Phosphoric Acid Catalyzed Formation of Hydrogen-Bonded o‑Quinone Methides. Enantioselective Cycloaddition with β‑Dicarbonyl Compounds toward Benzannulated Oxygen Heterocycles

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.