Asymmetric Synthesis Involving Reversible Photodimerization of a Prochiral Flavonoid Followed by Crystallization

Abstract

Asymmetric synthesis involving photochemical dimerization of a prochiral flavonoid derivative in solution without any chiral source was achieved. Irradiation of ethyl 6‐bromochromonecarboxylate in solution efficiently gave a C2‐chiral anti‐head‐to‐head dimer in excellent chemical yield with good quantum efficiency (Φ365 = 0.15). X‐ray crystallographic analysis revealed that the dimer crystallized as a conglomerate of C2 space group. The crystalline dimer precipitated upon irradiation of the monomer in solution, and indirect racemization of the dimer through a reversible photoreaction in solution and selective crystallization simultaneously occurred to give the C2‐chiral dimer in optically active form with up to 80 % ee. Optically active photoproducts could be obtained by simply irradiating achiral materials in solution without an external chiral source.