Yonarolide A,an unprecedented furanobutenolide-containing norcembranoid derivative formed by photoinduced intramolecular [2+2] cycloaddition

Eight polycyclic furanobutenolide-containing norcembrane diterpenoids featuring C19 frameworks (1–8) were rapidly recognized and isolated from the Hainan soft coral Sinularia sp. by the HSQC-based small molecule accurate recognition technology. Yonarolide A (1a), featuring an unprecedented 5/6/4/4/7 pentacyclic ring skeleton, was surprisingly obtained as a transformed product by leaving compound 1 under indoor natural light, and was further proved to be a 2 + 2] cycloaddition product of 1 by photochemical reaction. The absolute stereochemistry of 1a and the three known norcembrane diterpenoids 1, 4, and 7 were determined by using X-ray diffraction (XRD) analyses. Further, with the aid of XRD analysis, the structure of scabrolide B (2), which was previously reported of possessing 5/6/7 tricyclic skeleton, was firmly revised as 2a with the rare inelegane skeleton featured by the highly oxygenated 5/7/6 tricyclic carbocycle.