Reaction Products of β-Aminopropioamidoximes Nitrobenzenesulfochlorination: Linear and Rearranged to Spiropyrazolinium Salts with Antidiabetic Activity

Nitrobenzenesulfochlorination of β-aminopropioamidoximes leads to a set of products depending on the structure of the initial interacting substances and reaction conditions. Amidoximes, functionalized at the terminal C atom with six-membered N-heterocycles (piperidine, morpholine, thiomorpholine and phenylpiperazine), as a result of the spontaneous intramolecular heterocyclization of the intermediate reaction product of an SN2 substitution of a hydrogen atom in the oxime group of the amidoxime fragment by a nitrobenzenesulfonyl group, produce spiropyrazolinium ortho- or para-nitrobenzenesulfonates. An exception is ortho-nitrobenzenesulfochlorination of β-(thiomorpholin-1-yl)propioamidoxime, which is regioselective at room temperature, producing two spiropyrazolinium salts (ortho-nitrobezenesulfonate and chloride), and regiospecific at the boiling point of the solvent, when only chloride is formed. The para-Nitrobezenesulfochlorination of β-(benzimidazol-1-yl)propioamidoxime, due to the reduced nucleophilicity of the aromatic β-amine nitrogen atom, is regiospecific at both temperatures, and produces the O-para-nitrobenzenesulfochlorination product. The antidiabetic screening of the new nitrobezenesulfochlorination amidoximes found promising samples with in vitro α-glucosidase activity higher than the reference drug acarbose. 1H-NMR spectroscopy and X-ray analysis revealed the slow inversion of six-membered heterocycles, and experimentally confirmed the presence of an unfavorable stereoisomer with an axial N–N bond in the pyrazolinium heterocycle.


Introduction
Heterocycles with potential bioactive properties are of great interest, first of all, for medical chemists working in the field of heterocyclic compounds synthesis. Among the new drugs approved by the FDA in 2021, almost 50% are substances with nitrogen-containing heterocycles [1]. Pyrazoline derivatives, as prominent representatives of nitrogen-containing heterocycles, became the subject of a report on the world market of diphenylpyrazolines from Market Strides (global aggregator and publisher of market intelligence research Two comprehensive reviews on functionalized spiropyrazolines were published in 2013 and 2019 [10,11]. The most common methods for the construction of the 1,2-and 2,3-spiro-1-pyrazolines involve the formation of a new ring on an existing carbo-or heterocycle, having exocyclic C=C double bonds. The essential steps in the formation of spiropyrazoline systems in these cases are 1,3-cycloaddition reactions of nitrogen-containing molecules (nitrilimines [12][13][14][15] and diazoalkanes [16,17]) to double bonds, or a condensation reaction of substituted chalcones with hydrazine or its derivatives in an acidic or alkaline medium [18,19].
The most common methods for the construction of the 1,2-and 2,3-spiro-1-pyrazolines involve the formation of a new ring on an existing carbo-or heterocycle, having exocyclic C=C double bonds. The essential steps in the formation of spiropyrazoline systems in these cases are 1,3-cycloaddition reactions of nitrogen-containing molecules (nitrilimines [12][13][14][15] and diazoalkanes [16,17]) to double bonds, or a condensation reaction of substituted chalcones with hydrazine or its derivatives in an acidic or alkaline medium [18,19].

Synthesis and Spectra
The interaction of β-aminopropioamidoximes (1-5) with ortho-, para-nitrobenzenesulfochlorides in СHCl3 was carried out at room temperature and by heating the reaction mixture to the solvent boiling point. A change in the electronic properties of the sulfochlorinating agent, the transition from tosyl chloride to ortho-, para-nitrobenzenesulfochlores lead to an increase in the reaction time at room temperature from 15-20 h in the case of tosylation [4] to 38-120 h for para-benzenesulfochlorination, and up to 25-104 h for orthobenzenesulfochlorination.

Synthesis and Spectra
The interaction of β-aminopropioamidoximes (1-5) with ortho-, para-nitrobenzenesulfochlorides in CHCl 3 was carried out at room temperature and by heating the reaction mixture to the solvent boiling point. A change in the electronic properties of the sulfochlorinating agent, the transition from tosyl chloride to ortho-, para-nitrobenzenesulfochlores lead to an increase in the reaction time at room temperature from 15-20 h in the case of tosylation [4] to 38-120 h for para-benzenesulfochlorination, and up to 25-104 h for ortho-benzenesulfochlorination.

Scheme 6. Nitrobenzenesulfochlorination of β-aminopropioamidoximes.
It is assumed that, in the case of the nitrobenzenesulfochlorination of β-aminopropioamidoximes 1-4, the O-nitrobenzenesulphonates of β-propioamidoximes formed as intermediate B, due to the thermodynamic advantage, rearranging into spiropyrazoline nitrobenzenesulphonates (6-12, 13a, 14). Only in the case of the benzimidazole derivative, the intermediate B remains stable and produces O-para-nitrobenzenesulphonate 10.
The stoichiometry of the reaction does not provide for the formation of a hydrate. We believe that the preparation of hydrate 13b can be explained by the prolonged contact of the mother liquor during the preparation of the single crystals of the ortho-nitrophenylsulfochlorination product of amidoxime 3 with atmospheric moisture.

Scheme 6. Nitrobenzenesulfochlorination of β-aminopropioamidoximes.
It is assumed that, in the case of the nitrobenzenesulfochlorination of β-aminopropioamidoximes 1-4, the O-nitrobenzenesulphonates of β-propioamidoximes formed as intermediate B, due to the thermodynamic advantage, rearranging into spiropyrazoline nitrobenzenesulphonates (6-12, 13a, 14). Only in the case of the benzimidazole derivative, the intermediate B remains stable and produces O-para-nitrobenzenesulphonate 10.

Scheme 6. Nitrobenzenesulfochlorination of β-aminopropioamidoximes.
It is assumed that, in the case of the nitrobenzenesulfochlorination of β-aminopropioamidoximes 1-4, the O-nitrobenzenesulphonates of β-propioamidoximes formed as intermediate B, due to the thermodynamic advantage, rearranging into spiropyrazoline nitrobenzenesulphonates (6-12, 13a, 14). Only in the case of the benzimidazole derivative, the intermediate B remains stable and produces O-para-nitrobenzenesulphonate 10.
The stoichiometry of the reaction does not provide for the formation of a hydrate. We believe that the preparation of hydrate 13b can be explained by the prolonged contact of the mother liquor during the preparation of the single crystals of the ortho-nitrophenylsulfochlorination product of amidoxime 3 with atmospheric moisture. Salts of spiropyrazolinium compounds 13a and 13b isolated at the orthonitrobenzenesufochlorination of β-(thiomorpholine-1-yl)propioamidoxime.
The stoichiometry of the reaction does not provide for the formation of a hydrate. We believe that the preparation of hydrate 13b can be explained by the prolonged contact of the mother liquor during the preparation of the single crystals of the ortho-nitrophenylsulfochlorination product of amidoxime 3 with atmospheric moisture.
When establishing the structure of nitrobenzenesulfochlorination products, a difference was noted in the values of the mobility index R f for the products produced by the nitrobenzenesulfochlorination of amidoximes with six-membered nitrogenous heterocycles in the β-position (6-9, 11-14) and for the para-nitrobenzenesulfoderivative of β-(benzimidazol-1-yl)propioamidoxime (10) (R f 0.01-0.12 and 0.75). Compounds 7 and 13b were previously described in [7] and [26,27], respectively (Table 1). In the IR spectra of compounds 6-12, 13a, and 14, there are two pairs of bands related to the characteristic stretching vibrations of a strong intensity of the NO 2 and SO 2 groups at 1515-1549 (as) cm −1 and 1349-1377 (sy) cm −1 , and 1207-1240 (as) cm −1 and 1024-1197 (sy) cm −1 , respectively, whereas, in the IR spectrum of compound 13b, there are no bands of stretching vibrations for the bonds of the NO 2 and SO 2 groups.
It is interesting that, in the 1 H-NMR spectra of compounds 6-9, 13a, and 13b, it was possible to fix the diastereotopic nature of the geminal protons of the methylene groups located at the ammonium nitrogen atom, which produce pairs of multiplet signals with an intensity of 2 protons at δ: 3.  (14). Evidently, the effect of the slow rotation of β-heterocycles with the possibility of fixing the equatorial and axial protons is observed here. In addition, the diastereotopicity of these protons may be associated with the presence of an asymmetry axis inherent in the spiro compounds.
The signals of Csp 3 and Csp 2 carbon atoms in the 13 C-NMR spectra of compounds 6-14 are present in the characteristic regions.
We obtained a quantum-chemical confirmation of the advantageousness of the formation of spirocyclic tosylation and para-nitrobenzenesulfochlorination products of βaminopropioamidoximes (1-4), with negative values of the Gibbs energy of the chemical reaction in the range of −119.99-−163.57 kJ/mol and the disadvantage of the formation of a spirostructure for β-(benzimidazole-1-yl)propioamidoxime (5), with positive values of the Gibbs energy for the tosylation and para-nitrobenzenesulfochlorination products as 45.87 and 20.02 kJ/mol, respectively [28].
It should be noted that we attempted to react β-(benzimidazol-1-yl)propioamidoxime (5) with ortho-nitrobenzenesulfonate several times, under the described conditions (CHCl 3 , DIPEA, r.t. and 70 • C), but, each time, a resinous reaction mixture was obtained. functions (inhibition) is critical to the treatment regimen. This implies that new drug candidates with a potent inhibition of the α-glucosidase and α-amylase would be valuable to drug discovery and development for diabetes mellitus [30].

The In Vitro
The in vitro antidiabetic activity of spiropyrazolilammonium ortho-, para-nitrobenzenesulfonates and chloride 6-9, 11-14, and the product of O-para-nitrobenzenesulfochlorination of β-(benzimidazol-1-yl)propioamidoxime (10) was assessed via the degree of inhibition of the activity of α-amylase and α-glucosidase by the studied substances, compared to the standard drug acarbose ( Table 2). The table shows four series of experiments with different experimental values of the in vitro activity of acarbose in relation to α-amylase and α-glucosidase. All the tested compounds had an average inhibitory activity against α-amylase, the values of which are less than the activity of acarbose. In regard to α-glucosidase, the products of para-nitrobenzenesulfochlorination 8 and 10 had a pronounced inhibitory activity (61.0% and 67.1%). In the same series of experiments, the average inhibitory activity of 36.5% and 48.1% is shown by the compounds 7 and 9. The reference drug acarbose exhibited the standard inhibitory activity of 58.9% and 50.3% in terms of α-glucosidase and α-amylase, respectively. In two other series of experiments, the tested ortho-nitro derivatives (11)(12)(13)(14) did not show activity against α-amylase and α-glucosidase, comparable to the activity of acarbose.
The apparent difference in the antidiabetic activity of the subgroups of compounds 6-10 and 11-14 can be explained by the difference in their chemical structure. The first subgroup is derivative of para-nitrophenylsulfonic acid; the second subgroup is an orthonitrophenylsulfonic acid derivative. It is likely that binding to the sites of α-amylase and α-glucosidase enzymes responsible for increasing blood sugar is more efficient for para-nitrobenzenesulfonic acid derivatives (6-10), while ortho-nitrophenylsulfonic acid derivatives (10)(11)(12)(13)(14) are less efficient for the target reference antidiabetic drug acarbose.
The absence of α-glucosidase activity in representatives of the subgroup of compounds 11-14 may be associated with the previously described trend of a general decrease in antidiabetic activity in the series of ortho-nitrophenylsulfonic acid derivatives (10)(11)(12)(13)(14), whereas the absence of α-glucosidase activity for compound 6 may be a random variable.
As a result of the ortho-nitrobenzenesulfochlorination of β-thiomorpholin-1-ylpropioamidoxime, nitrophenylsolfonate 13a and chloride monohydrate 13b of the corresponding spirocation, similar to the one previously reported [29], can be obtained depending on the reaction conditions. No rearrangement was detected for benzimidazol-1-yl-containing product 10 in accord with the B3LYP/6-31++G(d,p) calculations of the standard Gibbs free energies of reaction [28,29] found for N-substituted aminopyrazoles [31,32]. All salts contain one cation and one anion in the asymmetric unit ( Figure S1, ESI). The quality of XRD data allowed us to locate all of the hydrogen atoms on difference Fourier maps, and undoubtedly confirmed that none of the sulfonate groups contained any hydrogen atoms.
The molecular structures of these compounds are depicted on Figure S1 (ESI), and the main geometry parameters of the cations and anions are listed in Table 3. The X-CH 2 bond distances increase and CH 2 -X-CH 2 angles decrease, passing from O to NPh; CH 2 and S. Valence angles at the positively charged N1 atom are close to ideal 109.5 • values, however only N-CH 2 bond distances within the 6-membered ring are typical for a single N-C bond. In 10, a similar N1-CH 2 bond is equal to 1.458(2) Å, due to the mesomeric effect of the benzimidazole substituent. The N-N bond in these salts varies from 1.460(2) to 1.470(2) Å, which is longer than 1.42-1.43 Å that is found for N-substituted aminopyrazoles [32,33]. Overall the conformations of the cations in 7, 9, 11,13a, and 13b correspond to one conformation on the six-membered ring towards the five-membered ring, while 6, 8, 12, and 14 are the first examples of the inverted chair conformation (Figure 3) of this ring confirmed by means of X-ray diffraction. The difference in the two conformations manifests itself in the 1 H-NMR spectra (see above), and also through the N1-C1 bond length, which is generally shorter in cations with 'novel' conformations. In our opinion, the overall conformation of the molecule and elongation of bond distances in the pyrazole ring can be attributed to the anomeric effect [33] of the (hetero)atom in the six-membered ring. The length of the N2=C bond in all the spirocations is nearly the same as the N2=C bond distance of 1.302(2) in 10.  (5) 1 The deviation of the C(1) atom from the mean plane formed by N-N=C-C atoms in the 5-membered pyrazole ring. 2 The twist angle between the mean planes of the phenyl ring and the NO 2 group for the anion.  A prominent variation in the biological properties of these compounds can be accounted for the possibility of these cations to realize different conformations and to take part in different types of H bonds [34]. In these solids, the only donor for the H bonds is the amino group, while the sulfonate groups of the anions and heteroatoms of the cations compete to act as acceptors of H bonding. As a result, H-bonded chains are observed in 2amino-1,5-diazathiospiro[4.5]-dec-1-ene-5-ammonium-containing salts and tetramers in six other salts (Figure 4). The D 3 3(9) tetramers in 6, 7, and 11 are formed through two N-H…N interactions and two N-H…O ones. Similar D 3 3(13) tetramers in 12 and 14 are obtained through a similar N-H…O cation…anion interactions, but the cations are shifted A prominent variation in the biological properties of these compounds can be accounted for the possibility of these cations to realize different conformations and to take part in different types of H bonds [34]. In these solids, the only donor for the H bonds is the amino group, while the sulfonate groups of the anions and heteroatoms of the cations compete to act as acceptors of H bonding. As a result, H-bonded chains are observed in 2-amino-1,5-diazathiospiro[4.5]-dec-1-ene-5-ammonium-containing salts and tetramers in six other salts (Figure 4). The D 3 3 (9) tetramers in 6, 7, and 11 are formed through two N-H . . . N interactions and two N-H . . . O ones. Similar D 3 3 (13) tetramers in 12 and 14 are obtained through a similar N-H . . . O cation . . . anion interactions, but the cations are shifted along each other to allow for the bonding with a heteroatom of the six-membered ring. The R 4 4 (12) cycles in 9 and the H-bonded chains in 8 and 13 are formed by the interactions of the amino group with sulfonate groups of two neighboring molecules. The G a d (n) notation of H-bonded architectures is presented in terms of [33], where G represents the type of pattern (C for chain, S for intramolecular hydrogen bonds, R for ring, and D for finite), a is the number of acceptors, d is the number of donors, and n the number of atoms in the pattern. Note that, despite the similar crystal parameters and space group of 11-13 (Table S1, ESI), these compounds cannot be regarded as isostructural, as they realize different packing and intermolecular (including H-bonding) interactions. Crystal packing demonstrates that, for these spirocations, heteroatoms of the six-membered cycle can take part in H bonding, and different conformations can be stabilized by means of intermolecular bonding.
atoms in the 5-membered pyrazole ring.
A prominent variation in the biological properties of these compounds can be accounted for the possibility of these cations to realize different conformations and to take part in different types of H bonds [34]. In these solids, the only donor for the H bonds is the amino group, while the sulfonate groups of the anions and heteroatoms of the cations compete to act as acceptors of H bonding. As a result, H-bonded chains are observed in 2amino-1,5-diazathiospiro [4.5]-dec-1-ene-5-ammonium-containing salts and tetramers in six other salts (Figure 4). The D 3 3(9) tetramers in 6, 7, and 11 are formed through two N-H…N interactions and two N-H…O ones. Similar D 3 3(13) tetramers in 12 and 14 are obtained through a similar N-H…O cation…anion interactions, but the cations are shifted along each other to allow for the bonding with a heteroatom of the six-membered ring. The R 4 4(12) cycles in 9 and the H-bonded chains in 8 and 13 are formed by the interactions of the amino group with sulfonate groups of two neighboring molecules. The G a d(n) notation of H-bonded architectures is presented in terms of [33], where G represents the type of pattern (C for chain, S for intramolecular hydrogen bonds, R for ring, and D for finite), a is the number of acceptors, d is the number of donors, and n the number of atoms in the pattern. Note that, despite the similar crystal parameters and space group of 11-13 (Table  S1, ESI), these compounds cannot be regarded as isostructural, as they realize different packing and intermolecular (including H-bonding) interactions. Crystal packing demonstrates that, for these spirocations, heteroatoms of the six-membered cycle can take part in H bonding, and different conformations can be stabilized by means of intermolecular bonding. (6) (11)

Synthesis
The reagents were purchased from different chemical suppliers and were purified Before use. FT-IR spectra were obtained on a Thermo Scientific Nicolet 5700 FTIR instrument (Thermo Fisher Scientific, Inc., Waltham, MA, USA) in KBr pellets. The 1 H-and 13 C-NMR spectra of compounds 6-10 were acquired on a Bruker Avance III 500 MHz NMR spectrometer (Bruker, BioSpin GMBH, Rheinstetten, Germany) and 1 H-and 13  The signals of the residual undeuterated solvents were used as a reference for the 1 H-NMR (2.50 ppm) and 13C-NMR (39.5 ppm) spectra. Elemental analysis was carried out on a CE440 elemental analyzer (Exeter Analytical, Inc., Shanghai, China). The melting points were determined in glass capillaries on a PTP(M) apparatus (Khimlabpribor, Klin, Russia). The reaction progress and purity of the obtained products were controlled using Sorbfil (Sorbpolymer, Krasnodar, Russia) TLC plates coated with CTX-1A silica gel, grain size 5-17 μm, containing UV-254 indicator. The eluent for TLC analysis was a mixture of benzene-EtOH, 1:3. The solvents for the synthesis, recrystallization, and TLC analysis (ethanol, 2-PrOH, benzene, DMF, and acetone) were purified according to the standard techniques.

Synthesis
The reagents were purchased from different chemical suppliers and were purified Before use. FT-IR spectra were obtained on a Thermo Scientific Nicolet 5700 FTIR instrument (Thermo Fisher Scientific, Inc., Waltham, MA, USA) in KBr pellets. The 1 H-and 13 C-NMR spectra of compounds 6-10 were acquired on a Bruker Avance III 500 MHz NMR spectrometer (Bruker, BioSpin GMBH, Rheinstetten, Germany) and 1 H-and 13 C-NMR spectra of compounds 11-14 on a Jeol JNM-ECA 500 (Jeol, Tokyo 196-8558, JAPAN), (500 and 126 MHz, respectively).
The signals of the residual undeuterated solvents were used as a reference for the 1 H-NMR (2.50 ppm) and 13C-NMR (39.5 ppm) spectra. Elemental analysis was carried out on a CE440 elemental analyzer (Exeter Analytical, Inc., Shanghai, China). The melting points were determined in glass capillaries on a PTP(M) apparatus (Khimlabpribor, Klin, Russia). The reaction progress and purity of the obtained products were controlled using Sorbfil (Sorbpolymer, Krasnodar, Russia) TLC plates coated with CTX-1A silica gel, grain size 5-17 µm, containing UV-254 indicator. The eluent for TLC analysis was a mixture of benzene-EtOH, 1:3. The solvents for the synthesis, recrystallization, and TLC analysis (ethanol, 2-PrOH, benzene, DMF, and acetone) were purified according to the standard techniques.

Screening
The in vitro antidiabetic activity of samples 6-14 was assessed by the degree of inhibition of α-amylase and α-glucosidase activity. Pure DMSO was used as a solvent. The final concentration of the sample substances was 10 mg/mL. A total of 100 µL of α-amylase or α-glucosidase (1 U/mL) and 200 µL of the test sample solution (10 mg/mL) were added to 500 µL of phosphate buffer (0.1 M; pH 6.8). The resulting mixture was incubated for 15 min at +37 • C, and 200 µL of P-NPG solution (5 mM) was added.
Then, the resulting mixture was incubated again at +37 • C for 20 min. The reaction was stopped by the addition of 500 µL of sodium carbonate (0.1 M). Since the samples showed too much absorbance at 405 nm, they were diluted 5 times with 5 mL of water and 1 mL of sodium carbonate solution (0.1 M). A solution of α-amylase or α-glucosidase (1 U/mL) was used as a blank. As a negative control, 200 µL of pure DMSO was used in triplicate. As a reference drug, acarbose was obtained at a concentration of 10.0 mg/mL (positive control).
Simultaneously, a negative control was placed without the addition of the test compounds. All the samples were examined in triplets. The inhibitory activity was expressed as a percentage (%) of the degree of inhibition of α-glucosidase, in comparison to the negative control.

Single-Crystal X-ray Diffraction
The X-ray diffraction data of 8, 9, and 11-14 were collected on a Bruker Apex II diffractometer (Bruker AXS, Inc., Madison, WI, USA) equipped with an Oxford Cryostream cooling unit and a graphite monochromated Mo anode (λ = 0.71073 Å). The intensities of the reflections for 10 were collected at 100 K at the "Belok" beamline of the Kurchatov Synchrotron Radiation Source (NRC "Kurchatov Institute", Moscow, Russia), at the wavelength of 0.745 Å using a MAR CCD 165 detector. The image integration was performed using the iMosflm software [35]. The integrated intensities were empirically corrected for the absorption using the Scala program [36]. The crystal structures were solved using SHELXT [37] program and refined with SHELXL [38] using OLEX2 software [39]. The structures were refined by the full-matrix least-squares procedure against F 2 . Non-hydrogen atoms were refined anisotropically. The H(C) positions were calculated, the H(N) and H(O) atoms were located on difference Fourier maps and refined using the riding model. The details of the experiment and the crystal parameters are presented in Tables S1 and S2 (Electronic Supporting Information).
The para-Nitrobenzenesulfochlorination of β-(benzimidazol-1-yl)propioamidoxime produces the O-para-nitrobenzenesulfochlorination product. The reaction time when the reaction mixture is heated is reduced by 2-3 times. The in vitro screening of the library of nitrobenzenelsulfochlorination products for antidiabetic activity reveals two samples with high α-glucosidase activity exceeding the activity of the acarbose standard: products of para-nitrobenzeneulfochlorination of β-(thiomorpholin-1-yl)and β-(benzimidazol-1yl)propioamidoximes. The arsenal of the physicochemical and spectral methods made it possible to establish the structural features of the studied spiropyrazolinium organic salts. Thus, in DMSO-d 6 solutions in the 1 H NMR spectra, the slow inversion of sixmembered nitrogen-containing β-heterocycles can be observed. These spiropyrazoline derivatives can be interesting objects in dynamic NMR spectroscopy, which would allow for the rotational barriers of six-membered heterocycles to be measured. It is assumed that the bulky substituted arylsulfonate anion anchors the spiroheterocycles in the most thermodynamically favorable chair-like position. According to X-ray diffraction data, the axial location of the N-N bond in the spiropyrazoline heterocycles is unambiguously determined. The NMR and XRD data demonstrate that two various conformations of spirocation are present both in solution and in solids. The cation can take part in different types of intermolecular interactions, depending on the conformation and the nature of the six-membered cycle.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27072181/s1, Figure S1: Asymmetric units of the X-rayed compounds in a representation of atoms with thermal ellipsoids (p = 50%); Table S1: Crystallographic data and the experimental details for compounds 6 and 8-10; Table S2: Crystallographic data and the experimental details for compounds 11-14; Compounds 6 and 8-14 are registered in CCDC with the numbers 2154973-2154980. Crystallographic information files are available from the Cambridge Crystallographic Data Center upon request (http://www.ccdc.cam.ac.uk/structures).