Abstract: Maoyancaosu (m-su), a new coumarinolignoid, was isolated from the seeds of Euphorbia lunulata Bge. and Euphorbia esula L. which are two co-occurring species with no conspicuous morphological differences. Biogenetic considerations and spectroscopic analysis (NMR, MS, IR) led to the revision of the original structure (A) of m-su to (B₁). The new structure (B₁) was confirmed by synthesis and comparison of relevant data (mp, Rf, IR). The two substituents at C-1' and C-2' of the dioxane ring in m-su were found to be equatorial and trans to each other from the coupling constant (8Hz)of the two axial hydrogens. It is devoid of any optical activity (589—365 nm) and hence a racemic mixture (R,R+S,S).In our hands the silver oxide for the coupling of Ⅰ and Ⅱ was used in ten-fold excess in order to achieve a moderate yield, in contrast with the equimolar quantity reported in the literature. It is of interest to note that the unnatural isomer (Ⅲ₂, 32%) was actually obtained in much higher yield in the biomimetic synthesis at the expense of the natural one (Ⅲ₁, 10%), apparently because of the reluctance of the electron-poor 7-OH (as compared with 6-OH) of esuletin (Ⅱ) in parting with an electron to form the required free radical. In the plant, esculin (6-glucoside of Ⅱ) or its equivalent is probably involved in the biogenetic pathway, where blocking of the 6-OH will account for the occurrence of m-su (B₁) as the exclusive isomer.