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1.
《Tetrahedron: Asymmetry》2014,25(23):1532-1536
An asymmetric synthesis of the C8–C16 fragment with several stereogenic centers of amphidinolide R, is described. The key reaction, Sharpless asymmetric epoxidation has provided the basis for generation of the required stereocenters. The target fragment was accomplished in a convergent manner in nine steps (longest linear synthesis of the sequence) and 32% overall yield was obtained starting from 2-butene-1,4-diol.  相似文献   

2.
《Fluid Phase Equilibria》1999,165(2):157-168
A simple method is developed to estimate mixture critical temperatures (Tc), pressures (Pc), and densities (ρc) as a function of overall composition (X) from near critical region experimental coexistence data. This three-step method is applied to four mixtures, CO2–C3H8, CO2nC4H10, C2H6–C3H8, and C3H8nC4H10. Isothermal liquid–vapor coexistence data, which includes temperature, vapor pressure, coexisting densities (ρ and ρv), and coexisting compositions for the more volatile component (x1v and x1) are used. In the first step, the difference of the saturated liquid and vapor densities (ρρv) is fitted to an empirical function in ((PcP)/Pc) to obtain Pc. Then P/Pc and ((ρ+ρv)/2ρc) are simultaneously fitted to functions of a polynomial in (X1−(x1v+x1)/2) yielding estimates of ρc and X1. Finally, the discrete estimated critical data points are fitted with an equation to provide a continuous representation of the critical lines. The method is successfully tested for the mixtures, CO2–C3H8 and CO2nC4H10, for which there is a reasonable amount of isothermal data. The procedure is then applied to the mixtures, C2H6–C3H8 and C3H8nC4H10, for which there are sparse data. For all four mixtures, the critical temperature line, Tc vs. X1, matches literature values within ±0.5%. The critical pressure line, Pc vs. X1, and critical density line, ρc vs. X1, match literature values, in general, within ±2%.  相似文献   

3.
A convergent synthesis of bullatanocin is envisaged by the union of C18–C34, C3–C17 and the butenolide subunits. The synthesis of the C3–C17 and C18–C34 subunits is disclosed that takes advantage of the chirality of tartaric acid for 1,2-asymmetric induction, chloro sulfides for carbon chain elongation and [2,3] sigmatropic shift for the preparation of 1,4-diol moiety via efficient 1,3-chirality transfer. The THF ring is elaborated by intramolecular displacement.  相似文献   

4.
《Tetrahedron letters》1987,28(28):3253-3256
The middle (C10–C17) and right (C18–C30) segments of the polyether antibiotic salinomycin were stereoselectively synthesized from D-glucose, D-mannitol and ethyl L-lactate. Coupling of the two segments followed by construction of the bisketal ring system gave the C10–C30 segment, which was already converted to salinomycin by Kishi.  相似文献   

5.
The electronic structure of UCoC2, a di-carbide with the C–C units is examined from ab initio with an assessment of the properties of chemical bonding. The energy–volume equation of state shows large anisotropy effects due to C–C alignment along tetragonal c-axis leading to high linear incompressibility. Relevant features of selective bonding of uranium and cobalt with carbon at two different Wyckoff sites and strong C–C interactions are remarkable. The vibrational frequencies for C⋯C stretching modes indicate closer behavior to aliphatic C–C rather than CC double bond. A ferromagnetic ground state is proposed from the calculations.  相似文献   

6.
Differences in the system constants of the solvation parameter model and retention factor correlation plots for varied solutes are used to study the retention mechanism on XBridge C8, XBridge Phenyl and XTerra Phenyl stationary phases with acetonitrile–water and methanol–water mobile phases containing from 10 to 70% (v/v) organic solvent. These stationary phases are compared with XBridge C18 and XBridge Shield RP18 characterized in an earlier report using the same protocol. The XBridge stationary phases are all quite similar in their retention properties with larger difference in absolute retention explained by differences in cohesion and the phase ratio, mainly, and smaller changes in relative retention (selectivity) by the differences in individual system constants and their variation with mobile phase type and composition. None of the XBridge stationary phases are selectivity equivalent but XBridge C18 and XBridge Shield RP18 have similar separation properties, likewise so do XBridge C8 and XBridge Phenyl, while the differences between the two groups of two stationary phases is greater than the difference within either group. The limited range of changes in selectivity is demonstrated by the high coefficient of determination (>0.98) for plots of the retention factors for varied compounds on the different XBridge phases with the same mobile phase composition.  相似文献   

7.
8.
Okadaic acid, a potent and selective inhibitor of Protein Phosphatases 1 and 2A (PP1 and PP2A), is widely used as a probe for various biochemical processes. We describe herein two innovative methods for the synthesis of the terminal C28–C38 fragment of the natural polyether. Suárez photochemical oxidative cyclization and electrochemical oxidation of malonates to their ketals equivalents have been successfully applied for the assembly of the key spiroketal core.  相似文献   

9.
An efficient and facile approach to synthesize imidazoles from amidines and arylketone via oxidative coupling of sp3 C–H bond and N–H bond is reported. This strategy exhibits high performance in terms of regioselectivity with moderate to high yields by using easily available materials, and provides an alternative method to synthesize multi-substituted imidazole skeletons.  相似文献   

10.
Alkylarenes, obtained from abundant hydrocarbon feedstock sources, are an attractive starting material for the formation of complex molecular architectures. Conventional activation strategies of the relatively inert sp3-hybridized benzylic C–H bonds usually require relatively harsh conditions and are difficult to apply to the synthesis of fine chemicals. The present review describes recent strategic advances for the activation of benzylic C–H bonds for the catalytic formation of C–C bonds. In particular, two activation methods, i.e., strategies that generate benzylic radicals or benzyl anions, are discussed.  相似文献   

11.
《Tetrahedron》2019,75(33):4570-4576
Synthesis of the C1–C13 fragment of eribulin mesylate has been accomplished. It features a highly stereoselective construction of a trans-dihydropyran framework using three key reactions: (1) Sharpless epoxidation, (2) regioselective ring opening, and (3) olefin metathesis.  相似文献   

12.
The [H,S,Cl] potential-energy surface has been investigated at the self-consistent field (SCF), complete active space self-consistent field (CASSCF), second-order M?ller–Plesset, coupled-cluster single-double and perturbative triple excitation, [CCSD(T)]/6-31G(d,p), 6-31G(2df,2pd), and correlation-consistent polarized valence triple zeta (cc-pVTZ) levels of theory. CCSD(T)/ cc-pVTZ results predict a very stable HSCl species, an isomer HClS, 51.84 kcal/mol higher in energy, and a transition state 57.68 kcal/mol above HSCl. Independent of the level of theory, results with the smaller 6-31G(d,p) basis set turned out to be poor, especially for HClS. Vibrational analysis indicates that both species can be easily differentiated if isolated. Bonding differences between these molecules are illustrated by contour plots of valence orbitals. Viewed classically, bonding in HClS involves a dative bond. Transition-state rate constants, and equilibrium constants for the HSCl ↔ HClS isomerization have been estimated for various temperatures (200–1000 K). At 298.15 K, the forward rate is predicted to be 7.95 × 10−29 s−1, and the equilibrium constant to be 2.31 × 10−38. Tunneling corrections vary from 1.57 at 298.15 K to 1.05 at 1000 K. Activation energies have been obtained by a two-points linear fit to the Arrhenius equation. Received: 7 May 1999 / Accepted: 22 July 1999 / Published online: 4 October 1999  相似文献   

13.
14.
《Tetrahedron: Asymmetry》1999,10(8):1539-1549
Starting from racemic 2α-methyl-8-oxabicyclo[3.2.1]oct-6-en-3-one the spongistatin E segment has been prepared in nine steps (2.3 steps per stereogenic center) with umpolung of anomeric reactivity at C37. This 3,5-syn-diol sequence completes our methodology to all stereoisomers of 3,5,7-trihydroxy heptanoic ester building blocks functionalized for α-oxyanion chemistry.  相似文献   

15.
We performed molecular dynamics simulations to analyze the initial stage of the thermal degradation of polyethylene, which is dominated by the random scission reaction. The simulations were initiated from structures that were taken from previously equilibrated snapshots of the amorphous polymer and of a free-standing thin film. Isolated chains were also used for comparison. Our systems were coupled to a thermal heat bath, and the effect of different coupling constants was studied. Rate of random scission increases as the strength of the temperature coupling increases. Rates of reaction are almost similar in thin films and the bulk, whereas the rates are much faster in isolated chains. Expansion of the free-standing thin film accompanies degradation, producing fragments of various sizes. Chains of higher molecular weights than the initial chains can be produced due to recombination of fragments during the expansion of thin films. The polydispersity index of the resulting fragments is higher in thin films compared to the bulk. The bonds at the low density portion of the thin films have a higher probability of being broken.  相似文献   

16.
X-ray fluorescence, X-ray phase analysis, and transmission Mössbauer and NGR spectrometry are used to study the formation, phase, and elemental composition of Fe–Ti particles. The interaction between Fe(III) ions and dispersed titanium in an aqueous solution containing chloride ions and HF is studied. It is shown that the resulting Fe–Ti samples are a set of core–shell microparticles with titanium cores coated with micro- and nanosized α-Fe nucleation centers with the thinness outer layer of iron(III) oxide characterized by a developed surface.  相似文献   

17.
Many drug candidates are poorly soluble. The formation of solid dispersion can improve their solubility, consequently their bioavailability. For this purpose, the use of eutectic mixtures is well known in the pharmaceutical field. At the eutectic composition, both components are in reduced particle size and well dispersed. In this work, we focus on the combination of paracetamol, aspirin, and caffeine, which is highly effective for the treatment of migraine headache pain. We have reinvestigated the paracetamol–caffeine, aspirin–caffeine, and paracetamol–aspirin phase equilibria diagrams taking into account the polymorphism of caffeine, paracetamol, and aspirin. The three phase diagrams are determined using X-ray diffraction and the differential scanning calorimetry from the binary mixtures. It is concluded that the paracetamol–caffeine and aspirin–caffeine systems are similar and exhibit two invariants, one eutectic and one metatectic. The paracetamol–aspirin phase diagram reveals the formation of one eutectic. The composition of the three eutectics formed is confirmed by the related Tamman’s triangles. No compound formation is found in the three systems.  相似文献   

18.
Two novel analogues of triptolide were synthesized using triptolide as the starting material through reductive opening of epoxy ring, hydration and olefin epoxidation, and related ketones have also been afforded by oxidation of them with IBX or Jone‘s reagent.  相似文献   

19.
《Tetrahedron: Asymmetry》2014,25(2):187-192
An efficient and stereoselective synthesis of the entire C27–C45 moiety of lagunamide A has been achieved from 1-[(4S)-4-benzyl-2-thioxothiazolidin-3-yl]propan-1-one in six steps with 22% overall yield. The key step in the synthesis is an asymmetric acetal aldol reaction featuring the enantioselective addition of a chiral thiazolidinethione-derived titanium enolate to an acetal to establish the stereochemistry at C39.  相似文献   

20.
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