CPX has been obtained by two related ways: 1) Nitrosation of 6-amino-1,3-dipropyluracil (I) with NaNO2 in acetic acid gives the 5-nitrosouracil derivative (II), which is reduced with H2 over Pd/C in ethanol to yield 5,6-diamino-1,3-dipropyluracil (III). Acylation of (III) with cyclopentanecarboxylic acid (IV) by heating up to the mixture's melting point or by treatment with diisopropylcarbodiimide (DIC) gives the amide (V), which is finally cyclized by means of NaOH in refluxing water or refluxing methanol/water. 2) Acylation of the 5-nitrosouracil (II) with cyclopentanecarbonyl chloride (VI) by means of lithium hexamethyldisilazane (LHMDS) in THF gives the amide (VII), which is submitted to a reductive cyclization with Sn(OAc)2 in the same solvent.