Synthesis of asymmetric [ 75 Se]selenoethers via carbodiimides

A convenient radiosynthesis of asymmetric [ 75 Se]selenoethers was developed using 1,3‐disubstituted [ 75 Se]selenoureas as intermediates. These were prepared from appropriate carbodiimides and hydrogen [ 75 Se]selenide, which could only be generated from carrier‐added (c.a.) [ 75 Se]selenite in aqueous solution using phosphinic acid as reducing agent. Optimization of this initial labelling step with dicyclohexylcarbodiimide and polymeric N ‐cyclohexylcarbodiimide‐ N ′‐methyl polystyrene resulted in radiochemical yields (RCY) of 73 and 55% (bound on the polymer), respectively, within 45 min. Treatment of [ 75 Se]selenoureas with alkylbromides led to corresponding [ 75 Se]selenouronium salts in nearly quantitative yields. Hydrolysis under basic conditions provided the [ 75 Se]selenolates and a second alkylation yielded asymmetric [ 75 Se]selenoethers. Thus, within 90 min benzylmethyl[ 75 Se]selenide, benzylbutyl[ 75 Se]selenide, benzylisopropyl[ 75 Se]selenide and 1‐phenyl‐l‐(methyl[ 75 Se]seleno)ethane were synthesized with respective RCY of about 59, 55, 10 and 60%. Furthermore, the 75 Se‐labelled alkylating agent 3‐(methyl[ 75 Se]seleno)‐1‐propanyl p ‐toluenesulfonate and [ 75 Se]selenomethionine were obtained with radiochemical yields of 51 and 41%, respectively. Copyright © 2001 John Wiley & Sons, Ltd.