Stereoselective Construction of Halogenated Quaternary Carbon Centers by Brønsted Base Catalyzed [4+2] Cycloaddition of α‐Haloaldehydes

• Published in: Angewandte Chemie International Edition 2018-02, Vol.57 (7), p.1913-1917
• Main Authors: Li, Qiang, Zhou, Liang, Shen, Xu‐Dong, Yang, Kai‐Chuan, Zhang, Xiang, Dai, Qing‐Song, Leng, Hai‐Jun, Li, Qing‐Zhu, Li, Jun‐Long
• Format: Article
• Language: English
• Subjects: Access control; Amines; Carbon; Construction; Cycloaddition; Dehalogenation; enolates; Halogenation; halogens; organocatalysis; spirocompounds
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201711813

Asymmetric construction of halogenated quaternary carbon centers under mild reaction conditions remains challenging. Reported here is an unprecedented and highly stereoselective Brønsted base catalyzed [4+2] cycloaddition between either α‐chloro‐ or α‐bromoaldehydes and cyclic enones. The key intermediate, an α‐halogenated enolate, is susceptible to dehalogenation and can be stabilized and stereochemically controlled using bifunctional tertiary amines. This method provides facile access to a collection of optically pure bicyclic dihydropyrans having three contiguous stereocenters, including a halogen‐bearing quaternary carbon center. Of note, the product can be transformed in situ into densely functionalized spirocyclopropanes in a highly efficient and stereoselective manner. Hello Halo! An unprecedented highly enantioselective, Brønsted base catalyzed [4+2] cycloaddition of α‐haloaldehydes and electron‐deficient cyclic enones has been developed. A series of fused bicyclic dihydropyrans featuring a halogenated quaternary stereocenter was synthesized in excellent yields with up to >99 % ee under mild reaction conditions. These products were further transformed into spirocyclopropanes having two vicinal all‐carbon quaternary centers.