carba Nicotinamide Adenine Dinucleotide Phosphate: Robust Cofactor for Redox Biocatalysis.

Here we report a new robust nicotinamide dinucleotide phosphate cofactor analog (carba-NADP ) and its acceptance by many enzymes in the class of oxidoreductases. Replacing one ribose oxygen with a methylene group of the natural NADP was found to enhance stability dramatically. Decomposition experiments at moderate and high temperatures with the cofactors showed a drastic increase in half-life time at elevated temperatures since it significantly disfavors hydrolysis of the pyridinium-N-glycoside bond.

Enzymatic and Chemoenzymatic Three-Step Cascades for the Synthesis of Stereochemically Complementary Trisubstituted Tetrahydroisoquinolines.

Chemoenzymatic and enzymatic cascade reactions enable the synthesis of complex stereocomplementary 1,3,4-trisubstituted tetrahydroisoquinolines (THIQs) with three chiral centers in a step-efficient and selective manner without intermediate purification. The cascade employs inexpensive substrates (3-hydroxybenzaldehyde and pyruvate), and involves a carboligation step, a subsequent transamination, and finally a Pictet-Spengler reaction with a carbonyl cosubstrate. Appropriate selection of the carboligase and transaminase enzymes enabled the biocatalytic formation of (1R,2S)-metaraminol.

Biocatalytic trifluoromethylation of unprotected phenols.

Organofluorine compounds have become important building blocks for a broad range of advanced materials, polymers, agrochemicals, and increasingly for pharmaceuticals. Despite tremendous progress within the area of fluorination chemistry, methods for the direct introduction of fluoroalkyl-groups into organic molecules without prefunctionalization are still highly desired. Here we present a concept for the introduction of the trifluoromethyl group into unprotected phenols by employing a biocatalyst (laccase), tBuOOH, and either the Langlois' reagent or Baran's zinc sulfinate.

Vinylation of Unprotected Phenols Using a Biocatalytic System.

Readily available substituted phenols were coupled with pyruvate in buffer solution under atmospheric conditions to afford the corresponding para-vinylphenol derivatives while releasing only one molecule of CO2 and water as the by-products. This transformation was achieved by designing a biocatalytic system that combines three biocatalytic steps, namely the C-C coupling of phenol and pyruvate in the presence of ammonia, which leads to the corresponding tyrosine derivative, followed by deamination and decarboxylation. The biocatalytic transformation proceeded with high regioselectivity and afforded exclusively the desired para products.

Asymmetric Biocatalytic Amination of Ketones at the Expense of NH3 and Molecular Hydrogen.

A biocatalytic system is presented for the stereoselective amination of ketones at the expense of NH3 and molecular hydrogen. By using a NAD(+)-reducing hydrogenase, an alanine dehydrogenase, and a suitable ω-transaminase, the R- as well as the S-enantiomer of various amines could be prepared with up to >99% ee and 98% conversion.

Stereoselective synthesis of γ-hydroxynorvaline through combination of organo- and biocatalysis.

An efficient route for the synthesis of all four diastereomers of PMP-protected α-amino-γ-butyrolacton to access γ-hydroxynorvaline was established. The asymmetric key steps comprise an organocatalytic Mannich reaction and an enzymatic ketone reduction. Three reaction steps could be integrated in a one-pot process, using 2-PrOH both as solvent and as reducing agent.

Synthesis of (R)- or (S)-valinol using ω-transaminases in aqueous and organic media.

Valinol is part of numerous pharmaceuticals and has various other important applications. Optically pure valinol (ee >99%) was prepared employing different ω-transaminases from the corresponding prochiral hydroxy ketone. By the choice of the enzyme the (R)- as well as the (S)-enantiomer were accessible.

Synthesis of pharmaceutically relevant 17-α-amino steroids using an ω-transaminase.

An efficient and sustainable biocatalytic route for the synthesis of important 17-α-amino steroids has been developed using an ω-transaminase variant from Arthrobacter sp. Optimisation of the reaction conditions facilitated the synthesis of these valuable synthons on a preparative scale, affording excellent isolated yields and stereocontrol.

Biocatalytic synthesis of enantiopure building blocks for pharmaceuticals.

Biocatalytic transformations have emerged as a viable alternative to other asymmetric chemical methods due to the intrinsic high stereoselectivity of the enzymes and the mild reaction conditions. Just a decade ago, the reaction scope of applicable biotransformations for organic synthesis was limited to a handful of reaction types. Tremendous progress has been made in the meantime so that this review presents only a small selection of the broad range of possible biotransfromations for organic synthesis available today.

Asymmetric Preparation of -, -, and -Amines Employing Selected Biocatalysts.

This account focuses on the application of ω-transaminases, lyases, and oxidases for the preparation of amines considering mainly work from our own lab. Examples are given to access α-chiral primary amines from the corresponding ketones as well as terminal amines from primary alcohols via a two-step biocascade. 2,6-Disubstituted piperidines, as examples for secondary amines, are prepared by biocatalytical regioselective asymmetric monoamination of designated diketones followed by spontaneous ring closure and a subsequent diastereoselective reduction step.

Concise Chemoenzymatic Three Step Total Synthesis of Isosolenopsin Through Medium Engineering.

A short and efficient total synthesis of the alkaloid isosolenopsin and its enantiomer has been achieved. In the key step, a ω-transaminase catalyzed the regioselective mono-amination of the diketone pentadecane-2,6-dione which was obtained in a single step via Grignard reaction. Initial low conversions in the biotransformation could be overcome by optimisation of the reaction conditions employing suitable cosolvents.

Chemoenzymatic synthesis of all four diastereomers of 2,6-disubstituted piperidines through stereoselective monoamination of 1,5-diketones.

The regioselectivity of various enantiocomplementary ω-transaminases was evaluated for the stereoselective monoamination of designated 1,5-diketones; excellent conversions, enantio- and regioselectivities were observed. The resulting amino-ketones underwent spontaneous intramolecular ring closure to afford Δ1-piperideines, which served as precursors for the cis- and anti-piperidine scaffold as demonstrated for the synthesis of the alkaloids dihydropinidine and epi-dihydropinidine. Key to the success of accessing the trans-piperidines was a Lewis acid mediated conformational change of the Δ1-piperideines in the reduction step.

Dynamic kinetic resolution of 2,3-dihydrobenzo[b]furans: chemoenzymatic synthesis of analgesic agent BRL 37959.

An efficient asymmetric synthesis of (S)-2,3-dihydrobenzo[b]furan-3-carboxylic acid (8 a) and (S)-5-chloro-2,3-dihydrobenzo[b]furan-3-carboxylic acid (8 b) was established. Key to the success was the highly stereoselective enzymatic kinetic resolution of the corresponding methyl or ethyl esters that was further developed into a dynamic process. As a reliable and fast tool for analysing the enantiomeric excess, HPLC coupled with a CD detector was utilized.

TTC-based screening assay for ω-transaminases: a rapid method to detect reduction of 2-hydroxy ketones.

A rapid TTC-based screening assay for ω-transaminases was developed to determine the conversion of substrates with a 2-hydroxy ketone motif. Oxidation of the compounds in the presence of 2,3,5-triphenyltetrazolium chloride (TTC) results in a reduction of the colourless TTC to a red-coloured 1,3,5-triphenylformazan. The enzymatic reductive amination of a wide range of various aliphatic, aliphatic-aromatic and aromatic-aromatic 2-hydroxy ketones can be determined by the decrease of the red colouration due to substrate consumption.

Indoline-3-carboxylic acid derived organocatalysts for the anti-Mannich reaction.

Mannich type reactions of a preformed aldimine with various carbonyl compounds were investigated with a series of functionalised indoline derivatives as catalysts: indoline-3-carboxylic acid, the diphenylcarbinol analogue and O-protected silyl ether analogues. All compounds were readily prepared in enantiopure form by using an enzymatic kinetic resolution as a key step (E≫100). The alcohol and ether catalysts failed to induce complete chirality transfer but did afford the Mannich bases in good yields and high diastereomeric ratios, whereas the acid catalyst gave the products in a highly diastereo- and enantioselective manner.