Labelling of Carbaboranyl Compounds with a Selenium Atom with a View to Applications in Boron‐Neutron‐Capture Therapy (BNCT) and Positron‐Emission Tomography (PET)

We synthesized 2′‐carbaboranyl‐2,5′‐bi‐1H‐benzimidazoles containing 10 B‐atoms and labeled with Se or the positron‐emitting radionuclide ⁷³Se (t_1/2=7.1 h), with a view to their application to cancer treatment by boron‐neutron‐capture therapy (BNCT) and to compound‐distribution measurements in vivo by positron‐emission tomography (PET). Thus, 2,2′‐{{2′‐{4‐[1,2‐dicarba‐closo‐dodecaboran(12)‐2‐ylmethoxy]phenyl}‐[2,5′‐bi‐1H‐benzimidazol]‐5‐yl}imino}bis[ethanol] ( 26c ) was obtained by the reaction of 2,2′‐[(3,4‐diaminophenyl)imino]bis[ethanol] ( 19 ) with ethyl 2‐{4‐[1,2‐dicarba‐closo‐dodecaboran(12)‐2‐ylmethoxy]phenyl}‐1H‐benzimidazole‐5‐carboximidate hydrochloride ( 25 ), as well as the analogues 26a and 26b (Scheme 6). Tosylation of compound 26c gave 4 regioisomers 27a – d , which, after selenation, produced 2′‐{4‐[1,2‐dicarba‐closo‐dodecaboran(12)‐2‐ylmethoxy]phenyl}‐5‐(tetrahydro‐2H‐1,4‐selenazin‐4‐yl)‐2,5′‐bi‐1H‐benzimidazole ( 29 ) in 42% yield (Scheme 7).