Solid-phase synthesis of an Htc-containing dimer analog of the autophosphorylation site of pp60(src) PTK: Effective acylation conditions for Htc residues

The difficulty during SPPS in acylating the secondary amino group of Htc, a locally constrained tyrosine, can be correlated with the steric hindrance of the amino acid or with the conformation of the growing peptide chain. Our experimental data indicate that the availability of the Htc amino group is associated with its steric hindrance rather than a conformational effect of the peptide chain. An optimized solid phase automated protocol for Htc is reported. Under optimal conditions, Fmoc-amino acids with hindered side chains were incorporated in approximately 99% yield using HATU as coupling reagent. Unhindered side chain amino acid acylated the secondary amino group of Htc in good yield under classical HBTU/HOBt coupling conditions.