Solid Phase Synthesis of Peptide C-Terminal Thioesters by Fmoc/t-Bu Chemistry

Peptide C-terminal thioesters are key intermediates in a variety of applications, most notably the recently developed native chemical ligation methods for the total chemical synthesis of proteins. So far they have been prepared only by the use of the least prevalent Boc/benzyl solid-phase method on ad hoc prepared resin supports. We describe here a novel method for the solid phase synthesis of thioesters by the most prevalent Fmoc/t-Bu method. The method is based on the use of a 3-carboxypropanesulfonamide safety-catch linker, which is fully stable to repetitive exposure to the basic conditions needed for Fmoc cleavage. Activation with diazomethane or iodoacetonitrile followed by displacement with a suitable thiol produces the thioester in good to excellent yields. The method is also compatible with Boc/benzyl chemistry. Moreover, all the necessary reagents are commercially available.