clemizole clemizole clemizole is an h1 antagonist. clemizoleclinical dataatc codenoneidentifiers iupac name 1--2-(pyrrolidin-1-ylmethyl)benzimidazole cas number442-52-4pubchem cid2782chemspider2680uniit97cb3796lkeggd01705chebichebi:52140chemblchembl1407943niaid chemdb033090comptox dashboard (epa)dtxsid0046939 echa infocard100.006.486 chemical and physical dataformulac19h20cln3molar mass325.84 g·mol−13d model (jsmol)interactive image smiles clc1ccc(cc1)cn3c(nc2ccccc23)cn4cccc4 inchi inchi=1s/c19h20cln3/c20-16-9-7-15(8-10-16)13-23-18-6-2-1-5-17(18)21-19(23)14-22-11-3-4-12-22/h1-2,5-10h,3-4,11-14h2key:cjxaexpplwqrfr-uhfffaoysa-n synthesis benzimidazoles substituted with an alkylamine at position 2 have a venerable history as h1 antihistaminic agents. the standard starting material for many benzimidazoles consists of phenylenediamine, or its derivatives. clemizole synthesis: reaction of that compound with chloroacetic acid can be rationalized by invoking initial formation of the chloromethyl amide. imide formation with the remaining free amino group closes the ring to afford 2-chloromethyl benzimidazole (3). displacement of halogen with pyrrolidine affords the alkylation product. the proton on the fused imidazole nitrogen is then removed by reaction with sodium hydride. treatment of the resulting anion with α,4-dichlorotoluene gives the h1 antihistaminic agent clemizole (5).