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- W3048960319 endingPage "11720" @default.
- W3048960319 startingPage "11699" @default.
- W3048960319 abstract "A full account of the Brønsted acid catalyzed, enantioselective synthesis of 4H-chromenes and 1H-xanthen-1-ones from o-hydroxybenzyl alcohols and β-dicarbonyl compounds is provided. The central step of our strategy is the BINOL–phosphoric acid catalyzed, enantioselective cycloaddition of β-diketones, β-keto nitriles, and β-keto esters to in situ generated, hydrogen-bonded o-quinone methides. Upon acid-promoted dehydration, the desired products were obtained with generally excellent yields and enantioselectivity. Detailed mechanistic studies including online-NMR and ESI-MS measurements were conducted to identify relevant synthetic intermediates. A reversible formation of a dimer from the starting alcohol and the reactive o-quinone methide in an off-cycle equilibrium was observed, providing a reservoir from which the o-quinone methide can be regenerated and introduced into the catalytic cycle again. Reaction progress kinetic analysis was utilized to determine kinetic profiles and rate constants of the reaction uncovering o-quinone methide formation as the rate-limiting step. In combination with Hammett plots, these studies document the relationship between o-quinone methide stabilization by electronic effects provided by the substituents and the reaction rate of the described process. In addition, DFT calculations reveal a concerted yet highly asynchronous [4 + 2]-cycloaddition pathway and an attractive CH−π interaction between the catalyst’s tBu group and the o-quinone methide as an important stereochemical control element." @default.
- W3048960319 created "2020-08-18" @default.
- W3048960319 creator A5003452138 @default.
- W3048960319 creator A5005127669 @default.
- W3048960319 creator A5073000929 @default.
- W3048960319 creator A5078412901 @default.
- W3048960319 creator A5078656621 @default.
- W3048960319 creator A5091082166 @default.
- W3048960319 date "2020-08-10" @default.
- W3048960319 modified "2023-10-08" @default.
- W3048960319 title "Phosphoric Acid Catalyzed Formation of Hydrogen-Bonded <i>o</i>-Quinone Methides. Enantioselective Cycloaddition with β-Dicarbonyl Compounds toward Benzannulated Oxygen Heterocycles" @default.
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