Photocatalytic E→Z Isomerization of β-Ionyl Derivatives

Livingstone, Keith and Tenberge, Marius and Pape, Felix and Daniliuc, Constantin G. and Jamieson, Craig and Gilmour, Ryan (2019) Photocatalytic E→Z Isomerization of β-Ionyl Derivatives. Organic Letters, 21 (23). pp. 9677-9680. ISSN 1523-7060

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    An operationally simple E → Z isomerization of activated dienes, based on the β-ionyl motif intrinsic to retinal, is reported using inexpensive (−)-riboflavin (vitamin B2) 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.