Peptide N-alkylamides by solid phase synthesis*

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Abstract
Three new resins have been developed that allow for the solid phase synthesis of C-terminal peptide N-alkylamides using Boc amino acids, usual side chain protecting groups and hydrogen fluoride cleavage and deprotection. These resins were prepared by reacting the appropriate alkylamine (NH2 CH3, NH2 CH2 CH3, NH2 CH2 CF3) to Merrifield's 1% divinylbenzene cross-linked chloromethylated polystyrene resin. The application of these resins to the synthesis of C-terminal GnRH N-alkylamides illustrates the versatility of this approach. GnRH analogs were tested for their ability to release LH from cultured rat anterior pituitary cells. [D Glu6, Pro9-NHCH2 CH3]-GnRH was synthesized for the first time using the solid phase approach and found to be three times more potent than [D Glu6]-GnRH. Other analogs including [D Trp6, Pro9-NHCH2 CH3]-GnRH, [D Ala6, Pro9-NHCH2 CF3]-GnRH and related peptides were found to be equipotent and to have the same properties (HPLC retention times, amino acid analysis and specific rotation) as the corresponding peptides synthesized using less amenable strategies; yields were equivalent or better than those reported earlier.