Catalytic Asymmetric Formal [3+3] Cycloaddition of an Azomethine Ylide with 3-Indolylmethanol: Enantioselective Construction of a Six- Membered Piperidine Framework

A catalytic asymmetric formal [3+3] cycloaddition of 3-indolylmethanol and an in situ-generated azomethine ylide has been established to construct a chiral six-membered piperidine framework with two stereogenic centers. This approach not only represents the first enantioselective cycloaddition of isatin-derived 3-indolylmethanol, but also has realized an unusual enantioselective formal [3+3] cycloaddition of azomethine ylide rather than its common [3+2] cycloadditions. Besides, this protocol combines the merits of a multicomponent reaction and organocatalysis, which efficiently assembles a variety of isatin-derived 3-indolylmethanols, aldehydes, and amino esters into structurally diverse spiro[indoline-3,4-pyridoindoles] with one all-carbon quaternary stereogenic center in high yields and excellent enantioselectivities (up to 93% yield, >99% enantiomeric excess (ee)). Although the diastereoselectivity of the reaction is generally moderate, most of the diastereomers can be separated by using column chromatography followed by preparative TLC.