Four-dimensional orthogonal solid-phase synthesis of new scaffolds based on cyclic tetra-β-peptides

A four-dimensional orthogonal protecting scheme that involves the acid-labile BAL linker in conjunction with Fmoc, Alloc and pNb protecting groups, which can be removed by β-elimination, allyl transfer, and reductive hydrolysis, respectively, allows the solid-phase preparation of scaffolds based on a cyclic tetra-β-peptide with free amino side chains ready for further elaboration.