A chemo-enzymatic cascade for the production of enantiopure aminoalcohols

Abstract

The increasing demand for “greener” processes made enzymatic or chemo-enzymatic cascades a very popular approach. Combining different reactions in one pot without intermediate purification steps represents numerous advantages compared to the classic approach. In classic organic synthesis, longer operation time is needed and lower yields are typically reached when subsequent reactions are run separately. In cascade reactions, the elimination of work-up steps in between reactions reduces the amount of waste improving the environmental impact of a given process. Furthermore, a combination of chemical and enzymatic catalysts allows to access the complete set of reactions available for organic synthesis. The combination of individual reaction steps in one pot, however, poses also challenges as reaction conditions are often not directly compatible and requires extensive optimization for a successful cascade. In this thesis, a novel chemo-enzymatic cascade for the synthesis of enantiopure aliphatic amino alcohols starting from non-terminal alkenes was established. Optically active amino alcohols are important building blocks of pharmaceuticals as well as agrochemicals and find also application in cosmetic or food industry. The new synthetic route is based on the combination of three reaction steps: 1) chemical or enzymatic epoxidation of alkenes, 2) enzymatic ring opening of formed epoxides by halohydrin dehalogenases (HHDHs) using azide as nucleophile and 3) chemical hydrogenation of the azido alcohol to the corresponding amino alcohol. Identification of suitable catalysts exhibiting opposite enantioselectivity in the epoxidation and regioselectivity in epoxide ring opening enabled the selective synthesis of both enantiomers of the amino alcohol products. The individual reactions were separately optimized first. Afterwards, a fine tuning of respective reaction conditions was performed to achieve sufficient isolated yields and enantiopurity in the cascade runs on preparative scale. Additionally, the catalytic scope of HHDHs for application in organic synthesis was further expanded by in depth characterization of the regio- and enantioselectivity of 22 enzymes in the epoxide ring opening of racemic, vicinally di-substituted trans-epoxides. Moreover, using rational protein design, first mutants of the halohydrin dehalogenase HheG from Ilumatobacter coccineus could be obtained that display enhanced enantioselectivity in epoxide ring opening.