Synthesis of Saturated N-Heterocycles

Catalytic Asymmetric Synthesis of α-Arylpyrrolidines and Benzo-fused Nitrogen Heterocycles

Herein, we report a practical two-step synthetic route to α-arylpyrrolidines through Suzuki-Miyaura cross-coupling and enantioselective copper-catalyzed intramolecular hydroamination reactions. The excellent stereoselectivity and broad scope for the transformation of substrates with pharmaceutically relevant heteroarenes render this method a practical and versatile approach for pyrrolidine synthesis. Additionally, this intramolecular hydroamination strategy facilitates the asymmetric synthesis of tetrahydroisoquinolines and medium-ring dibenzo-fused nitrogen heterocycles.

Successive Nucleophilic and Electrophilic Allylation for the Catalytic Enantioselective Synthesis of 2,4-Disubstituted Pyrrolidines

Successive nucleophilic and electrophilic allylation mediated by the bis-Boc-carbonate derived from 2-methylene-1,3-propane diol enables formation of enantiomerically enriched 2,4-disubstituted pyrrolidines. An initial enantioselective iridium-catalyzed transfer hydrogenative carbonyl C-allylation is followed by Tsuji-Trost N-allylation using 2-nitrobenzenesulfonamide. Subsequent Mitsunobu cyclization provides the N-protected 2,4-disubstituted pyrrolidines.

Enantioselective synthesis of gem-disubstituted N-Boc diazaheterocycles via decarboxylative asymmetric allylic alkylation

An enantioselective synthesis of diverse N4-Boc-protected α,α-disubstituted piperazin-2-ones using the palladium-catalyzed decarboxylative allylic alkylation reaction has been achieved. Using a chiral Pd-catalyst derived from an electron deficient PHOX ligand, chiral piperazinones are synthesized in high yields and enantioselectivity. The chiral piperazinone products can be deprotected and reduced to valuable gem-disubstituted piperazines. This reaction is further extended to enable the enantioselective synthesis of α,α-disubstituted tetrahydropyrimidin-2-ones, which are hydrolyzed into corresponding chiral β2,2-amino acids.

A methanol and protic ionic liquid Ugi multicomponent reaction path to cytotoxic α-phenylacetamido amides

The Ugi four component reaction of an aldehyde, amine, isocyanide and an ethanoic acid was effected smoothly in protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) to afford analogues of α-phenylacetamido amides in good to excellent isolated yields. The corresponding reactions in [BMIM][PF₆] and the protic ionic liquid ethanolammonium nitrate (ETAN) failed. Microwave irradiation in EAN facilitated rapid access to three focused libraries, based on the parent isocyanide: cyclohexyl isocyanide, benzyl isocyanide and ethyl isocyanoacetate. Analysis of the structure activity relationship data suggested the presence of a bulky moiety originating from the isocyanide (cyclohexyl and benzyl) enhanced cytotoxicity. Removal of the acetylenic H-atom from the ethanoic acid moiety was detrimental to cytotoxicity. The most active analogues produced, N-(2-cyclohexylamino)-1-(4-methoxyphenyl)-2-oxoethyl-N-(3,5-dimethoxyphenyl)propiolamide, returned average GI₅₀ values of ≤1 μM across the cancer cell lines evaluated. Combined, these data suggest that analogues of this nature are interesting potential anti-cancer development leads.

The Ugi reaction (aldehyde, amine, isocyanide and an ethanoic acid) in the protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) gave excellent yields of α-phenylacetamido amides.

Nickel-catalyzed borrowing hydrogen annulations: access to diversified N-heterocycles

An efficient nickel-catalyzed synthesis of diversified N-heterocycles via atom-economic borrowing hydrogen annulations of easily available alcohols and amines.

Construction of N-Heterocycles through Cyclization of Tertiary Amines

N-Heterocycles have been found in a large number of natural products, drug molecules, and bioactive compounds, and they thereby play a vital role in diverse research disciplines including drug discovery, organic synthesis, chemical biology, and material science. To this end, the development of new methods and strategies for the construction of N-heterocyclic frameworks is arguably one of the most dynamic and significant research areas in organic synthesis. One of these powerful approaches to the synthesis of N-heterocycles is to establish cyclization reactions based on the transformation of tertiary amines, which has emerged as an attractive research topic. In this Minireview, the significant achievements in the construction of N-heterocycles through cyclization of tertiary amines are highlighted and a comprehensive overview of the rational design, development, and application of these synthetic methods is presented.

Redox-triggered cascade dearomative cyclizations enabled by hexafluoroisopropanol

An unprecedented cascade dearomative cyclization through hydrogen-bonding-assisted hydride transfer is realized. The aggregate effect of HFIP enables the rapid buildup of polycyclic amines directly from phenols and o-aminobenzaldehydes via a cascade dearomatization/rearomatization/dearomatization sequence. This unique transformation addressed the drawbacks of hydride transfer-involved redox-neutral reactions.

Mechanistic investigation into the C(sp3)-H acetoxylation of morpholinones

The study of a selective palladium(ii)-catalyzed C(sp³)–H acetoxylation reaction on a class of cyclic alkyl amines is reported.

The study of a selective palladium(ii)-catalyzed C(sp³)–H acetoxylation reaction on a class of cyclic alkyl amines is reported. Computational modelling and kinetic studies were used to provide support for a mechanism involving selective C–O bond formation from a γ-aminoalkyl-Pd(iv) intermediate. The C–O bond forming step was computed to occur by a dissociative ionization mechanism followed by an S_N2 process involving external acetate attack at the C–Pd(iv) bond. This pathway was computed to be of lowest energy with no competing C–N products observed. Additionally, with a few modifications to reaction conditions, preliminary studies showed that this process could be rendered enantioselective in the presence of a non-racemic BINOL-phosphoric acid.

Metal-free multicomponent cascade reactions of homopropargylic amines and acyl chlorides as well as potassium thiocyanate and diiodine: an access to thiazine imides

A novel metal-free multicomponent cascade reaction was developed for the construction of thiazine imides. This four-component cascade reaction had advantages of mild reaction conditions, wide substrate scope and good atom economy. Four new bonds were formed in one pot via a 6-exo-dig iodothiolation cyclization of homopropargylic amines. The corresponding E-configurational thiazine imide products possess an exocyclic vinyliodide functional group.

Pd(ii)-Catalyzed aerobic 1,2-difunctionalization of conjugated dienes: efficient synthesis of morpholines and 2-morpholones

A novel and efficient methodology concerning the Pd(ii)-catalyzed intermolecular difunctionalization of conjugated dienes is reported to synthesize a series of functionalized morpholines and 2-morpholones. Widely distributed and easily obtained β-amino alcohols and α-amino acids, as starting nitrogen and oxygen sources, are successfully applied in the difunctionalization of conjugated dienes respectively. The majority of the desired products were obtained in moderate to excellent yields. Oxygen was successfully employed as a terminal oxidant. Further transformation of the generated products allowed for the expansion of structural diversity.

Asymmetric Synthesis of Chiral 1,2-Amino Alcohols and Morpholin-2-ones from Arylglyoxals

Chiral 1,2-amino alcohols are privileged scaffolds with important applications as drug candidates and chiral ligands. Although various methods for the preparation of this structural motif have been reported, these methods are limited because of the use of precious metals and ligands. Here, we report a practical and high yielding synthesis of chiral 1,2-amino alcohols using arylglyoxals and pseudoephedrine auxiliary. This reaction is catalyzed by a Brønsted acid and provides morpholinone products in high yields and selectivities. The morpholine ring was converted into 1,2-amino alcohols in a two-step protocol.

Redox-Annulations of Cyclic Amines with 2-(2-Oxoethyl)malonates

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-(2-oxoethyl)malonates in the presence of catalytic amounts of benzoic acid. These reactions install a fully saturated five-membered ring and provide access to structures closely related to the natural products crispine A and harmicine.

Second-Generation Palladium Catalyst System for Transannular C-H Functionalization of Azabicycloalkanes

This article describes the development of a second-generation catalyst system for the transannular C-H functionalization of alicyclic amines. Pyridine- and quinoline-carboxylate ligands are shown to be highly effective for increasing the reaction rate, yield, and scope of Pd-catalyzed transannular C-H arylation reactions of azabicyclo[3.1.0]hexane, azabicyclo[3.1.1]heptane, azabicyclo[3.2.1]octane, and piperidine derivatives. Mechanistic studies reveal that the pyridine/quinoline-carboxylates play a role in impeding both reversible and irreversible catalyst decomposition pathways. These ligands enable the first reported examples of the transannular C-H arylation of the ubiquitous tropane, 7-azanorbornane, and homotropane cores. Finally, the pyridine/quinoline-carboxylates are shown to promote both transannular C-H arylation and transannular C-H dehydrogenation on a homotropane substrate.

Enantioselective Synthesis of α-(Hetero)aryl Piperidines through Asymmetric Hydrogenation of Pyridinium Salts and Its Mechanistic Insights

Enantioselective synthesis of α-aryl and α-heteroaryl piperidines is reported. The key step is an iridium-catalyzed asymmetric hydrogenation of substituted N-benzylpyridinium salts. High levels of enantioselectivity up to 99.3:0.7 er were obtained for a range of α-heteroaryl piperidines. DFT calculations support an outersphere dissociative mechanism for the pyridinium reduction. Notably, initial protonation of the final enamine intermediate determines the stereochemical outcome of the transformation rather than hydride reduction of the resultant iminium intermediate.

Expedient syntheses of N-heterocycles via intermolecular amphoteric diamination of allenes

Saturated 1,4-diazo heterocycles including piperazines, 1,4-diazepanes, and 1,4-diazocanes, are highly important for therapeutic development, but their syntheses are often tedious. We describe here an amphoteric diamination strategy to unite readily available 1,2-, 1,3- or 1,4-diamine derivatives with electron-deficient allenes via a formal [n + 2] (n = 4, 5, 6) cyclization mode to produce the corresponding 1,4-diazo heterocycles in just one step. This strategy features mild reaction conditions, high functional group tolerance, and scalability (gram scale). The reagents used are cheap and readily available and no transition metal catalysts are needed. More sophisticated products containing trifluoromethyl group or bicyclic ring systems can be accessed via a one-pot procedure as well. Our mechanistic studies support that formation of mono-iodinated or chlorinated diamine intermediates is important for the desired transformation and the commonly proposed chloride-iodide exchange process and a radical N−C bond formation is unlikely when the combination of NCS/KI is used.

Saturated N-heterocycle rings are common structural motifs of many FDA-approved drugs. Here, the authors show that a metal-free amphoteric diamination of allenes produces valuable six-, seven- and eight-membered N-heterocycles with high diversity and tolerance of functional groups on the ring.

Mild and efficient synthesis of indoles and isoquinolonesviaa nickel-catalyzed Larock-type heteroannulation reaction

A simple and mild approach for the preparation of substituted indoles and isoquinolonesviaa nickel-catalyzed Larock-type heteroannulation reaction is reported.

Transfer hydrogenation of nitrogen heterocycles using a recyclable rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica

Transfer hydrogenation of unsaturated nitrogen heterocycles using a rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica (BPy-PMO) is described.

Ce(iii)-Containing tungstotellurate(vi) with a sandwich structure: an efficient Lewis acid–base catalyst for the condensation cyclization of 1,3-diketones with hydrazines/hydrazides or diamines

A new Ce³⁺ modified Dawson-like polyoxometalate cluster was synthesized, which showed good Lewis acid and Lewis base activities in the condensation reaction.

Divergent synthesis of functionalized pyrrolidines and γ-amino ketones via rhodium-catalyzed switchable reactions of vinyl aziridines and silyl enol ethers

Rhodium-catalyzed enantiospecific intermolecular [3+2] cycloadditions and ring-opening reactions of vinylaziridines with silyl enol ethers have been developed to synthesize functionalized pyrrolidines and γ-amino ketones, respectively.

Copper-catalyzed regioselective dehydrogenative alkoxylation of morpholinonyl alkenols: application to the synthesis of spirotricyclic dihydropyrans and of trans-fused bicyclic morpholines

A base-free, regioselective, and atom-economical approach to morpholine-fused dihydropyrans, by Cu-catalyzed intramolecular dehydrogenative alkoxylation of allylic morpholinols, is described.

Synthesis of Polycyclic Imidazolidinones via Amine Redox-Annulation

α-Ketoamides undergo redox-annulations with cyclic secondary amines, such as 1,2,3,4-tetrahydroisoquinoline, pyrrolidine, piperidine, and morpholine. Catalytic amounts of benzoic acid significantly accelerate these transformations. This approach provides polycyclic imidazolidinone derivatives in typically good yields.

Divergent Access to Functionalized Pyrrolidines and Pyrrolines via Iridium-Catalyzed Domino-Ring-Opening Cyclization of Vinyl Aziridines with β-Ketocarbonyls

A useful synthesis of five-membered N-heterocycles has been developed through an iridium-catalyzed domino-ring-opening cyclization of vinylaziridines with β-ketocarbonyls. α-Substituted 1,3-dicarbonyls reacted with vinylaziridines to give 2-methylenepyrrolidines bearing two adjacent sp³-carbon centers with moderate to excellent diastereoselectivity, while the reaction of α-unsubstituted 1,3-dicarbonyls afforded 2-pyrrolines in good yield.

Preparation of Polyfunctional Naphthyridines by Cobalt-Catalyzed Cross-Couplings of Halogenated Naphthyridines with Magnesium and Zinc Organometallics

CoCl₂ (5%) catalyzes cross-couplings of various halogenated naphthyridines with alkyl- and arylmagnesium halides. Also, arylzinc halides undergo smooth cross-couplings with various naphthyridines in the presence of CoCl₂·2LiCl (5%) and sodium formate (50%), leading to polyfunctional arylated naphthyridines. Two of these arylated naphthyridines are highly fluorescent, with quantum efficiencies reaching 95% and long excited-state lifetimes of up to 12 ns.

Direct α-C-H bond functionalization of unprotected cyclic amines

Cyclic amines are ubiquitous core structures of bioactive natural products and pharmaceutical drugs. Although the site-selective abstraction of C-H bonds is an attractive strategy for preparing valuable functionalized amines from their readily available parent heterocycles, this approach has largely been limited to substrates that require protection of the amine nitrogen atom. In addition, most methods rely on transition metals and are incompatible with the presence of amine N-H bonds. Here we introduce a protecting-group-free approach for the α-functionalization of cyclic secondary amines. An operationally simple one-pot procedure generates products via a process that involves intermolecular hydride transfer to generate an imine intermediate that is subsequently captured by a nucleophile, such as an alkyl or aryl lithium compound. Reactions are regioselective and stereospecific and enable the rapid preparation of bioactive amines, as exemplified by the facile synthesis of anabasine and (-)-solenopsin A.

A new route to N-aromatic heterocycles from the hydrogenation of diesters in the presence of anilines

The hydrogenation of dicarboxylic acids and their esters in the presence of anilines provides a new synthesis of heterocycles. [Ru(acac)3] and 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos) gave good to excellent yields of the cyclic amines at 220 °C. When aqueous ammonia was used with dimethyl 1,6-hexadienoic acid, ε-caprolactam was obtained in good yield. A side reaction involving alkylation of the amine by methanol was suppressed by using diesters derived from longer chain and branched alcohols. Hydrogenation of optically pure diesters (dimethyl (R)-2-methylbutanedioate and dimethyl (S)-2-methylbutanedioate) with aniline afforded racemic 3-methyl-1-phenylpyrrolidine in 78% yield.

Acetic Acid Promoted Redox Annulations with Dual C-H Functionalization

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-alkylquinoline-3-carbaldehydes as well as the corresponding 4-alkyl isomers and pyridine analogues. These processes involve dual C-H bond functionalization. Acetic acid is used as a cosolvent and acts as the sole promoter of these transformations.

Enantioselective Iridium-Catalyzed Allylic Cyclizations

A method for the enantioselective synthesis of carbo- and heterocyclic ring systems enabled through the combination of Lewis acid activation and iridium-catalyzed allylic substitution is described. The reaction proceeds with branched, allylic alcohols and carbon nucleophiles as well as heteronucleophiles to give a diverse set of ring systems in good yields and with high enantioselectivities. The utility of the method is highlighted by the asymmetric syntheses of erythrococcamides A and B.

Lewis Acid Induced Toggle from Ir(II) to Ir(IV) Pathways in Photocatalytic Reactions: Synthesis of Thiomorpholines and Thiazepanes from Aldehydes and SLAP Reagents

Redox neutral photocatalytic transformations often require careful pairing of the substrates and photoredox catalysts in order to achieve a catalytic cycle. This can limit the range of viable transformations, as we recently observed in attempting to extend the scope of the photocatalytic synthesis of N-heterocycles using silicon amine protocol (SLAP) reagents to include starting materials that require higher oxidation potentials. We now report that the inclusion of Lewis acids in photocatalytic reactions of organosilanes allows access to a distinct reaction pathway featuring an Ir(III)*/Ir(IV) couple instead of the previously employed Ir(III)*/Ir(II) pathway, enabling the transformation of aromatic and aliphatic aldehydes to thiomorpholines and thiazepanes. The role of the Lewis acid in accepting an electron-either directly or via coordination to an imine-can be extended to other classes of photocatalysts and transformations, including oxidative cyclizations. The combination of light induced reactions and Lewis acids therefore promises access to new pathways and transformations that are not viable using the photocatalysts alone.

Phenacyl azides as efficient intermediates: one-pot synthesis of pyrrolidines and imidazoles

Phenacyl azides: versatile substrates to produce various diastereoselective pyrrolidines and imidazoles in a one-pot manner through azomethine ylide formation with excellent yields.

Phenanthroline-based metal–organic frameworks for Fe-catalyzed Csp3–H amination

We report here the synthesis of a robust and highly porous Fe-phenanthroline-based metal–organic framework (MOF) and its application in catalyzing challenging inter- and intramolecular C–H amination reactions. For the intermolecular amination reactions, a FeBr₂-metalated MOF selectively functionalized secondary benzylic and allylic C–H bonds. The intramolecular amination reactions utilizing organic azides as the nitrene source required the reduction of the FeBr₂-metalated MOF with NaBHEt₃ to generate the active catalyst. For both reactions, Fe or Zr leaching was less than 0.1%, and MOFs could be recycled and reused with no loss in catalytic activity. Furthermore, MOF catalysts were significantly more active than the corresponding homogeneous analogs. This work demonstrates the great potential of MOFs in generating highly active, recyclable, and reusable earth abundant metal catalysts for challenging organic transformations.