Enantioselective reduction biocatalysis

Within biocatalysis, redox reactions especially are often carried out in biphasic systems using crude cell systems, as the recycling of expensive cofactors is vital. The organic phase, which holds the rather insoluble substrate (and products), can be replaced by a more harmless IL. By this means, baker s yeast was used by Howarth and co-workers for enantioselective reduction of ketones [37]. In a 10 1 mixture of [BMIM][PF6] and water, reactions proceeded for a range of substrates giving yields and selectivities very comparable to those obtained by conventional methods [Eq. (8)]. 

Regio- and enantioselective reduction of diketones have been conducted successfully by biocatalysis. Examples of the reduction of diketones to hydroxyl ketones and diols are shown in Scheme 33.15. 

Biocatalysis in organic synthesis has a longstanding history and wide applicability in the enantioselective reduction of ketones (150, 151], Many enzymatic protocols provide reliable, scalable, and inexpensive routes to optically active alcohols consequently, such transformations have been extensively employed in industrial applications. A few representative examples of enzymatic reductions of ketones on large scale, giving hydroxy acids 243 [147, 152) and 245 (153) with exceptional enantioselectivities and yields, are shown below (Equations 18 and 19). 

Since stereoselectivities of biocatalytic reductions are not always satisfactory, modification of biocatalysis are necessary for practical use. This section explains how to find, prepare, and modify the suitable biocatalysts, how to recycle the coenzyme, and how to improve productivity and enantioselectivity of the reactions. 

Microbial reduction has been recognized for decades as a laboratory method of preparing alcohols from ketones with exquisite enantioselectivity. The baker s yeast system represents one of the better known examples of biocatalysis, taught on many undergraduate chemistry courses. Numerous other microorganisms also produce the ADH enzymes (KREDs) responsible for asymmetric ketone reduction, and so suitable biocatalysts have traditionally been identified by extensive microbial screening. Homann et have... 

Buchholz, S., and Groger, H. 2006. Enantioselective biocatalytic reduction of ketones for the synthesis of optically active alcohols. In Patel, R. N. (Ed.), Biocatalysis in the Pharmaceutical and Biotechnology Industries (pp. 757-790). Boca Raton FL CRC Press. 

The nitrile hydrates employed are selective and stop at the amide stage. However, they display no relevant enantioselectivity. The enantioselectivity in the processes is always introduced by an amidase (see for instance Schemes 6.27 and 6.28) in a second hydrolysis step. Overall the syntheses are, remarkably, often purely catalytic and combine chemical catalysis for reductions with biocatalysis for hydrolyses and the introduction of stereoinformation (Scheme 6.38). 

Together with enantioselective hydrolysis/acylation reactions, enantioselective ketone reductions dominate biocatalytic reactions in the pharma industry [10], In addition, oxidases [11] have found synthetic applications, such as in enantioselective Baeyer-Villiger reactions [12] catalyzed by, for example, cyclohexanone monooxygenase (EC 1.14.13) or in the TEMPO-mediated oxidation of primary alcohols to aldehydes, catalyzed by laccases [13]. Hence, the class of oxidoreductases is receiving increased attention in the field of biocatalysis. Traditionally they have been perceived as difficult due to cofactor requirements etc, but recent examples with immobilization and cofactor regeneration seem to prove the opposite. 

All these reported examples used whole cells, and therefore no biocatalytic characterization of the enzymes was performed along these proof-of-concept experiments. By virtue of the potential importance that these enzymes might have for practical applications, the first studies on enzyme isolation and biochemical characterization from different Streptomyces sp. have been recently reported [38,39]. Although reaction rates and yields are still low for these wUd-type biocatalysts, excellent enantioselectivities were observed for new substrates, thus representing a further evidence of the potential that these enzymes may have in organic synthesis (Scheme 2.12). For future research and development, ideally a designer bug for the imine reduction in an enantioselective fashion may be a powerful further application of biocatalysis to add to its already broad portfolio of options. 

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