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991.
Palmer FN Lach F Poriel C Pepper AG Bagley MC Slawin AM Moody CJ 《Organic & biomolecular chemistry》2005,3(20):3805-3811
Various approaches to the tyrosine-derived fragment of the marine secondary metabolite diazonamide A are described. Initial efforts were focused on the originally proposed structure of the natural product, and a feasibility study established that a model 4-aryltryptamine could be readily prepared. Protected 4-bromotryptamine underwent Pd0-catalyzed coupling with the boronic acid derived from 2-bromophenyl allyl ether by Claisen rearrangement, O-methylation and lithiation-boration. The resulting biaryl was elaborated into an alpha-diazo-beta-ketoester, dirhodium(II)-catalyzed reaction of which with N-Z-valinamide gave the desired tryptamine-oxazole following cyclodehydration of the intermediate ketoamide. A potential precursor to the benzofuran ring of the original structure of diazonamide A was prepared in eight steps from N-Z-tyrosine tert-butyl ester. Iodination, O-protection and Stille coupling gave the cinnamyl alcohol 25, converted via the bromide into the allyl aryl ether 27. Subsequent Claisen rearrangement and oxidative cleavage of the alkene gave the lactol 29, converted into the desired benzofuranone 31. The revision in the structure of diazonamide A to 2 resulted in the targeting of an alternative tyrosine-derived model benzofuranone 41 synthesized in four steps from N-Z-tyrosine methyl ester 36 by a route involving Claisen rearrangement of cinnamyl ether 37. Poor yields in this sequence prompted an investigation into the intramolecular Heck reaction as a route to benzofuranone 50. Coupling of 3-iodotyrosine 44 with 2-phenylbutenoic acid 48 gave ester 49 that readily underwent intramolecular Heck reaction to give benzofuranone 50, albeit with poor stereocontrol. 相似文献
992.
Regioselective and stereospecific intramolecular alkylative arylation of unsaturated oxabicyclic diol 6, mediated by Lewis acid or strong protic acid to give the tetracyclic products 7a and 7b, as shown above, represents the first example of an electrophilic (cationic in character) ring-opening-cyclization of oxabicyclic alkene. This constitutes the key cyclization step for a long-standing and potentially useful diene approach for the synthesis of estrone and analogous steroid structures. [structure: see text] 相似文献
993.
Z. G. Aliev L. O. Atovmyan S. S. Kataev V. V. Zalesov 《Chemistry of Heterocyclic Compounds》2007,43(3):377-381
Reactions between 2-pyridylamides of Z-4-aryl-2-hydroxy-4-oxobut-2-enoic acids with diazomethane have been used to synthesize
3-(2-aryl-2-oxoethyl)-3-methoxy-2-oxo-2,3-dihydroimidazo[1,2-a]pyridines, which form hydrochlorides with hydrochloric acid.
The structure of the latter has been demonstrated by XRD for the hydrochloride of 3-methoxy-2-oxo-3-(2-phenyl-2-oxoethyl)-2,3-dihydroimidazo[1,2-a]pyridine.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 464–468, 2007. 相似文献
994.
P. Thuéry M. Nierlich C. Bressot V. Lamare J. F. Dozol Z. Asfari J. Vicens 《Journal of inclusion phenomena and macrocyclic chemistry》1995,23(4):305-312
The crystal structures of two binuclear complexes between caesium and 1,3-calix[4]-bis-crowns have been determined. Cs2
Bis-benzoC6(NO3)2. 3CHCl3 (1) in whichBis-benzoC6 is 1,3-calix[4]-bis-benzo-crown-6, crystallizes in the orthorhombic system: space groupPca2
1 a=19.513(10),b=15.382(5),c=23.708(9) Å,V=7116(5) Å3,Z=4. Refinement led to a final conventionalR value of 0.065 for 2321 reflections. The structure of (1) is analogous to those already reported withBis-C6, (in whichBis-C6 is, 1,3-calix[4]-bis-crown-6) and NO
3
–
as a counter-ion. Cs2
Bis-C6(NCS)2 (2) crystallizes in the monoclinic system: space, groupC2 a=36.57(2),b=11.47(1),c=13.65(1) Å, =109.03(5)°.,V=5415(6) Å3,Z=4. Refinement led to a final conventionalR value of 0.063 for 2227 reflections. Compound (2) is made of dimers bridged by a disordered NCS– ion. The crown ether chain conformations are discussed.
Supplementary data relating to this article (atomic coordinates, anisotropic displacement parameters, bond distances and angles and observed and calculated structure factors) are deposited with the British Library as Supplementary Publication No. SUP 82199 (52 pages). 相似文献
995.
It is probable that different phenomena attributed to mechanical stirring during the Belousov-Zhabotinskii (BZ) reaction are caused by atmospheric oxygen and, as far as our experiments show, they have nothing to do with dissipative structures or with the turbulence of the reacting mixture.
, , , -, .相似文献
996.
Washing conditions of phosphate slurry from which Ni–P–O–Te catalyst had been obtained had practically no effect on the initial H
surf.
+
concentration but affected substantially the degree of acidification of the catalyst surface during the first stage of ethylbenzene oxidehydrogenation, and thereby the catalyst activity.
, Ni–P–O–Te, H . + , .相似文献
997.
Z. M. Enikeeva 《Chemistry of Natural Compounds》1998,34(6):699-705
With the aim of enhancing the cytostatic properties of the initial alkaloid, new aziridine and bis(chloroethyl)amine derivatives
of colchicine have been synthesized by the direct interaction of colchicine with chloroethylamine hydrochloride and also via
the mono- and diethanolamine derivatives. The structures of the compounds obtained have been studied by spectral methods.
An increased radiomodifying and antitumoral activity and a decreased toxicity of the substances synthesized as compared with
the initial colchicine has been shown. Results obtained in the National Cancer Institute of the USA from the study of the
cytostatic activity of the bis(chloroethyl)amino derivative of 60 tumor lines are presented.
Institute of Oncology and Radiology, Academy of Sciences of the Republic of Uzbekistan. Translated from Khimiya Prirodnykh
Soedinenii, No. 6, pp. 782–789, November–December, 1998. 相似文献
998.
Snezana S.?MiticEmail author Gordana Z.?Miletic Aleksandra N.?Pavlovic Snezana B.?Tosic 《Monatshefte für Chemie / Chemical Monthly》2004,135(8):927-936
Summary. A new selective, sensitive, and simple kinetic method is developed for the determination of trace amounts of chromium (VI). The method is based on the catalytic effect of Cr(VI) on the reaction of sodium pyrogallol-5-sulphonate (PS) with hydrogen peroxide. The reaction is followed spectrophotometrically by tracing the oxidation product at 437nm within 1min after addition of H2O2. The optimum reaction conditions are PS (1.32·10–3mol·dm–3), H2O2 (0.32mol·dm–3), HClO4 (2.6·10–3mol·dm–3) at 25°C. Following this procedure, chromium (VI) can be determined with a linear calibration graph up to 0.25ng·cm–3 and a detection limit of 0.024ng·cm–3, based on the 3 criterion. The interference effect of several species was also investigated and it was found that the most common cations and anions do not interfere with the determination. The developed procedure was successfully applied to the determination of Cr(VI) and total Cr in river waters and total Cr in herbal samples. 相似文献
999.
The complexes (RN=CH-CH=NR)Co(NO)(CO) with R = isopropyl, 2,6-diisopropylphenyl, or p-tolyl are chemically and electrochemically reducible to radical anions at potentials which strongly depend on R. The DFT calculated structure for the neutral compound with R = iPr agrees with the experiment, and the computed structure of the anion radical reveals changes according to a reduction of the R-DAB ligand. EPR results confirm an (R-DAB)-based singly occupied molecular orbital in [(RNCHCHNR)Co(NO)(CO)](.-), with minor but detectable contributions from NO as supported by IR spectroelectrochemistry and as quantified by DFT spin density calculations. The calculations indicate increasingly stabilized CO, NO, and RNCHCHNR pi* acceptor orbitals, in that order. On the basis of TD-DFT (time-dependent density functional theory) calculations, the lowest-lying excited states are assigned to metal-to-(R-DAB) charge transfer transitions while bands due to the metal-to-nitrosyl charge transfer occur at higher energies but still in the visible region. Resonance Raman studies were used to probe these assignments. 相似文献
1000.
The dipole moments of 2-alkylaminoand 2-alkylnitramino-3(or 5)-methyl-4-nitropyridines and their N-oxides have been measured, as well as calculated, using vectorial summation of the group moments and by ab initio method. The estimated interaction dipole moments, int, have been discussed in terms of the electronic and steric effects and intramolecular hydrogen bonding. Introduction of the NHCH3 group to position 2 in 4-nitropyridine strengthens the conjugation between the nitro group and ring nitrogen. The lack of such strengthening in 4-nitropyridine-N-oxides is explained as being due to formation of intramolecular hydrogen bond between the N-oxide group and the hydrogen of alkylamino group. Introduction of the nitramino group does not lead to marked modification of the charge distribution in a molecule. This fact may be explained by much weaker electron-donating ability of this group in comparison with the alkylamino group. 相似文献