A Nonconcerted Intramolecular Diels-Alder Reaction of Chiral Allenic-Acid Derivatives

The reaction between the chiral allenic acid (+)-(S)- 1 and the carbodiimides 2a – d and the keten-imine 6 gives, under mild conditions, the tricyclic compounds 3 – 5 , 7 , and 8 . Low diastereoselectivity and a partial loss of optical activity are observed. A stepwise mechanistic pathway via a biradical intermediate is postulated.