Stereoselective Aldol Additions Catalyzed by Dihydroxyacetone Phosphate-Dependent Aldolases in Emulsion Systems: Preparation and Structural Characterization of Linear and Cyclic Iminopolyols from Aminoaldehydes

The potential of dihydroxyacetone phosphate (DHAP)‐dependent aldolases to catalyze stereoselective aldol additions is, in many instances, limited by the solubility of the acceptor aldehyde in aqueous/co‐solvent mixtures. Herein, we demonstrate the efficiency of emulsion systems as reaction media for the class I fructose‐1,6‐bisphosphate aldolase (RAMA) and class II recombinant rhamnulose‐1‐phosphate aldolase from E. coli (RhuA)‐catalyzed aldol addition between DHAP and N‐benzyloxycarbonyl (N‐Cbz) aminoaldehydes. The use of emulsions improved the RAMA‐catalyzed aldol conversions by three to tenfold relative to those in conventional DMF/water mixtures. RhuA was more reactive than RAMA towards the N‐Cbz aminoaldehydes regardless of the reaction medium. With (S)‐ or (R)‐Cbz‐alaninal, RAMA exhibited preference for the R enantiomer, while RhuA had no enantiomeric discrimination. The linear N‐Cbz aminopolyols thus obtained were submitted to catalytic intramolecular reductive amination to afford the corresponding iminocyclitols. This reaction was diastereoselective in all cases examined; the face selectivity was controlled by the stereochemistry of the newly formed hydroxyl group originating from the aldehyde. Characterization of the resulting iminocyclitols allowed the assessment of the diastereoselectivity of the enzymatic aldol reactions with respect to the N‐protected aminoaldehyde. RAMA formed single diastereoisomers from N‐Cbz‐glycinal and from both enantiomers of N‐Cbz‐alaninal, while 14 % of the epimeric product was observed from N‐Cbz‐3‐aminopropanal. Diastereoselectivity from RhuA was lower than that observed from RAMA. Interestingly, a single diastereoisomer was formed from (S)‐Cbz‐alaninal, whereas only a 34 % diastereomeric excess was observed from its enantiomer (i.e., (R)‐Cbz‐alaninal). Emulsion systems are useful reaction media for enzyme‐catalyzed aldol reactions. The L‐fructose‐1,6‐bisphosphate aldolase (RAMA) and recombinant L‐rhamnulose‐1‐phosphate aldolase from E. coli (RhuA) catalyzed stereoselective aldol additions between DHAP and N‐Cbz aminoaldehydes (see scheme). A description of the reactivity of the aldolases in emulsions and a detailed stereochemical analysis of both linear and cyclic iminocylitols is presented.