INFLUENCE OF SOLID SUPPORT, SOLVENT AND COUPLING REAGENT ON THE HEAD-TO-TAIL CYCLIZATION OF RESIN-BOUND PEPTIDES

Head-to-tail cyclization of peptides attached to insoluble supports by means of the side chains of aspartic acid or glutamic acid allows the rapid synthesis of cyclic peptides of widely varied amino acid sequen ce. However; two side reactions dramatically reduce the yield of the d esired compound; these are (1) interpeptide condensation that creates dinners, trimers and higher-order oligomers and (2) racemization of th e C-terminnl residue. A comparison was made between benzo-triazoyloxy- tris-(dimethyl-amino)phosphonium hexafluorophosphate (BOP) and diisopr opylcarbodiimide (DIPCDI) as cyclization reagents on kieselguhr-suppor ted polydimethylacrylamide, chloromethyl, PolyHipe, and two commercial ly available polystyrene supports using DMF, CH2Cl2 and THF as cycliza tion solvents. Both of the side reactions are dependent on the amino a cid sequence, the reagent used to couple the ends of the peptide and t he solid support.