Insect neuropeptide antagonist. Part II. Synthesis and biological activity of backbone cyclic and precyclic PBAN antagonists

A new approach for the design and synthesis of pheromone biosynthesis activating neuropeptide (PBAN) agonists and antagonists using the backbone cyclization and cycloscan concepts is described. Two backbone cyclic (BBC) libraries were synthesized: library I (Ser library) was based on the active C-terminal hexapeptide sequence Tyr-Phe-Ser-Pro-Arg-Leu-NH₂ of PBAN1-33NH₂; whereas library II (D-Phe library) was based on the sequence of the PBAN lead linear antagonist Arg-Tyr-Phe-D-Phe-Pro-Arg-Leu-NH₂. In both libraries the Pro residue was replaced by the BBC building unit N^α-(ω-aminoalkyl) Gly having various lengths of alkyl chain. The peptides of the two libraries were tested for agonistic and antagonistic activity. Four precyclic peptides based on two of the BBC antagonists were also synthesized; their activity revealed that a negative charge at the N-terminus of the peptide abolished antagonistic activity. We also describe the use of the reagent SiCl₃l for selective deprotection of the Boc group from the building unit prior to on-resin amino-end to backbone-nitrogen (AE-BN) cyclization, during solid-phase synthesis with Fmoc chemistry.