Application of Engineered Amine Oxidases for the Synthesis of Chiral Amines

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Abstract

The development of cost-effective and sustainable catalytic methods for the production of enantiomerically pure chiral amines is a key challenge facing the pharmaceutical and fine chemical industries. There is an increasing demand for broadly applicable synthetic methods which deliver the desired amine product in high yield and enantiomeric excess (e.e.). Previously we have described the development of variants of monoamine oxidase from Aspergillus niger (MAO-N) which are able to mediate the complete conversion of racemic amines to the corresponding enantiomerically pure products in a single step. In this thesis we report a panel of MAO-N variants (D5, D9 and D11) developed in our laboratory, which are able to mediate the deracemisation of primary, secondary and tertiary amines with broad structural features. In particular, we have synthesized and subjected to deracemisation a broad range of tetrahydroisoquinolines and tetrahydro-β-carbolines checking enantioselectivity and enantiopreference of our biocatalysts. A relation between lipophilicity of the substituents and enantiopreference of the enzyme has been identified. We have also engineered a new MAO-N variant (D11) with a greatly increased substrate scope and enhanced tolerance for bulky substrates. Application of this engineered biocatalyst is highlighted by the asymmetric synthesis of the generic drugs Solifenacin and Levocetirizine as well as a number of important classes of biologically active alkaloid natural products. We also report a novel MAO-N mediated asymmetric oxidative Pictet-Spengler approach to the synthesis of (R)-harmicine.Another challenge facing the chemist in the new millennium is the development of cleaner and more efficient chemical processes. To this aim the combination of two or more catalytic systems to complete a series of cascade reactions is considered particularly appealing. We have reported a concurrent redox cascade for the deracemisation of pyrrolidines and tetrahydroisoquinolines using our monoamine oxidase-N with a biotinylated Ir-complex within streptavidin (SAV). To achieve the final goal it is necessary to shield the metal inside a host to avoid the mutual inactivation of the two catalysts. We have also described the combination of MAO-N with berberine bridge enzyme (BBE) for the synthesis of berbines (tetrahydroprotoberberines), which represent a sub-class of tetrahydro-isoquinoline alkaloids found in various plants. This bi-enzymatic cascade allows the synthesis of these structures achieving a theoretical 100% yield instead of the 50% given by the kinetic resolution using BBE itself.

Keyword(s)

Biocatalysis; MAO-N

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