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排序方式: 共有109条查询结果,搜索用时 78 毫秒
61.
Ikeda H Matsumori N Ono M Suzuki A Isogai A Nagasawa H Sakuda S 《The Journal of organic chemistry》2000,65(2):438-444
Aflastatin A (1) is a specific inhibitor of aflatoxin production by Aspergillus parasiticus. It has the novel structure of a tetramic acid derivative with a long alkyl side chain. The absolute configurations of 29 chiral centers contained in 1 were chemically elucidated in this study. First, four small fragment molecules were prepared from 1 or its methyl ether (2), and their absolute structures were assigned as N-methyl-D-alanine, (2S,4R)-2, 4-dimethyl-1,6-hexanediol dibenzoate, (R)-3-hydroxydodecanoic acid, and (R)-1,2,4-butanetriol tribenzoate. Next, an acyclic fragment molecule 3 with 13 chiral centers was obtained from 1 by NaIO(4) oxidation, and its relative stereochemistry was elucidated by J-based configuration analysis. By analyzing coupling constants of (3)J(H,H) and (2,3)J(C,H) and ROE data, the relative configuration of 3 was verified. Finally, by further J-based configuration analysis using a fragment molecule 7 prepared from 2 with 28 chiral carbons, all relative configurations in the alkyl side chain of 1 were clarified. By connecting these relative configurations with the absolute configurations of first four fragment molecules, the absolute stereochemistry of 1 was fully determined. 相似文献
62.
Okahashi Ken Takeuchi Miyuki Zhou Yaxin Ono Yuko Fujisawa Shuji Saito Tsuguyuki Isogai Akira 《Cellulose (London, England)》2021,28(10):6373-6387
Cellulose - A TEMPO-oxidized cellulose nanofibril (TEMPO-CNF)/water dispersion was mixed with an aqueous solution of hydroxypropyl cellulose (HPC), hydroxyethyl cellulose (HEC), methyl cellulose... 相似文献
63.
Yamato Kyouhei Yoshida Yutaka Kumamoto Yoshiaki Isogai Akira 《Cellulose (London, England)》2022,29(5):2839-2853
Cellulose - The carboxy groups abundantly and densely present on 2,2,6,6-tetramehylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TEMPO-CNFs) have been chemically modified to... 相似文献
64.
Galina Rodionova Tsuguyuki Saito Marianne Lenes Øyvind Eriksen Øyvind Gregersen Hayaka Fukuzumi Akira Isogai 《Cellulose (London, England)》2012,19(3):705-711
TEMPO-oxidized cellulose nanofibers (TOCN) were obtained from commercial Norway spruce and mixed Eucalyptus cellulose pulps
using TEMPO/sodium bromide (NaBr)/sodium hypochlorite (NaClO) system at pH 10 and 22 °C. After reaction, the fibrillated TEMPO-oxidized
celluloses were used for preparation of self-standing films and casting of laminate films on 50 μm thick polyethylene terephthalate.
Significant differences between N. spruce and Eucalyptus TOCN were registered. The tensile strength of the films showed a
maximum value for spruce samples oxidized with addition of 10 mmol g−1 of NaClO. Oxygen permeability decreased with increasing oxidation levels, being lower for N. spruce TOCN compared to Eucalyptus. 相似文献
65.
Masayuki Hirota Naoyuki Tamura Tsuguyuki Saito Akira Isogai 《Cellulose (London, England)》2012,19(2):435-442
A softwood bleached kraft pulp (SBKP) and cotton lint cellulose were fully or partially mercerized, and these along with celluloses
and commercially available regenerated cellulose fiber and beads were oxidized by 4-acetamido-TEMPO/NaClO/NaClO2 at 60 °C and pH 4.8. Weight recovery ratios and carboxylate contents of the oxidized celluloses were 65–80% and 1.8–2.2 mmol g−1, respectively. Transparent and viscous dispersions were obtained by mechanical disintegration of the TEMPO-oxidized celluloses
in water. These aqueous dispersions showed birefringence between cross-polarizers, indicating that mostly individualized cellulose
nanoelements dispersed in water were obtained by these procedures. Transmission electron microscopy observation showed that
the cellulose nanoelements prepared from mercerized SBKP, repeatedly mercerized SBKP, mercerized cotton lint cellulose, regenerated
cellulose beads and 18% NaOH-treated SBKP, i.e. partially mercerized SBKP, had similar morphologies and sizes, 4–12 nm in
width and 100–200 nm in length. The 18% NaOH-treated SBKP was converted to cellulose nanoelements consisting of both celluloses
I and II. 相似文献
66.
Primary aliphatic amines were introduced into most of (>95%) carboxyl groups densely present on the TEMPO-oxidized cellulose
nanofibril (TOCN) surfaces via carboxyl/amine salt formation in a water-isopropyl alcohol mixture. The carbon number of alkyl
chain lengths introduced into TOCN varied using n-decyl-, n-dodecyl, n-tetradecyl-, n-hexadecyl- and n-octadecyl-amines. When n-dodecyl-, n-tetradecyl- and n-hexadecyl-amines were used for neutralization of carboxyl groups, the TOCN-COOH/amine salts were dispersed at the individual
nanofibril level in pure isopropyl alcohol (IPA) by ultrasonication treatment, and gave stable and transparent IPA dispersions
with birefringence when observed between cross polarizers. Flexible, highly transparent and surface-hydrophobic self-standing
films of n-dodecylamine-treated TOCN-COOH (TOCN-COOH/C12-amine salt) were prepared by casting and drying of the dispersion. However, the introduction of abundant long-alkyl chains
on the TOCN surfaces via the amine salt formation resulted in clearly lower mechanical and oxygen barrier properties of the
films than those of TOCN-COONa films. 相似文献
67.
Koga H Tokunaga E Hidaka M Umemura Y Saito T Isogai A Kitaoka T 《Chemical communications (Cambridge, England)》2010,46(45):8567-8569
Topochemical synthesis of gold nanoparticles (AuNPs) was achieved on crystalline cellulose single nanofibers (CSNFs), which were tailored from native cellulose. Exposed AuNPs@CSNFs composite exhibited an excellent catalytic efficiency: the turnover frequency of the AuNPs@CSNFs was up to 840 times that of conventional polymer-supported AuNPs, for a model aqueous reduction reaction. Our novel strategy provides a promising solution to realize efficient use of limited noble metals using natural bioresources. 相似文献
68.
Takehiko Uematsu Yoshiaki Matsui Shusuke Kakiuchi Akira Isogai 《Cellulose (London, England)》2011,18(4):1129-1138
Carboxymethyl cellulose (CMC)-rich cellulose sheets were prepared with a cationic retention aid, poly[N,N,N-trimethyl-N-(2-methacryloxyethyl)ammonium chloride] (PTMMAC), using a papermaking technique. When 5% PTMMAC and 5% CMC were added to
cellulose slurries, approximately 94% of the polymers were retained in the sheets by formation of polyion complexes between
the two polymers. When the PTMMAC/CMC/cellulose sheets were soaked in solutions consisting of ethanol, water and calcium chloride
(EtOH/H2O/CaCl2) with a weight ratio of 75:24:1, almost all PTMMAC and CMC molecules remained in the sheets, forming the structures of PTMMAC-N+Cl− and CMC-COO−Ca2+Cl− without dissolution of these molecules in the soaking solution. Thus, PTMMAC, CMC and calcium contents in the sheets were
able to be determined on the basis of these PTMMAC and CMC structures from analytical data such as nitrogen, calcium and chlorine
contents. The trade-off properties between sufficient wet strength in use and water-disintegrability after use can be added
to the PTMMAC/CMC/cellulose sheets by selecting weight ratios of the EtOH/H2O/CaCl2 solution used as the impregnation liquid. 相似文献
69.
Takuya?Isogai Tsuguyuki?Saito Akira?IsogaiEmail author 《Cellulose (London, England)》2011,18(2):421-431
A softwood bleached kraft pulp (SBKP) was subjected to electro-mediated oxidation in water with TEMPO or 4-acetamido-TEMPO
without any chlorine-containing oxidant. Solid recovery ratios of water-insoluble fractions of the oxidized SBKPs were more
than 80%, and C6-carboxylate contents increased up to approximately 1 mmol g−1 after oxidation for 48 h. Significant amounts of C6-aldehyde groups (0.17–0.38 mmol g−1) were also formed in the oxidized SBKPs. The degree of polymerization decreased from 2,200 to 520 and 1,400 by the oxidation
for 48 h with TEMPO at pH 10 and 4-acetamido-TEMPO at pH 6.8, respectively. The original cellulose I crystal structure and
crystallinity of SBKP were maintained after the oxidation, indicating that all C6-oxidized groups were selectively formed
on crystalline cellulose microfibril surfaces. The oxidized SBKPs with carboxylate contents of more than 0.9 mmol g−1 were convertible to individual cellulose nanofibrils in yields of more than 80% by disintegration in water. 相似文献
70.
Minoru Kimura Zi-Dong Qi Hayaka Fukuzumi Shigenori Kuga Akira Isogai 《Cellulose (London, England)》2014,21(5):3193-3201
Nitrogen adsorption was used to characterize mesoporous structures in never-dried softwood cellulose fibers. Distinct inflections in desorption isotherms were observed over the relative vapor pressure (P/P0) range of 0.5–0.42 for never-dried cellulose fibers and partially delignified softwood powders. The reduction in N2 adsorption volume was attributed to cavitation of condensed N2 present in mesopores formed via lignin removal from wood cell walls during delignification. The specific surface areas of significantly delignified softwood powders were ~150 m2 g?1, indicating that in wood cell walls 16 individual cellulose microfibrils, each 3–4 nm in width, form one cellulose fibril bundle surrounded with a thin layer of lignin and hemicelluloses. Analysis of N2 adsorption isotherms indicates that mesopores in the softwood cellulose fibers and partially delignified softwood powders had peaks ranging from 4 to 20 nm in diameter. 相似文献