Search results for "Lewis"
showing 10 items of 271 documents
Aminosäureester als chirale Hilfsgruppen in Lewis-Säure-katalysierten Diels-Alder-Reaktionen
1990
Amino Acid Esters as Chiral Auxiliaries in Lewis Acid Catalyzed Diels-Alder Reactions Cyclopentadiene, cyclohexadiene and alkyl-substituted butadienes react with N-acryloyl-, N-crotonoyl- and N-(4-nitro-cinnamoyl)-(S)-proline benzyl and allyl ester in the presence of Lewis acids to give the cycloadducts in high yields and with stereoselectivities of up to 97:3 for the endo isomers. The sense of the asymmetric induction in these reactions can be directed by the use of differently coordinating catalysts (e.g. TiCl4: (2R):(2S) = 96:4; EtAlCl2: (2R):(2S) = 10:90 at 0°C). In the TiCl4-catalyzed reactions with the fumaric acid derivatives the ratio of the diastereomers reaches values of up to 112…
Mechanism of Anionic Polymerization of (Meth)acrylates in the Presence of Aluminum Alkyls, 6. Polymerization of Primary and Tertiary Acrylates
1998
The kinetics of the polymerization of n-butyl acrylate initiated by lithiated ester enolates in the presence of aluminum alkyls was investigated in toluene and in toluene/Lewis base mixed solvents at −78 °C. In pure toluene, curved time−conversion plots, incomplete monomer conversion, and broad molecular weight distributions (Mw/Mn ≈ 2) are observedin the absence of aluminum alkyls the molecular weight distributions are significantly broader (Mw/Mn > 14). High monomer conversions and narrower molecular weight distributions (Mw/Mn ≈ 1.5) are obtained when using Lewis bases (e.g., methyl pivalate) as cosolvents. The polymerization of tert-butyl acrylate rapidly reaches full monomer conversion…
Calculations of organic-solvent dispersions of single-wall carbon nanotubes
2005
The aqueous solubility Sw and 1-octanol–(Po), cyclohexane–(Pch), and chloroform–(Pcf) water partition coefficients are calculated. The properties of single-wall carbon nanotubes (SWNT) are related to the indices (n, m) designating the chiral vector. SWNTs are classified in zigzag (n, 0), armchair (n, n), and chiral (n, m). These properties permit classifying SWNTs according to (n, m). Correlations between (n2 + nm + m2)1/2, log Sw, log Po, log Pch, and log Pcf show that (n, m) are adequate indices. The (10,10) is the most abundant SWNT, presenting consistency between great kinetic stability, small Sw, and great {Po, Pch, Pcf}. The room temperature solubility of SWNTs is investigated in a va…
Transient CD15-positive endothelial phenotype in the human placenta correlates with physiological and pathological fetoplacental immaturity
2013
Abstract Objective Placental growth and villous maturation are critical parameters of placental function at the end of pregnancy. A failure in these processes leads to the development of placental dysfunction, as well as fetal and neonatal mortality and morbidity. The aim of the study was to determine the relevant diagnostic markers associated with pathological placental development. Study design Forty tissue samples from normal placentas of different gestational age and 68 pathological term placentas with defective villous maturation (GDM, idiopathic IUFD, preeclamsia, HELLP syndrome) comprised the comparative immunohistochemical study (CD15, CD45 and CD34). Positive immunohistochemical re…
Atypical CIDP: diagnostic criteria, progression and treatment response. Data from the Italian CIDP Database
2019
ObjectivesA few variants of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) have been described, but their frequency and evolution to typical CIDP remain unclear. To determine the frequency and characteristics of the CIDP variants, their possible evolution to typical CIDP, and treatment response.MethodsWe applied a set of diagnostic criteria to 460 patients included in a database of Italian patients with CIDP. Clinical characteristics and treatment response were reviewed for each patient. The Kaplan-Meier curve was used to estimate the progression rate from atypical to typical CIDP.ResultsAt the time of inclusion, 376 (82%) patients had a diagnosis of typical CIDP while 84 …
CD15 immunostaining improves placental diagnosis of fetal hypoxia
2020
Fetal hypoxic events with unclear predictive value are a common indication for placenta examination. We evaluated whether the use of CD15 immunostaining can improve the assessment of severity and duration of fetal hypoxia.We compared placentas (37-42 gestational weeks) from stillborns/newborns with birth asphyxia (BA) and non-hypoxic newborns. Placental findings were studied in following groups: (1) acute BA (n = 11) due to placental abruption, (2) non-acute BA (n = 121) due to non-acute conditions, (3) non-BA (n = 46) in pregnancies with preeclampsia and gestational diabetes, and (4) controls (n = 30).A high expression of CD15 in feto-placental resistance vessels (FRVs) was present in non-…
Blood group related antigens in ocular cicatricial pemphigoid
2004
Aim: To study the MUC5AC and the blood group related antigen expression in ocular cicatricial pemphigoid (OCP) according to the distribution of Lewis and secretor phenotypes in OCP patients compared to normal subjects. Methods: Immunostaining was performed on conjunctival biopsy specimens from 22 consecutive patients suffering from OCP, using monoclonal antibodies (Mabs) directed against the peptidic core MUC5AC mucin (anti-M1/MUC5AC Mabs) and against the saccharide moieties (anti-blood group related antigens). These latter included anti-Lea, anti-Leb, anti-sialyl Lea, and H type 2 Mabs, which immunoreact with Lewis positive and non-secretor (Lea), Lewis positive and secretor (Leb), Lewis p…
Unsaturated syn- and anti-1,2-Amino Alcohols by Cyclization of Allylic Bis-trichloroacetimidates. Stereoselectivity Dependence on Substrate Configura…
2014
Disubstituted allylic bis-imidates undergo Lewis acid catalyzed or spontaneous cyclization to oxazolines, which are precursors of unsaturated amino alcohols. Stereoselectivity of the cyclization is mainly determined by the substrate configuration. Highly selective cis-oxazoline formation is achieved starting from anti-E-bis-imidates while trans-oxazoline predominantly forms from anti-Z-bis-imidates. On the basis of DFT calculations, the stereoselectivity trends can be explained by the formation of the energetically most stable carbenium ion conformation, followed by the cyclization via most favorable bond rotations.
Efficient Addition of Acid Enediolates to Epoxides
2004
We report new conditions to facilitate the addition of dianions of carboxylic acids to epoxides as an alternative method to the use of aluminum enolates. These conditions require the use of a sub-stoichiometric (10%) amount of amine for dianion generation and the previous activation of the epoxide with LiCl. Other Lewis acids have been shown to be less effective. Yields are good but only low diastereoselectivity is attained, which has not been controlled despite attempts at optimization. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
Low-loading asymmetric organocatalysis
2011
Asymmetric organocatalysis is now recognized as the third pillar of asymmetric synthesis. Recent years have witnessed increasing interest towards the use of highly active and stereoselective organocatalysts. This critical review documents the advances in the development of chiral organocatalysts which are systematically used in ≤3 mol% loading in all the sub-areas of the field, namely aminocatalysis, Brønsted acids and bases, Lewis acids and bases, hydrogen bond-mediated catalysis, phase transfer and N-heterocyclic carbene catalyses (194 references).