87. Contra-thermodynamic E → Z isomerization of cinnamamides via selective energy transfer catalysis

A bio-inspired, photocatalytic E → Zisomerization of cinnamides is reported using inexpensive (−)-riboflavin (vitamin B₂) under irradiation at λ = 402 nm. This operationally simple transformation is compatible with a range of amide derivatives including –NR₂, –NHSO₂R and N(Boc)₂ (up to 99:1 Z:E). Selective energy transfer from the excited state photocatalyst to the starting E-isomer ensures that directionality is achieved: The analogous process with the Z-isomer is inefficient due to developing allylic strain causing chromophore deconjugation. This is supported by X-ray analysis and Stern-Volmer photo-quenching studies. Preliminary validation of the method in manipulating the conformation of a simple model Leu-enkephalin penta-peptide is disclosed via the incorporation of a cinnamamide-based amino acid.