2-Amino-6-chloropurine (I) is converted into the 2-fluoro analogue (II) by diazotization in the presence of fluoroboric acid. Subsequent alkylation of (II) with EtOH under Mitsunobu conditions provides the 9-ethyl purine (III).
Reaction of phosphinylmethyl phosphonate (IV) with 1-iodo-4-nitrobenzene (V) in the presence of Pd(PPh3)4 affords the nitrophenyl phosphinoyl derivative (VI). Reduction of the nitro group of (VI) to the corresponding aniline (VII) is accomplished by treatment with SnCl2 in ethanol. Then, hydrolysis of the triethyl ester (VII) under acidic conditions furnishes phosphonic acid (VIII). Displacement of the 6-chloro group of purine (III) with the aminophenyl phosphonic acid (VIII) in hot DMSO gives rise to the 6-anilino purine (IX). Finally, displacement of the remaining halogen atom of (IX) with trans-1,4-diaminocyclohexane (X) leads to the title 2,6-diaminopurine compound.