New nor-ent-halimane and nor-clerodane diterpenes from Callicarpa integerrima with anti-MRSA activity

Two new nor-ent-halimane diterpenes and three previously unreported nor-clerodane diterpenes, designated callicaintides A−E (1−5), were isolated from Callicarpa integerrima. Compounds 1 and 2 feature a distinctive 5/6-membered ring system, while compounds 3−5 are characterized by progressively truncated carbon skeletons, containing 18, 17, and 16 carbons, respectively. In addition, four known compounds 6−9 were also identified. Their structures were elucidated using advanced spectroscopic techniques, including nuclear magnetic resonance (NMR), high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), ultraviolet (UV), infrared radiation (IR), optical rotatory dispersion (ORD), DP4⁺ analysis and electronic circular dichroism (ECD), supported by quantum chemical calculations. Compounds 1−9 were evaluated for their anti-MRSA activity. Among them, compound 6 demonstrated significant anti-MRSA activity, with a minimum inhibitory concentration (MIC) of 16 μg·mL⁻¹.