The question of whether rare 10,11- seco -lathyranes are natural products or artifacts is thoughtfully considered after a Brønsted acid-mediated chemical conversion of naturally abundant 5/11/3 lathyrane type diterpenes into 10,11- seco -lathyranes was developed. Benefiting from this concise route, a series of 10,11- seco -lathyrane products ( 1 - 14 ) were smoothly synthesized. The conversion may involve an acid promoted cyclopropane ring opening accompanied by a double bond shift with final trapping of carbocation. The ease of this chemical conversion under mildly acidic conditions may imply that the 10,11- seco -lathyranes isolated to date are artifacts. This work not only develops a new modular synthetic strategy for efficient constructing rare 10,11- seco -lathyranes, but also provides a promising bioactive diterpene with excellent effect against the NO production on LPS-induced BV-2 cells.