Stereoselective synthesis of novel conformationally restricted β- and γ-amino acids

Borane-Catalyzed Carbazolation Reactions of Aryldiazoacetates

Herein, we report on the tris(pentafluorophenyl)borane-catalyzed reaction of carbazole heterocycles with aryldiazoacetates. We could demonstrate that selective N-H functionalization occurs in the case of an unprotected carbazole, other N-heterocycles, and secondary amines in good yields. In contract, the protected carbazole undergoes C-H functionalization at the C-3 position in a good yield. The application of both approaches was studied in 41 examples with up to a 97% yield.

N-Hydroxyphthalimidyl diazoacetate (NHPI-DA): a modular methylene linchpin for the C-H alkylation of indoles

Despite the extensive studies on the reactions between conventional diazocompounds and indoles, these are still limited by the independent synthesis of the carbene precursors, the specific catalysts, and the required multi-step manipulation of the products. In this work, we explore redox-active carbenes in the expedited and divergent synthesis of functionalized indoles. NHPI-DA displays unusual efficiency and selectivity to yield insertion products that can be swiftly elaborated into boron and carbon substituents that are particularly problematic in carbene-mediated reactions.

Multi C-H Functionalization Reactions of Carbazole Heterocycles via Gold-Catalyzed Carbene Transfer Reactions

Herein we describe a multiple C-H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C-H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one-pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.

Synthesis, oligomerization and catalytic studies of a redox-active Ni4-cubane: a detailed mechanistic investigation

A robust tetrameric nickel complex [Ni₄((O_al⁻)₂L-Me)₄(s)₄] (3) (s = solvent) with cubane-like Ni₄O₄ core topology was isolated as a light greenish-orange crystalline solid in excellent yield. The mechanism of formation of 3 involving the two chloride-containing precursors [Ni₄((O_al⁻)₂L-Me)₄(s)₄]·2MeOH (1) and [Ni₄((O⁻)₂L-Me)₃((O_al⁻)(OH)L-Me)Cl] (2) was studied by ESI mass spectrometry and confirmed by the solid state isolation and single-crystal X-ray diffraction. The challenging ligand fields containing mono/di-anionic O₂N donating atoms and/or chloride ions stabilized the pentacoordinate Ni(ii) ions in 1–2 upon controlling the experimental conditions. Complexes 1–3 have been characterized by NMR, UV-Vis and mass spectrometric analysis. Complex 3 was found to be redox active by cyclic voltammetry (CV) studies. Theoretical calculations were carried out to shed light on the effects of ligand fields on the stability of complexes 1–3. Complex 3 was found to be a potential catalyst for the diastereoselective cyclopropanation of heteroarenes with good to excellent yields. The ESI mass spectrometric analysis revealed the existence of solution dynamics and oligomerization of 3 in solution. Mechanistic investigation of the catalytic cycle revealed that complex 3 and its various oligomers bind to the diazoester employed, followed by dissociative insertion of the respective carbene moieties to the C2–C3 double bond of the involved aromatic heterocycle, leading to the diastereoselective cyclopropanation.

A robust tetrameric nickel complex [Ni₄((O_al⁻)₂L-Me)₄(s)₄] (s = solvent) with cubane-like Ni₄O₄ core topology identified as the efficient catalyst for the diastereoselective cyclopropanation of aromatic heterocycles.

Stoichiometric Photochemical Carbene Transfer by Bamford-Stevens Reaction

The photolysis of diazoalkanes is a timely strategy to conduct carbene-transfer reactions under mild and metal-free reaction conditions, and has developed as an important alternative to conventional metal-catalyzed carbene-transfer reactions. One of the major limitations lies within the rapidly occurring side reaction of the carbene intermediate with remaining diazoalkane molecules that result in the use of an excess of the reaction partner and thus impacts on the reaction efficiency. Herein, we describe a protocol that takes advantage of the in situ generation of donor-acceptor diazoalkanes by Bamford-Stevens reaction. Following this strategy, the concentration of the diazoalkane reaction partner can be minimized to reduce unwanted side reactions and to now conduct photochemical carbene transfer reactions under stoichiometric reaction conditions. We have explored this approach in the C-H and N-H functionalization and cyclopropanation reaction of N-heterocycles and could demonstrate the applicability of this method in 51 examples.

Tryptamine Synthesis by Iron Porphyrin Catalyzed C-H Functionalization of Indoles with Diazoacetonitrile

The functionalization of C-H bonds with non-precious metal catalysts is an important research area for the development of efficient and sustainable processes. Herein, we describe the development of iron porphyrin catalyzed reactions of diazoacetonitrile with N-heterocycles yielding important precursors of tryptamines, along with experimental mechanistic studies and proof-of-concept studies of an enzymatic process with YfeX enzyme. By using readily available FeTPPCl, we achieved the highly efficient C-H functionalization of indole and indazole heterocycles. These transformations feature mild reaction conditions, excellent yields with broad functional group tolerance, can be conducted on gram scale, and thus provide a unique streamlined access to tryptamines.

Gold-catalyzed stereoselective dearomatization/metal-free aerobic oxidation: access to 3-substituted indolines/oxindoles

An unprecedented dearomatization of indoles with diazoesters has been developed via cationic gold(i) catalysis. The functionalization selectively occurs at the C3-position to deliver methylene indole derivatives in good yields with excellent Z-selectivity, demonstrating unusual reactivity and selectivity compared with other noble metal catalysis. Importantly, simply followed by silica gel adsorption, an unprecedented metal-free aerobic oxidation occurs for indoles bearing N-electron donating substituents, providing a novel and efficient approach towards 3-substituted indolin-2-ones with a newly formed quaternary stereocenter in excellent stereoselectivity. Notably, these processes afford direct and selective access to a variety of valuable intermediates from abundant feedstock chemicals.

Stereoselective cis-Vinylcyclopropanation via a Gold(I)-Catalyzed Retro-Buchner Reaction under Mild Conditions

A highly stereoselective gold(I)-catalyzed cis-vinylcyclopropanation of alkenes has been developed. Allylic gold carbenes, generated via a retro-Buchner reaction of 7-alkenyl-1,3,5-cycloheptatrienes, react with alkenes to form vinylcyclopropanes. The gold(I)-catalyzed retro-Buchner reaction of these substrates proceeds by simple heating at a temperature much lower than that required for the reaction of 7-aryl-1,3,5-cycloheptatrienes (75 °C vs 120 °C). A newly developed Julia–Kocienski reagent enables the synthesis of the required cycloheptatriene derivatives in one step from readily available aldehydes or ketones. On the basis of mechanistic investigations, a stereochemical model for the cis selectivity was proposed. An unprecedented gold-catalyzed isomerization of cis- to trans-cyclopropanes has also been discovered and studied by DFT calculations.

Total Synthesis of Lundurine and Related Alkaloids: Synthetic Approaches and Strategies

This review focuses on the total synthesis of lundurines A-C. Their main structural feature is a unique cyclopropa[b]indole core that has been found only in these alkaloids. In addition to this characteristic structure, the biological activity makes them as attractive synthetic targets. However, almost two decades passed from their isolation and structural determination in 1995 to their first total synthesis. The first part of this review summarizes the synthetic approaches to the tri- and tetracyclic ring systems of lundurine as well as an inter- and intramolecular cyclopropanation strategy that gives the cyclopropa[b]indole core. The second part presents a detailed description of four total syntheses that were reported from 2014 to 2016. In addition, the asymmetric total synthesis of the related alkaloids grandilodine C and lapidilectine B is described.

Copper-catalyzed enantioselective C–H functionalization of indoles with an axially chiral bipyridine ligand

A copper-catalyzed enantioselective functionalization of indoles was developed with an axially chiral bipyridine ligand C4-ACBP with up to 95% ee.

Cobalt(II) Porphyrin-Catalyzed Intramolecular Cyclopropanation of N-Alkyl Indoles/Pyrroles with Alkylcarbene: Efficient Synthesis of Polycyclic N-Heterocycles

A protocol on chemoselective cobalt(II) porphyrin-catalyzed intramolecular cyclopropanation of N-alkyl indoles/pyrroles with alkylcarbenes has been developed. The reaction enables the rapid construction of a range of nitrogen-containing polycyclic compounds in moderate to high yields from readily accessible materials. These N-containing polycyclic compounds can be converted into a variety of N-heterocycles with potential synthetic and biological interest. Compared to their N-tosylhydrazone counterparts, the use of bulky N-2,4,6-triisopropylbenzenesulfonyl hydrazones as carbene precursors allows cyclopropanation to occur under milder reaction conditions.

Direct functionalization of indoles: copper-catalyzed malonyl carbenoid insertions

Indoles, when treated with dimethyl diazomalonate under catalysis by Cu(acac)(2), undergo C-H insertion reactions regioselectively depending on the substitution pattern on the indole moiety. Indoles where the 3-position is substituted give high yields of the C2-H insertion product. Microwave conditions are also disclosed which show comparable yields with reduced reaction times.

Evolution of a strategy for total synthesis of the marine fungal alkaloid (+/-)-communesin F

A new synthetic strategy for construction of the heptacyclic marine fungal alkaloid (+/-)-communesin F has been devised. Key reactions include an intramolecular Heck cyclization of a tetrasubstituted alkene to generate a tetracyclic enamide bearing one of the quaternary carbon centers (C7) of the alkaloid, an intramolecular reductive cyclization of an N-Boc aniline onto the oxindole moiety to form a pentacyclic framework containing the southern aminal, a stereoselective N-Boc-lactam enolate C-allylation to introduce the second quaternary carbon center (C8), and an azide reduction/N-Boc-lactam-opening cascade leading to the northern aminal.

Intermolecular reactions of electron-rich heterocycles with copper and rhodium carbenoids

This tutorial review describes the reactions of the electron-rich heterocycles pyrrole, furan, indole and benzofuran with copper and rhodium carbenoids. Two main reaction pathways are possible, involving either a concerted non-synchronous cyclopropanation or zwitterionic intermediates. A diverse range of products are possible and the outcome is very dependent on the structure of the heterocycle and the carbenoid. To emphasize this point the carbenoids are considered in terms of three classes: acceptor, acceptor-acceptor and donor-acceptor carbenoids. Unusual catalytic asymmetric transformations can be achieved with this chemistry while the asymmetric induction is strongly influenced by how the carbenoid can approach the heterocycles. This tutorial review gives an overview of the general features that govern the chemistry of metal carbenoids with heterocycles and presents a mechanistic rationale for the range of products that can be formed.

Gold(i)-catalysed arylation of 1,6-enynes: different site reactivity of cyclopropyl gold carbenes

Gold(I)-catalysed addition of electron-rich arenes and heteroarenes to 1,6-enynes gives two different types of products by reaction of the intermediate cyclopropyl gold carbenes at the cyclopropane or at the carbene.