Unusual visible-light photolytic cleavage of tertiary amides during the synthesis of cyclolignans related to podophyllotoxin

Kamil Lisiecki, Krzysztof K. Krawczyk, Piotr Roszkowski, Jan K. Maurin, Armand Budzianowski, Zbigniew Czarnocki

Tetrahedron, 
Volume 73, Issue 44, 2 November 2017, Pages 6316-6328
DOI: 10.1016/j.tet.2017.09.021

Keywords:
Lignan, Podophyllotoxin, Photochemistry, Visible-light photolysis, Amide cleavage

Abstract

During the attempted photochemical,cyclization of 2,3-bisbenzylidene-y-hydroxybutyric acid cyclic amide ester, it was observed that a gamma-butyrolactone ring was formed, which was concurrent with the release of the amine fragment from the amide. The process occurs with high yield giving rise to the formation of beta-apopicropodophyllin and its regioisomer. Additional experiments confirmed the photochemical nature of this transformation, and that it is independent from the photocyclization of the benzylidene groups a typical reactivity for the members of the fulgide family. In contrast to the latter UV-driven cyclization, the photochemical cleavage of the amide was proven to proceed under irradiation with visible light.

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