Enantioselective Phenolic α-Oxidation Using H2O2 via an Unusual Double Dearomatization Mechanism

Oxidation Triggered Supramolecular Chirality

Topochemical reactions normally occurring in the solid and crystalline state exhibit solvent-free and catalyst-free properties, with high atom economy properties, which have been widely applied in materials science and polymer synthesis. Herein, we explore the potential of topochemical reactions for controlling the emergence of supramolecular chirality and the precise fabrication of chiroptical materials. Boronic acid pinacol esters (BPin) were conjugated to naphthalimides containing an inherent chiral cholesteryl group linked by alkyl or benzene spacers. The BPin segments were oxidized by H2O2 to form hydroxyl groups, which enhanced luminescence, reduced steric effects, and increased amphiphilicity. The inherent liposomal aggregates underwent in situ oxidation and transformed into 1D nanoarchitectures, exhibiting macroscopic chirality, active Cotton effects, and circularly polarized luminescence. Oxidation could also initiate an intimate interplay between the building units and the guest molecule, by which the chirality and chiroptical evolution in the multiple component chiral assembly system were realized.

Dearomatization of Cyclic Diphenylhydrazines: Harnessing the o-Semidine Rearrangement for the Synthesis of Spirocyclic Tetrahydroquinolines

The synthesis of novel tetrahydroquinoline-containing spirocycles has been achieved through an acid-promoted dearomatization of cyclic diarylhydrazines. The reaction, proceeding through a dearomative o-semidine rearrangement, furnishes a stable, yet reactive spirocyclohexadieniminium ion, which can further be used as an electrophile or a diene in a one-pot sequence. These transformations efficiently produce novel diazaspirocycles and allow for further synthetic elaboration of the cyclohexadienamine products.

Palladium-Catalyzed Intermolecular Asymmetric Dearomative Annulation of Phenols with Vinyl Cyclopropanes

Herein, we report the Pd(0)-catalyzed intermolecular asymmetric dearomative [3 + 2] annulation of phenols with vinyl cyclopropanes via in situ generated ortho-quinone methide intermediates. A series of highly functionalized spiro-[5,6] bicycles which bear three contiguous stereogenic centers including one all-carbon quaternary were obtained with excellent stereoselectivities. Density functional theory (DFT) calculations indicate that the reactions were controlled by thermodynamics.

One-Pot Transformation of Salicylaldehydes to Spiroepoxydienones via the Adler-Becker Reaction in a Continuous Flow

The Adler-Becker reaction is a useful approach for the oxidative dearomatization of salicylic alcohols to spiroepoxydienones and has been applied in the total synthesis of several natural products. Despite the advantages, the substrate and product instability under the reaction conditions can decrease the reaction efficiency, leading to lower yields. Herein, we report the Adler-Becker reaction in a continuous flow for the transformation of reduced salicylaldehydes into spiroepoxydienones in a one-pot approach. For that, a heterogeneous oxidant based on periodate is developed, leading to an efficient continuous flow process, with higher productivity and shorter reaction times, when compared with batch conditions.

Divergent oxidative dearomatization coupling reactions to construct polycyclic cyclohexadienones

Highly selective divergent oxidative dearomatization coupling reactions, in which the chemoselectivity is controlled by catalysts and bases, are reported herein. Three different kinds of polycyclic cyclohexadienones are produced from the same reactants (41 examples, 85-99% yield). Our method marks a novel copper- and palladium-catalyzed C-H oxidative dearomatization of phenolic derivatives.

Asymmetric Intramolecular Dearomatization of Nonactivated Arenes with Ynamides for Rapid Assembly of Fused Ring System under Silver Catalysis

Arene dearomatization is a straightforward method for converting an aromatic feedstock into functionalized carbocycles. Enantioselective dearomatizations of chemically inert arenes, however, are quite limited and underexplored relative to those of phenols and indoles. We developed a method for diazo-free generation of silver-carbene species from an ynamide and applied it to the dearomatization of nonactivated arenes. Transiently generated norcaradiene could be trapped by intermolecular [4 + 2] cycloaddition, synthesizing polycycles with five consecutive stereogenic centers. This protocol constitutes the first highly enantioselective reaction based on the diazo-free generation of silver-carbene species. Mechanistic investigations revealed a dearomatization followed by two different classes of pericyclic reactions, as well as the origin of the chemo- and enantioselectivity.

Atypical Dearomative Spirocyclization of β-Naphthols with Diazoacetamides Using a Silver Catalyst

A chemoselective dearomatization of the less reactive benzenoid unit in β-naphthol was developed. Spirocyclization with a reductant constructs a pivotal structure for drug candidates. One-pot oxidative conversion enabled the tandem dearomatization of β-naphthol, producing conjugated tetraenone variants. The potential utility of the product as an F^--selective anion sensor was also demonstrated. Theoretical studies revealed the intermediacy of silver-carbenoid species leading to chemoselective spirocyclization over arene cyclopropanation.

Silver-Catalyzed, Chemo- and Enantioselective Intramolecular Dearomatization of Indoles to Access Sterically Congested Azaspiro Frameworks

An asymmetric dearomatization of indoles bearing α-diazoacetamide functionalities was developed for synthesizing high-value spiro scaffolds. A silver phosphate chemoselectively catalyzed the sterically challenging dearomatization, whereas more typically used metal catalysts for carbene transfer reactions, such as a rhodium complex, were not effective and instead resulted in a Büchner ring expansion or cyclopropanation. Mechanistic studies indicated that the spirocyclization occurred through a silver-assisted asynchronous concerted process and not via a silver-carbene intermediate. Analyses based on natural bond orbital population and a distortion/interaction model indicated that the degree of C-Ag mutual interaction is crucial for achieving a high level of enantiocontrol. In addition, an oxidative disconnection of a C(sp³)-C(sp²) bond in the product provided unconventional access to the corresponding chiral spirooxindole.

Divergent syntheses of spirooxindoles from oxindole-embedded four-membered synthon via cycloaddition reactions

The construction of five and six membered heterocycle fused spirooxindoles was achieved via the [4 + 1] and formal [4 + 2] cycloadditions between our rationally designed four-membered synthons and pyridinium methylides and α-bromoacetophenones, respectively.

Phenolic Oxidation Using H2O2 via in Situ Generated para-Quinone Methides for the Preparation of para-Spiroepoxydienones

Phenols are attractive starting materials for the preparation of highly substituted cyclohexane rings via dearomative processes. Herein we report an efficient preparation of dearomatized 1-oxaspiro[2.5]octa-4,7-dien-6-ones (para-spiroepoxydienones) via the nucleophilic epoxidation of in situ generated para-quinone methides from 4-(hydroxymethyl)phenols using aqueous H₂O₂. The developed protocol bypasses the need for stoichiometric bismuth reagents or diazomethane, which are frequently deployed for p-spiroepoxydienone preparation. The p-spiroepoxydienones are further elaborated in numerous downstream complexity-building transformations.