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Determination of absolute configuration using vibrational circular dichroism spectroscopy: Phenyl glycidic acid derivatives obtained via asymmetric epoxidation using oxone and a keto bile acid
No full textThe (+)-enantiomers of the o-Br, m-F and p-CH3 derivatives of trans phenyl glycidic acid have been obtained from the
corresponding trans cinnamic acid derivatives using Oxone and the tri-keto bile acid dehydrocholic acid. Vibrational circular dichroism
(VCD) spectroscopy of their methyl esters has been used to determine their absolute configurations. In each case, the absolute
configurations of both methyl ester and parent acid were shown to be (2S,3R)-(+)/(2R,3S)-()
The determination of absolute configuration using vibrational circular dichroism spectroscopy: chiral substituted tiepanes
No full textDetermination of Absolute Configuration Using Ab Initio Calculation of Optical Rotation
No full textAb initio Density Functional Theory (DFT) calculations of transparent
spectral region, discrete frequency specific rotations were used to assign the absolute
configurations (ACs) of: 1, 2H-naphtho[1,8-bc]thiophene 1-oxide; 2, m-F-phenyl glycidic
acid methyl ester; 3, o-Br-phenyl glycidic acid methyl ester; 4, p-CH3-phenyl glycidic
acid methyl ester; 5, 2-(1-hydroxyethyl)-chromen-4-one; and 6, 6-Br-2-(1-hydroxyethyl)-
chromen-4-one. The ACs of 5 and 6 were previously determined via X-ray crystallography
to be: 5, R(−)/S(+); 6, R(+)/S(−). The ACs obtained using []D are the same for
both 5 and 6: R(+)/S(−). We conclude that the previously reported AC of 5 is incorrec
Determination of Absolute Configuration Using Vibrational Circular Dichroism Spectroscopy: The Chiral Sulfoxide 1-Thiochroman-S-Oxide
No full textWe have determined the absolute configuration of the chiral sulfoxide 1-thiochroman S-oxide 1 using vibrational circular dichroism (VCD) spectroscopy. The VCD spectrum of a CCl4 solution of 1 was analyzed using density functional theory (DFT), which predicts three stable conformations of 1, separated by <1 kcal/mol. The VCD spectrum predicted using the DFT/GIAO methodology for the equilibrium mixture of the three conformations of (S)-1 is in excellent agreement with the experimental spectrum of (+)-1. The absolute configuration of 1 is therefore (R)-(-)/(S)-(+). (+)-1 and (-)-1 of high enantiomeric excess (e.e.) were synthesized in high yields via asymmetric oxidation of 1-thiochroman 2 using Ti(iso-PrO)4/(R,R)-1,2-diphenylethane-1,2-diol/H2O/ tert-butyl hydroperoxide and Ti(iso-PrO)4/L-diethyl tartrate/H2O cumene hydroperoxide, respectively. © 2001 Elsevier Science Ltd. All rights reserved
Determination of the absolute configuration of a chiral oxadiazol-3-one calcium channel blocker, resolved using chiral chromatography, via concerted density functional theory calculations of its vibrational circular dichroism, electronic circular dichroism, and optical rotation
No full textThe chiral oxadiazol-3-one 2 has recently been shown to exhibit myocardial calcium entry channel blocking activity, substantially higher than that of diltiazem. To determine the enantioselectivity of this activity, the enantiomers of 2 have been resolved using chiral chromatography. The absolute configuration (AC) of 2 has been determined by comparison of density functional theory (DFT) calculations of its vibrational circular dichroism (VCD) spectrum, electronic circular dichroism (ECD) spectrum, and optical rotation (OR) to experimental VCD, ECD, and OR data. All three chiroptical properties yield identical ACs; the AC of 2 is unambiguously determined to be S(+)/R(-)
Determinazione della Configurazione Assoluta del (+)-1-Tiocromano-S-ossido Mediante Dicroismo Circolare Vibrazionale: Limiti ed Ambiguità di Assegnamenti Configurazionali Basati sul Confronto Diretto di Spettri DCE
No full textConformational Analysis Using Infrared and Vibrational Circular Dichroism Spectroscopies: The Chiral Cyclic Sulfoxides 1-Thiochroman-4-one S-Oxide, 1-Thiaindan S-Oxide and 1-Thiachroman S-Oxide
No full textWe report the conformational analysis of three chiral cyclic sulfoxides, 1-thiochroman-4-one S-oxide (1), 1-thiaindan S-oxide (2), and 1-thiochroman S-oxide (3). Vibrational unpolarized absorption (IR) and vibrational circular dichroism (VCD) spectroscopies are employed, in combination with ab initio density functional theory (DFT). DFT calculations predict two, two, and three conformations with energies spanning < 1 kcal/mol in 1, 2, and 3, respectively. Harmonic IR and VCD spectra of the conformations of 1-3, predicted using DFT, are a sensitive function of conformational structure, and conformationally averaged IR and VCD spectra are Consequently more complex than the spectra of individual conformations. Predicted IR and VCD spectra are compared to experimental spectra of CCl4 and CS2 solutions of 1-3 in the mid-IR spectral region. Calculated and experimental spectra are in good agreement, permitting assignment of very nearly all experimental bands. The analysis unambiguously confirms the presence of the conformations of 1-3 predicted by DF
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Determination of the absolute configurations of chiral organometallic complexes via density functional theory calculations of their vibrational circular dichroism spectra: The chiral chromium tricarbonyl complex of N-pivaloyl-tetrahydroquinoline
No full textThe racemate of the chiral tricarbonyl-η6-arene-chromium(0) complex, tricarbonyl-η6-N-pivaloyl-tetrahydroquinoline-chromium(0), 1, has been synthesized and resolved using chromatography on a (R,R)-Whelk-O1 column. The Absolute Configuration (AC) of 1 has been determined using vibrational circular dichroism (VCD) spectroscopy. The VCD spectrum of 1 has been predicted using the Stephens equation for vibrational rotational strengths, implemented using density functional theory (DFT) in the gaussian program. Using the B3PW91 functional and the 6-311++G (2d,2p) basis set, the predicted VCD spectrum of S-1 is in excellent agreement with the experimental VCD spectrum of (+)-1, leading unambiguously to the AC S-(+). It is concluded that VCD is a useful technique for determining the ACs of chiral organometallic complexes, given the use of optimum functionals and basis sets
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