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11.
The reaction of chiral sulfur ylides with aldehydes and ketones has emerged as a useful asymmetric process for the synthesis of epoxides. Processes employing either catalytic or stoichiometric amounts of sulfides have been developed. Although a large number of chiral sulfur ylides have been tested in the epoxidation process, only a few have delivered high diastereo- and enantio- selectivity. This review examines the factors that influence stereocontrol (steric hindrance of the sulfide, ylide conformation, ylide face selectivity, reversibility of betaine formation, solvent, and metal salts). This analysis leads to the conclusion that high reversibility in betaine formation leads to high diastereoselectivity but low enantioselectivity, and non-reversible betaine formation leads to low diastereoselectivity and high enantioselectivity (provided that other criteria are met). To achieve both high diastereoselectivity and high enantioselectivity simultaneously, requires non-reversible formation of the anti-betaine and reversible formation of the syn-betaine. Thus, factors that influence the degree of reversibility in betaine formation are critically important since with subtle changes in reaction conditions (solvent, temperature, metal ions) both high enantio- and diastereoselectivity can often be achieved. 相似文献
12.
Adams LA Aggarwal VK Bonnert RV Bressel B Cox RJ Shepherd J de Vicente J Walter M Whittingham WG Winn CL 《The Journal of organic chemistry》2003,68(24):9433-9440
A simple and high-yielding method for the preparation of cyclopropane amino acids is described. The novel method involves the one-pot cyclopropanation of readily available dehydroamino acids using aryl and unsaturated diazo compounds generated in situ from the corresponding tosylhydrazone salts. It was found that thermal 1,3-dipolar cycloaddition followed by nitrogen extrusion gave the cyclopropane amino acid derivatives with good E selectivity, while reactions in the presence of meso-tetraphenylporphyrin iron chloride gave predominantly the corresponding Z isomers. The synthetic utility of this process was demonstrated in the synthesis of (+/-)-(Z)-2,3-methanophenylalanine [(+/-)-(Z)-1], the anti-Parkinson (+/-)-(E)-2,3-methano-m-tyrosine [(+/-)-(E)-2], and the natural product (+/-)-coronamic acid [(+/-)-3]. 相似文献
13.
S. A. Pai K. V. Lohithakshan P. D. Mithapara S. K. Aggarwal 《Journal of Radioanalytical and Nuclear Chemistry》2000,246(3):623-628
Synergistic extraction of hexavalent uranium and plutonium as well as trivalent americium was studied in HNO3 with thenoyl, trifluoro-acetone (HTTA)/1-phenyl, 3-methyl, 4-benzoyl pyrazolone-5 (HPMBP) in combination with neutral donors viz, DPSO, TBP, TOPO (mono-functional) and DBDECMP, DHDECMP, CMPO (bi-functional) with wide basicity range using benzene as dileunt. A linear correlation was observed when the equilibrium constant log Ks for the organic phase synergistic reaction of both U(VI) and Pu(VI) with either of the chelating agents HTTA or HPMBP was plotted vs. the basicity (log Kh) of the donor (both mono- and bi-functional) indicating bi-functional donors also behave as mono-functional. This was supported by the thermodynamic data (G
0, H
0, S
0) obtained for these systems. The organic phase adduct formation reactions were identified for the above systems from the thermodynamic data. In the Am(III) HTTA system log K
s
values of bi-functional donors were found to be very high and deviate from the linear plot (log K
s
vs. log K
h
) obtained for mono-functional donors, indicating that they function as bi-functional for the Am(III)/HTTA system studied. This was supported by high +ve S
0 values obtained for this system. 相似文献
14.
The lithiation of trans- and cis-stilbene oxides (+/-)-1 and 8 has been investigated. While with 8, lithiation occurred exclusively at the benzylic position, with the trans isomer (+/-)-1, ortho-lithiation competed with alpha-lithiation depending upon the experimental conditions. The configurational stability of the alpha-lithiated cis- and trans-stilbene oxides (+/-)-2 and (+/-)-9, respectively, was proved as well as that of scalemic stilbene oxide (R,R)-2. 相似文献
15.
S. K. Aggarwal D. Alamelu P. M. Shah N. N. Mirashi 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(3):771-774
Determination of 241Am/243Am ratios is required for vanous purposes including assay of Am by isotope dilution techniques. Alpha-spectrometry on electrodeposited
sources is a preferred technique for this determination. However, there is an inherent problem of tail contribution which
necessitates the use of suitable algorithms to account for the same. Recently, in the frame of a Coordinated Research Program
(CRP) of the International Atomic Energy Agency (IAEA), WinALPHA software has been developed which is a combination of an
asymmetrical Gaussian for the main part of the peak and a low energy function. Therefore, it was of interest to compare the
use of this algorithm with the routinely used method, in our laboratory, based on geometric progression (G. P.) decrease.
Since, there are no reference materials available commercially for 241Am/243Am ratios, synthetic mixtures covening a wide range (0.3 to 2.0) of 241Am/243Am α-activity ratios were used and un-ignited electrodeposited sources were prepared for α-spectrometry. The α-spectra obtained using PIPS detector, were evaluated using the two algonthms The 241Am/243Am α-activity ratios obtained were also compared with those determined by thermal ionization mass spectrometry (TIMS). An agreement
of about 1% was obtained in the 241Am/243Am ratios determined by the two methods and also by using the two algorithms for α-spectrum evaluation. 相似文献
16.
17.
Ashok Kumar Singh Sameena Mehtab Udai P. Singh Vaibhave Aggarwal 《Electroanalysis》2007,19(11):1213-1221
New polymeric membrane cadmium‐ion selective sensors have been prepared by incorporating nitrogen and sulfur containing tridentate ligands as the ionophores into the plasticized PVC membranes. Poly(vinyl chloride) (PVC) based membranes of potassium hydrotris[N‐(2,6‐xylyl)thioimdazolyl) borate] (KTt2,6‐xylyl) and potassium hydrotris(3‐phenyl‐5‐methylpyrazolyl) borate (KTpPh,Me) with sodium tetraphenyl borate (NaTPB) as an anionic excluder and dibutylphthalate (DBP), tributylphthalate (TBP), dioctylsebacate (DOS), and o‐nitrophenyloctyl ether (o‐NPOE) as plasticizing solvent mediators were investigated in different compositions. KTt2,6‐xylyl was found to be a selective and sensitive ion carrier for Cd(II) membrane sensor. A membrane composed of KTt2,6‐xylyl:NaTPB:PVC:DBP with the % mole ratio 2.3 : 1.1 : 34.8 : 61.8 (w/w) works well over a very wide concentration range (7.8×10?8–1.0×10?2 M) with a Nernstian slope of 29.4±0.2 mV/decades of activity between pH values of 3.5 to 9.0 with a detection limit of 4.37×10?8 M. The sensor displays very good discrimination toward Cd(II) ions with regard to most common cations. The proposed sensor shows a short response time for whole concentration range (ca. 8 s). The effects of the cationic (tetrabutylammonium chloride, TBC), anionic (sodium dodecyl sulfate, SDS) and nonionic (Triton X‐100) surfactants were investigated on the potentiometric properties of proposed cadmium‐selective sensor. The proposed sensor based on KTt2,6‐xylyl ionophore has also been used for the direct determination of cadmium ions in different water samples and human urine samples. 相似文献
18.
S. B. Manohar S. K. Aggarwal S. M. Deshmukh A. R. Parab P. P. Burte H. C. Jain Satya Prakash 《Journal of Radioanalytical and Nuclear Chemistry》1981,63(1):145-153
A gamma-spectrometric method using an intrinsic high resolution germanium detector has been developed for the determination
of isotope ratios of plutonium from samples in solution form. The method is based on the assay of low energy gamma-rays of238Pu,239Pu,240Pu and241Pu and does not require the use of branching intensities or the knowledge of detection efficiencies for different gamma rays.
Since low energy gamma-rays are used, the effect of241Am has also been studied. It is found that results are not affected up to 0.5 wt% of241Am in plutonium samples. An accuracy of 3% is achievable in the determination of240Pu/239Pu and241Pu/239Pu atom ratios as demonstrated by carrying out measurements on isotopic standards of plutonium. 相似文献
19.
Summary
N-benzamidosalicylaldimine (H2L) complexes of CuII, NiII, CoII, FeII, MnII. VOIV and TiOIV have been prepared. The ligand probably coordinates to the metal from the hydroxyl, carbonyl and imino groups. 相似文献
20.
Leonard I. Wassenaar Liang-Feng Han Thomas Schiefer Gustav Kainz Luis Araguas-Araguas Pradeep K. Aggarwal 《Isotopes in environmental and health studies》2018,54(3):274-287
Tritium (3H) is an essential tracer of the Earth's water cycle; yet widespread adoption of tritium in hydrologic studies remains a challenge because of analytical barriers to quantification and detection of 3H by electrolytic pre-concentration. Here, we propose a simple tritium electrolytic enrichment system based on the use of solid polymer electrolyte membranes (PEMs) that can be used to enrich 3H in 250–3000?mL environmental water samples to a 10-mL final volume. The IAEA PEM-3H system reported here can produce high enrichment factors (>70-fold) and, importantly, removes some of the deterrents to conventional 3H enrichments methods, including the use of toxic electrolysis and neutralization chemicals, spike standards, a complex electrolysis apparatus that requires extensive cooling and temperature controls, and improves precision by eliminating the need for tracking recovery gravimetrics. Preliminary results with varying operating conditions show 3H enrichments to 70-fold and higher are feasible, spanning a wide range of tritium activities from 5 to 150 TU with a precision of ~4.5?%. Further work is needed to quantify inter-sample memory and to establish lower 3H detection limits. The IAEA PEM-3H system is open source, with 3-D CAD and design files made freely available for adoption and improvement by others. 相似文献