Direct synthesis of N-functionalized indoles through isomerization of azomethine ylides.

An unexpected isomerization of azomethine ylides generated in situ from isatin with indoline-2-carboxylic acid has been disclosed, providing direct access to N-functionalized indole scaffolds. This protocol has good functional group tolerance and provides various 3-(1H-indol-1-yl)indolin-2-one derivatives in moderate to high yields simply by using alcohol as the solvent, with no additional additive being required.

Electrochemical bromocyclization enables 3,5-diversification of heterocyclic indolines.

Electrophilic bromocyclization reactions are widely used as key steps in the synthesis of diverse functionalized tetrahydrofuroindolines and hexahydropyrroloindolines. However, the direct dibromination variants of these reactions for the synthesis of 3,5-dibromoindolines remain undeveloped. Here, we report a protonic-acid-promoted electrooxidative protocol for the dearomative C3,C5-dibromocyclizations of tryptophol and tryptamine derivatives. This electrosynthetic approach, which enables direct selective construction of heterocyclic 3a,5a-dibromoindolines with inexpensive, non-hazardous NaBr as both the electrolyte and Br source, provides a convenient, practical method for the late-stage 3,5-diversification of heterocyclic indolines.

Copper-Catalyzed Oxygenative Skeletal Rearrangement of Tetrahydro-ß-carbolines Using H2 O and O2 as Oxygen Sources.

Herein, we report an unprecedented skeletal rearrangement reaction of tetrahydro-ß-carbolines enabled by copper-catalyzed single-electron oxidative oxygenation, in which H2 O and O2 act as oxygen sources to generate a unique 2-hydroxyl-3-peroxide indoline intermediate. The synthetic reactivity of 2-hydroxyl-3-peroxide indoline species was demonstrated by a unique multi-step bond cleavage and formation cascade. Using a readily available copper catalyst under open-air conditions, highly important yet synthetically difficult spiro[pyrrolidone-(3,1-benzoxazine)] products were obtained in a single operation. The synthetic utility of this methodology is demonstrated by the efficient synthesis of the natural products donaxanine and chimonamidine, as well as the 3-hydroxyl-pyrroloindoline scaffold, in just one or two steps.

Copper-Catalyzed Cyclization/Dimerization of Tryptamines with O2/Air as the Sole Oxidant: Direct Access to Complex Bispyrrolidino[2,3-b]indoline.

The first transition metal catalytic one-step synthesis of the 3a, 3a'-bispyrrolidino [2,3-b] indoline scaffold via tandem cyclization/dimerization of tryptamines has been realized with the environmentally friendly O2/air as the sole oxidant. Different from the traditional direct oxidation of indole "N-H" group by excess amount of metal salts, a copper-catalyzed oxidative cyclization reaction is developed for the formation of the radical pyrrolidinoindoline intermediate in the current strategy. The robustness and practicality of this methodology is demonstrated by the step-economic, divergent total synthesis of natural products (±)-folicanthine and meso-folicanthine.

Design, Synthesis of Novel Tetrandrine-14-l-Amino Acid and Tetrandrine-14-l-Amino Acid-Urea Derivatives as Potential Anti-Cancer Agents.

Tetrandrine, a dibenzyltetrahydroisoquinoline alkaloid isolated from the root of the traditional Chinese medicinal plant Stephania tetrandra S. Moore, a member of the Menispermaceae, showed anti-cancer activity by inhibiting cell proliferation, preventing cell cycle progress and induction of cell death and autophagy. In this study, twelve tetrandrine-l-amino acid derivatives and twelve tetrandrine-14-l-amino acid-urea derivatives were designed and synthesized, using C14-aminotetrandrine as raw material. Then the preliminary in vitro anti-cancer activities of these derivatives against human breast cancer cell line MDA-MB-231, human leukemia cell lines HEL and K562 were evaluated. The in vitro cytotoxicity results showed that these derivatives exhibited potent inhibitory effects on cancer cell growth, and the primary structure-activity relationships were evaluated. Notably, compound 3f exhibited satisfactory anticancer activity against all three cancer cell lines, especially the HEL cell line, with the IC50 value of 0.23 µM. Further research showed that 3f could induce G1/S cycle arrest and apoptosis in a dose- and time- dependent manner on the leukemia cell line HEL. The results suggested that 3f may be used as a potential anti-cancer agent for human leukemia.

Synthesis of Flavone Derivatives via N-Amination and Evaluation of Their Anticancer Activities.

Seventeen new flavone derivatives substituted at the 4'-OH position were designed, synthesized and evaluated for their anticancer and antibacterial activities. Among them, compounds 3, 4, 6f, 6e, 6b, 6c and 6k demonstrated the most potent antiproliferative activities against a human erythroleukemia cell line (HEL) and a prostate cancer cell line (PC3). The results also showed that the IC50 value of compounds 3, 4, 6f, 6e, 6b, 6c and 6k were close to that of the anticancer drug cisplatin (DDP) and lower than that of apigenin. All of the derivatives did not present antibacterial activities. The structure-activity relationships evaluation showed that the configuration of methyl amino acid might affect their biological activities.

Potent Cytotoxicity of Novel L-Shaped Ortho-Quinone Analogs through Inducing Apoptosis.

Twenty-seven L-shaped ortho-quinone analogs were designed and synthesized using a one pot double-radical synthetic strategy followed by removing methyl at C-3 of the furan ring and introducing a diverse side chain at C-2 of the furan ring. The synthetic derivatives were investigated for their cytotoxicity activities against human leukemia cells K562, prostate cancer cells PC3, and melanoma cells WM9. Compounds TB1, TB3, TB4, TB6, TC1, TC3, TC5, TC9, TC11, TC12, TC14, TC15, TC16, and TC17 exhibited a better broad-spectrum cytotoxicity on three cancer cells. TB7 and TC7 selectively displayed potent inhibitory activities on leukemia cells K562 and prostate cancer cells PC3, respectively. Further studies indicated that TB3, TC1, TC3, TC7, and TC17 could significantly induce the apoptosis of PC3 cells. TC1 and TC17 significantly induced apoptosis of K562 cells. TC1, TC11, and TC14 induced significant apoptosis of WM9 cells. The structure-activity relationships evaluation showed that removing methyl at C-3 of the furan ring and introducing diverse side chains at C-2 of the furan ring is an effective strategy for improving the anticancer activity of L-shaped ortho-quinone analogs.

Copper/Iodine Co-catalyzed Oxygenative Transannulation of Tryptamines Enables Direct Synthesis of Donaxaridine and Its Derivatives.

We report a general copper/iodine co-catalyzed oxygenative transannulation strategy using readily available tryptamines. Molecular oxygen and water are used as oxygen sources and provide direct access to the donaxaridine scaffold and its derivatives. This methodology is applied to the efficient synthesis of the natural products donaxaridine, chimonamidine, donaxanine, donaxarine, and aline in just one or two steps. The tryptamines, albeit with oxy-sensitive dialkyl N-H groups, are selectively oxidized through a single-electron transfer dioxygenation process.

Copper-Catalyzed Aerobic Selective Oxidation of Tetrahydrocarbolines.

We report a safe and convenient open-flask copper-catalyzed selective oxidation/functionalization methodology for tetrahydrocarbolines and tetrahydro-ß-carbolines that employs atmospheric O2 as the terminal oxidant. The system is applicable to oxidative rearrangement of tetrahydro-ß-carbolines, tetrahydrocarboline oxidation to α-alkoxy carbazoles and spirooxindoles, and Witkop oxidation. Mechanistic experiments indicated that a single-electron oxidation process is responsible for the tunable selectivity control. This copper-catalysis protocol represents a significant advance in the field of indole oxidation.

Copper-Catalyzed Selective Oxidative Cross-Coupling of Tryptophols and Tryptamines To Access Heterocyclic 3a,3a'-Bisindolines.

The first example of cyclization cross-coupling of tryptophols and tryptamines has been realized by copper catalysis with air or oxone as the terminal oxidant, resulting in the direct construction of a new class of heterocyclic 3a,3a'-bisindolines in moderate to good yields with high chemoselectivities. A series of mechanistic control experiments were also conducted, indicating that the copper catalyst selectively coordinates with the nitrogen moiety of the tryptamine to initiate the oxidation, and a nucleophilic-alkylation process is proposed for the carbon-carbon bond-forming in the reaction. The novel synthetic strategies and molecular skeletons outlined in this work provide new ideas and concepts for the design of other useful reaction and potential drugs.

Copper-catalyzed aerobic oxidative radical alkoxycyclization of tryptamines to access 3-alkoxypyrroloindolines.

We report a copper-catalyzed alkoxycyclization of tryptamine derivatives with O2 as the sole oxidant, leading to a variety of C3a-alkoxypyrroloindolines in good yields with high diastereoselectivities. This reaction involves an interesting double catalytic cycle in which copper-catalyzed carboamination cyclization is favored to form the C-3 radical pyrrolidinoindoline intermediate, then a copper-catalytic radical alkoxylation reaction proceeds smoothly.

Oxazoline-/Copper-Catalyzed Alkoxyl Radical Generation: Solvent-Switched to Access 3a,3a'-Bisfuroindoline and 3-Alkoxyl Furoindoline.

We report the first example of oxazoline-/copper-catalyzed alcohol oxidation to generate the alkoxyl radical under additive-free conditions. The resulting alkoxyl radical addition to alkene enables useful C-O bond-forming and selective C(sp3)-C(sp3) radical-radical dimerization/radical-trapping reactions, providing direct access to the 3a,3a'-bisfuro[2,3-b]indoline scaffold for the first time and a wide range of 3-alkoxyl furoindolines with high efficiency.