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Solid phase synthesis of cyclic peptides: model studies involving i-(i+4) side chain-to-side chain cyclisation.

Conditions for the synthesis of i-(i + 4) side chain-to-side chain head-to-tail Lys-->Glu and Glu-->Lys linked cyclic peptides related to hypoglycaemic analogues of human growth hormone hGH [6-13] have been examined. The success of the cyclisation reaction with the corresponding resin-bound, partially protected linear peptides was found to be both reagent as well as sequence dependent, with competing inter-chain oligomerisation predominating in some cases. The results also indicated that protection with the bulky Fmoc group of the amino acid residues immediately adjacent to the side chain-deprotected Lys and Glu residues, which participate in the cyclisation reaction, enhanced the rate of lactam formation.