Peptide Stapling with Anion-π Catalysts.

We report design, synthesis and evaluation of a series of naphthalenediimides (NDIs) that are bridged with short peptides. Reminiscent of peptide stapling technologies, the macrocycles are conveniently accessible by a chromogenic nucleophilic aromatic substitution of two bromides in the NDI core with two thiols from cysteine sidechains. The dimension of core-bridged NDIs matches that of one turn of an alpha helix. NDI-stapled peptides exist as two, often separable atropisomers. Introduction of tertiary amine bases in amino-acid sidechains above the pi-acidic NDI surface affords operational anion-pi catalysts. According to an enolate chemistry benchmark reaction, anion-pi catalysis next to peptides occurs with record chemoselectivity but weak enantioselectivity. Catalytic activity drops with increasing distance of the amine base to the NDI surface, looser homocysteine bridges, mismatched, shortened and elongated alpha-helix turns, and acyclic peptide controls. Elongation of isolated turns into short alpha helices significantly increases activity. This increase is consistent with remote control of anion-pi catalysis from the alpha-helix macrodipole.