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41.
Diastereoselective C2-elongation processes of N-mono- 1a-c and N,N-diprotected 1d-fl-alaninals, using vinylmagnesium bromide and chloride, are described. A substantial difference between effects of the N-protecting groups replacing either one or two amino protons was observed. 相似文献
42.
Wojciech Kroszczyski Ewa Olszewska Piotr Saaski Janusz Jurczak 《Helvetica chimica acta》2004,87(6):1488-1492
A simple and effective method to deprotect of sterically hindered steroid esters is described. Deprotection was carried out in MeOH in the presence of a catalytic amount of Et3N under high‐pressure conditions. Enzymatic, anionite, and high‐pressure methods are compared. 相似文献
43.
Małgorzata Baśko Przemyslaw Kubisa 《Journal of polymer science. Part A, Polymer chemistry》2007,45(14):3090-3097
Cationic copolymerization of L,L ‐lactide (LA) and ε‐caprolactone (CL) initiated by low molecular weight diols in the presence of acid catalyst gives corresponding copolyesters terminated at both ends with hydroxyl groups in practically quantitative yield. Copolymerization proceeds by Activated Monomer mechanism. LA is consumed preferentially and at the later stages of copolymerization the reaction mixture is enriched with CL. In spite of that, random distribution of both units is observed and end‐groups are mainly ? LA‐OH groups and not ? CL‐OH groups. This is explained by the fact that to reach high conversion of both comonomers the relatively long reaction times are required and at those conditions transesterification reaction becomes significant. Thus the microstructure of copolymers and the nature of the end‐groups is governed by transesterification rather then by the kinetics of comonomers incorporation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3090–3097, 2007 相似文献
44.
Janusz Jurczak Andrzej Konowal Zbigniew Krawczyk Andrzej Ejchart 《Magnetic resonance in chemistry : MRC》1981,15(2):193-196
The diastereomeric differences (Δδ) were measured by 13C NMR spectroscopy for a series of 11 (?)-ω-camphanic esters of secondary alcohols of known absolute configuration. This method cannot be used for the determination of the absolute configuration, but it can be applied to the estimation of the enantiomeric purity of chiral alcohols. 相似文献
45.
Kinga Suwińska Marek Pietraszkicwicz Janusz Lipkowski Janusz Jurczak Giovanni. D Andreetti Gabriele Bocelli 《Journal of Molecular Structure》1981,75(1):121-127
The chiral crown ether was prepared according to the method described in the literature and its complex with potassium iodide was obtained. The crystal data are as follows: a = 10.315(4), b = 9.265(4), c = 15.88(2) Å, β = 100.70(7)°. V = 1494 Å,3 Z = 2, D0 = 1.55 Mg m?3 space group symmetry P21.The structure was determined by the heavy atom method using 2214 independent observed reflections. The conformation of the macrocyclic ring is discussed. 相似文献
46.
47.
Brewer CA Brizuela F Wachulak P Martz DH Chao W Anderson EH Attwood DT Vinogradov AV Artyukov IA Ponomareko AG Kondratenko VV Marconi MC Rocca JJ Menoni CS 《Optics letters》2008,33(5):518-520
We have demonstrated near-wavelength resolution microscopy in the extreme ultraviolet. Images of 50 nm diameter nanotubes were obtained with a single ~1 ns duration pulse from a desktop-size 46.9 nm laser. We measured the modulation transfer function of the microscope for three different numerical aperture zone plate objectives, demonstrating that 54 nm half-period structures can be resolved. The combination of near-wavelength spatial resolution and high temporal resolution opens myriad opportunities in imaging, such as the ability to directly investigate dynamics of nanoscale structures. 相似文献
48.
P. Gluziński S. Kasprzyk J. W. Krajewski P. Sałański T. Stankiewicz J. Jurczak 《Journal of chemical crystallography》1991,21(3):357-363
1,8-Diaza-11,14-dioxacyclohexadeca-2,7-dione (3) has been synthesized under high-pressure conditions, whereas 1,10-diaza-4,7-dioxacyclohexadeca-2,9-dione (6) has been obtained under thermal conditions. Both compounds have been investigated in the solid state and in solution by X-ray structural and NMR methods, respectively. The crystals of3 are orthorhombic, space groupPbca; the crystals of6 are triclinic, space groupP¯1. Both structures were solved by direct methods, and refined by a full-matrix, least-squares procedure giving for3
R=0.045 and for 6R=0.056 (R
w
=0.065). From the NMR investigations in solution a twofold symmetry of both molecules (probably in intermediate conformations) is suggested; no such symmetry was found in crystalline state. The structure of6 exhibits a positional disorder of one O-heteroatoms with the population ratio 0.510.49. The arrangement of the macroring with respect to the carbonyl groups was found to betransoid in3 andcisoid in6. 相似文献
49.
Pawel Bechler Ronald DeVore Anna Kamont Guergana Petrova Przemyslaw Wojtaszczyk 《Transactions of the American Mathematical Society》2007,359(2):619-635
Let be the space of functions of bounded variation on with . Let , , be a wavelet system of compactly supported functions normalized in , i.e., , . Each has a unique wavelet expansion with convergence in . If is the set of indicies for which are largest (with ties handled in an arbitrary way), then is called a greedy approximation to . It is shown that with a constant independent of . This answers in the affirmative a conjecture of Meyer (2001).
50.
Cationic copolymerization of 3-membered cyclic ethers (oxiranes) with 5-membered cyclic ether (tetrahydrofuran) leads to linear copolyethers containing significant amount of cyclic fraction. When the copolymerization is conducted in the presence of diols, telechelic copolymers are formed by the process in which oxirane is incorporated into copolymer irreversibly by Activated Monomer mechanism, while tetrahydrofuran is incorporated reversibly by Active Chain End mechanism. Thus both kinetic and thermodynamic factors can be used to control the rate of the incorporation of comonomers. Studies of the kinetics of the competing reactions leading to formation of copolymer, studies of copolymer microstructure by 13C-NMR and analysis of the composition of the cyclic fraction, led to the correlation between the copolymer microstructure and the amount of cyclic fraction formed. The dependence of the cyclic fraction content on the reaction conditions was therefore explained. 相似文献