排序方式: 共有134条查询结果,搜索用时 15 毫秒
91.
Akai S Tsujino T Fukuda N Iio K Takeda Y Kawaguchi K Naka T Higuchi K Akiyama E Fujioka H Kita Y 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(21):6286-6297
The asymmetric total synthesis of the potent antitumor antibiotic fredericamycin A ((S)-1) was achieved by the intramolecular [4+2] cycloaddition of the silylene-protected styrene derivative (S)-7 followed by the aromatic Pummerer-type reaction of the sulfoxide (S)-5. Although we had already succeeded in the total synthesis of racemic 1 by the same approach, synthesis of its asymmetric version was more complicated than we had expected due to the difficulties involved in constructing the quaternary carbon center and the tendency of this center to undergo facile racemization. Racemization of this center during the installation of the acetylene moiety on the dione (R)-8 was the most serious aspect. Systematic studies of its DE-ring analogue (R)-25 revealed that racemization of the quaternary carbon center proceeded by a retro-aldol-aldol reaction of the initial adduct, (1R)-39 a-Li, and that the degree of racemization was dependent on the reaction temperature. The racemization process could be completely depressed by keeping the reaction temperature at -78 degrees C. The construction of the stereogenic quaternary carbon center was achieved by the lipase-catalyzed desymmetrization of the prochiral 1,3-diol 9 a bearing the DEF-ring moiety. These studies enabled us to attain the asymmetric total synthesis of (S)-1 while completely retaining the chiral integrity created by the enzymatic reactions. 相似文献
92.
Fournier PA Savoie J Stenne B Bédard M Grandbois A Collins SK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(28):8690-8695
In asymmetric olefin metathesis reactions, the addition of halide additives is often required to augment enantioselectivities, despite the fact that the additives result in catalysts with diminished reactivities. The preparation of new chiral Ru-based catalysts was accomplished by exploiting previously reported mechanistic studies. The catalysts possess a high level of reactivity and successfully induce high levels of asymmetry in desymmetrization reactions without the use of halide additives. 相似文献
93.
Ren-Zhe Li Hua Tang Dr. Ke R. Yang Li-Qiang Wan Xia Zhang Jie Liu Zhengyan Fu Prof. Dawen Niu 《Angewandte Chemie (International ed. in English)》2017,56(25):7213-7217
A copper/borinic acid dual catalytic reaction enabled the enantioselective propargylation of aliphatic polyols. Readily available reagents and catalysts were used in this transformation, which displayed good to excellent chemo- and stereoselectivity for a broad array of substrates. The method was also applicable to the desymmetrization of meso 1,2-diols to furnish products with three stereogenic centers and a terminal alkyne group in one operation. 相似文献
94.
Construction of a Chiral Silicon Center by Rhodium‐Catalyzed Enantioselective Intramolecular Hydrosilylation 下载免费PDF全文
Dr. Yuki Naganawa Tomoya Namba Mayu Kawagishi Prof. Dr. Hisao Nishiyama 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(26):9319-9322
Rhodium‐catalyzed enantioselective desymmetrizing intramolecular hydrosilylation of symmetrically disubstituted hydrosilanes is described. The original axially chiral phenanthroline ligand (S)‐BinThro (Binol‐derived phenanthroline) was found to work as an effective chiral catalyst for this transformation. A chiral silicon stereogenic center is one of the chiral motifs gaining much attention in asymmetric syntheses and the present protocol provides cyclic five‐membered organosilanes incorporating chiral silicon centers with high enantioselectivities (up to 91 % ee). The putative active RhI catalyst takes the form of an N,N,O‐tridentate coordination complex, as determined by several complementary experiments. 相似文献
95.
Enantioselective Desymmetrization of Prochiral Cyclohexanones by Organocatalytic Intramolecular Michael Additions to α,β‐Unsaturated Esters 下载免费PDF全文
Adam D. Gammack Yamagata Dr. Swarup Datta Kelvin E. Jackson Linus Stegbauer Dr. Robert S. Paton Prof. Dr. Darren J. Dixon 《Angewandte Chemie (International ed. in English)》2015,54(16):4899-4903
A new catalytic asymmetric desymmetrization reaction for the synthesis of enantioenriched derivatives of 2‐azabicyclo[3.3.1]nonane, a key motif common to many alkaloids, has been developed. Employing a cyclohexanediamine‐derived primary amine organocatalyst, a range of prochiral cyclohexanone derivatives possessing an α,β‐unsaturated ester moiety linked to the 4‐position afforded the bicyclic products, which possess three stereogenic centers, as single diastereoisomers in high enantioselectivity (83–99 % ee) and in good yields (60–90 %). Calculations revealed that stepwise C? C bond formation and proton transfer via a chair‐shaped transition state dictate the exclusive endo selectivity and enabled the development of a highly enantioselective primary amine catalyst. 相似文献
96.
Copper‐Catalyzed Intramolecular Desymmetric Aryl CO Coupling for the Enantioselective Construction of Chiral Dihydrobenzofurans and Dihydrobenzopyrans 下载免费PDF全文
Wenqiang Yang Yangyuan Liu Shasha Zhang Prof. Dr. Qian Cai 《Angewandte Chemie (International ed. in English)》2015,54(30):8805-8808
O‐Heterocyclic structures such as 2,3‐dihydrobenzofurans are key motifs in many natural compounds and pharmaceuticals. Enantioselective formation of chiral dihydrobenzofurans and analogues was achieved through a copper‐catalyzed desymmetrization strategy with a chiral cyclic 1,2‐diamine. A broad range of substrates are compatible with this CuI‐diamine catalytic system and afford the desired coupling products with chiral tertiary or quaternary carbon centers in high yields and good to excellent enantioselectivities under mild conditions. 相似文献
97.
Tao He Li-Chuan Liu Wen-Peng Ma Bin Li Prof. Dr. Qing-Wei Zhang Prof. Dr. Wei He 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(71):17011-17015
Catalytic, enantioselective synthesis of stereogenic silicon compounds remains a challenge. Herein, we report a rhodium-catalyzed regio- and enantio-selective intermolecular hydrosilylation of alkene with prochiral dihydrosilane. This new method features a simple catalytic system, mild reaction conditions and a wide functional group tolerance. 相似文献
98.
Renata Koodziejska Renata Studziska Agnieszka Tafelska-Kaczmarek Hanna Pawluk Dominika Mlicka Alina Wo
niak 《Molecules (Basel, Switzerland)》2021,26(6)
In this study, we examined the Aureobasidium pullulans strains DSM 14940 and DSM 14941 included in the Blossom Protect™ agent to be used in the bioreduction reaction of a symmetrical dicarbonyl compound. Both chiral 2-hydroxy-1,2-diphenylethanone antipodes were obtained with a high enantiomeric purity. Mild conditions (phosphate buffer [pH 7.0, 7.2], 30 °C) were successfully employed in the synthesis of (S)-benzoin using two different methodologies: benzyl desymmetrization and rac-benzoin deracemization. Bioreduction carried out with higher reagent concentrations, lower pH values and prolonged reaction time, and in the presence of additives, enabled enrichment of the reaction mixture with (R)-benzoin. The described procedure is a potentially useful tool in the synthesis of chiral building blocks with a defined configuration in a simple and economical process with a lower environmental impact, enabling one-pot biotransformation. 相似文献
99.
Dr. Nivesh Kumar Dr. Reddy Rajasekhar Reddy Dr. Ahmad Masarwa 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(34):8008-8012
An efficient and general method for the chemoselective synthesis of unsymmetrical gem-diborylalkanes is reported. This method is based on a late-stage desymmetrization through nucleophilic “trifluorination”, providing chiral gem-diborylalkanes bearing a trifluoroborate group. The reaction offers a highly modular and diastereoselective approach towards the synthesis of gem-diborylcyclopropanes. The utility of the gem-diborylalkane building blocks was demonstrated by selective post-functionalization of the trifluoroborate group. These functionalizations include inter- and intra- Pd-catalyzed Suzuki–Miyaura coupling reactions. 相似文献
100.
Synthesis of the Strychnos Alkaloid (−)‐Strychnopivotine and Confirmation of its Absolute Configuration 下载免费PDF全文
Gaëtan Maertens Prof. Sylvain Canesi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(21):7090-7093
The first enantioselective synthesis of (?)‐strychnopivotine from a known and inexpensive phenol has been achieved in 15 steps. The strategy is based on a new diastereoselective aza‐Michael‐enol‐ether cascade desymmetrization of a dienone, guided by a removable lactic acid‐derived chiral auxiliary. Synthesis involves a phenol dearomatization, a conjugated silicon addition, a stereoselective double reductive amination, and two Heck‐type carbopalladations as key steps. The absolute configuration of the natural compound, which, to date, has been uncertain, was confirmed by using circular dichroism (CD) spectroscopy and X‐ray analyses. 相似文献