Palladium-Catalyzed Modular Synthesis of Substituted Piperazines and Related Nitrogen Heterocycles

Palladium-Catalyzed Heteroannulation of Salicylamides with Propargyl Carbonates: Synthesis of 1,4-Benzoxazepin-5-ones

Herein, we report a palladium-catalyzed method to synthesize 1,4-benzoxazepin-5-ones using salicylamides and propargyl carbonates. The heteroannulation provides a wide range of products in good to excellent yields with broad functional group tolerance. In addition, H2O is used as a low-cost, abundant, and safe solvent, which is important in terms of sustainability.

The medicinal chemistry of piperazines: A review

The versatile basic structure of piperazine allows for the development and production of newer bioactive molecules that can be used to treat a wide range of diseases. Piperazine derivatives are unique and can easily be modified for the desired pharmacological activity. The two opposing nitrogen atoms in a six-membered piperazine ring offer a large polar surface area, relative structural rigidity, and more acceptors and donors of hydrogen bonds. These properties frequently result in greater water solubility, oral bioavailability, and ADME characteristics, as well as improved target affinity and specificity. Various synthetic protocols have been reported for piperazine and its derivatives. In this review, we focused on recently published synthetic protocols for the synthesis of the piperazine and its derivatives. The structure-activity relationship concerning different biological activities of various piperazine-containing drugs has also been highlighted to provide a good understanding to researchers for future research on piperazines.

Synthesis, Kinetics and Computational Explorations of 4-Phenylpiperazine Bearing N-(Aryl)-3-substituted-benzamides as Auspicious Tyrosinase Inhibitors

In the aimed research study, a new series of N-(aryl)-3-[(4-phenyl-1-piperazinyl)methyl]benzamides was synthesized, which was envisaged as tyrosinase inhibitor. The structures of these newly designed molecules were verified by IR, 1H-NMR, 13C-NMR, EI-MS and CHN analysis data. These molecules were screened against tyrosinase and their inhibitory activity explored that these 3-substituted-benzamides exhibit good to excellent potential, comparative to the standard. The Kinetics mechanism was investigated through Lineweaver-Burk plots which depicted that molecules inhibited this enzyme in a competitive mode. Moreover, molecular docking was also performed to determine the binding interaction of all synthesized molecules (ligands) with the active site of tyrosinase enzyme and the results showed that most of the ligands exhibited efficient binding energy values. Therefore, it is anticipated that these molecules might serve as auspicious therapeutic scaffolds for treatment of the tyrosinase associated skin disorders.

Synthesis of 1,4-Diazacycles by Hydrogen Borrowing

We report the syntheses of 1,4-diazacycles by diol-diamine coupling, uniquely made possible with a (pyridyl)phosphine-ligated ruthenium(II) catalyst (1). The reactions can exploit either two sequential N-alkylations or an intermediate tautomerization pathway to yield piperazines and diazepanes; diazepanes are generally inaccessible by catalytic routes. Our conditions tolerate different amines and alcohols that are relevant to key medicinal platforms. We show the syntheses of the drugs cyclizine and homochlorcyclizine in 91% and 67% yields, respectively.

Palladium-catalyzed hydroamination of vinylidenecyclopropane-diester with pyrroles and indoles: an approach to azaaromatic vinylcyclopropanes

A new method for the synthesis of azaaromatic vinylcyclopropanes (VCPs) has been disclosed in this paper by using pyrrole or indole derivatives as nucleophilic reagents to react with vinylidenecyclopropane-diesters (VDCP-diesters) in the presence of a Pd catalyst, a phosphine ligand and a base under mild conditions in up to 98% yield with a reasonable substrate scope. This reaction is one of the few examples of hydroamination at electron-rich allene's β-position. A plausible reaction mechanism has also been proposed through a zwitterionic π-propargyl N-palladium species according to the previous work and the obtained deuterium labeling experimental result.

Palladium(0)-catalysed regioselective cyclisations of 2-amino(tosyl) benzamides/sulphonamides: the stereoselective synthesis of 3-ylidene-[1,4]benzodiazepin-5-ones/benzo[f][1,2,5]thiadiazepine-1,1-dioxides

The Pd(0) catalysed cyclisation reactions between tert-butyl propargyl carbonates and 2-aminotosyl benzamides or sulphonamides deliver 1,4-benzodiazepin-5-ones or sultam derivatives, key components of many biologically active compounds. But 2-amino benzamides/sulphonamides require propargyl carbonates substituted at acetylenic carbon to undergo the reaction resulting in the stereoselective formation of the said products.

Palladium catalyzed divergent cycloadditions of vinylidenecyclopropane-diesters with methyleneindolinones enabled by zwitterionic π-propargyl palladium species

A palladium-catalyzed divergent synthesis of spirooxindoles fused with a five- or a six-membered ring by a cycloaddition reaction of vinylidenecyclopropane-diesters with methyleneindolinones was disclosed. This protocol features an in situ generated unprecedented zwitterionic π-propargyl palladium species in cycloaddition reactions and a switchable process between (3+2) and (4+2) cycloadditions by changing the phosphine ligand.

Recent advances in annulation reactions based on zwitterionic π-allyl palladium and propargyl palladium complexes

Carbocyclic and heterocyclic compounds could be constructed through the palladium-catalyzed annulation reactions of zwitterionic π-allyl palladium or propargyl palladium complexes with unsaturated electrophiles.

Construction of Highly Functionalized Piperazinones via Post-Ugi Cyclization and Diastereoselective Nucleophilic Addition

A novel method for the generation of uniquely functionalized piperazinones by utilizing post-Ugi functionalization is described. The method involves an Ugi reaction with aminoacetaldehyde dimethyl acetal, followed by acid-mediated cyclization to generate the iminium precursor that was subjected to nucleophilic addition in a diastereoselective manner. The method was also employed to synthesize trans-dragmacidine C and praziquantel-like molecules.

Benefits and applications of microwave-assisted synthesis of nitrogen containing heterocycles in medicinal chemistry

Nitrogen containing heterocycles are of immense research interest because they are often found as naturally occurring bioactive compounds.

Practical and scalable synthesis of orthogonally protected-2-substituted chiral piperazines

A synthetic route to orthogonally protected, enantiomerically pure 2-substituted piperazines is described. Starting from α-amino acids, within four steps chiral 2-substituted piperazines are obtained. The key transformation involves an aza-Michael addition between an orthogonally bis-protected chiral 1,2-diamine and the in situ generated vinyl diphenyl sulfonium salt derived from 2-bromoethyl-diphenylsulfonium triflate. Further validation using different protecting groups as well as synthesis on multigram scale was performed. The method was also applied to the construction of chiral 1,4-diazepanes and 1,4-diazocanes. Additionally, the method was utilized in a formal synthesis of chiral mirtazapine.

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.

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.

Decarboxylation-promoted Pd-catalyzed asymmetric propargylic [3 + 2] annulation for the enantioselective construction of a quaternary stereocenter in 2,3-dihydrofurans

The enantioselective construction of a quaternary stereocenter in 2,3-dihydrofuran frameworks has been realized via the palladium-catalyzed asymmetric [3 + 2] cycloaddition of tertiary propargylic carbonates with β-ketoesters enabled by a chiral ferrocene/benzimidazole-based bidentate P,N-ligand. The reaction was significantly promoted by loss of CO₂ to irreversibly form π-propargylpalladium or allenylpalladium intermediates. This protocol features a good tolerance of functional groups in both tertiary propargylic carbonates and β-ketoesters, thereby delivering a variety of highly functionalized chiral 2,3-dihydrofurans bearing a quaternary stereocenter at the 2-position and an exocyclic double bond at the 3-position in good chemical yields and high enantioselectivities (up to 98% ee).

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Copper-catalyzed [1,2]-rearrangements of allylic iodides and aryl α-diazoacetates

The [1,2]- and [2,3]-rearrangements of iodonium ylides are synthetically useful reactions for the generation of functionalized α-iodoesters. Allylic iodides are coupled with α-diazoesters in the presence of a copper catalyst and a ligand to generate iodonium ylides, which undergo metal-mediated rearrangements. By fine-tuning the structure of the ligand, we have reversed the regioselectivity of copper-catalyzed reactions of iodonium ylides from [2,3]- to [1,2]-rearrangements with the use of alternate bipyridine ligands. The preference for [1,2]-rearrangements was further improved by using bulky aryl α-diazoester substrates. Several α-iodoesters with a diverse range of functional groups were generated in good yields (up to 88% yield) and high regioselectivities (up to >95:5 regioisomeric ratio). A deuterium-labeled substrate was utilized to gain insight into the mechanism of the reaction.