Biocatalytic strategy toward asymmetric β-hydroxy nitriles and γ-amino alcohols

doi:10.1016/j.tetlet.2011.03.009

Tetrahedron Letters

Volume 52, Issue 19, 11 May 2011, Pages 2440-2442

https://doi.org/10.1016/j.tetlet.2011.03.009 Get rights and content

Abstract

A library of 20 bakers’ yeast reductases, that are overexpressed in Escherichia coli, were screened against a variety of β-keto nitriles. Enzymes from the aldose reductase and the short chain dehydrogenase family displayed activity toward these substrates. All of the seven substrates were reduced with high enantioselectivities and in some cases both antipodes could be synthesized in high ees. These whole-cell reactions afforded gram quantities of asymmetric compounds that could ultimately lead to scaleable and simple synthesis to new drug analogs of serotonin reuptake inhibitors and β-adrenergic blocking agents.

Graphical abstract

Introduction

Asymmetric β-hydroxy nitriles are excellent chiral precursors due to their reactive diversity (Scheme 1). These asymmetric products have been used to make a variety of chiral natural products and pharmaceuticals.1, 2, 3 The most popular example is the utilization of β-hydroxy nitriles 1 or γ-amino alcohols 2 in the synthesis of the serotonin reuptake inhibitors4, 5, 6 and β-adrenergic blocking agents.7, 8, 9

Chiral organic10, 11, 12 and inorganic13, 14, 15, 16, 17, 18, 19, 20 catalysts have both been investigated in the synthesis of these molecules. The main disadvantage to these strategies is that the catalysts often involve a difficult synthesis and have toxicity issues especially when considering the use of heavy metals for the synthesis of pharmaceuticals. In addition, high enantioselectivity is often difficult to achieve.

Biocatalytic strategies utilizing isolated enzymes have also been a strategy toward asymmetric β-hydroxy nitriles 1.21, 22, 23 The most popular examples use a lipase or nitrilase to catalyze the kinetic resolution of the racemic β-hydroxy nitrile.4, 24, 25, 26, 27 However, these enzymes are inherently limited to a 50% yield when resolving the racemate. The reduction of β-keto nitriles using isolated keto reductases has recently been investigated; however, the cofactor must be supplied or a cofactor regeneration system used when working with pure enzymes.28, 29

Whole-cell biocatalytic strategies have also been investigated for the reduction of β-keto nitriles. This is advantageous because the cell will supply the cofactors needed for the reduction resulting in affordable and simple reaction conditions. However, studies using Curvularia lunata and bakers’ yeast (Saccharomyces cerevisiae) to reduce these substrates have been plagued by a dominant alkylating mechanism (Scheme 2).30, 31, 32, 33, 34, 35

Bakers’ yeast (S. cerevisiae) has been a popular biocatalytic tool that has been investigated to achieve a variety of reductions.³⁶ Its ability to reduce an assortment of ketone substrates is ultimately due to the many keto reductases this organism contains.³⁷ Unfortunately, this large number of reductases often leads to a mixture of products formed by competing enzymes. To circumvent this problem, GST-reductase chimeras were engineered and placed into Escherichia coli creating a bakers’ yeast reductase library.37, 38, 39 We have used this system to screen the stereospecificity of a single reductase for a given substrate by use of the pure fusion protein or in whole-cells.39, 40, 41

Section snippets

Results and discussion

This manuscript reports the screening of this heterologous library for its ability to asymmetrically reduce a variety of β-keto nitriles. The simplicity of this system lies in the fact that it uses whole-cells to supply the cofactors, minimizes competing reactions by overexpressing a single yeast reductase, and is scaleable thus it is well suited for the synthetic chemist who desires gram quantities.

The asymmetric reduction of a variety of alkyl and aryl substituted β-keto nitriles were

Conclusions

Asymmetric β-hydroxy nitriles are chiral precursors which have been applied to the synthesis of serotonin reuptake inhibitors4, 5, 6 and β-adrenergic blocking agents.7, 8, 9 We have shown a simple and scaleable approach to make a variety of asymmetric β-hydroxy nitriles while avoiding the alkylated product often seen with whole-cell biocatalysis. In most cases these products are reduced with very high stereospecificity and there are also examples in which both antipodes can be synthesized by

Acknowledgments

This work was supported by NSF-RUI grant CHE-0848708 from the Organic and Macromolecular Chemistry Program and NSF-MRI CHE-0923153. Additional thanks to Jon Stewart at the University of Florida for donating the enzyme library in these studies.

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Biocatalytic strategy toward asymmetric β-hydroxy nitriles and γ-amino alcohols

Abstract

Graphical abstract

Introduction

Section snippets

Results and discussion

Conclusions

Acknowledgments

Tetrahedron

Bioorg. Med. Chem. Lett.

Tetrahedron: Asymmetry

Tetrahedron Lett.

Tetrahedron: Asymmetry

Tetrahedron: Asymmetry

Tetrahedron: Asymmetry

Tetrahedron

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron: Asymmetry

16

Adv. Appl. Microbiol.

Tetrahedron: Asymmetry

Synth. Commun.

Medicinal Chemistry: The role of Organic Chemistry in Drug Research of Beta Blockers

J. Med. Chem.

Bull. Korean Chem. Soc.

Org. Lett.

Org. Chem.