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F. Weygand et al.' have reported an excellent method of racemization test with GLC (gas-liquid-chromato graphy) by using the coupling reaction between TfaPro-Val-OH and H-Pro-OMe. In an attempt to pursue this method further, we wished to synthesize Tfa-L-Pro-L-VaI-OtBu (I) and Tfa-L-Pro-D-Val-OtBu (II) which then should be converted to the corresponding Tfa-Pro-Val-OH. However, in the condensation product from Tfa-LPro-OH and H-L-Val-O,Bu by the method of DCCD/ HOBt,2) we have always detected a small amount of the by-product by GLC-analysis at a tR corresponding to the enantiomer of the L-D-compound. In this case, the Tfa-L-Pro-OH used was an ether-soluble fraction ob tained from a reaction mixture of L-Pro and 2 2.5 eq mole Tfa-anhydride. Also in the condensation product between Tfa-L-Pro-OH and H-D-Val-OtBu, a small amount of by-product was always detected at the position corresponding to the enantiomer of the L-Lcompound (Fig. 1). (GLC: Hitachi Perkin Elmer K 53; column (1 m x 3 m/m) packed with OV-17 (5 %) impregnated on chromosorb M AW HMDS; FIDdetector; carrier gas N2 1.5 atm; column temperature, 170°C; tR of Tfa-L-Pro-L-Val-OtBu=6.40 min and tR of its epimer, L-D-Compound =5.40 min). The fact that those by-products were the unexpected stereoisomers e.g. D-L-enantiomer in the former case and D-Denantiomer in the latter, respectively, was confirmed from the following investigation: 1) The elementary analyses of both condensation products agreed with the composition of Tfa-Pro-Val-OtBu. 2) The pattern of GLC/MS-analyses of the both mainand sub-peaks were identical and all the fragments satisfied Tfa-Pro-ValOtBu structure (m/e: 367(M+), 310 (M+-C(CH3)3), 293, 266 (M+-COOC(CH,),)). In order to distinguish to which enantiomer, D-D or L-L, the sub-peak in the latter condensation product above should be assigned, in other words, to decide in which amino acid, Pro or Val, the racemization had

Racemization of L-proline during trifluoroacetylation. Detection of unexpected stereoisomers in trifluoroacetylprolyl-valine t-butyl ester.