Asymmetric Total Syntheses and Structure Revisions of Eurotiumide A and Eurotiumide B, and Evaluation of their Fluorescent Properties as Natural Probes

Asymmetric total syntheses and structure revisions of dihydroisocoumarin-type natural products, eurotiumide A and eurotiumide B have been described. The key features of these total syntheses are the asymmetric Shi epoxidation, regio and stereo-selective epoxide opening, C1 insertion/lactonization cascade reaction for constructing 4-methoxyisochroman-1-one skeleton. We confirmed the structures and configurations of eurotiumide A and B on the basis of X-ray crystallographic analysis of the key intermediate, and revealed that eurotiumide A and B have cis and trans configurations at H3/H4 positions, which are the opposite relationship of the stereochemistry from the previous report, respectively. The absolute configurations of them are also determined. These natural products exhibited highly fluorescence in several solvents with large Stokes shifts involving the excited-state intramolecular proton transfer mechanism, which is supported by time-dependent density functional theory. Eurotiumide A also emitted fluorescence in Bacillus cereus