An Expeditious Synthesis of Methyl Jasmonate

We present an efficient three‐step, two‐pot synthesis of methyl jasmonate ( trans ‐ 1 ) based on Diels–Alder cycloaddition of cyclopent‐2‐enone ( 2 ) and chloroprene (= 2‐chlorobuta‐1,3‐diene; 3d ) in either CHCl 3 or CH 2 Cl 2 , catalyzed by SnCl 4 (0.2 mol‐equiv.) at 20° (75% yield). Subsequent ozonolysis of a cis / trans 55 : 45 mixture of the cycloadduct 4d in either CH 2 Cl 2 or AcOEt at − 78°, followed by addition of Me 2 S and MeOH in the presence of NaHCO 3 , afforded, in 64% yield, a cis / trans 40 : 60 mixture of the known aldehyde 5c . The latter was reacted at − 50° under salt‐free conditions with the propyl Wittig reactant to furnish 1 as a cis / trans 20 : 80 mixture (( E / Z ) 3 : 97). Alternatively, a cis / trans 7 : 93 mixture (( E / Z ) 4 : 96) was obtained in 88% yield from epimerized 5c (AcOH, H 2 O, 40°; 99%) under usual Wittig conditions at − 20°.