5-Azidoimidacloprid and an Acyclic Analogue as Candidate Photoaffinity Probes for Mammalian and Insect Nicotinic Acetylcholine Receptors

The 5-azido analogue of the major insecticide imidacloprid, 1-(5-azido-6-chloropyridin-3-ylmethyl)-2-nitroiminoimidazolidine (<BO>1</BO>), and an acyclic analogue, N-(5-azido-6-chloropyridin-3-ylmethyl)-N‘-methyl-N‘ ‘-nitroguanidine (<BO>2</BO>), were prepared in good yields as candidate photoaffinity probes for mammalian and insect nicotinic acetylcholine receptors (nAChRs). The essential intermediate was 5-azido-6-chloropyridin-3-ylmethyl chloride (<BO>3</BO>) prepared in two ways:  from 6-chloro-5-nitronicotinic acid by selective reduction and then diazotization, and from N-(6-chloropyridin-3-ylmethyl)morpholine by an electrophilic azide introduction with lithium diisopropylamide followed by chlorine substitution of morpholine with ethyl chloroformate. Coupling of <BO>3</BO> with 2-nitroiminoimidazolidine gave <BO>1</BO>. Conversion of <BO>3</BO> to <BO>2</BO> was achieved in good yields via the hexahydrotriazine intermediate <BO>14</BO>. Fortuitously, the azido substituent in <BO>1</BO> and <BO>2</BO> increases the affinity 7−79-fold for rat brain and recombinant α4β2 nAChRs (K<INF>i</INF>s 4.4−60 nM competing with [³H](−)-nicotine) while maintaining high potency on both insect nAChRs (Drosophila and Myzus) (K<INF>i</INF>s 1−15 nM competing with [³H]imidacloprid). Azidopyridinyl compounds <BO>1</BO> and <BO>2</BO> are therefore candidate photoaffinity probes for characterization of both mammalian and insect receptors.