An efficient Ir-catalyzed biomimetic method for photoisomerization of cyclopropane in lathyrane-type Euphorbia diterpenes is reported. Lathyrane diterpenes featuring the trans-fused cyclopropane (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 isomers. Moreover, 5a could further convert into another trans-isomer 5b. The present work provides a convenient route for easy access of naturally rare 12(Z)-trans-cyclopropane lathyranes.