Diastereomeric purity, determination

Addition of lithiated Ar-(4-methylphenylsulfonyl)-S-phenyl-S -(2-propenyl)sulfoximine to acyclic enones gives exclusively 1,4-a-adducts in high diastereomeric purity. The configuration of the adduct (R2 = R3 = C6H5) has been determined by a single crystal X-ray structure determination27. 

The mechanism of this process remains unknown. The reaction employing nitronate containing auxiliary(138b), produces the corresponding optically active nitronate (139b), but its optical and diastereomeric purity was not determined (117). 

In principle, separation of resonances of diastereomeric compounds (such as dl and meso isomers) may be increased simply through use of an appropriate achiral solvent. Chiral solvents may in some cases be especially effective in producing a separation, particularly if the diastereomers differ in configuration about a center that is amenable to analysis by the CSA method. Kaehler and Rehse (89) give a detailed account of conditions necessary for measurement of the ratio of meso- and dZ-tartaric acid employing A,N-dimethyl PEA. Bomyl acetate used as solvent for l,2-difluoro-l,2-dichloroethane (90) allows measurement of the diastereomeric composition. Paquette and co-workers (91,92), using TFAE, were able to determine the diastereomeric purity of the recrystallized adducts 47 of... 

The optical purity was determined by chiral GC, using a 20 % permethylated cyclodextrin column, after esterification of the pure product with (CF3C0)20 in dry CH2CI2 (65 °C isothermal carrier gas N2, pressure 70 kPa). T r = 26.305 min (>99%, (3R,45)-3-allyl-4-hydroxy-2-pentanone). The enantiomeric purity was estimated to be >99 % and the diastereomeric purity >99 %. 

S,4/J,5/ )-/J-(./V-Acetylbenzylamino)-2,2,5-trimethyl-l,3-dioxolane-4-propanal can be prepared in a similar manner54 in 63% yield and with a diastereomeric purity of 90%, as determined by H NMR. Analysis of the H-NMR spectrum of the cyclization product obtained from this aldehyde by treatment with hydrochloric acid in anhydrous methanol unambiguously establishes the syn configuration of the vicinal hydroxy and benzylamino group. Thus, the stereochemical course of benzylamine addition to both 2-alkenylthiazolium salts is the same. 

To assess the stereospecificity of the Grignard and organolithium reactions with menthyl phosphinates, the diastereomeric purity of starting menthyl esters was estimated by pmr spectroscopy (see Sect. 2.2) and, in most cases, highest reported rotations were used to estimate the. enantiomeric purity of the derived optically active phosphine oxides The method of preference for determining the enantiomeric purity of a phosphine oxide, even in those cases in which a value for the rotation of optically pure material is reported, involves stereospecific reduction of the phosphine oxide with hexa-chlorodisilane (see Sect. 2.4) to the corresponding phosphine, followed by quatemization with 2-phenyl-2-methoxy-ethyl bromide and pmr analysis of the diastereomeric phosphonium bromides (Eq. (1)) > This method for determining optical purity, shown ) to be applicable... 

A NMR study on the formation of diastereoisomeric inclusion complexes between fluorinated amino acid derivatives and a-CD in 10% D2O solution shows that the chemical shifts of the D-amino acid derivatives included by a-CD are upheld from those of their L analogues [77]. The shift difference between the diastereoisomers formed with D and L enantiomers can be used for chiral analysis and optical purity determinations. For example, the interaction of -CD with propanolol hydrochloride produces diastereomeric pairs. The protons of the antipode give NMR signals which differ in chemical shifts in D2O solution at 400 MHz. The intensity of the resonance signals for each diastereoisomer has been used for optical purity determination. By adding racemate to pure (—) isomer, this technique is able to measure optical purity of propanolol hydrochloride in water down to the level of 1%. 

The diastereomeric purities of 2a were determined by capillary gas chromatography or H-NMR studies (quantitative determination of the molar ratio 2a/2b). The enantiomeric purities of 2a were established, affer derivatization with (S)-2-methoxy-2-(trifluoromethyl)-2-phenylacetyl chloride [(5)-MTPA-Cl], by H- or F-NMR studies [quantitative determination of the molar ratio (Si5, C7 )-2a/(SiR,CS)-2a]. The enantiomeric purities of 3a (after hydrolytic conversion into 2a) were determined analogously to those of 2a. The absolute configurations at the SiCH(OH)C carbon atoms of the enantiomers of 2a were established, after derivatization with (5 -MTPA-Cl, by the NMR-spectroscopic correlation method described in [7]. The relative configurations of 2a and 2b were assigned by comparison with the diastereoisomers of 1-methyl-1-phenyl-l-sila-2-cyclohexanol (in this context, see [8]). 

OPTIONAL. Obtain the NMR spectrum of the aziridine. Establish from this spectrum and the NMR data obtained in Experiment [F2] if any unreacted dibro-mochalcone still contaminates the aziridine sample that has been purifiedpr use in the preparation of the photochromic target molecule. Determine the diastereomeric purity of your product, and determine that the aziridine product is indeed trans substituted. 

Racemic mixtures of sulfoxides have often been separated completely or partially into the enantiomers. Various resolution techniques have been used, but the most important method has been via diastereomeric salt formation. Recently, resolution via complex formation between sulfoxides and homochiral compounds has been demonstrated and will likely prove of increasing importance as a method of separating enantiomers. Preparative liquid chromatography on chiral columns may also prove increasingly important it already is very useful on an analytical scale for the determination of enantiomeric purity. 

Thus, the enantiomeric contents in a pair of sulphoxides can be determined by the NMR chemical shifts in the methine or methylene protons in the two diastereomeric complexes which are stabilized by the hydrogen bond between the hydroxyl and the sulphinyl groups147-151 (Scheme 13). Similarly, the enantiomeric purity and absolute configurations of chiral sulphinate ester can be determined by measuring the H NMR shifts in the presence of the optically active alcohols152. 

As already mentioned, chiral cations are involved in many areas of chemistry and, unfortunately, only few simple methods are available to determine their optical purity with precision. In the last decades, NMR has evolved as one of the methods of choice for the measurement of the enantiomeric purity of chiral species [ 110,111 ]. Anionic substances have an advantage over neutral reagents to behave as NMR chiral shift agents for chiral cations. They can form dia-stereomeric contact pairs directly and the short-range interactions that result can lead to clear differences in the NMR spectra of the diastereomeric salts. 

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