Nudifloids O-R, four undescribed diterpenoids from Callicarpa nudiflora with anti-inflammatory activity

Four undescribed 3,4-seco-labdane diterpenoid derivatives (nudifloids O-R) along with 12 known terpenoids were isolated from the aerial part of Callicarpa nudiflora Hook. et Arn. Nudifloids O and P (compounds 1 and 2) represent the first examples of 3,4-seco-labdane diterpenoid coupled with 2-methylene-3-butenal likely via the Diels-Alder reaction, forming a rare cyclohexene moiety. The structures including absolute configurations were elucidated using comprehensive spectral data, calculated ¹³C NMR-DP4+ probability analysis, and electronic circular dichroism. Putative biosynthetic pathways for nudifloids O and P were proposed. All isolates were evaluated for their inhibitory activities on NO and IL-1β production in the LPS-stimulated RAW 264.7 macrophage cells. Nudifloids P (compound 2) showed potent inhibitory activities against IL-1β and NO production.