Meroapplanins A–E: Five Meroterpenoids with a 2,3,4,5-Tetrahydropyridine Motif from Ganoderma applanatum

Meroapplanins A–E (1–5) with a 2,3,4,5-tetrahydropyridine fragment were isolated from the fruiting bodies of Ganoderma applanatum. Their structures were elucidated by comprehensive spectroscopic analyses. Their absolute configurations were established based on the X-ray diffraction, electronic circular dichroism (ECD), and calculated nuclear magnetic resonance (NMR) with DP4+ analysis. A plausible biosynthetic pathway for 1–5 was proposed. Furthermore, compounds 1–5, together with optically pure compounds 1a–4a and 1b–4b, were evaluated for their protective effects in PC12 cells damaged by H2O2. The results showed that 3b had protective activity with a cell viability of 82.58 ± 1.31%, compared with the model group (cell viability: 65.27 ± 1.48%).