Two new premyrsinane-type diterpenes (<b>2</b> and <b>3</b>) as diastereomers were synthesized from lathyrane-type diterpene euphorbia factor L<sub>3</sub> (<b>1</b>) for the first time via an efficient Fe(acac)<sub>3</sub>-catalyzed skeleton conversion process. This conversion features a biogenetically inspired strategy that relies on a concise reductive olefin coupling involving intramolecular Michael addition with free radicals. The structures of <b>2</b> and <b>3</b> were elucidated by a combination of the interpretation of their spectroscopic data and single-crystal X-ray diffraction analysis. The premyrsinane diterpenes <b>2</b> and <b>3</b> exhibited cytotoxic activity against the 4T1 breast cancer cell line, while the parent compound euphorbia factor L<sub>3</sub> (<b>1</b>) was inactive. The current results not only confirmed the biogenetic relationship between lathyranes and premyrsinanes for the first time but also suggested a novel method for the preparation of naturally rare premyrsinane diterpenes with high bioactivity from the more abundant natural lathyrane diterpenes.