Photocatalytic E → Z Isomerization of β-Ionyl Derivatives

An operationally simple E -> Z isomerization of activated dienes, based on the beta-ionyl motif intrinsic to retinal, is reported using inexpensive (-)-riboflavin (vitamin B-2) under irradiation at 402 nm. Selective energy transfer from photoexcited (-)-riboflavin to the starting E-isomer enables geometrical isomerization. Since the analogous process with the Z-isomer is inefficient, microscopic reversibility is circumvented, thereby enabling a directional isomerization to generate the contra-thermodynamic product (up to 99% yield, up to 99:1 Z/E). Prudent choice of photocatalyst enables chemoselective isomerization to be achieved in both inter- and intramolecular systems. The principles established from this study, together with a molecular editing approach, have facilitated the development of a regioselective isomerization of a truncated triene based on the retinal scaffold.