Asymmetric whole-cell bioreduction of an α,β-unsaturated aldehyde (citral): competing prim-alcohol dehydrogenase and C–C lyase activities

Asymmetric bioreduction of (E/Z)-3,7-dimethyl-2,6-octadienal (citral) using the enoate reductase activity of whole cells of yeasts, bacteria and fungi, gave the α,β-saturated aldehyde (R)-3,7-dimethyl-6-octenal (citronellal), which constitutes an important flavour component, in up to ⩾95% ee. Depending on the microorganism, various amounts of prim-alcohols (nerol/geraniol and citronellol) were formed due to the action of competing prim-alcohol dehydrogenases. Citral lyase activity—leading to the loss of a C2-fragment (acetaldehyde) forming sulcatone—and oxidation of the aldehyde moiety yielding the carboxylic acid (geranic/neric acid) were detected as additional metabolic activities.

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(R)-3,7-Dimethyl-6-octenal (citronellal)C10H18OEe >95%[α]D20=+10.1 (c 1.0, EtOH)Source of chirality: asymmetric bioreductionAbsolute configuration: (R)