Amination of 5-Spiro-Substituted 3-Hydroxy-1,5-dihydro-2H-pyrrol-2-ones

The 3-hydroxy-1,5-dihydro-2H-pyrrol-2-one motif is a valuable scaffold in drug discovery. The replacement of the 3-oxy fragment in 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones-based compounds with a 3-amino one (3-amino analogs of 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones, 3-amino-1,5-dihydro-2H-pyrrol-2-ones) can play a crucial role in their biological effect. Thus, approaches to 3-amino-1,5-dihydro-2H-pyrrol-2-ones are of significant interest. We developed an approach to 5-spiro-substituted 3-amino-1,5-dihydro-2H-pyrrol-2-ones that could not be obtained using previously reported approaches (reactions of 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones with amines). The developed approach is based on the thermal decomposition of 1,3-disubstituted urea derivatives of 5-spiro-substituted 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones, which were prepared via their reaction with carbodiimides.

As a part of our long-standing interest in the synthesis of spiro compounds bearing the HDP moiety [17], we wanted to prepare their 3-amino analogs. However, we were unable to synthesize the desired compounds from HDPs and amines [1,2,7,[9][10][11][12], and therefore, herein, we report an alternative approach to ADPs (Scheme 2).

Results and Discussion
Initially, we prepared HDPs bearing a rigid spiro substituent at the C 5 position, compounds 1a-g, by the reaction of compounds 2a-g with dicyclohexylurea (Scheme 3) [17][18][19]. This reaction proceeded smoothly and yielded target compounds 1a-g in excellent yields (isolated yields of 86-98%). The spectral data of compounds 1a-g were in good agreement with other similar spiro HDPs reported by our group earlier [18,19]. Compounds 1a-g were isolated by simple filtration from the reaction mixture and were sufficiently pure to be used in further experiments without additional purification. Then, we performed a series of reactions of compound 1a with cyclohexylamine (Table 1) in order to obtain the required cyclohexylamino derivative 3a. The examined conditions (Table 1) were selected according to the previously reported substitution reactions of hydroxyl groups in various non-spiro HDPs (bearing hydrogen and a substituent at the C 5 position) with amines [1,2,7,[9][10][11][12]. Although the examined conditions were productive in the reported cases [1,2,7,[9][10][11][12], in our study, none of them led to the desired results (Table 1). We observed that the reaction mixtures contained unconverted compound 1a (93-99%), cyclohexylamine and small quantities of unidentified side products. The target compound 3a was only observed in trace amounts when 1,4-dioxane was used as the solvent (Table 1, Entries 10,11).
We suppose that such inaction of compound 1a in the reaction with cyclohexylamine could be explained by the fact that the desired transformation proceeded via the participation of the corresponding HDP A in keto form, which underwent a condensation reaction with amine to afford the target 3-amino derivative of HDP B (Scheme 4) [12]. Compound 1a (Scheme 4, structure A, R 2 + R 3 = spiro) had less conformational flexibility and less propensity for tautomerization to its keto form due to the influence of the rigid spiro-substituent at the C 5 position of the pyrrol-2-one moiety than the previously reported [1,2,7,[9][10][11][12] non-spiro HDPs bearing hydrogen and a substituent at the C 5 position (Scheme 4, structure A, R 2 = H, R 3 = Alk, Ar, H). Therefore, we decided to develop a new strategy based on the ability of urea and alkyl/arylureas to decompose with the formation of ammonia/amines and isocyanates (Scheme 5) [20,21]. For the implementation of our strategy, we obtained compounds 4a,b, 1,3-dialkylurea derivatives of compound 1a, via the reaction of compound 1a with inexpensive and commercially available dialkylcarbodiimides 5a,b (Scheme 6). The reaction with dicyclohexylcarbodiimide (DCC) 5a proceeded smoothly at a reagent ratio of 1:1 to give the target compound 4a in an excellent yield (93%). However, the reaction with diisopropylcarbodiimide (DIC) 5b at a reagent ratio of 1:1 yielded the target compound 4b in a lower yield (73%). Such a decrease, when changing DCC to DIC, could be because the boiling point of DIC (145-146 • C) [22] is lower than DCC (151-152 • C at 10 mmHg) [22], and DIC partially evaporated from the reaction mixture. Increasing the DIC ratio to two equivalents increased the isolated yield of compound 4b to 88%. Compounds 4a,b were isolated by filtration from the reaction mixture and were sufficiently pure to be used in further experiments without additional purification. Then, the proposed method was successfully implemented to the synthesis of compounds 4c-n (Scheme 6). This approach worked well with various aryl substituents in compounds 1 (Scheme 6). The involvement of aryl and primary alkyl-bearing carbodiimides 5c,d in this reaction was successful too (Scheme 6). Scheme 6. Reaction of compounds 1a-g with carbodiimides 5a-d.
Probably, the formation of compounds 4a-n proceeded through the pathway of acyl transfer in isourea intermediate C (Scheme 7), which is a common concurrent process for the Steglich esterification [23]. It should be emphasized that although compounds 1a-g contain two different hydroxy groups, enolic and phenolic ones, the reaction with carbodiimides 5a-d proceeded exclusively at the enolic hydroxy group, which could be explained by its higher acidity in comparison with the phenolic one, and, consequently, higher reactivity in reactions with carbodiimides [24]. Having synthesized derivatives 4a-n, we investigated the possibility of their thermal decomposition to afford the desired amino derivatives of HDPs, compounds 3a-n ( Table 2). We found that compounds 4a-n readily decomposed at their melting point temperatures to afford the desired compounds 3a-n (84-98%). The proposed method was suitable for various aroyl substituents and R' substituents in a urea moiety of compounds 4a-n ( Table 2).
Preliminary antimicrobial assays [25] of compounds 1a-g and 4a,c-h were carried out (detailed data are given in Supporting Materials). Unfortunately, we found that the tested compounds did not show any significant antimicrobial activity (against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Candida albicans, Cryptococcus neoformans var. grubii) in vitro.

Materials and Methods
3.1. General Information 1 H and 13 C NMR spectra (Supplementary Materials) were acquired on a Bruker Avance III 400 HD spectrometer (Bruker BioSpin AG, Fällanden, Switzerland) (at 400 and 100 MHz, respectively) in CDCl 3 (stab. with Ag) or DMSO-d 6 using the HMDS signal (in 1 H NMR) or solvent residual signals (in 13 C NMR, 77.00 for CDCl 3 , 39.51 for DMSO-d 6 ; in 1 H NMR, 7.26 for CDCl 3 , 2.50 for DMSO-d 6 ) as internal standards. IR spectra were recorded on a Perkin-Elmer Spectrum Two spectrometer (PerkinElmer, Waltham, MA, USA) from mulls in mineral oil. Melting points were measured on a Khimlabpribor PTP apparatus (Pribor-T, Saratov, Russia) or a Mettler Toledo MP70 apparatus (Mettler-Toledo GmbH, Greifensee, Switzerland). Elemental analyses were carried out on a Vario MICRO Cube analyzer (Elementar, Langenselbold, Germany). The reaction conditions were optimized using UPLC-UV-MS (Waters ACQUITY UPLC I-Class system (USA); Acquity UPLC BEH C18 column, grain size of 1.7 µm; acetonitrile-water as eluents; flow rate of 0.6 mL/min; ACQUITY UPLC PDA eλ Detector (Thermo Fisher Scientific, Waltham, MA, USA) (wavelength range of 230-780 nm); Xevo TQD mass detector (Agilent, Santa Clara, CA, USA); electrospray ionization (ESI); positive and negative ion detection; ion source temperature of 150 • C; capillary voltage of 3500-4000 V; cone voltage of 20-70 V; vaporizer temperature of 200 • C). The single-crystal X-ray analyses of compounds 3b, 3c, and 4h were performed on an Xcalibur Ruby diffractometer (Agilent Technologies, Cheadle, UK). The empirical absorption correction was introduced by the multi-scan method using SCALE3 ABSPACK algorithm [26]. Using OLEX2 [27], the structures were solved with the SHELXS-97 [28] program and refined by the full-matrix least-squares minimization in the anisotropic approximation for all non-hydrogen atoms with the SHELXL (accessed on 1 March 2018) [29] program. Hydrogen atoms bound to carbon were positioned geometrically and refined using a riding model. The hydrogen atoms of OH and NH groups were refined freely with isotropic displacement parameters. The contribution of the solvent electron density (for compounds 3b and 4h) was removed using the SQUEEZE routine in PLATON [30].
Thin-layer chromatography (TLC) was performed on Merck silica gel 60 F 254 plates using EtOAc/toluene, 1:5 v/v, toluene, EtOAc as eluents. Starting compounds 2a-g were obtained according to reported procedures [31] from oxalyl chloride (purchased from commercial vendors) and heterocyclic enamines (obtained according to reported procedures [31] from commercially available reagents). Toluene for procedures involving compounds 2a-g was dried over Na before use. All other solvents and reagents were purchased from commercial vendors and used as received. Procedures involving compounds 2a-g were carried out in oven-dried glassware.

General Procedure to Compounds 4a-n
A suspension of the corresponding compound 1 (1.8 mmol) and the corresponding carbodiimide 5 (1.8 mmol for 5a,c,d; 3.6 mmol for DIC 5b) in 20 mL of toluene was refluxed for 2 h. Then, the resulting precipitate was filtered off to afford the desired compound 4.

General Procedure to Compounds 3a-n
The corresponding compound 4 (0.5 mmol) was put into an oven-dried tube, pressed slightly, and then, it was heated at 160-290 • C (the temperature for each compound is given in Table 2; caution: R NCO evolves during the reaction). The reaction mixture was cooled to room temperature and scrubbed with hexane (about 10 mL) to give the appropriate compound 3.