Isoacyl dipeptides are remarkable new tools for enhancing synthetic efficiency in Fmoc SPPS that consist of a Boc-protected serine or threonine derivative in which the hydroxyl group is acylated by an Fmoc-amino acid. They offer the same benefits as pseudoproline dipeptides, but with the added advantage that the depsipeptides obtained directly from the TFA cleavage reaction are often markedly more soluble than the native target sequence. This property allows insoluble aggregated sequences such as β-amyloid to be purified in the soluble isoacyl form prior to conversion to the insoluble native form. Isoacyl dipeptides can also be used in the C-terminal dipeptide sequence of protected peptide fragments to prevent epimerization during fragment coupling. Isoacyl dipeptides are best coupled using DIPCDI/HOBt in DCM.