Miskov‐Pajic V, Willig F, Wanner DM, Frey W, Peters R. Enantiodivergent [4+2] Cycloaddition of Dienolates by Polyfunctional Lewis Acid/Zwitterion Catalysis.
Angew Chem Int Ed Engl 2020;
59:19873-19877. [PMID:
32697020 PMCID:
PMC7693193 DOI:
10.1002/anie.202009093]
[Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2020] [Indexed: 12/23/2022]
Abstract
Diels-Alder reactions have become established as one of the most effective ways to prepare stereochemically complex six-membered rings. Different catalysis concepts have been reported, including dienophile activation by Lewis acids or H-bond donors and diene activation by bases. Herein we report a new concept, in which an acidic prodiene is acidified by a Lewis acid to facilitate deprotonation by an imidazolium-aryloxide entity within a polyfunctional catalyst. A metal dienolate is thus formed, while an imidazolium-ArOH moiety probably forms hydrogen bonds with the dienophile. The catalyst type, readily prepared in few steps in high overall yield, was applied to 3-hydroxy-2-pyrone and 3-hydroxy-2-pyridone as well as cyclopentenone prodienes. Maleimide, maleic anhydride, and nitroolefin dienophiles were employed. Kinetic, spectroscopic, and control experiments support a cooperative mode of action. High enantioselectivity was observed even with unprecedented TONs of up to 3680.
Collapse