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91.
Kobayashi Y Kumar GB Kurachi T Acharya HP Yamazaki T Kitazume T 《The Journal of organic chemistry》2001,66(6):2011-2018
By using the convenient protocol for conversion of 2-substituted furans into 4-oxo-2-alkenoic acids ((i) NBS, (ii) NaClO(2)), macrosphelide B (2) was synthesized from furyl alcohol 5 (>98% ee) and acid 6 (99% ee). The protocol was first applied to the PMB ether of 5 to afford acid 13b. On the other hand, DCC condensation of acid 6 with 5 gave 16 after deprotection of the TBS group. Condensation was again carried out between 13b and 16 to furnish the key ketone 17, which upon reduction with Zn(BH(4))(2) afforded anti alcohol 18 stereoselectively (15:1). After protection/deprotection steps, the furan 18 was converted to seco acid 3 by using the furan oxidation protocol mentioned above, and lactonization of 3 with Cl(3)C(6)H(2)COCl, Et(3)N, and DMAP afforded 22 (MOM ether of 2), which upon deprotection with TFA produced 2. Transformation of 22 to macrosphelide A (1) was then investigated. Although the chelation-controlled reduction of 22 should afford the desired anti alcohol 24, Zn(BH(4))(2) at <-90 degrees C gave a 2 approximately 1:1 mixture of anti/syn alcohols. On the contrary, reduction with NaBH(4) in MeOH at -15 degrees C produced the syn isomer 23 with >10:1 diastereoselectivity. Mitsunobu inversion of the resulting C(14)-hydroxyl group and deprotection of the MOM group with TFA afforded 1. Similarly, reduction of 2 with NaBH(4) afforded the C(14)-epimer of 1 stereoselectively. The observed stereoselectivity in the reductions of 22 and 2 could be explained on the basis of computer-assisted calculation, which showed presence of the low-energy conformers responsible for the stereoselective reduction. In addition, conversion of 2 to 1 was established, for the first time. 相似文献
92.
Electron spin resonance imaging of mouse B16 melanoma 总被引:2,自引:0,他引:2
H Katsuda T Kobayashi H Saito T Matsunaga M Ikeya 《Chemical & pharmaceutical bulletin》1990,38(10):2838-2840
An X-band electron spin resonance (ESR) imaging apparatus with a pin-hole TE102 mode cavity and a rapid scan coil was constructed. Using this apparatus, ESR imaging of melanin in mouse B16 melanoma was observed for the first time. The ESR spectrum of B16 melanoma is similar to that of natural melanin extracted from sepia officinalis in microwave power dependence. 相似文献
93.
Masayoshi Watanabe Tadashi Iida Kohei Sanui Naoya Ogata Tadahiko Kobayashi Zentaro Ohtaki 《Journal of polymer science. Part A, Polymer chemistry》1984,22(6):1299-1307
Novel polyamides that contain tetrathiafulvalene (TTF) moieties were synthesized by direct polycondensation of Δ2,2′ -bi-1,3-dithiole-4,4′(5′)-dicarboxylic acid with aromatic diamines in the presence of triphenylphosphine, hexachloroethane, and pyridine. Complexes of these polyamides with bromine were TTF cation radical () salts which resulted from the charge transfer of TTF moieties to bromine. The electrical conductivity of undoped polyamides increased with a decrease in diamine length. By doping the bromine the conductivity of the polyamides was enhanced by 3–5 orders of magnitude and reached 10?5–;10?9S cm?1. Maximum conductivity was observed at the doping ratio of [Br]/[repeat unit] < 1. The electrical conduction in the bromine complexes may be attributed to the hopping of odd electrons of between TTF moieties. 相似文献
94.
Fumiyo Saito Ryo Takeuchi Tomoyuki Kamino Kouji Kuramochi Kengo Sakaguchi Susumu Kobayashi Masashi Tsuda 《Tetrahedron letters》2004,45(43):8069-8071
A total synthesis of the proposed structure of plakevulin A was accomplished. However, the NMR spectral data of the synthetic plakevulin A were not identical of those of the reported compound. We next converted the synthetic plakevulin A into 1-dihydrountenone A. The 1H and 13C NMR spectral data of 1-dihydrountenone A were identical with those of reported plakevulin A except for the peaks derived from levulinic acid. Thus, we repurified sample of the natural product and confirmed that the natural sample contained 1-dihydrountenone A and levulinic acid in the ratio of one to one. We also found that not plakevulin A but 1-dihydountenone A possessed the inhibitory activity against mammalian DNA polymerases α and β. 相似文献
95.
Tsai P Ichikawa K Mailer C Pou S Halpern HJ Robinson BH Nielsen R Rosen GM 《The Journal of organic chemistry》2003,68(20):7811-7817
Apparent rate constants, at acidic pH and neutral pH for the reaction of a family of ester-containing 5-carboxyl-5-methyl-1-pyrroline N-oxides with superoxide (O2*-) were estimated, using ferricytochrome c as a competitive inhibitor. It was of interest to note that the rate constants were similar among the different nitrones and not that significantly different from that found for 5-(diethoxyphosphoryl)-5-dimethyl-1-pyrroline N-oxide. At acidic pH, the rate constant for spin trapping O2*- was 3-fold greater than that at physiological pH. Subsequent experiments determined the half-life of aminoxyls, derived from the reaction of these nitrones with O2*-. The EPR spectra were modeled by using a global analysis method. The results clearly demonstrated that EPR spectra of all the aminoxyls were inconsistent with a model that included a single gamma-hydrogen splitting. A better interpretation modeled them as two diastereomers with identical nitrogen splittings and slightly different beta-hydrogen splittings. Detailed line width analyses slightly favored an equal line width-unequal population ratio for the two diastereomers. 相似文献
96.
This review covers the literature on simple indole alkaloids and those with a nonrearranged monoterpenoid unit from the beginning up to the end of 2004. Newly isolated alkaloids, structure determinations, total synthesis and biological activities are included. 相似文献
97.
Three-component reactions of aldehydes, ammonia, and allylboronates were found to provide homoallylic primary amines in high yields with high chemo- and stereoselectivities. A two-step, one-pot, stereoselective synthesis of an uncommon alpha-amino acid, alloisoleucine, was achieved utilizing this reaction. 相似文献
98.
A simple post-column derivatization method for the fluorometric determination of biguanides (buformin and phenformin) in serum by high-performance liquid chromatography is described. The serum was treated with 4% perchloric acid to precipitate proteins, and the supernatant was directly injected into the column. Synthesized 9,10-phenanthrenequinonesulphonate (PSQ) was used as a fluorogenic reagent and added to the mobile phase. Biguanides were separated within 10 min on a Radial-Pak microBondapak C18 cartridge (10 microns, 10 cm x 8 mm I.D.) by reversed-phase ion-pair chromatography. They were then allowed to react with PQS in an alkaline stream and detected fluorometrically. This method was applied to the analysis of serum from patients with diabetes mellitus. 相似文献
99.
A Chromolith Performance octadecylsilyl (ODS) monolithic silica column (Merck) was compared with a conventional microparticulate ODS-bonded silica column in the high-performance liquid chromatography separation of natural polyprenols. A system comprising two connected monolithic columns afforded an equivalent separation at half the analysis time of the conventional method. Furthermore, ten connected columns achieved a tremendously high-resolution separation, in which the complicated series of homologous polyprenols with geometric isomerism were fully separated. 相似文献
100.
Manabe K Iimura S Sun XM Kobayashi S 《Journal of the American Chemical Society》2002,124(40):11971-11978
Dehydration reactions in water have been realized by a surfactant-type catalyst, dodecylbenzenesulfonic acid (DBSA). These reactions include dehydrative esterification, etherification, thioetherification, and dithioacetalization. In these reactions, DBSA and substrates form emulsion droplets whose interior is hydrophobic enough to exclude water molecules generated during the reactions. Detailed studies on the esterification revealed that the yields of esters were affected by temperature, amounts of DBSA used, and the substrates. Esters were obtained in high yields for highly hydrophobic substrates. On the basis of the difference in hydrophobicity of the substrates, unique selective esterification and etherification in water were attained. Furthermore, chemospecific, three-component reactions under DBSA-catalyzed conditions were also found to proceed smoothly. This work not only may lead to environmentally benign systems but also will provide a new aspect of organic chemistry in water. 相似文献