Stereodivergent synthesis of 2-oxo-oligopyrrolidines by an iterative coupling strategy

Natural linear polyamines play diverse roles in physiological processes by interacting with receptors at the cellular level. Herein, we describe the stereodivergent synthesis of oligopyrrolidines, which are conformationally constrained polyamines. We synthesized dimeric and trimeric 2-oxo-oligopyrrolidines using an iterative coupling strategy. The key to our success is an iridium-catalyzed trans/cis-selective nucleophilic addition and subsequent threo/erythro-stereoselective reduction. The synthesized pyrrolidines show varying cytotoxicities against a human cancer cell line depending on the number of rings and their stereochemistry.

Aza-Achmatowicz rearrangement coupled with intermolecular aza-Friedel-Crafts enables total syntheses of uleine and aspidosperma alkaloids

Aspidosperma and uleine alkaloids belong to the large family of monoterpene indole alkaloids with diverse biological activities and thus have attracted extensive synthetic interest. Reported is the development of a new synthetic strategy that allows direct C3-C2' linkage of indoles with functionalized 2-hydroxypiperidines to construct the core common to all aspidoserma and uleine alkaloids. Such indole-piperidine linkage is enabled by coupling aza-Achmatowicz rearrangement (AAR) with indoles via an intermolecular aza-Friedel-Crafts (iAFC) reaction. This AAR-iAFC reaction proceeds under mild acidic conditions with wide tolerance of functional groups (33 examples). The synthetic application of the AAR-iAFC method was demonstrated with collective total syntheses of 3 uleine-type and 6 aspidosperma alkaloids: (+)-3-epi-N-nor-dasycarpidone, (+)-3-epi-dasycarpidone, (+)-3-epi-uleine, 1,2-didehydropseudoaspidospermidine, 1,2-dehydroaspidospermidine, vincadifformine, winchinine B, aspidospermidine, and N-acetylaspidospermidine. We expect that this AAR-iAFC strategy is applicable to other monoterpene indole alkaloids with the C3-C2' linkage of indoles and piperidines.

Asymmetric Deoxygenative Functionalization of Secondary Amides with Vinylpyridines Enabled by a Triple Iridium-Photoredox-Chiral Phosphoric Acid System

An enantioselective deoxygenative functionalization of secondary amides with vinylpridines is developed by merging relay iridium catalysis and cooperative photoredox-chiral Brønsted acid catalysis, affording a series of valuable chiral amines in moderate to good yields with good enantioselectivities. The intriguing multiple catalytic system invoking triple-catalysis was found to be the key to the success of the current reactions, which may stimulate further development of catalytic methodologies for asymmetric deoxygenative transformations of amides.

Iridium-Catalyzed Reductive (3+2) Annulation of Lactams Enabling the Rapid Total Synthesis of (±)-Eburnamonine

A reductive (3+2) annulation of lactams through iridium-catalyzed hydrosilylation and photoredox coupling with α-bromoacetic acid was developed. The iridium-catalyzed hydrosilylation of the lactam carbonyl group and subsequent elimination provide a transient cyclic enamine, which undergoes iridium-catalyzed photoredox coupling with α-bromoacetic acid in a one-pot process. The developed conditions show high functional-group tolerance and provide cyclic N,O-acetals containing a quaternary carbon center. The resulting N,O-acetals undergo a variety of acid-mediated nucleophilic addition reactions via iminium ions to give substituted cyclic amines. The developed sequence including reductive (3+2) annulation and acid-mediated nucleophilic addition was successfully applied to the four-step total synthesis of (±)-eburnamonine.

Asymmetric Synthesis of Arboduridine

The first asymmetric total synthesis of the monoterpenoid indole alkaloid arboduridine has been accomplished. The tricyclic A/B/D ring system was constructed by an enantioselective Michael reaction followed by intramolecular nucleophilic addition. Intramolecular α-amination of a ketone forged the piperidine ring, while a Horner-Wadsworth-Emmons (HWE) reaction was used to form the pyrrolidine ring. A reduction cyclization cascade led to formation of the tetrahydrofuran ring.

Catalytic enantioselective reductive alkynylation of amides enables one-pot syntheses of pyrrolidine, piperidine and indolizidine alkaloids

The primary objective in synthetic organic chemistry is to develop highly efficient, selective, and versatile synthetic methodologies, which are essential for discovering new drug candidates and agrochemicals. In this study, we present a unified strategy for a one-pot, catalytic enantioselective synthesis of α-alkyl and α,α'-dialkyl pyrrolidine, piperidine, and indolizidine alkaloids using readily available amides and alkynes. This synthesis is enabled by the identification and development of an Ir/Cu/N-PINAP catalyzed highly enantioselective and chemoselective reductive alkynylation of α-unbranched aliphatic amides, which serves as the key reaction. This reaction is combined with Pd-catalyzed tandem reactions in a one-pot approach, enabling the collective, catalytic enantioselective total syntheses of eight alkaloids and an anticancer antipode with 90-98% ee. The methodology's enantio-divergence is exemplified by the one-step access to either enantiomer of alkaloid bgugaine.

Reactivity Umpolung of Tertiary Amide Enabled by Catalytic Reductive Stannylation

Reactivity umpolung is an important concept in organic chemistry. Established reactivity umpolung mainly focuses on the aldehyde and umpolung of amide carbonyl group is not known. In this report, we describe a process to obtain the umpolung reactivity of tertiary amide. This process hinges on the efficient reductive stannylation catalyzed by Ir/silane and facile Sn-Li exchange. By leveraging this umpolung reactivity, drug Fluoxetine was derivatized to 12 different analogues via reacting with various electrophiles and four biologically active molecules were prepared concisely. This unlocked umpolung reactivity of tertiary amide is expected to find applications to synthesize complex amines from amides.

Sequential KOtBu/FeCl3-catalyzed reductive phosphonylation of tertiary amides for the synthesis of α-amino phosphonates and phosphines

A simple, mild and efficient sequential KO^tBu/FeCl₃-catalyzed reductive phosphonylation of tertiary amides is herein described. This process first involved the KO^tBu-catalyzed selective semi-reduction of tertiary amides to hemiaminal intermediates by TMDS (1,1,3,3-tetramethyldisiloxane) and then the FeCl₃-catalyzed nucleophilic addition of the hemiaminal intermediates to phosphonates, which allowed the straightforward synthesis of α-amino phosphonates in moderate to good yields. This method applied well to amides and lactams that bear no strong acidic α-hydrogens, and various functional groups, including methoxy, methylthio, cyano, halogen, and heterocycles, could be tolerated.

Synthesis of isolable β-chloroenamines from N-alkoxylactams with organometallic reagents

An efficient approach to access isolable β-chloroenamines via nucleophilic addition/dehydration of α-chloro N-alkoxylactam with organolithium and Grignard reagents is reported. This approach is amenable to the synthesis of β-chloroenamines by incorporating various C(sp) and C(sp²) units, such as alkyne, aryl, and heteroaryl moieties. The sequential reaction has a broad substrate scope and can be carried out for a scalable synthesis of β-chloroenamines. Control experiments suggested that both chloro and alkoxy groups act as inductive electron-withdrawing substituents to improve the stability of the enamines.

Multicatalysis protocol enables direct and versatile enantioselective reductive transformations of secondary amides

The catalytic asymmetric geminal bis-nucleophilic addition to nonreactive functional groups is a type of highly desirable yet challenging transformation in organic chemistry. Here, we report the first catalytic asymmetric reductive/deoxygenative alkynylation of secondary amides. The method is based on a multicatalysis strategy that merges iridium/copper relay catalysis with organocatalysis. A further combination with the palladium-catalyzed alkyne hydrogenation allows the one-pot enantioselective reductive alkylation of secondary amides. This versatile protocol allows the efficient synthesis of four types of α-branched chiral amines, which are prevalent structural motifs of active pharmaceutical ingredients. The protocol also features excellent enantioselectivity, chemoselectivity, and functional group tolerance to be compatible with more reactive functional groups such as ketone and aldehyde. The synthetic utility of the method was further demonstrated by the late-stage functionalization of two drug derivatives and the concise, first catalytic asymmetric approach to the κ-opioid antagonist aticaprant.

Ir-Catalyzed Chemoselective Reductive Condensation Reactions of Tertiary Amides with Active Methylene Compounds

The catalytic reductive condensation reactions of tertiary amides with active methylene compounds leading to multifunctionalized non-N-containing products is described. The reactions proceed through sequential iridium-catalyzed hydrosilylation of the amides followed by acid-mediated condensation with the active methylene compounds. This scalable method is broad in scope and shows remarkable chemoselectivity for the amide group in the presence of several sensitive or even more reactive functionalities such as ester, cyano, nitro, silyl dienol ether, and ketone.

KOtBu-Mediated Reductive Cyanation of Tertiary Amides for Synthesis of α-Aminonitriles

A simple, mild, catalyst-free, and efficacious KO^tBu-mediated reductive cyanation reaction of tertiary amides under hydrosilylation conditions has been described. A series of α-aminonitriles is obtained in moderate to high yield with good functional group tolerance. The reaction works well with a readily available amide substrate, a cheap and versatile base KO^tBu, and a commercially available hydrosilane (EtO)₃SiH and is convenient for workup and purification.

Difluoroalkylation of Tertiary Amides and Lactams by an Iridium-Catalyzed Reductive Reformatsky Reaction

An iridium-catalyzed, reductive alkylation of abundant tertiary lactams and amides using 1-2 mol % of Vaska's complex (IrCl(CO)(PPh₃)₂), tetramethyldisiloxane (TMDS), and difluoro-Reformatsky reagents (BrZnCF₂R) for the general synthesis of medicinally relevant α-difluoroalkylated tertiary amines is described. A broad scope (46 examples), including N-aryl- and N-heteroaryl-substituted lactams, demonstrated an excellent functional group tolerance. Furthermore, late-stage drug functionalizations, a gram-scale synthesis, and common downstream transformations proved the potential synthetic relevance of this new methodology.

Total synthesis of (±)-vinoxine: construction of the bridged pyrido[1,2-a]indole skeleton via Tf2O-mediated Bischler-Napieralski reaction and stereoselective radical cyclization

The total synthesis of (±)-vinoxine was achieved featuring the assembly of a multi-substituted tetrahydropyrido[1,2-a]indole skeleton through the Tf₂O-mediated Bischler-Napieralski reaction. The characteristic diazabicyclo[3.3.1]nonane skeleton was stereoselectively constructed via radical cyclization based on the one stereochemistry of the C3 position. The established methodology provides new options for the synthesis of natural products and pharmaceuticals containing the multi-substituted pyrido[1,2-a]indole skeleton.

Deoxygenative Cross-Coupling of Aromatic Amides with Polyfluoroarenes

Considering the ubiquitous nature and ready synthesis of amides, and the great significance of organofluorine-containing species, the cross-coupling of amides and polyfluoroarenes, leading to new carbon-carbon bond-forming methodologies, would find useful applications in synthesis, late-stage functionalization, and rapid generation of molecular diversity. Herein, we present a novel synthesis of α-polyfluoroaryl amines via Sm/SmI₂ -mediated deoxygenative cross-coupling of aromatic amides with polyfluoroarenes through direct C-H functionalization. The structural and functional diversity of these readily available precursors provides a versatile and flexible strategy for the streamlined synthesis of α-polyfluoroaryl amines. Combining experimental and theoretical studies, a novel plausible mechanism of the α-aminocarbene-mediated C-H insertion has been revealed, which may stimulate future work for the development of novel methods in amine synthesis.

Catalyst-Controlled Regiodivergent Synthesis of α/β-Dipeptide Derivatives via N-Allylic Alkylation of O-Alkyl Hydroxamates with MBH Carbonates

A controllable and regiodivergent N-allylation reaction involving readily available O-alkyl hydroxamates derived from natural α-amino acids has been developed, allowing regiospecific access to α/β-dipeptides containing α-unsaturated β-amino acids moieties in moderate to good yields. The regioselectivity could be conveniently switched by alternation of the catalysts and solvents.

Tandem Catalysis Enabled Highly Chemoselective Deoxygenative Alkynylation and Alkylation of Tertiary Amides: A Versatile Entry to Functionalized α-Substituted Amines

We report here the highly chemoseive catalytic reductive alkynylation and reductive alkylation of tertiary amides to give propargylamines and α-branched amines, respectively. The method features a tandem iridium (Vaska’s complex)-catalyzed...

Metal-free synthesis of secondary amides using N-Boc-O-tosylhydroxylamine as nitrogen source via Beckmann rearrangement

An efficient methodology for the direct synthesis of secondary amides from ketones has been developed by using N-Boc-O-tosylhydroxylamine (TsONHBoc).

Iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for synthesis of α-tertiary amines

A protocol for the synthesis of α-tertiary amines was developed by iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides. Nucleophilic 1,2-addition of organolithium reagents to carboxamides forms anionic tetrahedral carbinolamine (hemiaminal) intermediates, which are subsequently treated with bromotrimethylsilane (Me₃SiBr) followed by organomagnesium (Grignard) reagents, organolithium reagents or tetrabutylammonium cyanide, affording α-tertiary amines. Employment of (trimethylsilyl)methylmagnesium bromide as the 2^nd nucleophile allowed for aza-Peterson olefination of the resulting α-tertiary (trimethylsilyl)methylamines with acidic work-up, resulting in the formation of 1,1-diarylethylenes.

Tf2O/TTBP (2,4,6-Tri-tert-butylpyrimidine): An Alternative Amide Activation System for the Direct Transformations of Both Tertiary and Secondary Amides

Ten types of Tf₂O/TTBP-mediated amide transformation reactions were investigated. The results showed that compared with pyridine derivatives 2,6-di-tert-butyl-4-methylpyridine (DTBMP) and 2-fluoropyridine (2-F-Pyr.), TTBP can serve as an alternative amide activation system for the direct transformation of both secondary and tertiary amides. For most surveyed examples, higher or comparable yields were generally obtained. In addition, Tf₂O/TTBP combination was used to promote the condensation reactions of 2-(tert-butyldimethylsilyloxy)furan (TBSOF) with both tertiary and secondary amides, the one-pot reductive Bischler-Napieralski-type reaction of tertiary lactams, and Movassaghi and Hill's modern version of the Bischler-Napieralski reaction. The value of the Tf₂O/TTBP-based methodology was further demonstrated by the concise and high-yielding syntheses of several natural products.

Asymmetric Deoxygenative Alkynylation of Tertiary Amides Enabled by Iridium/Copper Bimetallic Relay Catalysis

A variety of inert tertiary amides have been successfully transformed into synthetically important chiral propargylamines in high yields with good to excellent enantioselectivities via a relayed sequence of Ir catalyzed partial reduction and Cu/GARPHOS catalyzed asymmetric alkynylation with terminal alkynes. The reaction was readily extended to some drug molecules and the transformations of representative products have been demonstrated, thus attesting the practical utilities and the robust nature of the protocol.

Continuous Flow Acylation of (Hetero)aryllithiums with Polyfunctional N,N-Dimethylamides and Tetramethylurea in Toluene

The continuous flow reaction of various aryl or heteroaryl bromides in toluene in the presence of THF (1.0 equiv) with sec -BuLi (1.1 equiv) provided at 25 °C within 40 sec the corresponding aryllithiums which were acylated with various functionalized N,N-dimethylamides including easily enolizable amides at -20 °C within 27 sec, producing highly functionalized ketones in 48-90% yield (36 examples). This method was well suited for the preparation of α-chiral ketones such as naproxene and ibuprofen derived ketones with 99% ee . A one-pot stepwise bis-addition of two different lithium organometallics to 1,1,3,3-tetramethyurea (TMU) provided unsymmetrical ketones in 69-79% yield (9 examples).

Merging Electron Transfer with 1,2-Metalate Rearrangement: Deoxygenative Arylation of Aromatic Amides with Arylboronic Esters

Amides are essentially inert carboxyl derivatives in many types of chemical transformations. In particular, deoxygenative C-C bond formation of amides to synthetically important amines is a long-standing challenge for synthetic chemists due to the inertness of the resonance-stabilized amide C=O bond. Herein, it is disclosed that by merging electron-transfer-induced activation with 1,2-metalate rearrangement, a wide range of aromatic amides react smoothly with arylboron reagents, affording a series of biologically relevant diarylmethylamines as deoxygenative C-C bond cross-coupling products. With its simplicity and versatility, this reaction shows great promise in the synthesis of amines from amides, which may open up new avenues in retrosynthetic planning and find widespread use in academia and industry.

Reductive Functionalization of Amides in Synthesis and for Modification of Bioactive Compounds

In this review, applications of the amide reductive functionalization methodology for the synthesis and modification of bioactive compounds are covered. A brief summary of the different protocols is presented in the introduction, followed by the synthetic application of these in late-stage functionalization, leading to known pharmaceuticals or to their derivatives, including bioisosteres, with potential higher activity as the main axis of the article. The synthetic approach to natural products based on amide reduction is also discussed; however, this is given in a condensed form focusing on recent or as yet unexplored applications due to a number of recently published excellent reviews covering this topic. The aim of this review is to illustrate the potential of reductive functionalization of amides as an elegant and useful tool in the synthesis of bioactive compounds and inspire further work in this field.

Chemoselective Reactions of Isocyanates with Secondary Amides: One-Pot Construction of 2,3-Dialkyl-Substituted Quinazolinones

A facile method for the preparation of 2,3-dialkyl-substituted quinazolinones from readily available N-arylamides and commercial isocyanates was developed. This one-pot procedure involves the chemoselective activation of the secondary amide with Tf₂O/2-Br-Pyr, the sequential addition of isocyanate, and cyclization. The mild reaction is general for a wide range of substrates and can be run on a gram scale.

Enantioselective Reductive Cyanation and Phosphonylation of Secondary Amides by Iridium and Chiral Thiourea Sequential Catalysis

The combination of transition-metal catalysis and organocatalysis increasingly offers chemists opportunities to realize diverse unprecedented chemical transformations. By combining iridium with chiral thiourea catalysis, direct enantioselective reductive cyanation and phosphonylation of secondary amides have been accomplished for the first time for the synthesis of enantioenriched chiral α-aminonitriles and α-aminophosphonates. The protocol is highly efficient and enantioselective, providing a novel route to the synthesis of optically active α-functionalized amines from the simple, readily available feedstocks. In addition, the reactions are scalable and the thiourea catalyst can be recycled and reused.

Multi-Photocatalyst Cascades: Merging Singlet Oxygen Photooxygenations with Photoredox Catalysis for the Synthesis of Alkaloid Frameworks

The development of photocascades that rapidly transform simple and readily accessible furan substrates into polycyclic alkaloid frameworks or erythrina natural products is described. Each of the sequences developed makes use of photocatalyzed energy transfer processes, which generate singlet oxygen, to set up the substrates for the second photocatalyzed reaction, wherein electron transfer generates carbon-centered radicals for the cyclizations that give the final complex frameworks. A chemical switch has been developed that can "switch off" one photocatalyst; thus, allowing a second photocatalyst to take over control of the sequence. As a corollary, this strategy represents the first time it has been possible to use multiple photocatalysts in photocascades, and, as such, it expands significantly the reactions that can be included in such cascades and the order in which they can be initiated.

A Novel Class of 7-Membered Heterocyclic Compounds

The work presented herein describes the synthesis of a formerly inaccessible class of heterocyclic compounds. The reaction relies on α‐phthalimido‐amides, which are readily prepared from amino acids in 2 simple reactions steps. Under amide activation conditions in which classical keteniminium ions are not formed, the nitrile solvent is incorporated into the new fused 7‐membered ring system. Due to the absence of a keteniminium intermediate, the stereogenic information in the α‐position is fully retained.

Transition-Metal-Free Reductive Functionalization of Tertiary Carboxamides and Lactams for α-Branched Amine Synthesis

A new method for the synthesis of α-branched amines by reductive functionalization of tertiary carboxamides and lactams is described. The process relies on the efficient and controlled reduction of tertiary amides by a sodium hydride/sodium iodide composite, in situ treatment of the resulting anionic hemiaminal with trimethylsilyl chloride and subsequent coupling with nucleophilic reagents including Grignard reagents and tetrabutylammonium cyanide. The new method exhibits broad functional-group compatibility, operates under transition-metal-free reaction conditions, and is suitable for various synthetic applications on both sub-millimole and on multigram scales.

Chemodivergent transformations of amides using gem-diborylalkanes as pro-nucleophiles

Amides are versatile synthetic building blocks and their selective transformations into highly valuable functionalities are much desirable in the chemical world. However, the diverse structure and generally high stability of amides make their selective transformations challenging. Here we disclose a chemodivergent transformation of primary, secondary and tertiary amides by using 1,1-diborylalkanes as pro-nucleophiles. In general, selective B-O elimination occurs for primary, secondary amides and tertiary lactams to generate enamine intermediate, while tertiary amides undergo B-N elimination to generate enolate intermediate. Various in situ electrophilic trapping of those intermediates allows the chemoselective synthesis of α-functionalized ketones, β-aminoketones, enamides, β-ketoamides, γ-aminoketones, and cyclic amines from primary, secondary, tertiary amides and lactams. The key for these transformations is the enolization effect after the addition of α-boryl carbanion to amides.

Amides are versatile synthetic building blocks, however the general stability of the amide bond makes its selective transformation challenging. Here, the authors report a chemodivergent transformation of primary, secondary and tertiary amides by using 1,1-diborylalkanes as pro-nucleophiles.

Synthesis of Novel Heterocycles by Amide Activation and Umpolung Cyclization

Herein, we report a metal-free synthesis of cyclic amidines, oxazines, and an oxazinone under mild conditions by electrophilic amide activation. This strategy features an unusual Umpolung cyclization mode and enables the smooth union of α-aryl amides and diverse alkylazides, effectively rerouting our previously reported α-amination transform.