Photoinduced Irreversible Intramolecular Proton Transfer
of Arnebinones B, D, and E: The Case of Photoenolization at the p‑Benzoquinone-CH2/CH‑π System
Arnebinones B, E, and D (1–3)
have been found to be sensitive to light, generating complex and diverse
proton transfer products when triggered by light. A unique two-step
irreversible intramolecular proton transfer of 1 produced
five scalemic mixtures, of which four possessed intriguing dual planar
chirality. The unprecedented orientation epimerization equilibrium
of the intra-annular double bond was first observed and researched
in the homologous meroterpenoids by HPLC monitoring and DFT calculations.
A “p-benzoquinone-CH2/CH-π”
moiety in the structure was the common key feature for the occurrence
of this type of photoenolization reaction. The product transformation
processes and universality of this photoinduced irreversible proton
transfer reaction were analyzed together with the cytotoxic activities
of arnebinones B, D, and E, and their photoreaction products.