High Stereoselectivity in Lewis‐Acid‐Catalyzed and Uncatalyzed Diels–Alder Reactions of the Fumarate of (S)‐Ethyl Lactate

Enantiomeric diastereomers can be selectively obtained from the fumarate 1 and dienes. For instance, the uncatalyzed Diels-Alder reaction of 1 with cyclopentadiene gives preferentially the diastereomer 2a (2a : 2b = 98 :2); with TiCl4 as catalyst, on the other hand, 2b is the main product (2a : 2b = 5 :95). Analogous results were obtained for less reactive dienes [R*OH = (S)-ethyl lactate].