Structure and Reactivity of Indolylmethylium Ions: Scope and Limitations in Synthetic Applications

Short Total Synthesis of (+)-Colletotryptins B-D and Mucronatin B Derivative

The short and first total synthesis of (+)-colletotryptins B-D, ent-colletotryptin A, and diastereomer of mucronatin B, which are a group of natural 3-(indol-2-yl)-3-(indol-3-yl)-1,2-propanediol (IIPDO) analogues containing two stereogenic centers at the C8' and C9' positions, isolated from endophytic fungus Colletotrichum sp. SC1355 and Tetrapterys mucronata, respectively, has been successfully accomplished in two and three steps with overall yields ranging from 28 to 54%. Key features of this synthesis include an innovative Bi(OTf)3-catalyzed stereoselective transindolylation of (S)-3,3'-di(1H-indol-3-yl)propane-1,2-diol. The operational simplicity, environmentally friendly catalyst, and broad functional group tolerance of this modular strategy render it suitable for adoption in both academic and industrial settings.

Relevant Developments in the Use of Three-Component Reactions for the Total Synthesis of Natural Products. The last 15 Years

Multicomponent reactions (MCRs) offer a highly useful and valuable strategy that can fulfill an important role in synthesizing complex polysubstituted compounds, by simplifying otherwise long sequences and increasing their efficiency. The total synthesis of selected natural products employing three-component reactions as their common strategic MCR approach, is reviewed on a case-by-case basis with selected targets conquered during the last 15 years. The revision includes detailed descriptions of the selected successful sequences; relevant information on the isolation, and bioactivity of the different natural targets is also briefly provided.

Nucleophilicity of 4-(Alkylthio)-3-imidazoline Derived Enamines

Imidazolidine-4-thiones (ITOs) are cyclic, secondary amines that were considered as potential prebiotic organocatalysts for light-driven α-alkylations of aldehydes by bromoacetonitrile (BAN). Recent studies showed that the initially supplied ITOs represent the pre-catalyst because they undergo S-alkylation with BAN to give 4-(alkylthio)-3-imidazolines (TIMs). Given that the same reagent mix that undergoes light-driven α-alkylations is also effective in the dark, we synthesized ten ITO- or TIM-derived enamines of aldehydes and characterized their nucleophilic reactivities by kinetic studies in acetonitrile. The experimental second-order rate constants k2 for reactions of enamines with benzhydrylium ions (reference electrophiles) were evaluated by the Mayr-Patz equation, lg k2 (20 °C)=sN (N+E). The determined nucleophilicities N (and sN ) reveal the reactivity profiles of these enamines under prebiotically relevant conditions as well as their potential for use in organocatalytic synthesis.

Azo Coupling of Indoles Revisited: Synthesis of Biindolyl Photoswitches via the Azo-Coupling/C-H Functionalization Domino Approach

When azo coupling of aryldiazonium salts with indoles was carried out in aprotic nonpolar solvent on air, a pseudo-three-component reaction has been discovered. Azo coupling is followed by a nucleophilic addition of a second indole unit to the indolium intermediate; aromatization and oxidation are achieved under air.

Radical-Polar Crossover Enabled Triple Cleavage of CF2Br2: A Multicomponent Tandem Cyclization to 3-Fluoropyrazoles

The elaboration of step-economy and catalytic approaches for accessing diverse fluorinated heterocyclics is highly desirable. Described herein is a radical-polar crossover enabled three-component cyclization to polysubstituted fluoropyrazoles by using CF2Br2 as a novel C1F1 synthon. Mechanistic experiments revealed that the in situ generation of the reactive intermediate gem-difluorovinylimine ion is the key to this transformation. This protocol unlocks the novel reactivity of CF2Br2 and adds significant synthetic values to fluorine chemistry.

Visible-Light-Driven Multicomponent Cyclization by Trapping a 1,3-Vinylimine Ion Intermediate: A Direct Approach to Pyrimido[1,2-b]indazole Derivatives

Here we develop a novel visible-light-driven three-component cyclization by trapping an 1,3-vinylimine ion intermediate for the direct synthesis of pyrimido[1,2-b]indazole derivatives from bromodifluoroacetic acid derivatives, enaminones, and 3-aminoindazoles under mild conditions. Notably, the robust methodology provides a valuable opportunity for the introduction of aliphatic substituents and enables good compatibility of complex bioactive molecules. Furthermore, this is the first example of photoinduced multicomponent reaction by employing bromodifluoroacetic acid derivatives as a C1-synthon.

Short Total Synthesis of (±)-Gelliusine E and 2,3'-Bis(indolyl)ethylamines via PTSA-Catalyzed Transindolylation

A first and short total synthesis of the marine sponge 2,3'-bis(indolyl)ethylamine (2,3'-BIEA) alkaloid (±)-gelliusine E was performed in both a three-step divergent approach and a one-pot three-component approach with an overall yield of up to 58%. A key feature of the novel strategy is PTSA-catalyzed transindolylation of the readily synthesized 3,3'-BIEAs with tryptamine derivatives. The structure of the isolated natural product is revised as protonated (±)-gelliusine E (4'). By design, this modular route allows the rapid synthesis of other members of the 2,3'-BIEA family, for example, (±)-6,6'-bis-(debromo)-gelliusine F and analogues with step economy, operational simplicity, and reduced waste. Furthermore, their cytotoxicity in breast cancer cells was investigated.

Three-Component Reactions of 3-Arylidene-3H-Indolium Salts, Isocyanides and Amines

A multicomponent reaction of isocyanides with aryl(indol-3-yl)methylium salts and amines has been found. A series of aryl(indol-3-yl)acetimidamides was obtained in up to 96% yields. In the case of ethyl isocyanoacetate, the reaction is followed by cyclization to form 3,5-dihydro-4H-imidazol-4-one derivatives.

FeCl3·6H2O as a Mild Catalyst for Nucleophilic Substitution of Symmetrical Bis(indoyl)methanes

In this paper, unsymmetrical bis(indolyl)methane (BIM) and 3-alkylindole derivatives are smoothly synthesized from symmetrical BIMs with a variety of nucleophiles including heteroaromatic/aromatic compounds, allylsilane and alkynylsilane. FeCl₃·6H₂O is found to be a mild and highly effective catalyst for this nucleophilic substitution reaction in which N-methyl-2-phenylindole behaves as a good leaving group in the C_sp³-C_sp² bond cleavage reaction. The operational ease, nonexpensive and environmentally friendly catalyst, mild reaction conditions, broad functional group tolerance, and scalability of this reaction strategy are advantages of the present procedure.

Methyl Anion Affinities of the Canonical Organic Functional Groups

Calculated methyl anion affinities are known to correlate with experimentally determined Mayr E parameters for individual organic functional group classes but not between neutral and cationic organic electrophiles. We demonstrate that methyl anion affinities calculated with a solvation model (MAA*) give a linear correlation with Mayr E parameters for a broad range of functional groups. Methyl anion affinities (MAA*), plotted on the log scale of Mayr E, provide insights into the full range of electrophilicity of organic functional groups. On the Mayr E scale, the electrophilicity toward the methyl anion spans 180 orders of magnitude.

Trapping of chiral enolates generated by Lewis acid promoted conjugate addition of Grignard reagents to unreactive Michael acceptors by various electrophiles

Here we show trapping of chiral enolates with carbenium ions, Michael acceptors, and bromine. Silyl ketene aminals, disilyl acetals, and aza-enolates were obtained via Lewis acid mediated enantioselective conjugate addition of Grignard reagents to unsaturated amides, carboxylic acids and alkenyl heterocycles.

Three-Component Reaction of 3-Arylidene-3H-Indolium Salts, Isocyanides, and Alcohols

A novel isocyanide-based multicomponent synthesis of alkyl aryl(indol-3-yl)acetimidates has been established. Starting from aryl(indol-3-yl)methylium tetrafluoroborates, aromatic isocyanides and alcohols, the imidates were obtained in moderate to very good yields. Consecutive four-component synthesis of the above mentioned imidates from N-alkylindoles, aromatic aldehydes, aromatic isocyanides and alcohols was also proposed. In addition, it was shown that in the presence of water, aryl(indol-3-yl)methylium tetrafluoroborates reacted with isocyanides to furnish aryl(indol-3-yl)acetamides.

Substrate-controlled divergent synthesis of polycyclic indoloazepines and indolodiazepines via 1,5-hydride shift/7-cyclization cascades

Novel and practical acid-catalyzed cyclization of 2-indolyl aryl carbinols via tandem dehydration/1,5-hydride shift/7-cyclization sequences has been developed. By appropriate variation of the substrate, diverse polycyclic-fused indoles were synthesized in good yield, thus demonstrating the high efficiency, transition-metal-free nature, and high step/atom economy of the synthesis strategy.

Sulfonyl Azoles in the Synthesis of 3-Functionalized Azole Derivatives

Sulfonyl indoles, as well as related azolyl derivatives, have been recently introduced in synthesis as stable precursors of reactive indolenine intermediates. This personal account reports on the discovery of sulfonyl azoles and their practical utilization in many synthetic processes for the preparation of functionalized 3-substituted indoles, indazoles, and pyrroles. The indolenine intermediates obtained by treatment of sulfonyl azoles with Brønsted bases or Lewis acids can be considered as vinylogous imino derivatives that can be made to react with different nucleophilic reagents. These include organometallic reagents, reducing agents, stabilized carbanions, and heteronucleophiles. The controlled and mild conditions for the generation of indolenines from sulfonyl azoles make these substrates particularly useful in asymmetric synthesis, exploiting organo- or metal-catalyzed processes. Although less exploited, sulfonyl indoles can also be involved in photochemical processes for the preparation of polycyclic derivatives.

Brønsted acid-catalyzed regioselective reactions of 2-indolylmethanols with cyclic enaminone and anhydride leading to C3-functionalized indole derivatives

An abnormal regioselective substitution of 2-indolylmethanols with nucleophiles has been established to afford C3-functionalized indole derivatives in high yield and regiospecificity (40 examples, up to 99% yield).