419 research outputs found
Unusual Nucleophilic Addition of Grignard Reagents in the Synthesis of 4-Amino-pyrimidines
Pyrimidines have always received considerable attention because of their importance in synthesis and elucidation of biochemical roles, in particular that of vitamin B1. Herein, we describe a reaction pathway in a Grignard reagent-based synthesis of substituted pyrimidines. A general synthesis of α-keto-2-methyl-4-amino pyrimidines and their C6-substituted analogues from 4-amino-5-cyano-2-methylpyrimidine is reported. The presence of the nitrile substituent in the starting material also results in an unusual reaction pathway leading to C6-substituted 1,2-dihydropyrimidines. Grignard reagents that give normal pyrimidine products under standard reaction conditions can be 14 switched to give dihydropyrimidines by holding the reaction at 0 °C before quenching
Upper-rim monofunctionalisation in the synthesis of triazole- and disulfide-linked multicalix[4]- and -[6]arenes.
Covalently linked multiple calixarenes are valued in supramolecular chemistry. We report an easy and versatile synthetic route to covalently linked double and triple calix[4]arene and calix[6]arenes by a novel DMFâcontrolled selective alkylation of a convenient and readily available upperârim dimethylaminomethylâsubstituted tetrahydroxy calix[4]arene and â[6]arenes. Synthetic routes to upperârim functionalised redox active disulfideâlinked doubleâ, tetraâ and peptidohybridâcalixarenes employing either redox chemistry (CH2SH) or thiolates (CH2Sâ) are also opened up from the same key starting material
A Gravitational Instability-Driven Viscosity in Self-Gravitating Accretion Disks
We derive a viscosity from gravitational instability in self-gravitating
accretion disks, which has the required properties to account for the observed
fast formation of the first super-massive black holes in highly redshifted
quasars and for the cosmological evolution of the black hole-mass distribution.Comment: 14 pages, 1 figure, ApJ Letters (in press
Synthesis of novel stilbeneâcoumarin derivatives and antifungal screening of monotes kerstingii-specialized metabolites against fusarium oxysporum
Fusarium is one of the most toxigenic phytopathogens causing diseases and reduced agricultural productivity worldwide. Current chemical fungicides exhibit toxicity against non-target organisms, triggering negative environmental impact, and are a danger to consumers. In order to explore the chemical diversity of plants for potential antifungal applications, crude extract and fractions from Monotes kerstingii were screened for their activity against two multi-resistant Fusarium oxysporum strains: Fo32931 and Fo4287. Antifungal activity was evaluated by the determination of minimum inhibitory concentration (MIC) by broth dilution of fermentative yeasts using kinetic OD600 nm reading by a spectrophotometer. The n-butanol fraction showed the best activity against Fo4287. We screened eleven previously reported natural compounds isolated from different fractions, and a stilbeneâcoumarin 5-[(1E)-2-(4-hydroxyphenyl)ethenyl]-4,7-dimethoxy-3-methyl-2H-1-benzopyran-2-one (1) was the most active compound against both strains. Compound 1 was employed as a nucleophile with a selection of electrophilic derivatizing agents to synthesize five novel stilbeneâcoumarin analogues. These semisynthetic derivatives showed moderate activity against Fo32931 with only prenylated derivative exhibiting activity comparable to the natural stilbeneâcoumarin (1), demonstrating the key role of the phenolic group
Could bright gamma-ray burst optical transients have been recorded historically?
The brightest optical flash from a gamma-ray burst (GRB) was, briefly, a
naked-eye object. Several other GRBs have produced optical transients only
slightly fainter. We argue that, based upon the recently accumulated data from
hundreds of GRB transients, many such optical events should have been visible
to the unaided eye in the course of human history. The most likely repositories
of such observations are historical records from the Orient, and we have
located and discuss a number of candidates. We also consider the value of such
observations, should any very likely ones be uncovered, to modern astrophysics.Comment: 9 pages, 3 figure
Asymmetric Oxidation of Enol Derivatives to α-Alkoxy Carbonyls Using Iminium Salt Catalysts: A Synthetic and Computational Study
We report herein the first examples of asymmetric oxidation of enol ether and ester substrates using iminium salt organocatalysis, affording moderate to excellent enantioselectivities of up to 98% ee for tetralone-derived substrates in the α-hydroxyketone products. A comprehensive density functional theory study was undertaken to interpret the competing diastereoisomeric transition states in this example in order to identify the origins of enantioselectivity. The calculations, performed at the B3LYP/6-31G(D) level of theory, gave good agreement with the experimental results, in terms of the magnitude of the effects under the specified reaction conditions, and in terms of the preferential formation of the (R)-enantiomer. Just one of the 30 characterized transition states dominates the enantioselectivity, which is attributed to the adoption of an orientation relative to stereochemical features of the chiral controlling element that combines a CH interaction between a CH 2 group in the substrate and one of the aromatic rings of the biaryl section of the chiral auxiliary with a good alignment of the acetoxy group with the other biaryl ring, and places the smallest substituent on the alkene (a hydrogen atom) in the most sterically hindered position
Synthesis and photophysical properties of iron-carbonyl complex-coumarin conjugates as potential bimodal IR-fluorescent probes
An expedient synthesis of the first examples of iron-carbonyl complex-coumarin conjugates is reported. 7-Amino/7-hydroxycoumarin derivatives have been readily derivatized through an easily implemented single-step reaction involving the tricarbonyl(η5-cyclohexadienyl)iron(1+) cation [(C6H7)Fe(CO)3]+. The scope and limitations of this N-/O-alkylation reaction were also investigated. The fluorescence properties of these novel metal-carbonyl complexes have been studied and support their further use as valuable building blocks in the design of bimodal contrast agents for combined vibrational and fluorescence imaging
Planar chiral pseudo-isocoumarins by copper catalysed desymmetrisation
A new copper catalysed desymmetrisation reaction of bisalkynyl ferrocenecarboxylic acids results in the first asymmetric synthesis of planar chiral pseudo-isocoumarins in up to 64â% enantiomeric excess and up to 99â% yield. The absolute configuration of the chiral heterocycles is proven by chemical correlation and circular dichroism spectroscopy, opening the way for the rational extension of this method to produce bioactive isocoumarin-fused ferrocene derivatives
Trypanocidal and leishmanicidal activity of six limonoids
Six limonoids [kotschyienone A and B (1, 2), 7-deacetylgedunin (3), 7-deacetyl-7-oxogedunin (4), andirobin (5) and methyl angolensate (6)] were investigated for their trypanocidal and leishmanicidal activities using bloodstream forms of Trypanosoma brucei and promastigotes of Leishmania major. Whereas all compounds showed anti-trypanosomal activity, only compounds 1â4 displayed anti-leishmanial activity. The 50% growth inhibition (GI 50) values for the trypanocidal and leishmanicidal activity of the compounds ranged between 2.5 and 14.9Â ÎŒM. Kotschyienone A (1) was found to be the most active compound with a minimal inhibition concentration (MIC) value of 10Â ÎŒM and GI 50 values between 2.5 and 2.9Â ÎŒM. Only compounds 1 and 3 showed moderate cytotoxicity against HL-60 cells with MIC and GI 50 values of 100Â ÎŒM and 31.5â46.2Â ÎŒM, respectively. Compound 1 was also found to show activity against intracellular amastigotes of L. major with a GI 50 value of 1.5Â ÎŒM. The results suggest that limonoids have potential as drug candidates for the development of new treatments against trypanosomiasis and leishmaniasis
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