Minimal side-chain protection can be a successful strategy in solid-phase peptide synthesis

The N-terminal tyrosine residue of Met-enkephalin could be readily incorporated without protection of its phenolic hydroxylgroup. Furthermore, the HF-cleaved product contained fewer impurities than that derived from hydroxyl-protected material. Despite the presence of Tyr in the center of the chain, an LH-RH antagonist, [D-Phe2, D-Trp3, D-Phe6]-LH-RH, could also be made in normal yield by incorporation of free Boc-Tyr. Syntheses of the same model peptide without protection of the Ser residue and protection of the Arg residue as the guanidine HCl salt also gave excellent yields of analog. Finally, the LH-RH inhibitor and a highly active agonist, [D-Leu6, desGly-NH2(10)]-LH-RH ethylamide, were synthesized without protection of Tyr, Ser and Arg, which enabled free peptides to be generated directly by ammonolysis and ethylaminolysis, respectively, without HF treatment. In all examples, no evidence emerged to suggest reaction of side-chains during synthesis.