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https://www.readbyqxmd.com/read/28445710/stereoselective-preparation-of-quaternary-2-vinyl-sphingosines-and-ceramides-and-their-effect-on-basal-sphingolipid-metabolism
#1
Raquel Calderón, Nerea Mercadal, José Luis Abad, Xavier Ariza, Antonio Delgado, Jordi Garcia, Aleix Rodríguez, Gemma Fabriàs
The dicyclohexylborane-mediated addition of allene 1 to (E)-2-tridecenal affords a quaternary protected 2-amino-2-vinyl-1,3-diol in good yield as a single diastereomer. This compound is readily transformed into the four stereoisomers of the quaternary (E)-2-vinyl analogs of sphingosine. The metabolic fate and the effect of these compounds on the basal sphingolipid metabolism in human A549 lung adenocarcinoma cells has been studied, together with the ceramide analog of the most relevant vinylsphingosine derivative...
April 23, 2017: Chemistry and Physics of Lipids
https://www.readbyqxmd.com/read/28445064/enantioselective-michael-addition-of-photogenerated-o-quinodimethanes-to-enones-catalyzed-by-chiral-amino-acid-esters
#2
Xiaoqian Yuan, Shupeng Dong, Zhen Liu, Guibing Wu, Chuncheng Zou, Jinxing Ye
The first example of a photoexcitated amine-catalyzed process for asymmetric Michael addition of o-quinodimethanes to enones is described. In the presence of simple chiral amino acid esters, a variety of Michael adducts were generally obtained in good yields and excellent stereoselectivities. This strategy can be successfully applied to 3-substituted-2-cyclohexenones and provides an asymmetric access to all-carbon quaternary centers. Furthermore, the high stereocontrol was explained by means of density-functional theory (DFT) calculations...
April 26, 2017: Organic Letters
https://www.readbyqxmd.com/read/28445063/role-of-configuration-at-c6-in-catalytic-activity-of-l-proline-derived-bifunctional-organocatalysts
#3
Hui Jin, Soo Min Cho, Juyeol Lee, Do Hyun Ryu
l-Proline-derived chiral bifunctional (thio)urea organocatalysts epi-PTU and epi-PU were newly synthesized, and their catalytic performances were compared with their C6 epimeric catalysts PTU and PU in various Michael reactions of nitrostyrene in terms of reactivities and stereoselectivities. The experimental results indicate that a proper relative stereochemistry at C2 and C6 in l-proline-derived bifunctional organocatalysts is important for successful catalysis and that catalysts (PTU and PU) with the 2S,6R configuration are much more efficient...
April 26, 2017: Organic Letters
https://www.readbyqxmd.com/read/28444796/determination-of-volatile-flavour-profiles-of-citrus-spp-fruits-by-sde-gc-ms-and-enantiomeric-composition-of-chiral-compounds-by-mdgc-ms
#4
Joon Ho Hong, Naeem Khan, Nargis Jamila, Young Shin Hong, Eun Yeong Nho, Ji Yeon Choi, Cheong Mi Lee, Kyong Su Kim
INTRODUCTION: Citrus fruits are known to have characteristic enantiomeric key compounds biosynthesised by highly stereoselective enzymatic mechanisms. In the past, evaluation of the enantiomeric ratios of chiral compounds in fruits has been applied as an effective indicator of adulteration by the addition of synthetic compounds or natural components of different botanical origin. OBJECTIVE: To analyse the volatile flavour compounds of Citrus junos Sieb. ex Tanaka (yuzu), Citrus limon BURM...
April 25, 2017: Phytochemical Analysis: PCA
https://www.readbyqxmd.com/read/28443915/photochemical-studies-on-bis-sulfide-and-sulfone-tethered-polyenic-derivatives
#5
Simon Guélen, Max Blazejak, Lise-Marie Chamoreau, Arnaud Huguet, Sylvie Derenne, François Volatron, Virginie Mouriès-Mansuy, Louis Fensterbank
This study focusses on the [2 + 2]-photocycloaddition of a symmetric polyenic system tethered by an aryl bis-sulfide or sulfone platform. Using direct irradiation or photosensitization, no expected ladderane product was isolated. In most cases, only tricyclic products including a single cyclobutane moiety were formed. Irradiation of bis-acrylic precursors in water with encapsulation by a host (cyclodextrin or cucurbituril) provided a stereoselective access to valuable cyclobutyl adducts.
April 26, 2017: Organic & Biomolecular Chemistry
https://www.readbyqxmd.com/read/28443907/lanthanide-amide-catalyzed-one-pot-functionalization-of-isatins-synthesis-of-spiro-cyclopropan-1-3-oxindoles-and-2-oxoindolin-3-yl-phosphates
#6
Cheng Peng, Jiaojiao Zhai, Mingqiang Xue, Fan Xu
Lanthanide amide-catalyzed one-pot reaction of isatins, dialkyl phosphite, and activated alkenes was developed and a series of spiro[cyclopropan-1,3'-oxindoles] were obtained in moderate to excellent yields. The reaction is stereoselective and the two substituents at the 2 and 3-positions of the cyclopropane in the major product are in the trans configuration. A four-step mechanism involving the Pudovik reaction, Brook rearrangement, Michael addition, and intramolecular nucleophilic substitution is proposed, and an umpolung strategy was used in the process...
April 26, 2017: Organic & Biomolecular Chemistry
https://www.readbyqxmd.com/read/28443891/-3-3-sigmatropic-rearrangement-of-allenic-alcohols-stereoselective-synthesis-of-1-3-diene-2-ol-sulfonates
#7
Yuyang Zhao, Yurong Wang, Zhanshou Gu, Zhiming Wang
An efficient synthetic pathway to 1,3-diene-2-ol sulfonates involving the [3,3]-sigmatropic rearrangement of allenic alcohols with sulfonic acids under mild reaction conditions is described. These products can easily undergo reduction or transition-metal catalyzed cross-coupling reactions to yield a series of stereodefined multisubstituted 1,3-dienes.
April 26, 2017: Organic & Biomolecular Chemistry
https://www.readbyqxmd.com/read/28443661/engineering-of-rumb-toward-a-green-catalyst-for-carbene-insertion-reactions
#8
Matthew W Wolf, David A Vargas, Nicolai Lehnert
The small, stable heme protein myoglobin (Mb) was modified through cofactor substitution and mutagenesis to develop a new catalyst for carbene transfer reactions. The native heme was removed from wild-type Mb and several Mb His64 mutants (H64D, H64A, H64V), and the resulting apoproteins were reconstituted with ruthenium mesoporphyrin IX (RuMpIX). The reconstituted proteins (RuMb) were characterized by UV-vis and circular dichroism spectroscopy and were used as catalysts for the N-H insertion of aniline derivatives and the cyclopropanation of styrene derivatives...
April 26, 2017: Inorganic Chemistry
https://www.readbyqxmd.com/read/28441490/asymmetric-synthesis-of-cis-3-4-dihydrocoumarins-via-4-2-cycloadditions-catalyzed-by-amidine-derivatives
#9
Jing-Hai Jin, Xiang-Yu Li, Xiaoyan Luo, John Stephen Fossey, Wei-Ping Deng
A highly efficient chiral amidine derivative-catalyzed tandem Michael addition/lactonisation of carboxylic acids and o-quinone methides (o-QMs) has been developed, which enables the asymmetric synthesis of cis-3,4-dihydrocoumarins bearing contiguous tertiary stereogenic centers in high yields with excellent stereoselectivities.
April 25, 2017: Journal of Organic Chemistry
https://www.readbyqxmd.com/read/28441024/stereoselective-synthesis-and-evaluation-of-c6%C3%A2-substituted-5a-carbasugar-analogues-of-sl0101-as-inhibitors-of-rsk1-2
#10
Mingzong Li, Yu Li, Katarzyna A Ludwik, Zachary M Sandusky, Deborah A Lannigan, George A O'Doherty
A convergent synthesis of 5a-carbasugar analogues of the n-Pr-variant of SL0101 is described. The analogues were synthesized in an effort to find compounds with potent in vivo efficacy in the inhibition of p90 ribosomal s6 kinase (RSK1/2). The synthesis derived the desired C-4 L-rhamnose stereochemistry from quinic acid and used a highly selective cuprate addition, NaBH4 reduction, Mitsunobu inversion, and alkene dihydroxylation to install the remaining stereochemistry. A Pd-catalyzed cyclitolization stereoselectively installed the aglycon at the anomeric position...
April 25, 2017: Organic Letters
https://www.readbyqxmd.com/read/28440882/highly-selective-olefin-assisted-pd-ii-catalyzed-oxidative-alkynylation-of-enallenes
#11
Jan-E Bäckvall, Jeffrey Henry, Daniels Posevins, Bin Yang, Youai Qiu
An olefin-assisted, palladium-catalyzed oxidative alkynylation of enallenes for regio- and stereoselective synthesis of substituted trienynes has been developed. The reaction shows a broad substrate scope and good tolerance for various functional groups on the allene moiety, including carboxylic acid esters, free hydroxyls, imides, and alkyl groups. Also, a wide range of ter¬minal alkynes with electron-dona¬ting and electron-withdrawing aryls, heteroaryls, alkyls, trimethylsilyl, and free hydroxyl groups are tolerated...
April 25, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28440647/controllable-diastereodivergent-synthesis-of-pyrrolo-2-1-a-isoquinolines-via-catalytic-intramolecular-acylsulfenylation-of-activated-alkenes
#12
Wei Liu, Shan-Tao Du, Shu-Yue Wang, Wei-Wei Liao
A controllable stereoselective synthesis of tetrahydropyrrolo[2,1-a]isoquinoline derivatives bearing a sulfur moiety was demonstrated with high diastereoselectivity through a catalytic intramolecular acylsulfenylation of activated alkenes. This approach involved a catalytic thia-Michael addition triggered intramolecular aldol-type tandem sequence. Both cis- and trans-products can be readily prepared in moderate to high yields with excellent diastereoselectivities in a catalytically atom-economic fashion under the optimized mild reaction conditions...
April 25, 2017: Journal of Organic Chemistry
https://www.readbyqxmd.com/read/28440624/acid-base-catalysis-in-glycosidations-a-nature-derived-alternative-to-the-generally-employed-methodology
#13
Peng Peng, Richard R Schmidt
Inverting glycosyltransferases enforce in the active site an intramolecular, acid-base catalyzed glycosidation that, due to proximity of the donor anomeric carbon and the acceptor hydroxyl group, follows an SN2-type reaction. Spacers, tethering donor and acceptor via nonreacting functional groups, led in intramolecular glycosidations to excellent yields and, independent of the donor anomeric configuration, to either the α- or the β-anomer. The requirement of a demanding protecting group pattern confines the application of this efficient method...
April 25, 2017: Accounts of Chemical Research
https://www.readbyqxmd.com/read/28440592/sequential-reactions-of-alkynes-on-an-ir-iii-single-site
#14
Miguel A Sierra, Mar Gómez-Gallego, Luis Casarrubios, Carmen Ramírez de Arellano, Elena A Giner, Alba D Merinero
Sequential insertion of terminal alkynes on Ir(III) cyclometallated complexes allow the formation of novel metallapolycycles in controlled and efficient manner. ortho-Methylarylethynyl derivatives led to an unprecedented cascade combination of three fundamental processes (C-C bond formation, Csp3-H activation and reductive elimination) on a single Ir(III) center, in a process compatible with functionalized biomolecules and photoactive substrates. The reaction with tert-butylacetylene led to a [6,5,4]-polycycle that incorporates an iridacyclobutenylidene in its structure...
April 25, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28437118/blue-light-photocatalytic-glycosylation-without-electrophilic-additives
#15
Peng Wen, David Crich
Photocatalytic formation of glycosidic bonds employing stable and readily accessible O-glycosyl derivatives of 2,2,6,6-tetramethylpiperidin-1-ol is presented that employs an iridium-based photocatalyst and blue LEDs. The reaction proceeds at room temperature and in the absence of additives other than 4 Å molecular sieves. Stereoselectivities are modest but nevertheless dependent on the anomeric configuration of the donor, suggesting a substantial degree of concerted character.
April 24, 2017: Organic Letters
https://www.readbyqxmd.com/read/28436677/stereoselective-total-synthesis-of-carolacton
#16
Tapan Kumar Kuilya, Rajib Kumar Goswami
A short and convergent strategy for the stereoselective total synthesis of biologically active natural product carolacton has been accomplished. Our synthesis highlights the Urpi acetal aldol, Crimmins aldol, Ireland-Claisen rearrangement, TiCl4-assisted aldol followed by β-hydroxy elimination to construct C7-C8 olefin, and ring-closing metathesis as the key steps for achieving the target molecule with an overall yield of 18.8%.
April 24, 2017: Organic Letters
https://www.readbyqxmd.com/read/28436670/enantio-and-stereoselective-construction-of-atisane-scaffold-via-organocatalytic-intramolecular-michael-reaction-and-diels-alder-reaction
#17
Hiroko Sekita, Kyohei Adachi, Ippei Kobayashi, Yusuke Sato, Masahisa Nakada
An enantio- and stereoselective construction of the atisane scaffold via organocatalytic intramolecular Michael reaction and Diels-Alder reaction is described. The organocatalytic intramolecular Michael reaction has been found to stereoselectively generate a trans-stereodiad comprising an all-carbon quaternary and a tertiary stereogenic centers. Use of the chiral secondary amine bearing thiourea with benzoic acid as additive is the key to obtaining the desired product with excellent ee in synthetically acceptable yield...
April 24, 2017: Organic Letters
https://www.readbyqxmd.com/read/28436669/stereoselective-synthesis-of-exocyclic-tetrasubstituted-vinyl-halides-via-ru-catalyzed-halotropic-cycloisomerization-of-1-6-haloenynes
#18
Barry M Trost, Christopher A Kalnmals
Herein, a ruthenium-catalyzed cycloisomerization that transforms 1,6-haloenynes into 5-membered carbo- and heterocycles that bear exocyclic, stereodefined, tetrasubstituted vinyl halides is reported. The reaction is insensitive to air and water, tolerates a variety of functional groups, and proceeds with good to excellent stereoselectivity and yield.
April 24, 2017: Organic Letters
https://www.readbyqxmd.com/read/28436530/solvent-free-uncatalyzed-asymmetric-ene-reactions-of-n-tert-butylsulfinyl-3-3-3-trifluoroacetaldimines-a-general-approach-to-enantiomerically-pure-%C3%AE-trifluoromethyl-tryptamines
#19
Giuseppe Mazzeo, Giovanna Longhi, Sergio Abbate, Martina Palomba, Luana Bagnoli, Francesca Marini, Claudio Santi, Jianlin Han, Vadim A Soloshonok, Emilio Di Crescenzo, Renzo Ruzziconi
A novel approach to regioselectively substituted and stereoselectively α-trifluoromethylated tryptamines is reported based on the ene reaction of Boc-protected 3-methyleneindolines with optically pure (R)- or (S)-tert-butanesulfinyltrifluoroacetaldimine. Boc- and sulfinylamido-protected α-trifluoromethyltryptamines are obtained in 60-70% yield and 85/15 dr by just heating equimolar amounts of the two reaction partners at 80-90 °C for 2-3 h without a solvent. The absolute configuration of the amino α-carbon has been assigned based on the vibrational circular dichroism (VCD) spectral analysis...
April 24, 2017: Organic & Biomolecular Chemistry
https://www.readbyqxmd.com/read/28436515/exploring-the-reversal-of-enantioselectivity-on-a-zinc-dependent-alcohol-dehydrogenase
#20
Miguel A Maria-Solano, Adrian Romero-Rivera, Sílvia Osuna
Alcohol Dehydrogenase (ADH) enzymes catalyse the reversible reduction of prochiral ketones to the corresponding alcohols. These enzymes present two differently shaped active site pockets, which dictate their substrate scope and selectivity. In this study, we computationally evaluate the effect of two commonly reported active site mutations (I86A, and W110T) on a secondary alcohol dehydrogenase from Thermoanaerobacter brockii (TbSADH) through Molecular Dynamics simulations. Our results indicate that the introduced mutations induce dramatic changes in the shape of the active site, but most importantly they impact the substrate-enzyme interactions...
April 24, 2017: Organic & Biomolecular Chemistry
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