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41.
The ring opening of N-tosylaziridines with trimethylsilylated nucleophiles, catalyzed by N,N,N',N'-tetramethylethylenediamine, led to the production of beta-functionalized sulfonamides in good to excellent yields with high regioselectivity. [reaction: see text] 相似文献
42.
Riboflavin-sensitized photodynamic modification of collagen led to significant formation of cross-linked molecules. Sodium azide or l,4-diazabicyclo(2,2,2)octane, which are known to be singlet oxygen quenchers, and catalase could not inhibit the modification. Surprisingly, the collagen modification was accelerated in the presence of superoxide dismutase. The aggregation was accompanied by the loss of tyrosine and histidine residues in the collagen. An inhibitory effect of dissolved oxygen on the modification of collagen was observed. Similarly, the loss of tyrosine residues in the irradiated collagen was inhibited in the presence of dissolved oxygen. Dityrosine formation was also observed with the loss of tyrosine. These results indicate that photodynamic modification of tyrosine probably contributes to the riboflavin-sensitized cross-linking of collagen through the formation of dityrosine. 相似文献
43.
Hayasaka Y Waters EJ Cheynier V Herderich MJ Vidal S 《Rapid communications in mass spectrometry : RCM》2003,17(1):9-16
Two proanthocyanidin (PA) fractions, one (Sdp3) with the mean degree of polymerization (mDP) of 3 and the other (Sdp9) with mDP of 9, were obtained from a Vitis vinifera cv. Shiraz grape seed extract. The PA fractions were directly analyzed by electrospray ionization mass spectrometry (ES-MS) and negative ion mass spectra were recorded. The mass spectrum of Sdp3 exhibited only singly charged ions corresponding to the molecular mass of PA with a degree of polymerization (DP) up to 9 (nonamers). In contrast, Sdp9 yielded rather complex mass spectra featuring ions with single [M - H](-), double [M - 2H](2-) and triple [M - 3H](3-) charge representing the molecular masses of PAs up to a DP of 28. In addition, the degree of galloylation per procyanidin (DG) was observed to be up to 5 (pentagallates) in Sdp3 and 8 (octagallates) in the Sdp9. This is the first evidence obtained by mass spectrometry for the distribution of grape seed PAs with such a high degree of polymerization and a broad diversity of galloylation. ES-MS data together with the complementary information provided by acid hydrolysis provides a detailed picture of the composition of grape seed PAs. 相似文献
44.
Oxyoxazolidinones, which are prepared from secondary O-acylmandelamides by treatment with TBSOTf, underwent the RCM reaction in the presence of Grubbs catalyst to give oxazoloazepines or oxazoloazecines, which were readily converted into N-unsubstituted seven-membered or eight-membered lactams by the reductive treatment. 相似文献
45.
Biju V Makita Y Sonoda A Yokoyama H Baba Y Ishikawa M 《The journal of physical chemistry. B》2005,109(29):13899-13905
The formation of narrow size dispersed and nanometer size aggregates (clusters) of cadmium selenide (CdSe) quantum dots (QDs) and their temperature-sensitive photoluminescence (PL) spectral properties close to room temperature (298 K) are discussed. CdSe QDs formed stable clusters with an average diameter of approximately 27 nm in the absence of coordinating solvents. Using transmission electron microscopy (TEM) imaging, we identified the association of individual QDs with 2-5 nm diameters into clusters of uniform size. A suspension of these clusters in different solvents exhibited reversible PL intensity changes and PL spectral shifts which were correlated with temperature. Although the PL intensity of CdSe QDs encapsulated in host matrixes and the solid state showed a response to temperature under cryogenic conditions, the current work identified for the first time QD clusters showing temperature-sensitive PL intensity variations and spectral shifts at moderate temperatures above room temperature. Temperature-sensitive reversible PL changes of clusters are discussed with respect to reversible thermal trapping of electrons at inter-QD interfaces and dipole-dipole interactions in clusters. Reversible luminescence intensity variations and spectral shifts of QD clusters show the potential for developing sensors based on QD nanoscale assemblies. 相似文献
46.
p-Nitrobenzenediazonium tetrafluoroborate dissolved in dimethylsulfoxide (DMSO) at 50° forms p-nitrophenol in 88–90% yield. The phenolic oxygen atom originates exclusively from the oxygen atom of DMSO as demonstrated by the use of 18O-labelled DMSO. The first-order rate of dediazoniation is the same under N2 as it is in the presence of air. The rate is little influenced by the addition of benzene or iodobenzene. However, the products formed in the presence of these additives are significantly different. UV. spectra and the reactivity of diazonium salt solutions in DMSO when mixed with reagents in aqueous solution demonstrate that a relatively stable charge-transfer complex is formed between the diazonium ion and DMSO. The product analyses and the kinetic and spectral results of dediazoniation in DMSO with and without additives are consistent with a mechanism in which the rate-limiting step is the formation of a p-nitrophenyl radical from the charge-transfer complex. p-Nitrophenol and the products with benzene and iodobenzene are formed in subsequent fast competition steps. In the presence of small amounts of pyridine the dediazoniation is much faster and follows a different kinetic law. Pyridine effectively competes with DMSO in the reaction with diazonium ions. 相似文献
47.
The 1,3-dipolar cycloaddition of polymer-supported azomethine ylides to dipolarophiles gave pyrrolidine derivatives in good yields. The azomethine ylides were generated from resin-bound alpha-silylimines via a 1,2-silatropic shift. The features of this method are not only a traceless synthesis but also a unique solid-phase route to pyrrolidines with extensive diversity. [reaction: see text] 相似文献
48.
Yoji Maeda D. S. Shankar Rao S. Krishna Prasad S. Chandrasekhar Sandeep Kumar 《Liquid crystals》2013,40(11):1679-1690
The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 500 MPa using high pressure optical and DTA measurements. The known enantiotropic phase transitions of HHTT, i.e. crystal (Cr)-helical phase (H), H-hexagonal columnar phase (Colh) and Colh-isotropic liquid (I) were observed up to 32 MPa. Application of hydrostatic pressures above 32 MPa results in the H and Colh phases becoming monotropic, depending upon the applied pressure. The H phase was observed as a monotropic phase in the pressure region between 32 and about 180 MPa. Thus, the I →Colh →H →Cr transition sequence appeared only on cooling under these pressures, while the Cr →Colh →I transition occurred on heating. Further increases in pressure above a second limiting value leads to the Colh phase becoming monotropic. Thus the I →Colh →Cr transition sequence appeared on cooling, while the Cr →I transition was observed on heating. The T vs. P phase diagram based on the data obtained in the heating mode contains two triple points; one is estimated as 40 MPa, 77.2°C for the Cr-H-Colh triple point and the other is extrapolated as 285 MPa, 118.3°C for the Cr-Colh-I triple point. These triple points define the upper limits for the appearance of the stable H and Colh phases, respectively. 相似文献
49.
The phase behaviour of a thermotropic cubic mesogen of 1,2-bis(4′-n-tetradecyloxybenzoyl)hydrazine BABH-14 was studied under hydrostatic pressure using a polarising optical microscope equipped with a high-pressure optical cell, and the P–T phase diagram was constructed. BABH-14 shows the Cr–Cub–I transition sequence under atmospheric and lower pressures, but the Cub phase is replaced completely by the high-pressure SmC, SmC(hp), phase under higher pressures. There is a narrow intermediate-pressure region between the low- and high-pressure regions, in which the Cr–SmC(hp)–Cub–I phase sequence is recognised. The SmC(hp)–Cub transition line has a positive slope with pressure and there are two triple points: one is for the Cr, Cub and SmC(hp) phases and the other is for the I, Cub and SmC(hp) phases. Comparing the phase sequence of BABH-14 with those for BABH-8 and -10, the pressure-induced inversion of the phase sequence between the cubic and SmC phases occurs in the BABH-n homologous compounds. Another new phenomenon is the formation of the monotropic cubic phase on cooling in the intermediate- and high-pressure regions, and an intriguing phenomenon of the cubic phase appearing twice, i.e. I–Cub–SmC(hp)– Cub–Cr phase transition, occurs in the intermediate-pressure region. 相似文献
50.
Yoshinobu Morino Ikumasa Hidaka Yoji Oderaotoshi Mitsuo Komatsu Satoshi Minakata 《Tetrahedron》2006,62(52):12247-12251
A new protocol for the cyclization of N-alkenylamides using chloramine-T and iodine is described. When N-alkenylsulfonamides are treated with chloramine-T and iodine, three- to six-membered N-heterocycles are obtained with complete stereoselectivity. The method is compatible with the cyclization of the allylbenzamide or allylbenzthioamide to afford an oxazoline or thiazoline derivative, respectively. Mechanistic studies indicate that the chloramine-T/I2 system functions as an effective iodonium species. 相似文献