New isopimaradiene diterpenoids from kaempulchraol E via Rhizopus oryzae fungal transformation

Abstract Fungal transformation of the diterpenoid kaempulchraol E (1) by Rhizopus oryzae KX685359 afforded two trihydroxlated isopimaradienes: 2α,6β,14β-trihydroxy-isopimara-8(9),15-diene (2) and 2α,6β,14α-trihydroxy-isopimara-8(9),15-diene (3) in addition to the known metabolite roscorane B (4). Chemical structures were established by spectroscopic techniques including: HRMS, FT-IR and 1D- & 2D-NMR. The structure of 2 was confirmed by single-crystal, X-ray diffraction analysis. Bio-transformed metabolites as well as the substrate originally isolated from Kaempheria galanga were tested against two cancer cell lines: HSC-2 and B16-BL6. Compounds 4 showed greater toxicity than the original plant derived diterpene (IC50 54.08 ± 0.05 versus 96.05 ± 0.03 μM, respectively) when assayed against the HSC-2 cell line. This study confirms the role of fungal transformations in developing novel natural products with efficacious biological activities.