Antibacterial Activities of 4-Substituted-2-[(E)-{(1S,2R)/(1R,2S)-1-Hydroxy-1-Phenylpropan-2-Ylimino}Methyl]Phenol

Article abstract of DOI:10.1111/j.1747-0285.2011.01263.x

A series of norephedrine-based Schiff bases (1a-6a and 1b-6b) were synthesized by reacting substituted salicylaldehydes with d-norephedrine or l-norephedrine. The structure of these compounds was confirmed by elemental analyses and spectroscopic techniques. The molecular structures of 5a and 6a have been determined by X-ray crystallography, which revealed that the compounds are in the oxoamino form, with bent intramolecular N-H···O (N···O≈2.58A) hydrogen bonds and that they are associated in dimers bridged by linear intermolecular O-H···O (O···O≈2.69A) hydrogen bonds. The density functional theory calculations on 5a confirmed that the oxoamino form is more stable than the phenolimino form by 12.2kcal/mol. All the compounds were evaluated for their antibacterial activity using resazurin dye as indicator by twofold dilution method against four bacteria namely, Bacillus subtilis (NCIM2718), Staphylococcus aureus (NCIM5021), Escherichia coli (NCIM2931), and Proteus vulgaris (NCIM2813).

Full text of DOI:10.1111/j.1747-0285.2011.01263.x

ª 2011 John Wiley & Sons A/S
doi: 10.1111/j.1747-0285.2011.01263.x
Chem Biol Drug Des 2012; 79: 177–185
Research Article
Antibacterial Activities of 4-Substituted-2-[(E )-
{(1S,2R)/ (1R,2S)-1-Hydroxy-1-Phenylpropan-2-
Ylimino}Methyl]Phenol
Manoharan Muthu Tamizh1, Devarayan
test has been reported (1). Schiff and Mannich bases of isatin
derivatives have been synthesized and their antibacterial (against
26 of pathogenic bacteria), antifungal (against eight pathogenic
fungi), and anti-HIV activity (replication of HIV-1 (IIIB) in MT-4 cells)
have been reported (2). Schiff bases also exhibit antiinflammatory,
antiproliferative, and antipyretic activities (3–5). Various references
Kesavan1, Ponnurengam Malliappan
Sivakumar2, Kurt Mereiter3, Mohan
Deepa1, Karl Kirchner4, Mukesh Doble2 and
Ramasamy Karvembu1,*
1Department of Chemistry, National Institute of Technology,
explain the importance of imine group in bioactive molecules (6–
14). In the present study, Schiff bases have been synthesized from
norephedrine and substituted salicylaldehydes. Earlier l-norephedrine
was used in the synthesis of oxazaborolidines, which exhibited anti-
bacterial activity against Streptococcus mutans, an organism associ-
ated with the tooth decay (8). Virtual screening for quorum sensing
inhibitors which included ephedrine to eradicate, the biofilm formed
by Pseudomonas aeruginosa has been performed (15). The 5-chloro
salicylaldehyde-based Schiff bases were also evaluated for their
antibacterial activity against four bacteria and three of the fungi
(16). We herein report the synthesis of 4-substituted-2-[(E)-
{[(1S,2R) ⁄ (1R,2S)-1-hydroxy-1-phenylpropan-2-ylimino}methyl]phenols
(Schiff bases) (Table 1) from d-norephedrine ⁄ l-norephedrine and
substituted salicylaldehyde, and their antibacterial activity against
two Gram-positive and two Gram-negative bacteria.
Tiruchirappalli 620 015, India
2Department of Biotechnology, Indian Institute of Technology,
Chennai 600 020, India
3Institute of Chemical Technologies and Analytics, Vienna University
of Technology, A-1060 Vienna, Austria
4Institute of Applied Synthetic Chemistry, Vienna University of
Technology, A-1060 Vienna, Austria
*Corresponding author: R. Karvembu, kar@nitt.edu
A
series of norephedrine-based Schiff bases
(1a–6a and 1b–6b) were synthesized by reacting
substituted salicylaldehydes with d-norephedrine
or l-norephedrine. The structure of these com-
pounds was confirmed by elemental analyses and
spectroscopic techniques. The molecular struc-
tures of 5a and 6a have been determined by X-ray
crystallography, which revealed that the com-
pounds are in the oxoamino form, with bent intra-
Materials and Methods
˚
molecular N-HÆÆÆO (NÆÆÆO ꢀ 2.58 A) hydrogen bonds
and that they are associated in dimers bridged by
All the chemicals used were of analytical grade. Solvents were
purified and dried according to standard procedure (17). Salicylalde-
hyde was purchased from Loba Chemie and was used after double
distillation. The 2-hydroxy-5-methylbenzaldehyde, 2-hydroxy-5-
methoxybenzaldehyde, 5-chlorosalicylaldehyde, 2-hydroxy-5-nitro-
benzaldehyde, 5-bromo-2-hydroxybenzaldehyde, d-norephedrineÆHCl,
and l-norephedrineÆHCl were purchased from Sigma-Aldrich and
were used without further purification. Microbial strains Bacillus
subtilis (NCIM2718), Staphylococcus aureus (NCIM5021), Escherichia
coli (NCIM2931), and Proteus vulgaris (NCIM2813) were obtained
from National Chemical Laboratory, Pune, India. Media components,
including broth and agar, and control drugs (ampicillin and tetracy-
cline) were purchased from Himedia, Mumbai, India.
˚
linear intermolecular O-HÆÆÆO (OÆÆÆO ꢀ 2.69 A) hydro-
gen bonds. The density functional theory calcula-
tions on 5a confirmed that the oxoamino form is
more stable than the phenolimino form by
12.2 kcal/mol. All the compounds were evaluated
for their antibacterial activity using resazurin dye
as indicator by twofold dilution method against
four bacteria namely, Bacillus subtilis (NCIM2718),
Staphylococcus aureus (NCIM5021), Escherichia
coli (NCIM2931), and Proteus vulgaris (NCIM2813).
Key words: antibacterial studies, norephedrine, Schiff base, spec-
tral studies
Received 14 March 2011, revised 31 August 2011 and accepted for
publication 16 October 2011
Physical measurements
The FT-IR spectra were recorded on a PerkinElmer Spectrum One FT–
IR spectrometer as KBr pellets in the frequency range of 400–4000
per cm. The C, H, and N contents were determined by Thermoflash
EA1112 series elemental analyzer. Specific rotation was recorded
with a Rudolph Autopol IV polarimeter (589 nm and CHCl3). The
Schiff bases are known therapeutical agents. They are intermedi-
ates in various therapeutically active chemical entities. The antitu-
bercular activity of D-mannitol-based Schiff bases against
Mycobacterium tuberculosis H37Rv using alamar blue susceptibility
177

Muthu Tamizh et al.
Table 1: Crystal Data, data collection, and structure refinement
parameters for 5a and 6a
APEX-II CCD diffractometer (Bruker AXS GmbH, Karlsruhe, Germany)
using graphite-monochromated Mo Ka radiation (k = 0.71073 ꢁ).
After data integration with program SAINT, absorption corrections
with the multiscan method and program SADABS were applied.a The
structures were solved by direct methods (SHELXS-97) and refined by
full-matrix least squares fitting based on F2 using the program SHEL-
XL-97 (18). All non-hydrogen atoms were refined with anisotropic dis-
placement parameters. All H atoms were located by different Fourier
syntheses and were then included in the refinement mostly with ide-
alized geometry riding on the atoms to which they were bonded. Only
the N-bonded hydrogen atoms were refined in x, y, z using a SADI
distance restraint for the N-H distance. In addition, the crystal struc-
ture of compound 4a, the Cl analogue of the Br compound 5a, was
determined and found to be isostructural with 5a. Due to crystal
quality problems and larger R-values, this structure is not reported in
the present work, but structural data can be obtained on request.
Parameters
5a
6a
Empirical formula
Formula weight
Color
C16 H16 Br N O2
334.21
Yellowish orange
Block
C16 H16 N2 O4
300.31
Orange
Habit
Block
Crystal dimension
0.28 · 0.16
· 0.11 mm
Triclinic
0.60 · 0.50
· 0.40 mm
Triclinic
Crystal system
Space group
a (ꢁ)
b (ꢁ)
c (ꢁ)
a (ꢀ)
b (ꢀ)
P1
P1
7.1016 (14)
9.2799 (18)
12.299 (2)
69.075 (2)
80.006 (2)
75.075 (2)
728.5 (2)
2
6.0305 (5)
10.2143 (9)
12.2730 (11)
92.325 (1)
100.757 (1)
92.917 (1)
740.77 (11)
2
c (ꢀ)
Volume (ꢁ3)
Z
Computational details
Temperature (K)
DC (Mg ⁄ m3)
Absorption
coefficient (mm)1
F (000)
100 (2)
1.523
2.822
100 (2)
1.346
0.098
Calculations were performed using the GAUSSIAN 03 software pack-
age,b and the B3LYP functional (19–21) without symmetry con-
straints. The functional includes a mixture of Hartree–Fock (22)
exchange with DFT (23) exchange-correlation, given by Becke's three
parameter functional with the Lee, Yang, and Parr correlation func-
tional, which includes both local and non-local terms. For all atoms,
the 6-31g** basis set was employed. Frequency calculations were
performed to confirm the nature of the stationary points yielding no
imaginary frequency for the minima (24–30). A scaling factor of
0.9614 was applied for the IR frequencies. The NMR chemical shift
calculations were obtained with the GIAO method at the B3LYP ⁄ 6-
31g** level (31). The TMS, calculated at the same level of theory,
was used as reference to scale the absolute shielding value.
)
340
316
k (ꢁ)
0.71073
2.98–30.13
x, u
)9 £ h £ 10,
)13 £ k £ 12,
)17 £ l £ 17
11045 ⁄ 8163 ⁄ 8019
0.71073 ꢁ
2.68–30.00
x, u
)8 £ h £ 8,
)14 £ k £ 14,
)16 £ l £ 17
10928 ⁄ 8245 ⁄ 8089
h range (ꢀ)
Scan type
Index ranges
Reflns collected ⁄
unique ⁄ obs.
Rint
0.0144
0.0174
Refinement method
Full-matrix least
squares on F2
Bruker Kappa
APEX-II CCD
Multiscan, program
SADABS
Full-matrix least
squares on F2
Bruker Kappa
APEX-II CCD
Multiscan, program
SADABS
Diffractometer
Synthesis of chiral Schiff bases
Absorption correction
The d-norephedrine.HCl or l-norephedrine.HCl (0.5 g, 2.66 mmol)
and potassium carbonate (0.368 g, 2.66 mmol) were added to
20 mL of absolute ethanol and stirred for 0.5 h at 27 ꢀC. To the
resulting solution, 5-substituted salicylaldehyde (2.66 mmol) was
added and stirred for 4 h at 27 ꢀC. The reaction was monitored by
TLC. After completion of reaction, the resulting solution was evapo-
rated to dryness, then dissolved in dichloromethane, and filtered
through Whatman filter paper (no. 1). The filtrate was evaporated
to dryness. An oily residue was obtained for 1a, 1b, 2a, 2b, 3a,
and 3b and for others solid was obtained.
Final R indices
0.0292 ⁄ 0.0744
0.0321 ⁄ 0.0908
R1 ⁄ wR2 [I > 2r (I)]
R1 ⁄ wR2 (all data)
Goodness-of-fit on F2
Dqmax ⁄ min, e ꢁ)3
Data ⁄ restraints ⁄
parameters
0.0297 ⁄ 0.0748
1.057
0.54 and )0.26
8163 ⁄ 4 ⁄ 371
0.0327 ⁄ 0.0916
1.056
0.37 and )0.18
8245 ⁄ 58 ⁄ 409
ESI-MS was recorded by a Bruker Daltonics HCT-Ultra ETDII ion trap
mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany). The
1H and 13C{1H} NMR spectra were recorded in CDCl3 solution on a
Bruker AVANCE III 500 MHz spectrometer (Bruker BioSpin AG, Fꢀllan-
Antibacterial evaluation
1
den, Switzerland) using TMS as internal standard. The H-13C HSQC
In vitro antibacterial inhibitory activities of the Schiff bases were
determined by twofold dilution assay (32) with modifications from
the reported method (33) using resazurin as an indicator dye. Muller
Hinton broth was used to grow the bacterial strains to a final inoc-
ulum size of 5 · 105 CFU ⁄ mL. The Schiff bases were dissolved in
absolute ethanol to a concentration of 10 mg ⁄ mL (34). Schiff base
solutions were added to successive wells in a 96-well microtiter
plate and incubated with the organisms for 18 h at 37 ꢀC. Growth
and sterility controls were maintained during the experiments. Com-
pounds without the microorganism were also kept in the 96-well
spectrum was obtained using the standard Bruker pulse programs.
Melting points were determined with Khera digital melting point
apparatus (Khera Instuments Pvt. Ltd., New Delhi, India).
X-ray crystallography
Single crystals of 5a and 6a suitable for X-ray diffraction studies
were grown at room temperature from ethanol ⁄ hexane solution by
slow evaporation. Diffraction data were collected on a Bruker Kappa
178
Chem Biol Drug Des 2012; 79: 177–185

Antibacterial Chiral Schiff Bases
microtiter plates to check the precipitation of compounds during the
course of the experiments. Ten microliters of 0.01% resazurin solu-
tion was added to each well and incubated for 2 h. A blank array
with ethanol alone and its effect on the growth of the microorgan-
isms was also studied. The color change was assessed visually,
with the highest dilution remaining blue (inhibition of growth) indi-
cating minimum inhibitory concentration (MIC). Ampicillin and tetra-
cycline were used as control drugs.
C10. Schiff bases 2a and 2b exhibit a singlet at 2.26 ppm, and 3a
and 3b show a singlet at 3.75 ppm correspond to methyl and
methoxy protons, respectively. Remaining aromatic protons in nor-
ephedrine moiety appeared as
a multiplet in the region
7.20–7.39 ppm. In the 13C NMR spectra of all the compounds,
azomethine carbon resonance is observed in the 163.35–
164.38 ppm range. The methyl carbon present in all the Schiff
bases appeared in the region 17.97–18.01 ppm. The resonance for
phenolic C-O carbon appeared in the region 158.18–160.36 ppm for
1a–5a and 1b–5b, and for 6a and 6b, it appeared at 170.28
and 169.97 ppm respectively. Peaks due to aliphatic C-O and C-N
carbons are observed in the region 77.09–77.66 and 69.73–
70.21 ppm, respectively. Methyl (2a and 2b) and methoxy carbons
(3a and 3b) of Schiff bases exhibit a resonance at 20.33 and
55.95 ppm, respectively. In 5a and 5b, signals due to protons pres-
ent in C3 and C5 are merged in the region 7.21–7.37 ppm, where
signals of aromatic protons of norephedrine moiety are also pres-
Results and Discussions
The reaction between equimolar ratio of 5-substituted salicylaldehyde
and d-norephedrine or l-norephedrine in the presence of base (K2CO3)
in ethanol yielded the new 4-substituted-2-[(E)-{[(1S,2R) ⁄ (1R,2S)-1-
hydroxy-1-phenylpropan-2-ylimino}methyl]phenols in good yield. They
were found to be soluble in CH2Cl2, CH3CN, C6H6, DMSO, DMF, and
C2H5OH. The analytical and ESI-MS data for these compounds are in
good agreement with the proposed molecular formula.
1
ent. To confirm the presence of protons in C3 and C5, H-13C HSQC
was recorded for 5a which establishes the connectivity of carbons,
namely C3 (134.97) and C5 (133.50) with its protons (Figure 1).
NMR spectra
1
In H NMR spectrum of all the compounds, a singlet observed in
the region 8.07–8.20 ppm has been assigned to azomethine proton
(-CH=N-). A quintet observed in the region 3.0–3.5 ppm in all the
Schiff bases is assigned to hydrogen present in C9 (for atom num-
bering see Table 1). Two doublets observed in the region 1.28–1.37
and 4.75–4.83 ppm with the proton count of three and one, respec-
tively, in all the Schiff bases are assigned to proton(s) on C8 and
IR spectra
The IR spectra of all the compounds exhibit a strong C=N stretching
vibration around 1630–1653 per cm. A band appeared in the region
1565–1612 per cm in all the compounds may be assigned to aro-
matic C=C stretching vibration (35). The bands in the region 1322–
1327 and 1033–1052 per cm have been assigned to phenolic m(C-O)
Figure 1: 1H-13C HSQC NMR spectrum of 5a.
Chem Biol Drug Des 2012; 79: 177–185
179

Muthu Tamizh et al.
and aliphatic m(C-O), respectively (35,36). The m(C-N) stretching fre-
quency was appeared in the region 1226–1277 per cm. A broad
band appeared around 3400 per cm was assigned to merged ali-
phatic as well as phenolic m(O-H). For compounds 6a and 6b, anti-
symmetric (out-of-phase) stretching vibration of the aromatic nitro
group gives rise to a strong IR band at 1545 and 1547 per cm,
respectively. The symmetric (in-phase) stretching vibration of the
nitro group present in 6a and 6b exhibits a strong band at 1323
and 1324 per cm, respectively.
are listed in Table 2. The molecular structures and the adopted
atom numbering are presented in Figures 2 and 3, respectively.
Both compounds crystallize in triclinic lattices of space group sym-
metry P1 (no. 1) with unit cell contents of each two independent
molecules linked to dimers by a pair of O-HÆÆÆO hydrogen bonds.
Schiff bases of salicylaldehydes are known for their intramolecular
short hydrogen bonds between O and N and the associated tautom-
erism between hydroxyimino (O-HÆÆÆN) and oxoamino (OÆÆÆH-N) form,
this last being also interpretable as an ortho-quinoid or a zwitter
ionic form (O1)ÆÆÆH-N1+) (37,38). In the solid state, 5a and 6a adopt
the oxoamino form with a short C(1)-O(1) bond of ꢁ1.29 ꢁ and an
alternating long–short–long bond length pattern in the phenyl ring
C(1) through C(6) related to an ortho-quinoid structure. These fea-
tures become clearly evident when the ring bond lengths of 5a and
6a (Table 2) are compared with those of a Schiff base in the
X-ray crystallography
The compounds 5a and 6a were structurally determined by X-ray
crystallography. The crystallographic and measurement data are
shown in Table 1 and representative bond lengths and bond angles
Table 2: Selected bond lengths (ꢁ) and angles (ꢀ) for compounds 5a and 6a
5a
6a
Bond lengths
Molecule 1
Molecule 2
Bond lengths
Molecule 1
Molecule 2
O(1)-C(1)
O(2)-C(10)
N(1)-C(7)
N(1)-C(9)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(4)-Br(1)
1.288 (3)
1.410 (2)
1.294 (3)
1.469 (2)
1.437 (3)
1.440 (3)
1.379 (3)
1.411 (3)
1.374 (3)
1.403 (3)
1.429 (3)
1.526 (3)
1.553 (3)
1.520 (3)
1.894 (2)
1.290 (3)
1.409 (2)
1.292 (3)
1.473 (3)
1.436 (3)
1.441 (3)
1.376 (3)
1.408 (3)
1.373 (3)
1.405 (3)
1.435 (3)
1.519 (4)
1.542 (3)
1.520 (3)
1.897 (2)
O(1)-C(1)
O(2)-C(10)
N(1)-C(7)
N(1)-C(9)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(4)-N(2)
N(2)-O(3)
N(2)-O(4)
1.279 (1)
1.422 (1)
1.292 (1)
1.462 (1)
1.429 (1)
1.449 (1)
1.367 (1)
1.407 (1)
1.380 (1)
1.401 (1)
1.433 (1)
1.524 (1)
1.545 (1)
1.515 (1)
1.444 (1)
1.236 (1)
1.234 (1)
1.288 (1)
1.415 (1)
1.295 (1)
1.473 (1)
1.430 (1)
1.443 (1)
1.368 (1)
1.408 (1)
1.378 (1)
1.402 (1)
1.437 (1)
1.521 (1)
1.544 (1)
1.515 (1)
1.446 (1)
1.232 (1)
1.235 (1)
Bond angles
Bond angles
C(2)-C(1)-C(6)
C(1)-C(6)-C(5)
C(1)-C(6)-C(7)
C(5)-C(6)-C(7)
C(6)-C(7)-N(1)
C(7)-N(1)-C(9)
N(1)-C(9)-C(8)
N(1)-C(9)-C(10)
C(8)-C(9)-C(10)
O(2)-C(10)-C(9)
O(2)-C(10)-C(11)
C(9)-C(10)-C(11)
115.8 (2)
121.6 (2)
120.4 (2)
118.1 (2)
122.2 (2)
127.8 (2)
113.6 (2)
106.1 (2)
112.5 (2)
109.8 (2)
111.0 (1)
112.0 (2)
115.8 (2)
121.8 (2)
120.0 (2)
118.3 (2)
121.8 (2)
127.6 (2)
112.6 (2)
107.4 (2)
113.2 (2)
110.6 (2)
110.4 (2)
111.4 (2)
C(2)-C(1)-C(6)
C(1)-C(6)-C(5)
C(1)-C(6)-C(7)
C(5)-C(6)-C(7)
C(6)-C(7)-N(1)
C(7)-N(1)-C(9)
N(1)-C(9)-C(8)
N(1)-C(9)-C(10)
C(8)-C(9)-C(10)
O(2)-C(10)-C(9)
O(2)-C(10)-C(11)
C(9)-C(10)-C(11)
O(3)-N(2)-O(4)
O(3)-N(2)-C(4)
O(4)-N(2)-C(4)
116.5 (1)
121.0 (1)
120.5 (1)
118.4 (1)
121.4 (1)
127.3 (1)
108.1 (1)
106.2 (1)
112.9 (1)
105.2 (1)
112.0 (1)
111.9 (1)
122.2 (1)
118.5 (1)
119.3 (1)
117.1 (1)
120.6 (1)
120.5 (1)
118.9 (1)
120.7 (1)
128.5 (1)
112.5 (1)
105.4 (1)
113.7 (1)
107.4 (1)
112.9 (1)
113.1 (1)
123.0 (1)
118.1 (1)
118.9 (1)
Dihedral angles
Dihedral angles
C7-N1-C9-C10
N1-C9-C10-C11
C9-C10-C11-C12
)118.1 (2)
)160.5 (2)
)126.6 (2)
)126.2 (3)
)161.9 (2)
)126.3 (2)
C7-N1-C9-C10
N1-C9-C10-C11
C9-C10-C11-C12
129.3 (1)
173.8 (1)
)90.5 (1)
)137.9 (1)
)166.7 (1)
)91.4 (1)
180
Chem Biol Drug Des 2012; 79: 177–185

Antibacterial Chiral Schiff Bases
Figure 2: Molecular structure of compound 5a showing the two crystallographically independent molecules with hydrogen bonds and
50% displacement ellipsoids.
Figure 3: Molecular structure of compound 6a showing the two crystallographically independent molecules with hydrogen bonds and
50% displacement ellipsoids.
hydroxyimino form, for example, 2-(N-salicylidene)amino-2-methyl-
propan-1,3-diol (39), where following bond lengths (ꢁ) were found
[values in brackets are corresponding mean values of 5a and 6a,
e.s.d.s 0.002 ꢁ]: O(1)-C(1) = 1.356(2) [1.286], C(1)-C(2) = 1.392(2)
[1.433], C(2)-C(3) = 1.383(2) [1.372], C(3)-C(4) = 1.388(3) [1.409], C(4)-
C(5) = 1.385(2) [1.376], C(5)-C(6) = 1.399(2) [1.403], C(6)-C(1) =
1.409(2) [1.443], C(6)-C(7) = 1.456(2) [1.434], C(7)-N(1) = 1.274(2)
[1.293], N(1)-C(8) = 1.477(2) [1.469]. The bond angles in both com-
pounds are unremarkable except for the ring bond angles C(6)-C(1)-
C(9), which are with values of about 115ꢀ, distinctly smaller than
all other ring bond angles. The bond angles C(7)-N(1)-C(9) are about
128ꢀ and thus, by about 10ꢀ larger than for corresponding com-
pounds in hydroxyimine form. The hydrogen bonds are compiled in
Table 3. The intramolecular N-HÆÆÆO bonds are very short
(NÆÆÆO ꢀ 2.58 ꢁ) and distinctly bent (140–150ꢀ). The intermolecular
O-HÆÆÆO are also relatively short (OÆÆÆO ꢀ 2.69 ꢁ) but essentially lin-
ear (164–179ꢀ). It is interesting to note that the O-HÆÆÆO bonds in
both 5a and 6a link pairs of symmetry independent molecules to
dimers in a pseudosymmetric fashion (Figures 2 and 3). In 5a, the
two independent molecules are related by a non-crystallographic
C2-axis that extends at x ꢀ 1 ⁄ 2, z ꢀ 1 ⁄ 2 infinitely parallel to the
b-axis to form columns. As the crystallographic a- and c-axes are
not perpendicular to the b-axis, in three-dimensional space instead
of a monoclinic lattice of space group P2, a truly triclinic lattice of
space group symmetry P1 is formed with different neighborhoods
for the two independent kinds of molecules. Interestingly, also 6a
shows pseudosymmetry – here, however, Ci with a pseudoinversion
at x, y, z ꢀ 1 ⁄ 2, 1 ⁄ 2, 1 ⁄ 2 – which relates approximately the two
independent molecules except for their groups C(8)H3 and O(9)H (cf.
Figure 3). These latter groups define the handedness of the two
Chem Biol Drug Des 2012; 79: 177–185
181

Muthu Tamizh et al.
Table 3: Hydrogen bonds in 5a and 6a (ꢁ, ꢀ). Both crystal
structures contain each two pseudosymmetry-related independent
molecules (unprimed and primed atom labels).
than the monomeric form by 26.2 kcal ⁄ mol. Structure A, where the
two 'benzoquinone' rings show p-p-stacking, is slightly disfavored
over B by 3.2 kcal ⁄ mol. The gas phase structures agree very well
with solid-state structures of 5a and 6a, as determined by X-ray
crystallography, where it was found that structure A is formed in
the case of 5a, whereas for 6a, the structural type B is observed.
The calculated IR spectra of monomeric I and II display strong
peaks of the C=N stretching vibration at 1669 and 1633 per cm,
respectively, whereas the dimeric compounds A and B give rise to
signals at 1640 and 1635 per cm, respectively. The scaled calcu-
lated frequencies mC=N show a reasonably good agreement with the
D-HÆÆÆA
D-A
D-H
HÆÆÆA
D-HÆÆÆA
Compound 5a
N(1)-H(1n) ÆÆÆO(1)
O(2)-H(2o) ÆÆÆO(1¢)
N(1¢)-H(1¢n) ÆÆÆO(1¢)
O(2¢)-H(2¢o) ÆÆÆO(1)
Compound 6a
2.593 (2)
2.670 (2)
2.576 (2)
2.671 (2)
0.92 (2)
0.84
0.92 (2)
0.84
1.76 (3)
1.83
1.74 (3)
1.83
149 (3)
178
150 (3)
179
N(1)-H(1n) ÆÆÆO(1)
O(2)-H(2o) ÆÆÆO(1¢)
N(1¢)-H(1n¢) ÆÆÆO(1¢)
O(2¢)-H(2o¢) ÆÆÆO(1)
2.580 (1)
2.701 (1)
2.570 (1)
2.714 (1)
0.86 (1)
0.84
0.86 (1)
0.84
1.859 (14)
1.86
1.819 (15)
1.90
140 (2)
174
145 (2)
164
1
experimentally observed value for 5a being 1633 per cm. In the H
NMR spectrum of A and B, the C=NH proton exhibits a signal at
about 13.7 ppm (cf. 13.32 in 5a).
Antibacterial activity
independent but homochiral molecules and consequently, violate the
Ci pseudosymmetry most (a true inversion would invert the chirality
of the symmetry-related molecule). However, different from 5a, the
Ci pseudosymmetry in 6a is not limited to one dimension but cov-
ers the triclinic lattice in three dimensions (Appendix S1).
The synthesized compounds were tested for antibacterial activity
against E. coli, P. vulgaris, S. aureus, and B. subtilis by twofold dilu-
tion method. Antibacterial activity was expressed in terms of MIC
in lM concentration (Table 4). Compounds 1a, 5a, 2b, 3b, and 5b
were the most active and 2a and 3a were the least active against
E. coli. A strong correlation [correlation coefficient (r) = 0.96] exists
between the antibacterial activity of the most active comounds (1a,
5a, 2b, 3b, and 5b) and ClogP (measure of lipophilicity or hydro-
phobicity of a compound). Compounds 1a, 2a, 5a, 2b, 5b, and
6b were the most active and compounds 1b and 4b were the
least active against P. vulgaris. Once again, a strong positive corre-
lation exists between the antibacterial activity and ClogP of the
most active compounds (1a, 2a, 5a, 2b, 5b, and 6b) (r = 0.99).
Compounds 1a, 2b, 3b, and 6b were the most active and
DFT/B3LYP calculations
The DFT calculations reveal that molecules of the type (I) (Figure 4)
undergo an intra-molecular proton transfer from the phenolic OH to
the azomethine nitrogen resulting in a quinoid structure (II) with
the latter being more stable by a free energy of 12.2 kcal ⁄ mol.
However, II is able to form dimeric structures A and B which are
connected via strong intra- and inter-molecular hydrogen bonds with
another molecule as depicted in Figure 5. These are more stable
CH3
CH3
Br
Br
N
N
OH
H
OH
O
O
H
II
I
Figure 4: Possible configurations of 5.
Figure 5: DFT ⁄ B3LYP optimized structures of two molecules of compound 5a connected via strong NH and OH hydrogen bonds.
182 Chem Biol Drug Des 2012; 79: 177–185

Antibacterial Chiral Schiff Bases
Table 4: In vitro antimicrobial activity (minimum inhibitory concentration, lM) of the compounds and standard reagents
Minimum inhibitory concentration (lM)
Gram-negative
Gram-positive
Substitution
E. coli
P. vulgaris
B. subtilis
S. aureus
Structure
Compound
at R
ClogP
NCIM 2931
NCIM2813
NCIM2718
NCIM5021
R
1a
2a
3a
4a
5a
6a
H
2.374
2.873
2.483
3.191
3.341
2.339
0.031
0.232
0.219
0.216
0.094
0.208
0.015
0.058
0.110
0.108
0.094
0.104
0.031
0.232
0.219
0.216
0.187
0.416
0.031
0.232
0.438
0.216
0.094
0.416
CH3
OCH3
Cl
Br
NO2
(R)
H3C
N
OH
R
OH
(S)
1b
2b
3b
4b
5b
6b
H
2.374
2.873
2.483
3.191
3.341
2.339
0.122
0.058
0.055
0.108
0.094
0.104
0.245
0.058
0.110
0.216
0.094
0.013
0.061
0.029
0.027
0.108
0.187
0.026
0.979
0.464
0.876
0.863
0.748
0.204
CH3
OCH3
Cl
Br
NO2
(S)
H3C
N
OH
OH
(R)
–
–
Ampicillin
Tetracycline
–
–
0.005
0.001
0.168
0.001
0.168
0.001
0.084
0.002
compounds 2a, 3a and 6a were the least active against B. subtil-
is. A negative correlation exists between the antibacterial activity
of the least active compounds (2a, 3a, and 6a) and ClogP
(r = )0.66). Compounds 1a, 4a, 5a, and 6b were the most active
and 1b, 3b, and 4b were the least active against S. aureus. A
negative correlation exists between the antibacterial activity and
ClogP and the activity of least active compounds (1b, 3b, and 4b)
(r = )0.68). Enantiomers may have either similar or different phar-
macodynamic profiles. In our case, active compounds 1a and 1b
(enantiomers) have similar pharmacodynamic profiles whereas in
the cases of 3a, 5a, 2b, 4b, and 6b, antibacterial activity is
higher in a single stereoisomer and other exert lower activity. Simi-
lar observation has already been made (40,41). In general, com-
pounds 1a, 5a, 2b, and 6b were the most active and compounds
1b, 4b, and 3a were the least active in this series against all
strains. Most active compounds showed a positive correlation with
ClogP especially against Gram-negative organisms (E. coli and
P. vulgaris). The least active compounds showed a negative correla-
tion with ClogP, especially against Gram-positive organisms (B. sub-
tilis and S. aureus). This study clearly shows that increasing the
hydrophobicity increases the antibacterial activity against Gram-neg-
ative organisms and decreases the antibacterial activity against
Gram-positive organisms. Earlier reports related to acetophenones
(42), chalcones (43), and 1,3,5-triphenyl-2-pyrazolines (44) as anti-
bacterial agents showed that hydrophilic and lipophilic balance of
the molecules influence their antibacterial activity. The mode of
action of the Schiff bases may involve the formation of the hydro-
gen bond through the azomethine group (C=N) with the active cen-
ters of the cell constituents resulting in the interference with
normal cell process (45). Interestingly, the activity of some of the
compounds reaches the effectiveness of conventional bactericide
ampicillin, particularly against P. vulgaris and B. subtilis. However,
activity did not reach the effectiveness of tetracycline, against all
bacteria tested.
Conclusion
Novel 4-substituted 2-[(E)-{[(1S,2R) ⁄ (1R,2S)-1-hydroxy-1-phenylpro-
pan-2-ylimino}methyl] phenols have been prepared from d-norephe-
drine or l-norephedrine and substituted salicylaldehydes, and
characterized by analytical, spectral, and crystallographic tech-
niques. Single crystal X-ray crystallography and DFT studies showed
that compounds are in the quinoid form and exist as dimers at
least in solid state. The antibacterial activity of the synthesized
compounds was evaluated using twofold dilution technique against
four bacteria using resazurin as indicator. Compounds 1a, 5a, 2b,
and 6b were the most active and compounds 3a, 1b, and 4b
were the least active against all the four tested organisms. This
study clearly indicates that hydrophobicity of the Schiff bases influ-
ences their antibacterial activity.
Acknowledgments
R.K. gratefully acknowledges Technical Education Quality Improve-
ment Programme (TEQIP) for awarding grant to visit Vienna Univer-
sity of Technology, Austria. We thank SAIF, IIT-M, Chennai for
recording NMR spectra.
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Supporting Information
Additional Supporting Information may be found in the online ver-
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Appendix. S1. CCDC 805343 & CCDC 727008 contains the sup-
plementary crystallographic data for 5a and 6a. Chacterization data
of 1a to 6a and 1b to 6b are provided. These data can be
obtained free of charge from The Cambridge Crystallographic Data
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185