The first examples of diterpene lactones with an unusual 2-nor-tetrahydro-2H-pyran-2-one nucleus, eufislactones A (1) and B (2), were isolated from the roots of Euphorbia fischeriana, together with a new (3) and fifteen known biosynthetic congeners (4–18). Compound 1 possesses an unprecedented 2,3-seco-2-nor-ent-atisane diterpenoid skeleton, and 2 features a new 2,3-seco-2-nor-ent-isopimarane diterpenoid core. Their structures incorporating absolute configurations were elucidated via the comprehensive spectroscopic analyses, quantum chemical calculations, electronic circular dichroism (ECD) calculations, and sin-gle-crystal X-ray diffraction analyses. Biogenetically, this compound was constructed by the plausible monomeric precursor, ent-atis-16-ene-3,14-dione (6) and ent-isopimara-8(14),15-dien-3-one (17), via key Baeyer-Villiger oxidation, decarboxylation, and recyclization to form an unique 2-nor-tetrahydro-2H-pyran-2-one core. Our bioassays have revealed that eufislactone A (EFA, 1) dis-played significant inhibitory effect on the osteogenic differentiation of human valvular interstitial cells (VICs), highlighting its poten-tial as a preventive agent against the progression of human calcific aortic valve disease (CAVD).