Synthesis and characterization of novel methyl (3)5-(n-boc-piperidinyl)-1h-pyrazole-4-carboxylates

Article abstract of DOI:10.3390/molecules26133808

Series of methyl 3-and 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates were developed and regioselectively synthesized as novel heterocyclic amino acids in their N-Boc protected ester form for achiral and chiral building blocks. In the first stage of the

Full text of DOI:10.3390/molecules26133808

molecules
Article
Synthesis and Characterization of Novel Methyl
3)5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates
(
Gita Matulevi cˇ i u¯ t e˙ 1 , Egl e˙ Arba cˇ iauskien e˙ * , Neringa Kleizien e˙ , Vilija Kederien e˙ , Greta Ragait e˙ ,
,2
2,
1
2
1
1
1
1
3
1,
Migl e˙ Dagilien e˙ , Aurimas Bieliauskas , Vaida Miliši u¯ nait e˙ , Frank A. Sløk and Algirdas Ša cˇ kus *
1
Institute of Synthetic Chemistry, Kaunas University of Technology, K. Baršausko g. 59,
LT-51423 Kaunas, Lithuania; gita.matuleviciute@ktu.lt (G.M.); neringa.kleiziene@ktu.lt (N.K.);
greta.ragaite@ktu.lt (G.R.); migle.dagiliene@ktu.lt (M.D.); aurimas.bieliauskas@ktu.lt (A.B.);
vaida.milisiunaite@ktu.lt (V.M.)
2
Department of Organic Chemistry, Kaunas University of Technology, Radvil e˙ n u˛ pl. 19,
LT-50254 Kaunas, Lithuania; vilija.kederiene@ktu.lt
Vipergen ApS, Gammel Kongevej 23A, V DK-1610 Copenhagen, Denmark; fas@vipergen.com
3
*
Correspondence: egle.arbaciauskiene@ktu.lt (E.A.); algirdas.sackus@ktu.lt (A.Š.); Tel.: +370-37-451-401 (A.Š.)
Abstract: Series of methyl 3- and 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates were developed
and regioselectively synthesized as novel heterocyclic amino acids in their N-Boc protected ester
form for achiral and chiral building blocks. In the first stage of the synthesis, piperidine-4-carboxylic
and (R)- and (S)-piperidine-3-carboxylic acids were converted to the corresponding
which were then treated with N,N-dimethylformamide dimethyl acetal. The subsequent reaction
of -enamine diketones with various N-mono-substituted hydrazines afforded the target 5-(N-Boc-
β-keto esters,
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
β
piperidinyl)-1H-pyrazole-4-carboxylates as major products, and tautomeric NH-pyrazoles prepared
from hydrazine hydrate were further N-alkylated with alkyl halides to give 3-(N-Boc-piperidinyl)-
Citation: Matulevi cˇ i u¯ t e˙ , G.;
Arba cˇ iauskien e˙ , E.; Kleizien e˙ , N.;
Kederien e˙ , V.; Ragait e˙ , G.; Dagilien e˙ ,
M.; Bieliauskas, A.; Miliši u¯ nait e˙ , V.;
Sløk, F.A.; Ša cˇ kus, A. Synthesis and
Characterization of Novel Methyl
1
H-pyrazole-4-carboxylates. The structures of the novel heterocyclic compounds were confirmed by
1H-, 13C-, and N-NMR spectroscopy and HRMS investigation.
15
Keywords: heterocyclic amino acids; pyrazoles; piperidines; β-keto esters; enamines; hydrazines;
(3)5-(N-Boc-piperidinyl)-1H-pyrazole-
building blocks
4-carboxylates. Molecules 2021, 26,
3
808. https://doi.org/10.3390/
molecules26133808
1
. Introduction
Heterocyclic amino acids are becoming very important in modern drug discovery [1–5]
For instance, (RS)-piperidine-3-carboxylic acid (DL-nipecotic acid) is one of the most po-
tent inhibitors of neuronal and glial -aminobutyric acid (GABA) uptake in vitro ]. (S)-
Academic Editor: Mohammad Alam
.
Received: 17 May 2021
Accepted: 19 June 2021
Published: 22 June 2021
γ
[6
Pyrrolidinyl-2-carboxylic acid (L-proline) has been found to act as an agonist of the glycine
receptor and of both the N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic gluta-
mate receptors [7].
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Heterocyclic amino acids are also important scaffolds and building blocks for the
preparation of heterocyclic systems, hybrids, and peptides [
has been applied as a scaffold in the preparation of pyrrolizidine [12
8
–11]. For example, L-proline
–
14], pyrrolo[1,2-c][1,3]
oxazole [15], pyrrolo[2,1-c][1,4]benzodiazepine [16], and benzo[f]pyrrolo[1,2-a][1,4]
diazepine derivatives [17], while nipecotic and isonipecotic acids have given deriva-
tives of heterospirocyclic 3-amino-2H-azirines [18,19]. Moreover, L-proline is a building
block for N-(3-mercapto-2-D-methylpropanoyl)-L-proline, named captopril, which is used
to regulate blood pressure [20]. D-Nipecotic acid, a building block for (R)-1-[4,4-bis-(3-
methyl-2-thienyl)-3-butenyl]-3-piperidine carboxylic acid, named (R)-tiagabine, which
amplifies neurotransmission of GABA, the predominant inhibitory neurotransmitter in the
Copyright: © 2021 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 (https://
creativecommons.org/licenses/by/
brain [21–23]. New derivatives of nipecotic acid, guvacin, and homo-β-proline are very
potent and selective analogs of GABA uptake inhibitors [24–26].
The heterocyclic tripeptide Gly-Pro-Glu
I, containing an L-proline residue, is a neuro-
4
.0/).
protective compound for the control of neurodegenerative processes such as Parkinson’s
Molecules 2021, 26, 3808. https://doi.org/10.3390/molecules26133808
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 3808
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disease [27
,28], while a proline peptidomimetic, faldaprevir II, was used as an experi-
mental drug to treat hepatitis (Figure 1) [29–31]. The synthetically prepared derivative of
the tripeptide (pyro)Glu-His-Pro(NH2) III has specific activity as a hypothalamic gland
thyrotropin-releasing hormone [32]. Many aromatic heterocyclic amino acids, such as
[
5-amino-4-(tert-butoxycarbonyl)thiophen-2-yl]acetic acid, provide synthetic peptides, in-
cluding enantiopure cyclic tetraamide IV 33], which are similar to compounds in marine
plants that exhibit resistance to infection or antitumor effects [34].
[
Figure 1. Examples of biologically active peptides derived from heterocyclic amino acids: Gly-Pro-Glu
pyro)Glu-His-Pro(NH ) III, cyclic tetraamide IV, and VPC00628 V.
I, faldaprevir II,
(
2
Heterocyclic amino acids have been applied widely as building blocks for the
preparation of DNA-encoded chemical libraries, including heterocyclic hybrid and
peptide compounds [35–39]. In general, a DNA-encoded library of target component
molecules should have a high degree of structural and functional diversity, taking
into account diversity-oriented synthesis (DOS) [40]. For example, a highly specific
and potent p38α kinase tripeptide-type inhibitor (VPC00628) V containing the residue
of 3-amino-1-phenyl-1H-pyrazole-4-carboxylic acid has been identified directly from a
multimillion-membered DNA-encoded molecule library that was prepared using high-
fidelity yoctoReactor (yR) technology [41].
We recently reported an efficient protocol for synthesizing highly functionalized
amino acid building blocks by combining pyrazole, indazole, and indole carboxylates
with N-Boc-3-iodoazetidine [42]. Moreover, we synthesized 4-(N-Boc-cycloaminyl)-1,3-
thiazole- and 4-(N-Boc-cycloaminyl)-1,3-selenazole-5-carboxylates as novel heterocyclic
chiral amino acid-like derivatives [43
N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates as heterocyclic amino acid-like derivatives
for novel achiral and chiral building blocks from piperidine-4-carboxylic and (R)- and
S)-piperidine-3-carboxylic acids.
,44]. Herein, we report the efficient synthesis of 3(5)-
(
(
2. Results and Discussion
Numerous methods for forming pyrazole ring systems have been developed. The
most common synthetic method for the production of pyrazoles is the condensation of the
corresponding hydrazine derivative, which acts as a double nitrogen nucleophile, with three
carbon units containing compounds such as 1,3-dicarbonyl and 2,3-unsaturated carbonyl,
or enamine [45
both regioisomers of 4,5-substituted N-phenylpyrazoles from
phenylhydrazine, and the regiochemistry of the reaction was protic or aprotic solvent depen-
–47]. Rosa et al. [48] developed a simple and efficient method for preparing
β-enamino diketones and

Molecules 2021, 26, 3808
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dent. A patent [49] was obtained for the synthesis of 4-(piperidin-4-yl)-N-phenylpyrazole
derivatives from β-enamino diketones with 4-fluoro- and 4-methoxyphenylhydrazines.
Our strategy for the synthesis of methyl 3(5)-(N-Boc-piperidinyl)-1H-pyrazole-4-
carboxylates according to the enamine method is described in Schemes 1 and 2, and
Figure 2. The synthetic sequence started with preparing
N-Boc protected piperidine acids 1a with 2,2-dimethyl-1,3-dioxane-4,6-dione (Meldrum’s
acid) in the presence of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride
EDC HCl) and 4-dimethylaminopyridine (DMAP), and further methanolysis of Meldrum’s
acid adduct [50 51]. Compounds 2a were treated with N,N-dimethylformamide dimethyl
acetal (DMF·DMA) to obtain β-enamino diketones 3a–c [49].
β-keto esters 2a–c by treating
–
c
(
·
,
–c
Scheme 1. Synthesis of starting β-enamino diketones 3a–c.
Scheme 2. Synthesis of compounds 5a–o and 6a–o.
In the next step, we investigated the formation of 3(5)-substituted-1H-pyrazoles
0
5
and 6 via the key intermediates
4
and
4
(
Scheme 2). Optimization of the coupling reaction
conditions was undertaken, choosing 3a and phenylhydrazine as a model system (Table 1).
An investigation of the reaction course and regioselectivity was carried out in various
1
solvents, and the LC/MS and H-NMR spectral data of the crude reaction mixture of
intermediate compound 4a and products 5a
, 6a were analyzed after 1 and 18 h (Table 1).
EtOH was used as a polar protic (Table 1, entry 1), ACN as a polar aprotic (Table 1, entry 2),
and CCl as a nonpolar solvent (Table 1, entry 3). As a result, the reaction in EtOH provided
4
high regioselectivity (99.5%) and good yield (78%) of 5a and just traces of its regioisomer 6a
(Table 1, entry 1). Similarly, the reaction in ACN resulted in 5a as the main product (75%),
and 6a was obtained with a 3% yield (Table 1, entry 2). The poorest yield and regioselectivity

Molecules 2021, 26, 3808
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were observed when the reaction mixture was stirred in CCl . In this case, 5a formed as a
4
major product with 54% yield, and regioisomer 6a was obtained with 9% yield (Table 1
entry 3). During optimization of the reaction conditions in different solvents, H-NMR
,
1
analysis of the crude reaction mixture after 1 h also showed the formation of intermediate
compound 4a, which was successfully isolated for structure elucidation. The regioisomer
0
6a
formed as a minor isomer via intermediate 4 a which resulted from the nucleophilic
attack of a secondary amino group of phenylhydrazine on β-enamino diketone 3a.
Figure 2. Synthesized compounds 5a–o.
Table 1. Solvent effect on the formation of compounds 4a, 5a, and 6a.
Entry
Solvent *
EtOH
ACN
t (h)
4a/5a/6a **
52.4/46.8/0.8
27.6/68.2/4.2
58.3/27.5/14.2
t (h)
18
4a/5a/6a **
0/99.5/0.5
5a, Yield (%) ***
1
2
3
1
1
1
78
75
54
18
0/95.2/4.8
CCl4
18
18.3/67.9/13.8
1
*
All reaction mixtures were stirred at room temperature. ** Ratio was determined by H-NMR spectral data from crude sample. *** After
purification by column chromatography.
In the case of intermediate compound 4a, the key information for structure elucidation
was obtained from the 15N-NMR data. In the H- N HMBC spectrum of 4a, the N shift
1
15
15
of δ −241.2 ppm was assigned to nitrogen N , due to the correlation with the neighbor-
a
0
0
1
15
ing protons 2 (6 )-H (
δ
6.81 ppm) from the phenyl moiety (Figure 3). The H- N HSQC
-H ( 6.24 ppm) had one-bond connectivity with the
experiment indicated that proton N
a
δ
aforementioned nitrogen N
a
at δ −241.2 ppm, while proton N -H (
δ 11.72 ppm) generated
b
a cross peak with nitrogen N at δ −275.1 ppm. The formation of compound 4a was also
b
0
0
confirmed by a NOESY experiment, which exhibited NOEs between the 2 (6 )-H protons at
δ
6.81 ppm and the enamine proton at δ 8.28 ppm. However, the configuration of the (2E or
2Z)-isomer of compound 4a is not yet known.
Discrimination between regioisomeric compounds 5a and 6a was based on data
1
13
1
15
1
1
from H- C HMBC, H- N HMBC, and H- H NOESY experiments (Figure 3). The
1
15
H- N HMBC experiment of the major regioisomer 5a revealed three-bond correlations
between the piperidine 4 -H proton at
0
00 00
δ 3.10 ppm and the phenyl group 2 (6 )-H protons

Molecules 2021, 26, 3808
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at δ 7.34 ppm, with the pyrazole N-1 “pyrrole-like” nitrogen at
δ
–160.3 ppm [52
,
53]. The
1
1
00 00
H- H NOESY spectrum of 5a exhibited NOEs between the phenyl group 2 (6 )-H protons
0
and the 4 -H proton from the piperidine moiety.
1
13
1
15
1
1
1
15
1
Figure 3. Relevant H- C HMBC, H- N HMBC, H- H NOESY, and H- N HSQC correlations and H-NMR (ital-
ics), 13C-NMR, and 15N-NMR (bold) chemical shifts of intermediate compounds 4a
5a (major regioisomer), and 6a
minor regioisomer).
,
(
The second regioisomer 6a was easily identified by utilizing a similar approach. The
minor regioisomer 6a exhibited a strong three-bond connectivity between the piperidine
0
proton 4 -H (
δ
3.43 ppm) and the pyrazole N-2 “pyridine-like” nitrogen at
δ
–81.5 ppm,
7.68 ppm) showed three-bond connectivity
with the pyrazole N-1 “pyrrole-like” nitrogen at –165.7 ppm. Moreover, the pyrazole
-H proton in the H- C HMBC spectrum showed a three-bond connectivity with the
0
0
00
while the phenyl group protons 2 (6 )-H (
δ
δ
1
13
5
0
0
phenyl group C-1 carbon at
assignments was obtained from the H- H NOESY 6a spectrum, showing only the NOEs
between the phenyl group 2 (6 )-H protons and the pyrazole 5-H proton (δ 8.34 ppm).
The optimal conditions for the regioselective synthesis of methyl 5-(N-Boc-piperidinyl)-
δ 139.3 ppm. Finally, confirmation of these regiochemical
1
1
0
0
00
1
H-pyrazole-4-carboxylate 5a were applied to the synthesis of other pyrazoles to evaluate
the scope of the methodology (Figure 2). -Enamino diketone 3a was coupled with dif-
ferent phenylhydrazines to give corresponding products 5b with fair to good yields.
No obvious effect of the phenylhydrazine substituent on the reaction yield was observed.
A reaction of -enamino diketone 3a with methylhydrazine provided a corresponding
tert-butyl 4-[4-(methoxycarbonyl)-1-methyl-1H-pyrazol-5-yl]piperidine-1-carboxylate 5i
with a 51% yield. To our delight, the reactions of chiral -enamino diketones 3b with
β
–
h
β
β
,c
phenyl-, (4-methylphenyl)- or [3-(trifluoromethyl)phenyl]hydrazines formed products 5j
also with good yields. While analyzing the LC/MS and H-NMR spectral data of crude
–
o
,
1
cyclization reaction mixtures, the formation of the regioisomeric 6b
–
o
was observed at
trace amounts. The structure of compounds 5b–o was determined by analogous NMR
spectroscopy experiments as described above.
Next, having
β-enamino diketone 3a, we also performed a cyclocondensation reac-
tion with hydrazine hydrate under the conditions described above, and the formation of

Molecules 2021, 26, 3808
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tautomeric 3(5)-substituted NH-pyrazole
Figure 4).
7
was established by NMR analysis (Scheme 3,
Scheme 3. Two tautomers, 7a and 7b, of 3(5)-substituted NH-pyrazole (7) and regioisomers 5i, 6i, and compound 8.
Figure 4. Relevant H- C HMBC, H- N HMBC, and H- H NOESY correlations and H-NMR (italics), 13C-NMR,
1
13
1
15
1
1
1
1
5
and N-NMR (bold) chemical shifts of compound
7 (tautomeric equilibrium of 7a and 7b) and regioisomers 5i, 6i in
◦
CDCl (25 C).
3
The prototropic tautomerism of NH-pyrazoles is well documented in many scientific
studies, including with the use of multinuclear dynamic NMR spectroscopy [54 56]. In
–
general, the annular tautomerism of 3(5)-1H-pyrazoles in solution under normal conditions
is a very rapid process on the NMR time scale, and the determination of tautomeric ratios
can usually be achieved only at low temperatures [57]. We carried out NMR studies of
◦
at 25 C in a diluted CDCl solution (Figure 4). The H-NMR spectrum of
3
1
compound
compound
[
7
7
revealed a narrow singlet of the pyrazole ring proton resonating at
δ
7.96
3.83
3(5)-H] and two singlets for methyl ester and Boc moiety protons in the area of
δ
13
(
OCH ) and 1.47 [C(CH ) ] ppm, respectively. The C-NMR spectrum provided important
3 3 3
information; as expected, the characteristic signal of the pyrazole C-4 carbon at
remained sharp, while the other two signals of pyrazole ring carbons 3(5)-C resonated
at 138.7 and 153.6 ppm and appeared broadened. It is known that the broadening of
δ 110.1 ppm
δ
NMR spectral lines very often reflects dynamic structural transformations of molecules
in solution [58]. Therefore, the observed broadness of relevant C-3 and C-5 pyrazole
carbon signals is due to the coalescence of individual signals to average signals, indicating
tautomeric equilibrium of
exhibited NOEs not only with the pyrazole ring proton at 7.96 ppm but also with the 3 -H
7
(
7a and 7b). In addition, the pyrazole NH proton (δ 11.52 ppm
)
0
piperidine protons at 1.70 ppm, which is only possible in the case of annular tautomerism 7.

Molecules 2021, 26, 3808
7 of 22
It was not possible to obtain relevant information for the nitrogen atoms of the pyrazole ring
N-1 and N-2 from the 15N-NMR spectral data since H- N HSQC and HMBC experiments
1
15
showed no direct or long-range correlations with appropriate protons.
Tautomeric compound
7 was alkylated with alkyl iodides (Scheme 3). It is known
that N-alkylation of asymmetrically ring-substituted 1H-pyrazoles generally results in
the formation of a mixture of regioisomeric N-substituted products [59]. Treatment of
compound
mixture of regioisomers 5i and 6i in a ratio of about 1:5 and a total yield of 74%. However,
alkylation of 1H-pyrazole-4-carboxylate with ethyl iodide under analogous conditions
afforded compound 8 as the sole product with a good 87% yield.
Discrimination of regioisomeric compounds 5i and 6i were based on H- C HMBC,
7 with methyl iodide in the presence of KOH in DMF gave an inseparable
7
1
13
1
15
1
1
1
15
H- N HMBC, and H- H NOESY spectral data (Figure 4). In the H- N HMBC spectra
15
of minor regioisomer 5i, a N shift of δ −178.3 ppm was assigned to the “pyrrole-like”
0
nitrogen N-1 due to the correlation of this signal with a piperidine ring proton 4 -H
1
13
(
δ 3.54 ppm). The H- C HMBC experiment exhibited a three-bond correlation of the
1
1
1
-CH protons with a pyrazole quaternary carbon C-5 at δ 148.7 ppm. Moreover, the H- H
3
NOESY spectrum of 5i exhibited NOEs between the methyl group protons (1-CH3) at
0
1
15
3
.92 ppm and the piperidine proton 4 -H at δ 3.54 ppm. In the H- N HMBC spectra of
the major regioisomer 6i, an appropriate correlation between the piperidine ring proton
0
4
-H (δ 3.36 ppm) and the “pyridine-like” pyrazole N-2 nitrogen which resonated at
1
13
δ −77.3 ppm could be observed. The H- C HMBC spectral data of compound 6i provided
a strong three-bond correlation of 1-CH protons with pyrazole protonated carbon C-5
3
at
δ 134.6 ppm. Finally, the regiochemistry of compound 6i was confirmed by a NOESY
experiment, which exhibited NOEs between the 1-CH protons and pyrazole proton
3
5
-H (δ 7.78 ppm). The structure of compound 8 was determined by analogous NMR
spectroscopy experiments as described above.
After the successful synthesis of 3(5)-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates,
we further prepared several pyrazole carboxylic acids (Scheme 4). In particular, achiral
pyrazole-4-carboxylic acid 9a was prepared from the corresponding ester 5a under the
basic conditions (2N NaOH, methanol, reflux). The same hydrolysis conditions were
applied to the production of chiral pyrazole-4-carboxylic acids (R)-9b and (S)-9c from
esters 5j and 5k, respectively.
Pyrazole carboxylic acid amides, including anilides, have been known to play an
important role in agrochemical research as fungicides [60,61]. Pyrazole-4-carboxylic acids
9a–c
were used to obtain new anilide compounds (Scheme 4). First, 9a reacted with aniline
in the presence of EDC
Moreover, chiral pyrazole anilide (R)-10b (100% ee) was obtained from carboxylic acid 9b
·
HCl, DMAP, and dichloromethane to give pyrazole anilide 10a
.
,
while the corresponding chiral anilide (S)-10c (96% ee) was synthesized from carboxylic
acid 9c. The enantiomeric purity of prepared anilides 10b was evaluated by chiral HPLC
,c
analysis. As an example, HPLC analysis of enantiomeric samples of anilides 10b,c is shown
in Figure 5.
Figure 5. Chiral HPLC analysis of (
a
) anilide 10b and (
b
) anilide 10c. Conditions: CHIRAL ART Amylose-SA (100
×
4.6 mm
I.D.; S-3
µ
m; chiral selector amylose tris(3,5-dimethylphenylcarbamate); YMC); mobile phase: ACN/(H2O + 0.1% HCOOH
◦
(30:70→70:30 in 10 min); T = 36 C; flow rate 1.0 mL/min.

Molecules 2021, 26, 3808
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Scheme 4. Reagents and conditions: (i) 2N NaOH, MeOH, reflux, 5 h; (ii) aniline, EDC
DCM, 0 C to r.t., 48 h.
·
HCl, DMAP,
◦
3. Materials and Methods
3.1. General Information
All starting materials were purchased from commercial suppliers and were used as
received. Flash column chromatography was performed on Silica Gel 60 Å (230–400 m,
µ
Merck KGaA, Darmstadt, Germany). Thin-layer chromatography was carried out on Silica
Gel plates (Merck Kieselgel 60 F254) and visualized by UV light (254 nm). Melting points
were determined on a Büchi M-565 melting point apparatus and were uncorrected. The IR
spectra were recorded on a Bruker Vertex 70v FT-IR spectrometer (Bruker Optik GmbH,
Ettlingen, Germany) using neat samples and are reported in the frequency of absorp-
–1
+
tion (cm ). Mass spectra were obtained on a Shimadzu LCMS-2020 (ESI ) spectrometer
(
Shimadzu Corporation, Kyoto, Japan). High-resolution mass spectra were measured on
+
Bruker MicrOTOF-Q III (ESI ) apparatus (Bruker Daltonik GmbH, Bremen, Germany).
Optical rotation data were recorded on a UniPol L SCHMIDT+HAENSCH polarime-
ter (concentration of compound (g/100 mL) was included in calculations automatically
(
Windaus-Labortechnik GmbH & Co. KG, Clausthal-Zellerfeld, Germany). HPLC anal-
ysis was carried out on Shimadzu LC-2030C apparatus with CHIRAL ART Amylose-SA
m; chiral selector amylose tris(3,5-dimethylphenylcarbamate);
(
100 × 4.6 mm I.D.; S-3
µ
1
13
15
YMC, Shimadzu USA Manufacturing, Inc., Canby, OR, USA). The H-, C-, and N-NMR
◦
spectra were recorded in CDCl solutions at 25 C on a Bruker Avance III 700 (700 MHz for
3
1
13
15
H, 176 MHz for C, 71 MHz for N, Bruker BioSpin AG, Fallanden, Switzerland) spec-
1
13
15
trometer equipped with a 5 mm TCI H- C/ N/D z-gradient cryoprobe, and a Bruker
Avance III 400 (400 MHz for H, 101 MHz for C, 40 MHz for N, (Bruker BioSpin AG)
1
13
15

Molecules 2021, 26, 3808
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spectrometer using a 5 mm directly detecting BBO probe. The chemical shifts (
δ
) expressed
in ppm, were relative to tetramethylsilane (TMS). The 15N-NMR spectra were referenced
to neat, external nitromethane (coaxial capillary). Full and unambiguous assignment of
1
13
15
the H-, C- and N-NMR resonances was achieved using a combination of standard
NMR spectroscopic techniques [62] such as DEPT, COSY, gs-HSQC, gs-HMBC, NOESY and
1
13
1
15
1
,1-ADEQUATE experiments [63]. H-, C-, and H- N HMBC NMR spectra, and HRMS
data of all new compounds are provided in Supplementary Materials as Figures S1–S99.
3.2. Synthesis of tert-Butyl 3- and
4-[(2)-3-(Dimethylamino)-2-(methoxycarbonyl)prop-2-enoyl]piperidine-1-carboxylates (3a–c)
To a solution of the corresponding l-(tert-butoxycarbonyl)piperidinecarboxylic acid
◦
(
(
(
1a–
c
) (4 g, 17.4 mmol) in DCM (24 mL) cooled to 0 C temperature Meldrum’s acid
2.77 g, 19.2 mmol) was added followed by DMAP (4.26 g, 34.9 mmol). Then EDC
3.68 g 19.2 mmol) was added in portions over 10 min. The reaction mixture was gradually
·
HCl
,
warmed to r.t. and stirred for 16 h. The reaction solution was diluted with DCM (10 mL),
washed with 1 M KHSO (2 15 mL) and brine (20 mL). The organic layer was dried with
×
4
anhydrous sodium sulfate, filtered, and concentrated under reduced pressure. Then the
residue was dissolved in MeOH (20 mL) and left under reflux for 5 h. The solvent was
evaporated in vacuo. A solution of crude β-keto ester (2a–c) (4.7 g, 16.4 mmol) and N,N-
dimethylformamide dimethyl acetal (4.4 mL, 32.8 mmol) in dioxane (24 mL) was stirred
◦
at 100 C. After 5 h the solvent was removed under reduced pressure. Crude compounds
3a–c were carried forward without any further purification.
3.3. Synthesis Procedure for the Preparation of Compounds 4a, 5a, and 6a
Method I. Compound 3a (500 mg, 1.5 mmol) was dissolved in EtOH (15 mL) and
treated with phenylhydrazine (160 mg, 1.5 mmol). The reaction mixture was stirred at
r.t. for 18 h. After removal of the solvent in vacuo, the residue was purified by flash
column chromatography (SiO , eluent: acetone/n-hexane, 1:7, v/v) to provide compound
2
5a (441 mg, 78%).
Method II. The reaction of compound 3a (500 mg, 1.5 mmol) with phenylhydrazine
(160 mg, 1.5 mmol) in ACN (15 mL), was carried out and purified as described in Method I
and afforded compounds 5a (424 mg, 75%) and 6a (17 mg, 3%).
Method III. The reaction of compound 3a (500 mg, 1.5 mmol) with phenylhydrazine
(
160 mg, 1.5 mmol) in CCl (15 mL) was carried out as described in Method I, and the
4
resulted residue was purified by gradient flash chromatography on silica gel (acetone/n-
hexane, 1:15 1:7, v/v) to yield compounds 4a (79 mg, 14%), 5a (305 mg, 54%) and 6a
→
(
51 mg, 9%).
3
.3.1. tert-Butyl 4-[(2E(Z))-2-(methoxycarbonyl)-3-(2-phenylhydrazinyl)prop-2-
enoyl]piperidine-1-carboxylate (4a)
1
Yellowish oil. H-NMR (700 MHz, CDCl ):
δ 1.46 (s, 9H, C(CH ) ), 1.53–1.60 (m, 2H,
3 3
3
Pip 3,5-H), 1.75–1.83 (m, 2H, Pip 3,5-H), 2.76–2.87 (m, 2H, Pip 2,6-H), 3.70 (tt, J = 11.7 Hz,
3
.5 Hz, 1H, Pip 4-H), 3.73 (s, 3H, OCH ), 4.06–4.26 (m, 2H, Pip 2,6-H), 6.24 (s, 1H, N
a
H),
3
6
.81 (d, J = 8.1 Hz, 2H, Ph 2‘,6‘-H), 6.99 (t, J = 7.4 Hz, 1H, Ph 4‘-H), 7.28 (t, J = 7.8 Hz, 2H,
1
3
Ph 3‘,5‘-H), 8.28 (d, J = 10.8 Hz, 1H, 3E(Z)-H), 11.76 (d, J = 10.8 Hz, 1H, N H). C-NMR
b
(
4
176 MHz, CDCl3):
5.5 (Pip 4-C), 51.1 (OCH ), 79.3 (
δ
28.5 (2
×
CH , Pip 3,5-C and C(
C
H ) ), 43.8 (2
×
CH , Pip 2,6-C),
2
3 3
2
C
(CH ) ), 99.2 (2E(Z)-C), 113.6 (2
×
CH, Ph 2‘,6‘-C), 122.4
3
3
3 3
(
Ph 4‘-C), 129.5 (2
×
CH, Ph 3‘,5‘-C), 146.3 (Ph 1‘-C), 154.8 (COOC(CH ) ), 162.4 (3E(Z)-C),
3
15
1
66.6 (COOCH ), 203.5 (C=O). N-NMR (71 MHz, CDCl3): δ −275.1 (N H),
−
a
241.2 (N H).
3
b
−
1
IR (FT-IR, νmax, cm ): 3438(N-H), 2928, 1717 (C=O), 1690 (C=O), 1242, 767. MS m/z
−
H] , 95%). HRMS (ESI ) for C H N NaO ([M + Na] ) calcd 426.1999,
21 29 3 5
+
+
(%): 402 ([M
−
found 426.2001.

Molecules 2021, 26, 3808
10 of 22
3
4
.3.2. tert-Butyl
-[4-(methoxycarbonyl)-1-phenyl-1H-pyrazol-5-yl]piperidine-1-carboxylate (5a)
◦
1
Yellowish crystals, mp 151–153 C. H-NMR (700 MHz, CDCl ):
δ
1.46 (s, 9H, C(CH3)3),
3
1
.53–1.61 (m, 2H, Pip 3,5-H), 2.28 (qd, J = 12.7 Hz, 4.5 Hz, 2H, Pip 3,5-H), 2.49–2.69 (m,
2
2
3
4
H, Pip 2,6-H), 3.10 (tt, J = 12.4 Hz, 3.6 Hz, 1H, Pip 4-H), 3.84 (s, 3H, OCH ), 4.02–4.27 (m,
H, Pip 2,6-H), 7.31–7.37 (m, 2H, Ph 2,6-H), 7.47–7.54 (m, 3H, Ph 3,4,5-H), 8.03 (s, 1H, Pyr
3
-H). 13C-NMR (176 MHz, CDCl3):
δ
28.4 (C(
C
H ) ), 28.6 (2
×
CH , Pip 3,5-C), 35.1 (Pip
3 3
2
-C), 44.1 (2
×
CH , Pip 2,6-C), 51.2 (OCH ), 79.4 (
C
(CH ) ), 111.7 (Pyr 4-C), 126.6 (2
×
CH,
2
3
3 3
Ph 2,6-C), 129.3 (2
×
CH, Ph 3,5-C), 129.4 (Ph 4-C), 139.2 (Ph 1-C), 142.8 (Pyr 3-C), 149.8
(
(
(
(
Pyr 5-C), 154.7 (
N-Boc), 160.3 (Pyr N-1),
C=O), 1255, 765. MS m/z (%): 386 ([M + H] , 99%). HRMS (ESI ) for C H N NaO
[M + Na] ) calcd 408.1894, found 408.1894.
C
OOC(CH ) ), 163.5 (
C
OOCH3). 15N-NMR (71 MHz, CDCl ): δ −294.5
3
3
3
−
1
−
−
76.0 (Pyr N-2). IR (FT-IR, νmax, cm ): 2979, 1712 (C=O), 1674
+
+
21
27
3
4
+
3
4
.3.3. tert-Butyl
-[4-(methoxycarbonyl)-1-phenyl-1H-pyrazol-3-yl]piperidine-1-carboxylate (6a)
◦
1
Brownish crystals, mp 134–136 C. H-NMR (700 MHz, CDCl ):
δ
1.47 (s, 9H, C(CH3)3),
3
1
.83 (qd, J = 12.2 Hz, 4.2 Hz, 2H, Pip 3,5-H), 1.93–2.01 (m, 2H, Pip 3,5-H), 2.83–2.97 (m, 2H,
Pip 2,6-H), 3.43 (tt, J = 11.6 Hz, 3.7 Hz, 1H, Pip 4-H), 3.85 (s, 3H, OCH ), 4.12–4.29 (m, 2H,
3
Pip 2,6-H), 7.30–7.35 (m, 1H, Ph 4-H), 7.43–7.49 (m, 2H, Ph 3,5-H), 7.65–7.70 (m, 2H, Ph 2,6
-
H), 8.34 (s, 1H, Pyr 5-H). 13C-NMR (176 MHz, CDCl3):
δ
28.6 (C( CH2,
C
H ) ), 31.0 (2
×
3 3
Pip 3,5-C), 35.0 (Pip 4-C), 44.0 (2
-C), 119.5 (2 CH, Ph 2,6-C), 127.3 (Ph 4-C), 129.7 (2
39.3 (Ph 1-C), 155.0 ( OOC(CH ) ), 159.0 (Pyr 3-C), 163.8 (
CDCl3): δ −292.6 (N-Boc),
708 (C=O), 1692 (C=O), 1537, 753. MS m/z (%): 386 ([M + H] , 95%). HRMS (ESI ) for
×
CH , Pip 2,6-C), 51.4 (OCH ), 79.4 (C(CH ) ), 112.8 (Pyr
2
3
3 3
4
1
×
×
CH, Ph 3,5-C), 131.2 (Pyr 5-C),
15
C
COOCH3). N-NMR (71 MHz,
3
3
−
81.5 (Pyr N-2). IR (FT-IR, νmax, cm ): 2949,
1
−
165.7 (Pyr N-1),
−
+
+
1
+
C H N NaO ([M + Na] ) calcd 408.1894, found 408.1894.
21
27
3
4
3.4. Synthesis of tert-Butyl 4-[4-(methoxycarbonyl)-1H-pyrazol-5-yl]piperidine-1-carboxylates (5b–o)
Compounds 5b were obtained from -enamino diketones 3a (500 mg, 1.5 mmol)
–o
β
–c
and appropriate hydrazines (1.5 mmol) in EtOH (15 mL) by the procedure which was used
for the preparation of compound 5a (Method I).
3
.4.1. tert-Butyl
4
-[4-(methoxycarbonyl)-1-(4-methylphenyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5b)
Compound 3a was coupled with p-tolylhydrazine hydrochloride. The obtained
residue was purified by column chromatography (SiO , eluent: acetone/n-hexane, 1:7,
2
◦
v/v) to provide compound 5b as yellowish crystals. Yield 411 mg (70%), mp 133–135 C.
1
H-NMR (400 MHz, CDCl3):
δ 1.44 (s, 9H, C(CH ) ), 1.49–1.61 (m, 2H, Pip 3,5-H), 2.23
3 3
(
qd, J = 12.6 Hz, 4.3 Hz, 2H, Pip 3,5-H), 2.44 (s, 3H, CH ), 2.49–2.72 (m, 2H, Pip 2,6-H),
3
3
7
.10 (tt, J = 12.4 Hz, 3.6 Hz, 1H, Pip 4-H), 3.83 (s, 3H, OCH ), 4.00–4.29 (m, 2H, Pip 2,6-H),
3
.17–7.23 (m, 2H, Ph 2,6-H), 7.27–7.32 (m, 2H, Ph 3,5-H), 8.00 (s, 1H, Pyr 3-H). C-NMR
1
3
(
4
101 MHz, CDCl3):
δ
21.4 (CH ), 28.6 (C( H ) ), 28.9 (2 CH , Pip 3,5-C), 35.3 (Pip 4-C),
C
×
3
3 3
2
4.4 (2 × CH2, Pip 2,6-C), 51.4 (OCH ), 79.6 (
C
(CH ) ), 111.8 (Pyr 4-C), 126.5 (2
×
CH,
3
3 3
Ph 2,6-C), 130.0 (2
×
CH, Ph 3,5-C), 137.0 (Ph 1-C), 139.7 (Ph 4-C), 142.9 (Pyr 3-C), 150.1
(
(
(
(
Pyr 5-C), 155.0 (
C
OOC(CH ) ), 163.8 (
C
OOCH3). 15N-NMR (41 MHz, CDCl ): δ −294.5
3
−
3
3
−
1
N-Boc), 160.6 (Pyr N-1),
−
75.8 (Pyr N-2). IR (FT-IR, νmax, cm ): 2980, 1703 (C=O), 1688
+
+
C=O), 1255, 779. MS m/z (%): 400 ([M + H] , 99%). HRMS (ESI ) for C H N NaO
22
29
3
4
+
[M + Na] ) calcd 422.2050, found 422.2051.
3
.4.2. tert-Butyl
-[4-(methoxycarbonyl)-1-(3-methylphenyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5c
4
)
Compound 3a was coupled with m-tolylhydrazine hydrochloride. The obtained
residue was purified by column chromatography (SiO , eluent: acetone/n-hexane, 1:9,
2
◦
1
v/v) to provide compound 5c as white crystals. Yield 310 mg (53%), mp 123–124 C. H-

Molecules 2021, 26, 3808
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NMR (400 MHz, CDCl3):
δ 1.45 (s, 9H, C(CH ) ), 1.51–1.60 (m, 2H, Pip 3,5-H), 2.26 (qd,
3 3
J = 12.7 Hz, 4.4 Hz, 2H, Pip 3,5-H), 2.42 (s, 3H, CH ), 2.50–2.71 (m, 2H, Pip 2,6-H), 3.08 (tt,
3
J = 12.4 Hz, 3.6 Hz, 1H, Pip 4-H), 3.83 (s, 3H, OCH ), 4.04–4.26 (m, 2H, Pip 2,6-H), 7.10
3
(
d, J = 7.9 Hz, 1H, Ph 6-H), 7.16 (s, 1H, Ph 2-H), 7.30 (d, J = 8.0 Hz, 1H, Ph 4-H), 7.37 (t,
J = 7.7 Hz, 1H, Ph 5-H), 8.01 (s, 1H, Pyr 3-H). 13CNMR (101 MHz, CDCl3):
21.4 (CH ), 28.6
C( H ) ), 28.8 (2 CH , Pip 3,5-C), 35.3 (Pip 4-C), 44.3 (2 CH , Pip 2,6-C), 51.4 (OCH ),
9.6 (
39.4 (Ph 1-C), 139.8 (Ph 3-C), 142.9 (Pyr 3-C), 150.0 (Pyr 5-C), 155.0 (
δ
3
(
7
1
(
C
×
×
3
3
2
2
3
C(CH ) ), 111.8 (Pyr 4-C), 123.6 (Ph 6-C), 127.4 (Ph 2-C), 129.1 (Ph 5-C), 130.3 (Ph 4-C),
3 3
COOC(CH ) ), 163.8
3 3
15
C
OOCH ). N-NMR (41 MHz, CDCl3): δ −159.9 (Pyr N-1),
−
76.2 (Pyr N-2). IR (FT-IR,
3
−
1
+
νmax, cm ): 2979, 1703 (C=O), 1688 (C=O), 1243, 779. MS m/z (%): 400 ([M + H] , 100%).
HRMS (ESI ) for C H N NaO ([M + Na] ) calcd 422.2050, found 422.2050.
+
+
22
29
3
4
3
.4.3. tert-Butyl
-[1-(3-fluorophenyl)-4-(methoxycarbonyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5d
4
)
Compound 3a was coupled with (3-fluorophenyl)hydrazine hydrochloride. The ob-
tained residue was purified by column chromatography (SiO , eluent: acetone/n-hexane,
2
◦
1
:8, v/v) to provide compound 5d as yellowish crystals. Yield 433 mg (73%), mp 134–136 C.
1
H-NMR (400 MHz, CDCl3):
δ 1.45 (s, 9H, C(CH ) ), 1.51–1.64 (m, 2H, Pip 3,5-H), 2.27
3 3
(qd, J = 12.7 Hz, 4.4 Hz, 2H, Pip 3,5-H), 2.49–2.72 (m, 2H, Pip 2,6-H), 3.08 (tt, J = 12.3 Hz,
3
.6 Hz, 1H, Pip 4-H), 3.83 (s, 3H, OCH ), 3.96–4.32 (m, 2H, Pip 2,6-H), 7.05–7.17 (m, 2H,
3
Ph 2,6-H), 7.19–7.25 (m, 1H, Ph 4-H), 7.43–7.53 (m, 1H, Ph 5-H), 8.02 (s, 1H, Pyr 3-H).
1
3
C-NMR (101 MHz, CDCl3):
δ
28.6 (C(
C
H ) ), 28.8 (2
×
CH , Pip 3,5-C), 35.3 (Pip 4-C),
3 3
2
4
4.3 (2 × CH2, Pip 2,6-C), 51.5 (OCH ), 79.7 (
C(CH ) ), 112.3 (Pyr 4-C), 114.6 (d, J = 23.8 Hz,
3
3 3
Ph 2-C), 116.8 (d, J = 20.9 Hz, Ph 4-C), 122.5 (d, J = 3.3 Hz, Ph 6-C), 130.7 (d, J = 9.0 Hz, Ph
5
-C), 140.7 (d, J = 9.7 Hz, Ph 1-C), 143.3 (Pyr 3-C), 150.1 (Pyr 5-C), 154.9 (COOC(CH ) ),
3 3
15
1
62.8 (d, J = 249.7 Hz, Ph 3-C), 163.5 (
C
OOCH ). N-NMR (41 MHz, CDCl3): δ −161.2
3
−
1
(Pyr N-1),
−
74.5 (Pyr N-2). IR (FT-IR, νmax, cm ): 2980, 1711 (C=O), 1674 (C=O), 1243, 867.
+
+
+
MS m/z (%): 404 ([M + H] , 99%). HRMS (ESI ) for C H FN NaO ([M + Na] ) calcd
21
26
3
4
426.1800, found 426.1799.
3
4
.4.4. tert-Butyl
-[1-(2-fluorophenyl)-4-(methoxycarbonyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5e
)
Compound 3a was coupled with (2-fluorophenyl)hydrazine hydrochloride. The ob-
tained residue was purified by column chromatography (SiO , eluent: acetone/n-hexane,
2
◦
1
:8, v/v) to provide compound 5e as yellowish crystals. Yield 367 mg (62%), mp 114–116 C.
1
H-NMR (400 MHz, CDCl3):
δ 1.43 (s, 9H, C(CH ) ), 1.48–1.58 (m, 1H, Pip 3-H), 1.60–1.76
3 3
(m, 1H, Pip 5-H), 2.03–2.30 (m, 2H, Pip 3,5-H), 2.44–2.71 (m, 2H, Pip 2,6-H), 2.88–3.05
(
m, 1H, Pip 4-H), 3.83 (s, 3H, OCH ), 3.95–4.29 (m, 2H, Pip 2,6-H), 7.22–7.32 (m, 2H, Ph
3
1
3
3
,6-H), 7.34–7.42 (m, 1H, Ph 5-H), 7.47–7.55 (m, 1H, Ph 4-H), 8.06 (s, 1H, Pyr 3-H). C-NMR
28.6 (C( H ) and 2 CH , Pip 3,5-C), 35.6 (Pip 4-C), 44.1 (2 CH2,
Pip 2,6-C), 51.4 (OCH ), 79.6 ( (CH ) ), 111.8 (Pyr 4-C), 116.9 (d, J = 19.6 Hz, Ph 3-C), 125.0
(101 MHz, CDCl3):
δ
C
×
×
3 3
2
C
3
3 3
(d, J = 4.0 Hz, Ph 6-C), 127.4 (d, J = 12.6 Hz, Ph 1-C), 129.7 (Ph 5-C), 131.8 (d, J = 7.7 Hz,
Ph 4-C), 143.7 (Pyr 3-C), 151.4 (Pyr 5-C), 154.9 (COOC(CH ) ), 157.5 (d, J = 252.5 Hz, Ph
3 3
2
-C), 163.6 (
C
OOCH3). 15N-NMR (41 MHz, CDCl ): δ −292.8 (N-Boc),
−
172.7 (Pyr N-1),
3
−
1
−
73.8 (Pyr N-2). IR (FT-IR, νmax, cm ): 2980, 1716 (C=O), 1682 (C=O), 1275, 770. MS m/z
+
+
+
(
%): 404 ([M + H] , 96%). HRMS (ESI ) for C H FN NaO ([M + Na] ) calcd 426.1800,
21 26 3 4
found 426.1800.
3
.4.5. tert-Butyl
4
-[4-(methoxycarbonyl)-1-(4-methoxyphenyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5f)
Compound 3a was coupled with (4-methoxyphenyl)hydrazine hydrochloride. The
obtained residue was purified by column chromatography (SiO , eluent: acetone/n-hexane,
2
◦
1
:8, v/v) to provide compound 5f as orange crystals. Yield 366 mg (60%), mp 151–153 C.
1
H-NMR (400 MHz, CDCl3):
δ 1.44 (s, 9H, C(CH ) ), 1.48–1.59 (m, 2H, Pip 3,5-H), 2.20
3 3
(qd, J = 12.7 Hz, 5.1 Hz, 2H, Pip 3,5-H), 2.49–2.73 (m, 2H, Pip 2,6-H), 3.09 (tt, J = 12.4 Hz,

Molecules 2021, 26, 3808
12 of 22
3
2
3
4
4
.6 Hz, 1H, Pip 4-H), 3.83 (s, 3H, COOCH ), 3.87 (s, 3H, OCH ), 3.99–4.30 (m, 2H, Pip
3 3
,6-H), 6.98 (d, J = 8.8 Hz, 2H, Ph 3,5-H), 7.23 (d, J = 8.8 Hz, 2H, Ph 2,6-H), 7.99 (s, 1H, Pyr
-H). 13C-NMR (101 MHz, CDCl3):
28.6 (C( H ) ), 28.9 (2 CH , Pip 3,5-C), 35.3 (Pip
-C), 44.2 (2 CH , Pip 2,6-C), 51.4 (COO H ), 55.7 (OCH ), 79.6 (
-C), 114.5 (2
50.2 (Pyr 5-C), 154.9 (
δ
C
3
×
3 3
2
×
C
C(CH ) ), 111.6 (Pyr
3 3
2
3
×
CH, Ph 3,5-C), 128.0 (2
×
CH, Ph 2,6-C), 132.4 (Ph 1-C), 142.7 (Pyr 3-C),
15
1
C
OOC(CH ) ), 160.3 (Ph 4-C), 163.8 (
C
OOCH ). N-NMR (41 MHz,
3
3
3
−
75.4 (Pyr N-2). IR (FT-IR, νmax, cm ): 2981, 1715 (C=O),
1
CDCl3): δ −161.4 (Pyr N-1),
682 (C=O), 1245, 780. MS m/z (%): 416 ([M+H] , 100%). HRMS (ESI ) for C H N NaO
5
−
+
+
1
22
29
3
+
(
[M+Na] ) calcd 438.1999, found 438.2000.
3
.4.6. tert-Butyl
-[4-(methoxycarbonyl)-1-(3-methoxyphenyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5g)
4
Compound 3a was coupled with (3-methoxyphenyl)hydrazine hydrochloride. The
obtained residue was purified by column chromatography (SiO , eluent: acetone/n-hexane,
2
◦
1
:9, v/v) to provide compound 5g as yellowish crystals. Yield 330 mg (54%), mp 69–71 C.
1
H-NMR (400 MHz, CDCl3):
δ 1.45 (s, 9H, C(CH ) ), 1.50–1.61 (m, 2H, Pip 3,5-H), 2.27 (qd,
3 3
J = 12.7 Hz, 4.3 Hz, 2H, Pip 3,5-H), 2.51–2.71 (m, 2H, Pip 2,6-H), 3.11 (tt, J = 12.4 Hz, 3.7 Hz,
1
H, Pip 4-H), 3.83 (s, 6H, OCH and COOCH ), 3.99–4.29 (m, 2H, Pip 2,6-H), 6.85–6.93 (m,
3
3
2
H, Ph 2,4-H), 7.00–7.06 (m, 1H, Ph 6-H), 7.35–7.43 (m, 1H, Ph 5-H), 8.01 (s, 1H, Pyr 3-H).
13
C-NMR (101 MHz, CDCl3):
δ
28.6 (C(
H ), 55.7 (OCH ), 79.6 (C(CH ) ), 112.0 (Pyr 4-C),
3 3
C
H ) ), 28.8 (2
×
CH , Pip 3,5-C), 35.3 (Pip 4-C),
3 3
2
4
4.3 (2 CH , Pip 2,6-C), 51.4 (COO
×
C
2
3
3
1
12.5 (Ph 2-C), 115.5 (Ph 4-C), 118.8 (Ph 6-C), 130.1 (Ph 5-C), 140.4 (Ph 1-C), 143.0 (Pyr 3-C),
1
50.0 (Pyr 5-C), 155.0 (
C
OOC(CH ) ), 160.4 (Ph 3-C), 163.7 (
C
OOCH ). IR (FT-IR, νmax
,
3
3
3
−
1
+
cm ): 2979, 1702 (C=O), 1686 (C=O), 1109, 778. MS m/z (%): 400 ([M + H] , 100%). HRMS
+
+
(
ESI ) for C H N NaO ([M + Na] ) calcd 438.1999, found 438.2000.
22 29 3 5
3
.4.7. tert-Butyl 4-{4-(methoxycarbonyl)-1-[3-(trifluoromethyl)phenyl]-1H-pyrazol-5-
yl}piperidine-1-carboxylate (5h)
Compound 3a was coupled with [3-(trifluoromethyl)phenyl]hydrazine hydrochloride.
The obtained residue was purified by column chromatography (SiO , eluent: acetone/n-
2
hexane, 1:5, v/v) to provide compound 5h as yellowish crystals. Yield 506 mg (76%), mp
◦
13–115 C. H-NMR (700 MHz, CDCl ): δ 1.46 (s, 9H, C(CH ) ), 1.55–1.62 (m, 2H, Pip
3 3 3
1
1
3
3
6
,5-H), 2.24–2.37 (m, 2H, Pip 3,5-H), 2.50–2.70 (m, 2H, Pip 2,6-H), 3.05 (tt, J = 12.3 Hz,
.6 Hz, 1H, Pip 4-H), 3.86 (s, 3H, OCH ), 4.06–4.29 (m, 2H, Pip 2,6-H), 7.54–7.57 (m, 1H, Ph
3
-H), 7.65 (br s, 1H, Ph 2-H), 7.68 (t, J = 7.9 Hz, 1H, Ph 5-H), 7.79 (br s, 1H, Ph 4-H), 8.06
(
s, 1H, Pyr 3-H). 13C-NMR (176 MHz, CDCl ): δ 28.4 (C(CH ) ), 28.8 (2 × CH , Pip 3,5-C),
3
3 3
2
35.4 (Pip 4-C), 44.1 (2
×
CH , Pip 2,6-C), 51.4 (OCH ), 79.6 (C(CH ) ), 112.5 (Pyr 4-C), 123.3
2
3
3 3
(q, J = 271.5 Hz, CF ), 123.7 (q, J = 3.7 Hz, Ph 2-C), 126.2 (q, J = 3.7 Hz, Ph 4-C), 129.7 (Ph
3
5
-C), 130.1 (Ph 6-C), 132.1 (q, J = 33.3 Hz, Ph 3-C), 139.8 (Ph 1-C), 143.5 (Pyr 3-C), 150.1
(
(
(
Pyr 5-C), 154.8 (
N-Boc),
C
OOC(CH ) ), 163.3 (
C
OOCH3). 15N-NMR (71 MHz, CDCl ): δ −294.7
3
3
3
−
1
−
163.3 (Pyr N-1),
−
76.0 (Pyr N-2). IR (FT-IR, νmax, cm ): 2980, 1714 (C=O), 1686
+
+
+
C=O), 1169, 1062. MS m/z (%): 354 ([M-Boc + H] ), 454 ([M + H] ), 99%. HRMS (ESI ) for
+
C H F N NaO ([M + Na] ) calcd 476.1768, found 476.1768.
22
26
3
3
4
3
4
.4.8. tert-Butyl
-[4-(methoxycarbonyl)-1-methyl-1H-pyrazol-5-yl]piperidine-1-carboxylate (5i)
Compound 3a was coupled with methylhydrazine. The obtained residue was purified
by column chromatography (SiO , eluent: acetone/n-hexane, 1:3, v/v) to provide compound
2
◦
i as white crystals. Yield 242 mg (51%), mp 147–149 C. H NMR (400 MHz, CDCl ):
3
1
5
δ
3
1.48 (s, 9H, C(CH ) ), 1.59–1.70 (m, 2H, Pip 3,5-H), 2.15 (qd, J = 12.7 Hz, 4.4 Hz, 2H, Pip
3
3
,5-H), 2.68–2.89 (m, 2H, Pip 2,6-H), 3.53 (tt, J = 12.6 Hz, 3.7 Hz, 1H, Pip 4-H), 3.79 (s, 3H,
13
OCH ), 3.90 (s, 3H, CH ), 4.10–4.44 (m, 2H, Pip 2,6-H), 7.81 (s, 1H, Pyr 3-H). C NMR (101
3
3
MHz, CDCl3):
δ
28.6 (C(
C
H ) ), 28.6 (2
×
CH , Pip 3,5-C), 34.2 (Pip 4-C), 38.7 (CH ), 44.5
3 3
2
3
(
(
2
×
CH , Pip 2,6-C), 51.2 (OCH ), 79.8 (
C
(CH ) ), 111.3 (Pyr 4-C), 141.5 (Pyr 3-C), 148.8
2
3
3 3
15
Pyr 5-C), 154.9 (
COOC(CH ) ), 164.0 (
COOCH3). N NMR (41 MHz, CDCl3): δ −293.2
3
3

Molecules 2021, 26, 3808
13 of 22
(
(
+
N-Boc),
−
176.9 (Pyr N-1),
−
75.1 (Pyr N-2). IR (FT-IR, νmax, cm 1): 2980, 1705 (C=O), 1688
−
+
+
C=O), 1234, 779. MS m/z (%): 324 ([M + H] , 100%). HRMS (ESI ) for C H N NaO ([M
16
25
3
4
+
Na] ) calcd 346.1737, found 346.1737.
3
.4.9. tert-Butyl
(
3R)-3-[4-(methoxycarbonyl)-1-phenyl-1H-pyrazol-5-yl]piperidine-1-carboxylate (5j)
Compound 3b was coupled with phenylhydrazine. The obtained residue was pu-
rified by column chromatography (SiO , eluent: acetone/n-hexane, 1:7, v/v) to provide
2
20
1
compound 5j as as brownish oil. Yield 379 mg (67%), [
α
]D = 6.4 (c 1.12, MeOH). H-NMR
(
(
3
400 MHz, CDCl3):
qd, J = 13.1 Hz, 4.1 Hz, 1H, Pip 4-H), 2.74–2.87 (m, 1H, Pip 6-H), 2.88–3.01 (m, 1H, Pip
-H), 3.54–3.69 (m, 1H, Pip 2-H), 3.86 (s, 3H, OCH ), 3.92–4.17 (m, 2H, Pip 2,6-H), 7.36–7.42
δ 1.40 (s, 10H, C(CH ) and Pip 5-H), 1.63–1.78 (m, 2H, Pip 4,5-H), 2.46
3 3
3
13
(
m, 2H, Ph 2,6-H), 7.48–7.54 (m, 3H, Ph 3,4,5-H), 8.05 (s, 1H, Pyr 3-H). C-NMR (101 MHz,
CDCl3): 25.3 (Pip 5-C), 27.4 (Pip 4-C), 28.5 (C( H ) ), 36.0 (Pip 3-C), 43.7 (Pip 6-C), 46.1
Pip 2-C), 51.5 (OCH ), 79.6 ( (CH ) ), 112.3 (Pyr 4-C), 126.5 (2
δ
C
3 3
(
C
×
CH, Ph 2,6-C), 129.4
3
3 3
(
(
(
Ph 4-C), 129.5 (2
×
CH, Ph 3,5-C), 139.0 (Ph 1-C), 143.3 (Pyr 3-C), 148.1 (Pyr 5-C), 154.8
1
5
C
OOC(CH ) ), 163.9 (
C
OOCH ). N-NMR (41 MHz, CDCl3): δ −159.4 (Pyr N-1),
−
76.1
3
3
3
−
1
Pyr N-2). IR (FT-IR, νmax, cm ): 2975, 1716 (C=O), 1687 (C=O), 1261, 1099, 765. MS m/z
+
+
+
+
(%): 286 ([M-Boc+H] ), 386 ([M + H] ), 95%. HRMS (ESI ) for C H N NaO ([M + Na] )
21 27 3 4
calcd 408.1894, found 408.1893.
3
.4.10. tert-Butyl
(
3S)-3-[4-(methoxycarbonyl)-1-phenyl-1H-pyrazol-5-yl]piperidine-1-carboxylate (5k)
Compound 3c was coupled with phenylhydrazine. The obtained residue was puri-
fied by column chromatography (SiO , eluent: acetone/n-hexane, 1:11, v/v) to provide
2
20
1
compound 5k as brownish oil. Yield 436 mg (77%), [
α
]D
=
−
6.4 (c 0.73, MeOH). H-NMR
(
(
400 MHz, CDCl ): 1.40 (s, 10H, C(CH ) and Pip 5-H), 1.60–1.80 (m, 2H, Pip 4,5-H), 2.46
3
3 3
q, J = 13.3 Hz, 1H, Pip 4-H), 2.80 (s, 1H, Pip 6-H), 2.94 (s, 1H, Pip 3-H), 3.48–3.79 (m, 1H,
Pip 2-H), 3.86 (s, 3H, OCH ), 3.91–4.30 (m, 2H, Pip 2,6-H), 7.32–7.46 (m, 2H, Ph 2,6-H),
3
13
7
(
(
.46–7.56 (m, 3H, Ph 3,4,5-H), 8.05 (s, 1H, Pyr 3-H). C-NMR (101 MHz, CDCl3):
Pip 5-C), 27.4 (Pip 4-C), 28.5 (C( H ) ), 36.0 (Pip 3-C), 44.0 (Pip 6-C), 46.0 (Pip 2-C), 51.5
OCH ), 79.6 ( (CH ) ), 112.3 (Pyr 4-C), 126.5 (2
δ 25.3
C
3
3
C
×
CH, Ph 2,6-C), 129.4 (Ph 4-C), 129.5
OOC(CH ) ),
3
3 3
(
1
2 × CH, Ph 3,5-C), 139.1 (Ph 1-C), 143.3 (Pyr 3-C), 148.1 (Pyr 5-C), 154.7 (
C
3
3
−
63.9 (COOCH ). IR (FT-IR, νmax, cm ): 2979, 1717 (C=O), 1684 (C=O), 1408, 1259, 757.
3
1
+
+
+
MS m/z (%): 386 ([M + H] , 96%). HRMS (ESI ) for C H N NaO ([M + Na] ) calcd
21
27
3
4
408.1894, found 408.1892.
3
.4.11. tert-Butyl
(
3R)-3-[4-(methoxycarbonyl)-1-(4-methylphenyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (5l)
Compound 3b was coupled with p-tolylhydrazine hydrochloride. The obtained residue
was purified by column chromatography (SiO , eluent: acetone/n-hexane, 1:7, v/v) to
2
20
provide compound 5l as brownish oil. Yield 458 mg (78%), [
α
]D = 4.1 (c 0.62, MeOH).
1
H-NMR (400 MHz, CDCl3):
δ 1.40 (s, 10H, C(CH ) and Pip 5-H), 1.61–1.74 (m, 2H, Pip
3 3
4
,5-H), 2.43 (s, 4H, Pip 4-H and CH ), 2.79 (s, 1H, Pip 6-H), 2.93 (s, 1H, Pip 3-H), 3.48–3.72
3
(
2
2
m, 1H, Pip 2-H), 3.85 (s, 3H, OCH ), 3.90–4.18 (m, 2H, Pip 2,6-H), 7.22–7.33 (m, 4H, Ph
3
,3,5,6-H), 8.03 (s, 1H, Pyr 3-H). 13C-NMR (101 MHz, CDCl3):
7.4 (Pip 4-C), 28.5 (C(
(CH ) ), 112.1 (Pyr 4-C), 126.3 (2
δ 21.4 (CH ), 25.3 (Pip 5-C),
3
C
H ) ), 36.0 (Pip 3-C), 43.9 (Pip 6-C), 45.8 (Pip 2-C), 51.5 (OCH ), 79.5
3
3
3
(
C
×
CH, Ph 2,6-C), 130.0 (2
×
CH, Ph 3,5-C), 136.6 (Ph
3
3
1
-C), 139.6 (Ph 4-C), 143.1 (Pyr 3-C), 148.1 (Pyr 5-C), 154.7 (
COOC(CH ) ), 164.0 (
C
OOCH ).
3
3
3
1
5
−1
N-NMR (41 MHz, CDCl3): δ −157.9 (Pyr N-1),
−
74.6 (Pyr N-2). IR (FT-IR, νmax, cm ):
+
+
2
9
976, 1710 (C=O), 1692 (C=O), 1148, 824. MS m/z (%): 300 ([M-Boc + H] ), 400 ([M + H] ),
+
+
7%. HRMS (ESI ) for C H N NaO ([M + Na] ) calcd 422.2050, found 422.2052.
22
29
3
4

Molecules 2021, 26, 3808
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3
.4.12. tert-Butyl (3S)-3-[4-(methoxycarbonyl)-1-(4-methylphenyl)-1H-pyrazol-5-
yl]piperidine-1-carboxylate (5m)
Compound 3c was coupled with p-tolylhydrazine hydrochloride. The obtained residue
was purified by column chromatography (SiO , eluent: acetone/n-hexane, 1:9, v/v) to
2
20
provide compound 5m as brownish oil. Yield 475 mg (81%), [
α
]D
=
−
4.3 (c 0.86, MeOH).
1
H-NMR (400 MHz, CDCl3):
δ 1.40 (s, 10H, C(CH ) and Pip 5-H), 1.61–1.77 (m, 2H, Pip
3 3
4
,5-H), 2.43 (s, 4H, Pip 4-H and CH ), 2.79 (s, 1H, Pip 6-H), 2.93 (s, 1H, Pip 3-H), 3.48–3.76
3
(
2
2
m, 1H, Pip 2-H), 3.85 (s, 3H, OCH ), 3.90–4.26 (m, 2H, Pip 2,6-H), 7.21–7.32 (m, 4H, Ph
3
,3,5,6-H), 8.03 (s, 1H, Pyr 3-H). 13C-NMR (101 MHz, CDCl3):
7.4 (Pip 4-C), 28.5 (C(
(CH ) ), 112.1 (Pyr 4-C), 126.3 (2
δ 21.4 (CH ), 25.3 (Pip 5-C),
3
C
H ) ), 36.1 (Pip 3-C), 43.5 (Pip 6-C), 46.4 (Pip 2-C), 51.5 (OCH ), 79.5
3
3
3
(C
×
CH, Ph 2,6-C), 130.0 (2
×
CH, Ph 3,5-C), 136.6 (Ph 1-C),
3
3
1
39.6 (Ph 4-C), 143.1 (Pyr 3-C), 148.1 (Pyr 5-C), 154.7 (
COOC(CH ) ), 164.0 (
C
OOCH ). IR
3
3
3
−
1
+
(
FT-IR, νmax, cm ): 2930, 1714 (C=O), 1688 (C=O), 1261, 822. MS m/z (%): 400 ([M + H] ,
+
+
9
5%). HRMS (ESI ) for C H N NaO ([M + Na] ) calcd 422.2050, found 422.2051.
22 29 3 4
3
.4.13. tert-Butyl (3R)-3-{4-(methoxycarbonyl)-1-[3-(trifluoromethyl)phenyl]-1H-pyrazol-
-yl}piperidine-1-carboxylate (5n)
5
Compound 3b was coupled with [3-(trifluoromethyl)phenyl]hydrazine hydrochloride.
The obtained residue was purified by column chromatography (SiO , eluent: acetone/n-
2
2
0
hexane, 1:5, v/v) to provide compound 5n as brownish oil. Yield 526 mg (79%), [α]D = 9.9
1
(
c 1.31, MeOH). H-NMR (700 MHz, CDCl ):
δ
1.39 (br s, 10H, C(CH ) and Pip 5-H),
3
3 3
1
.68–1.77 (m, 2H, Pip 4,5-H), 2.50 (qd, J = 12.9 Hz, 3.9 Hz, 1H, Pip 4-H), 2.72–3.00 (m, 2H,
Pip 3,6-H), 3.52–3.75 (m, 1H, Pip 2-H), 3.87 (s, 3H, OCH ), 3.90–4.21 (m, 2H, Pip 2,6-H),
3
7
.52–7.69 (m, 2H, Ph 5,6-H), 7.71 (br s, 1H, Ph 2-H), 7.73–7.82 (m, 1H, Ph 4-H), 8.07 (s, 1H,
13
Pyr 3-H). C-NMR (176 MHz, CDCl3):
δ
25.2 (Pip 5-C), 27.3 (Pip 4-C), 28.3 (C(
CH ) ), 35.9
3 3
(
(
Pip 3-C), 43.9 (Pip 6-C), 45.7 (Pip 2-C), 51.5 (OCH ), 79.6 ( (CH ) ), 112.8 (Pyr 4-C), 123.3
C
3
3 3
q, J = 272.4 Hz, CF ), 123.5 (Ph 2-C), 126.1 (Ph 4-C), 129.4 (Ph 5-C), 130.0 (Ph 6-C), 132.2 (Ph
3
3
-C), 139.3 (Ph 1-C), 143.7 (Pyr 3-C), 148.1 (Pyr 5-C), 154.7 (
C
OOC(CH ) ), 163.5 (
C
OOCH ).
3
3
3
1
5
−1
N-NMR (41 MHz, CDCl3): δ −162.0 (Pyr N-1),
−
76.1 (Pyr N-2). IR (FT-IR, νmax, cm ):
+
+
2
9
951, 1717 (C=O), 1688 (C=O), 1130, 1098. MS m/z (%): 354 ([M-Boc + H] ), 454 ([M + H] ),
+
+
6%. HRMS (ESI ) for C H F N NaO ([M + Na] ) calcd 476.1768, found 476.1769.
22
26
3
3
4
3
.4.14. tert-Butyl (3S)-3-{4-(methoxycarbonyl)-1-[3-(trifluoromethyl)phenyl]-1H-pyrazol-
-yl}piperidine-1-carboxylate (5o)
5
Compound 3c was coupled with [3-(trifluoromethyl)phenyl]hydrazine hydrochlo-
ride. The obtained residue was purified by column chromatography (SiO , eluent: ace-
2
tone/n-hexane, 1:11, v/v) to provide compound 5o as brownish oil. Yield 420 mg (63%),
2
0
1
[
α]D = −9.8 (c 0.85, MeOH). H-NMR (700 MHz, CDCl ):
δ 1.39 (br s, 10H, C(CH ) and
3
3 3
Pip 5-H), 1.68–1.77 (m, 2H, Pip 4,5-H), 2.50 (qd, J = 12.9 Hz, 3.9 Hz, 1H, Pip 4-H), 2.72–3.00
(
m, 2H, Pip 3,6-H), 3.52–3.75 (m, 1H, Pip 2-H), 3.87 (s, 3H, OCH ), 3.90–4.21 (m, 2H, Pip
3
2
1
3
,6-H), 7.52–7.69 (m, 2H, Ph 5,6-H), 7.71 (br s, 1H, Ph 2-H), 7.73–7.82 (m, 1H, Ph 4-H), 8.08 (s,
25.2 (Pip 5-C), 27.3 (Pip 4-C), 28.3 (C( H ) ),
5.9 (Pip 3-C), 43.9 (Pip 6-C), 45.7 (Pip 2-C), 51.5 (OCH ), 79.6 ( (CH ) ), 112.8 (Pyr 4-C),
H, Pyr 3-H). 13C-NMR (176 MHz, CDCl3):
δ
C
3 3
C
3
3 3
1
1
(
23.3 (q, J = 272.4 Hz, CF ), 123.5 (Ph 2-C), 126.1 (Ph 4-C), 129.4 (Ph 5-C), 130.0 (Ph 6-C),
3
32.2 (Ph 3-C), 139.3 (Ph 1-C), 143.7 (Pyr 3-C), 148.1 (Pyr 5-C), 154.7 (COOC(CH ) ), 163.5
3 3
−
COOCH ). IR (FT-IR, νmax, cm ): 2951, 1717 (C=O), 1688 (C=O), 1130, 1099. MS m/z
3
1
+
+
+
(
%): 454 ([M + H] , 100%). HRMS (ESI ) for C H F N NaO ([M + Na] ) calcd 476.1768,
22 26 3 3 4
found 476.1772.
3.5. Synthesis of tert-Butyl 4-[4-(methoxycarbonyl)-1H-pyrazolyl]piperidine-1-carboxylate (7)
Compound 3a (500 mg, 1.5 mmol) was dissolved in EtOH (15 mL) and treated with 55%
hydrazine hydrate solution (74 mg, 1.5 mmol). Reaction mixture was stirred at r.t. for 18 h.
After removal of the solvent in vacuo, the residue was purified by flash column chromatog-
raphy (SiO , eluent: acetone/n-hexane, 1:7, v/v) to provide compound
7 as white crystals.
2

Molecules 2021, 26, 3808
15 of 22
◦
1
Yield 272 mg (60%), mp 128–130 C. H-NMR (700 MHz, CDCl ):
δ
1.47 (s, 9H, C(CH3)3),
3
1.64–1.78 (m, 2H, Pip 3,5-H), 1.92–1.99 (m, 2H, Pip 3,5-H), 2.77–2.95 (m, 2H, Pip 2,6-H), 3.53
(
t, J = 12.0 Hz, 1H, Pip 4-H), 3.83 (s, 3H, OCH ), 4.10–4.36 (m, 2H, Pip 2,6-H), 7.96 (s, 1H,
3
Pyr 3(5)-H), 11.52 (s, 1H, Pyr NH). 13C-NMR (176 MHz, CDCl3):
δ 28.5 (C(CH ) ), 30.8
3 3
(
1
2 × CH2, Pip 3,5-C), 33.8 (Pip 4-C), 44.2 (2
×
CH , Pip 2,6-C), 51.2 (OCH ), 79.8 (
C
(CH ) ),
2
3
3 3
1
5
10.1 (Pyr 4-C), 138.7 and 153.6 (Pyr 3(5)-C), 154.9 (
C
OOC(CH ) ), 164.1 (
COOCH3). N-
3
3
−
1
NMR (71 MHz, CDCl3): δ −292.7 (N-Boc). IR (FT-IR, νmax, cm ): 3208 (N-H), 2980, 1706
+
−
(
C=O), 1655 (C=O), 1434, 1165, 763. MS m/z (%): 210 ([M-Boc + H] ) 308 ([M − H] ), 97%.
+
+
HRMS (ESI ) for C H N NaO ([M + Na] ) calcd 332.1581, found 332.1581.
15
23
3
4
3
.6. Synthesis of tert-Butyl 4-[4-(methoxycarbonyl)-1H-pyrazolyl]piperidine-1-carboxylates (5i
A solution of compound (100 mg, 0.3 mmol), KOH (27 mg, 0.5 mmol), and alkyl
iodide (1 mmol) in DMF (0.75 mL) was stirred at r.t. for 4 h. The reaction mixture was
diluted with EtOAc (10 mL), washed with H O (2 15 mL) and brine (15 mL). The
, 6i, 8)
7
×
2
organic layer was dried with anhydrous sodium sulfate, filtered, and concentrated under
reduced pressure. The crude product was purified by flash chromatography using an
eluent—Hex/Me CO in the appropriate ratio.
2
3
4
.6.1. tert-Butyl
-[4-(methoxycarbonyl)-1-methyl-1H-pyrazol-3-yl]piperidine-1-carboxylate (6i) and
tert-Butyl 4-[4-(methoxycarbonyl)-1-methyl-1H-pyrazol-5-yl]piperidine-1-carboxylate (5i
)
Compound
7 was coupled with iodomethane. The obtained residue was purified
by column chromatography (SiO , eluent: acetone/n-hexane, 1:4, v/v) to provide an
2
1
inseparable mixture of regioisomers 6i
:
5i (5:1) as white crystals. Yield 77 mg (74%). H-
NMR (700 MHz, CDCl ) (two isomers are seen in the spectra ratio ~ 5:1 (6i
:
5i)):
δ 1.44 (s, 9H,
3
C(CH ) , (6i)), 1.47 (s, 9H, C(CH ) , (5i)), 1.59–1.76 (m, 2H, Pip 3,5-H, (6i and 5i)), 1.82–1.95
3
3
3 3
(m, 2H, Pip 3,5-H, (6i)), 2.14 (qd, J = 12.7 Hz, 4.5 Hz, 2H, Pip 3,5-H, (5i)), 2.69–2.95 (m, 2H,
Pip 2,6-H, (6i and 5i)), 3.36 (tt, J = 11.8 Hz, 3.6 Hz, 1H, Pip 4-H, (6i)), 3.54 (t, J = 12.6 Hz
,
1
3
7
2
H, Pip 4-H, (5i)), 3.79 (s, 3H, OCH , (6i)), 3.80 (s, 3H, OCH , (5i)), 3.85 (s, 3H, CH , (6i)),
3 3 3
.92 (s, 3H, CH , (5i)), 4.03–4.34 (m, 2H, Pip 2,6-H, (6i and 5i)), 7.78 (s, 1H, Pyr 5-H, (6i)),
3
13
.82 (s, 1H, Pyr 3-H, (5i)). C-NMR (176 MHz, CDCl3):
δ
28.6 (2
×
CH , Pip 3,5-C, (5i)),
2
8.6 (C( H ) , (6i and 5i)), 31.2 (2 CH , Pip 3,5-C, (6i)), 34.1 (Pip 4-C, (5i)), 34.8 (Pip 4-C,
C
×
3
3
2
(
6i)), 38.7 (CH , (5i)), 39.2 (CH , (6i)), 43.8 (2
×
CH , Pip 2,6-C, (6i)), 44.7 (2
×
CH , Pip
2
3
3
2
2
,6-C, (5i)), 51.2 (OCH , (6i)), 51.2 (OCH , (5i)), 79.3 (C(CH ) , (6i)), 79.8 (C(CH ) , (5i)),
3 3 3 3 3 3
1
10.7 (Pyr 4-C, (6i)), 111.3 (Pyr 4-C, (5i)), 134.6 (Pyr 5-C, (6i)), 141.4 (Pyr 3-C, (5i)), 148.7
(
1
(
(
Pyr 5-C, (5i)), 154.8 (
C
OOC(CH ) , (6i)), 154.9 (
C
OOC(CH ) , (5i)), 158.1 (Pyr 3-C, (6i)),
3
3
3 3
15
63.9 (
C
OOCH , (6i)), 164.0 (
C
−
OOCH , (5i)). N-NMR (71 MHz, CDCl3): δ −294.3 (N-Boc,
3
3
5i)),
−
183.8 (Pyr N-1, (6i)),
178.3 (Pyr N-1, (5i)),
−
77.3 (Pyr N-2, (6i)),
−
76.7 (Pyr N-2,
4
+
+
+
5i)). MS m/z (%): 324 ([M + H] , 100%). HRMS (ESI ) for C H N NaO ([M + Na] )
16
25
3
calcd 346.1737, found 346.1737.
3
4
.6.2. tert-Butyl
-[1-ethyl-4-(methoxycarbonyl)-1H-pyrazol-3-yl]piperidine-1-carboxylate (8)
Compound was coupled with iodoethane. The obtained residue was purified by
7
column chromatography (SiO , eluent: acetone/n-hexane, 1:5, v/v) to provide compound
8
2
◦
1
as white crystals, yield 95 mg (87%), mp 77–78 C. H-NMR (700 MHz, CDCl ):
δ
1.43–1.50
3
(
m, 12H, C(CH ) and CH CH ), 1.72 (qd, J = 12.5 Hz, 4.1 Hz, 2H, Pip 3,5-H), 1.85–1.93 (m,
3 3 2 3
2
H, Pip 3,5-H), 2.80–2.90 (m, 2H, Pip 2,6-H), 3.37 (tt, J = 11.8 Hz, 3.6 Hz, 1H, Pip 4-H), 3.79
1
3
(s, 3H, OCH ), 4.08–4.23 (m, 4H, Pip 2,6-H and CH CH ), 7.82 (s, 1H, Pyr 5-H). C-NMR
3
2
3
(
176 MHz, CDCl3):
δ
15.2 (CH CH ), 28.6 (C(
C
H ) ), 31.1 (2
×
CH , Pip 3,5-C), 34.9 (Pip
2
3
3 3
2
4
1
-C), 44.4 (2
33.1 (Pyr 5-C), 155.2 (
CDCl3): δ −168.7 (Pyr N-1),
678 (C=O), 1219, 768. MS m/z (%): 338 ([M + H] , 99%). HRMS (ESI ) for C H N NaO
4
×
CH , Pip 2,6-C), 47.3 (CH CH ), 51.3 (OCH ), 79.9 (C(CH ) ), 110.5 (Pyr 4-C),
2 2 3 3 3 3
15
COOC(CH ) ), 157.8 (Pyr 3-C), 163.9 (COOCH ). N-NMR (71 MHz,
3 3 3
−
1
−
82.9 (Pyr N-2). IR (FT-IR, νmax, cm ): 2980, 1715 (C=O),
+
+
1
17
27
3
+
(
[M + Na] ) calcd 360.1894, found 360.1894.

Molecules 2021, 26, 3808
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3.7. Synthesis of 5-[1-(tert-Butoxycarbonyl)piperidinyl]-1H-pyrazole-4-carboxylic acids (9a–c)
Corresponding ester (5a, 5j, 5k) (300 mg, 0.78 mmol) was dissolved in MeOH (0.1 mM
)
and treated with 2 N NaOH (4 equiv). The solution was stirred under reflux for 5 h. After
removal of the solvent in vacuo, the residue was dissolved in water (15 mL), washed
with EtOAc (2
×
15 mL), acidified with 1 M KHSO (pH = 1), and washed with EtOAc
4
(
2 × 15 mL). The extracts were combined and dried over sodium sulfate, filtered, and
concentrated to dryness to give desired compounds which were directly used in the next
step without further purification.
3
.7.1. 5-[1-(tert-Butoxycarbonyl)piperidin-4-yl]-1-phenyl-1H-pyrazole-4-carboxylic Acid (9a)
◦
1
Brownish crystals, yield 240 mg (83%), mp 190–192 C. H-NMR (400 MHz, CDCl ):
3
δ
3
2
3
1.46 (s, 9H, C(CH ) ), 1.52–1.70 (m, 2H, Pip 3,5-H), 2.26 (qd, J = 12.7 Hz, 4.3 Hz, 2H, Pip
,5-H), 2.48–2.76 (m, 2H, Pip 2,6-H), 3.12 (tt, J = 12.5 Hz, 3.6 Hz, 1H, Pip 4-H), 3.97–4.30 (m,
3
3
H, Pip 2,6-H), 7.31–7.38 (m, 2H, Ph 2,6-H), 7.47–7.57 (m, 3H, Ph 3,4,5-H), 8.09 (s, 1H, Pyr
-H), 9.39 (br s, 1H, OH). 13C-NMR (101 MHz, CDCl3):
δ
28.6 (C(
C
H ) ), 28.8 (2 × CH ,
3 3
2
Pip 3,5-C), 35.4 (Pip 4-C), 44.3 (2 × CH2, Pip 2,6-C), 79.8 (
2 × CH, Ph 2,6-C), 129.5 (2 CH, Ph 3,5-C), 129.7 (Ph 4-C), 139.3 (Ph 1-C), 143.9 (Pyr
-C), 150.9 (Pyr 5-C), 155.0 (
δ −159.6 (Pyr N-1), 75.8 (Pyr N-2). IR (FT-IR, νmax, cm ): 2852, 1675 (C=O), 1547, 1424,
64. MS m/z (%): 370 ([M
calcd 394.1737, found 394.1738.
C(CH ) ), 111.5 (Pyr 4-C), 126.8
3 3
(
3
×
15
COOC(CH ) ), 168.4 (COOH). N-NMR (41 MHz, CDCl3):
3 3
−
1
−
−
H] , 95%). HRMS (ESI ) for C H N NaO ([M + Na] )
20 25 3 4
+
+
7
−
3
.7.2. 5-[(3R)-1-(tert-Butoxycarbonyl)piperidin-3-yl]-1-phenyl-1H-pyrazole-4-carboxylic
Acid (9b)
◦
α
]D20 = 10.4 (c 1.10, MeOH). H-
1
Brownish crystals, yield 254 mg (88%), mp 86–88 C, [
NMR (400 MHz, CDCl3): 1.41 (s, 10H, C(CH ) and Pip 5-H), 1.60–1.80 (m, 2H, Pip 4,5-H),
.38–2.52 (m, 1H, Pip 4-H), 2.64–2.86 (m, 1H, Pip 6-H), 2.86–3.13 (m, 1H, Pip 3-H), 3.45–3.75
m, 1H, Pip 2-H), 3.87–4.23 (m, 2H, Pip 2,6-H), 7.36–7.44 (m, 2H, Ph 2,6-H), 7.49–7.57 (m,
H, Ph 3,4,5-H), 8.15 (s, 1H, Pyr 3-H). 13C-NMR (101 MHz, CDCl3):
25.2 (Pip 5-C), 27.4
Pip 4-C), 28.5 (C( H ) ), 36.1 (Pip 3-C), 43.5 (Pip 6-C), 46.3 (Pip 2-C), 79.7 ( (CH ) ), 111.8
Pyr 4-C), 126.5 (2
-C), 148.8 (Pyr 5-C), 154.7 (
δ −158.1 (Pyr N-1), 75.9 (Pyr N-2). IR (FT-IR, νmax, cm ): 2930, 1686 (C=O), 1412, 1147,
64. MS m/z (%): 370 ([M
calcd 394.1737, found 394.1738.
δ
3 3
2
(
3
δ
(
(
3
C
×
C
3
3
3 3
CH, Ph 2,6-C), 129.6 (3
×
CH, Ph 3,4,5-C), 138.9 (Ph 1-C), 144.1 (Pyr
OOC(CH ) ), 168.3 (COOH). N-NMR (41 MHz, CDCl3):
15
C
3
3
−
1
−
−
H] , 97%). HRMS (ESI ) for C H N NaO ([M + Na] )
20 25 3 4
+
+
7
−
3
.7.3. 5-[(3S)-1-(tert-Butoxycarbonyl)piperidin-3-yl]-1-phenyl-1H-pyrazole-4-carboxylic
Acid (9c)
◦
]D20
Yellowish crystals, yield 243 mg (84%), mp 88–90 C, [
H-NMR (400 MHz, CDCl3): 1.41 (s, 10H, C(CH ) and Pip 5-H), 1.62–1.78 (m, 2H, Pip
,5-H), 2.45 (qd, J = 12.7 Hz, 7.3 Hz, 1H, Pip 4-H), 2.68–2.87 (m, 1H, Pip 6-H), 2.92–3.08 (m,
H, Pip 3-H), 3.46–3.76 (m, 1H, Pip 2-H), 3.87–4.21 (m, 2H, Pip 2,6-H), 7.36–7.45 (m, 2H, Ph
α
=
−
10.5 (c 1.0, MeOH).
1
δ
3 3
4
1
2
δ
7
13
,6-H), 7.47–7.57 (m, 3H, Ph 3,4,5-H), 8.15 (s, 1H, Pyr 3-H). C-NMR (101 MHz, CDCl3):
25.2 (Pip 5-C), 27.4 (Pip 4-C), 28.5 (C( H ) ), 36.1 (Pip 3-C), 43.5 (Pip 6-C), 46.3 (Pip 2-C),
9.7 ( (CH ) ), 111.7 (Pyr 4-C), 126.5 (2
C
3
3
C
×
CH, Ph 2,6-C), 129.6 (3
×
CH, Ph 3,4,5-C), 138.9
3
3
(
Ph 1-C), 144.1 (Pyr 3-C), 148.9 (Pyr 5-C), 154.7 (
C
OOC(CH ) ), 168.3 (COOH). IR (FT-IR,
νmax, cm ): 2930, 1687 (C=O), 1412, 1148, 765. MS m/z (%): 370 ([M
ESI ) for C H N NaO ([M + Na] ) calcd 394.1737, found 394.1739.
3
3
−
1
−
−
H] , 97%). HRMS
+
+
(
20 25 3 4
3.8. Synthesis of tert-Butyl 3- and
4-[4-(Phenylcarbamoyl)-1H-pyrazol-5-yl]piperidine-1-carboxylates (10a–c)
To a solution of the appropriate pyrazole-4-carboxylic acids (9a
and DMAP (7 mg, 0.05 mmol) in DCM (0.1 mM) cooled to 0 C temperature EDC
114 mg, 0.59 mmol) and aniline (50 mg, 0.54 mmol) were added. The reaction mixture was
–
c
) (200 mg, 0.54 mmol)
◦
·
HCl
(

Molecules 2021, 26, 3808
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left at r.t. for 48 h. The solvent was removed under reduced pressure and the crude product
was purified by flash chromatography using an eluent—Hex/Me CO (6:1, v/v).
2
3
4
.8.1. tert-Butyl
-[1-phenyl-4-(phenylcarbamoyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (10a)
◦
1
White crystals, yield 192 mg (80%), mp 187–189 C. H-NMR (400 MHz, CDCl ):
δ
1.42
3
(
s, 9H, C(CH ) ), 1.55–1.76 (m, 2H, Pip 3,5-H), 2.18–2.33 (m, 2H, Pip 3,5-H), 2.43–2.71 (m,
3
3
2
7
3
H, Pip 2,6-H), 3.15 (tt, J = 12.4 Hz, 3.5 Hz, 1H, Pip 4-H), 3.99–4.24 (m, 2H, Pip 2,6-H),
.10–7.18 (m, 1H, NHPh 4-H), 7.33–7.39 (m, 4H, NHPh 3,5-H and NPh 2,6-H), 7.48–7.54 (m,
H, NPh 3,4,5-H), 7.54–7.61 (m, 2H, NHPh 2,6-H), 7.67 (s, 1H, NH), 7.90 (s, 1H, Pyr 3-H).
13
C-NMR (101 MHz, CDCl3):
δ
28.6 (C(
C
H ) ), 29.6 (2
×
CH , Pip 3,5-C), 35.4 (Pip 4-C),
3 3
2
4
4.3 (2 CH , Pip 2,6-C), 79.6 (
×
C
(CH ) ), 116.0 (Pyr 4-C), 120.5 (2
×
CH, NHPh 2,6-C),
CH, NHPh 3,5-C), 129.5 (2 CH,
NPh 3,5-C), 129.7 (NPh 4-C), 138.0 (NHPh 1-C), 138.9 (Pyr 3-C), 139.6 (NPh 1-C), 149.2 (Pyr
2
3 3
1
24.6 (NHPh 4-C), 126.8 (2
×
CH, NPh 2,6-C), 129.2 (2
×
×
1
5
5
−
-C), 154.9 (
159.8 (Pyr N-1),
m/z (%): 347 ([M-Boc + H] , 99%). HRMS (ESI ) for C H N NaO ([M + Na] ) calcd
C
OOC(CH ) ), 161.8 (CONH). N-NMR (41 MHz, CDCl ): δ −252.4 (NH),
3
3
3
−
76.7 (Pyr N-2). IR (FT-IR, νmax, cm ): 3390, 1671 (C=O), 1435, 748. MS
1
−
+
+
+
26
30
4
3
469.2210, found 469.2209.
3
.8.2. tert-Butyl
(
3R)-3-[1-phenyl-4-(phenylcarbamoyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (10b)
◦
]D20
White crystals, yield 197 mg (82%), mp 199–201 C, [
α
=
−
20.1 (c 0.87, MeOH).
1
H-NMR (400 MHz, CDCl3):
-H), 1.70–1.83 (m, 1H, Pip 4-H), 2.48 (qd, J = 12.9 Hz, 4.0 Hz, 1H, Pip 4-H), 2.69–3.05 (m, 2H,
Pip 3,6-H), 3.49–3.81 (m, 1H, Pip 2-H), 3.88–4.24 (m, 2H, Pip 2,6-H), 7.10–7.18 (m, 1H, NHPh
-H), 7.33–7.45 (m, 4H, NHPh 3,5-H and NPh 2,6-H), 7.46–7.59 (m, 5H, NHPh 2,6-H and
δ 1.40 (s, 10H, C(CH ) and Pip 5-H), 1.57–1.70 (m, 1H, Pip
3 3
5
4
1
3
NPh 3,4,5-H), 7.72 (s, 1H, NH), 7.92 (s, 1H, Pyr 3-H). C-NMR (101 MHz, CDCl3):
δ 25.2
(
(
Pip 5-C), 27.9 (Pip 4-C), 28.5 (C(
C
H ) ), 36.3 (Pip 3-C), 43.8 (Pip 6-C), 46.6 (Pip 2-C), 79.5
3
3
C
(CH ) ), 116.5 (Pyr 4-C), 120.6 (2
×
CH, NHPh 2,6-C), 124.7 (NHPh 4-C), 126.5 (2 × CH
,
3
3
NPh 2,6-C), 129.3 (2
1
×
CH, NHPh 3,5-C), 129.5 (3
39.1 (NPh 1-C), 139.3 (Pyr 3-C), 147.0 (Pyr 5-C), 154.8 (COOC(CH ) ), 161.8 (CONH). N-
×
CH, NPh 3,4,5-C), 137.9 (NHPh 1-C),
1
5
3 3
NMR (41 MHz, CDCl ): δ −252.5 (NH),
−
158.7 (Pyr N-1),
−
77.1 (Pyr N-2). IR (FT-IR, νmax,
3
cm ): 3400 (N-H), 1677 (C=O), 1405, 1137, 751. MS m/z (%): 447 ([M + H]+, 96%). HRMS
−
1
+
+
(
ESI ) for C H N NaO ([M + Na] ) calcd 469.2210, found 469.2216. The enantiomeric
26 30 4 3
excess was determined by HPLC with a CHIRAL ART Amylose-SA column, t = 6.5 min
R
(
100%), ee = 100%.
3
.8.3. tert-Butyl
(
3S)-3-[1-phenyl-4-(phenylcarbamoyl)-1H-pyrazol-5-yl]piperidine-1-carboxylate (10c)
◦
α
]D20 = 19.9 (c 0.70, MeOH).
White crystals, yield 161 mg (67%), mp 199–201 C, [
H-NMR (400 MHz, CDCl3): 1.39 (s, 10H, C(CH ) and Pip 5-H), 1.57–1.67 (m, 1H, Pip
-H), 1.71–1.80 (m, 1H, Pip 4-H), 2.48 (qd, J = 12.8 Hz, 4.0 Hz, 1H, Pip 4-H), 2.69–3.02
1
δ
3 3
5
(
(
m, 2H, Pip 3,6-H), 3.45–3.83 (m, 1H, Pip 2-H), 3.88–4.20 (m, 2H, Pip 2,6-H), 7.10–7.18
m, 1H, NHPh 4-H), 7.33–7.44 (m, 4H, NHPh 3,5-H and NPh 2,6-H), 7.46–7.61 (m, 5H,
NHPh 2,6-H and NPh 3,4,5-H), 7.75 (s, 1H, NH), 7.92 (s, 1H, Pyr 3-H). 1 C-NMR (101
3
MHz, CDCl3):
-C), 46.6 (Pip 2-C), 79.5 (
NHPh 4-C), 126.5 (2 CH, NPh 2,6-C), 129.2 (2
NPh 3,4,5-C), 138.0 (NHPh 1-C), 139.1 (NPh 1-C), 139.3 (Pyr 3-C), 147.0 (Pyr 5-C), 154.6
δ
25.2 (Pip 5-C), 27.9 (Pip 4-C), 28.5 (C(
CH ) ), 36.3 (Pip 3-C), 43.9 (Pip
3 3
6
(
C
(CH ) ), 116.5 (Pyr 4-C), 120.6 (2
×
CH, NHPh 2,6-C), 124.6
3
3
×
×
CH, NHPh 3,5-C), 129.5 (3 CH,
×
1
5
(
C
OOC(CH ) ), 161.8 (CONH). N-NMR (41 MHz, CDCl ): δ −252.4 (NH),
−
158.6 (Pyr
3
3
3
−
1
N-1), 77.0 (Pyr N-2). IR (FT-IR, νmax, cm ): 3402 (N-H), 1677 (C=O), 1405, 1137, 751.
MS m/z (%): 347 ([M-Boc + H] ), 447 ([M + H]+), 99%. HRMS (ESI ) for C H N NaO
3
[M + Na] ) calcd 469.2210, found 469.2210. The enantiomeric excess was determined by
−
+
+
26
30
4
+
(
HPLC with a CHIRAL ART Amylose-SA column, t = 6.5 min (1.8% minor enantiomer),
R
tR = 9.2 min (98.2% major enantiomer), ee = 96 %.

Molecules 2021, 26, 3808
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4
. Conclusions
In summary, we developed a new regioselective process for synthesizing 3- or 5-
N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates as achiral and chiral heterocyclic building
(
blocks. Regioselective synthesis of targeted building blocks was obtained starting from
piperidine-4-carboxylic and (R)- and (S)-piperidine-3-carboxylic acids conversion to the
corresponding
β-enamino diketones via formation of intermediate
β-keto esters. Further
investigation of the reaction of
β-enamino diketones with various aryl and alkyl hydrazines
in various solvents at room temperature proved the regioselective formation of 5-(N-Boc-
piperidinyl)-1H-pyrazole-4-carboxylates in ethanol compared to polar aprotic or nonprotic
solvents. Regioisomeric 3-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates were obtained
by treating β-enamino diketone with hydrazine hydrate and subsequent alkylation of
tautomeric 3(5)-substituted NH-pyrazole with alkylhalides. Furthermore, we demon-
strated that 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates can be successfully applied
to the synthesis of tert-butyl 3- and 4-[4-(phenylcarbamoyl)-1H-pyrazol-5-yl]piperidine-1-
carboxylates by basic hydrolysis and the subsequent reaction of obtained carboxylic acids
with aniline in the presence of EDC HCl and DMAP. The structures of all synthesized
·
compounds were confirmed by detailed NMR spectroscopy and HRMS investigations.
Supplementary Materials: The following are available online. Figure S1: 1H-NMR (700 MHz,
1
3
CDCl ) spectrum of compound 4a, Figure S2: C-NMR (176 MHz, CDCl ) spectrum of compound 4a
,
3
3
1
15
Figure S3: H- N HMBC (71 MHz, CDCl ) spectrum of compound 4a, Figure S4: HRMS (ESI-TOF) of
compound 4a, Figure S5: H-NMR (700 MHz, CDCl ) spectrum of compound 5a, Figure S6: C-NMR
176 MHz, CDCl ) spectrum of compound 5a, Figure S7: H- N HMBC (71 MHz, CDCl ) spectrum
3 3
of compound 5a, Figure S8: HRMS (ESI-TOF) of compound 5a, Figure S9: H-NMR (700 MHz, CDCl )
spectrum of compound 6a, Figure S10: C-NMR (176 MHz, CDCl ) spectrum of compound 6a
Figure S11: H- N HMBC (71 MHz, CDCl ) spectrum of compound 6a, Figure S12: HRMS (ESI-TOF)
of compound 6a, Figure S13: H-NMR (400 MHz, CDCl ) spectrum of compound 5b, Figure S14:
C-NMR (101 MHz, CDCl ) spectrum of compound 5b, Figure S15: H- N HMBC (41 MHz,
3
1
13
3
1
15
(
1
3
1
3
,
3
1
15
3
1
3
13
1
15
3
CDCl ) spectrum of compound 5b, Figure S16: HRMS (ESI-TOF) of compound 5b, Figure S17:
3
1
13
H-NMR (400 MHz, CDCl ) spectrum of compound 5c, Figure S18: C-NMR (176 MHz, CDCl3)
spectrum of compound 5c, Figure S19: N-NMR (41 MHz, CDCl ) spectrum of compound 5c
Figure S20: HRMS (ESI-TOF) of compound 5c, Figure S21: H-NMR (400 MHz, CDCl ) spectrum of
compound 5d, Figure S22: C-NMR (101 MHz, CDCl ) spectrum of compound 5d, Figure S23: H-
3
1
5
,
3
1
3
1
3
1
3
15
N HMBC (41 MHz, CDCl ) spectrum of compound 5d, Figure S24: HRMS (ESI-TOF) of compound
3
, Figure S25: 1H-NMR (400 MHz, CDCl ) spectrum of compound 5e, Figure S26: C-NMR
13
5
(
d
3
1
15
101 MHz, CDCl ) spectrum of compound 5e, Figure S27: H- N HMBC (41 MHz, CDCl ) spectrum
3 3
of compound 5e, Figure S28: HRMS (ESI-TOF) of compound 5e, Figure S29: H-NMR (400 MHz,
1
1
3
CDCl ) spectrum of compound 5f, Figure S30: C-NMR (176 MHz, CDCl ) spectrum of compound
3
3
1
5
5
f
, Figure S31: N-NMR (41 MHz, CDCl ) spectrum of compound 5f, Figure S32: HRMS (ESI-TOF)
of compound 5f, Figure S33: H-NMR (400 MHz, CDCl ) spectrum of compound 5g, Figure S34:
3
1
3
13
C-NMR (101 MHz, CDCl ) spectrum of compound 5g, Figure S35: HRMS (ESI-TOF) of compound
3
, Figure S36: 1H-NMR (700 MHz, CDCl ) spectrum of compound 5h, Figure S37: C-NMR
13
5
(
g
3
1
15
176 MHz, CDCl ) spectrum of compound 5h, Figure S38: H- N HMBC (71 MHz, CDCl ) spectrum
3 3
of compound 5h, Figure S39: HRMS (ESI-TOF) of compound 5h, Figure S40: H-NMR (400 MHz,
1
1
3
CDCl ) spectrum of compound 5i, Figure S41: C-NMR (101 MHz, CDCl ) spectrum of compound
3
3
1
15
5
i
, Figure S42: H- N HMBC (41 MHz, CDCl ) spectrum of compound 5i, Figure S43: HRMS (ESI-
TOF) of compound 5i, Figure S44: H-NMR (400 MHz, CDCl ) spectrum of compound 5j, Figure S45:
3
1
3
13
1
15
C-NMR (101 MHz, CDCl ) spectrum of compound 5j, Figure S46: H- N HMBC (41 MHz, CDCl )
3
3
spectrum of compound 5j, Figure S47: HRMS (ESI-TOF) of compound 5j, Figure S48: 1H-NMR
1
3
(
400 MHz, CDCl ) spectrum of compound 5k, Figure S49: C-NMR (101 MHz, CDCl ) spectrum
3 3
of compound 5k, Figure S50: HRMS (ESI-TOF) of compound 5k, Figure S51: H-NMR (400 MHz,
1
1
3
CDCl ) spectrum of compound 5l, Figure S52: C-NMR (101 MHz, CDCl ) spectrum of compound 5l
,
3
3
1
15
Figure S53: H- N HMBC (41 MHz, CDCl ) spectrum of compound 5l, Figure S54: HRMS (ESI-TOF)
of compound 5l, Figure S55: H-NMR (400 MHz, CDCl ) spectrum of compound 5m, Figure S56:
3
1
3
13
C-NMR (101 MHz, CDCl ) spectrum of compound 5m, Figure S57: HRMS (ESI-TOF) of compound
3
, Figure S58: 1H-NMR (700 MHz, CDCl ) spectrum of compound 5n, Figure S59: C-NMR
13
5
m
3

Molecules 2021, 26, 3808
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1
15
(
176 MHz, CDCl ) spectrum of compound 5n, Figure S60: H- N HMBC (41 MHz, CDCl ) spectrum
3 3
1
of compound 5n, Figure S61: HRMS (ESI-TOF) of compound 5n, Figure S62: H-NMR (700 MHz,
CDCl ) spectrum of compound 5o, Figure S63: C-NMR (176 MHz, CDCl ) spectrum of compound
5o, Figure S64: HRMS (ESI-TOF) of compound 5o, Figure S65: H-NMR (700 MHz, CDCl ) spectrum
3
of compound 7, Figure S66: C-NMR (176 MHz, CDCl ) spectrum of compound 7, Figure S67:
3
, Figure S68: HRMS (ESI-TOF) of compound
Figure S69: H-NMR (700 MHz, CDCl ) spectrum of compounds 6i and 5i, Figure S70: C-NMR
1
3
3
3
1
1
3
1
1
H- H NOESY (CDCl ) spectrum of compound
7
7,
3
1
13
3
1
15
(
176 MHz, CDCl ) spectrum of compounds 6i and 5i, Figure S71: H- N HMBC (71 MHz, CDCl )
3
3
spectrum of compounds 6i and 5i, Figure S72: HRMS (ESI-TOF) of compound 6i and 5i, Figure S73:
1
13
H-NMR (700 MHz, CDCl ) spectrum of compound
8
, Figure S74: C-NMR (176 MHz, CDCl3)
3
1
15
spectrum of compound
Figure S76: HRMS (ESI-TOF) of compound
8
, Figure S75: H- N HMBC (71 MHz, CDCl ) spectrum of compound 8,
3
1
8
, Figure S77: H-NMR (400 MHz, CDCl ) spectrum of
3
1
3
1
compound 9a, Figure S78: C-NMR (101 MHz, CDCl ) spectrum of compound 9a, Figure S79: H-
3
15
N HMBC (41 MHz, CDCl ) spectrum of compound 9a, Figure S80: HRMS (ESI-TOF) of compound
3
, Figure S81: 1H-NMR (400 MHz, CDCl ) spectrum of compound 9b, Figure S82: C-NMR
13
9
a
3
1
15
(
101 MHz, CDCl ) spectrum of compound 9b, Figure S83: H- N HMBC (41 MHz, CDCl ) spectrum
3 3
1
of compound 9b, Figure S84: HRMS (ESI-TOF) of compound 9b, Figure S85: H-NMR (400 MHz,
CDCl ) spectrum of compound 9c, Figure S86: C-NMR (101 MHz, CDCl ) spectrum of compound
9c, Figure S87: HRMS (ESI-TOF) of compound 9c, Figure S88: H-NMR (400 MHz, CDCl ) spectrum
3
1
3
3
3
1
1
3
of compound 10a, Figure S89: C-NMR (101 MHz, CDCl ) spectrum of compound 10a, Figure S90:
3
1
15
H- N HMBC (41 MHz, CDCl ) spectrum of compound 10a, Figure S91: HRMS (ESI-TOF) of
3
1
compound 10a, Figure S92: H-NMR (400 MHz, CDCl ) spectrum of compound 10b, Figure S93:
3
13
1
15
CNMR (101 MHz, CDCl ) spectrum of compound 10b, Figure S94: H- N HMBC (41 MHz, CDCl )
3
3
1
spectrum of compound 10b, Figure S95: HRMS (ESI-TOF) of compound 10b, Figure S96: H-NMR
1
3
(
400 MHz, CDCl ) spectrum of compound 10c, Figure S97: C-NMR (101 MHz, CDCl ) spectrum of
3 3
1
15
compound 10c, Figure S98: H- N HMBC (41 MHz, CDCl ) spectrum of compound 10c, Figure S99:
3
HRMS (ESI-TOF) of compound 10c.
Author Contributions: Conceptualization, F.A.S. and A.Š.; methodology, N.K.; validation, N.K., G.M.
and G.R.; formal analysis, A.B., G.R. and M.D.; investigation, G.M., V.M. and M.D.; data curation,
V.M.; writing—original draft preparation, A.Š., A.B., E.A. and G.M.; writing—review and editing,
N.K. and V.K.; visualization A.B., A.Š. and E.A.; resources, F.A.S., supervision, A.Š. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by Vipergen ApS (Copenhagen, Denmark).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding authors.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.
References
1
.
Singh, P.; Samanta, K.; Das, S.K.; Panda, G. Amino acid chirons: A tool for asymmetric synthesis of heterocycles. Org. Biomol.
Chem. 2014, 12, 6297–6339. [CrossRef] [PubMed]
2
.
Sardina, F.J.; Rapoport, H. Enantiospecific Synthesis of Heterocycles from
CrossRef] [PubMed]
Blaskovich, M.A.T. Unusual Amino Acids in Medicinal Chemistry. J. Med. Chem. 2016, 59, 10807–10836. [CrossRef] [PubMed]
Žukauskaite, A.; Moretto, A.; Peggion, C.; De Zotti, M.; Ša cˇ kus, A.; Formaggio, F.; De Kimpe, N.; Mangelinckx, S. Synthesis and
Conformational Study of Model Peptides Containing N-Substituted 3-Aminoazetidine-3-carboxylic Acids. Eur. J. Org. Chem.
014, 2014, 2312–2321. [CrossRef]
Vale, N.; Ferreira, A.; Matos, J.; Fresco, P.; Gouveia, M.J. Amino Acids in the Development of Prodrugs. Molecules 2018, 23, 2318.
CrossRef]
α-Amino Acids. Chem. Rev. 1996, 96, 1825–1872.
[
3
4
.
.
˙
2
5
.
.
[
6
Bonina, F.P.; Arenare, L.; Palagiano, F.; Saija, A.; Nava, F.A.; Trombetta, D.; De Caprariis, P. Synthesis, stability, and pharmacological
evaluation of nipecotic acid prodrugs. J. Pharm. Sci. 1999, 88, 561–567. [CrossRef]

Molecules 2021, 26, 3808
20 of 22
7
8
9
1
.
.
.
Henzi, V.; Reichling, D.B.; Helm, S.W.; MacDermott, A.B. L-proline activates glutamate and glycine receptors in cultured rat
dorsal horn neurons. Mol. Pharmacol. 1992, 41, 793–801.
Wang, M.; Rakesh, K.; Leng, J.; Fang, W.-Y.; Ravindar, L.; Gowda, D.C.; Qin, H.-L. Amino acids/peptides conjugated heterocycles:
A tool for the recent development of novel therapeutic agents. Bioorg. Chem. 2018, 76, 113–129. [CrossRef]
Walsh, C.T.; O’Brien, R.V.; Khosla, C. Nonproteinogenic Amino Acid Building Blocks for Nonribosomal Peptide and Hybrid
Polyketide Scaffolds. Angew. Chem. Int. Ed. 2013, 52, 7098–7124. [CrossRef]
0. Cerminara, I.; Chiummiento, L.; Funicello, M.; Guarnaccio, A.; Lupattelli, P. Heterocycles in Peptidomimetics and Pseudopeptides:
Design and Synthesis. Pharmaceuticals 2012, 5, 297–316. [CrossRef]
1
1
1. Hollis, S.J. Heterocyclic in Peptide Chemistry. Ph. D. Thesis, The Open University, Milton Keynes, UK, 2000.
2. Kothapalli, Y.; Puthukanoori, R.K.; Alapati, S.R. Enantiospecific first total synthesis of 7a(S)-p-hydroxyphenopyrrozin. Tetrahedron
Lett. 2012, 53, 1891–1893. [CrossRef]
1
3. Lin, C.-H.; Hong, B.-C.; Lee, G.-H. Asymmetric synthesis of functionalized pyrrolizidines by an organocatalytic and pot-economy
strategy. RSC Adv. 2016, 6, 8243–8247. [CrossRef]
1
4. Luna-Freire, K.R.; Scaramal, J.P.S.; Resende, J.A.L.C.; Tormena, C.F.; Oliveira, F.L.; Aparicio, R.; Coelho, F. An asymmetric
substrate-controlled Morita–Baylis–Hillman reaction as approach for the synthesis of pyrrolizidinones and pyrrolizidines.
Tetrahedron 2014, 70, 3319–3326. [CrossRef]
1
1
1
1
1
2
5. Dockerty, P.; Edens, J.G.; Tol, M.B.; Angeles, D.M.; Domenech, A.; Liu, Y.; Hirsch, A.K.H.; Veening, J.-W.; Scheffers, D.-J.; Witte,
M.D. Bicyclic enol cyclocarbamates inhibit penicillin-binding proteins. Org. Biomol. Chem. 2016, 15, 894–910. [CrossRef]
6. Lu, X.; Shi, L.; Zhang, H.; Jiang, Y.; Ma, D. Assembly of N-substituted pyrrolo[2,1-c][1,4]benzodiazepine-5,11-diones via copper
catalyzed aryl amination. Tetrahedron 2010, 66, 5714–5718. [CrossRef]
7. Wang, H.; Jiang, Y.; Gao, K.; Ma, D. Facile synthesis of 1,4-benzodiazepin-3-ones from o-bromobenzylamines and amino acids via
a cascade coupling/condensation process. Tetrahedron 2009, 65, 8956–8960. [CrossRef]
8. Villalgordo, J.M.; Heimgartner, H. Synthesis of a novel heterospirocyclic 3-(N-methyl-N-phenylamino)-2H-azirine and its use as
an amino acid equivalent in the preparation of a model tripeptide. Tetrahedron 1993, 49, 7215–7222. [CrossRef]
9. Heimgartner, H.; Strässler, C.; Linden, A. Synthesis of Tripeptides Containing Heterocyclic α-Amino Acids by Using Heterospiro-
cyclic 3-Amino-2H-azirines. Heterocycles 2018, 97, 333. [CrossRef]
0. Shimazaki, M.; Hasegawa, J.; Kan, K.; Nomura, K.; Nose, Y.; Kondo, H.; Ohashi, T.; Watanabe, K. Synthesis of captopril starting
from an optically active.BETA.-hydroxy acid. Chem. Pharm. Bull. 1982, 30, 3139–3146. [CrossRef]
2
2
2
1. Luer, M.S.; Rhoney, D.H. Tiagabine: A Novel Antiepileptic Drug. Ann. Pharmacother. 1998, 32, 1173–1180. [CrossRef]
2. Leach, J.P.; Brodie, M.J. Tiagabine. Lancet 1998, 351, 203–207. [CrossRef]
3. Chorghade, M.S.; Ellegaard, P.; Lee, E.C.; Petersen, H.; Sørensen, P.O. Synthesis of Desmethyl Tiagabine. Heterocycles 1994, 37, 783.
[
CrossRef]
4. Andersen, K.E.; Sørensen, J.L.; Lau, J.; Lundt, B.F.; Petersen, H.; Huusfeldt, P.O.; Suzdak, P.D.; Swedberg, M.D.B. Synthesis of
Novel -Aminobutyric Acid (GABA) Uptake Inhibitors. 5.1Preparation and Structure Activity Studies of Tricyclic Analogues of
2
2
γ
−
Known GABA Uptake Inhibitors. J. Med. Chem. 2001, 44, 2152–2163. [CrossRef] [PubMed]
5. Seth, A.; Sharma, P.A.; Tripathi, A.; Choubey, P.K.; Srivastava, P.; Tripathi, P.N.; Shrivastava, S.K. Design, synthesis, evaluation and
molecular modeling studies of some novel N-substituted piperidine-3-carboxylic acid derivatives as potential anticonvulsants.
Med. Chem. Res. 2018, 27, 1206–1225. [CrossRef]
2
2
6. Schaarschmidt, M.; Höfner, G.; Wanner, K.T. Synthesis and Biological Evaluation of Nipecotic Acid and Guvacine Derived
1
,3-Disubstituted Allenes as Inhibitors of Murine GABA Transporter mGAT1. ChemMedChem 2019, 14, 1135–1151. [CrossRef]
7. Saura, J.; Curatolo, L.; Williams, C.E.; Gatti, S.; Benatti, L.; Peeters, C.; Guan, J.; Dragunow, M.; Post, C.; Faull, R.L.M.; et al.
Neuroprotective effects of Gly-Pro-Glu, the N- terminal tripeptide of IGF-1, in the hippocampus in vitro. NeuroReport 1999, 10,
1
61–164. [CrossRef]
2
8. Minelli, A.; Conte, C.; Cacciatore, I.; Cornacchia, C.; Pinnen, F. Molecular mechanism underlying the cerebral effect of Gly-Pro-Glu
tripeptide bound to l-dopa in a Parkinson’s animal model. Amino Acids 2012, 43, 1359–1367. [CrossRef]
2
3
9. Kanda, T.; Yokosuka, O.; Omata, M. Faldaprevir for the Treatment of Hepatitis C. Int. J. Mol. Sci. 2015, 16, 4985–4996. [CrossRef]
0. White, P.W.; Llinàs-Brunet, M.; Amad, M.; Bethell, R.C.; Bolger, G.; Cordingley, M.G.; Duan, J.; Garneau, M.; Lagacé, L.; Thibeault,
D.; et al. Preclinical Characterization of BI 201335, a C-Terminal Carboxylic Acid Inhibitor of the Hepatitis C Virus NS3-NS4A
Protease. Antimicrob. Agents Chemother. 2010, 54, 4611–4618. [CrossRef]
3
1. Bilodeau, F.; Bailey, M.D.; Bhardwaj, P.K.; Bordeleau, J.; Forgione, P.; Garneau, M.; Ghiro, E.; Gorys, V.; Halmos, T.; Jolicoeur, E.S.;
et al. Synthesis and optimization of a novel series of HCV NS3 protease inhibitors: 4-Arylproline analogs. Bioorg. Med. Chem. Lett.
2
013, 23, 4267–4271. [CrossRef]
2. Folkers, K.; Enzmann, F.H. Tripeptidine Having the Activity of the Thyrotropin Releasing Hormone. U.S. Patent 3,746,697,
7 July 1973.
3
3
1
3. Ngo, H.T.; Berndt, H.; Wilsdorf, M.; Lentz, D.; Reissig, H.-U. Linear and Cyclic Hybrids of Alternating Thiophene-Amino
Acid Units: Synthesis and Effects of Chirality on Conformation and Molecular Packing. Chem. A Eur. J. 2013, 19, 15155–15165.
[CrossRef]
3
4. Raja, R.; Hemaiswarya, S.; Ganesan, V.; de Carvalho, I.S. Recent developments in therapeutic applications of Cyanobacteria. Crit.
Rev. Microbiol. 2015, 42, 1–12. [CrossRef]

Molecules 2021, 26, 3808
21 of 22
3
3
3
3
3
4
4
5. Shi, B.; Zhou, Y.; Huang, Y.; Zhang, J.; Li, X. Recent advances on the encoding and selection methods of DNA-encoded chemical
library. Bioorg. Med. Chem. Lett. 2017, 27, 361–369. [CrossRef]
6. Usanov, D.L.; Chan, A.I.; Maianti, J.P.; Liu, D.R. Second-generation DNA-templated macrocycle libraries for the discovery of
bioactive small molecules. Nat. Chem. 2018, 10, 704–714. [CrossRef]
7. Madsen, D.; Azevedo, C.; Micco, I.; Petersen, L.K.; Vest Hansen, N.J. An overview of DNA-encoded libraries: A versatile tool for
drug discovery. Prog. Med. Chem. 2020, 59, 181–249. [CrossRef]
8. Blakskjaer, P.; Heitner, T.; Vest Hansen, N.J. Fidelity by design: Yoctoreactor and binder trap enrichment for small-molecule
DNA-encoded libraries and drug discovery. Curr. Opin. Chem. Biol. 2015, 26, 62–71. [CrossRef]
9. Hansen, M.H.; Blakskjær, P.; Petersen, L.K.; Hansen, T.H.; Hojfeldt, J.; Gothelf, K.V.; Vest Hansen, N.J. A Yoctoliter-Scale DNA
Reactor for Small-Molecule Evolution. J. Am. Chem. Soc. 2009, 131, 1322–1327. [CrossRef]
0. Galloway, W.R.; Isidro-Llobet, A.; Spring, D.R. Diversity-oriented synthesis as a tool for the discovery of novel biologically active
small molecules. Nat. Commun. 2010, 1, 80. [CrossRef]
1. Petersen, L.K.; Blakskjær, P.; Chaikuad, A.; Christensen, A.; Dietvorst, J.; Holmkvist, J.; Knapp, S.; Korinek, M.; Larsen, L.K.;
Pedersen, A.; et al. Novel p38
MedChemComm 2016, 7, 1332–1339. [CrossRef]
2. Iškauskiene, M.; Ragaite, G.; Sløk, F.A.; Ša cˇ kus, A. Facile synthesis of novel amino acid-like building blocks by N-alkylation of
heterocyclic carboxylates with N-Boc-3-iodoazetidine. Mol. Divers. 2020, 24, 1235–1251. [CrossRef]
3. Malinauskiene, V.; Kveselyte, A.; Dzedulionyte, K.; Bieliauskas, A.; Burinskas, S.; Sløk, F.A.; Ša cˇ kus, A. L-Proline and related
α MAP kinase inhibitors identified from yoctoReactor DNA-encoded small molecule library.
4
4
˙
˙
˙
˙
˙
chiral heterocyclic amino acids as scaffolds for the synthesis of functionalized 2-amino-1,3-selenazole-5-carboxylates. Chem.
Heterocycl. Compd. 2018, 54, 469–473. [CrossRef]
4
4. Sackus, A.; Kveselyte, A.; Malinauskiene, V.; Dzedulionyte, K.; Bieliauskas, A.; Burinskas, S.; Krikstolaityte, S.; Slok, F.A.
Heterocyclic amino acids as scaffolds for the synthesis of functionalized chiral 1,3-thiazole and 1,3-selenazole derivatives.
In Proceedings of the 256th National Meeting and Exposition of the American-Chemical-Society (ACS), Boston, MA, USA,
1
9–23 August 2018; p. 586.
4
4
4
4
5. Fustero, S.; Sánchez-Roselló, M.; Barrio, P.; Fuentes, A.S. From 2000 to Mid-2010: A Fruitful Decade for the Synthesis of Pyrazoles.
Chem. Rev. 2011, 111, 6984–7034. [CrossRef] [PubMed]
6. Karrouchi, K.; Radi, S.; Ramli, Y.; Taoufik, J.; Mabkhot, Y.N.; Al-Aizari, F.A.; Ansar, M. Synthesis and Pharmacological Activities
of Pyrazole Derivatives: A Review. Molecules 2018, 23, 134. [CrossRef] [PubMed]
7. Ansari, A.; Ali, A.; Asif, M.; Shamsuzzaman, S. Review: Biologically active pyrazole derivatives. New J. Chem. 2016, 41, 16–41.
[
CrossRef]
8. Souza, T.F.; Silva, M.J.V.; Silva, R.G.M.; Gonçalves, D.S.; Simon, P.A.; Jacomini, A.P.; Basso, E.A.; Moura, S.; Martins, M.A.P.;
Back, D.F.; et al. Regiochemical Control of Pyrazoles by Solvent and -Enamino Diketone Structure: Regioselective Synthesis of
,5-Disubstituted N -Phenylpyrazoles. Asian J. Org. Chem. 2017, 6, 627–633. [CrossRef]
9. Gao, W.; Lau, T.; Pan, S.; Phillips, D.P.; Wang, X. Compounds and Compositions as TGR5 Agonists. WO Patent WO2012/082947A1,
4 December 2011.
0. Brooks, D.W.; Lu, L.D.-L.; Masamune, S. C-Acylation under Virtually Neutral Conditions. Angew. Chem. Int. Ed. 1979, 18, 72–74.
CrossRef]
1. Thomassigny, C.; Le Bouc, G.; Greck, C. Asymmetric syntheses of functionalized pyrrolizidin-3-ones. Arkivoc 2012, 2012, 231.
β
4
4
5
5
5
5
1
[
[CrossRef]
2. Semenov, V.; Samultsev, D.O.; Krivdin, L.B. Substitution effects in the 15N NMR chemical shifts of heterocyclic azines evaluated
at the GIAO-DFT level. Magn. Reson. Chem. 2018, 56, 767–774. [CrossRef]
3. Chernyshev, K.A.; Semenov, V.A.; Krivdin, L.B. Quantum-chemical calculations of NMR chemical shifts of organic molecules:
VIII. Solvation effects on 15N NMR chemical shifts of nitrogen-containing heterocycles. Russ. J. Org. Chem. 2013, 49, 379–383.
[
CrossRef]
4. Claramunt, R.M.; López, C.; Maria, M.D.S.; Sanz, D.; Elguero, J. The Use of NMR Spectroscopy to Study Tautomerism. ChemInform
007, 38, 169–206. [CrossRef]
5. Secrieru, A.; O’Neill, P.M.; Cristiano, M.L.S. Revisiting the Structure and Chemistry of 3(5)-Substituted Pyrazoles. Molecules 2019
5, 42. [CrossRef]
6. Arba cˇ iauskiene, E.; Krikštolaityte
5
5
5
2
,
2
˙
˙
, S.; Mitrulevi cˇ iene
˙
, A.; Bieliauskas, A.; Martynaitis, V.; Bechmann, M.; Roller, A.; Ša cˇ kus, A.;
Holzer, W. On the Tautomerism of N-Substituted Pyrazolones: 1,2-Dihydro-3H-pyrazol-3-ones versus 1H-Pyrazol-3-ols. Molecules
018, 23, 129. [CrossRef]
2
5
7. Aguilar-Parrilla, F.; Cativiela, C.; Diaz-De-Villegas, M.D.; Limbach, H.-H.; Smith, J.A.S.; Toiron, C. The tautomerism of 3(5)-
phenylpyrazoles: An experimental (1H, 13C, 15N NMR and X-ray crystallography) study. J. Chem. Soc. Perkin Trans. 2 1992, 2,
1
737–1742. [CrossRef]
5
5
8. Tarabová, D.; Šoralová, S.; Breza, M.; Fronc, M.; Holzer, W.; Milata, V. Use of activated enol ethers in the synthesis of pyrazoles:
Reactions with hydrazine and a study of pyrazole tautomerism. Beilstein J. Org. Chem. 2014, 10, 752–760. [CrossRef]
9. Huang, A.; Wo, K.; Lee, S.Y.C.; Kneitschel, N.; Chang, J.; Zhu, K.; Mello, T.; Bancroft, L.; Norman, N.J.; Zheng, S.-L. Regioselective
Synthesis, NMR, and Crystallographic Analysis of N1-Substituted Pyrazoles. J. Org. Chem. 2017, 82, 8864–8872. [CrossRef]

Molecules 2021, 26, 3808
22 of 22
6
6
0. Liu, X.; Qiao, L.; Zhai, Z.; Cai, P.; Cantrell, C.L.; Tan, C.; Weng, J.; Han, L.; Wu, H. Novel 4-pyrazole carboxamide derivatives
containing flexible chain motif: Design, synthesis and antifungal activity. Pest. Manag. Sci. 2019, 75, 2892–2900. [CrossRef]
1. Gewehr, M.; Dietz, J.; Grote, T.; Blettner, C.; Grammenos, W.; Hünger, U.; Müller, B.; Schieweck, F.; Schwögler, A.; Lohmann,
J.K.; et al. Pyrazole Carboxylic Acid Anilides, Method for the Production Thereof and Agents Containing Them for Controlling
Pathogenic Fungi. WO Patent WO2006087343A1, 15 February 2006.
6
6
2. Braun, S.; Kalinowski, H.O.; Berger, S. 150 and more Basic NMR Experiments: A Practical Course, 2nd ed.; ACS: New York, NY,
USA, 2000.
3. Buevich, A.V.; Williamson, R.T.; Martin, G.E. NMR Structure Elucidation of Small Organic Molecules and Natural Products:
Choosing ADEQUATE vs HMBC. J. Nat. Prod. 2014, 77, 1942–1947. [CrossRef]