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Opposite Configuration (opposite + configuration)
Selected AbstractsAsymmetric Cyclopropanation of Optically Active (1-Diethoxyphosphoryl)vinyl p -Tolyl Sulfoxide with Sulfur Ylides: A Rationale for DiastereoselectivityEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2005Wanda H. Midura Abstract The title sulfoxide (S)-(+)- 1a was found to react with sulfur ylides affording the corresponding cyclopropanes in high yields. With fully deuterated dimethyl(oxo)sulfonium methylide, (CD3)2S(O)CD2, the cyclopropanation reaction occurred in a highly diastereoselective manner producing the cyclopropane 4a - d2 as a major diastereomer in which the newly formed quaternary ,-carbon atom is chiral due to isotopic substitution (CH2 vs. CD2). The diastereomer 4b - d2, having the opposite configuration at the ,-carbon atom, was obtained starting form the 2,2-dideuterio substituted vinyl sulfoxide, (S)-(+)- 1a - d2, and the nondeuterated ylide. The diastereomeric ratio in both reactions was found to be ca. 10:1. The reaction of (S)-(+)- 1a with diphenylsulfonium isopropylide yielded the cyclopropane (+)- 7 as a single diastereomer. X-ray structural studies of the crystalline 1-phosphorylvinyl sulfoxide 9 as well as density functional calculations (B3LYP/6-31G*) on (1-phosphoryl)vinyl sulfoxides revealed the origin of the experimentally observed diastereoselectivities and allowed us to propose a transition state model for the cyclopropanation reaction of chiral 1-phosphorylvinyl sulfoxides. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Structural characterization of cyclic kallidin analogues in DMSO by nuclear magnetic resonance and molecular dynamicsJOURNAL OF PEPTIDE SCIENCE, Issue 1 2005Elisabetta Schievano Abstract The conformational properties in DMSO of two head-to-tail cyclic analogues of kallidin ([Lys0]-bradykinin, KL) as well as those of the corresponding linear peptides were studied by NMR and molecular dynamics (MD) simulations. The modifications in the sequence were introduced at position 6, resulting in the four peptides, [Tyr6]-KL (YKL), [Trp6]-KL (WKL), cyclo-([Tyr6]-KL) (YCKL) and cyclo-([Trp6]-KL) (WCKL). The linear WKL analogue was significantly more potent than kallidin on rat duodenum preparations, whereas YKL was significantly less potent. Both cyclic peptides, YCKL and WCKL displayed similar activity, lower than that of the linear analogues and also of cyclo-KL. The two linear analogues display high conformational flexibility in DMSO. In the predominant conformer, for both peptides, all three X-Pro bonds adopt a trans configuration. Three out of four conformers present in YCKL and WCKL were completely assigned. The configurations at the X-Pro bonds are the same for the two analogues. All cyclic conformers show a cis configuration in at least one X-Pro bond and always opposite configuration for the two consecutive X-Pro bonds. The NOE-restrained MD calculations resulted in the detection of several elements of secondary structure in each of the conformers. Such elements are described and their possible relevance to biological activity is discussed. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd. [source] Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2001The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyanoethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoximato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoximato)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co,C*D(CH3)CN. The crystals of the two cobalt complexes were irradiated with a xenon lamp for 72,h and 27,d, respectively. The unit-cell dimensions were gradually changed with retention of the single-crystal form. The crystal structures after irradiation were determined by neutron diffraction. In each crystal the chiral 1-cyanoethyl group of one of the two crystallographically independent molecules was partly inverted to the opposite configuration, whereas that of the other molecule kept the original configuration. The C*,D bond in the inverted group was completely conserved in the process of the inversion of the chiral alkyl group. This suggests that the inversion of the chiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radical after the Co,C bond cleavage by photo-irradiation so that the opposite side of the radical faces the Co atom. This is followed by recombination of the Co,C bond to form the inverted 1-cyanoethyl group. [source] Chiral Induction, Memory, and Amplification in Porphyrin Homoaggregates Based on Electrostatic InteractionsCHEMPHYSCHEM, Issue 6 2009LiXi Zeng Dr. Abstract Supramolecular chirality in two configurational homoaggregates of anionic meso -tetrakis(4-sulfonatophenyl)porphyrin (TPPS) can be induced by D - and L -alanine in acidic water (see picture). The chirality can be further memorized and enforced through strong electrostatic interactions between TPPS aggregates and achiral poly(allylamine) [PAA]. Supramolecular chirality in two configurational homoaggregates of anionic meso -tetrakis(4-sulfonatophenyl)porphyrin (TPPS) can be induced by D - and L -alanine (Ala) in acidic water, respectively. The induced supramolecular chirality can be further memorized and enforced, even after complete removal of Ala or in the presence of excess Ala with the opposite configuration, through strong electrostatic interactions with achiral poly(allylamine) [PAA]. The ionic chiral interactions between TPPS and Ala or PAA are characterized by means of UV/Vis absorption and circular dichroism spectrometry. Fluorescence spectroscopy and atomic force microscopy are used as complementary techniques. On the basis of the comprehensive experimental results, a possible mechanism for chiral induction, memory, and amplification of TPPS homoaggregates by chiral amino acids and achiral PAA is proposed. Thus, we demonstrate a novel strategy to realize chiral memory in supramolecular systems by polyelectrolytes through hierarchical electrostatic self-assembly. [source] Bifunctional Enantioselective Ligands of Chiral BINOL Derivatives for Asymmetric Addition of Alkynylzinc to AldehydesCHINESE JOURNAL OF CHEMISTRY, Issue 2 2008Xiao-Wei ZOU Abstract Four analogous binaphthyl compounds (R) -3a - 3d containing (R)-3,3,-bis(2-pyridyl) groups were synthesized by the conjugation of (R)-2,2,-dimethoxy-1,1,-binaphthyl-3,3,-diboronic acid [(R)- 2] with 2-bromopyridine, 2-bromo-5-methylpyridine, 2-chloro-4-fluoropyridine and 2-chloro-3-(trifluoromethyl)pyridine via Pd-catalyzed Suzuki reactions, respectively. The application of the four chiral ligands in combination with Et2Zn and Ti(Oi -Pr)4 to the asymmetric addition of phenylacetylene to various aldehydes has been studied. The results show that (R)- 3a and (R)- 3b are not good catalysts for the alkynylzinc addition to aldehydes, (R)- 3d shows good enantioselectivity only for the alkynylzinc addition to aliphatic aldehydes, and (R)- 3c exhibits excellent enantioselectivity for phenylethynylzinc addition to both aromatic and aliphatic aldehydes. All the four chiral ligands produced the opposite configuration of the propargylic alcohols to that of the chiral ligands. [source] Dual enantioselective Diels,Alder process in the cyclization of chiral acrylamide with dienesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2004Doo Young Jung Abstract Diels,Alder cycloadditions of chiral acrylamides with cyclopentadiene or 2, 3-dimethyl butadiene proceed with high diastereofacial selectivity. Either endo-R or endo-S products have been obtained depending upon the structures of acrylamides and Lewis acids used. The endo form was exclusively obtained over the exo form. The dependence of the mechanism of formation of opposite configurations of endo-R or endo-S products on the Lewis acids is discussed. Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||