Biocatalytic site- and stereoselective carbonyl desaturation for late-stage functionalization of cyclic ketones.

Direct carbonyl desaturation to prepare α,β-unsaturated carbonyl compounds is an important area of research in organic synthesis owing to the significance of these molecules in medicinal chemistry and chemical biology. Despite numerous methods developed for this transformation, approaches that enable precise control over the site selectivity of the reaction on substrates containing multiple potential reactive sites remain rare, limiting their applications in late-stage functionalization of complex molecules. Here we report the engineering of 'ene'-reductases for the direct carbonyl desaturation of diverse cyclic ketones to their corresponding enones with excellent site divergence. This study leverages the distinctive ability of 'ene'-reductases to differentiate the stereochemical environments of hydrogens at the carbonyl β-positions. The synthetic utility of this biocatalytic platform is further demonstrated through the successful late-stage dehydrogenation on terpenoids with complementary site selectivity to existing methods. In addition, the method could efficiently prepare chiral enones with a β-all carbon quaternary stereogenic centre via biocatalytic desaturative kinetic resolution. Mechanistic studies elucidate key enzyme-substrate interactions responsible for the enzyme-controlled site divergence.