Synthetic enzyme cascades for the synthesis of amino alcohols and tetrahydroisoquinolines

  • Synthetische Enzymkaskaden zur Herstellung von Aminoalkoholen und Tetrahydroisochinolinen

Erdmann, Vanessa; Rother, Dörte (Thesis advisor); Blank, Lars Mathias (Thesis advisor)

Aachen (2018, 2019)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

The focus of this PhD thesis was on the enlargement of the product platform of chiral, vicinal amino alcohols. The amino alcohols 1-amino-1-phenylpropan-2-ol (APP), a building block for chiral fine chemicals, and methoxamine and metaraminol, substances with sympathomimetic function that work as α-adrenergic receptor agonists and vasopressors, were made accessible with high optical purities by the use of 2-step enzyme cascades combining carboligation- and transamination steps. It was possible to produce three of four APP-stereoisomers ((1R,2R)-,(1R,2S)-, and (1S,2R)-APP) from (R)- and (S)-2-hydroxypropiophenone (HPP) with high conversions and excellent stereoselectivities by transamination, catalysed by transaminases from Arthrobacter sp. (AsTA) and Bacillus megaterium (BmTA). Additionally, a 2-step cascade to (1S,2R)-APP was developed in which the substrates benzaldehyde and acetaldehyde were converted in a first carboligation step, catalysed by a benzaldehyde lyase from Pseudomonas fluorescens, to (R)-HPP. Subsequently, (R)-HPP was converted by AsTA to (1S,2R)-APP (overall conversion 92 %, ee >99 %, de 95 %). Furthermore, the four possible methoxamine isomers were produced successfully in 1-pot 2-step cascade reactions with excellent optical purities (isomeric content (ic) 94-99 %) and medium to high overall cascade conversions (59-80 %). For that, the substrates 2,5-dimethoxybenzaldehyde and pyruvate were converted towards the single methoxamine isomers using different catalyst combinations of (R)- and (S)-selective ThDP-dependent enzymes (acetohydroxyacid synthase from Escherichia coli (EcAHAS-I) and pyruvate decarboxylase variant from Acetobacter pasteurianus (ApPDC-E469G-I468A-W543F)) and transaminases (BmTA and AsTA). Subsequently, the optimised reaction conditions of the methoxamine cascade were transferred towards a 2-step (1R,2S)-metaraminol synthesis starting from 3-hydroxybenzaldehyde and pyruvate (overall conversions 88 %, ic 97 %).Additionally, a supercritical fluid chromatography (SFC) method was developed in which two chiral HPLC columns were connected in a row. This made it possible to increase the separation efficiency so that the individual isomers could be separated from each other without prior derivatisation of the samples. Furthermore, the 2-step cascade towards metaraminol was used as basis for follow-up synthesis towards tetrahydroisoquinolines (THIQ), which were produced from metaraminol by either norcoclaurine synthases or phosphate mediation. Thereby, the tetrahydroisoquinolines (1S,3S,4R)-1-benzyl-3-methyl-1,2,3,4-tetrahydroisoquinoline-4,6-diol (overall conversion 88 %, ic 97 %) and (1S,3S,4R)-1-(2-bromophenyl)-3-methyl-1,2,3,4-tetrahydroisoquinoline-4,6-diol (overall conversion 77 %, ic 97 %), which have three chiral centres, were made accessible by enzymatic- and chemoenzymatic 3-step cascade reactions. The configuration at the C1 stereocentre of the chemoenzymatic THIQ product is stereocomplementary to the product of the enzymatic 3-step-cascade.

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