A Lewis-acid-catalyzed method for the substrate-directed formation of peptide bonds has been developed, and this powerful approach is utilized for the new "remote" activation of carboxyl groups under solvent-free conditions. The presented method has the following advantages: 1) the high-yielding peptidesynthesis uses a tantalum catalyst for any amino acids; 2) the reaction proceeds without any racemization;
When dimethylmethylthiosulfonium triflate (DMTST) is added to a tert-butyl-protected thiol (RSt-Bu), the unsymmetrical disulfide RSSMe is formed. At this point the free thiol (RSH) can be obtained by the addition of trimethylphosphine and water. Most byproducts are small organic molecules that are easily removed.