Efficient Synthesis of 3,3′-Mixed Bisindoles via Lewis Acid Catalyzed Reaction of Spiro-epoxyoxindoles and Indoles

Stereo/regio-selective access to substituted 3-hydroxy-oxindoles with anti-proliferative assessment and in silico validation

The manuscript focuses on a highly stereo-/regioselective approach for synthesizing a diverse array of substituted-3-hydroxy-2-oxindoles. The synthesized compounds were subsequently subjected to anti-proliferative assessment against various cell lines, including colorectal carcinoma, ovarian cancer, and human metastatic melanoma cancer. The structural characterization of the synthesized scaffolds was unambiguously confirmed using X-ray diffraction analysis. Among the synthesized compounds, one compound demonstrated exceptional potency within the series. It exhibited 1.2, 2.12, and 1.55 times greater potency than cisplatin against the HCT116, OVCAR10, and 1205Lu cell lines, respectively. These results were further supported by in vitro caspase-mediated apoptosis studies. Molecular docking studies of these compounds on the target VEGFR2 protein revealed their binding capability.

Enantioselective Synthesis of Quaternary Oxindoles: Desymmetrizing Staudinger-Aza-Wittig Reaction Enabled by a Bespoke HypPhos Oxide Catalyst

This paper describes a catalytic asymmetric Staudinger-aza-Wittig reaction of (o-azidoaryl)malonates, allowing access to chiral quaternary oxindoles through phosphine oxide catalysis. We designed a novel HypPhos oxide catalyst to enable the desymmetrizing Staudinger-aza-Wittig reaction through the PIII/PV═O redox cycle in the presence of a silane reductant and an IrI-based Lewis acid. The reaction occurs under mild conditions, with good functional group tolerance, a wide substrate scope, and excellent enantioselectivity. Density functional theory revealed that the enantioselectivity in the desymmetrizing reaction arose from the cooperative effects of the IrI species and the HypPhos catalyst. The utility of this methodology is demonstrated by the (formal) syntheses of seven alkaloid targets: (-)-gliocladin C, (-)-coerulescine, (-)-horsfiline, (+)-deoxyeseroline, (+)-esermethole, (+)-physostigmine, and (+)-physovenine.

Epoxides: Small Rings to Play with under Asymmetric Organocatalysis

Optically pure epoxides are recognized as highly valuable products and key intermediates, useful in different areas from pharmaceutical and agrochemical industries to natural product synthesis and materials science. The predictable fate of the ring-opening process, in terms of stereoselectivity and often of regioselectivity, enables useful functional groups to be installed at vicinal carbon atoms in a desired manner. In this way, products of widespread utility either for synthetic applications or as final products can be obtained. The advent of asymmetric organocatalysis provided a new convenient tool, not only for their preparation but also for the elaboration of this class of heterocycles. In this review, we focus on recent developments of stereoselective organocatalytic ring-opening reactions of meso-epoxides, kinetic resolution of racemic epoxides, and Meinwald-type rearrangement. Examples of asymmetric organocatalytic processes toward specific synthetic targets, which include ring opening of an epoxide intermediate, are also illustrated.

Phosphine-Catalyzed Asymmetric Allylic Alkylation of Achiral MBH Carbonates with 3,3'-Bisindolines: Enantioselective Construction of Quaternary Stereogenic Centers

The asymmetric allylic alkylation (AAA) of achiral Morita-Baylis-Hillman (MBH) carbonates with 3,3'-bisindolines under the catalysis of amino-acid-derived bifunctional phosphines was accomplished. With the AAA approach introduced herein, challenging 3,3'-bisindolines bearing an all-carbon quaternary stereocenter (C3a) have been constructed in good yields with good to excellent enantioselectivties. In addition, the synthetic value of this protocol was demonstrated by the facile synthesis of the core skeleton of gliocladin C.

Sequential one-pot synthesis of N-sulfonyl spiroaziridine oxindoles from spiroepoxy oxindoles

The N-sulfonyl spiroaziridine oxindole is a recently developed versatile precursor in the synthesis of a wide range of 3,3-disubstituted spirooxindoles. It is usually prepared in three steps from isatin and needs costly and hardly available sulfinimides and hazardous peracid. A sequential and one-pot direct strategy for the synthesis of terminal N-sulfonyl spiroaziridine oxindoles has been developed under ambient conditions with excellent yields (up to 95%) from easily accessible spiroepoxy oxindoles by regioselective amination with aqueous ammonia and a subsequent ring enclosure reaction of the resulting 1,2-amino alcohol using easily available sulfonyl chloride and a base. Other salient features of the protocol include inexpensive substrate requirement and the ease of isolation of the desired product by performing single column chromatographic purification after two consecutive steps.

Design, Synthesis, Antiproliferative Evaluation, and Molecular Docking Studies of N-(3-Hydroxyindole)-Appended β-Carbolines/Tetrahydro-β-Carbolines Targeting Triple-Negative and Non-Triple-Negative Breast Cancer

The present manuscript pertains to the design and synthesis of a series of 3-hydroxyindole-substituted β-carbolines/tetrahydro-β-carbolines with an aim to explore their antiproliferative structure-activity relationship against breast cancer. The conjugate with an optimum combination of a flexible tetrahydro-β-carboline core, a tertiary alcoholic group along with a chloro substituent on the indole ring, proved to be the most active compound. It displayed IC50 values of 13.61 and 22.76 μM against MCF-7 (ER+) and MDA-MB-231 (ER-) cells, respectively. The docking studies were found to be consistent with experimental results owing to the stronger binding affinity of the synthesized conjugates via hydrophobic and H-bonding interactions.

Efficient chemical fixation and defixation cycle of carbon dioxide under ambient conditions

Chemical fixation of CO₂ as a C1 feedstock for producing value-added products is an important post-combustion technology reducing the CO₂ emission. As it is an irreversible process, not considered for the CO₂ capture and release. Overall, these chemical transformations also do not help to mitigate global warming, as the energy consumed in different forms is much higher than the amount of CO₂ fixed by chemical reactions. Here we describe the development of re-generable chemical fixation of CO₂ by spiroaziridine oxindole, where CO₂ is captured (chemical fixation) under catalyst-free condition at room temperature both in aqueous and non-aqueous medium even directly from the slow stream of flue gas producing regioselectively spirooxazolidinyl oxindoles, a potential drug. The CO₂-adduct is reversed back to the spiroaziridine releasing CO₂ under mild conditions. Further both the fixation-defixation of CO₂ can be repeated under near ambient conditions for several cycles in a single loop using a recyclable reagent.

Asymmetric Ring-Opening of Epoxides Catalyzed by Metal–Salen Complexes

The asymmetric ring-opening of epoxides is an important reaction in organic synthesis, since it allows for the enantioselective installation of two vicinal functional groups with specific stereochemistry within one step from a highly available starting material. An effective class of catalysts for the asymmetric ring-opening of epoxides is metal–salen complexes. This review summarizes the development of metal–salen catalyzed enantioselective desymmetrization of meso-epoxides and kinetic resolution of epoxides with various nucleophiles, including the design and application of both homogeneous- and heterogeneous epoxide-opening catalysts as well as multi-metallic covalent and supramolecular catalytic systems.

Enantioselective synthesis of mixed 3,3'-bisindoles via a phosphine-catalyzed umpolung γ-addition of 3'-indolyl-3-oxindoles to allenoates

An enantioselective umpolung γ-addition reaction of 3'-indolyl-3-oxindoles to allenoates catalyzed by amino acid-derived bifunctional phosphine catalysts has been developed. A wide range of chiral mixed 3,3'-bisindole scaffolds containing an all-carbon quaternary stereogenic center were obtained in high yields and with excellent enantioselectivities. 3,3'-Bisindoles are valuable synthetic intermediates, the employment of which led to formal total syntheses of (+)-Chimonanthine, (+)-Folicanthine and (-)-Calycanthine, as well as facile creation of useful pyrrolidinoindoline core structure.

Oxidative cyanation of 2-oxindoles: formal total synthesis of (±)-gliocladin C

Efficient oxidative direct cyanations of 3-alkyl/aryl 2-oxindoles using Cyano-1,2-BenziodoXol-3(1H)-one (CBX) (2a) have been reported under 'transition metal-free' conditions to synthesize a wide variety of 3-cyano 3-alkyl/aryl 2-oxindoles sharing an all-carbon quaternary center under additive-free conditions. The application of this process is shown by the formal total synthesis of (±)-gliocladin C (11c) in a few steps.

Syntheses and reactivity of spiro-epoxy/aziridine oxindole cores: developments in the past decade

This review highlights various reactions to afford spiro-epoxy/aziridine oxindoles and their potential synthetic transformations.

Controlling the regioselectivity of the ring opening of spiro-epoxyoxindoles for efficient synthesis of C(3)–N(1′)-bisindoles and C(3)–N(1′)-diindolylmethane

A strategy for the construction of both C(3)–N(1′) bisindoles and C(3)–N(1′) diindolylmethane has been explored via tuning of the nucleophilicity of indoline/indole to spiro-epoxyoxindole.

Tryptophol and derivatives: natural occurrence and applications to the synthesis of bioactive compounds

This report presents some fundamental aspects related to the natural occurrence and bioactivity of tryptophol as well as the synthesis of tryptophols and their utilization for the preparation of naturally occurring alkaloid metabolites embedding the indole system.

A Novel Sc(OTf)3 -Catalyzed (2+2+1)-Cycloannulation/Aza-Friedel-Crafts Alkylation Sequence toward Multicyclic 2-Pyrrolines

The rapid assembly of molecular complexity continues to be at the forefront of novel reaction development. In the pursuit of that goal, we herein report a novel Sc(OTf)₃ -catalyzed, one-pot multicomponent reaction that furnishes complex multicyclic 2-pyrrolines with excellent overall yields and perfect diastereocontrol. This process is based on our previously established (2+2+1)-cycloannulation of in situ generated 1-azaallyl cations, 1,3-dicarbonyls and primary amines. The newly formed and highly reactive aminal moiety is readily substituted with indoles and pyrroles both as external and internal π-nucleophiles to provide densely functionalized N-heterocycles with four new σ-bonds and two vicinal quaternary stereogenic centers. In addition, DFT calculations have been conducted to further characterize the intermediate 1-azaallyl cations.

Homologation chemistry with nucleophilic α-substituted organometallic reagents: chemocontrol, new concepts and (solved) challenges

The transfer of a reactive nucleophilic CH2X unit into a preformed bond enables the introduction of a fragment featuring the exact and desired degree of functionalization through a single synthetic operation. The instability of metallated α-organometallic species often poses serious questions regarding the practicability of using this conceptually intuitive and simple approach for forming C-C or C-heteroatom bonds. A deep understanding of processes regulating the formation of these nucleophiles is a precious source of inspiration not only for successfully applying theoretically feasible transformations (i.e. determining how to employ a given reagent), but also for designing new reactions which ultimately lead to the introduction of molecular complexity via short experimental sequences.

Synthesis of CF3-containing spiro-epoxyoxindoles via the Corey–Chaykovsky reaction of N-alkyl isatins with Ph2S+CH2CF3OTf−

CF₃-containing spiro-epoxyoxindoles were successfully prepared via Corey–Chaykovsky reaction of N-alkyl isatins with the ylide generated from Ph₂S⁺CH₂CF₃OTf⁻ with almost exclusive diastereoselectivity.

Regio- and stereospecific Friedel–Crafts alkylation of indoles with spiro-epoxyoxindoles

A highly efficient strategy for the regio- and stereospecific Friedel–Crafts alkylation of indoles with spiro-epoxyoxindoles has been developed in the mixed solvents of HFIP/H₂O (1 : 9) without the use of catalysts.

An asymmetric acetate-Mannich reaction of chiral isatin derived ketimines and its applications

A highly efficient TMSOTf-mediated asymmetric acetate-Mannich reaction of isatin derived tert-butylsulfinyl ketimines and S-phenyl thioacetate was developed to afford the direct synthesis of indole-based β^3,3-amino acid thioester with excellent selectivity (dr > 98 : 2). Syntheses of (+)-AG-041R and 3-aminopyrroloindoline have been accomplished utilizing the developed method.

Regioselective synthesis of 2,3′-biindoles mediated by an NBS-induced homo-coupling of indoles

An efficient method for the synthesis of 2,3′-biindole and [3,2-a]carbazole derivatives via an NBS-induced homo-coupling of indoles with high regioselectivity.