced the targets in high yield by the stepwise
approach we proceeded to developed a convenient multi-compo-
nent variant of this chemistry that could potentially be employed
to produce larger libraries of 1,4-disubstituted-triazole-oxazolin-
Overall the isolated yields for the one-pot reactions were mod-
est, 39–61%. However, it should be noted that the reactions were
clean when monitored by TLC. The yields were negatively
impacted as result of using impure starting materials 2a–c. While
the starting materials could be purified from the closely migrating
isocyanurate by silica gel chromatography, the process was ardu-
ous. Surprisingly, recrystallization was also ineffective at removing
the contaminant. Therefore, partially purified starting materials
(ca. 70% pure) 2a–c were used. After the trizole formation step of
Table 1
Yield of C-5 (bromomethyl)-oxazolidinones for the TBAB catalyzed condensation of
1,2-epoxides with isocyanatesa
Isocyanate
Solvent
Time (h)
Yield 2a–c (%)
R2
O
1a
1a
1a
1a
1b
1c
Tolueneb
Xylenesb
Xylenesc
Xylenesc
Tolueneb
Xylenesb
1.5
1.5
1.5
3.0
1.0
3.0
3
38
33
60
53
65
NaN3
30 min
DMSO
O
CuI
R1
N
O
R1
N
O
1 h
N
N
Br
N
R2
a
1.3–1.9 mol % TBAB, reactions in toluene run at 85–90 °C, reactions in xylenes
2a-c
4-15
run at 140 °C.
b
c
Isocyanate–oxirane (1:1–1.1).
Isocyanate–oxirane (1:1.5).
Scheme 3. Three-component reaction to form 1,4-disubstituted-(1H-1,2,3)-tria-
zole-oxazolidinones.
O
H3C
TBAI (cat.)
N
O
+
2b
NaN3
DMF (52 %)
N3
3b
O
H3C
10 mol% Na ascorbate
10 mol% Cu(II) SO4 - 5H2O
N
O
+
3b
R
N
N
1:1 t-butanol/water
N
R
R = (CH2)7CH3
R = (CH2)2OH
R = C6H5
4: R = (CH2)7CH3 (100%)
5: R = (CH2)2OH (62%)
6: R = C6H5 (91%)
Scheme 2. Stepwise reaction to form 1,4-disubstituted-(1H-1,2,3)-triazole-oxazolidinones.

4870
J. A. Demaray et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4868–4871
Table 2
coccus faecalis ATCC. Mycobacterium smegmatis was inoculated into
Middlebrook 7H9 containing 0.2% glycerol and ADC enrichment
while the other strains were inoculated into Mueller–Hinton broth.
The inocula of all strains were allowed to incubate for approxi-
mately 24 h at 37 °C and 180 rpm. Mycobacterium smegmatis was
plated on Middlebrook 7H10 agar containing 0.5% glycerol and
OADC enrichment. While the other strains were plated on nutrient
Structures and isolated yields for 1,4-disubstituted-(1H-1,2,3)-triazole-oxazolidinon-
es prepared using the three-component reaction between 5-(bromomethyl)-oxazo-
lidinones, sodium azide, and alkynes
Entry
R1
R2
Yield (%)
4
40
2a
agar. Aliquots (10 lL) of oxazolidinone derivatives dissolved in
DMSO (1 mg/mL) were applied to 6 mm diameter sterile paper
disks. INH and vancomyocin (1 mg/mL) were used as positive con-
trols for M. smegmatis and other Gram-positive bacteria, respec-
tively. The M. smegmatis and other Gram-positive bacteria plates
were incubated for 48 and 24 h, respectively, at 37 °C and then
analyzed. DMSO was used as a negative control. The diameters of
the zones of inhibition (DZIs) are summarized in Table 3A for com-
pound 13 and compound 18, both of which showed significant
antibacterial activity.
Minimum inhibitory concentrations (MICs) of 13 and com-
pound 18 were also determined against the same strains using a
broth macrodilution assay using either Middlebrook 7H9 contain-
ing 0.2% glycerol and ADC enrichment or Mueller–Hinton broth.20
5
2a
2a
2a
57
58
59
40
40
40
40
52
61
48
39
OH
OH
OH
6
7
Si
Si
Si
CH3
8
2b
9
2b
2b
2b
The highest concentration for each compound tested was 32 lg/
mL, and each subsequent tube was a twofold dilution of the previ-
ous. The M. smegmatis tubes were incubated for 48 h at 37 °C while
the other strains were incubated for 24 h and then analyzed. Thiaz-
olyl blue tetrazolium bromide (MTT) was dissolved in MeOH
10
11
12
13
14
15
(10 mg/mL) and added to the solution (20 lL) to aid in bacteria
visualization.21 The MICs are summarized in Table 3B for 13 and
compound 18.
Discussion. In summary, we have successfully prepared a series
of C-5 substituted (bromomethyl)-oxazolidinones from a bromide
catalyzed cycloaddition of aryl isocyanates with epibromohydrin
in moderate yields. The C-5 bromomethyl moiety could be dis-
placed with azide via an SN2 reaction with NaN3. These 3-N-aryl
C-5 azido substituted oxazolidin-2-ones could be transformed in
situ using a ‘one-pot’ protocol to afford 1,4-disubstituted C-5
substituted-(1H-1,2,3)-triazole-oxazolidin-2-ones in moderate
yields. The choice of solvent was critical for accessing the 4-tri-
methylsilyl substituted derivatives and the overall sequence is
highly amenable to the combinatorial generation of oxazolidinone
libraries by varying either the isocyanate component or the alkyne
component.
COCH3
2c
2c
2c
2c
The compound library resulting from the one-pot method was
screened for activity against a small panel of Gram-positive
the one-pot reaction the compound polarity showed a marked in-
crease and flash chromatography became straightforward.
Small or sp2 hybridized groups at the 4-position of the triazole
in triazole-oxazolidinones have been reported to posses higher
antimicrobial activities.9 Therefore, we replaced 4-TMS moiety of
triazoles 7, 10, and 15 with hydrogen by treatment with TBAF to
afford the mono-substituted triazoles 16–18, respectively,
Scheme 4.
Table 3
Antibacterial activity of triazole-oxazolidinones
Antibiotic
Ms
Bs
Ef
A. Diameters of zones of inhibition (DZI), mma
Biology. The compound library was screened using a disk
diffusion assay on the Gram-positive bacterial reference strains,19
M. smegmatis ATCC 14468, Bacillus subtilis ATCC 6633, and Entero-
13
18
INH
Van
DMSO
15
25
17
N.T.
N.A.
17
26
N.T.
15
N.A.
N.A.
N.A.
N.T.
14
N.A.
B. Minimal inhibitory concentrations (MICs), l
g/mLb
O
O
13
16
>32
4
N.T.
0.5
>32
16
N.T.
1
TBAF
R
R
N
O
18
1
N
O
N
N
INH
Van
4
N.T
N
N
N
N
SiMe3
Abbreviations: Ms, Mycobacterium smegmatis ATCC 14468; Bs, Bacillus subtilis ATCC
6633; Ef, Enterococcus faecalis ATCC 29212.
a
The DZIs were determined by the Kirby–Bauer disk method. Solutions of 1 mg/
16: R = Ph
17: R =
18: R =
7: R = Ph
11: R =
15: R =
mL for 13, compound 18, and INH were prepared in DMSO. Vancomycin was pre-
pared to the same concentration in water. Ten microliters of each solution was then
applied to a susceptibility disk. N.T. indicates no data was collected, N.A. indicates
p
p
-CH3-Ph
-COCH3-Ph
p
p
-CH3-Ph
-COCH3-Ph
no activity.
b
MICs are given in lg/mL.
Scheme 4. Deprotection of 4-TMS-substituted triazole-oxazolidinones.

J. A. Demaray et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4868–4871
4871
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bacterial reference strains. Notably, racemic 3-(4-acetyl-phenyl)-
5-(1H-1,2,3-triazol-1-yl)methyl)-oxazolidin-2-one (18) was a po-
tent inhibitor of the growth of M. smegmatis ATCC 14468 and pos-
sessed an MIC fourfold higher that the INH. The trends in activity
were predictable based on known structure–activity relationships
for the oxazolidine compound class. The 3-(4-acetylphenyl) substi-
tuent, derived from the isocyanate reagent, is same 3-substituent
found in DuP-721,4 while the higher activity of the mono-substi-
tuted (1H-1,2,3)-triazol fulfills the requirement for a small substi-
tuent in the 4-position of triazole-oxazolidinones.9 More
significant is the observation that the 5-(1H-1,2,3-triazol-1-yl)-
methyl moiety was a particularly good C-5 substituent for achiev-
ing activity in a mycobacterial strain.
Acknowledgments
This work was supported by the University of Toledo, the Ohio
Science and Engineering Alliance (OSEA) through a Glenn–Stokes
Summer Research internship (to M.N.D.), and by the National Sci-
ence Foundation, Ohio Consortium for Undergraduate Research:
Research Experiences to Enhance Learning (REEL), NSF
(CHE0532250).
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Supplementary data
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Supplementary data associated with this article can be found, in
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