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

1,3-Difunctionalization of [1.1.1]propellane through iron-hydride catalyzed hydropyridylation

Current methodologies for the functionalization of [1.1.1]propellane primarily focus on achieving 1, 3-difunctionalized bicyclo[1.1.1]pentane or ring-opened cyclobutane moiety. Herein, we report an innovative approach for the 1, 3-difunctionalization of [1.1.1]propellane, enabling access to a diverse range of highly functionalized cyclobutanes via nucleophilic attack followed by ring opening and iron-hydride hydrogen atom transfer. To enable this method, we developed an efficient iron-catalyzed hydropyridylation of various alkenes for C - H alkylation of pyridines at the C4 position, eliminating the need for stoichiometric quantities of oxidants or reductants. Mechanistic investigations reveal that the resulting N-centered radical serves as an effective oxidizing agent, facilitating single-electron transfer oxidation of the reduced iron catalyst. This process efficiently sustains the catalytic cycle, offering significant advantages for substrates with oxidatively sensitive functionalities that are generally incompatible with alternative approaches. The strategy presented herein is not only mechanistically compelling but also demonstrates broad versatility, highlighting its potential for late-stage functionalization.

Revisiting the 1,3-azadiene-succinic anhydride annulation reaction for the stereocontrolled synthesis of allylic 2-oxopyrrolidines bearing up to four contiguous stereocenters

Polysubstituted 2-oxopyrrolidines bearing at least two contiguous stereocenters constitute the core of several pharmaceuticals, including clausenamide (antidementia). Here, we describe a flexible annulation strategy, which unites succinic anhydride and 1,3-azadienes to produce allylic 2-oxopyrrolidines bearing contiguous stereocenters. The approach is chemoselective, efficient, modular, scalable, and diastereoselective. The scalable nature of the reactions offers the opportunity for post-diversification, leading to incorporation of motifs with either known pharmaceutical value or that permit subsequent conversion to medicinally relevant entities.

Deoxygenative Nucleophilic Phosphonation and Electrophilic Alkylation of Secondary Amides: A Facile Access to Quaternary α-Aminophosphonates

The widespread occurrence and synthetic accessibility of amides render them valuable precursors for the synthesis of diverse nitrogen-containing compounds. Herein, we present a metal-free and streamlined synthetic strategy for the synthesis of quaternary α-aminophosphonates. This approach involves sequential deoxygenative nucleophilic phosphonation and versatile electrophilic alkylation of secondary amides in a one-pot fashion. Notably, this method enables the direct bis-functionalization of secondary amides with both nucleophiles and electrophiles for the first time, with simple derivatization leading to valuable free α-aminophosphonates by hydrolysis. The protocol has the advantages of operational simplicity, broad functional-group compatibility, environmental friendliness, and scalability to multigram quantities.

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.

Direct Amino-α-C-H Heteroarylation of Amides under Electrochemical Conditions

An electrochemical hydrogen atom transfer (HAT) strategy for the direct amino-α-C-H heteroarylation of amides is described. The cheap TMSN3 acts as a hydrogen atom transfer reagent. A series of heteroarenes including quinoxalin-2(1H)-ones, 4-methylquinoline, isoquinoline, 2-methylquinoxaline, benzothiazole, etc., and various readily available amides/lactams were suitable. The reaction has the characteristics of a wide range of substrates, good regioselectivity, chemical oxidant-free conditions, etc.

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.

Skeletal metalation of lactams through a carbonyl-to-nickel-exchange logic

Classical metalation reactions such as the metal-halogen exchange have had a transformative impact on organic synthesis owing to their broad applicability in building carbon-carbon bonds from carbon-halogen bonds. Extending the metal-halogen exchange logic to a metal-carbon exchange would enable the direct modification of carbon frameworks with new implications in retrosynthetic analysis. However, such a transformation requires the selective cleavage of highly inert chemical bonds and formation of stable intermediates amenable to further synthetic elaborations, hence its development has remained considerably challenging. Here we introduce a skeletal metalation strategy that allows lactams, a prevalent motif in bioactive molecules, to be readily converted into well-defined, synthetically useful organonickel reagents. The reaction features a selective activation of unstrained amide C-N bonds mediated by an easily prepared Ni(0) reagent, followed by CO deinsertion and dissociation under mild room temperature conditions in a formal carbonyl-to-nickel-exchange process. The underlying principles of this unique reactivity are rationalized by organometallic and computational studies. The skeletal metalation is further applied to a direct CO excision reaction and a carbon isotope exchange reaction of lactams, underscoring the broad potential of metal-carbon exchange logic in organic synthesis.

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.

Mild Divergent Semireductive Transformations of Secondary and Tertiary Amides via Zirconocene Hydride Catalysis

The mild catalytic partial reduction of amides to imines has proven to be a challenging synthetic transformation, with many transition metals directly reducing these substrates to amines. Herein, we report a mild, catalytic method for the semireduction of both secondary and tertiary amides via zirconocene hydride catalysis. Utilizing just 5 mol % of Cp2ZrCl2, the reductive deoxygenation of secondary amides is demonstrated to furnish a diverse array of imines in up to 94% yield with excellent chemoselectivity and without the need for glovebox handling. Moreover, a novel reductive transamination of tertiary amides is also achievable when the catalytic protocol is carried out in the presence of a primary amine at room temperature, providing access to an expanded assortment of imines in up to 98% yield. Through slight procedural tuning, the single-flask conversion of amides to imines, aldehydes, amines or enamines is feasible, including multicomponent syntheses.

Preparation of novel acyl pyrazoles and triazoles by means of oxidative functionalization reactions

Novel acyl pyrazoles and acyl triazoles have been prepared by means of the oxidative amidation of aldehydes in the presence of the requisite azole. Yields range from modest to good in both cases, and some limitations of the substrate scope have been discovered. Acyl pyrazoles were prepared by treatment of a mixture of aldehyde and pyrazole with an oxoammonium salt bearing the nitrate anion. In the case of acyl triazoles, the oxidative functionalization was performed using sodium persulfate as a terminal oxidant in the presence of a catalytic quantity of a nitroxide.

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.

Site-Selective Amide Functionalization by Catalytic Azoline Engrafting

Direct peptide and protein activation is a challenging transformation because of the stabilizing effect of the amide group. While enzymes can be considered as prototypical systems that have evolved to achieve high selectivity and specificity, small-molecule catalysts that functionalize the amide group may accommodate a much larger selection of substrates but currently remain scarce. Here, by combining the desired features from both catalytic regimes we designed an artificial cyclodehydratase, a catalytic system for the site-selective modification of peptides and natural products by engrafting heterocycles into their scaffolds. The catalytic system features a molybdenum(VI) center that was decorated with a sterically congested tripod ligand. The optimized catalyst can introduce azolines into small molecules, natural products, and oligopeptides with high efficiency and minimal waste. We further demonstrate the utility of the new protocol in the direct functionalization of a single amide group in the presence of up to seven other chemically similar positions and in the direct conversion of these groups into amines and thioamides. This new mechanistic paradigm may address an unmet need for a general method for the selective and sustainable functionalization of peptides and natural products.

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.

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...