Search results for "Chiral"

Chiroptical Phenomena in Reverse Micelles: The Case of (1R,2S)-Dodecyl (2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium Bromide (DMEB)

2014

(1R,2S)-Dodecyl(2-hydroxy-1-methyl-2-phenylethyl)dimethylammonium bromide (DMEB) aggregates dispersed in carbon tetrachloride have been investigated by Fourier transform infrared (FT-IR), vibrational circular dichroism (VCD) and 1H nuclear magnetic resonance (NMR) spectroscopy at various surfactant concentration and water-to-surfactant molar ratio. Experimental data indicate that, even at the lowest investigated concentration and in absence of added water, DMEB molecules associate in supramolecular assemblies. At higher DMEB concentration the aggregates can confine water molecules, making it plausible to think that DMEB form reverse micelles and that water molecules are quite uniformly dist…

Stereoselective synthesis of 1-aminoalkanephosphonic acids with two chiral centers and their activity towards leucine aminopeptidase

2003

The stereoselective synthesis of 1-amino-2-alkylalkanephosphonic acids, namely, compounds bearing two chiral centers, was achieved by the condensation of hypophosphorous acid salts of (R)(+) or (S)(-)-N-alpha-methylbenzylamine with the appropriate aldehydes in isopropanol. Simultaneous deprotection and oxidation by the action of bromine water provided equimolar mixtures of the RS:RR and SR:SS diastereomers of desired acids. They appeared to act as moderate inhibitors of kidney leucine aminopeptidase with potency dependent on the absolute configuration of both centers of chirality.

Optically active titanium complexes containing a tridentate linked amido-cyclopentadienyl ligand

2000

Optically active titanium complexes Ti{η5:η1-C5R4SiMe2NC6H10 (OCH2Ph)-2}Cl2 (R = H, Me), containing a cyclopentadienyl ligand linked to the chiral trans-2-benzyloxycyclohexylamido group, were synthesized and characterized in both enantiomerically pure forms. A single crystal X-ray structure analysis of (−)-(R,R)-Ti{η5:η1-C5H4SiMe2NC6H10(OCH2Ph)-2}Cl2 shows a structure in which the benzyloxy group in the amido sidechain is not interacting with the titanium center. Upon activation with n-butyllithium, these complexes hydrogenate acetophenone N-benzylimine with low enantioselectivity. Chirality 12:472–475, 2000. © 2000 Wiley-Liss, Inc.

Left, Right, or Both? On the Configuration of the Phenanthroindolizidine Alkaloid Tylophorine from Tylophora indica

2013

The alkaloid (-)-tylophorine was isolated from a sample of Tylophora indica, and the crude extract was analyzed by HPLC/MS(n) and chiral HPLC/MS. While the literature states that the naturally occurring form of this alkaloid is the R-enantiomer and that its S-antipode is usually not found in nature, we confirmed the hypothesis of Govindachari and Nagarajan that natural levorotatory tylophorine is indeed a nearly racemic mixture with a slight excess of the R-enantiomer.

Comparative enantioseparation of planar chiral ferrocenes on polysaccharide‐based chiral stationary phases

2022

Planar chiral ferrocenes are well-known compounds that have attracted interest for application in synthesis, catalysis, material science, and medicinal chemistry for several decades. In spite of the fact that asymmetric synthesis procedures for obtaining enantiomerically enriched ferrocenes are available, sometimes, the accessible enantiomeric excess of the chiral products is unsatisfactory. In such cases and for resolution of racemic planar chiral ferrocenes, enantioselective high-performance liquid chromatography (HPLC) on polysaccharide-based chiral stationary phases (CSPs) has been used in quite a few literature articles. However, although moderate/high enantioselectivities have been ob…

Neoclerodane Diterpenoids from Scutellaria pontica

1997

Seven novel neoclerodane diterpenoids, scupontins A-G, have been isolated from the Me2CO extract of the aerial parts of Scutellaria pontica (1-7), together with the known neoclerodanes scutalbin A and scutalpin M. Structures 1-7 were established by exhaustive NMR spectroscopic studies and chemical transformations. Scupontins A-D(1-4, respectively) and scupontins E (5)andF(6) possess unusual [(3¢S,3¢¢S)-3¢-[(3¢¢-acetoxybutyryl)oxy]butyryloxy and [(3¢S,3¢¢S,3¢¢¢S)- 3¢-[[3¢¢-[(3¢¢¢-hydroxybutyryl)oxy]butyryl]oxy]butyryl]oxy substituents, respectively, attached to the C-19 position of the neoclerodane framework. In the case of the 6R,7â-dibenzoate derivative 7 (scupontin G) its absolute config…

Enantioselective copper-aminopyridine-catalyzed aza-Henry reaction with chelating N-(2-pyridyl)sulfonyl imines

2012

This article describes a copper-catalyzed aza-Henry reaction. Copper complexes of camphor-derived aminopyridines catalyze the addition of nitromethane to N-(2-pyridyl)sulfonyl aldimines to give the corresponding β-nitrosulfonamides with good yields and variable enantiomeric excesses (up to 83%). An example of transformation of these compounds into N-(2-pyridyl)sulfonyl-α-amino acids and deprotection of the sulfonamide with Mg–MeOH is provided. Chirality 24:441–450, 2012. © 2012 Wiley Periodicals, Inc.

NMR enantiodifferentiation of triphenylphosphonium salts by chiral hexacoordinated phosphate anions

2003

BINPHAT anion—rather than TRISPHAT—is an efficient NMR chiral shift reagent for triphenylphosphonium salts containing stereogenic centers on the aliphatic side-chain. BINPHAT—rather than TRISPHAT—anion is an efficient NMR (1H and 31P) chiral shift reagent for chiral triphenylphosphonium salts.

3.17 Synthetically Derived Auxiliaries: Phosphorus Derivatives

2012

This chapter aims to give an overall view of chiral-pool based asymmetric synthesis of organophosphorus compounds more particularly bearing the chirality on the phosphorus atom (P-stereogenic). The enantiomerically pure organophosphorus compounds can be obtained either by resolution of the racemate or by stereoselective synthesis using particularly ephedrine or sparteine as the main chiral auxiliaries and borane as a P(III)-protecting group.

2018

Abstract We show that a Wilson-type discretization of the Gross–Neveu model, a fermionic N -flavor quantum field theory displaying asymptotic freedom and chiral symmetry breaking, can serve as a playground to explore correlated symmetry-protected phases of matter using techniques borrowed from high-energy physics. A large- N study, both in the Hamiltonian and Euclidean formalisms, yields a phase diagram with trivial, topological, and symmetry-broken phases separated by critical lines that meet at a tri-critical point. We benchmark these predictions using tools from condensed matter and quantum information science, which show that the large- N method captures the essence of the phase diagram…