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921.
Intramolecular Diels-Alder reactions of 2-azadiene models have been studied quantum chemically at the B3LYP/6-31G level in order to elucidate the stereochemical features of the cyclization step involved in the biosynthesis of paraherquamide A and VM99955. These cycloadditions take place through concerted transition states associated with [4 + 2] processes. Analysis of the energies along the competitive paths reveals that while the cycloadditions of the oxindoles present a large anti selectivity, the indoles show a low syn selectivity for the formation of the C20 stereogenic center that is larger for the reduced tertiary amide form. The presence of the C14 methyl of the beta-methylproline ring produces a low hindrance along the reaction coordinate for the syn approach of the isoprene framework, in agreement with the low facial selectivity found experimentally. An analysis of the electrophilicity and activation parameters for experimental models of the inter- and intramolecular Diels-Alder reactions reveals several significant factors controlling these biosynthetic cyclizations. The results are in reasonable agreement with the available experimental data.  相似文献   
922.
Reactions of cis-dialkoxy-bis(acetylacetonato)titanium(IV), [(acac)2Ti(OR)2] (R = Et, Pr i ) with alkoxyalkanols (ROCH2CH2OH) (R = Me, Et, n-Bu) in 1:1 and 1:2 molar ratios in refluxing benzene under anhydrous conditions yield [(acac)2Ti(OR)2–n (OCH2CH2OR) n ] (n = 1 or 2) complexes, which were purified by distillation under reduced pressure. On the basis of i.r. and n.m.r. (1H- and 13C-) spectral studies, a cis-octahedral environment around TiIV is proposed. On keeping the distilled dark brown-red viscous liquid [(acac)2Ti(OEt)(OCH2CH2OBu)] for 2 weeks, orange yellow crystals of [(acac)2TiO]2 were obtained. A single crystal X-ray diffraction study suggests the product is a new modification of [(acac)2TiO]2.  相似文献   
923.
Evidence of the interaction among the [PbAc]+ and[PbEDTA]-2 with the -cyclodextrin (-CD)is shown, using a 24 factorial design. The factors used werethe [Pb(II)], [-CD], [Ac-] and [EDTA] concentrationsat pH = 6.2. The statistical results indicate that the changes inchemical response for these species are associated to theinteraction among these factors. Spectrophotometric andelectrochemical studies were undertaken to assess the interactions.The equilibrium constants for inclusion complexes were determinedFor -CD-[PbAc]+ and -CD-[PbEDTA]-2.  相似文献   
924.
The phosphorescence spectra and triplet state lifetimes of palladium octaethylporphyrin (PdOEP), palladium octaethylchlorin (PdOEC) and palladium 2,3-dimethyloctaethylisobacteriochlorin (PdOEiBC) in n-octane Shpolskii matrices at 77 K are reported. The lifetime and T(1)/S(0) origin energy of each complex are: PdOEP, 1.90+/-0.04 ms, 15162 cm(-1); PdOEC, 0.43+/-0.03 ms, 12547 cm(-1) and PdOEiBC, 0.59+/-0.03 ms, 12863 cm(-1).  相似文献   
925.
926.
The lithium diisopropylamide-mediated 1,2-elimination of 1-bromocyclooctene to provide cyclooctyne is investigated using approximately 50 potentially hemilabile polyethers and amino ethers. Rate laws for selected ligands reveal chelated monomer-based pathways. The dependence of the rates on ligand structure shows that anticipated rate accelerations based on the gem-dimethyl effect are nonexistent and that substituents generally retard the reaction. With the aid of semiempirical and DFT computational studies, the factors influencing chelation are discussed. It seems that severe buttressing within chelates of the substitutionally rich ligands precludes a net stabilization of the chelates relative to nonchelated (eta(1)-solvated) forms. One ligand-MeOCH(2)CH(2)NMe(2)-appears to promote elimination uniquely by a higher-coordinate monomer-based pathway.  相似文献   
927.
[reaction: see text] An efficient asymmetric synthesis of the vasopeptidase inhibitor BMS-189921 was accomplished. Two short enantioselective syntheses of the common key intermediate (S)-alpha-aminoazepinone 6b were developed. Olefin 3 was converted to 6b via asymmetric hydrogenation. Alternatively, enyne 12 was converted to racemic alpha-aminoazepinone 15b, which was transformed to 6b by a practical dynamic resolution.  相似文献   
928.
Ram RN  Meher NK 《Organic letters》2003,5(2):145-147
[reaction: see text] 2,2,2-Trichloroethyl carboxylates undergo highly efficient dechlorinative Surzur-Tanner rearrangement with 2 equiv of a 1:1 molar mixture of CuCl and bpy in boiling DCE to give 1-chloroethenyl carboxylates in which copper appears to play an important role, probably by coordinating the initial radical or as a Lewis acid catalyst.  相似文献   
929.
This work reports the use of and -cyclodextrin-modified carbon paste electrodes (CPE-CD and CPE-CD) to determine simultaneously Pb(II) and Cd(II) by means of the electrochemical technique known as anodic stripping voltammetry (ASV). Both modified electrodes displayed good resolution of the oxidation peaks of the said metals. Statistic analysis of the results strongly suggests that the CPE-CD exhibited a better analytical response that the CPE-CD, while the detection limits obtained for Pb(II) were 6.3×10–7 M for the CPE-CD and 7.14×10–7 M for the CPE-CD, whereas for Cd(II) they were 2.51×10–6 M for the CPE-CD and 2.03×10–6 M for the CPE-CD.  相似文献   
930.
Apparent molar volumes of glycine, DL--alanine, L-valine, L-leucine, and L-phenylalanine in 0.5, 1.0, 2.0, 3.5, and 5.0 m B (mol-kg–1) aqueous solutions of glycerol have been obtained from solution densities at 25°C using precise vibrating-tube digital densimeter. The estimated partial molar volumes at infinite dilution V o 2 have been used to obtain the corresponding transfer volumes tr V 2 o from water to different glycerol–water mixtures. The transfer volumes are positive for glycine and DL--alanine, and both positive and negative for the other amino acids over the concentration range studied. Interaction coefficients have been obtained from McMillan–Mayer approach and the data have been interpreted in terms of solute–cosolute interactions.  相似文献   
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