The sex pheromone of the longtailed mealybug, identified as 2-(1, 5, 5-trimethylcyclopent-2- en-1-yl) ethyl acetate, represents the first example of a new monoterpenoid skeleton. A [2, 3]- sigmatropic rearrangement was used in a key step during construction of the sterically congested tetraalkylcylopentene framework.