An efficient Ir-catalyzed biomimetic method for photoisomerization of cyclopropane in lathyrane-type Euphorbia diterpenes is reported. Lathyrane diterpenes featuring the trans-fusedcyclopropane (1a–5a) were successfully prepared from cis-cyclopropane lathyranes (1–5) in excellent yields by this stereochemical permutations strategy, which first verified the biogenesis relationship between the lathyrane