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Selectively S-Protected Cysteine Peptides. III. Synthesis of a [Ala12]-Sheep Insulin A-Chain Octapeptides with 6–11-Disulfide Ring

Studying the problems of the selective sulphur protection in cysteine peptides, a octapeptide A6–13 with 6–11-disulphide ring from the [Ala12]-sheep insulin A-chain was prepared by conventional synthesis using the S-Acm and S-Dpm group. The synthesis of the octapeptide Boc-Cys(Acm)-Cys(Dpm)-Ala-Gly-Val-Cys(Acm)-Ala-Leu-OH 1 was carried out with the fragments Boc-Cys(Acm)-Cys(Dpm)-Ala-Gly-OH 4 and HCl · H-Val-Cys(Acm)-Ala-Leu-OH 8 by means of DCCI/HOOBt coupling method. Fragments 4 and 8 were synthetisized step by step without carboxyl protecting group from the carboxyterminal end using the Boc-amino protecting group. Treatment of 1 with iodine in methanol gives formation of the octapeptide 2 with 6–11 – disulphide ring. The Dpm-thiol protecting group is completely stable during this step.

Mass spectra of S-Dpm and S-Acm protected compounds are discussed.