2-(N'-alkylidenehydrazino)adenosines: potent and selective coronary vasodilators

The reaction of aliphatic aldehydes and ketones with 2-hydrazinoadenosine under relatively mild conditions (at room temperature or in refluxing methanol) formed 2-(N'-alkylidenehydrazino)-adenosines, 5-22, in good yields. Two kinds of adenosine receptors regulate cardiac and coronary physiology. In supraventricular tissues an A1AR coupled to muscarinic K channels mediates the negative chronotropic, dromotropic, and inotropic actions of adenosine, and an inhibitory A1AR coupled to adenylate cyclase mediates the "antiadrenergic" action of adenosine. One or more kinds of A2 receptors mediate coronary vasodilation. Bioassays employing a guinea pig heart Langendorff preparation showed that 5-22 weakly retard impulse conduction through the AV node (negative dromotropic effect), but several analogues were very active coronary vasodilators. The coronary vasoactivity of the (n-alkylidene- and of the (isoalkylidenehydrazino)adenosines paralleled the length of the alkyl chain, the EC50s of the of the most active n-pentylidene (8) and isopentylidene (18) congeners being 1 nM. The EC50s of the cyclohexylmethylene (9), cyclohexylethylidene (10), and cyclohex-3-enylmethylene (12), analogues were likewise < 1 nM, but the cyclohex-1-enylmethylene congener 12 was 10 times less active than 9. The unselective adenosine receptor antagonist 8-(p-sulfophenyl)theophylline (0.1 mM) raised the EC50s of the negative dromotropic effects of 8, 9, and 18 by 5-28-fold and the EC50s of coronary vasodilation of 22-90-fold. Catalytic reduction of 9 increased the hydrophobicity and changed the UV spectrum, suggesting reduction of the --CH = N-- bond. The product darkened on exposure to air and so was not characterized further. A new method for preparing 2',3',5'-tri-O-acetyl-2,6-dichloropurine riboside, a precursor in the synthesis of 2-hydrazinoadenosine, consists of the addition of tert-butyl nitrite to a mixture of 2',3',5'-tri-O-acetyl-6-chloroguanosine and CuCl in CHCl3 saturated with Cl2.