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Stereochemistry
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
No! Specific rotation is a physical property of a molecule that is determined from the interaction of the enantiomers with plane-polarized light. Absolute configuration is the spatial arrangement of atoms around a stereogenic center and prioritizing those atoms is an interpretive property, not a physical property. An enantiomer with a specified absolute configuration could have either a (+) or a (–) specific rotation.What are the sequence rules used to determine absolute configuration?
New quaterphenyls laterally substituted by methyl group and their influence on the self-assembling behaviour of ferroelectric bicomponent mixtures
Published in Liquid Crystals, 2022
Marzena Tykarska, Katarzyna Kurp, Paulina Zieja, Jakub Herman, Sergei Stulov, Alexej Bubnov
Compound Q3 also contains a chiral carbon atom with an absolute configuration (S), but the chiral centre is connected to the rigid core with the ester group; this is the reason why it forms a right-handed structure. The handedness of the SmC* helical structure in all three quatraphenyls is in a proper agreement with the Gray-McDonell rule [40]. In the resulting mixture with 6F2OBi used as a base, even a small amount of Q3 compound used as dopant causes the decrease of the helical pitch in the SmC* phase (Figure 8c). It was not possible to measure the pitch of mixtures 0.4–0.8 mole fraction of Q3 dopants because the SmC* phase was over the measuring range of the used temperature programmer. In this system, it was possible to measure the helical pitch in the SmC*A phase too. The increase of the amount of Q3 causes the decrease of the helical pitch for concentrations of 0.2–0.6 mole fraction and next the increase of the pitch for concentrations of 0.8–0.9 mole fraction.
The use of residual dipolar couplings for conformational analysis of non-steroidal anti-inflammatory drugs dissolved in weakly ordering media
Published in Liquid Crystals, 2018
Maria Enrica Di Pietro, Giorgio Celebre, Rosachiara Antonia Salvino, Christie Aroulanda, Denis Merlet, Giuseppina De Luca
Profens are chiral bioactive compounds, whose individual enantiomers are characterised by different pharmacological activity [64,65]. Several studies have shown that the anti-inflammatory activity is largely stereospecific for S-enantiomers [66]. However, since the enantiomer of absolute configuration R is generally inactive or can convert irreversibly into its enantiomer, many drugs are marketed as racemic mixtures. Naproxen represents an exception, because it is internationally sold as S-enantiomer due to the potential toxicity of the R-isomer [67]. These compounds are characterised by the presence of two or more degrees of flexibility that make the description of the torsional potential challenging, compared to the single rotor case seen before. However, encouraged by the results obtained for the DFL and in order to verify if the AP–DPD approach was able to describe more complex conformational situations in weakly ordering media, we have undertaken the study of these compounds. S-naproxen and R-flurbiprofen were chosen as model compounds for molecules which display two rotations that are topologically far enough to be considered as independent, while the study of S-ibuprofen and S-ketoprofen is addressed to deal with more complex conformational equilibria including coupled torsions.
Helix parameters in bi- and multicomponent mixtures composed of orthoconic antiferroelectric liquid crystals with three ring molecular core
Published in Liquid Crystals, 2014
One of the methods to obtain orthoconic antiferroelectric materials with long pitch is by mixing compounds with the opposite absolute configuration at a chiral carbon atom (‘R’ + ‘S’). The biggest disadvantage of this method is the destabilisation of antiferroelectric phase as well as the decrease of mixture spontaneous polarisation. Second method of obtaining long pitch material is by adding chiral compounds to the mixture of achiral anticlinic compounds. Unfortunately, the number of achiral compounds forming anticlinic phase is very limited.[19,20] Furthermore, achiral anticlinic structures are characterised by high viscosity. The third method is to mix compounds of different helical twist sense having different structure of chiral chain, namely differing in the distance of the chiral centre from rigid core by odd or even number of atoms.[21] The problem is that such compounds do not want to mix with each other very well.