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Zinc‐Catalyzed Asymmetric Formal [4+3] Annulation of Isoxazoles with Enynol Ethers by 6π Electrocyclization: Stereoselective Access to 2 H ‐Azepines
Author(s) -
Zhu XinQi,
Wang ZeShu,
Hou BoShang,
Zhang HaoWen,
Deng Chao,
Ye LongWu
Publication year - 2020
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201912534
Subject(s) - annulation , chemistry , catalysis , enantioselective synthesis , stereoselectivity , combinatorial chemistry , organocatalysis , stereospecificity , substrate (aquarium) , atom economy , organic chemistry , oceanography , geology
6π electrocyclization has attracted interest in organic synthesis because of its high stereospecificity and atom economy in the construction of versatile 5–7‐membered cycles. However, examples of asymmetric 6π electrocyclization are quite scarce, and have to rely on the use of chiral organocatalysts, and been limited to pentadienyl‐anion‐ and triene‐type 6π electrocyclizations. Described herein is a zinc‐catalyzed formal [4+3] annulation of isoxazoles with 3‐en‐1‐ynol ethers via 6π electrocyclization, leading to the site‐selective synthesis of functionalized 2H‐azepines and 4H‐azepines in good to excellent yields with broad substrate scope. Moreover, this strategy has also been used to produce chiral 2H‐azepines with high enantioselectivities (up to 97:3 e.r.). This protocol not only is the first asymmetric heptatrienyl‐cation‐type 6π electrocyclization, but also is the first asymmetric reaction of isoxazoles with alkynes and the first asymmetric catalysis based on ynol ethers.