Norcrohalane A, an Unusual C ₁₅ <i>nor</i> ‐Halimane‐Type Diterpenoid from <i>Croton crassifolius</i>

An unusual C₁₅ nor-halimane-type diterpenoid, norcrohalane A (1), a new nor-lasserane-type sesquiterpenoid, norcrolasane A (2), along with four known compounds (3-6) were isolated from the roots of Croton crassifolius. Compound 1 represents the first example of a C₁₅ nor-halimane-type diterpenoid, which features a rare 6/6/6 tricyclic ring system, and the A ring is aromatic. Compound 2 is a rare nor-lasserane-type sesquiterpenoid. Their structures and absolute configurations were established by a combination of mass spectrometry, nuclear magnetic resonance, electronic circular dichroism calculation, and single-crystal X-ray diffraction analyses. The anti-neuroinflammatory activities of 1-6 were evaluated for their inhibitory effects on nitric oxide (NO) production in lipopolysaccharide-induced BV2 cells, and compounds 1 and 2 showed moderate inhibition on NO production with IC₅₀ values of 26.82 ± 0.61 and 17.35 ± 0.78 µM, respectively.