[6-chloro-3-pyridylmethyl-3H] neonicotinoids as high-affinity radioligands for the nicotinic acetylcholine receptor: Preparation using NaB3H4 and LiB3H4

NaB3H4 and LiB3H4 at 78% and 97% isotopic enrichments, respectively, were used in the synthesis of 3H-labeled 1-(6-chloro-3-pyridyl)-methyl-2-nitromethyleneimidazolidine (CH-IMI) and N′-[(6-chloro-3-pyridyl)methyl]-N″-cyano-N′-methylacetamidine (acetamiprid) (two very potent insecticides) and of 1-(6-chloro-3-pyridyl)methyl-2-iminoimidazolidine (desnitro-IMI) (a metabolite of the commercial insecticide imidacloprid). 6-Chloronicotinoyl chloride was treated with either NaB3H4 in methanol or LiB3H4 in tetrahydrofuran and the resulting alcohol transformed to 2-chloro-5-chloromethylpyridine, which was then coupled to N-cyano-N′-methylacetamidine to give [3H]acetamiprid (45 Ci/mmol). 2-Chloro-5-chloro[3H]methylpyridine was also reacted with ethylenediamine and the product was either refluxed in absolute ethanol with 1,1-bis(methylthio)-2-nitro-ethylene to provide [3H]CH-IMI or reacted in toluene with a solution of cyanogen bromide to produce [3H]desnitro-IMI (each 55 Ci/mmol).