Synthesis of Casein-Related Peptides and Phosphopeptides. X. A Modified Method for the Synthesis of Ser(P)-Containing Peptides Through 4-Bromobenzyl Phosphate Protection

Abstract

The three 4-halobenzyl phosphoramidite reagents di (4-fluorobenzyl) N,N- diisopropylphosphor-amidite, di (4-chlorobenzyl) N,N- diisopropylphosphoramidite and di (4-bromobenzyl) N,N-diethylphosphoramidite were prepared and used for the efficient phosphite-triester phosphorylation of isobutyl alcohol. While all three 4-halobenzyl groups were cleaved at similar rates from the 4-halobenzyl phosphorotriesters by 4 M HCl/dioxan or 50% CF₃CO₂H/CH₂Cl₂, the 4-bromobenzyl group had greater stability than either the 4-fluorobenzyl or 4-chlorobenzyl groups in formic acid or 1 M HCl /acetic acid solutions. The protected 4-bromobenzyl derivative, Boc-Ser(PO₃BrBzl₂)-OH, was prepared by a novel one-step procedure which featured di-4-bromobenzyl N,N- diethylphosphoramidite as phosphitylating agent. This derivative was used for the synthesis of Boc-Glu ( OBzl )-Ser(PO₃BrBzl₂)- Leu-OBzl by the Boc mode of peptide synthesis with 98% formic acid being used for the cleavage of the Boc group. Palladium-catalysed hydrogenolysis of the protected Ser(PO₃BrBzl₂) tripeptide in formic acid gave zwitterionic Glu -Ser(P)-Leu in high overall yield.