Nucleosides and Nucleotides. 184. Synthesis and Conformational Investigation of Anti-Fixed 3-Deaza-3-halopurine Ribonucleosides1,2

A versatile synthetic route to 3-deaza-3-haloinosines 6 − 8 and 46, -adenosines 15 − 17 and 64, and -guanosines 25, 26, and 52 is described. 3-Deaza-3-chloro-, -bromo-, and -iodopurine ribonucleosides can be synthesized by treating the 3-deazapurine ribonucleosides with N -halosuccinimides. For the synthesis of 3-deaza-3-fluoropurine ribonucleosides, 5-formylimidazole-4-carboxamide ribosides 34 and 35 prepared from 5-iodoimidazole-4-carboxamide derivatives 29 and 31 were used as the key intermediates. The reaction of 34 and 35 with lithium (trimethylsilyl)acetylide and sodium cyanide, respectively, followed by appropriate manipulations gave 3-deaza-3-fluoroinosine derivative 46 and 3-deaza-3-fluoroguanosine derivative 52 . 3-Deaza-3-fluoroadenosine ( 64 ) was also synthesized by selective reductive deamination of 4,6-diamino-7-fluoroimidazo[4,5- c ]pyridine derivative 51, followed by deprotection. A conformational analysis using 1 H NMR and NOE experiments showed that the introduction of halogens (chloro, bromo, and iodo) into the 3-position of 3-deazapurine ribonucleosides forced fixation of the glycosyl torsion angle in the anti region, but did not abnormally influence sugar puckering. On the other hand, 3-deaza-3-fluoropurine ribonucleosides should rotate freely around the glycosyl bond.