Asymmetric alkylations of electron-rich arenes such as indoles are of great importance for the synthesis of many natural products and pharmaceuticals.[1] Hence, different approaches have been undertaken to develop catalytic enantioselective additions of indoles to a,b-unsaturated carbonyl compounds. To date, these have been based on the application of chiral transition-metal complexes [2] or secondary amines, the latter of which function through covalent activation, forming intermediary iminium ions. [3] In this context the use of b,g-unsaturated a-keto esters is of particular interest since they not only exhibit a higher reactivity but also can be functionalized readily to the corresponding amino acids or a-hydroxy acids.Given the frequent occurrence of the indole core structure in biologically active substances and natural products [4] together with the possibility of activating carbonyl functionalities with chiral Brønsted acids, [5][6] the development of an enantioselective, metal-free, noncovalently catalyzed Friedel-Crafts alkylation of indoles appeared to be of great significance. This would not only be the first example of such an organocatalyzed transformation, but more importantly it would give simple and direct access to optically pure a-keto and a-amino acids. We report here on the development of such a reaction, a highly enantioselective Brønsted acid catalyzed addition of indoles to a,b-unsaturated carbonyl compounds.In continuing studies on the Bønsted acid catalyzed asymmetric Nazarov cyclization of divinyl ketones [5] [Eq. (1)], we assumed that an enantioselective FriedelCrafts alkylation of indoles through the noncovalent activation of a-keto esters using N-triflylphosphoramides [Eq. (2)] should also be feasible. Therefore, our investigations started with the examination of the Brønsted acid catalyzed addition of N-methylindole (1 a) to the a-keto ester 2 a. While no reaction was observed when weak Brønsted acids, such as carbonic acids or diphenyl phosphate, were used, catalytic amounts of N-triflylphosphoramide 5 a resulted in product formation. However, in addition to the desired 1,4-addition product 3 a, the bisindole 4 a was isolated as the main product (Scheme 1).The Lewis or Brønsted acid catalyzed formation of bisindoles starting from aldehydes, ketones, and 1,2-diketones is well known, [7] and several naturally occurring alkaloids contain this structural element.[8] However, the remarkable regioselectivity observed in the reaction of indoles with b,gunsaturated a-keto esters favoring the 1,2-addition with the generation of bisindole 4 a has not previously been reported. Figure 1 shows the X-ray crystal structure of 4 a. In contrast to all previously reported bisindoles, 4 a exhibits atropisomerism as a result of the rotation barrier about the bonds to the quaternary carbon bond. The bisindole atropisomers are not only observed in the X-ray crystal structure but can also be Scheme 1. Brønsted acid catalyzed reaction of N-methylindole (1 a) with a-keto ester 2 a to form bisindole 4 a.