1 Introduction
In the pharmaceutical and fine chemistry, α-substituted β ketoesters are widely used as intermediates. For example, ethyl-α-(2’-furfuryl) acetoacetate is used with iridium in organic light-emitting diode screens (Saha, Rozenberg, & Lemcoff, 2015). Ethyl-2-pentylacetoacetate is used to synthesize porphyrins (Mikhalitsyna et al., 2012) which are important components in supramolecular chemistry. Similarly, ethyl 2-[(4-chlorophenyl)methyl]-3-oxobutanoate and ethyl 2-benzylacetoacetate have pharmaceutical uses. Their downstream products, including pyridine benzimidazole derivatives, have been used to develop anti-tuberculosis and schistosomiasis drugs (Okombo et al., 2017; Pieroni et al., 2011). Currently, the main strategy to synthesize α-substituted β ketoesters is through alkylation of 1,3-dicarbonyl compounds, during which nucleophiles attack highly reactive carbons, leading to the formation of carbon-carbon bonds (Shirakawa & Kobayashi, 2007). For the nucleophilic substitution reactions, halogenated alkanes are traditional substrates. However, these substrates have several disadvantages, such as salt treatment and effluent disposal restrictions due to halogen removal. From the perspectives of atomic economic efficiency and product processing, alcohols and enols have been identified as ideal substrates. For example, Makoto Yasuda and co-workers (Yasuda, Somyo, & Baba, 2006) reported that indium can be used as a catalyst to generate carbon-carbon bonds between alcohols and active methylene in toluene. In this method the byproduct, water, requires comparatively easy post-treatment. The disadvantage of this strategy is that catalytic activation of alcohols is difficult due to the poor leaving ability of the hydroxyl group. Thus, significant amounts of a Brønsted acid or of Lewis acid must be employed to promote the reactions.
Enzymes are green catalysts widely used in organic synthesis due to their excellent chemoselectivity, stereoselectivity, and mild reaction conditions (Yang, Hechavarria Fonseca, & List, 2004; Yang, Hechavarria Fonseca, Vignola, & List, 2005; Ouellet, Tuttle, & MacMillan, 2005; Tuttle, Ouellet, & MacMillan, 2006; Shoda, Uyama, Kadokawa, Kimura, & Kobayashi, 2016). Some researchers promote the reduction of products of Knoevenagel condensation to synthesize α-substituted β ketoesters. For example, Jimenez et al. catalyzed Knoevenagel condensation between aryl aldehydes and malononitrile in methanol and then reduced the unsaturated alkenes using whole cells of Penicillium citrinum CBMAI 1186, with excellent yields (~98%) ( Jimenez, Ferreira, Birolli, Fonseca, & Porto, 2016). However, the reaction requires separation and purification between two sub-steps. Generally, cascade reactions contain several reactions in a single pot, lowers consumption of energy, eliminates intermediate separation and purification steps when compared with traditional stepwise synthesis (Kroutil & Rueping, 2014; Schrittwieser et al., 2013; Pellissier, 2013; Oroz-Guinea & García-Junceda, 2013). Remarkable developments have recently occurred in the use of heterogeneous, homogeneous, organic, and biological catalysts in cascade systems (Pellissier, 2012; Wende & Schreiner, 2012; Albrecht, Jiang, & Jørgensen, 2011; Ricca, Brucher, & Schrittwieser, 2011; Grondal, Jeanty, & Enders, 2010; Climent, Corma, & Iborra, 2009). Recently, researchers found that metal nanoparticles such as Ni and Pd could promote Knoevenagel condensation (Javad Kalbasi, Mesgarsaravi, & Gharibi, 2019; H. Wang et al., 2019). Sequentially, H2 was used to reduce condensation compounds in one pot to produce saturated α-substituted β ketoesters. But the condensation and reduction must be operated in sequence, and difficult removing of the metal catalyst also restricted the application in pharmaceutical synthesis. In most of one-pot systems, several catalysts work together to accomplish the desired reaction. Multi-step reactions using a single catalyst have rarely been studied. Bifunctional catalyst, Pd-ZIF-8/rGO, was used as an efficient catalyst for the Knoevenagel condensation-reduction tandem reaction (Scheme 1). However, both conversion and selectivity were lower towards less active methylene compounds, such as diethyl malonate and ethyl cyanoacetate. Meanwhile, condensation and reduction are still required to operate one by one (Cuetos, Bisogno, Lavandera, & Gotor, 2013; Willemsen, van Hest, & Rutjes, 2013; Foulkes, Malone, Coker, Turner, & Lloyd, 2011; Kraußer et al., 2011; Tenbrink, Seßler, Schatz, & Gröger, 2011).
In this work, we show that ene-reductases (ERs) are effective biocatalysts, which directly catalyzed the synthesis of saturated α-substituted β ketoesters 3a-k from readily available aldehydes 1a-k and 1,3-diketones. This one-pot two sub-steps reaction which underwent the steps as Knoevenagel condensation to form α, β-unsaturated intermediates 2a-k, which were immediately reduced to the saturated products by the same single enzyme, NerA. Without the use of precious metals, heating and hazardous H2, the tandem reaction was simply performed in an aqueous phase at room temperature, yield and selectivity as high as 95% and 100%, respectively. As shown in Scheme 1, this cascade reaction fulfills the requirements of green chemistry and provides an environmentally friendly method for synthesizing saturated α-substituted β ketoesters.
2 Material and Experimental Section