The alkylation of 6-chloro-2-fluoropurine (I) with isopropanol (II) under Mitsunobu conditions gives 6-chloro-2-fluoro-9-isopropylpurine (III), which is regioselectively alkylated at the 6 position with 4-methoxybenzylamine (IV) to yield 2-fluoro-9-isopropyl-N6-(4-methoxybenzyl)adenine (V). Finally, the 2-fluoro position of (V) is aminated with perhydroazepine (VI) in n-butanol at high temperature in a sealed tube to afford the target trisubstituted purine.