TY - JOUR. T1 - An arene-alkene photocycloaddition-radical cyclization cascade. T2 - The first synthesis of cis,cis,cis,trans-[5.5.5.5]-fenestranes. AU - Wender, Paul A.. AU - Dore, Timothy. AU - DeLong, Mitch A.. PY - 1996/10/21. Y1 - 1996/10/21. N2 - The first cis,cis,cis,trans-[5.5.5.5]-fenestranes have been synthesized in three steps using a novel arene-alkene photocycloaddition-radical cyclization cascade, which creates overall 5 new rings and 8 stereogenic centers. A lead tetraacetate mediated ring expansion of a 5,5-fused ring system, discovered in the course of exploring the chemistry of these new materials, is also described.. AB - The first cis,cis,cis,trans-[5.5.5.5]-fenestranes have been synthesized in three steps using a novel arene-alkene photocycloaddition-radical cyclization cascade, which creates overall 5 new rings and 8 stereogenic centers. A lead tetraacetate mediated ring expansion of a 5,5-fused ring system, discovered in the course of exploring the chemistry of these new ...
The Nazarov cyclization is a common synthetic tool in the synthesis of cyclopentenone rings. The reaction is characterized by a divinyl ketone starting reagent that undergoes a 4π-electron electrocyclization after addition of a Lewis/Brønsted acid; forming the cyclopentadienyl cation that undergoes cyclization. An extension of this methodology involves using a divinyl imine starting reagent. This cyclization is less favoured due to the electron donating ability of the nitrogen atom, causing the pentadienyl cation to be relatively stable and unwilling to undergo cyclization. To improve this cyclization, the imine is replaced with the electron withdrawing nitronate functionality. It is hypothesized that by doing so the addition of an acid will form the cyclopentadienyl cation that will readily undergo Nazarov cyclization. The synthesis of an aryl vinyl silyl nitronate was optimized and the compound was tested with a number of different acid catalysts to determine if the nitronate Nazarov ...
Effect of primary polymer chain rigidity on intramolecular cyclization and intramolecular crosslinking in free-radical crosslinking monomethacrylatedimethacrylate copolymerizations., The effluent analysis of several pyrrone and polyimide precursors during cyclization., Reduction and cyclization to form poly(benzimidazole amide imide) copolymers., Thermal degradation of polymers. Part XIX. The thermal cyclization and characterization of quinazolone pre-polymers, Cationic cyclization of cis-1 4-polybutadiene I. Synthesis and technological evaluation of the polymer, ThreeFourFive-Component Diastereoselective Cyclization of Fischer Carbene Complexes Lithium Enolates and Allylmagnesium Bromide., Evolution of aggregation structure of polyacrylonitrile fibers in the cyclization reaction., Epoxy resins based on aromatic glycidylamines. III. Kinetics of intramolecular cyclization of N N-bis(2-hydroxy-3-chloropropyl)aniline, Cyclization during polyesterification Isolation of an 18-member ring compound from
A proposed universal monoterpene synthases (cyclases) cyclization mechanism topologically requires an isomerization step of the substrate geranyl diphosphate (GPP) to the formation of the proposed universal intermediate linalyl diphosphate (LPP), which has never been directly observed. The study described here was designed to investigate the cyclization mechanism by studying a model enzyme, (4R)-(+)-limonene synthase ((+)-LS), a monoterpene cyclase that produces (+)-limonene by using substrate GPP. The study synthesized two novel difluorinated substrate analogues, 8,9-difluorogeranyl diphosphate (DFGPP) and 8,9-difluorolinalyl diphosphate (DFLPP), which were designed to retard the cyclization step in order to observe the intermediate at the enzyme active site. Both analogues are proved to be substrates for (+)-LS. Compared to GPP, which has a kcat value of 0.0905 +/- 0.0070 s-1, DFGPP has a reduced kcat value at 0.00144 +/- 0.00002 s-1, showing a 69-fold slower rate in the catalytic reaction. ...
TY - JOUR. T1 - Systematic investigations on fused π-system compounds of seven benzene rings prepared by photocyclization of diphenanthrylethenes. AU - Fujino, Shota. AU - Yamaji, Minoru. AU - Okamoto, Hideki. AU - Mutai, Toshiki. AU - Yoshikawa, Isao. AU - Houjou, Hirohiko. AU - Tani, Fumito. N1 - Publisher Copyright: © 2017 The Royal Society of Chemistry and Owner Societies.. PY - 2017. Y1 - 2017. N2 - We studied the photoproducts of 1-(n-phenanthryl)-2-(m-phenanthryl)ethenes (nEm; n, m = 1, 3 and 9) for understanding photocyclization patterns based on NMR spectroscopy. The crystal structures of the photoproducts were analyzed by X-ray crystallography, and the photophysical features of the photocyclized molecules were investigated based on emission and transient absorption measurements. Phenanthrene derivatives substituted at the 1- and 3-positions were prepared for synthesizing nEm by photocyclization of stilbene derivatives. We obtained four types of primary photoproducts ([email protected]) from the ...
The iodine-catalyzed Prins cyclization of homoallylic alcohols and aldehydes was investigated under metal-free conditions and without additives. Anhydrous conditions and inert atmosphere are not required. The reaction of 2-(3,4-dihydronaphthalen-1-yl)propan-1-ol and 21 aldehydes (aliphatic and aromatic) in CH2Cl2 in the presence of 5 mol % of iodine gave 1,4,5,6-tetrahydro-2H-benzo[f]isochromenes in 54%-86% yield. Under similar conditions, the Prins cyclization of six alcohols containing an endocyclic double bond (primary, secondary, or tertiary) led to dihydropyrans in 52%-91% yield. The acyclic homoallylic alcohols gave 4-iodo-tetrahydropyran in 29%-41% yield in the presence of 50 mol % of iodine. This type of substrate is the main limitation of the methodology. The relative configuration of the products was assigned by NMR and X-ray analysis. The mechanism and the ratio of the products are discussed, based on DFT calculations.
Title: Metal-mediated Heterocyclization: Synthesis of Heterocyclic Compounds Containing More Than One Heteroatom Through Carbon-Heteroatom Bond Forming Reactions. VOLUME: 14 ISSUE: 8. Author(s):K. C. Majumdar, B. Roy, P. Debnath and A. Taher. Affiliation:Department of Chemistry, University of Kalyani, Kalyani- 741 235, India.. Abstract: Heterocyclic compounds are versatile building blocks in organic synthesis. These are important part in various natural products and bioactive compounds. Several methodologies have been developed to synthesize these heterocyclic compounds. This review article covers an updated (published during 2005 to 2008) summary of transition-metal catalyzed approaches for the syntheses of commonring heterocycles containing more than one heteroatom via the selective formation of carbon-heteroatom bonds. The five-membered ring heterocycles with two nitrogen atoms include imidazoles, pyrazoles, indazoles, imidazoindolones etc. and heterocycles with one nitrogen and one oxygen ...
TY - JOUR. T1 - Ammonia synthons for the multicomponent assembly of complex γ-lactams. AU - Tan, Darlene Q.. AU - Martin, Kevin S.. AU - Fettinger, James C.. AU - Shaw, Jared T.. PY - 2011/4/26. Y1 - 2011/4/26. N2 - The synthesis of γ-lactams that are unsubstituted at the 1-position (nitrogen) as well as their subsequent N-functionalization is reported. A recently discovered four-component reaction (4CR) is employed with either an ammonia precursor or a protected form of ammonia that can be deprotected in a subsequent synthetic step. These methods represent the first multicomponent assembly of complex lactam structures that are unsubstituted at nitrogen. In addition, two methods for the introduction of nitrogen substituents that are not possible through the original 4CR are reported. X-ray crystallographic analysis of representative structures reveals conformational changes in the core structure that will enable future deployment of this chemistry in the design and synthesis of diverse ...
A highly efficient method for the construction of pyrrolidinyl spirooxindoles with 3-isothiocyanato oxindoles and chalcones via a Michael/cyclization cascade reaction has been developed by using bifunctional cinchona-derived squaramide organocatalysts. A series of complex pyrrolidinyl spirooxindoles could be
It seemed plausible that the biosynthesis of Bistellettadine A (9) was proceeding by intermolecular dimerization of the monomeric carboxylic acid corresponding to 7. Barry B. Snider of Brandeis University found (Org. Lett. 2010, 12, 828. ) that that intermolecular dimerization did proceed efficiently, but to give a 5:4 mixture of diastereomers. In contrast, the linked diester 7 cyclized with exclusive diastereocontrol. The product 8 was readily carried on to Bistellettadine A 9. This raises the possibility that a chiral template, attached either covalently or through salt formation, could be designed that would direct the absolute configuration of the cycloaddition.. ...
In this review James C. Collins and Keith James from the Scripps Research Institute critically analyse macrocyclization reactions published over the last three years. Based upon on this analysis, and first-hand experience of macrocyclization, Collins and James propose a macrocyclization efficiency index, Emac, as a means of determining the true efficiency of a macrocyclization reaction.. Emac takes into account both reaction yield and concentration, which Collins & James state addresses the key factors that determine the practicality of using a given reaction in a drug discovery context. In other words, the Emac for a reaction indicates the likelihood of being able to conduct the reaction on a sufficiently large scale whilst remaining resource efficient.. This index also allows comparison of a large number of literature macrocyclization reactions and identifies those which deliver the powerful combination of high yield and high practicality. Collins and James hope that those who are actively ...
The (3+2)-cycloaddition reaction involving oxyallyl cations has proven to be a versatile and efficient approach for the construction of five-membered carbo- and heterocycles, which are prevalent frameworks in natural products and pharmaceuticals. The following article will provide a brief summary of recent disclosures on this process featuring chemo-, regio- and diastereoselective oxyallyl cycloadditions with both electron-rich and electron-deficient 2π partners.
The unprecedented combination of a palladium(ii) complex with a chiral Bu-QUOX ligand and a chiral phosphoric acid enables the highly efficient asymmetric oxidative tandem cyclization of N-(2,2-disubstituted hex-5-en-1-yl)acrylamides, providing a straightforward method to access chiral 6,5-bicyclic aza-heter
List of : One-pot reductive cyclization to antitumor quinazoline precursorsOn resonance interactions in methyl 4-X-benzoates and the effect of 2,6-dimethyl substitution. 13C or 17O NMR chemical shifts as effective indicators of π-electron-density distribution?Open-chain and cyclic amidrazones forming persistent radicals. An electrochemical and quantum chemical studyOne-pot synthesis of stable phosphorus ylides using aldehyde phenylhydrazone derivativesOne-pot synthesis of 5-carboxanilide-dihydropyrimidinones using etidronic acid
A cobalt-rhodium heterobimetallic nanoparticle-catalyzed reductive cyclization of 2-(2-nitroaryl)acetonitriles to indoles proceeds without any additives under mild conditions. The catalytic can be reused more than ten times without loss of catalytic activity.. ...
SYNTHESIS OF FUSED PYRAZOLES VIA INTRAMOLECULAR CYCLIZATION OF APPROPRIATELY ortho -SUBSTITUTED NITROARENES AND NITROHETEROARENES
Home » Visible light photocatalysis of radical cation Diels-Alder cycloadditions: preparation of tris(2,2-bipyrazyl)ruthenium(II) bis(tetrakis(3,5-bis(trifluoromethyl)phenyl)borate ...
TY - JOUR. T1 - SilverCatalyzed Cascade Carboxylation and Cyclization of Trimethyl(2-methylenebut-3-yn-1-yl)silane Derivatives. AU - Sekine, Kohei. AU - Sadamitsu, Yuta. AU - Yamada, Tohru. PY - 2015/11/20. Y1 - 2015/11/20. N2 - C-C bond-forming carboxylation and cyclization of trimethyl(2-methylenebut-3-yn-1-yl)silane derivatives and carbon dioxide was developed. Silver catalysts and CsF promoted the reaction to afford the corresponding 2-furanone and 2-pyrone derivatives in good-to-high yields. The structure of the 2-furanone was confirmed by single-crystal X-ray crystallography, which revealed that the geometry of the exo-olefin was that of a Z-isomer. When an aromatic ring-substituted alkyne was used, 2-furanone derivatives were selectively obtained via 5-exo-dig cyclization, whereas the reaction of alkyl-substituted alkynes produced 2-pyrone derivatives with high selectivity.. AB - C-C bond-forming carboxylation and cyclization of trimethyl(2-methylenebut-3-yn-1-yl)silane derivatives and ...
Oxidative photoinduced electron transfer (PET) reactions have been performed with various silyl enol ethers and silyloxy-2H-chromones bearing an olefinic or silylacetylenic side chain, The reactions result in regioselective ring closure with the formation of bi- to tetracyclic ring systems with a well-defined ring juncture, e.g. perhydrophenanthrenones 13 or benzo-annellated xanthenones 24. Our investigations have focussed on the optimization of this cyclization method with regard to irradiation time and product yield. The irradiation times could be reduced by using the cosensitized PET method. Modifying the substrate at the silyl group led to enhanced yields. In addition, we found that solvent and pressure dependences are important tools, allowing control of the regiochemistry. Both the synthesis of 6-endo products by radical cationic reaction pathways, as well as 5-exo ring closure by radical intermediates was achieved. Mechanistic details, including findings from deuterium labelling ...
TY - JOUR. T1 - Enantioselective organo-SOMO cascade cycloadditions. T2 - A rapid approach to molecular complexity from simple aldehydes and olefins. AU - Jui, Nathan T.. AU - Lee, Esther C.Y.. AU - MacMillan, David W.C.. PY - 2010/7/28. Y1 - 2010/7/28. N2 - A highly selective, radical-mediated (4 + 2) coupling reaction of aldehydes and conjugated olefins has been achieved through asymmetric SOMO-catalysis. A radical-polar crossover mechanism is proposed wherein olefin addition to a transient enamine radical cation and oxidation of the resulting radical furnishes a cation which is vulnerable to nucleophilic addition. A range of aromatic aldehydes are shown to couple with styrenes and dienes to provide cyclic products with high chemical efficiency, regioselectivity, and stereoselectivity.. AB - A highly selective, radical-mediated (4 + 2) coupling reaction of aldehydes and conjugated olefins has been achieved through asymmetric SOMO-catalysis. A radical-polar crossover mechanism is proposed ...
ESI Induced Gas-Phase Nazarov Cyclization of (2E)-3-(2-Methoxyphenyl)-1-(thiophen-2-yl)-prop-2-en-1-one and its Oxygen Analogue ...
I. The acid catalyzed reactions of certain 1,5-epoxyolefins. The monoxides of a number of 1,5-dienes were prepared and treated with a variety of acidic reagents known to induce cyclization of the parent diolefins. In all cases the major reaction observed was the isomerization of the epoxide to a carbonyl function. No cyclic products were observed. These results are discussed in terms of the general requirements for cyclization.. II. The alkylative cyclization of 1,5-hexadiene The alkylation of 1,5 -hexadiene with t-butyl chloride in the presence of aluminum trichloride was studied. The major monomeric reaction products were cyclic, and both the t-butylcyclohexane and neopentylcyclopentane skeletons were identified among them. Formation of the latter ring system appeared to be slightly favored over formation of the six-membered ring. This preference is undoubtedly due to both steric and electronic factors.. III. The acid catalyzed cyclization of cis, cis- and trans, trans-2,6-octadiene. Both cis, ...
In organic chemistry, an electrocyclic reaction is a type of pericyclic rearrangement where the net result is one pi bond being converted into one sigma bond or vice versa. These reactions are usually categorized by the following criteria: Reactions can be either photochemical or thermal. Reactions can be either ring-opening or ring-closing (electrocyclization). Depending on the type of reaction (photochemical or thermal) and the number of pi electrons, the reaction can happen through either a conrotatory or disrotatory mechanism. The type of rotation determines whether the cis or trans isomer of the product will be formed. The Nazarov cyclization reaction is a named electrocyclic reaction converting divinylketones to cyclopentenones. A classic example is the thermal ring-opening reaction of 3,4-dimethylcyclobutene. The cis isomer exclusively yields cis,trans-2,4-hexadiene whereas the trans isomer gives the trans,trans diene: This reaction course can be explained in a simple analysis through the ...
A novel cascade reaction involving oxidative dearomatization/semipinacol rearrangement of indol-2-yl cyclobutanols is explored with N-sulfonyloxaziridine (Davis oxaziridines) as oxidant, generating various 2-spiroquaternary 3-oxindoles in modest to good yields. This method might be useful in the synthesis of indole-based alkaloids bearing 2-spiroquaternary carbon centers. PTS=p-toluenesulfonic acid monohydrate. ...
These two SOS volumes present the broad scope of metal-catalyzed reactions for the synthesis of cyclic compounds, from long-established methods to state-of-the-art approaches. The power of these reactions is highlighted by selected examples of application in natural product/drug synthesis.. Significant advances in metal-catalyzed reactions, especially cyclizations, have dramatically improved the efficiency of organic synthesis over the last three decades. To date, these transformations are widely used in the area of synthesis of both natural products and therapeutic agents. Science of Synthesis: Metal-Catalyzed Cyclization Reactions presents the most commonly used and significant metal-catalyzed reactions for modern organic synthesis. The basic principles, the current state of the art, scope, limitations, and mechanism of these reactions are also discussed. Typical examples of target synthesis are provided to help inspire further applications.. Volume 1 covers intramolecular coupling reactions ...
Significant advances in metal-catalyzed reactions, especially cyclizations, have dramatically improved the efficiency of organic synthesis over the last three decades. To date these transformations are widely used in the area of synthesis of both natural products and therapeutic agents. Science of Synthesis: Metal-Catalyzed Cyclization Reactions presents the most commonly used and significant metal-catalyzed reactions for modern organic synthesis. The basic principles, the current state of the art, scope, limitations, and mechanism of these reactions are also discussed. Typical examples of target synthesis are provided to help inspire further applications ...
Thieme eBooks, Significant advances in metal-catalyzed reactions, especially cyclizations, have dramatically improved the efficiency of organic synthesis over the last three decades. To date these transformations are widely used in the area of synthesis of both natural products and therapeutic agents. Science of Synthesis: Metal-Catalyzed Cyclization Reactions presents the most commonly used and s...
Synthesis of Novel Steroid-Peptoid Hybrid Macrocycles by Multiple Multicomponent Macrocyclizations Including Bifunctional Building Blocks MiBs. . Biblioteca virtual para leer y descargar libros, documentos, trabajos y tesis universitarias en PDF. Material universiario, documentación y tareas realizadas por universitarios en nuestra biblioteca. Para descargar gratis y para leer online.
Chen, H. [陳浩國]. (2009). Silver catalyzed enyne cyclization reactions. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_ ...
The importance of dynamics in simple reactions is made yet again in a recent study by Doubleday and Houk in 1,3-dipolar cycloadditions.1 They looked at the reaction of acetylene or ethylene with either nitrous oxide, diazonioazanide, or methanediazonium. The transition state for these 6 reactions all show a concerted reaction. The transition vector has three major components; (a) symmetric formation/cleavage of the two new σ bonds, (b) bending of the dipolar component, or (c) symmetric bending of the hydrogens of ethylene or acetylene.. Classical trajectories were traced from the transition state back to reactant and forward to product. In the approach of the two fragments, the dipole bend vibrates, but then after the TS, it needs to bend quickly to close the 5-member ring. This means that the bending mode effectively has to turn a corner in phase space, and without energy in this mode, the molecules will simple bounce off of each other. Analysis of the reactants indicates significant ...
Synlett, 2009, 1208-1218. Anodic electrochemistry is a powerful tool for generating radical cation intermediates and initiating new cyclization reactions. Like most electrochemical reactions, the transformations involve umpolungs. In this review, recent studies examining the intramolecular coupling of radical cations derived from enol ethers, vinyl sulfides, and ketene acetals with carbon, oxygen, and nitrogen trapping groups are discussed to highlight their synthetic potential.. ...
Steric effects are important in synthesis. Whilst steric hindrance is well known in hindering reactions, steric effects can also be employed to accelerate reactions, in particular cycloaddition reactions, and even to promote reactions that otherwise do not occur. A survey is included in previous work on steric effects in chemical reactions, principally cycloadditions. This includes a brief discussion on the importance of orientation and solvent effects on Diels-Alder cyclisations and ene reactions. The effect of substituents on the cyclisation of N-allyl furfurylamines has been studied. It was shown that bulky N-protecting groups enhance cyclisation, an effective buttress being the trityl (triphenylmethyl) group. The latter has the added advantage of being particularly easy to remove. A study of some ene reactions has also been carried out and steric acceleration of these processes has also been observed. A novel reaction involving an intermolecular cycloaddition followed by a sterically ...
The dissertation herein describes the first total synthesis of the complex Daphniphyllum alkaloid, (-)-calyciphylline N. This alkaloid was chosen as the target in our synthetic program due its unprecedented structure, the inherent challenges associated with its synthesis, and limited reports of synthetic studies towards members of this family of natural products. An initial unsuccessful approach is discussed, followed by a revised, ultimately successful approach. Highlights of the synthesis include a substrate controlled, intramolecular Diels-Alder reaction to build the bicyclo[2.2.2]octane core and set four contiguous stereocenters, a transannular enolate alkylation to secure ring D, a highly efficient Stille carbonylation/Nazarov cyclization sequence to construct ring E with concomitant activation of a hindered silyl group towards Fleming-Tamao oxidation, and an unprecedented, homogeneous hydrogenation of an exceptionally hindered diene ester to complete the eastern hemisphere, a structural motif that
Autor: Glas, Adrian et al.; Genre: Zeitschriftenartikel; Im Druck veröffentlicht: 2015-01; Keywords: cyclic peptides; hydrophobic cross-links; ring-closing metathesis; protein-protein interactions; Pseudomonas aeruginosa; Titel: Constraining Peptide Conformations with the Help of Ring-Closing Metathesis
TY - JOUR. T1 - Synthetic studies of cis-3a-aryloctahydroindole derivatives by copper- catalyzed cyclization of N-allyltrichloroacetamides. T2 - Facile construction of benzylic quaternary carbons by carbon-carbon bond-forming reactions. AU - Iwamatsu, Sho Ichi. AU - Matsubara, Kouki. AU - Nagashima, Hideo. PY - 1999/12/24. Y1 - 1999/12/24. N2 - Cyclization of N-(2-aryl-2-cyclohexen-1-yl)trichloroacetamides by a copper catalyst was investigated. It is crucially important for successful cyclization under mild conditions that alkoxycarbonyl groups are introduced to the nitrogen atom of the N-allyltrichloroacetamides as well as that CuCl- (bipyridine) is used as the catalyst. Three compounds, N-(2-phenyl-2- cyclohexen-1-yl)-, N-[2-(3,4-dimethoxyphenyl)-2-cyclohexen-l-yl]-, and N-[2- (3,4-methylenedioxyphenyl)-2-cyclohexen-1-yl]trichloroacetamides, where the Cbz or MeO2C- group was attached to the nitrogen atom, were instantly converted to the corresponding trichlorinated cis-3a-aryloctahydroindol-2- ...
This video was recorded at CHEM 125b: Freshman Organic Chemistry II. Cyclic conjugation that arises when p-orbitals touch one another can be as important for transition states as aromaticity is for stable molecules. It is the controlling factor in pericyclic reactions. Regiochemistry, stereochemistry, and kinetics show that two new sigma bonds are being formed simultaneously, if not symmetrically, in the 6-electron Diels-Alder cycloaddition. Although thermal dimerization of thymine residues in DNA is forbidden, photochemistry allows the 4-electron cycloaddition. Electrocyclic ring opening or closing chooses a conrotatory Möbius pathway, or a disrotatory Hückel pathway, according to the number of electron pairs involved and whether light is used in the process. Dewar benzene provides an example of a very unstable molecule that can be formed photochemically and then persists because of unfavorable orbital overlap in the transition state for ring opening ...
Extra info for Chemistry of Heterocyclic Compounds: The Purines, Supplement 1, Volume 54. Sample text. C. D. E. F. G. 2. 3. 4. 5. . . . . . . . . . 22 Cyclization with Formic Acid (H33) . . . . . . . . . . . . . . Cyclization with Dithioformic Acid (H38) . . . . . . . . . . . . Cyclization with Other Carboxylic Acids (H41) . . . . . . . . . . . Cyclization with Acid Anhydrides (H44) . . . . . . . . . . . . Cyclization with Acid Chlorides (H47) . . . . . . . . . . . . . Cyclization with Orthoesters and Diethoxymethyl Acetate (H50) . It is postulated that 63 arises from attack by a second molecule of thiourea on the 5-thioureidopyrimidine initially formed, displacing the 4-amino group; the product (64)then cyclizes with elimination of g ~ a n i d i n e . ~ ~ 59 60 61 0. Cyclizationwith Cyanates, lsocyanates and Derivatives (H61) Under reflux conditions methyl isocyanate and 4,5-diamino-l,2,3,6-tetrahydro-2-oxo-6-thioxopyrimidinein pyridine (3 h) give the corresponding 5-(N- * Removal of an ...
The enantiopure γ-amino alcohols 7 and 18 are prepared by using the diastereoselective michael addition of lithium N-benzyl ( R )-α-methylbenzylamide to α,β-unsaturated esters as a key step. The michael addition of 7 or 18 to an alkynone 8 followed by an intramolecular cyclization afford the cyclic enzmine 10 or 20. which are subjected to the diastereoselective hydrogenation, and the subsequent transformations provide 6-epi-alkaloid 223A and alkaloid 223A, respectively ...
Photochemical cleavage reactions have found a widespread used in biological applications that require the photorelease of biologically active molecules such as proteins, peptides, neurotransmitters, and nucleotide phosphates. This research focuses on the design of photoremovable protecting groups which can be utilized to release these biomolecules by photolysis. These biomolecules are attached to the photoremovable protecting group at the sites of functional groups that are present within these substrates. Such functional groups are carboxylates, phosphates, thiolates, or phenolates, which upon exposure to light, are released as anions of varying basicities. The photochemical reaction involved is an electrocyclic ring closure between aromatic groups that are bridged by a carboxamide linkage. The key intermediate produced by electrocyclization is thought to have zwitterionic character. This zwitterionic intermediate is believed to expel the leaving group as the anion. One of the aromatic groups attached
The research developed in this Doctoral Thesis comprises from the preparation of polyaromatic compounds to the synthesis of natural product derivatives, highlighting the proficiency of homogeneous gold catalysis for the construction of molecular diversity.. A novel approach for the synthesis of a new trindane-based crushed fullerene C60 was developed by a threefold palladium-catalyzed cross-coupling of four suitably functionalized C15 trindene fragments. The trindane C15 skeleton of the central motif was constructed through a triple gold(I)-catalyzed oxidative cyclization.. The scope of the intramolecular gold(I)-catalyzed formal [2+2+2] cycloaddition reaction was extended to O-protected homopropargylic and allylic oxo-1,5-enynes. Under the optimized reaction conditions, the cyclization of (Z)- and (E)- isomers takes place with moderate to excellent yield and increased selectivity in most of the cases, providing access to octahydro-1H-indenes skeletons.. The mastery of gold(I) complexes for the ...
Molecular complexity is one of the key reasons why fragment-based lead discovery should work. As described in 2001 by Mike Hann and colleagues at GlaxoSmithKline, the idea is that very small, simple molecules are likely to be able to bind to many different sites on many different proteins; think of the water molecule as being an extreme example of this. As molecules become larger and more complex, they are less likely to bind to any given site on a protein, though if they are complementary to a site the potency will be greater. Similarly, more complex molecules are more likely to have a single binding mode than smaller, less-decorated molecules, which could assume multiple orientations at a single site. These intuitive ideas were supported by a simple computational model that suggested that there is an optimum complexity where molecules would be simple enough that they would bind to several different targets (and thus be useful in a screening collection) while still being complex enough to bind ...
TY - BOOK. T1 - Solid-Phase Iminium Cyclization Reactions for the Synthesis of Natural Product-Like Diketopiperazines. AU - Petersen, Rico. PY - 2012. Y1 - 2012. N2 - The development of methodology for the solid-phase synthesis of fused 2,5-diketopiperazines with an emphasis on structural and stereochemical control, has been accomplished through two different approaches. The first approach was based on a highly trans-stereoselective (82% d.e.) intramolecular N-acyliminium ion cyclization of N-terminal peptide glyoxylamides, assembling the complete molecular skeleton in a single step. The second approach aimed for the use of a cis-stereoselective intermolecular Pictet-Spengler cyclization to construct the first part of the ringsystem and a subsequent diketopiperazine formation to close the second ring. In the first approach, four different methods for accessing the glyoxylamide cyclization precursor were developed; oxidative cleavage of a serine, dihydroxylation/oxidative cleavage of an ...
TY - JOUR. T1 - Carbene and silicon routes as methods for the generation and dipolar cycloaddition reactions of methyl nitrile ylide. AU - Padwa, Albert. AU - Gasdaska, John R.. AU - Tomas, Miguel. AU - Turro, Nicholas J.. AU - Cha, Yuan. AU - Gould, Ian. PY - 1986. Y1 - 1986. N2 - Methyl nitrile ylide was generated by both a photochemical and desilylation route and has been characterized by UV spectroscopy and by its kinetic and cycloaddition behavior. The dipole is formed by the addition of singlet methylene to acetonitrile and shows a strong transient absorption at 280 nm which is quenched by the addition of standard dipolarophiles. The relative rates of formation of 1,3-dipolar adducts and the relative rates of quenching of the transient absorption are in excellent agreement. Generation of the dipole from a silylthioimidate followed by dipolar cycloaddition also gave rise to related cycloadducts. The ratio of cycloadducts obtained from an unsymmetrically substituted dipolarophile was found ...
Stereogenic-at-Mo monoalkoxide and monoaryloxide complexes promote enyne ring-closing metathesis (RCM) reactions, affording the corresponding endo products with high selectivity (typically ,98:,2 endo:exo). All catalysts can be prepared and used in situ. Five-, six-, and seven-membered rings are obtained through reactions with enyne substrates that bear all-carbon tethers as well as those that contain heteroatom substituents. The newly developed catalytic protocols complement the related exo-selective Ru-catalyzed processes. In cases where Ru-based complexes deliver exo and endo products nondiscriminately, such as when tetrasubstituted cyclic alkenes are generated, Mo-catalyzed reactions afford the endo product exclusively. The efficiency of synthesis of N- and O-containing endo diene heterocycles can be improved significantly through structural modification of Mo catalysts. The modularity of Mo-based monopyrrolides is thus exploited in the identification of the most effective catalyst variants. ...
TY - JOUR. T1 - Multicomponent reactions of phosphines, diynedioates, and aryl aldehydes generated furans appending reactive phosphorus ylides through cumulated trienoates as key intermediates. T2 - A phosphine α-addition-δ-evolvement of an anion pathway. AU - Deng, Jie Cheng. AU - Chuang, Shih-Ching. PY - 2014/11/7. Y1 - 2014/11/7. N2 - Multicomponent reactions of phosphines, diynedioates, and aryl aldehydes have been demonstrated, providing trisubstituted furans appending reactive phosphorus ylides, through cumulated trienoates as key intermediates. The proposed trienoate intermediates, 1,5-dipolar species formed via nucleophilic α-attack of phosphines toward diynedioates (α-addition-δ-evolvement of an anion, abbreviated αAδE), undergo addition to aryl aldehydes followed by 5-endo-dig cyclization, proton transfer, and resonance to give trisubstituted furans. Furthermore, the phosphorus ylides are oxidized to α-keto ester furans and utilized as Wittig reagents.. AB - Multicomponent ...
Title:Organocatalyzed Transient Dienamine-Mediated Diels-Alder Reactions between α,β-Unsaturated Ketones and Alkenes. VOLUME: 15 ISSUE: 5. Author(s):Iosune Arrastia, Ana Arrieta and Fernando P. Cossio*. Affiliation:Donostia International Physics Center, San Sebastian/Donostia, Donostia International Physics Center, San Sebastian/Donostia, Donostia International Physics Center, San Sebastian/Donostia. Keywords:Organocatalysis, diels-Alder reactions, cycloadditions, michael additions, pericyclic reactions, stepwise mechanisms.. Abstract:Organocatalyzed (4+2) cycloadditions (Diels-Alder reactions) are reviewed in this manuscript. Among the different catalytic alternatives, reactions involving α,β-unsaturated ketones and alkenes are considered. In these processes, substituted cyclohexanones are obtained via HOMO activation generated by amine organocatalysis that generate transient dienamine species. Both concerted and stepwise mechanisms are described. In most cases, excellent regio-, ...
An enantioselective total synthesis of zampanolide has been accomplished using a novel DDQ/Brønsted acid promoted cyclization as the key reaction. The synthesis features cross-metathesis to construct the trisubstituted olefin and a ring-closing metathesis to form the macrolactone. The final N-acyl aminal formation was stereoselectively accomplished by an organocatalytic reaction.
Denitrogenative 6-endo-dig azide-yne cyclization of α-propargyloxy-β-haloalkylazides was enabled by gold catalysis, thus providing 2H-1,3-oxazines. This rare cyclization mode in gold-catalyzed reactions of azide-yne substrates was demonstrated to be facilitated and controlled by electronic and resonance effects of the alkyne substituents. Molecular transformations of the as-prepared 2H-1,3-oxazines were also investigated.
This Mouse Glutaminyl-peptide cyclotransferase-like protein (QPCTL) ELISA Kit employs a two-site sandwich ELISA to quantitate QPCTL.,QPCTL; FLJ20084; glutaminyl cyclase-like,QPCTL, also termed Iso-glutaminyl cyclase catalyzes the intramolecular cyclization of N-terminal glutamine residues into pyroglutamic acid with liberation of ammonia and the intramolecular cyclization of N-terminal glutamate residues into pyroglutamic acid with liberation of water. Glutaminyl cyclase (QPCT) catalyzes the intramolecular cyclization of N-terminal glutamine residues into pyroglutamic acid liberating ammonia. In contrast, the physiological function of the plant QC is less clear. In case of the enzyme from C. papaya, a role in the plant defence against pathogenic microorganisms was suggested. Putative QCs from other plants were identified by sequence comparisons. The physiological function of these enzymes, however, is still ambiguous.
Pascal begins with the assumption that terminal aryl substitution on the enediyne will both (a) inhibit the C1-C6 cyclization due to steric interactions and (b) the C1-C5 cyclization should be enhanced due to stabilization of the radical by the neighboring aryl group. He computed the activation energies of a series of analogues, some of which are listed in Table 1. The transition state structures are shown in Figure 1 for 1b and 1c. Phenyl substitution does accomplish both suggestions: the activation barrier for the Bergman cyclization increases by 4 kcal mol-1, while the barrier for the C1-C5 cyclization is lowered by nearly 6 kcal mol-1. Further substitution of the phenyl ring by either chloro or methyl groups brings the barriers into near degeneracy.. Table 1. RBLYP/6-31G(d) Activation energies (kcal mol-1) ...
TY - JOUR. T1 - Synthesis, spectroscopy, and applications of small nitrile oxides. AU - Pasinszki, Tibor. AU - Westwood, Nicholas P.C.. PY - 2011/6. Y1 - 2011/6. N2 - Small covalent nitrile oxides are important transient species in organic synthesis and widely used especially for dipolar cycloaddition reactions. Due to their instability, their isolation and characterization are challenging. This review focuses on the generation, spectroscopy, structural properties, and synthetic application of small nitrile oxides, XCNO, where the X substituent group is smaller than five atoms, viz. X= hydrogen, halogen, pseudohalogen, alkyl, vinyl, ethynyl, or nitro.. AB - Small covalent nitrile oxides are important transient species in organic synthesis and widely used especially for dipolar cycloaddition reactions. Due to their instability, their isolation and characterization are challenging. This review focuses on the generation, spectroscopy, structural properties, and synthetic application of small ...
Abstract: The mechanisms of cycloaddition reactions of singlet dibromocarbene and formaldehyde have been studied using density functional theory of quantum chemistry method at B3LYP/6-31G* level. The geometrical parameters, harmonic vibrational frequencies and energies of stationary points on the potential energy surface are calculated. The structures of the stationary points were optimized, and the intermediates and transition states were detected by the frequency analysis. The calculation results show that the reaction has four possible reaction pathways. According to the data of the activation energy, we predict that path c would be the major reactive channel of the cycloaddition reactions of singlet dibromocarbene with formaldehyde, which proceeds in three steps and the energy barrier for the key step is 13.7 kJ·mol-1. Key words: Density functional theory, Cycloaddition reaction, Transition states, Dibromocabene, B3LYP level ...
Diversity-oriented synthesis yields a new drug lead for treatment of chagas disease.: A phenotypic high-throughput screen using ∼100,000 compounds prepared usin
TY - JOUR. T1 - Trifluoromethanesulfonic acid-catalyzed tandem semi-pinacol rearrangement/alkyne-aldehyde metathesis reaction of arylpropagylsulfonamide- tethered 2,3-epoxycyclohexan-1-ols to spiropiperidines. AU - Lin, Ming Nan. AU - Wu, Shih Hui. AU - Yeh, Ming-Chang P.. PY - 2011/12/1. Y1 - 2011/12/1. N2 - A simple and efficient trifluoromethanesulfonic acid-catalyzed cycloisomerization of arylpropagylsulfonamide-tethered 2,3-epoxycyclohexan-1-ols is described. The cyclization proceeds via tandem semi-pinacol rearrangement/alkyne-aldehyde metathesis to afford spiropiperidines under mild reaction conditions.. AB - A simple and efficient trifluoromethanesulfonic acid-catalyzed cycloisomerization of arylpropagylsulfonamide-tethered 2,3-epoxycyclohexan-1-ols is described. The cyclization proceeds via tandem semi-pinacol rearrangement/alkyne-aldehyde metathesis to afford spiropiperidines under mild reaction conditions.. KW - alkyne/aldehyde metathesis. KW - semi-pinacol rearrangement. KW - ...
We developed an efficient, stereoselective synthetic method for the diketopiperazine moiety of neoechinulin A and its derivatives. The intramolecular cyclization at 80 ºC proceeded with minimal racemization of the stereogenic center at C-12 on neoechinulin A, even though the cyclization at 110 ºC caused partial racemization. In contrast with these results, the cyclization on diketopiperazine of 8,9-dihydroneoechinulin A derivatives did not cause epimerization of the stereogenic centers, even at 110 °C. We examined the structure-activity relationships for the cytoprotective activity against cytotoxicity induced by 3-morpholinosydnonimine (SIN-1) in nerve growth factor (NGF)-differentiated PC12 cells. The C-8/C-9 double bond, but not the stereogenic center derived from alanine, was found to play a key role in the cytoprotective activity.
TY - JOUR. T1 - Regioselective synthesis of thiophene fused sultam derivatives via iodocyclization approach and their application towards triazole linker. AU - Barange, Deepak Kumar. AU - Kavala, Veerababurao. AU - Kuo, Chun Wei. AU - Wang, Cheng Chuan. AU - Rajawinslin, R. R.. AU - Donala, Janreddy. AU - Yao, Ching Fa. PY - 2014/10/14. Y1 - 2014/10/14. N2 - An efficient regioselective synthesis of 4-iodo-2,3-disubstituted-2H-thieno[3,2-e][1,2]thiazine-1,1-dioxide derivatives via iodocyclization approach using iodine under mild reaction condition described herein. This coupling-iodocyclization strategy tolerated a variety of functional groups such as alkyl, cycloalkyl, phenyl producing the six-membered heterocyclic ring selectively. The resulting 4-iodo-2,3-disubstituted-2H-thieno[3,2-e][1,2]thiazine-1,1-dioxide was coupled with a variety of boronic acids (Suzuki coupling) and activated alkenes (Heck coupling). The iodo group was utilized for Sonogashira coupling followed by efficient ...
ADP-ribosyl cyclases catalyze the transformation of nicotinamide adenine dinucleotide (NAD+) into the calcium-mobilizing nucleotide second messenger cyclic adenosine diphosphoribose (cADP-ribose) by adenine N1-cyclization onto the C-1 position of NAD+. The invertebrate Aplysia californica ADP-ribosyl cyclase is unusual among this family of enzymes by acting exclusively as a cyclase, whereas the other members, such as CD38 and CD157, also act as NAD+ glycohydrolases, following a partitioning kinetic mechanism. To explore the intramolecular cyclization reaction, the novel nicotinamide 2-fluoroadenine dinucleotide (2-fluoro-NAD+) was designed as a sterically very close analogue to the natural substrate NAD+, with only an electronic perturbation at the critical N1 position of the adenine base designed to impede the cyclization reaction. 2-Fluoro-NAD+ was synthesized in high yield via Lewis acid catalyzed activation of the phosphoromorpholidate derivative of 2-fluoroadenosine 5-monophosphate and coupling
TY - JOUR. T1 - Hypervalent iodine in synthesis 93. A facile synthesis of 2-substituted imidazo[1,2-a]pyrimidines by cyclocondensation of alkynyl(phenyl)iodonium salts and 2-aminopyrimidine. AU - Zhi, Liu. AU - Chen, Zhen-Chu. AU - Zheng, Qinguo. PY - 2003/9. Y1 - 2003/9. N2 - Simple stirring of a mixture of the alkynyl(phenyl)iodonium salts 1 with 2-aminopyrimidine 2 in chloroform under reflux for two hours in the presence of K2CO3 gave, after workup, the 2-substituted imidazo[1,2-]pyrimidines 3 in moderate to good yields. A possible mechanism for the formation of 3 involves the intramolecular cyclization of the intermediate alkylidenecarbene 6.. AB - Simple stirring of a mixture of the alkynyl(phenyl)iodonium salts 1 with 2-aminopyrimidine 2 in chloroform under reflux for two hours in the presence of K2CO3 gave, after workup, the 2-substituted imidazo[1,2-]pyrimidines 3 in moderate to good yields. A possible mechanism for the formation of 3 involves the intramolecular cyclization of the ...
Oxidative cyclizations of a variety of heteroatom nucleophiles onto unactivated olefins are catalyzed by palladium(II) and pyridine in the presence of molecular oxygen as the sole stoichiometric oxidant in a nonpolar solvent (toluene). Reactivity studies of a number of N-ligated palladium complexes show that chelating ligands slow the reaction. Nearly identical conditions are applicable to five different types of nucleophiles: phenols, primary alcohols, carboxylic acids, a vinylogous acid, and amides. Electron-rich phenols are excellent substrates, and multiple olefin substitution patterns are tolerated. Primary alcohols undergo oxidative cyclization without significant oxidation to the aldehyde, a fact that illustrates the range of reactivity available from various Pd(II) salts under differing conditions. Alcohols can form both fused and spirocyclic ring systems, depending on the position of the olefin relative to the tethered alcohol; the same is true of the acid derivatives. The racemic ...
In an effort to make diverse molecular complexity from simple hydrocarbons, tricarbonyl(cyclohexadienyl)iron(+1) cation was prepared in two steps from 1, 3-cycloxehadiene. Reactivity of the symmetric iron cation with heteroatom nucleophiles and stabilized carbon nucleophiles was studied. Nucleophilic attack of potassium phthalimide at the dienyl terminus of the cation followed by oxidative decomplexation with Ce4+ provided the ligand N-(2,4-cyclohexadiene-1-yl)phthalimide. A series of stereochemically diverse polyhydroxyl aminocyclohexane aminocyclitols derivatives and a number of racemic and optically active hydroxy-and polyhydroxy 1,3-diaminocyclohexane derivatives have been synthesized from N-(2,4-cyclohexadiene-1-yl)phthalimide. The relative stereochemistries of the compounds ware assigned on the basis of the 1H NMR data as well as X-ray single crystal diffraction analysis. In a similar attempt tricabonyl(η5-6-styrylcyclohepta-2,4-diene-1-yl)iron(+1) cation was synthesized in three steps from 1,
TY - JOUR. T1 - Selective [3+2] Cycloadditions of β-Amino-α,β-unsaturated Pentacarbonylcarbenechromium Complexes to Alkynes-A New Approach to Functionally Substituted Cyclopentadienes. AU - Flynn, Bernard L.. AU - Funke, Frank J.. AU - Silveira, Claudio C.. AU - Meijere, Armin De. PY - 1995/10/1. Y1 - 1995/10/1. N2 - (3-Amino-1-ethoxyalkenylidene)pentacarbonylchromium complexes 2 react with alkynes in pyridine or acetonitrile to give highly substituted 5-amino-3-ethoxycyclopentadienes 3 as formal [3+2]-cycloadducts selectively in high yields rather than the usually favored [2+2+1]-cycloadducts, 5-methylene-2-cyclopenten-1-ones.. AB - (3-Amino-1-ethoxyalkenylidene)pentacarbonylchromium complexes 2 react with alkynes in pyridine or acetonitrile to give highly substituted 5-amino-3-ethoxycyclopentadienes 3 as formal [3+2]-cycloadducts selectively in high yields rather than the usually favored [2+2+1]-cycloadducts, 5-methylene-2-cyclopenten-1-ones.. KW - 1-Metalla-1,3,5-hexatrienes. KW - ...
Medvedev, et al. have examined the cyclization step in the formation of Spinosyn A, which is catalyzed by the putative Diels-Alderase enzyme SpnF.1 This work follows on the computational study done by Houk, Singleton and co-workers,2 which I have discussed in this post (Dynamics in a reaction where a [6+4] and [4+2] cycloadditons compete). In fact, I recommend that you read the previous post before continuing on with this one. In summary, Houk, et al. found that a single transition state connects reactant 1 to both 2 and 3. The experimental product with the enzyme SpnF is 3. In the absence of enzyme, Houk, et al. suggest that reactions will cross the bispericyclic transition state TS-BPC (TS1 in the previous post) leading primarily to 2, which then undergoes a Cope rearrangement to get to product 3. Some molecules will follow pathways that go directly to 3 ...
Conjugated dienes exist in two conformation called s-cis and s-trans. They interconvert freely in open-chain dienes, by single bond rotation, but may not be able to in some cyclic cases. For the Diels-Alder reaction to occur, the s-cis conformation needs to be present, so the two carbons of the alkene can connect to both of them simultaneously. The reaction occurs in a single step during which the pi electrons reorganize through a cyclic transition state, forming two new single bonds to the ends of the diene. It is one of the reactions among a larger class called cycloaddition, and cycloaddition falls into a broader class called pericyclic - reactions which involve no intermediate, only a cyclic reorganization of electrons. Good yields of Diels-Alder products are obtained when the dienophile (alkene) has electron-withdrawing groups, often carbonyl groups, ester groups or cyano groups. Maleic anhydride is often used as a dienophile. ...
Oxidosqualene cyclases are a group of enzymes that cyclize 2,3-oxidosqualene to any of over 80 triterpene skeletons. Sequence correlation suggests these enzymes share a common overall structure and subtle changes in active site residues are responsible for differences in the structure of the cyclization product. Further characterization of conserved residues has lead to a better understanding of the mechanisms through which regio- and stereo-selective cyclization occurs. This work has resulted in the cloning of three new oxidosqualene cyclases. By cloning triterpene synthases from far-diverged organisms, new sequence data has been added for correlation and has provided new information regarding evolution of the enzymes from a common ancestor. Arabidopsis thaliana lupeol synthase was cloned and the full-length cDNA expressed in yeast. Degenerate PCR primers designed from regions of highly conserved sequence were used to clone cycloartenol synthase and a presently uncharacterized triterpene ...
Polyesters have been used for many biomedical applications ranging from sutures to drug delivery vehicles. However, their bulk degradation results in an accumulation of acidic byproducts, which is detrimental to the human body. In order to circumvent this problem, as well as to impart new properties and functions to polymers for biomedical applications, poly(ester amide)s (PEAs) have been proposed as a different class of biodegradable polymers. However, up to date, there exists no way to stimulate the degradation of these polymers. The Gillies research group has previously incorporated self-immolative spacers into polymers and has been able to stimulate their degradation by adding the appropriate trigger. The objective of this thesis was to incorporate amino acids capable of 1,5-cyclization into the PEA backbone such that upon activation of the functional moiety, a 1,5-cyclization was induced, leading to degradation of the PEA backbone. PEAs containing L-2,4-diaminobutyric acid and DL-homocysteine were
A carbohydrate-based synthesis of both cis- and trans-fused bicylic ethers has been achieved by radical cyclization of epoxides using a transition-metal radical source. Thus, 6-exo radical cyclizations of the carbohydrate derivatives using [bis(cyclopentadienyl) titanium(III)]chloride (Cp2TiCl) as the radical source has resulted in corresponding cis- and trans-fused bicyclic ethers. While the trans-fused compound has allowed stereoselective radical cyclization, the cis-fused has ended up only in a mixture of isomers. The functionalities present in the bicyclic compounds are potential intermediates as multifunctional conformationally rigid scaffolds ...
Enantioselective approaches, where the stereocontrol of the cyclization is not governed by a stereocentre already in the cyclization precursor, have received much less attention than the diastereoselective, racemic or achiral version of the PS reaction. It was shown in 1996 that the PS reaction could be effected with superstoichiometric Lewis acid - in this case the cyclization of the (Z)-nitrone derived from Nb-hydroxytryptamine using diisopinocampheylchloroborane (Ipc2BCl) to give the corresponding tetrahydro-β-carboline products (Scheme Nakagawa 1998).(10.1016/0957-4166(96)00134-6 and 10.1021/jo980810h and 10.3987/REV-98-SR1) The racemic cyclization could be effected with Bronsted acid and a number of achiral Lewis acids, but it was found that Ipc2BCl gave high yields and ee with reduced temperatures. Lowering the quantity of Ipc2BCl to 0.5 eq. caused a significant reduction of yield, while attempts to alter the Lewis acidity by substitution of the chloride (with e.g. fluoride or triflate) ...
Enantioselective approaches, where the stereocontrol of the cyclization is not governed by a stereocentre already in the cyclization precursor, have received much less attention than the diastereoselective, racemic or achiral version of the PS reaction. It was shown in 1996 that the PS reaction could be effected with superstoichiometric Lewis acid - in this case the cyclization of the (Z)-nitrone derived from Nb-hydroxytryptamine using diisopinocampheylchloroborane (Ipc2BCl) to give the corresponding tetrahydro-β-carboline products (Scheme Nakagawa 1998).(10.1016/0957-4166(96)00134-6 and 10.1021/jo980810h) The racemic cyclization could be effected with Bronsted acid and a number of achiral Lewis acids, but it was found that Ipc2BCl gave high yields and ee with reduced temperatures. Lowering the quantity of Ipc2BCl to 0.5 eq. caused a significant reduction of yield, while attempts to alter the Lewis acidity by substitution of the chloride (with e.g. fluoride or triflate) did not improve yield or ...
TY - JOUR. T1 - Microwave-assisted deacylation of unactivated amides using ammonium-salt-accelerated transamidation. AU - Shimizu, Yuhei. AU - Morimoto, Hiroyuki. AU - Zhang, Ming. AU - Ohshima, Takashi. PY - 2012/8/20. Y1 - 2012/8/20. N2 - Easy does it! The chemoselective oxidative ?-C(sp3)H alkylation/cyclization reaction of N-benzyl carbamates using simple mono-, di-, and trisubstituted olefins provides functionalized N-heterocycles such as oxazinones (see picture). A TEMPO oxoammonium salt serves as the oxidant, making it possible to carry out the reaction at low temperatures. Neither a metal catalyst nor preactivation in the ?-position to the nitrogen group are needed.. AB - Easy does it! The chemoselective oxidative ?-C(sp3)H alkylation/cyclization reaction of N-benzyl carbamates using simple mono-, di-, and trisubstituted olefins provides functionalized N-heterocycles such as oxazinones (see picture). A TEMPO oxoammonium salt serves as the oxidant, making it possible to carry out the ...
Although the [2 + 2] cycloaddition of alkenes and ketenes to form cyclobutanones is one of the few general methods for the construction of four-membered rings, the intramolecular version of this reaction had been little explored when we started work in this area in the early 1980s.109 These reactions differ from most other pericyclic reactions in that the ketene is a reactive intermediate, which will either rapidly form a cyclobutanone or decompose. The yield is determined by the relative rate of these two processes. Alkenyloxyketenes were chosen for the initial exploratory work because the alkenyloxy carboxylic acids were easily prepared by alkylation of unsaturated alcohols with α-bromo carboxylic acids. The electronic effects of the alkyl substituents on the alkene control the regiochemistry of the cycloaddition and the yield depends on the balance between the nucleophilicity of the alkene and steric hindrance.83,90,109 Leading bond formation occurs between the electrophilic carbonyl carbon ...
Microwave-assisted organic synthesis in near-critical water (NCW) in the 270-300 °C temperature range has been investigated in a dedicated multimode microwave reactor utilizing heavy-walled quartz reaction vessels. Several different known transformations such as the hydrolysis of esters or amides, the hydration of alkynes, Diels-Alder cycloadditions, pinacol rearrangements, and the Fischer indole synthesis were successfully performed in microwave-generated NCW without the addition of an acid
Looking for Electrocyclic? Find out information about Electrocyclic. The interconversion of a linear π-system containing n π-electrons and a cyclic molecule containing π-electrons which is formed by joining the ends of the... Explanation of Electrocyclic
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A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds Reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements ...
Fabio Aricò has started working in the field of Green Chemistry in 2005 after being awarded his Ph.D. at Reading University and post-doc position at UCLA, Los Angeles, CA. He is now associate professor of organic chemistry at Ca Foscari University, Venice, Italy. His research interest within Green Chemistry is to develop innovative reaction pathways focussing on chlorine-free synthetic approaches via dialkyl carbonate (DAC) chemistry. Fabio Aricò has exploited dialkyl carbonates chemistry in the phosgene-free synthesis of carbonates and carbamates with potential applications as monomers for polycarbonate/polyurethane. He also contributed to demonstrate that DACs are highly selective alkylating agents that can be used in substitution of alkyl halides and dimethyl sulphate for alkylation of numerous nitrogen and oxygen substrates. He has reported on the DAC-mediated cyclization reactions can be used as sacrificial molecules in cyclization reaction instead of halogenated compound. As a result, ...
Three synthetic methodologies are studied in details in this dissertation. For 1,5-hydride shift of alkenyl sulfoximine methodology, the reaction mechanism was studied using deuterium labeling. An uncommon 6-endo-trig 1,5-hydride shift process was discovered. The scope and limitation were studied using N-alkyl, N-allyl, and N-benzyl-substituted S-alkenyl sulfoximines. N-H-S-alkyl sulfoximines, four- and six-membered heterocyclic rings and a new class of chiral dienes were obtained. In [4+3] cycloaddition and ring opening chapter, we demonstrated an ene-like reaction using a symmetric oxyallylic cation can provide α-substituted cyclopentenones. Enantio pure products are potentially accessible by this method. [4+3] Cycloaddition of the symmetric oxyallylic cation with substituted furans, and the ring-opening process of the resulting 8-oxabicyclo[3.2.1]oct-6-en-3-one were also studied. The reaction conditions and scope were investigated. An acid-catalyzed mechanism was proposed for the ...
The thermal cyclisation of enediynes to benzene-1,4-diyl diradicals (Bergman cyclisation) is affected by geometrical and electronic conditions. While the effect of ring strain or conformational constrains on the cyclisation temperature has been investigated in detail, electronic contributions have been less studied. Often geometrical and electronic contributions cannot be clearly distinguished. In most cases metal ion chelation does involve both. In this review we have summarised clear-cut observations of electronic substituents effects on the thermal enediyne reactivity. The effects of substituents in the vinylic and terminal alkyne position, the influence of benzo-fusion and hetarene fusion, as well as the changes induced by heteroatoms in the enediyne skeleton, are within the scope of this review. With the exception of more complex heterocyclic hetarene-fused enediynes the experimental data of electronic substituent effects on the thermal Bergman cyclisation of enediynes follow theoretical ...
The Diels-Alder reaction is the reaction between a conjugated diene and an alkene (dienophile) to form unsaturated six-membered rings. Since the reaction involves the formation of a cyclic product via a cyclic transition state, it is also referred to as a cycloaddition. The Diels-Alder reaction is an electrocyclic reaction, which involves [4+2]‑cycloaddition of 4 π-electrons of the conjugated diene and 2 π-electrons of the dienophile (an alkene or alkyne). The reaction involves the formation of new σ-bonds, which are energetically more stable than the π-bonds. This reaction has great synthetic importance and was discovered by two German chemists, Otto Diels and Kurt Alder in 1928. They were awarded the Nobel Prize in 1950.1 ...
Sumanirole (PNU-95,666) is a highly selective D2 receptor full agonist, the first of its kind to be discovered. It was developed for the treatment of Parkinsons disease and restless leg syndrome. While it has never been approved for medical use it is a highly valuable tool compound for basic research to identify neurobiological mechanisms that are based on a dopamine D2-linked (vs. D1, D3, D4, and D5-linked) mechanism of action. In 2004, Pfizer announced the end of their clinical development program for sumanirole, citing recent studies that failed to sufficiently distinguish sumanirole from currently available therapies. Ropinirole Romero AG, et al. Synthesis of the selective D2 receptor agonist PNU-95666E from D-phenylalanine using a sequential oxidative cyclization strategy. Journal of Organic Chemistry. 1997; 62(19):6582. McCall RB, Lookingland KJ, Bédard PJ, Huff RM (September 2005). Sumanirole, a highly dopamine D2-selective receptor agonist: in vitro and in vivo pharmacological ...
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. Furthermore, scalability of the reaction up to 15 g scale and derivatization of the carbocyclic product has been demonstrated, including the formation of a rare trans-bicyclo[4.3.0]nonene scaffold via a ring-closing metathesis. The structure of this and all carbocyclic products were confirmed by X-ray crystallographic analysis. ...