Synthesis and Biological Evaluation of 4-Purinylpyrrolidine Nucleosides

The synthesis of several novel carbocyclic purine nucleosides that incorporate a nitrogen in place of carbon 3 of the cyclopentyl moiety are described. These analogues are all derived from the key stereochemically defined intermediate N-(tert-butoxycarbonyl)-O-[(4-methoxyphenyl)diphenylmethyl]-trans-4-hydroxy-D-prolinol (19), which was accessible in 61.1% overall yield for a five-step sequence starting from cis-4-hydroxy-D-proline. The heterocyclic bases, 6-chloropurine and 2-amino-6-chloropurine, are efficiently introduced onto the pyrrolidine ring via a Mitsunobu-type coupling procedure with triphenylphosphine and diethyl azodicarboxylate. Standard transformations and removal of protecting groups gave the cis-adenine (26), hypoxanthine (27), 2,6-diaminopurine (28), and guanine (29) D-prolinol derivatives. In addition, a related sequence from trans-4-hydroxy-L-proline provided the enantiomeric L-prolinol guanine derivative (36). Lastly, the 6-(dimethylamino) purine analogue, 37, was coupled to N-(benzyl-oxycarbonyl)-p-methoxy-L-phenylalanine to provide, after deprotection, the novel puromycin-like analogue 39. The analogues 26-29, 36, and 39 were all evaluated for antitumor and, except for 39, for antiviral activity. These compounds failed to appreciably inhibit the growth of P388 mouse leukemia cells in vitro at concentrations up to 100 μg/mL. In addition, they did not exhibit noticeable activity against the human immunodeficiency virus or herpes simplex virus type 1 at concentrations as high as 100 μM. The adenine analogue, 26, did, however, prove to be a substrate for adenosine deaminase. It possessed an affinity for the enzyme only 50% less than that of adenosine with a K_i= 85 μM.