Ring-Opening of Epoxides with Amines for Synthesis of β -Amino Alcohols in a Continuous-Flow Biocatalysis System

: An efficient method for the preparation of β -amino alcohols catalyzed by lipase TL IM from Thermomyces lanuginosus in a continuous-flow reactor was developed. The eco-friendly biocatalyst combined with continuous-flow reaction technology displayed high efficiency in the synthesis of β -amino alcohols. The benign reaction conditions (35 °C) and short residence time (20 min), together with the use of low cost and readily available starting materials, make this synthetic approach a promising alternative to current β -amino alcohol synthesis.


Introduction
β-Amino alcohols are vital intermediates for the preparation of various bioactive compounds, chiral auxiliaries, unnatural amino acids and organocatalysts [1][2][3][4][5][6]. Moreover, β-amino alcohols play an increasingly key role in pharmaceuticals, where they are used as cardiovascular drugs (I), anti-asthma drugs (II), antimalarial drugs (III), antihypertensive drugs (IV), β-blockers, antibacterial and hypoglycemic drugs, etc. [7][8][9][10] (Figure 1). Given the significance of β-amino alcohols in organic synthesis and pharmaceutical development, the development of a flexible synthetic pathway to this moiety is desperately desirable. The asymmetric ring-opening (ARO) reaction of epoxides with amines is the most straightforward route to prepare β-amino alcohols. In recent years, several catalysts such as Lewis acids [11][12][13], metal-organic frameworks [14] and Bismuth (III) salts [15][16][17][18] have been reported for ring-opening reactions. However, these approaches usually require the use of costly reagents, high catalyst loadings, poor regioselectivity and also entail side effects, such as the rearrangement of epoxides to allyl alcohols under strong alkaline conditions or polymerization under strongly acidic conditions. Therefore, it is urgent to develop a simple, efficient and environmentally benign synthetic method. Biocatalysis shows good selectivity in organic synthesis, hence making it an environmentally friendly alternative to chemical synthesis [19][20][21][22]. Lipases have been extensively explored in Michael reactions, esterification and Knoevenagel-Michael cascade reactions to synthesize a variety of vital molecules related to foods, pharmaceuticals, flavorings and fine chemicals [23][24][25][26][27][28]. However, so far the synthesis of β-amino alcohols from the ring-opening of epoxides with aromatic amines catalyzed by enzymes has rarely been studied. Lipase from Aspergillus oryzae has been reported to be used in the synthesis of β-amino alcohols through a one-pot reaction along with amines, epichlorohydrin and aromatic phenols. This reaction needs to be carried out under the synergistic action of phase transfer catalyst 1-butyl-3-methylimidazolium chloride salt ([BMIM]Cl) [29]. Pankaj [30] used Candida rugosa lipase to catalyze the ring-opening of epoxides with amines in diisopropyl ether (DIPE), and 70%-90% yields could be obtained in 8-12 h. Compared to chemical catalysis, enzymatic reactions are relatively mild, but they need extra auxiliary reagents or a longer reaction time to achieve the anticipated yield (Scheme 1).

Scheme 1.
Strategies for the preparation of β-amino alcohols by lipase.
Recently, continuous-flow technology has been widely used in enzymatic reactions, and the combination of biological catalytic technology and flow chemistry opens influential novel process windows [31][32][33][34][35]. With the advantages of high specific surface area, good heat transfer performance and efficient mixing, continuous-flow reactors can increase the reaction efficiency and selectivity [36][37][38]. To explore an efficient and novel synthesis protocol for β-amino alcohols, we first reported a continuous-flow method for the preparation of β-amino alcohol derivatives catalyzed by lipase from Thermomyces lanuginosus in continuous-flow reactors (Scheme 1). The purpose of our research is to investigate the effect of different reaction parameters on this enzymatic flow preparation method. We would like to rapidly establish relevant chemical compound libraries through this novel synthetic method for future pharmaceutical screening.

Effect of Reaction Media and Catalyst
First, we explored the enzymatic synthesis of β-amino alcohols by a ring-opening reaction in continuous-flow reactors (Table 1). We chose the reaction of epichlorohydrin with aniline as the model reaction to study (Scheme 2). For the β-amino alcohol synthesis reaction, the enzyme and the reaction medium are significant elements that influence the enzyme activity and reaction process. We conducted a blank control trial and found that this reaction could not proceed without the participation of lipase. Next, two different enzymes, Subtilisin and Lipozyme TL IM, were screened for the ring-opening of epichlorohydrin with aniline in different organic solvents. It was observed that the reaction could be promoted by increasing the polarity of the solvent. Lipozyme TL IM in methanol could catalyze the nucleophilic molecules to form the main product 3a with the best yield of 85.2% (Entry 2, Table 1). As a result, we synthesized the β-amino alcohols catalyzed by Lipozyme TL IM in methanol.

Effect of Reaction Temperature
Temperature has an important effect on the stability and catalytic activity of the enzyme [39]. In the interest of obtaining the optimum temperature, we adjusted it from 30 to 50 °C and the yield of products was determined at a different temperature with 30 min residence time. As shown in Figure 2, it is obvious that the yield increased from 30 to 35 °C. If the reaction temperature continued to increase, the yield decreased. Therefore, the optimum temperature of synthesizing β-amino alcohol is 35 °C.

Effect of Residence Time
The residence time is an important parameter in the continuous processes because it describes the contact time between the reagents and the biocatalyst, which affects the conversion of the reaction. Hence, the influence of residence time on the reaction was studied by changing the residence time from 10 to 30 min, and the results are shown in Figure 3. With the increase in residence time, interactions between the substrate and enzyme increased, which led to a higher yield of β-amino alcohol. After 20 min, the optimum yield was obtained under a flow rate of 31.4 μL min −1 . Hence, 20 min was chosen as the best residence time for the subsequent study.

Effect of Substrate Ratio
In enzyme catalysis reaction, the relative content of substrates is the key to determine the composition of the final product and the efficiency of enzyme catalysis [40]. The influence of substrate ratio on reaction efficiency was studied by using different molar ratios of aniline to epichlorohydrin with a 30 min residence time ( Figure 4). As the molar ratio of substrate increased, the catalytic efficiency of β-amino alcohol synthesis was improved. The highest yield of 91.3% was achieved when a molar ratio of aniline to epichlorohydrin of 1:1 was used. However, further changes in the molar ratio reduced the yield. Hence, aniline/epichlorohydrin = 1:1 is the optimal ratio for preparing β-amino alcohol.

The Effect of Enzyme Reusability on the Reaction
Enzyme reusability is a vital aspect of an enzymatic reaction due to its economic significance. The yield of β-amino alcohol 3a was examined over 10 reaction cycles to determine its reusability. As shown in Figure 5, an increase in the number of cycles is accompanied by a progressive decrease in yields, and a 43.4% yield was obtained during the tenth cycle. This result indicates that Lipozyme TL IM has sufficient reusability.

The Effect of Aromatic Amine Structure on the Reaction
We further researched the influence of substrate structure on the synthesis of β-amino alcohols ( Figure 6). As illustrated in Table 2, the yield of p-toluidine to epichlorohydrin was much higher (92.7%, entry 5) than 4-chloroaniline (70.2%, entry 6) under the same conditions, which indicates that the introduction of an electron donor can effectively improve the reactivity of the amine, and the amines containing electron withdrawing groups such as halogen are not conducive to the ring-opening reaction. In addition, we also found that aniline (91.3%, entry 1) has a better reactivity than N-methylaniline (73.2%, entry 2) because the steric hindrance of the secondary amine slowed down nucleophilic attack. Due to the steric effect, the amine was inclined to nucleophilic attack at the carbon atoms with less steric hindrance and the major product 3 was formed ( Table 2). These results were confirmed by NMR. Lastly, to study the applications of this innovative β-amino alcohol synthesis reaction catalyzed by lipase from Thermomyces lanuginosus in a continuous-flow reactor, six amines (aniline, N-methylaniline, o-toluidine, m-toluidine, p-toluidine, 4-chloroaniline) and three epoxides (epichlorohydrin, styrene oxide, cyclohexene oxide) were subjected to the optimized conditions (Scheme 3). Eighteen β-amino alcohol compounds were prepared in excellent yields by a ring-opening reaction catalyzed by Lipozyme TL IM under a continuous-flow reactor ( Table 2).

Experimental Setup and Experiment Conditions
The equipment configuration that was used for the synthesis of β-amino alcohols by a ring-opening reaction is shown in Figure 7 as well as in Figures

Thin-Layer Chromatography (TLC)
TLC analysis with ethyl acetate/petroleum ether 1/4 (v/v) as the eluent. The results were detected by UV irradiation at 254 nm.

Nuclear Magnetic Resonance (NMR)
The products were characterized by 1 H NMR and 13 C NMR. 1 H (500 MHz) and 13 C (126 MHz) NMR spectra were obtained on a Bruker-AVANCE Ⅲ 500 MHz NMR spectrometer. The sample was dissolved in CDCl3 or DMSO-d6 and with 22 °C temperature.

Conclusions
In conclusion, we presented an attractive method for the synthesis of β-amino alcohols in a continuous-flow reactor. Lipozyme TL IM from Thermomyces lanuginosus was first used in the β-amino alcohol synthesis reaction. The combination of an environmentally friendly biocatalyst and continuous-flow technology displayed high efficiency in our research. We explored the influences of reaction medium, temperature, substrate ratio, residence time and reactant structure on the ring-opening reaction. Using this method, 18 β-amino alcohols were quickly prepared. The striking characteristics of this method include the green reaction medium (methanol), short period of residence time (20 min), mild reaction temperature (35 °C) and the flexible and easy to control biocatalysis reaction process, which blaze a new road for the synthesis of β-amino alcohols. The use of continuous-flow technology to prepare β-amino alcohol derivatives is an innovation that allows the application of enzyme reactors in the biotransformation of β-amino alcohol derivatives.