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