Biosynthesis of Dodecano‐4‐lactone in Ripening Fruits: Crucial Role of an Epoxide‐Hydrolase in Enantioselective Generation of Aroma Components of the Nectarine (Prunus persica var. nucipersica) and the Strawberry (Fragaria ananassa)

Early steps in the biosynthesis of dedecano‐4‐lactone in ripening nectarines (Prunus persica var. nucipersica) and strawberries (Fragaria ananassa) were studied using cis‐9,10‐([18O]epoxy)heptadecanoic acid (5), instead of the natural C18‐precursor epoxystearic acid 1. In nectarines, an epoxide hydrolase catalyzed, in a highly regio‐ and enantioselective reaction, the attack of H2O at C(9) of epoxy acid 5 yielding preferentially (9R,10R)‐dihydroxyheptadecanoic acid 6a as an early intermediate and, after further metabolization, undecano‐4‐lactone 7a. In strawberries, the same transformation exhibited only a moderate regio‐ and enantioselectivity. Besides the previously established function of phytogenic epoxide hydrolases involved in generating oxygenated fatty‐acid derivatives, which are used in cutin monomer production and plant defence, a new rǒle for these enzymes now emerges in the enantioselective production of aroma components of ripening fruits. The cis‐9,10‐([18O]epoxy)heptadecanoic acid (≥ 98% 18O; 5) was obtained in good overall yield (68%) from isopropyl (9Z)‐heptadec‐9‐enoate using 18O2 gas in a free‐radical chain reaction in the presence of 2‐methylbutanal.