Lipase-catalyzed transesterification of primary alcohols: resolution of 2-ethylhexan-1-ol and 2-ethylhex-5-en-1-ol

Abstract

Both the (R) and (S) enantiomers of 2-ethylhex-5-en-1-ol (R)-(−)-1, (S)-(+)-1 and 2-ethylhexan-1-ol (R)-(−)-2, (S)-(+)-2 were prepared in good yields and in high enantiomeric excess via lipase-catalyzed transacetylation of racemic alcohols 1 and 2 with vinyl acetate. Various experimental conditions (lipase PSL or PLF, solvent and temperature ranging from 30 to −30 °C) are used and their influence on the enantioselectivity and on the reaction rate is evaluated. The (S) unsaturated alcohol 1 is specifically esterified by lipase PS in CH₂Cl₂ at 0 °C with a very high enantiomeric ratio (E ≈ 750) thus allowing the production of both enantiomers (R)-(−)-1 and (S)-(+)-1 with fairly good yields from the same enzymatic transformation. The comparison of unsaturated and saturated primary alcohols 1 and 2 shows that, for a similar atomic framework, the enantioselectivity of PSL is greatly enhanced by the presence of a terminal double bond. On the other hand, an enhancement of PSL enantioselectivity without loss of catalytic activity is achieved by decreasing the temperature to −30 °C thus giving access to both enantiomers.