Synthesis and biological activity of new 2,3-dihydro-1H-cyclopenta[b]- quinoline derivatives as acetylcholinesterase inhibitors

Article abstract of DOI:10.2174/157018012801319454

In this study we present synthesis and biological evaluation of derivatives of 4-fluorobenzoic acid and 2,3-dihydro-1H-cyclopenta[b]quinoline towards inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Synthesis of acquired molecul

Full text of DOI:10.2174/157018012801319454

Letters in Drug Design & Discovery, 2012, 9, 645-654
645
Synthesis and Biological Activity of New 2,3-dihydro-1H-cyclopenta[b]-
quinoline Derivatives as Acetylcholinesterase Inhibitors
Paweꢀ Szymaꢁski*,1, Magdalena Markowicz1, Marek Bajda*,2, Barbara Malawska2 and
Elꢂbieta Mikiciuk-Olasik1
1Department of Pharmaceutical Chemistry and Drug Analyses, Medical University, ul. Muszyꢀskiego 1, 90-151 Lodz,
2
Poland; Department of Physicochemical Drug Analysis, Chair of Pharmaceutical Chemistry, Faculty of Pharmacy,
Jagiellonian University Medical College, 30-688 Krakow, Medyczna 9, Poland
Received May 09, 2012: Revised May 26, 2012: Accepted May 30, 2012
Abstract: In this study we present synthesis and biological evaluation of derivatives of 4-fluorobenzoic acid and 2,3-
dihydro-1H-cyclopenta[b]quinoline towards inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE).
Synthesis of acquired molecules involved the reaction of condensation between the activated 4-fluorobenzoic acid and
amino derivatives of 2,3-dihydro-1H-cyclopenta[b]quinoline. Biological testing towards the inhibition of cholinesterases
was conducted according to the Ellman’s spectrophotometric method. Compounds 4b and 4e were found to be less active
in comparison with tacrine. However, compounds 4d, 4g and 4h showed similar activity to tacrine. Compounds 4a, 4c and
4f were more active towards inhibition of AChE than tacrine. Every synthesized compound displayed higher selectivity
towards AChE and lower selectivity towards BChE in comparison with tacrine apart from compound 4b. Compound 4b
was characterized by similar selectivity towards AChE and higher selectivity towards BChE than tacrine.
Keywords: Acetylcholinesterase inhibitors, Alzheimer’s disease, Cyclopenta[b]quinoline, Ellman method, Fluorobenzoic acid,
molecular docking.
INTRODUCTION
treatment. The first widely used AChEI, which has positive
influence on cognitive function, was tacrine. Second
XXI st century medicine still faces numerous issues
which at the current state of knowledge seem to be
unsolvable. One of them is Alzheimer’s disease (AD) which
is characterized by a progressive neurodegeneration of the
brain. Initially the symptoms (mainly memory deterioration)
are almost imperceptible and frequently are considered as a
common process connected with advanced age. But with
time passing by the signs become more apparent. Typical
AD symptoms include: progressive cognitive deterioration,
neuropsychiatric and behavioral disturbances and
impairment of everyday activities. [1,2] Pathologic lesions
include cellular degeneration and neuronal loss which
involve the cerebral cortex, initially affecting more temporal
and cortical regions than the parietal and occipital cortices
[3].
generation
AChEIs
(donepezil,
rivastigmine
and
galantamine) demonstrate greater efficacy in AD treatment.
[4-6] Due to adverse side effects of these drugs, to date, the
therapeutic strategies for the treatment of AD have been
mainly focused on the restoration of cholinergic
functionality. Furthermore, AChEIs do not only increase the
level of ACh but also have other functions in pathogenesis
and development of AD such as modulatory effects on
cerebral blood flow, amyloid cascade, and tau
phosphorylation of the inflammatory process [7]. Thus,
novel compounds, such as Schiff bases of isatin [8],
bisquaternary isoquinolinium derivatives [9], 7H-
thiazolo[3,2-b]-1,2,4-triazin-7-one derivatives [10] that
might increase the level of Ach, appear to be a promising
strategy to be explored by scientists. Synthesis of the
analogues of the approved drugs: donepezil, rivastigmine
and galantamine is still of interest for many research groups
[11,12]. The most promising novel compounds that belong to
tacrine derivatives are presented in Fig. (1). [7]
AD is associated with alterations of certain
neurotransmitter systems in the central nervous system. The
most significant changes concern a deficit of cholinergic
transmission. The acetylcholine deficiency leads to the
cholinergic treatment strategy whose aim is to increase the
concentration of this neurotransmitter. Medicines based on
this mechanism, acetylcholinesterase inhibitors (AChEIs),
are best-developed and widely approved drugs for AD
Development of new therapies for AD is extremely
important. Thus, in continuation of our research program
[13], we present study concerning new derivatives of 2,3-
dihydro-1H-cyclopenta[b]quinoline and fluorobenzoic acid.
Synthesized compounds were obtained by introducing the
side chain in position 9 into the structure of 2,3-dihydro-1H-
cyclopenta[b]quinoline. After that we analyzed those
molecules towards inhibition of AChE and BChE.
*Address correspondence to these authors at the Department of
Pharmaceutical Chemistry and Drug Analyses, Medical University, ul.
Muszyꢁskiego 1, 90-151 Lodz, Poland; Tel: +48-42-677-92-90;
Fax: +48-42-677-92-50; E-mail: pawel.szymanski@umed.lodz.pl
Department of Physicochemical Drug Analysis, Chair of Pharmaceutical
Chemistry, Faculty of Pharmacy, Jagiellonian University Medical College,
30-688 Krakow, Medyczna 9, Poland; Tel: +48-12-620-54-65;
E-mail: marek.bajda@uj.edu.pl
We introduced fluorobenzoic acid into the structure of
synthesized compound because nowadays the number of
drug candidates with one or more fluorine atoms is gradually
1875-628X/12 $58.00+.00
© 2012 Bentham Science Publishers

646 Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7
Szymaꢀski et al.
NH2
HN
N
O
N
R
N
N
R2
HN
(CH2)n
O
S
N
HN
HN
(CH2)n
(CH2)n
HN
HN
N
N
N
A
C
B
O
(CH2)n
NH
HN
N
S
S
R
D
Fig. (1). Analogues of tacrine –AChE inhibitors: A 6-Hydrazino-N-[ꢀ-(1,2,3,4-tetrahydroacridin-9-ylamino)alkilo]nicotinamide [8], B
Tacrine-thiadiazolidinone Hybrids[11], C Bis-tacrines [12], D Tacrine and Lipoic Acid as a hybrid derivatives [13].
increasing and seems to become commonplace.
Incorporation of fluorine atoms into drug candidate structure
enables to obtain enhanced binding interactions, metabolic
stability, changes in physical properties, and selective
reactivity. [14]
[16]. This molecule was obtained by the cyclization of
anthranilic acid with cyclopentanone in POCl3. In the next
step 1,ꢀ-diamines were attached to the heterocycle, in
similar reaction as presented in our previous paper [13].
Various 1,ꢀ-diamines with different length of the carbon
chain
reacted
with
9-chloro-2,3-dihydro-1H-
Therefore, scientific teams search for highly selective
and potent compounds with fluorine atoms in their structure
as AChE inhibitors. Hu et al. synthesized a number of
homodimeric tacrine congeners and estimated their effects
on rat AChE and human BChE. They found that bis-(6-
fluoro)-tacrines were more potent in inhibiting rat AChE
than tacrine and the unsubstituted bis-tacrine. [15] Thus, we
cyclopenta[b]quinoline and generated compounds (2a-2h)
(Scheme 1) [17-22]. Afterwards we coupled 4-fluorobenzoic
acid to compounds (2a-2h) in the presence of 2-chloro-4,6-
dimethoxy-1,3,5-triazine (CDMT). In this step the
carboxylic group of 4-fluorobenzoic acid was activated by
CDMT and N-methylmorpholine in tetrahydrofuran (THF).
Subsequently, compounds (2a-2h) were added in small
volume of THF to the solution of activated fluorobenzoic
acid [23,24]. The mixture was mixed for 24 h. The resulting
compounds (3a-3h) were then converted into hydrochlorides
(4a-4h) by crystallization from HCl in ether (Scheme 2).
Barreiro et al. synthesized a series of derivatives of THA,
where the A ring was replaced by a pyrazol ring with a
phenyl substituent at different positions [25]. Unfortunately,
all tested compounds appeared to be less active than tacrine.
Also tacrine analogues such as pyrano[2,3-b]quinoline,
decided to synthesize
a
series of 2,3-dihydro-1H-
cyclopenta[b]quinolone derivatives which were substituted
by 4-fluorobenzoic acid.
INVESTIGATIONS AND RESULTS
Synthesis
The synthesis of 9-chloro-2,3-dihydro-1H-cyclopenta[b-
]quinoline were described in patent of Han-Zhoung Zhang
2a
: n=2
2d
: n=5
2g
: n=8
2b: n=3 2e: n=6 2h: n=9
NH2
2c
: n=4
2f
: n=7
(CH2)n
O
HN
Cl
a
b
OH
NH2
N
N
1
2a-2h
Scheme 1. Synthesis of compounds 1, 2a – 2h. Reagents: a: cyclopentanone, POCl3, reflux; b: 1,ꢀ-diamine, phenol, NaI, reflux.

2,3-dihydro-1H-cyclopenta[b]quinoline Derivatives as Acetylcholinesterase Inhibitors
Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7 647
2a, 3a, 4a: n=2
2b, 3b, 4b: n=3
2c, 3c, 4c: n=4
2d, 3d, 4d: n=5
2e, 3e, 4e: n=6
2f, 3f, 4f: n=7
2g, 3g, 4g: n=8
2h, 3h, 4h: n=9
O
O
HN
(CH2)n
NH2
HN
(CH2)n
(CH2)n
HN
F
HN
HN
F
HCl
a
b
*
N
N
N
4a-4h
3a-3h
2a-2h
Scheme 2. Synthesis of compounds 3a – 3h and 4a – 4h. Reagents: a: 4-fluorobenzoic acid, CDMT, methylmorpholine, THF; b: HCl/ether.
Table 1. IC50 Values for Activities Towards Acetylcholinesterase and Butyrylcholinesterase
Compounds
AChE IC50,
BChE IC50,
Selectivity
for AChEc
Selectivity
for BChEd
[nM] ±SEMa
[nM] ±SEMb
4a
1.07 ±0.07
10.80 ±0.50
0.49 ±0.012
5.83 ±0.25
4.59 ±1.50
4.3
0.4
0.2
2.3
4b
4.70 ±0.30
5.91 ±0.60
4c
12.1
2.4
0.1
4d
14.00 ±1.40
559.00 ±4.32
717.00 ±7.07
699.00 ±1.16
744.00 ±9.89
2.44 ±0.60
0.4
4e
153.00 ±5.16
2.77 ±0.19
5.20 ±0.90
5.48 ±0.20
5.46 ±1.00
3.7
0.3
4f
258.8
134.4
135.8
0.4
0.004
0.008
0.007
2.2
4g
4h
tacrine
a Inhibitor concentration (means ± SEM of three experiments) for 50% inactivation of AChE
b Inhibitor concentration (means ± SEM of three experiments) for 50% inactivation of BChE
cSelectivity for AChE is defined as IC50(BChE)/IC50(AChE)
dSelectivity for BChE is defined as IC50(AChE)/IC50(BChE)
pyrano[3,2-e]pyridine and [1,8]naphthyridine synthesized by
Marco and co-workers were less active in comparison with
tacrine [26,27].
AChE was compound 4f. Apart from 4b, every synthesized
compound was less selective towards BChE than tacrine.
Within this study we describe the synthesis of 8
derivatives of 2,3-dihydro-1H-cyclopenta[b]quinoline which
differed in the number of carbon atoms in chain linking 2,3-
dihydro-1H-cyclopenta[b]quinoline with 4-fluorobenzoic
acid.
Molecular Modeling Studies
Docking studies showed the binding mode of novel
compounds. Each derivative presented similar conformation
in the active center. The most interesting compound was 4c
due to its highest activity against acetylcholinesterase.
GoldScore function for this compound was equaled 70.14
(AChE IC50 0.49 nM) in comparison with 51.42 for tacrine
(IC50 5.46 nM) and 80.72 for bis-(7)-tacrine (IC50 0.2 nM
[15]). It occurred in extended conformation, interacting with
catalytic and peripheral active site of AChE (Fig. 2). The
four carbon linker provided the best fit of the outermost
moieties to both active sites. The cyclopentaquinoline
fragment was engaged in formation of a characteristic
sandwich due to ꢀ-ꢀ stacking with Trp84 and Phe330. Its
protonated form created H-bond with His440 carbonyl
group. The tether was organized in the middle of the gorge
and it formed hydrophobic interactions with Tyr334. The
linker amine and amide groups could create weak hydrogen
bonds with carbonyl group of Ser81 and hydroxyl substituent
in Tyr121, respectively. The benzamide phenyl ring
Biological Activity
The inhibitory activity of the synthesized compounds
towards
cholinesterases
was
evaluated
by
spectrophotometric Ellman’s method [28]. According to the
procedure pioneered by Ellman, we acquired the following
data concerning anticholinesterase activity of the newly
synthesized compounds (Table 1). Furthermore, values of
the relative inhibitory effects towards acetylcholinesterase
(ratio IC50 BChE/AChE) and towards butyrylcholinesterase
(ratio IC50 AChE/BChE) are presented in Table 1.
Every synthesized compound, apart from 4b was
characterized by higher selectivity towards AChE than
tacrine. The most active compound 4c was 55-fold more
selective for AChE than tacrine. The most selective towards

648 Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7
Szymaꢀski et al.
Fig. (2). Binding mode of compound 4c with acetylcholinesterase.
interacted with Trp279 and Tyr70, forming the other
sandwich. It was also engaged in hydrophobic interactions
with Tyr121. Derivative 4c appeared in bent conformation in
the active gorge of butyrylcholinesterase (Fig. 3). For this
compound ChemScore function equaled 41.38 (BChE IC50
5.91 nM) in comparison with 52.63 for tacrine (IC50 2.44
nM). The cyclopentaquinoline moiety created ꢀ-ꢀ stacking
with Trp82, was engaged in hydrophobic interaction with
Trp430, Tyr440 and Met437 and could form weak H-bond
with carbonyl group of His438. The tetramethylene linker
was located near Tyr332. The fluorobenzamide moiety was
placed in hydrophobic pocket which was built of Phe329,
Leu286, Trp231, Val288 and Phe398. The compound 4c was
the most promising inhibitor among the whole series because
of the highest potency against AChE and high activity
against BChE. It fitted well into the active gorge of both
cholinesterases, especially acetylcholinesterase.
EXPERIMENTAL SECTION
Chemistry
Anhydrous Na2SO4 was used to dry organic solutions
during work-up and the removal of solvents was carried out
under vacuum with a rotary evaporator. Progress of the
reactions was monitored by TLC using 25 DC-Alufolien
Kieselgel 60F254 (Merck), and detection was done by a UV
Lamp (254 nm). Column chromatography was performed
using silica gel 60 (200-400 mesh, Merck). IR spectra were
recorded in KBr using a Mattson Infinity Series FT-IR
Fig. (3). Binding mode of compound 4c with butyrylcholinesterase.

2,3-dihydro-1H-cyclopenta[b]quinoline Derivatives as Acetylcholinesterase Inhibitors
Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7 649
1
spectrophotometer. H NMR spectra were recorded with a
Varian
197.0, 185.0; MS-HR (FAB) Calc. for C15H19 N3: 241.1579
Found: 241.1579.
Mercury 300 MHz spectrometer, using
tetramethylsilane as an internal standard. Mass spectra were
performed by the Centre of Molecular and Macromolecular
Studies in Lodz (Polish Academy of Sciences).
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)butane-1,4-
diamine (2c)
A mixture of 1 (0.71g, 3.5 mmol), 1,6-diaminohexane
(0.81g, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2c as an oil. Compound 2c:
yield 52%; IR (film) ꢀ (cm-1): 1566.8 (NH2), 2865.0
(CH2CH2CH2), 2934.1(CH2CH2CH2), 3304.1 (ArN); 1H
NMR (CD3OD) (ꢁ ppm.): 8.0 (1H, d, J = 7.6 Hz, ArH), 7.7
(1H, d, J = 7.3 Hz, ArH), 7.5 (1H, t, J = 6.8 Hz, ArH), 7.4
(1H, t, J = 8.3 Hz, ArH), 3.6 (2H, t, J = 6.3 Hz, NHCH2), 3.2
(2H, t, J = 7.1 CH2NH2), 2.9 (2H, t, J = 7.8 Hz, CH2), 2.7
(2H, t, J = 7.1 Hz CH2), 2.1 (2H, p, J = 7.6, 7.6 Hz CH2), 1.6
-1.8 (4H, m, CH2);MS (FAB) m/z (M+1) 256.2, 197.0,
185.0; MS-HR (FAB) Calc. for C16H21N3: 255.1735 Found:
255.1733.
9-chloro-2,3-dihydro-1H-cyclopenta[b]quinoline (1)
To a mixture of anthranilic acid (7.4 g, 53.9 mmol) and
cyclopentanone (1.64 ml, 53.9 mmol) 30 ml of POCl3 was
carefully added in an ice bath. The mixture was heated under
reflux for 2 h, then cooled at room temperature, and
concentrated under reduced pressure to give slurry. The
residue was diluted with ethyl acetate (50 ml), neutralized
with aqueous K2CO3 (30 ml), and washed with brine (2 x 20
ml). The organic layer was dried over MgSO4 and
concentrated to dryness under reduced pressure to give a
brown solid. Recrystallization from acetone gave the desired
product 1 as yellow solid. Compound 1: mp 85-87° C; yield
54%; IR (KBr) ꢀ (cm-1): 766.2 (Cl), 1607.2 (Ar), 2955.1
(CH2CH2CH2), 2920.7(CH2CH2CH2), 3417.8(ArN); 1H
NMR (CDCl3) (ꢁ ppm.): 8.1 (1H, d, J = 8.3 Hz, ArH), 7.9
(1H, d, J = 8.5 Hz, ArH), 7.6 (1H, t, J = 6.9 Hz, ArH), 7.4
(1H, t, J = 6.9 Hz, ArH), 3.0 (2H, d, J = 5.9 Hz, CH2), 2.9
(2H, d, J = 5.0 Hz, CH2), 1.8 (2H, t, J = 6.9 CH2).
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)penthane-
1,5-diamine (2d)
A mixture of 1 (0.71g, 3.5 mmol), 1,5-diaminopentane
(0.82 ml, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2d as an oil. Compound 2d:
yield 75%; IR (film) ꢀ (cm-1): 1567.9 (NH2), 2856.5
(CH2CH2CH2), 2931.8(CH2CH2CH2), 3310.4 (ArN); 1H
NMR (CDCl3) (ꢁ ppm.): 7.8 (1H, d, J = 7.3 Hz, ArH), 7.6
(1H, d, J = 7.5 Hz, ArH), 7.5 (1H, t, J = 6.9 Hz, ArH), 7.3
(1H, t, J = 6.9 Hz, ArH), 4.6 (1H, s, NH), 3.5 (2H, m,
NHCH2), 3.1 (2H, t, J = 6.9 Hz, CH2), 3.0 (2H, t, J = 7.7 Hz,
CH2NH2), 2.6 (2H, t, J = 6.7 Hz, CH2), 2.1 (2H, p, J = 3.9,
3.8 Hz, CH2), 1.9 (2H, s, NH2), 1.6 (2H, p, J = 7.3, 6.5 Hz,
CH2), 1.3 - 1.5 (4H, br, CH2); MS (FAB) m/z (M+1) 270.3,
197.1, 185.0; MS-HR (FAB) Calc. for C17H23N3: 269.1892
Found: 270.1971 (M+1).
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)ethane-1,2-
diamine (2a)
A mixture of 1 (0.71g, 3.5 mmol), 1,2-diaminoethane
(0.47 ml, 7 mmol), phenol (1.5g), and NaI (0.07g) was
carefully heated at 180°C for 2 h and then cooled at room
temperature. The mixture was diluted with ethyl acetate (50
ml) and made basic with 10% KOH solution (30 ml). The
organic layer was washed with water (20 ml) and brine (2 x
20 ml), and then dried over MgSO4 and concentrated under
reduced pressure. The resulting residue was purified on silica
gel chromatography (CH2Cl2:CH3OH:NH3=10:4.6:0.5) to
afford 2a as an oil. Compound 2a: yield 64%; IR (KBr) ꢀ
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)hexane-
1,6-diamine (2e)
(cm-1):
1570.4
(NH2),
2856.4
(CH2CH2CH2),
A mixture of 1 (0.71g, 3.5 mmol), 1,6-diaminohexane
(0.82g, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2e as an oil. Compound 2e:
yield 64%; IR (film) ꢀ (cm-1): 1567.5 (NH2), 2855.0
(CH2CH2CH2), 2928.4(CH2CH2CH2), 3350.4 (ArN); 1H
NMR (CD3OD) (ꢁ ppm.): 8.0 (1H, d, J = 8.3 Hz, ArH), 7.7
(1H, d, J = 8.3 Hz, ArH), 7.5 (1H, t, J = 7.8 Hz, ArH), 7.3
(1H, t, J = 7.8 Hz, ArH), 3.6 (2H, t, J = 7.1 Hz, NHCH2), 3.2
(2H, t, J = 7.1 Hz, CH2), 3.0 (4H, t, J = 7.8 Hz, CH2NH2),
2.6 (2H, t, J = 6.8 Hz, CH2), 2.1 (2H, p, J = 7.6, 7.3 Hz,
CH2), 1.6 - 1.7 (2H, m, CH2), 1.3 - 1.5 (6H, br, CH2); MS
(FAB) m/z (M+1) 284.3, 197.0, 185.0; MS-HR (FAB) Calc.
for C18H25N3: 283.2048 Found: 283.2043.
2924.5(CH2CH2CH2), 2950.0 (CH2CH2CH2), 3355.7
(ArN); 1H NMR (CDCl3) (ꢁ ppm.): 7.8 (1H, d, J = 8.3 Hz,
ArH), 7.7 (1H, d, J = 8.3 Hz, ArH), 7.5 (1H, t, J = 6.9 Hz,
ArH), 7.3 (1H, t, J = 8.1 Hz, ArH), 5.4 (1H, s, NH), 3.6 (2H,
d, J = 5.4 Hz, NHCH2), 3.1 (2H, t, J = 7.3 Hz CH2NH2), 2.8 -
3.0 (4H, m, CH2), 2.6 (2H, p, J = 7.5, 7.5 Hz CH2), 1.6 (2H,
s, NH2), MS (FAB) m/z (M+1) 228.1, 197.0, 185.0; MS-HR
(FAB) Calc. for C14H17N3: 227.1422 Found: 227.1420.
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)propane-
1,3-diamine (2b)
A mixture of 1 (0.71g, 3.5 mmol), 1,3-diaminopropane
(0.58 ml, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2b as an oil. Compound 2b:
yield 72%; IR (film) ꢀ (cm-1): 1568.8 (NH2), 2870.4
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)heptane-
1,7-diamine (2f)
1
(CH2CH2CH2), 2951.9 (CH2CH2CH2), 3348.7 (ArN); H
A mixture of 1 (0.75g, 3.5 mmol), 1,7-diaminoheptane
(0.92g, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2f as an oil. Compound 2f:
yield 72%; IR (film) ꢀ (cm-1): 1567.9 (NH2), 2853.9
NMR (CD3OD) (ꢁ ppm.): 8.0 (1H, d, J = 7.6 Hz, ArH), 7.7
(1H, d, J = 7.6 Hz, ArH), 7.6 (1H, t, J = 7.1 Hz, ArH), 7.4
(1H, t, J = 6.8 Hz, ArH), 3.7 (2H, t, J = 7.1 Hz, NHCH2), 3.3
(2H, t, J = 7.1 CH2NH2), 3.0 (2H, t, J = 7.1 Hz, CH2), 2.8
(2H, t, J = 7.1 Hz CH2), 2.1 (2H, p, J = 7.6, 7.8 Hz CH2), 1.9
(2H, p, J = 7.1, 7.1 Hz, CH2); MS (FAB) m/z (M+1) 242.2,
1
(CH2CH2CH2), 2927.9(CH2CH2CH2), 3294.4 (ArN); ; H
NMR (CDCl3) (ꢁ ppm.): 7.9 (1H, d, J = 7.5 Hz, ArH), 7.7
(1H, t, J = 8.3 Hz, ArH), 7.5 (1H, t, J = 7.1 Hz, ArH), 7.4
(1H, t, J = 6.9 Hz, ArH), 4.6 (1H, s, NH), 3.6 (2H, m,

650 Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7
Szymaꢀski et al.
NHCH2), 3.3 (2H, t, J = 7.5 Hz, CH2), 3.0 (2H, t, J = 7.7 Hz,
CH2NH2), 2.6 (2H, t, J = 6.7 Hz, CH2), 2.1 (2H, p, J = 7.7,
7.3 Hz, CH2), 1.7 (2H, s, NH2), 1.5 - 1.6 (2H, m, CH2CH2),
1.2 - 1.5 (8H, br, CH2CH2); MS (FAB) m/z (M+1) 298.3,
197.0, 185.0; MS-HR (FAB) Calc. for C19H27N3: 297.2205
Found: 298.2291 (M+1).
(2H, t, J = 8.1 Hz, CH2), 2.3 (2H, p, J = 7.8, 7.8 Hz, CH2);
MS (FAB) m/z (M+1) 350.1, 185.0; MS-HR (FAB) Calc. for
C21H20 FN3O: 349.1590 Found: 350.1669 (M+1).
N-[3-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)propyl]-4-fluorobenzamide (3b)
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)octane-1,8-
diamine (2g)
(CDMT) (1.76g, 10.2 mmol), 4-fluorobenzoic acid (1.43g,
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2b (2.60g, 10.2 mmol) in THF (10 ml) were combined as
above to afford 3b as yellow solid. Compound 3b: mp 112-
114°C; yield 58%; IR (KBr) ꢀ (cm-1): 765.0 (F), 1585.8
(NH), 1661.5 (NHCO), 2736.9 (CH2CH2CH2), 2786.8
(CH2CH2CH2), 3239.7 (CON); 1H NMR (CD3OD) (ꢁ
ppm.): 8.3 (1H, d, J = 8.5 Hz, ArH), 7.8 – 7.9 (4H, m, ArH),
7.7 (1H, d, J = 9.3 Hz, ArH), 7.6 (1H, t, J = 7.3 Hz, ArH),
7.2 (1H, t, J = 6.8 Hz, ArH), 3.9 (2H, t, J = 7.1 Hz, NHCH2),
3.6 (2H, t, J = 7.1 CH2NH2), 3.4 (2H, t, J = 7.3 Hz, CH2), 3.1
(2H, t, J = 8.1 Hz CH2), 2.3 (2H, p, J = 7.8, 7.3 Hz, CH2),
2.0 (2H, p, J = 6.1, 6.6 Hz, CH2); MS (FAB) m/z (M+1)
364.2, 185.0; MS-HR (FAB) Calc. for C22H22 FN3O:
363.1747 Found: 364.1825 (M+1).
A mixture of 1 (0.71g, 3.5 mmol), 1,8-diaminooctane
(1.00g, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2g as an oil. Compound 2g:
yield 68%; IR (film) ꢀ (cm-1): 1567.2 (NH2), 2853.2
1
(CH2CH2CH2), 2925.5(CH2CH2CH2), 3329.5 (ArN); ; H
NMR (CDCl3) (ꢁ ppm.): 7.8 (1H, d, J = 8.3 Hz, ArH), 7.5
(1H, t, J = 7.3 Hz, ArH), 7.3 (1H, t, J = 7.5 Hz, ArH), 4.5
(1H, s, NH), 3.5 (2H, m, NHCH2), 3.1 (2H, t, J = 7.1 Hz,
CH2), 3.0 (2H, d, J = 7.7 Hz, CH2NH2), 2.6 (2H, t, J = 6.5
Hz, CH2), 1.5 - 1.6 (4H, br, CH2, NH2), 1.1 - 1.5 (10H, br,
CH2); MS (FAB) m/z (M+1) 312.5, 197.1, 185.1; MS-HR
(FAB) Calc. for C20H29N3: 311.2361 Found: 312.2444
(M+1).
N’-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-yl)nonane-
1,9-diamine (2h)
N-[4-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)buthyl]-4-fluorobenzamide (3c)
A mixture of 1 (0.71g, 3.5 mmol), 1,9-diaminononane
(1.10g, 7 mmol), phenol (1.5g), and NaI (0.07g) was
combined as above to afford 2h as an oil. Compound 2h:
yield 66%; IR (film) ꢀ (cm-1): 1568.0 (NH2), 2852.5
(CH2CH2CH2), 2925.7 (CH2CH2CH2), 3294.4 (ArN); ; 1H
NMR (CDCl3) (ꢁ ppm.): 7.8 (1H, d, J = 7.3 Hz, ArH), 7.6
(1H, d, J = 7.5 Hz, ArH), 7.5 (1H, t, J = 6.9 Hz, ArH), 7.2
(1H, t, J = 6.9 Hz, ArH), 4.6 (1H, s, NH), 3.5 (2H, m,
NHCH2), 3.1 (2H, t, J = 7.1 Hz CH2), 3.0 (2H, t, J = 7.7 Hz,
CH2NH2), 2.6 (2H, t, J = 6.9 Hz CH2), 2.1 (2H, p, J = 7.7,
7.7 Hz CH2), 1,7 (2H, s, NH2), 1.5 - 1.6 (2H, m, CH2CH2),
1.1 - 1.3 (14H, br, CH2CH2); MS (FAB) m/z (M+1) 326.3,
185.0; MS-HR (FAB) Calc. for C21H31N3: 325.2518 Found:
326.2609 (M+1).
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
(CDMT) (1.76g, 10 mmol), 4-fluorobenzoic acid (1.43g,
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2c (3.03 g, 10.2 mmol) in THF (10 ml) were combined as
above to afford 3c as yellow solid. Compound 3c: mp 176-
178°C; yield 64%; IR (KBr) ꢀ (cm-1): 762.2 (F), 1561.1
(NH), 1634.7 (NHCO), 2871.8 (CH2CH2CH2), 2934.4
(CH2CH2CH2), 3297.1 (CON); 1H NMR (CDOD) (ꢁ ppm.):
8.3 (1H, d, J = 8.1 Hz, ArH), 8.2 (1H, d, J = 7.8 Hz, ArH),
7.7 - 7.9 (3H, m, ArH), 7.5 - 7.6 (2H, m, ArH), 7.1 (1H, t, J
= 8.5 Hz, ArH), 3.7 - 4.1 (4H, m, CH2), 3.2 - 3.4 (4H, m,
CH2), 2.2 (2H, p, J = 7.1, 7.3 Hz, CH2), 1.8 - 1.9 (4H, m,
CH2) ;MS (FAB) m/z (M+1) 378.3, 185.0; MS-HR (FAB)
Calc. for C23H24FN3O: 377.1903 Found: 378.1982 (M+1).
N-[2-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)ethyl]-4-fluorobenzamide (3a)
N-[5-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)penthyl]-4-fluorobenzamide (3d)
To a stirred mixture of 2-chloro-4,6-dimethoxy-1,3,5-
triazine (CDMT) (1.76g, 10.2 mmol), 4-fluorobenzoic acid
(1.43 g, 10.2 mmol) in THF (8 ml), N-methylmorpholine
(1.12 ml, 10.2 mmol) was added dropwise at such a rate as to
keep temperature at -5°C to 0°C. Stirring has been continued
at 0°C for 1- 4 h until all CDMT has been consumed. Then
to the crude mixture, obtained as described above 2a (2.32 g,
10.2 mmol) in THF (10 ml), at -5°C to 0°C, was added.
Stirring has been continued at 0°C for 2 h, and then for 12 h
at room temperature. Precipitate formed and was isolated by
filtration. Recrystallization from ethyl acetate afforded the
desired product 3a as yellow solid. Compound 3a: mp 220-
222°C; yield 60%; IR (KBr) ꢀ (cm-1): 765.4 (F), 1587.4
(NH), 1633.4 (NHCO), 2846.0 (CH2CH2CH2), 2929.5
(CH2CH2CH2), 3264.4 (CON); 1H NMR (CD3OD) (ꢁ
ppm.): 8.3 (1H, d, J = 8.1 Hz, ArH), 7.8 – 7.9 (4H, m, ArH),
7.7 (1H, d, J = 7.1 Hz, ArH), 7.6 (1H, t, J = 7.1 Hz, ArH),
7.2 (1H, t, J = 8.8 Hz, ArH), 4.1 (2H, t, J = 5.6 CH2NH2), 3.7
(2H, t, J = 6.1 Hz, CH2), 3.4 (2H, t, J = 7.1 Hz CH2), 3.1
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
(CDMT) (1.76g, 10 mmol), 4-fluorobenzoic acid (1.43g,
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2d (2.75g, 10.2 mmol) in THF (10 ml) were combined as
above to afford 3d as yellow solid. Compound 3a: mp 100-
103°C; yield 62%; IR (KBr) ꢀ (cm-1): 766.6 (F), 1584.1
(NH), 1632.5 (NHCO), 2862.8 (CH2CH2CH2), 2933.7
(CH2CH2CH2), 3261.5 (CON); 1H NMR (CDOD) (ꢁ ppm.):
8.3 (1H, d, J = 8.5 Hz, ArH), 7.6 - 7.8 (5H, m, ArH), 7.6
(1H, t, J = 1.5 Hz, ArH), 7.1 (1H, t, J = 6.6 Hz, ArH), 3.8
(2H, t, J = 7.6 Hz CH2), 3.4 (2H, m, CH2), 3.1 (2H, t, J = 7.8
Hz, CH2NH2), 2.9 (2H, m, CH2), 2.3 (2H, p, J = 7.8, 7.6 Hz,
CH2), 1.8 (4H, m, CH2), 1.7 (2H, m, CH2), 1.5 (2H, m,
CH2);MS (FAB) m/z (M+1) 392.3, 185.0; MS-HR (FAB)
Calc. for C24H26FN3O: 391.2060 Found: 392.2128 (M+1).
N-[6-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)hexyl]-4-fluorobenzamide (3e)
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
(CDMT) (1.76g, 10.2 mmol), 4-fluorobenzoic acid (1.43g,

2,3-dihydro-1H-cyclopenta[b]quinoline Derivatives as Acetylcholinesterase Inhibitors
Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7 651
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2e (2.89 g, 10.2 mmol) in THF (10 ml) was combined as
above to afford 3e as yellow solid. Compound 3e: mp 125-
127°C; yield 38%; IR (KBr) ꢀ (cm-1): 764.9 (F), 1584.2
(NH), 1632.7 (NHCO), 2857.3 (CH2CH2CH2), 2932.7
(CH2CH2CH2), 3222.6 (CON); 1H NMR (CD3OD) (ꢁ
ppm.): 8.3 (1H, d, J = 8.1 Hz, ArH), 7.8 - 7.9 (3H, m, ArH),
7.7 (1H, d, J = 7.8 Hz, ArH), 7.6 (1H, t, J = 7.3 Hz, ArH),
7.2 (1H, t, J = 6.8 Hz, ArH), 3.8 (2H, t, J = 7.1 Hz CH2), 3.7
(2H, t, J = 6.3 Hz CH2), 3.4 (2H, t, J = 6.1 Hz, CH2NH2), 3.2
(2H, t, J = 7.8 Hz, CH2), 2.3 (2H, p, J = 7.6, 7.6 Hz, CH2),
1.8 (2H, m, CH2), 1.7 (2H, m, CH2), 1.4 - 1.5 (4H, m, CH2);
MS (FAB) m/z (M+1) 406.4, 183.1; MS-HR (FAB) Calc. for
C25H28FN3O: 405.2216 Found: 406.2293 (M+1).
(NH), 1680.4 (NHCO), 2866.5 (CH2CH2CH2), 2926.9
(CH2CH2CH2), 3217.5 (CON); 1H NMR (CD3OD) (ꢁ
ppm.): 8.3 (1H, d, J = 8.5 Hz, ArH), 7.8 - 7.9 (3H, m, ArH),
7.7 (1H, d, J = 7.1 Hz, ArH), 7.6 (1H, t, J = 7.1 Hz, ArH),
7.2 (1H, t, J = 8.5 Hz, ArH), 3.9 (2H, m, CH2), 3.8 (2H, t, J
= 7.3 Hz CH2), 3.4 (2H, t, J = 8.3 Hz, CH2NH2), 3.2 (2H, t, J
= 7.6 Hz, CH2), 2.3 (2H, p, J = 7.8, 7.6 Hz, CH2), 1.8 (2H,
m, CH2), 1.6 (2H, m, CH2), 1.3 - 1.4 (10H, m, CH2); MS
(FAB) m/z (M+1) 448.3, 185.0; MS-HR (FAB) Calc. for
C28H34FN3O: 447.26859 Found: 448.2763 (M+1).
N-[2-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)ethyl]-4-fluorobenzamide hydrochloride (4a).
A solution of HCl in ether has been prepared by bubbling
HCl into ether (50 ml) at a moderate rate for 20 min. 3a
(0.20 g, 0.5183 mmol) was dissolved in ether (2 ml),
HCl/ether (4 ml) was added, and the reaction mixture was
stirred at room temperature. After 2 min, the solution
became cloudy and precipitate formed. The precipitate was
isolated by filtration and the solid was washed with ether and
dried in vacuum. Compound 4a: mp >280°C; yield 28%; IR
(KBr) ꢀ (cm-1): 764.0 (F), 1563.6 (NH), 1634.3 (NHCO),
2851.0 (CH2CH2CH2), 2931.8 (CH2CH2CH2), 3263.6
(CON); 1H NMR (DMSO) (ꢁ ppm.): 14.0 (1H, s, HCl), 8.9
(1H, m, NH), 8.5 (1H, d, J = 8.3 Hz, ArH), 7.8 - 7.9 (5H, m,
ArH), 7.6 (1H, t, J = 7.3 Hz, ArH), 7.3 (1H, t, J = 9.1 Hz,
ArH), 3.9 (2H, m, CH2), 3.6 (2H, m, CH2), 3.5 (3H, m, NH,
CH2), 3.1 (2H, t, J = 8.1 Hz, CH2), 2.1 (2H, m, CH2).
N-[7-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)heptyl]-4-fluorobenzamide (3f).
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
(CDMT) (1.76g, 10.2 mmol), 4-fluorobenzoic acid (1.43g,
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2f (3.03 g, 10.2 mmol) in THF (10 ml) was combined as
above to afford 3f as yellow solid. Compound 3c: mp 105-
107°C; yield 40%; IR (KBr) ꢀ (cm-1):1565.2.4 (NH), 1632.4
(NHCO), 2847.5 (CH2CH2CH2), 2933.0 (CH2CH2CH2),
1
3259.7 (CON); H NMR (CD3OD) (ꢁ ppm.): 8.3 (1H, d, J =
8.1 Hz, ArH), 7.8 - 7.9 (3H, m, ArH), 7.7 (1H, d, J = 7.8 Hz,
ArH), 7.6 (1H, t, J = 7.3 Hz, ArH), 7.2 (1H, t, J = 6.8 Hz,
ArH), 3.8 (2H, t, J = 7.3 Hz CH2), 3.7 (2H, t, J = 6.8 Hz
CH2), 3.4 (2H, m, CH2NH2), 3.2 (2H, t, J = 7.8 Hz, CH2), 2.3
(2H, p, J = 7.8, 7.8 Hz, CH2), 1.8 (2H, m, CH2), 1.6 (2H, m,
CH2), 1.4 - 1.5 (6H, m, CH2); MS (FAB) m/z (M+1) 420.4,
183.0; MS-HR (FAB) Calc. for C26H30FN3O: 419.2373
Found: 420.2446 (M+1).
N-[3-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)propyl]-4-fluorobenzamide hydrochloride (4b).
A 3b (0.20 g, 0.4831 mmol) were combined as above to
afford 4b as yellow solid. Compound 4b: mp >280°C; yield
24%; IR (KBr) ꢀ (cm-1): 751.3 (F), 1563.7 (NH), 1667.8
(NHCO), 2848.4 (CH2CH2CH2), 2932.0 (CH2CH2CH2),
N-[8-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)octyl]-4-fluorobenzamide (3g).
1
3265.9 (CON); H NMR (DMSO) (ꢁ ppm.): 14.0 (1H, s,
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
HCl), 8.8 (1H, m, NH), 8.5 (1H, d, J = 8.9 Hz, ArH), 7.8 -
7.9 (5H, m, ArH), 7.6 (1H, t, J = 6.5 Hz, ArH), 7.3 (1H, t, J
= 8.9 Hz, ArH), 3.7 – 3.8 (2H, m, CH2), 3.3 - 3.4 (3H, m,
NH, CH2), 3.2 (2H, t, J = 7.7 Hz, CH2), 3.1 (2H, t, J = 7.9
Hz, CH2), 2.1 (2H, m, CH2), 1.9 (2H, m, CH2).
(CDMT) (1.76g, 10.2 mmol), 4-fluorobenzoic acid (1.43 g,
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2g (3.18 g, 10.2 mmol) in THF (10 ml) was combined as
above to afford 3g as yellow solid. Compound 3d: mp 121-
123°C; yield 35%; IR (KBr) ꢀ (cm-1): 764.9 (F), 1565.2
(NH), 1680.0 (NHCO), 2868.4 (CH2CH2CH2), 2928.9
(CH2CH2CH2), 3227.0 (CON); 1H NMR (CD3OD) (ꢁ
ppm.): 8.3 (1H, d, J = 8.1 Hz, ArH), 7.8 - 7.9 (3H, m, ArH),
7.7 (1H, d, J = 7.1 Hz, ArH), 7.6 (1H, t, J = 7.1 Hz, ArH),
7.2 (1H, t, J = 6.8 Hz, ArH), 3.9 (2H, m, CH2), 3.7 (2H, t, J
= 7.3 Hz CH2), 3.4 (2H, m, CH2NH2), 3.2 (2H, t, J = 7.8 Hz,
CH2), 2.3 (2H, p, J = 7.8, 7.8 Hz, CH2), 1.8 (2H, m, CH2),
1.6 (2H, m, CH2), 1.4 - 1.5 (8H, m, CH2); MS (FAB) m/z
(M+1) 434.4, 185.0; MS-HR (FAB) Calc. for C27H32FN3O:
433.2529 Found: 434.2603 (M+1).
N-[4-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)butyl]-4-fluorobenzamide hydrochloride (4c).
A 3c (0.20 g, 0.4673 mmol) were combined as above to
afford 4c as brown solid. Compound 4c: mp >280°C; yield
33%; IR (KBr) ꢀ (cm-1): 762.6 (F), 1560.8 (NH), 1632.4
(NHCO), 2851.2 (CH2CH2CH2), 2936.1 (CH2CH2CH2),
1
3308.8 (CON); H NMR (DMSO) (ꢁ ppm.): 13.9 (1H, s,
HCl), 8.7 (1H, m, NH), 8.4 (1H, d, J = 8.7 Hz, ArH), 7.7 -
7.9 (4H, m, ArH), 7.5 -7.6 (2H, m, ArH), 7.2 - 7.3 (1H, m,
ArH), 3.7 – 3.9 (2H, m, CH2), 3.2 - 3.5 (3H, m, NH, CH2),
3.0 (2H, t, J = 7.9 Hz, CH2), 2.8 – 2.9 (2H, m, CH2), 2.0 –
2.1 (2H, m, CH2), 1.6 – 1.7 (4H, m, CH2).
N-[9-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)nonyl]-4-fluorobenzamide (3h).
N-[5-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)penthyl]-4-fluorobenzamide hydrochloride (4d)
A
mixture of 2-chloro-4,6-dimethoxy-1,3,5-triazine
(CDMT) (1.76g, 10.2 mmol), 4-fluorobenzoic acid (1.43 g,
10.2 mmol), N-methylmorpholine (1.12 ml, 10.2 mmol) and
2h (3.32 g, 10.2 mmol) in THF (10 ml) was combined as
above to afford 3h as yellow solid. Compound 3e: mp 128-
130°C; yield 36%; IR (KBr) ꢀ (cm-1): 749.4 (F), 1565.5
A 3d (0.20 g, 0.5109 mmol) were combined as above to
afford 4d as brown solid. Compound 4d: mp >300°C; yield
30%; IR (KBr) ꢀ (cm-1): 764.3 (F), 1568.7 (NH), 1632.2
(NHCO), 2737.4 (CH2CH2CH2), 2937.1 (CH2CH2CH2),
1
3243.0 (CON); H NMR (DMSO) (ꢁ ppm.): 14.2 (1H, s,

652 Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7
Szymaꢀski et al.
HCl), 8.8 (1H, m, NH), 8.5 (1H, d, J = 5.4 Hz, ArH), 7.7 -
7.9 (5H, m, ArH), 7.5 -7.6 (1H, m, ArH), 7.2 (1H, t, J = 8.7
Hz, ArH), 3.5 – 3.6 (5H, m, NH, CH2), 3.3 (2H, t, J = 7.3,
CH2), 3.1 (2H, t, J = 7.9, CH2), 2.1 (2H, p, J = 7.7, 7.3, CH2),
1.7 (2H, m, CH2), 1.6 (2H, p, J = 7.1, 6.7, CH2), 1.4 (2H, m,
CH2).
Enzyme Studies
All new derivatives of 2,3-dihydro-1H-cyclopenta[b]qu-
inoline were examined towards AChE and BChE inhibition.
We used spectrophotometric Ellman’s method with own
modifications. All measurements were conducted in one and
the same conditions. Determination of the inhibitory activity
was carried in the environment of phosphate buffer (0.1M,
pH 8.0) at 37° C. The content of every single sample was as
follows: 5,5’-dithiobisnitrobenzoic acid (DTNB, 0.05 ml,
0.5M), AChE (5 units/ml), acetylthiocholine iodide
(substrate) and analyzed, newly synthesized inhibitor. The
volume of every probe was equal and it totaled 3 ml. The
absorbance was measured after 1 minute incubation of all
ingredients. The inhibition curves of every compound were
obtained by usage of 7 concentrations of acetylthiocholine
iodide. Every procedure was repeated three times. All
measurements were conducted with the accompaniment of
sample without inhibitor in order to obtain the 100% of
AChE activity. The only alteration in estimation of the drugs
inhibitory activity towards butyrylcholinesterase (BChE)
was utilization of BChE (5 units/ml) instead of AChE. The
drug concentration that contributes to the inhibition of 50%
AChE or BChE activity (value of IC50) was determined by
non-linear and linear regression. All reagents: DTNB,
enzymes (acetylcholinesterase from Electrophorus electricus
(electric eel) and butyrylcholinesterase from equine serum)
and acetylthiocholine iodide were purchased from Sigma-
Aldrich.
N-[6-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)hexyl]-4-fluorobenzamide hydrochloride (4e).
A 3e (0.20 g, 0.4932 mmol) was combined as above to
afford 4e as yellow solid. Compound 4e: mp 240-242°C;
yield 28%; IR (KBr) ꢀ (cm-1): 762.3.4 (F), 1566.7 (NH),
1633.6
(NHCO),
2854.6
(CH2CH2CH2),
2929.9
(CH2CH2CH2), 3196.1 (CON); 1H NMR (DMSO) (ꢁ ppm.):
14.0 (1H, s, HCl), 8.7 (1H, m, NH), 8.5 (1H, m, ArH), 7.8 -
7.9 (3H, m, ArH), 7.6 -7.7 (2H, m, ArH), 7.2 (1H, t, J = 8.9
Hz, ArH), 3.7 (4H, m, NH CH2, CH2), 3.2 -3.3 (3H, m, NH,
CH2), 3.1 (2H, t, J = 7.9, CH2), 2.1 (2H, m, CH2), 1.7 (2H,
m, CH2), 1.4 (2H, m, CH2), 1.2 (4H, m, CH2).
N-[7-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)heptyl]-4-fluorobenzamide hydrochloride (4f).
A 3f (0.20 g, 0.4757 mmol) was combined as above to
afford 4f as yellow solid. Compound 4f: mp 179-181°C;
yield 25%; IR (KBr) ꢀ (cm-1): 764.0 (F), 1562.7 (NH),
1633.7
(NHCO),
2853.3
(CH2CH2CH2),
2928.0
(CH2CH2CH2), 3227.8 (CON); 1H NMR (DMSO) (ꢁ ppm.):
14.0 (1H, s, HCl), 8.7 (1H, m, NH), 8.5 (1H, d, J = 7.3 Hz,
ArH), 7.8 - 7.9 (5H, m, ArH), 7.6 (1H, t, J = 6.8 Hz, ArH),
7.3 (1H, t, J = 8.9 Hz, ArH), 3.7 (2H, m, NHCH2), 3.2 -3.3
(5H, m, NH, CH2), 3.1 (2H, t, J = 7.9, CH2), 2.1 (2H, m,
CH2), 1.6 (2H, m, CH2), 1.5 (2H, m, CH2), 1.2 (6H, m, CH2).
Molecular Modeling
Structures of inhibitors were created by Corina on-line
(Molecular Networks) and then Gasteiger-Marsili charges
were assigned by Sybyl 8.0 (Tripos). Ligands were docked
to both cholinesterases with application of GoldSuite 4.1
(CCDC). Before docking acetylcholinesterase from 2CKM
and butyrylcholinesterase from 1P0I crystal complex were
prepared. Protonation at Nꢂ of histidine residues were
assigned, the hydrogen atoms were added, ligand and water
molecules were deleted. The binding site was defined as all
amino acid residues within 10 Å from bis-(7)-tacrine for
AChE and 20 Å from the glycerol molecule, present in the
active gorge of BChE. A standard set of genetic algorithm
with population size 100, number of operations 100000 and
clustering with a tolerance of 1Å were applied to find 10
poses for each ligand, sorted by GoldScore (AChE) and
ChemScore (BChE) function. The results were visualized by
PyMOL 0.99rc6 (DeLano Scientific LLC).
N-[8-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)octyl]-4-fluorobenzamide hydrochloride (4g).
A 3g (0.20 g, 0.4613 mmol) was combined as above to
afford 4g as brown solid. Compound 4g: mp 174-176°C;
yield 44%; IR (KBr) ꢀ (cm-1): 764.0 (F), 1562.4 (NH),
1632.8
(NHCO),
2855.1
(CH2CH2CH2),
2924.3
(CH2CH2CH2), 3218.1 (CON); 1H NMR (DMSO) (ꢁ ppm.):
14.0 (1H, s, HCl), 8.7 (1H, m, NH), 8.5 (1H, d, J = 8.1 Hz,
ArH), 7.8 - 7.9 (5H, m, ArH), 7.6 (1H, t, J = 6.8 Hz, ArH),
7.3 (1H, t, J = 8.9 Hz, ArH), 3.7 (2H, m, NHCH2), 3.2 -3.3
(5H, m, NH, CH2), 3.1 (2H, m, CH2), 2.1 (2H, m, CH2), 1.6
(2H, m, CH2), 1.5 (2H, m, CH2), 1.2 -1.4 (8H, m, CH2).
N-[9-(2,3-dihydro-1H-cyclopenta[b]quinolin-9-
ylamino)nonyl]-4-fluorobenzamide hydrochloride (4h).
A 3h (0.20 g, 0.4468 mmol) was combined as above to
afford 4h as brown solid. Compound 4h: mp 118-120°C;
yield 32%; IR (KBr) ꢀ (cm-1): 759.2 (F), 1588.0 (NH),
DISCUSSION AND CONCLUSION
Enhancement of cholinergic function in the central
nervous system in the course of AD is of extreme
importance. This goal might be achieved by use of
acetylcholinesterase inhibitors such as tacrine, the first
AChEI drug approved by FDA for clinical use, rivastigmine,
galantamine and donepezil. However, there is still a great
need to search for new compounds with anticholinesterase
activity. In this study we described synthesis of a series of
tacrine-fluorobenzoic acid hybrids.
1644.0
(NHCO),
2849.9
(CH2CH2CH2),
2925.3
(CH2CH2CH2), 3226.9 (CON); 1H NMR (DMSO) (ꢁ ppm.):
13.9 (1H, s, HCl), 8.7 (1H, m, NH), 8.5 (1H, d, J = 7.5 Hz,
ArH), 7.8 - 7.9 (5H, m, ArH), 7.6 (1H, t, J = 8.5 Hz, ArH),
7.3 (1H, t, J = 8.9 Hz, ArH), 3.7 (2H, m, NHCH2), 3.2 -3.3
(5H, m, NH, CH2), 3.1 (2H, t, J = 7.7 Hz, CH2), 2.2 (2H, m,
CH2), 1.7 (2H, m, CH2), 1.5 (2H, m, CH2), 1.2 -1.4 (10H, m,
CH2).

2,3-dihydro-1H-cyclopenta[b]quinoline Derivatives as Acetylcholinesterase Inhibitors
Letters in Drug Design & Discovery, 2012, Vol. 9, No. 7 653
In our earlier work we also presented derivatives of
fluorobenzoic acid but on the basis of the 1,2,3,4-
tetrahydroacridin-9-ylamine. All novel compounds described
in this study were characterized, in comparison with tacrine,
by 4-fold higher inhibitory activity towards AChE. In case of
BChE inhibition, slightly higher IC50 values were obtained
for all constituents than for tacrine. These three synthesized
compounds were differing from each other only in the length
of the aliphatic chain. According to the results of the study,
introduction of longer aliphatic chain contributed to the
remarkable increase in compounds’ inhibitory activity
suggesting that it was good way to introduce such alteration
[17].
high level of radioligand accumulation both in the striatum
and the olfactory tubercle which are AChE-rich regions [32].
On the basis of the current stage of knowledge the next
step of our study will be labeling of compounds presented in
this work with 18F atoms and then evaluating them as
radioligands for the in vivo mapping of AChE.
In summary, results of our synthesis and analysis suggest
that obtained hybrids of tacrine and 4-fluorobenzoic acid
may be considered to be a kind of novel anti-Alzheimer's
drug candidates. This kind of new potential drugs could be
used in the treatment of cholinergic deficit occurring in AD,
inhibiting both enzymes AChE and BChE which could
restore the acetylcholine level.
In comparison with our previous study, the exchange of a
six membered ring for a five membered ring significantly
influenced the activity and interaction with active site.
However, in the current study we did not record such
coherent and straightforward results. We cannot observe that
together with the length of aliphatic chain between
CONFLICT OF INTEREST
Declared none.
fluorobenzoic
acid
and
2,3-dihydro-1H-
ACKNOWLEDGEMENTS
cyclopenta[b]quinolone the activity of synthesized
compounds is increased. Among obtained compounds the
most active towards AChE were compounds 4c, 4a and 4f.
They were 11-, 5- and 2-fold more active than tacrine,
respectively. The 11-fold more active than tacrine,
compound 4c is more selective towards AChE in comparison
with tacrine. Compounds 4d, 4g and 4h exhibited similar to
tacrine value of IC50.
Financial support by grant (N N405 669940) from
National Science Centre in Poland is gratefully
acknowledged.
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Isosteres:
A
New Class of Potent and Selective