Improved solid phase synthesis of peptide carboxyamidomethyl (Cam) esters for enzymatic segment condensation

Abstract

Peptide C-terminal carboxyamidomethyl (Cam-)esters are pivotal building blocks for enzymatic segment condensation and their yield and purity are crucial for the overall efficiency of this strategy. Although a few methods for their preparation have been disclosed, the solid phase synthesis of peptide C-terminal Cam-esters is not straightforward. Herein, we describe two novel method types for their synthesis in high yield and good purity. The first type is based on the coupling of hydroxyl protected glycolic acid to a solid support, followed by ester synthesis using an N-protected amino acid and dicyclohexyl carbodiimide with catalytic 4-dimethylaminopyridine. The second type is based on the synthesis of amino acid carboxymethyl ester building blocks, which are coupled to the solid support using standard coupling reagents and procedures. The latter procedure is easily implemented in peptide synthesizer protocols and applicable to all standard Fmoc-protected amino acid building blocks.