Resins for the Fmoc-/tBu-Synthesis of Peptide Acids

The TFA-labile Wang resin is a standard support for batch synthesis of peptide acids following the Fmoc-/tBu-protection scheme.^1,2 It is easily cleaved by treatment with 95% TFA. Special attention should be given to the attachment of the first amino acid residue to this benzyl alcohol type support because the activation step can lead to a substantial degree of racemization. For the incorporation of the C-terminal amino acids, the following coupling methods are recommended: carbodiimide/dimethyl aminopyridine (DMAP) in dichloromethane,³ 2,6-dichlorobenzoyl chloride/pyridine in dimethyl formamide (DMF),⁴ 1- hydroxybenzotriazole (HOBt)/N,N´-dicyclohexylcarbodiimide (DCC) in DMF (catalytic amounts of DMAP optional),⁵ 1-(2- mesitylenesulfonyl)-3-nitro-1H-1,2,4-triazole (MSNT) in the presence of N-methylimidazole,⁶ or the Mitsunobu coupling reaction.^7,8