Quasi-irreversible oxidation of sec-alcohols was achieved via biocatalytic hydrogen transfer reactions using alcohol dehydrogenases employing selected ketones as hydrogen acceptors, which can only be reduced but not oxidized. Thus, only 1 equiv of oxidant was required instead of a large excess. For the oxidation of both isomers of methylcarbinols a single nonstereoselective short-chain dehydrogenase/reductase from Sphingobium yanoikuyae was identified and overexpressed in E. coli.

The combination of an oxidation and a reduction in a cascade allows performing transformations in a very economic and efficient fashion. The challenge is how to combine an oxidation with a reduction in one pot, either by running the two reactions simultaneously or in a stepwise fashion without isolation of intermediates. The broader availability of various redox enzymes nowadays has triggered the recent investigation of various oxidation–reduction cascades.

Water is omnipresent and unreactive. How to speed up water addition and even make it selective are highlighted in this perspective.

Imine reductases (IREDs) have recently become a primary focus of research in biocatalysis, complementing other classes of amine‐forming enzymes such as transaminases and amine dehydrogenases. Following in the footsteps of other research groups, we have established a set of IRED biocatalysts by sequence‐based in silico enzyme discovery. In this study, we present basic characterisation data for these novel IREDs and explore their activity and stereoselectivity using a panel of structurally diverse cyclic imines as substrates. Specific activities of >1 U/mg and excellent stereoselectivities (ee>99 %) were observed in many cases, and the enzymes proved surprisingly tolerant towards elevated substrate loadings. Co‐expression of the IREDs with an alcohol dehydrogenase for cofactor regeneration led to whole‐cell biocatalysts capable of efficiently reducing imines at 100 mM initial concentration with no need for the addition of extracellular nicotinamide cofactor. Preparative biotransformations on gram scale using these ‘designer cells’ afforded chiral amines in good yield and excellent optical purity.

The acyl transferase from Mycobacterium smegmatis (MsAcT) catalyses transesterification reactions in aqueous media because of its hydrophobic active site. Aliphatic cyanohydrin and alkyne esters can be synthesised in water with excellent and strikingly opposite enantioselectivity [(R);E>37 and (S);E>100, respectively]. When using this enzyme, the undesired hydrolysis of the acyl donor is an important factor to take into account. Finally, the choice of acyl donor can significantly influence the obtained enantiomeric excesses.magnified image

Deracemization, that is, the transformation of a racemate into a single product enantiomer with theoretically 100 % conversion and 100 % ee, is an appealing but also challenging option for asymmetric synthesis. Herein a novel chemo-enzymatic deracemization concept by a cascade is described: the pathway involves two enantioselective oxidation steps and one non-stereoselective reduction step, enabling stereoinversion and a simultaneous kinetic resolution. The concept was exemplified for the transformation of rac-benzylisoquinolines to optically pure (S)-berbines. The racemic substrates were transformed to optically pure products (ee>97 %) with up to 98 % conversion and up to 88 % yield of isolated product.

This article reviews the progress in chemo-enzymatic alkaloid synthesis over the last 25 years, focusing on recent developments that have led to significant improvements in terms of step-economy and yield.

A biocatalytic redox-neutral reaction cascade was designed for the deracemisation of racemic mandelic acid to yield optically pure l-phenylglycine employing three enzymes. The cascade consisted of three steps: a racemisation, an enantioselective oxidation and a stereoselective reductive amination. The enantioselective oxidation of dmandelic acid to the corresponding oxo acid was coupled with the stereoselective reductive amination of the latter; thus the oxidation as well as the reduction reactions were performed simultaneously. The formal hydrogen abstracted in the first stepthe oxidation -was consumed in the reductive amination allowing a redox-neutral cascade due to a cascade-internal cofactor recycling. The enantiomers of the starting material were interconverted by a racemase (mandelate racemase) ensuring that in theory 100% of the starting material can be transformed. Using this set-up racemic mandelic acid was transformed to optically pure l-phenylglycine (ee > 97%) at 94% conversion without the requirement of any additional redox reagents in stoichiometric amounts.