Systematic synthesis of purine 8,5'-imino and substituted imino cyclonucleosides

To achieve a systematic synthesis of purine 8,5'-imino and substituted imino cyclonucleosides, 2',3'-O-isopropylidene-purinenucleosides substituted with a methylamino (4a,b), benzyl-amino (4c,d,g and h) and allylamino group (4e,f,i and j) at the C8 were synthesized. With these substrates in hand, extensive 8,5'-cyclization reactions were carried out using diphenyl carbonate/Et3N (Method A), N,N'-carbonyldiimidazole (Method B) and the Mitsunobu reaction (Method C) to give 8,5'-substituted imino cyclonucleosides (5a,c,d,e,f and g). The yields of cyclization by Method C are generally higher than by the other two methods. 5a, b,c,d,e,f,g and h were deprotected to the corresponding mother compounds 8 through one or two steps. In guanosine series, a new cyclic system comprising an 8,5'-carbamate ester bridge (6a-c) has been introduced.