Late-Stage Chemo- and Enantioselective Oxidation of Indoles to C3-Monosubstituted Oxindoles

Catalytic asymmetric preparation of chiral 3-monosubstituted oxindoles represents a significant challenge in synthetic chemistry due to the ease of racemization of the tertiary stereocenter through enolization. Here, we describe a general titanium-catalyzed chemo- and enantioselective indole oxidation to produce a diverse set of chiral 3-monosubstituted oxindoles with up to 96% yield, 99% ee, and with a substrate/catalyst ratio of 10,000 by using the combination of a simple titanium(salan) catalyst with green and atom-economic terminal oxidant H2O2. The mild approach tolerates a broad range of functional groups, enabling late-stage asymmetric diversification of a series of commercial drugs and natural products together with late-stage asymmetric construction of a wide set of enzyme antagonists, all of which are difficult to achieve through existing methods.

Systematic Development of Vanadium Catalysts for Sustainable Epoxidation of Small Alkenes and Allylic Alcohols

The catalytic epoxidation of small alkenes and allylic alcohols includes a wide range of valuable chemical applications, with many works describing vanadium complexes as suitable catalysts towards sustainable process chemistry. But, given the complexity of these mechanisms, it is not always easy to sort out efficient examples for streamlining sustainable processes and tuning product optimization. In this review, we provide an update on major works of tunable vanadium-catalyzed epoxidations, with a focus on sustainable optimization routes. After presenting the current mechanistic view on vanadium catalysts for small alkenes and allylic alcohols' epoxidation, we argue the key challenges in green process development by highlighting the value of updated kinetic and mechanistic studies, along with essential computational studies.

The Total Synthesis of Raistrickindole A

The total synthesis of raistrickindole A has been achieved, thereby confirming the proposed structure as an N-hydroxylated DKP. In the first but less selective approach, the DKP was built up by cyclization of a diastereoisomerically mixed N-hydroxylated dipeptide. In the second approach, the same DKP was constructed stereoselectively by the intramolecular Mitsunobu reaction of a hydroxamic acid. The synthesis was completed by a stereoselective oxidative cyclization.

4-Alkyl-3-azidomethyl-2-ethoxy-2,5-dihydro-5H-1,2-oxaphosphole 2-Oxides: Synthesis and 1,3-Cycloaddition

Starting from phosphorylated allenes, a three-steps synthesis of a new class of organic azides with a 1,2-oxaphospholene carbon skeleton has been developed. The series of obtained 4-alkyl-3-azidomethyl-2-ethoxy-2,5-dihydro-5H-1,2-oxaphosphole 2-oxides were utilized in the 1,3-cycloaddition with alkyl 2-[1-(propyn-2-yl)-1H-indol-3-yl]-2-oxoacetates for the synthesis of conjugates, which are potentially active cytostatics.

Adiponectin-Secretion-Promoting Cyclic Peptide-Polyketide Hybrids from a Halophyte-Associated Fungus, Colletotrichum gloeosporioides JS0417

Three new cyclic peptide-polyketide hybrids (1-3) and two new chaetiacandin-type polyketides (4 and 5) along with nine known compounds were isolated from cultures of a halophyte-associated fungus, Colletotrichum gloeosporioides JS0417. Spectroscopic analysis revealed that 1-3 were cyclic depsipeptides where 3,5,11-trihydroxy-2,6-dimethyldodecanoic acid was linked to two amino acids through amide and ester bonds to form a 12-membered ring. Relative and absolute configurations for the peptides were determined with spectroscopic analysis and chemical reactions. The cyclic depsipeptides 2 and 6 were determined to act as strong adiponectin-secretion-promoting modulators with potential to treat metabolic diseases associated with hypoadiponectinemia. Notably, a known compound, tryptophol, significantly inhibited PGE₂ synthesis and also promoted adiponectin secretion, exhibiting a similar biological activity profile to aspirin, but with greater potency. The presence of an isoleucine moiety and non-glycosylation may be important for biological activity of the cyclic peptide-polyketide hybrids, and non-methoxylation of the side chain may influence activity of the indole derivatives.

Metal-Free C-C/C-N/C-C Bond Formation Cascade for the Synthesis of (Trifluoromethyl)sulfonylated Cyclopenta[b]indolines

A bis(triflyl)ethylation [triflyl = (trifluoromethyl)sulfonyl] inserted into a sequential cyclization cascade resulted in the direct formation of gem-bis(triflyl)ated cyclopenta[b]indolines from anilide-derived allenols and alkenols. This catalyst- and irradiation-free sequence facilitated the efficient preparation of functionalized tricyclic indoline cores bearing two contiguous stereocenters. The formed cyclopenta[b]indolines can be easily transformed into a wide variety of triflylated indolines, including the tetracycle ring system found in polyveoline.

Oxidative Indole Dearomatization for Asymmetric Furoindoline Synthesis by a Flavin-Dependent Monooxygenase Involved in the Biosynthesis of Bicyclic Thiopeptide Thiostrepton

The interest in indole dearomatization, which serves as a useful tool in the total synthesis of related alkaloid natural products, has recently been renewed with the intention of developing new methods efficient in both yield and stereoselective control. Here, we report an enzymatic approach for the oxidative dearomatization of indoles in the asymmetric synthesis of a variety of furoindolines with a vicinal quaternary carbon stereogenic center. This approach depends on the activity of a flavin-dependent monooxygenase, TsrE, which is involved in the biosynthesis of bicyclic thiopeptide antibiotic thiostrepton. TsrE catalyzes 2,3-epoxidation and subsequent epoxide opening in a highly enantioselective manner during the conversion of 2-methyl-indole-3-acetic acid or 2-methyl-tryptophol to furoindoline, with up to >99 % conversion and >99 % ee under mild reaction conditions. Complementing current chemical methods for oxidative indole dearomatization, the TsrE activity-based approach enriches the toolbox in the asymmetric synthesis of products possessing a furoindoline skeleton.

A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2via decarboxylation

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Catalytic asymmetric preparation of pyrroloindolines: strategies and applications to total synthesis

Pyrroloindolines are widely present in natural products. In this review, we summarize state-of-the-art of catalytic asymmetric synthesis of pyrroloindolines, as well as related applications to natural products total synthesis.

Vanadium(V) Complex-Catalyzed One-Pot Synthesis of Phenanthridines via a Pictet-Spengler-Dehydrogenative Aromatization Sequence

Phenanthridine and its derivatives are important structural motifs that exist in natural products, biologically active compounds, and functional materials. Here, we report a mild, one-pot synthesis of 6-arylphenanthridine derivatives by a sequential cascade Pictet-Spengler-dehydrogenative aromatization reaction mediated by oxovanadium(V) complexes under aerobic conditions. The reaction of 2-(3,5-dimethoxyphenyl)aniline with a range of commercially available aryl aldehydes provided the desired phenanthridine derivatives in up to 96% yield. The ability of vanadium(V) complexes to function as efficient redox and Lewis acid catalysts enables the sequential reaction to occur under mild conditions.

Electrooxidation enables highly regioselective dearomative annulation of indole and benzofuran derivatives

The dearomatization of arenes represents a powerful synthetic methodology to provide three-dimensional chemicals of high added value. Here we report a general and practical protocol for regioselective dearomative annulation of indole and benzofuran derivatives in an electrochemical way. Under undivided electrolytic conditions, a series of highly functionalized five to eight-membered heterocycle-2,3-fused indolines and dihydrobenzofurans, which are typically unattainable under thermal conditions, can be successfully accessed in high yield with excellent regio- and stereo-selectivity. This transformation can also tolerate a wide range of functional groups and achieve good efficiency in large-scale synthesis under oxidant-free conditions. In addition, cyclic voltammetry, electron paramagnetic resonance (EPR) and kinetic studies indicate that the dehydrogenative dearomatization annulations arise from the anodic oxidation of indole into indole radical cation, and this process is the rate-determining step.

Enriching the chemical space of polycyclic heterocycles is of high value in chemical biology. Here, the authors report an electrooxidative protocol for the regioselective dearomative annulation of indoles and benzofurans under undivided cell conditions and obtain five- to eight-membered fused heterocycles.

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.

General Approach To Construct Azepino[2,3-b:4,5-b']diindoles, Azocino[2,3-b:4,5-b']diindoles, and Azonino[2,3-b:4,5-b']diindoles via Rh(II)-Catalyzed Reactions of 3-Diazoindolin-2-imines with 3-(Bromoalkyl)indoles

Rh(II)-catalyzed reactions of 3-diazoindolin-2-imines with 3-(2-bromoethyl)indoles, 3-(3-bromopropyl)indoles, and 3-(4-bromobutyl)indoles, followed by treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in a one-pot operation furnished azepino[2,3-b:4,5-b']diindoles, azocino[2,3-b:4,5-b']diindoles, and azonino[2,3-b:4,5-b']diindoles, respectively. Structural uniqueness of the products, broad substrate scope, mild reaction conditions, and readily available starting materials are the merits of this approach.

Palladium(0)-Catalyzed Intermolecular Asymmetric Cascade Dearomatization Reaction of Indoles with Propargyl Carbonate

An intermolecular asymmetric cascade dearomatization reaction of indole derivatives with propargyl carbonate was developed. The challenges associated with both the chemoselectivity between the carbon and nitrogen nucleophile and the enantioselective control during the formation of an all-carbon quaternary stereogenic center were well addressed by a Pd catalytic system derived from the Feringa ligand. A series of enantioenriched multiply substituted fused indolenines were provided in good yields (71-86 %) with excellent enantioselectivity (91-96 % ee) and chemoselectivity (3/4>19:1 in most cases).

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.

Carbene-Catalyzed Construction of Carbazoles from Enals and 2-Methyl-3-oxoacetate Indoles

Direct and rapid construction of carbazoles has been successfully developed via carbene-catalyzed oxidative formal [4 + 2] annulation of enals with 2-methyl-3-oxoacetate indoles. This metal-free reaction features a broad substrate scope, features good functional-group tolerance, proceeds under mild conditions, and can be easily scaled up.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Polycyclic Indoles

An intermolecular Pd-catalyzed allylic dearomatization reaction of polycyclic indoles with substituted allylic carbonates was realized in the presence of a newly synthesized chiral phosphoramidite ligand. Various polycyclic indoline and indolenine derivatives were successfully synthesized in excellent yields (up to 99%) with excellent enantioselectivity (up to 98% ee). The obtained products could undergo versatile transformations, increasing the application potential of the method in organic synthesis.

Direct C3-arylations of 2-indolylmethanols with tryptamines and tryptophols via an umpolung strategy

A Brønsted acid-catalyzed direct C3-arylation of 2-indolylmethanols with tryptamines and tryptophols has been established, leading to a series of potentially bioactive 2,3′-biindole derivatives with a broad substrate scope and generally good yields (38 examples, up to 96% yield).

Chemoselective N–H functionalization of indole derivatives via the Reissert-type reaction catalyzed by a chiral phosphoric acid

An asymmetric N-alkylation of indole derivatives via the Reissert-type reaction was realized in the presence of 10 mol% chiral phosphoric acid.

Transition-Metal-Free Stereocomplementary Cross-Coupling of Diols with Boronic Acids as Nucleophiles

The transition-metal-free diastereoselective C(sp²)-C(sp³) cross-coupling between unprotected diols and boronic acids or potassium organotrifluoroborates is reported. Depending on the reaction conditions, the syn or the anti reaction products are obtained in a stereocomplementary fashion. This type of coupling is developed with alkenyl-, heteroaryl- and arylboron compounds as carbon nucleophiles.

Chiral Phosphoric Acid Catalyzed Intramolecular Dearomative Michael Addition of Indoles to Enones

An enantioselective intramolecular dearomative Michael addition of indolyl enones is presented. In the presence of catalytic amount of chiral phosphoric acid, various enantioenriched spiro-indolenines bearing a quaternary stereogenic center were obtained with good yields and enantioselectivity (up to 97% ee) under mild reaction conditions.

Catalytic asymmetric chemodivergent arylative dearomatization of tryptophols

The first catalytic asymmetric arylative dearomatization of tryptophols has been established. By using quinone imine ketals as aryl group surrogates and modulating the reaction conditions, the cascade reaction of tryptophols with quinone imine ketals afforded two series of arylative dearomatization products in generally high yields, and excellent diastereo- and enantioselectivities.

Pd(ii)-Catalyzed oxidative dearomatization of indoles: substrate-controlled synthesis of indolines and indolones

A Pd(ii)-catalyzed oxidative dearomatization of indoles provides a straightforward access to diverse fused indolines and indolones bearing C2-oxygenated quaternary stereocenters.

Catalytic asymmetric C2-nucleophilic substitutions of C3-substituted indoles with ortho-hydroxybenzyl alcohols

Catalytic asymmetric C2-nucleophilic substitutions of C3-substituted indoles such as tryptophols or tryptamines with ortho-hydroxybenzyl alcohols have been established, leading to the generation of triaryl methane products in generally high yields and excellent enantioselectivities.

Efficient Enantioselective Synthesis of Oxahelicenes Using Redox/Acid Cooperative Catalysts

An efficient and enantioselective synthesis of oxa[9]helicenes has been established via vanadium(V)-catalyzed oxidative coupling/intramolecular cyclization of polycyclic phenols. A newly developed vanadium complex cooperatively functions as both a redox and Lewis acid catalyst to promote the present sequential reaction and afford oxa[9]helicenes in good yields with up to 94% ee.