Synthesis of New Di- and Triamides as Potential Organocatalysts for Asymmetric Aldol Reaction in Water.

New di- or triamide organocatalysts derived from (L)-proline were synthesized and successfully used in the direct asymmetric aldol reaction of aliphatic ketones and aromatic aldehydes in water at 0oC in the presence of benzoic acid as co-catalyst. (S)-methyl-2-((S)-3-hydroxy-2-((S)-3-pyrrolidine-2-carboxamido)propanamido)-3-phenylpropanoate (7c) as organocatalyst showed best results under these reaction conditions, and good diastereoselectivities (up to 99%), enantioselectivities (up to 98%) and yields (up to 91%) were observed.


Introduction
The aldol reaction is an important tool in organic chemistry to synthesize attractive intermediates. This carbon-carbon bond formation reaction yields the β-hydroxyketones which are important precursors for pharma-ceuticals and natural products. Especially, the asymmetric version of the reaction has been extensively studied and used for the synthesis of valuable intermediates, highly functionalized and complex molecules with important biological activities. The most considerable asymmetric method is the using of asymmetric organocatalysis which are non-metal, small, easily synthesizable organic compounds with low toxicity and mostly natural products. [1][2][3] Among these natural organocatalysts, L-proline is the corner stone of natural amino acid organocatalysts and used successfully to catalyze various organic reactions. [4][5][6][7] But, this small organic compound has some drawbacks such as low solubility, low yields and low enantioselectivity with aromatic aldehydes as organocatalyst in asymmetric aldol condensation. 8 To overcome these drawbacks, some derivatives of proline had been succesfully synthesized and used in asymmetric aldol reaction and new modifications on proline are still under investigation. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Especially, proline based amides have been used succesfully for this reaction. [25][26][27][28][29][30] These derivatives have some advantages such as easy preperation, high stability and the presence of important functional groups. The catalytic effect of these catalysts based on secondary amine group on the pyrrolidine ring which form enamine to activate carbonyl group and the hyrogen bond donors which improve the activation of electrophile and selectivity. These advantages make them one of the most popular organocatalysts for organic syn-thesis and the design and investigation of new proline based amides as organocatalysts for various organic reactions is still under investigation by several research groups. 31,32 In our continuing research on the synthesis of chiral organocatalysts and their investigation in direct asymmetric aldol reaction, we have investigated the catalytic potential of some proline amide derivatives and also 1,2,3,4-tetrahydroisoquinoline and thiazolidine-4-carboxylic acid amide derivatives. [33][34][35][36] Now we herein report the synthesis of new proline based amides (3a-d and 7ac) ( Figure 1) and their application in asymmetric direct aldol reaction.

Results and Discussion
The reaction of Boc-protected L-proline with aminobenzamide derivatives (1a-d) which were synthesized by the reaction of isatoic anhydride with some amines and subsequent deprotection of products (2a-d) gave new pro- line based diamides (3a-d) (Scheme 1). Aminobenzamides (1a-d) were known in the literature 37,38 and the IR, 1 H NMR and MS data of the compounds 1a-d were in accordance with those data. The characterization of unknown compounds 2a-d and 3a-d were performed from their spectral data.
Compounds 7a-c were synthesized by a reaction sequence as amidation, hydrolysis, second amidation and deprotection reactions starting from Boc-protected proline and some amino acid esters and amine (Scheme 2). All new compounds were in accordance with their spectral data.
The reaction of p-nitrobenzaldehyde with cyclohexanone under diferent conditions was chosen as a model reaction to investigate the catalytic activities of amide compounds 3 a-d and 7 a-c, in direct asymmetric aldol reactions. First, the reaction was carried out with new amide compounds in water or dichloromethane (DCM), or without any solvent in the presence of benzoic acid (BA) as co-catalyst at room temperature. The results are shown in the Table 1. Compound 7c showed the best catalytic activity with good diastereoselectivity (90%), enantioselectivity (91%) and yield (89%). The poor activities of 3 a-d can be interpreted that the planar phenyl rings prevent the appropriate arrangement in the transition state due to the sterical effect and thus these compounds did not show good asymmetric induction. Compounds 7a and 7b also showed lower selectivities due to the steric effect of phenyl group. The best asymmetric induction was obtained with 7c, which has less steric effect around proline NH and prolinamide NH. It is also thought that the OH group is effective in asymmetric induction through hydrogen bond formation.
Then, various co-catalysts containing 4-nitrobenzoic acid (4-NBA), (2R, 3R)-(+)-tartaric acid (2R, 3R-TA), acetic acid (AcOH) and benzoic acid (BA) were also tested at 0 °C and room temperature to determine the optimum conditions with the best organocatalyst 7c. As it can be seen from the Table 2, the best results were obtained at 0 °C in the presence of BA as co-catalyst in water.
With these promising results, the substrates in the reaction were broadened with different aliphatic ketones and aromatic aldehydes in the presence of organocatalyst 7c under the optimum conditions (Table 3). All aldol products are known in the literature, and their structures  are in agreement with the literature data. 39,40 The diastereomeric ratios and enantiomeric excesses were determined by chiral HPLC analysis of the products by using literature methods. [41][42][43]

Conclusion
In conclusion, we have designed and synthesized new di-or triamide organocatalysts derived from (L)-proline and successfully used in the direct asymmetric aldol reaction of aliphatic ketones and aromatic aldehydes in water. Among the catalysts investigated in this study, catalyst 7c gave the best diastereoselectivities (up to 99%), enantioselectivities (up to 98%) and yields (up to 91%) when different aliphatic ketones and aromatic aldehydes with electron withdrawing groups were used. Furthermore, these catalysts showed their best catalytic activities in water which is also an important contribution to green chemistry requirements.

General
All reagents were of commercial quality and reagent quality solvents were used without further purification. IR spectra were determined on a Perkin Elmer, Spectrum One FT-IR spectrometer and Bruker Tensor 27 spectrometer. NMR spectra were recorded on Bruker Avance III 500 MHz and Varian-INOVA 500 MHz NMR spectrometer. Chemical shifts δ are reported in ppm with TMS as internal standart and the solvents were CDCl 3 and CD 3 OD. Column chromatography was conducted on silica gel 60 (40-63 μM). TLC was carried out on aluminum sheets precoated with silica gel 60F 254 (Merck). GC-MS spectrum was recorded on Agilent 6890N-GC-System-5973 IMSD spectrometer. LC-MS (QTOF) spectra were obtained on Agilent G6530B model TOF/Q-TOF Mass Spectrometer. Optical rotations were measured with Bellingham Stanley ADP-410 Polarimeter. Chiral HPLC analyses were performed with Shimadzu HPLC (Daicel Chiralpak AD and AD-H columns) equipped with SPD-20A detector and isopropanol/hexane mixtures as the eluent. The protection of L-proline was carried out according to the literature procedure. 44 Spectroscopic data of this compound were in accordance with its structure.

General Procedure for the Synthesis of Aminobenzamid Derivatives (1a-d)
The corresponding amines (1.15 mmol) were added to isatoic anhydride (1.00 mmol), dissolved in ethyl acetate and stirred at room tempearature for 5 hours (for compound 1a at 60 °C for 1.5 hours). The precipitates were filtered and the crude products were purified by crystallization or column chromatography.

General Procedure for Amidation Reactions
1-Hydroxy-1H-benzotriazole (HOBt, 1.00 mmol) was added to the stirred solution of Boc-protected acid (0.92 mmol) in dry THF. After 10 min stirring at 0 °C under nitrogen, dicyclohexylcarbodiimide (DCC) (1.00 mmol) was added. The mixture was stirred at 0 °C for 1 h, and the amine (1.02 mmol) was added. In the case of amino acid ester hydrochloride, to the suspension of amino acid ester hydrochloride in dry THF, triethylamine (0.5 mL) was added, and stirred at room temperature for 1 h. This solution was then added to the first mixture. The reaction mixture was then stirred at room temperature for 24 h, and the reaction monitored by TLC. The formed precipitate was removed by filtration, and filtrate evaporated under vacuum. The residue was dissolved in 50 mL of ethyl acetate, and resultant solution washed successively with

General Procedure for Hydrolysis to Synthesize 5a and 5c
To the solution of compounds 4a and 4c (1.00 mmol) in methanol (5 mL), 2 N aqueous NaOH was added at 0 °C to adjust to pH 11. The reaction mixture was stirred at 0 °C for 3 h, and at room temperature for 24 h, and then adjusted to pH 2 with aqueous solution of KHSO 4 . The solution was evaporated under vacuum to remove methanol, and extracted with ethyl acetate (30 mL × 3). The combined organic layer was successively washed with brine (20 mL × 2) and dried with anhydrous Na 2 SO 4 . After filtration, the filtrate was evaporated to provide compounds 5a and 5c which was used without any further purification.

General Procedure for Deprotection and Synthesis of 3a-d and 7a-c
To the solution of N-Boc protected compounds 2a-d and 6a-c (1.00 mmol) in dry DCM (15 mL) at 0 o C, TFA (27.00 mmol) was added dropwise over 5 min with stirring. The reaction mixture was stirred at 0 o C for 1 h, and at room temperature for 2 h, then 2M K 2 CO 3 was added to the reaction mixture to adjust basic pH. The organic phase was washed with water, dried over MgSO 4 , filtered and evaporated to give the pure compounds 3a-d and 7a-c.

General procedure for aldol reaction catalyzed by organocatalyst 7c
The catalyst 7c (0.10 mmol) and benzoic acid (0.10 mmol) were stirred in water at 0 °C for 10 min. Then, aldehyde (1.00 mmol) and ketone (10.00 mmol) were added, and the reaction mixture was stirred at 0 °C until the reaction completed. After the evaporation of water, the crude products were purified by column chromatography, eluted by EtOAc/hexane mixture. The enantioselectivity was determined by chiral HPLC with a Chiralpak AD and AD-H columns (UV detection set at 254 nm, i-PrOH/hexane as eluent).