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The conversion of saturated fatty acids to high value chiral hydroxy-acids and lactones poses anumber of synthetic challenges:t he activation of unreactive CÀHb onds and the need for regio-and stereoselectivity.H ere the first example of aw ild-type cytochrome P450 monooxygenase (CYP116B46 from Tepidiphilus thermophilus) capable of enantio-and regioselective C5 hydroxylation of decanoic acid 1 to (S)-5hydroxydecanoic acid 2 is reported. Subsequent lactonization yields (S)-d-decalactone 3,ahigh value fragrance compound, with greater than 90 %ee. Docking studies providearationale for the high regio-and enantioselectivity of the reaction.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

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The squalene hopene cyclase from Alicyclobacillus acidocaldarius (AacSHC) is a highly efficient enzyme catalyst for stereoselective Brønsted acid catalysis. We engineered AacSHC to catalyze the selective Prins cyclization of citronellal. Four active site variants were identified for the diastereoselective cyclization of (S)‐citronellal to stereoisomers (−)‐iso‐isopulegol, (+)‐isopulegol and (−)‐neo‐isopulegol, respectively. The replacement of active site residues resulted in two triple variants that catalyzed the transformation of (R)‐citronellal to give the isomers (+)‐neo‐isopulegol and (−)‐isopulegol with up to >99 % de, respectively. The newly designed library of functionally diverse active site geometries exhibits high selective control during citronellal cyclization, leading exclusively to a single diastereomer of the desired isopulegol. Whereas the cyclization of citronellal with chemical catalysts was observed to produce the isopulegol isomer with the lowest energy, the reaction with AacSHC variants proceeded with higher product selectivity. The results of this study show that variants of AacSHC are excellent catalysts for the highly selective formation of isopulegol stereoisomers.