Search results for "diastereomer"
showing 10 items of 149 documents
Selective derivatisation of resorcarenes: Part 7. The reason for the diastereoselectivity of Mannich reactions with chiral amines
2001
The formation of tetrakis(benzo-3,4-dihydro-1,3-oxazines) 2 by condensation of resorcarenes 1 with various primary amines and an excess of formaldehyde is not only regioselective but, in the case of (R)- or (S)-1-phenylethylamine and their para-substituted (Br, Me) derivatives, also completely diastereoselective. The reaction with the structurally similar (R)-1-aminoindane and (S)-1-(1-naphthyl)ethylamine, on the other hand, produced only a slight excess of one of the two possible epimers 2 and with (R)-1-cyclohexylethylamine no diastereoselectivity was found at all. The presence and the ratio of both diastereomeric structures can be clearly determined by NMR spectroscopy. Inspection of par…
Diastereoselective formation of homochiral flexible perylene bisimide cyclophanes and their hybrids with fullerenes†‡
2021
Cyclophanes of different ring sizes featuring perylene-3,4:9,10-tetracarboxylic acid bisimide (PBI) linked by flexible malonates were designed, synthesized, and investigated with respect to their structural, chemical and photo-physical properties. It is predominantly the number of PBIs and their geometric arrangement, which influence dramatically their properties. For example, two-PBI containing cyclophanes reveal physico-chemical characteristics that are governed by strong co-facial π–π interactions. This is in stark contrast to cyclophanes with either three or four PBIs. Key to co-facial π–π stackings are the flexible malonate linkers, which, in turn, set up the ways and means for diaster…
On the enhancement of stereoselection by cooperation between chiral auxiliaries. Asymmetric diels-alder reactions with fumaric acid bis ((S)-proline …
1989
Abstract Fumaric acid bis ((S)-proline benzyl ester) amide reacts with cyclopentadiene in thermal and Lewis acid catalyzed Diels-Alder reactions to give the cycloadducts with high yields and diastereomeric ratios up to 100:1.
Reversal of asymmetric induction in stereoselective strecker synthesis on galactosyl amine as the chiral matrix
1988
Abstract The reversal of the direction of asymmetric induction in Lewis acid catalyzed Strecker synthesis using the 2,3,4,6-tetra-O-pivaloyl-β-D-galactopyranosyl amine 1 is described. In isopropanol or tetrahydrofuran 1 had given ( R )-diastereomers of the corresponding α-amino nitriles preferably. However, in chloroform in the presence of heterogeneous zinc chloride the same auxiliary alternatively leads to an excess of the ( S )-diastereomers.
Resolution of β-aminophosphines with chiral cyclopalladated complexes
2005
Abstract Resolution of the racemic chiral β-aminophosphines Ph 2 PCH 2 CH(Ph)NH(Ar) ( L 1 for Ar = C 6 H 5 and L 2 for Ar = 2,6-C 6 H 3 i Pr 2 ) has been investigated by use of different cyclopalladated complexes as chiral agents. The resulting complexes afford diastereomeric adducts in a 1:1 ratio. After successive crystallizations from ethanol, a d.e. of 98% was achieved for one aminophosphine palladium complex, while no significant d.e. was obtained after crystallizations from chlorinated solvents. The X-ray structure analysis has pointed out intermolecular hydrogen interactions N–H⋯Cl between the P,N ligand and the chloride ion, which are responsible for the formation and stabilization …
Entropy-controlled diastereoselectivity in the photocyclization of rigid derivatives of o-allylaniline.
2002
Two rigid derivatives of o-allylaniline, namely 8-allyl-2-phenyl-1,2,3,4-tetrahydroquinoline (1b) and 7-(trans-2-cinnamyl)-2-methylindoline (1c), have been chosen as suitable systems to study the potential stereoselectivity of the photocyclization process. Photolysis of 1b leads to a mixture of diastereomeric lilolidines 4 (trans/cis), while 1cproduces a mixture of 4 (trans/cis) and the tetrahydropyrrolo[3,2,1-hi]indole derivatives 5 (trans/cis). To disclose whether the diastereoselectivity could be entropy dependent, photolysis of 1b and 1c has been performed at several temperatures. In both cases, linear relationships have been observed when ln(k(t)/k(c)) (the relative reaction rate const…
Synthesis and X-ray structure of metalated Rhodium(II) catalysts with a chiral phospholane
2001
The reaction of Rh2(O2CR)4 (R = CH3, CF3) with the chiral phosphane (2S,5S)-2,5-dimethyl-1-phenylphospholane (PC*H), results in the formation of two diastereoisomers of Rh2(O2CCR)2(PC*)2, with (P) and (M) configuration. These can easily be isolated by chromatographic methods to obtain enantiomerically pure RhII compounds. Preliminary catalytic studies have shown that they induce moderate asymmetry in the cyclization of 5-aryl-1-diazo-2-pentanones and 1-diazo-5-hexen-2-one. X-ray analysis of the (M) diastereoisomer with formula Rh2(O2CCCH3)2(PC*)2 is reported. The crystallographic parameters are as follows: space group P21212 (orthorhombic) with a = 12.1347(11) A, b = 14.5870(13) A, c = 9.81…
Di‐ and Trivalent Ytterbium Complexes Containing Linked Amino‐ and Amidocyclopentadienyl Ligands
2003
Reactions of the aminocyclopentadienes (C5Me4H)SiMe2NHR (R = Et, allyl, nPr, tBu) with [YbI2(THF)2] in the presence of two equivalents of potassium 1,2-diphenylethenide in THF at room temperature gave the diamagnetic half-sandwich complexes [Yb(η5-C5Me4SiMe2NHR)Ln(μ-I)]2 (L = THF, n = 2, L = DME, n = 1). The tert-butylamido derivative [Yb(η5-C5Me4SiMe2NHtBu)(THF)2(μ-I)]2 was characterized by X-ray structural analysis as a dinuclear complex containing a nonchelating aminocyclopentadienyl ligand. Deprotonation of the aminocyclopentadiene (C5H4tBu)SiMe2NHtBu with two equivalents of potassium 1,2-diphenylethenide in THF, followed by reaction with [YbI2(THF)2] at 60 °C, gave the trivalent ytterb…
9‐Oxabicyclo[6.1.0]nonine
1989
Semiempirische quantenmechanische Rechnungen (MNDO) und Kraftfeldrechnungen (MM2) zeigen, das fur die drei konstitutionsisomeren cis-9-Oxabicyclo[6.1.0]nonine 1–3 jeweils zwei energiearme, zueinander diastereomere Konformere a und b existieren. Der synthetische Zugang zu den hoch gespannten Verbindungen 1–3 gelingt mit der Selenadiazolmethode oder durch Dehydrobromierung (Schemata 3, 4 und 6). Die NMR-spektroskopische Konformationsanalyse bestatigt bei 2 das vorhergesagte Konformerengleichgewicht; bei 1 gehen selbst bei –60°C die beiden Konformeren durch Umklappen des Achtrings schnell ineinander uber, und bei 3 ist praktisch nur eine Konformation populiert. 9-Oxabicyclo[6.1.0]nonynes Semie…
Stereochemistry of terpene derivatives. Part 5: Synthesis of chiral lactones fused to a carane system—insect feeding deterrents
2006
Abstract Chiral iodo- 9, bromo- 10 and hydroxylactones 12 condensed with the carane system were obtained. In each case, the synthetic pathway led to an enantiomerically pure diastereoisomer to generate two stereogenic centers. Iodo- 9 and bromolactone 10 possess a γ-lactone group while the hydroxylactone 12 possesses a δ-lactone moiety situated trans to the gem-dimethylcyclopropyl ring. The structures of the products were confirmed by X-ray crystallography. These lactones were tested for antifeedant activity against storage pest insects.