Study on a three-step rapid assembly of zolpidem and its fluorinated analogues employing microwave-assisted chemistry

Article abstract of DOI:10.3390/molecules25143161

We developed an efficient microwave-assisted three-step synthesis of zolpidem and its fluorinated analogues 1-3. The procedure relays on the utilization of easily accessible and inexpensive starting materials. Our protocol shows superior performance in terms of yield and purity of products, compared to conventional heating systems. Notably, the total time needed for reaction accomplishment is significantly lower comparing to oil bath heating systems. Finally, we have performed a detailed study on the preparation of zolpidem tartrate salt I, and we assessed its particle-sizes using a polarizing microscope. Our goal was to select the appropriate method that generates the acceptable particle-size, since the solid-size directly influences solubility in biological fluids and further bioavailability. We believe that the disclosed procedure will help to produce a lab-scale quantity of zolpidem and its fluorinated derivatives 1-3, as well as zolpidem tartrate salt I, with suitable fine-particle size for further biological experimentation.

Full text of DOI:10.3390/molecules25143161

molecules
Article
Study on a Three-Step Rapid Assembly of Zolpidem
and Its Fluorinated Analogues Employing
Microwave-Assisted Chemistry
Nikola Fajkis , Monika Marcinkowska * , Beata Gryzło, Anna Krupa and
Marcin Kolaczkowski
Faculty of Pharmacy, Jagiellonian University Medical College, 9 Medyczna Street, 30-688 Kraków, Poland;
nikola.fajkis@doctoral.uj.edu.pl (N.F.); beata.gryzlo@uj.edu.pl (B.G.); a.krupa@uj.edu.pl (A.K.);
marcin.kolaczkowski@uj.edu.pl (M.K.)
* Correspondence: monika.marcinkowska@uj.edu.pl
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 25 June 2020; Accepted: 8 July 2020; Published: 10 July 2020
Abstract: We developed an efficient microwave-assisted three-step synthesis of zolpidem and its
fluorinated analogues . The procedure relays on the utilization of easily accessible and inexpensive
1–3
starting materials. Our protocol shows superior performance in terms of yield and purity of
products, compared to conventional heating systems. Notably, the total time needed for reaction
accomplishment is significantly lower comparing to oil bath heating systems. Finally, we have
performed a detailed study on the preparation of zolpidem tartrate salt I, and we assessed its
particle-sizes using a polarizing microscope. Our goal was to select the appropriate method that
generates the acceptable particle-size, since the solid-size directly influences solubility in biological
fluids and further bioavailability. We believe that the disclosed procedure will help to produce a
lab-scale quantity of zolpidem and its fluorinated derivatives 1–3, as well as zolpidem tartrate salt I,
with suitable fine-particle size for further biological experimentation.
Keywords: zolpidem; GABA-A receptor; imidazo[1,2-a]pyridine; microwave supported synthesis
1. Introduction
Zolpidem
annual prescription rate of nearly 39 million [
subtle alternative to benzodiazepines, which could rapidly promote sleep, while retaining the negligible
risk of impaired daily functioning [ ]. Zolpidem binds selectively to GABA-A ( -aminobutyric acid)
receptors containing the 1 subunit, and further mediates synaptic GABA-ergic signaling [ ]. During
the past decade, zolpidem has featured in several curious case reports that documented the reversal of
brain stroke symptoms [ ], improvement of motor dysfunction in Parkinson’s disease patients [ ] or
the effective reduction of chronic pain [ ]. All these unique features are attributed to the enhanced
synaptic GABA-ergic currents in specific brain regions. In addition, our group found that zolpidem
], as well as its fluorinated analogues 10 11] (Figure 1), display prominent antipsychotic
activity in animal models of schizophrenia. These findings opened new prospects for the therapeutic
utility of selective 1-GABA-A receptor ligands in the treatment of various pathologic conditions,
besides insomnia. In this regard, a plethora of experiments have been recently conducted by many
research groups, using zolpidem as a pharmacological tool to probe the activity of 1-GABA-A
1
is one of the most widely prescribed hypnotic medications in the United States, with an
1
,2
]. It was developed in the late 1980s [ ] and 1990s as a
3
4
γ
α
5
6
7
8
1
[
9
2, 3 [ ,
α
α
receptor. As zolpidem is a short-acting ligand characterized by fast elimination from the organism,
this makes it merely appropriate for acute or short-term biological experiments. The fluorinated
analogues of zolpidem, developed in our lab, are characterized by promising metabolic stability and
Molecules 2020, 25, 3161; doi:10.3390/molecules25143161
www.mdpi.com/journal/molecules

Molecules 2020, 25, 3161
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a longer duration of biological activity [10
,
11], thus, are relevant tools for the purpose of chronic
treatment studies. Consequently, zolpidem
1, and its fluorinated analogues 2, 3, are highly desirable
pharmacological tools for biological investigations, and the development of synthetic methods that
would give fast and easy access to 1–3 are highly warranted.
Figure 1. Chemical structure of zolpidem 1 and its fluorinated analogues 2 and 3.
Recent advances in heterocyclic chemistry revealed that it is possible to obtain zolpidem and
possibly its fluorinated analogues by applying diverse one-pot protocols or intramolecular double
aza-Michael addition of 2-aminopyridines to Morita-Baylis-Hillman (MBH) acetates [12]. However,
these methods require usage of special technological devices, employ hazardous reagents and necessitate
fluorinated starting materials, which are not easily available from commercial sources, and thereby
require extensive preparation [13]. Alternatively, zolpidem and its analogues might be prepared by
lengthy six to eight step procedures, which generate large amounts of waste, require expensive starting
materials, give moderate yields and are narrowed in scope [14].
Recently we reported a practical three-step synthesis of 2–3, based on the incorporation of
N,N-dimethylaminoacetamide backbone into position 3 of the 2-phenylimidazo[1,2-a]-pyridine
framework [10] (Scheme 1). The main advantage of this pathway is the direct use of
α
-bromoacetophenones and fluoropyridine-2-amines, which represent relatively cheap and the most
facilely available starting materials in a broad range of fluorination degrees. The method proved to be
viable, but unexpectedly moderately yielding, somewhat lengthy and providing products with high
amount of trace impurities. Mounting evidence shows that the application of microwave-assisted
techniques significantly accelerate reaction rates, provide higher yields and product purities, as well
as lowering the processing cost. Furthermore, when it applies minimal solvent amounts, it helps
to achieve cleaner reactions [15–17]. Thus, we reasoned that a slight modification of our three-step
protocol and employment of microwave irradiation would be particularly advantageous, in terms of
accelerated reaction time and higher yields and purities of final products.
Herein, we report a fast, high yielding, experimentally simple and less cumulative energy
demanding (CED) synthesis of zolpidem
1 and its fluorinated analogues 2, 3. The developed protocol
portrayed in Scheme 1 takes advantage of microwave-enhanced chemistry, is operatively simple,
reactions take place smoothly and effectively. Our microwave-assisted synthesis shows superior
performance compared to the reactions and yields achieved with conventional heating. An additional
challenging issue was the preparation of zolpidem salt with tartaric acid, which accounts for suitable
solubility in biological fluids and provides adequate bioavailability [18]. Therefore, we attempted
to develop an effective procedure for achieving zolpidem tartrate salt and performed a detailed
characterization of the achieved product using a polarizing microscope.

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Scheme 1. Schematic representation of the protocol developed under microwave irradiation. Reagents
and conditions: (i) NaHCO3, toluene, 110 ◦C, MW, 30 min, 82–90% (ii) N,N-dimethyl-2-oxoacetamide,
CH3COOH, 65 ◦C, MW, 1 h, 71–82% (iii) a: PBr3, tetrahydrofuran (THF), 50 ◦C, MW, 45 min, b: workup;
Na2CO3, H2O, rt, 71–82%.
2. Results and Discussion
2.1. Synthesis
We began the optimization process from the preparation of 2-phenylimidazo[1,2-a]pyridine
cores
4–6 (step 1, Table 1) via the condensation of suitably substituted 2-aminopyridines and
α
-bromoacetophenones, applying microwave-enhanced chemistry. Despite the apparent simplicity of
the process, the initial results were not very satisfactory, as we found that the reactions were highly
sensitive to the type of solvent used. Therefore, we decided to study the process and screen several
different microwave absorbing solvents (Table 1). Considering an economic standpoint, we selected
5-methylpyridin-2-amine and 2-bromo-1-(p-tolyl)ethan-1-one as the model reaction for optimization.
We found that with the use of high microwave absorbing, polar solvents such as methanol did
not induce conversion to the desired products (Table 1). Quite surprisingly, the use of nonpolar
solvent, such as toluene, exhibited a remarkable enhancement of the reactivity. Clearly the microwave
irradiation accelerated the condensation process in the presence of low microwave absorbing solvent
such as toluene. We suspect that the improved performance in toluene mainly depended on the
formation of ‘hot spots’, produced by increased microwave absorption at the surface of the insoluble
NaHCO3 particles [16,19,20].
Next, using the optimal conditions set in the microwave apparatus, we performed the condensation
among fluorinated 2-aminopyridines and α-bromoacetophenones, and found that fluorinated starting
materials were also amenable substrates for this transformation with good product yields (71–91%,
Table 2). As the multicomponent reactions performed under microwave heating are highly dependent
on the concentration [16] of the reaction mixtures, as an ultimate factor, we decided to compare the
reaction yields heated in 0.1 M (0.3 mmol/3 mL) and 0.06 M (0.3 mmol/5 mL) of toluene. In this
particular case, for all reactions, higher yields were observed in more condense settings; 0.1 M
(0.3 mmol/3 mL). Overall, the optimal conditions found for
4
–
6
were: 1 equiv. pyridin-2-amine and
◦
α
-bromoacetophenones, 1.5 eq of sodium bicarbonate heating at 100 C in toluene (concentration:
0.1 M) for 40 min (detailed description provided in the experimental section). Notably, the synthesis of

Molecules 2020, 25, 3161
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2-phenylimidazo[1,2-a]pyridines in conventional oil-bath experiments, typically requires overnight
heating under the reflux. Transferring the synthesis under microwave conditions not only accelerated
the reaction time, but also provided higher product purities and remarkably higher isolated yields
(Table 2).
Table 1. Pilot study evaluating the effect of various organic solvents on microwave-supported
condensation between 5-methylpyridin-2-amine and 2-bromo-1-(p-tolyl)ethan-1-one.
NH2
1.5 eq NaHCO3
O
N
+
N
N
Br
solvent
MW
1 eq
1 eq
(4)
Entry
Solvent
Temperature (◦C)
Time (minutes)
Yield (%) a
1
2
3
4
5
6
7
8
ethanol
methanol
acetone
dioxane
acetonitrile
THF
78
64
56
101
82
66
30
30
30
30
30
30
30
30
82.9
0
84
88.5
46.3
90.7
77.3
95.4
chloroform
toluene
61
110
The substrates were reacted in equimolar amounts (0.3 mmol) in the presence of 1.5 eq of sodium bicarbonate
under microwave irradiation using closed vessel system and the progress of the reaction was monitored by HPLC,
the temperature was adjusted to the specific solvent boiling point (solvent amount: 0.5 mL). a Conversion determined
by the HPLC.
Table 2. Comparison of the performance of condensations between 2-aminopyridines and
α-bromoacetophenones under microwave radiation versus traditional oil-bath heating (Step 1).
1.5 eq NaHCO3
NH2
N
O
R2
R2
+
N
N
Br
R1
R1
toluene, 110OC
1 eq
1 eq
(4-6)
Conventional Method
Microwave Assisted Method b
a
Yield (%) c
Compound
R1, R2
Time (h) Yield (%) c
Time
Solvent Amount (mL)
R1 = CH3
R2 = CH3
5
3
71
90
4
5
6
70
78
85
40 min
12
12
12
R1 = CH3
R2 = F
5
3
75
82
40 min
40 min
R1 = F
R2 = F
a
5
3
86
89
b
Conditions under conventional heating: 1 eq = 3 mmol, 10 mL of toluene; Conditions: 1 eq = 3 mmol;
Isolated yield.
c
The second step involves functionalization of 2-phenylimidazo[1,2-a]pyridine cores at the
3 position (Step 2, Table 3). This transformation has been described in the literature only
occasionally, and, in some cases, limited experimental information was available. In general,
α-hydroxyacetamide intermediates 7–9 can be obtained by reacting 2-phenylimidazo[1,2-a]pyridines
with hemi-hydrate of N,N-dimethylglyoxylamide [21], acetal of N,N-dimethyglyoxamides [22
]
or N,N-dimethyl-2,2-dichloroacetamide (in a two-step reaction [23]). However, these reagents

Molecules 2020, 25, 3161
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are unstable, require in situ preparation in a very laborious procedures and give the
desired -hydroxyacetamide intermediates in very low yields. Previously, we reported
α
that a stable form of N,N-dimethylglyoxylamide might be achieved by direct oxidation of
N,N,N0,N0-tetramethyl-L-tartramide [10]. Next, N,N-dimethylglyoxylamide can be reacted with
suitable 2-phenylimidazo[1,2-a]pyridines to form
heating starting materials in the presence of acetic acid. The procedure proved to be advantageous
upon traditional heating system, however the -hydroxyacetamide intermediates were isolated
α-hydroxyacetamide intermediates 7–9, simply by
α
7–9
after 12 h with 50–62% yield, together with relevant amounts of acetate by-products. When we
applied equal conditions in microwave reactors, the product could be obtained easily after 1h, however,
the acetate by-products were also forming virtually instantaneously. To overcome these difficulties,
we sought to perform the reaction, reducing the amount of acetic acid to the minimum amount required,
to catalyze the process and heat the reaction in MW. Gratifyingly, employing these settings we were
able to suppress almost entirely the formation of ester by-products, and to obtain
α-hydroxyacetamide
intermediates within 1h, as the major products in very good yields 81–88%. Moreover, we found
7–9
that the desired products could be isolated easily from the reaction mixture by filtrating the precipitated
product (details provided in the experimental section).
Table 3. Functionalization of 2-phenylimidazo[1,2-a]pyridines with of N,N-dimethylglyoxylamide
under MW compared with traditional heating systems conditions.
O
H
N
2.5 eq
N
N
(10)
O
R2
N
N
R2
R1
N
CH3COOH, toluene,
65oC
OH
R1
O
1 eq
(4-6)
(7-9)
Microwave Assisted Method b
Conventional Method a
Compound
R1, R2
Time (h)
Yield (%) c
Time (h)
Yield (%) c
R1 = CH3
R2 = CH3
7
8
9
12
65
1
81
R1 = CH3
R2 = F
12
12
62
57
1
86
88
R1 = F
R2 = F
1
a
c
b
Conditions under conventional heating: 1 eq = 3 mmol, 10 mL of toluene.
Isolated yield.
Conditions: 1 eq = 3 mmol.
With the
α
-hydroxyacetamide precursors
7–9 in hand, we proceeded with the synthesis of the
final products 1–3. The final dehydroxylation was performed in MW, in the presence of phosphorus
tribromide added in an excess to the reaction mixture (Table 4). During the reaction a precipitable salt
is formed which can be collected by filtration and further hydrolyzed during the workup, to give the
final products 1–3. This process has been described previously to occur using various solvents (polar
or halogenated solvents) in the traditional heating systems. We decided to transfer the experimental
setting and perform the reaction under MW heating, starting with the screening of a◦variety of solvents.
We observed that, when the reaction was carried out in dichloroethane (at 120 C), the substrates
reacted vigorously, forming high amounts of trace impurities (according to HPLC and NMR), which are
difficult to identify and inseparable from the main product. Moreover, after several purification
attempts the final products still appeared as intense yellow solids, which is not with agreement with

Molecules 2020, 25, 3161
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the previous literature reports, which describe it as white solids. A possible explanation of the low
performance of PBr3 in dichloroethane is related with the fact that dichloroethane tend to decompose
under a high temperature, forming a mixture of various products [24,25], which further react with the
remaining components of the reaction mixture, leaving a high amount of impurities. To circumvent
this undesired event, we opted to perform the reaction, replacing dichloroethane with non-halogenated
solvents. Satisfactorily, we found that the use of polar solvents such as tetrahydrofuran (THF) or
dioxane in MW proved to be extremely rewarding, which resulted in strikingly clean reactions and a
high purity of the final products 1–3. The substrates reacted smoothly affording the final products
in very clean reactions with good yields and high purity. We observed that performing the reaction
in the presence of THF, heating for 45 min at 50 ◦C in MW proved to be slightly more effective over
◦
reactions heated at 110 C in dioxane (Table 4). These differences were particularly noticeable for
2-phenylimidazo[1,2-a]pyridines bearing 6-methyl 1 and 40-F 3.
We attempted also to transform the intermediate alcohols to final acetamides exploring other
conditions. Several different dehydroxylation procedures were tested, including I2/PPh3
Pd/HCOOH [27], TMSCl/NaI [ ]. However, upon all investigated conditions, dehydroxylation
took place to a very small extent and the desired acetamides were always obtained as minor products
in a mixture with the carboxylic acids delivered by hydrolysis of , along with other unidentified
[26],
2
7–9
side-products. Nevertheless, the dehydroxylation applying phosphorus tribromide in THF, in our
hands, proved to be the most fruitful. The final products can be obtained within 45 min, in very clean
reactions with good yields, while potentially hazardous unreacted materials can be easily removed
during the filtration step and work up.
Altogether, performing the three-step synthesis of 1–3 under microwave heating resulted in clean
reactions, increased yields, significantly reduced the amount of solvents and effectively shortened the
time needed for transformation and reduced concurrent side reactions.
Table 4. Dehydroxylation of
α
-hydroxyacetamide precursors (
7–
9) in the presence of PBr3 and MW in
comparison to traditional heating.
N
N
R2
i. 2.6 eq PBr3, solvent, heat
ii. Na2CO3, H2O, r.t.
R2
N
N
R1
R1
OH
O
O
N
N
1 eq
(7-9)
(1-3)
Conventional Method a
Microwave Assisted Method b
Compound
R1, R2
Time
Yield (%) c
Time
Solvent
Yield (%) c
R1 = CH3
R2 = CH3
THF
dioxane
71
58
45
min
1
2
3
6
70
R1 = CH3
R2 = F
THF
dioxane
82
75
45
min
12
12
70
R1 = F
R2 = F
THF
dioxane
76
58
45
min
43
a Conditions under conventional heating: 1 eq = 2 mmol, dichloroethane 3 mL, 120 ◦C. b Conditions: 1 eq = 0.15 mmol
,
3 mL of solvent, temperature THF 50 ◦C, dioxane 90 ◦C. c Isolated yield.
2.2. Preparation of Water-Soluble Salts
With the lab-scale quantities of zolpidem 1, we next proceeded with the preparation of
corresponding organic salts. We chose zolpidem as a model compound for investigating the formation
of suitable salt, because it has been recently widely used by researchers as a pharmacological tool
for biological investigation. Zolpidem free base 1 has been reported to be almost insoluble in water

Molecules 2020, 25, 3161
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(~0.1mg/mL) [28
responses [30]. This inconvenient feature has prompted the pharmaceutical industry towards the
composition of suitable salts characterized by improved solubility [18 31]. In fact, zolpidem appears
in various pharmaceutical compositions as a salt with natural L(+)tartaric acid, where the molar
ratio of zolpidem and tartaric acid in the salt is 2:1 [28 32] (Table 5, ). Zolpidem tartrate possesses
,29], which, in turn, may result in poor bioavailability and inconsistent in vivo
,
,
I
I
advantageous solubility in water (18mg/mL) and other similar pharmaceutically acceptable solvents.
Based on the literature reports, we proceeded with the preparation of zolpidem tartrate (2:1) by mixing
both substrates in methanol and allowing to settle the crystals overnight (Method A). Direct filtration
of the resulting crystals afforded the desired salt (I) with high purity, however, with a considerably
low yield. In the second attempt, we straightforwardly evaporated the reaction mixture (Method B).
However, we could confidently predict that such a procedure resulted in a notably higher yield; also,
the salt contained higher amounts of various impurities, inseparable by further crystallization attempts.
After this explorative attempt, we changed the strategy and decided to accelerate the formation of
crystals by subsequent addition of a precipitant (diethyl ether), which had been previously employed
in our labs to give excellent results [10,33,34]. The addition of Et2O proved to be particularly rewarding,
as the salt was separated in a good yield and excellent purity (Table 5).
Table 5. Preparation of zolpidem tartrate (I) by diverse protocols.
H
N
N
OH
O
L(+)tartaric acid
N
N
1
3
O
2
4
O
methanol, reflux 2h,
crystallization
O
O
O
OH
N
N
(1)
2
(I)
Standard Crystalization
(Method A)
Direct Evaporation
(Method B)
Crystalization + Et2O
(Method C)
Purity %
Yield %
99%
45%
88%
99%
73%
100%
2.3. Evaluation of the Particles’ Size
An additional aspect that may equally influence compound’s solubility and bioavailability is
the particle’s size, shape and texture. These features are crucial factors, from the technological point
of view, while thinking of developing first dosage forms for preclinical studies [35]. In general,
fine non-agglomerated particles have a higher surface area, and therefore a higher solubility and
dissolution rate than coarse particles or aggregates. As a result, many slowly dissolving, poorly
soluble drugs are commercially available in the micronized form [35]. Yet, the micronization of
hydrophobic particles may also cause their aggregation, which finally makes solubility enhancement
impossible. Apart from solubility concerns, the crystal habit determines also other important features
of compounds, such as powder flow, or sedimentation and caking in suspensions, which should be
taken into account upon a new dosage form development [18,31]. During the experimentation process,
we noticed that the crystals of formed salt (I) can be altered, depending on the applied conditions.
While a direct crystallization afforded thick compact agglomerates formed of primary lath particles
(Method A), an alternative method based on addition of Et2O (Method C) resulted in irregular fine
primary particles, which formed less coherent and much smaller aggregates. It was assumed that the
selection of the most favorable morphology of crystals could be crucial to prepare formulations for
biological studies. We reasoned that, otherwise, one would risk to acquire apparently negative results
associated with poor solubility and bioavailability, rather than the real lack of biological activity.

Molecules 2020, 25, 3161
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Thus, the crystal habit of developed particles was analyzed in a more detailed way, using polarized
light microscopy. The images of the particles obtained by Method A and C are shown in Figure 2.
The product developed in Method B based on a direct evaporation of the reaction mixture was omitted
due to the large number of impurities.
As can be seen from Figure 2, all examined products showed characteristic birefringence
testifying their crystallinity. However, the crystals varied significantly in both size and shape.
Overnight crystallization from methanol (Method A) resulted in agglomerated lath particles of about
10–20
µm in length (Figure 2A,B). In contrast, the addition of the precipitant (Et2O) delivered mostly
non-agglomerated fine particles of less than 5
µm in length (Figure 2C,D). Since the tendency to form
agglomerates and the particle size may have a fundamental impact on the properties of biologically
active molecules, controlling their solubility and bioavailability, we believe that Method B can be a
promising approach to deliver a high quality zolpidem tartrate, which may be used as a pharmacological
probe for various biological studies.
Figure 2. Images of zolpidem tartrate taken using polarizing light microscopy. (
crystalizing from methanol (Method A): prior to analysis crystals were dissolved in drop of methanol
which was evaporated on microscope plate; ( ) crystals obtained in Method A placed directly on
microscope plate without recrystallization; ( ) salt obtained by adding Et2O (Method C): prior to
analysis, the crystals were dissolved in drop of methanol, which was evaporated on a microscope plate;
) crystals obtained in Method C placed directly on a microscope plate, without recrystallization.
A) Salt obtained by
B
C
(
D
Images were taken using RT series camera (Opta-Tech, Warsaw, Poland) and polarized light microscope
Hund Wetzlar type H600/12 (Helmut Hund GmbH, Wetzlar, Germany).

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3. Materials and Methods
3.1. General Methods
Unless otherwise indicated, all the starting materials were obtained from commercial suppliers
(Merck, Alfa Aesar or Fluorochem) and were used without further purification. Analytical thin-layer
chromatography (TLC) was performed on Merck Kieselgel 60 F254 (0.25 mm) pre-coated aluminum
sheets (Merck, Darmstadt, Germany). Visualization was performed with a 254 nm UV lamp. Column
chromatography was performed using silica gel (particle size 0.063–0.200 mm; 70–230 Mesh ATM)
purchased from Merck. The UPLC-MS or UPLC-MS/MS analyzes were run on UPLC-MS/MS system
comprising Waters ACQUITY® UPLC® (Waters Corporation, Milford, MA, USA), coupled with Waters
TQD mass spectrometer (electrospray ionization mode ESI with tandem quadrupole). Chromatographic
separations were carried out using the ACQUITY UPLC BEH (bridged ethyl hybrid) C18 column:
2.1 × 100 mm and 1.7
conditions using 95% to 0% of eluent A over 10 min, at a flow rate of 0.3 mL/min. Eluent A: water/formic
acid (0.1%, v/v); eluent B: acetonitrile/formic acid (0.1%, v/v). A total of 10 l of each sample were
injected, and chromatograms were recorded using a Waters e PDA detector. The spectra were
µ
m particle size. The column was maintained at 40 ◦C and eluted under gradient
µ
λ
analyzed in the range of 200–700 nm, with a 1.2 nm resolution, and at a sampling rate of 20 points/s.
MS detection settings of Waters TQD mass spectrometer were as follows: source temperature 150 ◦C,
desolvation temperature 350 ◦C, desolvation gas flow rate 600 L/h, cone gas flow 100 L/h, capillary
potential 3.00 kV and cone potential 20 V. Nitrogen was used for both nebulizing and drying. The data
were obtained in a scan mode ranging from 50 to 1000 m/z at 0.5 s intervals; 8 scans were summed up
to obtain the final spectrum. Collision activated dissociation (CAD) analyses were carried out with
the energy of 20 eV, and all the fragmentations were observed in the source. Consequently, the ion
spectra were obtained in the range from 50 to 500 m/z. MassLynx V 4.1 software (Waters) was used
for data acquisition. Standard solutions (1 mg/mL) of each compound were prepared in a mixture
comprising analytical grade acetonitrile/water (1/1, v/v). The UPLC/MS purity of all the test compounds
1
and key intermediates was determined to be >95%. H NMR, 13C NMR, and19F NMR spectra were
obtained in a Joel spectrometer (Jeol, 500 MHz), in CDCl3, CD3OD or DMSO operating at 500 MHz
(1H NMR), 126 MHz (13C NMR), and 471 MHz (19F NMR). The chemical shifts are reported in ppm
and were referenced to the residual solvent signals (CHLOROFORM-d 1H: 7.26 ppm, 13C: 77.16 ppm;
CD3OD 1H: 3.31 ppm, 13C: 49.00 ppm), coupling constants are reported in hertz (Hz). The J values are
expressed in Hertz. Signal multiplicities are represented by the following abbreviations: s (singlet),
br.s (broad singlet), d (doublet), dd (doublet of doublets), dt (doublet of triplets), dtd (doublet of
triplet of doublets), t (triplet), td (triplet of doublets), tdd (triplet of doublet of doublets), q (quartet),
quin (quintet), m (multiplet). This is in agreement with the structural data reported in the literature [10].
The spectral data of final compounds were previously reported by our group [10], however, they were
recorded on Varian 300 MHz apparatus, therefore, we decided to record them again using Jeol, 500 MHz.
The microwave-assisted reactions were performed using a CEM Discover System 908010 microwave
apparatus (Matthews, NC, USA), operating at a frequency of 2.45 GHz with continuous irradiation
power from 0 to 200 W. Tetrahydrofuran (THF) was distilled under nitrogen immediately before use.
The drying agent used for THF was sodium/benzophenone ketyl. Dioxane dry, toluene dry and
methanol dry were purchased from commercial sources (Sigma Aldrich, Poznan, Poland). Images
of zolpidem tartrate I were taken using an RT series camera (Opta-Tech, Warsaw, Poland) and the
polarized light microscope Hund Wetzlar type H600/12 (Helmut Hund GmbH, Wetzlar, Germany).
3.2. Compound Characterization
6-methyl-2-(p-tolyl)imidazo[1,2-a]pyridine (4). To a solution of 5-methylpyridin-2-amine (314 mg, 3 mmol,
1 eq) in toluene (3 mL), 4-methylbenzoyl bromide (636 mg, 3 mmol, 1 eq) and sodium hydrogen
carbonate (378 mg, 4.5 mmol, 1.5 eq) was added. The mixture was irradiated at 110 ◦C for 40 min in
CEM-Discover system, operating at a frequency of 2.445 GHz, with continuous irradiation power from

Molecules 2020, 25, 3161
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0 to 200 W. After that time, the reaction mixture was extracted with EtOAc, dried over Na2SO4 and
concentrated under reduced pressure. Purification by column chromatography using (Hex:EtOAc 7:3)
as eluent, gave
1H NMR (500 MHz, CDCl3)
7.24 (d, J = 8.0 Hz, 2 H), 7.01 (dd, J = 1.7, 9.2 Hz, 1 H), 2.39 (s, 3 H), 2.31 (d, J = 1.1 Hz, 3 H); 13C NMR
4
(585 mg, 90%) as a light yellow powder mp 200.8–203.7 ◦C, Rf = 0.31 (Hex:EtOAc 7:3);
δ
= 7.89 (s, 1 H), 7.85–7.82 (m, 2 H), 7.74 (s, 1 H), 7.54 (d, J = 9.2 Hz, 1 H),
(126MHz, CDCl3) δ = 145.6, 144.7, 137.7, 131.0, 129.5, 127.9, 125.9, 123.4, 122.1, 116.8, 107.6, 21.4, 18.2;
EI MS m/z: 223.0 [M + H+]. Alternatively, the reaction performed using 5 mL of toluene afforded the
product 4 with 71% yield.
2-(4-fluorophenyl)—6-methylimidazo[1,2-a]pyridine (5). To a solution of 5-methylpyridin-2-amine (314 mg,
3 mmol, 1 eq) in toluene (3 mL), 4-fluorobenzoyl bromide (648 mg, 3 mmol, 1 eq) and sodium hydrogen
carbonate (378 mg, 4.5 mmol, 1.5 eq) was added. The mixture was irradiated at 110 ◦C for 40 min in
the CEM-Discover system, operating at a frequency of 2.445 GHz, with continuous irradiation power
from 0 to 200 W. The reactor was cooled down to room temperature and depressurized. The reaction
mixture was extracted with EtOAc, dried over Na2SO4 and concentrated under reduced pressure.
Purification by column chromatography using (Hex:EtOAc 7:3) as eluent gave 5 (560 mg, 82%) as a light
yellow powder; mp 198.9–202.5 ◦C; Rf = 0.31 (Hex:EtOAc 7:3); 1H NMR (500 MHz, CHLOROFORM-d)
δ = 7.88 (dd, J = 5.4, 9.0 Hz, 3 H), 7.69 (s, 1 H), 7.50 (d, J = 9.2 Hz, 1 H), 7.09 (t, J = 8.8 Hz, 2 H), 7.00
(dd, J = 1.6, 9.2 Hz, 1 H), 2.30 (s, 3 H); 13C NMR (126MHz, CHLOROFORM-d)
δ
= 163.4 (d, J = 248.7
Hz), 144.9 (d, J = 24.0 Hz), 130.3, 128.0, 127.7 (d, J = 3.8 Hz), 123.4, 122.2, 116.9, 115.8 (d, J = 3.1 Hz),
107.6, 18.2; 19F NMR (471MHz, CHLOROFORM-d) 114.4 (s, 1 F); EI MS m/z: 227.0 [M + H+];
δ
=
−
Alternatively, the reaction performed using 5 mL of toluene afforded the product 5 with 75% yield.
6-fluoro-2-(4-fluorophenyl)imidazo[1,2-a]pyridine (6). To a solution of 5-fluoropyridin-2-amine (336 mg,
3 mmol, 1 eq) in toluene (3 mL), 4-fluorobenzoyl bromide (648 mg, 3 mmol, 1 eq) and sodium hydrogen
carbonate (378 mg, 4.5 mmol, 1.5 eq) was added. The mixture was irradiated at 110 ◦C for 40 min in
the CEM-Discover system, operating at a frequency of 2.445 GHz, with continuous irradiation power
from 0 to 200 W. The reactor was cooled down to room temperature and depressurized. The reaction
mixture was extracted with EtOAc, dried over Na2SO4 and concentrated under reduced pressure.
Purification by column chromatography using (Hex:EtOAc 7:3) as eluent gave
brown powder; mp 171.2–175.3 ◦C; Rf = 0.45 (Hex:EtOAc 7:3); EI MS m/z: 231.0 [M + H+]; 1H NMR
(500 MHz, CHLOROFORM-d) = 8.07–8.00 (m, 1 H), 7.93–7.83 (m, 2 H), 7.79 (s, 1 H), 7.64–7.53 (m,
1 H), 7.15–7.05 (m, 3 H); 13C NMR (126MHz, CHLOROFORM-d)
= 163.0 (d, J = 245.9 Hz), 153.3 (d,
6 (616 mg, 89%) as a
δ
δ
J = 236.4 Hz), 146.3, 143.5, 129.7 (d, J = 3.5 Hz), 127.8 (d, J = 7.1 Hz), 117.9 (d, J = 9.6 Hz), 116.9 (d,
J = 25.3 Hz), 115.8 (d, J = 21.7 Hz), 112.2 (d, J = 40.1 Hz), 109.2; 19F NMR (471MHz, CHLOROFORM-d)
δ = −113.6 (s, 1 F),
−
140.32 (s, 1 F); EI MS m/z: 231.0 [M + H+]. Alternatively, the reaction performed
using 5 mL of toluene afforded the product 6 with 86% yield.
2-hydroxy-N,N-dimethyl-2-(6-methyl-2-(p-totyl)imidazo[1,2- ]pyridin-3-yl)acetamide (7). To a solution of
α
6-methyl-2-(p-tolyl)imidazo[1,2- ]pyridine (672 mg, 3 mmol, 1 eq) in toluene (4 mL) and acetic
α
4
acid (1 mL), N,N-dimethyl-2-oxoacetamide (756 mg, 7.5 mmol, 2.5 eq) was added. The mixture
◦
was irradiated at 65 C for 60 min in the CEM-Discover system, operating at a frequency of 2.445
GHz, with continuous irradiation power from 0 to 200 W. The reactor was cooled down to room
temperature and depressurized. The reaction mixture was extracted with EtOAc, dried over Na2SO4
and concentrated under reduced pressure. The product was separated from remaining mixture by
filtration and the remaining residue was purified by column chromatography using (Et2O:DCM:
MeOH 2:7.5:0.5) as eluent to give
(Et2O:DCM:MeOH 2:7.5:0.5); 1H NMR (500 MHz, CHLOROFORM-d)
(m, 2 H), 7.53 (d, J = 9.2 Hz, 1 H), 7.29–7.24 (m, 2 H), 7.06 (dd, J = 1.7, 9.2 Hz, 1 H), 5.69 (s, 1 H), 2.84 (s,
3 H), 2.38 (s, 3 H), 2.34–2.32 (m, 3 H), 2.30 (s, 3 H); 13C NMR (126 MHz, CHLOROFORM-d)
=170.4,
7
(790 mg, 81%) as a white powder; mp 170.2–172.3 ◦C; Rf = 0.73
= 7.96–7.92 (m, 1 H), 7.66–7.63
δ
δ
146.0, 144.7, 138.3, 130.1, 129.6, 128.9, 128.4, 122.9, 122.6, 117.0, 115.7, 64.0, 36.8, 36.2, 18.5; EI MS m/z:
323.9 [M + H+].

Molecules 2020, 25, 3161
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2-(2-(4-fluorophenyl)-6-methylimidazo[1,2-
α]pyridin-3-yl)-2-hydroxy-N,N-dimethylacetamide (8). To a
solution of 6-fluoro-2-(4-fluorophenyl)imidazo[1,2-a]pyridine
5 (684 mg, 3 mmol, 1 eq) in toluene
(4 mL) and acetic acid (1 mL), N,N-dimethyl-2-oxoacetamide (758 mg, 7.5 mmol, 2.5 eq) was added.
The mixture was irradiated at 65 ◦C for 60 min in the CEM-Discover system, operating at a frequency
of 2.445 GHz, with continuous irradiation power from 0 to 200 W. The reactor was cooled down to
room temperature and depressurized. The reaction mixture was extracted with EtOAc, dried over
Na2SO4 and concentrated under reduced pressure. The product was separated from remaining
mixture by filtration and the remaining residue was purified by column chromatography using (Et2O:
◦
DCM:MeOH 2:7.5:0.5) as eluent to give
8
(851 mg, 86%) as a white powder; mp 183.5–185.2 C);
Rf = 0.67 (Et2O:DCM:MeOH 2:7.5:0.5); 1H NMR (500 MHz, CHLOROFORM-d)
δ
= 7.94 (d, J = 1.1 Hz,
1 H), 7.82–7.66 (m, 2 H), 7.52 (d, J = 9.2 Hz, 1 H), 7.18–7.12 (m, 2 H), 7.09 (dd, J = 1.7, 9.2 Hz, 1 H), 5.63
(s, 1 H), 2.84 (s, 3 H), 2.34 (s, 3 H), 2.31 (d, J = 1.1 Hz, 3 H); 13C NMR (126MHz, CHLOROFORM-d)
δ
= 170.1, 163.0 (d, J = 248.9 Hz), 144.9 (d, J = 24.9 Hz), 130.8, 130.0, 129.9 (d, J = 3.7 Hz), 128.7, 122.8,
117.0, 115.9 (d, J = 3.2 Hz), 115.7, 63.8, 36.8, 36.1, 29.4, 18.5; 19F NMR (471 MHz, CHLOROFORM-d)
δ = −113.2 (s, 1 F); EI MS m/z: 328.0 [M + H+].
2-6-fluoro-2-(4-fluorophenyl)imidazo[1,2-
α
]pyridin-3-yl)-2-hydroxy-N,N-dimethylacetamide (9). To a solution
of 2-(4-fluorophenyl)-6-methylimidazo[1,2-a]pyridine
6
(696 mg, 3 mmol, 1 eq) in toluene (4 mL) and
acetic acid (1 mL), N,N-dimethyl-2-oxoacetamide (758 mg, 7.5 mmol, 2.5 eq) was added. The mixture
◦
was irradiated at 65 C for 60 min in the CEM-Discover system, operating at a frequency of 2.445
GHz, with continuous irradiation power from 0 to 200 W. The reactor was cooled down to room
temperature and depressurized. The reaction mixture was extracted with EtOAc, dried over Na2SO4 and
concentrated under reduced pressure. The product was separated from remaining mixture by filtration
and the remaining residue was purified by column chromatography using (Et2O: DCM:MeOH 2:7.5:0.5)
as eluent to give
2:7.5:0.5); 1H NMR (500 MHz, CHLOROFORM-d)
= 5.2, 9.7 Hz, 1 H), 7.22–7.12 (m, 3 H), 5.66 (s, 1 H), 2.89 (s, 3 H), 2.36 (s, 3 H); 13C NMR (126 MHz,
CHLOROFORM-d) = 169.3, 163.3 (d, J = 248.8 Hz), 153.2 (d, J = 239.3 Hz), 146.2, 143.3, 130.7 (d,
9
(921 mg, 88%) as a brown powder; mp 172.2–174.5 ◦C; Rf = 0.70 (Et2O:DCM:MeOH
= 8.20–8.13 (m, 1 H), 7.77–7.70 (m, 2 H), 7.61 (dd, J
δ
δ
J = 7.6 Hz), 129.5 (d, J = 3.4 Hz), 118.1 (d, J = 8.4 Hz), 117.6 (d, J = 2.1 Hz), 116.1 (d, J = 21.7 Hz), 112.4,
112.1, 63.6, 36.8, 36.1; 19F NMR (471 MHz, CHLOROFORM-d)
m/z: 331.9 [M + H+].
δ
=
−
126.6 (s, 1 F), 138.2 (s, 1 F); EI MS
−
N,N-dimethyl-2-(6-methyl-2-(p-totyl)imidazo[1,2-
2-hydroxy-N,N-dimethyl-2-(6-methyl-2-(p-totyl)imidazo[1,2-a]pyridin-3-yl)acetamide
α
]pyridin-3-yl)acetamide (1). To a solution of
(50 mg, 0.151
7
mmol, 1 eq) in THF (3 mL), phoshorus tribromide (0.039 mL, 0.393 mmol, 2.6 eq) was added. The mixture
was irradiated at 90 ◦C for 45 min in the CEM-Discover system, operating at a frequency of 2.445 GHz,
with continuous irradiation power from 0 to 200 W. The reactor was cooled down to room temperature
and depressurized. Next, hexane, water and a saturated solution of sodium hydrogen carbonate was
added. The reaction mixture was extracted with EtOAc, dried over Na2SO4 and concentrated under
reduced pressure. Purification by column chromatography using (Et2O:DCM:MeOH 2:7.5:0.5) as
eluent gave
2:7.5:0.5); 1H NMR (500 MHz, CHLOROFORM-d)
7.53 (d, J = 9.2 Hz, 1 H), 7.28–7.23 (m, 2 H), 7.06 (dd, J = 1.7, 9.2 Hz, 1 H), 4.09 (s, 2 H), 2.84 (s, 3 H), 2.38
(s, 3 H), 2.34–2.32 (s, 3 H), 2.30 (s, 3 H); 13C NMR (126 MHz, CHLOROFORM-d)
= 170.4, 146.0, 144.7,
1
(33 mg, 71%) as a light green powder; mp 184.2–186.8 ◦C, Rf = 0.48 (Et2O: DCM:MeOH
= 7.94 (d, J = 1.1 Hz, 1 H), 7.65 (d, J = 8.6 Hz, 2 H),
δ
δ
138.3, 130.9, 129.6, 128.9, 128.4, 122.9, 122.6, 117.0, 115.7, 36.8, 36.2, 30. 6, 21.4, 18.5, EI MS m/z: 308.0 [M
+ H+]. Alternatively, reaction performed in dioxane (3mL) gave 1 with 58% yield.
2-(2-(4-fluorophenyl)-6-methylimidazo[1,2-
solution of 2-(2-(4-fluorophenyl)-6-methylimidazo[1,2-
amide
α
]pyridin-3-yl)-2-hydroxy-N,N-dimethylacetamide (2). To a
α
]pyridin-3-yl)-2-hydroxy-N,N-dimethylacet
8
(50 mg, 0.153 mmol, 1 eq) in dioxane (3 mL), phoshorus tribromide (0.039 mL, 398 mmol, 2.6
eq) was added. The mixture was irradiated at 90 ◦C for 45 min in the CEM-Discover system, operating
at a frequency of 2.445 GHz, with continuous irradiation power from 0 to 200 W. The reactor was

Molecules 2020, 25, 3161
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cooled down to room temperature and depressurized. Next, hexane, water and a saturated solution of
sodium hydrogen carbonate was added. The reaction mixture was extracted with EtOAc, dried over
Na2SO4 and concentrated under reduced pressure. Purification by column chromatography using
(Et2O:DCM:MeOH 2:7.5:0.5) as eluent gave
= 0.70 (Et2O:DCM:MeOH 2:7.5:0.5); 1H NMR (500 MHz, CHLOROFORM-d)
(m, 3 H), 7.15–7.03 (m, 3 H), 4.03 (s, 2 H), 2.96–2.92 (m, 3 H), 2.90 (s, 3 H), 2.36–2.28 (m, 3 H); 13C NMR
(126 MHz, CHLOROFORM-d) = 168.2, 162.7 (d, J = 247.8 Hz), 143.7 (d, J = 23.8 Hz), 130.0, 129.8,
128.0 (d, J = 3.8 Hz), 122.2, 116.6, 115.6 (d, J = 3.0 Hz), 113.9, 37.6, 36.0, 30.1, 18.6; 19F NMR (471 MHz,
CHLOROFORM-d)
114.1 (s, 1 F); EI MS m/z: 312.0 [M + H+]. Alternatively, reaction performed in
2
(38 mg, 82%) as a brown powder; mp 187.2–189.1 ◦C); Rf
= 7.93 (s, 1 H), 7.63–7.49
δ
δ
δ
=
−
THF (3 mL) gave 2 with 75% yield.
2-(6-fluoro-2-(4-fluorophenyl)imidazo[1,2-α]pyridin-1-yl)-N,N-dimethylacetamide (3). To a solution of
2-6-fluoro-2-(4-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-2-hydroxy-N,N-dimethylacetamide
9
(50 mg,
0.151mmol, 1 eq) in dioxane (3 mL), phoshorus tribromide (0.039 mL, 0.393 mmol, 2.6 eq) was
◦
added. The mixture was irradiated at 90 C for 45 min in the CEM-Discover system, operating
at a frequency of 2.445 GHz, with continuous irradiation power from 0 to 200 W. The reactor was
cooled down to room temperature and depressurized. Next, hexane, water and saturated solution of
sodium hydrogen carbonate was added. The reaction mixture was extracted with EtOAc, dried over
Na2SO4 and concentrated under reduced pressure. Purification by column chromatography using
(Et2O:DCM:MeOH 2:7.5:0.5) as eluent gave
= 0.70 (Et2O:DCM:MeOH 2:7.5:0.5); 1H NMR (500 MHz, CHLOROFORM-d)
7.63–7.59 (m, 4 H), 4.05 (s, 2 H), 2.97 (s, 3 H), 2.94 (s, 3 H), 13C NMR (126 MHz, CHLOROFORM-d)
167.8, 163.1 (d, J = 248.6 Hz), 152.7 (d, J = 236.8 Hz), 146.2, 144.3, 130.4 (d, J = 7.3 Hz), 129.7 (d, J = 8.4
Hz), 118.2 (d, J = 9.7 Hz), 117.2 (d, J = 2.1 Hz), 115.7 (d, J = 16.2 Hz), 111.4, 111.3, 37.6, 35.9, 29.7; 19
NMR (235.3 MHz, CDCl3) 112.7 (s, 1 F),
138.0; EI MS m/z: 316.0 [M + H+]. Alternatively, reaction
performed in THF (3mL) gave 3 with 58% yield.
3
(36 mg, 76%) as a yellow powder; mp 183.2–185.4 ◦C; Rf
= 8.09–8.05 (m, 3 H),
δ
δ
=
F
δ:
−
−
N,N-dimethyl-2-(6-methyl-2-(p-tolyl)imidazo[1,2-a]pyridin-3-yl)acetamide 2,3-dihydroxysucci
nate (I) Method C.
To a solution of zolpidem base
1
(50 mg, 0.162 mmol, 2 eq) in dry methanol 2 ml, tartaric acid
(12 mg, 0.081 mmol, 1 eq) was added, and the resulting mixture was stirred (520 rpm) for 2 h at
reflux. After that time, the mixture was cooled to room temperature, next, 2 mL of diethyl ether was
added in one portion. The additi◦on of diethyl ether induced slow precipitation. The resulting mixture
was placed in a freezer (at 14 C) overnight, and the next day, 3 mL of diethyl ether was added,
−
and the precipitate was washed with a small amount of diethyl ether (2 mL) to give
yield (90 mg). The spectral data matched those previously reported [36]; white solid, mp 195–197 ◦C;
1H NMR (500 MHz, DMSO-d6)
= 8.02 (d, J = 1.1 Hz, 1H), 7.55 (d, J = 8.4 Hz, 2H), 7.52 (d, J = 9.0 Hz,
1H), 7.25–7.21 (m, 2H), 7.08 (dd, J = 1.7, 9.2 Hz, 1 H), 4.30 (s, 1H), 4.12 (s, 2H), 3.10 (s, 3H), 2.81 (s, 3H),
2.32 (s, 3H), 2.28 (s, 3H); 13C NMR (126 MHz, CHLOROFORM-d)
= 173.2, 169.1, 146.1,144.3, 137.7,
I with 73% of
δ
δ
130.8, 128.2, 127.9, 122.4, 120.1, 117.6, 115.2, 71.8, 37.1, 35.4, 29.8, 21.1, 18.2; EI MS m/z: 308.0 [M + H+].
Sample preparation for microscope imagining: 1mg of zolpidem tartrate I was placed on a
microscope plate, dissolved in a drop of methanol and evaporated directly on the microscope plate.
The images were taken immediately after the solvent was evaporated using polarized light microscope
Hund Wetzlar type H600/12 (Helmut Hund GmbH, Wetzlar, Germany).
4. Conclusions
We have disclosed a practical procedure for delivering lab-scale quantities of zolpidem 1 and its
fluorinated
effective preparation of the final products
accessible and inexpensive starting materials, namely 2-aminopyridines and
Compared to conventional heating systems, our protocol shows superior performance in terms of yield
2–
3
analogues, incorporating microware supported chemistry. The procedure allows for the
in a minimum number of steps, and it starts from easily
-bromoacetophenones.
1–3
α

Molecules 2020, 25, 3161
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and purity of products. Foremost, the application of microware chemistry substantially enhanced the
speed of reaction performance and, thus, the overall time needed to complete the entire synthesis
(from 36 h to 2.30 h). The disclosed method also utilizes a minimal amount of solvents in the reactions.
Finally, we revealed a suitable method for preparing zolpidem tartrate salt I, and found that addition
of a precipitant, ahead of crystallization, makes it possible to obtain a small-particle size of
for biological studies.
I, suitable
Author Contributions: Conceptualization: M.M., synthesis step 1: B.G., synthesis step 2, 3: M.M., N.F.,
salt preparation: N.F., particle size analysis: A.K., writing—original draft preparation: M.M., M.K., A.K. and N.F.
All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by National Science Centre, Poland grant: 2018/30/E/NZ7/00247 and
Jagiellonian University Medical College founds: N42/DBS/000055.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Harward, J.L.; Clinard, V.B.; Jiroutek, M.R.; Lingerfeldt, B.H.; Muzyk, A.J. Impact of a US Food and Drug
Administration Drug Safety Communication on Zolpidem Dosing: An Observational Retrospective Cohort.
Prim Care Companion CNS Disord. 2015, 17. [CrossRef] [PubMed]
2.
3.
4.
5.
Kaplan, J.P.; George, P. Imidazo[1, 2-A] Pyridine Derivatives and Their Application as Pharmaceuticals.
U.S. Patent 4,382,938A, 10 May 1983.
George, P.; Allen, J. Imidazopyridines, leur Preparation et leur Application en Therapeutique. FR 2,606,409B1,
5 June 1989.
Kaufmann, C.N.; Spira, A.P.; Alexander, G.C.; Rutkow, L. Trends in Prescribing of Sedative-Hypnotic
Medications in the USA: 1993–2010. Pharmacoepidemiol. Drug Saf. 2016, 25, 637–645. [CrossRef] [PubMed]
Scatton, B.; Claustre, Y.; Dennis, T.; Nishikawa, T. Zolpidem, a novel nonbenzodiazepine hypnotic II. Effects
on cerebellar cyclic GMP levels and cerebral monoamines. J. Pharmacol. Exp. Ther. 1986, 237, 659–665.
[PubMed]
6.
7.
Sutton, J.A.; Clauss, R.P. A review of the evidence of zolpidem efficacy in neurological disability after brain
damage due to stroke, trauma and hypoxia: A justification of further clinical trials. Brain Inj. 2017, 31,
1019–1027. [CrossRef]
Daniele, A.; Panza, F.; Greco, A.; Logroscino, G.; Seripa, D. Can a Positive Allosteric Modulation of GABAergic
Receptors Improve Motor Symptoms in Patients with Parkinson’s Disease? The Potential Role of Zolpidem
in the Treatment of Parkinson’s Disease. Park Dis. 2016, 2016, 2531812. [CrossRef]
Puthiyathu, M.; Greenspan, H.; Levitt, W.; Riccardi, G.; Montemurro, N.; Nandu, B.; Waseem, S. The Analgesic
Properties of Zolpidem. Psychiatr Ann. 2013, 43, 6–7. [CrossRef]
8.
9.
Mierzejewski, P.; Kolaczkowski, M.; Marcinkowska, M.; Wesolowska, A.; Samochowiec, J.; Pawlowski, M.;
Bienkowski, P. Antipsychotic-like effects of zolpidem in Wistar rats. Eur. J. Pharmacol. 2016, 773, 51–58.
[CrossRef]
10. Marcinkowska, M.; Kołaczkowski, M.; Kamin´ski, K.; Bucki, A.; Pawłowski, M.; Siwek, A.; Karcz, T.;
Mordyl, B.; Starowicz, G.; Kubowicz, P.; et al. Design, synthesis, and biological evaluation of fluorinated
imidazo[1,2-a]pyridine derivatives with potential antipsychotic activity. Eur. J. Med. Chem. 2016, 124,
456–467. [CrossRef]
11. Marcinkowska, M.; Kołaczkowski, M.; Kamin´ski, K.; Bucki, A.; Pawłowski, M.; Siwek, A.;
Tarcz, T.; Starowicz, G.; Bienkowski, P.; Mierzejewski, P.; et al. 3-Aminomethyl Derivatives of
2-Phenylimidazo[1,2-a]-pyridine as Positive Allosteric Modulators of GABAA Receptor with Potential
Antipsychotic Activity. ACS Chem. Neurosci. 2017, 8, 1291–1298. [CrossRef]
12. Nair, D.K.; Mobin, S.M.; Namboothiri, I.N.N. Synthesis of Imidazopyridines from the Morita–Baylis–Hillman
Acetates of Nitroalkenes and Convenient Access to Alpidem and Zolpidem. Org. Lett. J. Am. Chem. Soc.
2012, 14, 4580–4583.
13. Zhang, B.; Shan, G.; Ma, Q.; Xu, Q.; Lei, X. An improved and scalable synthesis of zolpidem via a
CuI/BINOL-mediated tandem reaction of imine and alkyne. Heterocycl. Commun. 2017, 23, 445–448.
[CrossRef]

Molecules 2020, 25, 3161
14 of 15
14. Sumatatha, Y.; Reddy, T.R.; Pratap, P. A simple and efficient synthesis of hypnotic agent, zolpidem and its
related substances. Arkivoc 2009, 2, 315–320.
15. Kappe, C.O.; Pieber, B.; Dallinger, D. Microwave Effects in Organic Synthesis: Myth or Reality? Angew. Chem.
Int. Ed. 2013, 52, 1088–1094. [CrossRef]
16. Kappe, C.O. Controlled microwave heating in modern organic synthesis. Angew. Chem. Int. Ed. Engl. 2004
,
43, 6250–6284. [CrossRef] [PubMed]
17. Da˛browska, S.; Chudoba, T.; Wojnarowicz, J.; Łojkowski, W. Current Trends in the Development of Microwave
Reactors for the Synthesis of Nanomaterials in Laboratories and Industries: A Review. Crystals 2018, 8, 379.
[CrossRef]
18. Sun, J.; Wang, F.; Sui, Y.; She, Z. Effect of particle size on solubility, dissolution rate, and oral bioavailability:
Evaluation using coenzyme Q10 as naked nanocrystals. Int. J. Nanomed. 2012, 7, 5733–5744.
19. Hoz A de la Díaz-Ortiz, Á.; Moreno, A. Microwaves in organic synthesis. Thermal and non-thermal
microwave effects. Chem. Soc. Rev. 2005, 34, 164–178. [CrossRef] [PubMed]
20. Horikoshi, S.; Osawa, A.; Abe, M.; Serpone, N. On the generation of hot-spots by microwave electric and
magnetic fields and their impact on a microwave-assisted heterogeneous reaction in the presence of metallic
Pd nanoparticles on an activated carbon support. J. Phys. Chem. 2011, 115, 23030–23035. [CrossRef]
21. Schloemer, G.C. Process for the Preparation Imidazo[1,2-A]pyridine-3-acetamides. U.S. Patent 6,861,525,
1 March 2015.
22. Eades, K. A Process for the Preparation of 2-Phenyl-Imidazo[1,2-A] Pyridine-3-Acetamides. U.S. Patent
6,384,226, 7 May 2002.
23. Rossey, G.; Long, D. Process for the Preparation of Imidazopyridines. U.S. Patent 4,794,185A, 27 December
1988.
24. Mousavipour, S.H.; Saheb, V.; Pirhadi, F.; Dehbozorgi, M.R. Experimental and theoretical study on the
kinetics and mechanism of thermal decomposition of 1,2-dichloroethane. J. Iran. Chem. Soc. 2007, 4, 279–298.
[CrossRef]
25. Prinsloo, J.J.; Van Berge, P.C.; Zlotnick, J. The catalytic decomposition of 1,2-dichloroethane with activated
carbon catalysts. J. Catal. 1974, 32, 466–469. [CrossRef]
26. Alvarez-Manzaneda, E.J.; Chahboun, R.; Cabrera Torres, E.; Alvarez, E.; Alvarez-Manzaneda, R.; Haidour, A.;
Lopez-Ramos, J.M. Reaction of allylic and benzylic alcohols and esters with PPh3/I2: One-pot synthesis of
β,γ-unsaturated compounds. Tetrahedron Lett. 2005, 46, 3755–3759. [CrossRef]
27. Fitzgerald, S.P.; McConnell, R.I. Immunoassay with Extended Detection Window. U.S. Patent 8,673,652,
18 March 2014.
28. Ettema, G.; Lemmens, J.; Peters, T.; Pìcha, F. Zolpidem Salts. NL Patent 1,014,634C1, 8 March 2000.
29. Ettema, G.J.B.; Lemmens, J.M.; Peters, T.H.A.; Picha, F. Zolpidem Salts. EP1,163,241B1, 15 June 2005.
30. Foda, N.H.; Ali, S.M. Chapter 11—Zolpidem Tartrate. In Profiles Drug Subst Excip Relat Methodol; Brittain, H.G.,
Ed.; Academic Press: Amsterdam, The Netherlands, 2012; pp. 413–438.
31. Krupa, A.; Descamps, M.; Willart, J.-F.; Strach, B.; Wyska, E.; Jachowicz, R.; Danède, F. High-Energy Ball
Milling as Green Process to Vitrify Tadalafil and Improve Bioavailability. Mol. Pharm. 2016, 13, 3891–3902.
[CrossRef]
32. Ettema, G.J.B.; Lemmens, J.M.; Peters, T.H.A.; Picha, F. Zolpidem Salt Forms. U.S. Patent 6,242,460B1, 5 June
2001.
´
33. Marcinkowska, M.; Kotan´ska, M.; Zagórska, A.; Sniecikowska, J.; Kubacka, M.; Siwek, A.; Bucki, A.;
Pawlowski, M.; Bednarski, M.; Sapa, J.; et al. Synthesis and biological evaluation of N-arylpiperazine
derivatives of 4,4-dimethylisoquinoline-1,3(2H,4H)-dione as potential antiplatelet agents. J. Enzyme Inhib.
Med. Chem. 2018, 33, 536–545. [CrossRef] [PubMed]
34. Jagodzinska, M.; Huguenot, F.; Candiani, G.; Zanda, M. Assessing the bioisosterism of the trifluoromethyl
group with a protease probe. Chem. Med. Chem. 2009, 4, 49–51. [CrossRef] [PubMed]
35. Khadka, P.; Ro, J.; Kim, H.; Kim, I.; Kim, J.T.; Kim, H.; Cho, M.J.; Yun, G.; Lee, J. Pharmaceutical particle
technologies: An approach to improve drug solubility, dissolution and bioavailability. Asian J. Pharm. Sci.
2014, 9, 304–316. [CrossRef]

Molecules 2020, 25, 3161
15 of 15
36. Hoffmanne, V.; L
ászló, P.; Reiter, J.; Simig, G.; Koncz, L.; Rivo, E.; Donath, G.; Nagy, K. Process for
the Preparation of 6-Methyl-2-(4-methyl-phenyl)-imidazo 1,2-a]pyridine-3-(n,n-dimethyl-acetamide) and
Intermediates. WO 2001/038327, 22 November 2001.
Sample Availability: Samples of the compounds are available from the authors.
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2020 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/).