The synthetic versatility of alkoxycarbonyl- and hydroxymethyl-piperazine-2,5-diones

Alkoxycarbonylpiperazine-2,5-diones are versatile precursors for the α-functionalisation of piperazine-2,5-diones. The alkoxycarbonyl group activates the α-carbon position to alkylation reactions and this provides a mild and selective method for the extension of the carbon framework of piperazine-2,5-diones. In addition, the alkoxycarbonyl group can be converted to the carboxy group, which in turn can be ‘deleted’ or manipulated for the installation of carbon and/or heteroatom substituents where desired, the latter via N-acyliminium chemistry. We also demonstrate that hydroxymethylpiperazine-2,5-diones complement carboxypiperazinediones as precursors for the generation of N-acyliminium ions.     

One-step diketopiperazine synthesis using phase transfer catalysis

A simple and efficient one-step procedure is described for the synthesis of a number of symmetrical 1,4-disubstituted piperazine-2,5-diones under phase transfer conditions. The reactions are carried out at room temperature, starting from a suitable N-chloroacetamide in the presence of an aqueous solution of sodium hydroxide. Piperazine-2,5-diones were obtained with excellent selectivity in yields of up to 90%.     

Condensation of 1,4-diacetylpiperazine-2,5-dione with aldehydes

Condensation of 1,4-diacetylpiperazine-2,5-dione with aldehydes has been applied to the synthesis of albonoursin and unsymmetrical 3,6-diarylidenepiperazine-2,5-diones. The reaction has been extended to 1,4-diacetyl-3,6-dimethylpiperazine-2,5-dione, which gives derivatives of 2-methyl-3- phenylserine. The mechanism and stereochemistry are discussed; cis 1-acetyl-3-isobutylidene- piperazine-2,5-dione has been isolated.     

Liquid-Crystalline Properties of Unsaturated Piperazine-2,5-dione Derivatives

The synthesis, characterization, and mesomorphic properties of a new type of liquid-crystalline compounds, (3Z,6Z)-3,6-bis(3,4-dialkoxybenzylidene)piperazine-2,5-diones are reported. These compounds were derived from unsaturated piperazine-2,5-dione as the core group, and were prepared by condensation reactions of 1,4-diacetylpiperazine-2,5-dione and 3,4-dialkoxybenzaldehydes. The products were characterized by ¹H- and ¹³C-NMR spectroscopy, and elemental analysis, and the phase behavior of these compounds was characterized and studied by differential scanning calorimetry (DSC) and polarization microscopy. The results indicate that these rod-like compounds exhibit smectic C (S_c) phases. However, for the derivatives with two flexible alkoxy side chains, highly ordered smectic G (S_G) phases were also formed and confirmed by X-ray powder diffraction. The liquid crystallinity of these molecules was attributed to the presence of intermolecular hydrogen bonds involving the NH groups of the heterocyclic rings. The correlation of phase behavior and molecular shape is also discussed.     

Efficient organocatalytic α-sulfenylation of substituted piperazine-2,5-diones

Organocatalytic α-sulfenylation of substituted piperazine-2,5-diones is reported through the use of cinchona alkaloids as Lewis bases and electrophilic sulfur transfer reagents. 1-Phenylsulfanyl[1,2,4]triazole, a novel sulfur transfer reagent, gave excellent product yields with a number of substituted piperazine-2,5-diones under mild conditions. Catalyst loading, stoichiometry of sulfur electrophile, temperature, and solvent were optimized to achieve high product yields.     

Intermolecular and intramolecular Diels-Alder cycloadditions of 3-ylidenepiperazine-2,5-diones and 5-acyloxy-2(1h)-pyrazinones

The 3-ylidenepiperazine-2,5-diones 16 and 39 and 5-acyloxy-2(1H)-pyrazinones 17 can serve as starting materials for the Diels-Alder reactions of alkenes and alkynes to the piperazine ring, under acidic conditions or in the presence of acetyl chloride, to afford tricyclic piperazine-2,5-diones 19, 20, 23-25, 27, 44, and 45. Intramolecular cycloadditions occur if 3-ylidenepiperazine-2,5-diones 30 and 32 are used as the starting materials. This procedure is a convenient path to bridged bicyclo[2.2.2]diazaoctane ring systems such as 31 and 33, the former being found in biologically active secondary mold metabolites, such as VM55599 (1) or brevianamide A (5), which have been isolated from various fungi. The synthesis of the indole compound 31 provided evidence for the proposed biochemical pathway with a Diels-Alder reaction as key step. Quantum chemical calculations have revealed that piperazinones with a cationic azadiene moiety are the most reactive species in Diels-Alder cycloadditions.     

Photochromic fluorescent indol-3-yl-substituted maleimides

New hetarylethenes, 3-(indol-3-yl)-4-thienyl(but-1-en-1-yl)-substituted pyrrole-2,5-diones containing coumarin or fluorene substituents on the pyrrole nitrogen atom, were synthesized by reactions of furan-2,5-diones with 9H-fluoren-2-amine and 6-amino-2H-chromen-2-one. Acyclic maleimide isomers showed fluorescence with quantum yields of 0.002 to 0.072. Their irradiation with UV light generates non-fluorescing cyclic isomer. The reverse ring opening occurs in the excited state.     

New highlights in the synthesis and reactivity of 1,4-dihydropyrazine derivatives

In the course of our investigations on the synthesis of original nitrogen heterocyclic derivatives, we were interested in the synthesis and study of original 1,4-dihydropyrazine rings. To this aim the desired bisvinylphosphate derivative was prepared from N-Boc piperazine-2,5-dione and then was engaged in palladium catalyzed reactions (reduction, Suzuki and Stille cross-coupling reactions). The 1,4-dihydropyrazine and the corresponding 2,5-disubstituted derivatives were obtained in fair to good yields and were then functionalized under anionic conditions. Aromatization into 1,4-pyrazines was investigated in a second stage.     

Highly diastereoselective desymmetrizations of cyclo(Pro,Pro): an enantioselective strategy toward phakellstatin and phakellin

[reaction: see text] Monoenolates of C(2)-symmetric, proline-derived piperazine-2,5-diones were generated and trapped with a variety of electrophiles to produce, in a highly diastereoselective fashion, functionalized diketopiperazines (DKPs). These reactions provide the basis for an asymmetric, desymmetrization strategy toward the marine alkaloids phakellstatin and phakellin. The relative stereochemistry of the functionalized DKPs was confirmed by single-crystal X-ray analysis and/or NOE experiments. Bis-functionalization of the DKPs was also found to proceed with high levels of diastereoselectivity.     

Synthesis and structure of novel (S)-1,6-dialkylpiperazine-2,5-diones and (3S,6S)-1,3,6-trialkylpiperazine-2,5-diones

Eleven (S)-1,6-dialkylpiperazine-2,5-diones and five (3S,6S)-1,3,6-trialkylpiperazine-2,5-diones were prepared in three steps from the corresponding (S)-α-amino acid esters comprising of reductive N-alkylation, N-acylation and cyclisation. The synthesis of (S)-1,6-dialkylpiperazine-2,5-diones has a broad scope allowing preparation of diketopiperazines with primary and secondary alkyl groups at N(1), while the synthesis of (3S,6S)-1,3,6-trialkylpiperazine-2,5-diones is limited to compounds with primary alkyl groups at N(1). Reductive alkylation of amino acid ester hydrochlorides by catalytic hydrogenation in the presence of a carbonyl compound proved to be a simple, efficient and general method for the preparation of stable (storable) α-alkylamino acid ester hydrochlorides. The structures of the novel compounds were determined by NMR and X-ray diffraction.     

Pictet Spengler-type reactions in 3-arylmethylpiperazine-2,5-diones. Synthesis of pyrazinotetrahydroisoquinolines

The behavior of aldehydes and acetals as N-alkylating agents of 1-acetyl-3-arylmethylpiperazine-2,5-diones and the subsequent cyclization of the N-alkylated products was studied. Use of paraformaldehyde in different reaction conditions gave 6-unsubstituted 3,6,11,11a-tetrahydro-2H-pyrazino[1,2-b]isoquinoline-1,4-diones and, in some cases, benzo[f]pyrazino[1,2-c]1,3-oxazepine-1,4-diones. Succesful reactions with benzaldehyde required a first activation of the lactam function and a catalyzed N-alkylation with the aldehyde dimethyl acetal. The isolated O,N-amidoacetals thus obtained were submitted to a diastereoselective Pictet Spengler-type reaction that worked with arenes at several degrees of ring activation and with thiophene to give 6-phenylpyrazinoisoquinolinediones and the corresponding thieno analog.     

A convenient procedure for the synthesis of chiral 6,7-dihydroxy-1-phenylamino-dihydro-1H-pyrrolo[1,2-a]imidazole-2,5(3H,6H)-diones

The cyclocondensation reactions between l-α-amino acid phenylhydrazides and 2,3-O-isopropylidene-l-erythruronolactone in the presence of a catalytic amount of p-toluenesulfonic acid afforded diastereomerically pure (3S,6R,7R,7aS)-3-substituted-6,7-isopropylidenedioxy-1-phenylamino-dihydro-1H-pyrrolo[1,2-a]imidazole-2,5(3H,6H)-diones, which were converted by acidic hydrolysis with MeOH–HCl into their corresponding optically active (3S,6R,7R,7aS)-3-substituted-6,7-dihydroxy-1-phenylamino-dihydro-1H-pyrrolo[1,2-a]imidazole-2,5(3H,6H)-diones in good yields.     

Synthetic approaches to bicyclomycin I. Prepration of monocylic intermediates by retrograde Michael cleavage of 6-alkyl-6-methoxyhexahydro-3H-thiazolo [3,4-a]pyrazine-5,8-diones.

The syntheses of N-protected-3-(hydroxypropyl)-3-methoxy-6-alkylidine piperazine-2,5-diones are described, in relation to an approach to bicyclomycin. A chemoselective Grignard reaction and a novel diastereoselective retrograde Michael cleavage highlight the sequence.     

New and efficient dehalogenative coupling and reduction of α-haloketones with active nickel complex. Facile syntheses of 1,4-diketones and bicyclo[4.2.O]oct-3-ene-2,5-diones

The active nickel complex generated in situ by reduction of NiBr₂(PPh₃)₂ with zinc in the presence of Et₄NI is a useful reagent for the dehalogenative coupling of phenacyl halides to 1,4-diaryl-1,4-diketones and for the dechlorination of 3,4-dichlorobicyclo[4.2.0]-octane-2,5-diones to bicyclo[4.2.0]oct-3-ene-2,5-diones.     

Novel asymmetric dihetarylethenes derived from N-isopropylindole and thiophene: synthesis and photochromic properties

Novel asymmetric dihetarylethenes, viz., 3-(1-isopropyl-5-methoxy-2-methyl-1H-indol-3-yl)-4-(3-thienyl)furan-2,5-dione and 1-alkyl/aryl-3-(1-isopropyl-5-methoxy-2-methyl-1H-indol-3-yl)-4-(3-thienyl)-1H-pyrrole-2,5-diones, were obtained. These dihetarylethenes exhibit photochromism in solutions. Replacement of the N-methyl group in the indole fragment by the N-isopropyl group imparts photochromic properties to the resulting furan-2,5-dione derivative. The open forms of (N-isopropylindol-3-yl)pyrrole-2,5-diones are characterized by lower quantum yields of fluorescence, and their cyclic forms are thermally more stable.     

THE PHOTOCHEMISTRY OF PIPERAZINE-2,5-DIONE IN AQUEOUS SOLUTION

Abstract The aqueous photochemistry of deoxygenated solutions of piperazine-2,5-dione was investigated and three products were isolated in low yield using preparative high-performance liquid chromatography: 3,3'-bis(piperazine-2,5-dione) ( 1 ), oxamide and N-methyloxamide. The structure of 1 was established by infrared, mass and high-field proton magnetic resonance spectroscopy, and by the synthesis of its meso diastereoisomer. A 1:2 mixture of meso and racemic diastereoisomers of 1 was formed in good yield in thermally or photochemically initiated reactions of piperazine-2,5-dione with t -butyl peroxide in aqueous t -butanol.
The rate of photolysis of aqueous piperazine-2,5-dione was markedly enhanced in the presence of iron(II) ions, copper(II) ions or powdered titanium dioxide. A diastereomeric mixture of 1, oxamide and N-methyloxamide were isolated from the reaction with iron(II) ions. The possibility that oxamides were formed in these reactions via reaction with atmospheric oxygen was eliminated in a series of control experiments.
The results suggest α-carbon radicals are important in the photolysis of peptides in fluid aqueous media, possibly as a consequence of photoinduced electron transfer. No significant adduct formation could be detected when mixtures of piperazine-2,5-dione and uracil or thymine were irradiated in frozen aqueous solution.     

Enantioselective organocatalytic α-sulfenylation of substituted diketopiperazines

The asymmetric organocatalytic α-sulfenylation of substituted piperazine-2,5-diones is reported, with cinchona alkaloids as chiral Lewis bases and electrophilic sulfur transfer reagents. Catalyst loadings, the type of sulfur transfer reagent, temperature, and solvent were investigated in order to optimize the reaction conditions. The effects of ring substitution and the type of catalyst on the yield and enantioselectivity of the reaction are reported.     

Synthesis of 2,6-bridged piperazine-3-ones by N-acyliminium ion chemistry

Several 2-substituted and 2,5-disubstituted piperazine-3,6-diones were synthesized starting from readily available alpha-amino acids. After activation of a lactam carbonyl via introduction of a methoxycarbonyl group onto nitrogen, this carbonyl was selectively reduced. Treatment of the resulting urethane with protic acid generated the corresponding N-acyliminium ion, which was trapped by a nucleophilic C2-side chain to provide 2,6-bridged piperazine-3-ones. Several aromatic, heteroaromatic, and nonaromatic side chains were used as pi-nucleophiles. In addition, the effect of the presence of a C5-methyl group on the stereochemical outcome of the cyclization was examined.     

From amines to diketopiperazines: a one-pot approach

An efficient one-pot synthesis is described for the preparation of 1,4-disubstituted piperazine-2,5-diones starting from a suitable amine and chloroacetyl chloride in the presence of an aqueous base. The resulting chloroacetamide is cyclised in situ by employing the phase-transfer (PT) catalyst, benzyltriethylammonium chloride (TEBA). The products are isolated in excellent yields of up to 90%.     

Rigid Scaffolds: Synthesis of 2,6-Bridged Piperazines with Functional Groups in all three Bridges

The activity of pharmacologically active compounds can be increased by presenting a drug in a defined conformation, which fits exactly into the binding pocket of its target. Herein, the piperazine scaffold was conformationally restricted by substituted C₂- or C₃-bridges across the 2- and 6-position. At first, a three-step, one-pot procedure was developed to obtain reproducibly piperazine-2,6-diones with various substituents at the N-atoms in high yields. Three strategies for bridging of piperazine-2,6-diones were pursued: 1. The bicyclic mixed ketals 8-benzyl-6-ethoxy-3-(4-methoxybenzyl)-6-(trimethylsilyloxy)-3,8-diazabicyclo[3.2.1]octane-2,4-diones were prepared by Dieckmann analogous cyclization of 2-(3,5-dioxopiperazin-2-yl)acetates. 2. Stepwise allylation, hydroboration and oxidation of piperazine-2,6-diones led to 3-(3,5-dioxopiperazin-2-yl)propionaldehydes. Whereas reaction of such an aldehyde with base provided the bicyclic alcohol 9-benzyl-6-hydroxy-3-(4-methoxybenzyl)-3,9-diazabicyclo[3.3.1]nonane-2,4-dione in only 10 % yield, the corresponding sulfinylimines reacted with base to give N-(2,4-dioxo-3,9-diazabicyclo[3.3.1]nonan-6-yl)-2-methylpropane-2-sulfinamides in >66 % yield. 3. Transformation of a piperazine-2,6-dione with 1,4-dibromobut-2-ene and 3-halo-2-halomethylprop-1-enes provided 3,8-diazabicyclo[3.2.1]octane-2,4-dione and 3,9-diazabicyclo[3.3.1]nonane-2,4-dione with a vinyl group at the C₂- or a methylene group at the C₃-bridge, respectively. Since bridging via sulfinylimines and the one-pot bridging with 3-bromo-2-bromomethylprop-1-ene gave promising yields, these strategies will be exploited for the synthesis of novel receptor ligands bearing various substituents in a defined orientation at the carbon bridge