PEG 400/cerium ammonium nitrate combined with microwave-assisted synthesis for rapid access to beta-amino ketones. an easy-to-use protocol for discovering new hit compounds

Article abstract of DOI:10.3390/molecules23040775

Compound libraries are important requirement in target-based drug discovery. In the present work, a small focused compound library based on β-aminoketone scaffold has been prepared combining microwave-assisted organic synthesis (MAOS) with polymer-assiste

Full text of DOI:10.3390/molecules23040775

molecules
Article
PEG 400/Cerium Ammonium Nitrate Combined with
Microwave-Assisted Synthesis for Rapid Access to
Beta-Amino Ketones. An Easy-to-Use Protocol for
Discovering New Hit Compounds
Giacomo Rossino 1,†, Maria Valeria Raimondi 2,†, Marta Rui 1, Marcello Di Giacomo 1
,
ID
Daniela Rossi 1 and Simona Collina 1,
*
ID
1
Drug Sciences Department, Medicinal Chemistry and Pharmaceutical Technology Section,
University of Pavia, 27100 Pavia, Italy; giacomo.rossino01@universitadipavia.it (G.R.);
marta.rui01@universitadipavia.it (M.R.); marcello.digiacomo@unipv.it (M.D.G.);
daniela.rossi@unipv.it (D.R.)
Department of Biological, Chemical and Pharmaceutical Sciences and Technologies, Medicinal Chemistry
and Pharmaceutical Technologies Section, University of Palermo, 90100 Palermo, Italy;
mariavaleria.raimondi@unipa.it
2
*
†
Correspondence: simona.collina@unipv.it; Tel.: +39-0382-987379
These Authors contributed equally to this work.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 2 March 2018; Accepted: 26 March 2018; Published: 28 March 2018
Abstract: Compound libraries are important requirement in target-based drug discovery. In the
present work, a small focused compound library based on β-aminoketone scaffold has been prepared
combining microwave-assisted organic synthesis (MAOS) with polymer-assisted solution phase
synthesis (PASPS) and replacing reaction workup standard purification procedures with solid phase
extraction (SPE). Specifically, the effects of solvent, such as dioxane, dimethylformamide (DMF),
polyethylene glycol 400 (PEG 400), temperature, irradiation time, stoichiometric ratio of reagents, and
catalysts (HCl, acetic acid, cerium ammonium nitrate (CAN)) were investigated to maximize both
conversion and yield. The optimized protocol generally afforded the desired products in satisfying
yields and purities. The designed library is a part of our current research on sigma 1 receptor
modulators, a valuable tool for the identification of novel potential hit compounds.
Keywords: Mannich reaction;
β-aminoketones; microwave-assisted organic synthesis; polymer-
assisted solution phase synthesis; solid phase extraction; drug discovery
1. Introduction
Identifying hit compounds is the first step in the complex drug-discovery process, and the
degree of structural diversity is an important element, enhancing the rate of success in finding
a potential lead candidate. In this context, β-amino carbonyl compounds represent a class of important
pharmacophores and useful building blocks for the synthesis of diverse classes of biologically active
molecules [1,2].
Numerous
medicinal chemistry, such as anti-inflammatory [
analgesic [ ], and anticancer activity [10 13], to cite just a few examples (Figure 1). Moreover,
acids are found in some important bioactive natural compounds and are widely employed in the
preparation of peptide-based drugs [14 17] (Figure 1). No less important, -amino ketones can be key
intermediates for the synthesis of pharmaceutically relevant compounds [18,19].
β
-amino ketones and their analogues exhibit potent activity of great interest in
], antibacterial [ ], antiviral [ ], antifungal [ ],
-amino
3,
4
5
,6
7
6,8
9
–
β
–
β
Molecules 2018, 23, 775; doi:10.3390/molecules23040775
www.mdpi.com/journal/molecules

Molecules 2018, 23, 775
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Figure 1. Some important β-amino ketones, both synthetic and natural, and their biological properties.
While many synthetic strategies to achieve β-amino carbonyl compounds can be found in the
recent literature (Figure 2), such as aza-Michael reaction [20], enamine-aldehyde cross-coupling
via N-heterocyclic carbenes [21], copper-catalyzed electrophilic amination of cyclopropanols [22],
Pd-catalyzed aminocarbonylation of alkenes [23], and hydrogenolysis of isoxazolines [24], the Mannich
multicomponent reaction (Figure 2) remains the most used procedure [2,25], and many improvements
to and implementations of the original protocol have been studied [19,26].
Figure 2. Alternative synthetic routes for accessing β-amino carbonyl compounds.
In particular, the three-component one-pot Mannich reaction allows the formation of β-amino
ketones, presenting general structure A or B (Figure 2) with great structural variability, depending on
the amine and aldehyde employed [27,28].
In light of these considerations and as part of our ongoing research, we herein focus on the
development of an efficient protocol based on the three-component one-pot Mannich reaction for the
preparation of a
β-amino ketone small library endowed with general formula A (Figure 2), consisting
of a tertiary amine bridged to an aromatic ring by a propylenic chain. The final aim is to discover

Molecules 2018, 23, 775
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new potential sigma receptor (SR) modulators [29
–
32]. We set up an efficient, clean, quick, and
scalable protocol based on microwave-assisted organic synthesis (MAOS), using cerium ammonium
nitrate (CAN) as a catalyst and polyethylene glycol 400 (PEG 400) as a solvent, combined with
polymer-assisted solid phase synthesis (PASPS). Purification of final compounds occurred by solid
phase extraction (SPE). Overall, our strategy led us to obtain the desired
and quickly.
β-amino ketones efficiently
2. Results and Discussion
Through this procedure, a small focused library of 36
of aryl-ketones with amines (Figure 3) was prepared. Relying on our long experience in the SR
field, both building blocks were selected by taking into account the state-of the-art structure activity
relationship (SAR) of SR ligands [33 34]. We exploited aromatic or heterocyclic methyl-ketones (1–6)
β-amino ketones derived from the coupling
1–
6
a–f
,
and cyclic (a, d, f) or benzyl acyclic (b, c, e) secondary amines (Figure 3).
Figure 3. Designed library.
2.1. Setup and Optimization of Synthetic Protocol
According to data in the literature concerning the different reactivities of secondary amines related
to their structures and experimental conditions in the Mannich reaction [25 35], we set up a novel protocol
using the cyclic and acyclic amines and as “building block” models. First, compounds 1a and 1b were
synthesized with conventional heating, applying an existing protocol (Figure 4, condition A), and were
properly purified [36]. Molar extinction coefficients of acetophenones 1a, and 1b were determined
(1.265
104, 6.327 103, and 6.703 103 L mol−1 cm−1, respectively) and high performance liquid
,
a
b
1,
×
×
×
·
·
chromatography-ultraviolet-photodiode array detector (HPLC-UV-PAD) methods were devecloped
to determine the percentage of conversion and purity of new compounds. Afterwards, based on our
own experience, we set up a microwave-assisted synthetic protocol (Figure 4) to obtain our
β-amino
ketones . Of note, MAOS has already been successfully employed in Mannich reactions [37,
1
–
6
,
a
–
f
38].
Microwave oven parameters (i.e., temperature, irradiation power, and time) were explored and different
solvents, such as dioxane, dimethylformamide (DMF), tetrahydrofuran (THF),◦methanol (MeOH),
◦
ethanol (EtOH) tested. Temperature and irradiation power varied from 35 C to 200 C and from 60 W to
200 W, respectively, as did irradiation time. Lastly, both type and amount of protic acidic additive were
evaluated (HCl, HBF4, HClO4, acetic acid). Unfortunately, no satisfying results were obtained. Therefore,
we considered the use of ceric ammonium nitrate (CAN) as a catalyst in PEG 400, as it had already
been used in a three-component Mannich reaction to access β-amino ketones of general structure B
(Figure 2) under conventional heating [39]. Accordingly, we employed this catalyst/solvent combination
in our microwave-assisted protocol to access the designed compounds of general structure A (Figure 2).

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Compounds 1a and 1b were obtained under microwave irradiation (60 W, 90 ◦C for 10 min) using 5%
mol of CAN in PEG 400. The HPLC analysis (see Appendix A) showed that the reaction was clean and
quick, affording the desired products with 80% conversion. Interestingly, using hydrochloride amines as
reagents led to the best results. A schematic comparison between the old and new protocols (path A and
path B, respectively) is shown in Figure 4.
Figure 4. Comparison between (A) traditional and (B) new protocol.
With these promising results, we moved forward to determine the effect of stoichiometric ratio
of reagents on both conversion percentage and crude purity. Results are reported in Tables 1 and 2.
Reactions conducted with excess amine (entries 2–4 and 10–13) led to high conversion percentages,
even if the products had lower purities. An opposite trend was observed using an excess of ketone
(entries 5–8 and 14–17). Accordingly, conditions of entries 6 and 15 (i.e., 2 equivalents (eq.) of ketone
and 1 eq. of amine) were considered the best compromise and were extended to the other substrate for
preparation of the whole library.
Table 1. Investigation of stoichiometric ratio of the reagents on conversion and purity of compound 1a
.
Entry
1 (eq.) a (eq.)
% Conversion
% Purity
1
2
3
4
5
6
7
8
1
1
1
1
1.5
2
1
1.5
2
2.5
1
1
1
1
80.0
76.7
79.9
78.5
67.1
72.4
77.2
67.7
82.5
77.8
67.5
60.7
68.9
97.7
96.1
95.8
2.5
3
Reagents and reaction conditions: cerium ammonium nitrate (CAN) (0.05 eq.), paraformaldehyde (1.0 eq.), PEG 400,
(MW: 90 ◦C, 60 W, 10 min).
Table 2. Investigation of stoichiometric ratio of the reagents on conversion and purity of compound 1b
.
Entry
1 (eq.) b (eq.)
% Conversion
% Purity
9
1
1
1
1
1
1.5
2
3
1
2
2.5
3
3.5
1
1
80.0
77.2
86.9
93.4
64.9
81.5
74.5
61.6
49.2
60.1
35.1
34.2
23.5
23.5
69.4
81.1
89.6
92.2
10
11
12
13
14
15
16
17
1
1
3.5
Reagents and reaction conditions: CAN (0.05 eq.), paraformaldehyde (1.0 eq.), PEG 400, (MW: 90 ◦C, 60 W, 10 min).
2.2. MW-Assisted Library Synthesis
The optimized protocol was then employed to synthesize the small focused library. The following
table reports the yields and purities of each compound.

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The developed protocol allowed production of the desired compounds (Table 3) with the exception
of dibenzylaminic ( 6c) and 4 acetyl-pyridinic (4a ) compounds (Figure 3). Of note, cyclic amines
) were well tolerated in this protocol; in particular, piperidines were the most versatile reagents,
1–
–f
(a, d, f
since they were able to react with the ketone counterpart, offering products in satisfying yields
(Table 3). Relying on the molecules endowed with an acyclic amine, different reactivity was seen:
N-benzylmethylamine > 3,4-dimethoxy-N-methylbenzylamine > dibenzylamine. The failure in the
reaction involving dibenzylamine may be attributed to its low basicity, which may slow down the
formation of the intermediate imine resulting from the reaction with formaldehyde. This result is not
surprising. Indeed, it has been shown that the reactivity in the Mannich reaction is strictly related to
the amine structure, and within a homologous series the reactivity may be different. An emblematic
example is diethylamine, which is unable to be transformed into β-amino ketone, whereas the superior
and inferior amine analogues react efficiently to give access to the desired products [35]. Regarding
the ketone building block, the procedure was successfully applied to aromatic ketones, presenting
benzene and thienyl nucleus, whereas no product was observed when ketone
moiety, was used. This behavior can be explained by considering the basic properties of the pyridine
ring, which reduced the reactivity of methyl ketone . We performed additional model reactions on
ketone under traditional conditions at high temperature and for long times to force the reaction.
4, bearing a pyridine
4
4
Again, the desired products (compounds 4a and 4b) were not isolated, supporting our hypothesis that
the low reactivity of 4 acetyl-pyridine compromised the reaction outcome.
Table 3. Yield and purity of compounds 1a–6f.
Compound
Ar
NR1R2
Yield % a
Purity % a
1a
1b
1d
1e
1f
4-benzylpiperidine
N-benzylmethylamine
piperidine
72
70
58
33
-
98
86
75
phenyl
3,4-dimethoxy-N-methylbenzylamine
66
morpholine
Traces (5)
2a
2b
2d
2e
2f
4-benzylpiperidine
N-benzylmethylamine
piperidine
32
34
50
38
46
69
81
75
54
71
naphtyl
biphenyl
3,4-dimethoxy-N-methylbenzylamine
morpholine
3a
3b
3d
3e
3f
4-benzylpiperidine
N-benzylmethylamine
piperidine
75
55
50
-
75
67
73
3,4-dimethoxy-N-methylbenzylamine
Traces (5)
61
morpholine
50
5a
5b
5d
5e
5f
4-benzylpiperidine
N-benzylmethylamine
piperidine
63
56
30
25
32
95
85
83
66
77
2-thienyl
3,4-dimethoxy-N-methylbenzylamine
morpholine
6a
6b
6d
6e
6f
4-benzylpiperidine
N-benzylmethylamine
piperidine
55
40
29
n.r.
6
76
37
57
-
5-bromo-2-thienyl
3,4-dimethoxy-N-methylbenzylamine
morpholine
38
Reagents and reaction conditions: ketone (2.0 eq.), amine (1.0 eq.), CAN (0.05 eq.), paraformaldehyde (1.0 eq.),
a
PEG 400, (MW: 90 ◦C, 60 W, 10 min). determined by LC-MS analysis. - no data. n.r. no reaction.

Molecules 2018, 23, 775
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3. Materials and Methods
Reactions performed under conventional heating were monitored by thin layer chromatography
(TLC) with Fluka silica gel 60 F254 (Merck KGaA, Darmstadt, Germany) and purified by automatic
flash chromatography with CombiFlash®RF (AlfaTech, Teledyne Isco, Inc., Genoa, Italy).
All reactions conducted under microwave irradiation were performed in a microwave mono-mode
oven specifically for organic synthesis (Discover® LabMate instrument, CEM Corporation, Matthews,
NC, USA). The obtained products were purified with Bond Elut SCX® cartridge (Varian, Walnut Creek,
CA, USA) and silica gel SPE cartridge (Varian, Walnut Creek, CA, USA).
UV spectra were recorded on a LAMBDA
MA, USA). HPLC analyses were carried out on a Jasco HPLC system (Jasco Europe S.r.l., Cremella, Italy),
consisting of a pump model PU 1580, a Reodyne 7125 injector (20 L sample loop), and an MD-1510
™ 25 UV/VIS spectrometer (Perkin Elmer Inc., Waltham,
µ
diode array detector, combined with a Spectra AS3000 autosampler. Experimental data were acquired
and interpreted with Borwin PDA and Borwin chromatograph software 1.5. Reversed-phase HPLC
analyses were carried out at room temperature on an XTerra RP18 column (3.5
µ
m, 4.6
×
50 mm)
(Waters, Milford, MA, USA) and a Hypersil ODS RP18 column (3 m, 4.6 100 mm) (VWR, Milano,
µ
×
Italy). The mobile phase was phosphate buffer (pH 7.8) added with acetonitrile as organic modifier;
the analysis was carried out using gradient elution (see Tables A1 and A2, Figures A1 and A2 in
Appendix A).
Electrospray ionization LC-MS analyses were performed with a single quadrupole AQA
ThermoQuest Finnigan (ThermoFinnigan, San Jose, CA, USA) or a Waters Micromass ZQ2000 (Waters,
Milford, MA, USA), employing an XBridge C8 column (3.5
µ
m, 4.6
×
50 mm) (Waters, Milford, MA, USA).
1H-NMR spectra were registered with a Brüker ARX 300 (300 MHz) (Bruker Daltonics, Billerica,
MA, USA). Chemical shifts are reported in parts per million (δ) downfield from tetramethylsilane
(TMS) as internal standard.
3.1. General Procedure for the Synthesis of β-Aminoketone 1a and 1b under Conventional Heating (Method A)
A solution of acetophenone (1.0 eq.), amine (2.0 eq.), paraformaldehyde (2.0 eq.), and HCl (2.0 eq.)
in absolute ethanol (2.25 mL) was refluxed for 24 h in N2 atmosphere under magnetic stirring. Then,
the reaction mixture was evaporated under reduced pressure and the residue purified by automated
flash chromatography (CombiFlash®RF) using a mixture of 80:20 hexane:diethyl ether, 0.1 NH3/MeOH
as eluent, and silica gel RediSep column (12 g) (particle size: 35–70 µm).
3.2. General Procedure for the Synthesis of β-Aminoketones 1a–6f under Microwave Heating (Method B)
A mixture of ketone (2.0 eq.), amine hydrochloride (1 eq.), paraformaldehyde (1 eq.), and CAN
(0.05 eq.) in PEG 400 (0.8 mL) was irradiated with a microwave power of 60 watts at 90 ◦C for 10 min.
The reaction workup was performed as follows: the mixture was quenched in 2 M NaOH, then
the solid was collected by centrifugation, dissolved in methanol or dichloromethane (depending on
the solubility of the compound), and purified using SCX cartridge, eluting with a solution of 0.3 M
NH3/MeOH in dichloromethane to remove the excess ketone. Finally, the product was isolated using
silica gel SPE cartridge, eluting with dichloromethane to remove the nonreacted amine. Then the
organic phase was evaporated to dryness.
3.3. Analytical Data of Prepared Compounds
3-(4-Benzylpiperidin-1-yl)-1-phenylpropan-1-one (1a). Yield: 44% (method A), 72% (method B). Yellow oil;
1H-NMR (300 MHz, CDCl3) (ppm): 1.62 (br s, 2H), 1.74 (d, 3H), 2.26 (br s, 2H), 2.58 (d, 2H), 2.95–3.26
(m, 4H), 3.43 (t, 2H), 7.10–7.25 (m, 3H), 7.25–7.33 (m, 2H), 7.43–7.53 (m, 2H), 7.53–7.63 (m, 1H), 7.92–8.03
(m, 2H); LC-MS: Purity 98%; RT 4.53 min. MH+ 308.14 [40].

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3-[Benzyl(methyl)amino]-1-phenylpropan-1-one (1b). Yield: 70%; Yellow oil; 1H-NMR (300 MHz, CDCl3)
(ppm): 2.34 (s, 3H), 3.00 (t, 2H), 3.29 (t, 2H), 3.67 (br s, 2H), 7.28–7.32 (m, 1H), 7.32–7.39 (m, 4H),
7.42–7.50 (m, 2H), 7.53–7.61 (m, 1H), 7.92–7.88 (m, 2H); LC-MS: Purity 86%; RT 3.84 min. MH+
254.11 [41].
1-Phenyl-3-(piperidin-1-yl)propan-1-one (1d). Yield: 58%; 1H-NMR (300 MHz, CDCl3) (ppm): 1.49–1.57
(m, 2H), 1.62–1.79 (m, 4H), 2.54–2.71 (m, 4H), 2.94 (t, 2H), 3.34 (t, 2H), 7.41–7.52 (m, 2H), 7.53–7.63 (m,
1H), 7.94–8.01 (m, 2H); LC-MS: Purity 75%; RT 3.12 min. MH+ 218.16 [42].
1
3-[(3,4-Dimethoxybenzyl)(methyl)amino]-1-phenylpropan-1-one (1e). Yield: 33%; Yellow oil; H-NMR
(300 MHz, CDCl3) (ppm): 2.34 (s, 3H), 2.94–3.06 (m, 2H), 3.21–3.33 (m, 2H), 3.71 (s, 2H), 3.88 (s, 6H),
6.65–6.77 (m, 2H), 6.77–6.87 (m, 2H), 7.42–7.53 (m, 2H), 7.53–7.62 (m, 1H), 7.91–8.05 (m, 1H); LC-MS:
Purity 66%; RT 3.79 min. MH+ 314.12.
3-(Morpholin-4-yl)-1-phenylpropan-1-one (1f). LC-MS: Purity 5%; RT 2.73 min. MH+ 220.11 [41]
3-(4-Benzylpiperidin-1-yl)-1-(naphthalen-2-yl)propan-1-one (2a). Yield: 32%; Yellow oil; 1H-NMR
(300 MHz, CDCl3) (ppm): 1.52–1.88 (m, 5H), 2.19–2.48 (m, 2H), 2.60 (d, 2H), 3.04–3.35 (m, 4H),
3.46–3.76 (m, 2H), 7.12–7.24 (m, 3H), 7.28–7.36 (m, 2H), 7.47–7.70 (m, 2H), 7.84–7.94 (m, 2H), 7.95–8.08
(m, 2H), 8.54 (s, 1H); LC-MS: Purity 69%; RT 4.99 min. MH+ 358.09 [40].
3-[Benzyl(methyl)amino]-1-(naphthalen-2-yl)propan-1-one (2b). Yield: 34%; 1H-NMR (300 MHz, CDCl3)
(ppm): 2.38 (s, 3H), 3.06 (t, 2H), 3.43 (t, 2H), 3.71 (s, 2H), 7.30–7.41 (m, 5H), 7.52–7.66 (m, 2H), 7.85–7.93
(m, 2H), 7.97 (d, 1H), 8.02 (dd, 1H), 8.47 (s, 1H); LC-MS: Purity 81%; RT 4.51 min. MH+ 304.11 [41].
1-(Naphthalen-2-yl)-3-(piperidin-1-yl)propan-1-one (2d). Yield: 50%; 1H-NMR (300 MHz, CDCl3) (ppm):
1.44–1.63 (m, 2H), 1.72–1.91 (m, 4H), 2.60–2.88 (m, 4H), 3.11 (t, 2H), 3.57 (br t, 2H), 7.46–7.68 (m, 2H),
7.83–8.11 (m, 4H), 8.53 (br s, 1H); LC-MS: Purity 75%; RT 4.04 min. MH+ 268.14 [42].
3-((3,4-Dimethoxybenzyl)(methyl)amino)-1-(naphthalen-2-yl)propan-1-one (2e). Yield: 38%; Yellow oil;
1H-NMR (300 MHz, CDCl3) (ppm): 2.32 (s, 3H), 2.98 (t, 2H), 3.34 (t, 2H), 3.60 (s, 2H), 3.85 (s, 6H), 6.8
(m, 3H), 7.70 (m, 2H), 7.98–8.15 (m, 4H), 8.50 (s, 1H); LC-MS: Purity 54%; RT 4.41 min. MH+ 364.08.
3-(Morpholin-4-yl)-1-(naphthalen-2-yl)propan-1-one (2f). Yield: 46%; 1H-NMR (300 MHz, CDCl3) (ppm):
2.76 (br s, 4H), 3.10 (br t, 2H), 3.51 (br t, 2H), 3.86 (br t, 4H), 7.53–7.66 (m, 2H), 7.86–7.94 (m, 2H),
7.96–8.01 (m, 1H), 8.04 (dd, 1H), 8.51 (br s, 1H); LC-MS: Purity 71%; RT 3.78 min. MH+ 270.09 [42].
3-(4-Benzylpiperidin-1-yl)-1-(biphenyl-4-yl)propan-1-one (3a). Yield: 75%; 1H-NMR (300 MHz, CDCl3)
(ppm): 1.67–1.84 (m, 5H), 2.58 (br d, 2H), 3.18–3.37 (m, 4H), 3.53–3.71 (m, 4H), 7.09–7.17 (m, 2H),
7.19–7.24 (m, 1H), 7.28–7.32 (m, 1H), 7.37–7.52 (m, 4H), 7.60–7.66 (m, 2H), 7.67–7.74 (m, 2H), 8.03–8.09
(m, 2H); LC-MS: Purity 75%; RT 5.21 min. MH+ 384.12.
1
3-[Benzyl(methyl)amino]-1-(biphenyl-4-yl)propan-1-one (3b). Yield: 55%; H-NMR (300 MHz, CDCl3)
(ppm): 2.40 (s, 3H), 3.02–3.15 (m, 2H), 3.39 (t, 2H), 3.75 (br s, 2H), 7.28–7.44 (m, 6H), 7.45–7.53 (m, 2H),
7.60–7.66 (m, 2H), 7.66–7.73 (m, 2H), 7.99–8.07 (m, 2H); LC-MS: Purity 67%; RT 4.83 min. MH+ 330.11.
1
1-(Biphenyl-4-yl)-3-(piperidin-1-yl)propan-1-one (3d). Yield: 50%; H-NMR (300 MHz, CDCl3) (ppm):
1.51–1.66 (m, 2H), 1.72–1.86 (m, 4H), 2.66–2.78 (m, 4H), 3.06 (t, 2H), 3.46 (br t, 2H), 7.38–7.52 (m, 3H),
7.61–7.66 (m, 2H), 7.67–7.73 (m, 2H), 8.03–8.10 (m, 2H); LC-MS: Purity 73%; RT 4.38 min. MH+ 294.16.
3-[(3,4-Dimethoxybenzyl)(methyl)amino]-1-(biphenyl-4-yl)propan-1-one (3e). LC-MS: Purity 5%; RT 4.73
min. MH+ 390.03.
1-(Biphenyl-4-yl)-3-(morpholin-4-yl)propan-1-one (3f). Yield: 50%; 1H-NMR (300 MHz, CDCl3) (ppm):
2.70–2.86 (m, 4H), 3.04–3.16 (m, 2H), 3.38–3.50 (m, 2H), 3.84–3.93 (m, 4H), 7.39–7.53 (m, 3H), 7.61–7.67
(m, 2H), 7.68–7.73 (m, 2H), 8.03–8.09 (m, 2H); LC-MS: Purity 61%; RT 4.18 min. MH+ 296.11 [40].

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3-(4-Benzylpiperidin-1-yl)-1-(thiophen-2-yl)propan-1-one (5a). Yield: 63%; 1H-NMR (300 MHz, CDCl3)
(ppm): 1.37–1.77 (m, 5H), 2.12 (t, 2H), 2.57 (d, 2H), 2.92 (t, 2H), 2.98–3.08 (m, 2H), 3.23 (t, 2H), 7.12–7.17
(m, 3H), 7.19–7.28 (m, 2H), 7.28–7.32 (m, 1H), 7.63–7.67 (m, 1H), 7.77 (dd, 1H); LC-MS: Purity 95%; RT
4.40 min. MH+ 314.05 [43].
1
3-[Benzyl(methyl)amino]-1-(thiophen-2-yl)propan-1-one (5b). Yield: 56%; H-NMR (300 MHz, CDCl3)
(ppm): 2.35 (s, 3H), 2.97–3.07 (m, 2H), 3.19–3.29 (m, 2H), 3.68 (s, 2H), 7.13 (dd, 1H), 7.29–7.40 (m, 5H),
7.64 (dd, 1H), 7.73 (dd, 1H); LC-MS: Purity 85%; RT 3.59 min. MH+ 260.08 [44].
1
3-(Piperidin-1-yl)-1-(thiophen-2-yl)propan-1-one (5d). Yield: 30%; H-NMR (300 MHz, CDCl3) (ppm):
1.33–1.61 (m, 2H), 1.70 (m, 4H), 2.41–2.69 (m, 4H), 2.94 (t, 2H), 3.27 (m, 2H), 7.01–7.19 (m, 1H), 7.63–7.68
(m, 1H), 7.78 (dd, 1H); LC-MS: Purity 83%; RT 2.77 min. MH+ 224.13 [42].
3-[(3,4-Dimethoxybenzyl)(methyl)amino]-1-(thiophen-2-yl)propan-1-one (5e). Yield: 25%; 1H-NMR (300 MHz,
CDCl3) (ppm): 2.38 (s, 3H), 2.97–3.05 (m, 2H), 3.13–3.25 (m, 2H), 3.75 (s, 3H), 3.87–3.91 (m, 5H), 6.61–6.89
(m, 3H), 7.15 (t, 1H), 7.62 (d, 1H), 7.73 (d, 1H); LC-MS: Purity 66%; RT 3.19 min. MH+ 320.04.
3-(Morpholin-4-yl)-1-(thiophen-2-yl)propan-1-one (5f). Yield: 32%; 1H-NMR (300 MHz, CDCl3) (ppm):
2.63–2.78 (m, 4H), 3.03 (br t, 2H), 3.29 (br t, 2H), 3.78–3.87 (m, 4H), 7.15 (dd, 1H), 7.67 (dd, 1H), 7.79
(dd, 1H); LC-MS: Purity 77%; RT 2.16 min. MH+ 226.08 [42].
3-(4-Benzylpiperidin-1-yl)-1-(5-bromothiophen-2-yl)propan-1-one (6a). Yield: 55%. Yellow oil.; 1H-NMR
(300 MHz, CDCl3) (ppm): 1.55–1.81 (m, 5H), 2.22–2.39 (m, 2H), 2.58 (d, 2H), 2.98–3.10 (m, 2H), 3.11–3.22
(m, 2H), 3.33 (t, 1H), 7.07–7.16 (m, 3H), 7.16–7.26 (m, 2H), 7.26–7.34 (m, 2H), 7.57 (d, 1H); LC-MS: Purity
76%; RT 4.83 min. MH+ 391.91 [43].
3-[Benzyl(methyl)amino]-1-(5-bromothiophen-2-yl)propan-1-one (6b). Yield: 40%. Yellow oil. 1H-NMR
(300 MHz, CDCl3) (ppm): 2.40 (s, 3H), 3.08 (d, 2H), 3.26 (br s, 2H), 3.78 (s, 2H), 7.10 (d, 1H), 7.31–7.44
(m, 5H), 7.50 (d, 1H); LC-MS: Purity 37%; RT 4.20 min. MH+ 339.93 [45].
1-(5-Bromothiophen-2-yl)-3-(piperidin-1-yl)propan-1-one (6d). Yield: 29%; 1H-NMR (300 MHz, CDCl3)
(ppm): 0.80–0.94 (m, 1H), 1.49–1.55 (m, 1H), 1.67–1.80 (m, 4H), 2.57–2.68 (m, 4H), 2.96 (t, 2H), 3.24 (br t,
2H), 7.10–7.13 (m, 1H), 7.54 (d, 1H); LC-MS: Purity 57%; RT 3.61 min. MH+ 303.97 [45].
1
1-(5-Bromothiophen-2-yl)-3-(morpholin-4-yl)propan-1-one (6f
)
.
Yield: 6%; H-NMR (300 MHz, CDCl3)
(ppm): 2.62–2.89 (m, 4H), 2.99–3.11 (m, 1H), 3.22–3.34 (m, 1H), 3.55–3.67 (m, 3H), 3.86 (br t, 3H), 7.13
(d, 1H), 7.54 (d, 1H); LC-MS: Purity 38%; RT 3.38 min. MH+ 305.91.
4. Conclusions
In summary, we have developed a rapid and easy-to-use microwave-assisted protocol based
on a combination of PEG 400/CAN, PASPS, and SPE, obtaining the desired products faster than
conventional procedures. The reaction optimized with respect to various parameters afforded most of
the desired products with good yield and satisfying purity. Our approach could be adapted to a new
library of compounds with different aromatic ketones. No less important, the obtained compounds
could serve as key intermediates for further functionalization at the ketone group to allow scaffold
modifications, suitable for disclosing novel potential hit compounds. We believe that simple reaction
procedures and substrate compatibility along with environmentally friendly conditions make our
protocol an important supplement to the existing methods.
Lastly, the small focused library we present aims at discovering new potential sigma 1 receptor
modulators as part of our ongoing research in this field. From this consideration came our decision
to add these products to the library of MuTaLig, an innovative ligand identification platform for the
drug-discovery process.
Acknowledgments: MIUR and University of Pavia are gratefully acknowledged for the doctoral and postdoctoral
fellowships to G.R. and M.R.

Molecules 2018, 23, 775
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Author Contributions: S.C. conceived the work, contributed to reviewing the manuscript, and was responsible
for the correctness of the whole study. M.V.R. and G.R. wrote the manuscript. D.R., M.V.R., and M.D.G. were
responsible for the experimental design and for data analysis of the whole study. G.R. and M.R. performed
the experiments.
Conflicts of Interest: The authors declare no conflict of interest.
Appendix A
Table A1. HPLC analysis for 1a. XTerra RP18 column (3.5 µm, 4.6 × 50 mm). Flow rate: 1 mL/min.
Time (Minutes)
% Phosphate Buffer
% Acetonitrile
0
3
90
90
60
60
5
10
10
40
40
95
10
10
10
13
20
25
35
90
90
Figure A1.
(A) Acetophenone (RT: 6.8 min); (B) compound 1a (RT: 16 min); (C) reaction mixture (MW)
acetophenone (2.0 eq.), 4-benzylpiperidine hydrochloride (1 eq.), paraformaldehyde (1 eq.).

Molecules 2018, 23, 775
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Table A2. HPLC analysis for 1b. Hypersil ODS RP18 column (3
µm, 4.6
×
100 mm). Flow rate: 2 mL/min.
Time (Minutes)
% Phosphate Buffer
% Acetonitrile
0
3
90
90
60
60
5
5
90
90
10
10
40
40
95
95
10
10
10
13
20
30
35
40
Figure A2.
(
A
) Compound 1b (RT: 12 min); (B) reaction mixture (MW) acetophenone (2.0 eq.),
N-benzylmethylamine hydrochloride (1 eq.), paraformaldehyde (1 eq.).
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).