BF3 etherate catalysed formation of [11C]methyl esters:: a novel radiolabeffing technique

A novel way of preparing C-11 labelled methyl esters using [C-11]methanol and either BF3 etherate or trimethylsilyl chloride as catalyst was investigated. Radiochemical yields with BF3 etherate were between 30 and 33% for [C-11]methyl benzoate and less than 1% for [C-11]methyl thio salicylate. No [C-11]methyl ester formation could be observed with trimethylsilyl chloride for all compounds investigated. This method is an alternative to using [C-11]methyl iodide in the presence of a base. It is particularly suited for carboxylic acids bearing functional groups which would compete for [C-11]methyl iodide, thus eliminating the need to introduce protecting groups. However, o-anisic acid formed [C-11]methyl salicylate in 33-30% decay corrected radiochemical yield due to hydrolytic cleavage of the methyl ether, and none of the desired [C-11]methyl 2-methoxy benzoate could be obtained. When salicylic acid was used as starting material, [C-11]methyl salicylate could only be obtained in 5-8% decay corrected radiochemical yield. Copyright (C) 2004 John Wiley Sons, Ltd.