Process development for the production of the ( S)-acid precursor of a novel elastase inhibitor (L-694,458) through the lipase-catalyzed kinetic resolution of a β-lactam benzyl ester

A limited screen of several commercially-available and internally-produced lipases and esterases identified the lipase PS-800 as a suitable biocatalyst for the resolution of a racemic β-lactam benzyl ester to the ( S)-acid. This β-lactam is a precursor to the elastase inhibitor L-694,458, an experimental drug targeted for the treatment of cystic fibrosis. Key to the development of a scalable process was the optimization of β-lactam and surfactant (Triton X-100) charges. These optimization studies yielded a 21-fold increase in the volumetric production of the ( S)-acid (from 0.38 g/ l to 8.0 g/ l) and a 10-fold improvement in the initial bioconversion rate (from 17 mg/( l·h) to 170 mg/( l·h)). Additionally, these studies achieved the control of the ( S)-acid enantiomeric excess (e.e.) which was improved from less than 65% to greater than 90%. Keys to an economical and high yielding process, both the recycling of the lipase (at levels of 90%) and the use of re-racemized unreacted ( R)-benzyl ester in multiple reaction cycles were successfully demonstrated. This process was scaled up in 2.0- l reactors and afforded gram quantities of greater than 90% e.e ( S)-acid, which upon purification was successfully used to synthesize the elastase inhibitor L-694,458.