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41.
Oxonia-cope prins cyclizations: a facile method for the synthesis of tetrahydropyranones bearing quaternary centers 总被引:1,自引:0,他引:1
A new cationic cascade reaction has been developed that produces 4-tetrahydropyranones in good yield. The reaction is based on the facile 2-oxonia Cope rearrangement of allyl-substituted oxocarbenium ions. In the presence of a more nucleophilic silyl enol ether, such systems rearrange and cyclize to produce tetrahydropyranones. The substrates were prepared by silyl ketene acetal addition to ketenes. The rearrangement is compatible with tetrasubstituted silyl enol ethers, which result in the diastereoselective introduction of quaternary centers at the C3 position of the tetrahydropyran ring. The oxonia-Cope Prins rearrangement is a versatile new route to tetrahydropyrans. 相似文献
42.
Tseng HR Vignon SA Celestre PC Stoddart JF White AJ Williams DJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(2):543-556
The template-directed syntheses, employing bisparaphenylene-[34]crown-10 (BPP34C10), 1,5-dinaphthoparaphenylene-[36]crown-10 (1/5NPPP36C10), and 1,5-dinaphtho-[38]crown-10 (1/5DNP38C10) as templates, of three [2]catenanes, whereby one of the two bipyridinium units in cyclobis(paraquat-p-phenylene) is replaced by a bipicolinium unit, are described. The crude reaction mixtures comprising the [2]catenanes all contain slightly more of the homologous [3]catenanes, wherein a "dimeric" octacationic cyclophane has the crown ether macrocycles encircling the alternating bipyridinium units with the bipicolinium units completely unfettered. X-ray crystallography, performed on all three [2]catenanes and two of the three [3]catenanes reveals co-conformational and stereochemical preferences that are stark and pronounced. Both the [3]catenanes crystallize as mixtures of diastereoisomers on account of the axial chirality associated with the picolinium units in the solid state. Dynamic (1)H NMR spectroscopy is employed to probe in solution the relative energy barriers for rotations by the phenylene and pyridinium rings in the tetracationic cyclophane component of the [2]catenanes. Where there are co-conformational changes that are stereochemically "allowed", crown ether circumrotation and rocking processes are also investigated for the relative rates of their occurrence. The outcome is one whereby the three [2]catenanes containing BPP34C10, 1/5NPPP36C10, and 1/5DNP38C10 exist as one major enantiomeric pair of diastereoisomers amongst two, four, and eight diastereoisomeric pairs of enantiomers, respectively. The diastereoisomerism is a consequence of the presence of axial chirality together with helical and/or planar chirality in the same interlocked molecule. These [2]catenanes constitute a rich reserve of new stereochemical types that might be tapped for their switching and mechanical properties. 相似文献
43.
The antioxidant activities of a number of N-phenyl nitrones containing phenolic functions were compared with N-methyl nitrones and conventional antioxidants in peroxide and TMTD sulphur-less vulcanizates. The N-phenyl nitrones were found to be effectively bound during vulcanization whereas the N-methyl nitrones were not. Unhindered phenols reacted to a higher degree and were relatively more effective than the typical hindered phenol antioxidant structures. A conventional bisphenol was effective in the sulphurless vulcanizate after solvent extraction but this is considered to be due to the formation of an insoluble zinc salt. Synergism was observed in the TMTD sulphurless vulcanizate both before and after extraction. 相似文献
44.
Fischer CJ Gafni A Steel DG Schauerte JA 《Journal of the American Chemical Society》2002,124(35):10359-10366
The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm(2)), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 micros, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 micros; Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612-2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc. 1995, 117, 7646-7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 micros decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins. 相似文献
45.
Photoelectron energy and angular distributions are measured for the 2+1 multiphoton ionization process H2 X1Σg+ (ν = 0,J) + 2hv → E,F1Σg+(νE,JE = J) + hν → H2+X2Σg+ (ν+) + e?, for νE = 0, 1, or 2 and for JE = 0 or 1 of the inner well of the double-minimum E,F state. Although a strong preference is found for ν+ = νE, the detailed H2+ vibrational distribution does not exhibit Franck-Condon behavior, and the photoelectron angular distributions vary markedly with both the JE value of the intermediate state and the ν+ value of the ion. 相似文献
46.
47.
A convergent total synthesis of anhydrolycorinone is detailed, enlisting sequential intramolecular Diels-Alder reactions of a suitably substituted 2-amino-1,3,4-oxadiazole defining a novel oxadiazole --> furan --> benzene Diels-Alder strategy. 相似文献
48.
McRee D 《Chemistry & biology》2003,10(4):295-297
The first X-ray structure of human carboxylesterase 1 (hCE1) and the structures of hCE1 with drug analogs bound reveal important molecular details of how the drugs cocaine, heroin, and tacrine are metabolized and cleared. 相似文献
49.
50.
[formula: see text] A highly diastereoselective, microwave-induced Claisen rearrangement of an appropriately substituted propargylic enol ether allows the formation of the sterically congested C8-C14 bond of azadirachtin. When combined with a radical-mediated cyclization of the corresponding allene, this sequence offers rapid entry to the framework of azadirachtin. 相似文献