Anticonvulsant and neurotoxicity evaluation of some novel kojic acids and allomaltol derivatives

Article abstract of DOI:10.1002/ardp.200900236

A series of new 3-hydroxy-6-hydroxymethyl/methyl-2-substituted 4H-pyran-4-ones were synthesized and prepared by the reaction of kojic acid or allomaltol with piperidine derivatives and formaline as potential anticonvulsant compounds. The structure of the synthesized compounds was confirmed using the elemental analysis results and the spectroscopic techniques such as IR, 1H-NMR, and ESI-MS. Anticonvulsant activities were examined by maximal electroshock (MES) and subcutaneous Metrazol (scMet)-induced seizure tests. Neurotoxicity was determined by the rotorod toxicity test. All these tests were performed in accordance with the procedures of the Antiepileptic Drug Development (ADD) program. According to the activity studies and at all doses, 3-hydroxy-2-[(4-hydroxy-4-phenylpiperidin-1-yl)methyl]-6-methyl-4H-pyran-4-one (compound 1), 2-{[4-(4-chlorophenyl)-3,6-dihydropyridin-1(2H)-yl]methyl}-3- hydroxy-6-methyl-4Hpyran-4-one (compound 6), 2-[(4-acetyl-4-phenylpiperidin-1- yl)methyl]-3-hydroxy-6-(hydroxymethyl)-4H-pyran-4-one (compound 11), and 2-{[4-(4-chlorophenyl)-3,6-dihydropyridin-1(2H)-yl] methyl}-3-hydroxy-6- hydroxymethyl-4H-pyran-4-one (compound 12) were found to have anticonvulsant activity against MES-induced seizures at 4 h. Also, 2-{[4-(4-bromophenyl)-4- hydroxypiperidin-1-yl]methyl}-3-hydroxy-6-(hydroxymethyl)-4H-pyran-4-one (compound 8) was determined to be the most active compound against scMet-induced seizures at all doses at 0.5 and 4 h. In the rotorod neurotoxicity screening, all compounds showed no toxicity at all doses.

Full text of DOI:10.1002/ardp.200900236

Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
M. Dilsiz Aytemir and ꢀ. C˛ alıs˛
173
Full Paper
Anticonvulsant and Neurotoxicity Evaluation of Some Novel
Kojic Acids and Allomaltol Derivatives
Mutlu Dilsiz Aytemir and ꢀnsal ꢁalıs˛
Hacettepe University, Faculty of Pharmacy, Department of Pharmaceutical Chemistry, Ankara, Turkey
A series of new 3-hydroxy-6-hydroxymethyl/methyl-2-substituted 4H-pyran-4-ones were synthe-
sized and prepared by the reaction of kojic acid or allomaltol with piperidine derivatives and
formaline as potential anticonvulsant compounds. The structure of the synthesized compounds
was confirmed using the elemental analysis results and the spectroscopic techniques such as IR,
1H-NMR, and ESI-MS. Anticonvulsant activities were examined by maximal electroshock (MES)
and subcutaneous Metrazol (scMet)-induced seizure tests. Neurotoxicity was determined by the
rotorod toxicity test. All these tests were performed in accordance with the procedures of the
Antiepileptic Drug Development (ADD) program. According to the activity studies and at all
doses, 3-hydroxy-2-[(4-hydroxy-4-phenylpiperidin-1-yl)methyl]-6-methyl-4H-pyran-4-one (com-
pound
1),
2-{[4-(4-chlorophenyl)-3,6-dihydropyridin-1(2H)-yl]methyl}-3-hydroxy-6-methyl-4H-
pyran-4-one (compound 6), 2-[(4-acetyl-4-phenylpiperidin-1-yl)methyl]-3-hydroxy-6-(hydroxy-
methyl)-4H-pyran-4-one (compound 11), and 2-{[4-(4-chlorophenyl)-3,6-dihydropyridin-1(2H)-yl]
methyl}-3-hydroxy-6-hydroxymethyl-4H-pyran-4-one (compound 12) were found to have anticon-
vulsant activity against MES-induced seizures at 4 h. Also, 2-{[4-(4-bromophenyl)-4-hydroxypiper-
idin-1-yl]methyl}-3-hydroxy-6-(hydroxymethyl)-4H-pyran-4-one (compound 8) was determined to
be the most active compound against scMet-induced seizures at all doses at 0.5 and 4 h. In the
rotorod neurotoxicity screening, all compounds showed no toxicity at all doses.
Keywords: Allomaltol / Anticonvulsant activity / Kojic acid / Mannich reaction / Synthesis /
Received: September 30, 2009; accepted: November 4, 2009
DOI 10.1002/ardp.200900236
drome. It is a group of signs and symptoms that custom-
arily occur together. Identification of the syndrome helps
to determine the appropriate therapy and the prognosis.
The maximal electroshock (MES)-induced seizure test is a
predictor of compounds that are active against tonic-
clonic (grand mal) seizures. The subcutaneous Metrazol
(scMet)-induced seizure test is used to detect compounds
useful in treating generalized absence (petit mal) seiz-
ures [5]. There is continuing demand for new anticonvul-
sant agents, as it has not been possible to control every
kind of seizure with the currently available anti-epileptic
drugs. To provide an improvement of the quality of life
for people suffering from epilepsy, it is essential to search
for new chemical entities with lower toxicity and fewer
side effects for the treatment.
Introduction
Epilepsy, characterized by recurrent unprovoked seiz-
ures, is the most prevalent neurological disorder, affect-
ing approximately 60 million people worldwide. More
than 30% of patients with epilepsy have inadequate con-
trol of seizures with available medical therapies. The
established conventional anti-epileptic drugs – though
widely prescribed – exhibited undesired side-effect pro-
files ranging from drowsiness to megaloblastic anemia
and failure to control seizures adequately [1–4].
Seizures that are arising from discharging lesions of
the cerebral cortex often occur as part of an epileptic syn-
Correspondence: Assoc. Prof. Mutlu D. Aytemir, PhD, University of Ha-
cettepe, Faculty of Pharmacy, Department of Pharmaceutical Chemistry,
06100 Sihhiye-Ankara, Turkey.
E-mail: mutlud@hacettepe.edu.tr
Fax: +90 312 311-4777
Previously, some Mannich base derivatives were pre-
pared in our laboratory and were tested for their anticon-
vulsant and antimicrobial activities [6–9]. Among these
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

174
M. Dilsiz Aytemir and ꢀ. C˛ alıs˛
Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
tional heterocyclic, an oxygen-containing ring with sev-
eral important centers enabling additional reactions like
oxidation and reduction, alkylation and acylation, nucle-
ophilic and electrophilic substitution reactions, a ring
opening of the molecule, and chelation. Therefore, many
researchers used it as starting material for the prepara-
tion of new compounds [11–15].
Allomaltol was synthesized from commercially avail-
able kojic acid in a two-step reaction according to the lit-
erature [16–17]. Chlorination of the 2-hydroxymethyl
moiety of kojic acid using thionyl chloride at room tem-
perature afforded chlorokojic acid, with the ring
hydroxyl being unaffected. Reduction of chlorokojic acid
with zinc dust in concentrated hydrochloric acid
resulted in the production of allomaltol [6–8, 18].
It is well known that hydroxypyranones can exist in
cationic and anionic forms due to the protonation or
deprotonation reactions, respectively. The hydroxyl
group that is directly bound to the pyranone ring was
probable more deprotonated than the hydroxymethyl
group [19]. Quantum mechanical investigations on tauto-
meric equilibria of kojic acid were performed. Because of
two intramolecular hydrogen bonds, the enolic structure
of neutral kojic acid is expected to be the most stable one.
One of these two bonds is located between the keto and
hydroxyl group and the other hydrogen bond can be
formed weakly between the hydroxymethyl moiety and
the intra-ring oxygen [20]. The details of crystal structure
analyses data of chlorokojic acid and allomaltol were
investigated and their molecular geometry was com-
pared to the geometry of their close analog kojic acid [21,
22]. In our previous studies, the structures of some Man-
nich bases of allomaltol derivatives were also determi-
nated by X-ray analysis. The conformations of the mole-
cules were determined by intra- and intermolecular
hydrogen bonds. Some weak intramolecular interactions
help to stabilize the structure [23, 24].
Figure 1. Structure of 2-[4-(4-chlorophenyl)piperazin-1-ylmeth-
yl]-3-hydroxy-6-methyl-4H-pyran-4-one I and 2-[4-(4-chlorophe-
nyl)piperazin-1-ylmethyl]-3-hydroxy-6-hydroxymethyl-4H-pyran-
4-one II.
compounds, 2-[4-(4-chlorophenyl)piperazin-1-ylmethyl]-3-
hydroxy-6-methyl-4H-pyran-4-one (I; Fig 1) was found to
be protective against MES-induced seizures at all doses at
0.5 h. By the way, 2-[4-(4-chlorophenyl)piperazin-1-
ylmethyl]-3-hydroxy-6-hydroxymethyl-4H-pyran-4-one (II;
Fig. 2) was determined to be the most active against MES-
induced seizures at 0.5 and 4 h. In connection with these
studies, we planned to modify the piperazine structure
on the final Mannich base in order to develop new anti-
convulsant agents. In this paper, synthesis and anticon-
vulsant activity of a series of 3-hydroxy-6-hydroxymethyl/
methyl-2-substituted 4H-pyran-4-one derivatives (1–12)
which are carrying various substituted piperidine deriv-
atives at the 2-position are described.
Anticonvulsant activities of the synthesized com-
pounds were examined by MES and scMet-induced seiz-
ure tests. The acute neurological toxicity was determined
in the rotorod test. All these tests were performed in
male mice according to the phase-I tests of the Antiepi-
leptic Drug Development (ADD) program which were
developed by National Institutes of Health (NIH) and
National Institute of Neurological Disorders and Stroke
(NINDS), both USA [5]. This program was used for the
screening of many compounds in various previous stud-
ies [6–10].
Results and discussion
Multicomponent reactions are the major parts of the
synthetic organic chemistry with advantages ranging
from lower reaction times and temperatures to higher
yields. Mannich-type reactions are three-component con-
densation reactions involving carbonyl compounds,
existing as keto-enol tautomeric forms, formaline, and a
primary or secondary amine. In 1912, Mannich and Kro-
sche were the first to prepare some Mannich bases only
Chemistry
Kojic acid (5-hydroxy-2-hydroxymethyl-4H-pyran-4-one)
provides a promising skeleton for the development of
new and more potent derivatives such as chlorokojic acid
(2-chloromethyl-5-hydroxy-4H-pyran-4-one), allomaltol (5-
hydroxy-2-methyl-4H-pyran-4-one), and pyromeconic acid
(3-hydroxy-4H-pyran-4-one) (Fig. 2). It contains a polyfunc-
Figure 2. Hydroxypyranone derivatives.
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
Novel Kojic Acids and Allomaltol Derivatives
175
Reagents: a) SOCl2; b) Zn/HCl/H2O; c) related substituted piperidine/HCHO/MeOH.
Scheme 1. Synthesis of compounds 1–12.
at the 6-position; this is the reactive position of kojic acid their chemical shifts, multiplicities, and coupling con-
1
[25]. Due to its phenol-like properties, kojic acid readily stants. The H-NMR spectra for all compounds demon-
undergoes aminomethylation in the Mannich reaction at strated the presence of the characteristic singlet peaks of
room temperature, ortho to the enolic hydroxyl group. It the 4H-pyran-4-one (H5) ring proton in accordance with
is reported that di-Mannich derivatives can be obtained the literature [17]. Also, singlet peaks of the 6-methyl or
in an acidic medium from kojic acid, formaline, and an 6-hydroxymethyl protons of the ring appeared in a
aromatic amine [25, 26]. Mannich bases of kojic acid region of 2.14 to 2.47 and 4.28 to 4.32 ppm, respectively.
derivatives, which show various biological activities, The methylene-group protons of compounds 1–12 were
were prepared and evaluated for their activities by differ- seen at 3.45 to 3.64 ppm as a singlet. 13C-NMR spectra of
ent researchers [6–9, 25, 27].
compounds 2 and 4 have typical peaks for 6-methyl and
In this study, twelve new 3-hydroxy-6-hydroxymethyl/ carbonyl of the pyranone ring at 20.2 to 20.4 and 174.4
methyl-2-substituted 4H-pyran-4-one derivatives were ppm, respectively. The characteristic peak for -CH2- was
synthesized as Mannich bases by the methodology shown observed at 54.3 to 56.1 ppm. The mass spectra of all com-
in Scheme 1. The basic substituent can be introduced at pounds showed [M+ + 1] and [M+ + 23] peaks.
the 6-position of allomaltol or kojic acid via a Mannich-
type reaction, using formaline and an appropriate substi- Anticonvulsant activity
tuted piperidine in methanol at room temperature. The In our former studies, evaluation of the anticonvulsant
reaction proceeded very rapidly.
activity of the Mannich bases of allomaltol or kojic acid
All the compounds were prepared as new products. derivatives were studied. When the effects of different
Formation of the desired Mannich bases was confirmed piperazine rings upon the activity were examined, kojic
on the basis of elemental analysis and the structures of acid derivatives were found to be more active than allo-
the compounds were supported by spectral data. The IR, maltol derivatives [6, 9]. Whenever mono-substituted
1H-NMR, 13C-NMR, 13C-DEPT, and ESI-MS are in agreement piperidine derivatives or morpholine rings (They were
with the proposed structures. The results of elemental also used instead of the piperazine ring) were used, the
analysis for C, H, and N were within l 0.4% of the theoret- anticonvulsant activity of these Mannich bases of allo-
ical values. Yields and melting points of the synthesized maltol derivatives decreased [7, 8].
compounds are presented in Table 1.
In the view of previous study results, here, two Man-
In the IR spectra, compounds 1–12 have O-H stretching nich base series were synthesized and compared to each
(st) vibrations at 3400–3300 cm– 1. All compounds were other for anticonvulsant activity. These two series differ
associated with C=O (acetyl), C=O (pyranone), C=C, and C- in the methyl (compounds 1–6) or hydroxymethyl (com-
O st at 1697, 1657–1618, 1594–1456, and 1232–1197 pounds 7–12) groups at the 6-positions of the substituted
cm– 1, respectively. In the nuclear magnetic resonance pyranone ring. Also, both Mannich bases, which have
spectra (1H-NMR), the signals of the respective protons of mono- or di-substituted piperidine rings, were designed
the synthesized compounds were verified on the basis of (Scheme 1). It is generally accepted that the lipid solubil-
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

176
M. Dilsiz Aytemir and ꢀ. C˛ alıs˛
Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
Table 1. Physicochemical data of the synthesized compounds.
Compound
R1
R2
M.p.
(8C)
Yield
(%)
Emperical
Formula
MW
(g/mol)
1
-CH3
192–193
154–155
121–122
52
88
81
C18H21NO4
315.36
394.26
349.81
2
3
-CH3
-CH3
C18H20BrNO4
C18H20ClNO4
4
5
6
7
-CH3
149–150
191–192
189–190
126–127
66
67
93
62
C19H20N2O3
C20H23NO4
C18H18ClNO3
C18H21NO5
324.37
341.40
331.79
331.36
-CH3
-CH3
-CH2OH
8
9
-CH2OH
-CH2OH
134–135
141–142
96
94
C18H20BrNO5
C18H20ClNO5
410.26
365.81
10
11
-CH2OH
-CH2OH
154–155
166–167
58
85
C19H20N2O4
C20H23NO5
340.37
357.40
12
-CH2OH
182–183
64
C18H18ClNO4
347.80
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
Novel Kojic Acids and Allomaltol Derivatives
177
Table 2. Phase-I anticonvulsant screening of the compounds.
MESa)
scMetb)
Toxicityc)
Compound
0.5 h (mg/kg)
4 h (mg/kg)
0.5 h (mg/kg)
100 300 30
4 h (mg/kg)
100 300 30
0.5 h (mg/kg)
4 h (mg/kg)
100 300
30
100 300 30
100 300 30
100 300 30
1
2
3
4
5
6
7
8
9
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
1/1
0/1
1/1
0/1
0/1
0/1
0/1
0/1
0/1
1/1
1/1
0/1
1/1
0/1
1/1
0/1
1/1
1/1
0/1
0/1
0/1
1/1
0/1
0/1
0/1
0/1
1/1
0/1
0/1
0/1
0/1
1/1
1/1
1/1
0/1
0/1
0/1
1/1
1/1
0/1
0/1
0/1
0/1
1/1
1/1
1/1
0/1
1/1
1/1
1/1
1/1
0/1
0/1
0/1
0/1
1/1
1/1
1/1
0/1
0/1
1/1
0/1
0/1
0/1
1/1
0/1
0/1
1/1
0/1
1/1
0/1
0/1
1/1
0/1
0/1
0/1
1/1
0/1
0/1
1/1
1/1
1/1
0/1
0/1
1/1
0/1
0/1
1/1
1/1
0/1
0/1
1/1
1/1
0/1
0/1
0/1
0/1
0/1
1/1
1/1
1/1
1/1
1/1
0/1
0/1
0/1
0/1
0/1
0/1
0/1
1/1
1/1
1/1
1/1
1/1
0/1
0/1
0/1
1/1
1/1
0/1
0/1
1/1
1/1
1/1
1/1
1/1
1/1
1/1
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
4/4
0/4
0/4
0/4
0/4
0/4
0/4
0/4
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
0/2
10
11
12
a)
Maximal electroshock.
Subcutaneous Metrazol.
b)
c)
Rotorod test.0/1: No activity at dose level; 1/1: noticeable activity at dose level; 0/4: number of animals exhibited toxicity/number
of animals tested for 0.5 h; 0/2: number of animals exhibiting toxicity/number of animals tested for 4 h.
ity of a drug is an important factor in connection with its pound 7 was established to be protective at 0.5 h at the
transfer into the central spinal fluid and the brain. Struc- 300 mg/kg dose. Besides this, compound 12 showed activ-
ture-activity relationship studies show that most of anti- ity at the 100 and 300 mg/kg doses after 0.5 h.
convulsant drugs contain a phenyl ring and a carbonyl,
In the MES-induced seizure test in this series, com-
or other electronegative groups adjacent to the phenyl pounds 1, 6, 11, and 12 were determined to protect
ring [1]. Also, substitution of different lipophylic phenyl against seizures at all doses at 4 h. While compounds 6
derivatives at the piperidine ring enables the penetration and 12 are carrying the same moiety at the 2-position,
of the blood-brain barrier. Therefore, in this study, the they have differences at the 6-position on the pyran-
effects of mono substitution of the phenyl group with 4(1H)-one ring. Also, compounds 1 and 11 which are car-
electron-withdrawing groups at the para-position were rying a hydroxyl and acetyl moiety at the 4-position on
examined. Moreover, the effect of six different piperidine the piperidine ring, respectively, were found to have anti-
rings on biological activity was investigated in both ser- convulsant activity at the same doses. Some compounds
ies. Different mono- or di-substitutions of the 4-position like 3 and 4 exhibited activity at the 300 mg/kg dose and
of the piperidine ring were also investigated for their compound 5 was also active at 100 and 300 mg/kg doses
influence on the activity.
at 4 h. Considering the results at 0.5 h, while compounds
The anticonvulsant activities of the compounds were 1, 2, 8, and 9 showed activity at the 300 mg/kg dose, com-
initially evaluated against MES and scMet seizure tests pounds 4 and 6 were protective against seizures at 100
induced at 0.5 and 4 h after administration with 30, 100, and 300 mg/kg doses.
and 300 mg/kg doses using male Swiss albino mice (20 l 2
In Mannich bases of allomaltol derivatives, 4-bromo-
g). Preliminary screening results are presented in Table 2.
phenyl and 4-chlorophenyl derivatives (compounds 2
With respect to the results of the anticonvulsant activ- and 3, respectively) were found to have lower activity
ity studies, 2-{[4-(4-bromophenyl)-4-hydroxypiperidin-1- than compound 1 which has a non-substituted phenyl
yl]methyl}-3-hydroxy-6-(hydroxymethyl)-4H-pyran-4-one
ring. Conversely, in Mannich bases of kojic acid deriv-
8 was the most active compound against scMet-induced atives, compound 8 which is carrying a 4-bromophenyl
seizures at all doses at 0.5 and 4 h. In the same test, both moiety was the most active compound against scMet-
of the non-substituted phenyl derivatives (compound 7 induced seizure tests in this series. While the presence of
and 10) were highly selective and were found to be active an electron-rich group attached to the phenyl ring
against -induced seizures at all doses after 4 h. Also, com- showed increased potency in scMet-induced seizure test
pound 9, which is carrying a 4-chlorophenyl moiety, has in kojic acid derivatives (compounds 8 and 9), this
same activity in the scMet test but is not selective. While potency was decreased for allomaltol derivatives. On the
compounds 2, 3, 11, and 12 showed activity at 4 h, com- other hand, when comparing the results of this study
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

178
M. Dilsiz Aytemir and ꢀ. C˛ alıs˛
Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
Thomas Hoover Capillary Melting Point Apparatus (Philadel-
phia, PA, USA) and were uncorrected. IR spectra were recorded
on a Perkin Elmer FT-IR Spectrometer 1720 X (Perkin Elmer, Bea-
between the two series, replacement of the hydroxy-
methyl with a methyl group at the 6-position of the pyra-
none ring increased the protective effect, because of the
two hydrogen bonds of kojic acid, which are located
between the keto and hydroxyl group and/or the
hydroxymethyl moiety and the intra-ring oxygen.
Besides this, compounds 6 and 12, which are carrying the
same moiety, have anticonvulsant activity.
In the series of kojic acid derivatives, the compounds
of 4-hydroxypiperidine with substituted 4-bromophenyl
and 4-chlorophenyl have higher activity against scMet-
induced seizure tests than MES-induced seizure tests.
Replacement of hydroxy with cyano or acetyl groups at
the 4-position of the piperidine ring has a similar protec-
tive effect when compared with 4-hydroxy-4-phenypiperi-
dine. None of the compounds showed neurotoxicity
according to the rotorod test at any of the doses studied.
consfield, UK) and Bruker Vector 22 IR as KBr disc (k, in cm– 1
;
1
Opus Spectroscopic Software Version 2.0). H-NMR spectra were
obtained on a Bruker DPX 400 MHz High Performance Digital FT
NMR (Bruker, Karlsruhe, Germany) and a Bruker Spectrospin
Avance DPX-400 MHz Ultra Shield Superconducting NMR
(Bruker, Rheinstetten, Germany) spectrophotometer using TMS
as an internal standard (chemical shift in d, ppm). 13C-NMR and
13C-DEPT spectra were recorded on a Bruker DPX 400 MHz High
Performance Digital FT NMR spectrophotometer. Mass analysis
was carried out with a Micromass ZQ LC-MS (Micromass, Man-
chester, UK) with Masslynx Software Version 4.1 by using ESI (+)
method. The elemental analyses were performed with a Leco
CHNS-932 (Leco, St. Joseph, MI, USA) at The Scientific & Techno-
logical Research Council of Turkey-Ankara Testing and Analyses
Laboratory (TꢀBITAK-ATAL). The purity of the compounds was
assessed by thin layer chromatography (TLC) on Kieselgel 60 F254
chromatoplates (Merck, Darmstadt, Germany).
2-Chloromethyl-5-hydroxy-4H-pyran-4-one (Chlorokojic
acid)
Conclusion
Chlorokojic acid was synthesized as described by Ellis et al. [17].
Yield: 76%; m.p.: 166–1678C.
A series of novel Mannich bases of kojic acid or allomal-
tol, namely 3-hydroxy-6-hydroxymethyl/methyl-2-(substi-
tuted piperidin-1-yl)methyl-4H-pyran-4-ones were synthe-
sized and studied for anticonvulsant activity in MES- and
scMet-induced seizure tests. In this series, for all doses at
0.5 and 4 h, compound 8 was determined to be the most
active against scMet-induced seizures. At all doses, com-
pounds 1, 4, 6, 7, 9, 10, and 11 were also protective. Com-
pound 1, 4, 11, and 12 were found to have a significantly
high anticonvulsant activity at 4 h against MES-induced
seizures. In the rotorod neurotoxicity screening none of
the compounds showed toxicity at any dose.
The results of this study revealed that the anticonvul-
sant activity of Mannich bases of allomaltol derivatives
(compounds 1–6) has effectively increased in comparison
with our previous studies [6–8]. As for the present study,
kojic acid derivatives were more active than allomaltol
derivatives. It can be suggested that the increase in activ-
ity depends on the structure of kojic acid, because it
includes two hydrogen bonds. Generally, most of the syn-
thesized compounds (1, 4, 6, 7, 8, 9, 10, 11, and 12) seemed
to be promising candidates for new anticonvulsant com-
pounds.
2-Methyl-5-hydroxy-4H-pyran-4-one (Allomaltol)
Chlorokojic acid (30 g, 0.187 mol, 1 equiv.) was added to 100 mL
of distilled water and heated to 508C with stirring. Zinc dust
(24.4 g, 0.375 mol, 2 equiv.) was added followed by the dropwise
addition of concentrated hydrochloric acid (56.1 mL, 3 equiv.)
over 1 h with vigorous stirring maintaining the temperature
between 70–808C. The reaction mixture was stirred for another
3 h at 708C. The excess zinc was removed by hot filtration and
the filtrate extracted with dichloromethane (36200 mL). The
combined organic extracts were dried over anhydrous sodium
sulphate, filtered, and concentrated in vacuum to yield the
crude product. Recrystallization from isopropanol afforded allo-
maltol as colourless plates (14.8 g, 63%). M.p.: 152–1538C (lit.:
153–1558C) [17]. IR (KBr disc) k [cm– 1]: 1640 (C=O st), 1587 (C=C
st), 1223, 1150 (C-O st); 1H-NMR (DMSO-d6) d [ppm]: 2.25 (s, 3H, 2-
CH3), 6.10 (s, 1H, H3), 6.30–7.15 (br, 1H, -OH), 7.80 (s, 1H, H6).
General preparation of Mannich bases of allomaltol
derivatives 1–6
Mannich bases were prepared by the reaction of substituted
piperidine derivatives (0.01 mol) and allomaltol (0.01 mol) in
methanol with 37% formaline. The mixture was stirred vigo-
rously for 15 to 25 min. The resulting precipitate was collected
by filtration and washed with cold methanol. The crude product
was recrystallized from appropriate solvents.
3-Hydroxy-2-[(4-hydroxy-4-phenylpiperidin-1-yl)methyl]-
6-methyl-4H-pyran-4-one 1
Experimental
Recrystallization from methanol gave a white powder. IR (KBr
disc) k [cm– 1]: 1662 (C=O st), 1588, 1464 (C=C st), 1232, 1197 (C-O
st); 1H-NMR (DMSO-d6) d [ppm]: 1.78 (d, J = 12.34 Hz, 2H, piperi-
dine), 2.21 (t, J = 12.60 Hz, 2H, piperidine), 2.47 (s, 3H, 6-CH3), 2.71
(t, J = 10.75 Hz, 2H, piperidine), 2.77 (d, J = 10.40 Hz, 2H, piperi-
dine), 3.53 (s, 1H, -OH), 3.73 (s, 2H, -CH2-), 6.43 (s, 1H, H5), 7.39 (t,
Chemistry
All chemicals used for the synthesis of the compounds were sup-
plied by Merck (Darmstadt, Germany) and Aldrich Chemical Co.
(Steinheim, Germany). Melting points were determined on a
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
Novel Kojic Acids and Allomaltol Derivatives
179
1H, phenyl H49), 7.50 (t, 2H, phenyl H39, H59), 7.67 (d, J = 7.49 Hz, 2H,
phenyl H29, H69); MS (ESI) m/z: 316 [M+ + 1] (100), 338 [M+ + 23] (57).
2-{[4-(4-Chlorophenyl)-3,6-dihydropyridin-1(2H)-
yl]methyl}-3-hydroxy-6-methyl-4H-pyran-4-one 6
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1655 (C=O st), 1585
(C=C st), 1217 (C-O st); 1H-NMR (DMSO-d6) d [ppm]: 2.18 (s, 3H, -6-
CH3), 1.78 (t, J = 1.60 Hz, 2H, piperidine), 2.21 (t, J = 5.60 Hz, 2H,
piperidine), 3.07 (d, J = 2.70 Hz, 2H, piperidine), 3.54 (s, 2H, -CH2-),
6.10 (t, 1H, -CH=C=), 6.16 (s, 1H, H5), 7.30 (d, J = 8.56 Hz, 2H, phe-
nyl H29, H69), 7.36 (d, J = 8.62 Hz, 2H, phenyl H39, H59); MS (ESI) m/z:
161 (100), 332 [M+ + 1] (36), 334 [M+ + 3] (12), 354 [M+ + 23] (44).
2-{[4-(4-Bromophenyl)-4-hydroxypiperidin-1-yl]methyl}-3-
hydroxy-6-methyl-4H-pyran-4-one 2
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1619 (C=O st), 1464
(C=C st), 1225 (C-O st); 1H-NMR (CDCl3) d [ppm]: 1.66 (d, J = 12.28
Hz, 2H, piperidine), 2.06 (t, J = 11.90 Hz, 2H, piperidine), 2.20 (s,
3H, 6-CH3), 2.59 (t, J = 13.18 Hz, 2H, piperidine), 2.76 (d, J = 11.22
Hz, 2H, piperidine), 3.37 (s, 1H, -OH), 3.61 (s, 2H, -CH2-), 6.09 (s,
1H, H5), 7.26 (d, J = 8.60 Hz, 2H, phenyl), 7.36 (d, J = 8.58 Hz, 2H,
phenyl); 13C-NMR (CDCl3 + DMSO-d6) d [ppm]: 20.44 (6-CH3), 38.40
and 49.71 (piperidine -CH2), 56.06 (-CH2-), 69.98 (piperidine =C=),
77.90 and 104.91 (phenyl =C=), 112.10 (C5), 120.64, 127.29 and
131.38 (phenyl -CH-), 144.26 (C3), 148.96 (C2), 165.13 (C6), 174.47
(C4); 13C-DEPT (CDCl3 + DMSO-d6) d: 20.44 (6-CH3), 38.40 and 49.71
(piperidine -CH2, –1.95, –1.98), 56.06 (-CH2-, –0.50), 112.10,
127.29, 131.38 (-CH); MS (ESI) m/z: 394 [M+] (98), 396 [M+ + 2] (100),
417 [M+ + 23] (25).
General preparation of Mannich bases of kojic acid
derivatives 7–12
Mannich bases were prepared by the reaction of substituted
piperidine derivatives (0.01 mol) and kojic acid (0.01 mol) in
methanol with 37% formaline. The mixture was stirred vigo-
rously for 15 to 25 min. The resulting precipitate was collected
by filtration and washed with cold methanol. The crude product
was recrystallized from appropriate solvents.
3-Hydroxy-6-(hydroxymethyl)-2-[(4-hydroxy-4-
phenylpiperidin-1-yl)methyl]-4H-pyran-4-one 7
2-{[4-(4-Chlorophenyl)-4-hydroxypiperidin-1-yl]methyl}-3-
hydroxy-6-methyl-4H-pyran-4-one 3
Recrystallization from methanol gave a white powder. IR (KBr
disc) k [cm– 1]: 1660 (C=O st), 1582, 1465 (C=C st), 1205 (C-O st); 1H-
NMR (DMSO-d6) d [ppm]: 1.58 (d, J = 12.60 Hz, 2H, piperidine),
1.95 (t, J = 12.56 Hz, 2H, piperidine), 2.59 (t, J = 10.85 Hz, 2H,
piperidine), 2.66 (d, J = 10.35 Hz, 2H, piperidine), 3.17 (s, 1H, -OH),
3.56 (s, 2H, -CH2-), 4.31 (s, 2H, HOCH2-), 6.34 (s, 1H, H5), 7.19 (t, J =
7.25 Hz, 1H, phenyl H49), 7.30 (t, J = 7.59 Hz, 2H, phenyl H39, H59),
7.46 (d, J = 7.54 Hz, 2H, phenyl H29, H69); MS (ESI) m/z: 332 [M+ + 1]
(100), 334 [M+ + 3] (3), 354 [M+ + 23] (28).
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1618 (C=O st), 1464
(C=C st), 1221 (C-O st); 1H-NMR (CDCl3) d [ppm]: 1.69 (d, J = 14.23
Hz, 2H, piperidine), 2.09 (t, J = 13.10 Hz, 2H, piperidine), 2.22 (s,
3H, 6-CH3), 2.65 (t, J = 11.96 Hz, 2H, piperidine), 2.82 (d, J = 9.05
Hz, 2H, piperidine), 3.40 (s, 1H, -OH), 3.63 (s, 2H, -CH2-), 6.12 (s,
1H, H5), 7.22–7.36 (m, 4H, phenyl); MS (ESI) m/z: 350 [M+ + 1]
(100), 352 [M+ + 3] (34), 372 [M+ + 23] (13).
1-[(3-Hydroxy-6-methyl-4-oxo-4H-pyran-2-yl)methyl]-4-
phenylpiperidine-4-carbonitrile 4
2-{[4-(4-Bromophenyl)-4-hydroxypiperidin-1-yl]methyl}-3-
hydroxy-6-(hydroxymethyl)-4H-pyran-4-one 8
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1621 (C=O st), 1497
(C=C st), 1217 (C-O st); 1H-NMR (DMSO-d6) d [ppm]: 1.90–1.99 (m,
4H, piperidine), 2.14 (s, 3H, 6-CH3), 2.36 (t, J = 11.37 Hz, 2H, piper-
idine), 2.88 (d, J = 12.03 Hz, 2H, piperidine), 3.48 (s, 2H, -CH2-),
6.13 (s, 1H, H5), 7.24 (t, 1H, phenyl ring H49), 7.31 (t, 2H, phenyl
ring H39, H59), 7.41 (d, J = 7.47 Hz, 2H, phenyl H29, H69); 13C-NMR
(DMSO-d6) d [ppm]: 20.29 (6-CH3), 36.29 and 50.95 (piperidine
-CH2), 42.58 (piperidine =C=), 54.31 (-CH2-), 112.12 (C5), 122.12
(phenyl =C=), 126.50, 128.91 and 129.87 (phenyl -CH-), 140.99
(-CN), 144.33 (C3), 147.29 (C2), 165.60 (C6), 174.43 (C4); 13C-DEPT
(DMSO-d6) d [ppm]: 20.29 (6-CH3), 36.29 and 50.95 (piperidine
-CH2, –1.69, –1.60), 54.31 (-CH2-, –0.90), 112.12, 126.50, 128.91,
129.87 (-CH); MS (ESI) m/z: 161 (100), 325 [M+ + 1] (10), 347 [M+ + 23]
(38).
Recrystallization from chloroform gave a white powder. IR (KBr
disc) k [cm– 1]: 1659 (C=O st), 1464 (C=C st), 1203 (C-O st); 1H-NMR
(DMSO-d6) d [ppm]: 1.57 (d, J = 12.71 Hz, 2H, piperidine), 1.93 (t, J =
12.36 Hz, 2H, piperidine), 2.57–2.69 (m, 4H, piperidine H29, H69),
3.18 (s, 1H, -OH), 3.57 (s, 2H, -CH2-), 4.32 (s, 2H, HOCH2-), 6.34 (s,
1H, H5), 7.42 (d, J = 8.48 Hz, 2H, phenyl H39, H59), 7.47 (d, J = 8.43 Hz,
2H, phenyl H29, H69); MS (ESI) m/z: 410 [M+] (100), 412 [M+ + 2] (95),
433 [M+ + 23] (34).
2-{[4-(4-Chlorophenyl)-4-hydroxypiperidin-1-yl]methyl}-3-
hydroxy-6-(hydroxymethyl)-4H-pyran-4-one 9
Recrystallization from chloroform gave a white powder. IR (KBr
disc) k [cm– 1]: 1660 (C=O st), 1581, 1486 (C=C st), 1203 cm– 1 (C-O
1
st); H-NMR (DMSO-d6) d [ppm]: 1.56 (d, J = 12.46 Hz, 2H, piperi-
dine), 1.90 (t, J = 12.65 Hz, 2H, piperidine), 2.50–2.67 (m, 4H,
piperidine), 3.17 (s, 1H, -OH), 3.56 (s, 2H, -CH2-), 4.31 (s, 2H,
HOCH2-), 4.93 (s, 1H, -OH), 6.33 (s, 1H, H5), 7.35 (d, J = 8.59 Hz, 2H,
phenyl H39, H59), 7.49 (d, J = 8.55 Hz, 2H, phenyl H29, H69); MS (ESI)
m/z: 226 (100), 366 [M+ + 1] (86), 368 [M+ + 3] (28), 388 [M+ + 23] (41).
2-[(4-Acetyl-4-phenylpiperidin-1-yl)methyl]-3-hydroxy-6-
methyl-4H-pyran-4-one 5
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1697 (C=O st, car-
bonyl), 1657 (C=O st, pyranone), 1587, 1485 (C=C st), 1215 (C-O
st); 1H-NMR (CDCl3) d [ppm]: 1.83 (s, 3H, -COCH3), 2.07 (t, J = 10.90
Hz, 2H, piperidine), 2.21 (s, 3H, 6-CH3), 2.38–2.43 (m, 4H, piperi-
dine), 2.78 (d, J = 11.46 Hz, 2H, piperidine), 3.53 (s, 2H, -CH2-), 6.12
(s, 1H, H5), 7.18–7.29 (m, 5H, phenyl); MS (ESI) m/z: 342 [M+ + 1]
(46), 347 [M+ + 23] (100).
1-{[3-Hydroxy-6-(hydroxymethyl)-4-oxo-4H-pyran-2-
yl]methyl}-4-phenylpiperidine-4-carbonitrile 10
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1654 (C=O st), 1594,
1468 (C=C st), 1209 (C-O st); 1H-NMR (DMSO-d6) d [ppm]: 2.02–2.12
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

180
M. Dilsiz Aytemir and ꢀ. C˛ alıs˛
Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
(m, 4H, piperidine), 2.52 (t, J = 11.94 Hz, 2H, piperidine), 2.99 (d,
J = 12.14 Hz, 2H, piperidine), 3.63 (s, 2H, -CH2-), 4.32 (s, 2H,
HOCH2-), 5.60–8.80 (br, 1H, -OH), 6.36 (s, 1H, H5), 7.36 (t, J = 7.20
Hz, 1H, phenyl H49), 7.43 (t, J = 7.58 Hz, 2H, phenyl H39, H59), 7.53 (d,
J = 7.59 Hz, 2H, phenyl H29, H69), 8.98 (s, 1H, -OH); MS (ESI) m/z: 177
(100), 341 [M+ + 1] (36), 363 [M+ + 23] (60).
(s.c.) on the back of the neck. The rotorod toxicity test was per-
formed on a 1-inch-diameter knurled wooden rod, rotating at
6 rpm (the rotorod used in phase-I test was made by Hacettepe
University Technical Department).
Maximal electroshock (MES)-induced seizure test
MES seizures were elicited with a 60-cycle alternating current of
50 mA intensity (5 to 7 times more than that required to elicit
minimal seizures) delivered for 0.2 s via corneal electrodes. A
drop of 0.9% saline was instilled into the eye prior to application
of the electrodes in order to prevent the death of the animal.
Abolition of the hind limb tonic extension component of the
seizure was defined as protection.
2-[(4-Acetyl-4-phenylpiperidin-1-yl)methyl]-3-hydroxy-6-
(hydroxymethyl)-4H-pyran-4-one 11
Recrystallization from chloroform/petroleum ether (40–608C)
gave a white powder. IR (KBr disc) k [cm– 1]: 1697 (C=O st, car-
bonyl), 1652 (C=O st, pyranone), 1588, 1469 (C=C st), 1198 cm– 1
(C-O st);1H-NMR (DMSO-d6) d [ppm]: 1.87 (s, 3H, -COCH3), 1.96 (t, J =
10.09 Hz, 2H, piperidine), 2.30 (t, J = 10.71 Hz, 2H, piperidine),
2.37 (d, J = 13.22 Hz, 2H, piperidine), 2.62 (d, J = 11.09 Hz, 2H,
piperidine), 3.45 (s, 2H, -CH2-), 4.28 (s, 2H, HOCH2-), 5.60–5.70 (br,
1H, -OH), 6.30 (s, 1H, H5), 7.25–7.40 (m, 5H, phenyl); MS (ESI) m/z:
358 [M+ + 1] (100), 380 [M+ + 23] (80).
Subcutaneous Metrazol (scMet)-induced seizure test
85 mg/kg of Metrazol (produces seizures in more than 95% of
mice) was administered as a 0.5% solution s.c. into the posterior
midline. The animal was observed for 30 min to decide whether
the failure of the threshold seizure (a single episode of clonic
spasms of at least 5 s duration) could be defined as protection.
2-{[4-(4-Chlorophenyl)-3,6-dihydropyridin-1(2H)-
yl]methyl}-3-hydroxy-6-hydroxymethyl-4H-pyran-4-one
Neurotoxicity
The rotorod test was used to evaluate neurotoxicity. The animal
was placed on a 1-inch-diameter knurled wooden rod rotating at
6 rpm. Normal mice remain on a rod rotating at this speed indef-
initely. Neurologic toxicity was defined as the failure of the ani-
mal to remain on the rod for 1 min.
12
Recrystallization from methanol gave a white powder. IR (KBr
disc) k [cm– 1]: 1652 (C=O st), 1558, 1456 (C=C st), 1199 (C-O st); 1H-
NMR (DMSO-d6) d [ppm]: 2.51 (br-s, 2H, piperidine), 2.73 (t, J = 5.48
Hz, 2H, piperidine), 3.16 (d, J = 1.95 Hz, 2H, piperidine), 3.64 (s,
2H, -CH2-), 4.31 (s, 2H, HOCH2-), 5.60–5.70 (br, 1H, -OH), 6.18 (t,
1H, -CH=C=, piperidine), 6.34 (s, 1H, H5), 7.30 (d, J = 8.48 Hz, 2H,
phenyl H39, H59), 7.36 (d, J = 8.51 Hz, 2H, phenyl H29, H69), 8.99 (s, 1H,
OH); MS (ESI) m/z: 177 (100), 348 [M+ + 1] (58), 350 [M+ + 3] (18), 370
[M+ + 23] (52).
This study was supported by TUBITAK (Project no: TBAG 2021)
and the Hacettepe University Research Center Office (Project
no: 03 02 301 001). We would like to thank Professor Erhan
Palaska for his help with the mass spectra analysis.
Anticonvulsant assay
The authors have declared no conflict of interest.
The compounds were tested for their anticonvulsant activity
against maximal electroshock (MES)- and subcutaneous Metra-
zol (scMet)-induced seizure threshold tests. The acute neurologi-
cal toxicity was determined in the rotorod test. All these tests
were performed in male mice according to the phase-I tests of
the Antiepileptic Drug Development (ADD) program which were
developed by National Institutes of Health (NIH), National Insti-
tute of Neurological Disorders and Stroke (NINDS) [5]. This pro-
gram was used for the screening of many compounds in various
previous studies [6–10]. Stimulator (Grass S88, Astro-Med. Inc.
Grass Instrument Division, W. Warwick, RI, USA), constant cur-
rent unit (Grass CCU1A, Grass Medical Instrument, Quincy, MA,
USA), and corneal electrodes were used for the evaluation of
anticonvulsant activity against MES test. All synthesized com-
pounds were suspended in 30% aqueous of PEG 400 and adminis-
tered to the mice intraperitoneally in a volume of 0.01 mL/g at
body weight. Twelve Swiss albino male mice (20 l 2 g) were used
for each compound (mice were obtained from the Hacettepe Uni-
versity Animal Farm) according to the ADD-NINDS program [5].
The animals were kept under standard conditions at an ambient
temperature of 25 l 28C and allowed free access to food and
water except at the time they were brought out of the cage. All
References
[1] A. Korolkovas in Essentials of Medicinal Chemistry, John
Wiley & Sons, New York 1988, Second Edition.
[2] C. L. P. Deckers, P. Genton, G. J. Sills, D. Schmidt, Epilepsy
Res. 2003, 53, 1–17.
[3] W. Lꢁscher, Trends Pharmacol Sci. 2002, 23, 113–118.
[4] B. Malawska, Curr. Top. Med. Chem. 2005, 5, 69–85.
[5] R. L. Krall, J. K. Penry, B. G. White, J. H. Kupferberg, E. A.
Swinyard, Epilepsia 1978, 9, 409–428.
[6] M. D. Aytemir, ꢀ. ꢂalı˛s, M. ꢃzalp, Arch. Pharm. 2004, 337,
281–288.
[7] M. D. Aytemir, ꢀ. ꢂalı˛s, FABAD J. Pharm. Sci. 2006, 31, 23–
29.
[8] M. D. Aytemir, ꢀ. ꢂalı˛s, HU J. Fac. Pharm. 2007, 27, 1–10.
[9] M. D. Aytemir, E. Septioglu, ꢀ. ꢂalı˛s, Arzneim.-Forsch./Drug
Res. 2010, 1, 22–29.
the experimental protocols were carried out with the permis-
,
[10] E. Septioglu, M. D. Aytemir, ꢀ. ꢂalı˛s, Arzneim.-Forsch./Drug
Res. 2005, 55, 259–264.
sion from Hacettepe University, Laboratory Animals Ethic Com-
mittee’ decision (17. 04. 2002 date 2002/ 24-3 number and 08. 04.
2003 date 2003/ 58-2 number). Control animals received 30%
aqueous PEG 400. Metrazol was administered subcutaneously
[11] J. Brtko, L. Rondahl, M. Fickova, D. Hudecova, et al., Cent.
Eur. J. Public Health 2004, 12, 16–18.
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com

Arch. Pharm. Chem. Life Sci. 2010, 343, 173–181
Novel Kojic Acids and Allomaltol Derivatives
181
[12] M. D. Aytemir, D. D. Erol, R. C. Hider, M. ꢃzalp, Turk. J.
Chem. 2003, 27, 757–764.
[20] K. Zborowski, R. Grybos, L. M. Proniewicz, J. Mol. Struct.
2003, 639, 87–100.
[13] M. Uher, V. Konecny, O. Rajniakova, Chem. Pap. –Chem.
Zvesti. 1994, 48, 282–284.
[21] J. Lokaj, J. Kozisek, B. Koren, M. Uher, V. Vrabel, Acta Cryst.
1991, C47, 193–194.
[14] W. Kandioller, C. G. Hartinger, A. A. Nazarov, J. Kasser, et
al., J. Organomet. Chem. 2009, 694, 922–929.
[22] K. Hryniewicz, K. Stadnicka, A. Pattek-Janczyk, J. Mol.
Struct. 2009, 919, 255–270.
[15] G. ꢃztꢄrk, D. D. Erol, M. D. Aytemir, T. Uzbay, Eur. J. Med.
Chem. 2002, 37, 829–834.
[23] N. O. Iskeleli, S. Isik, M. D. Aytemir, Acta Cryst. 2005, E61,
1947–1949.
[16] T. Yabuta, J. Chem. Soc. 1924, 123, 575–587.
[24] Y. Kꢁysal, S. Isik, M. D. Aytemir, Acta Cryst. 2004, E60, 112–
114.
[17] B. L. Ellis, A. K. Duhme, R. C. Hider, M. B. Hossain, et al., J.
Med. Chem. 1996, 39, 3659–3670.
[25] G. J. O'brien, M. Patterson, J. R. Meadow, J. Org. Chem. 1960,
25, 86–89.
[18] Z. D. Liu, S. Piyamongkol, D. Y. Liu, H. H. Khodr, et al., Bio-
org. Med. Chem. 2001, 9, 563–573.
[26] L. L. Woods, J. Am. Chem. Soc. 1946, 68, 2744–2745.
[19] A. Beelik, C. B. Purves, Can. J. Chem. Soc. 1955, 33, 1361–
[27] L. S. Dehkordi, Z. D. Liu, R. C. Hider, Eur. J. Med. Chem.
2008, 43, 1035–1047.
1374.
i 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.archpharm.com