New stereoconservative syntheses of beta,beta,beta- and gamma,gamma,gamma-trifluoro-alpha-amino, alpha-hydroxy, and alpha-mercapto acids and their incorporation into a peptide and depsipeptide fragment

Syntheses of beta,beta,beta- and gamma,gamma,gamma -trifluoro-alpha -amino, alpha -hydroxy and a-mercapto acids using hexafluoroacetone as protecting and activating reagent are described. The key step of the syntheses is the transformation of an omega -carboxy group into a trifluoromethyl group on t reatment with sulfur tetrafluoride. The carboxy activated species obtained are suitable for direct incorporation into peptide and depsipeptide fragmen ts, RP-HPLC experiments and Mosher's TMPA method (H-1 and F-19) demonstrate that the reaction sequence occurs without racemization.