AbstractA comparative study has been undertaken between Hmb‐protected amino acid and pseudoproline building block analogues for use in the solid phase synthesis of ‘difficult’ peptides. Both of these derivatives act by blocking inter‐ and intramolecular hydrogen bonding, which has been shown to be a major cause of poor synthesis/quality/efficiency. While the two were shown to result in substantial improvements in the purity of crude peptides, pseudoproline incoporation was found to be superior to Hmb backbone protection. This was due to slow and incomplete coupling of the amino acid immediately following the Hmb amino acid. Copyright © 1999 European Peptide Society and John Wiley&S