Spectroscopic and Computational Strategy for the Absolute Configuration Determination of Immunosuppressive Spiroketal and Cyclopentanone-Containing Sesterterpenoids from Colquhounia coccinea var. mollis

Eighteen new colquhounane sesterterpenoids (1–18), featuring spiroketal, cyclopentanone, and cyclopentane-1,3-dione moieties, were isolated fromColquhounia coccineavar. mollis. Their chemical structures, including the absolute configurations, were unambiguously determined through a combination of spectroscopic analysis, experimental and calculated ECD, DFT-based NMR chemical shift calculations, and DP4+ probability analysis, which were further corroborated by single crystal X-ray diffraction analysis of a pair of diastereomers colquhounoid G (1) and 5,13-dehydro-colquhounoid G (7). Additionally, Quantum Mechanical (QM)-based 1H iterative functionalized Spin Analysis (HifSA) was employed to correlate specific stereochemical configurations (C-4/C-16/C-20) with proton coupling patterns within the spiroketal and cyclopentanone moieties, providing a diagnostic tool for the rapid configurational analysis of such motifs. Furthermore, colquhounoids featuring the cyclopentane-1,3-dione moiety (14–17) were established as interconvertible C-16 epimers via HPLC analysis and chemical transformation. Compounds 7, 8, and 14 showed moderate immunosuppressive activity against cytokine IFN-γ secretion and T-cell proliferation.