Synthesis and biological evaluation of 2-Phenoxyacetamide analogues, a novel class of potent and selective monoamine oxidase inhibitors

Article abstract of DOI:10.3390/molecules191118620

Monoamine oxidases (EC 1.4.3.4; MAOs), a family of FAD-containing enzymes, is an important target for antidepressant drugs. In this paper, a series of 2-phenoxyacetamide analogues were synthesized, and their inhibitory potency towards monoamine oxidases A (MAO-A) and B (MAO-B) were evaluated using enzyme and cancer cell lysate. 2-(4-Methoxyphenoxy)acetamide (compound 12) (SI = 245) and (2-(4-((prop-2- ynylimino)methyl)phenoxy)acetamide (compound 21) (IC_50MAO-A = 0.018 μM, IC_50MAO-B = 0.07 μM) were successfully identified as the most specific MAO-A inhibitor, and the most potent MAO-A/-B inhibitor, respectively. The inhibitory activities of these two compounds in living cells were also further evaluated utilizing HepG2 and SHSY-5Y cell lysates.

Full text of DOI:10.3390/molecules191118620

Molecules 2014, 19, 18620-18631; doi:10.3390/molecules191118620
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Biological Evaluation of 2-Phenoxyacetamide
Analogues, a Novel Class of Potent and Selective Monoamine
Oxidase Inhibitors
Wei Shen 1, Shian Yu 1, Jiaming Zhang 1, Weizheng Jia 2 and Qing Zhu 2,*
1
Department of General Surgery, Jinhua Central Hospital, Jinhua 321000, China
2
College of Biological and Environmental Engineering, Zhejiang University of Technology,
18 Chaowang Road, Hangzhou 310014, China
* Author to whom correspondence should be addressed; E-Mail: chmzhuq@hotmail.com;
Tel./Fax: +86-571-8832-0781.
External Editor: Derek J. McPhee
Received: 11 August 2014; in revised form: 6 November 2014 / Accepted: 7 November 2014 /
Published: 14 November 2014
Abstract: Monoamine oxidases (EC 1.4.3.4; MAOs), a family of FAD-containing enzymes,
is an important target for antidepressant drugs. In this paper, a series of 2-phenoxyacetamide
analogues were synthesized, and their inhibitory potency towards monoamine oxidases A
(MAO-A) and B (MAO-B) were evaluated using enzyme and cancer cell lysate.
2-(4-Methoxyphenoxy)acetamide (compound 12) (SI = 245) and (2-(4-((prop-2-
ynylimino)methyl)phenoxy)acetamide (compound 21) (IC50MAO-A = 0.018 μM,
IC50MAO-B = 0.07 μM) were successfully identified as the most specific MAO-A inhibitor,
and the most potent MAO-A/-B inhibitor, respectively. The inhibitory activities of these
two compounds in living cells were also further evaluated utilizing HepG2 and SHSY-5Y
cell lysates.
Keywords: monoamine oxidase; inhibitors; phenoxyacetamides; inhibitory activity;
cell lysates

Molecules 2014, 19
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1. Introduction
Monoamine oxidases (EC 1.4.3.4; MAOs) are FAD-containing enzymes that bind tightly to the outer
mitochondrial membrane in the brain, liver, intestinal mucosa, and other organs and catalyze the
oxidative deamination of biogenic and xenobiotic amines. There are two types of isoenzymes, MAO-A
and MAO-B, which can be distinguished by their differential substrates, inhibitor selectivity, tissue
distribution, and primary DNA sequences [1]. MAO-A is located predominantly in catecholaminergic
neurons, while MAO-B is present in serotonergic neurons and glia. MAO-A catalyzes the oxidation
of 5-hydroxytryptamine (5-HT) and norephinephrine, whereas MAO-B deaminates dopamine and
2-phenylethylamine (2-PEA). Recent studies have shown that MAOs are important target enzymes
for antidepressant drugs [2]. In addition, the biotransformation of 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine (MPTP) into 1-methyl-4-phenylpyridinium (MPP+) mediated by MAO-B results in
the neurodegenerative symptoms of the Parkinsonism [3]. Therefore, the study of MAO inhibitors has
attracted increasing interest in recent years for their therapeutic effect on mental illness. For example,
selective MAO-A inhibitors such as iproniazid, clorgyline and moclobemide exhibit antidepressant and
antianxiety activity [4,5]. Selective MAO-B inhibitors such as selegiline, rasagiline and lazabemide are
used as adjuncts in the treatment of Parkinson’s and Alzheimer’s diseases [6]. Unfortunately, most of
the existing MAO inhibitors, such as iproniazid and tranylcypromine, have been shown to induce
hepatotoxicity and another important side effect, the “cheese reaction” [7,8]. Thus, the development of
new MAO inhibitors for neuro-related diseases is urgently needed.
In order to address the need for new MAO inhibitors with fewer side effects, our group evaluated
compounds previously discovered for their potential as MAOIs. Among them, we found safinamide,
which was reported to be a potent anti-MAO B agent, and milacemide, which was found to be a potent
MAO inhibitor and a prodrug for glycine [9,10]. Of note, both compounds possess an acetamide group
(Figure 1), prompting us to hypothesize that acetamide might play an important role in inhibition of
MAO activities. Therefore, we herein report the synthesis of a new series of 2-phenoxyacetamide
analogues and the evaluation of their inhibition of MAO-A and MAO-B activities.
Figure 1. Examples of known monoamine oxidase (MAO) inhibitors bearing an acetamide group.
2. Results and Discussion
2.1. Chemistry
To begin, 2-phenoxyacetamide analogues were synthesized by a nucleophilic reaction, outlined
in Scheme 1. Substituted phenols with electron donating (Me or OMe) or withdrawing groups (F, Cl,
COOCH3 or CHO) reacted with 2-chloroethanamide under basic conditions, creating aryloxycarboxylic
acetamides in excellent yields.

Molecules 2014, 19
Scheme 1. General synthetic method for 2-phenoxyacetamide analogues.
18622
Furthermore, in order to broaden the scope of such kind of compounds, more derivatives bearing
heterocycles and amines (compounds 19–26) were synthesized according to the procedures outlined in
Scheme 2. For example, compounds 19 and 20 were obtained in modest yield from the condensation of
compound 6 or 7 with ethylenediamine. The reaction of the different aldehydes and propargylamine
(or benzylamine) produced the corresponding schiff bases 21, 23, 25, followed by reduction to
compounds 22, 24, and 26 respectively, with four equivalents of NaBH4. Structures of target
compounds 1–26 were confirmed by their1H and 13C NMR spectra and high-resolution electron
impactmass spectra (HR-EIMS) (see SI).
Scheme 2. Synthetic methods for compounds 19–26.
2.2. Enzyme Inhibition Studies
All of these compounds were evaluated for their inhibitory abilities towards MAOs. The inhibition
activities against MAO-A and MAO-B of the synthesized compounds were investigated by our reported

Molecules 2014, 19
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method [11,12]. The IC50 values with MAO-A and MAO-B and the selectivity index (SI, IC50 of
MAO-B/IC50 of MAO-A) of all compounds are shown in Table 1.
Table 1. Monamine oxidase inhibitory activity of compounds 1–28 a.
IC50 (μM)
Item
R
X
SI b
MAO-A
MAO-B
778
1
2
Phenyl
2-Naphthalenyl
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
69
11.27
3.64
2.77
0.41
7.08
5.13
3.94
4.73
25.5
180
149
542
3
4-Fluoro-phenyl
92
255
4
4-Chloro-phenyl
490
202
5
2-Chloro-phenyl
98
694
6
4-Formyl-phenyl
2-Formyl-phenyl
3,4-Dimethyl-phenyl
2-Methyl-phenyl
4-Methyl-phenyl
2-Methoxy-phenyl
4-Methoxy-phenyl
o-Carboxyphenyl
o-Acid ester
89
457
7
142
559
8
113
534
9
26
663
10
11
12
13
14
15
16
17
18
3
541
96
775
8.07
245
4
217
980
177
0.82
0.91
1.51
9.07
3.87
1.58
8.30
3.84
4.22
1.74
5.99
14.43
2.59
1.19
3.82
4.64
108
98
4-(N-tert-Butyl O-acyl)amine-phenyl
4-(N-acetyl)amine- phenyl
Phenyl
196
296
61
553
166
642
3,4-Dimethyl-phenyl
S
292
366
19 4-(4,5-dihydro-1H-imidazol-2-yl)-phenyl
20 2-(4,5-dihydro-1H-imidazol-2-yl)-phenyl
O
O
O
O
O
O
O
O
O
O
61
506
186
714
21
22
23
24
25
26
27
28
4-((Prop-2-ynylimino)methyl)-phenyl
4-((Prop-2-ynylamino)methyl)-phenyl
4-((benzylimino)methyl)-phenyl
4-((benzylamino)methyl)-phenyl
2-((Prop-2-ynylimino)methyl)-phenyl
2-((Prop-2-ynylamino)methyl)-phenyl
2-((benzylimino)methyl)-phenyl
2-((benzylamino)methyl)-phenyl
Clorgyline
0.018
0.094
96
0.076
0.164
575
37
534
0.068
0.168
147
0.176
0.188
562
107
497
0.0011 (0.0014)
Pargyline
0.0035 (0.0038)
a Results are expressed as mean IC50; b SI = (IC50 of MAO-B)/(IC50 of MAO-A).
The results clearly show that almost all compounds take on higher inhibitory activity against MAO-A
than MAO-B (SI > 1). Particularly, the compound 12 containing para-methoxyl moiety exhibits the best
selectivity and the SI value is up to 245 as an MAO-A inhibitor. On the basis of the results in Table 1,
the addition of methyl and methoxy groups in the benzene ring appears to increase inhibitory activity
toward MAO-A rather than MAO-B. The dimethyl-substituted compound 8 significantly lost selectivity
and inhibitory activity against MAO-A, compared with the single substitute compounds 9 and 10.
One can also conclude that the addition of F or Cl at the ortho or para position in the phenyl ring
(compound 3–6) seemed to decrease inhibitory activity against MAO-A. In order to increase inhibitory

Molecules 2014, 19
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activity and selectivity, amino groups were added by modification of compounds 6 and 7 with different
amines to obtain compounds 21–26. Their IC50 values for MAO-A and B were found to be decreased
by approximately 100 fold. The compound 21 bearing propynylimino moiety posses the best activity,
and the IC50 value is up to 18 nM, which indicates that imine group may contribute to the binding
affinity for MAOs. From Table 1, it can be further figured that the 2-phenoxyl derivatives 1 and 8 have
better activities than the 2-thiophenyl compounds 17 and 18, respectively, indicating that the replacement
of 2-O with 2-S has very little affect on the potency of inhibition toward MAO-A and MAO-B.
Having demonstrated that 2-phenoxyacetamides displayed good inhibitory activity by enzymatic
assay, we also carried out experiments to detect their inhibitory potency on the native MAOs, using
lysates from human tumor cells as source of enzyme. Thus, lysates from SH-SY5Y (neuroblastoma
cells) and HepG2 (human hepatocellular liver carcinoma cell) cells, where possessing only endogenous
MAO-A and B activities, respectively [13,14], were utilized determine inhibitory of the newly synthesized
compounds. Compounds 12, 21, 22, 25 were chosen for comparison due to their excellent inhibitory
activities, and the results were shown in Table 2. Substantially, all four compounds were found to be
able to inhibit the native enzymes from raw samples, but display much less potent inhibitory in cell
lysates than enzymes. The compound 21 was still the most potent inhibitor in SH-SY5Y and HepG2
lysates, and 21, 22 and 25 displayed 2.3, 2.0 and 2.1 times more potent in SH-SY5Y lysate relative to
HepG2 lysate, respectively, in agreement with enzyme assays. However, it is noteworthy that these
three compounds significantly lost inhibitory activities in both of the lysates compared to 12. This is
most likely due to the instability of imine group in cell lysates.
Table 2. MAO inhibitory activity (μM) in different cell lysates a.
Enzymes/Cell Lysates
SH-SY5Y
12
5.4
21
22
25
0.18
0.018
0.41
0.076
0.26
0.094
0.53
0.164
0.88
0.068
1.81
0.176
MAO-A
4.0
N.D. b
HepG2
MAO-B
980
a Results are expressed as mean IC50. All experiments were repeated three times; b N.D. means not determined.
2.3. Reversibility of Inhibition
To determine whether this new series of compounds are reversible or irreversible inhibitors toward
MAOs, the reversibility of inhibition of compound 12 and Safinamide was further studied using repeated
washing method. As shown in Table 3, the enzyme activities were able to be recovered by more than
80% after repeated washing, indicating that inhibition of both MAO-A and MAO-B is reversible in
presence of these two compounds.
Taken together, all these findings suggest that the derivatives of acetamide may be developed as
potential lead compounds for the treatment of neurodegenerative diseases (MAOI-B), and affective
disorders (MAOI-A).

Molecules 2014, 19
Table 3. Reversibility of MAOs inhibition with 12 and Safinamide.
18625
MAO-A Inhibiton (%)
Before Washing After Washing
88.7% 12.5%
36.1% 9.8%
MAO-B Inhibiton (%)
Before Washing After Washing
Compound
12 (100 nM)
96.9%
91.3%
8.6%
7.1%
Safinamide (100 nM)
3. Experimental Section
3.1. Materials and Methods
Silica gel (100–200 mesh, Qingdao Haiyang Chemical Co., Qingdao, China) was used for flash
column chromatography. Analytical thin-layer chromatography was performed with GF254 silica gel.
1H-NMR spectra were recorded on a Bruker Avance 300 MHz apparatus. HR-ESI-MS and HR-EI
analysis were measured on a Bruker micrOTOF-QII and Waters GCT Premier GC-TOF-MS spectrometer,
respectively. All chemicals were purchased from the Eastern China Chemical Co (Hangzhou, China).
All fluorescent data was recorded on Spectrum M2 (Molecular Device Company). Human recombinant
Monoamine Oxidase A (M7316) and B (M7441) (5 mg/mL) were purchased from Sigma Aldrich.
Pierce BCA protein assay kit (23227) was purchased from Thermo Scientific.
3.2. General Procedure for the Synthesis of Derivatives of Acetamides [15]
3.2.1. General Procedure for the Preparation of Compounds 1–18
2-Chloroethanamide (1 mmol), K2CO3 (1.5 mmol) and KI (0.1 mmol) was added to a solution of
phenol (1 mmol) in dry DMF. The reaction mixture was stirred at room temperature for 5 h, and was
extracted with EtOAc twice. The organic layer was dried over anhydrous MgSO4, and the solvent
stripped out in a rotary evaporator to obtain the crude product, which was further purified using silica
gel chromatography to obtain a white solid.
1
Compound 1: H-NMR (CDCl3-d3): δ 4.57 (s, 2H), 5.95 (brs, 1H), 6.71 (brs, 1H), 6.96 (d, J = 7.8 Hz,
2H), 7.06 (t, J = 7.3 Hz, 1H), 7.35 (m, 2H). 13C-NMR (CDCl3-d3): δ 67.9, 113.7, 123.5, 129.4, 158.1,
171.6. HR-MS (EI) calculated for C8H9NO2 [M+] 151.0633 found 151.0628.
Compound 2: 1H-NMR (DMSO-d6): δ 4.71 (s, 2H), 5.97 (brs, 1H), 6.61 (brs, 1H), 6.88 (d, J = 8.0 Hz,
13
1H), 7.45 (m, 4H), 7.88 (m, 1H), 8.39 (m, 1H). C-NMR (DMSO-d6): δ 67.7, 106.2, 110.7, 122.8,
124.5, 125.7, 126.0, 126.5, 127.3, 134.4, 153.1, 169.5. HR-MS (EI) calculated for C12H11NO2 [M+]
201.0790 found 202.0775.
1
Compound 3: H-NMR (DMSO-d6): δ 4.82 (s, 2H), 5.99 (s, 2H), 6.92 (s, 2H), 7.78 (m, 2H) 8.11 (m,
13
2H). C-NMR (DMSO-d6): δ 68.1, 115.2 (J = 6.9 Hz), 116.5 (J = 23.2 Hz), 152.4, 155.1, 171.6
(J = 238.1 Hz). HR-MS (EI) calculated for C8H9FNO2 [M+] 170.0617 found 170.0602.

Molecules 2014, 19
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1
Compound 4: H-NMR (DMSO-d6): δ 4.75 (s, 2H), 6.18 (s, 2H), 6.88 (m, 4H) 7.24 (m, 2H).
13C-NMR (DMSO-d6): δ 68.1, 115.7, 117.1, 132.4, 154.7, 171.4. HR-MS (EI) calculated for
C8H9ClNO2 [M+] 185.0244, 187.0214, found 185.0236, 187.0221.
Compound 5: 1H-NMR (DMSO-d6): 4.75 (s, 2 H), 6.12, 6.35 (s, 2H), 6.99 (m, 2 H), 7.32 (m, 3H). 13C-
NMR (DMSO-d6): δ 68.1, 122.1, 122.6, 129.3, 132.4, 170.4. HR-MS (EI) calculated for C8H9ClNO2
[M+] 185.0244, 187.0214, found 185.0250, 187.0238.
1
Compound 6: H-NMR (DMSO-d6): δ 4.74 (s, 2 H), 6.00 (brs, 1H), 6.52 (brs, 1H), 7.00 (d, J = 7.8 Hz,
2H), 7.89 (d, J = 7.8 Hz, 2H), 9.93 (s, 1H). 13C-NMR (DMSO-d6): δ 68.5, 115.6, 131.0, 133.5, 164.2,
191.3. HR-MS (EI) calculated for C9H9NO3 [M+] 179.0582, found 179.0584.
Compound 7: 1H-NMR (DMSO-d6): δ 4.75 (s, 2 H), 6.00 (brs, 1H), 6.52 (brs, 1H), 7.54 (m, 4H), 9.88
13
(s, 1H). C-NMR (DMSO-d6): δ 69.7, 115.2, 122.9, 126.1, 133.5, 136.6, 163.2, 170.7. HR-MS (EI)
calculated for C9H9NO3 [M+] 179.0582, found 179.0579.
Compound 8: 1H-NMR (CDCl3-d3): δ 2.31 (s, 3H), 2.37 (s, 3H), 7.58 (s, 2H), 5.67 (brs, 1H), 6.62 (brs,
1H), 7.07 (m, 3H). 13C-NMR (CDCl3-d3): δ 18.1, 19.6, 69.2, 1112.3, 115.9, 128.6, 129.9, 140.3, 157.6,
170.3. HR-MS (EI) calculated for C10H13NO2 [M+] 179.0946, found 179.0934.
1
Compound 9: H-NMR (CDCl3-d3): δ 2.18 (s, 3H), 4.58 (s, 2H), 5.67 (brs, 1H), 6.62 (brs, 1H), 7.12
13
(m, 4H). C-NMR (CDCl3-d3): δ 15.6, 69.6, 113.5, 123.1, 126.7, 1267.2, 132.9, 157.3, 169.8. HR-MS
(EI) calculated for C9H11NO2 [M+] 115.0790, found 115.0779.
Compound 10: 1H-NMR (CDCl3): δ 2.28 (s, 3H), 4.58 (s, 2H), 5.61 (brs, 1H), 6.60 (brs, 1H), 6.81 (d, J
13
= 8.5 Hz, 2H), 7.08 (d, J = 8.5 Hz, 2H). C-NMR (CDCl3-d3): δ 55.7, 68.0, 114.7, 115.8, 151.4,
154.1, 171.6. HR-MS (EI) calculated for C9H11NO2 [M+] 165.0790, found 165.0802.
1
Compound 11: H-NMR (DMSO-d6): δ 3.92 (s, 3H), 4.57 (s, 2H), 5.95 (brs, 2H), 7.00 (m, 4H).
13C-NMR (DMSO-d6): δ 55.3, 109.2, 116.7, 120.6, 122.3, 146.9, 150.5, 170.1. HR-MS (EI) calculated
for C9H11NO3 [M+] 181.0739, found 181.0742.
Compound 12: 1H-NMR (DMSO-d6): δ 3.97 (s, 3H), 4.57 (s, 2H), 5.81 (brs, 1H), 6.70 (brs, 1H), 6.93
13
(m, 4H). C-NMR (DMSO-d6): δ 20.4, 67.4, 69.2, 114.5, 130.2, 131.5, 155.2, 171.5. HR-MS (EI)
calculated for C9H11NO3 [M+] 181.0739, found 181.0721.
1
Compound 13: H-NMR (DMSO-d6): δ 4.74 (s, 2H), 5.67 (brs, 1H), 6.62 (brs, 1H), 7.50 (m, 4H),
12.09 (brs, 1H). 13C-NMR (DMSO-d6): δ 69.6, 115.2, 117.2, 122.1, 132.6, 136.5, 158.4, 166.2, 170.3.
HR-MS (EI) calculated for C9H9NO4 [M+] 195.0532, found 195.0526.
Compound 14: 1H-NMR (DMSO-d6): δ 3.38 (s, 3H), 4.78 (s, 2H), 5.67 (brs, 1H), 6.62 (brs, 1H), 7.49
(m, 4H), 10.09 (brs, 1H). 13C-NMR (DMSO-d6): δ 51.2, 69.6, 114.9, 117.1, 122.5, 132.6, 136.5, 158.8,
166.0, 170.3. HR-MS (EI) calculated for C10H11NO4 [M+] 209.0688, found 209.0697.

Molecules 2014, 19
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Compound 15: 1H-NMR (DMSO-d6): δ 1.40 (s, 9H), 4.75 (s, 2H), 7.38 (m, 4H). 13C-NMR (DMSO-d6): δ
28.9, 69.6, 79.2, 112.5, 120.3, 122.6, 127.7, 148.2, 148.9, 153.0, 170.1. HR-MS (EI) calculated for
C13H18N2O4 [M+] 266.1267, found 266.1258.
Compound 16: 1H-NMR (DMSO-d6): δ 2.11 (s, 3H), 4.76 (s, 2H), 7.03 (m, 4H). 13C-NMR (DMSO-d6): δ
25.1, 70.2, 113.1, 120.9, 121.6, 126.8, 129.5, 169.6, 170.1. HR-MS (EI) calculated for C10H12N2O3
[M+] 208.0848, found 208.0857.
Compound 17: 1H-NMR (CDCl3-d3): δ 3.63 (s, 2H), 6.10 (brs, 1H), 6.70 (brs, 1H), 7.27 (m, 4H). 13C-
NMR (CDCl3-d3): δ 38.2, 127.3, 130.1, 130.3, 136.8, 174.3. HR-MS (EI) calculated for C8H9NOS
[M+] 167.0405, found 167.0412.
1
Compound 18: H-NMR (CDCl3-d3): δ 2.30 (s, 3H), 2.38 (s, 3H), 4.58 (s, 2H), 5.67 (brs, 1H), 6.62
13
(brs, 1H), 7.03 (m, 3H), C-NMR (CDCl3-d3): δ 21.9, 38.2, 127.6, 131.9, 135.2, 174.9. HR-MS (EI)
calculated for C9H11NOS [M+] 181.0561, found 181.0554.
3.2.2. Preparation of Compounds 19 and 20
Ethane-1,2-diamine (1.10 mmol) was added to a solution of compounds 6 or 7 (1 mmol) in dry
CH2Cl2. The reaction mixture was stirred at room temperature for 40 min, then NBS (1 mmol) was
added and the reaction was stirred overnight. The resultant precipitate was extracted with chloroform
and washed successively with water. The organic layer was dried over anhydrous sodium sulfate, and
the solvent stripped out in a rotary evaporator to yield the crude product. The residue was further
purified using silica gel chromatography to obtain compound 19 or 20.
Compound 19: 1H-NMR (DMSO-d6): δ 3.80 (s, 4H), 3.96 (brs, 1H), 4.82 (s, 2H), 5.97 (brs, 1H), 6.82
13
(brs, 1H), 6.91 (d, J = 8.8 Hz, 2H), 7.77 (d, J = 8.8 Hz, 2H). C-NMR (DMSO-d6): δ 46.9, 71.5,
112.5, 130.2, 157.2, 164.8, 167.5. HR-MS (EI) calculated for C11H13N3O2 [M+] 219.1008, found 219.1023.
1
Compound 20: H-NMR (DMSO-d6): δ 3.80 (m, 4H), 4.83 (s, 2H), 5.67 (brs, 1H), 6.62 (brs, 1H), 7.04
13
(m, 2H), 7.43 (m, 1H), 8.00 (m, 1H). C-NMR (DMSO-d6): δ 46.9, 71.5, 110.1, 112.9, 120.6, 128.4,
132.0, 157.2, 164.8, 167.5. HR-MS (EI) calculated for C11H13N3O2 [M+] 219.1008, found 219.1018.
3.2.3. Preparation of Compounds 21, 23 or 25
Propargylamine or benzylamine (1.1 mmol) and MgSO4 (0.2 g) were added to a solution of
compound 6 or 7 (1 mmol) in dry THF (10 mL). The reaction mixture was stirred at room temperature
for 4 h. After completion of the reaction, the solid was removed by filtration and the solvent was
distilled, followed by the addition of water and EtOAc. The organic layer was dried over MgSO4,
filtered, and the solvent removed under vacuum. The residue was chromatographed to obtain the
products 21, 23 or 25.
1
Compound 21: H-NMR (DMSO-d6): δ 2.83 (s, 1H), 4.22 (m, 2H), 4.59 (s, 2H), 5.18 (m, 2H), 5.82
(brs, 1H), 6.50 (brs, 1H), 6.89 (d, J = 8.9 Hz, 2H), 7.08 (d, J = 8.9 Hz, 2H), 8.61 (s, 1H).

Molecules 2014, 19
18628
13C-NMR (DMSO-d6): δ 48.2, 67.7, 75.2, 79.1, 113.0, 134.0, 132.5, 171.2, 160.6, 190.2. HR-MS (EI)
calculated for C12H12N2O2 [M+] 216.0899, found 216.0873.
Compound 23: 1H-NMR (DMSO-d6): δ 4.50 (s, 2H), 4.77 (s, 2H), 5.85 (brs, 1H), 6.56 (brs, 1H), 6.92
(d, J = 8.5 Hz, 2H), 7.28 (m, 5H), 7.72 (d, J = 8.5 Hz, 2H), 8.31 (s, 1H). 13C-NMR (DMSO-d6): δ 65.1,
69.9, 115.1, 125.2, 125.8, 126.7, 127.2, 128.4, 129.0, 131.3, 138.9, 161.0, 162.8, 170.3. HR-MS (EI)
calculated for C16H16N2O2 [M+] 268.1212, found 268.1225.
Compound 25: 1H-NMR (DMSO-d6): δ 2.80 (s, 1H), 4.36 (s, 2H), 4.65(s, 2H), 6.92 (m, 2H), 7.35 (m,
13
1H), 7.89 (m, 1H), 8.65 (s, 1H). C-NMR (DMSO-d6): δ 44.8, 70.3, 71.1, 81.2, 114.4, 121.2, 130.6,
132.3, 132.3, 142.5, 165.5, 190.6. HR-MS (EI) calculated for C12H12N2O2 [M+] 216.0899,
found 216.0908.
3.2.4. Preparation of Compound 22, 24 or 26
Compound 21, 21 or 25 (1 mmol) and NaBH4 (3 mmol) were dissolved in absolute EtOH under
mechanical stirring for 15 min. The ethanolic filtrate was evaporated to dryness, dissolved in DCM,
and washed with water. The organic layer was dried over MgSO4, filtered, and evaporated to dryness
to obtain compounds 22, 24 or 26.
1
Compound 22: H-NMR (DMSO-d6): δ 2.65 (m, 1H), 3.72 (s, 2H), 4.65(s, 2H), 6.86 (d, J = 8.6 Hz,
13
2H), 7.24 (d, J = 8.6 Hz, 2H). C-NMR (CDCl3-d3): δ 38.8, 52.6, 68.1, 72.8, 83.4, 113.6, 132.1,
131.0, 156.9, 169.2. HR-MS (EI) calculated for C12H14N2O2 [M+] 218.1055, found 218.1070.
Compound 25: 1H-NMR (DMSO-d6): δ 2.80 (s, 1H), 4.36 (s, 2H), 4.65(s, 2H), 6.92 (m, 2H), 7.35 (m,
13
1H), 7.89 (m, 1H), 8.65 (s, 1H). C-NMR (DMSO-d6): δ 44.8, 70.3, 71.1, 81.2, 114.4, 121.2, 130.6,
132.3, 132.3, 142.5, 165.5, 190.6. HR-MS (EI) calculated for C12H12N2O2 [M+] 216.0899,
found 216.0908.
1
Compound 26: H-NMR (DMSO-d6): δ 2.65 (s, 1H), 3.47 (m, 2H), 3.78 (s, 2H), 4.85 (s, 2H), 5.79
13
(brs, 1H), 6.64 (brs, 1H), 6.90 (m, 1H), 7.23 (m, 3H). C-NMR (CDCl3-d3): δ 39.7, 46.9, 69.3, 72.1,
83.4, 112.4, 119.8, 123.1, 127.6, 128.2, 157.6, 169.5. HR-MS (EI) calculated for C12H14N2O2 [M+]
218.1055, found 218.1050.
3.3. Biochemistry
3.3.1. MAOs Inhibitory Assay
MAO IC50 values were determined using our reported fluorescence-based microplate assay [11].
Briefly, MAO-A (or -B) and inhibitors (10 nM–1000 μM) were incubated in borate buffer (50 mM,
pH 8.4) at 37 °C for 3 h, followed by the addition of 50 μM 4-methyl-7-(3-aminopropoxy) coumarin
(Km 62 μM or 82 μM for MAO-A and MAO-B, respectively). The fluorescent signals were measured
at λex 360 nm and λem 460 nm on a Spectrum-M2 spectrofluorometer.
IC50 values were calculated by nonlinear regression analysis using the four-parameter equation
[Y = max − (max − min)/(1 + (X/IC50)N] where Y is the observed fluorescence, X is the concentration

Molecules 2014, 19
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of inhibitor, max is the fluorescence in the absence of inhibitor (0% control), min is the fluorescence
in the presence of inhibitor (MAO activity was completed inhibited, 100% control), IC50 is the
concentration of inhibitor that gave 50% increase in fluorescence, and N is the empirical Hill slope.
Curve fitting was performed using commercial software (IDBS ActivityBase XE). Each IC50 was
calculated based on 5 points ranged from 10 nM to 1 mM (10, 20, 40, 80 nM, 0.1, 0.2, 0.4, 0.8, 1, 2, 4,
8, 20, 40, 80, 200, 400, 800, 1000 μM) based on its value of IC50.
3.3.2. Cell Lysates Preparation
Cells were cultured at 37 °C and 5% CO2 in RPMI-1640 media containing 100 units/mL penicillin,
100 μg/mL streptomycin, and 10% FBS. For lysis, cells were resuspended at 5 × 107 cells/mL in lysis
buffer (42.3 mM PBS, 126 mM NaCl, 1.27 mM MgCl2, 0.85 mM EDTA, 1 mM PMSF, 0.95% Triton
X-100, 9.5% glycerol, 4% 25× complete protease inhibitor, pH 7.4). The cells were vortexed to mix
and keep on ice for 30 min, and then sonicated to break the cells (180 watts). The cell lysate was then
clarified by ultracentrifugation at 4 °C for 30 min at 45,000. Total protein concentration was determined
via a Bradford Protein Assay Kit (Bio-Rad) and cell lysates were stored at −80 °C until further use.
3.3.3. Reversibility Experiments
Reversibility of the MAO inhibition with derivatives of acetamides was evaluated by a reported
method [16]. MAO-A or B were incubated with 21 (50 nM) or safinamide (100 nm) as the reference in
a sodium phosphate buffer (PBS, 50 mM, pH 7.4) at 37 °C. After incubating for 2 h, an aliquot was
stored at 4 °C for the measurement of MAO-A and -B activity. The remaining reaction solution was
placed in an Ultrafree-0.5 centrifugal tube with a 30 kDa Biomax membrane (Millipore) and
centrifuged at 9000 g for 20 min at 4 °C. The enzyme retained in the membrane was resuspended in a
sodium phosphate buffer and centrifuged two successive times. Then, the enzyme retained in the
membrane was resuspended in PBS (100 mL) and an aliquot of this suspension was used for MAO-A
and -B activity measurement. In order to define 100% MAO activity, control experiments were
performed simultaneously by replacing the inhibitors with appropriate dilutions of the vehicles. The
corresponding values of percent MAO isoforms inhibition (with and without repeated washing) were
calculated based on the control experiments.
4. Conclusions
In conclusion, we have synthesized a series of 2-phenoxyacetamide analogues and evaluated their
monoamine oxidase (MAO) A/B inhibitory activity and selectivity in vitro. The results show that most
of the synthesized compounds are potent and selective inhibitors of MAO-A rather than of MAO-B.
Of particular importance is compound 12, which has exhibited excellent inhibitory activity selectivity
toward MAO-A, and compound 21, which displayed the highest MAO-A inhibitory potency. Future
studies aimed at designing new MAO inhibitors should focus on addition of acetamide, which we have
established as a unique functional group capable of potently inhibition MAOs.

Molecules 2014, 19
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Acknowledgments
This work was financially supported by Zhejiang Natural Science Fund (No. LY12B02018),
Project of Science Technology Department of Zhejiang Province (2014C33141) and Project of Science
Technology Department of Jinhua City (2013-3-003).
Author Contributions
W.S., S.Y., J.Z. and W.J. performed research and analyzed the data; Q.Z. designed research and
wrote the paper. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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