Catalytic Photochemical Enantioselective α-Alkylation with Pyridinium Salts

We have developed a chiral amine catalyzed enantioselective α-alkylation of aldehydes with amino acid derived pyridinium salts as alkylating reagents. The reaction proceeds in the presence of visible light (390 nm) and in the absence of a photocatalyst via a light activated charge-transfer complex. We apply this photochemical stereoconvergent process to the total synthesis of the lignan natural products (–)-enterolactone and (–)-enterodiol. Mechanistic studies support the ground-state complex-ation of the reactive components followed by divergent charge-transfer processes involving catalyst-controlled radical chain and in-cage radical recombination steps.