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.