Squaramide-Catalyzed Asymmetric Mannich Reaction between 1,3-Dicarbonyl Compounds and Pyrazolinone Ketimines: A Pathway to Enantioenriched 4-Pyrazolyl- and 4-Isoxazolyl-4-aminopyrazolone Derivatives

A series of N-Boc ketimines derived from pyrazolin-5-ones have been used as electrophiles in enantioselective Mannich reactions with different 1,3-dicarbonyl compounds. This method provides a direct pathway to access the 4-amino-5-pyrazolone derivatives bearing a quaternary substituted stereocenter and containing two privileged structure motifs, the β-diketone and pyrazolinone substructures. The adducts were obtained in excellent yields (up to 90%) and enantioselectivities (up to 94:6 er) by employing a very low loading of 2 mol% of a quinine-derived bifunctional squaramide as an organocatalyst for a wide range of substrates. In addition, the utility of the obtained products was demonstrated through one step transformations to enantioenriched diheterocyclic systems (4-pyrazolyl-pyrazolone and 4-isoxazolyl-pyrazolone), potentially promising candidates for drug discovery.


Introduction
The enantioselective synthesis of nitrogen-containing heterocycles bearing stereogenic centers has received substantial attention in recent years due to their ubiquity in the cores of natural products and bioactive molecules [1][2][3]. Among the different types of nitrogencontaining heterocycles, pyrazole and pyrazolone derivatives are a privileged class of compounds that possess a broad spectrum of applications as pharmaceutical and agrochemical products, as well as material science [4,5]. Medicinal chemistry researchers have synthesized drug-like pyrazolone candidates that exhibit significant pharmacological activities including antimicrobial, antitumor, CNS (central nervous system) effect, anti-inflammatory activities, and so on. For this reason, significant efforts have been made in recent years to develop new methods for the asymmetric synthesis of the structurally diverse pyrazolone derivatives, especially employing the reactivity of pyrazolin-5-one core [6][7][8][9][10]. However, the asymmetric synthesis of 4,4-disubstituted pyrazolones bearing a nitrogen at C-4 is challenging given the predictable biological activity of these molecules. Several examples are found in the literature that describe the preparation of pyrazolones bearing a tetrasubstituted center via the α-amination reaction of 4-substituted pyrazolones [11][12][13]. Alternatively, the asymmetric reaction of pyrazole-4,5-dione ketimines with different nucleophilic reagents is another straightforward method for the construction of the 4-aminopyrazolone core with a quaternary carbon center. Recently, some organocatalytic asymmetric transformations based on ketimines derived from pyrazolin-5-ones have been reported including Strecker [14] and aza-Friedel Crafts [15,16] reactions.
The asymmetric Mannich reaction is a crucial method for the formation of new C-C bond including β-amino carbonyl compounds [17]. In 2017, Enders group was the first Later, Yuan's group reported the asymmetric decarboxylative Mannich reaction of β-ketoacids with pyrazole-4,5-dione ketimines catalyzed by a quinine-derived squaramide to access chiral β-amino ketone-pyrazolones bearing a tetrasubstituted center at C-4 position in excellent yields and, generally, good enantioselectivities (Scheme 1b) [19]. In 2019, Du and coworkers developed the diastereo-and enantioselective Mannich reaction of 3-fluorooxindole to N-aryl pyrazole-4,5-dione-derived ketimines using a dihydroquininederived squaramide as an organocatalyst. The desired products containing an aminopyrazolone-oxindole scaffold and an asymmetric fluorine atom were obtained in high to excellent yields, with excellent enantio-and good to excellent diastereoselectivities (Scheme 1c) [20]. Unfortunately, the same reaction carried out with N-Boc-protected pyrazolinone ketimine under the optimized reaction conditions provided the target product with low yield and diastereo-and enantioselectivity. In the same year, Shao reported the enantiodivergent Mannich reaction of N-Boc ketimines derived from pyrazolin-5-ones with propionaldehyde promoted by acyclic chiral secondary aminocatalyst leading to the corresponding adducts in good yields with both high diastereoselectivity and enantioselectivity (Scheme 1d) [21].

Results and Discussion
First, we investigated the reaction of N-Boc ketimine 1a with 2,4-pentanedione (2a) as the model reaction in the presence of 10 mol% of bifunctional organocatalysts C1 and C2, both derived from quinine, in toluene at room temperature (Table 1). With thiourea C1 as a catalyst, the reaction gave the desired product 3a in 74% yield with 68:32 er (entry 1). To our delight, squaramide catalyst C2 provided the product 3a in excellent yield and with an increase in the er value to 80:20 (entry 2). Screening of different solvents including DCM, CHCl 3 , DCE, Et 2 O, THF, 1,4-dioxane, and ethyl acetate showed that toluene was better than other solvents (entry 2 vs. entries [3][4][5][6][7][8][9]. In contrast to these results, the reaction in acetonitrile gave the opposite enantiomer with the same enantiomeric ratio (compare entries 2 and 10).
Then, we analyzed the influence of the H-bonding donor group by comparing quininederived squaramide C2 (bearing a phenethyl substituent) with C3 ((bis(trifluoromethyl) benzyl derivative) and C4 ((bis(trifluoromethyl)phenyl derivative) squaramides (entries 2 and 11-12). The catalyst C4 where the squaramide unit is directly attached to an aryl group provided better enantioselectivity (84:16 er) than squaramides C2 and C3 in lower reaction time. Additional trials performed under the same reaction conditions with cinchonidine derived-squaramide C5 and hydroquinine-derived squaramide C6 did not lead to any increase in enantioselectivity (see entries [13][14]. Quinidine-derived squaramide C7, a pseudoenantiomer of C4, also effectively catalyzed this reaction but gave the opposite enantiomer of 3a with a similar yield and lower selectivity (71:29 er, entry 15). A significant decrease in enantioselectivity was also observed by using L-valine derived-squaramide C8 (64:36 er, entry 16).
Next, the catalyst loading of C4 was reduced to 5 and 2 mol%, and no erosion in chemical yield or enantioselectivity was observed after 2 h of reaction time (entry 18). Lowering the reaction temperature to −18 • C resulted in a longer reaction time and no improvement in the value of er (entry 19). The ratio of nucleophile can be decreased from 2 equivalents to 1.1 equivalents without changing the enantioselectivity (entry 20). In light of the above screening experiments, the best reaction conditions for the enantioselective Mannich reaction were established: 1.1 equiv of diketone in toluene with 2 mol% C4 at room temperature.  With the optimized reaction conditions in hand, various N-Boc pyrazolinone ketimines 1a-g were reacted with different dicarbonyl compounds 2a-c to produce the corresponding adducts 3a-l. The results are collected in Scheme 2. With the optimized reaction conditions in hand, various N-Boc pyrazolinone ketimines 1a-g were reacted with different dicarbonyl compounds 2a-c to produce the corresponding adducts 3a-l. The results are collected in Scheme 2.

Scheme 2.
Substrate scope for the asymmetric Mannich reaction. a Reactions were carried out by using 1 (0.1 mmol), 2 (0.11 mmol), and catalyst C4 (2 mol%) in 1 mL of PhMe at room temperature. Yields correspond to isolated compound after flash chromatography. The er values were determined by chiral HPLC analysis. b Determined from the mother liquor after recristallyzation from hexane-EtOAc. c 5 mol% catalyst was used. d 10 mol% catalyst was used.
The imines 1b and 1c bearing ethyl and isopropyl substituent (R) at the C-3 position worked well in the reaction with pentane-2,4-dione and gave the expected products 3b and 3c in good yield with 84:16 and 88:12 er, respectively. It was observed that the increase in steric bulk of the alkyl group resulted in lower reactivity, although it provided better The imines 1b and 1c bearing ethyl and isopropyl substituent (R) at the C-3 position worked well in the reaction with pentane-2,4-dione and gave the expected products 3b and 3c in good yield with 84:16 and 88:12 er, respectively. It was observed that the increase in steric bulk of the alkyl group resulted in lower reactivity, although it provided better enantiocontrol. However, in the reaction of N-Boc ketimine 1d bearing a tert-butyl group at the C-3 position, no product was observed after seven days of reaction, presumably due to increased steric hindrance. In the case of a phenyl group at the same position, the corresponding product 3e was obtained with very good yield and enantioselectivity (94:6 er) after 48 h of reaction time. Nevertheless, a small decrease in enantioselectivity (90:10 er) was observed when the N-Boc ketimine 1l, N-methyl substituted, was reacted with 2,4-pentanedione under similar reaction conditions. When using N-Boc ketimines with different aryl groups at the N-1 position, whether it be electron-withdrawing (1f) or electron-donating (1g), good yields and slightly higher enantioselectivities were obtained for 3f and 3g. It is important to note that recrystallization of adducts 3a and 3f from hexane-EtOAc allowed for obtaining enantioenriched 3a and 3f (er ≥ 95:5) from the mother liquor in 68% yield.
After exploring a series of pyrazolinone ketimines, the substrate scope of β-diketones was further extended. 3,5-Heptanedione (2b) readily reacted with ketimine 1a leading to 3h in good yield and moderate enantioselectivity (85:15 er), but the reaction of 1a with dibenzoylmethane (2c) giving 3i was slower and more enantioselective. Both diketones 2b and 2c reacted with the less reactive ketimine 1e in the presence of 10 mol% catalyst C4, providing adducts 3j and 3k in moderate yield but with good enantioselectivity (93:7 er).
Next, the practical synthetic utility of this Mannich reaction was demonstrated by the transformation of adducts 3 into a series of 4-pyrazolyl-pyrazolone derivatives 4 with potential pharmacological interest (Scheme 3).
Condensation of adducts 3a-c,e-g with a two-fold excess of hydrazine monohydrate in methanol proceeded easily at room temperature furnishing the pyrazole derivatives 4a-c,e-g in good yields (60-82%). However, adduct 3j, prepared from heptane-3,5-dione, reacted under the same reaction conditions to give compound 4j in only moderate yield (40%). Chiral HPLC analysis of the final pyrazoles 4 showed that the enantiomeric ratio was maintained with respect to the starting compounds, with no erosion of the enantiomeric purity during the transformation. The adduct 4a was achieved enantiomerically pure (er > 99:1) after recrystallization from hexane-ethyl acetate.
Unexpectedly, in the reaction of dibenzoylmethane derivative 3i with hydrazine monohydrate, the corresponding condensation product (4i) was not the final product; instead, the chiral β-amino ketone-pyrazolinone derivative 5i was isolated in 52% yield, after cleavage of the benzoyl group of 3i. This unwanted reaction has not been observed in the reactions of the adducts derived from the pentane-2,4-dione (3a-c,e-g) and heptane-3,5dione (3j). Fortunately, the comparison of specific rotation and HPLC retention times of 5i with those described in literature [19] allowed us to determine the absolute configuration (S) of adduct 3i by chemical correlation. The absolute configuration of products 3 and 4 is expected to be the same by analogy assuming a common reaction pathway.
A plausible mechanism of this well-known deacylation process [22] is described in Scheme 4. The nucleophilic attack of hydrazine hydrate on the carbonyl group of 3i leads to intermediate A, which undergoes a debenzoylation process to furnish the β-amino ketone-pyrazolinone derivative 5i.
2b and 2c reacted with the less reactive ketimine 1e in the presence of 10 mol% catalys C4, providing adducts 3j and 3k in moderate yield but with good enantioselectivity (93:7 er).
Next, the practical synthetic utility of this Mannich reaction was demonstrated by the transformation of adducts 3 into a series of 4-pyrazolyl-pyrazolone derivatives 4 with po tential pharmacological interest (Scheme 3).   Condensation of adducts 3a-c,e-g with a two-fold excess of hydrazine monohydrate in methanol proceeded easily at room temperature furnishing the pyrazole derivatives 4a-c,e-g in good yields (60-82%). However, adduct 3j, prepared from heptane-3,5-dione, reacted under the same reaction conditions to give compound 4j in only moderate yield (40%). Chiral HPLC analysis of the final pyrazoles 4 showed that the enantiomeric ratio was maintained with respect to the starting compounds, with no erosion of the enantiomeric purity during the transformation. The adduct 4a was achieved enantiomerically pure (er > 99:1) after recrystallization from hexane-ethyl acetate.
Unexpectedly, in the reaction of dibenzoylmethane derivative 3i with hydrazine monohydrate, the corresponding condensation product (4i) was not the final product; instead, the chiral β-amino ketone-pyrazolinone derivative 5i was isolated in 52% yield, after cleavage of the benzoyl group of 3i. This unwanted reaction has not been observed in the reactions of the adducts derived from the pentane-2,4-dione (3a-c,e-g) and heptane-3,5-dione (3j). Fortunately, the comparison of specific rotation and HPLC retention times of 5i with those described in literature [19] allowed us to determine the absolute configuration (S) of adduct 3i by chemical correlation. The absolute configuration of products 3 and 4 is expected to be the same by analogy assuming a common reaction pathway.
A plausible mechanism of this well-known deacylation process [22] is described in Scheme 4. The nucleophilic attack of hydrazine hydrate on the carbonyl group of 3i leads to intermediate A, which undergoes a debenzoylation process to furnish the β-amino ketone-pyrazolinone derivative 5i. Scheme 4. Plausible mechanism of debenzoylation.
To further illustrate the synthetic potential of this methodology, the asymmetric Mannich addition products 3a,e were treated with 4-chlorophenylhydrazine and hydroxylamine hydrochloride in refluxing ethanol to afford their corresponding 4-chlorophenylpyrazoles (6a,e) and isoxazoles (7a,e), respectively, in moderate to good yields To further illustrate the synthetic potential of this methodology, the asymmetric Mannich addition products 3a,e were treated with 4-chlorophenylhydrazine and hydroxylamine hydrochloride in refluxing ethanol to afford their corresponding 4-chlorophenylpyrazoles (6a,e) and isoxazoles (7a,e), respectively, in moderate to good yields (Scheme 5). Again, there is no erosion of the enantiomeric purity during the transformations. To further illustrate the synthetic potential of this methodology, the asymmetric Mannich addition products 3a,e were treated with 4-chlorophenylhydrazine and hydroxylamine hydrochloride in refluxing ethanol to afford their corresponding 4-chlorophenylpyrazoles (6a,e) and isoxazoles (7a,e), respectively, in moderate to good yields (Scheme 5). Again, there is no erosion of the enantiomeric purity during the transformations. In addition, on the basis of our results and those previously reported [18,19], we proposed the formation of the ternary complex depicted in Figure 1 to rationalize the stereochemistry of the products. The H-bonding activation of N-Boc ketimine 1 by the squaramide moiety of catalyst C4 facilitates the nucleophilic attack of the diketone enolate from the re-face of the imine group, leading to the formation of adduct 3 with the (S) configuration. In addition, on the basis of our results and those previously reported [18,19], we proposed the formation of the ternary complex depicted in Figure 1 to rationalize the stereochemistry of the products. The H-bonding activation of N-Boc ketimine 1 by the squaramide moiety of catalyst C4 facilitates the nucleophilic attack of the diketone enolate from the re-face of the imine group, leading to the formation of adduct 3 with the (S) configuration.

General Information
1 H NMR (500 MHz, 400 MHz) and 13C NMR (126 MHz, 101 MHz) spectra were recorded in CDCl3 or DMSO-d6 as solvent (Laboratory of Instrumental Techniques, University of Valladolid). Chemical shifts for protons are reported in ppm from TMS with the residual CHCl3 resonance as internal reference. Chemical shifts for carbons are reported in ppm from TMS and are referenced to the carbon resonance of the solvent. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quadruplet, quint = quintuplet, sext = sextuplet, sept = septuplet, m = multiplet, br s = broad signal), coupling constants in Hertz, and integration. Specific rotations were measured on a PerkinElmer 341 digital polarimeter using a 5 mL cell with a 1 dm path length and a sodium lamp, and concentration is given in g per 100 mL. Infrared spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer and are reported in frequency of absorption (only the structurally most important peaks are given). Melting points were obtained with a micro melting point Leica Gallen III apparatus and are un-

General Information
1 H NMR (500 MHz, 400 MHz) and 13C NMR (126 MHz, 101 MHz) spectra were recorded in CDCl 3 or DMSO-d 6 as solvent (Laboratory of Instrumental Techniques, University of Valladolid). Chemical shifts for protons are reported in ppm from TMS with the residual CHCl 3 resonance as internal reference. Chemical shifts for carbons are reported in ppm from TMS and are referenced to the carbon resonance of the solvent. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quadruplet, quint = quintuplet, sext = sextuplet, sept = septuplet, m = multiplet, br s = broad signal), coupling constants in Hertz, and integration. Specific rotations were measured on a PerkinElmer 341 digital polarimeter using a 5 mL cell with a 1 dm path length, and a sodium lamp, and concentration is given in g per 100 mL. Infrared spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer and are reported in frequency of absorption (only the structurally most important peaks are given). Melting points were obtained with a micro melting point Leica Gallen III apparatus and are uncorrected.
Flash chromatography was carried out using silica gel (230-240 mesh). Chemical yields refer to pure isolated substances. TLC analysis was performed on glass-backed plates coated with silica gel 60 and an F254 indicator, and visualized by either UV irradiation or by staining with phosphomolybdic acid solution. Chiral HPLC analysis was performed on a JASPO HPLC system (JASCO PU-2089 pump and UV-2075 UV/Vis detector) equipped with a quaternary pump, using a Chiralpak AD-H, Chiralpak IA, Lux-Amylose-2 and Lux-i-Amylose-3 analytical columns (250 × 4.6 mm). UV detection was monitored at 254 nm. ESI mass spectra were obtained on an Agilent 5973 inert GC/MS system.

General Procedure for the Synthesis of Mannich Products 3a-k by Enantioselective Mannich Reaction of N-Boc Ketimines with β-Diketones
To a mixture of N-Boc ketimine 1a-g (0.1 mmol), catalyst C4 (0.002 mmol, 0.02 equiv) in 1.0 mL of toluene, β-diketone 2a-c (0.11 mmol, 1.1 equiv) was added at room temperature, and the reaction mixture was stirred in a Wheaton vial. The progress of the reaction was monitored by TLC analysis. After the completion of the reaction, the solvent was removed under reduced pressure. The crude reaction mixture was purified by flash column chromatography to afford the corresponding product 3a-k. The enantiomeric excess was determined by chiral-phase HPLC analysis using mixtures of hexane/isopropanol as eluent.