The mapped reactions were then corrected for stereochemistry if a chiral center not participating in the reaction changed. This is usually the case when an isomeric SMILES was retrieved only for either the reactant or product, if the correct information was missing in the trivial name, or for wrong entries in the molecular databases used for name retrieval. Fig. depicts an example where the wrong stereoisomer was retrieved for the product, and thus corrected. In principle, we do not know whether the stereoinformation is correct in the reactant or product. We choose to keep the stereoisomer of the reactant (concerning the chiral center which is not in the reaction center), since many product entries in BRENDA are erroneous. We note that this offers no guarantee to obtain a correct reaction, but was beneficial in a large number of cases upon manual examination.