Astracondensatol D: A 6/6/5/6 Cycloartane Triterpenoid from <i>Astragalus condensatus</i>

Astracondensatol D (1), a pentacyclic triterpenoid featuring an uncommon 6/6/5/6-fused ring system, along with its precursor astracondensatol E (2), and two simplified 20(27)-octanorcycloastragenol derivatives (3 and 4) were isolated from Astragalus condensatus for the first time. Classical NMR spectroscopic data, integrated with NMR and DP4+ calculations, unambiguously determined their absolute stereostructures. X-ray crystallography provided independent confirmation of the structure of compound 3. The plausible biosynthetic pathway, encompassing the Wagner-Meerwein rearrangement and retro-aldol reaction, was proposed for their formation and supported by computational analysis.