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

During the attempted photochemical cyclization of 2,3-bisbenzylidene-g-hydroxybutyric acid cyclic amide ester, it was observed that a g-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 b-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 e a typical reactivity for the members of the fulgide family. In contrast to the latter UV-driven cyclization, the hotochemical cleavage of the amide was proven to proceed under irradiation with visible light.