Synthetic enzyme cascades for the synthesis of amino alcohols and tetrahydroisoquinolines

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