Oxidative Rearrangement Coupling Reaction for the Functionalization of Tetrahydro-β-carbolines with Aromatic Amines

Activation of enamine by photoexcited organocatalyst assisted singlet oxygen: synthesis of oxazoles and quinoxalines

Herein, a novel transition-metal-free thiol-based donor-acceptor organophotocatalyst-assisted, singlet-oxygen-mediated tandem oxidative cyclization for the synthesis of substituted oxazoles in moderate-to-good yields is described. The developed method demonstrates applicability for the synthesis of various substituted quinoxalines in good-to-excellent yields. The metal-free methodology shows a practical route for the synthesis of oxazole and quinoxaline derivatives, which are privileged moieties prevalent in various biologically active compounds and natural products. To the best of our knowledge, both the thiol photocatalyst and synthesis of oxazoles by visible-light irradiation are reported for the first time.

Switchable Skeletal Rearrangement of Hexahydro-4H-indol-4-ones: Divergent Synthesis of Dihydroxy-4H-cyclopenta[b]pyridin-4-ones and 8-Alkenyl Oxepane-2,6-diones

An unprecedented base-controlled selective skeletal rearrangement reaction of hexahydro-4H-indol-4-ones has been developed. In this protocol, highly functionalized dihydroxy-4H-cyclopenta[b]pyridin-4-ones and 8-alkenyl oxepane-2,6-diones were prepared with a broad substrate scope and high chemoselectivity in moderate to excellent yields selectively by modulating LiOH and Et3N. In addition, the newly formed 8-alkenyl oxepane-2,6-dione scaffolds could be easily further derivatized to 5-(pyrrol-2-yl)dihydrofuran-2(3H)-ones through a rare intramolecular rearrangement reaction.

Synthesis, structure-activity relationship, and biological evaluation of quinolines for development of anticancer agents

Tetrahydro-β-carbolines (THβCs) are a kind of natural alkaloids with multiple pharmaceutical activities. Herein, a focused compound library derived from THβCs was synthesized and their anticancer activities were studied in several cancer cell lines. Among them, three compounds showed considerable anticancer activities with low micromolar to submicromolar IC₅₀ values. The abilities to induce apoptosis and alter mitochondrial membrane potential levels, which are comparable to those of the commercial anticancer drug adriamycin, were confirmed by one representative compound (21) on the B16/F10 cell line. Our preliminary structure-activity relationship studies indicated that alkylamines with suitable lengths are very important for potency improvement.

Lewis Acid-Relayed Singlet Oxygen Reaction with Enamines: Selective Dimerization of Enamines to Pyrrolin-4-ones

Singlet oxygen (¹O₂)-mediated oxidation represents an attractive strategy for incorporation of oxygen atoms from air under mild and environmentally benign conditions. However, the ¹O₂ reaction with enamine suffers from fragmentation, leading to very unsuccessful transformation. Here, Lewis acid is introduced to intercept [2 + 2] or "ene" reaction intermediates of the ¹O₂ reaction and enables oxidative dimerization of enamines to produce pyrrolin-4-ones in good to excellent yields. Mechanistic studies reveal the formation of the imino ketone intermediate from the interaction of ¹O₂ and enamine, which is able to interact with Lewis acid, relaying the ¹O₂ reaction in enamine chemistry. For the first time, selective cross-dimerization of two different enamines is achieved. Due to the advantages of mild conditions, high chemoselectivity, and up to 99% yield, a promising strategy has been developed for synthesizing aza-heterocycles under ambient conditions, which can be further applied for the synthesis of imidazolone, quinoxaline, and highly functionalized imine.

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-β-carbolines that employs atmospheric O₂ as the terminal oxidant. The system is applicable to oxidative rearrangement of tetrahydro-β-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.

Synthesis of 3,3′-bisindoles via demethylenation

This work provides a mild demethylenation for the synthesis of 3,3′-bisindoles.

Direct C-H Functionalization of Tetrahydro-γ-Carbolines at α-Position

The direct C-H functionalization of tetrahydro-γ-carbolines (THγCs) at α-position has been presented. This mild and simple strategy allows coupling of THγCs with various anilines and indoles with satisfactory to excellent...

Harnessing selective PET and EnT catalysis by chlorophyll to synthesize N-alkylated quinoline-2(1H)-ones, isoquinoline-1(2H)-ones and 1,2,4-trioxanes

Photocatalytic syntheses of quinoline-2(1H)-ones, isoquinoline-1(2H)-ones and 1,2,4-trioxanes were achieved by selective photo-induced electron transfer (PET) and energy transfer (EnT), respectively, by chlorophyll under visible light irradiation. Quinoline-2(1H)-ones, isoquinoline-1(2H)-ones and 1,2,4-trioxanes are biologically potent scaffolds and their syntheses following mild reaction protocols are highly sought after. This work showcases the divergent photocatalytic roles of chlorophyll viz., electron transfer in the case of quinolines or isoquinolines and energy transfer with allyl alcohols as substrates, affording their aerobic oxidation under green reaction conditions. The mechanistic investigations affirm that the catalytic cycle follows the electron-transfer pathway in carrying out the oxidation of N-alkyl(iso)quinolinium salts. Furthermore, the method provides an environmentally benign, simple reaction strategy for organic transformations of (N)-heterocycles.

TCCA-mediated oxidative rearrangement of tetrahydro-β-carbolines: facile access to spirooxindoles and the total synthesis of (±)-coerulescine and (±)-horsfiline

Multi-reactive centered reagents are beneficial in chemical synthesis due to their advantage of minimal material utilization and formation of less by-products. Trichloroisocyanuric acid (TCCA), a reagent with three reactive centers, was employed in the synthesis of spirooxindoles through the oxidative rearrangement of various N-protected tetrahydro-β-carbolines. In this protocol, low equivalents of TCCA were required to access spirooxindoles (up to 99% yield) with a wide substrate scope. Furthermore, the applicability and robustness of this protocol were proven for the gram-scale total synthesis of natural alkaloids such as (±)-coerulescine (1) and (±)-horsfiline (2) in excellent yields.

Three-reactive centered reagent (TCCA) mediated construction of spirooxindoles through an oxidative rearrangement of various N-protected tetrahydro-β-carbolines and total synthesis of natural alkaloids (±)-coerulescine and (±)-horsfiline.

Diverse Functionalization of Tetrahydro-β-carbolines or Tetrahydro-γ-carbolines via Oxidative Coupling Rearrangement

We report herein diverse functionalization of tetrahydro-β-carbolines (THβCs) or tetrahydro-γ-carbolines (THγCs) via oxidative coupling rearrangement. The treatment of THβCs or THγCs with t-BuOOH (TBHP) afforded 3-peroxyindolenines, followed by HCl catalyzed indolation to form unexpected 2-indolyl-3-peroxyindolenines. Further rearrangement of these peroxides allows for rapid access to a skeletally diverse chemical library in good to excellent yields.

Visible-Light Cascade Photooxygenation of Tetrahydrocarbazoles and Cyclohepta[b]indoles: Access to C,N-Diacyliminium Ions

Tetrahydrocarbazoles and perhydrocyclohepta[b]indoles undergo a catalytic cascade singlet oxygenation in alkaline medium, which leads to chiral tricyclic perhydropyrido- and perhydroazepino[1,2-a]indoles in a single operation. These photooxygenation products are new synthetic equivalents of uncommon C,N-diacyliminium ions and can be functionalized with the aid of phosphoric acid organocatalysis.

Pyrrolo [3,4-b]-quinolin-9-amine compound FZU-0038-056 suppresses triple-negative breast cancer partially through inhibiting the expression of Bcl-2

Triple-negative breast cancer (TNBC) has a poorer prognosis than other subtypes of breast cancer; however, it lacks effective targeted therapies clinically. In this study, we found FZU-0038-056, a novel compound derived from last-stage functionalization of tetrahydro-β-carboline scaffold, showed the most potent anti-cancer activity against TNBC cells among the 42 synthesized derivatives. We found FZU-0038-056 significantly induces apoptosis in HCC1806 and HCC1937 TNBC cells. FZU-0038-056 reduces the expression levels of several anti-apoptosis proteins, including Bcl-2, Mcl-1 and XIAP. Furthermore, we found FZU-0038-056 induces apoptosis partially through inhibiting the expression of Bcl-2. Finally, we found FZU-0038-056 significantly suppresses HCC1806 xenograft tumor growth in nude mice without affecting their body weight. Therefore, FZU-0038-056 has the potential to be a new anticancer agent for treating human TNBC.

Construction of Bisindolines via Oxidative Coupling Cyclization

Herein, we report a general approach for the efficient construction of three-dimensional bisindolines via oxidative coupling cyclization in an intermolecular manner. This reaction is featured by its operational simplicity, metal-free conditions, lack of protecting group, and high selectivity. Notably, a wide range of anilines are suitable in this intermolecular cyclization, furnishing corresponding bisindolines in up to 98% yield.

Facile access to evodiakine enabled by aerobic copper-catalyzed oxidative rearrangement

Oxidation as a fundamentally important method for the synthesis of complex structures is difficult to achieve in a selective manner. Evodiakine, a complex natural product possessing an unprecedented ring system (6/5/5/7/6), has a high oxidation state without a practical solution. Herein, we report the first synthesis of evodiakine via aerobic copper-catalyzed late-stage functionalization of evodiamine.

Stabilization of Transient 3-Chloroindolenines Enables Diverse Functionalization

Transient intermediates as highly reactive species are difficult to manipulate due to their poor stability. Stabilization of unstable intermediates for functionalization is an attractive approach, but the practical applications are still rare. Herein, we explore a strategy that could effectively stabilize labile 3-chloroindolenines and significantly improve the lifetime from seconds to weeks. This chemistry was utilized to enable the synthesis of 55 diverse compounds which are unable to be achieved by traditional approach.

Oxidation of Tetrahydro-β-carbolines by Persulfate

The development of persulfate-mediated oxidation of tetrahydro-β-carbolines is reported. This mild reaction facilitates the formation of a variety of 2-formyl N-substituted tryptamines and the related derivatives as key intermediates in moderate to excellent yields. The method is applicable to direct last-stage oxidation of two interesting pharmaceuticals, Cialis and evodiamine.

Manipulation of Water for Diversified Functionalization of Tetrahydro-β-carbolines (THβCs) with Indoles

Water plays a crucial role in organic synthesis. However, diversified functionalization manipulated by water is still rare and remains unexplored. Herein, we report the first water-manipulated protocol to achieve the diversified functionalization of tetrahydro-β-carbolines (THβCs) in an open flask at room temperature that exhibit a broad functional-group tolerance. More water leads to monoarylation, while less water leads to diarylation. Further one-step transformation afforded oxidized bis(indolyl)methanes, eudistomin U, and the related derivatives in satisfactory yields.

Visible-light-induced oxidative ring expansion of indoles with amidines

An efficient and mild visible-light-enabled reaction involving the oxidative ring expansion of indoles with amidines in the aqueous phase at room temperature is developed.

Visible-Light-Induced Organic Photochemical Reactions through Energy-Transfer Pathways

Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible-light-induced photochemical reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.

Palladium-Catalyzed Ligand-Controlled Regioselective Nucleophilic Aromatic Substitution of 1-(Chloromethyl)naphthalenes with Arylacetonitriles

The palladium-catalyzed reaction of 1-(chloromethyl)naphthalenes 1 with (hetero)arylacetonitriles 2 gives either para- or ortho-acylated naphthalenes (3 or 4) in good to high yields. The regioselectivity can be controlled by the ligand of a palladium catalyst. A sterically bulky ligand, ^tBuPPh₂, affords para-acylated products 3, whereas a sterically less bulky ligand, Me₂PPh, provides ortho-acylated products 4. Further, direct substitution product 5 at the benzylic position is not obtained essentially, although such a reaction at the benzylic position is favorable in ordinary nucleophilic substitutions. In this paper, it was revealed that the benzylpalladium intermediate could react through a different mode (η³-benzylpalladium intermediate or η¹-benzylpalladium intermediate) in nucleophilic aromatic substitution. In addition to the interesting mechanistic aspect, the present reaction provides a facile synthetic method for a wide range of diaryl ketones, some of which are not easily available through the previously known procedures.

Biomimetic Oxidative Coupling Cyclization Enabling Rapid Construction of Isochromanoindolenines

Herein, we report a biomimetic oxidative coupling cyclization strategy for the highly efficient functionalization of tetrahydrocarbolines (THCs). This process enables rapid access to complex isochromanoindolenine scaffolds in moderate to excellent yields. The reaction proceeds smoothly and rapidly (complete within minutes) in an open flask. This operationally simple protocol is scalable and compatible with a wide range of functional groups. Late-stage functionalization of a pharmacologically relevant molecule is also demonstrated.

Highly diastereoselective oxa-[3+3] cyclization with C,N-cyclic azomethine imines via the copper-catalyzed aerobic oxygenated CC bond of indoles

Herein, a copper-catalyzed highly diastereoselective aerobic oxygenated [3+3] cyclization of 3-substituted indoles with C,N-cyclic azomethine imines using oxygen as the sole oxidant under mild conditions has been developed.