PubMed · 9737324
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K Weerasuriya. 1998-09-05. Mibefradil: the sole exception.. https://doi.org/10.1016/s0140-6736(05)60724-6
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2-Ferrocenyl- and 2-phenyl-N-n-butylbenzimidazoles were synthesized to evaluate the influence of the benzimidazole functional group upon their directed lithiation. The regiochemistry of lithiation was studied, as well as the effect of stabilization of the lithiated species by diamine coordination using tetramethyl- ethylenediamine and (-)-sparteine. The lithiations were followed by reaction with a variety of electrophiles to give the disubstituted 2-ferrocenyl- and 2-phenyl-N-n-butylbenzimidazoles compounds. This study showed that despite a simple n-butyl function on the benzimidazole, directed lithiation was readily achieved with high regiocontrol on the ferrocenyl and phenyl groups. (-)-Sparteine failed to provide asymmetric induction in the ferrocene system, and its inefficiency is explained by intramolecular coordination of the lithiated species by the benzimidazole nitrogen, which is preferred over sparteine coordination.
An effective route to benzimidazo[2,1-b]quinazolin-12(5H)-ones from commercially available o-aryl isothiocyanate esters and o-phenylenediamines is reported. This method accommodates a variety of substituents on either starting material and proceeds under microwave irradiation in the presence of barium hydroxide, conditions that do not hydrolyze methyl ester substituents. The pharmacologically pertinent benzimidazoquinazolinone heterocycle is delivered in excellent yield and purity via both solution- and solid-phase protocols, the latter involving traceless release from the resin.
The key step in the synthesis of new five, six and seven-membered alicyclic ring [1,2-a]-fused bioreductive benzimidazolequinones was radical cyclisation. Six and seven-membered tributyltin hydride-mediated homolytic aromatic substitutions of nucleophilic N-alkyl radicals onto the benzimidazole-2-position occurred in high yields (63-70 %) when quaternising the pyridine-like 3-N of imidazole with camphorsulfonic acid and using large excesses of the azo-initiator, 1,1'-azobis(cyclohexanecarbonitrile), to supplement the non-chain reaction. Elaboration of benzimidazoles to the benzimidazolequinones occurred in excellent yields. The IC50 values for the cytotoxicity of benzimidazolequinones towards the human skin fibroblast cell line GM00637 were in the nanomolar range, as determined by using the MTT assay. The benzimidazolequinones were much more cytotoxic than indolequinone analogues. 1,2,3,4-Tetrahydropyrido[1,2-a]benzimidazole-6,9-dione was the most potent compound prepared being more than 300 times more cytotoxic than the clinically used bioreductive drug, mitomycin C. The latter benzimidazolequinone was more potent under hypoxic conditions (associated with solid tumors), being 4.4 times more cytotoxic than under aerobic conditions, while mitomycin C was 1.8 times more selective towards hypoxia. The cyclopropane fused pyrido[1,2-a]benzimidazolequinone, 1a,2,3,9b-tetrahydro-1H-cyclopropa[3,4]pyrido[1,2-a]benzimidazole-5,8-dione was less cytotoxic and selective than the five-membered ring analogue, 1,1a,8,8a-tetrahydrocyclopropa[3,4]pyrrolo[1,2-a]benzimidazole-3,6-dione. Modifying the structure of the most potent pyrido[1,2-a]benzimidazolequinone by attaching methyl substituents onto the quinone moiety increased reductive potentials and decreased cytotoxicity and selectivity towards hypoxia.