ANTIMICROBIAL ACTIVITY OF SOME NEW OXADIAZOLE
DERIVATIVES
Zuhair Muhi-eldeen1, Ghada Juma’a 2, Elham Al-kaissi 1 , Lina Nouri 3,
1
College of Pharmacy, University of Petra, Amman – Jordan
2
College of Pharmacy, University of Kufa, Kufa, Iraq
3
College of Pharmacy, University of Baghdad, Baghdad, Iraq
Department of Pharmaceutics, School of Pharmacy, Petra University, P.O. Box
961343 Amman, Jordan, Fax 00962-6-5715551, Tel. 00962-6-05715546
E. mail: kaielham@hotmail.com
1
ABSTRACT
Alkyl, alkenyl, sulfonyl, thiocarbamates and Mannich derivatives were synthesized and
characterized through IR, NMR, and Elemental analysis. It is of interest to report the
isomerization rearrangement of propynyl to allene group in Mannich reaction under basic
condition.
The most promising compound as antibacterial agent was 5-(pyridyl)-1, 3, 4-oxadiazole2-benzylthiocarbamates.
Keywords: Mannich derivative, Antibacterial, Isomerization, Propynyl, Allene
2
INTRODUCTION
A large number of oxadiazole derivatives have been prepared and many of these
compounds have shown a wide spectrum of antimicrobial activity (Lohray et al., 2004a,
b; Weidner et al., 2004; Paget et al., 2003). The observation that some oxadiazoles with
different substituents at different location on the heterocyclic ring resulted in fungicidal
(Geban et al., 1999; Gulay et al., 2002a) and antibacterial agents (Thomasco et al.,2003;
Srivastava et al., 2003; Gulay et al., 2002b; Heerding et al., 2001) of various potencies.
Despite a number of antimicrobial agents available for treatment of microbial infections,
emergence of multi-drug resistant organism has posed a great challenge to the scientists
(Neu, 1992; Bax et al., 2000; Cohn, 1992; Struelens, 1998; Finch et al., 2003; von Eiff et
al., 2001; Kloos and Mannerman, 1994) for continuous looking for new antimicrobial
agents. This promoted our interest to synthesize a new series of substituted 2- mercapto –
5 – phenyl or 5 – (4 – pyridyl) – 1,3,4 – oxadiazole derivatives in which the mercapto
group was converted to alkyl, alkenyl , alkynyl, sulfon, thiocarbamate and mannich
derivatives as a new compounds of possible antimicrobial activity and to investigate the
influence of unsaturation on their activity profile.
3
MATERIALS AND METHODS
Experimental
Melting point was determined by using a calibrated Thomas-Hoover melting
apparatus. IR spectra were recorded using a Perkin – Elmer 257 spectrophotometer;
NMR spectra were carried out on Varian EM-390, 90MHZ spectrometer using
tetramethylsilane as the internal reference.
Microanalyses were performed in the Laboratories of Dr. BERNHARDT, Mulheim, West
Germany and in the Laboratories of the Oil Exploration Company; Iraq.
The analyses are shown in (Table 4, 5, 6) or included in the experimental part and
indicated only by symbols of the elements analyzed, the result obtained had a maximum
deviation of ± 0.4% from the theoretical value.
Starting material
Ethylisonicotinate and isonicotinic hydrazides were prepared as previously described
by Burrus and British pharmaceutical codex. Benzoic acid hydrazide was synthesized
according to method reported in Vogel (Vogel, 1978).
5-Subtituted-2-mercapto-1, 3, 4-oxadiazoles (1, 2)
To a solution containing 400ml of 95% ethanol and (0.1 mole,5.6g) of potassium
hydroxide (dissolved in 15ml of water), (0.1 mole) of the appropriate hydrazide was
added. After the solution occurred (6.6ml, 0.11 moles) of carbon disulfide was added and
the mixture was refluxed for 3 hrs. The solution then was concentrated to a small volume
and the residue was dissolved in water. A precipitate was obtained by adding the solution
to ice containing hydrochloric acid. The solid was filtered off, dried and recrystallized
from ethanol.
4
The IR spectra showed a weak SH stretching absorption at 2600 cm-1.
Melting point for 5-phenyl and 5-(4-pyridyl)-2-mercapto-1, 3, 4-oxadiazoles 1 and 2
were 270 – 271 oC and 218 – 220 oC respectively and are consistent with their previously
reported melting points (Young and Wood, 1955).
2- Alkyl/alkenyl/aralkyl-thio-5-substituted-1, 3, 4-oxadiazoles (3-9)
To a stirred solution of 1 or 2 (0.01 mole) in 20ml absolute ethanol, was added
(0.01mole, 0.56g) potassium hydroxide, the solution was refluxed for 30 minutes. Then
the appropriate alkyl or allyl halide (0.01 mole) was added dropwise to the stirred
reaction mixture, which was then refluxed for two hrs. After cooling, the mixture was
filtered and the filtrate poured into ice- cold water.
The crude product was collected and recrystallized from ethanol-water. The oily alklated
products, were separated as oil drops after pouring the mixture on ice-cold water.
The aqueous mixture was then extracted with ether, the ether layer dried over anhydrous
sodium sulfate, the ether was evaporated and the product was purified from acetone. The
physical data, the prepared compounds and their IR spectral analyses were reported in
(Table 1 and 4) respectively.
2- Alkyl/aralkyl sulfonyl-5-substituted-1, 3, 4-oxadiazoles (10-14)
A solution of 0.7g potassium permanganate in 8ml water was added to a stirred ice-cold
suspension of 0.002 mole of the appropriate 2- alkyl thio -5- substituted -1,3,4oxadiazoles 3-9 in 5ml glacial acetic acid. Cooling and stirring was continued for four
hrs, then the reaction temperature was brought to 0 oC and saturated sodium sulfite
solution was added gradually with stirring until the color of the permanganate
disappeared. The reaction mixture was then filtered and the precipitate washed with water
5
and crystallized from ethanol-water. The physical data of the prepared compounds and
their IR spectral analyses were reported in (Table 2 and 5) respectively.
5-(4-pyridyl)-1, 3, 4-oxadiazolyl-2-thiocarbanates (15, 16)
A mixture of 5-(4-pyridyl)-2-mercapto-1,3,4-oxadiazole (0.005 mole) absolute ethanol
(20ml), potassium hydroxide (0.005 mole, 0.28g), and anhydrous sodium carbonate
(0.005 mole, 0.42g) was refluxed for 30 minutes. The mixture was then cooled to room
temperature, and the appropriate chloroformate (0.005 mole) was added dropwise with
continuous stirring, the mixture was heated at (30-35 oC) for one hr, cooled to room
temperature and poured into ice-water. The precipitated product was filtered, washed
with water, and recrystallized from ethanol-water. Yields melting point and IR spectral
analyses were reported in (Table 3 and 6).
5-phenyl-1, 3, 4-oxadiazolyl-2-thiocarbamates (17)
To a solution of 0.01 mole of 5-phenyl-2-mercapto-1, 3, 4-oxadiazole in 20 ml absolute
ethanol was added (0.84 mole, 0.01g) of anhydrous sodium carbonate, then (0.01 mole)
of the appropriate chloroformate was added dropwise. The reaction mixture was refluxed
for 1-2 hrs; cooled to room temperature and poured on 100 ml ice water. The precipitated
carbonate was filtered off, washed with water and crystallized from ethanol-water.
Physical data of the prepared compounds, and the IR spectral analyses were shown in
(Table 3 and 6).
2-Allenyl thio-5-substituted-1, 3, 4-oxadiazole (18)
To a stirred solution of 1 (0.01 mole) in 20 ml absolute ethanol, (0.01 mole, 0.56g) of
potassium hydroxide was added, the solution was refluxed for 30 minutes, then
propargylbromide (0.011 mole) was added dropwise and refluxed for 2 hrs.
6
After cooling to room temperature, the mixture was filtered and the filtrate poured into
ice-cold water. The precipitate was filtered off and recrystallized from ethanol-water. The
physical data were shown in Table 1.
2-(2-propynyl)-thio-5-phenyl-1,3,4-oxadiazole (18)
To a solution of 2-mercapto-5-phenyl-1, 3, 4-oxadiazole (0.01 mole) in ethanol (50
ml), propargyl bromide (0.012 mole, 2 ml) was added dropwise.
When the addition was completed, the reaction mixture was stirred for 3 hrs at room
temperature. Evaporation of the ethanol under reduced pressure yielded a crude product
which was extracted with ethyl acetate and dried over anhydrous sodium sulphate. The
ethanol was removed under reduced pressure. The crude solid was crystallized from ethyl
acetate-ethanol mixture as a white solid in 65% yield.
The IR spectra showed the following absorption bands (KBr, cm-1) 3270 (=CH), 3020
(ArH), 2120 (C=C), the NMR spectra showed the following characteristic chemical shifts
(CDCI3, δ) 3.5 (doublet, 2H, CH2-C=, Jz =2,2) 205 (triplet, IH, =CH, Jz = 2,2). Other
signals in the spectra are consistent with various protons in the aromatic ring.
2-(4-pyrrolidino-2-butynyl) thio-5-phenyl-1, 3, 4-oxadiazole (19)
A mixture of 5-phenyl-(2-propynyl)-1,3,4- oxadiazole (0.003 mole), paraformaldehyde
(0.0033 mole), pyrrolidine (0.003 mole) and a catalytic amount of cuprous chloride in 10
ml per oxide-free dioxone. The mixture was stirred at room temperature for 10 minutes
then was heated at 70 – 75 oC for three hours. Concentration of the reaction mixture under
reduced pressure gave brown syrup, which was suspended in water and neutralized with
sodium carbonate solution.
7
Extraction of the syrup with ethyl acetate, dried over anhydrous sodium sulfate. The
chromatography on silica gel resulted in the desired Mannich product. The IR spectra
showed the following characteristic absorption bands (KBr, cm-1) 3050 (CH, ArH), 2100
(very weak, C=C); The NMR spectra showed the following characteristic chemical shifts
(DMSO, δ).
4.5 (triplet, 2H, S-CH2-C≡, J= 2.2Hz) 3.4 (triplet, 2H, ≡C-CH2-N, J=2.2Hz).
Other signals in the NMR spectra were consistent with the protons in aromatic ring and
the pyrrolidine.
ANTIMICROBIAL ACTIVITY
All tested compounds were assayed for their antimicrobial activity according to the
macrodilution method of the National Committee for Clinical Laboratory Standards
(NCCLS, 1998; NCCLS, 2002) recommendations, against three standard bacterial
strains, Staphylococcus aureus( ATCC 29213), Escherichia coli (ATCC 25922) and
Pseudomonas aeruginosa (ATCC 27953). The compounds were evaluated against
clinical isolated Candida albicans for their antifungal activity. The evaluation was done
using dilution method as shown in (Table 7). Increasing concentrations of the compounds
were incorporated into Muller Hinton broth (BBL Microbiology Systems) for bacteria
and Sabouraud’s broth for Candida.
MIC breakpoints for defining susceptibility were in accordance with the description by
National Committee for Clinical laboratory Standards (NCCLS, 2002).
8
RESULTS AND DISCUSSION
The new compounds (Table 1, 2, 3) and compounds (18, 19, 20) were prepared as
depicted in Scheme 1. 2 – mercapto – 5 – substituted – 1, 3, 4 – oxadiazoles 1, 2 were
prepared as previously described by Young and Wood in 1955.
The potassium salts of 1, 2 were converted to their corresponding thioethers 3-9 through
reaction with the appropriate alkyl halides (Table 1). Oxidation of the thioethers upon
treatment with potassium permanganate in glacial acetic acid yielded the corresponding
sulfonyl derivatives 10-14 (Table 2).
Treatment of 1, 2 with chloroformate in the presence of sodium carbonate afforded the
corresponding thiocarbamates 15-17 (Table 3). The physical constant, IR, and elemental
analyses were consistent with assigned structures (Table 4, 5, 6). Treatment of 1 with
alcoholic sodium or potassium hydroxide afforded 2 – allenyl thio – 5 – phenyl – 1, 3, 4 –
oxadiazole 8.
The formation of the allene may result from the isomerization of the kinetically
controlled product 18 into the thermodynamically controlled product 8 as illustrated in
scheme 2.
The ir and elemental analysis were in agreement with the assigned structures, whoever
alkylation of 1 with propargyl bromide in absolute ethanol afforded the 2 – (2-propynyl)
– thio – 5 – phenyl – 1,3,4 – oxadiazole 19 .
Compound were subjected to mannich reaction utilizing paraformaldehyde, pyrrolidine
as secondary amine and a catalytic quantity of cuprous chloride in peroxide – free
dioxane yielded the N – 2 – (4 – pyrrolidine – 2 – butynyl) thio – 5 – phenyl – 1,3,4 –
oxadiazole 19.
9
The IR, NMR and elemental analysis were consistent with prepared compound. It is of
interest to report here that in our previous publications ( Muhi-eldeen, et al., 1982; Muhieldeen, et al., 1985).
The alkylation of 2 – mercapto – 5 – substituted - 1, 3, 4 – thiadiazoles or
3 – mercapto – 4,5 – disubtituted – 1,2,4 – triazoles in basic ethanolic solution yielded the
2 – propynyl thio rather than the allenyl thio – derivatives.
These results may suggest that the oxygen atom in the heterocyclic ring exert a greater
electronic polarization on the neighboring mercapto group than sulfur or nitrogen within
the heterocyclic ring and that resulted in the isomerization of 2 – propynyl to allene
group.
ANTIMICROBIAL ACTIVITY
The lowest concentration which inhibited growth was considered as the MIC. The
synthesized compounds showed low antibacterial activity against Ps. aeruginosa but
good activity against Staph. aureus and E. coli. Only thiocarbamate 16 and 17 were
effective against Gram positive and Gram negative bacteria with less activity against
fungi. These results promote our interest to investigate further the thiocarbamate series.
.
10
Table 1: Physical data of the 2-alkyl, alkenyl thio-5-substituted-1, 3, 4-oxadiazoles
N
R
Compound
3
4
5
6
7
8
9
R
R1
4-Pyridyl
Phenyl
-CH=C-CH2
allyl
ethyl
isopropyl
Phenyl-ethyl
-CH=C=CH2
isopropyl
%
yie
-ld
m.p.
C◦
86
83
50
32
80
66
41
113-115
69
75-77
49-50
56-58
76
oily
N
SR1
O
Elemental Analyses
Chemical
Formula
%Calculated
C
H
N
C
H
N
C10H7N3OS
C10H9N3OS*
C9H9N3OS
C10H11N3OS§
C15H13N3OS
C11H8N2OS
C11H12N2OS
55.29
52.71
52.16
50.20
63.57
3.22
3.97
4.37
4.63
4.62
18.8
18.41
20.27
17.56
14.83
55.55
53.25
51.68
50.75
62.99
3.33
3.98
4.26
4.59
4.22
18.32
18.91
14.74
* These compounds contain ½ mole of H2O
§ These compounds contain 1 mole of H2O
11
%Found
Table 2: Physical data of the 2-alkylsulfonyl -5-substituted-1, 3, 4-oxadiazoles
N
R
Compound
10
11
12
13
14
R
4-Pyridyl
Phenyl
-
R1
ethyl
isopropyl
Phenyl-ethyl
isopropyl
Phenyl-ethyl
%
yield
22
19
46
64
47
N
SO2R1
O
m.p.
C◦
123-124
137
165-166
115
148
Elemental Analysis
Chemical
Formula
C9H9N3O3S
C10H11N3O3S
C15H13N3O3S
C11H12N3O3S
C16H14N2O3S
12
%Calculated
% found
C
H
N
C
H
N
45.16
47.43
57.14
52.37
61.13
3.79
4.35
4.13
4.79
4.49
17.57
16.6
13.33
11.11
8.91
44.90
48.16
57.33
51.92
60.69
3.69
4.48
4.37
4.78
4.4
17.2
16.5
13.87
11.75
9.15
Table 3: Physical data of 5-substituted-1, 3, 4-oadiazolyl -2-thiocarbamates
O
N
R
N
S-C-OR2
O
Elemental Analyses
Compound
R
15
16
17
4-Pyridyl
-
R2
Isobutyl
Benzyl
Et
%
yield
m.p.
C◦
Chemical
Formula
60
50
39
92
106-107
132-133
C12H13N3O3S
C15H11N3O3S
C11H10N2O3S
13
%Calculated
%Found
C
H
C
H
51.60
57.50
53.00
4.69
3.54
4.00
51.85
58.02
53.35
4.50
3.30
3.99
Table 4: Selected infrared data of 5-substituted-2-alkyl/alkenyl/aralkyl thio-1, 3, 4oxadiazoles
N
R
R
R1
4-Pyridyl
N
S R1
O
allyl
C=N
Stretching
CM-1
1550 (m)*
C-C
Stretching
CM-1
1620 (m)
4-Pyridyl
isopropyl
1550 (m)
1620 (m)
Phenyl
isopropyl
1560 (s)
1620 (s)
Other bands
CM-1
1420 (m)-S-CH2- in plane
bending
1000 (m), 840 (s), 710 (s), out
of plane olefinic C-H bending
1070 (m) in plane C-H
bending
900 (w) – CH3 rocking
3100 (w), 3000 (s) C-H
stretching
1375, 1360 doublet (s) gem
dimethyl bending
930 (w)-CH3 rocking
780 (s), 710 (s) aromatic C=C
bending of mono-substituted
ring
* Abbreviations: (s) Strong, (m) Medium, (w) Weak.
14
Table 5: Selected infrared data of 2-alkyl/aralkyl/sulfonyl-5-substituted- 1, 3, 4oxadiazoles
N
R
R
R1
4-Pyridyl
N
SO2R1
O
ethyl
SO2
symmetric
stretching
CM-1
1150 (s)*
SO2
symmetric
stretching
CM-1
1150 (s)
Phenyl
isopropyl
1150 (s)
1150 (s)
Phenyl
Phenyl
1150 (s)
1140 (s)
* Abbreviations: (s) Strong, (m) Medium, (w) Weak.
15
Other bands
CM-1
1580 (m) C=C ring
stretching
1540 (s) C=N ring
stretching
1420 (s) –S-CH2- in plane
bending
1610 (s) aromatic C=C
stretching
1210 (m), 1060 (s), ring C-H
in plane bending
720 (s), 650 (m), aromatic
C-H out of plane bending
1610 (m) aromatic C=C
stretching
1200 (w) ring C-H in plane
bending
710 (s), 700 (s), 630 (s)
aromatic C-H out of plane
bending
Table 6: Selected infrared data of 5-substituted- 1, 3, 4 - oxadiazolyl – 2 –
thiocarbamates
N
R
O
R
R2
C=O
M-1
isobuty
l
1770(s)*
4-Pyridyl
benzyl
4-Pyridyl
Phenyl
O
N
S - C - OR2
-C=Cstretchin
g CM-1
1660 (m)
1780 (s)
ethyl
1620 (s)
1775(s)
doublets
Other bands
CM-1
1600 (m) C=N stretching
1470 (s) CH2 scissoring (in
plane bending)
1290 (s) asymmetric C-O-C
stretching
1470 (s) CH2 scissoring
950 (m), 980 (m) =C-H
bending
700 (s), 750 (s), aromatic C-H
bending
1580 (m) C=N stretching
980 (s), 950 (w) =C-H
bending
730 (s), 700 (s) aromatic C-H
bending
* Abbreviations: (s) Strong, (m) Medium, (w) Weak.
16
Table7: MIC (g/ml) of selected oxadiazole derivatives
compound
3
8
10
13
15
16
17
20
Staph.
aureus
64
64
32
32
32
8
8
64
E. coli
64
64
64
64
64
8
32
64
Ps.
aeruginosa
128
128
128
128
128
16
64
128
17
Candida
albicans
64
64
64
64
64
64
32
64
REFERENCES
Bax, R., Mullan, N., and Verhuef, J., 2000.The millennium bugs-the need for and
development of new antibacterial. International Journal of Antimicrobial Agents.
16, 51-59.
Cohn , M., 1992. Epidemiology of drug resistance: for a post-antimicrobial era,
Science. 5-257,1050.
Finch R., Greenwood, D., Norrby, S.R., and Whitley, R., 2003. Antibiotic and
chemotherapy: Anti-infective agent and their use in therapy. 8th ed. Churchill
Livingstone: Philadelphia, USA., p1000.
Geban, O., Ertepinar, H., Yurtsever, M., Ozden, S., Gomos, F., 1999. QSAR study on
antibacterial and antifungal activities of some 3,4-disubstituted-1,2-oxa(thia)-diazole5(4H)-ones(thiones)
using
physiochemical,
quantumchemical
and
structural
parameters. Eur. J. Med. Chem. 34(9),753-758.
Gülay, S., Palaska, E., Ekizoĝlu, M., Ozalp, M., 2002. Synthesis and antimicrobial
activity of 1,3,4-oxadiazole derivatives. II Farmaco. 57(7), 539-542.
Gulay, S., Erhan, P., Melike, E., Meral, O., 2002. Synthesis and antibacterial activity of
some 1,3,4-oxadiazole derivitaives.3rd international Symposium on Pharmaceutical
Chemistry, Istanbul, Turkey. 57(7), 539-542.
Heerding, D. A., Chan, G., DeWolf, W. E., at al., 2001.1,4-Disubstituted imidazoles are
potential antibacterial agents functioning as inhibitors of enoyl acyl carrier pprotein
reductase (Fab1).Bioorganic & Medicinal Chemistry Letters. 11(16), 2061-2065.
18
Kloos, W. E., and Bannerman, T. L., 1994. Update on clinical significance on coagulasenegative staphylococci. Clin. Microbiol. Rev. 7,117-140.
Lohray, B. B. Lohray, V. B., Srivastava, B. K., Kapadnis, P. B., and Pandya, P., 2004.
Noval tetrahydro-thieno pyridyl oxazolidinone: an antibacterial agent. Bioorganic &
Medicinal Chemistry. 12, 4557-4564.
Lohray, B. B., Lohray, V. B., Srivastava, B. K., Gupta, S., Solanki, M., Kapadnis, P. B.,
Takale, V., and Pandya, P., 2004. Oxazolidinone: search for highly potent
antibacterial. 14, 3139-3142.
Muhi-Eldeen, Z., Al-Jawad, F., Eldin, S., Abdul-Kader, S., Gantous, H., and Garabet, M.,
1982. Synthesis and biological evaluation of 2-(4-tert-amino2-butynyl) thio-5—
aryl-1,3,4-thiadiazoles Eur. J. Med. Chem. 17, 479-481.
Muhi-Eldeen, Z., Eldin, S., Al-Jawad, F., Hatim, A. F., Kalil, D., and Sameh, I., 1985.
Synthesis and biological evaluation of 5-aryl-3-(4-tert- amino-2-butynyl)-1,3,4thiadiazol-2(3H)-ones. Eur. J. Med. Chem. 20(1), 33-36.
National Committee for Clinical Laboratory Standards(NCCLS).1998. Performance
Standards for Antimicrobial Susceptibility Testing. Standard (M100-128).
Villanova, Pa. 5th ed.
National Committee for Clinical Laboratory Standards (NCCLS). 2002. Reference
Method for Broth Dilution Antifungal Susceptibility Testing for Yeasts. Approved
Standard (M27-A2). 2nd ed.
Neu, H., 1992.The crisis in antibiotic resistance, Science. 257, 1064-1073.
19
Paget, S.D., Foleno, B. D., Boggs, C. M., Goldschmidt, R. M., Hlasta, D. J., Weidnerwells, M. A., Werblood, H. M., Wira, E., Bush, K., and Macielag, M. J., 2003.
Synthesis and antibacterial activity of Pyrroloaryl- Substituted Oxazolidinones.13,
4173-4177.
Srivastava, R. M., De Almeida, L. A., Viana, O.S., Da Costa, S. M. J., Catanho, M. T. J.
A., De Morais, J. O. F., 2003. Antiinflammatory property of 3-aryl-5-(n-propyl)1,2,4-oxadiazole and antimicrobial property of 3-aryl-5-(n-propyl)-4,5,-dihydro1,2,4-0xadiazoles: their synthesis and spectroscopic studies. Bioorganic & Medicinal
Chemistry. 11(8), 1821-1827.
Struelens, M., 1998. The epidemiology of antimicrobial resistance in hospital
acquired infections: problems and possible solution. British Medical Journal. 317,
652-654.
Thomasco, L. M., Gadwood, R. C., Weaver, E. A., Ochoada, J. M., Ford, C. W.,
Zurenko, G. E., Hamel, J. C., Stapert, D., Moerman, J. K., Schaadt, R. D., and Yagi,
B. H., 2003. The synthesis and antibacterial activity of 1,3,4-Thiodiazole Phenyl
Oxazolidinone Analogues. Bioorganic & Medicinal Chemistry letters.13, 4193-4196.
Vogel, A., 1978.Text book of Practical organic chemistry. 4th ed. London, Longman
Group Limited. p1204.
von Eiff, C., Proctor, R. A., and Peters, G., 2001. Coagulase-negative staphylococci.
Pathogens have major role in nosocomial infections. Postgrad Med. 110(64), 69-70,
73-76.
20
Weidner-Wells, M., Werblood, H. M., Goldschmidt, R., Bush, K., Foleno, B. D., Hilliard,
J. J., Melton, J., Wira, E., and Macielag, M. J., 2004. The synthesis and antimicrobial
evaluation of new series of isoxazolinyl oxazolidinones. 14, 3060-3072
Young, R.W., Wood, K.H.; 1955.The Cyclization of 3-Acyldithiocarbazate Esters.
J. Am. Chem. Soc. 77, 400-403.
.
21