Synthesis and characterization of benzothiazolyl-substituted anils

Article abstract of DOI:10.1002/mrc.2568

New Schiff bases containing a hydroxynaphthyl ring and substituted benzothiazolyl groups have been synthesized. High-resolution NMR spectra confirmed that these anils exist as enol-keto tautomers in solution. The results from NMR data demonstrated that the proportion of enol tautomer exceeded 90% in these substituted anils. Some compounds exhibited thermochromism in solid state. Copyright

Full text of DOI:10.1002/mrc.2568

Research Article
Received: 26 October 2009
Revised: 6 December 2009
Accepted: 9 December 2009
Published online in Wiley Interscience: 11 January 2010
(www.interscience.com) DOI 10.1002/mrc.2568
Synthesis and characterization
of benzothiazolyl-substituted anils
Taracad K. Venkatachalam,a∗ Gregory K. Pierensb and David C. Reutensa
New Schiff bases containing a hydroxynaphthyl ring and substituted benzothiazolyl groups have been synthesized. High-
resolution NMR spectra confirmed that these anils exist as enol–keto tautomers in solution. The results from NMR data
demonstrated that the proportion of enol tautomer exceeded 90% in these substituted anils. Some compounds exhibited
c
thermochromism in solid state. Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Keywords: synthesis; benzothiazoles; Schiff bases; thermochromism
Introduction
The FTIR spectra were similar for most of the anils with the
presence of a broad band between 3500 and 3400 cm−1 due to
eithertheOHof theenolortheNHgroupsassociatedwiththeketo
tautomer (Table 2). Peaks at 1720 and 1625 cm−1 were observed,
indicative of C O or C N stretching. The observed peaks in the
IR spectra suggest the presence of both enol and keto tautomeric
forms.
Schiff bases derived from hydroxynaphthaldehyde have at-
tracted many researchers due to the wide application of these
compounds as photochromic/thermochromic precursors.[1–6]
Recently, Asiri et al. prepared Schiff bases derived from 2-
hydroxynaphaldehyde or 2-hydroxybenzaldehyde and amino
piperizine or pyridine compounds.[2] Based on the spectral charac-
teristics, the authors concluded that compounds derived from
the 2-hydroxylbenzaldehyde showed preference toward enol
tautomers, whereas the 2-hydroxylnaphthaldehyde-substituted
Schiff bases showed the presence of both keto and enol tau-
tomers. Recently, Sashidara et al. showed formation of the keto-
enamine tautomer with chromenedicarboxylaldehyde-related
Schiff bases.[3] Using high-field NMR, they were able to determine
the structure of these keto-enamines with accuracy. Benzothiazole
and thiazole compounds may find biomedical applications. For
example, 11C-labeled (N-methyl-[11C]-2-(4ꢁ-methylaminophenyl)-
6-hydroxybenzothiazole has been used to image amyloid in
patients with Alzheimer’s disease.[7–18] We describe the synthesis
and characterization of anils bearing both hydroxynaphthyl and
substituted benzothiazolyl groups.
Generally, the keto–enol tautomers pass through equilibrium,
and Scheme 2 illustrates the complex equilibrium between each
of the species based on the present compound reported.
It can be seen from the scheme that the proton from the
enol compound is transferred to the nitrogen atom forming
a zwitterionic intermediate. This zwitterionic intermediate can
rearrange to form a keto tautomer by shifting the double
bond to the aromatic moiety. In an acidic environment, the
keto tautomer gets protonated at the nitrogen atom forming
a quaternary ammonium species. This quaternary ammonium
species subsequently transfers the proton to the oxygen atom of
the keto tautomer forming a protonated enolic species. Finally,
this proton is released to form the original enolic compound.
Depending on the structure and substituents present either in
benzothiazole or naphthylene moiety; this equilibrium can be
altered to form the more stable compound. Additionally, both
temperature and solvent conditions can change the stability of
one tautomer to another.
Results and Discussion
In essence, the keto–enol tautomerism is a complex phe-
nomenon, which exists especially in certain Schiff bases. Further-
more, in our structural feature of the anil, we can anticipate a
strong hydrogen bond between the proton and the oxygen or ni-
trogen atom, depending on whether it is enol or keto form, adding
stability for the tautomers by forming a stable six-membered ring
The synthesis of benzothiazolyl-substituted Schiff base was
accomplished following Scheme 1.
In brief, appropriate amino-substituted benzothiazolyl com-
pounds were condensed with 2-hydroxy naphthaldehyde in
absolute ethanol to furnish the required anils. Hydroxy naph-
thaldehyde was chosen to facilitate further functionalization of
these Schiff bases. These anils were obtained as crystalline solids
with colors varying from bright yellow to orange at room temper-
ature (Table 1).
The UV/VIS spectrum of these anils in both methanol and
chloroform showed an intense absorption peak at approximately
405–420 nm (Table 2). Additional absorption peaks were also
observed around 340 nm. The long wavelength absorption
observed is due to the presence of naphthyl rings in the structure
of these anils.
∗
Correspondence to: Taracad K. Venkatachalam, Centre for Advanced Imaging,
The University of Queensland, Brisbane, Queensland 4072, Australia.
E-mail: t.venkatachalam@uq.edu.au
a
Centre for Advanced Imaging, The University of Queensland, Brisbane,
Queensland 4072, Australia
b
Centre for Magnetic Resonance, The University of Queensland, Brisbane,
Queensland 4072, Australia
c
Magn. Reson. Chem. 2010, 48, 210–218
Copyright ꢀ 2010 John Wiley & Sons, Ltd.

Benzothiazolyl-substituted anils
SH
PPA
N
S
+
HOOC
NH2
NH
2
NH2
220C/ 10h
CHO
EtOH/8h/reflux
HO
S
N
N
HO
R
S
N
CHO
R
N
S
HO
EtOH
NH2
+
HO
N
Reflux/ 8h
Scheme 1. Synthetic scheme for preparation of benzothiazolyl-substituted anils.
(III and IV), which, in principle, should enhance the stability of the
tautomers. This is shown below in Scheme 3.
appears typically in a range from δ = 7.9 to 8.9 ppm in most
of the reported compounds. An energy minimization calculation
using within ChemBio3D Ultra using the MMFF94 force field
with 5000 iterations with a convergence criteria of 0.01 RMS
for the gradient resulted in a planar molecule. We propose that
this planar conformation of our compounds accounts for high
chemical shift value of this proton due to the ring-current effect
from the naphthyl ring. Depending on the groups attached to
the aromatic ring, the chemical shift of the neighboring proton
showed considerable change. For example, the unsubstituted
benzothiazolyl proton (H18) appeared at δ = 7.95 ppm, whereas
the monofluoro substitution shielded this proton (δ = 7.54 ppm).
Similarly, when a CF3 group was introduced, this proton signal
appeared at δ = 8.14 ppm. Furthermore, this proton showed
coupling to the fluorine atoms in the CF3 group resulting in a
complex multiplet. All other 1H chemical shifts are consistent with
those reported in the literature for other anils.[2]
We also analyzed the 13C NMR spectra for these benzothiazolyl-
substituted anils and found the following trend (Table 3). The
(N)2-C-S carbon signal appeared at δ = 168–171 ppm for all
derivatives. This deshielding of the benzothiazolyl carbon is due to
the presence of three bonded heteroatoms. The aromatic C–OH
carbon signals appeared in the range of δ = 166–168 ppm in all
the compounds providing evidence that these compounds exist
predominatelyasenoltautomersinsolution;theC Osignalofthe
keto tautomer generally appears δ > 170 ppm. It has also been
found that the imine carbon (N CH, C11) showed a chemical
shift value of δ = 160–161 ppm in all compounds. Finally, the
(C1) carbon showed the largest shielding, δ = 109–110 ppm. In
essence, the 13C chemical shifts obtained for these compounds
strengthen our hypothesis that the enol–enamine structure was
predominant for these benzothiazolyl-substituted anils. The other
signals were consistent with the structure of the compounds.
The stabilization of this structure with a six-membered ring
causes a planar configuration for these compounds. Additionally,
the hydrogen bond can easily be formed from structure I when
compared with structure II, and the likely reason for such a
trend lies in the fact that structure I retains the aromaticity
on the naphthyl ring through the conjugation between the
benzothiazolyl and naphthalene rings. On the other hand, the
possibility of maintaining the aromaticity on the naphthyl ring
is reduced considerably in structure II. Due to the presence
of the planar structure, the electron delocalization can take
place conveniently to the entire aromatic rings present in
these anils. In this context, several reports have appeared
describing the keto–enol tautomerism of naphthyl-substituted
Schiff bases.[19–31] Based on the literature precedence of such
an equilibrium system, we examined the NMR spectra of these
anils to evaluate the nature of species present in solution, and
the following paragraphs highlight the data obtained for all
compounds.
The 1H NMR spectra of the anils containing substituted
benzothiazolyl rings in their structure were examined (Table 3).
The 1H NMR spectrum showed a broad signal at δ = 14.40 ppm.
This broad signal was assigned to the hydroxyl proton of the
enol tautomer. The extreme high-frequency shift exhibited by this
proton is ascribed due to the formation of a hydrogen bond to the
nitrogenatom asdiscussedpreviously. Inthepreviousreports,[1–3]
the presence of hydrogen bonding in the enol tautomer has been
deduced from the chemical shift of the OH protons with a broad
singlet at approximately δ = 14 ppm. Based on this literature,
we suggest that our anils exist predominantly as enol tautomer.
The signal at δ = 10.19 ppm corresponds to the olefinic proton
(
CH) next to the nitrogen atom. It is known that this proton
c
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2010, 48, 210–218
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T. K. Venkatachalam, G. K. Pierens and D. C. Reutens
Table 1. Physical properties of the anilsa
Structure of compounds
Color of the product
Orange crystals
Keto (%)
1
Enol (%)
99
N
N
S
F
OH
1
Brownish yellow crystals
6
94
N
S
N
OH
2
Golden shiny yellow crystals
Orange yellow crystals
3
2
97
98
CH3
CH3
N
S
N
N
OH
3
N
N
S
OH
4
Deep yellow crystals
6
94
N
N
S
F3C
OH
5
a The percentages of keto/enol tautomers were calculated from the relative 1H signal integration ratios between keto and enol tautomers. Aromatic
protons were used for these calculations.
Table 1 shows the percentage of keto–enol compounds in
solution, whichwascalculatedfromintegratingthecorresponding
proton peaks. In some cases, the aromatic signals were used
to estimate this relative values. Caution should be exercised
here because the percentage of keto form of some compounds
is very low, and, therefore, the percentages reported are only
approximate values. We also used the method of Olivieri[22] and
Alarcon et al.[32] to calculate approximate equilibrium constants
for the tautomeric proton exchange and found that compound 1
showed a value of 0.70, whereas the others showed values in the
range of 0.90–0.82. Interestingly, compound 5 showed a value
of 0.62. We emphasize that our object was not to quantify the
percentageofenol/ketoformbutrathertoobtainapuretautomer.
The effect from changing solvents was examined using one of
the anils. The enol–keto equilibrium was monitored by observing
chemical shift changes. Figure 1 presents the 1H NMR spectra
obtained in three different solvents at room temperature. In
chloroform and benzene, we were able to observe the hydrogen-
bondedOHpeakatδ = ∼14.5 ppm,whereasindimethylsulfoxide-
d6 (DMSO-d6) this peak was very broad. This may be due to
exchange processes due to the presence of the residual water
in DMSO-d6. The spectrum was run with commercial deuterated
DMSO-d6 and no effort was made to remove the residual water
from solvent expected due to the fast exchange of OH protons in
polar solvent. The signal of the olefinic proton ( CH) was found
to shift only slightly in all these solvents. However, we observed
a marked shielding of the aromatic protons occurred in benzene
compared with chloroform and DMSO-d6. This may be rationalized
by to the polarity difference between benzene and DMSO-d6. It
is also noteworthy that the benzene spectrum seems to be much
cleaner when compared with the other two solvents. Notably, the
peaks at δ = 10.8 and 13.3 ppm are totally absent, suggesting
that one of the tautomer is preferentially soluble in benzene when
compared with the other two solvents. We propose that this could
be used to our advantage to separate the keto–enol tautomers
although detailed work is warranted.
Weexaminedthetemperatureinfluenceonthe1Hchemicalshift
for compound 2 in benzene. On varying the temperature between
283 and 313 K, we observed that the hydrogen-bonded OH signal
shiftedby0.18 ppm.At283 Kthispeakappearedatδ = 14.53 ppm,
c
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Benzothiazolyl-substituted anils
Table 2. Physical data of benzothiazolyl-substituted anils
Compound
Physical data
1
2
3
UV (MeOH): 422, 348, 255 nm: IR (KBr); 3400-3200 (b, OH), 1747, 1600, 1567, 1480, 1450, 1312, 1255, 1193, 1144, 1120, 1033, 981,
848, 818, 741 cm−1. M.p. 195-6 ◦C. Rf 0.75, chloroform/hexane mixture (80 : 20), single yellow spot: Mass spectra: low
resolution (LRESIMS): calculated for C18H11FN2OS: 322 (M+), found (−ve mode): 321 (M-1); high resolution (HRESIMS):
calculated for C18H11FN2OS Na: 345.0468, found (+ve mode): 345.0471.
UV (MeOH), 408, 349, 258 (sh), 228 nm: IR (KBr): 3563 ◦(b, OH), 3285, 1830, 1623, 1556, 1488, 1428, 1401, 1363, 1326, 1255, 1228,
1194, 1146, 1093, 1034, 823, 757 cm−1. M.p. 162-3 C. Rf 0.55, chloroform/hexane mixture (80 : 20), single yellow spot. Mass
spectra: low resolution (LRESIMS): Calculated for C14H10N2OS: 253 (M+), found (+ve mode): 255 (M+2), 277 (M+ + 1+Na);
high-resolution (HRESIMS): calculated for C14H10N2OS Na: 277.0406, found (+ve mode): 277.0403.
UV (CHCl3): 411, 320, 279, 259, 244 nm: IR (KBr): 3300-3200 (b, OH), 1632, 1577, 1518, 1478, 1431, 1349, 1310, 1243, 1203, 1174,
1132, 1079, 1014, 967, 828, 792 cm−1. M.p. 217-8 ◦C. (i) Rf, 0.11, chloroform/hexane mixture (80 : 20), single yellow spot. (ii) Rf
0.27 chloroform/hexane mixture (90 : 10), single yellow spot. Mass spectra: low resolution (LRESIMS): calculated for
C24H22N3OS: 401 (M+), found (+ve mode): 402.1 (M+2); high-resolution (HRESIMS): calculated for C24H22N3OS: 400.1489,
found (−ve mode): 400.1488.
4
5
UV (MeOH): 423, 356 nm; UV (CHCl3): 424, 347, 290, 239 nm: IR (KBr): 3550 (b, OH), 1747, 1625, 1622, 1560, 1477, 1441, 1390,
1322, 1257, 1224, 1156, 1090, 835, 759 cm−1. M.p. 212-3 ◦C. Rf 0.66, chloroform/hexane mixture (80 : 20), single yellow spot.
Mass spectra: low resolution (LRESIMS): calculated for C18H12N2OS: 304 (M+), found (−ve mode): 303.1 (M-1); high resolution
(HRESIMS): calculated for C18H11N2OS: 303.0598, found (−ve mode): 303.0599.
UV (MeOH); 425, 345, 262 nm; ◦IR (KBr): 3430 (b), 2923, 1651, 1622, 1601, 1563, 1488, 1455, 1413, 1320, 1285, 1167, 1107, 1050,
1022, 822 cm−1. M.p. 180-1 C. Rf 0.88, chloroform/hexane mixture (80 : 20), single yellow spot. Mass spectra: low resolution
(LRESIMS): calculated for C19H10F3N2OS: 372 (M+), found (−ve mode): 371.1 (M-1); High resolution (HRESIMS): calculated for
C19H10F3N2OS: 371.0471, found (−ve mode): 371.0471.
-O
HO
N
S
N
S
+
N
N
H
O
H+
N
S
N
H
H+
HO
O
H
N
S
N
S
+
N
N+
H
H
Scheme 2. Scheme showing the keto–enol tautomeric equilibrium of anils.
H
H
N
N
N
S
N
S
H O
H O
I
II
H
H
N
N
S
N
N
S
H O
H
O
III
IV
Scheme 3. Illustration of the keto–enol tautomerism with hydrogen bonds.
c
Magn. Reson. Chem. 2010, 48, 210–218
Copyright ꢀ 2010 John Wiley & Sons, Ltd.
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T. K. Venkatachalam, G. K. Pierens and D. C. Reutens
Table 3. 1H and 13C chemical shifts of benzothiaozlyl-substituted anils
Compound
Compound
1H
1
2
4
5
3
13C
1
2
4
5
3
1
–
–
14.5
7.2
–
13.72
7.21
8.12
7.91
7.42
7.66
8.31
–
–
–
1
2
109
109.7
164.9
119.8
136.9
129.4
124.1
128.4
119.7
133.1
127.9
159.8
109.9
165.1
121.9
138.2
129.2
124.3
129
109.6
167.3
120.1
138.8
129.6
124.6
128.9
119.8
133.2
127.9
161.8
109.2
2-OH
3
14.4
14.4
14.45
166.4
120.1
138.1
129.5
124.5
128.7
119.7
133.1
128
164.4
7.19
7.18
–
3
98.8
4
7.92
7.79
7.43
7.62
8.28
–
7.9
7.96
7.8
7.44
7.62
8.3
–
–
4
152.3
5
7.78
6.29
5
104.4
6
7.4
7.2
6
134.4
7
7.58
8.47
7
160.9
8
8.27
–
8
119.6
132.5
127.7
163.2
–
9
–
–
9
151.4
10
11
12
13
14
15
16
17
18
19
20
21
others
–
–
–
–
–
10
11
12
13
14
15
16
17
18
19
20
21
others
121.5
10.11
–
10.11
9.99
–
10.19
–
8.13
161.4
–
123.1
–
–
–
135.1
–
–
–
8.13
168.3
–
171.1
169.2
171.1
123.1
–
–
–
–
7.35
121.5
–
7.71
7.24
–
–
–
–
159.6
148.2
–
141.4
133.8
151.3
139.1
153.7
167.6
–
–
–
–
118.1
–
–
–
–
8.08
–
–
–
–
–
128.7
7.54
–
–
7.95
7.39
7.54
8.08
8.14
–
7.51
108.1
160.2
115.2
123.7
108.1
126.7
125.1
122.4
109.6
161.9
126.3
–
7.39
125.1
121.6
7.23
7.92
–
7.72
8.05
7.92
119.1
–
–
123.6
130.7
25: 3.44 26: 1.24
22: 126.9
22: 154.2 25: 44.8 26: 12.7
15.0 14.5 14.0 13.5 13.0 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0
f1 (ppm)
Figure 1. 1H NMR spectra of compound 1 in different solvents at room temperature (solvent used: lower panel, benzene; middle panel, chloroform; top
panel, DMSO-d6).
whereas at 313 K this peak appeared at δ = 14.34 ppm. The
deshieldingcausedbylowering thetemperatureisconsistentwith
a strengthening of the hydrogen bond. However, the CH peak for
theolefinicprotonshowsonlyaminorshiftof0.03 ppm,inkeeping
with the lower sensitivity of olefinic protons chemical shifts
to temperature changes than the chemical shifts of hydrogen-
bonded protons. For compound 1 in chloroform, the hydrogen-
bonded NH protons intensity at δ = 14.4 ppm decreased with
increasing temperature with a concomitant increase in the peak
intensity of the signal at δ = 13.1 ppm (Fig. 2). We also observed
that there was slight shift of the aromatic proton signals in this
solvent as the temperature was altered. In summary, the hydrogen
c
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Benzothiazolyl-substituted anils
14.5 14.4 14.3 14.2 14.1 14.0 13.9 13.8 13.7 13.6 13.5 13.4 13.3 13.2 13.1 13.0
f1 (ppm)
Figure 2. 1H NMR spectra of compound 1 in chloroform at various temperatures.
H
N
H
N
O
N
S
N
S
N
H
N
H
O
H
O
S
H
enol(OH-form)
Cis-keto (NH-form)
trans-keto (NH-form)
Scheme 4. Proton transfer equilibrium of the anils.
bonding characteristics was less pronounced due to the breaking
of this bond at high temperatures.
(iii) a change of crystal packing due to temperature and (iv)
due to rotation of the bonds. Also deepening of the color
due to temperature may be indicative of a higher population
of the keto tautomer. The existence of keto forms in the solid
state was demonstrated by Inabe et al.[21] for N,N-bis(2-hydroxy-
1-naphthylmethylene)-1,4-phenylenediamine. They rationalized
the existence of these two forms due to delocalization of the
electrons, which imparts stabilization for these two structures.
Based on literature precedence, we examined the thermochromic
property of these newly synthesized anils. Figure 3 shows the
thermochromic property exhibited by these anils. Compound
1 changed slightly from orange red to light orange at lower
temperatures. Compounds 2 and 3 did not show much change in
their color on lowering the temperature. However, compound 4
showed considerable change in color at −40 ◦C. The compound
converted into a yellow product in solid state on lowering the
temperature. Thecolorchangeimpartedbytheseanilsmayalsobe
attributed due to the structural variation associated with the anils.
Compound 2 has only a thiazolyl moiety, whereas compounds 1
and 4 have the benzothiazolyl moiety in their structure. Although
compound3containsthebenzothiazolylringinthestructureithas
anextendedchainandperhapsdoesnotshowanythermochromic
effect under our experimental conditions.
Thermochromism
ManySchiffbasesexhibitthermochromism.Carleset al.[19] showed
that salicylidene Schiff bases exhibit thermochromism in the
solid state. Oagawa et al.[20] also studied the properties of
crystalline forms of certain anils exhibiting thermochromism
using X-ray crystallography. According to Cohen et al.[12] there
are two crystal forms, which exhibit either photochromism or
thermochromism. In general, a yellow compound was found
to show photochromic property, whereas the red compounds
exhibited the thermochromic property. Furthermore, Cohen
et al.[33,34] also proposed an intramolecular proton transfer
mechanism. Accordingly, Scheme 4 shows the mechanism for
the proton transfer.
In the above scheme, the enol form is hydrogen-bonded
to the nitrogen, which is in equilibrium with the cis-keto-
NH form, whereas it has been converted into the keto form
while retaining the configuration as cis. On the other hand,
there is also another possible tautomer that can be formed
depending on the conditions. This tautomer exist as a trans-
keto form where the hydrogen atom resides on the nitrogen
without forming a hydrogen bond with the C O group. There
are four possible pathways through which a compound can
exhibit thermochromism which includes (i) proton transfer, (ii)
electron delocalization depending on planarity of the molecule,
In summary, we have synthesized anils containing substituted
benzothiazolyl groups. 1H and 13C NMR spectroscopy suggest
the preponderance of the enol forms. Solvent change affected
the chemical shifts of hydrogen-bonded OH groups. Variable-
c
Magn. Reson. Chem. 2010, 48, 210–218
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T. K. Venkatachalam, G. K. Pierens and D. C. Reutens
Figure 3. Thermochromism of anils at various temperatures.
temperature 1H NMR gave evidence for the weakening of
the hydrogen-bonded OH peak at higher temperatures. Some
benzothiazolyl-substituted anils also showed thermochromism.
(20 : 80) solvent mixture. Low-resolution electrospray ionization
mass spectrometry (LRESIMS) measurements were carried out on
a Bruker Esquire HCT (high capacity 3D ion trap) instrument with
a Bruker ESI source. Some of the spectra were done in positive ion
mode (+ve) and others in negative ion mode (−ve) depending
on the mode that produced the best result. High-resolution
electrospray ionization (HRESIMS) accurate mass measurements
were carried out on a Bruker MicrOTOF-Q (quadrupole-Time
of Flight) instrument with a Bruker ESI source. Accurate mass
measurements were carried out with external calibration using
sodium formate as reference calibrant.
Experimental
All chemicals were obtained from Sigma-Aldrich and used without
further purification. The deuterated solvents were obtained
from Cambridge Isotopic Laboratories and used without further
purification. The NMR spectra were acquired in both chloroform
and dimethyl sulfoxide-d6 on a Bruker 900 MHz spectrometer
fitted with a cryoprobe, and the chemical shifts are reported
relative to chloroform peak at δ = 7.27 ppm or DMSO-d6 at
General procedure for synthesis of anil derivatives: In a 100 ml
round bottom flask, the appropriate benzothiazole compound
(0.05 mol), the respective aldehyde (0.05 mol) and 50 ml of
absolute ethanol were placed. The contents were stirred and
the resulting solution was refluxed over an oil bath for 8 h. The
precipitated anils were filtered under vacuum, washed with a
copious amount of ethanol and dried in vacuum. The compounds
were further purified by crystallization using chloroform solvent.
1H NMR and 13C NMR peaks were assigned using COSY, TOCSY,
HSQC and HMBC experiments. Sample acquisition parameters are
as follows for the various experiment, 1H NMR (SW = 20 ppm; O1 =
8 ppm; D1 = 1 s and 32 scans acquired), 13C NMR (SW = 230 ppm;
O1 = 105 ppm; D1 = 1 s and 128 scans acquired), COSY/TOCSY
(NS = 8 and TD = 256 or 512, mixing time = 70 ms), HSQC (F1
δ = 2.54 ppm, respectively, for 1H NMR and δ = 77 ppm for 13
C
NMR and δ = 39.5 ppm in the case of chloroform and DMSO-d6,
respectively. For each sample, the probe was matched and tuned
and the proton 90◦ pulse measure (∼9 µs). UV/Vis spectra were
obtained with a Lambda UV spectrometer using either methanol
or chloroform as solvent. Infrared spectra were taken as KBr disc
on a Lambda FTIR spectrometer. Melting points were measured
on a Fisher-Jones electrothermal melt apparatus, benzoic acid
used as a calibration standard at 122-3 ◦C. During melting point
determination, we observed that the compounds changed into
dark red color and eventually formed a dark red liquid on melting.
The Rf values were measured by spotting the compound on a
silica gel (60 mesh) plate (aluminum based) supplied by Merck,
Germany. The plates were developed using hexane/chloroform
1
SW2 = 180 ppm, O2 = 80 ppm, J coupling = 145 Hz, D1 = 1 s
and TD = 256, 16 scans acquired) and HMBC (F1 SW2 = 220 ppm,
O2 = 110 ppm, long-range coupling = 8 Hz, D1 = 1 s and TD
c
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Copyright ꢀ 2010 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2010, 48, 210–218

Benzothiazolyl-substituted anils
Figure 4. Structures of synthesized compounds along with the atom numbering.
= 256, 64 scans acquired). Figure 4 shows the structures of the
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Acknowledgements
This research was funded by a Program Grant from the National
Health and Medical Research Council of Australia. We thank the
Queensland NMR Network (QNN) for granting access to the Bruker
900 MHz spectrometer.
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