New Heterocyclic Product Space for the Castagnoli–Cushman Three-Component Reaction

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.

Identification of BACE-1 inhibitors through directed C(sp3)-H activation on 5-oxo-pyrrolidine-3-carboxylic acid derivatives

Convenient synthesis of stereochemically dense 5-oxo-pyrrolidines was obtained from succinic anyhdride and imines by combining the Castagnoli-Cushman reaction with directed Pd-catalyzed C(sp3)-H functionalization, taking advantage of the developing carboxylic group properly derivatized with 8-aminoquinoline as a directing group. These fully substituted 5-oxopyrrolidines were found to be able to inhibit BACE-1 enzyme with sub-micromolar activity, thanks to the interaction of the key aryl appendage introduced by C(sp3)-H activation within BACE-1 S2' subsite.

The Castagnoli–Cushman Reaction

Since the first reports of the reaction of imines and cyclic anhydrides by Castagnoli and Cushman, this procedure has been applied to the synthesis of a variety of lactams, some of them with important synthetic or biological interest. The scope of the reaction has been extended to the use of various Schiff bases and anhydrides as well as to different types of precursors for these reagents. In recent years, important advances have been made in understanding the mechanism of the reaction, which has historically been quite controversial. This has helped to develop reaction conditions that lead to pure diastereomers and even homochiral products. In addition, these mechanistic studies have also led to the development of new multicomponent versions of the Castagnoli–Cushman reaction that allow products with more diverse and complex molecular structures to be easily obtained.

Dicarboxylic Acid Monoesters in β- and δ-Lactam Synthesis

A N-(2-methoxy-2-oxoethyl)-N-(phenylsulfonyl)glycine monomethyl ester of the respective dicarboxylic acid was involved in a reaction with imines promoted by acetic anhydride at an elevated temperature. Instead of the initially expected δ-lactam products of the Castagnoli–Cushman-type reaction, medicinally important 3-amino-2-azetidinones were obtained as the result of cyclization, involving a methylene group adjacent to an acid moiety. In contrast, replacing alcohol residue with hexafluoroisopropyl in the same substrate made another methylene group (adjacent to the ester moiety) more reactive to furnishing the desired δ-lactam in the Castagnoli–Cushman fashion.

Trifluoroethanol Promoted Castagnoli-Cushman Cycloadditions of Imines with Homophthalic Anhydride

Lactams are essential compounds in medicinal chemistry and key intermediates in the synthesis of natural products. The Castagnoli–Cushman reaction (CCR) of homophthalic anhydride with imines is an exciting method for accessing cyclic densely substituted lactam products. Most CCRs need to be catalyzed or heated. Herein, we report a new, efficient, metal and catalyst-free CCR for the synthesis of poly-substituted 3,4-lactams utilizing the unique properties of trifluoroethanol (TFE). This procedure provides high-speed and smooth access to a broad range of densely substituted 3,4-lactams in good yields and a 100% atom-economical fashion.

Serendipitous synthesis of 2-alkenyl- and 2-aryl-4-thiazolidinones using dithiodiglycolic anhydride

Dithiodiglycolic anhydride undergoes an efficient formal cycloaddition with imines to afford functionalized 4-thiazolidinones, without complications arising from the anhydride-imine reaction or the sulfa-Michael reaction (in the case of 1,3-azadienes).

Mechanistic Investigation of Castagnoli-Cushman Multicomponent Reactions Leading to a Three-Component Synthesis of Dihydroisoquinolones

The mechanisms for the three- and four-component variants of the Castagnoli-Cushman reaction (CCR) have been investigated. A series of crossover experiments were conducted to probe the structure and reactivity of known amide-acid intermediates for the three- and four-component variants of the CCR (3CR and 4CR, respectively). Control experiments paired with in situ reaction monitoring with infrared spectroscopy for the 4CR align with a mechanism in which amide-acids derived from maleic anhydride can reversibly form free amine and cyclic anhydride. Although this equilibrium is unfavorable, the aldehyde present can trap the primary amine through imine formation and react with the enol form of the anhydride through a Mannich-like mechanism. This detailed mechanistic investigation coupled with additional crossover experiments supports an analogous mechanism for the 3CR and has led to the elucidation of new 3CR conditions with homophthalic anhydride, amines, and aldehydes for the formation of dihydroisoquinolones in good yields and excellent diastereoselectivity. This work represents the culmination of more than a decade of mechanistic speculation for the 3- and 4CR, enabling the design of new multicomponent reactions that exploit this novel mechanism.

Functionalized Biodegradable Polymers via Termination of Ring-Opening Polymerization by Acyl Chlorides

Aliphatic polyesters are an important class of polymeric materials for biomedical applications due to their versatile and tunable chemistry, biocompatibility and biodegradability. A capability of direct bonding with biomedically significant molecules, provided by the presence of the reactive end functional groups (FGs), is highly desirable for prospective polymers. Among FGs, N-hydroxysuccinimidyl activated ester group (NHS) and maleimide fragment (MI) provide efficient covalent bonding with -NH- and -SH containing compounds. In our study, we found that NHS- and MI-derived acyl chlorides efficiently terminate living ring-opening polymerization of ε-caprolactone, L-lactide, ethyl ethylene phosphonate and ethyl ethylene phosphate, catalyzed by 2,6-di-tert-butyl-4-methylphenoxy magnesium complex, with a formation of NHS- and MI-functionalized polymers at a high yields. Reactivity of these polymers towards amine- and thiol-containing model substrates in organic and aqueous media was also studied.

Enantiomeric Resolution and Absolute Configuration of a Chiral δ-Lactam, Useful Intermediate for the Synthesis of Bioactive Compounds

During the past several years, the frequency of discovery of new molecular entities based on γ- or δ-lactam scaffolds has increased continuously. Most of them are characterized by the presence of at least one chiral center. Herein, we present the preparation, isolation and the absolute configuration assignment of enantiomeric 2-(4-bromophenyl)-1-isobutyl-6-oxopiperidin-3-carboxylic acid (trans-1). For the preparation of racemic trans-1, the Castagnoli-Cushman reaction was employed. (Semi)-preparative enantioselective HPLC allowed to obtain enantiomerically pure trans-1 whose absolute configuration was assigned by X-ray diffractometry. Compound (+)-(2R,3R)-1 represents a reference compound for the configurational study of structurally related lactams.

Fluoral Hydrate: A Perspective Substrate for the Castagnoli-Cushman Reaction

The reactivity of azomethines based on trifluoroacetaldehyde hydrate in the Castagnoli-Cushman reaction (CCR) was researched. The impact of the nature of anhydrides explored on the reaction route was determined. The preparative procedures for the synthesis of N-substituted (3R*,4R*)-1-oxo-3-(trifluoromethyl)-1,2,3,4-tetrahydroisoquinoline-4-carboxylic and (1R*,2R*)-4-oxo-2-(trifluoromethyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepine-1-carboxylic acids in gram scales were elaborated. It was shown that the trifluoromethyl group remarkably decreased the reactivity of azomethines in CCR. The mechanism for the formation of different products depending on the anhydride's nature was proposed.

One-step route to tricyclic fused 1,2,3,4-tetrahydroisoquinoline systems via the Castagnoli-Cushman protocol

The Castagnoli-Cushman reaction of 3,4-dihydroisoquinolines with glutaric anhydride, its oxygen and sulfur analogues was investigated as a one-step approach to the benzo[a]quinolizidine system and its heterocyclic analogs. An extension towards the pyrrolo[2,1-a]isoquinoline system was achieved with the use of succinic anhydride. The results are evidence of an unexplored method for the access of the aforementioned tricyclic annelated systems incorporating a bridgehead nitrogen atom. The structures and relative configurations of the new compounds were established by means of 1D and 2D NMR techniques. The reactions between 1-methyldihydroisoquinoline and glutaric, diglycolic and succinic anhydrides yielded unexpected isoquinoline derivatives containing an exocyclic double bond. The compounds prepared bear the potential to become building blocks for future synthetic bioactive molecules.

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.

Further Insight into the Castagnoli-Cushman-type Synthesis of 1,4,6-Trisubstituted 1,6-Dihydropyridin-2-(3H)-ones from 3-Arylglutaconic Acid Anhydrides

The earlier reported three-component Castagnoli-Cushman-type synthesis of 1,4,6-trisubstituted 1,6-dihydropyridin-2-(3H)-ones from 3-arylglutaconic acids, primary amines, and aromatic aldehydes has been further investigated. It was shown to proceed via 3-arylglutaconic anhydrides, which, in turn, were found to give superior results in the two-component reactions with imines. The initial formation of the Castagnoli-Cushman carboxylic acids was shown to be the case, and their decarboxylation was found to follow a complex, "forked" pathway, which was confirmed by deuterium incorporation experiments.

Synthesis of morpholine derivatives using the Castagnoli-Cushman reaction as BACE1 inhibitors: Unexpected binding activity of cyclic thioamides

The Castagnoli-Cushman reaction between diglycolic anhydride and imines was applied for the synthesis of morpholine derivatives containing a thioamide or an amidino group. Enzyme inhibition assays towards BACE1 revealed an unexpected role of the cyclic thioamide group in providing inhibition in the micromolar range. Molecular docking calculations showed the thioamido group interacting with catalytic aspartic acid, and calculated BBB permeability indicated this molecular scaffold as a promising hit for further optimization.

Direct access to vicinally functionalized and N-trifluoroacetylated (bicyclic)ketopiperazines using a readily affordable N-heterocyclic anhydride

Functionalized 2-piperazinones play essential roles as conformationally-constrained peptidomimetics by mimicking inverse γ-turns in peptides. Here, we describe an efficient, scalable, stereoselective, modular, and chemoselective annulation protocol between a novel N-trifluoroacetyl anhydride and several reactive partners, including lactim ethers, imidoyl chlorides, aryl aldimines, and 1,3-azadienes, leading to vicinally functionalized (bicyclic) 2-piperazinones.

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

Reaction behaviour of arylamines with nitroalkenes in the presence of bismuth(iii) triflate: an easy access to 2,3-dialkylquinolines

We report the reaction behaviour of arylamines with nitroalkenes in the presence of bismuth(iii) triflate (10 mol%) and diacetoxyiodobenzene (10 mol%).

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.

Organocatalytic Mukaiyama Mannich Reactions of 2,5-Bis(trimethylsilyloxy)furan

The organocatalytic synthesis of densely substituted mono- and bis-γ-lactams involving the Mukaiyama Mannich addition of 2,5-bis(trimethylsilyloxy)furan to imines is described. Use of a ditoluenesulfonylimide catalyst produces γ-lactams from monoaddition, whereas a more acidic catalyst (triflic acid) produces fused bis-lactams from double addition. Optimized organocatalytic conditions allow for the selective synthesis of either desired core as well as the one-pot, multicomponent assembly of the trisubstituted monolactams from aldehydes, amines, and bis-trimethylsilyloxyfuran. An examination of chiral acids found these organocatalysts to be highly active and diastereoselective in the monoaddition reaction, albeit with no enantioselectivity.

Synthesis of DNA-coupled isoquinolones and pyrrolidines by solid phase ytterbium- and silver-mediated imine chemistry

DNA-encoded libraries of chemically synthesized compounds are an important small molecule screening technology. The synthesis of encoded compounds in solution is currently restricted to a few DNA-compatible and water-tolerant reactions. Encoded compound synthesis of short DNA-barcodes covalently connected to solid supports benefits from a broad range of choices of organic solvents. Here, we show that this encoded chemistry approach allows for the synthesis of DNA-coupled isoquinolones by an Yb(iii)-mediated Castagnoli-Cushman reaction under anhydrous reaction conditions and for the synthesis of highly substituted pyrrolidines by Ag(i)-mediated 1,3-dipolar azomethine ylide cycloaddition. An encoding scheme for these DNA-barcoded compounds based on a DNA hairpin is demonstrated.

Facile Access to Fe(III)-Complexing Cyclic Hydroxamic Acids in a Three-Component Format

Cyclic hydroxamic acids can be viewed as effective binders of soluble iron and can therefore be useful moieties for employing in compounds to treat iron overload disease. Alternatively, they are analogs of bacterial siderophores (iron-scavenging metabolites) and can find utility in designing antibiotic constructs for targeted delivery. An earlier described three-component variant of the Castagnoli—Cushman reaction of homophthalic acid (via in situ cyclodehydration to the respective anhydride) was extended to involve hydroxylamine in lieu of the amine component of the reaction. Using hydroxylamine acetate and O-benzylhydroxylamine was key to the success of this transformation due to greater solubility of the reagents in refluxing toluene (compared to hydrochloride salt). The developed protocol was found suitable for multigram-scale syntheses of N-hydroxy- and N-(benzyloxy)tetrahydroisoquinolonic acids. The cyclic hydroxamic acids synthesized in the newly developed format have been tested and shown to be efficient ligands for Fe³⁺, which makes them suitable candidates for the above-mentioned applications.

One-shot access to isoquinolone and (hetero)izidinone architectures using cyclic α-chloro eneformamides and cyclic anhydrides

A site-selective annulation reaction between cyclic α-chloro eneformamides and cyclic anhydrides which provides direct and modular access to pharmaceutically pertinent izidinones and isoquinolones, is presented.

Diversity-Oriented Synthesis and Chemoinformatic Analysis of the Molecular Diversity of sp3-Rich Morpholine Peptidomimetics

Diversity-Oriented Synthesis (DOS) consists of generating structurally diverse compounds from a complexity-generating reaction followed by cyclization steps and appendage diversity. DOS has gathered interest to systematically explore the chemical space by generating high-quality small-molecule collections as probes to investigate biological pathways. The generation of heterocycles using amino acid and sugar derivatives as building blocks is a powerful approach to access chemical and geometrical diversity thanks to the high number of stereocenters and the polyfunctionality of such compounds. Our efforts in this field are focused on the generation of diversity-oriented molecules of peptidomimetic nature as a tool addressing protein-protein interactions, taking advantage of amino acid- and sugar-derived polyfunctional building blocks to be applied in couple-pair synthetic approaches. In this paper, the combination of diversity-oriented synthesis and chemoinformatics analysis of chemical space and molecular diversity of heterocyclic peptidomimetics are reported, with particular interest toward carbohydrate- and amino acid-derived morpholine scaffolds with a higher fraction of sp3 carbon atoms. Also, the chemoinformatic analysis of chemical space and molecular diversity of 186 morpholine peptidomimetics is outlined.

Unusually Reactive Cyclic Anhydride Expands the Scope of the Castagnoli-Cushman Reaction

In the course of synthesizing and testing various "azole-including" cyclic anhydrides in the Castagnoli-Cushman reaction with imines, a remarkably reactive, pyrrole-based anhydride has been identified. It displayed a remarkably efficient reaction with N-alkyl and N-aryl imines, in particular, with "enolizable" α-C-H imines which typically fail to react with a majority of known cyclic anhydrides. The reactivity of this anhydride has been justified by an efficient resonance stabilization of its enol form. This finding expands the existing arsenal of highly reactive cyclic anhydrides and further confirms the importance of anhydride enolization for an efficient Castagnoli-Cushman reaction.

Bicyclic Piperazine Mimetics of the Peptide β-Turn Assembled via the Castagnoli-Cushman Reaction

5-Oxopiperazine-2-carboxamides and respective carboxylic acids (obtained by the Castagnoli-Cushman reaction of protected iminodiacetic anhydride) were converted into cis- and trans-configured bicyclic piperazines containing two stereogenic centers. The latter are not only well-established mimetics of peptide β-turn but also attractive, high-F_sp3 cores for drug design in general. The methodology was applied to the preparation of ring-expanded version of bicyclic piperazines not described in the literature.

Cyclic Hydroxamic Acid Analogues of Bacterial Siderophores as Iron-Complexing Agents prepared through the Castagnoli-Cushman Reaction of Unprotected Oximes

The first application of multicomponent chemistry (the Castagnoli-Cushman reaction) toward the convenient one-step preparation of cyclic hydroxamic acids is described. Cyclic hydroxamic acids are close analogues of bacterial siderophores (iron-binding compounds) and form stable complexes with Fe³⁺ ions as confirmed by spectrophotometric measurements. These compounds are potential components for the design of chelating agents for iron overload disease therapy, as well as siderophore-based carrier systems for antibiotic delivery across the bacterial cell wall.

Skeletal Diversity in Combinatorial Fashion: A New Format for the Castagnoli-Cushman Reaction

A new format for the Castagnoli-Cushman reaction of structurally diverse dicarboxylic acids, amines, and aldehydes in the presence of acetic anhydride as dehydrating agent is described. The reaction is distinctly amenable to parallel format: the combinatorial array of 180 reactions delivered 157 products of >85% purity without chromatographic purification (of this number, 143 compounds had >94% purity). The new method offers a convenient preparation of the skeletally and peripherally diverse, lead- and druglike γ- and δ-lactam carboxylic acids with high diastereoselectivity in combinatorial fashion.

A speedy route to sterically encumbered, benzene-fused derivatives of privileged, naturally occurring hexahydropyrrolo[1,2-b]isoquinoline

A series of 15 benzene-fused hexahydropyrrolo[1,2-b]isoquinolonic acids with substantial degree of steric encumbrance has been prepared via a novel variant of the Castagnoli–Cushman reaction of homophthalic anhydride (HPA) and various indolenines. The employment of a special kind of a cyclic imine component reaction allowed, for the first time, isolating a Mannich-type adduct between HPA and an imine component which has been postulated but never obtained in similar reactions.

Brønsted acid-mediated cyclization-dehydrosulfonylation/reduction sequences: An easy access to pyrazinoisoquinolines and pyridopyrazines

An efficient and alternative synthetic approach has been developed to prepare various N-(arylethyl)piperazine-2,6-diones from 4-benzenesulfonyliminodiacetic acid and primary amines using carbonyldiimidazole in the presence of a catalytic amount of DMAP at ambient temperature. Piperazine-2,6-diones are successfully transformed to pharmaceutically useful pyridopyrazines or pyrazinoisoquinolines and ene-diamides via an imide carbonyl group activation strategy using a Brønsted acid. Subsequent dehydrosulfonylation reactions of the ene-diamides, in a one pot manner, smoothly transformed them to substituted pyrazinones. A concise synthesis of praziquantel (1) has also been achieved through this method.

Synthesis of indolo[4,3-bc]phenanthridine-6,11(2H,12H)-diones using the Schiff base-homophthalic anhydride cyclization reaction

A novel indolophenanthridine ring system has been synthesized via the Schiff base-homophthalic anhydride cyclization followed by thionyl chloride-mediated dehydrogenation and intramolecular Friedel-Crafts acylation. This adds to the array of heterocyclic systems that are available through the cycloaddition reaction of imines with cyclic dicarboxylic acid anhydrides. The cytotoxicities of the indolophenanthridines were investigated in human cancer cell cultures, and the results documented significant antitumor activity in a variety of human cancer cell lines. This provides a new heterocyclic scaffold for anticancer drug design.