Hydroxynitrile Lyase-Catalyzed Enantioselective Conversion of Ketones into Cyanohydrins

Abstract

In this thesis I have addressed several issues related to the HNL-catalyzed preparation of cyanohydrins. I first demonstrated in Chapter 2 that immobilized HNL as sol-gels and as commercially available Cross Linked Enzyme Aggregates (CLEA®) improved several features of the biocatalyst such as solvent stability, and substrate or product inhibition/deactivation. In particular, MeCLEA was remarkably stable towards the deleterious effect of organic solvent and the enzymatic reaction could be carried out in organic media. The CLEA immobilization strategy is nonetheless enzyme-dependent and I successfully developed the biocatalyst LuCLEA for optimum catalytic performances in organic media as described in Chapter 3. This enantioselective and recyclable biocatalyst appeared to be particularly effective for the preparation of 2-butanone cyanohydrin. In Chapter 4, I used benzaldehyde as a model substrate to develop multistep strategies towards cyanohydrin derivatives based on HNL-CLEA catalysis in organic solvents. The reaction could be carried out in one pot or with limited downstream processing/purification of the cyanohydrin intermediate. In the case of ketones such as acetophenone where unfavourable thermodynamics limit the practical conversion, all attempts to derivatize the cyanohydrin in situ in order to shift the equilibrium were not successful. Cyanohydrins from ketones can indeed be considered as tertiary alcohols which require relatively reactive reagents for derivatization. Under these conditions the biocatalyst was rendered inactive. Since no in situ derivatization method could de designed to enable complete conversion of unreactive ketones, kinetic resolution as a means to produce chiral cyanohydrin was explored in Chapter 5. I established enzymatic activity for a previously unreported ?,?-unsaturated ketone and showed that kinetic resolution was more suitable than the direct synthetic route for the preparation of the corresponding chiral cyanohydrin. As an extension of this work I also described the rearrangement of a similar ?,?-unsatuared cyanohydrin acetate into the corresponding tetronic acid derivative. Chapter 6 concludes this thesis with straightforward synthetic procedures towards racemic cyanohydrins from unreactive ketones in order to improve the overall cost efficiency of the kinetic resolution approach.