Modular Regiodivergent Synthesis of Benzo-Fused Isocoumarins by a Cyclopropane Aromatization Strategy

Switchable skeletal editing of quinolines enabled by cyclizative sequential rearrangements

The rapid diversification of core ring structures in complex molecules through switchable skeletal editing is valuable in the drug discovery process. However, controllable methods for chemically divergent modifications of azaarene frameworks using common substrates are challenging, despite the potential to maximize structural diversity and complexity. Here we report the tunable skeletal editing of quinolines through Brønsted acid-catalysed multicomponent reactions of quinoline N-oxides, dialkyl acetylenedicarboxylates and water to generate nitrogen-containing heteroaromatic compounds together with linear compounds in a modular fashion. Specifically, in a one-pot procedure, after cyclization and sequential rearrangement processes, the quinoline N-oxides are easily converted into unique 2-substituted indolines. These then undergo acid-promoted fragmentation to give indoles, base-facilitated ring-opening to afford 2-alkenylanilines and oxidative cyclization to yield isoquinolinones. Catalytic asymmetric skeletal editing of quinolines is also realized, providing enantioenriched benzazepines bearing quaternary stereocentres, and late-stage skeletal modification of quinoline cores in several drugs is demonstrated.

Stereoselective Route to 2,3,4,5-Tetraaryltetrahydrofurans via Efficient Cascade Synthesis of 2,3-Dihydrofurans with Four Different (Hetero)Aryl Substituents

Tf2NH-mediated regio- and diastereoselective skeletal metamorphosis of 1-alkynyl-2,3-diaryl-2-methoxycarbonylcyclopropyl ketones readily gave 2,3,4,5-tetraaryl-2,3-dihydrofurans. Controllable reduction and removal of the ester group then precisely afforded four types of the eight possible tetrahydrofuran (THF) diastereomers with four different (hetero)aryl substituents. Further, deuterium-labeling results revealed an unprecedented hydrogen transfer from the formyl C-H bond in tBuOK-promoted decarbonylation under tBuOH-free conditions.

Catalytic Dearomative [1,5]-Sigmatropic Carbon Shift of Heterole-Fused Norcaradienes Enabled Concise Helicenation

In contrast to the locked fluxionality of norcaradienes fused by benzene, unexplored less aromatic heterole-fused norcaradienes, creatively generated by intramolecular hydroarylation of heteroaryl alkynylcyclopropanes, reserve a balancing fluxionality that permits a dearomative [1,5]-sigmatropic carbon shift of norcaradienes akin to the reduced aromaticity of heterole. This "walk" shift was confirmed by the isolation of a cycloheptatriene species derived from ring-expansion of a dearomatized alkynylated heterole-fused norcaradiene. A following ester-directed ring-opening rearomatization of these dearomatized heterole-fused norcaradienes gives the products featuring migratory acylmethyls that are competent for helicenation with the neighboring (hetero) arenes via (formal) dehydrative alkenylation. Such balancing reactivity of heterole-fused norcaradienes will open up the opportunity for the development of controllable reactions of fused norcaradienes.

Scorpion-Shaped Hybrid Double Helicenes via Orthogonal Alkyne Annulation Reactions

Scorpion-shaped hybrid double helicenes, consisting of a [5] or [6] carbohelicene and an aza[4]helicene, have been successfully constructed by orthogonal alkyne annulations via an aryl C-I bond and amido N-H bond from polyaromatic ring-fused iodoisocoumarins. In spite of the unexpected instability upon aerobic oxidation upon ambient visible light irradiation over several days, both ultraviolet-visible absorption and photoluminescence spectra along with density functional theory calculations of these helicenes have been studied, which rely heavily on the bent polyaromatic ring-fused quinolizinone conjugate skeleton. In addition, the Stokes shifts of hybrid double helicenes are generally larger than those of the structurally similar mono-carbohelicenes.

Intramolecular 1,2-Aroyl Migration in Spiro Donor-Acceptor Cyclopropanes: Formation of 1,4-Naphthoquinones and 1-Naphthols as Ring-Expansion Products

Most of the known rearrangement reactions of donor-acceptor cyclopropanes (DACs) involve the migration of cationic carbon atom to anionic carbon or heteroatoms in 1,3- or 1,4-positions. In the present work, we observed that spiro DACs based on 1,3-indanedione or 1-indanone moiety undergo intramolecular 1,2-aroyl migration when treated with titanium(IV) chloride to afford 1,4-naphthoquinones and α-naphthols readily. The reactions take place through the formation of putative 1,3-dipolar intermediates, followed by cleavage and migration of the aroyl group to the adjacent carbon to afford the ring-expansion products.

Ni-Catalyzed 1,5-Sigmatropic Ester Shift on Cyclopentadiene Rings: Regioselective Conversion of 5,5-Disubstituted Cyclopentadienes to CH2-Cyclopentadienes

Described herein is a nickel(II)-catalyzed regioselective rearrangement of 5,5-disubstituted cyclopentadienes to fully functionalized CH2-cyclopentadienes via successive 1,5-sigmatropic shifts of the ester group on the quaternary carbon and hydrogen under mild basic conditions. The obtained CH2-cyclopentadienes were also readily applied in the preparation of highly functionalized dibenzo[e,g]azulene derivatives in two steps.

Regiodivergent Ring-Expansion of Oxindoles to Quinolinones

The development of divergent methods to expedite structure-activity relationship studies is crucial to streamline discovery processes. We developed a rare example of regiodivergent ring expansion to access two regioisomers from a common starting material. To enable this regiodivergence, we identified two distinct reaction conditions for transforming oxindoles into quinolinone isomers. The presented methods proved to be compatible with a variety of functional groups, which enabled the late-stage diversification of bioactive oxindoles as well as facilitated the synthesis of quinolinone drugs and their derivatives.

Regio- and Stereospecific Hydrative Cloke-Wilson Rearrangement

The Cloke-Wilson rearrangement of unsymmetrical β-diketone-derived cyclopropanes inevitably yields a mixture of two 4-acylated 2,3-dihydrofuran regiomers. By using alkynes as masked acyls, Tf2NH-promoted Cloke-Wilson rearrangement of polysubstituted 1-(1-alkynyl)cyclopropyl ketones followed by alkyne hydration is described, regioselectively affording 2,3-dihydrofurans bearing 4-acyls nonequivalent to that involved in the Cloke-Wilson rearrangement. The 2,3-dihydrofuran rings with cis 2,3-diaryls are unexpectedly more stable than their trans diastereomers under the reaction conditions, guaranteeing the regiospecificity of this hydrative Cloke-Wilson rearrangement with high fidelity.

Aromatic Homo-Nazarov-Type Cyclization of Benzonorcaradienes: Stereoselective Synthesis of Hydrochrysenes

Formal homo-Nazarov cyclization of benzonorcaradienes produced by intramolecular hydroarylation of arylated alkynylcyclopropanes promoted by TfOH has been described, providing stereoselective access to highly substituted hydrochrysenes. An unprecedented 1,2-acyl migration occurred for the 2-heteroaroyl substrates, thus giving the same products as their 3-heteroaroyl analogs. Moreover, these products could be readily oxidized by air to fully π-conjugated chrysenes after decarboxylation.

Brønsted acid promoted substrate-dependent regiodivergent alkynylcyclopropane–cyclopentadiene rearrangement assisted by the internal carbonyl group

Facile Brønsted acid promoted alkynylcyclopropane–cyclopentadiene rearrangement, avoiding the five-membered cycloallene intermediate assisted by the internal carbonyl groups, has been developed.