Synthesis of 2,3-Dialkylated Tartaric Acid Esters via Visible Light Photoredox-Catalyzed Reductive Dimerization of α‑Ketoesters

A mild transition-metal-free
protocol to prepare 2,3-dialkylated
tartaric acid esters has been developed by taking advantage of a visible
light photoredox-catalyzed reductive dimerization of α-ketoesters
with a combination of an organic dye photocatalyst and a Hantzsch-type
1,4-dihydropyridine hydrogen donor. A broad range of functional groups
including cyclopropane, alkene, alkyne, 4-methoxybenzyl ether, acetal,
silyl ether, carbamate, cyclic ether, cyclic thioether, bromoalkane,
and N-alkoxyphthalimide are well-compatible. By employing
the visible light photoredox-catalyzed reductive coupling and the
subsequent optical resolution, both enantioenriched diastereomers
of 2,3-dialkylated tartaric acid could be acquired conveniently.