Biscroyunoid A, an Anti-Hepatic Fibrotic 19-nor-Clerodane Diterpenoid Dimer with a C-16–C-12′ Linkage from Croton yunnanensis

Biscroyunoid A (1), a 19-nor-clerodane diterpenoid dimer featuring a unique C-16–C-12′ linkage and containing an unusual 4,7-dihydro-5H-spiro[benzofuran-6,1′-cyclohexane] motif, together with its biosynthetic precursor, croyunoid A (2), were isolated from Croton yunnanensis. Their structures were determined by spectroscopic, computational, and single-crystal X-ray diffraction methods. Compound 1 exerted an antihepatic fibrosis effect in LX-2 cells via inhibition of TGFβ-Smad2/3 signaling.