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Elise Malek-Adamian, Dale Guenther, Shigeo Matsuda, Saúl Martínez-Montero, Ivan Zlatev, Joel Harp, Mihai Burai Patrascu, Donald J Foster, Johans J Fakhoury, Lydia Perkins, Nicolas Moitessier, Rajar Manoharan, Nate Taneja, Anna Bisbe, Klaus Charisse, Martin A Maier, Kallanthottathil G Rajeev, Martin Egli, Muthiah Manoharan, Masad J Damha
We designed novel 4'-modified 2'-deoxy-2'-fluorouridine (2'-F U) analogues with the aim to improve nuclease resistance and potency of therapeutic siRNAs by introducing 4'-C-methoxy (4'-OMe) as the alpha (C4'α) or beta (C4'β) epimers. The C4'α epimer was synthesized by a stereoselective route in six steps; however, both α and β epimers could be obtained by a non-stereoselective approach starting from 2'-F U. (1)H-NMR analysis and computational investigation of the α-epimer revealed that the 4'-OMe imparts a conformational bias towards the North-East sugar pucker, due to intramolecular hydrogen bonding and hyperconjugation effects...
September 22, 2017: Journal of the American Chemical Society
Dhanarajan Arunprasath, Balasubramanian Devi Bala, Govindasamy Sekar
An efficient diastereoselective synthesis of α-tetralone-fused spirooxindoles is reported. The Pd-catalyzed domino reaction proceeds through a carbene migratory insertion followed by a 6-endo-trig mode of conjugate addition sequence from easily accessible isatin-derived N-tosylhydrazones and 2'-iodochalcones. The versatility of the protocol has been showcased by high functional group tolerance, broad substrate scope, and extension to an expedient synthesis of spiroacenaphthylenes. NMR reaction profiling and deuterium-labeling investigations provide insight into the mechanistic pathway...
September 22, 2017: Organic Letters
Tanja Knaus, Luca Cariati, Marcelo F Masman, Francesco G Mutti
The direct and efficient conversion of alcohols into amines is a pivotal transformation in chemistry. Here, we present an artificial, oxidation-reduction, biocatalytic network that employs five enzymes (alcohol dehydrogenase, NADP-oxidase, catalase, amine dehydrogenase and formate dehydrogenase) in two concurrent and orthogonal cycles. The NADP-dependent oxidative cycle converts a diverse range of aromatic and aliphatic alcohol substrates to the carbonyl compound intermediates, whereas the NAD-dependent reductive aminating cycle generates the related amine products with >99% enantiomeric excess (R) and up to >99% conversion...
September 22, 2017: Organic & Biomolecular Chemistry
Hujun Xie, Yang Li, Lihong Wang, Jian Kuang, Qunfang Lei, Wenjun Fang
Quantum chemistry calculations have been employed to study the mechanisms of nickel-catalyzed Suzuki-Miyaura cross-coupling reactions of benzylic carbamates with arylboronic esters, and the energy profiles have been computed to evaluate possible origins for the generation of different stereochemistry products. It has been demonstrated that the mechanism can be divided into three steps: oxidative addition, transmetallation and reduction elimination. Transmetallation is the rate-limiting step for the whole reaction cycle, and oxidative addition controls the stereoselectivity of the resulting products...
September 22, 2017: Dalton Transactions: An International Journal of Inorganic Chemistry
Stephanie A Murray, Michael Z Liang, Simon J Meek
A copper-catalyzed three-component linchpin coupling method for the stereoselective union of readily available epoxides and allyl electrophiles is disclosed. Transformations employ [B(pin)]2-methane as a conjunctive reagent, resulting in the formation of two C-C bonds at a single carbon center bearing a C(sp3) organoboron functional group. Products are obtained in 42-99% yield, and up to >20:1 dr. The utility of the approach is highlighted by stereospecific transformations entailing allylation, tandem cross coupling, and application to the synthesis 1,3-polyol motifs...
September 21, 2017: Journal of the American Chemical Society
Carlos Palo-Nieto, Abhijit Sau, M Carmen Galan
Au(I) in combination with AgOTf enables the unprecedented direct and α-stereoselective catalytic synthesis of deoxyglycosides from glycals. Mechanistic investigations suggest that the reaction proceeds via Au(I)-catalysed hydrofunctionalization of the enol ether glycoside. The room temperature reaction is high yielding and amenable to a wide range of glycal donors and OH nucleophiles.
September 21, 2017: Journal of the American Chemical Society
Nikita M Podvalnyy, Nelly N Malysheva, Maria V Panova, Alexander I Zinin, Alexander O Chizhov, Anna V Orlova, Leonid O Kononov
A series of novel sialyl donors containing O-trifluoroacetyl (TFA) groups at various positions was synthesized. The choice of protecting groups in sialyl donors was based on hypothesis that variations in ability of different acyl groups to act as hydrogen bond acceptors would influence the supramolecular structure of reaction mixture (solution structure), hence the outcome of sialylation. These glycosyl donors were examined in the model glycosylation of the primary hydroxyl group of 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose in comparison with sialyl donors without O-TFA groups...
September 8, 2017: Carbohydrate Research
Jin-Long Pan, Chao Chen, Zhi-Gang Ma, Jia Zhou, Li-Ren Wang, Shu-Yu Zhang
An efficient and convenient palladium-catalyzed direct intermolecular silylation of C(sp(2))-H bonds by using disilanes as the silicon source with the assistance of a readily removable bidentate directing group is reported. This strategy provided a regio- and stereoselective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility. Silylation of the isolated palladacycle intermediate revealed the Z-stereoselective pathway. Moreover, the practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product...
September 21, 2017: Organic Letters
Salma Kassem, Alan T L Lee, David A Leigh, Vanesa Marcos, Leoni I Palmer, Simone Pisano
It has been convincingly argued that molecular machines that manipulate individual atoms, or highly reactive clusters of atoms, with Ångström precision are unlikely to be realized. However, biological molecular machines routinely position rather less reactive substrates in order to direct chemical reaction sequences, from sequence-specific synthesis by the ribosome to polyketide synthases, where tethered molecules are passed from active site to active site in multi-enzyme complexes. Artificial molecular machines have been developed for tasks that include sequence-specific oligomer synthesis and the switching of product chirality, a photo-responsive host molecule has been described that is able to mechanically twist a bound molecular guest, and molecular fragments have been selectively transported in either direction between sites on a molecular platform through a ratchet mechanism...
September 20, 2017: Nature
Baochao Yang, Kuaikuai Lin, Yingbo Shi, Shuanhu Gao
Stereoselective construction of polycyclic rings with all-carbon quaternary centers, and vicinal all-carbon quaternary stereocenters, remains a significant challenge in organic synthesis. These structures can be found in a wide range of polycyclic natural products and drug molecules. Here we report a Ti(Oi-Pr)4-promoted photoenolization/Diels-Alder (PEDA) reaction to construct hydroanthracenol and related polycyclic rings bearing all-carbon quaternary centers. This photolysis proceeds under mild conditions and generates a variety of photo-cycloaddition products in good reaction efficiency and stereoselectivity (48 examples), and has been successfully used in the construction of core skeleton of oncocalyxones, tetracycline and pleurotin...
September 20, 2017: Nature Communications
Yang-Zi Liu, Shao-Jing Shang, Wu-Lin Yang, Xiaoyan Luo, Wei-Ping Deng
A highly efficient asymmetric 1,3-dipolar cycloaddition of azomethine ylides to β,β-disubstituted β-fluoro-acrylates catalyzed by a chiral N,O-ligand/Cu(CH3CN)4BF4 system is reported, affording chiral densely-substituted pyrrolidines with four contiguous stereocenters, including one fluorinated quaternary stereocenter at the 3-position, in good to excellent yields (up to 99%), with excellent levels of diastereo- and enantioselectivities (dr >20:1; ee up to 99%).
September 21, 2017: Journal of Organic Chemistry
David R Williams, Seth A Bawel, Richard N Schaugaard
A novel annelation strategy has been devised for stereoselective synthesis of tetrahydrocarbazoles. The pathway features a regio- and stereocontrolled condensation of indole and its substituted derivatives with electron-deficient 1,3-dienes via a Michael-Mannich reaction sequence. An extension of this method to include cross-conjugated allenes as substrates also results in a Michael-Mannich-Michael cascade, incorporating 2 equiv of indole with increasing product complexity. The formal 4π + 2π cyclization describes a concise route to polycyclic alkaloids of this family...
September 20, 2017: Organic Letters
Xun Yang, Fraser F Fleming
Metalated nitriles are exceptional nucleophiles capable of forging highly hindered stereocenters in cases where enolates are unreactive. The excellent nucleophilicity emanates from the powerful inductive stabilization of adjacent negative charge by the nitrile, which has a miniscule steric demand. Inductive stabilization is the key to understanding the reactivity of metalated nitriles because this permits a continuum of structures that range from N-metalated ketenimines to nitrile anions. Solution and solid-state analyses reveal two different metal coordination sites, the formally anionic carbon and the nitrile nitrogen, with the site of metalation depending intimately on the solvent, counterion, temperature, and ligands...
September 20, 2017: Accounts of Chemical Research
Andreas Boelke, Lucien D Caspers, Boris J Nachtsheim
The first directing-group-mediated C-H alkenylation with alkenyl-λ(3)-iodanes as electrophilic alkene-transfer reagents has been developed. The application of free aromatic amines as challenging but synthetically valuable directing groups in combination with an Ir(III) catalyst enabled the synthesis of highly desirable 1,3-dienes in excellent yields of up to 98% with high to perfect (Z,E) stereoselectivity. A broad substrate scope and further synthetic modifications are demonstrated.
September 19, 2017: Organic Letters
Muhammet Uyanik, Takeshi Yasui, Kazuaki Ishihara
Highly enantioselective oxidative dearomatization of 2-naphthol derivatives was achieved for the first time by using conformationally flexible organoiodine catalysts derived from 2-aminoalcohol as a chiral source. Moreover, with the use of these catalysts, excellent enantioselectivities were also achieved for 1-naphthol derivatives, which had previously been obtained with only lower enantioselectivities. Furthermore, the product obtained from the present reaction could be transformed to a highly functionalized spirolactone in high yield and with excellent stereoselectivity...
September 19, 2017: Journal of Organic Chemistry
Hiroaki Iwamoto, Yu Ozawa, Koji Kubota, Hajime Ito
The copper(I)-catalyzed regio- and stereoselective intramolecular alkylation of propargyl ethers and amines bearing an alkyl electrophilic moiety has been developed. The reaction showed high functional group tolerance and gave highly functionalized alkenylboronates bearing heterocyclic rings, which are versatile synthetic intermediates in organic chemistry. The borylation products can be transformed into multi-substituted alkenes through stereospecific transformations. Mechanistic studies showed that the chemo- and stereoselectivity of copper(I)-catalyzed borylation depends on the type of leaving group...
September 19, 2017: Journal of Organic Chemistry
Ahria Roushanbakhti, Yifan Liu, Paul Winship, Michael Tucker, Wasim Akhtar, Daryl Walter, Gail Wrigley, Timothy James Donohoe
Catalytic oxidative cyclisation reactions have been employed for the synthesis of the E and F rings of the complex natural product target pectenotoxin 4. The choice of metal catalyst (cobalt or osmium based) allowed for the formation of THF rings with either trans or cis stereoselectivity. Fragment union using a modified Julia reaction then enabled the synthesis of an advanced synthetic intermediate containing the EF and G rings of the target.
September 19, 2017: Angewandte Chemie
Sven Tobisch
This first comprehensive computational study of the copper-mediated formal aminoboration utilising an electrophilic strategy has identified the most accessible pathway for the productive catalysis. The mechanistic picture derived from smooth energy profiles acquired by employing a reliable computational protocol applied to a realistic catalyst model conforms to all available experimental data. The high degree of chemo- and stereoselectivity achieved in syn-borylcupration and Umpolung electrophilic amination is instrumental to the exclusive generation of the (syn)-beta-aminoalkylborane product...
September 19, 2017: Chemistry: a European Journal
Xiong Li, Lingling Hu, Junhao Jia, He Gu, Yuanliang Jia, Xiaochuan Chen
A new short approach to (-)-lepadins A-C has been developed based on a stereocontrolled Diels-Alder reaction employing a chiral dienophile. With this approach, (-)-lepadin B is synthesized from 5-deoxy-d-ribose in 13 steps with 14.8% overall yield. The cis-decahydroquinoline core containing five stereocenters could be rapidly constructed via stereoselective cycloaddition and subsequent five-step one-pot hydrogenation-cyclization.
September 19, 2017: Organic Letters
Kamal Kumar Bhardwaj, Reena Gupta
Pure enantiomers can be produced by chemical and enzymatic methods. Enzymatic methods to produce pure enantiomers are more advantageous. High enantiomeric excess is the advantage of enzymatic method, resulting from the inherent selectivity of the enzyme. Lipase (EC enzymes are widely used for the synthesis of chirally pure enantiomers because of their stereoselectivities which can satisfy the growing demand for obtaining optically active compounds. Various methods are used for the synthesis of chirally pure enantiomers, however kinetic resolution and asymmetric synthesis are more common...
September 15, 2017: Journal of Oleo Science
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