Asymmetric Carbohydroxylation of Alkenes Using Photoenzymatic Catalysis

Alkene difunctionalizations enable the synthesis of structurallyelaborated products from simple and ubiquitous starting materialsin a single chemical step. Carbohydroxylations of olefins representa family of reactivity that furnish structurally complex alcohols.While examples of this type of three-component coupling have beenreported, catalytic asymmetric examples remain elusive. Here, we reportan enzyme-catalyzed asymmetric carbohydroxylation of alkenes catalyzedby flavin-dependent "ene"-reductases to produce enantioenrichedtertiary alcohols. Seven rounds of protein engineering reshape theenzyme's active site to increase activity and enantioselectivity.Mechanistic studies suggest that C-O bond formation occursvia a 5-endo-trig cyclization with the pendant ketone to afford an & alpha;-oxy radical which is oxidized and hydrolyzed to form the product.This work demonstrates photoenzymatic reactions involving "ene"-reductasescan terminate radicals via mechanisms other than hydrogen atom transfer,expanding their utility in chemical synthesis.