Search results for "Stereoselectivity"
showing 10 items of 325 documents
Thermische Cycloadditionen von 1,2‐Dihydropentalen
1989
1,2-Dihydropentalen (1) ist ein gekreuzt konjugiertes 6π-Elektronensystem, das in thermischen Cycloadditionen als 2π-, 4π- oder 6π-Komponente fungieren kann. Es werden die Additionen an Tetraphenylcyclopentadienon (1+23), N-Phenylmaleinimid (1+45) und Tropon (1+68 bzw. 10) beschrieben. Auf der Basis von MNDO-Rechnungen werden Peri-, Regio- und Stereoselektivitat anhand von primaren und sekundaren Orbitalwechselwirkungen diskutiert. Thermal Cycloaddition of 1,2-Dihydropentalene 1,2-Dihydropentalene (1) is a cross-conjugated 6π-electron system, that can act in thermal cycloaddition processes as a 2π, 4π, or 6π component. The addition reactions with tetraphenylcyclopentadienone (1+23), N-pheny…
4,4′‐Distyrylazobenzol als Chromophor
1992
4,4′-Distyrylazobenzene as Chromophore 4,4′-Distyrylazobenzenes are prepared from 4,4′-dimethylazobenzene. Whereas the Siegrist method furnishes stereoselectively the (E,E,E) configuration, a Wittig olefination leads to a mixture of (Z,E,Z), (Z,E,E) and (E,E,E) isomers which can be easily separated. Irradiation of the intense absorption at λ ≈ 400 nm causes a selective (E) (Z) isomerization of the N = N bond of all three isomers. The reverse reaction takes part in the photochemical equilibration, moreover, it is a thermal process with a relatively low activation barrier.
Synthesis of oligomeric chains with 9,10‐dihydroanthracene units by carbanion alkylation
1988
Deprotonation of 9,10-dihydroanthracene (2) affords the monoanion 6 which is subjected to alkylation reactions with mono and bifunctional electrophiles. Crucial intermediates in syntheses using 6 are 9-(3-bromopropyl)-9,10-dihydroanthracene (7) and 1,3-bis(9,10-dihydro-9-anthryl)propane (9) since they provide access to linear oligomers in which 9,10-dihydroanthracene units are linked by trimethylene groups. The alkylation processes of these species can be extended to the structurally related polymer 4. The regio- and stereoselectivity of the alkylation reactions are investigated by 1H- and 13C-NMR spectroscopy. Oligomere Ketten mit 9,10-Dihydroanthracen-Einheiten durch Carbanionalkylierung …
P-Chirogenic Secondary Phosphine Oxides: New Stereoselective Synthesis and Applications
2015
Die Stereochemie der Spaltung chiraler acyclischer quartärer Phosphoniumsalze zu tertiären Phosphinoxiden mit Alkalialkoholat
1975
Die beobachtete Brutto-Stereochemie der Spaltung chiraler acyclischer quartarer Phosphoniumsalze 1–5 zu tertiaren Phosphinoxiden 6–10 mittels Natriumathylat ist — mit einer Ausnahme — prinzipiell die gleiche wie bei der Spaltung der betreffenden Salze mit Natrium-hydroxid. Die Stereoselektivitat ist dagegen bei der Spaltung mit Alkoholat meist deutlich geringer als bei der Reaktion mit Natriumhydroxid. Stereochemistry of the Alkali Alcoholate-induced Fission of Chiral Acyclic Quaternary Phosphonium Salts to Tertiary Phosphine Oxides The stereochemical course of the sodium ethanolate-induced fission of chiral acyclic quaternary phosphonium salts 1–5 to tertiary phosphine oxides 6–10 is (with…
Diels-Alder Reactions of (1H-Indol-3-yl)-enacetamides and -endiacetamides: A Selective Access to Acetylamino-Functionalized [b]Annelated Indoles and …
1991
Diels-Alder reactions of the (1H-indol-3-yl)-enacetamides and -endiacetamides 1a–d with some carbodieno-philes and 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione give rise to the novel amino-functionalized carbazole; 4–6 and 8 (Scheme 3). Ethenetetracarbonitrile reacts with 1b to furnish the Michael-type adduct 7 (Scheme 3). Structural aspects of the starting materials 1, which exhibit above all 3-vinyl-1H-indole reactivity, are discussed with regard to the prediction of a Diels-Alder process.
Regio- and Stereoselective Synthesis of Spiropyrrolizidines and Piperazines through Azomethine Ylide Cycloaddition Reaction.
2015
A series of original spiropyrrolizidine derivatives has been prepared by a one-pot three-component [3 + 2] cycloaddition reaction of (E)-3-arylidene-1-phenyl-pyrrolidine-2,5-diones, l-proline, and the cyclic ketones 1H-indole-2,3-dione (isatin), indenoquinoxaline-11-one and acenaphthenequinone. We disclose an unprecedented isomerization of some spiroadducts leading to a new family of spirooxindolepyrrolizidines. Furthermore, these cycloadducts underwent retro-1,3-dipolar cycloaddition yielding unexpected regioisomers. Upon treatment of the dipolarophiles with in situ generated azomethine ylides from l-proline or acenaphthenequinone, formation of spiroadducts and unusual polycyclic fused pip…
ChemInform Abstract: Photochemical Transformations of Some Neoclerodane and Labdane Diterpene Ketones.
2010
Some neoclerodane (2–4) and labdane (5 and 6) diterpene ketones having a β- or γ-hydroxy function, an α-epoxy group, or an α-olefinic double bond have been irradiated at λ = 313 nm in dry solvents (benzene or methanol). Fruticolone (2) yielded the naturally occurring 5,6-seco-neoclerodan-6,1α-olide derivative fruticolide (1), while hispanolone (5) gave the δ-lactone 8, both transformations involving a Norrish type I photoreaction. The α,β-unsaturated ketone derivative 4 was transformed into 7 by a stereoselective intramolecular [2+2] cycloaddition reaction involving its furanic 13(16)-double bond. Finally, the α-epoxy ketone 6 was rearranged to the 10(98)abeo-labda-7,9-dione derivative 10. …
Combining organocatalysis and lanthanide catalysis: a sequential one-pot quadruple reaction sequence/hetero-Diels-Alder asymmetric synthesis of funct…
2016
A stereoselective one-pot synthesis of functionalized complex tricyclic polyethers has been achieved using the combination of secondary amine and lanthanide catalysis. This one-pot quadruple reaction/Hetero-Diels–Alder sequence gave good yields (per step) as well as excellent diastereo- and enantioselectivities. Furthermore, the particular combination of lanthanide complexes with organocatalysis is one of the first examples described for sequential catalysis.
Stereoselective Total Synthesis and Absolute Configuration of the Natural Decanolides (−)-Microcarpalide and (+)-Lethaloxin. Identity of (+)-Lethalox…
2005
[reaction: see text] Convergent, stereoselective syntheses of the pharmacologically active, naturally occurring lactones (-)-microcarpalide and (+)-lethaloxin have been achieved from the commercially available, chiral reagents (R)-glycidol, (S,S)-tartaric acid, and d-ribose as the starting materials. These syntheses have further served to establish the hitherto unknown absolute configuration of (+)-lethaloxin and to show its identity with (+)-pinolidoxin.