Multienzymatic Stereoselective Reduction of Tetrasubstituted Cyclic Enones to Halohydrins with Three Contiguous Stereogenic Centers

The asymmetric hydrogenation of conjugated
tetrasubstituted alkenes with transition-metal catalysts is a challenging
reaction, especially for substrates bearing a halide substituent.
We describe a two-step multienzymatic reduction of a series of α-halo
β-alkyl tetrasubstituted cyclic enones, affording halohydrins
with three contiguous stereogenic centers, in good yield and with
a high stereoselectivity. The reduction is catalyzed by a stereospecific
ene-reductase (OYE2-3 or NemA) and a highly enantioselective alcohol
dehydrogenase (ADH). The use of two enantiodivergent ADHs allows the
control of the diastereoselectivity. The absolute stereochemical configurations
of the products have been determined from the analysis of single-crystal
structures (Flack’s parameter). The enantiomeric excess (ee) has been determined by derivatization of the products
with (R) Mosher’s acid. Lastly, we extended
our methodology also to a nonhalogenated substrate: the α-methyl
ketoisophorone was reduced by two distinct enantiodivergent ene-reductases
(flavin mononucleotide- and F420-dependent), affording
each enantiomer of the saturated ketone with ee >
98%.