[3+2]-Cycloaddition of azomethine ylides to 5-methylidene-3-aryl-2-сhalcogen-imidazolones: access to dispiro indolinone-pyrrolidine-imidazolones

A synthesis of dispiro derivatives from 5-methylidene-2-chalcogenimidazolones and azomethine ylides generated from isatins and N-substituted α-amino acids has been developed.


Introduction
2-Сhalcogen-imidazolones (hydantoins and thiohydantoins) are well-known classes of heterocyclic compounds with proven biological activity, characterized by a variety of approaches to their synthesis and application, which are well described in the literature [1,2]. Compounds of these structural classes exhibit a wide spectrum of pharmacological activity against various significant diseases, such as cancers, microbial infections, metabolic diseases and epilepsy [3]. Several hydantoin and thiohydantoin derivatives, such as phenytoin, nitrofurantoin and enzalutamide are currently in clinical use.
A convenient feature of the hydantoin and thiohydantoin scaffolds is the possibility to easily introduce various functional groups into these molecules. In particular, arylmethylidene-and indolylidene-substituted thiohydantoins and hydantoins can be functionalized by 1,3-dipolar cycloaddition reactions at the exocyclic C=C double bond to obtain derivatives of interest due

Results and discussion
In this study, we developed a regio-and diastereoselective synthesis of dispiro derivatives of 5-methylidene hydantoins and thiohydantoins based on 1,3-dipolar cycloaddition to these dipolarophiles of azomethine ylides generated in situ from isatins and N-substituted glycines. The general reaction scheme and structures of the obtained products are shown in scheme 1.
Initially, we optimized the reaction conditions for 1,3-dipolar cycloaddition using sarcosine and isatine as the 1,3-dipole precursors and N(3)-Ph-substituted methylidene hydantoin 1 or thiohydantoin 2a [13] as dipolarophiles, and by varying the solvent. To avoid undesirable dimerization of methylidene derivatives 1 and 2a [14], they were introduced into a solution preheated to the boiling temperature. The yields of the products obtained by reactions in various solvents are presented in table 1. The resulting spiro derivatives 3a and 4a were isolated from the reaction mixtures by column chromatography. The optimal reaction conditions were as follows: initially, sarcosine was dissolved in the respective solvent, after which the solution was heated to reflux, dipolarophile 1 or 2a and isatin were added in one portion, and then the mixture was refluxed with stirring for 4 h.
In toluene and tetrahydrofuran (THF), we did not observe complete conversion of the starting compounds 1 and 2а, based on thin layer chromatography (TLC) data. The best yields were obtained when the reaction was carried out in methanol for spiro hydantoin 3a, and in ethanol for thiohydantoin 4а, thus these two solvents were used in further syntheses (scheme 1). It can be assumed that, in the case of more reactive methylidene hydantoins, the polarity of the solvent plays a more important role; therefore, the yields of the products in the more polar methanol are higher than Antimalarial activity [6 ] Anticonvulsant activity [7,8]  royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 9: 211967 3 in ethanol. Less reactive methylidentityhydantoins seem to react more efficiently with increasing temperature, and the higher-boiling ethanol appears to be a better solvent. It was found that 1,3-dipolar cycloaddition reactions with compound 1 successfully proceeded not only with sarcosine, but also with more sterically hindered amino acids such as N-ethylglycine and N-(4-methoxy)benzylglycine (N-PMB-glycine) (scheme 1; compounds 3 h-l). However, when amino acids with bulky substituents at the nitrogen atom, such as N-(4-chloro)phenylglycine, isopropylglycine and tert-butylglycine, were introduced into the reaction, target product 3 did not form. In contrast with N-(4-chloro)phenylglycine, N-PMB-glycine proved successful in this type of reaction, thus it can be concluded that an additional CH 2 -group is probably required between the nitrogen atom and the bulky substituent in order to reduce steric hindrance.
Azomethine ylides generated from N-substituted isatins, such as N-methylisatin can be successfully introduced into the reaction.
Comparing the yields of 1,3-dipolar cycloaddition reactions to 5-methylidene hydantoins and thiohydantoins with the previously described reactions of arylmethylidene hydantoins and thiohydantoins [4,5,11], it can be noted that the yields of reactions with methylene derivatives 1 and 2 are somewhat lower than those of the corresponding arylmethylidene analogues. This can be explained by the side reaction of dimerization of compounds 1 and 2, which easily takes place in an acidic medium for methylidene derivatives of hydantoins [14], but is not characteristic for their arylmethylidene derivatives.
In all reactions, we observed the formation of 1,3-dipolar cycloaddition products as single diastereomers with the relative 2 0 R Ã ,4S Ã -configuration; other possible stereoisomeric products were not detected in the nuclear magnetic resonance (NMR) spectra of the reaction mixtures, even in trace amounts.
NMR spectroscopy data confirmed the structure of the resulting products. Application of twodimensional techniques (heteronuclear multiple bond correlation (HMBC), heteronuclear single quantum coherence (HSQC) and nuclear Overhauser effect spectroscopy (NOESY)) provided complete resonance signal assignments in the spectra of compounds 3f and 4i (see electronic supplementary material, information, pp. S13, S44). The observed long-range 1 H-13 C correlations corresponded to those of dispiro products. Thus, the HMBC spectrum of 3f (electronic supplementary material, information, p. S13) reveals interactions of the spiro carbon, which connects the pyrrolidine and isatin fragments (δC 77.5 ppm, C 6 D 6 ), with methylene protons (δH 3.30 and 2.50 ppm) and NCH 3 protons (δH 2.01 ppm) of the pyrrolidine ring, and ortho-proton of the benzoic ring (δH 7.66 ppm). Additional connectivity between this carbon and the NH-group, related to the isatin was observed for compound 4i (δC 73.6 ppm, δH 10.68 ppm). In addition, other spiro carbons display long-range interactions with protons of both pyrrolidine and hydantoin rings.
The stereochemistry of the resulting products was established using the data from the NOE study for compound 3f and X-ray structural analysis for compounds 3j and 4b. Thus, in the 1 H-1 H NOESY spectrum of compound 3f, the cross-peaks corresponding to the interactions of the N-CH 3 protons (δH 2.67 ppm) and the proton in the ortho-position of the benzoic ring of the isatin fragment (δH 6.21 ppm), and the proton of the NH group of the hydantoin fragment (δH 7.01 ppm) were detected (figure 2). This confirms the location of these structural fragments on one side of the plane of the central pyrrolidine fragment. The structures of the obtained dispirohydantoin 3j and thiohydantoin 4b were also confirmed by X-ray diffraction analysis. The molecular structures of compounds 3j and 4b are shown in figure 3. The five-membered imidazolone rings in these molecules are practically flat; the angles between the planes of the spiro-connected imidazolone, pyrrolidine and indolinone fragments are close to 90°.
The stereochemistry of the reaction can be explained by considering the mechanism of 1,3-dipolar cycloaddition, as shown in scheme 2. This mechanism is similar to that described for 5-arylmethylidene hydantoins and thiohydantoins [4] and includes the following stages (scheme 2):    royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 9: 211967 formation of the iminium intermediate as a result of sarcosine attacking the carbonyl group of isatin, the subsequent cyclization of the iminium intermediate into a spiro-lactone and the elimination of CO 2 from the latter with the generation of the 1,3-dipole, which regio-and diastereoselectively attacks the C=C double bond of 5-methylidene-2-сhalcogen-imidazolone. It should be noted that the attack of the dipole can proceed from both above and below the double bond plane, resulting in a mixture of enantiomers. Possible routes of the azomethine ylide approach to the dipolarophile moiety are shown in scheme 2. Based on the structure of the products, we can conclude that the cycloaddition reaction takes place through the exo transition state [11] (scheme 2), possibly due to increase in the transition state energy, as a result of electrostatic repulsion of the cis-located carbonyl groups of azomethine ylide and dipolarophile, in the other approaches. It should be noted that the 1 H, 13 C and 19 F NMR spectra of ortho-substituted products 4i and4j show two sets of resonance signals (electronic supplementary material, Information, pp. S43, S45 and S47).This can be explained by the hindered rotation around the single bond С(Ar)-N(imidazolone) of the hydantoin fragment, and consequently, the existence of compounds 4i and 4j in the form of atropisomers (axially chiral heterocyclic analogues of biaryl derivatives similar to those described for ortho-substituted 5-alkyl-2-thiohydantoins) [15][16][17][18]. The axially chiral compounds 4i and 4j may appear in stereoisomeric forms (figure 4) with a transoid (a) and cisoid (b) arrangement of the highest substituent at C5 and halogen atom in the aryl fragment of the 2-thioimidazolone cycle.
The chemical shift differences of the ortho-protons of the benzene ring are especially evident for the two atropisomers of compounds 4i and 4j (approx. 1 ppm, approx. 6.3 and 7.3 ppm), and significant differences were also observed for the protons of the NH and N-Me groups (electronic supplementary material, Information, p. S43).
The barrier of internal rotation around the N-C bond was estimated using dynamic 1 H NMR study of compound 4i with an atropisomer population ratio of ca 1 : 1.4 at ambient temperature. The temperature dependence of the proton spectra at set intervals from 22°C to 80°C corresponds to slow and intermediate rates of exchange. The line shape analysis of the N-Me-groups signals [19] (electronic supplementary material, Information, p. S45) resulted in activation barriers ΔG ≠ 300 = 19.0 and 19.2 kcal mol −1 for the two directions. These values are comparable to the barrier found for ortho-F-substituted Scheme 2. The proposed reaction mechanism.
royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 9: 211967 2-thioimidazolone derivatives (19.8 kcal mole −1 ) [15] also using dynamic NMR study. One also draws attention to the strong temperature dependence of the chemical shifts of the NH protons of compound 4i in DMSO (electronic supplementary material, Information, p. S45), which is most likely associated with intermolecular processes. The question of which of the atropisomers A or B is predominant cannot be unambiguously answered based on the available data. However, some assumptions can be made; the difference in the chemical shifts of the ortho-protons for the two isomers of compound 4i is about 1 ppm (electronic supplementary material, Information, p. S43), while the proton is more shielded in the dominant isomer (δortho 6.4 ppm for the major stereoisomer and 7.35 ppm for the minor one). It should be noted that the average value of this chemical shift (6.88 ppm, in this case we averaged without the weighting factor) is close to δortho = 6.84 ppm for compound 4а, in which the rotation of the phenyl group is free. The influence of fluorine on the chemical shift can be neglected since the increment of this substituent in benzene is −0.03 ppm [20]; thus, the shielding of ortho-protons in the 3-aryl-2thiohydantoin series may be considered as a common feature. It can be assumed that the influence of the magnetic anisotropy of C=S and C=O exocyclic bonds on the chemical shifts is insignificant and/ or the same, since according to X-ray diffraction data (figure 3), the plane of the aromatic substituent is moved in relation to the plane of the heterocycle; however, the preferred arrangement of the ortho-protons in relative proximity around one of these two exocyclic multiple bonds is preserved.
It can be assumed that a significant part of the difference between the 4i atropisomers is due to the magnetic anisotropy of the isatin moiety. According to the X-ray diffraction data, one of the ortho-protons is located almost above the centre of the isatin core. The estimated distance between the ortho-protons and the isatin fragment of approximately 4 Å corresponds to an NMR signal shift of 0.3-0.4 ppm.
According to this hypothesis, the dominant isomer 4i is the structure in which the ortho-proton is located together with isatin on one side relative to the thiohydantoin plane, that is, isomer B in figure 3. In this case, although the ortho-fluorine atom in the major isomer of 4i should also be more shielded, it should be taken into account that 19 F nuclei have an incomparably wider range of chemical shifts than protons, and the difference in chemical shifts of 1.7 ppm observed for isomers A and B does not seem vital.
We also established that the para-methoxybenzyl group at the nitrogen atom of the central pyrrolidine ring of compound 3l could be successfully removed by the action of a mixture of triflic and trifluoroacetic acids (scheme 3). Considering the higher solubility of the resulting compound 5 in water, this reaction is promising for further development of bioavailable spiro structures of this type. Notably, compound 5, royalsocietypublishing.org/journal/rsos R. Soc. Open Sci. 9: 211967 which has no hydrophobic substituents attached to the pyrrolidine nitrogen atom, unlike compounds 3 and 4, was isolated from the aqueous phase in the form of a trifluoroacetate salt.
It should be noted that we were unable to conduct the reaction of 5-methylidene-2chalcogenimidazolones with azomethine ylides generated from glycine and isatin. Even though there are examples in the literature of the addition of azomethineylides formed from alpha-amino acids and isatin in the presence of various additives [21,22]. In the absence of additives, the Schiff base formed in the course of the reaction is inefficiently attached to double bonds and is of no synthetic value. Thus, debenzylation of compound 3l may be a convenient alternative method for the preparation of pyrrolidine-imidazolones, which do not contain substituents at the pyrrolidine nitrogen atoms.

General information
All common reagents were purchased from commercial suppliers and were used as received. The melting points were uncorrected. 1 H (400 MHz), 13 C (101 MHz) and 19 F (376 MHz) NMR spectra were recorded on Bruker Avance 400 and Agilent MR-400 spectrometers in DMSO-d 6 and C 6 D 6 . Chemical shifts were measured relative to residual solvent signals and referenced in parts per million to tetramethylsilane (TMS). High-resolution mass spectra (HRMS) were recorded on an OrbitrapElite (Thermo Scientific) mass spectrometer with electrospray ionization (ESI) and an orbital trap. To inject solutions with a concentration of 0.1 to 9 mg ml −1 (in 1% formic acid in acetonitrile), direct injection into the ion source using a syringe pump (0.3 ml min −1 ) was used. The spray voltage was ±3.5 kV, and the temperature of the capillary was 275°C.
Starting compounds 1 and 2a-i were synthesized according to the methods described in [13]. CCDC 2052348 and 2052655 contain electronic supplementary material, crystallographic data for this study. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.