Synthesis of Saturated N-Heterocycles

The Role of the Catalyst on the Reactivity and Mechanism in the Diels–Alder Cycloaddition Step of the Povarov Reaction for the Synthesis of a Biological Active Quinoline Derivative: Experimental and Theoretical Investigations

An experimental and theoretical study of the reactivity and mechanism of the non-catalyzed and catalyzed Povarov reaction for the preparation of a 4-ethoxy-2,3,4,4a-tetrahydro-2-phenylquinoline as a biological active quinoline derivative has been performed. The optimization of experimental conditions indicate that the use of a catalyst, namely Lewis acid with an electron-releasing group, creates the best experimental conditions for this kind of reaction. The chemical structure was characterized by the usual spectroscopic methods. The prepared quinoline derivative has been also tested in vitro for antibacterial activity, which displays moderate inhibitory activity against both Escherichia coli and Staphylococcus aureus. The antioxidant activity was investigated in vitro by evaluating their reaction with 1,1-diphenyl-2-picrylhydrazyl DPPH radical, which reveals high reactivity. The computational study was performed on the Diels–Alder step of the Povarov reaction using a B3LYP/6-31G(d,p) level of theory. The conceptual DFT reactivity indices explain well the reactivity and the meta regioselectivity experimentally observed. Both catalysts enhance the reactivity of the imine, favoring the formation of the meta regioisomers with a low activation energy, and they change the mechanism to highly synchronous for the Lewis acid and to stepwise for the Brønsted acid. The reaction of imine with allyl alcohol does not give any product, which requires high activation energy.

Enantioselective hydrogenation of annulated arenes: controlled formation of multiple stereocenters in adjacent rings

We report a method for the enantioselective hydrogenation of annulated arenes using 4H-pyrido[1,2-a]pyrimidinones as substrates. The method selectively generates multiple stereocenters in adjacent rings leading to architecturally complex motifs, which resemble bioactive molecules. The mechanistic study of the stereochemical outcome revealed that the catalyst is able to overcome substrate stereocontrol providing all-cis-substituted products predominantly. In a sequential protocol, a matching interaction between catalyst and substrate stereocontrol is achieved that facilitates diastereo- and enantioselective access to trans-products.

α-C-H Bond Functionalization of Unprotected Alicyclic Amines: Lewis-Acid-Promoted Addition of Enolates to Transient Imines

Enolizable cyclic imines, obtained in situ from their corresponding lithium amides by oxidation with simple ketone oxidants, are readily alkylated with a range of enolates to provide mono- and polycyclic β-aminoketones in a single operation, including the natural product (±)-myrtine. Nitrile anions also serve as competent nucleophiles in these transformations, which are promoted by BF3 etherate. β-Aminoesters derived from ester enolates can be converted to the corresponding β-lactams.

Diversification of Unprotected Alicyclic Amines by C-H Bond Functionalization: Decarboxylative Alkylation of Transient Imines

Despite extensive efforts by many practitioners in the field, methods for the direct α-C-H bond functionalization of unprotected alicyclic amines remain rare. A new advance in this area utilizes N-lithiated alicyclic amines. These readily accessible intermediates are converted to transient imines through the action of a simple ketone oxidant, followed by alkylation with a β-ketoacid under mild conditions to provide valuable β-amino ketones with unprecedented ease. Regioselective α'-alkylation is achieved for substrates with existing α-substituents. The method is further applicable to the convenient one-pot synthesis of polycyclic dihydroquinolones through the incorporation of a SN Ar step.

Traceless Redox-Annulations of Alicyclic Amines

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-(nitromethyl)benzaldehyde. Benzoic acid acts as a promoter in these reactions, which involve concurrent amine α-C-H bond and N-H bond functionalization. Subsequent removal of the nitro group provides access to tetrahydroprotoberberines not accessible via typical redox-annulations. Also reported are decarboxylative annulations of ortho-(nitromethyl)benzaldehyde with proline and pipecolic acid.

Diastereoselective Synthesis of Nonplanar 3-Amino-1,2,4-oxadiazine Scaffold: Structure Revision of Alchornedine

Herein, we report the diastereoselective synthesis of a 3-amino-1,2,4-oxadiazine (AOXD) scaffold. The presence of a N-O bond in the ring prevents the planar geometry of the aromatic system and induces a strong decrease in the basicity of the guanidine moiety. While DIBAL-H appeared to be the most efficient reducing agent because it exhibited high diastereoselectivity, we observed various behaviors of the Mitsunobu reaction on the resulting β-aminoalcohol, leading to either inversion or retention of the configuration depending on the steric hindrance in the vicinity of the hydroxy group. The physicochemical properties (pK_a and log D) and hepatic stability of several AOXD derivatives were experimentally determined and found that the AOXD scaffold possesses promising properties for drug development. Moreover, we synthesized alchornedine, the only natural product with the AOXD scaffold. Based on a comparison of the analytical data, we found that the reported structure of alchornedine was incorrect and hypothesized a new one.

SLAP reagents for the photocatalytic synthesis of C3/C5-substituted, N-unprotected selenomorpholines and 1,4-selenazepanes

Herein, we disclose the first set of unique selenium-containing SLAP (SiLicon Amine Protocol) reagents for the direct synthesis of C3/C5-substituted selenomorpholines and 1,4-selenazepanes from diverse (hetero)aldehydes under mild photocatalytic conditions. Enantiomerically pure 1,2-amino alcohol/α-amino acid versions of these heterocycles were also synthesized. Further, we have shown the late-stage modification of certain biologically active agents using the developed seleno-SLAP reagents.

Copper-Catalyzed Sequential Cyclization/Migration of Alkynyl Hydrazides for Construction of Ring-Expanded N-N Fused Pyrazolones

A copper-catalyzed sequential cyclization/migration reaction of alkynyl hydrazides for the synthesis of ring-expanded N-N fused pyrazolones was developed. Control experiments indicate that the copper-ligand complex plays an essential role in the reaction. This approach features a broad scope including some functional group tolerance as well as a nucleophilic addition/1,3-migration/formal 1,2-migration sequence. This protocol provides simple manipulation and less waste due to high yield and atom economy. The synthetic utility of N-N fused pyrazolones was also demonstrated by further transformations.

Transition metal-free domino acyl substitution/Michael addition of alkenyl Grignard reagents to lactam esters: synthesis of lactam-bearing homoallylic ketones

A solvent-controlled protocol for the direct and transition metal-free addition of alkenyl Grignard reagents to vicinally functionalized sp³-rich morpholinones has been developed, leading to the chemo and regioselective synthesis of lactam-bearing homoallylic ketones. The addition of lithium chloride proved to be essential. In cases where a new stereocenter is generated, the doubly branched homoallylic ketones are obtained in unexpectedly high diastereoselectivities. Efforts to extend the methodology to other heterosubstituted lactams revealed some important reactivity and selectivity differences.

A solvent-controlled and heteroatom-dictated protocol for the transition metal-free addition of alkenyl Grignard reagents to lactam esters has been developed, leading to the chemo- and regioselective synthesis of lactam-tethered homoallylic ketones.

Synthesis and Evaluation of Antimicrobial Activities of Novel N-Substituted Indole Derivatives

Indole motifs are one of the most significant scaffolds in the discovery of new drugs. We have described a synthesis of new N-substituted indole derivatives (1-3), and their in vitro antimicrobial activities were investigated. The synthesis of titled compounds has been demonstrated by utilizing commercially available starting materials. The antibacterial and antifungal activities were performed using new strains of bacteria Staphylococcus aureus, Escherichia coli, and Candida albicans using the disc diffusion method. Notably, the compound 4-(1-(2-(1H-indol-1-yl) ethoxy) pentyl)-N,N-dimethyl aniline (1) was found to be most potent than the other analogues (2 and 3), which has shown higher inhibition than the standard drug chloramphenicol.

Rapid functionalization of multiple C-H bonds in unprotected alicyclic amines

The synthesis of valuable bioactive alicyclic amines containing variable substituents in multiple ring positions typically relies on multistep synthetic sequences that frequently require the introduction and subsequent removal of undesirable protecting groups. Although a vast number of studies have aimed to simplify access to such materials through the C-H bond functionalization of feedstock alicyclic amines, the simultaneous introduction of more than one substituent to unprotected amines has never been accomplished. Here we report an advance in C-H bond functionalization methodology that enables the introduction of up to three substituents in a single operation. Lithiated amines are first exposed to a ketone oxidant, generating transient imines that are subsequently converted to endocyclic 1-azaallyl anions, which can be processed further to furnish β-substituted, α,β-disubstituted, or α,β,α'-trisubstituted amines. This study highlights the unique utility of in situ-generated endocyclic 1-azaallyl anions, elusive intermediates in synthetic chemistry.

Functionalization of Piperidine Derivatives for the Site-Selective and Stereoselective Synthesis of Positional Analogues of Methylphenidate

Rhodium‐catalyzed C−H insertions and cyclopropanations of donor/acceptor carbenes have been used for the synthesis of positional analogues of methylphenidate. The site selectivity is controlled by the catalyst and the amine protecting group. C−H functionalization of N‐Boc‐piperidine using Rh₂(R‐TCPTAD)₄, or N‐brosyl‐piperidine using Rh₂(R‐TPPTTL)₄ generated 2‐substitited analogues. In contrast, when N‐α‐oxoarylacetyl‐piperidines were used in combination with Rh₂(S‐2‐Cl‐5‐BrTPCP)₄, the C−H functionalization produced 4‐susbstiuted analogues. Finally, the 3‐substituted analogues were prepared indirectly by cyclopropanation of N‐Boc‐tetrahydropyridine followed by reductive regio‐ and stereoselective ring‐opening of the cyclopropanes.

Rapid, two-pot procedure for the synthesis of dihydropyridinones; total synthesis of aza-goniothalamin

A fast, protecting-group-free synthesis of dihydropyridinones has been developed. Starting from commercially available aldehydes, a novel one-pot amidoallylation gave access to diene compounds in good yields. Ring-closing metathesis conditions were then employed to produce the target dihydropyridinones efficiently and in high yields.

Hexafluoroisopropanol-Mediated Redox-Neutral α-C(sp3)-H Functionalization of Cyclic Amines via Hydride Transfer

Hexafluoroisopropanol has been demonstrated as the versatile promoter for redox-neutral α-C(sp³)-H functionalization of cyclic amines via the cascade [1,5]-hydride transfer/cyclization strategy. A wide range of cyclic amines are functionalized into bioactive tetrahydroquinolines, quinazolines, benzoxazines, and benzotriazepines in moderate to excellent yields. This protocol features additive-free conditions, operational simplicity, and wide substrate scope.

Redox-Annulations of Cyclic Amines with ortho-Cyanomethylbenzaldehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with ortho-cyanomethylbenzaldehydes. These amine α-C-H bond functionalization reactions are promoted by acetic acid. The resulting β-aminonitriles can be converted to the corresponding β-aminoalcohols in diastereoselective fashion.

Copper-catalyzed alkenylation of novel N-iodoarylated allylic ketopiperazinonates with unactivated alkenes

Novel N-iodoarylated allylic ketopiperazines have been engaged in a CuBr-catalyzed alkenylation protocol with simple alkenes, leading to epimerization-free, efficient, and regioselective synthesis of N-stilbeno-ketopiperazines

Skeletal remodelling of α-substituted cyclic eneformamides to α-ketonyl cyclic amines

A modular dehomologation strategy, which skeletally remodels cyclic α-substituted eneformamides to one-carbon shorter α-ketonyl saturated cyclic amines is described.

Modular synthesis and transition metal-free alkynylation/alkenylation of Castagnoli–Cushman-derived N,O- and N,S-heterocyclic vinyl chlorides

A modular and functional group-tolerant protocol for the transition metal-free coupling of novel N,O- and N,S-heterocyclic vinyl chlorides with terminal acetylenes and styrenes has been developed, leading to the epimerization-free synthesis of fully carbofunctionalized dihydro-1,4-oxazines/thiazines. Bicyclic morpholines have also been prepared through the interrogation of newly synthesized cross-conjugated dienes in Diels–Alder reactions. The use of environmentally benign reaction media endows the current strategy with a practical advantage.

A functional group-tolerant and transition metal-free coupling of novel N,O- and N,S-heterocyclic vinyl chlorides, which affords fully carbosubstituted dihydro-1,4-oxazines/thiazines as well as bicyclic morpholines, is described.

Transition-metal and oxidant-free approach for the synthesis of diverse N-heterocycles by TMSCl activation of isocyanides

A highly efficient TMSCl-mediated addition of N-nucleophiles to isocyanides has been achieved. This transition-metal and oxidant-free strategy has been applied to the construction of various N-heterocyles such as quinazolinone, benzimidazole and benzothiazole derivatives by the use of distinct amino-based binucleophiles. The notable feature of this protocol includes its mild reaction condition, broad functional group tolerance and excellent yield.

A highly efficient TMSCl-mediated addition of N-nucleophiles to isocyanides has been achieved.

Highly Enantioselective Hiyama Cross-Coupling via Rh-Catalyzed Allylic Arylation of Racemic Allyl Chlorides

Highly enantioselective Hiyama cross-coupling reactions have been achieved through rhodium(I)-catalyzed dynamic kinetic asymmetric transformations between aryl siloxanes and cyclic racemic allyl halides. This process affords valuable enantiomerically enriched aryl-substituted cyclic allyl products and is compatible with heterocyclic allyl chloride electrophiles.

Highly enantioselective rhodium-catalyzed cross-coupling of boronic acids and racemic allyl halides

Although Csp²-Csp² Suzuki-Miyaura couplings (SMCs) are widely used in small-molecule synthesis, related methods that allow the incorporation of Csp³-hybridized coupling partners, particularly in an asymmetric manner, are less developed. This protocol describes catalytic asymmetric SMC reactions that provide access to enantiomerically enriched cyclic allylic products. The method couples racemic allyl halide starting materials with sp²-hybridized boronic acid derivatives and is compatible with heterocyclic coupling partners. These reactions are catalyzed by a rhodium-ligand complex and typically display very high levels of enantioselectivity (>95% enantiomeric excess (ee)). In this protocol, we detail a procedure using a dihydropyridine-derived allyl chloride for the synthesis of (-)-(S)-tert-butyl-3-(4-bromophenyl)-3,6-dihydropyridine-1(2H)-carboxylate, an intermediate in the synthesis of the anticancer drug niraparib. This procedure affords 1.17 g (86% yield) of the coupling product with 96% ee. The initial experimental setup of the reaction takes 45-50 min, and the reaction is complete within 4-5 h.

Stereocontrolled Synthesis of Functionalized Azaheterocycles from Carbocycles through Oxidative Ring Opening/Reductive Ring Closing Protocols

Fluorine-containing organic scaffolds are of significant interest in medicinal chemistry. The incorporation of fluorine into biomolecules can lead to remarkable changes in their physical, chemical, and biological properties. There are already many drugs on the market, which contain at least one fluorine atom. Saturated functionalized azaheterocycles as bioactive substances have gained increasing attention in pharmaceutical chemistry. Due to the high biorelevance of organofluorine molecules and the importance of N-heterocyclic compounds, selective stereocontrolled procedures to the access of new fluorine-containing saturated N-heterocycles are considered to be a hot research topic. This account summarizes the synthesis of functionalized and fluorine-containing saturated azaheterocycles starting from functionalized cycloalkenes and based on oxidative ring cleavage of diol intermediates followed by ring expansion with reductive amination.

Traceless Electrophilic Amination for the Synthesis of Unprotected Cyclic β-Amino Acids

Electrophilic aminations involve an umpolung of a nitrogen atom, providing an alternate, distinctive synthetic strategy. The recent advent of various designed O-substituted hydroxylamines has significantly advanced this research field. An underappreciated issue is atom economy of the transformations: The necessary activating group on the oxygen atom is left in coproduced waste. Herein, we describe Rh-catalyzed electrophilic amination of substituted isoxazolidin-5-ones for the synthesis of unprotected, cyclic β-amino acids featuring either benzo-fused or spirocyclic scaffolds. Using the cyclic hydroxylamines allows for retaining both nitrogen and oxygen functionalities in the product. The traceless, redox neutral process proceeds on a gram scale with as little as 0.1 mol % catalyst loading. In contrast to related electrophilic aminations in the literature, a series of mechanistic experiments suggests a unique pathway involving spirocyclization, followed by the skeletal rearrangement. The insights provided herein shed light on a nuanced reactivity of the active species, Rh-nitrenoid generated from the activated hydroxylamine, and extend the knowledge on electrophilic aromatic substitutions.

α-Functionalization of Cyclic Secondary Amines: Lewis Acid Promoted Addition of Organometallics to Transient Imines

Cyclic imines, generated in situ from their corresponding N-lithiated amines and a ketone hydride acceptor, undergo reactions with a range of organometallic nucleophiles to generate α-functionalized amines in a single operation. Activation of the transient imines by Lewis acids that are compatible with the presence of lithium alkoxides was found to be crucial to accommodate a broad range of nucleophiles including lithium acetylides, Grignard reagents, and aryllithiums with attenuated reactivities.

Modular, stereocontrolled Cβ-H/Cα-C activation of alkyl carboxylic acids

The union of two powerful transformations, directed C-H activation and decarboxylative cross-coupling, for the enantioselective synthesis of vicinally functionalized alkyl, carbocyclic, and heterocyclic compounds is described. Starting from simple carboxylic acid building blocks, this modular sequence exploits the residual directing group to access more than 50 scaffolds that would be otherwise extremely difficult to prepare. The tactical use of these two transformations accomplishes a formal vicinal difunctionalization of carbon centers in a way that is modular and thus, amenable to rapid diversity incorporation. A simplification of routes to known preclinical drug candidates is presented along with the rapid diversification of an antimalarial compound series.

Green electrochemical strategy for one-step synthesis of new catechol derivatives

In this paper, we present promising results in the green electrochemical oxidation of catechol in the presence of three different thiol nucleophiles at the surface of a glassy carbon electrode in an aqueous solution using cyclic voltammetry (CV). The outcome indicated the synthesis of some new heterocyclic compounds functionalized with phenolic, triazole, triazine and pyrimidine groups. The effects of repetitive cycling, nucleophile concentrations and sweep rates were explored to get more information about the systems. The voltammetric data showed that the electro-generated o-benzoquinone is a quite reactive intermediate, which in aqueous solutions can quickly participate in a Michael-addition reaction with any one of the nucleophiles to form the corresponding products. The structures of all newly electro-synthesized compounds were confirmed by elemental analyses and FT-IR, 1H NMR, 13C-NMR and MS spectra. The one-pot synthesis strategy led to new organics with high purities and good yields under green conditions without harmful reagents.

Photoredox Radical/Polar Crossover Enables Construction of Saturated Nitrogen Heterocycles

Photoredox-mediated radical/polar crossover (RPC) processes offer new avenues for the synthesis of cyclic molecules. This process has been realized for the construction of medium-sized saturated nitrogen heterocycles. Photocatalytically generated alkyl radicals possessing pendant leaving groups engage imines in C-C bond formation, and subsequent reduction of the intermediate nitrogen-centered radical triggers anionic ring closure. With the aid of visible light irradiation, substituted pyrrolidines, piperidines, and azepanes can be prepared under mild, redox-neutral conditions.

Redox-Annulations of Cyclic Amines with Electron-Deficient o-Tolualdehydes

Amines such as 1,2,3,4-tetrahydroisoquinoline undergo redox-neutral annulations with 2-methyl-3,5-dinitrobenzaldehyde and closely related substrates. Acetic acid serves as the solvent and sole promoter of these transformations which involve dual C-H functionalization.