Synthesis of Analogues of (−)-Cytisine for in Vivo Studies of Nicotinic Receptors Using Positron Emission Tomography

[formula: see text] 9-Substituted analogues of (-)-cytisine were synthesized in high yields via palladium-mediated couplings of either 9-(-)-bromocytisine and organostannanes or 9-(-)-trimethylstannylcytisine and fluorobromobenzene. The protection of the amine with a nitroso group and the use of PdCl2(PPh3)2 to carry out the Stille reaction allowed the rapid synthesis of 9-(4'-[18F]fluorophenyl)cytisine (18F: t1/2 = 109.7 min), a new promising radioligand (radiochemical yield: 10% from [18F]KF, 150 min, four steps) for positron emission tomography studies of alpha 4 beta 2 nicotinic receptors.