1,720,992 research outputs found
Parallel solution phase synthesis of 4-dialkylamino-2-methylsulfonyl-6-vinylpyrimidines
No full textA simple and straightforward methodology for the parallel, solution-phase synthesis of novel 4-dialkylamino-2-methylsulfonyl-6-vinylpyrimidines 9a-j has been developed. Starting from 2-methylthio-6-[2-(p-toluensulfonyloxy)ethan-1-yl]-4(3H)-pyrimidinone (6), a three-step procedure (namely, tosylate substitution by amines, base-catalyzed rearrangement, and sulfide to sulfone oxidation) using a Buchi Sincore synthesizer gave the final products in high yield after simple ethyl acetate extraction and without further purification. Interestingly, when the final oxidation step was performed on 4-arylpiperazine derivatives 8g-j, the corresponding highly polar piperazine N-oxides 9g-j were obtained, which conversely needed chromatographic purification in order to give the pure products
Microwave-assisted intramolecular huisgen cycloaddition of azido alkynes derived from α-amino acids
No full textThe intramolecular version of the Huisgen cycloaddition is a potentially useful reaction for the stereocontrolled preparation of 1,5-disubstituted and 1,4,5-trisubstiututed triazoles. When α-azido propargyl esters derived from α-amino acids are submitted to [3 + 2] cycloaddition, the expected 4H-[1,2,3]triazolo[5,1-c][1,4]oxazin-6-ones are not formed; rather, an oligomeric cyclic polyester is obtained via a prevailing intermolecular cycloaddition. We have discovered that propargyl α-azido amides undergo metal-free intramolecular Huisgen cycloaddition in MeCN/H2O under microwave dielectric heating. This reaction provides access to new condensed triazoles that can be considered as conformationally constrained peptidomimetics. Moreover, the following microwave-assisted lactam ring opening provides 1,4-disubstituted and 1,4,5-trisubstituted triazole amino acids. The same kind of compounds are obtained from the ester cycloadduct by reaction with primary amines in the presence of AlMe3. In order to interpretate this unpredictable behavior, an ab initio study of the reaction pathway was undertaken using GAMESS(US) at the B3LYP/6-31G** level of theory. Different relaxed potential energy profiles were obtained for esters and amides, suggesting that the cis-arrangement of the-CO=N-could account for the amide reactivity
Avoiding hot-spots in Microwave-Assisted Pd/C catalysed reactions by using the biomass derived solvent γ-Valerolactone
Full text linkHerein, we report the use of γ-valerolactone as a new biomass-derived reaction medium for microwave assisted organic synthesis. The interaction of this solvent with microwaves and its heating profile under microwave irradiation has been fully characterized for the first time, demonstrating its stability and the applicability in microwave assisted Pd/C catalysed reactions avoiding the arcing phenomena frequently observed in these conditions. The use of γ-valerolactone demonstrated to be compatible with aliphatic and aromatic amines in the hydrogen transfer Pd/C mediated synthesis of benzimidazoles
An improved synthesis of solid-supported reagents (SSRs) for selective acylation of amines by microwave irradiation
No full textMicrowave-assisted acylation of a solid-supported pyrimidine linker with different acyl chlorides gave polymer-bound
4-acyloxypyrimidines, which in turn were used as SSRs for rapid and selective acylation of amines under microwave irradiatio
Synthesis of D- and L-2,3-trans-3,4-cis-4,5-trans-3,4-dihydroxy-5-hydroxymethylproline and tripeptides containing them
No full textEnantiomerically pure (-)-(1R,4R,5R,6S)- and (+)-(1S,4S,5S,6R)-7-(tert-butoxycarbonyl)-5,6-exo-isopropylidenedioxy-7-azabicyclo[2.2.1]hept-2-one ((-)-3 and (+)-3) have been obtained from the Diels-Alder adduct of N-(tert-butoxycarbonyl)pyrrole and 2-bromo-1-(p-toluenesulfonyl)acetylene, including the Alexakis optical resolution of ketone ()-3 via formation of cyclic aminals with (1R,2R)diphenylethylenediamine. Compounds (-)-3 and (+)-3 were converted into D- and L-2,3-trans-3,4-cis-4,5-trans-N-(tert-butoxycarbonyl)-5-hydroxymethyl-3,4-isopropylidenedioxyprolines (-)-4 and (+)4, respectively. Applying the Boc and Fmoc strategies of peptide synthesis, these compounds were used to construct two tripeptides containing the D- or L-2,3-trans-3,4-cis-4,5-trans-3,4-dihydroxy-5-hydroxymethylproline
In Water Markovnikov Hydration and One-Pot Reductive Hydroamination of Terminal Alkynes under Ruthenium Nanoparticle Catalysis
No full textIn the presence of Shvo's catalyst [2,3,4,5-tetraphenyl-1,3-cyclopentadien-1-ol, ruthenium(II) complex], a small quantity of aniline and TGPS-750-M surfactant, terminal alkynes were hydrated under microwave (MW) dielectric heating in water as solvent with high conversion and good yield. The reaction gives the Markovnikov product exclusively, unlike that commonly observed with ruthenium complexes. Under the influence of MWs, Ru nanoparticles embedded in the nanomicelle environment were formed, acting as the effective hydration catalyst. Introducing sodium formate in the aqueous phase and using a stoichiometric amount of amines, the Ru nanoparticle nanomicelle catalyst gave the first example of one-pot single-step hydroamination of alkynes with the formation of the corresponding secondary amines. The reaction is characterized by low environmental impact as TGPS-750-M is required in low amount, and organic solvents employed only for product separation or catalyst recycling
Microwave assisted hydroaminomethylation of alkenes
No full textHydroaminomethylation of terminal alkenes can be regioselectively carried out in less than 30 min with secondary amines in EtOH under MW irradiation using (PPh3)3RhCO(H) and Xantphos or Biphephos as ligands. When primary amines were employed, the corresponding enamines were obtained in good yields. Tris-benzyl allylglycine was transformed into different (basic) benzylated α-amino acids. Moreover, the benzyl protection was removed in few minutes under MW irradiation with Pd(OH)2 under H2 atmosphere. © 2007 Elsevier Ltd. All rights reserved
Microwave-Assisted Carbonylation and Cyclocarbonylation of Aryl Iodides under Ligand Free Heterogeneous Catalysis
No full text"Chemical Equation Presented" Carbonylation reaction is a very effective transformation for the synthesis of esters, amides, and heterocyclic compounds. Heterogeneous catalyzed carbonylation reactions can be carried out using the association of Pd/C and microwave dielectric heating. Alkoxy carbonylation can be performed, with stoichiometric amounts of different primary and secondary alcohols in DMF in the presence of DBU as the base. Analogously, iodobenzene, CO, and amines can be transformed into the corresponding amides in good yields after a simple filtration to remove the catalyst. Pd/C was also successfully employed in microwave-assisted cyclocarbonylation of o-iodoaniline with acyl chlorides to give benzoxazinones. Pd/ C can be recycled two times without, a considerable difference in the reaction yields. © 2010 American Chemical Society
Metal Catalysis with Microwaves in Organic Synthesis: a Personal Account
No full textIn the last 25 years, the influence of microwave (MW) irradiation in chemical synthesis has intrigued chemists for the exceptional results obtained in shortening reaction times. Inspired by recent papers that demonstrate the existence of a non-thermal effect in some MW assisted reactions, we examine herein a series of reactions catalysed by homogeneous and heterogeneous metals occurring under MW irradiation. The review contains examples of hydroformylation, hydrogenation (including enantioselective versions with 98 % ee), hydrogenolysis, hydrogen transfer and borrowing hydrogen reactions carried out under MWs and metal catalysis. An increase in the influence of MW irradiation was often observed in reactions performed with heterogeneous catalysts such as Pd/C. In these cases, a “microwave catalysis” can be possible. The increase in the (apparent) reaction rate requires the contribution of MW irradiation that modifies the catalyst surface, driving the reaction progress as occurring in other kinds of catalysis
Solution-phase parallel synthesis of S-DABO analogues
No full textA simple and straightforward methodology for the parallel, solution-phase synthesis of a new series of S-DABO derivatives 1 and 2, bearing aromatic substituents at the C2 and C6 positions, has been developed. Starting from potassium ethyl malonates 3, thiouracil intermediates 5 were prepared through parallel synthesis and isolated as pure products by simple extraction with ethyl acetate. Selective S-benzylation of 5 was achieved in few minutes under microwave irradiation to give the title compounds 1, which were oxidized in parallel to the corresponding sulfones 2. Some of the new compounds 1 showed potent inhibitory activity against HIV-1 RT
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