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.

Stereoselective Photodimerization of 3-Arylindenones in Solution and in the Solid State

“…3-Arylindenones 1 with π-stacking could play an important role leading to anti -HH adducts 2 . A similar stereoselective photoreaction from the triplet excited state was also reported in the photodimerization of chromonecarboxylic esters. − …”

Photoracemization‐Based Viedma Ripening of a BINOL Derivative

“…[18] Up to now,afew successful examples of resolution through dynamic crystallization of prochiral molecules racemizing under UV light starting from completely achiral conditions have been reported. [19,20] Here, we aim to extend the application of Viedma ripening to molecules racemizing by photocatalysis, and focus the attention on chiral BINOL derivatives. This class of compounds is widely used as catalysts in organic synthesis, as templates for chiral recognition and as building blocks for polymers and other molecules.…”

“…The application of UV light to molecules having sufficient absorbance in the UV region can lead to racemization by bond breakage or by exciting the molecular orbitals, allowing for rearrangements that are forbidden in the ground state . Up to now, a few successful examples of resolution through dynamic crystallization of prochiral molecules racemizing under UV light starting from completely achiral conditions have been reported . Here, we aim to extend the application of Viedma ripening to molecules racemizing by photocatalysis, and focus the attention on chiral BINOL derivatives.…”

Asymmetric Diels–Alder Reaction Involving Dynamic Enantioselective Crystallization

“…Recently, asymmetric synthesis involving a reversible reaction and crystal chirality has been developed. Stereoselective crystallization via stereoisomerization from meso to dl -isomers, aza-Michael addition reactions, photoisomerization followed by ring-closing–opening reactions, Strecker-type reactions, and reversible photodimerization reactions have been reported. These reactions were initiated from prochiral precursors, and high optically active crystalline products were conveniently provided under absolutely achiral conditions.…”