Absolute configuration solvation

Optically active 2,2,2-trifluorophenylethanol, when used as NMR solvent, causes enantiomeric spectral dissimilarities for chiral episulphoxides the relative field positions of non-equivalent NMR resonances are analyzed with respect to the absolute configuration of the solvated compounds220. 

In conclusion, the most important result is that the use of permethylated cyclodextrin as chiral solvating agent for NMR spectroscopy not only affords a simple and practical way for the determination of the stereochemical purities of trisubstituted allenes, but also allows one to simultaneously determine their absolute configuration. Indeed, TRIMEB induced only positive complexation shifts of all the allene protons, which are greater for the (S )-enantiomer than for the (R)-enantiomer, independent of the structure of the allene. This empirical correlation seems to be reliable since it has been satisfied by a large number of trisubstituted allenes. 

The detailed structure of the small bacteriocin (isolated from the culture broth of the Gram-negative bacterium Rhizobium leguminosarum) as N-[(3R)-hydroxy-7-ns-tetradecenoyl]-L-HSL 14 was elucidated largely by the 1-D and 2-D and 13C spectroscopy. The absolute configuration of both asymmetric carbon atoms in the molecule was determined by the use of chiral solvating agents (S)-(+)- and (R)-(-)-2,2,2-trifhioro-l-(9-anthryl)ethanol [11]. 

In general, the observation of opposite senses of nonequivalence for substituents on opposite faces of the plane defined by primary and secondary interactions will be the hallmark of a normal solvation model. Deviations are of no consequence for enantiomeric purity determinations but should raise questions concerning the validity of the usual model for the assignment of absolute configuration based on the observed senses of nonequivalence. Since knowledge of solute structure often allows anticipation of such third interactions, ... 

Compounds of unknown chirality to which a chiral reference is added in order to form a covalent derivative, a salt, or even a solvate. In the latter case, a compound of known chirality is incorporated by way of cocrystallization in the crystal structure of a compound of unknown absolute configuration, or vice versa. 

Axillarine (40) was isolated from C. axillaris Ait. by Crout.20 The crystal structure of the ethanol solvate of axillarine hydrobromide has been determined by X-ray diffraction methods.21 The absolute configurations of the four chiral centres in the necic acid portion have been established. The overall shape of axillarine is somewhat similar to fulvine (41),22 which also has an 11-membered macrocyclic... 

Pirkle s reagent, a chiral solvating agent, was used to determine the absolute configuration of the annonaceous butenolides by the NMR method <02CEJ5662>. 

Achiral lanthanide chelates have been used in conjunction with other chiral NMR deriva-tizing or solvating agents. The addition of the lanthanide enhances enantiomeric discrimination and/or causes shifts in the spectrum that show a characteristic trend with absolute configuration. 

The equilibria that describe the 1 1 interactions of a CSA and a pair of enantiomeric solutes (Equation 2.16) is similar to the one used to explain shift reagents (Equation 2.11). An important distinction is that we assumed, for the sake of simplicity, that the CSR was a single species. For chiral solvating agents, that assumption is not necessary, because it is fact. As a result, the analysis of the geometry of the diastereomeric solvates, (+)-CSA R and (+)-CSA-5, often allows determination of absolute configuration. 

As was the case with chiral shift reagents, preferential population of one diastereomer over the other (Kr Ks) is not a prerequisite for induced anisochrony of enantiotopic groups. Additionally, since the CSA is diamagnetic, it may be used in excess over the analyte. A five-fold excess is usually sufficient to drive the equilibria of Equation 2.16 to the outside, such that the solute is present only as its two diastereomeric solvates. Since the observed spectra are time-averages of all the species in solution, this chemical trick simplifies analysis of absolute configuration by focussing on the diastereomeric solvates alone. 

For the analysis of compounds that are chiral by virtue of isotopic substitution, NMR is the method of choice, since energetic differences between diastereomeric complexes are not required for induced anisochrony. When it works, NMR is also one of the simplest and fastest techniques available. For monofunctional or weakly basic solutes, chiral shift reagents are more likely to succeed, whereas chiral solvating agents are simpler (when they work) and are better for the assignment of absolute configuration. 

The determination of the absolute configuration of a chiral substance is a very important part of the characterization of that molecule. That is also true for chiral solvents. Circular dichroisms induced in the UV spectra of metal complexes solvated by a chiral solvent can be used for the determination of the absolute configuration of the solvent [Br 80]. This method is attractive in that the experiments are easy to carry out. 

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