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81.
The kinetics and thermodynamics of atropisomerism within the protected kedarcidin chromophore aglycon 8, as well as a series of ansa-bridged synthetic intermediates leading to 8, were determined by 1H NMR spectroscopy. The data show that the ratio of atropisomeric forms of chloropyridine-bridged ansa intermediates is subject to wide variation with seemingly subtle structural variation. The vinyl bromide 4, for example, in the first X-ray structure determination of a kedarcidin ansa-bridged system, was found to exist as a single atropisomer in the solid state, but a nearly equal mixture (K = 0.70) of isomers in solution (t1/2 for isomer interconversion approximately 0.2 s at 20 degrees C). The aglycon 8, a 2.2:1 mixture of atropisomers, was found to undergo direct unimolecular biradical-forming cycloaromatization at ambient temperature in a mixture of 1,4-cyclohexadiene-benzene, without nucleophilic activation. The product 9 was formed as a single atropisomer (k = 2 x 10-4 s-1, t1/2 = 58 min, 81% yield), suggesting that the rate of atropisomerism within 8 is rapid with respect to cycloaromatization. The rate of cycloaromatization of 8 was found to be highly solvent-dependent, being more rapid in the presence of a good hydrogen-atom donor, consistent with the earlier model studies of Hirama et al. that showed that certain nine-membered cyclic (Z)-enediynes may equilibrate with their biradical cycloaromatization products. Incubation of 8 with beta-mercaptoethanol, under conditions mimicking experiments leading to DNA cleavage with kedarcidin, showed no evidence for nucleophilic activation. The product of direct cycloaromatization (9) was isolated instead. The evidence suggests that kedarcidin, like the enediyne agent C-1027, is capable of spontaneous thermal biradical formation without prior chemical activation. 相似文献
82.
The microbial degradation of deoxycholic acid 1 by Pseudomonas NCIB 10590 has been studied and two major products have been isolated and identified as 12β-hydroxyandrosta-1,4-dien-3,17-dione 2 and 12α-hydroxypregna-1,4-dien-3-one-20-carboxylic acid 9. Three minor products were isolated and evidence is given for the following structures: 12α-hydroxyandrosta-1,4-dien-3,17-dione 4, 12β-hydroxyandrosta-4-en-3,17-dione 7 and 12?, 17?-dihydroxyandrosta-1,4-dien-3-one 8. 相似文献
83.
Synthetic approaches to 6
-9-methoxy-5, 11-dimethylpyrido[4,3-
] fluorene are described, thus 2-[1-(4-pyridyl)ethyl]6-methoxyindanone has been reacted with vinylmagnesium bromide to yield 2-[1-(4-pyridyl)ethyl]-6 methoxy-1-vinyl-1-indanol. However, on dehydration this compound rearranges to 2-[1-(4-pyridyl)ethyl]-3-ethylidene-5-methoxyindene rather than the required isomer 3-ethylidine-2-[1-(4-pyridyl)ethyl]-5-methoxyindane needed fro pericyclic ring-closure to the pyrido[4,3-
]fluorene system. Reaction of
-2-[1-methyl-1-(4-pyridyl)methylene]-6-methoxyindanone with ethylene triphenylphosphorane similarly gives 6-methoxy-spiro-2-[1-(2,3-dimethyl-3-(4-pyridyl)-cyclopropyl) indanone rather than the desired indane. 相似文献
84.
Mobilities of H+ and H? in He and in H2, and of H+2 and H+3 in He, are calculated from ion-neutral potentials derived from theory and ion-beam scattering. Agreement with experiment is reasonable except for H? in H2 and H+2 in He, which present unexplained puzzles. 相似文献
85.
Synthesis of amides from coupling esters with a range of primary amines can be conveniently achieved in moderate to excellent yields (69-99%) using an air-stable adduct of trimethylaluminium (AlMe3)2·DABCO (DABCO is 1,4-diazobicyclo[2.2.2]octane), referred to as DABAL-Me3. Reactions can be run without requiring the exclusion of atmospheric oxygen or the drying of solvents. 相似文献
86.
Cowley AR Downs AJ Himmel HJ Marchant S Parsons S Yeoman JA 《Dalton transactions (Cambridge, England : 2003)》2005,(9):1591-1597
The novel adduct 1,1,3,3-tetramethylguanidine-gallane, (Me2N)2CN(H).GaH3, has been prepared by the reaction of [(Me2N)2CNH2]+Cl- with LiGaH4 in Et2O solution. Its spectroscopic properties indicate a monomeric species with an unusually strong coordinate link between the imido function and GaH3, an inference confirmed by the crystal structure at 150 K which also reveals significant secondary interactions through non-classical N-H...H-Ga bridges. Despite the intrinsic strength of the Ga-N bond, however, vaporisation at ca. 310 K occurs with partial dissociation, and decomposition via more than one pathway proceeds at temperatures >330-350 K to give a variety of products, including the free base, Me2NH, H2, and a novel gallium-nitrogen compound composed of a Ga4N4 cubane-like core bridged on three edges by -N{C(NMe2)2}GaH2- units. 相似文献
87.
The basicity of 1,3-di-tert-butylimidazol-2-ylidene (1) was measured in THF against three hydrocarbon indicators. Both ion pairs and free ions were found and the corresponding equilibrium constants were measured. Homoconjugation was not found in either THF or DMSO. The carbene is effectively more basic in DMSO by several pK units, probably because of hydrogen bonding of 1-H(+) to DMSO. Model ab initio computations are consistent with these results. 相似文献
88.
Summary The organofunctional trisiloxanes Me3SiOSiMe(R)OSiMe3 [R=(CH2)2PPh2, (CH2)3C5H4N, (CH2)3CN, (CH2)2Ph, (CH2)2SPh, CH=CH2 and CH2CH=CH2] have been reacted with metal halide and-carbonyl moieties in order to determine the coordination preferences of materials being used as models for metallated longchain linear functionalised polysiloxanes. The products [Me3SiOSiMe(R)OSiMe3]3MLn [R=(CH2)2PPh2, MLn=RhCl],cis-[Me3SiOSiMe(R)OSiMe3]2MLn [R=(CH2)2PPh2 or (CH2)3C5H4N, MLn=Mo(CO)4],trans-[Me3SiOSiMe(R)OSiMe3]2MLn[R=(CH2)2PPh2, MLn=NiCl2, PdCl2, PtCl2 and [Rh(CO)Cl] and [Me3SiOSiMe(R)OSiMe3]MLn [R=(CH2)2PPh2, MLn=Mo(CO)3(2,2-bipyridine); R=(CH2)2Ph, MLn=Mo(CO)3; R=(CH2)3C5H4N, (CH2)3CN, or (CH2)2SPh, MLn=Rh(CO)2Cl; R=CH=CH2 or CH2CH=CH2, MLn=Fe(CO)4] have been isolated and characterised spectroscopically in the course of these studies. 相似文献
89.
90.
The preparation of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexaazatricyclo [13.3.1.16,10]eicosane (2) by the reaction of the nickel(II) complex of N-(2-aminoethyl)-1,3-diaminopropane with formaldehyde in MeOH solution is described. The crystal structure of [NiL](ClO4)2 has been determined. The nickel atom is four coordinate and planar with Ni-N bond lengths of 1.969(4) and 1.928(3)Å in a centrosymmetric structure. The basic diazacyclam ring system has a trans III configuration with the two additional six-membered rings fused in a chair conformation.The kinetics of the metal exchange:for the nickel complexes (1) and (2) have been studied in detail. Under the experimental conditions employed, with copper(II) in at least a tenfold excess, the reaction is independent of the copper(II) concentration. The copper(II) effectively scavenges the free ligand as the nickel(II) complex dissociates. For the nickel complex (1) k = 2 × 10–4 s–1 at 60°C and H = 126 ± 5 kJmol–1 and S298 = 61 ± 15 JK–1mol–1. For the complex (2), k = 1.8 × 10–4 s–1 at 60°C and H = 99 ± 6 kJmol–1 and S298 = –21 ± 10 JK–1 mol–1. 相似文献