A one-pot oxidation/cycloaddition cascade synthesis of 2,4-diaryl chromans via ortho-quinone methides

Visible-Light-Induced ortho-Quinone Methides Formation in Situ Using ortho-Alkylarenols as Precursors for Tandem [4 + 2] Cycloaddition Reaction

Reported herein is the generation of ortho-quinone methides (o-QMs) via metal-free visible-light-induced oxidation of ortho-alkylarenols, as well as their subsequent reaction with olefins to afford chromans in good to excellent yields (up to 91%). The key is the selective activation of the benzylic C(sp3)-H bond of ortho-alkylarenols via single electron transfer (SET) and the formation of o-QMs via hydrogen atom transfer (HAT).

One-Pot Synthesis of 3,4-Dihydrocoumarins via C-H Oxidation/Conjugate Addition/Cyclization Cascade Reaction

The 3,4-dihydrocoumarin derivatives were obtained from 2-alkyl phenols and oxazolones via C-H oxidation and cyclization cascade in the presence of silver oxide (Ag2O) and p-toluenesulfonic acid as a Brønsted acid catalyst. This approach provides a one-pot strategy to synthesize the multisubstituted 3,4-dihydrocoumarins with moderate to high yields (64-81%) and excellent diastereoselectivity (>20:1).

Chemoenzymatic o-Quinone Methide Formation

Generation of reactive intermediates and interception of these fleeting species under physiological conditions is a common strategy employed by Nature to build molecular complexity. However, selective formation of these species under mild conditions using classical synthetic techniques is an outstanding challenge. Here, we demonstrate the utility of biocatalysis in generating o-quinone methide intermediates with precise chemoselectivity under mild, aqueous conditions. Specifically, α-ketoglutarate-dependent non-heme iron enzymes, CitB and ClaD, are employed to selectively modify benzylic C-H bonds of o-cresol substrates. In this transformation, biocatalytic hydroxylation of a benzylic C-H bond affords a benzylic alcohol product which, under the aqueous reaction conditions, is in equilibrium with the corresponding o-quinone methide. o-Quinone methide interception by a nucleophile or a dienophile allows for one-pot conversion of benzylic C-H bonds into C-C, C-N, C-O, and C-S bonds in chemoenzymatic cascades on preparative scale. The chemoselectivity and mild nature of this platform is showcased here by the selective modification of peptides and chemoenzymatic synthesis of the chroman natural product (-)-xyloketal D.

A Strategy To Obtain o-Naphthoquinone Methides: Ag(I)-Catalyzed Cyclization of Enynones for the Synthesis of Benzo[ h]chromanes and Naphthopyryliums

A new strategy to obtain o-NQM intermediates through a ring-formation strategy by Ag(I)-catalyzed cyclization of 2-alkenylphenyl alkynyl ketones and its [4 + 2] annulations with styrenes has been developed. This reaction features high efficiency, mild reaction conditions, as well as flexible substitutions and atom economy. The obtained benzo[ h]chromane products were further oxidized to naphthopyryliums, which displayed tunable photophysical properties.

A chiral scandium-complex-catalyzed asymmetric inverse-electron-demand oxa-Diels–Alder reaction of o-quinone methides with fulvenes

A highly efficient asymmetric [4+2] cycloaddition of o-quinone methides with fulvenes has been realized using a chiral N,N′-dioxide/Sc(iii) complex as the catalyst. The corresponding optically active chromane derivatives were obtained in high yields with excellent dr and ee values.

Donor–acceptor cyclopropanes as ortho-quinone methide equivalents in formal (4 + 2)-cycloaddition to alkenes

A new reactivity of donor–acceptor cyclopropanes as synthetic equivalents of o-quinone methides was revealed for 2-hydroxyarylcyclopropane diesters in their formal (4 + 2)-cycloaddition to alkenes yielding chromanes.