Picrocylide A: A Cyclobutane-Containing Apocarotenoid-Derived Meroterpenoid from Neopicrorhiza scrophulariiflora with Hepatoprotective Effect

Picrocylide A (1), an apocarotenoid-derived meroterpenoid characterized by a rare 4/9/6 tricyclic skeleton, was isolated from the rhizomes of Neopicrorhiza scrophulariiflora, along with its putative biosynthetic precursor, 14-hydroxyaeginetic acid (2). The structure was elucidated by 1D/2D nuclear magnetic resonance spectroscopy and high-resolution electrospray ionization mass spectroscopy. The absolute configuration was determined by experimental and calculated electronic circular dichroism spectra matching and GIAO ¹³C NMR chemical shift calculation coupled with DP4+ probability analysis. A plausible biosynthetic pathway for compound 1 is also proposed. Additionally, compound 1 exhibited potent hepatoprotective activity against N-acetyl-p-aminophenol (APAP)-induced HepG2 cell damage with an EC₅₀ value of 10.65 μM.