Racemization resonance integral

The coupling of the selone to racemic carboxylic acids with dicyclohexylcarbodiimide in dichloromethane with 4-dimethylaminopyridine at 0°C for 0.5-1 hour affords the acylated derivatives. Similar results are obtained using the acid chloride and triethyl amine. Figure 13 (upper part) shows the selenium spectrum for the diastereomeric. Y-acyl derivatives of the oxazo-lidine-2-selone with (A,A)-5-methylheptanoic acid. Clearly the shift difference of about (5 = 0.1 is sufficient for integration of the two singlets. The lower part of the figure shows the remarkably different 77Se resonances observed for four species derived from partially deuterated phenylacetic acid. The diastereomeric mixture of the monodeuteriated substance is easily detected (AS = 0.07, 5.0 Hz). 

Volatile peptide derivatives, for instance trifluoroacetyl-L-valyl-L-valine methyl ester or benzyloxycarbonyl-L-leucyl-L-phenylalanyl-L-valine tert.butyl ester can be separated from their diastereoisomers that contain a D-residue by vapor phase chromatography. Also, through the examination of nmr spectra of relatively simple peptides the extent of racemization that occured during their preparation can be determined without separation of the diastereoisomers, because the difference in the chemical shifts of some selected resonances is sufficient for integration. Thus the areas under the well separated peaks of the alanine methyl protons in acetyl-L-phenylalanyl-L-alanine methyl ester and in acetyl-D-phenylalanyl-L-alanine methyl ester can be integrated and the values used to determine the extent of racemization of the phenylalanine residue during coupling. 

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