Asymmetric biocatalytic hydrocyanation of pyrrole carboxaldehydes

The asymmetric hydrocyanation of pyrrole-2- and -3-carboxaldehydes substituted with either methyl, benzyl or phenyl in the 1-position catalyzed by the hydroxynitrile lyases from Hevea brasiliensis (HbHNL) and Prunus amygdalus (PaHNL) is reported. The products could be isolated—after O-silylation—with moderate to good enantiomeric purity although the carbonyl activity of the substrates was found to be very low, which is supported by quantum-chemical calculations. Structural effects concerning substrate size and regiochemistry are discussed considering docking calculations based on the X-ray crystal structures of the two enzymes. From these calculations one particular amino acid residue (Trp-128) in the active site of HbHNL could be identified, which plays a major role for the appropriate binding of structurally demanding carbonyl compounds.