Dynamic Reductive Kinetic Resolution (DYRKR): Enzymatically "deracemizing" substrates and setting absolute & relative stereochemistry: From SsADH to CaADH and beyond

The Berkowitz group has a longstanding interest in exploiting enzymes as biocatalysts for stereocontrolled synthesis. Lipases were front and center in early efforts, setting the absolute stereochemistry in the total synthesis of (-)-podophyllotoxin by the desymmetrization of an advanced meso-intermediate (DOI: 10.1021/jo991582%2B). Lipases also provided for enantioenriched, quaternary, α-vinylic amino acids, via “reverse transesterification." More recently, the group has been actively exploiting alcohol dehydrogenase (ADH) enzymes in stereocontrolled synthesis, including in the selective reduction of ω-keto esters to pharmaceutical building blocks, and of ketone building blocks for (-)-oseltamivir (Tamiflu; DOI: 10.1021/acs.joc.1c00326). We focus here on systems in which DYRKR can be exploited to effectively “deracemize” a substrate. One prominent such example involved SsADH from the archeon Sulfolobus solfataricus (see also: DOI: org/10.1002/cctc.202400835) to deracemize (±)-profenals to the “stereochemically correct” (S)-profenols (DOI: org/10.1021/ja910778p). In this presentation we focus on DYRKR studies with Clostridium acetobutylicum ADH (CaADH). Using DYRKR with appropriate (±)-α-chloro-β-keto esters, we use the enzyme to set both stereocenters of new and known aryl isoserine side chains for the Taxotère family of tubulin-binding chemotherapeutics [20 examples, another 16 are obtained by organometallic “tailoring” reactions]. We also report here the first X-ray structure of CaADH. Key features: (i) the NADP+ cofactor is bound in an unproductive anti-conformation, with the amide carbonyl occupying the ketone binding pocket, and (ii) there is a highly flexible loop near the active site. We present our perspective on the dynamic nature of CaADH through simulated annealing and molecular dynamics simulations, and put forward a halogen bonding model as a potential mechanism for the remarkable (S)-C-Cl bond selectivity displayed.