Substrate Structure and Solvent Hydrophobicity Control Lipase Catalysis and Enantioselectivity in Organic Media

The lipase from Candida cylindracea catalyzes the enantioselective esterification of 2-hydroxy acids in nearly anhydrous organic solvents with primary alcohols as nucleophiles. The nature of the 2-hydroxy acid and organic reaction medium affects the efficiency of catalysis and the enantioselectivity. Straight-chain 2-hydroxy acids are highly reactive and give nearly 100% enantioselectivities in esterification reactions with 1-butanol. Slight branching with a methyl group adjacent to the 2-hydroxy moiety in toluene causes a substantial loss (up to 200-fold) in the lipase's catalytic efficiency with a concomitant loss in enantioselectivity. Losses in catalytic efficiency and enantioselectivity are also observed when the lipase is employed in hydrophilic organic media such as dioxane or tetrahydrofuran as compared to hydrophobic solvents such as toluene. With straight-chain substrates, the lipase is over 100-fold more active in toluene than in tetrahydrofuran or dioxane, while optimal enantioselectivity is observed in toluene. The loss in enantioselectivity in hydrophilic solvents is mainly due to a drop in the catalytic efficiencies of the 5 isomers, as the R isomers’ catalytic efficiencies remain largely unchanged. In highly apolar solvents, such as cyclohexane, enantioselective relaxation occurs due to an increase in the reactivity of the R isomers relative to that of their 5 counterparts. These findings enabled a rational selection of substrates and solvents for a two-step, chemoenzymatic synthesis of optically active 1,2-diols to be carried out, the first step being the aforementioned enantioselective esterification of 2-hydroxy acids followed by reduction with LiAl(OCH3)3H to give the optically active 1,2-diol. Diols such as (S)-(+)- 1,2-propanediol, (S)-(-)-l,2-butanediol, (S)-(-)-1,2-hexanediol, and (S)-(-)-4-methyl-l,2-pentanediol were produced in high optical purities (at least 98% enantiomeric excess (ee)).