All of the designed compounds was synth

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 AL 8810 synthesis 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. On the other hand, imidazolylcyclopropane derivatives and having a 2-substituted benzimidazolyl group showed moderate binding affinities for both the HR and HR. Replacement of 4-chlororbenzylamino group of with a benzimidazolyl group () led to increase the affinity for the HR more than 5-fold ( = 186 nM for ; >1,000 nM for ), whereas the replacement did not significantly affect the HR affinity ( = 295 nM for ; 118 nM for ). Notably, the values of , which have a benzimidazolyl group instead of 4-chlorophenyl group of , to the HR (52 nM) and the HR (166 nM) were comparable to those of a well-known non-selective H/H antagonist, thioperamide (H, 51 nM; H, 124 nM; its structure is shown in ), although their values were bigger than those of (H, 8.4 nM; H, 7.6 nM). The indole/benzimidazole-piperazine derivatives and are the highly HR selective antagonists; however, this series of -cyclopropane derivatives – with an indole, benzimidazole, or piperazine structure did not show the HR selectivity. In summary, to develop newly HR selective ligands, we hybridized our cyclopropane-based conformationally restricted histamine analogues with indole/benzimidazole-piperazine derivatives, which are representative H selective antagonists. Whereas most of the synthesized derivatives showed no binding affinities for both the HR and HR, compound showed the H/H binding affinities comparable to thioperamide. The present study suggests that the binding modes of the cyclopropane-based ligands in the HR might be entirely different from those of the indole/benzimidazole-piperazine derivatives.