Selective Synthesis and Structural Elucidation of <i>S</i>-Acyl- and <i>N</i>-Acylcysteines

N-(Acyl)-1H-benzotriazoles 6a−f react with l-cysteine 5 at 20 °C to give exclusively (i) N-acyl-l-cysteines 8a−e in the presence of triethylamine in CH3CN−H2O (3:1), but (ii) S-acyl-l-cysteines 7a−e in CH3CN−H2O (5:1) in the absence of base. Structures 7b, 7d and 8b, 8d are supported by 2D NMR spectroscopic methods including gDQCOSY, gHMQC, gHMBC, and 1H−15N CIGAR-gHMBC experiments. The structure of compound 8d was also supported by single-crystal X-ray diffraction.