Chiral Phosphoric Acid-Catalyzed Asymmetric Cascade Reaction Of C(3) Substituted Indoles and Methyl Vinyl Ketone

Fused indolines bearing a chiral quaternary carbon center at the C(3) position represent very important moieties widely existed in natural products and biologically active compounds. Various approaches to access these important scaffolds have been developed. Among them, asymmetric dearomatization\cyclization cascade reaction is the most concise and effective method by using indole derivatives as starting material. However, the documented reports mainly employed tryptamine or tryptophol derivatives, providing indoline products, such as pyrroloindolines or furoindolines. Therefore, the development of other type of indole derivatives allowing structural diversity is highly desirable. In this paper, an efficient asymmetric Michael addition\cyclization cascade reaction of indole derivatives with methyl vinyl ketone (MVK) was developed. After screening various phosphoric acids in the reaction of indole derivative (la) with MVK, this cascade reaction delivered the dearomative product (2a) in 77% yield and 95% enantiomeric excess in the presence of (R)-SPINOL-derived chiral phosphoric acid (R)-4c with 5 A molecular sieves as additive in CHCl3 at room temperature. Under the optimized reaction conditions, a wide range of substituted indole derivatives bearing both electron-donating and electron-withdrawing groups have been tested. In all cases, the cascade dearomatization reaction proceeded smoothly to afford their corresponding indolo[2,3-b]quinoline products in moderate to good yields and excellent enantioselectivity. The absolute configuration of the products was then determined as (5aR,10bR) by an X-ray crystallographic analysis of a single crystal of enantiopure 2m. Moreover, this catalytic system was also feasible in a gram-scale reaction without erosion of enantiomeric excess. 1.25 g of product 2a can be prepared under the identical conditions in 91% yield and 93% ee.