Enzymatic Cascade in a Simultaneous, One-Pot Approach with In Situ Product Separation for the Asymmetric Production of (4S,5S)-Octanediol

Microbial alcohol dehydrogenases: recent developments and applications in asymmetric synthesis

Alcohol dehydrogenases are a well-known group of enzymes in the class of oxidoreductases that use electron transfer cofactors such as NAD(P)+/NAD(P)H for oxidation or reduction reactions of alcohols or carbonyl compounds respectively. These enzymes are utilized mainly as purified enzymes and offer some advantages in terms of green chemistry. They are environmentally friendly and a sustainable alternative to traditional chemical synthesis of bulk and fine chemicals. Industry has implemented several whole-cell biocatalytic processes to synthesize pharmaceutically active ingredients by exploring the high selectivity of enzymes. Unlike the whole cell system where cofactor regeneration is well conserved within the cellular environment, purified enzymes require additional cofactors or a cofactor recycling system in the reaction, even though cleaner reactions can be carried out with fewer downstream work-up problems. The challenge of producing purified enzymes in large quantities has been solved in large part by the use of recombinant enzymes. Most importantly, recombinant enzymes find applications in many cascade biotransformations to produce several important chiral precursors. Inevitably, several dehydrogenases were engineered as mere recombinant enzymes could not meet the industrial requirements for substrate and stereoselectivity. In recent years, a significant number of engineered alcohol dehydrogenases have been employed in asymmetric synthesis in industry. In a parallel development, several enzymatic and non-enzymatic methods have been established for regenerating expensive cofactors (NAD+/NADP+) to make the overall enzymatic process more efficient and economically viable. In this review article, recent developments and applications of microbial alcohol dehydrogenases are summarized by emphasizing notable examples.

Interdisciplinary development of an overall process concept from glucose to 4,5-dimethyl-1,3-dioxolane via 2,3-butanediol

To reduce carbon dioxide emissions, carbon-neutral fuels have recently gained renewed attention. Here we show the development and evaluation of process routes for the production of such a fuel, the cyclic acetal 4,5-dimethyl-1,3-dioxolane, from glucose via 2,3-butanediol. The selected process routes are based on the sequential use of microbes, enzymes and chemo-catalysts in order to exploit the full potential of the different catalyst systems through a tailor-made combination. The catalysts (microbes, enzymes, chemo-catalysts) and the reaction medium selected for each conversion step are key factors in the development of the respective production methods. The production of the intermediate 2,3-butanediol by combined microbial and enzyme catalysis is compared to the conventional microbial route from glucose in terms of specific energy demand and overall yield, with the conventional route remaining more efficient. In order to be competitive with current 2,3-butanediol production, the key performance indicator, enzyme stability to high aldehyde concentrations, needs to be increased. The target value for the enzyme stability is an acetaldehyde concentration of 600 mM, which is higher than the current maximum concentration (200 mM) by a factor of three.

Process intensification in continuous flow biocatalysis by up and downstream processing strategies

In this review, we focus on the holistic continuous enzymatic production and put special emphasis on process intensification by up- and downstream processing in continuous flow biocatalysis. After a brief introduction, we provide an overview of current examples of enzyme immobilization as an upstream process for flow biocatalysis. Thereafter, we provide an overview of unit operations as downstream processing strategies, namely continuous (i) liquid-liquid extraction, (ii) adsorptive downstream processing, and (iii) crystallization and precipitation. Eventually, we present our perspectives on future trends in this research field.

Application of Biobased Solvents in Asymmetric Catalysis

The necessity of more sustainable conditions that follow the twelve principles of Green Chemistry have pushed researchers to the development of novel reagents, catalysts and solvents for greener asymmetric methodologies. Solvents are in general a fundamental part for developing organic processes, as well as for the separation and purification of the reaction products. By this reason, in the last years, the application of the so-called green solvents has emerged as a useful alternative to the classical organic solvents. These solvents must present some properties, such as a low vapor pressure and toxicity, high boiling point and biodegradability, and must be obtained from renewable sources. In the present revision, the recent application of these biobased solvents in the synthesis of optically active compounds employing different catalytic methodologies, including biocatalysis, organocatalysis and metal catalysis, will be analyzed to provide a novel tool for carrying out more ecofriendly organic processes.