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Eighteen new colquhounane sesterterpenoids ( 1 – 18 ), featuring spiroketal, cyclopentanone, and cyclopentane-1,3-dione moieties, were isolated from Colquhounia coccinea var. 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 1 H 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.
Fu et al. (Wed,) studied this question.
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