Natural and Semisynthetic Tigliane Diterpenoids with New Carbon Skeletons from <i>Euphorbia dracunculoides</i> as a Wnt Signaling Pathway Inhibitor

Euphordraculoates A (1) and B (2), featuring tigliane diterpenoids with two new carbon skeletons, were characterized as metabolites of Euphorbia dracunculoides and semisynthetic products, respectively. Their structures were determined by spectroscopic analyses and X-ray crystallography. The respective biosynthetic and chemical formation mechanisms for 1 and 2 from a known tigliane 3 was proposed. The detailed decarbonization mechanism from 3 to 2 was further explored by 18O-labeling experiment. Compound 2 could inhibit Wnt pathway in a dose- and time-dependent manner.