Exploring Diversified Abietane Diterpenoids from Leucosceptrum canum with Immunosuppressive and Antifungal Activities

Thirty-nine structurally diversified abietane diterpenoids, including 20 new ones (1-20) and 19 known ones (21-39), were isolated from Leucosceptrum canum. Their structures were elucidated by extensive NMR spectroscopic analysis, ECD calculations, and X-ray diffraction. Notably, compounds 1 and 2 possess the rare 5/6/6- and 5/6-fused ring systems, respectively. Compound 4 is a trinor-abietane diterpenoid featuring a novel 18(4→3)-abeo-9,10-seco-15,16,17-trinor-abietane skeleton, while compounds 11-20 and 38-39 are abietane diterpenoids with distinct 9,13-endoperoxide stereoconfigurations. Compounds 1, 3, 11-20, and 38-39 exhibited extensive immunosuppressive activity by inhibiting the proliferation of T cells and the production of cytokine IFN-γ, with IC₅₀ values ranging from 3.75 to 14.96 μM, and the preliminary structure-activity relationship was summarized. Compounds 4, 34, and 38-39 exhibited moderate antifungal activities against Curvularia lunata, Fusarium graminearum, and F. oxysporum, with IC₅₀ values ranging from 39.4 to 106.6 μM.