The Adaptation of Diastereomeric S-Prolyl Dipeptide Derivatives to the Quantitative Estimation of R- and S-Leucine Enantiomers

Abstract

Our need for a convenient method of analytical estimation of the precise enantiomeric composition of R- and S-leucine mixtures has prompted us to examine in detail the preparation and GC separation of N-TFA-S-prolyl-R(and S)-leucine methyl ester diastereomers (II, R = isobutyl). Contrary to unsupported assertions in the literature, (11–13) both the formation and utilization (in the presence of triethylamine) of N-TFA-S(-)prolyl chloride (III), the dipeptide forming reagent, may be attended by extensive racemization. Methods have therefore been developed whereby III may be prepared and utilized to synthesize II without detectible racemization. A series of known composition mixture of the essentially optically pure diastereomers of II was then prepared and subjected to GC analyses under conditions permitting baseline separation of the two diastereomeric peaks, while peak areas were measured with an electronic digital integrator. Under these conditions it was found that the known enantiomeric compositions could be duplicated experimentally to an absolute error of only 0.0 to 0.6% over the entire composition range.