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Luis R Domingo, Mar Ríos-Gutiérrez, Patricia Pérez
The zw-type [3+2] cycloaddition (32CA) reactions of C,N-dialkyl nitrones with a series of ethylenes of increased electrophilic character have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Both, reactivity and selectivities are rationalised depending on the polar character of the reaction. Due to the strong nucleophilic character of C,N-dialkyl nitrones, the corresponding zw-type 32CA reactions are accelerated with the increased electrophilic character of the ethylene, which also plays a crucial role in the reaction mechanism, thus determining the regio- and stereoselectivities experimentally observed...
January 19, 2018: Journal of Organic Chemistry
Yusuke Higashino, Taisuke Okamoto, Kazuki Mori, Takashi Kawasaki, Masahiro Hamada, Noriyuki Nakajima, Akiko Saito
Proanthocyanidins, also known as condensed tannins or oligomeric flavonoids, are found in many edible plants and exhibit interesting biological activities. Herein, we report a new, simple method for the stereoselective synthesis of procyanidin B6, a (+)-catechin-(4-6)-(+)-catechin dimer, by Lewis acid-catalyzed intramolecular condensation. The 5-O-t-butyldimethylsilyl (TBDMS) group of 5,7,3'4'-tetra-O-TBDMS-(+)-catechin was regioselectively removed using trifluoroacetic acid, leading to the "regio-controlled" synthesis of procyanidin B6...
January 18, 2018: Molecules: a Journal of Synthetic Chemistry and Natural Product Chemistry
Guang-Zu Wang, Rui Shang, Yao Fu
It is reported that Pd(PPh3)2Cl2 in combination with 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) under irradiation of blue LEDs efficiently catalyzes a decarboxylative Heck reaction of vinyl arenes and vinyl heteroarenes with aliphatic N-(acyloxy)phthalimides at room temperature. A broad scope of secondary, tertiary, and quaternary carboxylates, including α-amino acid derived esters, can be applied as amenable substrates with high stereoselectivity. The experimental observation was explained by excitation-state reactivity of the palladium complex under irradiation to induce single-electron transfer to activate N-(acyloxy)phthalimides, and to suppress undesired β-hydride elimination of alkyl palladium intermediates...
January 18, 2018: Organic Letters
Akhil Padarti, Dongeun Kim, Hyunsoo Han
A distinctly different approach for the vinylogous aldolation of aldehydes is described, which exploits 2-oxonia-Cope rearrangement reactions between two readily available partners, a set of rationally designed chiral homoallylic alcohol synthons and aldehydes, under simple conditions. In these processes, chirality transfer from the former to the latter is nearly perfect, giving rise to excellent enantio- and diastereoselectivity without the regioselectivity issue associated with traditional vinylogous aldol reactions...
January 18, 2018: Organic Letters
Kamil Lisiecki, Zbigniew Czarnocki
The first total synthesis of (+)-epigalcatin was completed in a highly stereoselective manner starting from piperonal, 3,4-dimethylbenzaldehyde, and diethyl succinate. l-Prolinol was used as a chiral auxiliary. The crucial step in this procedure involves the construction of the cyclolignan framework by continuous-flow photocyclization of a chiral atropisomeric 1,2-bisbenzylidenesuccinate amide ester.
January 18, 2018: Organic Letters
Laia Vicens, Miquel Costas
The stereoselective oxidation of hydrocarbons is one of the most challenging reactions for synthetic chemists. However, this transformation is one of the most common reactions in nature. Metalloenzymes that catalyze this transformation are taken as inspiration for the development of new catalysts. There are several examples in the literature where either peptides or metal catalysts are used in the stereoselective oxidation reaction, but the synergistic combination of both systems is still a non-explored field...
January 18, 2018: Dalton Transactions: An International Journal of Inorganic Chemistry
Amrita Saha, Aditya Bhattacharyya, Ranadeep Talukdar, Manas K Ghorai
Two novel synthetic protocols for the syntheses of highly functionalized five-membered carbocyclic enaminonitriles and β-enaminoesters have been developed via domino ring-opening cyclization (DROC) and DROC/decarboxylative tautomerization of activated cyclopropanes with malononitrile pronucleophiles, respectively. Both of the efficient strategies (yield up to 93%) have been generalized with various donor-acceptor and acceptor cyclopropanes as well as with malononitrile derivatives. The stereospecific variants of the two SN2-type DROC strategies have also been developed by employing enantiopure donor-acceptor (DA) cyclopropanes to synthesize the corresponding nonracemic products with excellent stereoselectivities (dr up to >99:1, ee up to >99%)...
January 17, 2018: Journal of Organic Chemistry
Xinlei Li, Changjuan Chen, Jincai Wu
Polyesters, especially poly(lactide) (PLA), are used widely as biodegradable and biocompatible materials, yet their controllable synthesis, especially the stereoselective synthesis of polyesters, is still a challenge. Recently some excellent Lewis pair catalysts for ring-opening polymerization (ROP) of lactide and related cyclic esters have emerged. This review article will highlight the key advances in the ROP catalyzed by Lewis pair compounds with the aim of encouraging the wider application of Lewis pair catalysts in the polymerization of lactide and related cyclic esters...
January 17, 2018: Molecules: a Journal of Synthetic Chemistry and Natural Product Chemistry
Joan Guasch, Irene Giménez, Ignacio Funes-Ardoiz, Isabel Matheu, Feliu Maseras, Sergio Castillón, Yolanda Díaz, Miguel Bernús
Regio- and stereoselective oxyamination of dienes through a tandem rhodium catalysed aziridination-nucleophilic opening affords racemic oxazolidinone derivatives, which undergo a kinetic resolution acylation process using ABCs (amidine-based catalysts) achieving s values up to 117. This protocol was applied to the enantioselective synthesis of sphingosine.
January 17, 2018: Chemistry: a European Journal
André H B Dourado, Fabián C Pastrián, Susana I Córdoba DE Torresi
Proteins have been the subject of electrochemical studies. It is possible to apply electrochemical techniques to obtain information about their structure due to the presence of five electroactive amino acids that can be oriented to the outside of the peptidic chain. These amino acids are L-Tryptophan (L-Trp), L-Tyrosine (L-Tyr), L-Histidine (L-His), L-Methionine (L-Met) and L-Cysteine (L-Cys); their electrochemical behavior being subject of extensive research, but it is still controversial. No spectroscopic investigations have been reported on L-Trp, and due to the short life time of the intermediates, ex situ techniques cannot be employed, leading to a never-ending discussion about possible intermediates...
January 11, 2018: Anais da Academia Brasileira de Ciências
Amrita Mondal, Priyanka Kundu, Moumita Jash, Chinmay Chowdhury
An efficient method has been developed for the stereoselective synthesis of 4-(diarylmethylidene)-3,4-dihydroisoquinolin-1(2H)-ones 7 through tandem Heck-Suzuki coupling at rt using easily available substrates. DBU easily converted the exocyclic double bond of these compounds to endo, furnishing 8 and 9. Reduction of the carbonyl group of 7 was smoothly carried out with borane dimethyl sulphide. Subsequent treatment with KOtBu provided an easy access to 4-substituted isoquinolines 10a if carried out in refluxing 1,4-dioxane, while reaction in DMF at rt led to the incorporation of an extra hydroxyl group at the benzylic position of the isoquinolines to give 10b...
January 17, 2018: Organic & Biomolecular Chemistry
Saroj K Rout, Michael P Friedmann, Roland Riek, Jason Greenwald
The prebiotic replication of information-coding molecules is a central problem concerning life's origins. Here, we report that amyloids composed of short peptides can direct the sequence-selective, regioselective and stereoselective condensation of amino acids. The addition of activated DL-arginine and DL-phenylalanine to the peptide RFRFR-NH2 in the presence of the complementary template peptide Ac-FEFEFEFE-NH2 yields the isotactic product FRFRFRFR-NH2, 1 of 64 possible triple addition products, under conditions in which the absence of template yields only single and double additions of mixed stereochemistry...
January 16, 2018: Nature Communications
Emi Saito, Yuri Matsumoto, Akihiko Nakamura, Yuki Namera, Masahisa Nakada
The preparation and reactions of ortho-benzoquinone monohemiaminals are described. The oxidative dearomatization of phenols bearing amino alcohol groups induced N-cyclization to afford ortho-benzoquinone monohemiaminals. The N-cyclization stereoselectively affords the product when a chiral amino alcohol is used as the substituent. The chiral ortho-benzoquinone monohemiaminal undergoes stereoselective Diels-Alder reactions with electron-deficient alkenes, as expected, confirming the promising utility of ortho-benzoquinone monohemiaminals...
January 17, 2018: Organic Letters
Christinne Hedberg, Morten Estrup, Espen Z Eikeland, Henrik Helligsø Jensen
A novel Ferrier-type carbocyclization is reported. It involves a carbohydrate-derived lactone acetal synthesized from methyl α-D-glucopyranoside, which upon treatment with excess vinylmagnesium bromide provides a highly substituted carbocyclic product as a single stereoisomer. The yield is greatly increased when N,N,N',N'-tetramethylethylenediamine is added to the reaction mixture. Optimized reaction conditions have been applied to lactone acetals derived from other carbohydrates. Based on the obtained results, a possible reaction mechanism has been proposed...
January 16, 2018: Journal of Organic Chemistry
Suman Pramanik, Supriya Rej, Shun Kando, Hayato Tsurugi, Kazushi Mashima
Salt-free stereoselective synthesis of silyl enol ethers was achieved by treating α-halo carbonyl compounds with 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-dihydropyrazine. In this reaction, easily removable trimethylsilyl halides and 2,3,5,6-tetramethylpyrazine were generated as the reaction byproducts. Due to the inertness of the reaction byproducts, we found a one-pot transformation of the in situ-generated silyl enol ethers into various α-functionalized carbonyls by the reaction with Togni-II reagent or aldehydes...
January 17, 2018: Journal of Organic Chemistry
Naohiro Uemura, Hiroki Ishikawa, Naoki Tamura, Yasushi Yoshida, Takashi Mino, Yoshio Kasashima, Masami Sakamoto
Photodimerization of 3-arylindenones in solution and in the solid-state was examined. Irradiation of 3-arylindenones in benzene exclusively gave C2-symmetric anti-HH cyclobutane dimers in good yields. In contrast, photolysis in the solid-state afforded syn-HH cyclobutane dimers efficiently, which was considerably influenced by the molecular arrangement in the crystal lattice.
January 17, 2018: Journal of Organic Chemistry
Jacek G Sośnicki, Tomasz J Idzik, Aleksandra Borzyszkowska, Gabriela Maciejewska, Łukasz Struk
An efficient synthesis of bromofunctionalized 2,6-methano- and 1,5-methano-benzomorphanones, starting from easily available 6-benzyl-3,6-dihydropyridin-2(1H)-ones, is described. Furthermore, the synthesis of bridged benzomorphanones with hitherto not known polycyclic systems containing 2- or 3-azabicyclo[4.1.0]heptane units is developed upon treatment of both 2,6- and 1,5-methanobromobenzomorphans with t-BuOK. The effects of substituents on the diversity and stereoselectivity of both transformations are studied...
January 16, 2018: Journal of Organic Chemistry
Jianguo Yang, Hanjie Mo, Haijian Wu, Dongdong Cao, Chengmin Pan, Zhiming Wang
A highly regio- and stereoselective Pd-catalyzed cascade reaction of allenoate adducts has been developed. Various allenoate adducts of phosphine-catalyzed addition of allenoates are found to be efficient substrates for the preparation of 2,3-dihydrobenzofuran. It is the first example of allenoate adducts used as ideal and practical precursors of hard-to-get functionalized 1,3-butadienes for heterocycle synthesis.
January 16, 2018: Chemical Communications: Chem Comm
Jaehoon Sim, Hyunkyung Park, Juhee Lim, Inah Yoon, Changjin Lim, Hongchan An, Hwayoung Yun, Hyun Jin Choi, Young-Ger Suh
The first total synthesis of (-)-dendroside C aglycon, consisting of a 1,4,5-tri-cis-guaiane skeleton, from a versatile hydroazulene intermediate has been accomplished. The key features of the syntheses include the stereoselective preparation of the unusual cis-hydroazulene core via a sequence of a unique Dieckmann condensation of the bicyclic lactone system, which was concisely prepared by the tandem conjugate addition and intramolecular allylic alkylation of a butenolide precursor, and construction of the characteristic tricyclic skeleton by a carbene-mediated cyclopropanation...
January 16, 2018: Organic Letters
A Castán, R Badorrey, J A Gálvez, P López-Ram-de-Víu, M D Díaz-de-Villegas
Novel bifunctional pyrrolidine-based organocatalysts for the asymmetric Michael addition of carbonyl compounds to nitroolefins have been synthesised from homoallylamines, which are easily obtained from (R)-glyceraldehyde as a chiral precursor. Under optimal reaction conditions, these bifunctional organocatalysts showed a high catalytic efficiency (almost quantitative yield in most cases) and stereoselectivity in the Michael addition reactions of a variety of aldehydes (up to 98 : 2 dr and 97% ee) and ketones (up to 98 : 2 dr and 99% ee) to nitroolefins...
January 16, 2018: Organic & Biomolecular Chemistry
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