Modification of boc-protected CAN508 via acylation and suzuki-miyaura coupling

Article abstract of DOI:10.3390/molecules23010149

The cyclin-dependent kinase inhibitor, CAN508, was protected with di-tert-butyl dicarbonate to access the amino-benzoylated pyrazoles. The bromo derivatives were further arylated by Suzuki-Miyaura coupling using the XPhos Pd G2 pre-catalyst. The coupling

Full text of DOI:10.3390/molecules23010149

molecules
Article
Modification of Boc-Protected CAN508 via Acylation
and Suzuki-Miyaura Coupling
Martin Pisár 1, Eva Schütznerová 2, Filip Hancˇík 1, Igor Popa 3, Zdeneˇk Trávnícˇek 3
and
ID
Petr Cankarˇ 1,
*
ID
1
Department of Organic Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12,
771 46 Olomouc, Czech Republic; martin.pisar01@upol.cz (M.P.); Filda.Hancik@seznam.cz (F.H.)
Institute of Molecular and Translation Medicine, Faculty of Medicine, Palacký University, Hneˇvotínská 5,
779 00 Olomouc, Czech Republic; eva.schutznerova@upol.cz
Department of Inorganic Chemistry, Faculty of Science, Palacký University, 17. listopadu 1192/12,
771 46 Olomouc, Czech Republic; igor.popa@upol.cz (I.P.); zdenek.travnicek@upol.cz (Z.T.)
Correspondence: petr.cankar@upol.cz; Tel.: +42-058-563-4437
2
3
*
Received: 8 December 2017; Accepted: 11 January 2018; Published: 12 January 2018
Abstract: The cyclin-dependent kinase inhibitor, CAN508, was protected with di-tert-butyl
dicarbonate to access the amino-benzoylated pyrazoles. The bromo derivatives were further arylated
by Suzuki-Miyaura coupling using the XPhos Pd G2 pre-catalyst. The coupling reaction provided
generally the para-substituted benzoylpyrazoles in the higher yields than the meta-substituted ones.
The Boc groups were only utilized as directing functionalities for the benzoylation step and were
hydrolyzed under conditions of Suzuki-Miyaura coupling, which allowed for elimination of the
additional deprotection step.
Keywords: pyrazole; Boc-protection; Suzuki-Miyaura reaction; acylation; CDK inhibitor;
XPhos Pd G2
1. Introduction
Previously, we reported an acylation study [1] to substitute regioselectively the amino group
of cyclin-dependent kinase (CDK) [ ] inhibitor
2
,
3
1
(CAN508) [ ]. Pyrazole was protected with
4
2
the 2,4-dimethoxybenzyl group (Dmb) and with Boc (Figure 1). The Dmb was selectively and fully
introduced at the pyrazole endocyclic nitrogen using 2,4-dimethoxybenzylhydrazine. The additional
ortho-methoxy group in Dmb enabled milder deprotection conditions in comparison to the described
p-methoxybenzyl group (PMB) [1,5–8].
N NH
N NBoc
NH2
N NDmb
NH2
H2N
NH2
H2N
H2N
N
N
N
N
N
N
OH
OBoc
OBoc
1
3
2
(CAN508)
Figure 1. Derivatives of pyrazole
1 protected at the endocyclic pyrazole nitrogen and hydroxyl group.
Molecules 2018, 23, 149; doi:10.3390/molecules23010149
www.mdpi.com/journal/molecules

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Despite the methodology using Dmb is unequivocally regioselective, there is still a disadvantage
associated with the introduction of Dmb via a 3-step synthesis. To simplify the protecting methodology,
it is desirable to prepare pyrazole 3 (Figure 1).
Protecting groups are often used as auxiliary tools in the synthesis of biological active
compounds [9]. For example, the Boc protection of the endocyclic pyrazole nitrogen was used in the
synthesis of TBK1 inhibitor Tozasertib [10], selective CDK4 inhibitors based on the pyrazol-3-yl urea
scaffold [11], CDK2 inhibitor PHA-533533 [12], or selective SGK1 inhibitors [13].
The regioselective protection of pyrazoles is inherently complicated due to tautomerism of
a pyrazole heterocyclic system [14]. Moreover, if a pyrazole system is in conjugation with peripheral
functional groups, the tautomeric system is more complex, which results in even more difficult
regioselective protection.
The Boc-protection of a 3(5)-aminopyrazole system represents an illustrative example, where
both endocyclic nitrogens and the amino groups can be protected concurrently [15,16]. The protection
usually led to a mixture of mono- and di-substituted regio-isomers, which are difficult to purify.
To avoid that, several methods were developed: (i) the acylation of aminopyrazoles with 2 equiv.
of acyl reagents followed with saponification of the acyl group attached at the endocyclic pyrazole
nitrogen [17]; (ii) the oxidation-reduction sequence of the amino group including Boc-protection of the
endocyclic pyrazole nitrogen [18]; and (iii) the direct Boc-protection under alkaline conditions [16].
Furthermore, since the X-ray crystallography of the CAN508-CDK2 complex indicated a possible
extension of structure optimization to improve the CDK inhibition activity [
use pyrazole as a starting intermediate for synthetic Route A to give directly unprotected pyrazoles
12 (Scheme 1).
4], we initiated a concept to
3
Route A can be potentially an alternative to conventional Route B and offer other synthetic
strategies in drug development of pyrazole derivatives with an unprotected endocyclic nitrogen.
The synthetic sequence (Route A) employing acylation before the Suzuki-Miyaura coupling (SMC) [19
]
exposes the catalyst in the SMC reaction to more functionalized reactants/products with a potentially
higher coordination activity.
The classical catalysts with triphenylphosphine ligands are not robust and are frequently inhibited
or decomposed, when such unprotected functionalized pyrazoles are used. The sterically-bulky
phosphines were also inefficient [20
Route B) or the pyrazole endocyclic nitrogen is substituted to enable a coupling reaction with
reasonable yields [22].
,21]. Consequently, the SMC reaction has to precede the acylation
(

Molecules 2018, 23, 149
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Scheme 1. Possible synthetic routes leading to pyrazoles 12.
In the last decade, the development of ligands and their use as pre-catalysts significantly broaden
the scope of the SMC coupling with various nitrogen-containing heterocyclic systems [23 25]. Recently,
we have taken an advantage of these scope extensions and reported studies with the challenging
–
aminopyrazole substrates [20,21,26], which encourage us to attempt to synthesize pyrazoles 12 via
Route A.
In our endeavours to find more efficient CDK inhibitors based on the structure of pyrazole
(CAN508) [14 27 29], we decided to use the Boc-protection of pyrazole , subsequent benzoylation
of the Boc-protected pyrazole , and the final arylation under SMC conditions to further explore the
1
,
–
1
3
robustness of a catalyst originated from the XPhos Pd G2 pre-catalyst.
2. Results and Discussion
2.1. Boc-Protection and Benzoylation
We found that the reaction of pyrazole
nitrogen [ ]. To redirect the acylation reaction toward the amino group it is necessary to protect
the endocyclic pyrazole nitrogen and the hydroxyl group. Preliminary experiments with di-tert-butyl
dicarbonate provided mono-protected pyrazole and di-protected derivative , and showed
a significant role of a base (Scheme 2, Route A). While the protection with di-tert-butyl dicarbonate
in anhydrous dimethylformamide (DMF) gave pure mono-Boc protected derivative , a reaction in
. Another equivalent
1 with acyl chlorides proceeded at the endocyclic
1
5
3
5
pyridine resulted in a mixture of mono- and di-Boc protected derivatives
5 and 3
of di-tert-butyl dicarbonate provided solely pyrazole 3.
To verify that the first mono-Boc protection took place at the diaminopyrazole moiety, we
chose an alternative indirect synthetic approach involving an additional protecting group (Scheme 2,

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Route B). At first, hydrazone
4
was protected with a triisopropylsilyl (TIPS) group to give compound
6
,
which subsequently underwent cyclyzation with hydrazine to yield pyrazole
pyrazole was treated with di-tert-butyl dicarbonate to obtain derivative
unmasking of the hydroxyl group with TBAF resulted in the target pyrazole 5.
7
. Then, TIPS-protected
7
8. The final selective
Scheme 2. Two independent routes leading to Boc-protected pyrazoles
conditions: ( ) N2H4.H2O (1.5 equiv.), MeOH, reflux, 4 h, for : 97%; ( ) Boc2O (1.05 equiv.), anhydrous
DMF, r.t., 18 h, for : 99%, for : 77%; ( ) for : Boc2O (2 equiv.), pyridine, r.t., 18 h, 94%; ( ) anhydrous
5 and 3. Reagents and
a
7
b
5
8
c
3
d
DMF in dichloromethane (DCM) (10%), imidazole (1.1 equiv.), TIPS-Cl (1.1 equiv.), r.t., 18h 59%;
(e) TBAF (1 equiv.), MeOH, r.t., 18 h (91%).
While the alternative synthesis of pyrazole
diaminopyrazole moiety, the single crystal X-ray analysis of
position of the Boc group at the pyrazole endocyclic nitrogen (Figure 2). X-ray crystallography
also confirmed the position of the second Boc group at the hydroxyl in pyrazole . Selected bond
lengths and angles of and can be found in the Supplementary file. Moreover, the X-ray analysis
5
confirmed the position of the Boc group at the
5
unequivocally determined the exact
3
3
5
also revealed that the crystal structures of both compounds are stabilized by hydrogen bonds of
the N–H···N, N–H···O and O–H···N types, situating the individual molecules into supramolecular
1D-polymeric chains (see Figures S1 and S2 in the Supplementary File). Each of the supramolecular
chains is connected through a variety of the non-covalent contacts (e.g., N–H···N, C–H···O and/or
C–H···N) with two neighboring ones, thus forming a supramolecular-layered structure (see Figures S3
and S4 in the Supplementary File).

Molecules 2018, 23, 149
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(b)
(a)
Figure 2. Molecular structures: ( ) pyrazole
a
3
(CCDC 1453321) and (b) pyrazole 5 (CCDC 1453320),
together with the atom numbering schemes. The thermal ellipsoids of the non-hydrogen atoms are
drawn at the 50% probability level.
To confirm the markedly preferred reactivity of the hydroxyl over the amino groups, we carried
out benzoylation of pyrazole 5 with benzoyl chloride (Scheme 3). This reaction resulted in pyrazol 9.
Subsequent deprotection with diluted trifluoroacetic acid (TFA) provided benzoylated pyrazol 10. The
predominant reactivity of the hydroxyl group highlighted the necessity of its protection.
Scheme 3. Benzoylation of pyrazole
5. Reagents and conditions: (a) BzCl (1.5 equiv.), triethylamine
(1.5 equiv.), dry DMF, DCM, r.t., 18 h (47%); (b) 10% TFA in DCM, r.t., 1.5 h (81%).
The acylation of pyrazole
3 with benzoyl chlorides provided amino-benzoylated pyrazoles 11a–c
(Table 1). Since benzoylated pyrazoles 11a and 11b were accompanied with an increased amount
of impurities, the crude products were directly deprotected in the next step without isolation. The
subsequent cleavage of both Boc groups was carried out initially under conventional conditions
with diluted TFA to yield 12a (Table 1, entry 1). In view of the following Suzuki-Miyaura coupling,
tripotassium phosphate was additionally used to deprotect pyrazoles 11b–c in the very good to
excellent yields (Table 1, entries 2 and 3).

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Table 1. Benzoylation of pyrazole 3 and subsequent deprotection 1.
Entry
Pyrazole
Ar
Benzoylation [%] 2 Deprotection [%] 2
1
2
3
11a/12a
11b/12b
11c/12c
Phenyl
3-Bromophenyl
4-Bromophenyl
-
-
84
92
80
95
1 Reagents and conditions: (a) ArCOCl, pyridine, r.t., 3 h; (b) 10% TFA in DCM, r.t., 2.5 h; (c) K3PO4, 1,4-dioxane/H2O
2
(4:1), 100 ◦C, 1.5 h. Isolated yield.
2.2. Suzuki-Miyaura Coupling
In order to extend the diversity of pharmacologically relevant amino benzoylated pyrazoles 12b–c,
we used our previously reported SMC conditions for dinitropyrazoles [26]. Preliminary deprotection
experiments with tripotassium phosphate showed that already two thirds of 11c were converted
after 30 min into 12c and after 1 h the cleavage was almost complete. The first coupling experiments
with p-tolylboronic acid enabled for providing biphenyl 12d in a very good yield (Table 2, entry
1), which indicated the robustness of the catalyst [30]. The comparable yields were obtained with
benzene and 4-methoxybenzeneboronic acids (entries 2 and 3). The ortho-substitution did not affect
the reactivity negatively; it was possible to isolate 12g in the excellent yield (entry 4). However,
the electron-withdrawing nitro group decreased the yield (entry 5). Electron-rich furan-3-yl and
thiophen-3-yl boronic acids reacted with 11c in the very good or excellent yields (entries 6 and 7).
Table 2. Arylation of pyrazole 11c by Suzuki-Miyaura coupling 1.
Entry
Pyrazole
Ar
Yield [%] 2
1
2
3
4
5
6
7
12d
12e
12f
12g
12h
12i
Tolyl
Phenyl
4-MeO-phenyl
2-MeO-phenyl
4-NO2-phenyl
Furan-3-yl
88
82
86
93
45 3
86
12j
Thiophen-3-yl
96
1
Reagents and conditions: XPhos Pd G2 (5 mol %), K3PO4 (4 equiv.), (hetero)arylboronic acid (2 equiv.),
1,4-dioxane/water 4:1, 100 ◦C, 24 h.
Isolated yield.
After 12 h, the same amount of 4-nitroboronic acid
2
3
and XPhos Pd G2 was added and the reaction was proceeded for the next 24 h.

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Since the benzoylation of pyrazole 11b was accompanied by the mono-Boc impurity, and it was
found that the catalyst is already exposed to a substantial excess of unprotected pyrazoles 12b and
12c in the earliest stage of the coupling, we decided to perform the coupling reaction directly with
unprotected pyrazole 12b. Preliminary attempts showed a more difficult coupling reaction at the meta
position. The HPLC analyses usually confirmed the presence of unreacted pyrazole 12b, the desired
coupling product, and other unknown impurities. The yields were considerably lower and only
pyrazoles 12k–n were possible to isolate (Table 3).
Table 3. Arylation of pyrazole 12b by Suzuki-Miyaura coupling 1.
Entry
Pyrazole
Ar
Yield [%] 2
1
2
3
4
12k
12l
12m
12n
4-MeO-phenyl
2-MeO-phenyl
Furan-3-yl
41
65
50
60
Thiophen-3-yl
1
Reagents and conditions: XPhos Pd G2 (5 mol %), K3PO4 (4 equiv.), (hetero)arylboronic acid (2 equiv.),
1,4-dioxane/water 4:1, 100 ◦C, 24 h. Isolated yield.
2
3. Experimental Section
General
All starting materials are commercially available. Commercial reagents were used without
purification. Melting points were determined on a Boetius stage and are uncorrected. Flash column
chromatography was performed on silica gel (pore size 60 Å, 40–63 mm particle size). The LC/MS
analyses were carried out on a UHPLC-MS system consisting of UHPLC chromatography Accela
with photodiode array detector and triple quadrupole mass spectrometer TSQ Quantum Access (both
◦
Thermo Scientific, Santa Clara, CA, USA), using Nucleodur Gravity C18 column at 30 C and flow rate
of 800
µ
L/min (Kinetex, Phenomenex, 2.6
µ
m, 2.1
×
50 mm, Torrance, CA, USA). The mobile phase
was (A) 0.01M ammonium acetate in water, and (B) acetonitrile, linearly programmed from 10% to
80% B over 2.5 min, kept for 1.5 min. The column was reequilibrated with a 10% of solution B for
1 min. The atmospheric-pressure chemical ionization (APCI) source operated at discharge current
of 5 µ
A, vaporizer temperature of 400 ◦C, and capillary temperature of 200 ◦C. The HRMS analyses
were carried out on HRMSdExactive (Orbitrap) MS, Thermo Scientific, Santa Clara, CA, USA. The 1H-
and 13C-NMR spectra were measured in DMSO-d6 on a Bruker Avance 300 FT NMR spectrometer
(Billerica, MA, USA) and on a Varian 400 MHz FT NMR spectrometer (Palo Alto, CA, USA). The single
crystal X-ray data of 3 (CCDC 1453321) and 5 (CCDC 1453320) were obtained using an Xcalibur2
diffractometer equipped with a Sapphire2 CCD detector (Oxford Diffraction Ltd., Abingdon, UK), and
with MoKα radiation (monochromator Enhance, Oxford Diffraction Ltd) and ω-scan technique at 120 K.
Additional details regarding structure determinations, such as crystal data and structure refinements,
selected bond lengths and angles of covalent as well as non-covalent contacts are summarized in the
Supplementary File.

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Tert-butyl 3,5-diamino-4-((4-((tert-butoxycarbonyl)oxy)phenyl)diazenyl)-1H-pyrazole-1-carboxylate (3): To
a solution of pyrazole
1 (218 mg, 1.0 mmol) [4] in dry pyridine (6 mL), di-tert-butyl dicarbonate
(0.460 mL, 2.0 mmol) diluted with dry pyridine (2 mL) was added dropwise at 2–5 ◦C. The reaction
mixture was allowed to stir at room temperature for 18 h. After that, the solvent was removed under
reduced pressure and the residue was diluted in methanol (6 mL). The solution was added dropwise
to ice water (30 mL). The precipitate was filtered-off, washed with water, and dried in the air to give
as a yellow solid (392 mg, 94%); m.p. 148–150 ◦C; 1H-NMR (400 MHz, DMSO-d6)
= 7.82 (br s, 2H),
7.80 (d, J = 8.8 Hz, 2H), 7.23 (d, J = 8.8, 2H), 6.15 (br s, 2H), 1.54 (s, 9H), 1.48 (s, 9H) ppm; 13C-NMR
3
δ
(101 MHz, DMSO-d6)
δ = 27.7, 28.2, 83.7, 84.3, 113.6, 122.3, 122.3, 149.7, 150.1, 150.5, 150.6, 151.2,
151.6 ppm; HRMS (HESI, m/z) calcd. for C19H26N6O5 (418.45) [M + H]+ 419.2037, found 419.2037.
Tert-butyl 3,5-diamino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazole-1-carboxylate (5):
Reaction of unprotected pyrazole
1 with Boc-ahydride: To a solution of pyrazole 1 (218 mg, 1.0 mmol) [4] in
dry DMF (2 mL), di-tert-butyl dicarbonate (0.240 mL, 1.1 mmol) diluted with dry DMF (1 mL) was
added dropwise at 2–5 ◦C. The reaction mixture was allowed to stir at room temperature for 18 h and
then was added dropwise to ice water (15 mL). The precipitate was filtered-off, washed with water,
and dried in the air to give 5 as a yellow solid (314 mg, 99%).
TIPS deprotection of the hydroxyl group: To a solution of TIPS-protected pyrazole 8 (200 mg, 0.42 mmol)
in methanol (40 mL), tetrabutylammonium fluoride hydrate (TBAF; 110 mg, 0.42 mmol) was added.
The reaction mixture was allowed to stir at room temperature for 18 h. After that the solvent was
evaporated under reduced pressure and the residue was diluted in methanol (5 mL). The solution was
added dropwise to ice water (25 mL). The precipitate was filtered-off, washed with water, and dried in
◦
the air to give 5. Yield 122 mg (91%) as a yellow solid; m.p. 114–116 C; 1H-NMR (400 MHz, DMSO-d6)
δ
= 9.75 (br s, 1H), 7.63 (d, J = 8.8 Hz, 2H), 7.59 (br s, 2H), 6.79 (d, J = 8.8 Hz, 2H), 6.02 (br s, 2H), 1.53
(s, 9H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 28.2, 84.0, 112.6, 115.9, 123.0, 145.4, 146.4, 150.7, 152.1,
158.6 ppm; HRMS (HESI, m/z) calcd. for C14H18N6O3 (318.33) [M + H]+ 319.1515, found 319.1515.
(4-((Triisopropylsilyl)oxy)phenyl)carbonohydrazonoyl dicyanide (6): Hydrazone 4 (186 mg, 1.0 mmol) [1,4]
was dissolved in a mixture of dry DCM (5 mL) and DMF (0.5 mL), then imidazole (75 mg, 1.1 mmol)
and triisopropylsilyl chloride (TIPS-Cl; 235 µL, 1.1 mmol) were added under continuous stirring and
cooling on an ice-bath. The reaction mixture was stirred at room temperature for 18 h. After that,
the solvent was removed under reduced pressure and the residue was diluted in methanol (2 mL).
The solution was added dropwise to ice water (10 mL). The precipitate was filtered-off, washed with
water, and dried in the air. Crude hydrazone
solid. Yield 202 mg (59%); m.p. 136–138 ◦C; 1H-NMR (300 MHz, DMSO-d6)
2H), 6.91 (d, J = 9.0 Hz, 2H), 1.16–1.30 (m, 3H), 1.04 (d, J = 7.1 Hz, 18H) ppm; 13C-NMR (75 MHz,
6
was crystallized from methanol (3 mL) to give a yellow
δ
= 7.37 (d, J = 9.0 Hz,
DMSO-d6)
δ = 12.0, 17.7, 82.9, 110.3, 114.7, 118.04, 120.39, 135.51, 153.66 ppm; HRMS (HESI, m/z) calcd.
for C18H26N4OSi (342.51) [M − H]− 341.1792, found 341.1796.
4-((4-((Triisopropylsilyl)oxy)phenyl)diazenyl)-1H-pyrazole-3,5-diamine (
7): To a solution of hydrazone
6
(342 mg, 1.0 mmol) in methanol (15 mL), hydrazine hydrate (64%; 73
µL, 1.5 mmol) was added dropwise
at room temperature. The solution was heated at 65 ◦C for 4 h. After cooling to room temperature,
the solvent was evaporated under reduced pressure and the residue was diluted in methanol (2 mL).
The solution was added dropwise to ice water (10 mL). The precipitate was filtered-off, washed with
water, and dried in the air to give pyrazole
1H-NMR (400 MHz, DMSO-d6)
= 10.63 (s, 1H), 7.56 (d, J = 8.8 Hz, 2H), 6.85 (d, J = 8.8 Hz, 2H),
6.18 (br s, 2H), 5.63 (br s, 2H), 1.23 (sxt, J = 7.5 Hz, 3H), 1.06 (s, 9H), 1.04 (s, 9H) ppm; 13C-NMR
7
as a yellow solid. Yield 363 mg, 97%; m.p. 184–186 ◦C;
δ
(101 MHz, DMSO-d6)
δ = 2.5, 18.2, 114.0, 120.2, 122.2, 148.5, 154.9 ppm; HRMS (HESI, m/z) calcd.
for C18H30N6OSi (374.23) [M + H]+ 375.2323, found 375.2325.
Tert-butyl 3,5-diamino-4-((4-((triisopropylsilyl)oxy)-phenyl)diazenyl)-1H-pyrazole-1-carboxylate (8): To a
solution of pyrazole
7 (374 mg, 1.0 mmol) in dry DMF (6 mL), di-tert-butyl dicarbonate (0.240 mL,

Molecules 2018, 23, 149
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1.05 mmol) diluted with dry DMF (1 mL) was added dropwise at 2–5 ◦C. The reaction mixture was
allowed to stir at room temperature for 18 h and then was added dropwise to ice water (30 mL).
The precipitate was filtered-off, washed with water, and dried in the air to give
8
as a yellow solid.
= 7.69 (br s, 2H), 7.68 (d, J = 8.8 Hz,
2H), 6.89 (d, J = 8.8 z, 2H), 6.07 (br s, 2H), 1.53 (s, 9H), 1.24 (sxt, J = 7.5 Hz, 3H), 1.06 (s, 9H), 1.04
◦
Yield 365 mg, 77%, m.p. 88–90 C; 1H-NMR (400 MHz, DMSO-d6)
δ
(s, 9H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 12.5, 18.2, 28.2, 84.1, 113.0, 120.3, 122.9, 146.5, 148.0,
150.7, 152.1, 156.0 ppm; HRMS (HESI, m/z) calcd. for C23H38N6O3Si (474.28) [M + H]+ 475.2847,
found 475.2851.
Tert-butyl 3,5-diamino-4-((4-(benzoyloxy)phenyl)-diazenyl)-1H-pyrazole-1-carboxylate (
pyrazole (318 mg, 1.0 mmol) and triethylamine (209 L, 1.5 mmol) in anhydrous DMF (10 mL),
a solution of benzoyl chloride (175
9): To a solution of
5
µ
µ
L, 1.5 mmol) diluted with DCM (3 mL) was added dropwise at
2–5 ◦C. The reaction mixture was allowed to stir at room temperature for 18 h. After that, the reaction
mixture was added dropwise to ice water (50 mL). The precipitate was filtered-off, washed with water,
◦
9
as a yellow solid. Yield 199 mg 47%; m.p. 188–190 C; 1H-NMR (400 MHz,
and dried in the air to give
DMSO-d6) = 8.14 (d, J = 7.6 Hz, 2H); 7.88 (d, J = 8.8 Hz, 2H), 7.87 (br s, 2H), 7.74 (t, J = 7.5 Hz, 1H),
7.61 (t, J = 7.6 Hz, 2H), 7.35 (d, J = 8.8 Hz, 2H), 6.18 (br s, 2H), 1.54 (s, 9H) ppm; 13C-NMR (101 MHz,
DMSO-d6) = 28.2, 84.3, 113.7, 122.4, 122.8, 129.4, 129.5, 130.3, 134.5, 144.0, 150.5, 150.7, 151.3, 151.4,
165.1 ppm; HRMS (HESI, m/z) calcd. for C21H22N6O4 (422.44) [M + H]+ 423.1775, found 423.1774.
δ
δ
4-((3,5-Diamino-1H-pyrazol-4-yl)diazenyl)phenyl benzoate (10): Boc-protected pyrazole (0.5 mmol) was
9
dissolved in 10% TFA/DCM (15 mL). The reaction mixture was allowed to stir at room temperature
for 1 h. Then, the solvent was evaporated under reduced pressure. The residue was diluted with
water (20 mL), pH was adjusted with ammonia to 10, and the suspension was allowed to stir at room
temperature for 2 h. The precipitate was filtered-off, washed with water, and dried in the air to give
10 as a yellow solid. Yield 130 mg (81%); m.p. 196–198 ◦C; 1H-NMR (400 MHz, DMSO-d6)
δ = 10.75
(br s, 1H), 8.14 (d, J = 7.9 Hz, 2H), 7.74 (d, J = 8.8 Hz, 2H), 7.74 (tt, J = 1.3, 7.1 Hz, 1H), 7.60 (t, J = 7.8 Hz,
2H), 7.29 (dd, J = 2.0, 8.8 Hz, 2H), 6.30 (br s, 2H), 5.87 (br s, 2H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ
= 114.7, 121.7, 122.6, 129.4, 130.0, 130.2, 134.5, 144.5, 149.5, 151.5, 152.0, 165.1 ppm; HRMS (HESI, m/z)
calcd. for C16H14N6O2 (322.32) [M + H]+ 323.1251, found 323.1250.
General procedure for (11): To a solution of pyrazole (3.8 g, 9.1 mmol) in pyridine (13 mL) the
3
corresponding benzoyl chloride (9.1 mmol) was added in small portions under continuous stirring
◦
at 2–5 C. Then, the reaction mixture was allowed to stir at room temperature for 3 h. After that,
the solvent was evaporated under reduced pressure, the residue was diluted with methanol (15 mL),
and a methanolic solution (or suspension) was added dropwise into ice water (50 mL). The precipitate
was filtered-off, washed with water, and dried in the air to give 11.
Tert-butyl 5-amino-3-benzamido-4-((4-((tert-butoxycarbonyl)oxy)phenyl)diazenyl)-1H-pyrazole-1-carboxylate
(11a): The benzoylation was performed at the 1 mmol scale. Yield 481 mg as a yellow solid (a crude
product contains impurities). Then, the product was directly used for the Boc-deprotection step (please
see 12a).
Tert-butyl 5-amino-3-(3-bromobenzamido)-4-((4-((tert-butoxycarbonyl)oxy)phenyl)diazenyl)-1H-pyrazole-1-
carboxylate (11b): Yield 4.5 g (a crude product contains a mono-Boc protected pyrazole as an impurity).
Then, the product was directly used for the Boc-deprotection step (please see 12b).
Tert-butyl 5-amino-3-(4-bromobenzamido)-4-((4-((tert-butoxycarbonyl)oxy)phenyl)diazenyl)-1H-pyrazole-1-
carboxylate (11c): Yield 4.6 g (84%) as a yellow solid; m.p. 128–132 ◦C; 1H-NMR (400 MHz,DMSO-d6)
δ
= 10.87 (s, 1H), 8.08 (br s, 2H), 7.94 (d, J = 8.3 Hz, 2H), 7.77 (d, J = 8.3 Hz, 2H), 7.70 (d, J = 8.8 Hz, 2H),
7.27 (d, J = 8.8 Hz, 2H), 1.60 (s, 9 H), 1.49 (s, 9H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ
= 164.9, 151.0,
150.7, 150.3, 149.5, 146.2, 143.6, 132.6, 131.7, 129.9, 126.0, 122.0, 115.7, 85.5, 83.4, 27.6, 27.2 ppm; HRMS
(HESI, m/z) calcd. for C26H29BrN6O6 (601.46) [M − H]− 601.1227, found 601.1220.

Molecules 2018, 23, 149
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N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)benzamide (12a): Pyrazole 11a (0.5 mmol) was
dissolved in 10% TFA/DCM (15 mL). The reaction mixture was allowed to stir at room temperature
for 2.5 h. The solvent was evaporated under reduced pressure. The residue was dissolved in water
(20 mL) and pH was adjusted with aqueous ammonia to pH 10. The suspension was allowed to stir
at room temperature for 2 h. The product 12a was filtered-off, washed with water, and dried in the
air. Yield 148 mg (92%) as a yellow solid. The compound was identical with the pyrazole published
previously [1].
N-(5-Amino-4-[(4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl]-3-bromobenzamide (12b): A crude product
of Boc-protected pyrazole 11b (1.0 mmol) and K3PO4 (3.0 mmol) was dissolved in a solution of
1,4-dioxane/water (5 mL, 4/1). The reaction mixture was vigorously stirred at 100 ◦C for 1.5 h, then
cooled down to room temperature, neutralized with diluted HCl (1/5) to pH
water (20 mL). The precipitate was filtered-off, washed with water, and dried in the air. Yield 321 mg
(80%) as an orange solid; m.p. 249–252 ◦C; 1H-NMR (400 MHz, DMSO-d6)
= 11.81 (br s, 1H), 10.63
≈
7, and diluted with
δ
(br s, 1H), 9.77 (br s, 1H), 8.14 (t, J = 1.8 Hz, 1H), 7.97 (d, J = 7.8 Hz, 1H), 7.82 (dd, J = 1.5, 7.8 Hz, 1H),
7.54 (d, J = 8.7 Hz, 2H), 7.53 (t, J = 7.8 Hz, 1H), 6.81 (d, J = 8.7 Hz, 2H), 6.60 (br s, 2H) ppm; 13C-NMR
(101 MHz, DMSO-d6)
164.1 ppm; HRMS (HESI, m/z) calcd. for C16H13BrN6O2 (401.22) [M
δ
= 115.5, 116.2, 121.9, 122.5, 126.8, 130.4, 130.9, 134.6, 136.1, 143.3, 145.9, 158.1,
H]− 399.0205, found 399.0201.
−
N-(5-Amino-4-[(4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl]-4-bromobenzamide (12c): Prepared analogously as
12b. Yield 381 mg (95%) as an orange solid; m.p. 268–273 ◦C; 1H-NMR (400 MHz, DMSO-d6)
= 10.65
(br s, 3H), 7.99 (d, J = 8.7 Hz, 2H), 7.68 (d, J = 8.2 Hz, 2H), 7.52 (d, J = 9.2 Hz, 2H), 6.80 (d, J = 8.7 Hz,
2H), 6.28 (br s, 2H) ppm; 13C-NMR (101 MHz, DMSO-d6)
= 115.5, 116.7, 122.1, 124.4, 130.0, 131.1, 135.7,
δ
δ
144.2, 146.1, 157.6, 166.4 ppm; HRMS (HESI, m/z) calcd. for C16H13BrN6O2 (401.22) [M
found 399.0917.
−
H]− 399.0205,
General procedure of Suzuki-Miyaura coupling for (12d–n): A mixture of pyrazole 11c or 12b (0.25 mmol),
(hetero)arylboronic acid (0.5 mmol, 2 equiv.), and K3PO4 (212 mg, 1.0 mmol, 4 equiv.) in
1,4-dioxane/water (2.5 mL, 4/1) was bubbled with argon for 5 min in a 10 mL capped Supelco
vial . Then, the XPhos Pd G2 pre-catalyst (10 mg, 0.0125 mmol, 5 mol %) was added. The reaction
mixture was bubbled with argon again for 5 min, vigorously stirred at 100 ◦C for 24 h, and then was
cooled down to room temperature, neutralized with diluted HCl (1/5) to pH
water (10 mL). The precipitate was filtered-off, washed with water, and dried in the air. The crude
product was purified on a silica gel column (EtOAc, then EtOAc/MeOH = 5/1).
≈
7, and diluted with
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-40-methyl-[1,10-biphenyl]-4-carboxamide (12d):
◦
1
Yield 91 mg (88%) as an orange solid; m.p. 261–265 C; H-NMR (400 MHz, DMSO-d6)
δ = 11.85
(br s, 1H), 10.66 (br s, 1H), 9.77 (br s, 1H), 8.07 (d, J = 8.8 Hz, 2H), 7.85 (d, J = 8.3 Hz, 2H), 7.68
(d, J = 8.3 Hz, 2H), 7.58 (d, J = 8.8 Hz, 2H), 7.32 (d, J = 7.8 Hz, 2H), 6.83 (d, J = 8.8 Hz, 2H), 6.52
(br s, 2H), 2.36 (s, 3H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 20.7, 115.5, 116.0, 122.5, 126.5, 126.8,
128.3, 129.7, 132.3, 136.1, 137.7, 143.3, 145.9, 158.0, 165.0 ppm; HRMS (HESI, m/z) calcd. for C23H20N6O2
(412.45) [M − H]− 411.1570, found 411.1568.
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3◦-yl)-[1,10-biphenyl]-4-carboxamide (12e): Yield
1
82 mg (82%) as a bright orange solid; m.p. 249–253 C; H-NMR (400 MHz, DMSO-d6)
δ = 11.86
(br s, 1H), 10.67 (br s, 1H), 9.77 (s, 1H), 8.09 (d, J = 7.8 Hz, 2H), 7.88 (d, J = 8.2 Hz, 2H), 7.78 (d, J = 8.2 Hz,
2H), 7.59 (d, J = 8.7 Hz, 2H), 7.52 (t, J = 7.6 Hz, 2H), 7.43 (t, J = 7.3 Hz, 1H), 6.83 (d, J = 8.7 Hz, 2H),
6.54 (br s, 2H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 115.6, 116.0, 122.5, 126.9, 127.0, 128.3, 128.4,
129.1, 132.6, 139.1, 143.4, 145.9, 158.1, 165.0 ppm; HRMS (HESI, m/z) calcd. for C22H18N6O2 (398.43)
[M − H]− 397.1413, found 397.1406.
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-4 -methoxy-[1,1 -biphenyl]-4-carboxamide (12f):
Yield 92 mg (86%) as an orange-red solid; m.p. 220–224 ◦C; 1H-NMR (400 MHz, DMSO-d6)
= 1.85
(br s, 1H), 10.65 (br s, 1H), 9.77 (br s., 1H), 8.05 (d, J = 8.3 Hz, 2H), 7.83 (d, J = 8.3 Hz, 2H), 7.74
0
0
δ

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(d, J = 9.3 Hz, 2H), 7.59 (d, J = 8.8 Hz, 2H), 7.07 (d, J = 8.8 Hz, 2H), 6.83 (d, J = 8.8 Hz, 2H), 6.53
(br s, 2H), 3.82 (s, 3H) ppm; 13C-NMR (101 MHz, DMSO-d6)
= 55.2, 114.5, 115.5, 116.0, 122.5, 126.2,
δ
128.1, 128.3, 131.3, 131.8, 143.1, 145.9, 158.0, 159.5, 165.0 ppm; HRMS (HESI, m/z) calcd. for C23H20N6O3
(428.45) [M − H]− 427.1519, found 427.1513.
0
0
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-2 -methoxy-[1,1 -biphenyl]-4-carboxamide (12g):
Yield 100 mg (93%) as an orange solid; m.p. 188–192 C; 1H-NMR (400 MHz, DMSO-d6)
δ = 11.90 (br s,
◦
1H), 10.66 (br s, 1H), 9.79 (s, 1H), 8.02 (d, J = 8.2 Hz, 2H), 7.67 (d, J = 8.2 Hz, 2H), 7.58 (d, J = 8.7 Hz,
2H), 7.43–7.35 (m, 2H), 7.15 (dd, J = 0.9, 7.8 Hz, 1H), 7.07 (dt, J = 0.9, 7.3 Hz, 1H), 6.83 (d, J = 8.7 Hz,
2H), 6.52 (br s, 2H), 3.80 (s, 3H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 55.6, 111.9, 115.6, 116.0, 121.0,
122.5, 127.4, 128.8, 129.5, 129.6, 130.5, 132.1, 141.8, 144.1, 145.9, 156.2, 158.1, 165.2 ppm; HRMS (HESI,
m/z) calcd. for C23H20N6O3 (428.45) [M − H]− 427.1519, found 427.1515 [M − H]−.
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-40-nitro-[1,10-biphenyl]-4-carboxamide (12h):
After 12 h the same amount of 4-nitroboronic acid and XPhos Pd G2 was added and the reaction was
proceeded for next 24 h. Yield 50 mg (45%) as an orange solid; m.p. 187–192 ◦C; 1H-NMR (400 MHz,
DMSO-d6)
δ = 11.80 (br s, 1H), 10.66 (br s, 1H), 9.74 (br s, 1H), 8.35 (d, J = 8.7 Hz, 2H), 8.13 (d, J = 8.7 Hz,
2H), 8.08 (d, J = 9.2 Hz, 2H), 8.00 (d, J = 8.2 Hz, 2H), 7.57 (d, J = 8.2 Hz, 2H), 6.81 (d, J = 9.2 Hz, 2H), 6.61
(br s, 2H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 115.5, 122.5, 124.2, 127.5, 128.2, 128.5 134.0, 140.9,
145.5, 145.9, 147.1, 158.0, 164.8 ppm; HRMS (HESI, m/z) calcd. for C22H17N7O4 (443.42) [M + H]+
444.1420, found 444.1416.
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-4-(furan-3-yl)benzamide (12i): Yield 85 mg
(86%) as an orange solid; m.p. 266–271 ◦C; 1H-NMR (400 MHz, DMSO-d6)
δ = 11.86 (br s, 1H), 10.61
(br s, 1H), 9.79 (s, 1H), 8.36 (s, 1H), 8.01 (d, J = 8.2 Hz, 2H), 7.82 (d, J = 8.2 Hz, 2H), 7.80 (t, J = 1.6 Hz,
1H), 7.57 (d, J = 8.7 Hz, 2H), 7.10–7.07 (m, 1H), 6.83 (d, J = 8.7 Hz, 2H), 6.53 (br s, 2H) ppm; 13C-NMR
(101 MHz, DMSO-d6) δ = 108.7, 115.6, 116.0, 122.5, 125.2, 125.6, 128.3, 132.0, 135.5, 140.6, 143.9, 144.7,
145.9, 158.1, 165.0 ppm; HRMS (HESI, m/z) calcd. for C20H16N6O3 (388.39) [M + H]+ 389.1362,
found 389.1356.
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-4-(thiophen-3-yl)benzamide (12j): Yield 97 mg
(96%) as a yellow-orange solid; m.p. 272–274 ◦C; 1H-NMR (400 MHz, DMSO-d6)
δ
= 11.85 (br s, 1H),
10.63 (br s, 1H), 9.77 (s, 1H), 8.08 (dd, J = 1.6, 2.5 Hz, 1H), 8.03 (d, J = 8.2 Hz, 2H), 7.93 (d, J = 8.2 Hz
2H), 7.72–7.67 (m, 2H), 7.57 (d, J = 8.7 Hz, 2H), 6.82 (d, J = 9.2 Hz, 2H), 6.54 (br s, 2H) ppm; 13C-NMR
(101 MHz, DMSO-d6) = 115.5, 116.0, 122.4, 122.7, 126.1, 126.3, 127.5, 128.4, 132.1, 138.3, 140.4,
,
δ
145.9, 158.0, 164.9 ppm; HRMS (HESI, m/z) calcd. for C20H16N6O2S (404.45) [M
found 403.0975.
−
H]− 403.0977,
0
0
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-4 -methoxy-[1,1 -biphenyl]-3-carboxamide (12k):
Yield 43 mg (40%) as a dark orange solid; m.p. 192–196 ◦C; 1H-NMR (400 MHz, DMSO-d6) δ = 11.99
(br s, 1H), 10.70 (br s, 1H), 9.78 (br s, 1H), 8.21 (s, 1H), 7.90 (d, J = 7.8 Hz, 1H), 7.86 (d, J = 7.8 Hz, 1H),
7.73 (d, J = 8.7 Hz, 2H), 7.61 (t, J = 7.8 Hz, 1H), 7.53 (d, J = 8.7 Hz, 2H), 7.07 (d, J = 8.7 Hz, 2H), 6.78
(d, J = 8.7 Hz, 2H), 6.56 (br s, 2H), 3.82 (s, 3H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 55.3, 114.5, 115.5,
116.1, 122.5, 125.3, 126.1, 127.9, 128.1, 129.3, 129.6, 131.8, 134.5, 140.2, 140.8, 143.7, 145.9, 158.0, 159.3,
165.4 ppm; HRMS (HESI, m/z) calcd. for C23H20N6O3 (428.45) [M + H]+ 429.1675, found 429.1674.
0
0
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-2 -methoxy-[1,1 -biphenyl]-3-carboxamide (12l):
Yield 70 mg (65%) as a brown solid; m.p. 175–180 ◦C; 1H-NMR (400 MHz, DMSO-d6)
= 11.84 (br
δ
s, 1H), 10.74 (br s, 1H), 9.77 (br s, 1H), 8.06 (s, 1H), 7.93 (d, J = 7.8 Hz, 1H), 7.72 (d, J = 7.8 Hz, 1H),
7.60 (t, J = 7.6 Hz, 1H), 7.51 (d, J = 8.7 Hz, 2H), 7.41 (d, J = 7.3 Hz, 2H), 7.17 (d, J = 8.7 Hz, 1H), 7.09
(t, J = 7.6 Hz, 1H), 6.75 (d, J = 8.7 Hz, 2H), 6.50 (br s, 2H), 3.77 (s, 3H) ppm; 13C-NMR (101 MHz,
DMSO-d6) δ = 55.6, 111.8, 115.5, 120.9, 122.4, 126.3, 128.2, 128.5, 128.9, 129.5, 130.6, 132.9, 133.7, 138.5,
144.5, 145.8, 156.1, 158.0, 165.1 ppm; HRMS (HESI, m/z) calcd. for C23H20N6O3 (428.45) [M + H]+
429.1675, found 429.1672.

Molecules 2018, 23, 149
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N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-3-(furan-3-yl)benzamide (12m): Yield 49 mg
(50%) as an orange solid; m.p. 204–208 ◦C; 1H-NMR (400 MHz, DMSO-d6)
= 11.81 (br s, 1H), 10.58
δ
(br s, 1H), 9.76 (br s, 1H), 8.29 (t, J = 1.0 Hz, 1H), 8.21 (t, J = 1.6 Hz, 1H), 7.86 (dd, J = 1.6, 7.6 Hz,
2H), 7.80 (t, J = 1.6 Hz, 1H), 7.57 (t, J = 7.8 Hz, 1H), 7.53 (d, J = 8.7 Hz, 2H), 7.08–7.04 (m, 1H), 6.79
(d, J = 8.7 Hz, 2H), 6.56 (br s, 2H) ppm; 13C-NMR (101 MHz, DMSO-d6)
δ = 108.8, 115.5, 116.2, 122.5,
124.6, 125.3, 126.2, 128.9, 129.2, 132.4, 134.5, 139.9, 144.6, 145.9, 158.0, 165.4 ppm; HRMS (HESI, m/z)
calcd. for C20H16N6O3 (388.39) [M + H]+ 389.1362, found 389.1357.
N-(5-Amino-4-((4-hydroxyphenyl)diazenyl)-1H-pyrazol-3-yl)-3-(thiophen-3-yl)benzamide (12n): Yield 61 mg
(60%) as a bright orange solid; m.p. 278–282 ◦C; 1H-NMR (400 MHz, DMSO-d6)
δ
= 11.94 (br s, 1H),
10.66 (br s, 1H), 9.78 (br s, 1H), 8.32 (s, 1H), 8.02–8.00 (m, 1H), 7.96 (d, J = 7.8 Hz, 1H), 7.89 (d, J = 7.3 Hz
1H), 7.72–7.69 (m, 1H), 7.67 (d, J = 5.0 Hz, 1H), 7.60 (t, J = 7.8 Hz, 1H), 7.54 (d, J = 8.2 Hz, 2H), 6.78
(d, J = 8.7 Hz, 2H), 6.58 (br s, 2H) ppm; 13C-NMR (101 MHz, DMSO-d6)
= 115.5, 116.2, 121.9, 122.5,
,
δ
125.1, 126.3, 126.5, 127.5, 129.3, 129.5, 134.6, 135.5, 140.8, 143.5, 145.9, 158.0, 165.4 ppm; HRMS (HESI,
m/z) calcd. for C20H16N6O2S (404.45) [M − H]− 403.0977, found 403.0971.
4. Conclusions
To conclude, we developed the simple Boc-protection method of pyrazole
amino-benzoylated pyrazoles 11 and 12. The positions of the Boc groups in pyrazoles
1
(CAN508) to access
and were
3
5
unequivocally determined by single crystal X-ray analysis. Further modification of bromopyrazoles
11c and 12b via Suzuki-Miyaura coupling showed the relative robustness of the catalyst and proved
the possibility to carry out the coupling in the last step even with the unprotected amino-benzoylated
pyrazole system. The coupling reaction gave better yields with para-substituted bromobenzoylpyrazole
11c. Moreover, the use of Boc protecting groups enabled for directly synthesizing pyrazoles 12d–n
without an additional deprotection step.
Supplementary Materials: The following are available online, the copies of 1H and 13C-NMR spectra,
crystallographic data for structures and . CCDC 1453321 and CCDC 1453320 contain the supplementary
3
5
crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail:
deposit@ccdc.cam.ac.uk).
Acknowledgments: This work was supported by the Czech National Program for Sustainability
(project LO1304), the Operational Programme Enterprise and Innovations for Competitiveness (project
CZ.01.1.02/0.0/0.0/15_019/0004431), and the internal Palacky University IGA projects (IGA_PrF_2017_009
and IGA_LF_2017_028).
Author Contributions: M.P., E.S., and F.H. synthesized all compounds; I.P. measured and analyzed NMR spectra;
Z.T. performed the X-ray analysis and analyzed these data; P.C. conceived and designed the experiments and
wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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