Radiolabeling and efficient synthesis of tritiated 2-chloro-N-6(3-iodobenzyl)adenosine-5'-N-methyluronamide, a potent, selective A(3) adenosine receptor agonist

We recently reported that 2-substitution of N-6-benzyladenosine-5'-uronamides greatly enhances selectivity of agonists for rat A, adenosine receptors (J. Med. Chem. 1994, 37, 3614-3621). Specifically, 2-Chloro-N-6-(3-iodobenzyl)adenosine-5'-N-methyluron-amide (2-Cl-IB-MECA), which displayed a K-l value of 0.33 nM, is the most selective for A(3) receptors yet reported with selectivity versus A(1) and A(2a) receptors of 2500- and 1400-fold, respectively. In order to obtain pharmacological tools for the study of A(3) adenosine receptors, two routes for radiolabeling of 2-Cl-IB-MECA through incorporation of tritium at the 5'-methylamido group were compared. One route formed a 2',3'-protected nucleoside 5'-carboxylic acid (9), which was condensed with methylamine and deprotected. The more efficient synthesis started from D-ribose and provided 2-Cl-IB-MECA (12) in six steps with an overall yield of 5.6%. Tritium was introduced in the penultimate step by heating N-6-(3-iodobenzyl)-2-chloro-2',3'-di-O-acetyl-5'-(methoxycarbonyl) adenosine (17) with [H-3]methylamine in methanol at 60 degrees C for 2 h. The specific activity of [H-3]2-Cl-IB-MECA was 29 Ci/mmol with a radiochemical purity of 99%.