Enzymatic Reduction of Hydrophobic β,δ-Diketo Esters

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Abstract
The regio- and enantioselective reduction of two hydrophobic β,δ-diketo esters is presented. Enzymatic reduction of racemic tert-butyl 4-methyl-3,5-dioxohexanoate (rac-1) with alcohol dehydrogenase from Lactobacillus brevis (recLBADH) gave δ-hydroxy-β-keto ester syn-(4S,5R)-4 under dynamic kinetic resolution conditions (99.2% ee, syn:anti = 97:3, 66% isolated yield). The highly lipophilic tert-butyl-3,5-dioxoheptanoate (2) was reduced with the same sense of enantio- and regioselectivity by recLBADH. A biphasic system was applied in this case. The product, δ-hydroxy-β-keto ester (R)-9 (98.5% ee, 66% isolated yield), was converted into (R)-6-ethyl-5,6-dihydropyran-2-one [(R)-10], which is a naturally occurring fragrance.