Preparation of partially protected des-alanineB30 -insulin-B-chain-disulphide and its use in semisynthesis

The preparation of Nα1N29‐bis‐methylsulphonylethyloxycarbonyl‐des‐alanineB30‐B‐chain‐disulphide‐OΛ13OΛ21‐dimethyl ester is described. Starting with the di‐S‐sulphonate‐B‐chain it was prepared by i) removal of the C‐terminal amino acid by digestion with carboxypeptidase A, ii) reduction and oxidation to form the cyclic disulphide, iii) esterification of the three carboxyl groups (αCOOH = LysB29, ΛCOOH = GluB13 and GluB21) followed by selective tryptic hydrolysis of the α‐carboxyl ester, and iv) the protection of both amino groups (αNH2= PheB1, NH2= LysB29). This B‐chain derivative was activated selectively at the C‐terminal carboxyl group by formation of a mixed anhydride, and condensed with the dipeptide Thr‐Arg. All protecting groups were removed by treatment with alkali. The human‐B‐chain C‐terminal elongated with Arg was obtained in a yield of 30% based on the protected des‐AlaB30‐B‐chain derivative.