Synthesis of Casein-Related Peptides and Phosphopeptides. I. Solution-Phase Synthesis and 13C N.M.R.-Spectroscopy of the N-α-Acetyl Octapeptide N-Methylamide Corresponding to Region 14-21 of Bovine β-Casein A2

The octapeptide, Ac-Glu-Ser-Leu-Ser-Ser-Ser-Glu-Glu-NHMe (I), was synthesized by the solution-phase method by using the mixed anhydride coupling procedure for the fragment condensation of the Nα-acetyl tripeptide, Ac-Glu(OBut)-Ser(But)-Leu-OH, with the pentapeptide N-methylamide hydrochloride, Cl.H2-Ser(But)-Ser(But)-Ser(But)-Glu(OBzl)-Glu(OBzl)-NHMe, followed by palladium-catalysed hydrogenolysis of Ac-Glu(OBut)-Ser(But)-Leu-Ser(But)-Ser(But)- Ser(But) Glu(OBzl)Glu(OBzl NHMe in trifluoroacetic acid. The synthesis of the two peptide fragments was accomplished in high yields and purity by using the repetitive excess mixed anhydride procedure and the isobutoxycarbonyl mixed anhydride of acetic acid for the rapid and high yielding N-acetylation of the tripeptide fragment. 13C n.m.r. spectroscopy was routinely used to monitor the efficiency of the coupling steps and to confirm the structure of octapeptide (1), signal assignments being possible for both the protected tri- and penta-peptides.