Highly<i>Syn</i><i>π</i>-Facial Preference in the Diels-Alder Reactions of 1,2,3,4,5-Pentamethylcyclopentadienes Having Carboxy, Ethoxycarbonyl, and Cyano Substituents at 5-Positions
Diels-Alder reactions of 1,2,3,4,5-pentamethylcyclopentadienes having carboxy, alkoxycarbonyl, and cyano substituents at 5-positions with N-phenylmaleimide preferentially afforded the corresponding syn-attack products with the ratio of syn/anti = 80 : 20 to 100 : 0, while the diene having 5-hydroxymethyl moiety gave the anti-attack product exclusively.