Biocatalysis: Chiral cascades

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BIOCATALYSIS

Chiral cascades

Racemic or enantiomerically pure alcohols can be converted with high yield into enantiopure chiral amines in a one-pot redox-neutral cascade process by the clever combination of an alcohol dehydrogenase and an appropriate amine dehydrogenase.

Jian-bo Wang and Manfred T. Reetz

R R'

Chiral amines are needed for the production of a vast number of important compounds, especially in

the pharmaceutical industry where they serve as building blocks in the synthesis of many therapeutic drugs13. The standard routes to synthesize chiral amines involve the asymmetric reductive aminationof prochiral ketones or reduction of preformed ketimines using chiral transition metal catalysts, or the transformationof enantiomerically pure alcohols intothe respective amines by the Mitsunobu reaction1. However, the use of ammoniain direct formation of -chiral amines lacking protective groups is generally problematic because chiral transition metal catalysts that function efficiently under such conditions have not been developed, while the Mitsunobu reaction does not usually work well with NH3. Currently, the best approach is the stereoselective reductive amination of ketones using enzymes such as transaminases or amine dehydrogenases2,3.

Now, FrancescoG.Mutti, NicholasJ.Turner and collaborators have devised a new twist to the enzymatic approach for synthesizing -chiral amines starting not from ketones, but from alcohols4.

The basic idea is to combine an alcohol dehydrogenase (ADH) with an appropriate amine dehydrogenase (AmDH), the former mediating the oxidation of the alcohol with formation of the respective ketone and the latter catalysing enantioselective reductive amination in a redox self-sufficient cascade process that results in the formation of -chiral amines (Fig.1)4. The one-pot procedure requires only catalytic amounts of the coenzyme nicotinamide adenine dinucleotide (NAD+/NADH) which shuttles electrons in the form of hydride from the oxidative to the reductive step. As thereis no change in redox state between the substrate and product, this alleviates the need for additional oxidants and reductants and enables the process to be conducted in a single pot.

Two versions of this elegant hydrogen-borrowing concept are possible. In one embodiment, a racemic mixture of an

(R)- and (S)-alcohol is transformed stereoconvergently into the respective (R)-amine. Conceptually, this is possible if a mixture of (R)- and (S)-selective ADHs are used in the rst step required for intermediate ketone formation, in this version the second step is then catalysed by an (R)-selective AmDH. Alternatively, an enantiomerically pure alcohol [(R) or (S)] can be converted into the respective amine either with inversion or retention of conguration simply by choosing stereocomplementary ADH and AmDH enzymes. The substitution reaction also works when converting achiral primary alcohols into the respective achiral amines.

To demonstrate the versatility of their approach the researchers used ADHsfrom Aromatoleum sp., Lactobacillus sp. or Bacillus sp. and AmDH from Bacillus sp. In some cases mutants, previously produced by protein engineering, had to be employed to accommodate dierent starting substrates. In most examples, structurally dierent (R)- or (S)-alcohols in optically active form (Fig.1) were converted into the (R)-amine. The stereochemical result is either retention or inversion of conguration, depending upon the conguration of the starting alcohol.

In many cases excellent conversion was achieved (8095%), although in others the

transformations proved to be less efficient (765%). Very high enantioselectivity (9799% e.e.) was generally achieved. Similar conversion and enantioselectivity values were observed when starting from racemic alcohols, the products again having the (R)-conguration4.

Mutti, Turner and co-workers have opened a new door to the stereoselective synthesis of chiral amines having the (R)-conguration; however, to extendthis approach to produce the opposite enantiomeric amines, (S)-selective AmDHs would need to be used. Although the development of new enzymes is one area where this work could be extended, it seems only a matter of time before (S)-selective AmDHs become accessible through protein engineering techniques such as directed evolution. Improved variant enzymes with dierent substate specicity are also needed for reactants that are converted in low yield.

The new approach complements previous methods for synthesizing amines13. For

example, it is a metal-free alternativeto the Bckvall concept of combining a synthetic racemizing ruthenium-catalyst and an (R)-selective acylating lipase that enables the stereoconvergent formation of (R)-N-protected -chiral amines from racemic non-protected precursors5. The

OH

NH2

R R'

NAD+

H2O

ADH

NADH

AmDH

O

NH3/NH4+

R R'

Figure 1 | Scheme showing the one-pot amination of racemic alcohols to form either (R)- or (S)-aminesvia a dual-enzyme cascade4. In the rst step alcohol dehydrogenase (ADH) catalyses the oxidation of the alcohol to a ketone. Reductive amination of the ketone is then catalysed by amine dehydrogenase (AmDH). Electrons are cycled between the oxidative and reductive steps by NAD+/NADH in the form of a hydride.

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concept is also reminiscent of, but not identical to, biocatalytic deracemization of racemic alcohols6.

Organic chemists prot from an ever expanding toolbox of synthetic catalysts and biocatalysts. The optimal choice for a reaction depends upon a number of issues, including practical factors associated with downstream workup and (bio)process engineering7. Enzymes are oen assumedto be green catalysts because they are active under mild conditions, but in fact they are not always ecologically8 and economically superior to transition metal catalysts or organocatalysts. In order to assess whether a given biocatalytic process is truly green, the Sheldon E-parameters need to be computed for each case, which include such factors

as the type and amount of solvent required for extracting the products8. Nevertheless, developing new synthetic methods with dierent advantages (and disadvantages)can oer useful alternative routes to difficult products, and creating new synthetic choices is always a worthwhile endeavour.

Note added in proof: Aer this News & Views was written, a study (ref. 9) was published that independently describes the same concept with an emphasis on the conversion of racemic secondary alcohols into enantiopure amines.

Manfred T.Reetz and Jian-bo Wang are at the the Max-Planck-Institut fr Kohlenforschung, 45070 Mlheim, Germany. M.T.R.is also in the

Chemistry Department, Philipps-University, 35032, Marburg, Germany. e-mail:mailto:[email protected]

Web End [email protected]

References

1. Nugent, T.C. Chiral Amine Synthesis: Methods, Developments and Applications (John Wiley & Sons, 2010).

2. Kohls, H., Steen-Munsberg, F. & Hhne, M. Curr. Opin. Chem. Biol. 19, 180192 (2014).

3. Au, S.K., Bommarius, B.R. & Bommarius, A.S. ACS Catal.

4, 40214026 (2014).

4. Mutti, F.G., Knaus, T., Scrutton, N.S., Breuer, M. & Turner, N.J. Science 349, 15251529 (2015).

5. Paetzold, J. & Bckvall, J.E.J.Am. Chem. Soc. 127, 1762017621 (2005).

6. Schrittwieser, J.H., Sattler, J., Resch, V., Mutti, F.G. & Kroutil, W. Curr. Opin. Chem. Biol. 15, 249256 (2011).

7. Woodley, J.M. Curr. Opin. Chem. Biol. 17, 310316 (2013).8. Ni, Y., Holtmann, D. & Hollmann, F. ChemCatChem 6, 930943 (2014).

9. Chen, F. F., Liu, Y.-Y., Zheng, G.-W. & Xu, J.-H. ChemCatChem http://doi.org/f3jqvh

Web End =http://doi.org/f3jqvh (2015).

2015 NOBEL PRIZE IN PHYSIOLOGY OR MEDICINE

Punishing parasites

BOSTON GLOBE/CONTRIBUTOR/GETTY

YOSHIKAZU TSUNO/STAFF/GETTY

CHINAFOTOPRESS/CONTRIBUTOR/GETTY

The discovery of two medicines that have in the words of the Nobel Assembly at Karolinska Institutet revolutionized the treatment of parasitic diseases has resulted in the award of the 2015 Nobel Prize in Physiology or Medicine to WilliamC.Campbell, Satoshimura and YouyouTu (pictured leftright). Parasitic diseases aect a huge number of people, but they are more common in the poorest parts of the world. Campbell and mura were jointly awarded half of the prize for their work on a therapy against infections caused by roundworms. The other halfof the prize was awarded to Tu for her discovery of a new therapy against malaria.

mura, from Kitasato University, Japan, is an expert in culturing bacteria and isolating the chemical compounds

they produce. He isolated new strains of Streptomyces a group of soil-dwelling bacteria well-known for producing antibacterial compounds that showed promising activity against harmful microorganisms. He gave these strainsto a team led by Campbell an expert in parasite biology who was then working at the Merck Institute for Therapeutic Research in Rahway, NewJersey, USA. Campbell was able to implicate avermectins as partially responsible for the antiparasitic activity. Development of this series of compounds led to the production of the drug ivermectin, which is used to treat onchocerciasis (widely known as river blindness) and lymphatic lariasis (which causes elephantiasis).

YouyouTu led a research program to identify bioactive compounds

from traditional Chinese medicines.

Their search for antimalarial compounds motivated by the increasing ineectiveness of quinineand chloroquine eventually led tothe isolation of artemisinin from the wormwood plant (Artemisiaannua). Like many bioactive natural products, isolating sufficient material from the plant can be problematic. Therefore most artemisinin is now produced by a semi-synthetic method: the unusual peroxide structure thought to be responsible for the drugs mechanism of action is introduced bya chemical oxidation of artemisinic acid, which is itself produced by a specially engineered strain of yeast.

STEPHEN DAVEY

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