One-Pot Synthesis of Novel Chiral β-Amino Acid Derivatives by Enantioselective Mannich Reactions Catalyzed by Squaramide Cinchona Alkaloids

An efficient one-pot synthesis of novel β-amino acid derivatives containing a thiadiazole moiety was developed using a chiral squaramide cinchona alkaloid as organocatalyst. The reactions afforded chiral β-amino acid derivatives in moderate yields and with moderate to excellent enantioselectivities. The present study demonstrated for the first time the use of a Mannich reaction catalyzed by a chiral bifunctional organocatalyst for the one-pot synthesis of novel β-amino acid derivatives bearing a 1,3,4-thiadiazole moiety on nitrogen.

Our group has previously developed a highly enantioselective Mannich reaction catalyzed by cinchona alkaloid thiourea to produce novel β-amino acid ester and ketone derivatives containing benzoxazol and benzothiazole moieties, and β-amino ketones containing benzothiazole units [27][28][29]. In the present work, we report an enantioselective synthesis of β-amino acid derivatives bearing a 1, 3, 4-thiadiazole moiety on nitrogen by an asymmetric Mannich reaction catalyzed by a squaramide cinchona alkaloid catalyst. To the best of our knowledge, our study is the first one on the asymmetric synthesis of β-amino acid derivatives containing a 1,3,4-thiadiazole moiety on nitrogen in the presence of a squaramide organocatalyst.

Scheme 1. Synthesis of catalyst SQ.
In search of the optimum catalyst for the enantioselective synthesis of β-amino acids containing 1,3,4-thiadiazoles, we initially tested two commercially available cinchona alkaloid catalysts Q-1 and Q-2 ( Figure 1) in the catalytic one-pot asymmetric Mannich reaction of 2-amino-1,3,4-thiadiazole (1), benzaldehyde (2, R 1 = Ph) and dimethyl malonate (3, R 2 = Me) (Scheme 2).  The quinine catalyst Q-1 turned out to be a poor catalyst, affording a 37% yield and 11% ee, whereas the catalyst Q-2 performed slightly better, with 39% yield and 37% ee, respectively, when the reactions were carried out at 60 °C for 96 h (entry 1 and 2, Table 1). Since both Q-1 and Q-2 failed to achieve the desired high yield and enantioselectivity, we continued our search for a better catalyst and found that the cinchona alkaloid derivative (SQ) bearing both the hydrogen-bond donor squaramide and the hydrogen-bond acceptor tertiary amines delivered superior results. Compared with the quinine Q-1 and 9-amino (9-deoxyquinine) Q-2 catalysts, the SQ catalyst bearing strong electron-withdrawing trifluoromethyl substituents on its benzene ring achieved better yield (41%) and much higher enantioselectivity (91%) (entry 3, Table 1). The superior performance of SQ catalyst could be attributed to its ability to promote the reaction through double-hydrogen bond activation of the substrate. The effects of three important experimental parameters (solvent, catalyst loading, and reaction temperature) on the SQ-catalyzed reactions were examined to determine the optimal reaction conditions ( Table 2). We found that solvent significantly affected the reaction yield and ee of the final product. Among the four solvents tested, the best yield was obtained in methanol (53%) while the highest ee was achieved in toluene (91%). (entries 1-4, Table 2). The catalyst loading also affected the yield and ee of 4d. When the reaction was conducted at 60 °C in toluene, the highest yield (49%; entry 4, Table 2) and ee (91%; entry 4, Table 2) were obtained with the highest catalyst loading of 10 mol%. The yield and ee were also affected by the reaction temperature. When the reaction temperature was increased from room temperature to 60 °C, the reaction yield and ee increased by 12% and 11%, respectively (entries 4 and 5, Table 2). Taken together, the optimum result was achieved at 60 °C with 10 mol% catalyst loading in toluene. Having established the ideal reaction conditions, we explored the synthetic scope of the reaction with different aldehydes and malonates as substrates. The results were summarized in Table 3. The highest enantioselectivities were obtained with a phenyl R 1 group with higher than 45% yields (45% yield, 91% ee, entry 5; 52% yield, 99% ee, entry 6, Table 3). However, when the R 1 group was substituted phenyl, the yields dropped below 45% (except product 4e, 61% yield, entry 4, Table 3) and the ee values were < 60%, regardless of the substituent being an electron-withdrawing group (chlorine and trifluoromethyl) or an electron-donating group (methoxyl) (entries 1-4, Table 3). Therefore, the reaction showed highest enantioselectivity and best chemical yields with unsubstituted benzaldehyde. 2-Amino-1,3,4-thiadiazole (1) aldehyde 2, and dimethyl malonate (or diethyl malonate) 3 were mixed in the presence of catalyst SQ and stirred at 60 °C for 93-97 h. The in situ generation of imine was confirmed by thin-layer chromatography (TLC) and mass spectrometry (MS).

Scheme 3.
Proposed mechanism of the asymmetric Mannich reaction catalyzed by squaramide cinchona alkaloid.
We speculate that while the imine was activated by the squaramide moiety through hydrogen bonding, the intermediate transition state (enol form of the malonate) was activated by the basic nitrogen atom in the tertiary amine moiety of the catalyst, leading to a stable transition state (Scheme 3). These speculated interactions in our proposed mechanism might be responsible for the observed stereochemical outcome of the reaction and the enhanced reaction rate.

General
Unless otherwise stated, all reagents and reactants were purchased from commercial suppliers. Melting points were determined with a XT-4 binocular microscope (Beijing Tech Instrument Co., Beijing, China) without correction. 1 H-NMR, 13 C-NMR, and 19 F-NMR spectra were recorded on a JEOL ECX 500 NMR spectrometer at room temperature operated at 500 MHz for 1 H-NMR, 125 MHz for 13 C-NMR, and 470 MHz for 19 F-NMR using CDCl 3 as solvent and TMS as an internal standard. Infrared spectra were recorded in KBr on a Bruker VECTOR 22 spectrometer, and elemental analysis was performed on an Elemental Vario-III CHN analyzer. The progress of the reactions was monitored by TLC, and preparative TLC was performed on silica gel GF 254 . High-performance liquid chromatography (HPLC) analysis was performed on an Agilent 1100/1200 series instrument equipped with a quaternary pump using a Daicel Chiralpak IA Column (250 mm × 4.6 mm). UV absorption was monitored at 270 nm. Specific rotations were measured on a WZZ-2S digital polarimeter with a sodium lamp. The intermediate 3-(3,5-bis(trifluoromethyl)phenylamino)-4-methoxycyclobut-3-ene-1,2-dione was prepared according to a literature procedure [26].

Conclusions
In conclusion, we have developed a convenient one-pot synthesis of novel β-amino acid derivatives bearing a 1,3,4-thiadiazole moiety on nitrogen which have valuable applications in medicinal and chemical synthesis and studies using an enantioselective Mannich reaction catalyzed by the chiral squaramide cinchona alkaloid catalyst SQ. The desired β-amino acid derivatives were produced in moderate yields (39%-61%) and with moderate to excellent enantioselectivities (41%-99%). Further research aimed at investigating the mechanism and scope of the catalysts and the reactions, as well as the activity of the Mannich products against plant viruses, is underway and will be reported in due course.