6-Mercaptopurine monohydrate synthesis

Mercaptopurine, 6-purinthiol, is made from uric acid (30.1.2.5), which is synthesized from barbituric acid (30.1.2.1). Barbituric acid (30.1.2.1) is easily made by condensing urea with malonic ester and then nitrosylating it with nitrous acid. The nitrosoderivative (30.1.2.2) is reduced by hydrogen (obtained in situ by reacting tin with hydrochloric acid) to an amine (uramil) (30.1.2.3), and then reacted with isocyanic acid, which forms pseudouric acid (30.1.2.4). This undergoes cyclization to uric acid (30.1.2.5) when heated in the presence of hydrochloric acid. Upon reacting phosphorous pentachloride with uric acid, 2,6,8-trichloropurine (30.1.2.6) is formed. The three chlorine atoms in trichloropurine differ significantly in terms of reactivity for nucleophilic substitution. The chlorine atom at C6 is much more active than the chlorine atom at C2, and this is more active than the chlorine atom at C8, which allows subsequent manipulation by them. Interaction of 2,6,8-trichloropurine (30.1.2.6) with sodium hydroxide allows to replace the chlorine atom at C6, forming the dichloro-derivative (30.1.2.7), which is then reduced by hydriodic acid to hypoxanthine (30.1.2.8). Upon reaction with phosphorous pentasulfide, hypoxanthine is transformed into mercaptopurine (30.1.2.9).