Broadening the substrate scope of strictosidine synthases by site-directed mutagenesis

The condensation of ß-arylethylamines with carbonyl compounds (Pictet-Spengler reaction) is employed in the synthesis of tetrahydro-β-carboline and isoquinoline scaffolds which are common motifs in many naturally occurring alkaloids. These compounds exhibit a range of biological activities and are thus interesting targets for organic synthesis and medicinal chemistry. Nature’s equivalent to the Pictet-Spengler reaction is catalyzed by the so called Pictet-Spenglerases. Within this class of enzymes, strictosidine synthases (STRs, EC 4.3.3.2) have attracted attention [1-4]. These enzymes catalyse the formation of the 1,2,3,4-tetrahydro-β-carboline (S)-strictosidine, a key intermediate in the monoterpenoid indole alkaloid biosynthetic pathway in higher plants. Please click Additional Files below to see the full abstract

Catalytic Promiscuity of Transaminases : Preparation of Enantioenriched β-Fluoroamines by Formal Tandem Hydrodefluorination/Deamination

Transaminases are valuable enzymes for industrial biocatalysis and enable the preparation of optically pure amines. For these transformations they require either an amine donor (amination of ketones) or an amine acceptor (deamination of racemic amines). Herein transaminases are shown to react with aromatic β-fluoroamines, thus leading to simultaneous enantioselective dehalogenation and deamination to form the corresponding acetophenone derivatives in the absence of an amine acceptor. A series of racemic β-fluoroamines was resolved in a kinetic resolution by tandem hydrodefluorination/deamination, thus giving the corresponding amines with up to greater than 99 % ee. This protocol is the first example of exploiting the catalytic promiscuity of transaminases as a tool for novel transformations

Stereoselective Cascade to C3-Methylated Strictosidine Derivatives Employing Transaminases and Strictosidine Synthases

(<i>S</i>)-Strictosidine represents the first key intermediate in the biosynthesis of several pharmaceutically relevant monoterpenoid indole alkaloids. Optically pure C3-methyl-substituted strictosidine derivatives were prepared by setting up the two stereogenic centers at the β-carboline core via two enzymatic steps catalyzed by the enzymes transaminase and strictosidine synthase in a one-pot cascade fashion. The two enzymatic steps were performed simultaneously as well as in a stepwise fashion. The amination of the prochiral ketones led to optically pure amines with up to >98% enantiomeric excess. Depending on the enzyme used, the (<i>S</i>)- and (<i>R</i>)-enantiomers were prepared in most cases. Selected amines were then condensed with secologanin in a Pictet–Spengler reaction catalyzed by strictosidine synthase leading to diastereomerically pure products (>98% diastereomeric excess)

Asymmetric Amination of Tetralone and Chromanone Derivatives Employing ω‑Transaminases

Various (<i>S</i>)-selective and (<i>R</i>)-selective ω-transaminases were investigated for the amination of 1- and 2-tetralone and derivatives as well as of 3- and 4-chromanone. All ketones tested were aminated to give the corresponding enantiopure amines (<i>ee</i> > 99%) employing at least one of the enzymes investigated. In most of the cases the (<i>S</i>)- as well as the (<i>R</i>)-enantiomer was obtained in optically pure form. The amination of 3-chromanone was performed on a 100 mg scale leading to optically pure (<i>R</i>)-3-aminochromane (<i>ee</i> > 99%) with complete conversion and 78% isolated yield