Enantioselective synthesis of (2R, 3S)- and (2S, 3R)-4,4,4-trifluoro-N-Fmoc-O-tert-butyl-threonine and their racemization-free incorporation into oligopeptides via solid-phase synthesis

An efficient method for the enantioselective synthesis of (2R, 3S)- and (ZS, 3R)-4,4,4-trifluoro-N-Fmoc-O-tertbutyl-threonine on multigram scales was developed. Absolute configurations of the two stereoisomers were ascertained by X-ray crystallography. Racemization free coupling conditions for the incorporation of tfT into oligopeptides were then explored. For solution-phase synthesis, tfT racemization was not an issue under conventional coupling conditions. For solid-phase synthesis, the following conditions were identified to achieve racemization free synthesis: if tfT (3.0 equiv) was not the first amino acid to be linked to the resin (1.0 equiv), the condition is 2.7 equiv DIC/3.0 equiv HOBt as the coupling reagent at 0 C for 20 h; if tfT (3.0 equiv) was the first amino acid to be linked to the resin (1.0 equiv), then 1.0 equiv of CuCl2 needs to be added to the coupling reagent. 2007 Wiley Periodicals, Inc.