Synthesis and in vitro evaluation of 2-heteroarylidene-1-tetralone derivatives as monoamine oxidase inhibitors

Article abstract of DOI:10.1055/a-0620-8309

The present study investigates the human monoamine oxidase (MAO) inhibition properties of a series of twelve 2-hetero-arylidene-1-tetralone derivatives. Also included are related cyclohexylmethylidene, cyclopentylmethylidene and ben-zylidene substituted 1

Full text of DOI:10.1055/a-0620-8309

Original Article
Thieme
Amakali KlaudiaT et al. Synthesis and in vitro… Drug Res
2018; 00: 00–00
Synthesis and in vitro Evaluation of 2-heteroarylidene-1-tetralone
Derivatives as Monoamine Oxidase Inhibitors
Authors
Klaudia T. Amakali , Lesetja J. Legoabe , Anél Petzer1, 2, Jacobus P. Petzer1, 2
1
1
AꢀꢁTꢂAꢃT
Affiliations
1
Centre of Excellence for Pharmaceutical Sciences,
North-West University, Potchefstroom, South Africa
Pharmaceutical Chemistry, School of Pharmacy,
North-West University, Potchefstroom, South Africa
The present study investigates the human monoamine oxidase
(MAO) inhibition properties of a series of twelve 2-hetero-
arylidene-1-tetralone derivatives. Also included are related
cyclohexylmethylidene, cyclopentylmethylidene and ben-
zylidene substituted 1-tetralones. These compounds are re-
lated to the 2-benzylidene-1-indanone class of compounds
which has previously been shown to inhibit the MAOs, with
specificity for the MAO-B isoform. The target compounds were
synthesised by the Claisen-Schmidt condensation between
2
Key words
monoamine oxidase, inhibition, Parkinson’s disease,
Alzheimer’s disease, heteroarylidene-1-tetralone,
2
-benzylidene-1-tetralone
7
-methoxy-1-tetralone or 1-tetralone, and various aldehydes,
under acid (hydrochloric acid) or base (potassium hydroxide)
catalysis. The results of the MAO inhibition studies showed
that the 2-heteroarylidene-1-tetralone and related derivatives
are in most instances more selective inhibitors of the MAO-B
isoform compared to MAO-A. (2E)-2-Benzylidene-7-meth-
oxy-3,4-dihydronaphthalen-1(2H)-one (IC50 = 0.707 μM) was
found to be the most potent MAO-B inhibitor, while the most
potent MAO-A inhibitor was (2E)-2-[(2-chloropyridin-3-yl)
methylidene]-7-methoxy-3,4-dihydronaphthalen-1(2 H)-one
receivedꢀ
acceptedꢀ 19.04.2018
14.03.2018
Bibliography
DOI https://doi.org/10.1055/a-0620-8309
Online-Publication: 2018
Drug Res
©
Georg Thieme Verlag KG Stuttgart · New York
ISSN 2194-9379
(
IC50 = 1.37 μM). The effect of the heteroaromatic substituent
Correspondence
on MAO-B inhibition activity, in decreasing order was found to
be: cyclohexyl, phenyl > thiophene > pyridine, furane, pyrrole,
cyclopentyl. This study concludes that, although some 2-het-
eroarylidene-1-tetralone derivatives are good potency MAO
inhibitors, in general their inhibition potencies, particularly for
MAO-B, are lower than structurally related chalcones and 1-in-
danone derivatives that were previously studied.
Lesetja J. Legoabe
Centre of Excellence for Pharmaceutical Sciences
North-West University
Private Bag X6001
2
520 Potchefstroom
South Africa
Tel.: + 27/182/992 182
lesetja.legoabe@nwu.ac.za
Introduction
heimer’s disease [5, 6]. Examples of MAO-B inhibitors that are used
in the clinic for the treatment of Parkinson’s disease are selegiline
and rasagiline, while sembragiline is an example of a MAO-B inhib-
itor that is specifically being developed for the treatment of Alzhei-
mer’s disease (▶Fig. 1). MAO-A is a major serotonin metabolising
enzyme in the brain and is thus a drug target for the treatment of
depression [7]. MAO-A inhibitors that have been used for the treat-
ment of depressive illness include tranylcypromine, moclobemide
and toloxatone. An important consideration in MAO inhibitor de-
sign is the reversibility of inhibition. Irreversible MAO-A inhibitors
are used with caution due to a potentially fatal hypertensive event
that may occur when these medications are combined with
tyramine-containing food. This adverse event is termed the “cheese
The monoamine oxidases (MAOs) are enzymes responsible for the
metabolism of monoamine neurotransmitters such as serotonin,
norepinephrine and dopamine, thereby regulating their concen-
trations in the central and peripheral tissues [1]. The MAOs exist as
two isoforms, MAO-A and MAO-B, which are products of distinct
genes. Although the amino acid sequences of the MAO isoforms
are 70% identical they differ significantly in their three-dimension-
al structures, tissue distributions, and substrate and inhibitor spe-
cificities [2–4]. The MAOs also have different physiological roles.
MAO-B is a key enzyme for the metabolism of dopamine in the
human brain and represents a target for the development of drugs
for the treatment of Parkinson’s disease and more recently also Alz-
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

Original Article
Thieme
▶
Fig. 1 The structures of MAO-B specific inhibitors in clinical use for the treatment of parkinson’s disease (selegiline and rasagiline) and alzheimer’s
disease (sembragiline), and the structures of MAO-A inhibitors that have been used for the treatment of depression (tranylcypromine, moclobemide
and toloxatone).
reaction” and occurs when MAO-A is irreversibly inhibited [8–10].
Since tyramine is metabolised by MAO-A in the gastrointestinal
tract, irreversible inhibition of MAO-A leads to increased systemic
concentrations of tyramine. Tyramine is a sympathomimetic amine
and causes the release of noradrenaline from peripheral neurons
leading to an increase in blood pressure. Reversible MAO-A inhibi-
tors are considered to be safer in this regard and do not potentiate
the sympathomimetic effects of tyramine [11, 12]. For the design
of MAO inhibitors, a high degree of specificity for MAO-B or a re-
versible mode of MAO-A inhibition are therefore desirable charac-
teristics [13].
The current study further explores the structure-activity rela-
tionships (SARs) for chalcones as MAO inhibitors by introducing
conformational restriction and heteroaromatic substitution. This
will be done by cyclising the structure of chalcone to yield a series of
twelve 2-heteroarylidene-1-tetralone derivatives (4a–l) (▶Fig. 3).
These compounds are not only related to heterocyclic chalcones
that have been reported to act as MAO-B specific inhibitors, but
are also derivatives of the 2-benzylidene-1-indanone class of com-
pounds which has previously been shown to inhibit the MAOs, also
with specificity for the MAO-B isoform [19–21]. For example, series of
2-benzylidene-1-indanones were found to act as specific inhibitors
of human MAO-B with compound 5 exhibiting the highest potency
MAO-B inhibition [20]. Interestingly, some 2-benzylidene-1-indanone
derivatives are also MAO-A inhibitors as exemplified by compound
6 (IC50 =0.131μM). Similar to the open-chain chalcones, heterocy-
clic derivatives of 2-benzylidene-1-indanone also exhibit MAO in-
hibition. In this respect, compounds 7 (IC50 = 0.061 μM) and 8
(IC50 = 0.0044 μM) were found to be respectively the most potent
inhibitors of MAO-A and MAO-B[21].
Chalcones are α,β-unsaturated aromatic ketones that are es-
2
sentially planar due to the presence of carbon atoms with sp hy-
bridization. Traditional medicinal use of chalcone-containing plants
led to the discovery of their potential therapeutic applications.
Chalcones possess a broad spectrum of pharmacological activities
including antimalarial, anticancer, anti-inflammatory, antifungal,
antimicrobial, antioxidant and chemopreventive activities [14,15].
Of interest to us are reports that chalcones act as inhibitors of the
MAO enzymes. For example, Chimenti and coworkers (2009) found
that, with the appropriate substitution on the phenyl rings, syn-
thetic chalcone derivatives may act as high potency inhibitors of
the human MAOs. This is exemplified by chalcone (1), which inhib-
its human MAO-B with an IC50 value of 0.0044 µM (▶Fig. 2) [16].
This compound is highly specific for the MAO-B isoform and does
not inhibit MAO-A at concentrations up to 50 µM. Another study
has recently investigated the human MAO inhibition properties of
a series of furanochalcones and discovered potent MAO-B-selec-
tive inhibitors with the most active compound (2) exhibiting an IC50
value of 0.174 µM [17]. Other heterocyclic chalcones have also
been found to inhibit MAO-B potently and with high specificity as
exemplified by 3, which possesses an IC50 value of 0.067 µM [18].
For the purpose of this study, the 2-heteroarylidene-1-tetralone
derivatives (4a–l) will possess methoxy substitution (except 4d) on
the tetralone phenyl ring (ring A) while a variety of heterocycles (pyr-
idine, furan, pyrrole, thiophene) will be introduced as rings B. For
comparison, the effect of non-heteroaromatic systems (cyclohexyl,
cyclopentyl) and the phenyl ring as rings B will also be investigated.
Materials and Methods
Chemicalsꢀandꢀinstrumentation
All reagents and solvents were from Sigma-Aldrich and were used
without further purification. Column chromatography was carried
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

▶
Fig. 2 Examples of chalcone derivatives that act as MAO inhibitors.
out with silica gel 60 (Fluka, particle size 0.063–0.200mm) while
thin-layer chromatography (TLC) was performed on 0.20mm thick
aluminium sheets coated with silica gel 60 (Macherey-Nagel). H
and 13C nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker Avance III 600 spectrophotometer at 600 MHz and
and an Agilent 1200 series diode array detector. A Venusil XBP C18
column (4.60×150mm, 5μm) was used for separation and the mo-
bile phase consisted initially of 30 % acetonitrile and 70 % MilliQ
water at a flow rate of 1mL/min. At the start of each HPLC run a sol-
vent gradient program was initiated by linearly increasing the com-
position of the acetonitrile in the mobile phase to 85% acetonitrile
over a period of 5min. Each HPLC run lasted 15min and a time pe-
riod of 5min was allowed for equilibration between runs. A volume
of 20 µL of solutions of the test compounds in acetonitrile (1mM)
was injected into the HPLC system and the eluent was monitored
at wavelengths of 210, 254 and 300nm.
1
1
51 MHz, respectively, and CDCl or DMSO-d6 served as NMR sol-
3
vent. Chemical shifts are reported in parts per million (∂) and were
referenced to the residual solvent signal (CDCl : 7.26 and
3
1
7
3
7.16 ppm for H and 13C, respectively; DMSO-d6: 2.50 and
9.52ppm for H and 13C, respectively). Spin multiplicities are given
1
as s (singlet), d (doublet), t (triplet), dd (doublet of doublets), dt
doublet of triplets), td (triplet of doublets), ddd (doublet of dou-
blet of doublets) and m (multiplet). High resolution mass spectra
HRMS) were recorded on a Bruker micrOTOF-Q II mass spectrom-
(
Synthesisꢀandꢀcharacterisationꢀofꢀ4a–l
(
The 2-heteroarylidene-1-tetralone derivatives (4a–k) were synthe-
sised via the base-catalysed Claisen-Schmidt condensation reac-
tion of 1-tetralone or 7-methoxy-1-tetralone (2.27mmol), and the
appropriate aldehydes (2.497mmol). The reactants and potassium
hydroxide (4.54mmol) were dissolved in 5mL methanol and stirred
for 10–24 h at room temperature. Compound 4l was synthesised
via the Claisen-Schmidt condensation reaction using concentrated
hydrochloric acid (22mL) as a catalyst. The products were precipi-
tated by addition of a minimum of 20mL water and the crude prod-
ucts were collected by filtration and recrystallised from a suitable
solvent (e. g. ethanol).
eter in atmospheric-pressure chemical ionisation (APCI) mode.
Melting points (mp) were determined on a Buchi B-545 melting
point apparatus and are uncorrected. Fluorescence measurements
were conducted with a Varian Cary Eclipse fluorescence spectro-
photometer. Microsomes from insect cells containing recombinant
human MAO-A and MAO-B (5mg/mL), kynuramine dihydrobromide,
lazabemide and toloxatone were obtained from Sigma-Aldrich.
Chemical purity was determined by high performance liquid
chromatography (HPLC). HPLC analyses were performed using an
Agilent 1200 series HPLC system equipped with a quaternary pump
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

Original Article
Thieme
▶
Fig. 3 The 2-heteroarylidene-1-tetralone derivatives (4a–l) investigated in this study.
(
2E)-2-(Cyclohexylmethylidene)-7-methoxy-3,4-
1
crystals, mp 86.8–89.1 °C (ethanol). H NMR (CDCl ) δ 8.71 – 8.65
(
3
dihydronaphthalen-1(2ꢀH)-oneꢀ(4a)
m, 1 H), 7.75 – 7.67 (m, 2 H), 7.60 (d, J = 2.7 Hz, 1 H), 7.42 (d,
The title compound is a product of 7-methoxy-3,4-dihydronaph-
thalen-1(2 H)-one and cyclohexanecarbaldehyde in a yield of
J = 7.8 Hz, 1 H), 7.22 – 7.14 (m, 2H), 7.06 (dd, J= 8.3, 2.8 Hz, 1 H),
3.85 (s, 3 H), 3.59 – 3.52 (m, 2 H), 2.95 – 2.88 (m, 2 H); 13C NMR
1
1
0.6 %: light brown crystals, mp 94.0–95.1 °C (ethyl acetate). H
(CDCl ) δ ppm 188.16, 158.54, 155.36, 149.43, 139.18, 136.75,
3
NMR (CDCl ) δ ppm 7.56 (d, J=2.7Hz, 1H), 7.13 (d, J=8.3Hz, 1H),
136.34, 134.01, 133.53, 129.54, 126.96, 122.64, 121.67, 110.21,
3
+
7
2
1
1
1
.02 (dd, J = 8.3, 2.8 Hz, 1 H), 6.74 (d, J = 9.7 Hz, 1 H), 3.83 (s, 3 H),
.89 – 2.82 (m, 2 H), 2.75 (t, J = 6.0 Hz, 2 H), 2.42 – 2.31 (m, 1 H),
55.52, 27.93, 27.01. APCI-MS (m/z) calcd for C17H16NO [MH] ,
2
266.1176, found 266.1166. Purity (HPLC): 100 %.
.80 – 1.50 (m, 5 H), 1.37 – 1.16 (m, 5 H); 13C NMR (CDCl ) δ ppm
3
(
2E)-2-(Pyridin-3-ylmethylidene)-3,4-
87.86, 158.49, 145.13, 136.40, 134.50, 133.24, 129.40, 121.14,
dihydronaphthalen-1(2ꢀH)-oneꢀ(4d)
10.20, 55.51, 37.27, 32.24, 28.46, 25.95, 25.82, 25.63. APCI-MS
+
(
m/z) calcd for C18H23O . [MH] , 271.1693, found 271.1723. Pu-
The title compound is a product of 3, 4-dihydronaphthalen-1(2H)-
2
rity (HPLC): 99.5 %.
one and nicotinaldehyde in a yield of 29.7 %: yellow crystals, mp
1
7
1
2
7.5–77.7 °C (petroleum ether). H NMR (CDCl ) δ ppm 8.67 (s,
H), 8.59 – 8.49 (m, 1 H), 8.11 (d, J = 7.8 Hz, 1 H), 7.81 – 7.66 (m,
H), 7.48 (td, J = 7.5, 1.1 Hz, 1 H), 7.30 (ddd, J = 56.7, 10.9, 6.5 Hz,
3
(
2E)-2-(Cyclopentylmethylidene)-7-methoxy-3,4-
dihydronaphthalen-1(2ꢀH)-oneꢀ(4b)
The title compound is a product of 7-methoxy-3,4-dihydronaph-
3 H), 3.08 (dd, J = 9.1, 3.8 Hz, 2 H), 2.95 (t, J = 6.4 Hz, 2 H); 13C NMR
thalen-1(2 H)-one and cyclopentanecarbaldehyde in a yield of
(CDCl ) δ ppm 187.27, 150.49, 149.13, 143.08, 137.46, 136.77,
133.54, 133.11, 132.50, 131.70, 128.28, 128.24, 127.14, 123.30,
3
1
2
4.6 %: yellow oil (ethyl acetate). H NMR (CDCl ) δ ppm 7.56 (d,
3
+
J=2.7Hz, 1H), 7.13 (d, J=8.3Hz, 1H), 7.02 (dd, J=8.3, 2.8Hz, 1H),
28.71, 27.15. APCI-MS (m/z) calcd for C H NO [MH] , 236.1070,
1
6 14
6
1
1
3
.84 (d, J=9.8Hz, 1H), 3.82 (d, J=4.2Hz, 3H), 2.92 – 2.73 (m, 5H),
found 236.1062. Purity (HPLC): 98.5 %
.96 – 1.08 (m, 8 H); 13C NMR (CDCl ) δ ppm 187.67, 158.50,
3
(
2E)-7-Methoxy-2-(pyridin-3-ylmethylidene)-3,4-
45.29, 136.40, 134.50, 133.54, 129.40, 121.12, 110.22, 55.51,
8.90, 33.34, 28.36, 25.96, 25.61. APCI-MS (m/z) calcd for
dihydronaphthalen-1(2ꢀH)-oneꢀ(4e)
+
C H O [MH] , 257.1536, found 257.1552. Purity (HPLC): 97.1%.
The title compound is a product of 7-methoxy-3,4-dihydronaph-
17
21
2
thalen-1(2 H)-one and nicotinaldehyde in a yield of 72.2%: peach
(
2E)-7-Methoxy-2-(pyridin-2-ylmethylidene)-3,4-
1
crystals, mp 105.4–107.1°C (ethanol). H NMR (CDCl ) δ ppm 8.68
3
dihydronaphthalen-1(2ꢀH)-oneꢀ(4c)
(
s, 1H), 8.56 (d, J=2.9Hz, 1H), 7.76 (s, 1H), 7.71 (d, J=7.9Hz, 1H),
The title compound is a product of 7-methoxy-3,4-dihydronaph-
thalen-1(2 H)-one and picolinaldehyde in a yield of 45.3%: yellow
7.59 (d, J = 2.7 Hz, 1 H), 7.34 (dd, J = 7.7, 4.9 Hz, 1 H), 7.15 (d,
J=8.4Hz, 1H), 7.07 (dd, J=8.3, 2.8Hz, 1H), 3.85 (s, 3H), 3.06 (dd,
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

J = 9.1, 3.7 Hz, 2 H), 2.89 (t, J = 6.4 Hz, 2 H); 13C NMR (CDCl ) δ ppm
(2E)-7-Methoxy-2-(thiophen-2-ylmethylidene)-3,4-
dihydronaphthalen-1(2ꢀH)-oneꢀ(4j)
3
1
1
87.22, 158.71, 150.50, 149.12, 137.49, 136.76, 135.83, 133.91,
32.54, 129.53, 123.31, 121.83, 110.28, 55.54, 27.92, 27.36. AP-
The title compound is a product of 7-methoxy-3,4-dihydronaph-
+
CI-MS (m/z) calcd for C17H16NO , [MH] , 266.1176, found
thalen-1(2 H)-one and thiophene-2-carbaldehyde in a yield of
2
1
2
66.1176. Purity (HPLC): 98.6 %.
86.2 %: yellow crystals, mp 106.5–116.9 °C (ethanol). H NMR
(
CDCl ) δ ppm 8.02 (s, 1 H), 7.58 (d, J = 2.7 Hz, 1 H), 7.49 (d,
3
(
2E)-7-Methoxy-2-(pyridin-4-ylmethylidene)-3,4-
J = 5.0 Hz, 1 H), 7.38 (d, J = 3.4 Hz, 1 H), 7.19 – 7.08 (m, 2 H), 7.05
dd, J = 8.3, 2.8 Hz, 1 H), 3.85 (s, 3 H), 3.16 (dd, J = 9.3, 3.8 Hz, 2 H),
dihydronaphthalen-1(2ꢀH)-oneꢀ(4ꢀf)
(
The title compound is a product of 7-methoxy-3,4-dihydronaph-
thalen-1(2 H)-one and isonicotinaldehyde in a yield of 66.9%: yel-
2.95 (t, J = 6.5 Hz, 2 H); 13C NMR (CDCl ) δ ppm 187.05, 158.63,
139.13, 135.67, 134.42, 133.23, 131.76, 129.47, 129.33, 129.31,
127.54, 121.26, 110.27, 55.52, 27.27, 27.19. APCI-MS (m/z) calcd
3
1
low crystals, mp 136.9–137.6°C (ethanol). H NMR (CDCl ) δ ppm
3
+
8
.64 (d, J = 5.5 Hz, 2 H), 7.68 (s, 1 H), 7.58 (d, J = 2.7 Hz, 1 H), 7.25
for C16H15O S [MH] , 271.0787, found 271.0765. Purity (HPLC):
2
(
t, J = 5.7 Hz, 2 H), 7.15 (d, J = 8.4 Hz, 1 H), 7.07 (dd, J = 8.3, 2.8 Hz,
100 %.
1
2
1
5
2
H), 3.84 (s, 3 H), 3.03 (dd, J = 9.0, 3.7 Hz, 2 H), 2.88 (t, J = 6.4 Hz,
H); 13C NMR (CDCl ) δ ppm 187.14, 158.72, 149.97, 143.49,
(2E)-7-Methoxy-2-(thiophen-3-ylmethylidene)-3,4-
dihydronaphthalen-1(2ꢀH)-oneꢀ(4k)
3
38.84, 135.92, 133.77, 133.11, 129.57, 123.78, 121.97, 110.25,
+
5.52, 27.87, 27.32. APCI-MS (m/z) calcd for C17H16NO [MH] ,
The title compound is a product of 3,4-dihydronaphthalen-1(2 H)-
one and thiophene-3-carbaldehyde in a yield of 83.0%: cream crys-
2
66.1176, found 266.1189. Purity (HPLC): 100 %.
1
tals, mp 135.4–136.9 °C (ethanol). H NMR (CDCl ) δ ppm 7.82 (s,
3
(
2E)-2-[(2-Chloropyridin-3-yl)methylidene]-7-
1
3
H), 7.58 (d, J=2.8Hz, 1H), 7.50 (d, J=2.5Hz, 1H), 7.36 (dd, J=4.9,
.0 Hz, 1 H), 7.28 (dd, J = 5.0, 0.7 Hz, 1 H), 7.15 (d, J = 8.3 Hz, 1 H),
methoxy-3,4-dihydronaphthalen-1(2ꢀH)-oneꢀ(4ꢀg)
The title compound is a product of 7-methoxy-3,4-dihydronaph-
thalen-1(2 H)-one and 2-chloronicotinaldehyde in a yield of 75%:
7.05 (dd, J=8.3, 2.8Hz, 1H), 3.85 (s, 3H), 3.15 – 3.08 (m, 2H), 2.90
(t, J=6.5Hz, 2H); 13C NMR (CDCl ) δ ppm 187.55, 158.62, 137.36,
3
1
yellow crystals, mp 131.0–132.0 °C (ethanol). H NMR (CDCl ) δ
135.76, 134.38, 133.76, 130.52, 129.35, 128.96, 127.71, 125.83,
121.35, 110.25, 55.52, 27.59, 27.39. APCI-MS (m/z) calcd for
3
ppm 8.36 (dd, J=4.7, 1.5Hz, 1H), 7.75 (s, 1H), 7.65 – 7.59 (m, 2H),
+
7
.28 (dd, J = 7.5, 4.8 Hz, 1 H), 7.15 (d, J = 8.4 Hz, 1 H), 7.07 (dd,
C16H15O S [MH] , 271.0787, found 271.0805. Purity (HPLC):
2
J = 8.4, 2.8 Hz, 1 H), 3.85 (s, 3 H), 2.89 (s, 4 H); 13C NMR (CDCl ) δ
99.5 %.
3
ppm 187.00, 158.73, 151.37, 149.05, 138.76, 138.61, 135.96,
(
2E)-2-Benzylidene-7-methoxy-3,4-
1
2
3
33.77, 131.12, 131.04, 129.62, 122.01, 121.99, 110.25, 55.54,
+
dihydronaphthalen-1(2ꢀH)-oneꢀ(4ꢀl)
8.00, 27.49. APCI-MS (m/z) calcd for C17H15NO Cl [MH] ,
2
00.0786, found 300.0803. Purity (HPLC): 99.4 %
The title compound is a product of 7-methoxy-3,4-dihydronaph-
thalen-1(2 H)-one and benzaldehyde in a yield of 51.2%: pale yel-
low crystals, mp 121.5–122.7 °C (dichloromethane). H NMR
(
2E)-2-(Furan-2-ylmethylidene)-7-methoxy-3,4-
1
dihydronaphthalen-1(2ꢀH)-oneꢀ(4ꢀh)
(
CDCl ) δ ppm 7.85 (s, 1 H), 7.61 (d, J = 2.8 Hz, 1 H), 7.41 (dt,
3
The title compound is a product of 7-methoxy-3,4-dihydronaph-
J=15.2, 7.4Hz, 4H), 7.33 (t, J=7.0Hz, 1H), 7.15 (d, J=8.3Hz, 1H),
7.06 (dd, J = 8.3, 2.8 Hz, 1 H), 3.85 (s, 3 H), 3.09 (td, J = 6.7, 1.6 Hz,
thalen-1(2 H)-one and furan-2-carbaldehyde in a yield of 85.5 %:
1
2H), 2.90 – 2.84 (m, 2 H); 13C NMR (CDCl ) δ ppm 187.80, 158.62,
yellow crystals, mp 90.1–90.9 °C (petroleum ether). H NMR
3
(
(
CDCl ) δ ppm 7.60 – 7.52 (m, 3 H), 7.15 (d, J = 8.3 Hz, 1 H), 7.04
dd, J = 8.3, 2.8 Hz, 1 H), 6.69 (d, J = 3.4 Hz, 1 H), 6.50 (dd, J = 3.3,
.8Hz, 1H), 3.84 (s, 3H), 3.32 – 3.25 (m, 2H), 2.95 – 2.89 (m, 2H);
136.68, 135.92, 135.85, 135.43, 134.23, 129.84, 129.42, 128.49,
128.40, 121.50, 110.26, 55.52, 28.02, 27.34. APCI-HRMS m/z calcd
3
+
1
1
for C18H17O [MH] , 265.1223, found 265.1212. Purity (HPLC):
2
3
C NMR (CDCl ) δ ppm 187.27, 158.57, 152.48, 144.30, 136.22,
99.2 %.
3
1
5
2
34.29, 131.80, 129.36, 122.85, 121.27, 116.53, 112.19, 110.20,
+
DeterminationꢀofꢀIC50ꢀvalues
5.51, 27.49, 26.84. APCI-MS (m/z) calcd for C16H15O [MH] ,
3
55.1016, found 255.1011. Purity (HPLC): 100 %.
The IC50 values for the inhibition of human MAO-A and MAO-B were
determined as described in literature [22]. All enzymatic reactions
were carried out in 96-well microtiter plates in potassium phos-
phate buffer (100mM, pH 7.4) to a final volume of 200μL. The re-
actions contained MAO-A (0.0075 mg protein/mL) or MAO-B
(0.015mg protein/mL), the mixed MAO-A/B substrate kynuramine
(50 μM) and the test inhibitors (0.003–100μM). DMSO was added
to the reactions to yield a final concentration of 4 % (v/v) DMSO.
Control reactions were carried out in the absence of inhibitor and
also contained 4% DMSO. After initiating the reactions with the ad-
dition of the MAO enzyme, the reactions were incubated at 37 °C
for 20 min and terminated with the addition of 80μL NaOH (2 N).
At endpoint, the concentrations of the MAO generated 4-hydrox-
yquinoline in the reactions were measured by fluorescence spec-
(
2E)-7-Methoxy-2-(1ꢀH-pyrrol-2-ylmethylidene)-3,4-
dihydronaphthalen-1(2ꢀH)-oneꢀ(4i)
The title compound is a product of 7-methoxy-3,4-dihydronaph-
thalen-1(2 H)-one and 1 H-pyrrole-2-carbaldehyde in a yield of
3
1
5 %: yellow crystals, mp 186.1–190.2 °C (ethanol). H NMR
(
DMSO) δ ppm 11.53 (s, 1H), 7.70 (s, 1 H), 7.39 (d, J = 2.7 Hz, 1 H),
7
6
2
1
1
.27 (d, J=8.4Hz, 1H), 7.14 – 7.05 (m, 2H), 6.64 (d, J=2.7Hz, 1H),
.27 (s, 1H), 3.78 (s, 3H), 3.00 (t, J=6.2Hz, 2H), 2.87 (t, J=6.6Hz,
H); 13C NMR (DMSO) δ ppm 185.60, 158.10, 135.34, 134.36,
29.59, 128.55, 127.42, 126.95, 122.60, 120.02, 113.46, 110.90,
10.08, 55.23, 26.56, 26.40. APCI-MS (m/z) calcd for C16H15NO2
+
[
MH] , 254.1176, found 254.1182. Purity (HPLC): 99.1 %.
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

Original Article
Thieme
trophotometry (λex =310nm, λem =400nm) [23]. To quantify 4-hy-
droxyquinoline, a linear calibration curve was constructed from so-
lutions of authentic 4-hydroxyquinoline (0.0469–1.5μM). The rates
of kynuramine oxidation were calculated and the rate data were
fitted to the one site competition model incorporated into the Prism
software package (GraphPad). From the resulting sigmoidal curves
tralone derivatives, sigmoidal graphs (rate of enzyme catalysis ver-
sus logarithm of inhibitor concentration) were constructed with
the Prism 5 software package (GraphPad), from which IC50 values
were estimated.
The potencies by which the 2-heteroarylidene-1-tetralone de-
rivatives inhibit MAO-A and MAO-B are expressed as the IC50 values
and are given in ▶Table 1. The inhibitors exhibit moderate inhibi-
tory activities for MAO-A and MAO-B with the IC50 values mostly in
the micromolar range. With the exception of 4 g, the 2-heteroar-
ylidene-1-tetralone derivatives display little isoform selectivity.
Compound 4g is a MAO-A selective inhibitor and does not inhibit
MAO-B, even at a maximal tested concentration of 100 µM. This
compound also is the most potent MAO-A inhibitor of the series
with an IC50 value of 1.37 µM. The most potent MAO-B inhibitor of
the series is the phenyl substituted derivative, 4l (IC50 =0.707µM).
Although the IC50 values are for the most part within a narrow
range, some preliminary SARs may be derived: (1) Among the de-
rivatives evaluated, phenyl substitution (4l) is the most optimal for
MAO-B inhibition while the 2-chloro-3-pyridine moiety (4g) yields
weakest MAO-B inhibition. 2-Chloro-3-pyridine and phenyl substi-
tution, however, are the most optimal for MAO-A inhibition; (2)
The thiophene substituted derivatives (4j, 4k) are more potent
MAO-B inhibitors than the pyridine (4c–g) substituted compounds;
(3) Substitution with the non-aromatic cyclohexyl ring (4a) also
yields relatively potent MAO-B inhibition. Interestingly, the cyclo-
pentane derivative (4b) is comparatively weaker as a MAO-B inhib-
itor; (5) In general the effect of substitution on MAO-B inhibition
potency in decreasing order is: cyclohexyl, phenyl>thiophene>pyr-
idine, furane, pyrrole, cyclopentyl; (6) Methoxy substitution on ring
A (4e) yields more potent MAO inhibition compared to the unsub-
stituted homologue (4d).
(
catalytic rate versus the logarithm of the inhibitor concentration),
the IC50 values were estimated. All experiments were carried out
in triplicate and the IC50 values are expressed as mean ± standard
deviation (SD).
Results
Chemistry
The 2-heteroarylidene-1-tetralone derivatives were synthesised by
the Claisen-Schmidt condensation between 7-methoxy-1-tetralone
(
9) or 1-tetralone (10) and various aldehydes in reactions catalysed
by KOH or HCl with methanol serving as solvent (▶Fig. 4). The
structures of the products were verified by H NMR, 13C NMR and
mass spectrometry while the purities were estimated by HPLC. On
1
1
the H NMR spectra of the 2-heteroarylidene-1-tetralone deriva-
tives the anticipated signal of the vinylic proton of the α,β-
unsaturated moiety was observed at approximately 7.6–8.0ppm.
Where appropriate the methoxy CH group was evident as a sharp
3
singlet in the region of δ 3.78–3.85ppm. On the 13C NMR spectra,
signals in the regions of δ 185.6–188.2ppm and δ 55.2–55.5ppm
represent the carbonyl and methoxy carbons, respectively.
MAOꢀinhibitionꢀstudies
For the inhibition studies, the catalytic activities of MAO-A and
MAO-B were determined by measuring the production of 4-hydro-
quinoline (4-HQ) from the oxidation of kynuramine by the MAOs.
Discussion
4
-HQ is fluorescent in basic media and can thus be measured by
fluorescence spectrophotometry at endpoint, after alkalinisation
of the enzyme reactions. Recombinant human MAO-A and MAO-B
served as enzyme sources [23]. By measuring MAO activities in the
presence of various concentrations of the 2-heteroarylidene-1-te-
The chalcone class of compounds is well known as inhibitors of the
MAOs, with specificity for the MAO-B isoform. This is exemplified
by the study of Chimenti et al. (2009) in which a series of chalcone
derivatives substituted on both phenyl rings exhibited very good
▶
Fig. 4 Synthetic routes to the 2-heteroarylidene-1-tetralone derivatives.
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

▶
Table 1 IC50 values for the inhibition of human MAO-A and MAO-B by 2-heteroarylidene-1-tetralone derivatives 4a–l and reference inhibitors.
O
R1
R2
Iꢃ50 (μM±SD)†
ꢃompd.
ꢂ1
ꢂ2
MAO-A
MAO-B
ꢁI‡
4
a
7-OMe
4.59 ± 2.11
0.895 ± 0.066
5.1
4
b
7-OMe
7-OMe
7- H
32.3 ± 4.85
8.85 ± 0.981
23.4 ± 0.055
8.22 ± 0.402
5.82 ± 0.541
1.37 ± 0.209
6.31 ± 0.349
14.5 ± 1.60
6.03 ± 0.062
3.86 ± 0.608
1.96 ± 0.064
5.46 ± 0.978
3.37 ± 0.580
6.26 ± 0.241
2.69 ± 0.373
6.04 ± 0.355
NI§
5.9
2.6
3.7
3.1
0.96
–
4
4
4
4
4
4
4
4
4
4
c
d
e
f
N
N
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
7-OMe
N
N
g
h
i
N
Cl
4.53 ± 0.629
4.69 ± 0.245
1.75 ± 0.126
1.03 ± 0.144
0.707 ± 0.088
1.4
3.1
3.4
3.7
2.8
O
H
N
j
S
k
l
S
Lazabemide
Toloxatone
Chrysin
†
202¶
0.091¶
–
2220
–
3.92¶
0.770 ± 0.05
0.79 ± 0.09
0.97
All values are reported as mean± standard deviation (SD) of triplicate experiments; ‡ Selectivity index (SI) = IC50(MAO-A)/IC50(MAO-B); § No inhibition
observed at a maximal tested concentration of 100 µM; ¶ Inhibition values for the reference inhibitors were obtained from literature [23]
specificity for MAO-B with potencies in the micro to nanomolar
range. The most potent MAO-B inhibitor, compound (1)
unsaturated ketone moiety of chalcone, although limited, is im-
portant for establishing productive interactions with the MAO-B
active site. Alternatively, the structures of the 2-heteroarylidene-
1-tetralone derivatives are “frozen” in a conformation that is not
optimal for MAO-B binding compared to the open chain chalcones.
To illustrate this, the calculated three-dimensional structures
(MMFF94) of chalcone derivative 1 and a 2-heteroarylidene-1-te-
tralone derivative (4l) are shown in ▶Fig. 5. As shown chalcone 1
only deviates slightly from planarity compared to 4l, which is fro-
zen in a conformation where the tetralone moiety and phenyl are
perpendicular to each other. The flexibility of 1 compared to 4l may
also, in part, explain its ability to better bind to the MAO-B active
(
IC50 =0.0044μM) is considerably better than the derivatives of the
current study. Furanochalcones such as 2 (IC50 = 0.174 μM) and
other heterocyclic chalcones such as 3 (IC50 = 0.067 μM) [17, 18]
also are more potent MAO-B inhibitors than the 2-heteroarylidene-
1
-tetralone derivatives of this study. This suggests that introduc-
ing conformational restriction and heteroaromatic substitution
such as with derivatives 4a–l do not improve MAO inhibition com-
pared to open chain chalcones and heterocyclic chalcones. Al-
though the molecular basis for this behaviour is not clear, it may be
suggested that conformational freedom and rotation of the α,β-
Amakali KT et al. MAO and 2-heteroarylidene-1-tetralones... Drug Res

Original Article
Thieme
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