Mechanochemical Synthesis and Isomerization of N-Substituted Indole-3-carboxaldehyde Oximes ?

Article abstract of DOI:10.3390/molecules24183347

Performing solution-phase oximation reactions with hydroxylamine hydrochloride (NH₂OH·HCl) carries significant risk, especially in aqueous solutions. In the present study, four N-substituted indole-3-carboxaldehyde oximes were prepared from the corresponding aldehydes by solvent-free reaction with NH₂OH·HCl and a base (NaOH or Na₂CO₃) using a mechanochemical approach, thus minimizing the possible risk. In all cases, the conversion to oximes was almost complete. The focus of this work is on 1-methoxyindole-3-carboxaldehyde oxime, a key intermediate in the production of indole phytoalexins with useful antimicrobial properties. Under optimized conditions, it was possible to reach almost 95% yield after 20 min of milling. Moreover, for the products containing electron-donating substituents (-CH₃, -OCH₃), the isomerization from the oxime anti to syn isomer under acidic conditions was discovered. For the 1-methoxy analog, the acidic isomerization of pure isomers in solution resulted in the formation of anti isomer, whereas the prevalence of syn isomer was observed in solid state. From NMR data the syn and anti structures of produced oximes were elucidated. This work shows an interesting and possibly scalable alternative to classical synthesis and underlines environmentally friendly and sustainable character of mechanochemistry.

Full text of DOI:10.3390/molecules24183347

molecules
Article
Mechanochemical Synthesis and Isomerization of
N-Substituted Indole-3-carboxaldehyde Oximes †
Matej Baláž 1,
*
, Zuzana Kudlicˇková 2 , Mária Vilková 3 , Ján Imrich 3, L’udmila Balážová 4
and Nina Daneu 5
1
Department of Mechanochemistry, Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45,
04001 Košice, Slovakia
2
Department of Chemistry, Biochemistry and Biophysics, University of Veterinary Medicine and Pharmacy,
Komenského 73, 04181 Košice, Slovakia
NMR Laboratory, Faculty of Science, P. J. Šafárik University, Moyzesova 11, 04001 Košice, Slovakia
Department of Pharmacognosy and Botany, University of Veterinary Medicine and Pharmacy,
Komenského 73, 04181 Košice, Slovakia
3
4
5
Advanced Materials Department, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
Correspondence: balazm@saske.sk; Tel.: +421-55-792-2603
*
†
Dedicated to the memory of Assoc. prof. Dr. Peter Kutschy from Faculty of Science, P. J. Šaf
in Košice.
árik University
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Vjekoslav Štrukil and Alejandro Baeza Carratalá
Received: 2 July 2019; Accepted: 11 September 2019; Published: 14 September 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Performing solution-phase oximation reactions with hydroxylamine hydrochloride
(NH2OH HCl) carries significant risk, especially in aqueous solutions. In the present study, four
N-substituted indole-3-carboxaldehyde oximes were prepared from the corresponding aldehydes by
solvent-free reaction with NH2OH HCl and a base (NaOH or Na2CO3) using a mechanochemical
·
·
approach, thus minimizing the possible risk. In all cases, the conversion to oximes was almost
complete. The focus of this work is on 1-methoxyindole-3-carboxaldehyde oxime, a key intermediate
in the production of indole phytoalexins with useful antimicrobial properties. Under optimized
conditions, it was possible to reach almost 95% yield after 20 min of milling. Moreover, for the products
containing electron-donating substituents (-CH3, -OCH3), the isomerization from the oxime anti to syn
isomer under acidic conditions was discovered. For the 1-methoxy analog, the acidic isomerization of
pure isomers in solution resulted in the formation of anti isomer, whereas the prevalence of syn isomer
was observed in solid state. From NMR data the syn and anti structures of produced oximes were
elucidated. This work shows an interesting and possibly scalable alternative to classical synthesis
and underlines environmentally friendly and sustainable character of mechanochemistry.
Keywords: mechanochemistry; isomerization; oximation
1. Introduction
Aldoximes are a fundamental class of compounds employed for purification, protection, and
characterization of aldehydes [
compounds including amines [
the solution-phase reaction of hydroxylamine hydrochloride with an aldehyde in the presence of a
base [ ]. However, the use of this compound in an aqueous reaction in the presence of metal ions led to
a disastrous industrial accident [ ]. Therefore, developing safe syntheses (including oximations) with
low environmental impact are of critical importance to enabling sustainable chemical production. The
solvent-free solid-state reactions for oxime synthesis are gaining popularity [ 17]. Among various
solvent-free solid-state approaches, the addition of extra energy via microwave irradiation [18 19] or
1
]. They are important precursors for various nitrogen-based organic
2
], nitriles [ ], and nitro compounds [ ]. Typical preparation is through
3
4
5
6
7–
,
Molecules 2019, 24, 3347; doi:10.3390/molecules24183347
www.mdpi.com/journal/molecules

Molecules 2019, 24, 3347
2 of 19
mechanical force by grinding (mechanochemistry) [20
–23], are popular research topics. This energy
input accelerates reactions and can also lead to altered selectivity [24]. Research on mechanochemical
synthesis in general is increasing in frequency as it represents an environmentally friendly and “green”
method of synthesis. In recent years, significant advances have been achieved in both inorganic
and organic mechanochemistry [25
mechanochemically with good yields [24
into aldoximes [22,23].
–
35]. A wide array of organic reactions has been implemented
36 39]. This is also the case of the transformation of aldehydes
,
–
Although a broad range of aldehydes have been used in previous reports on the solid-state synthesis
of oximes [22 23 40 41], the reactions of N-substituted indole-3-carboxaldehydes, are unexplored.
Regarding 1-methoxyindole-3-carboxaldehyde, Somei’s method starting from indoline is typically
used for its synthesis [42 44]. One reason for our interest is that 1-methoxyindole-3-carboxaldehyde
,
, ,
–
oxime is a reaction intermediate in the long synthetic pathway to cruciferous phytoalexins that exhibit
antimicrobial and antiproliferative activity (the structures of some phytoalexins are provided in
Figure 1) [45–48].
Y
H CS
3
SCH
3
SCH
3
NH
N
N
SCH
3
S
N
N
N
OCH
3
OCH
OCH
3
3
Y=S 1-methoxybrassinin
Y=O 1-methoxybrassitin
1-methoxybrassenin A
sinalbin B
N
N
S
N
SCH
SCH
3
SCH
3
3
S
S
OH
OCH3
O
N
N
N
OCH
OCH
OCH
3
3
3
(2R,3R)-(-)-1-methoxyspirobrassinol
methyl ether
(R)-(+)-1-methoxyspirobrassinin 1-methoxyspirobrassinol
Figure 1. Structures of indole phytoalexins.
There are many reports on the solution-based syntheses of N-substituted indole-3-carboxaldehyde
oximes from their aldehydes [49–54]. Regarding the 1-methoxy analog, Hanley et al. firstly reported
the conversion of the corresponding aldehyde to oxime with the 50% yield [55]. The authors did
not investigate the isomerization of syn and anti oximes and merely used the reaction mixture for
further syntheses. In the same year, Kawasaki et al. performed the same reaction in pyridine with 98%
yield [56]. The individual syn and anti isomers were separated by column chromatography and the
possibility of mutual isomerization was shown. Later on, the same reaction was also performed by
Pedras et al. with quantitative yield [57].
Here, we report the simple solvent-free solid-state syntheses of four N-substituted
indole-3-carboxaldehyde oximes. Different ratios of isomers were found depending on reaction
conditions. The isomerization of crude reaction mixtures, as well as that of pure isomers was also
investigated showing different outcome depending on whether it was prepared by a solid-state route
or in solution. We demonstrate the simplification of one reaction step in the synthetic pathway to
indole phytoalexins [47,48] and their analogs [58,59].

Molecules 2019, 24, 3347
3 of 19
2. Results and Discussion
2.1. Mechanochemical Synthesis of 1-Methoxyindole-3-carboxaldehyde Oxime (Syn, Anti-1)
In the solution-phase syntheses of the 1-methoxyindole-3-carboxaldehyde aldoxime, sodium
carbonate [55], or pyridine [56] was used as a base. However, solid-state approaches have employed
sodium hydroxide for the substituted benzaldehydes aldoxime production [23]. In this study, the
versatile approach reported by Aakeroy et al. was used as a starting point for our system. However, we
found it is necessary to use high-energy ball milling to obtain acceptable yields. The general reaction
scheme, also for the non-substituted and other N-substituted analogs for which the results will be
reported in Section 2.4, is shown in Scheme 1.
Scheme 1. The general reaction scheme for the synthesis of N-substituted indole-3-carboxaldehyde
oximes from the corresponding aldehydes.
Initially, attempts to reproduce the procedure reported in [23] yielded unsatisfactory results,
most likely due to the electron-donating resonance effect of indole nitrogen which decreases the
electrophilicity of carbonyl carbon [60]. More details regarding this experiment are provided in the
ESI (Table S1) Optimization of the mechanochemical reaction under study was carried out through an
extensive variation of the reaction parameters (Table 1). Since excess hydroxylamine hydrochloride
has been used in the solution-based report on this reaction [55], we hypothesized that an increase in
the amount of hydroxylamine and hydroxide would lead to improved yields. In this optimization,
up to 5.2 equivalents of NH2OH HCl and 2.5 equivalents of base were used (Table 1, entries 1–2). In
·
both cases, the reaction went to completion according to NMR data. However, the reaction with 3.1
equivalents of hydroxylamine (Table 1, entry 1) exhibited slower kinetics (it was finished after 30 min)
and no product isomerization. Therefore, in the next experiment (Table 1, entry 3), 5 equivalents of
hydroxylamine were maintained, but an effort to further reduce amount of NaOH (to 2 eq) was made.
As seen in the table, complete conversion was again achieved. In order to investigate the effect of
altering the amount of hydroxide on the isomerization process, the same experiments under neutral
conditions (5 equivalents of NH2OH HCl and 5 equivalents of NaOH was realized (Table 1, entry 4)).
·
The reaction went to completion also in this case, just the isomers ratio was significantly different, but
this will be discussed later.

Molecules 2019, 24, 3347
4 of 19
Table 1. Basic experimental conditions used in the synthesis of 1-methoxyindole-3-carboxaldehyde
oxime and the reactions outcome. The different reaction conditions are marked by color: green color:
ball milling in Pulverisette 7 premium line mill, WC vessel (80 mL) and balls (in most cases 18 pieces,
∅
10 mm, weight of 1 ball 7.7 g); blue color: ball milling in Pulverisette 6 mill, agate vessel (250 mL) and
balls (50 pieces, 10 mm, weight of 1 ball 2.4 g). The ideal experiment is marked in bold. The milling
∅
time is reported for the final mixtures from which the NMR spectra were recorded. Until this time, the
reaction progress was monitored by TLC chromatography. The inconsistencies between the conversion
and isolated yield values come only from the ineffectiveness of manual extraction of reaction mixture
from the milling vessel.
Equivalents
Milling/Stirring
Conversion
from NMR
Syn:anti
Oxime Ratio
Isolated
Yield
Entry
Time (min)
NH2OH·HCl
NaOH
1
2
3.1
5
5
5
5
5
5.2
2.5
2.5
2
5
2
2
2.7
30
20
20
20
30
40
40
100
100
100
100
100
100
100
35:65
98:2
97:3
43:57
87:13
91:9
78
72
94
93
20
80
45
3 a
4 a
5 b
6 ac
7 d
30:70
Aging experiments
8
9
5
5
2
2
5
0.25
97 e
85 e
95:5
89:11
25
65
a
The powder from the vessel was thoroughly extracted with ethyl acetate, in order to isolate as much product as
b
possible. Liquid assisted grinding (LAG) through the addition of a few drops of methanol was performed,
= 0.022
η value
c
µ
L/mg. The reaction mixture was washed with distilled water and the products were isolated immediately
d
e
after its termination. Instead of NaOH, Na2CO3 was used as a base. The indicated conversion was not measured
immediately after the short milling time indicated in corresponding column, but 10 days after reaction and after
washing procedure, so there was plenty of time for solid-state reaction to progress also outside milling device (for
more details, see Figures 2 and 3). It was found that the reaction proceeds to some extent also in DMSO solution
(see Table S2). The great prevalence of anti isomer (on the contrary to the mechanochemical experiments) can be
seen. In order to minimize the contribution from solution reaction, the measurements were done quickly (ca. 2 min
after dissolution).
Unfortunately, the reaction mixtures extracted from tungsten carbide milling vessel were
significantly contaminated by particles coming from the milling media and chamber walls. The
effect of wear in the field of organic mechanochemistry has been studied in the past [61–66] and it has
been proposed that agate is the most suitable material from the contamination point of view [67]. Based
on this, it was hypothesized that performing the reaction in an agate milling vessel would produce
similar results with less wear. Moreover, it is known that liquid assisted grinding (LAG), the addition of
substoichiometric amounts of solvent to a milling reaction, can significantly reduce the time of milling
(thus also wear) and potentially also the stereochemical distribution of the reaction products [38,68–70].
Therefore the next experiment was performed in an agate vessel using LAG (Table 1, entry 5). Reaction
completion was achieved after 30 min of milling with no observable contamination. The neat reaction
(Table 1, entry 6) took slightly longer time (40 min), but the ratio of isomers was the same as when
using LAG. Based on these results LAG was not used in the further experiments. However, the
was just 0.022 g/mL, which means that the process was almost neat. The influence of LAG on this
process both in terms of solvent amount and type could be a topic for separate study in future.
η value
µ
Although the initial results with the agate milling material were positive, it was difficult to extract
the material from the milling vessel. Despite exhaustive mechanical efforts, only half of the reaction
mass could be mechanically extracted from the milling vessel. This is reflected in a low isolated yield
(Table 1, entry 5). Agate is prone to fracture and the reaction mixture was most probably lost in the
cracks in the vessel walls. However, an ethyl acetate extraction of the whole milling vessel facilitated
recovery of the reaction products (Table 1, entry 6).
For comparison, an experiment with sodium carbonate, a weaker base than sodium hydroxide,
but being less hygroscopic and easier to handle, was completed (Table 1, entry 7). As can be seen, a

Molecules 2019, 24, 3347
5 of 19
good yield was obtained upon switching bases. The amount of base and hydroxylamine was slightly
higher (2.7 and 5.2 eq, respectively), in order to simulate the conditions in [55]. The reaction was
completed in 40 min, so the necessary reaction time was longer in comparison with experiments
implementing NaOH.
In Figure S1, the 1H-NMR spectra of the purified reaction mixtures are provided, which show no
residual aldehyde for entries 1–7. The syn:anti oxime ratios presented in the Table 1 will be discussed
in detail in the following section. It follows from all these considerations that the ideal experimental
conditions are the ones applied for entry 3 in Table 1 (marked in bold). This experiment is also the
most favorable from the syn:anti oxime ratio point of view.
Additionally, events leading to increase in gas pressure in the chamber were detected during
the reaction using the special jar equipped with a pressure sensor. A detailed description of these
phenomena is provided in the ESI (Figure S2 and the description therein).
In general, the samples after ball milling were significantly contaminated with particles of milling
media. This was noted in the increased crude reaction mixture mass and color change (white to black).
Purification was achieved using a flash chromatography on silica gel and isolated yields are reported
in Table 1. The lowest yields (around 20%) were due to mechanical losses in extracting the product
from the agate milling vessel (Table 1, entry 5). The isolated yield from the tungsten carbide milling
vessel varies from 45% for the experiment with Na2CO3 to values above 75% for the experiments
with NaOH. When a small amount of ethyl acetate was used to extract the powder from the vessel
(Table 1, entries 3, 4, and 6), the isolated yields could be increased above 80% and 94% for the agate
and tungsten carbide milling vessels, respectively.
In order to compare the reactions conducted mechanically with standard synthetic approaches,
three reactions were performed in solution using the hydroxylamine and base ratios optimized for the
mechanochemical synthesis (Table S1, entries S1–S3). In all cases, the reactions were completed within
1 h. However, the ratio of isomers was different from the experiments utilizing ball milling, as will be
discussed later.
2.2. Aging Experiments for the Synthesis of 1-Methoxyindole-3-carboxaldehyde Oxime (Syn, Anti-1a)
In order to investigate whether the reaction can reach completion simply by a short activation via
milling followed by storage of the reaction mixture (aging), milling of the reagents was performed for
5 min in the agate vessel (Table 1, entry 8) and for 15 s in the WC vessel (Table 1, entry 9). Longer agate
milling times were used due to the lower density of silica versus tungsten carbide (2.65 g/cm3 versus
15.63 g/cm3).
The reaction progress can be easily followed by 1H-NMR spectroscopy (Figure S3), as the chemical
shifts of the protons of starting aldehyde (CHO; 9.88 ppm) are significantly different from the ones
for syn (H-8; 7.83 ppm) and anti (H-8; 8.21 ppm) oximes. In the agate experiment (Table 1, entry 8),
the starting aldehyde was almost completely consumed already after 1 day. The transformation of
aldehyde in the experiment using WC grinding (Table 1, entry 9) proceeded much more slowly as a
significant amount of the aldehyde was still present after 5 days. The anti-syn oxime isomerization
was observed in both systems and it will be described in detail later. Since it was found that the
reaction proceeds to some extent also in DMSO (Table S2), the NMR measurements were performed
immediately after dissolution of the reaction mixture. After washing, the final samples still contained
3% and 15% (respectively) of unreacted aldehyde (Table 1, entries 8 and 9). It was concluded, that the
surface energy of the reagents, which was increased by short milling through the increase of surface
area and generation of defects, was insufficient for the completion of the reaction. However, these
results suggest that there may be an optimal processing time that would result in a complete reaction.
2.3. Isomerization of 1-Methoxyindole-3-carboxaldehyde Oxime (Syn, Anti-1a)
It is well-known that oximes can be present as syn and anti (Z and E) isomers and they can undergo
isomerization [71]. In the case of 1-methoxyindole-3-carboxaldehyde oxime, the ratio of the isomers

Molecules 2019, 24, 3347
6 of 19
was not reported. To obtain these data, we performed an experiment according to the literature [55
]
and the obtained ratio of syn:anti oxime isomers was 40:60 (Table S1, last entry). In this work, we
turned our attention to the possible mutual isomerization of oxime isomers. It can be seen from Table 1
showing the results for the solvent-free synthesis that the syn:anti ratios were different, depending
on the experimental conditions used, however, in most reactions, the final content of syn isomer was
significantly higher, so the isomerization process must have taken place. In the solution-based reactions
performed for comparison, the prevalence of syn isomer was also evident, being higher when the
reaction mixture was left to isomerize (Table S1, entry S2). However, if a 1:1 hydroxylamine:hydroxide
ratio was used, the anti isomer was favored despite longer storage (Table S1, entry S3).
In the case of all solvent-free reactions performed with NaOH, there was an excess of the anti
isomer immediately after the reaction (according to TLC). However, when the residual non-reacted
hydroxylamine was not removed from the reaction mixture, the transformation from anti to syn isomer
occurred, and after few days, the anti isomer was nearly entirely converted to the syn one for the
experiments where a significant excess of the equivalents of NH2OH HCl over NaOH was used
·
(Table 1, entries 2, 3, 5, 6, 8 and 9). This was discovered by performing time-resolved TLC of a selected
experiment. Details can be found in the ESI (Figures S4 and S5).
This process can also take place when the reaction has not gone to completion. The two phenomena,
namely the completion of the studied reaction and the isomerization of the products can coincide. By
means of 1H-NMR spectroscopy, we followed these processes in the selected experiments (Figures 2
and 3).
Figure 2. Amounts of non-reacted aldehyde
1 and syn oxime 1a in selected experiments performed
with a Pulverisette 7 Premium Line mill in a WC jar determined from 1H-NMR spectra. Time of milling
for entry 3: 20 min and for entry 7: 40 min. In entry 3, 5 eq. NH2OH·HCl and 2 eq. NaOH were used.
In entry 7, 5.2 eq. NH2OH
the washed samples.
·HCl and 2.7 eq. Na2CO3 were used. The data reported after 10 days are for

Molecules 2019, 24, 3347
7 of 19
Figure 3. Amounts of non-reacted aldehyde
1 and syn oxime 1a in selected experiments performed in
Pulverisette 6 mill in agate chamber determined from 1H-NMR spectra. Time of milling for entry 5: 30
min, LAG and for entry 8: 5 min. In both entries, 5 eq. NH2OH
data reported after 10 days are for the washed samples.
·HCl and 2 eq. NaOH were used. The
In Figure 2, the influence of base type can be seen. The reaction went to completion during
the milling process in both cases (no aldehyde was detected). The residual hydroxylamine brought
about almost complete transformation of the anti isomer into the syn when NaOH was used (ideal
experiment, Table 1, entry 3). On the contrary, in the reaction with Na2CO3 (Table 1, entry 7), the
isomerization did not take place despite the exposure to residual hydroxylamine and sodium carbonate.
There are two types of reactions in the oxime synthesis: in situ deprotonation of NH2OH
base and reaction of NH2OH and the aldehyde. For the in situ deprotonation, the stoichiometric
ratio of NH2OH HCl and NaOH is 1:1, while for the reaction with Na2CO3, it is 2:1. In all realized
·HCl by
·
experiments with NaOH (except entry 4), the hydroxylamine:hydroxide stoichiometric ratio was
higher than 2:1, while for the experiments with Na2CO3, the hydroxylamine:carbonate ratio was 1.93:1
(almost stoichiometric). We assume that in the second case, complete deprotonation of NH2OH
occurred and the resulting reaction mixture was neutral, while in experiments with NaOH, less than
half of NH2OH HCl was deprotonated, thus resulting in the acidic conditions. Consequently, residual
HCl may cause isomerization. The isomerization of oximes from anti (E) to syn (Z) in the presence of
strong acids is well known [72 73]. To confirm the hypothesis outlined in this paragraph, additional
·HCl
·
,
experiments described later were performed.
In Figure 3, the selected experiments performed using the Pulverisette 6 mill in an agate vessel
are compared, namely the one applying LAG procedure (Table 1, entry 5) and the aging one (Table 1,
entry 8). When performing LAG in an agate vessel (red, Table 1, entry 5) for 30 minutes, the sample
still contained 7% unreacted aldehyde after the reaction, which was almost completely transformed
into oximes during the next 20 h. This experiment provided the anticipated result in terms of the
amount of syn isomer after 4 days, as 84% was measured. However, it seems that LAG enhances the
transformation to the syn isomer, as 3 h after the reaction, it is the major isomer detected, whereas in
the neat aging experiment (blue, Table 1, entry 8), the anti isomer was the major isomer after the same
amount of time. For the LAG experiment (red, Table 1, entry 5), the aldehyde was not detected at all

Molecules 2019, 24, 3347
8 of 19
at the end and the amounts of the oxime isomers were similar to that detected in the final 1H-NMR
measurement prior to washing.
For the aging experiment (Table 1, entry 8), a significant amount of non-reacted aldehyde was
detected after the reaction. It was subsequently consumed during storage, decreasing to less than 10%
after one day. However, a complete consumption was not observed. The anti-syn isomerization took
place and the anti isomer made up 12% of the product after 4 days. The final content of anti isomer
after workup was 8%, which is almost the same as in the case of the experiment using LAG procedure
(Table 1, entry 5).
The changes in content of aldehyde and syn isomer for the aging experiment performed in WC
(Table 1, entry 9) with time are presented in the ESI in Figure S6. The reaction did not go to completion
and syn isomer became greatly major.
After the synthesis and isomerization of oximes in the presence of non-reacted hydroxylamine were
investigated in the solid state, these processes were also studied in a solvent by NMR measurements in
DMSO. Whereas the presence of solvent facilitates the conversion to oximes (if not finished during
milling), the anti-syn isomerization is more rapid in the solid state. Detailed description of the results
can be found in the ESI (Table S3 and the text therein).
In the solution-based syntheses performed in the excess of hydroxylamine (Table S1, entries S1
and S2), syn isomer was major. Its content was higher when the mixture was left to isomerize (entry S2).
When the same reaction was performed with 5 eq. of both NH2OH HCl and NaOH (Table S1, entry S3),
·
we found that the amount of syn isomer after 1 h was only 34%. In this case the reaction mixture
was neutral and the anti isomer was isolated as the major product. The preference of anti isomer
was also found after repeating with Hanley’s [55] conditions (59%) and was similarly described by
Kawasaki and Somei’s [56] reaction in pyridine (62%). It seems that the main driving force behind the
isomerization is the acidity of the reaction mixture. In order to prove this, we subjected pure isomers
to both neutral and acidic conditions and studied the isomerization process (see the text below).
After the dissolution of individual isomers and subsequent storage in CDCl3 for 7 days (see ESI,
Figure S7), the susceptibility to isomerization of pure anti oxime into the syn one was discovered
(equilibrium ratio syn:anti 15:85). In contrast, we found that the syn isomer is more stable in CDCl3
solution and after 7 days syn:anti equilibrium ratio was 97:3. This is in contradiction with the results
of Kawasaki and Somei et al. [56] who observed the transformation of both oxime isomers into the
other one and attained an equilibrium significantly favoring the anti oxime (syn:anti 39:61) in both
CDCl3 and CHCl3. The isomerization did not occur in CD3OD. They planned to report details of this
phenomenon subsequently, but to the best of our knowledge, no report regarding this isomerization
was published until now.
In order to investigate the role of acidity in a mechanochemical process, pure isomers were milled
with 10 mol% of p-TsOH for 20 min (under the same conditions as in the ideal experiment (Table 1,
entry 3)). No isomerization was generally observed for syn isomer, while 39% of the anti isomer
was transformed into syn one in 1 h after milling and after 7 days, 75% was transformed (Figure 4,
corresponding NMR spectra can be found in the ESI in Figure S8). It seems that the syn isomer is stable
in the solid state under the acidic conditions.
To compare the behavior of individual isomers with the solid-state isomerization experiments
under the acidic conditions, the behavior of separated isomers in DMSO-d6 with 10 mol% p-TsOH
was investigated. In this case, each isomer changed to the other one and an equilibrium with ratio of
syn:anti ratio 40:60 was slowly achieved (Figure 5, corresponding NMR spectra can be found in the ESI
in Figure S9), being very close to those reported in [56]. From these results two major points regarding
isomerization under acidic conditions can be claimed: (i) on the contrary to the mechanochemical
approach, the isomerization of the syn isomer takes place in solution and (ii) whereas the formation
of anti isomer is favored in solution, syn isomer is preferred in the solid state. This brings us to the
specificity of mechanochemistry, which is known to provide different results than if the same reaction
is performed in a traditional way [24]. Further effort will be necessary to enlighten our observations.

Molecules 2019, 24, 3347
9 of 19
Figure 4. Amounts of syn oxime 1a after milling (Pulverisette 7 Premium Line mill, milling time 20 min)
of the pure individual oxime isomers with 10 mol% p-TsOH during storage in solid state (the content
was determined from 1H-NMR spectra presented in the ESI—Figure S8).
Figure 5. Amounts of syn oxime 1a during storage in DMSO containing 10 mol% p-TsOH starting from
pure isomers (the content was determined from 1H-NMR spectra presented in the ESI—Figure S9).
The detailed characterization of the pure 1-methoxyindole-3-carboxaldehyde oximes using various
methods can be found in the Electronic Supplementary Information (ESI), namely the complete set
of NMR spectra of both isomers (Figures S10–S19), proofs of configuration (syn and anti, Figure S20
and Table S4–S6), X-ray diffraction patterns (Figure S21), FT-infrared spectra (Figure S22), melting
temperature analysis, optical microscopy images (Figure S23) and transmission electron microscopy
images (Figure S24).

Molecules 2019, 24, 3347
10 of 19
2.4. Substrate Scope in the Oximation-Isomerization Reactions
In order to check if the proposed approach could be more universal, we have tested
it with different substituents on nitrogen. Namely non-substituted ( ), 1-methyl- ( ) and
) were used for the reactions using either NaOH or Na2CO3
2
3
1-acetyl-indole-3-carboxaldehyde (
4
as a base.
The results both in terms of reaction completion and isomerization are summarized in Table 2.
The isolated yields are reported after extraction with water (ethyl acetate was not applied in this
case). For this purpose, the reactions with 1-methoxyindole-3-carboxaldehyde were also repeated.
The reaction conditions and yields for the solution-based approaches reported in literature are also
provided for comparison.
Table 2. Conversions, isolated yields and syn:anti oxime ratios immediately after reaction and after
isomerization. The basic experimental conditions, together with yields reported in relevant literature
are provided for comparison. In a reference column, either a link to corresponding figures in the
supplementary information (NMR spectra, Figures S25–S35), or a literature reference is provided. For
all experiments, the conditions for entries 3 or 7 in Table 1 (when the reaction was performed with
NaOH or Na2CO3, respectively), were used.
Syn:anti Ratio
Conversion
from NMR
Isolated
Yield
N-R
Reference
After
After
Isomerization
Base
Reaction
NaOH
Na2CO3
Na2CO3
pyridine
Na2CO3
99
99
81:19
43:57
97:3
43:57
78
67
50
98
99
Figure S26
OCH3
[55]
[56]
[57]
60:40 a
NaOH
Na2CO3
NaOH
NaOH
Na2CO3
Na2CO3
100
100
100:0
100:0
100:0
100:0
95
76
91
62
78
98
Figure S27–S29
[49]
[50]
[51]
[52]
H
NaOH
Na2CO3
NaOH
98
100
53:47
32:68
74:26
33:67
85
71
86
Figure S30–S32
CH3
68:32 b
[50]
[52]
Na2CO3
100
NaOH
Na2CO3
NaOAc
NaOAc
100
100
19:70:12 c
2:50:48 c
34:45:21 c
0:0:100 c
96
71
88
84
Figure S33–S35
CO-CH3
[53]
[54]
a
b
The ratios were re-calculated from the reported yields. The configuration of isomers is not assigned in the paper,
c
just the ratio is reported. The ratios in this case are reported as follows: syn-4a:anti-4a:syn-2a (deacetylated analog).
In all cases the content of aldehyde is almost negligible, which means that the proposed
methodology can be used in a more universal fashion for the transformation of different N-substituted
indole-based aldehydes into corresponding oximes. The content of individual isomers was significantly
different both in terms of a base and substituent used.
For the reaction of 1-methoxy analog with NaOH, the washing with water leads to the significant
prevalence of the syn isomer already after the reaction. In comparison with the relevant studies, our
approach does not require high [57] or low temperature [55], or pyridine solvent [56].
The reactions of non-substituted aldehyde yielded only the syn isomer, regardless of the base
used, which is in accordance with the available literature reporting the formation of only one isomer,
although its configuration is not assigned therein [49,51,52]. The reaction times in the most relevant

Molecules 2019, 24, 3347
11 of 19
papers are comparable with our results, however, in most cases elevated temperature was necessary
for the reaction completion within a reasonable time [49,51].
1-methylindole-3-carboxaldehyde oxime has shown the similar trend as its methoxy-substituted
analog in terms of anti-syn isomerization when performing the reaction with NaOH. After the reaction,
the syn isomer was already major (53%) and further storage of reaction mixture resulted in an increase
of its content to 74% after more than 10 days. Its prevalence is lower than for the methoxy analog. The
anti:syn ratio for the reaction performed with Na2CO3 did not change during storage, similarly to the
methoxy analog. The content of the major anti isomer was 67%, which was higher than in the case of
methoxy-substituted product (57%).
In the case of the electron-withdrawing acetyl group, under the given experimental conditions,
in addition to the formation of the 4a syn and anti isomers, the product deacetylated to form the
syn-2a. The extent of deacetylation was greater when the reaction was performed with Na2CO3,
where the contents of deacetylated oxime 2a were 48% and 100% in the sample washed immediately
after reaction and after isomerization, respectively. The acidic conditions (in this experiment, the
excess of NH2OH HCl over NaOH was used, similarly to entry 3 in Table 1) did not result in as
·
much deacetylation, although a considerable amount was evident after isomerization (21%). The
anti-syn isomerization was observed when the reaction was performed in NaOH, however, after 10
days, the anti isomer was still the major isomer, so the process is much slower in comparison with the
experiments where electron-donating groups were present. The reaction with Na2CO3 resulted in the
formation of almost pure anti isomer (50%) and a significant amount of deacetylated syn-2a (48%). The
product was completely transformed into deacetylated analog syn-2a after 10 days. It seems that the
isomerization process is accompanied by deacetylation in this case. The relevant literature reports
used sodium acetate as a base and either higher temperature [54] or longer time [53] was necessary for
the reaction completion.
From the available NMR data (chemical shifts of carbonyl carbon) it is clear that whereas the
presence of acetyl group increases the electron deficiency on the carbonyl carbon [74], methyl and
methoxy groups compensate it by donating electrons [75,76]. Based on this, the following order can be
worked out (starting from the largest electron-donating effect on carbonyl carbon): methoxy > methyl
> H > acetyl. In the case of electron-donating substituents, the anti-syn isomerization occurs, being
more significant for the 1-methoxy-substituted oxime 1a than for the 1-methyl-substituted analog (3a),
thus following the proposed order. However, the non-substituted oxime 2a was present only in the syn
configuration already after the reaction, and the acetyl-substituted oxime 4a exhibited deacetylation
and slower anti-syn isomerization when compared to the analogs containing electron-donating groups.
Thus the results obtained for 2a and 4a do not follow the proposed order. Further research is necessary
to clarify the observed reactivity and isomerization but the substituent evidently plays a decisive
role here.
The isolated yields are higher for the reactions performed with NaOH, being almost quantitative.
In the case of reactions performed with Na2CO3, the mixture seems to stick to the chamber walls and it
is not possible to extract it quantitatively just by washing with water.
3. Materials and Methods
3.1. Chemicals
The following chemicals were used for the experiments: indoline (99%, Alfa Aesar, Haverhill, MA,
USA), Na2WO4
Slovakia), POCl3 (99%, Merck, Darmstadt, Germany) NH2OH
(98%, Centralchem, Slovakia) and Na2CO3 (98%, Centralchem, Slovakia), indole-3-carboxaldehyde
(97%, Merck, Germany).
·
2H2O (98.5%, Centralchem, Bansk
á
Bystrica, Slovakia), K2CO3 (98%, Centralchem,
HCl (99%, Centralchem, Slovakia), NaOH
·

Molecules 2019, 24, 3347
12 of 19
3.2. Preparation of Aldehydes
1-Methoxyindole-3-carboxaldehyde was prepared by a classical approach from indoline using a
Somei’s methodology [42–44] (Scheme 2):
Scheme 2. Somei’s methodology for the synthesis of 1-methoxyindol-3-carboxyaldehyde [42–44].
1-Methylindole-3-carboxaldehyde was prepared according to the procedure reported in [77].
1-Acetylindole-3-carboxaldehyde was prepared according to the procedure reported in [53].
3.3. Experimental Procedure for Mechanochemical Synthesis
In the experiments performed with tungsten carbide milling media, a planetary ball mill
Pulverisette 7 premium line (Fritsch, Idar-Oberstein, Germany) with a special 80 mL milling vessel
equipped with a sensor for measuring gas pressure and temperature (EasyGTM, Fritsch, Germany)
was used. Eighteen WC milling balls with diameter 10 mm were used and the whole mass of reactants
was 1.9699 g. In the case of milling in agate, a planetary ball mill Pulverisette 6 (Fritsch, Germany)
and an agate vessel of 250 mL volume were applied. In this case, 39 agate milling balls with diameter
10 mm were used and the whole mass of reactants was 1.3613 g. The number of balls and mass of
reactants in both cases was selected in order to maintain ball-to-powder ratio 71. The atmosphere in
both cases was air. All experiments were performed at a milling speed of 500 rpm. Further details
regarding the experimental conditions are listed in Table 1. After milling, the reaction mixtures were
either scraped from the milling chamber manually, or extracted with ethyl acetate (30 mL). The latter
was done for the entries 3, 4, and 6.
The mixtures were then washed with distilled water, in order to remove the residual hydroxylamine
hydrochloride. For 1.4 g of reaction mixture, 70 mL of distilled water was used. This ratio was
maintained also when a different mass was used. The washed mixtures were then filtered, dried at
75 ◦C, and stored.
When the progress of the isomerization process was targeted, the solid mixtures were left standing
in a closed glass vessel (not evacuated) for 7–8 days prior to washing.
For the experiments applying other aldehydes (non-substituted, 1-methyl and 1-acetyl), the
experimental conditions from entries 3 and 7 from Table 1 were applied. In some cases, some powder
was stuck to the lid of the milling chamber. This was excluded from further analysis. The powder after
the reaction was separated into two fractions: 0.5 g was extracted and stored for the study of possible
isomerization for more than 10 days and the rest of powder was immediately extracted from the
chamber using distilled water and washed under the conditions described earlier. The same was done
with the isomerizing powder after more than 10 days. In order to investigate the effect of immediate
water washing, this approach was used also for the 1-methoxy analog.
For the thin layer chromatography (TLC) analysis, the Silica gel 60 F254 aluminum sheets (Merck,
Germany) were used. A diethylether:hexane 4:1 mixture was used as the eluent. The relative content
of the isomers was evaluated densitometrically by comparing the intensities of the corresponding
spots using the software NIH ImageJ 1.48.
Column chromatography with Silica gel 60 (0.040–0.063 mm, Merck, Germany) on a glass column
with the diameter 20 mm and internal volume 400 mL was used for the separation of the isomers.

Molecules 2019, 24, 3347
13 of 19
For all experiments, the product, after washing with water, was purified from media wear
using flash chromatography over silica gel. The product absorbed on the silica gel was eluted using
hexane:ethyl acetate 1:1.
With the aim to separate and characterize both isomers of 1-methoxyindole-3-carboxaldehyde
oxime, the product of the milling process (Table 1, entry 4) was extracted with ethyl acetate, then
washed with water (25 mL) and brine (10 mL), dried over Na2SO4 and filtered. The washed product
obtained directly from the reaction vessel was dissolved in this filtrate and the solvent evaporated
with small amount of silica gel. The residue after evaporation was subjected to chromatographic
separation over silica gel (hexane/ethyl acetate 2:1). The individual isomers were then crystallized
(dichloromethane:hexane).
3.4. Experimental Procedure for Solution-Based Synthesis
To a solution of 1-methoxyindol-3-carboxaldehyde (0.25 mmol) in ethanol (0.6 mL) NH2OH HCl
·
(1.25 mmol, 5 eq.) and NaOH (0.5 mmol, 2 eq. for Table S1, Entry S1 and S2 and 1.25 mmol, 5 eq.
for Table S1, Entry S3) in water (0.32 mL) at 0 ◦C were added. The reaction mixture was stirred at
room temperature for 1 h (in the case of Entry S2, it was left without further processing at room
temperature for 7 days), then diluted with ethyl acetate (10 mL), washed with water and brine, dried
and evaporated.
3.5. Isolated Yield Calculation
The isolated yield was calculated as follows:
!
mi
mT
Isolatedyield(%) =
·100
(1)
where mi is the isolated mass of the product after extraction and subsequent purification by flash
chromatography and mT is the theoretical yield.
3.6. Spectral Data for the Products
3.6.1. Anti-1-methoxyindole-3-carboxaldehyde Oxime (anti-1a)
Yield: 0.2073 g, 31% (ref. [56] 39%). Melting point: 93–95 ◦C (dicholomethane/ hexane) (ref. [56
146–147 ◦C). White crystals. XRD: 2
11.3, 12.5, 13.8, 17.8, 20.8, 22.6, 27.8, 29.3, 34.9, 36.1, 42.2, 49.6,
]
θ
57.3◦. IR:
ν
3242, 3186, 3121, 2986, 2939, 2897, 1640, 1541, 1447, 1377, 1346, 1327, 1290, 1244, 1163, 1109,
1043 941, 806, 716 cm−1. H-NMR (600 MHz, CDCl3):
1
δ = 8.29 (s, 1H, H-8), 8.06 (d, 1H, J 8.1 Hz, H-4),
7.48 (s, 1H, H-2), 7.45 (d, 1H, J 8.3 Hz, H-7), 7.32 (ddd, 1H, J 8.2, 7.0, 1.1 Hz, H-6), 7.23 (ddd, 1H, J 8.0,
7.1, 1.0 Hz, H-5), 4.12 (s, 3H, CH3O). 13C-NMR (151 MHz, CDCl3): 145.4 (C-8), 132.6 (C-7a), 124.8 (C-2),
123.7 (C-6), 122.2 (C-4), 121.6 (C-5), 121.3 (C-3a), 108.4 (C-7), 105.6 (C-3), 66.3 (CH3O). 1H-NMR (600
MHz, DMSO): δ = 10.69 (s, 1H, H-10), 8.21 (s, 1H, H-8), 8.00 (d, 1H, J 8.1 Hz, H-4), 7.95 (s, 1H, H-2), 7.49
(d, 1H, J 8.2 Hz, H-7), 7.29 (ddd, 1H, J 8.2, 7.0, 1.2 Hz, H-6), 7.17 (ddd, 1H, J 8.0, 7.1, 1.0 Hz, H-5), 4.09
(s, 3H, CH3O). 13C-NMR (151 MHz, DMSO): 143.6 (C-8), 132.2 (C-7a), 125.9 (C-2), 123.3 (C-6), 122.0
1
(C-4), 121.0 (C-5), 120.6 (C-3a), 108.5 (C-7), 105.9 (C-3), 66.2 (CH3O) ppm. JC8H8 = 161.4 Hz. Rf value
(diethylether:hexane 4:1): 0.69.
3.6.2. Syn-1-methoxyindole-3-carboxyaldehyde Oxime (syn-1a)
Yield: 0.3519 g, 56% (ref. [56] 59%). Melting point: 137–139 ◦C (dicholomethane/ hexane) (ref. [56
]
98.5–99.5 ◦C). White crystals. XRD: 2
θ
11.9, 15.7, 16.5, 17.7, 18.6, 22.4, 24.0, 24.5, 30.5◦. IR:
ν
3151, 3063,
1
3018, 2935, 2824, 1639, 1512, 1327, 1229, 1177, 1096, 1024, 941, 746 cm−1. H-NMR (600 MHz, CDCl3):
δ
= 8.40 (s, 1H, H-2), 7.79 (s, 1H, H-8), 7.79 (d, 1H, J 7.9 Hz, H-4), 7.49 (d, 1H, J 8.2 Hz, H-7), 7.33 (t,
1H, J 7.5 Hz, H-6), 7.25 (t, 1H, J 7.3 Hz, H-5), 4.16 (s, 3H, CH3O). 13C-NMR (151 MHz, CDCl3): 139.5

Molecules 2019, 24, 3347
14 of 19
(C-8), 130.7 (C-7a), 128.5 (C-2), 123.2 (C-6), 123.2 (C-3a), 121.4 (C-5), 118.4 (C-4), 108.5 (C-7), 101.7 (C-3),
66.4 (CH3O). 1H-NMR (600 MHz, DMSO):
= 11.42 (s, 1H, H-10), 8.42 (s, 1H, H-2), 7.94 (d, 1H, J 8.0
δ
Hz, H-4), 7.83 (s, 1H, H-8), 7.52 (d, 1H, J 8.2 Hz, H-7), 7.30 (ddd, 1H, J 8.2, 7.1, 1.1 Hz, H-6), 7.19 (ddd,
1H, J 8.0, 7.0, 1.0 Hz, H-5), 4.13 (s, 3H, CH3O). 13C-NMR (151 MHz, DMSO): 137.5 (C-8), 130.1 (C-7a),
127.6 (C-2), 123.0 (C-6), 122.5 (C-3a), 120.8 (C-5), 118.9 (C-4), 108.4 (C-7), 102.2 (C-3), 66.4 (CH3O) ppm.
1JC8H8 = 172.2 Hz. Rf value (diethylether:hexane 4:1): 0.53.
3.6.3. Syn-indole-3-carboxyaldehyde Oxime (syn-2a)
Yield: 0.4972 g, 85%. Melting point: 179–181 ◦C (acetone/hexane) (ref. [50] 182 ◦C, chloroform).
White crystals. 1H-NMR (400 MHz, DMSO-d6):
δ = 11.58 (s, 1H, H-1), 11.19 (s, 1H, H-10), 8.23 (d,
1H, J = 2.7 Hz, H-2), 7.86 (d, 1H, J = 7.8 Hz, H-4), 7.79 (s, 1H, H-8), 7.44 (d, 1H, J = 8.0 Hz, H-7), 7.17
(ddd, 1H, J = 8.0, 7.0, 1.3 Hz, H-6), 7.10 (ddd, 1H, J = 8.0, 7.0, 1.2 Hz, H-5) ppm. 13C-NMR (100 MHz,
DMSO-d6):
δ
= 138.4 (C-8), 134.9 (C-7a), 130.5 (C-2), 126.2 (C-3a), 121.9 (C-6), 119.9 (C-5), 118.2 (C-4),
1
111.8 (C-7), 106.3 (C-3) ppm. JC8H8 = 169 Hz. Rf value (hexane:acetone 2:1): 0.19.
3.6.4. Syn and anti-1-methylindole-3-carboxyaldehyde Oxime (3a)
Melting point: 134–136 ◦C (dichloromethane/hexane). White crystals.
1
Spectral data for anti-3a: H-NMR (400 MHz, DMSO-d6):
δ = 10.50 (s, 1H, H-10), 8.21 (s, 1H, H-8),
7.96 (d, 1H, J = 7.9 Hz, H-4), 7.57 (s, 1H, H-2), 7.43 (d, 1H, J = 8.2 Hz, H-7), 7.21 (ddd, 1H, J = 8.2, 7.0,
1.2 Hz, H-6), 7.13 (m, 1H, H-5) ppm. 13C-NMR (100 MHz, DMSO-d6):
δ
= 144.0 (C-8), 137.2 (C-7a),
131.9 (C-2), 124.5 (C-3a), 121.8 (C-6), 121.4 (C-4), 120.1 (C-5), 109.9 (C-7), 108.5 (C-3), 32.5 (Me) ppm. Rf
value (hexane:ethylacetate 2:1): 0.42.
1
Spectral data for syn-3a: H-NMR (400 MHz, DMSO-d6):
δ = 11.22 (s, 1H, H-10), 8.22 (s, 1H, H-2),
7.83 (d, 1H, J = 7.8 Hz, H-4), 7.73 (s, 1H, H-8), 7.45 (d, 1H, J = 8.2 Hz, H-7), 7.21 (ddd, 1H, J = 8.2, 7.0,
1.2 Hz, H-6), 7.13 (m, 1H, H-5) ppm. 13C-NMR (100 MHz, DMSO-d6):
δ
= 138.5 (C-8), 135.3 (C-7a),
134.2 (C-2), 126.5 (C-3a), 122.2 (C-6), 119.9 (C-5), 118.2 (C-4), 109.9 (C-7), 105.6 (C-3), 32.6 (Me) ppm. Rf
value (hexane: ethylacetate 2:1): 0.31.
3.6.5. Anti and syn-1-acetylindole-3-carboxyaldehyde Oxime (4a)
White powder (in mixture with syn-2a).
1
Spectral data for anti-4a: H-NMR (400 MHz, DMSO-d6):
δ = 11.98 (s, 1H, H-10), 8.58 (s, 1H, H-2),
8.36 (d, 1H, J = 8.2 Hz, H-7), 7.97 (d, 1H, J = 7.8 Hz, H-4), 7.92 (s, 1H, H-8), 7.37 (m, 2H, H-5, H-6) ppm.
13C-NMR (100 MHz, DMSO-d6):
125.3 (C-6), 123.8 (C-5), 119.0 (C-4), 115.9 (C-7), 110.9 (C-3), 23.9 (Me) ppm. JC8H8 = 163.2 Hz. Rf value
δ
= 169.5 (C-11), 136.9 (C-8), 134.0 (C-7a), 130.4 (C-2), 128.5 (C-3a),
1
(hexane: ethylacetate 2:1): 0.33.
1
Spectral data for syn-4a: H-NMR (400 MHz, DMSO-d6):
δ = 11.15 (s, 1H, H-10), 8.36 (d, 1H, J =
8.2 Hz, H-7), 8.29 (s, 1H, H-8), 8.16 (s, 1H, H-2), 8.08 (d, 1H, J = 7.4 Hz, H-4), 7.37 (m, 2H, H-5, H-6)
ppm. 13C-NMR (100 MHz, DMSO-d6):
(C-3a), 125.6 (C-6), 124.0 (C-5), 122.1 (C-4), 115.9 (C-7), 115.3 (C-3), 23.8 (Me) ppm. JC8H8 = 174.4 Hz.
Rf value (hexane: ethylacetate 2:1): 0.28.
δ
= 169.9 (C-11), 143.3 (C-8), 135.7 (C-7a), 129.1 (C-2), 126.8
1
4. Conclusions
The successful mechanochemical conversion of four N-substituted indole-3-carboxaldehydes into
the corresponding oximes was achieved in a safe solvent-free mode, although the 1-acetyl-analog was
partly deacetylated during the reaction. This approach offers safe handling of dangerous hydroxylamine
hydrochloride. Under optimal conditions, the product containing nearly pure syn isomer can be
synthesized for the 1-methoxy analog after the solvent-free milling of the reagents for 20 min in a
tungsten carbide milling vessel at 500 rpm. When the residual hydroxylamine hydrochloride is not
immediately washed out after the mechanochemical process (the acidic conditions are maintained),

Molecules 2019, 24, 3347
15 of 19
the anti oxime is nearly completely transformed into the syn isomer within three days. The same
transformation was also observed for 1-methyl analog. However, if the conditions are neutral (balanced
stoichiometry between hydroxylamine and base), this transformation is not observed. The main
force behind the isomerization is the acidity of the reaction mixture, however, mechanochemistry
enhances this process. Under acidic conditions, the isomerization favors the opposite isomers for
the milled mixture and in solution. Whereas in the first case syn isomer is preferred, in solution an
equilibrium favoring the anti one is reached. The study of such isomerization is novel and represents
another stimulus for the organic mechanochemists. This study shows that mechanochemical syntheses
can be applied for the realization of an environmentally friendly syntheses of diastereoisomerically
pure compounds, which will facilitate the production of synthons for the development of new
pharmaceuticals. The environmental and personal risk is much lower in comparison with the
solvent-based synthesis methods.
Supplementary Materials: The following are available online. Supporting information for this paper contain:
NMR spectra reporting on the conversion of reaction and isomerization (both in neutral and acidic conditions);
information about pressure and temperature of gas in the milling chamber during the process; photographs of the
TLC plates of the isomerization mixture; changes in ratios of 1-methoxyindole-3-carboxaldehyde oxime isomers
calculated from the intensity of TLC spots with time for selected experiment; and characterization results of pure
1-methoxyindole-3-carboxaldehyde oxime isomers, namely 1H, 13C and 15N NMR spectra, XRD patterns, FTIR
spectra, melting temperatures description, optical microscopy analysis, and transmission electron microscopy
analysis. Also few appropriate references are provided [23,55,56,60,78–80].
Author Contributions: Conceptualization, M.B. and Z.K.; Investigation, M.B., Z.K., M.V., J.I., L’.B., and N.D.;
Methodology, M.B., Z.K., M.V., J.I., L’.B., and N.D.; Supervision, M.B.; Writing—original draft, M.B., Z.K., M.V.;
Writing—review and editing, M.B., Z.K., M.V., J.I., L’.B., and N.D., M.B. and Z.K. prepared samples and developed
the idea and concept of the paper. M.V. performed NMR measurements. L’.B. performed TLC spot analysis using
ImageJ software and optical microscopy measurements. N.D. performed TEM measurements. All authors read
and improved the final manuscript.
Funding: This research was funded by Slovak Agency for Science and Development [project no. APVV-18-0357]
and by the Slovak Grant Agency VEGA [project no. 2/0044/18].
Acknowledgments: The authors would like to express their gratitude to Richard G. Blair from University of
Central Florida and Ross F. Koby from Vanderbilt University (both USA) for the improvement of the English level
of the paper. The authors also acknowledge the performance of XRD measurements by Anna Zorkovsk
á from
Institute of Geotechnics, Slovak Academy of Sciences, Košice, Slovakia.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
References
1.
2.
3.
4.
Frutos, R.P.; Spero, D.M. Synthesis of protected, chiral alpha,alpha-disubstituted alpha-amino acids via a
Beckmann rearrangement. Tetrahedron Lett. 1998, 39, 2475–2478. [CrossRef]
Chandrasekhar, S.; Gopalaiah, K. Beckmann rearrangement in the solid state: Reaction of oxime
hydrochlorides. Tetrahedron Lett. 2001, 42, 8123–8125. [CrossRef]
Mendelsohn, B.A.; Lee, S.; Kim, S.; Teyssier, F.; Aulakh, V.S.; Ciufolini, M.A. Oxidation of Oximes to Nitrile
Oxides with Hypervalent Iodine Reagents. Org. Lett. 2009, 11, 1539–1542. [CrossRef]
Dave, P.R.; Forohar, F.; Axenrod, T.; Das, K.K.; Qi, L.; Watnick, C.; Yazdekhasti, H. Facile
preparation of 3,7-diazabicyclo [3.3.0]octane and 3,7,10-triheterocyclic [3.3.3]propellane ring systems from
1,5-diazacyclooctane 3,7-derivatives. J. Org. Chem. 1996, 61, 8897–8903. [CrossRef]
5.
Negi, S.;
Matsukura, M.;
Mizuno, M.;
Miyake, K.;
Minami, N. Synthesis of
(2R)-1-(4-chloro-2-pyridyl)-2-(2-pyridyl)ethylamine: A selective oxime reduction and crystallization-induced
asymmetric transformation. Synthesis 1996, 1996, 991–996. [CrossRef]
6.
7.
Iwata, Y.; Koseki, H.; Hosoya, F. Study on decomposition of hydroxylamine/water solution. J. Loss Prev.
Process Ind. 2003, 16, 41–53. [CrossRef]
Raja, R.; Sankar, G.; Thomas, J.M. Bifunctional molecular sieve catalysts for the benign ammoximation of
cyclohexanone: One-step, solvent-free production of oxime and epsilon-caprolactam with a mixture of air
and ammonia. J. Am. Chem. Soc. 2001, 123, 8153–8154. [CrossRef]

Molecules 2019, 24, 3347
16 of 19
8.
9.
Raja, R.; Thomas, J.M. Engineering active sites in bifunctional nanopore and bimetallic nanoparticle catalysts
for one-step, solvent-free processes. In Nanoporous Materials III; Elsevier: Amsterdam, The Netherlands, 2002;
Volume 141, pp. 317–328.
Bigdeli, M.A.; Nikje, M.M.A.; Jafari, S.; Heravi, M.M. Regioselective synthesis of syn-oximes using 3 angstrom
molecular sieves in a solventless system. J. Chem. Res.-S 2002, 20–21.
10. Mohammed, A.H.A.; Nagendrappa, G. A remarkably simple alpha-oximation of ketones to 1,2-dione
monooximes using the chlorotrimethylsilane-isoamyl nitrite combination. Tetrahedron Lett. 2003, 44,
2753–2755. [CrossRef]
11. Kiasat, A.R.; Kazemi, F.; Nourbakhsh, K. A clean conversion of carbonyl compounds to oximes using silica
gel supported hydroxylamine hydrochloride. Phosphorus Sulfur Silicon Relat. Elem. 2004, 179, 1193–1196.
[CrossRef]
12. Kiasat, A.R.; Kazemi, F.; Nourbakhsh, K. A convenient one-pot method of converting alcohols into oximes.
Phosphorus Sulfur Silicon Relat. Elem. 2004, 179, 1809–1812. [CrossRef]
13. Serna, P.; Lopez-Haro, M.; Calvino, J.J.; Corma, A. Selective hydrogenation of nitrocyclohexane to
cyclohexanone oxime with H-2 on decorated Pt nanoparticles. J. Catal. 2009, 263, 328–334. [CrossRef]
14. Fazaeli, R.; Aliyan, H. AlPW12O40 and AlPMo12O40 as Highly Effective and Eco-friendly Catalysts for
Aldoximation of Aldehydes Under Solvent-Free Conditions. Asian J. Chem. 2010, 22, 855–858.
15. Aghapour, G.; Mohamadian, S. Selective Tandem Synthesis of Oximes from Benzylic Alcohols Catalyzed
with 2, 3-Dichloro-5, 6-dicyanobenzoquinone. Bull. Korean Chem. Soc. 2012, 33, 1209–1212. [CrossRef]
16. Kong, D.L.; Liu, R.D.; Li, G.Z.; Zhang, P.W.; Wu, M.S. A Rapid, Convenient, Solventless Green Approach for
the Synthesis of alpha-Hydroxyphosphonates by Grinding. Asian J. Chem. 2014, 26, 1246–1248. [CrossRef]
17. Yadav, P.; Lal, K.; Rani, P.; Mor, S.; Kumar, A.; Kumar, A. Efficient synthesis and antimicrobial evaluation of
2-((1-substituted-1H-1,2,3-triazol-4-yl)-1-naphthaldehydes and their oxime derivatives. Med. Chem. Res.
2017, 26, 1469–1480. [CrossRef]
18. Alam, M.; Lee, D.U. Green synthesis, biochemical and quantum chemical studies of steroidal oximes. Korean
J. Chem. Eng. 2015, 32, 1142–1150. [CrossRef]
19. Hoelz, L.V.B.; Goncalves, B.T.; Barros, J.C.; da Silva, J.F.M. Solvent Free, Microwave Assisted Conversion of
Aldehydes into Nitriles and Oximes in the Presence of NH2OH center dot HCl and TiO2. Molecules 2010, 15,
94–99. [CrossRef]
20. Hong, Z.; Li, J.J.; Chen, G.; Jiang, H.J.; Yang, X.F.; Pan, H.; Su, W.K. Solvent-free mechanochemical synthesis of
arylcyanomethylenequinone oximes from phenylacetonitriles and 4-unsubstituted nitroaromatic compounds
using KF/nano-gamma-Al2O3 as catalyst. RSC Adv. 2016, 6, 13581–13588. [CrossRef]
21. Primozic, I.; Hrenar, T.; Baumann, K.; Kristo, L.; Krizic, I.; Tomic, S. Mechanochemical and Conformational
Study of N-heterocyclic Carbonyl-Oxime Transformations. Croat. Chem. Acta 2014, 87, 153–160. [CrossRef]
22. Mokhtari, J.; Naimi-Jamal, M.R.; Hamzeali, H.; Dekamin, M.G.; Kaupp, G. Kneading Ball-Milling and
Stoichiometric Melts for the Quantitative Derivatization of Carbonyl Compounds with Gas-Solid Recovery.
Chemsuschem 2009, 2, 248–254. [CrossRef]
23. Aakeroy, C.B.; Sinha, A.S.; Epa, K.N.; Spartz, C.L.; Desper, J. A versatile and green mechanochemical route
for aldehyde-oxime conversions. Chem. Commun. 2012, 48, 11289–11291. [CrossRef]
24. Hernandez, J.G.; Bolm, C. Altering Product Selectivity by Mechanochemistry. J. Org. Chem. 2017, 82,
4007–4019. [CrossRef]
25. James, S.L.; Adams, C.J.; Bolm, C.; Braga, D.; Collier, P.; Frišcˇic´, T.; Grepioni, F.; Harris, K.D.M.; Hyett, G.;
Jones, W.; et al. Mechanochemistry: Opportunities for new and cleaner synthesis. Chem. Soc. Rev. 2012, 41,
413–447. [CrossRef]
26. Baláž, P.; Achimovicˇová, M.; Baláž, M.; Billik, P.; Cherkezova-Zheleva, Z.; Criado, J.M.; Delogu, F.; Dutková, E.;
Gaffet, E.; Gotor, F.J.; et al. Hallmarks of mechanochemistry: From nanoparticles to technology. Chem. Soc.
Rev. 2013, 42, 7571–7637. [CrossRef]
27. Jones, W.; Eddleston, M.D. Introductory lecture: Mechanochemistry, a versatile synthesis strategy for new
materials. Faraday Discuss. 2014, 170, 9–34. [CrossRef]
28. Rightmire, N.R.; Hanusa, T.P. Advances in organometallic synthesis with mechanochemical methods. Dalton
Trans. 2016, 45, 2352–2362. [CrossRef]
29. Moores, A. Bottom up, solid-phase syntheses of inorganic nanomaterials by mechanochemistry and aging.
Curr. Opin. Green Sust. Chem. 2018, 12, 33–37. [CrossRef]

Molecules 2019, 24, 3347
17 of 19
30. Howard, J.L.; Cao, Q.; Browne, D.L. Mechanochemistry as an emerging tool for molecular synthesis: What
can it offer? Chem. Sci. 2018, 9, 3080–3094. [CrossRef]
31. Tan, D.; Friscic, T. Mechanochemistry for Organic Chemists: An Update. Eur. J. Org. Chem. 2018, 18–33.
[CrossRef]
32. Avila-Ortiz, C.G.; Péréz-Venegas, M.; Vargas-Carporali, J.; Juaristi, E. Recent applications of mechanochemistry
in enantioselective synthesis. Tetrahedron Lett. 2019, 60, 1749–1757. [CrossRef]
33. Bolm, C.; Hernandez, J.G. Mechanochemistry of Gaseous Reactants. Angew. Chem. Int. Ed. 2019, 58,
3285–3299. [CrossRef]
34. Qu, J.; Sha, L.; Wu, C.J.; Zhang, Q.W. Applications of Mechanochemically Prepared Layered Double
Hydroxides as Adsorbents and Catalysts: A Mini-Review. Nanomaterials 2019, 9, 80. [CrossRef]
35. Tan, D.; García, F. Main group mechanochemistry: From curiosity to established protocols. Chem. Soc. Rev.
2019, 48, 2274–2292. [CrossRef]
36. Hernandez, J.G.; Juaristi, E. Recent efforts directed to the development of more sustainable asymmetric
organocatalysis. Chem. Commun. 2012, 48, 5396–5409. [CrossRef]
37. Wang, G.W. Mechanochemical organic synthesis. Chem. Soc. Rev. 2013, 42, 7668–7700. [CrossRef]
38. Do, J.L.; Frišcˇic´, T. Mechanochemistry: A force of synthesis. ACS Central Sci. 2017, 3, 13–19. [CrossRef]
39. Margetic, D.; Strukil, V. Mechanochemical Organic Synthesis; Elsevier: Amsterdam, The Netherlands, 2016;
p. 386.
40. Damljanovic, I.; Vukicevic, M.; Vukicevic, R.D. A simple synthesis of oximes. Monatsh. Chem. 2006, 137,
301–305. [CrossRef]
41. Saikia, L.; Baruah, J.M.; Thakur, A.J. A rapid, convenient, solventless green approach for the synthesis of
oximes using grindstone chemistry. Org. Med. Chem. Lett. 2011, 1, 12. [CrossRef]
42. Somei, M.; Ohnishi, H.; Shoken, Y. The Chemistry of Indoles. XXVII.: A Practical Synthesis of the 1-Methoxy
Analog of an Ergot Alkaloid, (
[CrossRef]
±)-1-Methoxy-6, 7-secoagroclavine. Chem. Pharm. Bull. 1986, 34, 677–681.
43. Somei, M.; Kawasaki, T. A New and Simple Synthesis of 1-Hydroxyindole Derivatives. Heterocycles 1989, 29,
1251–1254. [CrossRef]
44. Somei, M. 1-Hydroxyindoles. Heterocycles 1999, 50, 1157–1211. [CrossRef]
45. Pedras, M.S.C.; Yaya, E.E.; Glawischnig, E. The phytoalexins from cultivated and wild crucifers: Chemistry
and biology. Nat. Prod. Rep. 2011, 28, 1381–1405. [CrossRef] [PubMed]
46. Chripkova, M.; Zigo, F.; Mojzis, J. Antiproliferative Effect of Indole Phytoalexins. Molecules 2016, 21, 1626.
[CrossRef] [PubMed]
47. Pedras, M.S.C.; Sarwar, M.G.; Suchy, M.; Adio, A.M. The phytoalexins from cauliflower, caulilexins A, B and
C: Isolation, structure determination, syntheses and antifungal activity. Phytochemistry 2006, 67, 1503–1509.
[CrossRef] [PubMed]
48. Pedras, M.S.C.; Zaharia, I.L. Sinalbins A and B, phytoalexins from Sinapis alba: Elicitation, isolation, and
synthesis. Phytochemistry 2000, 55, 213–216. [CrossRef]
49. Chakrabarty, M.; Sarkar, S.; Khasnobis, S.; Harigaya, Y.; Sato, N.; Arima, S. Study of the reactions of four
indolic 1-azadienes with a few enoic, ynoic, and azo dienophiles. Synth. Commun. 2002, 32, 2295–2306.
[CrossRef]
50. Ramon, R.S.; Bosson, J.; Diez-Gonzalez, S.; Marion, N.; Nolan, S.P. Au/Ag-Cocatalyzed Aldoximes to Amides
Rearrangement under Solvent- and Acid-Free Conditions. J. Org. Chem. 2010, 75, 1197–1202. [CrossRef]
51. Goyard, D.; Konya, B.; Chajistamatiou, A.S.; Chrysina, E.D.; Leroy, J.; Balzarin, S.; Tournier, M.; Tousch, D.;
Petit, P.; Duret, C.; et al. Glucose-derived spiro-isoxazolines are anti-hyperglycemic agents against type 2
diabetes through glycogen phosphorylase inhibition. Eur. J. Med. Chem. 2016, 108, 444–454. [CrossRef]
52. Zhong, X.; Chen, N.; Xu, J.X. A concise synthesis of cyclobrassinin and its analogues via a thiyl radical
aromatic substitution. New J. Chem. 2018, 42, 13549–13557. [CrossRef]
53. Budovska, M.; Pilatova, M.B.; Tischlerova, V.; Mojzis, J. Spirocyclization reactions and antiproliferative
activity of indole phytoalexins 1-methoxybrassinin and its 1-substituted derivatives. Arkivoc 2016, 198–234.
[CrossRef]
54. Pedras, M.S.C.; Okinyo-Owiti, D.P.; Thoms, K.; Adio, A.M. The biosynthetic pathway of crucifer phytoalexins
and phytoanticipins: De novo incorporation of deuterated tryptophans and quasi-natural compounds.
Phytochemistry 2009, 70, 1129–1138. [CrossRef] [PubMed]

Molecules 2019, 24, 3347
18 of 19
55. Hanley, A.B.; Parsley, K.R.; Lewis, J.A.; Fenwick, G.R. Chemistry of Indole Glucosinolates—Intermediacy of
Indol-3-Ylmethyl Isothiocyanates in the Enzymatic-Hydrolysis of Indole Glucosinolates. J. Chem. Soc. Perkin.
Trans. 1 1990, 1, 2273–2276. [CrossRef]
56. Kawasaki, T.; Somei, M. The first total syntheses of 9-methoxycarbazole-3-carboxaldehyde and
methoxybrassinin. Heterocycles 1990, 31, 1605–1607.
57. Pedras, M.S.C.; Okinyo, D.P.O. Syntheses of perdeuterated indoles and derivatives as probes for the
biosyntheses of crucifer phytoalexins. J. Labelled Compd. Radiopharm. 2006, 49, 33–45. [CrossRef]
58. Kutschy, P.; Salayova, A.; Curillova, Z.; Kozar, T.; Mezencev, R.; Mojzis, J.; Pilatova, M.; Balentova, E.;
Pazdera, P.; Sabol, M.; et al. 2-(Substituted phenyl)amino analogs of 1-methoxyspirobrassinol methyl ether:
Synthesis and anticancer activity. Bioorg. Med. Chem. 2009, 17, 3698–3712. [CrossRef] [PubMed]
59. Budovska, M. A novel palladium-catalyzed cyclization of indole phytoalexin brassinin and its 1-substituted
derivatives. RSC Adv. 2014, 4, 5575–5582. [CrossRef]
60. Sundberg, R.J. The Chemistry of Indoles in Organic Chemistry; Academic Press: New York, NY, USA, 1970;
p. 405.
61. Pankrushina, N.; Nikitina, I.; Chernjak, E.; Myz, C.; Shakhtshneider, T.; Boldyrev, V. Solvent-free
mechanochemical modification of lappaconitine and piroxicam. Mater. Manuf. Processes 2008, 23, 561–565.
[CrossRef]
62. Fulmer, D.A.; Shearouse, W.C.; Medonza, S.T.; Mack, J. Solvent-free Sonogashira coupling reaction via high
speed ball milling. Green Chem. 2009, 11, 1821–1825. [CrossRef]
63. Cook, T.L.; Walker, J.A.; Mack, J. Scratching the catalytic surface of mechanochemistry: A multi-component
CuAAC reaction using a copper reaction vial. Green Chem. 2013, 15, 617–619. [CrossRef]
64. Stefanic, G.; Krehula, S.; Stefanic, I. The high impact of a milling atmosphere on steel contamination. Chem.
Commun. 2013, 49, 9245–9247. [CrossRef] [PubMed]
65. Miklos, F.; Hum, V.; Fulop, F. Eco-friendly syntheses of 2,2-disubstituted- and 2-spiroquinazolinones. Arkivoc
2014, 2014, 25–37.
66. Do, J.L.; Mottillo, C.; Tan, D.; Strukil, V.; Friscic, T. Mechanochemical Ruthenium-Catalyzed Olefin Metathesis.
J. Am. Chem. Soc. 2015, 137, 2476–2479. [CrossRef] [PubMed]
67. Metro, T.X.; Bonnamour, J.; Reidon, T.; Duprez, A.; Sarpoulet, J.; Martinez, J.; Lamaty, F. Comprehensive
Study of the Organic-Solvent-Free CDI-Mediated Acylation of Various Nucleophiles by Mechanochemistry.
Chem. Eur. J. 2015, 21, 12787–12796. [CrossRef] [PubMed]
68. Friscic, T.; Reid, D.G.; Halasz, I.; Stein, R.S.; Dinnebier, R.E.; Duer, M.J. Ion- and Liquid-Assisted Grinding:
Improved Mechanochemical Synthesis of Metal-Organic Frameworks Reveals Salt Inclusion and Anion
Templating. Angew. Chem. Int. Ed. 2010, 49, 712–715. [CrossRef]
69. Jiang, Z.J.; Li, Z.H.; Yu, J.B.; Su, W.K. Liquid-Assisted Grinding Accelerating: Suzuki-Miyaura Reaction of
Aryl Chlorides under High-Speed Ball-Milling Conditions. J. Org. Chem. 2016, 81, 10049–10055. [CrossRef]
[PubMed]
70. Howard, J.L.; Sagatov, Y.; Repusseau, L.; Schotten, C.; Browne, D.L. Controlling reactivity through liquid
assisted grinding: The curious case of mechanochemical fluorination. Green Chem. 2017, 19, 2798–2802.
[CrossRef]
71. Nsikabaka, S.; Harb, W.; Ruiz-Lopez, M.F. The role of water on the acid-promoted E/Z isomerization of
oximes in aqueous solution. J. Mol. Struct. THEOCHEM 2006, 764, 161–166. [CrossRef]
72. Vasil’tsov, A.M.; Zhang, K.; Ivanov, A.V.; Ushakov, I.A.; Afonin, A.V.; Petrushenko, K.B.; Li, S.Y.; Ma, J.S.;
Mikhaleva, A.I.; Trofimov, B.A.; et al. 1-Vinylpyrrole-2-carbaldehyde oximes: Synthesis, isomerization, and
spectral properties. Mon. Chem. 2009, 140, 1475–1480. [CrossRef]
73. O’Ferrall, R.A.M.; O’Brien, D. Rate and equilibrium constants for hydrolysis and isomerization of (E)- and
(Z)-p-methoxybenzaldehyde oximes. J. Phys. Org. Chem. 2004, 17, 631–640. [CrossRef]
74. Liu, J.X.; Ma, S.M. Aerobic oxidation of indole carbinols using Fe(NO3)3.9H2O/TEMPO/NaCl as catalysts.
Org. Biomol. Chem. 2013, 11, 4186–4193. [CrossRef]
75. Fei, H.Y.; Yu, J.T.; Jiang, Y.; Guo, H.; Cheng, J. The ammonium-promoted formylation of indoles by DMSO
and H2O. Org. Biomol. Chem. 2013, 11, 7092–7095. [CrossRef] [PubMed]
76. Selvakumar, N.; Rajulu, G.G. Efficient total syntheses of phytoalexin and (+/ )-paniculidine B and C based
−
on the novel methodology for the preparation of 1-methoxyindoles. J. Org. Chem. 2004, 69, 4429–4432.
[CrossRef] [PubMed]

Molecules 2019, 24, 3347
19 of 19
77. Suzdalev, K.F.; Babakova, M.N. Synthesis of analogues of indole alkaloids from sea sponges—aplysinopsins
by the reaction of amines with (4Z)-4-[(1H-indol-3-yl)-methylene]-1,3-oxazol-5(4H)-ones. J. Heterocycl. Chem.
2016, 53, 1200–1206. [CrossRef]
78. Contreras, R.H.; Peralta, J.E. Angular dependence of spin-spin coupling constants. Prog. Nucl. Magn. Reson.
Spectrosc. 2000, 37, 321–425. [CrossRef]
79. Afonin, A.V.; Ushakov, I.A.; Vashchenko, A.V.; Simonenko, D.E.; Ivanov, A.V.; Vasil’tsov, A.M.; Mikhaleva, A.I.;
Trofimov, B.A. C–H...N and C–H...O intramolecular hydrogen bonding effects in the 1H, 13C and 15N NMR
spectra of the configurational isomers of 1-vinylpyrrole-2-carbaldehyde oxime substantiated by DFT
calculations. Magn. Reson. Chem. 2009, 47, 105–112. [CrossRef] [PubMed]
80. Palm, A.; Werbin, H. The infrared spectra of alpha and beta oximes. Can. J. Chem. 1953, 31, 1004–1008.
[CrossRef]
Sample Availability: Samples of the final compounds are available from the authors.
©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).