Synthesis of (2 S ,3 S )-3-Aroyl Pyroglutamic Acid Amides †

: A new methodology for the asymmetric synthesis of enantiomerically enriched 3-aroyl pyroglutamic acid derivatives has been developed through effective 5- exo -tet cyclization of N -chlo-roacetyl aroylalanines. The three-step sequence starts with the synthesis of N -substituted ( S , S )-2-amino-4-aryl-4-oxobutanoic acids via highly diastereoselective tandem aza -Michael addition and crystallization-induced diastereomer transformation (CIDT). Their N -chloroacetylation followed by base-catalyzed cyclization and ultimate acid-catalyzed removal of chiral auxiliary without loss of stereochemical integrity furnishes the target substituted pyroglutamic acids. Finally, several series of their benzyl amides were prepared as 3-aroyl analogs of known P2X7 antagonists.


Introduction
Pyroglutamic acid and its derivatives are a valuable class of compounds found in various natural products and pharmaceuticals, representing either an important chiral auxiliary or building block for the asymmetric synthesis of many biologically and pharmaceutically valuable compounds [1]. Moreover, pyroglutamic acid derivatives have recently appeared to be efficient antagonists of specific types of purinergic receptors. Their inhibition has a promising impact on treating neurodegenerative diseases such as Alzheimer, Huntington, and Parkinson's disease [2]. This knowledge has led to various studies of the mentioned receptors based on the pyroglutamic derivatives' inhibitive activity.
Among these, the inhibitive activity of two pyroglutamic compounds, GSK1370319A and GSK1482160, was confirmed by clinical tests [3]. Accordingly, various potential antagonists were designed. The affinity with receptors has been modulated by changing substituents in the first, second, and fourth position of the lactam ring (Scheme 1) [4]. Stereoselective preparation of 3-substituted analogs has not attracted significant attention. The preparation of enantiomerically pure 3-substituted pyroglutamic acid derivatives enables exploring the importance of substitution in the first and third positions of the lactam ring and their impact on biological activity. To our surprise, despite the numerous synthetic approaches towards substituted pyroglutamic acid derivatives and the widely recognized importance and utility of these compounds, the synthesis of 3-acyl substituted analogs is still relatively unexplored [5]. This led us to develop a novel approach toward synthesizing 3-aroyl pyroglutamic acid amides in a few steps (Scheme 2).

Results and Discussion
The initial step represents the aza-Michael addition of chiral mediators (2a,b) to appropriate aroyl acrylic acids (1a-d) in tandem with CIDT (crystallization-induced diastereomer transformation) (Scheme 3). This efficient methodology spread among our research group allows us to prepare aroyl alanines with high diastereoselectivity. The methodology is based on equilibration of stereoisomers in a solution, and continuous crystallization of a single isomer acts as a driving force for its gradual accumulation in the reaction suspension [6][7][8][9]. This enables isolation of products (3a-g) by simple filtration (Table 1).  OMe ---Recently, we published a strategy of N-acylation of amino acids in the presence of chloroacetyl chloride, which yielded the acetylated derivatives [10]. These intermediates represent suitable starting materials for cyclization in the presence of a base. It has been reported previously that N-chloroacetylated amino acids undergo 4-exo-tet cyclization to form β-lactams (6) (Scheme 4) [11]. Due to the occurrence of the enolizable ketone, we expected that the in situ formed enolate undergoes fast 5-exo-tet cyclization instead of the 4-exo-cyclization observed by González-Muniz [11] (Table 2).  According to Abdi et al. [3], the occurrence of bulky benzylic group on the nitrogen of lactam ring causes inhibitive activity decline towards target receptors. Because of that, we decided to find the suitable conditions for debenzylation of the functional groups descended from the former chiral mediator used in the initial step (Scheme 5). In the case of removing the 1-ethyl-4-methoxybenzylic group, the reaction was successful under conditions i. On the contrary, the removal of the 1-ethylbenzylic group under the same conditions did not proceed, and according to HPLC analysis, there was no conversion to product observed. However, reactions took place in the presence of p-TsOH in refluxing toluene to yield deprotected derivatives 9b,d,f without loss of stereochemical integrity (Table 3).  The second part of the synthetic approach represents the preparation of 3-aroyl-Nmethyl pyroglutamic acids (10a-d), which covered up a sequence of three reactions (Scheme 6). The first step demands the preparation of methyl esters (Ma-d) due to the low solubility of starting deprotected derivatives (9a-d). The establishment of the methyl group on lactam nitrogen requires a basic environment. Due to the enolizable functional group occurrence, the importance of a sufficient amount of base and low temperature was essential. The 3-aroyl-N-methylated pyroglutamic acids (10a-d) were obtained after three steps with satisfactory overall yields (Table 4).  Thiophen-2-yl - Another valuable source about the antagonism of P2X7 receptors delivered knowledge that disubstituted benzylamides of corresponding pyroglutamic acids are the most effective. We prepared four series of pyroglutamic acid amides that vary in substituents in the first and the third position of the lactam ring. These compounds were obtained as products of well-known acid amine coupling with corresponding activation agent-CDI (Scheme 7) [13]. The desired final products (12) were isolated in yields within the range of 34-84% (Table 5).  Me Thiophen-2-yl Me --

General Methods
Unless otherwise noted, all chemicals were purchased from commercial sources and used without further purification. Bulk solutions were evaporated under reduced pressure using a Büchi rotary evaporator. HPLC analyses were performed on Varian system using Phenomenex Phenyl-Hexyl 5 µm column. The used mobile phase is specified for each experiment. Due to the long-term use of columns, retention time values are only approximate. Column chromatography was carried out using Silica 60A, particle size 20-45 micron, Davisil, purchased from Fisher Chemical. All reactions were followed by thin-layer chromatography (TLC) where practical, using Macherey-Nagel's precoated TLC sheets POLYGRAM SIL G/UV254 visualized under UV light (254 nm) or by staining with aqueous basic potassium permanganate or cerium molybdate solutions as appropriate.
All 1 H and 13 C NMR spectra were recorded using a Varian INOVA 300 MHz and Varian VNMRS 600 MHz spectrometers. Chemical shifts (δ) are given in parts per million (ppm). The 1 H NMR chemical shift scale is referenced to TMS internal standard or solvent residual peak. The 13 C NMR chemical shift scale is referenced to the solvent peak. Coupling constants (J) are given in hertz (Hz). The multiplicity of 1 H NMR signals is reported as follows: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, bs = broad singlet.
Optical rotations were recorded using a JASCO P-2000 polarimeter; [α]D values are reported in deg cm 3 g −1 dm −1 ; concentration (c) is given in g/100 mL at 589 nm. HRMS were measured using Thermo Scientific mass spectrometer with Orbitrap analyzer and HESI and APPI ionization.

Michael addition
Starting aroyl acrylic acids 1a-d were prepared according to general procedures described in the literature [14][15][16]. The synthetic strategy and the general procedures are shown on phenyl derivatives (Ar = Ph). Accordingly, derivatives 9c,e were prepared.

N-Bebenzylation-conditions ii (2S,3S)-3-(3-Nitrobenzoyl)-5-oxopyrrolidine-2-carboxylic acid-9b
Carboxylic acid (8d, 200 mg, 0.582 mmol) was dissolved in toluene (5.8 mL) and p-TsOH (4.0 equiv, 2.33 mmol, 443 mg) was added. The resulting colorless solution was stirred under reflux for 6 h. Reaction was accompanied with color change from colorless to dark brown, indicated styrene polymer formation. Upon completion, the reaction mixture was cooled down to room temperature and poured into water (10 mL). The resulting mixture was extracted with EtOAc (3 × 10 mL). The collected organic layers were washed with 10% solution of K2CO3 (3 × 15 mL). The pH value of water phase was adjusted to 2-3 and extracted with EtOAc (3 × 20 mL). The extract was dried over MgSO4 and concentrated under reduced pressure. Crude product was triturated with a small amount of Et2O Accordingly, derivatives 9d,f were prepared.

Conflicts of Interest:
The authors declare no conflict of interest.