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Protective Groups (protective + groups)
Selected AbstractsTi-Mediated Synthesis of Aminocyclopropyl-Substituted CarbohydratesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 23 2005Christophe Laroche Abstract Carbohydrates bearing aminocyclopropyl moieties were conveniently prepared from the corresponding nitriles by titanium-mediated addition of Grignard reagents. A wide range of protective groups are tolerated. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Converting Core Compounds into Building Blocks: The Concept of Regiochemically Exhaustive FunctionalizationEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2005Elena Marzi Abstract In a model study, 3-fluorophenol and 3-fluoropyridine were converted into the each time four possible carboxylic acids by passing through the corresponding organometallic intermediates. As an attempt to generalize the findings reveals, a restricted set of principles and methods suffices to cope with all standard scenarios. The most valuable and versatile tools for the regiochemically exhaustive functionalization of a great variety of substrate patterns are the optionally site-selective metalation (either by reagent/substrate matching or by peripheral coordination control), the use of activating or congesting protective groups and the basicity gradient-driven heavy halogen migration (followed by halogen/metal permutation). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Regioflexibility in the Functionalization of Multiply Halogenated QuinolinesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2004Marc Marull Abstract 4-Bromo-6-fluoro-2-(trifluoromethyl)quinoline (1) and 4-bromo-7-fluoro-2-(trifluoromethyl)quinoline (13) were selected as model substrates to explore under what conditions regiochemically exhaustive functionalization reactions can be carried out. This goal was achieved by using trimethylsilyl entities and iodine atoms as the sole auxiliary substituents. The organolithium intermediates could be generated and the protective groups removed without impairing the bromine atom present at the 4-position. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Novel Palladium-on-Carbon/Diphenyl Sulfide Complex for Chemoselective Hydrogenation: Preparation, Characterization, and ApplicationADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2008Akinori Mori Abstract A diphenyl sulfide immobilized on palladium-on-carbon system, Pd/C[Ph2S], was developed to achieve the highly chemoselective hydrogenation of alkenes, acetylenes, azides, and nitro groups in the presence of aromatic ketones, halides, benzyl esters, and N-Cbz protective groups. Instrumental analyses of the heterogeneous catalyst demonstrated that diphenyl sulfide was embedded on Pd/C via coordination of its sulfur atom to palladium metal or physical interaction with graphite layers of the activated carbon. The catalyst could be recovered and reused at least five times without any significant loss of the reactivity. [source] Preparation of 4-[11C]methylmetaraminol, a potential PET tracer for assessment of myocardial sympathetic innervationJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 1 2003Oliver Langer Abstract The false adrenergic neurotransmitter [11C]meta -hydroxyephedrine ([11C]HED) is currently the PET tracer of choice for assessment of myocardial sympathetic innervation. The molecule is metabolised in the 4-position of the aromatic ring. The resulting radiolabelled metabolites need to be measured in order to obtain an arterial input function. Our aim was the development of a PET tracer with an increased metabolic stability relative to [11C]HED. We selected 4-methylmetaraminol as a candidate molecule for radiolabelling with 11C (t1/2 20.4 min). Our radiosynthetic approach towards 4-[11C]methylmetaraminol involved a palladium-catalyzed cross-coupling reaction of a protected 4-trimethylstannyl derivative of metaraminol with [11C]methyl iodide followed by removal of the protective groups. 4-[11C]methylmetaraminol was obtained in a final decay-corrected radiochemical yield of 20,25% within a synthesis time of 60,80 min. The specific radioactivity at the end of the synthesis ranged from 18,37 to GBq/,mol. The unlabelled reference molecule, 4-methylmetaraminol, was prepared in a 5-step synthesis starting from metaraminol. A biological evaluation of 4-[11C]methylmetaraminol is in progress and the results will be reported elsewhere. Copyright © 2002 John Wiley & Sons, Ltd. [source] Molecularly defined (L)-lactic acid oligomers and polymers: Synthesis and characterizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 18 2008Kenichi Takizawa Abstract The synthesis of (L)-lactide oligomers from dimer to 64mer via an exponential growth strategy is described. By careful selection of orthogonal protective groups, the synthesis were conducted using a t -butyldimethylsilyl (TBDMS) ether as the protective group of the hydroxyl group and benzyl (Bn) ester as the protective group of the carboxylic acid group. The yields of both the deprotection steps and coupling reactions using 1,3-dicyclohexylcarbodiimide or 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride were high (70,100%) and the absence of a requirement for conducting the majority of reactions under an inert atmosphere permitted a robust and efficient synthetic strategy to be developed. This allowed monodisperse dimer, tetramer, octamer, 16mer, 32mer, and 64mer materials to be prepared in gram quantities and fully characterized using mass spectrometry and size exclusion chromatography. Evaluation of the thermal and physical properties using thermogravimetric analysis, differential scanning calorimetry, and small angle X-ray scattering demonstrated a close correlation between the molecular structure of the well-defined Poly(lactide) oligomers and their physical properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5977,5990, 2008 [source] Synthesis and ring-opening (co)polymerization of L -lysine N -carboxyanhydrides containing labile side-chain protective groupsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 9 2003Juan Rodríguez Hernández Abstract This contribution describes the synthesis and ring-opening (co)polymerization of several L -lysine N -carboxyanhydrides (NCAs) that contain labile protective groups at the ,-NH2 position. Four of the following L -lysine NCAs were investigated: N, -trifluoroacetyl- L -lysine N -carboxyanhydride, N, -(tert -butoxycarbonyl)- L -lysine N -carboxyanhydride, N, -(9-fluorenylmethoxycarbonyl)- L -lysine N -carboxyanhydride, and N, -(6-nitroveratryloxycarbonyl)- L -lysine N -carboxyanhydride. In contrast to the harsh conditions that are required for acidolysis of benzyl carbamate moieties, which are usually used to protect the ,-NH2 position of L -lysine during NCA polymerization, the protective groups of the L -lysine NCAs presented here can be removed under mildly acidic or basic conditions or by photolysis. As a consequence, these monomers may allow access to novel peptide hybrid materials that cannot be prepared from ,-benzyloxycarbonyl- L -lysine N -carboxyanhydride (Z-Lys NCA) because of side reactions that accompany the removal of the Z groups. By copolymerization of these L -lysine NCAs with labile protective groups, either with each other or with ,-benzyl- L -glutamate N -carboxyanhydride or Z-Lys NCA, orthogonally side-chain-protected copolypeptides with number-average degrees of polymerization ,20 were obtained. Such copolypeptides, which contain different side-chain protective groups that can be removed independently, are interesting for the synthesis of complex polypeptide architectures or can be used as scaffolds for the preparation of synthetic antigens or protein mimetics. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1167,1187, 2003 [source] Well-defined glycopolymer amphiphiles for liquid and supercritical carbon dioxide applicationsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2001Weijun Ye Abstract Well-defined D -glucose-containing glycopolymers, poly(3- O -methacryloyl-1,2 : 5,6-di- O -isopropylidene- D -glucofuranose) (PMAIpGlc), and diblock copolymers of PMAIpGlc with poly(1,1-dihydroperfluorooctyl methacrylate) (PFOMA) were synthesized by living anionic polymerization in THF at ,78 °C with 1,1-diphenylhexyllithium in the presence of lithium chloride. The resulting polymers were found to possess predictable molecular weights and very narrow molecular weight distributions (MWD, Mw/Mn , 1.16). Removal of the acetal protective groups from the protected glycopolymer block copolymer was carried out using 90% trifluoroacetic acid at room temperature, yielding a hydrophilic block copolymer with pendant glucose moieties. Both protected (lipophilic/CO2 -philic) and deprotected (hydrophilic/CO2 -philic) fluorocopolymers were proved to be CO2 amphiphiles. Their solubility in CO2 was heavily influenced by the amphiphilic structure, such as the copolymer compositions and the polarities of sugar block. Light-scattering studies showed that, after removal of the protective groups, the deprotected block copolymer formed aggregate structures in liquid CO2 with an average micellar size of 27 nm. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3841,3849, 2001 [source] A Formal Total Synthesis of Eleutherobin Using the Ring-Closing Metathesis (RCM) Reaction of a Densely Functionalized Diene as the Key Step: Investigation of the Unusual Kinetically Controlled RCM StereochemistryCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2006Damiano Castoldi Dr. Abstract Asymmetric oxyallylation reactions and ring-closing metathesis have been used to synthesize compound 3, a key advanced intermediate used in the total synthesis of eleutherobin reported by Danishefsky and co-workers. The aldehyde 6, which is readily prepared from commercially available R -(,)-carvone in six steps in 30,% overall yield on multigram quantities, was converted into the diene 5 utilizing two stereoselective titanium-mediated Hafner,Duthaler oxyallylation reactions. The reactions gave the desired products (8 and 12) in high yields (73 and 83,%, respectively) as single diastereoisomers, with the allylic alcohol already protected as the p -methoxyphenyl (PMP) ether, which previous work has demonstrated actually aids ring-closing metathesis compared to other protective groups and the corresponding free alcohol. Cyclization under forcing conditions, using Grubbs' second-generation catalyst 13, gave the ten-membered carbocycle (E)- 14 in 64,% yield. This result is in sharp contrast to similar, but less functionalized, dienes, which have all undergone cyclization to give the Z stereoisomers exclusively. A detailed investigation of this unusual cyclization stereochemistry by computational methods has shown that the E isomer of the ten-membered carbocycle is indeed less thermodynamically stable than the corresponding Z isomer. In fact, the selectivity is believed to be due to the dense functionality around the ruthenacyclobutane intermediate that favors the trans -ruthenacycle, which ultimately leads to the less stable E isomer of the ten-membered carbocycle under kinetic control. During the final synthetic manipulations the double bond of enedione (E)- 16 isomerized to the more thermodynamically stable enedione (Z)- 4, giving access to the advanced key-intermediate 3, which was spectroscopically and analytically identical to the data reported by Danishefsky and co-workers, and thereby completing the formal synthesis of eleutherobin. [source] | ||||||||||