Electronic and steric factors governing syn-anti isomerism in the Diels-Alder reaction have been investigated employing 1,2,3,4,5-pentachlorocyclopentadiene as the plane asymmetric dienophile. The predominance of ti«?/-7-chloro isomers in many cases has been ascribed to forces arising from dipole-dipole, dipole-induced dipole, and London-dispersion interactions, the predominance of endo over exo isomers to secondary orbital interactions and the lack of anti-exo isomers to steric hinderance in the transition state.heptatriene9 and cyclooctatetraene10 gives 1 and 2, respectively. On the other hand 5-acetoxy-1,3 -cyclopen-1 (syn) 2 (syn)3 (anti)tadiene with ethylene gives 311 in which the dienophile(1) Presented in part at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1-5, 1968, and the Halocarbon Conference held at the Eastern Research Laboratory of the Dow Chemical Co., Wayland, Mass., May 5-8, 1968.(2) Taken from the thesis of Mrs. Hsu submitted to Mount Holyoke College in partial fulfillment of the requirements for the Ph.D. degree.

Die Diels‐Alder‐Reaktion des Cyclopentadiens (I) mit Fumarsäuredimethylester führt nur zum Bicyclohepten (II) (76,5% Ausbeute).