The natural phytotoxin coronatine, which is composed of two individual parts, coronafacic acid and coronamic acid, exhibits various promising biological activities similar to jasmonic acid. Interestingly, coronatine induces stomatal opening involving the swelling of guard cells in which jasmonic acid is not involved as an endogenous regulator. We established syntheses of four stereoisomers of coronatine employing the exo-selective Diels–Alder reaction as a key step. Remarkable differences in stomatal opening activity were observed between enantiomers of coronatine. This result strongly suggests that the stereo structure of coronatine is very important for its stomatal opening activity. In addition, SAR studies suggested that coronatine operates as a molecular mimic of jasmonyl-L-isoleucine in plant guard cells.