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排序方式: 共有241条查询结果,搜索用时 15 毫秒
41.
Highly dispersed Ni nanoparticles have been successfully immobilized by the zeolitic metal-organic framework ZIF-8 via sequential deposition-reduction methods, which show high catalytic activity and long durability for hydrogen generation from hydrolysis of aqueous ammonia borane (NH(3)BH(3)) at room temperature. 相似文献
42.
R Kobayashi K Hanaya M Shoji K Umezawa T Sugai 《Chemical & pharmaceutical bulletin》2012,60(9):1220-1223
In order to synthesize the potent nuclear factor (NF)-κB inhibitor, (2S,3S,4S)-dehydroxymethylepoxyquinomycin (DHMEQ), in a large scale, a new route for its corresponding racemic precursor, dihexanoyl (2R*,3R*,4R*)-DHMEQ, was developed. By employing both hydroquinone and benzoquinone intermediates, the total yield, reproducibility, and synthetic steps were improved and the synthetic cost was reduced. 相似文献
43.
Kazuaki Kuwata Rie Fujita Kengo Hanaya Shuhei Higashibayashi Takeshi Sugai 《Tetrahedron》2018,74(7):740-745
A formal total synthesis of (?)-hamigeran B was achieved in 17 steps from commercially available ethyl 2-oxocyclopentanecarboxylate. Carbonyl reductase-catalyzed asymmetric reduction and the subsequent chemical transformations furnished an enantiomerically pure synthetic intermediate, (R)-5-formyl-2-isopropyl-5-methylcyclopent-1-en-1-yl trifluoromethylsulfonate. Suzuki-Miyaura coupling with Gao's arylboronate [2-(2-formyl-3-methoxy-5-methylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane], under PdCl2(dppf)?CH2Cl2 catalysis, and the subsequent cyclization by way of intramolecular reductive SmI2-mediated 1,2-diol formation provided a tricyclic skeleton with a tetrasubstituted double bond between C-1 and C-9b. Upon hydrogenation of this double bond, the proper stereochemistry of the remaining chiral centers was established. Exclusive addition of the hydrogen atom from the β-face occurred, owing to the shielding of the α-face with a bulky TBS protective group on the C-4 alcohol. The hydrogenation products were transformed into Clive's synthetic precursor for (?)-hamigeran B. 相似文献
44.
A novel calixarene-based diimine, 1,3-bis(pyren-1-yliminomethyl)calix[4]arene (5), serves as a turn-on-type fluorescent sensor, which selectively detects Hg2+ in THF/H2O (99:1, v/v) in the presence of various other metal ions. Such selectivity is not seen with half salen 1 derived from salicylaldehyde and 1-aminopyrene. 1H NMR analysis reveals that it is a chemodosimetric sensor based on its hydrolysis mediated by Hg2+ to release 1-aminopyrene molecules as fluorescent chromophores. 相似文献
45.
Kengo Oka Shunsuke Fuchi Dr. Keita Komine Dr. Hayato Fukuda Prof. Dr. Susumi Hatakeyama Prof. Dr. Jun Ishihara 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(56):12862-12867
The catalytic asymmetric total synthesis of (−)-exiguolide, a complex 20-membered macrolide embedded with a bis(tetrahydropyran) motif, is reported. The convergent synthesis involves the construction of the C1–C11 tetrahydropyran segment via catalytic asymmetric allylation and Prins cyclization, and the formation of the C12–C21 phosphonate segment via catalytic asymmetric cyclocondensation reaction and Johnson–Claisen rearrangement. The synthesis of 15-epi-exiguolide is also described. 相似文献
46.
The mechanism of fragmentation processes in aqueous nanodroplets charged with ions is studied by molecular dynamics (MD) simulations. By using constant-temperature MD, the evaporation of the water is naturally taken into account and sequences of ion fragmentation events are observed. The size of the critical radius of the charged droplet just before the fragmentation and the distribution of the sizes of the fragments are estimated. Comparison of the Rayleigh critical radius for fragmentation and simulation data is within 0.23 nm. This seemingly small difference arises from a large difference in the number of water molecules that makes fragmentation an activated process as in the ion evaporation mechanism (IEM). This finding is in agreement with the predictions of Labowsky et al. [Anal. Chim. Acta 2000, 406, 105-118] for charged aqueous drops. The size of the daughter droplets is larger than the prediction of Born's theory by 0.1 to 0.15 nm. The nature and the dynamics of the intermediate states of the fragmentation process characterized by a bridge formed between the mother droplet and the evaporating ion or thorned structures where the ion sits on the tip are important for the outcome of the size-distribution of the fragments, while they are is missing in Born's theory. 相似文献
47.
Muranaka A Yoshida K Shoji T Moriichi N Masumoto S Kanda T Ohtake Y Kobayashi N 《Organic letters》2006,8(12):2447-2450
A series of supramolecular complexes formed between oxotitanium(IV) phthalocyanine and apple procyanidins have exhibited characteristic bisignate CD signals in the Q region (ca. 700 nm). The helicity of the oligomeric procyanidins is proposed to be left-handed on the basis of the CD analyses. [structure: see text] 相似文献
48.
We show that every oriented link diagram with a closed braid diagram as a sublink diagram can be deformed into a closed braid diagram by a deformation keeping the sublink diagram and, under a mild condition, the number of Seifert circles fixed. As an application, we give an upper bound for the braid index of the link obtained by reversing the orientation of its sublink by using only the information of an original link. 相似文献
49.
Kengo Akagawa 《Tetrahedron letters》2007,48(6):985-987
One-pot sequential acidic deacetalization and basic enantioselective aldol reaction were realized using Amberlite IR-120 (H+-form) and a resin-supported peptide catalyst, and the reusability of the catalysts was demonstrated. 相似文献
50.
Kengo Masuda Masaki Koshimizu Dr. Masanori Nagatomo Prof. Dr. Masayuki Inoue 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(1):230-236
(+)‐Ryanodine ( 1 ) is the ester derivative of 1H‐pyrrole‐2‐carboxylic acid and the complex terpenoid (+)‐ryanodol ( 2 ), which possesses eleven contiguous stereogenic centers on the ABCDE‐ring system. Compound 1 is known to be a potent modulator of intracellular calcium release channels, whereas the activity of 2 is significantly weaker. To chemically construct 1 , the multiple oxygen functional groups must be installed on the fused pentacycle in stereoselective fashions and the extremely hindered C3‐hydroxy group must be acylated in a site‐selective manner. First, the total synthesis of 2 was accomplished by introducing the five stereocenters from the previously prepared enantiopure ABDE‐ring 7 . Stereoselective construction of the C3‐secondary, C2‐ and C6‐tertiary alcohols was achieved by three nucleophilic reactions. The C9‐ and C10‐trisubstituted carbon centers were regio‐ and stereoselectively introduced by hydroboration/oxidation of the six‐membered C‐ring, which was formed by the ring‐closing metathesis reaction. Direct esterification of the C3‐alcohol with pyrrole‐2‐carboxylic acid proved unsuccessful; therefore, we developed a new, two‐step protocol for attachment of the pyrrole moiety. The C3‐hydroxy group was first converted into the less sterically cumbersome glycine ester, which was then transformed into the pyrrole ring through condensation with 1,3‐bis(dimethylamino)allylium tetrafluoroborate. This procedure resulted in the first total synthesis of 1 . 相似文献