Enantioselective total synthesis of cis-trikentrin B

Natural cis-trikentrin B was synthesized enantioselectively using, as key steps, an intramolecular Diels-Alder reaction of an allenic dienamide followed by aromatization to construct an indole ring and cleavage of bicyclo[2.2.1]heptene to launch a cis-dimethylcyclopentane ring. Diels-Alder adducts 9 and 10 were elaborated to provide allenic dienamide 25, and its intramolecular Diels-Alder reaction proceeded smoothly. Aromatization to an indole ring and stereoselective cleavage of the bicyclo[2.2.1]heptene ring were performed successfully. Introduction of an (E)-butenyl group via addition of propylmagnesium bromide and subsequent anti elimination of water gave natural cis-trikentrin B.