Two novel diterpenoids, one with a rearranged trans,trans-fused tricyclo[10.3.0.04,6]pentadecane framework (1) and the other with an unprecedented 15S configuration (2), were isolated from Euphorbia helioscopia. Their structures were elucidated by extensive analysis of HR-ESI-MS, NMR, quantum-chemical calculation, and X-ray crystallographic data. Biosynthetically, 1 has a unique "cyclopropane-shift-like" biogenesis involving an oxa-di-π-methane (ODPM) rearrangement, which inspired us to accomplish the biomimetic conversion of 3 to 1. Moreover, compound 1 displayed a potent immunosuppressive effect by inhibiting Kv1.3 voltage-gated channels.