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排序方式: 共有98条查询结果,搜索用时 15 毫秒
1.
2.
Terhi Oja Petri Tähtinen Nadine Dreiack Pekka Mäntsälä Jarmo Niemi Mikko Metsä-Ketelä Karel D. Klika 《Tetrahedron: Asymmetry》2012,23(9):670-682
Two new pairs of stereoisomeric alnumycin As, A2 {(2)-(1R,1′RS,4′SR,5′SR)} and A3 {(2)-(1R,1′RS,4′SR,5′RS)}, are described. Similar to alnumycin A1 {(2)-(1R,1′RS,4′RS,5′SR)}, each of these naturally occurring compounds is also a pair of C-1 inverse epimers. The relative configurations of the dioxane ring sidechains were assigned on the basis of 1H NMR NOE contacts and molecular modeling using density functional theory (DFT) at the M06-2X/6-31G(d) level of theory. The absolute configurations of C-1 and the determination of inverse epimeric relationships were achieved by experimental electronic circular dichroism (ECD) measurements, with both aspects confirmed by using the chiral derivatizing agent (CDA) Mosher′s acid chloride {α-methoxy-α-trifluorophenylacetyl chloride (MTPACl)} to effect enantiodifferentiation. The absolute configurations of the dioxane ring using the CDA could only be effected in the case of alnumycin A1, the results of which were in agreement with previous assignments. The dioxane ring conformational mobility and the likely interaction between the MTPA groups coupled with the structural novelty of the diols in the dioxane ring with respect to CDA analysis precluded an absolute configuration assignment for alnumycins A2 and A3 based on empirical comparisons or by computational analysis of through-space NMR shieldings (TSNMRS) emanating from the phenyl groups of the MTPA moieties. 相似文献
3.
A One‐Pot Synthesis of N‐Aryl‐2‐Oxazolidinones and Cyclic Urethanes by the Lewis Base Catalyzed Fixation of Carbon Dioxide into Anilines and Bromoalkanes
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Teemu Niemi Dr. Jesus E. Perea‐Buceta Dr. Israel Fernández Otto‐Matti Hiltunen Vili Salo Sari Rautiainen Dr. Minna T. Räisänen Prof. Timo Repo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(30):10355-10359
The multicomponent assembly of pharmaceutically relevant N‐aryl‐oxazolidinones through the direct insertion of carbon dioxide into readily available anilines and dibromoalkanes is described. The addition of catalytic amounts of an organosuperbase such as Barton's base enables this transformation to proceed with high yields and exquisite substrate functional‐group tolerance under ambient CO2 pressure and mild temperature. This report also provides the first proof‐of‐principle for the single‐operation synthesis of elusive seven‐membered ring cyclic urethanes. 相似文献
4.
Abstract We used canonical correlation analysis to examine the multivariate association between two distinct data sets commonly measured or calculated for approximately 600 chemicals: (1) measured or calculated values of select physieochemical properties (i.e., K ow, boiling point, heat of vaporization, molecular weight, water solubility, molecular volume, hydrogen bonding potential, and vapor pressure) and (2) calculated algorithmically-derived variables (i.e., topological and neighborhood indices derived from graph theory). Canonical correlation analysis identified eight highly significant associations between linear combinations of graph-theoretic variables and linear combinations of physicochemical properties. The set of graph theoretic variables was significantly related to all physieochemical properties, explaining 55% to 99% of the variation in these properties. 相似文献
5.
Rajeev K. Dubey Marja Niemi Kimmo Kaunisto Alexander Efimov Nikolai V. Tkachenko Helge Lemmetyinen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(21):6791-6806
Novel bay‐functionalized perylene diimides with additional substitution sites close to the perylene core have been prepared by the reaction between 1,7(6)‐dibromoperylene diimide 6 (dibromo‐PDI) and 2‐(benzyloxymethyl)pyrrolidine 5 . Distinct differences in the chemical behaviors of the 1,7‐ and 1,6‐regioisomers have been discerned. While the 1,6‐dibromo‐PDI produced the corresponding 1,6‐bis‐substituted derivative more efficiently, the 1,7‐dibromo‐PDI underwent predominant mono‐debromination, yielding a mono‐substituted PDI along with a small amount of the corresponding 1,7‐bis‐substituted compound. By varying the reaction conditions, a controlled stepwise bis‐substitution of the bromo substituents was also achieved, allowing the direct synthesis of asymmetrical 1,6‐ and 1,7‐PDIs. The compounds were isolated as individual regioisomers. Fullerene (C60) was then covalently linked at the bay region of the newly prepared PDIs. In this way, two separate sets of perylene diimide–fullerene dyads, namely single‐bridged (SB‐1,7‐PDI‐C60 and SB‐1,6‐PDI‐C60) and double‐bridged (DB‐1,7‐PDI‐C60 and DB‐1,6‐PDI‐C60), were synthesized. The fullerene was intentionally attached at the bay region of the PDI to achieve close proximity of the two chromophores and to ensure an efficient photoinduced electron transfer. A detailed study of the photodynamics has revealed that photoinduced electron transfer from the perylene diimide chromophore to the fullerene occurs in all four dyads in polar benzonitrile, and also occurs in the single‐bridged dyads in nonpolar toluene. The process was found to be substantially faster and more efficient in the dyads containing the 1,7‐regioisomer, both for the singly‐ and double‐bridged molecules. In the case of the single‐bridged dyads, SB‐1,7‐PDI‐C60 and SB‐1,6‐PDI‐C60, different relaxation pathways of their charge‐separated states have been discovered. To the best of our knowledge, this is the first observation of photoinduced electron transfer in PDI‐C60 dyads in a nonpolar medium. 相似文献
6.
Dilation theorems for Banach space valued stochastic processes and operator valued positive definite kernels are considered. It is shown, e.g., that a Banach space valued stochastic process X can be dilated to another process Y, if and only if the covariance kernel of Y is a majorant of the covariance kernel of X. Positive definite operator kernels having majorants of certain special type are characterized. 相似文献
7.
Priplata A Niemi J Salen M Harry J Lipsitz LA Collins JJ 《Physical review letters》2002,89(23):238101
Noise can enhance the detection and transmission of weak signals in certain nonlinear systems, via a mechanism known as stochastic resonance. Here we show that input noise can be used to improve motor control in humans. Specifically, we show that the postural sway of both young and elderly individuals during quiet standing can be significantly reduced by applying subsensory mechanical noise to the feet. We further demonstrate with input noise a trend towards the reduction of postural sway in elderly subjects to the level of young subjects. These results suggest that noise-based devices, such as randomly vibrating shoe inserts, may enable people to overcome functional difficulties due to age-related sensory loss. 相似文献
8.
A Minkowski space formalism of finite-temperature quantum field theory is used to compute static thermodynamic quantities in the one- and two-loop approximation in an elegant and straightforward way using a generalization of Weinberg's tadpole method of calculating effective potentials. Systematic diagrammatic techniques for low- and high-temperature expansions are developed. Renormalizability by zero-temperature symmetric counterterms is proven for all orders in the loop expansion and demonstrated explicitly to two loops. Many useful computational techniques applicable to general finite temperature calculations are explained. 相似文献
9.
A microbiological method was developed for group level identification of antibiotics in incurred kidney and muscle samples from cattle and pigs. The method was composed of six test bacterium-plate growth medium combinations and the result was recorded as a profile of growth inhibition zones. The sample profiles were compared to two sets of references: one constructed with standard antibiotic solution profiles, and the other with these combined with profiles of microbiologically and chemically identified residues from incurred samples. The algorithm employed in profile comparison located the minimal sum of absolute pairwise differences over the tests, with the addition of a number of experimentally observed intra-test criteria. Chemical identification and quantitation of incurred residues was based on liquid chromatography. The method identified penicillin G as a penicillinase sensitive penicillin, enrofloxacin and ciprofloxacin belonging to fluoroquinolone group, and oxytetracycline belonging to tetracycline group. Each of these residues was microbiologically identified below the Maximum Residue Limit (MRL) for kidney tissue. Combining sample profiles with the standard reference data set did not enhance the resolution. Microbiological and chemical identification test results were in good agreement. The results of this study show that a microbiological identification method is a useful tool in preliminary characterisation of antibiotic residues in animal tissues. 相似文献
10.
C. E. Nwankire V. J. Law A. Nindrayog B. Twomey K. Niemi V. Milosavljević W. G. Graham D. P. Dowling 《Plasma Chemistry and Plasma Processing》2010,30(5):537-552
Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper
presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system
was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power
and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge
with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume
of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide
and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen
molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures
between 319 and 347 K. 相似文献