A SYNTHESIS OF 9-(2-DEOXY-2-FLUORO-BETA-D-ARABINOFURANOSYL)ADENINE AND HYPOXANTHINE - AN EFFECT OF C3'-ENDO TO C2'-ENDO CONFORMATIONAL SHIFT ON THE REACTION COURSE OF 2'-HYDROXYL GROUP WITH DAST

O3',O5',N6-Tritrityladenosine (6), 3',5'-di-O-trityl-N1-benzylinosine (15), and 3',5'-di-O-tritylinosine (18) were prepared and subjected to nucleophilic reaction with DAST. Thus, 6 afforded 2'-beta-fluorine-substituted nucleoside 11 along with the isomeric 2-deoxy-2-(N-trityladenin-3-yl)-3,5-di-O-trityl-alpha-D-arabinofuranosyl fluoride (12). Nucleoside 15, under the same treatment with DAST, gave the desired 2'-fluoroarabino derivative 16 exclusively in high yield. Although 18 was converted into the 2'-beta-fluoro product 19 under the similar conditions, the yield was low. A plausible mechanism of formation of 12 is discussed. Deprotection of 11 and 16 afforded the desired 9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)adenine (1) and -hypoxanthine (2), respectively, in high yield. The conformational influence of sugar protecting groups on the rate of nucleophilic substitution against elimination is discussed. Treatment of O3',O5',N1-benzylinosine (20) with DAST afforded only the elimination products 9-(3',5'-di-O-benzyl-beta-D-erythro-pent-2-enofuranosyl)-1-benzylhypoxanthine (22) and 3-(benzyloxy)-2-[(benzyloxy)methyl]furan (23). On the other hand, 9-(3,5-di-O-trityl-beta-D-arabinofuranosyl)adenine (26) (prepared from 6 by triflyation followed by NaOAc treatment and deacetylation) afforded a mixture from which 2'-deoxy-2'-fluoroadenosine (27) and 9-(2-deoxy-3,5-di-O-trityl-D-erythro-pent-1-enofuranosyl)-N6-trityladenine (28) were isolated in 60 and 30% yield, respectively. O2',O5',N6-Tritrityladenosine (7) was selectively detritylated with HCO2H/Et2O to give O2',N6-ditrityladenosine (30), which, upon treatment with benzyl chloride/KOH, afforded 3',5'-di-O-benzyl-O2',N6-ditrityladenosine (31). 9-(3,5-Di-O-benzyl-beta-D-arabinofuranosyl)adenine (35) was prepared from 31 by further detritylation with CF3CO2H/CHCl3 and triflyation followed by NaOAc treatment and deacetylation of the product. Treatment of 35 with DAST followed by hydrogenolytic debenzylation afforded 2'-deoxy-2'-fluoroadenosine (3) in high yield. The three-step synthesis described herein, albeit about 10% overall yield, is far superior to the currently available multistep procedures which give the desired 2'-fluoroarabinosylpurines in much less overall yields.