Metal-Free Synthesis of 2-Substituted 3-(2-Hydroxyaryl)quinolines and 4-(2-Hydroxyaryl)acridines via Benzyne Chemistry

Chemodivergence in Fluorine Source-Controlled Cascade Reaction of Aryne Precursors to Synthesize Pyrrolo[3,4-b]indoles and 3-Arylated Maleimides

Reactions allowing chemodivergence prove to be attractive strategies in synthetic organic chemistry. We herein described a highly practical, transition-metal-free, highly regioselective and chemodivergent cascade reaction controlled by fluorine sources, which involved a [3 + 2] cycloaddition or C-arylation process between aryne precursors and 3-aminomaleimides. These two pathways led to a wide scope of structurally diverse pyrrolo[3,4-b]indoles (19 examples) and 3-arylated maleimides (25 examples) in good-to-excellent yields. Furthermore, the reaction could be scaled up, and several synthetic transformations were accomplished for the preparation of functionalized molecules and might provide new opportunities for the discovery of N-heterocyclic drugs.

Nitronate-aryne cycloaddition as a concise route to stereochemically complex fused benzisoxazolines and amino alcohols

The reaction of cyclic nitronates (isoxazoline N-oxides and 5,6-dihydro-4H-1,2-oxazine N-oxides) with Kobayashi's aryne precursors affords tricyclic benzene-fused nitroso acetals as a result of [3 + 2]-cycloaddition. The process is regio- and stereoselective in most cases and produces the target cycloadducts possessing up to four contiguous stereogenic centers. These nitroso acetals were shown to be convenient precursors of valuable polysubstituted aminodiols through catalytic hydrogenolysis of the N-O bonds. Also, the action of protic acids resulted in an unusual fragmentation of the cyclic nitroso acetal moiety through heterolytic N-O bond cleavage and Beckmann-type reaction. Using this acid-mediated reaction, the synthesis of a hitherto unknown hexahydrobenzo[4,5]isoxazolo[2,3-a]azepine scaffold was accomplished.

Deoxygenative Arylation of 5,6-Dihydro-4H-1,2-oxazine-N-oxides with Arynes

Six-membered cyclic nitronates (5,6-dihydro-4H-1,2-oxazine-N-oxides) react with Kobayashi's aryne precursors producing 3-(2-hydroxyaryl)-substituted 1,2-oxazines via deoxygenative C-H arylation. The process involves a hitherto unknown 1,3-dipolar cycloaddition of nitronate to the aryne to give an unusual tricyclic nitroso acetal, in which the N-O bond of the isoxazoline ring is selectively cleaved upon the action of a base (CsF) or an acid (TFA). The transient cycloadducts were isolated and characterized in some cases. The synthetic potential of the obtained 3-(2-hydroxyaryl)-substituted 1,2-oxazines was demonstrated by their stereoselective reduction to 1,4-amino alcohols and reductive 1,2-oxazine ring contraction to tetrahydrofuran derivatives.

Regioselective Synthesis of C3-Hydroxyarylated Pyrazoles

Pyrazoles are ubiquitous structures in medicinal chemistry. We report the first regioselective route to C3-hydroxyarylated pyrazoles obtained through reaction of pyrazole N-oxides with arynes using mild conditions. Importantly, this method does not require the C4 and C5 positions of the pyrazole to be functionalized to observe regioselectivity. Using this method, we completed the synthesis of a recently reported JAK 1/2 inhibitor. Our synthesis produces the desired product in 4 steps from commercially available starting materials.

Selective construction of functionalized chromeno[3,4-b]pyrroles and benzo[c]chromenesviaa K3PO4promoted three-component reaction

A K₃PO₄promoted three-component reaction of pivaloylacetonitrile (benzoylacetonitrile), dialkyl but-2-ynedioates and 2-aryl-3-nitrochromenes afforded functionalized chromeno[3,4-b]pyrroles.

Catalyst and Additive-Free Diastereoselective 1,3-Dipolar Cycloaddition of Quinolinium Imides with Olefins, Maleimides, and Benzynes: Direct Access to Fused N,N'-Heterocycles with Promising Activity against a Drug-Resistant Malaria Parasite

A convenient and eco-friendly synthesis of various fused N-heterocyclic compounds through catalyst and additive-free 1,3 dipolar cycloadditions of quinolinium imides with olefins, maleimides, and benzynes in excellent yields and diastereoselectivities is reported. The thermally controlled diastereoselective [3 + 2] cycloaddition reaction between quinolinium imides and olefins provided cis-isomers at low temperature and trans-isomers at high temperature. A reaction between quinolinium imides with substituted maleimides gave four-ring-fused N-heterocyclic compounds in high yields as a single diastereomer. The aryne precursors also provided four-ring-fused N,N'-heterocyclic compounds in high yields. The in vitro antiplasmodial activity of selected molecules revealed that this class of molecules possesses potential for ongoing studies against malaria.

Rational Design and Facile Synthesis of a Highly Tunable Quinoline-Based Fluorescent Small-Molecule Scaffold for Live Cell Imaging

Small-molecule fluorescent probes are powerful tools for chemical biology; however, despite the large number of probes available, there is still a need for a simple fluorogenic scaffold, which allows for the rational design of molecules with predictable photophysical properties and is amenable to concise synthesis for high-throughput screening. Here, we introduce a highly modular quinoline-based probe containing three strategic domains that can be easily engineered and optimized for various applications. Such domains are allotted for (1) compound polarization, (2) tuning of photophysical properties, and (3) structural diversity. We successfully synthesized our probes in two steps from commercially available starting materials in overall yields of up to 95%. Facile probe synthesis was permitted by regioselective palladium-catalyzed cross-coupling, which enables combinatorial development of structurally diverse quinoline-based fluorophores. We have further applied our probes to live-cell imaging, utilizing their unique two-stage fluorescence response to intracellular pH. These studies provide a full demonstration of our strategy in rational design and stream-lined probe discovery to reveal the diverse potential of quinoline-based fluorescent compounds.

Molecular diversity of the domino annulation reaction of 2-aryl-3-nitrochromenes with pivaloylacetonitriles

In the presence of triethylamine, the domino annulation reaction of two molecules of pivaloylacetonitrile with one molecule of 2-aryl-3-nitrochromene in tetrahydrofuran resulted in the unprecedented imino-substituted dihydrofuro[2,3-c]chromene derivatives in high yields.

Recent advances in molecular rearrangements involving aryne intermediates

A concise account on the recent developments in molecular rearrangements proceeding via the aryne intermediate is presented thereby shedding light on the synthetic potential of this versatile intermediate.