Asymmetric Organocatalytic Mannich Reaction in the Synthesis of Hybrid Isoindolinone-Pyrazole and Isoindolinone-Aminal from Functionalized α-Amidosulfone

The investigation of the reactivity of an α-amido sulfone derived from 2-formyl benzoate under organocatalytic conditions in the presence of acetylacetone allowed the synthesis of a new heterocyclic hybrid isoindolinone-pyrazole with high enantiomeric excess. Dibenzylamine was also used as a nucleophile to afford an isoindolinone with aminal substituent in 3-position in suitable selectivity. The use of Takemoto’s bifunctional organocatalyst not only led to observed enantioselectivity but was also important in accomplishing the cyclization step in both cases. Notably, this catalytic system proved to be particularly effective in comparison to widely used phase transfer catalysts.


Introduction
The value of some classes of compounds is often associated with the structural diversity and the presence of different functionalities and moieties. This feature is relevant in classes of heterocycles such as isoindolinones [1][2][3][4][5][6][7][8][9] and pyrazoles [10][11][12], whose biological activities are tuned by the linking of additional nitrogen-containing substituents or heterocyclic groups such as piperazines, piperidines, and so on ( Figure 1).

Introduction
The value of some classes of compounds is often associated with the st diversity and the presence of different functionalities and moieties. This feature is r in classes of heterocycles such as isoindolinones [1][2][3][4][5][6][7][8][9] and pyrazoles [10][11][12] biological activities are tuned by the linking of additional nitrogen-con substituents or heterocyclic groups such as piperazines, piperidines, and so on (Fi Therefore, the development of new synthetic methods to furnish new hybrid molecules is of paramount importance, even though additional difficulties can be encountered, especially when new stereocenters are formed [13]. In fact, asymmetric construction of the isoindolinone ring is a challenging research field, and many groups are involved [13]. It is worth noting that enantiopure isoindolinones have been reported to show enhanced biological properties, but often inefficient resolution of racemic mixtures or the use of chiral auxiliaries have been utilized to achieve this goal [13]. In this context, our group dedicated many efforts to the asymmetric synthesis of 3-substituted [14][15][16] and 3,3-disubstituted isoindolinones, [17,18] developing new cascade-type reactions often employing organocatalytic systems and producing several new enantioenriched products. Some of the obtained isoindolinones were also used in the total synthesis of biologically relevant compounds [5,15,18]. The reasons for the success of the developed methods also rely on the rational design and synthesis of suitable starting materials. Recently, readily available α-amido sulfones derived from 2-formyl benzoates revealed effective starting materials in the asymmetric synthesis of 3-nitromethyl isoindolinones, reaching enantioselectivities up to 98% ee and suitable yields in the process of asymmetric aza-Mannich/lactamization reaction [16]. Takemoto's neutral bifunctional organocatalyst [19] was not only essential to achieving high enantioselectivity but was also crucial for catalyzing the cyclization step of the process [13].
The easy access and high stability of these N-carbamoyl-α-amidosulfones gives several practical advantages avoiding, first of all, the use of the respective preformed imines, which were difficult to isolate, while the formation of the imines from α-amidosulfones can be easily carried out in situ in the presence of an inorganic base [20,21]. This approach is explored in asymmetric catalysis for the synthesis of functionalized chiral amines in nitro-Mannich reactions [16,20,21], while different nucleophiles [22,23] as dicarbonyl compounds [22] or amines are less investigated or not investigated at all. In these asymmetric transformations, chiral ammonium salts, employed as phase transfer catalysts, play a fundamental role in attaining high enantioselectivity [20,21,23], while neutral chiral organocatalysts have been less investigated [16,22]. The presence of easily removable N-Boc or N-Cbz groups is a further synthetic advantage of these methods [16,20,21].
Given the efficiency of the method in the synthesis of 3-nitromethyl isoindolinones [16] and the utility of the target compounds, the application of this methodology to different classes of nucleophiles is of paramount importance to attain new, differently decorated derivatives.
Therefore, in the present investigation, we describe the asymmetric synthesis of a new heterocyclic hybrid molecule as a consequence of the use of acetylacetone as a nucleophile in an asymmetric reaction with N-carbamoyl-α-amidosulfone derived from 2-formylbenzoate in the presence of Takemoto's bifunctional organocatalyst. The investigation of the reaction mechanism led to the development of a new synthetic route leading to the obtaining of a new hybrid isoindolinone-pyrazole in high enantioselectivity. The use of dibenzylamine as a nucleophile was also investigated for comparison. In both cases, Takemoto's bifunctional organocatalyst was more effective than benchmark chiral phase transfer catalysts.

Investigation of the Reactivity of the α-Amido Sulfone 1 under Asymmetric and Achiral Conditions
In the first set of experiments, α-amido sulfone 1 was reacted with acetylacetone in the presence of chiral neutral bifunctional organocatalysts and an ammonium salt in combination with an inorganic base ( Figure 2 and Table 1       We soon realized that the additional step is rapid, even below ambient tem while the cyclization is slow at room temperature, probably because of nucleophilicity of a NH belonging to a carbamate group. In the presence of Ta catalyst I, even after 72 h of stirring at room temperature, we recovered acyclic p with moderate enantioselectivity in DCM ( the experiment performing the purification at the end of the addition step at −40 • C, confirming the high enantioselectivity but with improved yield (Entry 4). Focusing only on the addition step, other catalytic systems were also tested for comparison as the PTC chiral ammonium salt II derived from quinine (Entry 5), cinchonine III (Entry 6), or epi-quinine bifunctional organocatalyst IV (Entry 7). Good enantioselectivity was also obtained with chiral ammonium salt II (Entry 5), but Takemoto's catalyst was the best performing one affording 2 with 88% ee. At this point, we thought that the increase in the temperature after the addition step could favor lactamization. To achieve this goal, we had to use a higher boiling solvent as 1,2-DCE in a mixture with DCM since 1,2-DCE has an unsuitable melting point of −35 • C, and DCM has an unsuitable boiling point of 40 • C. We first repeated the asymmetric addition step, confirming the goodness of the used mixture of solvents leading to similar results in terms of yield and enantioselectivity (Entry 8). Then, in a second experiment, we first performed the addition step at −40 • C until the reaction was complete, as checked by TLC, and then the reaction mixture was warmed up to 60 • C. This experiment was successful since the target cyclic isoindolinone was obtained in suitable yield but, unfortunately, as a racemate (Entry 9).
The reaction was also investigated under achiral conditions in the presence of K 2 CO 3 only (Table 2). In all the experiments, we recovered the acyclic product 2, even after prolonged heating at 60 • C and in different solvents such as toluene, DCM, or acetonitrile, confirming the importance of using Takemoto's catalyst I for the cyclization process. On the other hand, the addition process appears to be slow at −40 • C (Entries 1 and 2), explaining the high enantioselectivity (see Table 1) because of the lack of the background achiral reaction at low temperature. Mol. Sci. 2023, 24, x FOR PEER REVIEW 1,2-DCE in a mixture with DCM since 1,2-DCE has an unsuitable and DCM has an unsuitable boiling point of 40 °C. We first re addition step, confirming the goodness of the used mixture of so results in terms of yield and enantioselectivity (Entry 8). Then, we first performed the addition step at −40 °C until the reaction w by TLC, and then the reaction mixture was warmed up to 60 ° successful since the target cyclic isoindolinone was obtained unfortunately, as a racemate (Entry 9).
The reaction was also investigated under achiral conditions i only ( Table 2). In all the experiments, we recovered the acycli prolonged heating at 60 °C and in different solvents such as toluen confirming the importance of using Takemoto's catalyst I for the the other hand, the addition process appears to be slow at −4 explaining the high enantioselectivity (see Table 1) because of the achiral reaction at low temperature.

Investigation of the Cyclization of Intermediate 2
Cyclization was investigated directly on the purified enantioe 2. Several cyclization conditions were tested (

Investigation of the Cyclization of Intermediate 2
Cyclization was investigated directly on the purified enantioenriched acyclic product 2. Several cyclization conditions were tested (Table 3). Reaction performed at 60 • C in the presence of K 2 CO 3 only provided decomposition products, confirming the hypothesis that Takemoto's catalyst is important for promoting lactamization (Entries 1 and 2), probably with a mechanism as described in Figure 3. When a combination of K 2 CO 3 and Takemoto's catalyst was used, we were able to obtain the cyclic product, but unfortunately, as a racemate (Entries 3-7). The use of a catalytic amount of both Takemoto's catalyst and K 2 CO 3 at 40 • C was effective as well, but again racemization was observed (Entry 6). This experiment was repeated and stopped at about 50% conversion (Entry 7). Again, the cyclic product was racemic, while the unreacted acyclic intermediate maintained 88% ee.   All these experiments point out that racemization may occ probably via a retro aza-Michael reaction under basic condition 1. Removing the acidic proton of acetylacetone moiety should pr    All these experiments point out that racemization may probably via a retro aza-Michael reaction under basic cond 1. Removing the acidic proton of acetylacetone moiety shou Scheme 1. Proposed racemization process. All these experiments point out that racemization may occur on the cyclic product, probably via a retro aza-Michael reaction under basic conditions, as depicted in Scheme 1. Removing the acidic proton of acetylacetone moiety should prevent racemization.

Entry
Cat. I/K2CO3 (Eqv/Eqv) Solvent T (°C)  All these experiments point out that racemization probably via a retro aza-Michael reaction under basic 1. Removing the acidic proton of acetylacetone moiety Scheme 1. Proposed racemization process.

Asymmetric Synthesis of Isoindolinone-Pyrazole Hybrid
In order to confirm this hypothesis and overcome the racemization issue, our attention focused on the use of hydrazine, which should react selectively with acetylacetone to provide pyrazole ring, [24] a moiety also found in a very large number of bioactive compounds, [10][11][12] often combined with other heterocycles [12]. Therefore, the enantioenriched acyclic product 2 was treated with hydrazine under the conditions shown in Scheme 2, leading nicely to the pyrazole derivative 4 in a very high yield. Then, it was subjected to cyclization, under the conditions of Table 3, Entry 6, nicely furnishing the N-Boc protected isoindolinone 5. This derivative was, then, deprotected in the presence of TFA, without any decrease in enantiopurity as determined by HPLC on chiral stationary phase leading to NH-free hybrid isoindolinone-pyrazole 6 with unchanged enantiopurity of 89% ee. Absolute configuration (AC) of (+)-6 was found to be (R) by X-ray analysis performed on a suitable single crystal (Figure 4 and Figure S1 and Scheme 2). This finding also allowed us to assign the AC of all the previous intermediates and, therefore, of (+)-2, which was obtained with catalyst (R,R)-I. The sequence of reactions was also performed by readily available (S,S)-I, leading to similar results in terms of yields and enantiomeric purity for the opposite enantiomer (S)-2, demonstrating the goodness of our hypothesis and feasibility of the adopted strategy to prevent racemization.

FOR PEER REVIEW 6 of 11
finding also allowed us to assign the AC of all the previous intermediates and, therefore, of (+)-2, which was obtained with catalyst (R,R)-I. The sequence of reactions was also performed by readily available (S,S)-I, leading to similar results in terms of yields and enantiomeric purity for the opposite enantiomer (S)-2, demonstrating the goodness of our hypothesis and feasibility of the adopted strategy to prevent racemization.
The racemic hybrid compound was alternatively obtained by reaction of 3acetylacetone isoindolinone [25] 7 with hydrazine (Scheme 3, see Supplementary Materials for further experimental details). On the other hand, when both 2 and 7 were treated with phenylhydrazine, we recovered the starting materials unreacted.  finding also allowed us to assign the AC of all the previous inte of (+)-2, which was obtained with catalyst (R,R)-I. The seque performed by readily available (S,S)-I, leading to similar resu enantiomeric purity for the opposite enantiomer (S)-2, demonst hypothesis and feasibility of the adopted strategy to prevent rac Scheme 2. Asymmetric synthesis of isoindolinone-pyrazole hybrid.
The racemic hybrid compound was alternatively obta acetylacetone isoindolinone [25] 7 with hydrazine (Schem Materials for further experimental details). On the other hand, treated with phenylhydrazine, we recovered the starting mater

Reaction with Dibenzylamine (DBA)
The reactivity of 1 was next investigated in with the aim of producing a 3-substituted iso Several catalytic systems were tested as well (T Scheme 3. Synthesis of racemic isoindolinone-pyrazole hybrid.

Reaction with Dibenzylamine (DBA)
The reactivity of 1 was next investigated in the presence of the secondary amine DBA with the aim of producing a 3-substituted isoindolinone with an aminal functionality. Several catalytic systems were tested as well (Table 4 and Figure 2).  As noted from the data reported in Table 4, the best results were obtained again in the presence of Takemoto's catalyst I at −20 • C in toluene (Entry 7), while other catalytic systems such as the PTC II (Entry 4), cinchonine III (Entry 5) and the thiourea-epi-quinine IV (Entry 6) were less effective both in the preliminary screening at RT as well as at −20 • C (Entry 8). In this investigation, both (R,R)-I and (S,S)-I were used, leading the opposite enantiomers of 9 as analyzed by HPLC on chiral stationary phase. Interestingly, control experiments performed in the absence of K 2 CO 3 and with 2 eqv of DBA highlighted that the in-organic base is essential to obtain satisfactory yield and enantioselectivity (Entries 1 and 10). Other conditions were also investigated as different solvents (Entries 3 and 9), lower temperature (Entry 11), and other catalytic systems such as Maruoka's catalyst, a squaramide bifunctional organocatalyst and a PTC derived from (1R,2R)-diaminocyclohexane. All these catalysts were ineffective as well (for brevity, these data were reported in the Supplementary Materials, Table S1).
Also, in the presence of this nucleophile, the cyclization proved to be slow below RT but faster than previously observed with intermediate 2. In fact, further stirring the reaction mixture at RT was enough to obtain the expected cyclic product, usually in very suitable yields.

General Information
Unless otherwise noted, all chemicals, reagents, and solvents for the performed reactions are commercially available. α-Amidosulfone 1 was prepared according to a literature procedure [16]. All the reactions were monitored by thin layer chromatography (TLC) on precoated silica gel plates (0.25 mm) and visualized by fluorescence quenching at 254 nm. Flash chromatography was carried out using silica gel 60 (70-230 mesh, Merck, Darmstadt, Germany). Yields are given for isolated products showing one spot on a TLC plate, and no impurities were detectable in the NMR spectrum. The NMR spectra were recorded on Bruker DRX 600, 400, and 300 MHz spectrometers (600 MHz, 1 H, 150 MHz, 13 C; 400 MHz, 1 H, 100.6 MHz; 13 C, 300 MHz, 1 H, 75.5 MHz, 13 C, 250 MHz, 1 H, 62.5 MHz, 13 C). Internal reference was set to the residual solvent signals (δ H 7.26 ppm, δ C 77.16 ppm for CDCl 3 ). The 13 C NMR spectra were recorded under broad-band proton decoupling. Spectra are reported only for unknown compounds. The following abbreviations are used to indicate the multiplicity in NMR spectra: s-singlet, d-doublet, t-triplet, q-quartet, dd-doublet of doublets, m-multiplet, brs-broad signal. Coupling constants (J) are quoted in Hertz. High-resolution mass spectra (HRMS) were acquired using a Bruker SolariX XR Fourier transform ion cyclotron resonance mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 7T refrigerated actively shielded superconducting magnet. For ionization of the samples, electrospray ionization (ESI) or MALDI was applied. IR spectra were recorded on an IR Bruker Vertex 70 v spectrometer. Polarimeter Jasco P-2000 (Tokyo, Japan). The crystal was measured on a Bruker D8 QUEST diffractometer equipped with a PHOTON detector using Cu-Kα radiation (λ= 1.54178 Å). Data indexing (Table S2), reduction and absorption correction were performed using APEX3 software suite, version 2015. 5-2 [26-28]. The structure was solved using SHELXS [29] and refined by means of full matrix least-squares based on F 2 using the program SHELXL [30] and OLEX2 [31] as a GUI. The Flack parameter was defined accordingly to ref [32].

Conclusions
In this work, we have investigated the Mannich reaction of an α-amido sulfone derived from 2-formyl benzoate under organocatalytic conditions in the presence of acetylacetone and dibenzylamine as nucleophiles. The investigation of the reactivity of acetylacetone derivative allowed to synthesize a new heterocyclic hybrid molecule formed by an isoindolinone substituted in 3-position with a pyrazole ring in high enantioselectivity of 89% ee and an overall yield of about 70% in 4 synthetic steps. Moderate enantioselectivity was observed using dibenzylamine as a nucleophile leading to a 3-substituted isoindolinone with an aminal functionality. In both cases, Takemoto's bifunctional organocatalyst proved to be more effective than other catalytic systems as phase transfer catalysts (PTC). In this investigation, the bifunctional nature of the used catalyst also favored lactamization, as confirmed by a series of control experiments.