Practical Solid-Phase Parallel Synthesis of Δ5-2-Oxopiperazines via N-Acyliminium Ion Cyclization

Regioselective Cyclic Iminium Formation of Ugi Advanced Intermediates: Rapid Access to 3,4-Dihydropyrazin-2(1H)-ones and Other Diverse Nitrogen-Containing Heterocycles

Herein, advanced intermediates were synthesized through Ugi four-component reactions of isocyanides, aldehydes, masked amino aldehyde, and carboxylic acids, including N-protected amino acids. The presence of a masked aldehyde enabled acid-mediated deprotection and subsequent cyclization via the carbonyl carbon and the amide nitrogen. Utilizing N-protected amino acid as a carboxylic acid component, Ugi intermediates could be cyclized from two possible directions to target 3,4-dihydropyrazin-2(1H)-ones. Cyclization to the amino terminus (westbound) and to the carboxyl terminus (eastbound) was demonstrated. Deliberate selection of building blocks drove the reaction regioselectively and yielded diverse heterocycles containing a 3,4-dihydropyrazin-2(1H)-one core, pyrazin-2(1H)-one, and piperazin-2-one, as well as a tricyclic framework with a 3D architecture, 2,3-dihydro-2,6-methanobenzo[h][1,3,6]triazonine-4,7(1H,5H)-dione, from Ugi adducts under mild reaction conditions. The latter bridged heterocycle was achieved diastereoselectively. The reported chemistry represents diversity-oriented synthesis. One common Ugi advanced intermediate was, without isolation, rapidly transformed into various nitrogen-containing heterocycles.

Heterocycles as a Peptidomimetic Scaffold: Solid-Phase Synthesis Strategies

Peptidomimetics are a privileged class of pharmacophores that exhibit improved physicochemical and biological properties. Solid-phase synthesis is a powerful tool for gaining rapid access to libraries of molecules from small molecules to biopolymers and also is widely used for the synthesis of peptidomimetics. Small molecules including heterocycles serve as a core for hundreds of drugs, including peptidomimetic molecules. This review covers solid-phase synthesis strategies for peptidomimetics molecules based on heterocycles.

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Regioselective addition of Grignard reagents to N-acylpyrazinium salts: synthesis of substituted 1,2-dihydropyrazines and Δ5-2-oxopiperazines

The regioselective addition of Grignard reagents to mono- and disubstituted N-acylpyrazinium salts affording substituted 1,2-dihydropyrazines in modest to excellent yields (45–100%) is described. Under acidic conditions, these 1,2-dihydropyrazines can be converted to substituted Δ⁵-2-oxopiperazines providing a simple and efficient approach towards their preparation.

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 6-methyl-3,4-dihydropyrazinones using an Ugi 4-CR/allenamide cycloisomerization protocol

A two-step synthesis of 6-methyl-3,4-dihydropyrazinones from Ugi adducts and their transformation into pyrazino[2,1-a]isoindole and pyrazino[2,1-a]isoquinoline cores has been accomplished.

Synthesis and library construction of privileged tetra-substituted Δ5-2-oxopiperazine as β-turn structure mimetics

In this study, we developed an efficient and practical procedure for the synthesis of tetra-substituted Δ5-2-oxopiperazine that mimics the bioactive β-turn structural motif of proteins. This synthetic route is robust and modular enough to accommodate four different substituents to obtain a high level of molecular diversity without any deterioration in stereochemical enrichment of the natural and unnatural amino acids. Through the in silico studies, including a distance calculation of side chains and a conformational overlapping of our model compound with a native β-turn structure, we successfully demonstrated the conformational similarity of tetra-substituted Δ5-2-oxopiperazine to the β-turn motif. For the library construction in a high-throughput manner, the fluorous tag technology was adopted with the use of a solution-phase parallel synthesis platform. A 140-membered pilot library of tetra-substituted Δ5-2-oxopiperazines was achieved with an average purity of 90% without further purification.

Regioselective incorporation of backbone constraints compatible with traditional solid-phase peptide synthesis

A protected aldehyde was attached via a two-carbon spacer to a peptide backbone amide nitrogen during a traditional Merrifield solid-phase synthesis. Acid-mediated unmasking of the aldehyde triggered the regioselective formation of cyclic N-acyliminiums between the aldehyde and the neighboring peptide amide nitrogen. In the absence of an internal nucleophile, the cyclic iminiums formed dihydropyrazinones, a six-membered peptide backbone constraint between two peptide amides. In the presence of an internal nucleophile, tetrahydropyrazinopyrimidinediones or tetrahydroimidazopyrazinediones were formed via tandem N-acyliminium ion cyclization-nucleophilic addition. The outcome of this nucleophilic addition was dependent on the substituent on the nitrogen nucleophile.

The outcome of the oxidations of unusual enediamide motifs is governed by the stabilities of the intermediate iminium ions

We compare the results from the oxidation of two unusual "enediamide" motifs (3,4-dihydropyrazin-2(1H)-ones), where a double bond is flanked by two amides. In one case the oxidation led to a ring-opened product arising from the cleavage of the double bond, and in the other a rare cis-dioxygenated compound was obtained. Both products have been characterized by X-ray crystallography. The outcomes of the key reactions are rationalized based on calculated free energies of intermediates.

Solid-phase parallel synthesis of drug-like artificial 2H-benzopyran libraries

This review covers the construction of drug-like 2H-benzopyrans and related libraries using solid-phase parallel synthesis. In this context, the preparation of substituted benzopyrans such as mono-, di- and trisubstituted benzopyran derivatives and additional ring-fused benzopyrans such as benzopyranoisoxazoles, benzopyranopyrazoles, six-membered ring-fused benzopyrans, and polycyclic benzopyrans are highlighted.

Regioselective construction and screening of 1,3-disubstituted tetrahydroindazolones in enantiomerically pure pairs

In this paper, we describe a regioselective synthetic pathway for enantiopure 1,3-disubstituted tetrahydroindazolone derivatives via the condensation of 2-acylcyclohexane-1,3-dione with various alkyl- and arylhydrazines using the steric effects of a Boc-protected pyrrolidine ring. This synthetic method has a broad scope for substrate generality for various hydrazines with excellent regioselectivity. To maximize the molecular diversity, further diversifications of 1,3-disubstituted tetrahydroindazolones were pursued by systematic N-modification of the secondary amine of the pyrrolidine ring using solution-phase parallel synthesis with polymer-supported reagents. A library containing a total of 272 drug-like tetrahydroindazolones, including 85 enantiomeric pairs, was constructed; the average purity, without further purification, was 95%.

Solid-phase parallel synthesis of a tetrahydroindazolone library containing three unique core skeletons

We have developed a practical strategy for the regioselective synthesis of a 1-(hetero)aryl-3-substituted tetrahydroindazolone library. The condensation of in situ generated arylhydrazine on solid supports with 2-acylcyclohexane-1,3-diones ensured the efficiency of solid-phase parallel synthesis. In addition, we introduced three unique core skeletons containing nitrophenyl, anilyl, and pyridyl groups to maximize the molecular diversity through a diverse display of polar surface area in 3D chemical space. A 162-membered drug-like tetrahydroindazolone library was constructed in an average purity of 92% without further purification.

A synthetic route to highly substituted 1,2,3,4-tetrahydroisoquinolines via Yb(OTf)3 -catalyzed diastereoselective ring opening of bridged oxazolidines: asymmetric synthesis of 2-azapodophyllotoxin

We herein report a robust and efficient synthetic route to highly functionalized enantiopure 1,2,3,4-tetrahydroisoquinolines (THIQs) from Garner aldehyde. We utilized the inherent chirality of Garner aldehyde through 1,2- and 1,3-/1,4-asymmetric inductions iteratively to obtain 1,2,3,4-tetrasubstitued THIQs using rigid and isolable bridged oxazolidines without any external chiral sources. All possible stereoisomers of bridged oxazoliodines were efficiently synthesized from L- and D-Garner aldehydes and transformed into fully functionalized THIQs via diastereoselective ring opening with various nucleophiles in the presence of Yb(OTf)(3). This methodology furnished four out of eight possible diastereomers of 1,2,3,4-tetrasubstituted THIQs despite the electronic nature of substituents on the aryl rings. Finally, the enantioselective synthesis of 2-azapodophyllotoxin was achieved with an overall yield of 35.4% (eight steps) from D-Garner aldehyde using this synthetic route.

Discovery of novel benzopyranyl tetracycles that act as inhibitors of osteoclastogenesis induced by receptor activator of NF-κB ligand

A novel benzopyran-fused molecular framework 7ai was discovered as a specific inhibitor of RANKL-induced osteoclastogenesis using a cell-based TRAP activity assay from drug-like small-molecule libraries constructed by diversity-oriented synthesis. Its inhibitory activity was confirmed by in vitro evaluations including specific inhibition of RANKL-induced ERK phosphorylation and NF-κB transcriptional activation. 7ai can serve as a specific small-molecule modulator for mechanistic studies of RANKL-induced osteoclast differentiation as well as a potential lead for the development of antiresorptive drugs.

N-acyliminium intermediates in solid-phase synthesis

N-Acyliminium ions are powerful intermediates in synthetic organic chemistry. Examples of their use are numerous in solution-phase synthesis, but there are unmerited few reports on these highly reactive electrophiles in solid-phase synthesis. The present review covers the literature to date and illustrates the methods used to generate N-acyliminium intermediates on solid support and their further elaboration to a range of pharmacologically interesting peptidomimetics, heterocycles, and other small molecules.

Novel application of Leuckart–Wallach reaction for synthesis of tetrahydro-1,4-benzodiazepin-5-ones library

A novel and efficient strategy has been developed to synthesize privileged tetrahydro-1,4-benzodiazepines with excellent yields and purities; this synthetic pathway was established by the revitalization of the Leuckart-Wallach (LW) reaction via solid-phase synthesis.