Montecrinanes A-C: Triterpenes with an Unprecedented Rearranged Tetracyclic Skeleton from Celastrus vulcanicola. Insights into Triterpenoid Biosynthesis Based on DFT Calculations

Three new triterpenoids with an unprecedented 6/6/6/6‐fused tetracyclic carbon skeleton, montecrinanes A–C (1–3), were isolated from the root bark of Celastrus vulcanicola, along with known D:B‐friedobaccharanes (4–6), and lupane‐type triterpenes (7–12). The stereostructures of the new metabolites were elucidated based on spectroscopic (1D and 2D NMR) and spectrometric (HR‐EIMS and HR‐ESIMS) techniques. Their absolute configurations were determined by both NMR spectroscopy, with (R)‐(−)‐α‐methoxyphenylacetic acid as a chiral derivatizing agent, and biogenetic considerations. Biogenetic pathways for montecrinane and D:B‐friedobaccharane skeletons were proposed and studied by DFT methods. The theoretical results support the energetic feasibility of the putative biogenetic pathways, in which the 1,2‐methyl shift from the secondary baccharenyl cation represents a novel and key reaction step for a new montecrinane skeleton. Uncovering skeletons: Three new triterpenes (montecrinanes A–C) with an unprecedented 6/6/6/6‐fused tetracyclic carbon skeleton were isolated from the root bark of Celastrus vulcanicola. The biogenetic pathway for the montecrinane and D:B‐friedobaccharane skeletons were proposed through DFT calculations (see figure).