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

Eighteen new colquhounane sesterterpenoids (<b>1</b>-<b>18</b>), featuring spiroketal, cyclopentanone, and cyclopentane-1,3-dione moieties, were isolated from<i>Colquhounia coccinea</i>var. <i>mollis</i>. 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 (<b>1</b>) and 5,13-dehydro-colquhounoid G (<b>7</b>). 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 (<b>14</b>-<b>17</b>) were established as interconvertible C-16 epimers via HPLC analysis and chemical transformation. Compounds <b>7</b>, <b>8</b>, and <b>14</b> showed moderate immunosuppressive activity against cytokine IFN-γ secretion and T-cell proliferation.