Biomimetic studies using artificial system. IV. Biomimetic peptide synthesis by using multi-functionalized crown ethers as a novel enzyme model. A new concepts in mimicking of enzyme-catalyzed bond-forming reactions.

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Abstract
A novel approach to the mimicking of enzyme-catalyze bond-forming reactions has been examined using multifunctionalized chiral crown ethers. In addition to the 18-crown-6 moiety as a binding site, the hosts have one thiol and one thio ester with an N-protected .alpha.-amino acid or a peptide, and have successfully achieved peptide synthesis in an enzyme-mimetic reaction mode. This new method involves the following three key reactions. (1) Intra-complex thiolysis: the host carries out the rapid intra-complex thiolysis of .alpha.-amino thiolysis of .alpha.-amino acid ester salts to form the dithioester, corresponding to the assembly of two guests by the host. (2)Amide formation: intramolecular aminolysis occurs between the bound guests to form the amide bond. (3) Peptide chain elongation: as the thiol reactive group is regenerated, the above two reactions are repeated to elongate the peptide chain. Formal turnover of the enzyme model has been demonstrated by the synthesis of a tetrapeptide derivative by the repetition of the above processes.