Enantioselective Enzymatic Naphthoyl Ring Reduction

Birch reductions of aromatic hydrocarbons by means of single-electron-transfer steps depend on alkali metals, ammonia, and cryogenic reaction conditions. In contrast, 2-naphthoyl-coenzyme A (2-NCoA) and 5,6-dihydro-2-NCoA (5,6-DHNCoA) reductases catalyze two two-electron reductions of the naphthoyl-ring system to tetrahydronaphthoyl-CoA at ambient temperature. Using a number of substrate analogues, we provide evidence for a Meisenheimer complex-analogous intermediate during 2-NCoA reduction, whereas the subsequent reduction of 5,6-dihydro-2-NCoA is suggested to proceed via an unprecedented cationic transition state. Using vibrational circular dichroism (VCD) spectroscopy, we demonstrate that both enzymatic reductions are highly stereoselective in D₂ O, providing an enantioselective pathway to products inaccessible by Birch reduction. Moreover, we demonstrate the power of VCD spectroscopy to determine the absolute configuration of isotopically engendered alicyclic stereocenters.