Design and Application of Chiral Bifunctional 4-Pyrrolidinopyridines: Powerful Catalysts for Asymmetric Cycloaddition of Allylic N-Ylide

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Abstract

We designed bifunctional 4-pyrrolidinopyridines as powerful Lewis base catalysts. The catalyst structure features a 4-hydroxy-2-(hydroxydiphenylmethyl)pyrrolidine-1-formyl group at the pyridine’s C3 site and a chiral side arm at the C2 position of the p-pyrrolidine ring. An operationally simple three-step synthetic route allows for efficient and economical catalyst preparation. In comparison with other Lewis bases, the catalyst exhibits excellent efficiency and stereoselectivity in the asymmetric (3 + 2) cycloaddition of allylic N-ylide generated in situ from a pyrazolone-derived Morita–Baylis–Hillman carbonate, providing a powerful platform for the construction of chiral spiropyrazolone derivatives. The reaction mechanism was thoroughly studied. Control experiments and DFT calculations illustrated the origin of the chemoselectivity and the stereocontrol model. H-bonding is crucial for the control of enantioselectivity and diastereoselectivity. The catalyst was also successfully applied in other reactions, such as an acylative dynamic kinetic resolution to synthesize a chiral phthalidyl ester, including the prodrug Talmetacin.

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