Liquid Phase Peptide Synthesis by the Fragment Condensation on Soluble Polymer Support. I. Efficient Coupling and Relative Reactivity of a Peptide Fragment with Various Coupling Reagents

Abstract

Abstract The fragment condensation on the soluble polymer support using dicyclohexylcarbodiimide (DCCI) plus 1-hydroxy-1H-benzotriazole (HOBt), diethoxyphosphoryl cyanide (DEPC), and triphenylphosphine (Ph3P) plus di-2-pyridyl disulfide ((PyS)2) as coupling reagents gave rise to efficient coupling (86–99% yield) of the peptide fragment, t-butyloxycarbonyl-l-O-benzyl-tyrosylglycylglycyl-l-phenylalanine [Boc-l-Tyr(Bzl)–Gly–Gly–l-Phe–OH], to the amino free terminal of Leu anchored to the soluble polymer support, H–l-Leu–OCH2–resin. Relative reactivity ratios of Boc–amino acids and Boc–l-Tyr(Bzl)–Gly–Gly-l-Phe–OH with H–l-Leu–OCH2–resin in the coupling reaction using the three coupling reagent systems could be easily determined by use of amino acid ratios in the acid hydrolysates of the resulting peptide resins in which they were taken. The peptide chain length of Boc–l-Tyr(Bzl)–Gly–Gly–l-Phe–OH has no influence on the reactivity of C-terminal amino acid in the coupling reaction using DCCI plus HOBt or DEPC as a coupling reagent when Boc–l-Phe–OH was replaced with Boc–l-Tyr(Bzl)–Gly–Gly–l-Phe–OH. The coupling reagent, Ph3P plus (PyS)2, brought about a marked decrease in the reactivity of the fragment peptide acid.