To efficiently develop bioactive ligands even if

To efficiently develop bioactive ligands even if the structural information of the target protein is unknown, we have presented a three-dimensional structural diversity-oriented strategy based on structural properties of a chiral cyclopropane. Our strategy has allowed us to design and synthesize a series of cyclopropane-based conformationally restricted histamine analogues having imidazolylcyclopropane scaffold. Through the studies, we successfully identified a selective and potent antagonist for the H receptor (HR) (H,  = 5.3 nM; H,  = 127 nM) and a non-selective potent antagonist for the HR and the HR (H,  = 8.4 nM; H,  = 7.6 nM) shown in . However, as for HR selectivity, even the most potent HR-selective prostanoid receptors in the series of the imidazolylcyclopropane derivatives, the selectivity and potency were moderate (H,  > 1,000 nM; H,  = 118 nM). Thus, in this study, we aimed to convert the potent H/H dual antagonist and the moderate HR-selective antagonist into a highly HR-selective ligand. The indole/benzimidazole-piperazine derivatives have two nitrogen-containing ring moieties, an indole/benzimidazole () and piperazine (), in their structures (). We thought that these moieties, or , might make the molecules highly HR-selective. The imidazolylcyclopropane derivatives also have two nitrogen-containing moieties, an imidazole () and benzylamine (), which correspond to the or . Thus, we have designed cyclopropane derivatives – as H selective antagonists, which have the or moiety instead of the or in the imidazolylcyclopropane derivatives (). This molecular design might improve the HR selectivity of cyclopropane-based derivatives to give an insight into the binding modes of the cyclopropane-based compounds and the indole/benzimidazole-piperazine derivatives to the HR. All of the designed compounds – was synthesized from -cyclopropane aldehyde , which was prepared following a procedure we reported previously. Treatment of with 4-chlorobenzene-1,2-diamine in pyridine constructed benzimidazole structure of , which was subjected to deprotection and subsequent oxidation conditions to give aldehyde (). Reductive amination of with 4-chlorobenzylamine afforded . Wittig reaction of using MeOCHPPhCl and NaHMDS, followed by acidic hydrolysis and subsequent reductive amination provided . Indole derivatives and were synthesized using a method we reported. Reductive amination of with 4-chlorobenzylamine and subsequent protecting group manipulation gave (). Stepwise Dess–Martin and Pinnick oxidations of and the subsequent conversion to the corresponding acid azides, followed by Curtius rearrangement provided . After removal of the Fmoc group of , a piperazine structure was constructed with ditriflate to afford , which was treated with PhSH to give . Removal of the Boc group of gave . -methylation of using formaldehyde and subsequent deprotection of the amino group provided . Wittig reaction of with MeOCHPPhCl/NaHMDS, followed by acidic treatment gave the corresponding one-carbon elongated aldehyde, which was subjected to the same procedure described above produced and . Treatment of , which was prepared from , with benzene-1,2-diamine in pyridine to construct a benzimidazole structure and subsequent removal of the trityl group afforded (). Reductive amination of with 2-benzimidazolylmethylamine, which was prepared from benzene-1,2-diamine and glycine, followed by acidic hydrolysis gave . Binding affinities of – for the human HR subtype with [H]-methylhistamine and the human HR subtype with [H]histamine were evaluated using cell membranes expressing the human HR or HR according to the reported method (). Compounds –, which have an indole, benzimidazole, or piperazine structure instead of the imidazole of parent compounds and , failed to show any significant binding to both the HR and HR ( > 1,000 nM). These results indicate that the bulkier heterocycles, indole and benzimidazole, than imidazole might cause a steric repulsion in the binding to the HR and HR. The non-planar six-membered ring structure, piperazine, also might be sterically repulsive in the binding and might not be able to form appropriate hydrogen-bonds between the receptors due to the different position of nitrogen from that of imidazole. These results suggest that, when a series of -cyclopropane-based histamine analogues bind to the HR or HR, the space accommodating the imidazole moiety seems to be limited.