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901.
Insertion of monosubstituted allenes into stable oxapalladacycle I was studied. The aim of this work was to define steric and electronic parameters of allenes that would allow for a regio- and diastereoselective synthesis of 2,3-disubstituted 3,4-dihydro-2H-1-benzopyrans, which could not be prepared via related catalytic protocols. Allenes with electron-donating alkyl substituents R sterically unencumbered at the C-3 and C-4 carbons reacted with palladacycles I to afford benzopyrans IV in good yields (45-81%), exclusively as cis diastereomers. Less than 10% of the regioisomeric benzopyrans V was detected in the crude reaction mixtures. Methoxy 1,2-propadiene afforded benzopyran IV in 98% yield as the trans diastereomer in 92% de. In contrast, allenes with electron-withdrawing substituents yielded benzopyrans V with an E double bond exclusively. Nonracemic palladacycles featuring a palladium-bonded stereogenic carbon as the only element of asymmetry underwent the allene insertion with 63-93% retention of the stereochemical information, providing benzopyrans IV or V in 40-47% ee. These results demonstrated that O-bonded palladium enolates did not operate as predominant intermediates in the insertion process. The study highlights the configurational stability of carbon-bonded palladium ester enolates, especially notable in systems lacking chiral nonracemic auxiliary ligands. 相似文献
902.
Vitor F. Ferreira Antonio V. Pinto Maria C. R. F. Pinto Milton N. da Silva Janet M. S. Skakle Maria C. B. V. de Souza Solange M. S. V. Wardell 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(9):o560-o562
The central six‐membered ring in the title compound, C16H16O3, is almost planar (and almost coplanar with the aromatic ring), despite one of its C atoms being formally sp3 hybridized. The planarity is a consequence of the C atom at the centre of the spirocyclic system also being part of the three‐membered epoxide ring. The molecules are linked by π–π and C—H?π interactions. 相似文献
903.
Summary New MoVI-dioxodialkyl complexes, MoO2R2(bipy), R = CH2CH2Ph and p-MeC6H4CH2; bipy = 2,2-bipyridine, have been synthesized. The i.r. and the 1H-n.m.r. spectra of these complexes are noted. The structure of MoO2(o-MeC6H4CH2)2(bipy) was determined by X-ray analysis. Significant differences in the redox characteristics of these dioxodialkylcompounds are reflected in the contrasting patterns: whereas reduction of MoO2-(CH2CH2Ph) 2(bipy) is a reversible one-electron process, under similar conditions MoO2(p-MeC6H4CH2)2(bipy) and MoO2(o-MeC6H4CH2)2(bipy) are reduced irreversibly. Similarly, solutions of MoO2(CH2CH2Ph)2(bipy) remain unchanged but oxygenated organic products are formed from MoO2(p-MeC6H4CH2)2(bipy) and MoO2 (o-MeC6H4CH2)2(bipy). 相似文献
904.
Bernard N Chemla F Ferreira F Mostefai N Normant JF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(14):3139-3147
The carbometalation reaction of allyl- and crotylzinc bromide with metalated disubstituted homoallylic ethers gives tri- or tetrasubstituted 5-hexenyl ethers with excellent control of the diastereoselectivity. The stereochemistry of this reaction is discussed and has been attributed to an early transition state with chelation to the oxygen atom in the energetically favored conformer as the stereodetermining step. 相似文献
905.
Martins AM Ascenso JR Costa SM Dias AR Ferreira H Ferreira JA 《Inorganic chemistry》2005,44(24):9017-9022
[Ti[N(Ph)SiMe2]3-tacn]X complexes (X = Cl, 1; I, 2; PF6, 3; BPh4, 4) were studied by NMR and electron absorption and emission methods, which showed that these compounds exist in bromobenzene and dichloromethane solutions as ion pairs. The significant modifications observed in the proton resonances of tacn in C6D5Br, which follow the sequence BPh(4-) > or = PF(6-) > or = I- approximately Cl-, are a qualitative indication of the strength of the interactions that depend on the anion. The reaction of 2 with LiNMe2 led to [Ti(NPh)[NPh(SiMe2)]2-tacn], 5, that forms upon attack of Me2N- at one SiMe2 group. The formation of 5 is discussed on the basis of the interactions identified in solution. 相似文献
906.
Ana S. Abreu Luís S. Monteiro Isabel C.F.R. Ferreira Letícia M. Estevinho 《Tetrahedron》2004,60(51):11821-11828
Several benzo[b]thienyldehydrophenylalanines were synthesized from pure stereoisomers of the methyl ester of N-(tert-butoxycarbonyl)-β-bromodehydrophenylalanine as an extension of our previously reported method for the synthesis of dehydrotryptophan analogues to dehydrophenylalanine derivatives. The latter were obtained in high yields by N-deprotection and bromination of N,N-bis-(tert-butoxycarbonyl)-(Z)-dehydrophenylalanine using TFA and NBS. This was carried out in two steps or in a one pot procedure resulting in different E/Z ratios. These compounds were coupled under Suzuki cross-coupling conditions [Pd(PPh3)4, Na2CO3, DME/H2O] with several boronic benzo[b]thienyl acids in good to high yields maintaining the stereochemistry of the starting materials. The best yields were obtained when the boronic acid was in position 7 of the benzo[b]thiophene and with the E isomer of the brominated dehydrophenylalanine. In some cases it was possible to increase the lower yields by changing the Pd source to PdCl2(PPh3)2. A model dipeptide was prepared coupling a benzo[b]thienyldehydrophenylalanine with the methyl ester of alanine. Preliminary antimicrobial studies were performed with both isomers of one of the β, β-diaryldehydroalanines obtained. The results show that the compounds are selective and very active (very low MICs) against Gram positive bacteria (B. cereus and B. subtilis) the Z-isomer being more active. The compounds are also active against Candida albicans presenting similar MICs. 相似文献
907.
Dual amperometric biosensor device for analysis of binary mixtures of phenols by multivariate calibration using partial least squares 总被引:1,自引:0,他引:1
A simple and reliable method for rapid evaluation of mixtures of phenolic compounds (phenol/chlorophenol, cathecol/phenol, cresol/chlorocresol and phenol/cresol) using a dual amperometric device is described. This new approach is based on the difference between the sensitivity of laccase and tyrosinase for different phenolic compounds. A multichannel potentiostat was used to monitor simultaneously laccase- and tyrosinase-based biosensors, and the data were treated using the partial least squares (PLS) chemometric algorithm. This system showed an excellent efficiency for the resolution of the phenolic mixtures. For example, in the phenol/chlorophenol mixture it was studied the determination of individual species in a concentration range from 1.0×10−6 to 10.0×10−6 mol l−1 obtaining relative standard deviations of 3.5 and 3.1% for phenol and chlorophenol, respectively. The excellent correlation between the estimated and the real concentrations can also be observed by the correlation coefficients (0.9958 and 0.9981 for phenol and chlorophenol, respectively). These results show that proposed methodology can be successfully employed to the simultaneous determination of phenolic compounds in mixtures, even in more diluted solutions. 相似文献
908.
We show how to introduce the Foldy-Wouthuysen relativistic corrections in the multiple-scattering method, for the determination of the electronic energy levels of molecules. The present derivation begins from a variational expression where the Foldy-Wouthuysen hamiltonian is inserted, instead of the Schrödinger hamiltonian. The resulting secular equation becomes identical with the standard multiple-scattering secular equation, if one neglects the relativistic correction terms. 相似文献
909.
The density-functional approach to Molecular Orbital theory shows that the chemical bonding potential is better described by orbital electronegativities than by ionization energies. This results from the fact that the electronic relaxation connected with ionization is not significant for the homolytic breaking of chemical bonds. Electronegativity, on the other hand, is an eigenvalue corresponding to the average potential seen by an electron as a molecular orbital changes into monocentric (atomic) orbitals. 相似文献
910.
Haroldo Taurian Gasiglia Helena Okada 《Journal of Radioanalytical and Nuclear Chemistry》1995,199(4):295-304
Samarium-153-ethylenediaminetetramethylenephosphonate (153Sm-EDTMP) has been used to palliate pain resulting from bone cancer. This paper describes the preparation of153SmCl3 from irradiated natural samarium and the ability of153Sm to complex with EDTMP in liquid and in freeze-dried forms. The evaluation of radiolabeled EDTMP was done by paper chromatography. A rapid evaluation of free153SmCl3 and153Sm-EDTMP was developed using a miniaturized chromatographic system. 相似文献