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191.
Improved Erlenmeyer Synthesis with 5‐Thiazolone and Catalytic Mn(II) Acetate. Crystal Structure Confirmation of the Reaction Stereochemistry
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2‐Phenylthiazolin‐5‐one ( 5 , a thioazlactone) condenses with various aldehydes in the presence of the mild base Mn(II) acetate as catalyst in CH2Cl2 solution. This leads to the corresponding Erlenmeyer reaction products ( 6 ) in excellent yields in the case of aromatic aldehydes and moderate yields in others. The mildness of the reaction conditions is apparently enabled by the aromaticity of the (putative) intermediate thiazolone anion. The structure and stereochemistry (Z) of the product derived from i‐BuCHO was confirmed by single crystal X‐ray diffraction. This study overcomes key limitations of the classical Erlenmeyer synthesis and also introduces the relatively nontoxic Mn(II) acetate as a reagent in heterocyclic chemistry. 相似文献
192.
A variety of aliphatic and aromatic ketoaldehydes were reduced to the corresponding ketoalcohols with a mixture of sodium borohydride (1.2 equivalents) and sodium carbonate (sixfold molar excess) in water. Reactions were performed at room temperatures over (typically) 2 h, and yields of isolated products generally ranged from 70% to 85%. A bis-carbonate-borane complex, [(BH3)2CO2]2? 2Na+, possibly formed from the reagent mixture, is likely the active reductant. The moderated reactivity of this acylborane species would explain the chemoselectivity observed in the reactions. The readily available reagents and the mild aqueous conditions make for ease of operation and environmental compatibility, and make a useful addition to available methodology. 相似文献
193.
The products of the Henry nitroaldol reaction from nitromethane and several aldehydes were reduced to the corresponding nitroalkanes with (n-Bu)3SnH in water under microwave irradiation (80 °C/10 min), or dehydrated to the corresponding nitroalkenes with K2CO3 in water (generally 0–5 °C/20 min). Both “one-pot” reactions occur in excellent yields across a range of aliphatic and aromatic (including heteroaromatic) substrates. It seems likely that the deoxygenation of the nitroaldols occurs via coordination of an oxygen atom of the nitro group with a tin atom, which facilitates hydride delivery in the transition state. The elimination of water from the nitroaldols in mild base is likely driven by the stability of the conjugated nitroalkene products. The elimination required workup with 2 N HCl, which likely displaces a nitroalkane-nitroalkene equilibrium towards the latter. These extensions of the Henry reaction lead to products not easily obtained otherwise. 相似文献
194.
Srivari Chandrasekhar Vidyavathi Patro Lahu N. Chavan Rambabu Chegondi René Grée 《Tetrahedron letters》2014
An efficient and eco-friendly method for the synthesis of substituted pyrroles has been developed via ruthenium-catalyzed multicomponent reaction of ketone, amine, and ethylene glycol in PEG-400 as solvent medium without using any external ligand. The catalytic system and solvent can be recycled with the same, as well as different, ketones with minimum loss of Ru-catalyst activity. 相似文献
195.
Ratna Surya Alwi Ramakrishnan Gopinathan Avijit Bhowal Chandrasekhar Garlapati 《Molecules (Basel, Switzerland)》2020,25(24)
The adsorption response of eosin Y and indigo carmine acid dyes on activated carbon as a function of system temperature for a fixed concentration was investigated at various temperatures via adsorption isotherms and their thermodynamic quantities such as enthalpy, entropy, and Gibbs free energy changes. The adsorption data were exploited to develop a new adsorption isotherm. The new isotherm was developed with the spirit of solid–liquid phase equilibrium and regular solution theory. The proposed model has four adjustable constants and correlates adsorption isotherm in terms of the system temperature and melting temperature of the dye. The effect of pH on the removal of acid dyes was reported. The pH variation was observed to affect the adsorption efficiency. The removal of eosin Y and indigo carmine decreased from 99.4% to 82.6% and 92.38% to 79.48%, respectively, when the pH of the solution varied from 2 to 12. The thermodynamic analysis of the process reveals that the process of the removal of acid dyes is exothermic and spontaneous. Moreover, the kinetics parameters of the batch process are reported. 相似文献
196.
197.
The geometries of α- and β-silyl substituted vinyl radicals and of α, β-disilylvinyl radical have been optimised with the
STO-3G and the STO-3G * basis sets. The relative stabilities of various conformera have been determined at the UMP2/6-31G*
level. The stabilisation of vinyl radicals through α-silyl substitution is larger than that due to corresponding alkyl groups.
The presence of an α-silyl group also leads to a tendency towards linearisation of the vinyl radical centre and a corresponding
reduction in the inversion barrier. In marked contrast, the β-silyl effect is negligible. The geometric, conformational and
energetic consequences are insignificant. Overall, the silyl substituent effect at vinyl radicals is very different from that
computed earlier for the vinyl cations, but qualitatively similar to that found in carbanions. 相似文献
198.
199.
Chandrasekhar Roychoudhuri 《Optics Communications》1974,10(2):160-163
Suitably scanned Fabry-Pérot fringes were used to record and reconstruct a dynamic object. The technique can be used to increase the useful amplitude range of time averaged holography of moving objects and also as a convenient tool for multiplex holography. 相似文献
200.
Gerdt JP Miduturu CV Silverman SK 《Journal of the American Chemical Society》2008,130(45):14920-14921
Learning how native RNA conformations can be stabilized relative to unfolded states is an important objective, for both understanding natural RNAs and improving the design of artificial functional RNAs. Here we show that covalently attached double-stranded DNA constraints (ca. 14 base pairs in length) can significantly stabilize the native conformation of an RNA molecule. Using the P4-P6 domain of the Tetrahymena group I intron as the test system, we identified pairs of RNA sites where attaching a DNA duplex is predicted to be structurally compatible with only the folded state of the RNA. The DNA-constrained RNAs were synthesized and shown by nondenaturing polyacrylamide gel electrophoresis (native PAGE) to have substantial decreases in their Mg2+ midpoints ([Mg2+]1/2 values). These changes are equivalent to free energy stabilizations as large as DeltaDeltaGdegrees = -2.5 kcal/mol, which is approximately 14% of the total tertiary folding energy. For comparison, the sole modification of P4-P6 previously reported to stabilize this RNA is a single-nucleotide deletion (DeltaC209) that provides only 1.1 kcal/mol of stabilization. Our findings indicate that nature has not completely optimized P4-P6 RNA folding. Furthermore, the DNA constraints are designed not to interact directly and extensively with the RNA, but rather more indirectly to modulate the relative stabilities of folded and unfolded RNA states. The successful implementation of this strategy to further stabilize a natively folded RNA conformation suggests an important element of modularity in stabilization of RNA structure, with implications for how nature might use other molecules such as proteins to stabilize specific RNA conformations. 相似文献