F. Mesiti, A. Maruca, V. Silva et al.
European Journal of Medicinal Chemistry 213 (2021) 113183
compounds were in line with the general drug-likeness re-
quirements of the of CNS-active drugs (Table S3). In particular, the
estimated LogBB value suggested the ability of the quinoline de-
rivatives to cross the BBB and reach the CNS (Table S3).
7.76 (1H, ddd, J ¼ 8.1, 1.5, 0.5 Hz, H8), 7.55 (1H, ddd, J ¼ 8.1, 6.9,
1.5 Hz, H7), 7.51 (1H, ddd, J ¼ 8.0, 2.0, 1.0 Hz, H60), 7.39 (1H, dd,
J ¼ 8.0, 8.0 Hz, H50), 7.15 (1H, ddd, J ¼ 8.0, 2.0, 1.0 Hz, H40). 13C NMR
(100 MHz, DMSO‑d6)
d 176.2 (C4), 163.1 (CONH), 144.2 (C2), 140.1
(C10), 139.0 (C4a), 133.2 (C30), 133.0 (C6), 130.5 (C50), 125.8 (C8a),
125.4 (C5), 125.3 (C7), 123.0 (C40), 119.2 (C8), 119.0 (C2’), 118.0 (C6’),
110.1 (C3). MS/EI m/z (%): 89 (16), 116 (25), 127 (77), 129 (25), 172
(100), 298 (Mþ, 58) 300 (Mþ2, 19).
6. Conclusions
The synthesis, biological evaluation, and cytotoxicity profile of a
small set of N-phenyl-quinoline-3-carboxamides derivatives was
successfully accomplished. The screening assays against MAOs
isoforms showed that compounds 2 and 3 were not active at the
highest concentration tested and that compound 10 was the most
potent and selective IMAO-B (IC50 ¼ 5.30 0.74 nM and SI: ꢀ1887).
No significant cytotoxic effects in differentiated SH-SY5Y cells were
observed for the most active compounds in the range of concen-
tration tested.
Moreover, we found that the prototropic tautomerism of com-
pounds 2e4, which is blocked in compounds 8e10, markedly
influenced the MAO-B inhibitory activity. The unequivocally char-
acterisation of the type of tautomer present in solution was only
possible by using 2D NMR 1He15N HSQC and 1He15N HMBC tech-
niques. Computational studies and explicit water solvent MD sim-
ulations on enzyme-ligand complexes supported the experimental
data. Interestingly quinolones have a dissimilar binding pose in
MAO-B active site than chromones.
N-(3,4-Dichlorophenyl)-4-hydroxy-1,4-dihydroquinoline-3-
carboxamide (4). The compound was recrystallized from MeOH and
obtained in 35% yield. 1H NMR (400 MHz, DMSO‑d6)
d 12.68 (1H, s,
CONH), 8.86 (1H, s, H2), 8.33 (1H, dd, J ¼ 6.9, 1.1 Hz, H5), 8.20 (1H, d,
J ¼ 1.4 Hz, H20), 7.82 (1H, dd, J ¼ 6.9, 1.1 Hz, H50), 7.75 (1H, d,
J ¼ 8.2 Hz, H8). 7.61e7.56 (2H, m, H6, H60), 7.54 (1H, dd, J ¼ 8.2,
6.9 Hz, H7). 13C NMR (DMSO‑d6)
d 176.2 (C4), 163.4 (CONH), 144.5
(C2), 139.3 (C4a), 138.8 (C10), 133.1 (C6), 130.8 (C50), 125.9 (C8a),
125.4 (C5), 120.9 (C20), 119.7 (C8), 119.4 (C60), 110.0 (C3). C7, C30, C4’
were not possible to assign. MS/EI m/z (%): 77 (26), 103 (11), 115
(36), 131 (18), 159 (100), 160 (12), 319 (12).
7.1.2. Synthesis of ethyl-4-oxo-1,4-dihydroquinoline-3-carboxylate
(5)
A mixture of aniline (10.73 mmol) and DEEMM (10.73 mmol)
was heated at 120 ꢁC for 1 h. After cooling the solid obtained was
dissolved in diphenyl ether (32 mL). The reaction was refluxed for
2 h. After cooling, diethyl ether (25 mL) was added and the solid
was filtered and recrystallized from DMF (yield 80%). 1H NMR
To our knowledge, there have been no prior reports on the
characterisation of quinolone tautomers by 1He15N HSQC and
1He15N HMBC, which were shown to be valid and expedite tools for
drug discovery programs, in which tautomerism of the chemical
libraries is foreseen.
(400 MHz, DMSO‑d6) d: 12.31 (1H, s, NH), 8.55 (1H, s, H2), 8.16 (1H,
dd, J ¼ 7.6, 1.5, 0.6 Hz, H5), 7.71 (1H, dd, J ¼ 7.6, 6.9, 1.5 Hz, H7), 7.62
(1H, dd, J ¼ 7.6,1.5, 0.6 Hz, H8), 7.42 (1H, dd, J ¼ 7.6, 6.9 Hz, H6), 4.22
(2H, q, J ¼ 7.1 Hz, CH2), 1.29 (3H, t, J ¼ 7.1 Hz, CH3).
7. Experimental section
7.1.3. Synthesis of ethyl-1-methyl-4-oxo-1,4-dihydroquinoline-3-
carboxylate (6)
7.1. Synthesis of quinoline derivatives
Ethyl-4-oxo-1,4-dihydroquinoline-3-carboxylate
(5)
7.1.1. Synthesis of N-phenyl-4-hydroxyquinoline-3-carboxamides
(2e4)
(1.15 mmol) and DMF (5 mL) were placed in a round bottom flask.
Then, K2CO3 (1.72 mmol) was added, and the reaction was stirred at
room temperature in argon atmosphere. At that time, CH3I
(2.30 mmol) was introduced dropwise. The reaction was stirred at
60 ꢁC for 1 h. After, the mixture was diluted with water and
extracted with DCM (3 ꢂ 15 mL). The combined organic layers were
dried over anhydrous Na2SO4, and the solvent was evaporated
under reduced pressure. The crude product was crystallised from
General Procedure. To
a solution of 4-oxo-quinoline-3-
carboxylic acid (1) (1.05 mmol), TBTU (1.05 mmol), DIPEA
(2.1 mmol) in DMF (10 mL) the appropriate aromatic amine
(1.26 mmol) was added. The reaction was stirred at room temper-
ature overnight. Then, the mixture was diluted with DCM (50 mL),
extracted with H2O (3 ꢂ 15 mL), HCl 1 M (3 ꢂ 15 mL), and NaHCO3
saturated solution (3 ꢂ 15 mL). The combined organic phases were
dried over Na2SO4 anhydrous, filtered, and concentrated under
reduced pressure. The crude product was purified by
recrystallisation.
DCM (70% yield). 1H NMR (400 MHz, CDCl3)
d: 8.53 (1H, dd, J ¼ 8.0,
1.6, 0.6 Hz, H5), 8.46 (1H, s, H2), 7.70 (1H, dd, J ¼ 8.6, 7.2, 1.6 Hz, H7),
7.49e7.39 (2H, m, H8, H6) 4.40 (2H, q, J ¼ 7.1 Hz, CH2), 3.88 (3H, s,
NCH3), 1.42 (3H, t, J ¼ 7.1 Hz, CH3).
N-(3,4-Dimethylphenyl)-4-hydroxyquinoline-3-carboxamide
(2). The compound was recrystallized from MeOH and obtained in
7.1.4. Synthesis of 1-methyl-4-oxo-1,4-dihydroquinoline-3-
carboxylic acid (7)
80% yield. 1H NMR (400 MHz, DMSO‑d6):
d 12.90 (1H, s, OH), 12.35
(1H, s, CONH), 8.85 (1H, s, H2), 8.33 (1H, dd, J ¼ 8.4, 1.2 Hz, H5), 7.81
(1H, ddd, J ¼ 8.4, 6.9, 1.2 Hz, H6), 7.75 (1H, dd, J ¼ 8.4 Hz, 1.2 Hz, H8),
7.53 (1H, ddd, J ¼ 8.4, 6.9, 1.2 Hz, H7), 7.51 (1H, dd, J ¼ 8.0, 2.1 Hz,
H60), 7.47 (1H, d, J ¼ 2.1 Hz, H20), 7.11 (1H, d, J ¼ 8.0 Hz, H50), 2.23
(3H, s, 30CH3), 2.19 (3H, s, 40CH3). 13C NMR (100 MHz, DMSO- d6)
A suspension of ethyl-1-methyl-4-oxo-1,4-dihydroquinoline-3-
carboxylate (6) (4.41 mmol) in 10% aq. NaOH (12 mL) was
refluxed for 4 h. After cooling to room temperature, the mixture
was acidified with conc. HCl until pH 3. The resulting solid was
filtered and washed with water (95% yield). 1H NMR (400 MHz,
d
176.2 (C4), 162.4 (CONH), 143.9 (C2), 139.1 (C4a), 136.6 (C10), 136.5
DMSO‑d6) d: 15.26 (1H, s, COOH), 9.06 (1H, s, H2), 8.39 (1H, dd,
(C30), 132.8 (C6), 131.0 (C40), 129.8 (C50), 125.8 (C8a), 125.3 (C5),
125.1 (C7),120.6 (C20),119.1 (C8),116.8 (C60),110.7 (C3),19.5 (30CH3),
18.7 (40CH3). MS/EI m/z (%): 89 (8), 106 (13), 116 (13), 117 (6), 119
(12), 121 (100), 122 (9), 172 (25), 292 (Mþ, 6).
J ¼ 8.1, 1.5, 0.7 Hz, H5), 8.04e7.94 (2H, m, H7, H8), 7.70 (1H, dd,
J ¼ 8.1, 6.2, 1.8 Hz, H6), 4.11 (3H, s, NCH3).
7.1.5. Synthesis of N-phenyl-1-methyl-4-oxo-1,4-dihydroquinoline-
N-(3-Chlorophenyl)-4-hydroxyquinoline-3-carboxamide
The compound was recrystallized from MeOH and obtained in 48%
yield. 1H NMR (400 MHz, DMSO‑d6):
12.99 (1H, s, OH), 12.64 (1H,
(3).
3-carboxamides (8e10)
General
Procedure.
The
N-phenyl-1-methyl-4-oxo-1,4-
d
dihydroquinoline-3-carboxamides (8e10) were synthesised
following the general procedure previously described for com-
pounds 2e4, by using 1-methyl-4-oxo-1,4-dihydroquinoline-3-
s, CONH), 8.88 (1H, s, H2), 8.33 (1H, ddd, J ¼ 8.1, 1.5, 0.5 Hz, H5), 8.05
(1H, dd, J ¼ 2.0, 2.0 Hz, H20), 7.82 (1H, ddd, J ¼ 8.1, 6.9, 1.5 Hz, H6),
8