Hyperacmosin R, a New Decarbonyl Prenylphloroglucinol with Unusual Spiroketal Subunit from Hypericum acmosepalum .

Two previously undescribed polycyclic polyprenylated acylphloroglucinols, hyperacmosins R-S ( 1 - 2 ), were obtained from the aerial parts of Hypericum acmosepalum . Their structures were elucidated by extensive spectroscopic analysis and electronic circular dichroism calculation (ECD). Compound 1 featured an unprecedented 5,8-spiroketal subunit as well as the loss of C-2' carbonyl in the phloroglucinol ring. In addition, compounds 1 and 4 showed weak hepatoprotective activity against paracetamol-induced HepG2 cell damage at 10 μm. The plausible biosynthetic pathway of 1 was proposed via a retro-Clasisen reaction and decarboxylation.