Search results for "Cycloaddition"
showing 10 items of 392 documents
ChemInform Abstract: Toward an Understanding of the Mechanisms of the Intramolecular [5 + 2] Cycloaddition Reaction of γ-Pyrones Bearing Tethered Alk…
2000
The molecular mechanism for the intramolecular [5 + 2] cycloaddition reaction of β-silyloxy-γ-pyrones bearing tethered alkenes has been characterized using ab initio methods. A comparative study for this sort of cycloaddition carried out at different computational levels points out that the B3LYP/6-31G* calculations give similar barriers to those obtained with the MP3/6-31G* level. Analysis of the energetic results shows that the reaction takes place along a stepwise process: first, the migration of the neighboring silyl group to the carbonyl group of the γ-pyrone takes place to give a weak oxidopyrylium ylide intermediate, which by a subsequent concerted intramolecular [5 + 2] cycloadditi…
Reaktionen von Benzothiet mit Iminen
1993
Reactions of Benzothiete with Imines A great variety of compounds 3a–w containing CN double bonds was studied in the reaction with benzothiete (1). H, alkyl, aryl, OR, SR and NR groups can be attached to the imino function. The azomethines 3a–f, the thiazolines 3g–i, the lactim 3n, the oximes 3r–t, the isoxazolinone 3u and the azine 3v show [8π + 2π] cycloaddition reactions yielding derivatives of 1,3-benzothiazine (4a–i, n, r–v). In the case of 4n, s, t secondary reactions occur, initiated by elimination processes. [8π + 2π] Cycloadditions account also for the reactions 3e 10, 11, 3o 18(I), and 3q 19; however, CC double bonds are representing the 2π component in these examples. The thiazol…
Naphthalenophane durch doppelte Photocyclodimerisierung von Distyrylnaphthalinen
1989
Bei der Belichtung von Distyrylnaphthalinen entstehen in regiospezifischen Kopf-Kopf-Cycloadditionen Naphthalenophane. Je nach Stellung der Styryl-Gruppen beobachtet man vollkommen stereospezifische Prozesse (1d 2, 1g 3, 1h 4) und Dimerisierungen mit partiellen Stereoselektivitaten (1b 5–8, 1i 9, 10). Masgeblich sind die Rotamerenpopulation im Ausgangsprodukt 1 und die elektronischen und sterischen Effekte in den Excimeren. Prinzipiell sind Excimere mit paralleler Anordnung moglichst vieler π-Zentren bevorzugt — ein experimenteller Befund, der durch HMO-Rechnungen untermauert wird. Naphthalenophanes by Twofold Photocyclodimerizatio Reactions of Distyrylanaphthalenes Regiospecific head-to-he…
A clickable NHC–Au(i)-complex for the preparation of stimulus-responsive metallopeptide amphiphiles
2018
We report the synthesis of an alkyne functionalised NHC-Au(i)-complex which is conjugated with amphiphilic oligopeptides using a copper(i) catalysed cycloaddition. The resulting Au(i)-metalloamphiphiles are shown to self-assemble into charge-regulated stimulus-responsive supramolecular polymers in water via a weakly cooperative polymerisation mechanism.
Trapping Rare and Elusive Phosphinidene Chalcogenides
2017
Four-membered rings with a P2Ch2 core (Ch=S, Se) and phosphorus in the +3 oxidation state have been synthesized. The utility of these rings as a source of monomeric phosphinidene chalcogenides was probed by the addition of an N-heterocyclic carbene, resulting in a base-stabilized phosphinidene sulfide. Similarly, persistence of the phosphinidene selenide in solution was shown through cycloaddition chemistry with 2,3-dimethylbutadiene at elevated temperatures. The observed reactivity was explained by detailed computational work that established the conditions upon which the P2Ch2 rings can liberate phosphinidene chalcogenides. peerReviewed
Carbohydrates as Stereodifferentiating Auxiliaries
2017
Carbohydrates are inexpensive natural products. Although they contain numerous functional groups and stereogenic centers in one molecule, carbohydrates were recognized as stereodifferentiating auxiliaries much later than other classes of natural products as for example amino acids or terpenes. Apart from their polyfunctional nature carbohydrates are characterized by the anomeric and exo-anomeric effects which have distinct influence of the spatial orientation of substituents linked to the anomeric position. Based on these stereochemical properties carbohydrates were used as stereodifferentiating auxiliaries in numerous diastereoselective reactions, such as aldol addition, alkylation reactio…
New Studies and a Reinvestigation on [4+2] Cycloadditions of (−)-Thebaine: Asymmetrical Diels-Alder Reactions with a Conformationally Fixed Chiral Di…
1993
Some novel Diels-Alder reactions of the opium alkaloid (−)-thebaine (1) as an electron-rich diene and a reinvestigation of its reactions with cyclic and acyclic dienophiles are described. The π-facial selectivity has been studied on the basis of structural analyses of the cycloadducts. Some related results on [4 + 2] cycloaddition reactions of 1-methoxy-1,3-cyclohexadiene (11) are also reported.
Cyclodextrin–calixarene co-polymers as a new class of nanosponges
2014
Hyper-reticulated co-polymers jointly formed by cyclodextrin and calixarene units, which can be considered as a new class of nanosponges, were easily obtained by means of a click chemistry approach. In particular, we succeeded in preparing our materials by exploiting the copper-catalyzed 1,3-dipolar cycloaddition (CuAAC) reaction between heptakis-(6-deoxy)-(6-azido)-beta-cyclodextrin and (5,11,17,23-tetra-tert-butyl)-(25,26,27,28-tetra-propargyloxy)-calix-[4]-arene, mixed in different proportions. These materials were fully characterized by means of combined FT-IR, thermogravimetric, C-13 {H-1} CP-MAS NMR and nitrogen adsorption/desorption techniques. In particular, C-13 {H-1} CP-MAS spectr…
The role of the transfer group in the intramolecular [5+2] cycloadditions of substituted β-hydroxy-γ-pyrones: a DFT analysis
2005
The intramolecular [5+2] cycloaddition reactions of a series of 3-OR (R=SiMe3, H, CHO, Me) substituted β-hydroxy-γ-pyrones bearing tethered alkenes were studied using DFT methods at the B3LYP/6–31G* level. The role of the transfer R group was analyzed considering two alternative channels: (i) in channel a the process is initialized by the transfer of the R group with formation of an oxidopyrylium ylide intermediate, which is followed of an intramolecular [5+2] cycloaddition; (ii) in channel b, the process is initialized by the intramolecular [5+2] cycloaddition on the γ-pyrone followed by the transfer of the R group. Copyright © 2005 John Wiley & Sons, Ltd.
Asymmetric synthesis of polycyclic 3-fluoroalkylproline derivatives by intramolecular azomethine ylide cycloaddition
2019
The asymmetric synthesis of polycyclic fluorinated proline derivatives has been accomplished by means of an intramolecular azomethine ylide cycloaddition with fluorinated dipolarophiles. Thus, the reaction of benzaldehydes bearing a trifluoromethyl alkene in ortho position with an azomethine ylide precursor derived from chiral 2-amino indanol takes place with excellent levels of diastereoselectivity, with simultaneous generation of four stereocenters. In order to determine the influence of the fluorine moiety in the process, starting material bearing non-fluorinated alkenes were synthesised. In this case, a dramatic drop of selectivity occurred, which indicates that the role of the fluorine…