Assignment of absolute configuration of chiral carboxylic acids via exciton-coupled CD treatment: 4-phenylthioproline as a case study.

The 4-hydroxybenzoate chromophore was used as an exciton reporter group for the assignment of the absolute configuration of a chiral carboxylic acid of pharmaceutical interest, (-)-(2S,4S)-4-phenylthioproline, precursor of the ACE-inhibitor zofenopril. The nondegenerate coupling with the preexisting phenylthio chromophore was observed. A detailed conformational analysis, accomplished by means of (1)H-NMR NOESY and semiempirical PM3 computational methods, and quantitative CD calculations were necessary to substantiate the assignment based on a weak experimental couplet. Copyright 2002 Wiley-Liss, Inc.