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81.
Chiral 1-[9'-(2'-methoxyanthryl)]-2,2-dimesitylethenol (2), 1-[9'-(2'-fluoroanthryl)]-2,2-dimesitylethenol (3), and 1-[9'-(2'-fluoroanthryl)]-2,2- dimesitylvinyl acetate (4) were synthesized and their DNMR behavior in C6D5NO2 was studied. 3 and 4 were resolved on an amylose tris(3,5-dimethylphenylcarbamate) HPLC column to their enantiomers. Acetate 4 racemizes slowly in solution with DeltaG(e)(++), DeltaH(e)(++), and DeltaS(e)(++) values of 26.2, 27.6 kcal mol(-)(1), and 4.3 eu, respectively, as expected for a rotational betabeta'-2-ring flip process in a vinyl propeller and the racemization is unaffected by added TFA, Et3N, and EtOD. Although 3 racemizes almost 350 times faster, the racemization is catalyzed by TFA and shows bell-shape catalysis by Et3N and a KIE in a partially deuteriated solvent. From this and the DNMR data, it is concluded that 3 does not racemize via a rotational betabeta'-2-ring flip. Five nonflip routes are discussed for the racemization of 3, and it is concluded that only the one initiated by protonation at C1 does not contradict the experimental data. By analogy with the E/Z isomerization of the structurally related 2-(m-methoxymesityl)-1,2-dimesitylethenol 17, it is suggested that in the absence of added catalyst one or more enol molecule(s) catalyze the enantiomerization of another one. Only partial resolution was achieved for 2 and from the similarity of its behavior with that of 3, it is suggested that it racemizes by the same mechanism. 相似文献
82.
Jong S. Kim Menachem Lewin Bernard J. Bulkin 《Journal of Polymer Science.Polymer Physics》1986,24(8):1783-1789
Analysis is made of the asymmetry of the carbonyl stretching vibration of poly(ethylene terephthalate) and poly(propylene terephthalate) as a function of annealing frrom amorphous to crystalline polymer. An apparently anomalous behavior of carbonyl band half-width as a function of density for PPT is shown to be due to this asymmetry. The results are interpreted in terms of the conformational redistribution which occurs upon annealing. 相似文献
83.
We show that in the constructible universe, the two usual definitions of Butler groups are equivalent for groups of arbitrarily
large power. We also prove that Bext2(G, T) vanishes for every torsion-free groupG and torsion groupT. Furthermore, balanced subgroups of completely decomposable groups are Butler groups. These results have been known, under
CH, only for groups of cardinalities ≤ ℵω.
Partial support by NSF is gratefully acknowledged.
Partially supported by U.S.-Israel Binational Science Foundation. 相似文献
84.
The title compound reacts with the dienophile by attacking the face of a cyclo-hexadiene ring which is syn- to the lactone ring. 相似文献
85.
86.
Abstract— A sensitive optical method to monitor local environmental changes in biological systems is described. It utilizes the high optical amplification typical to any dye laser cavity by installing the investigated system inside the laser resonator. Very small changes in the optical density of a probe dye can be detected and related to the dye's state of aggregation or in another demonstration to its adsorption on differently charged micelles and liposomes. When further developed, this technique can prove to be a very sensitive method to measure membrane potentials, changes in molecular environment and molecular dynamics. The pulsed laser used is advantageous to a continuous source used in fluorescence and absorption methods reducing photodynamic damage. 相似文献
87.
Menachem Kojman Wieslaw Kubis Saharon Shelah 《Proceedings of the American Mathematical Society》2004,132(11):3357-3365
For an infinite cardinal , denotes the set of all cardinalities of nontrivial maximal almost disjoint families over .
Erdos and Hechler proved in 1973 the consistency of for a singular cardinal and asked if it was ever possible for a singular that , and also whether for every singular cardinal .
We introduce a new method for controlling for a singular and, among other new results about the structure of for singular , settle both problems affirmatively.
88.
Z-α-N-methylacetamidocinnamic acid and its methyl ester were hydrogenated with rhodhim(I) complexes containing (2R, 3R)-O-2,3-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane (DIOP) or its carbocyclic analogues: (1R,2R)-trans-1,2-bis(diphenylphosphinomethyl)cyclobutane or (1S,2S)-trans-1,2-bis(diphenylphosphinomethyl) cyclohexane. The N-acetyl-N-methylphenylalanine methyl ester reaction product was formed with an optical purity of 73% ee-(R) [(2R, 3R)-DIOP]; 43% ee-(R) [(1R, 2R)-cyclobutane analogue]; and 26% ee-(R) [(1S,2S])-cyclohexane analogue]. Similarly, N-acetyl-N-methylphenylalanine was formed with an optical purity of 87% ee-R [(2R, 3R)-DIOP] and 68% ee-(R) [(1R, 2R)-cyclobutane analogue]. 相似文献
89.
Cohen B Huppert D Solntsev KM Tsfadia Y Nachliel E Gutman M 《Journal of the American Chemical Society》2002,124(25):7539-7547
The aqueous phase of water/AOT reversed micelles having varying diameters was probed by a single free diffusing proton that was released form a hydrophilic photoacid molecule (2-naphthol-6,8-disulfonate). The fluorescence decay signals were reconstructed through the geminate recombination algorithm, accounting for the reversible nature of the proton-transfer reactions at the surface of the excited molecule and at the water/detergent interface. The radial diffusion of the proton inside the aqueous phase was calculated accounting for both the entropy of dilution and the total electrostatic energy of the ion pair, consisting of the pair-energy and self-energy of the ions. The analysis implied that micellar surface must be modeled with atomic resolution, assuming that the sulfono residue protrudes above the water/hydrocarbon interface by approximately 2 A. The analysis of the fluorescence decay curves implies that the molecule is located in a solvent with physical-chemical properties very similar to bulk water, except for the dielectric constant. For reversed micelles with r(max) > or = 16 A, the dielectric constant of the aqueous phase was approximately 70 and for smaller micelles, where approximately 60% of the water molecule is in contact with the van der Waals surface of the micelle, it is as low as 60. This reduction is a reflection of the increased fraction of water molecule that is in close interaction with the micelle surface. 相似文献
90.
Pnina Ashkenazi Menachem Kaftory Tuma Sayra Günther Maier David Ginsburg 《Helvetica chimica acta》1983,66(8):2709-2711
Two isomeric propellane epoxides 1a and 1b containing a cyclohexadiene ring are attacked by the title dienophile as predicted, syn to the five-membered ether ring whose α-hydrogens exert less repulsion than the α-epoxy hydrogens or the epoxide oxygen, respectively, of the cyclobutane ring. 相似文献