Native Chemical Ligation (NCL) is a powerful method for chemical protein synthesis. [1] NCL requires the coupling of peptide-^αthioester and Cys-peptide which has Cys residue at its N-terminus. It is known that Cys residue is relatively rare amino acid residue in naturally occurring protein, therefore several methods have been developed to overcome the site limitation of NCL. Desulfurization strategy is one of the robust approach of these, which originally can convert Cys to Ala residue. [2] Recently, the strategy is expanded to the other amino acid residues by using chemically prepared β-mercapto amino acid derivatives. However, in order to use these approaches, we have to prepare β-mercapto amino acid derivatives which usually require precise synthesis comprising several stereoselective reactions to suppress the epimerization of α position.

Here, we introduce a new concise synthesis of β-mercapto-phenylalanine derivative which contains optical resolution step by D- and L- acylase that can de-acetylate D-amino acid or L-amino acid respectively. In order to demonstrate our new synthesis, we have synthesized N-acetyl-β-mercato-phenylalanine derivative from racemic phenylalanine (namely D and L- phenylalanine) in 7 steps. The obtained β-mercapto-D,L- phenylalanine derivative was then subjected to the optical resolution by D- or L-acylase. As a result, each enzymatic reaction successfully afforded deacetylated product in ca. 50 % yield from racemic derivative. Furthermore, we could convert the product into N-Boc protected amino acid derivative which could be used for solid phase peptide synthesis. This new synthetic strategy takes only 10 steps from phenylalanine and it is noteworthy that each reaction does not require stereoselective reactions. We have already succeeded in the synthesis ofβ-mercapto-tyrosine derivative also by this strategy. In this presentation, we would like to introduce this new concise synthesis ofβ-mercapto-amino acid derivatives and discuss in detail.