Abstract
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.