Synthesis and biological evaluation of novel benzimidazol/ benzoxazolylethoxypiperidone oximes

Article abstract of DOI:10.1248/bpb.29.125

Some novel benzimidazol/benzoxazolylethoxypiperidone oximes were synthesized and their antibacterial activity against Staphylococcus aureus (NCIM-2492), Bacillus subtilis (NCIM-2439), Escherichia coli (NCIM-2345) and Pseudomonas aeruginosa (NCIM-2035) and

Full text of DOI:10.1248/bpb.29.125

January 2006
Biol. Pharm. Bull. 29(1) 125—130 (2006)
125
Synthesis and Biological Evaluation of Novel
Benzimidazol/Benzoxazolylethoxypiperidone Oximes
Srinivasan BALASUBRAMANIAN,1) Gopalakrishnan ARIDOSS, Paramasivam PARTHIBAN,
Chennan RAMALINGAN, and Senthamaraikannan KABILAN*
Department of Chemistry, Annamalai University; Annamalainagar–608 002, Tamil Nadu, India.
Received June 9, 2005; accepted October 20, 2005
Some novel benzimidazol/benzoxazolylethoxypiperidone oximes were synthesized and their antibacterial ac-
tivity against Staphylococcus aureus (NCIM-2492), Bacillus subtilis (NCIM-2439), Escherichia coli (NCIM-2345)
and Pseudomonas aeruginosa (NCIM-2035) and antifungal activity against Candida albicans (NCIM-C27), Can-
dida-6 (NCIM-C27), Candida-51 (NCIM-C27), Aspergillus niger (NCIM-590) and Aspergillus flavus (NCIM-539)
have been evaluated. Compounds 26 and 27 exerted potent in vitro antibacterial activity against Bacillus subtilis,
Escherichia coli and Staphylococcus aureus while compounds 26, 29 and 30 exhibited potent in vitro antifungal
activity against Candida albicans, Candida-51, and Aspergillus niger.
Key words piperidin-4-one; cyanoethylation; benzazole; oxime; antibacterial activity; antifungal activity
Piperidines are an important group of heterocyclic com- cophoric groups in the synthesized compounds towards their
pounds in the field of medicinal chemistry owing to the biological activities was evaluated.
fact that these can frequently be recognized in the structure
In the course of broad programme in developing biologi-
of numerous naturally occurring alkaloid and synthetic cally active molecules, we have recently reported the synthe-
compounds with interesting biological and pharmacolog- sis of 2,6-diarylpiperidin-4-one derivatives and evaluated
ical properties. Piperidones were also reported to possess their biological importance and also a reagent for effecting
analgesic,2,3) anti-inflammatory,3) central nervous system functional group interconversion.33—42)
(CNS),4—8) local anaesthetic,4,9) anticancer10) and antimicro-
bial activity.11) The earlier reports indicated that the signifi- MATERIALS AND METHODS
cant biological activities of piperidones were associated with
aromatic substituents at 2nd and/or 6th positions.6,11) Oximes
The course of reaction and purity were ascertained by per-
of the 2,6-disubstituted piperidones were also reported to ex- forming TLC. Melting points were determined in open capil-
hibit antimicrobial,12) analgesic,13) local anaesthetic13) and an- laries and are uncorrected. IR spectra were recorded in
tifungal activity.13)
Perkin-Elmer 297 spectrophotometer with KBr and only
The benzimidazole nucleus is widely accepted for its an- noteworthy absorption levels (reciprocal centimeter) are
tiallergic and antiasthmatic activity14,15) and its derivatives listed. 1H-NMR spectra were recorded at 400 MHz on Bruker
were found to possess diverse biological activities such amx 400 MHz spectrophotometer in CDCl3 using tetramethyl
as antiamoebic, microfilaricidal, antifungal and antiaryl- silane (TMS) as internal standard and 13C-NMR spectra were
themic.16—22) It is well known that in the treatment of para- recorded at 100 MHz on Bruker amx 400 MHz spectropho-
sitic infection, substituted benzimidazoles, mebendazole and tometer in CDCl3. Mass spectra were recorded on a VG ana-
febendazole were found to be efficient.23,24) Benzoxazole nu- lytical 7070E instrument equipped with VG 11-250 data ac-
cleus is also marked for its biological activity. 2-Substituted quisition system. Elemental analysis (C, H, and N) were car-
benzoxazoles have been shown to exert analgesic,25) fungi- ried out on a Carlo Erba Model 1106 and Perkin Elmer mod-
cidal, insecticidal, nematocidal,26) potent protease inhib- els 240 CHN analyzer. The results are within ꢀ0.4% of the
itory,27) anticancer activities28) and serve as topoisomerase I theoretical values.
poisons.29)
Unless, otherwise stated all the starting materials and
An essential part of the search for new leads in a drug de- reagents were of high-grade purchased from Aldrich, Fluka
signing programme is the synthesis of new molecules, which and Merck. All the solvents were distilled prior to use.
are novel yet resemble known biologically active molecules
The general synthetic scheme of the novel compounds
by virtue of the presence of some critical structural features. 25—30 is furnished in Fig. 1.
There are few small heterocyclic molecules, which act as
Chemistry Cyclic ketones normally undergo Baeyer-
highly functionalized scaffolds and are known pharma- Villiger oxidation (oxygen insertion reaction) to yield
cophores of a number of biologically active and medicinally lactones upon treatment with peracids.43—45) When 2,6-di-
useful molecules.30—32)
arylpiperidin-4-ones were subjected to Baeyer-Villiger type
Aiming at extending our knowledge in structure–activity of reaction by using meta chloroperbenzoic acid (m-CPBA),
relationship, we considered that it was valuable to synthesis a 1-hydroxy-2,6-diarylpiperidin-4-ones resulted instead of lac-
system which unites biolabile piperidone oximes and benz- tones. On treatment with acrylonitrile, substituted tetrahy-
imidazole and benzoxazole units respectively together to give drothiopyran-4-ones containing active hydrogen underwent
a compact structure of benzimidazolylethoxypiperidone cyanoethylation yielding 3-[2-cyanoethoxy] derivatives.46)
oximes and benzoxazolylethoxypiperidone oximes. The in- In 1-hydroxy-2,6-diarylpiperidin-4-ones, there are active
fluence of some structural variations at different pharma- methylenic hydrogens at C3 and C5 positions. Hence, expec-
∗ To whom correspondence should be addressed. e-mail: skabilan@rediffmail.com
© 2006 Pharmaceutical Society of Japan

126
Vol. 29, No. 1
matography with 4 : 1 benzene–pet.ether (bp 40—60 °C)
mixture yielded the product 7. The compounds 8—12 were
prepared similarly.
1-(2-Cyanoethoxy)-2,6-diphenylpiperidin-4-one (13)
A mixture of 1-hydroxy-2,6-diphenylpiperidin-4-one
7
(0.005 mol) and acrylonitrile (0.005 mol) in 50 ml of 1,4-
dioxane was taken in a 100 ml round bottom flask and cooled
in an ice-bath. A few crystals of resorcinol were added fol-
lowed by drop wise addition of Triton B with shaking. Then,
the contents were stirred for 9 h at 65—75 °C and concen-
trated. After cooling, the resulting solution was poured over
1 : 3 benzene–pet.ether mixture. The solid obtained was re-
crystallized from methanol to afford the product 13. The
compounds 14—18 were prepared similarly.
1-[2-(Benzimidazol-2-yl)ethoxy]-2,6-diphenylpiperidin-
4-one (19) To a mixture of 1-(2-cyanoethoxy)-2,6-di-
phenylpiperidin-4-one 13 (0.005 mol) and o-phenylenedi-
amine (0.005 mol), dilute hydrochloric acid (10%) was added
with constant shaking. The content of the flask was refluxed
on an oil-bath for 12 h. After the addition of 50 ml of water
to the reaction mass, it was filtered to remove impurities. To
isolate product as a base, the acid solution was treated with
aqueous ammonia (15 ml) and then poured into water. The
precipitated compound 19 was recrystallized twice from
ethanol. The compounds 20 and 21 were prepared similarly.
1-[2-(Benzoxazol-2-yl)ethoxy]-2,6-diphenylpiperidin-4-
one (22) A mixture of 1-(2-cyanoethoxy)-2,6-diphenyl-
piperidin-4-one 16 (0.005 mol), o-aminophenol (0.005 mol)
and dilute hydrochloric acid (10%) was taken in a 250 ml
round bottom flask and was allowed to reflux on an oil-bath
Fig. 1. Synthetic Scheme
tation of cyanoethylation to occur at these positions besides for 18 h. After cooling, the content in the flask was leached
at 1-hydroxyl group is quite normal. However in all the cases with 100 ml of diethyl ether and the separated ether extract
specifically the 1-hydroxy group alone underwent cya- was washed with five 25 ml portions of sodium hydroxide
noethylation34,36—38) to afford 1-[2-cyanoethoxy]-2,6-diaryl- (4 N) solution. The ether layer was separated, dried over cal-
piperidin-4-ones in good yields (60—74%) upon treatment cium chloride and freed of ether by distillation. The solid ob-
with acrylonitrile in the presence of catalyst Triton B.
tained was recrystallized twice from ethanol to yield the
Usually cyanoethylation47,48) is a base catalyzed reaction product 22. The compounds 23 and 24 were prepared simi-
and invariably requires an alkaline catalyst. But certain larly.
amines are quite exceptional. Oxides, hydroxides, alkoxides,
1-[2-(Benzimidazol-2-yl)ethoxy]-2,6-diphenylpiperidin-
alkali metal hydrides etc. are useful for this purpose. Solubil- 4-one Oxime (25) 1-[2-(Benzimidazol-2-yl)ethoxy]-2,6-
ity of the bases in organic solvents should be taken into ac- diphenylpiperidin-4-one 19 (0.05 mol), sodium acetate trihy-
count. Mono or multiple cyanoethylation depends upon the drate (0.15 mol) and hydroxylamine hydrochloride (0.06 mol)
proper choice of a catalyst with sufficient basicity to remove were refluxed for 30 min in ethanol medium. After refluxing,
the labile proton from the compound undergoing cyanoethy- the mixture was poured into ice water. The solid obtained
lation. Triton B is particularly employed here on account of was filtered and recrystallized from ethanol to afford 1-[2-
its basicity and its solubility in organic media. Cyanoethyla- (benzimidazol-2-yl)ethoxy]-2,6-diphenylpiperidin-4-one
tion requires cooling to avoid polymerization of acrylonitrile. oxime 25.
Inert solvents like benzene, dioxane, acetonitrile or pyridine
can be used to dissolve solid reactants or to moderate the re- Hz); 3.80—3.86 (3H, H6a, OCH2CH2, m); 3.47 (1H, H5e, d,
action. Jꢁ8.00 Hz); 2.52—2.73 (4H, H3a, H3e, OCH2CH2, m); 2.29
1H-NMR (CDCl3) d: 3.91 (1H, H2a, dd, 3Jꢁ11.45 Hz; 2.91
2,6-Diarylpiperidin-4-ones (1—6) were prepared by the (1H, H5a, d, Jꢁ8.00 Hz); 7.28—7.49 (14H, aryl protons, m);
condensation of appropriate ketones, aldehydes and ammo- 8.01 (1H, NOH, s). 13C-NMR (CDCl3) d: 70.173 (C2);
nium acetate in 1 : 2 : 1 ratio.49)
68.873 (C6); 41.288 (C3); 34.287 (C5); 157.108 (C4); 158.146
1-Hydroxy-2,6-diphenylpiperidin-4-one (7) A solution (Cb); 19.868 (OCH2CH2); 69.310 (OCH2CH2); 110.198,
of 2,6-diphenylpiperidin-4-one 1 (0.005 mol) and m-CPBA 115.008, 115.138, 120.976, 121.056, 127.742, 129.710,
(0.005 mol) in 40 ml of dry chloroform was stirred for 15 min 129.828, 130.592, 130.698, 137.268, 140.985, 141.827 (aryl
at 0 °C and kept aside for overnight at 20 °C. Then the mix- carbons). IR (KBr) (cmꢂ1): 3728 (N–OH), 3254 (N–H),
ture was extracted with chloroform and washed well with 2940, 2930, 2854, 2762 (C–H), 1664 (CꢁN), 1592, 1569,
10% sodium bicarbonate solution. The chloroform layer was 1490, 1422, 1400, 1370, 1345, 1328, 1308, 1291, 1240,
dried over anhydrous sodium sulphate and distilled off under 1156, 1124, 1070, 1030, 948, 901, 852, 816, 782, 750, 693,
reduced pressure. Purifications with silicagel column chro- 660, 605, 516, 497, 466. Mass m/z: 426 (Mꢃ), 252, 239, 222,

January 2006
127
208, 194, 144, 131, 117, 103 (100%), 91, 77, 65, 51. Melting
1-[2-(Benzoxazol-2-yl)ethoxy]-2,6-bis(p-chlorophenyl)-
point: 159 °C. Yield (%): 64. Anal. Calcd for C26H26N4O2: C, piperidin-4-one Oxime (29) 1H-NMR (CDCl3) d: 3.94
3
73.22; H, 6.15; N, 13.14. Found: C, 72.96; H, 6.16; N, 13.11. (1H, H2a, dd, Jꢁ11.45 Hz; 2.91 Hz); 3.84—3.89 (3H, H6a,
The compounds 26—30 were synthesized similarly.
OCH2CH2, m); 3.46 (1H, H5e, d, Jꢁ8.06 Hz); 2.52—2.75
1-[2-(Benzimidazol-2-yl)ethoxy]-2,6-bis( p-chloro- (4H, H3a, H3e, OCH2CH2, m); 2.29 (1H, H5a, d, Jꢁ8.26 Hz);
phenyl)piperidin-4-one Oxime (26) 1H-NMR (CDCl3) d: 7.13—7.43 (12H, aryl protons, m); 8.03 (1H, NOH, s).
3
3.94 (1H, H2a, dd, Jꢁ11.46 Hz; 2.91 Hz,); 3.81—3.88 (3H, 13C-NMR (CDCl3) d: 69.770 (C2); 68.272 (C6); 41.186
H6a, OCH2CH2, m); 3.46 (1H, H5e, d, Jꢁ8.00 Hz); 2.51— (C3); 34.085 (C5); 155.862 (C4); 163.613 (Cb); 27.644
2.74 (4H, H3a, H3e, OCH2CH2, m); 2.28 (1H, H5a, d, Jꢁ8.15 (OCH2CH2); 67.575 (OCH2CH2); 109.768, 114.204,
Hz); 7.18—7.50 (12H, aryl protons, m); 8.02 (1H, NOH, s). 118.976, 119.156, 129.361, 129.373, 129.883, 130.004,
13C-NMR (CDCl3) d: 69.783 (C2); 68.285 (C6); 41.163 133.120, 134.753, 134.922, 139.094, 139.953, 143.354 (aryl
(C3); 34.062 (C5); 155.909 (C4); 158.201 (Cb); 19.871 carbons). IR (KBr) (cmꢂ1): 3724 (N–OH), 2926, 2840, 2794,
(OCH2CH2); 69.321 (OCH2CH2); 110.212, 115.010, (C–H), 1664 (CꢁN), 1627, 1490, 1407, 1370, 1321, 1279,
115.142, 120.999, 121.089, 129.375, 129.380, 129.862, 1264, 1235, 1227, 1128, 1115, 1042, 1003, 952, 893, 820,
129.998, 134.741, 134.901, 137.269, 139.102, 139.970 (aryl 736, 690, 514, 488, 476. Mass (m/z): 495 (Mꢃ), 320, 307,
carbons). IR (KBr) (cmꢂ1): 3725 (N–OH), 3222 (N–H), 290, 276, 262, 182, 169, 146, 137, 132, 118, 111, 91, 75, 65,
2946, 2925, 2850, 2791 (C–H), 1666 (CꢁN), 1623, 1574, 53 (100%), 50. Melting point: 112—113 °C. Yield (%): 38.
1488, 1416, 1374, 1322, 1320, 1315, 1282, 1160, 1126, Anal. Calcd for C26H23N3O3Cl2: C, 62.91; H, 4.67; N, 8.47.
1062, 1007, 947, 895, 841, 824, 765, 743, 688, 671, 517, Found: C, 62.63; H, 4.65; N, 8.46.
492, 471. Mass (m/z): 494 (Mꢃ), 320, 307, 290, 276, 262,
1-[2-(Benzoxazol-2-yl)ethoxy]-2,6-bis( p-methoxy-
182, 169, 155, 145, 137, 117, 111, 91, 75, 65, 53 (100%), 50. phenyl)piperidin-4-one Oxime (30) 1H-NMR (CDCl3) d:
3
Melting point: 128 °C. Yield (%): 50. Anal. Calcd for 3.93 (1H, H2a, dd, Jꢁ11.49 Hz; 2.92 Hz); 3.81—3.87 (9H,
C26H24N4O2Cl2: C, 63.04; H, 4.88; N, 11.31; Found: C, H6a, aryl OCH3, OCH2CH2, m); 3.46 (1H, H5e, d, Jꢁ8.02
62.94; H, 4.9; N, 11.33.
Hz); 2.50—2.74 (4H, H3a, H3e, OCH2CH2, m); 2.27 (1H, H5a,
1-[2-(Benzimidazol-2-yl)ethoxy]-2,6-bis(p-methoxy- d, Jꢁ8.31 Hz); 6.90, 7.27—7.50 (4H, 8H, aryl protons); 8.01
phenyl)piperidin-4-one Oxime (27) 1H-NMR (CDCl3) d: (1H, NOH, s). 13C-NMR (CDCl3) d: 69.812 (C2); 68.411
3
3.93 (1H, H2a, dd, Jꢁ11.45 Hz; 2.99 Hz); 3.79—3.87 (9H, (C6); 41.501 (C3); 34.402 (C5); 156.664 (C4); 163.610 (Cb);
H6a, aryl OCH3, OCH2CH2, m); 3.44 (1H, H5e, d, Jꢁ8.00 27.676 (OCH2CH2); 67.612 (OCH2CH2); 109.743, 114.102,
Hz); 2.49—2.73 (4H, H3a, H3e, OCH2CH2, m); 2.26 (1H, H5a, 114.645, 114.879, 118.768, 118.936, 132.999, 133.126,
d, Jꢁ8.01 Hz); 6.87, 7.26—7.51 (4H, 8H, aryl protons); 8.00 135.132, 135.847, 143.380, 158.569, 158.940 (aryl carbons);
(1H, NOH, s). 13C-NMR (CDCl3) d: 69.828 (C2); 68.427 55.187 (aryl OCH3). IR (KBr) (cmꢂ1): 3717 (N–OH), 2925,
(C6); 41.485 (C3); 34.386 (C5); 156.712 (C4); 158.341 (Cb); 2851, 2784 (C–H), 1662 (CꢁN), 1639, 1464, 1376, 1365,
19.844 (OCH2CH2); 69.402 (OCH2CH2); 110.201, 114.894, 1528, 1234, 1191, 1134, 1042, 1023, 924, 857, 800, 742,
114.972, 114.980, 120.990, 120.996, 132.991, 133.110, 651, 600, 523, 512. Mass (m/z): 487 (Mꢃ), 312, 299, 282,
135.127, 135.833, 137.348, 158.562, 158.956 (aryl carbons); 268, 254, 178, 163, 147, 145, 132, 118, 107, 92, 75, 65, 53
55.182 (aryl OCH3). IR (KBr) (cmꢂ1): 3718 (N–OH), 3233 (100%), 50. Melting point: 126 °C. Yield (%): 56. Anal.
(N–H), 2940, 2928, 2831, 2786 (C–H), 1664 (CꢁN), 1635, Calcd for C28H29N3O5: C, 68.98; H, 6.0; N, 8.62. Found: C,
1570, 1462, 1438, 1380, 1361, 1314, 1250, 1193, 1136, 69.12; H, 5.88; N, 8.6.
1028, 920, 851, 796, 789, 740, 650, 524, 507, 441. Mass
To comprehend structure–activity relationship well, num-
(m/z): 486 (Mꢃ), 312, 299, 282, 268, 254, 178, 163, 151, berings of the target compound is shown in Fig. 2.
144, 133, 117, 107, 91, 75, 65, 53 (100%), 50. Melting point:
145 °C. Yield (%): 56. Anal. Calcd for C28H30N4O4: C, 69.12; PHARMACOLOGY
H, 6.22; N, 11.52. Found: C, 68.85; H, 6.19; N, 11.54.
1-[2-(Benzoxazol-2-yl)ethoxy]-2,6-diphenylpiperidin-4-
The bacterial strains Staphylococcus aureus (NCIM-
one Oxime (28) 1H-NMR (CDCl3) d: 3.91 (1H, H2a, dd, 2492), Bacillus subtilis (NCIM-2439), Escherichia coli
3Jꢁ11.48 Hz; 2.99 Hz); 3.84—3.87 (3H, H6a, OCH2CH2, m); (NCIM-2345), Pseudomonas aeruginosa (NCIM-2035), and
3.46 (1H, H5e, d, Jꢁ8.02 Hz); 2.50—2.74 (4H, H3a, H3e, antifungal strains Candida albicans (NCIM-C27), Candida-6
OCH2CH2, m); 2.28 (1H, H5a, d, Jꢁ8.53 Hz); 7.28—7.48 (NCIM-C27), Candida-51 (NCIM-C27), Aspergillus niger
(14H, aryl protons, m); 8.02 (1H, NOH, s). 13C-NMR
(CDCl3) d: 70.162 (C2); 68.863 (C6); 41.312 (C3); 34.317
(C5); 157.051 (C4); 163.604 (Cb); 27.648 (OCH2CH2);
67.572 (OCH2CH2); 109.758, 114.194, 118.938, 119.101,
127.754, 129.712, 129.820, 130.603, 130.714, 133.112,
140.979, 141.820, 143.134 (aryl carbons). IR (KBr) (cmꢂ1):
3720 (N–OH), 2922, 2850, 2765 (C–H), 1658 (CꢁN), 1600,
1455, 1428, 1390, 1372, 1346, 1286, 1254, 1236, 1154,
1103, 1077, 1033, 941, 902, 853, 785, 746, 687, 654, 602,
516, 500, 470. Mass (m/z): 427 (Mꢃ), 252, 239, 222, 208,
194, 145, 132, 118, 103 (100%), 91, 77, 65, 55, 51. Melting
point: 161 °C. Yield (%): 57. Anal. Calcd for C26H25N3O3: C,
Fig. 2. Structure of Target Compound
73.05; H, 5.9; N, 9.83. Found: C, 75.20; H, 5.88; N, 9.85.

128
Vol. 29, No. 1
Table 1. In Vitro Antibacterial Activity of Compounds 25—30
Minimum inhibitory concentration (MIC) in mg/ml
Staphylococcus Bacillus Escherichia Pseudomonas
(NCIM-590), Aspergillus flavus (NCIM-539) are procured
from National Chemical Laboratory, Pune, India.
In Vitro Antibacterial and Antifungal Activity The in
vitro activities of the compounds were tested in Sabourauds
dextrose broth (SDB) (Hi-media, Mumbai) for fungi and Nu-
trient broth (NB) (Hi-media, Mumbai) for bacteria by the
two-fold serial dilution method.50) The test compounds were
dissolved in DMSO (dimethylsulphoxide) to obtain 1 mg/ml
stock solutions. Seeded broth (broth containing microbial
spores) was prepared in NB from 24 h old bacterial cultures
on nutrient agar (Hi-media, Mumbai) at 37ꢀ1 °C while fun-
gal spores from 24 h—7 d old Sabourauds agar (Hi-media,
Mumbai) slant cultures were suspended in SDB. The colony
forming units (cfu) of the seeded broth were determined by
Compound
aureus
subtilis
coli
aeruginosa
25
26
27
28
29
50
25
12.5
200
50
100
12.5
50
50
12.5
25
200
25
100
25
12.5
100
12.5
50
100
50
25
50
12.5
200
25
25
50
25
50
50
25
30
Penicillin
Streptomycin
plating technique and adjusted in the range of 102—105 activity remarkably.
cfu/ml. The final inoculum size was 105 cfu/ml for antibacter-
In the case of benzimidazole derivative having para chloro
ial assay and 1.1—1.5ꢄ102 cfu/ml for antifungal assay. Test- substituent (compound 26) showed 2—4 fold increase in
ing was performed at pH 7.4ꢀ0.2. 0.2 ml of the solution of activity against Bacillus subtilis, Escherichia coli and
test compound was added to 1.8 ml of seeded broth to form Pseudomonas aeruginosa while only 50% improvement was
the first dilution. One milliliter of this was diluted with a fur- observed against Staphylococcus aureus. Moreover, in benz-
ther 1 ml of the seeded broth to give the second dilution and oxazole derivative (compound 29) 2—3 fold enhancement
so on till six such dilution were obtained. A set of assay was observed against Staphylococcus aureus, Bacillus sub-
tubes containing only inoculated broth was kept as control tilis whereas against Escherichia coli and Pseudomonas
and likewise solvent controls were also run simultaneously. aeruginosa one fold increase in activity was noted. In gen-
The tubes were incubated in BOD incubators at 37ꢀ1 °C for eral, the para substituted compounds 26 and 29 expressed
bacteria and 28ꢀ1 °C for fungi. The minimum inhibitory higher activity against the tested organisms when compared
concentrations (MICs) were recorded by visual observations to unsubstituted compounds 25 and 28.
after 24 h (for bacteria) and 72—96 h (for fungi) of incuba-
Replacement of chloro functionalities present at the para
tion. Penicillin, streptomycin and amphotericin B were used position of phenyl groups by methoxy groups in 26 and 29
as standards.
(compounds 27, 30) suppressed the activity against all the
tested organisms except Staphylococcus aureus and Es-
cherichia coli against which activity was improved due to
this replacement in compounds 26 and 29 respectively.
But, when compared to compound having keto functional-
RESULTS AND DISCUSSION
Structure–Activity Relationship Results. Antibacter-
ial Activity All the synthesized novel benzimidazol/benz- ity at C4, the antibacterial activity of the oximated compound
oxazolylethoxypiperidone oximes were evaluated in vitro for 27 was increased by 50% towards the tested bacterial strains
their activity to inhibit the growth of bacterial strains viz. and lies in the range of 12.5—50 mg/ml. Similarly compound
Staphylococcus aureus, Bacillus subtilis, Escherichia coli, 30 exerted the same trend in activity except against Es-
Pseudomonas aeruginosa. Penicillin and streptomycin were cherichia coli and Pseudomonas aeruginosa for which the
used as standard drugs whose minimum inhibitory concen- activity did not change appreciably.
tration (MIC) values are given in Table 1. To get clear com-
Antifungal Activity The in vitro antifungal activity of
prehension over antibacterial activity of the synthesized com- the novel compounds 25—30 was studied against the fungal
pounds, these were compared with that of their proceeding strains viz. Candida albicans, Candida-6, Candida-51, As-
compounds having keto functionality as reported by Rama- pergillus niger and Aspergillus flavus. Amphotericin B was
lingan et al.35,38)
used as standard drug whose minimum inhibitory concentra-
Unlike the compounds having keto functional group, the tion (MIC) values are furnished in Table 2.
present series of compounds 25—30, with oxime substitution
To mark the superiority of the synthesized compounds
in place of keto group (compounds 25—30) seems to play a 25—30 with oxime functionality towards antifungal activity,
vital role in the antibacterial activity against all the tested or- the results are compared with the results35,38) of compounds
ganisms. The compounds (with keto functionality) which having keto functional group instead of oxime at C4 of the
failed to show activity, exhibited moderate activity against heterocyclic ring moiety.
Escherichia coli, Pseudomonas aeruginosa, Staphylococcus
The compound 25 yielded by the oximation of the corre-
aureus and Bacillus subtilis at a MIC ranging from 100— sponding compound possessing keto group (which failed to
200 mg/ml due to oximation (compounds 25, 28). Compound exert in vitro antifungal activity even at a maximum concen-
25 significantly inhibits Staphylococcus aureus and Bacillus tration of 200 mg/ml towards Aspergillus niger and As-
subtilis bacterial strains at a MIC of 50 mg/ml. Likewise, pergillus flavus) exhibited well pronounced activity against
compound 28 also exhibited a higher activity against Candida-51, Aspergillus niger and Aspergillus flavus in the
Pseudomonas aeruginosa and recorded the same MIC.
range of 50—100 mg/ml whereas against Candida-6, the ac-
Substitution of chloro functionalities at the para position tivity was not improved appreciably. Likewise the antifungal
of phenyl rings present at C2 and C6 of the heterocyclic moi- activity of compound 28 was also significantly enhanced
ety in 25 and 28 (compounds 26, 29) enhanced antibacterial against Candida-6, Candida-51 and Aspergillus flavus while

January 2006
129
Table 2. In Vitro Antifungal Activity of Compounds 25—30
tivity for the respective organisms are significantly higher for
oximes (compounds 25—30) than the corresponding ke-
tones. Oximation of benzimidazol/benzoxazolylethoxypiperi-
dones changes requisite characteristics needed for exhibiting
better structure–activity correlate. Since, the oxime function-
ality carries a labile proton, which may in turn a key for suit-
ably tailoring the properties of the compounds in terms of
solubility, penetration and binding.
Minimum inhibitory concentration (MIC) in mg/ml
Compound
Candida
albicans
Aspergillus Aspergillus
Candida-6 Candida-51
niger
flavus
25
26
27
200
12.5
25
100
25
50
100
50
50
25
12.5
25
25
50
25
The pharmacological screening studies put evidence that
benzimidazolylethoxy piperidone oximes are proved to be
superior in exerting a marked antibacterial and antifungal
profile than the corresponding benzoxazolylethoxypiperidone
oximes. Furthermore, both the set of compounds with chloro
functions at the para position of the aryl moieties at C2 and
28
29
30
100
50
100
50
50
50
100
50
25
12.5
50
25
12.5
25
50
50
25
Amphotericin B
25
25
50
against Candida albicans and Aspergillus flavus, oximation C6 positions exhibited an elevated antibacterial and antifun-
has no effect and the antifungal potency of the parent com- gal activity than the corresponding methoxy substituted and
pounds having keto function was retained as such.
unsubstituted compounds. Indeed, these chloro substituted
Due to the introduction of chloro functionalities at the compounds showed a promising biological potency when
para position of the aryl groups at C2 and C6 of 25 and 28 compared to the standard drug.
(compounds 26, 29) instead of hydrogens, the MIC for the
tested organisms decreased by about 50% and lies in the able activity against Escherichia coli and Candida-51.
range of 12.5—50 mg/ml whereas for unsubstituted com- Thus, in future, this class of novel benzimidazol/benzoxa-
Moreover, all the tested compounds expressed a remark-
pounds, MIC ranges from 50 to 200 mg/ml. In fact, 2—8 fold zolylethoxypiperidone oximes may be used as templates to
increase in antifungal efficacy pertaining to compound 26 generate better drugs to combat bacterial and fungal infec-
was observed particularly against Candida-51, Candida-6 tions. Although much facinating biological activity of benza-
and Candida albicans compared to 25 whereas in the case of zole nucleus is already known, further explorations are sure
compound 29, only one fold increase in activity was ob- to uncover even more interesting and useful activities. It is
served against all the tested strains with respect to compound quite remarkable that benzazole nucleus is finding general
28.
acceptance as an essential pharmacophoric group. It is also
From the observation it is obvious that oximation has sig- anticipated that benzazole core will continue to find increas-
nificant role in enhancing the antifungal activity of the com- ing place in future drugs.
pounds having keto functional group.
Furthermore, introduction of methoxy functionalities in
Acknowledgements Authors are thankful to Prof. K.
place of hydrogen present at the para position of phenyl Pandiarajan, Head, Department of Chemistry for the facili-
groups at C2 and C6 of compounds 25 and 28 (compounds ties provided. One of the authors, S.K. is grateful to Univer-
27, 30) have registered a marked improvement in antifungal sity Grants Commission, New Delhi for financial support in
activity against all the tested organisms except to compound the form of Major Research Project. S.B. wishes to thank
30 which exerted the same activity of the compound having Council of Scientific and Industrial Research, New Delhi for
unsubstitution at aryl groups against Aspergillus niger.
the award of Senior Research Fellowship. The authors place
In fact, unlike chlorine substituted compound (compound sincere thanks to Prof. M. Vasudevan, JSS College of Phar-
29), the methoxy substituted compound (compound 30) ex- macy, Ooty, India for his kind help in conducting screening
erted one fold increase in activity against Candida-6 and As- studies.
pergillus flavus while against rest of the tested organisms, no
change in activity was noted compared to the corresponding
ketones.
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