Optical rotation chromatography

Harada et al. started from preparing inclusion complexes by adding an aqueous solution of PEG bisamine (PEG-BA) to a saturated aqueous solution of a-CD at room temperature and then allowing the complexes formed to react with an excess of 2,4-dinitrofluorobenzene. They examined the product by column chromatography on Sephadex G-50, with DMSO as the solvent, and obtained the elution diagram shown in Fig. 46. They identified the first, second, and third fraction, respectively, as the desired product, i.e., a polyrotaxane, dinitrophenyl derivatives of PEG, and uncomplexed a-CD, by measurement of both optical rotation and UV absorbance at 360 nm for the first, UV absorbance at 360 nm for the second, and optical rotation for the third. 

Initially, progress in this area was hampered by the lack of suitable analytical methods for chiral hydrocarbons. Early studies relied on optical rotation to determine enantiomeric excess (ee) values, but with the development of chiral gas chromatography (GC) and high-performance liquid chromatography (HPLC) columns, chromatographic methods have become more common. 

Introduction of nitrogen into the anulene ring (e.g. of 95) leads to a methano-azaanulene 107 121) with Q-symmetry which is therefore chiral (like its mono- or disubstituted derivatives)118). The low basicity of 107 (pKa 3.20) prevented its optical resolution by conventional methods (e.g. through salts with optically active acids). Excellent results were obtained, however, (as also in the case of the two isomeric carbocyclic methylesters 97 and 101 and of several derivatives of azaanulene) by chromatography on microcrystalline triacetyl cellulose in ethanol at 7 bar 1221 (see also Section 2.7.1). In many cases base line separations were accomplished to give both (optically pure) enantiomers. Enantiomeric relations were confirmed in all cases by recording the CD-spectra of both fractions. Some results of these separations are shown in Fig. 4 together with the optical rotations ([a]D in ethanol) of the enantiomers. 

Enantiomeric purity. In order to assess the efficiency of an enantioselective hydrolase-catalyzed reaction, it is imperative that one can accurately measure at least the conversion and the enantiomeric excesses of either the substrate or the product (see equations Equation 1, Equation 2, and Equation 3). Although optical rotation is sometimes used to assess enantiomeric excess, it is not recommended. Much better alternatives are various chromatographic methods. For volatile compounds, capillary gas chromatography on a chiral liquid phase is probably the most convenient method. Numerous commercial suppliers offer a large variety of columns with different chiral liquid phases. Hence it is often easy to find suitable conditions for enantioselective GC-separations that yield ee-values in excess of... 

Lee, Acree, and Shallenberger have in some cases carried out actual isolations of the anomeric trimethylsilyl per-O-trimethylsilyl glycosides by preparative gas chromatography and have then characterized the separated derivatives by optical rotation, elementary analysis, and NMR spectroscopy (30). These characterizations, which have been done for glucose, mannose, and galactose (see below) confirm the usefulness of the GLC technique. 

A different semisynthetic method involves the acylation of an amino alcohol with a peptide ester and the resulting amino alcohol is subsequently oxidized to the aldehyde 40 The acylation of H-Phe[CH2OH] with the peptide ester Z-Ala-Ala-Leu-OMe is carried out in 5% DMF/MeCN with the subtilisin distributed on the surface of macroporous silica gel. The resulting peptide alcohol is oxidized under mild conditions using anhydrous dimethyl sulfoxide and 20-fold excess of acetic anhydride with purification via flash chromatography 40] Z-Phe[CH2OH] has been oxidized under these conditions and the optical rotation indicates little epimerization as compared to literature values 11 40 ... 

Physical methods directly related to chemical composition have been used in efforts to detect adulterations of citrus products. Physical measurements include spectrophotometry (UV, fluorescence, colorimetry), gravimetric determinations (ash, specific gravity), mass spectrometry (isotope ratios), chromatography, and optical rotation. 

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