Guanosine (I) was protected as the triacetate ester (II) and then chlorinated with POCl3 in the presence of dimethylaniline and tetraethylammonium chloride to give the 6-chloro derivative (III). Diazotization of (III) with tert-butyl nitrite, followed by a modified Sandmeyer reaction provided dichloro purine (IV). Substitution of the 6-chloro of (IV) by 2,2-diphenylethylamine (V) in the presence of diisopropylethylamine in refluxing isopropanol furnished the chloroadenosine derivative (VI), and subsequent removal of the acetate protecting groups of (VI) by hydrolysis with Na2CO3 in aqueous MeOH yielded the adenosine derivative (VII).
Alternatively, condensation of 2,6-dichloropurine (VIII) with protected ribose (IX) at 140 C gave the ribofuranosyl purine (X). Subsequent displacement of the 6-chloro of (V) by amine (V), followed by hydrolysis of the benzoate esters afforded the intermediate (VII).
Protection of the 2',3'-diol group of adenosine (VII) as the corresponding acetonide (XII) was effected by treatment with 2,2-dimethoxypropane and p-toluenesulfonic acid. The primary alcohol group of (XII) was then oxidized to carboxylic acid (XIII) using KMnO4. After conversion of (XIII) to acid chloride by means of refluxing SOCl2, coupling with ethylamine furnished amide (XIV). Displacement of the 2-chloro of (XIV) by histamine (XV) in hot DMSO produced adduct (XVI). The acetonide group of (XVI) was finally deprotected using aqueous trifluoroacetic acid.