Add time:07/17/2019         Source:sciencedirect.com

Optically pure regioisomer of (S)-methyl 4-methyl-5-oxo-5-[1H-pyrazol-1-yl)pentanoate ((S)-2) and (R)-methyl 2-methyl-5-oxo-5-[1H-pyrazol-1-yl)pentanoate ((R)-3) were prepared from regioselective methanolysis of (S)-2-methylglutaric dipyrazolide ((S)-1) and (R)-2-methylglutaric dipyrazolide ((R)-1), respectively, in anhydrous methyl tert-butyl ether (MTBE) at 45 °C via Candida antarctica lipase B. The irreversible first-order kinetics for each substrate coupled with methanol inhibition was employed for obtaining the analytic solution, in which the kinetic parameters varied with the methanol concentration were estimated from the experimental data, and then compared and elucidated from the steric hindrance of methyl substituent to the chiral center and enzyme stereopreference when attacking the C-1 or C-5 carbonyl group. By using (R,S)-2-methylglutaric dipyrazolide ((R,S)-1) as the substrate, simultaneous preparation of (R)-3 and (S)-2 from the theoretical calculation was also addressed and compared with the experimental data reported previously for CALB-catalyzed esterification of (R,S)-2-methylpentanedioic anhydride with 2-methylpropano1 in organic solvents.

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