Solid-Phase Synthesis of Benzopiperazinones

Asymmetric Transfer Hydrogenation of 2-Substituted Quinoxalines with Regenerable Dihydrophenanthridine

The asymmetric hydrogenation of quinoxalines represents one of the most efficient approaches for the synthesis of optically active tetrahyroquinoxalines. In this paper, we demonstrate a metal-free asymmetric transfer hydrogenation of 2-substituted quinoxalines with regenerable dihydrophenanthridine under H2 using a combination of chiral phosphoric acid and achiral borane as catalysts. A wide range of optically active 2-substituted tetrahydroquinoxalines were produced in high yields with ≤98% ee.

Novel Approaches for the Solid-Phase Synthesis of Dihydroquinazoline-2(1H)-One Derivatives and Biological Evaluation as Potential Anticancer Agents

In the design of antineoplastic drugs, quinazolinone derivatives are often used as small molecule inhibitors for kinases or receptor kinases, such as the EGFR tyrosine kinase inhibitor gefitinib, p38MAP kinase inhibitor DQO-501, and BRD4 protein inhibitor PFI-1. A novel and convenient approach for the solid-phase synthesis of dihydroquinazoline-2(1H)-one derivatives was proposed and 19 different compounds were synthesized. Cytotoxicity tests showed that most of the target compounds had anti-proliferative activity against HepG-2, A2780 and MDA-MB-231 cell lines. Among them, compounds CA1-e and CA1-g had the most potent effect on A2780 cells, with IC₅₀ values of 22.76 and 22.94 μM, respectively. In addition, in an antioxidant assay, the IC₅₀ of CA1-7 was 57.99 μM. According to bioinformatics prediction, ERBB2, SRC, TNF receptor, and AKT1 were predicted to be the key targets and play an essential role in cancer treatment. ADMET prediction suggested 14 of the 19 compounds had good pharmacological properties, i.e., these compounds displayed clinical potential. The correct structure of the final compounds was confirmed based on LC/MS, ¹H NMR, and ¹³C NMR.

Enantioselective synthesis of trifluoromethylated dihydroquinoxalinones via palladium-catalyzed hydrogenation

A highly enantioselective palladium-catalyzed asymmetric hydrogenation of 3-(trifluoromethyl)quinoxalinones has been successfully developed, providing a general and facile access to chiral 3-(trifluoromethyl)-3,4-dihydroquinoxalinones with up to 99% ee.

Solid Phase Synthesis of Helically Folded Aromatic Oligoamides

Aromatic amide foldamers constitute a growing class of oligomers that adopt remarkably stable folded conformations. The folded structures possess largely predictable shapes and open the way toward the design of synthetic mimics of proteins. Important examples of aromatic amide foldamers include oligomers of 7- or 8-amino-2-quinoline carboxylic acid that have been shown to exist predominantly as well-defined helices, including when they are combined with α-amino acids to which they may impose their folding behavior. To rapidly iterate their synthesis, solid phase synthesis (SPS) protocols have been developed and optimized for overcoming synthetic difficulties inherent to these backbones such as low nucleophilicity of amine groups on electron poor aromatic rings and a strong propensity of even short sequences to fold on the solid phase during synthesis. For example, acid chloride activation and the use of microwaves are required to bring coupling at aromatic amines to completion. Here, we report detailed SPS protocols for the rapid production of: (1) oligomers of 8-amino-2-quinolinecarboxylic acid; (2) oligomers containing 7-amino-8-fluoro-2-quinolinecarboxylic acid; and (3) heteromeric oligomers of 8-amino-2-quinolinecarboxylic acid and α-amino acids. SPS brings the advantage to quickly produce sequences having varied main chain or side chain components without having to purify multiple intermediates as in solution phase synthesis. With these protocols, an octamer could easily be synthesized and purified within one to two weeks from Fmoc protected amino acid monomer precursors.

Design for carbon–carbon bond forming reactions under ambient conditions

The carbon–carbon (C–C) bond forms the ‘backbone’ of nearly every organic molecule, and lies at the heart of the chemical sciences! Let us explore designing of carbon–carbon frameworks at ambient conditions.

3-Alkyl-3-(alkylamino)indolin-2-ones via Base-Mediated C-Arylation of 2-Nitrobenzenesulfonamides

Resin-bound intermediates prepared from polymer-supported amino acid esters, 2-nitrobenezenesulfonyl chlorides, and alcohols were used to synthesize 3-alkyl-3-(alkylamino) indolin-2-ones. The key step of the reaction sequence was the formation of a quaternary carbon via the base-mediated C-arylation of 2-nitrobenzenesulfonamides. The cleavage of the acyclic precursors from the resin and subsequent reduction of the nitro group by Zn in acetic acid triggered the spontaneous cyclization of the arylated compounds to indolinones. The synthesis was carried out using simple commercially available building blocks under mild conditions and provided the 3,3-disubstituted indolinone derivatives with good overall yields however, the arylation reaction resulted in the epimerization of the quaternary carbon.

Microwave-Assisted Multicomponent Synthesis of Dihydroquinoxalinones on Soluble Polymer Support

A one-pot and two-step reaction of four components, including aldehydes, Fmoc-protected α-amino acid, isocyanide, and soluble polymer-supported 4-fluoro-3-amino benzoate ester, was developed for an efficient synthesis of dihydroquinoxalinones under microwave irradiation. Fmoc deprotection followed by intramolecular cyclization of the diamide gave a facile access to novel bicyclic quinoxalin-2-ones. On the basis of this approach, a new route to synthesize this privileged scaffold from the diamide intermediate was designed and accomplished with high yields. Use of multicomponent reaction (MCR) has been shown to display a good functional group tolerance, while the product isolation and purification is straightforward by precipitation and washings. Current library discusses the synthesis and diversification of 21 compounds produced using this strategy.

Polymer-supported stereoselective synthesis of tetrahydrobenzopyrazino-thiadiazinone dioxides via N-sulfonyl iminiums

The stereoselective synthesis of 1,2,11,11a-tetrahydrobenzo[e]pyrazino[1,2-b][1,2,4]thiadiazin-3(4H)-one 6,6-dioxides on a solid support via tandem N-sulfonyl iminium ion cyclization, followed by nucleophilic addition is reported. The synthesis proceeded with full control of stereoselectivity at the newly formed asymmetric carbon, under mild conditions, and using commercially available building blocks. The synthetic route provided high-purity crude products.

Solid-phase synthesis of diverse spiroisoxazolinodiketopiperazines

A convenient, efficient protocol to prepare diverse spiroisoxazolino-diketopiperazines via a parallel solid-supported synthesis was developed. The key steps are (1) a coupling reaction of an amino acid; (2) tosylation with concomitant β-elimination to form an α, β-unsaturated ester; (3) a 1,3-dipolar cycloaddition with an oxime to form isoxazoline rings; and (4) cyclic cleavage to release the product from the resin. All reaction steps and workup procedures were modified to allow the use of automated or semiautomated equipment. A 100-member demonstration library with two diversity sites was prepared in good purity and acceptable overall yields.

Piperazine amide linker for cyclative cleavage from solid support: traceless synthesis of dihydroquinoxalin-2-ones

A piperazine amide linker for cyclative cleavage from solid support and its use in the traceless solid-phase synthesis of dihydroquinoxalinones are described. Piperazine was attached to Wang resin via a carbamate linkage and acylated with Fmoc-amino acids. Following Fmoc group removal, resin-bound amines were reacted with 1-fluoro-2-nitrobenzenes. The nitro group of the resulting 2-nitroanilines was reduced, and acyclic precursors, in contrast to traditional ester-type linkage, remained attached to the resin. Target dihydroquinoxalinones were obtained either by acid- or microwave-mediated cyclative cleavage. The synthesis provided crude compounds of high purity and enabled the preparation of stable immobilized linear intermediates. The linker is suitable for combinatorial synthesis of compound libraries.

Enantioselective synthesis of benzimidazolyl quinoxalinones on soluble polymer support using focused microwave irradiation

Focused microwave irradiation has been applied to a multistep synthetic sequence of reactions designed to generate benzimidazolyl quinoxalinones using a soluble polymer support. They were obtained by the ipso-fluoro (S(N)Ar) displacement of the immobilized ortho-nitro fluoro benzimidazoles with chiral alpha amino esters under microwave irradiation. Intermediate chiral organic-polymer conjugates when subjected to neutral reduction underwent a spontaneous intramolecular ring closure. Cleavage of the polymer support, at room temperature, did not cause any significant racemization resulting in the generation of a chiral molecular library with two points of structural diversity.

Design and synthesis of some biologically interesting natural and unnatural products based on organosulfur and selenium chemistry

Organosulfur and selenium chemistry has provided fertile ground for the discovery of novel synthetic methodology and for the design of bioactive molecules with potential therapeutic applications. Thus, acetylenic sulfones have been employed in novel strategies for the synthesis of nitrogen heterocycles, including several biologically active alkaloids. The conjugate addition of nitrogen nucleophiles containing ester or chloroalkyl substituents to acetylenic sulfones was followed by base-mediated intramolecular alkylation or acylation to afford variously substituted piperidines, pyrrolizidines, indolizidines, quinolizidines, decahydroquinolines, and 4-quinolones. The products include the dendrobatid alkaloids (–)-pumiliotoxin C, indolizidines (–)-167B, 207A, 209B, and 209D, as well as (–)-(ent)-julifloridine, (–)-lasubine II, myrtine, and two recently discovered alkaloids from the medicinal plant Ruta chalepensis , which had not been previously synthesized. Acetylenic sulfones were also incorporated on solid supports and employed in the types of cyclizations mentioned above, as well as for Diels–Alder reactions and a large variety of 1,3-dipolar cycloadditions. Conjugate additions of tertiary cyclic α-vinyl amines to acetylenic sulfones generated zwitterions that underwent exceptionally facile formal aza-Cope rearrangements to afford ring-expanded macrocyclic amines. An iterative version was developed and used in the synthesis of motuporamine A and B. With respect to organoselenium chemistry, two classes of compounds are described that function as novel mimetics of the selenoenzyme glutathione peroxidase (GPx), which protects cells from oxidative stress caused by the formation of peroxides during aerobic metabolism. They include cyclic seleninates and spirodioxyselenuranes, both of which efficiently catalyze the reduction of peroxides with thiols and are of potential value in the mitigation of oxidative stress. Their aromatic derivatives are generally less effective catalysts, but substituent effects can be used to modulate their activities. The mechanism of their catalytic cycles has been elucidated and Hammett plots indicate that the oxidation of Se(II) to Se(IV) is the rate-determining step for both classes. A methoxy-substituted aromatic spirodioxyselenurane provided the fastest rate for a small-molecule selenium compound that we have observed to date for the reduction of hydrogen peroxide with benzyl thiol.

Microwave-Assisted Synthesis of Tetracyclic 2,5-Diketopiperazines on a Soluble Polymer Support: A Structural Analogue of Tadalafil

Structural analogues of tadalafil that contain two diversity points have been synthesized from a soluble polymer support employing a Pictet–Spengler reaction using focussed microwave irradiation. Polymer-bound deprotected tryptophan reacts with various aldehydes to generate the tetrahydro-β-carbolines on the support. Subsequently immobilized tetrahydro-β-carboline underwent a highly efficient intramolecular N-heterocyclization in a traceless fashion from various in-situ generated α-alkyl and heteroalkyl amides in two steps to generate tetracyclic 2,5-diketopiperazines in high purity. All the compounds were isolated as cis and trans isomers with good yields.

1,3-Dipolar cycloadditions of acetylenic sulfones in solution and on solid supports

Several representative acetylenic sulfones were immobilized on a polymer support derived from Merrifield resin by means of ester linkers that were used to couple free carboxylic acid groups on the solid support with benzylic hydroxyl functions on the arylsulfonyl moieties of the acetylenes. Several examples of reversed ester linkers, using Merrifield resin directly, were also successfully prepared. The 1,3-dipolar cycloadditions of the solid-supported acetylenic sulfones were investigated with a series of 1,3-dipoles, including benzyl azide, ethyl diazoacetate, diazomethane, as well as representative nitrile oxides, nitrile imines, nitrile ylides, nitrones, azomethine imines, azomethine ylides, munchnones, and sydnones. In general, analogous cycloadditions were also performed with acetylenic sulfones in solution phase for comparison. The cycloadditions typically afforded good to excellent yields of the desired products in both solution and solid phase, although the latter reactions sometimes required more vigorous conditions. Except in the case of benzyl azide and diazo compounds, where mixtures of regioisomers were obtained, the other 1,3-dipoles reacted with high regioselectivity and afforded essentially unique regioisomers. Cleavage of the products from the resin was smoothly effected by alkaline hydrolysis, while several attempts at reductive desulfonylation with sodium amalgam or samarium diiodide-HMPA resulted in N-O or C-O scission, in addition to cleavage from the polymer. The method provides access to a number of important classes of heterocycles, including variously substituted and functionalized triazoles, pyrazoles, 1,2-oxazoles, pyrroles, as well as their dihydro and bicyclic analogues. The success of the cycloadditions on polymer supports paves the way to future investigations of sequential transformations leading to libraries of useful heterocycles.

Evolution of natural product scaffolds by acyl- and arylnitroso hetero-diels-alder reactions: new chemistry on piperine

Piperine, a natural product containing a conjugated diene, was reacted with polymer-supported acyl- and arylnitroso dienophiles. The reactions with arylnitroso dienophiles were also carried out in solution. The oxazine rings formed by the corresponding hetero-Diels-Alder reactions were further transformed and novel acyclic as well as heterocyclic derivatives including pyrroles and quinoxalinones were prepared.

Cyclizations and cycloadditions of acetylenic sulfones on solid supports

Acetylenic sulfones attached to solid supports by means of ester linkers were employed in a variety of cyclization and cycloaddition reactions, followed by cleavage of the products from the resin by ester hydrolysis or reductive desulfonylation.

Synthesis of silicon traceless linker for solid-phase reaction

The silicon linker is the foremost traceless linker used in solid-phase reactions. Hydrogen fluoride (HF) or trifluoroacetic aicd (TFA) can remove the silicon linker with the silicon atom being replaced by a hydrogen atom. In this experiment, the linkers 1c and 2d, which are the most useful in solid-phase reactions, were synthesized. Linker 1c is composed of seven linearly linked carbons and linker 2d includes an oxygen atom in the linear carbon chain to increase the solvation capacity. The carboxylic acid component of linker 1c and 2d forms an amide or ester bond with resin. The synthesized linkers 1c and 2d could be utilized in constructing a chemical compound library that includes indole, benzodiazepine and phenothiazine (aromatic ring compounds).

Histogranin-Like Antinociceptive and Anti-Inflammatory Derivatives of o-Phenylenediamine and Benzimidazole

Histogranin (HN)-like nonpeptides were designed and synthesized using benzimidazole (compound 1) and o-phenylenediamine (compounds 2-7) as scaffolds for the attachment of phenolic hydroxyl and basic guanidino pharmacophoric elements present in HN. The benzimidazole derivative N-5-guanidinopentanamide-(2R)-yl-2-(p-hydroxybenzyl)-5-carboxybenzimidazole (1) and the o-phenylenediamine derivative N-5-guanidinopentanamide-(2S)-yl-2-N-(p-hydroxyphenylacetyl) phenylenediamine (2) were more potent analgesics than HN in both the mouse writhing (5.5 and 3.5 as potent as HN, respectively) and tail-flick (11.8 and 8.0 as potent as HN, respectively) pain assays. Improvements in the potencies and times of action of compound 2 in the mouse writhing test were obtained by attaching carboxyl (6)or p-Cl-benzoyl (7) groups at position 4 of the (2R) o-phenylenediamine derivative (5). In rats, compounds 2 (80 nmol i.t.), 6 (36 nmol i.t.), and 7 (18 nmol i.t.) were effective in blocking both persistent inflammatory pain in the formalin test and hyperalgesia in the complete Freund adjuvant assay. Compounds 2, 6, and 7, but not compound 1 at 10 nmol (i.c.v.) also mimicked the HN (60 nmol i.c.v.) blockade of N-methyl-D-aspartate (NMDA)-induced convulsions in mice. Finally, in primary cultures of rat alveolar macrophages, HN and compounds 1, 2, 6, and 7 (10(-8) M) significantly blocked lipopolysaccharide-induced cyclooxygenase-2 induction and prostaglandin E(2) secretion. These studies indicate that both derivatives of benzimidazole and o-phenylenediamine mimic the in vivo antinociceptive and in vitro anti-inflammatory effects of HN, but the HN protection of mice against NMDA-induced convulsions is mimicked only by the o-phenylenediamine derivatives.

The recent impact of solid-phase synthesis on medicinally relevant benzoannelated nitrogen heterocycles

Benzoannelated heterocycles such as benzodiazepines and indoles can be prepared efficiently through cyclization on solid supports, although no single approach is currently universal for the preparation of all benzoannelated N-heterocycle chemistries. In this review, a number of synthetic strategies for the generation of benzoannelated nitrogen heterocycles using resin-bound substrates have been described. Classical heterocycle forming reactions such as the Fischer indole, the Bischler-Napieralski tetrahydroisoquinoline, the Pictet-Spengler tetrahydro-beta-carboline, the Tsuge, the Nenitzescu and the Richter cinnoline reaction are presented. In addition, the Heck, Sonogashira, Wittig, Diels-Alder, and olefin metathesis reactions have been also used. Multicomponent reactions such as the Grieco three-component assembly have been exploited for the synthesis of heterocycles. Cyclative cleavage from the solid support is particularly suitable for the synthesis of heterocycles while particular emphasis has been focused on the synthesis of libraries and the use of combinatorial chemistry techniques. In addition, the most relevant pharmacological properties of benzoannelated nitrogen heterocycles are included.

Liquid-phase combinatorial synthesis of aminobenzimidazoles

Liquid-phase synthesis of 2-arylaminobenzimidazoles using soluble polymer support strategy is first described. The key step to benzimidazole skeleton is achieved in the presence of diisopropylcarbodimide (DICDI) and isothiocyanates. A mechanism study for one-pot cyclodesulfurization is also investigated on the support. A wide range of benzimidazole derivatives is synthesized in excellent yield and purity just by simple wash and precipitation.