Biocatalytic Cascades toward Iminosugar Scaffolds Reveal Promiscuous Activity of Shikimate Dehydrogenases

Iminosugar scaffolds are highly sought-after pharmaceutical targets, but their chemical synthesis is lengthy and can suffer from poor scalability and purification. Here we report protecting-group-free chemoenzymatic and biocatalytic cascades to synthesize iminosugars from sugar-derived aminopolyols in two steps. Using galactose oxidase variant F₂ followed by a chemical or enzymatic reduction provided an efficient one-pot route to these targets, with product formation >70%. Key to success of this strategy was the application of genome mining, which identified bacterial shikimate dehydrogenases as promiscuous iminosugar reductases. The cell-free protocols allowed for isolation of highly polar iminosugar products from biotransformations in a single step through development of a gradient-elution cation exchange purification. The two-step pathway provides a short synthetic route that can be used as a cell-free platform for broader iminosugar synthesis.

Preparation of iminosugars from aminopolyols via selective oxidation using galactose oxidase

Minimally protected aminopolyols are novel substrates for the galactose oxidase variant F₂. Site-selective oxidation proceeds at the terminal primary alcohol, followed by spontaneous cyclisation to afford stable hemiaminal/hemiacetal anomers of the piperidine and azepane scaffolds, with isolated yields of up to 94%. Simultaneous deprotection and reduction occured readily to afford valuable and biologically relevant iminosugars.

Recent advances in (chemo)enzymatic cascades for upgrading bio-based resources

Developing (chemo)enzymatic cascades is very attractive for green synthesis, because they streamline multistep synthetic processes. In this Feature Article, we have summarized the recent advances in in vitro or whole-cell cascade reactions with a focus on the use of renewable bio-based resources as starting materials. This includes the synthesis of rare sugars (such as ketoses, L-ribulose, D-tagatose, myo-inositol or aminosugars) from readily available carbohydrate sources (cellulose, hemi-cellulose, starch), in vitro enzyme pathways to convert glucose to various biochemicals, cascades to convert 5-hydroxymethylfurfural and furfural obtained from lignin or xylose into novel precursors for polymer synthesis, the syntheses of phenolic compounds, cascade syntheses of aliphatic and highly reduced chemicals from plant oils and fatty acids, upgrading of glycerol or ethanol as well as cascades to transform natural L-amino acids into high-value (chiral) compounds. In several examples these processes have demonstrated their efficiency with respect to high space-time yields and low E-factors enabling mature green chemistry processes. Also, the strengths and limitations are discussed and an outlook is provided for improving the existing and developing new cascades.