Microwave-assisted solid-phase synthesis of cephalosporin derivatives with antibacterial activity

Article abstract of DOI:10.1007/s007060070048

The reaction of heterocyclic acids with 7-amino-cephalosporanic acid adsorbed on basic alumina under microwave irradiation afforded the N-acylated cephalosporin analogues in satisfactory yield. All compounds were tested for their antibacterial activity; some of them showed significant antibacterial properties. Cefotaxime and cephalothin were used as reference drugs.

Full text of DOI:10.1007/s007060070048

Monatshefte f uÈ r Chemie 131, 937±943 (2000)
Microwave-Assisted Solid-Phase Synthesis
of Cephalosporin Derivatives with
Antibacterial Activity
Mazaahir Kidwai1 , Preeti Misra , Kumar R. Bhushan ,
;Ã
1
1
2
2
Rajendra K. Saxena , and Meena Singh
1
Department of Chemistry, University of Delhi, Delhi-110007, India
2
Department of Microbiology, University of Delhi, New Delhi-110021, India
Summary. The reaction of heterocyclic acids with 7-amino-cephalosporanic acid adsorbed on basic
alumina under microwave irradiation afforded the N-acylated cephalosporin analogues in
satisfactory yield. All compounds were tested for their antibacterial activity; some of them showed
signi®cant antibacterial properties. Cefotaxime and cephalothin were used as reference drugs.
Keywords. 7-Aminocephalosporanic acid; Solid support; Microwave irradiation; Antibacterial
activity.
Introduction
The discovery of cephalosporin C, the ®rst example of a naturally occurring
cephalosporin [1], was the beginning of a new era in the search for effective
antibacterials containing the ꢀ-lactam moiety. Since then many advances have been
made in the synthesis, chemical modi®cation, and biology of these molecules [2].
The increased chemical and biological stability of 7-substituted analogs (cepha-
lothin, cefotaxime) illustrates the advantages of structurally modi®ed derivatives.
A literature review shows that most of the synthetic strategies have been
patented. Coupling of 7-ACA (2) with an acid in the presence of triethyl amine
requires a reaction time of more than 4 hours [3]; a modi®cation using SO Á DMF
3
[4] at pH 7.5±8.0 still needs 2 hours. Usage of acetone-water/NaHCO and acetone
water/C H N [5] have also been reported. The application of coupling agents like
3
6
5
DCC [6] and triethylamine in dichloromethane [7] or in tetrahydrofuran [3] is not
desirable because of high cost and environment polluting solvents.
The environmentally friendly goal of synthesizing organic compounds without
using solvents has been the subject of many investigations in recent years taking
into account that drastic legistic restrictions on solvent pollution may require
increased application of solventless reaction conditions in the future [8]. With the
development of microwave ovens [9], reactions in dry media [10] have became
easier to perform. Since microwaves are only adsorbed by the reactants on the
à Corresponding author

938
M. Kidwai et al.
surface of common inorganic oxides such as silica and alumina [11], numerous
reagents supported on solid surfaces [12] have been effectively utilized for
conducting organic reactions under very safe and simple conditions using domestic
microwave ovens. In general, microwave-assisted reactions may lend themselves to
automation [13].
Though thousands of patents have been obtained for cephem derivatives,
continuous efforts to synthesize new cephem derivatives [14] and to develop
alternative synthetic methodologies are still in progress. In connection with the
interest for new cephalosporins we describe a microwave-assisted synthesis using
solid support [16±17] for the preparation of 7-substituted 8-oxo-5-thia-1-
azabicyclo[4.2.0]oct-2-ene-carboxylic acids. Their in vitro antibacterial pro®le
against various bacteria strains is also discussed.
Results and Discussion
Syntheses
Compounds 3 were synthesized starting from the heterocyclic acids 1 and 7-ACA 2
(
2
Scheme 1). 1a,g±j are commercially available, 1b±f were synthesized. Reaction of
with carboxylic acids 1a±j by adsorption on basic alumina under microwave
irradiation for 90±120 s afforded the 7-substituted cephalosporanic acid derivatives
a±j.
3
Scheme 1

Microwave-Assisted Synthesis of Cephalosporin Derivatives
939
Table 1. Reaction conditions and yields for compounds 3a±j
Conventional heating
Microwave irradiation
Yield/%
t/h
Yield/%
t/s
90
3
3
3
3
3
3
3
3
3
3
a
b
c
d
e
f
65
72
63
65
63
68
67
73
72
76
3
3
4
6
3
4
2
2
3
3
87
89
87
83
85
93
82
83
92
90
120
90
90
120
120
90
g
h
i
90
120
120
j
The structures attributed to the compounds are in accordance with the results of
1
elemental analyses and with IR and H NMR spectroscopic data.
The progress of the condensation of 1a±j with 2 was monitored by the absence
of starting material as detected by reverse phase HPLC. IR spectra of the
�
1
condensed products 3a±j displayed a band at 1700 cm due to the side chain
1
�
amide C=O group and a ꢀ-lactam C=O band at 1765 cm . The shifting of the
1
NH signal from 4.5 to 6.4 ppm H NMR spectrum also con®rmed the structure
of the products. For the cephem skeleton, two doublets at 4.74 and 4.95 ppm
with J ˆ 5.0 Hz were assigned to the cis-hydrogens attached to C-6 and C-7.
Pharmacological results
E. herbicola, Corynbacterium, Z. mobilis, E. Coli, Enterobactor, K. aerogens, B.
lichenformis, and P. vulgaries were used to determine the antibacterial activity by
the cup diffusion method [18]. Cefotaxime and cephalothin were used as standards.
All cephalosporin derivatives 3a±j showed signi®cant antibacterial activity against
gram-positive as well as gram-negative organisms. Compounds 3a,d,e,i,j were
most potent and comparable to cephalothin acid (Table 2); only weak activity was
observed with 3b,c,f,g,h.
In conclusion, to the best of our knowledge this is the ®rst report on the
synthesis of cephalosporins using solid support under microwave irradiation. The
salient feature of our approach is the combination of microwave technology with
solvent-free conditions, thus achieving a modernization and simpli®cation of the
classical procedure, avoiding volatile and toxic organic solvents as well as
corrosive mineral acids and bases. The results shown in Table 1 demonstrate the
versatility of the process, bringing down the reaction time from hours to minutes
with improved yield as compared to conventional heating [19].
Experimental
Melting points were determined by means of a Thomas-Hoover melting point apparatus and are
uncorrected. IR spectra (KBr pellets) were recorded on a Perkin-Elmer spectrophotometer model

940
M. Kidwai et al.
a
Table 2. In vitro antibacterial activity of compounds 3a±j
b
3
3
3
3
3
3
3
3
3
3
a
b
c
d
e
f
12.5
±
7.3
8.3
3.5
3.6
7.5
8.3
±
13.5
2.8
±
10.3
4.6
±
±
±
±
9.9
7.6
6.4
10.2
10.8
2.5
5.7
5.8
9.7
9.8
±
±
3.2
6.3
9.6
8.7
3.2
11.5
11.7
1.6
±
1.5
8.3
8.6
2.5
3.5
4.5
8.3
8.6
3.5
7.5
12.6
11.9
±
11.2
10.3
±
±
6.5
3.6
4.3
3.5
g
h
i
±
±
6.5
5.2
6.5
6.5
11.5
12.5
3.5
9.2
9.3
9.7
4.6
12.8
13.5
16.2
5.7
11.2
10.5
15.6
11.3
12.5
14.5
10.5
±
j
Cefotaxime 14.1
acid
14.7
15.3
Cephalothin 12.2
acid
7.9
8.7
15.4
13.5
±
±
10.3
a
b
Zones of inhibition (mm); no measurable activity
1
5
99. H NMR spectra were recorded on a Perkin-Elmer R-32 (90 MHz) instrument using TMS as
internal standard. Elemental analyses were performed by means of a Heraeus CHN rapid analyser;
their results agreed satisfactorily with the calculated values. A Padmini Essentia microwave oven,
Model Brownie, 2450 MHz, was used. For TLC analysis, silica gel coated Al plates (Merck) were
employed.
General procedure for the synthesis of 1b±f
To the solution of 5-methyl-1,3,4-thiadiazol-2-thiol, 5-phenyl-1,3,4-thiadiazol-2-thiol, 8-hydroxy-
3
quinoline, 2-hydroxy-4-methylquinoline, or benzothiazol-2-thiol (0.01 mol) in 5 cm acetone, 18 g
basic alumina [20] were added at room temperature. Ethyl bromo acetate (0.01 mol), adsorbed on
basic alumina in a similar manner, was dried, properly mixed with the ®rst batch, and irradiated in
the microwave oven for 4±5 min. After completion of the reaction the mixture was cooled and
3
extracted with acetone (4Â10 cm ). The acetone extracts were concentrated under reduced pressure
to yield the corresponding acids.
5-Methyl-1,3,4-thiadiazol-2-yl-sulfanyl acetic acid (1b; C5H6N2O2S2)
ꢀ
1
M.p.: 185±187 C; H NMR (CDCl3, ꢁ, 90 MHz): 2.65 (s, 3H, 5-CH3), 4.52 (s, 2H, SCH2) 10.1 (br,
1
H, OH) ppm; IR (KBr): ꢂ ˆ 3150 (OH), 1680 (C=O), 1520 (C=N) cm� 1
.
5-Phenyl-1,3,4-thiadiazol-2-yl-sulfanyl acetic acid (1c; C10H8N2O2S2)
ꢀ
1
M.p.: 170±172 C, H NMR (CDCl3, ꢁ, 90 MHz): 4.53 (s, 2H, SCH2), 7.0±7.5 (m, 5H, ArH), 10.13
(
br, 1H, OH) ppm; IR (KBr): ꢂ ˆ 3140 (OH), 1690 (C=O), 1520 (C=N) cm� 1
.

Microwave-Assisted Synthesis of Cephalosporin Derivatives
941
Quinolin-8-yloxy acetic acid (1d; C11H9NO3)
ꢀ
1
M.p.: 153±155 C; H NMR (CDCl3, ꢁ, 90 MHz): 4.36 (s, 2H, OCH2), 6.9±7.6 (m, 6H, ArH), 10.23
(
br, 1H, OH) ppm; IR (KBr): ꢂ ˆ 3150 (OH), 1690 (CˆO) cm� 1
.
4-Methylquinolin-2-yloxy acetic acid (1e; C12H11NO3)
ꢀ
1
M.p.: 173±175 C; H NMR (CDCl3, ꢁ, 90 MHz): 2.18 (s, 3H, 4-CH3), 4.49 (s, 2H, 2-OCH2), 6.7 (s,
H, 3-CH), 7.3 (t, 1H, 6-CH), 7.5 (t, 1H, 7-CH), 7.65 (d, 2H, 5- and 8-CH), 10.19 (br, 1H, OH) ppm;
1
IR (KBr): ꢂ ˆ 3150 (OH), 1675 (C=O) cm� 1.
Benzothiazol-2-yl-sulfanyl acetic acid (1f; C9H7NO2S2)
ꢀ
1
M.p.: 157±159 C; H NMR (CDCl3, ꢁ, 90 MHz): 4.42 (s, 2H, SCH2), 7.1±7.5 (m, 4H, ArH), 10.23
(
br, 1H, OH) ppm; IR (KBr): ꢂ ˆ 3130 (OH), 1690 (C=O) cm� 1
.
General procedure for the synthesis of 3a±j
3
Basic alumina (18 g) was added to a solution of 2.72 g 2 (0.01 mol) dissolved in 3 cm aqueous
ammonia at room temperature. In another beaker, 10 g basic alumina were added to a solution of 1a±
j (0.01 mol) in acetone. The mixture was dried at room temperature, and the two reactants were
thoroughly mixed using a mortar mixer and then placed in an alumina bath [21] inside the
microwave oven. Upon completion of the reaction (90±120 s) as monitored by HPLC examination,
the miture was cooled to room temperature, and the product was extracted into a mixture of water
3
and acetic acid (1:4; 4Â10 cm ). Removal of the solvent under reduced pressure afforded the product
which was puri®ed by crystallization from a mixture of acetone/acetic acid.
0
7
-((Tetrazol-1 -yl) acetylamino)-3-acetyloxymethyl-8-oxo-5-thia-1-azabicyclo
[
4.2.0]oct-2-ene-2-carboxylic acid (3a; C13H14N6O6S)
ꢀ
1
M.p.: 215±217 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.7 (s, 3H, COCH3), 4.2 (s, 2H, CH2), 4.35 (s,
2
H, OCH2), 4.56 (s, 2H, NCH2), 4.74 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.95 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.5
0
(
(
br s, 1H, CONH), 8.9 (s, 1H, 5 -CH) ppm; IR (KBr): ꢂ ˆ 1790 (C=O, ꢀ-lactam), 1680, 1640
�
1
CONH and COOH), 1575 (C=N) cm
.
0
0
0
0
0
7
-((5 -Methyl-1 ,3 ,4 -thiadiazol-2 -yl-sulfanyl)-acetylamino)-3-acetyloxymethyl-8-oxo-
5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (3b; C15H16N4O6S3)
ꢀ
1
0
M.p.: 206±208 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.7 (s, 3H, 5 -CH3), 2.9 (s, 3H, COCH3), 4.1 (s,
H, CH2), 4.3 (s, 2H, OCH2), 4.56 (s, 2H, SCH2), 4.7 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.9 (d, 1H,
2
J ˆ 5.0 Hz, 7-CH), 6.3 (br s, 1H, CONH) ppm; IR (KBr): ꢂ ˆ 1785 (C=O, ꢀ-lactam), 1675, 1642
�
1
(
CONH and COOH), 1520 (C=N) cm .
0
0
0
0
0
7
-((5 -Phenyl-1 ,3 ,4 -thiadiazol-2 -yl-sulfanyl)-acetylamnio)-3-acetyloxymethyl-8-oxo-
5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (3c; C20H18N4O6S3)
ꢀ
1
M.p.: 185±187 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.65 (s, 3H, COCH3), 4.1 (s, 2H, CH2), 4.42 (s,
2
H, OCH2), 4.53 (s, 2H, SCH2), 4.7 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.93 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.2
0
(
br s, 1H, CONH), 7.0±7.5 (m, 5H, 5 -ArH) ppm; IR (KBr): ꢂ ˆ 1788 (C=O, ꢀ-lactam), 1680, 1640
�
1
(
CONH and COOH), 1535 (C=N) cm
.

942
M. Kidwai et al.
0
7
-((Quinolin-5 -yl-oxy)-acetylamino)-3-acetyloxymethyl-8-oxo-5-thia-1-azabicyclo
[
4.2.0]oct-2-ene-2-carboxylic acid (3d; C21H19N3O7S)
ꢀ
1
M.p.: 192±194 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.4 (s, 3H, COCH3), 4.1 (s, 2H, CH2), 4.3 (s,
0
2
H, OCH2) 4.46 (s, 2H, 8 -OCH2), 4.56 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.73 (d, 1H, J ˆ 5.0 Hz, 7-CH),
.1 (br s, 1H, CONH), 6.7±7.3 (m, 6H, ArH) ppm; IR (KBr): ꢂ ˆ 1780 (C=O, ꢀ-lactam), 1670, 1630
6
�
1
(
CONH and COOH) cm .
0
0
7
((4 -Methylquinolin-2 -yl-oxy)-acetylamino)-3-acetyloxymethyl-8-oxo-5-thia-
1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (3e; C22H21N3O7S)
ꢀ
1
0
M.p.: 207±209 C; H NMR (DMSO-d6, ꢁ, 90MHz): 2.2 (s, 3H, 4 -CH3), 2.5 (s, 3H, COCH3), 4.2
0
(
s, 2H, OCH2), 4.56 (s, 2H, 2 -OCH2), 4.65 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.75 (d, 1H, J ˆ 5.0 Hz, 7-CH),
0 0 0 0
6
8
.2 (br s, 1H, CONH), 6.82 (s, 1H, 3 -CH), 7.26 (t, 1H, 6 -CH), 7.59 (t, 1H, 7 -CH), 7.61 (d, 2H, 5 and
0-ArH) ppm; IR (KBr): ꢂ ˆ 1785 (C=O, ꢀ-lactam), 1680, 1640 (CONH and COOH) cm� 1
.
0
7
-((Indol-3 -yl)-acetylamino)-acetyloxymethyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-
carboxylic acid (3f; C20H19N3O6S)
ꢀ
1
0
M.p.: 170-172 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.3 (s, 3H, COCH3), 3.45 (s, 2H, 3 -CH2), 4.2
(
s, 2H, CH2), 4.48 (s, 2H, OCH2), 4.7 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.8 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.3
0
(
br s, 1H, CONH), 7.3±7.6 (m, 4H, ArH), 9.0 (s, 1H, 2 -CH), 9.37 (br s, 1H, NH) ppm; IR (KBr):
ꢂ ˆ 1790 (C=O, ꢀ-lactam), 1680, 1635 (CONH and COOH) cm� 1.
0
7
-((Benzothiazol-2 -yl-sulfanyl)-acetylamino)-3-acetyloxymethyl-8-oxo-5-thia-1-
azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (3g; C20H19N3O6S2)
ꢀ
1
M.p.: 167±169 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.3 (s, 3H, COCH3), 4.06 (s, 2H, CH2), 4.28
(
s, 2H, OCH2), 4.45 (s, 2H, SCH2), 4.6 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.9 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.3
0
(
br s, 1H, CONH), 7.1±7.5 (m, 4H, ArH), 8.93 (s, 1H, 3 -CH) ppm; IR (KBr): ꢂ ˆ 1785 (C=O, ꢀ-
�
1
lactam), 1670, 1630 (CONH and COOH) cm
.
0
7
-((Pyridin-3 -yl)-formylamino)-3-acetyloxymethyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-
2-carboxylic acid (3h; C16H15N3O6S)
ꢀ
1
M.p.: 188±190 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.6 (s, 3H, COCH3), 4.2 (s, 2H, CH2), 4.3
(
s, 2H, OCH2), 4.6 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.75 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.50 (br s, 1H CONH),
6
.7±7.1 (m, 4H, ArH) ppm; IR (KBr): ꢂ ˆ 1795 (C=O, ꢀ-lactam), 1680, 1645 (CONH and COOH)
�
1
cm
.
0
7
2
-((Pyridin-4 -yl)-formylamino)-3-acetyloxymethyl-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-
-ene-2-carboxylic acid (3i; C16H15N3O6S)
ꢀ
1
M.p.: 173±175 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 2.6 (s, 3H, COCH3), 4.3 (s, 2H, CH2), 4.5 (s, 2H,
OCH2), 4.7 (d, 1H, J ˆ 5.0 Hz, 6-CH), 4.9 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.42 (br s, 1H, CONH), 6.8±7.3
(
m, 4H, ArH) ppm; IR (KBr): ꢂ ˆ 1790 (C=O, ꢀ-lactam), 1675, 1645 (CONH and COOH) cm� 1
.
0
0
0
7
-(N-Ethyl-6 -methyl-4 -oxo-naphthyridin-3 -yl)-formylamino)-3-acetyloxymethyl-
8-oxo-5-thia-1-azabicyclo[4.2.0]ot-2-ene-2-carboxylic acid (3j; C22H22N4O7S)
ꢀ
1
M.p.: 192±194 C; H NMR (DMSO-d6, ꢁ, 90 MHz): 16 (t, 3H, CH2CH3), 1.9 (t, 3H, CH3), 2.5 (s,
H, COCH3), 3.3 (q, 2H, NCH2), 4.4 (s, 2H, CH2), 4.6 (s, 2H, OCH2), 4.6 (d, 1H, J ˆ 5.0 Hz, 6-CH),
3

Microwave-Assisted Synthesis of Cephalosporin Derivatives
943
4
5
.8 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.3 (d, 1H, J ˆ 5.0 Hz, 7-CH), 6.3 (br s, 1H, CONH), 8.5±8.9 (m, 2H,
0
0
0
-CH and 7 -CH), 9.6 (s, 1H, 2 -CH) ppm; IR (KBr): ꢂ ˆ 1785 (C=O, ꢀ-lactam), 1670, 1640
�
1
(
CONH and COOH) cm .
Acknowledgements
P. Misra and K. R. Bhushan are thankful to the CSIR, New Delhi, for ®nancial support.
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[
[
[
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5
[
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Received February 23, 2000. Accepted March 28, 2000