Studies toward labeling cytisine with [11C]phosgene: rapid synthesis of a delta-lactam involving a new chemoselective lithiation-annulation method.

With the aim of the radiolabeling of cytisine, a potent agonist of nicotinic receptors, with [(11)C]phosgene, the rapid synthesis of a lactam model of our target has been studied. The key step of the delta-lactam formation is a new chemoselective lithiation-annulation method, under high dilution, of a suitable piperidinylcarbamoyl chloride. This precursor was obtained from (2-hydroxyethyl)piperidine in a linear synthetic sequence involving a Corey-Fuchs olefination of the corresponding aldehyde, followed by a selective reduction, using a diimide equivalent, of an iodoalkyne into a (Z)-iodopropene piperidine. This alkene served as main precursor to study the cyclization according to several procedures using phosgene as the required carbonylating reagent.