Solid-phase peptide synthesis without side-chain hydroxyl protection of threonine

A peptide containing four threonine residues was synthesised by the solid-phase method using fluorenyl-methoxycarbonylamino acid reactive esters or coupling by preactivation with 1-hydroxybenzotriazole and Castro's reagent. In two separate experiments the synthesis was carried out with or without protection of the side-chain hydroxyl group of threonine as the tert-butyl ether. Comparison of the crude peptides after deprotection and detachment from the synthesis resin suggests that side-chain protection of threonine is unnecessary under the synthetic conditions employed.