A NEW GENERAL SOLID-PHASE METHOD FOR THE SYNTHESIS OF BACKBONE-TO-BACKBONE CYCLIZED PEPTIDES

A model peptide with the sequences Ala-Pro-Lys(2ClZ)-Tyr(2BrZ) was syn thesized on a 4-methylbenzhydryl amine (MBHA) polystyrene resin using conventional Boc/benzyl protective group strategy. The amino acid alde hyde Boc-valinal was coupled by reductive alkylation with NaCNBH3 in a cidified DMF for 1 h. The secondary amine in the peptide-resin Boc-Val Psi[CH2NH]Ala-Pro-Lys(2ClZ)-Tyr(2BrZ)-MBHA was reductively alkylated by 3(4-methylbenzylthio)-propanal at 40 degrees C for 6 h, resulting t he peptide-resin BocVal CH2CH2-S-pMeBzl)]Ala-Pro-Lys(2ClZ)-Tyr(2BrZ)-M BHA. After the removal of the Boc group the synthesis was continued em ploying the above-mentioned methods, which led to the resin-bound pept ide Leu [CH2N(CH2CH2CH2S-pMeBzl)]Ser-Pro-Gly-Lys(2ClZ)-Val 2CH2CH2S-pM eBzl)]Ala-Pro-Lys(2ClZ)-Tyr(2BrZ)-MBHA. The peptide was cleaved from t he resin with hydrogen fluoride. Reversed-phase HPLC and plasma desorb tion mass spectrometry analysis showed that the expected peptide Leu P si[CH2N(CH2CH2CH2SH)]Ser-Pro-Gly-Lys-Val Psi[CH2N(CH2CH2CH2-SH)]Ala-Pr o-Lys-Tyr-NH2 was obtained as the major product with low levels of sid e products. Intramolecular oxidation of the thiols gave the backbone t o backbone cyclized peptide Leu Psi[CH2N(CH2CH2CH2S)]Ser-Pro-Gly-Lys-V al Psi[CH2N(CH2CH2CH2-S)]Ala-Pro-Lys-Tyr-NH2. (C) Munksgaard 1994.