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

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Becker, Marc R. and Morack, Tobias and Robertson, Jack and Metternich, Jan B. and Mück-Lichtenfeld, Christian and Daniliuc, Constantin and Burley, Glenn A. and Gilmour, Ryan (2020) Contra-thermodynamic E → Z isomerization of cinnamamides via selective energy transfer catalysis. Tetrahedron, 76 (51). 131198. ISSN 0040-4020 (https://doi.org/10.1016/j.tet.2020.131198)

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Abstract

A bio-inspired, photocatalytic E → Z isomerization 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.

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