New pseudopolymorphs of 5-fluorocytosine.

In order to better understand the interaction between the pharmaceutically active compound 5-fluorocytosine [4-amino-5-fluoropyrimidin-2(1H)-one] and its receptor, hydrogen-bonded complexes with structurally similar bonding patterns have been investigated. During the cocrystallization screening, three new pseudopolymorphs of 5-fluorocytosine were obtained, namely 5-fluorocytosine dimethyl sulfoxide solvate, C(4)H(4)FN(3)O.C(2)H(6)OS, (I), 5-fluorocytosine dimethylacetamide hemisolvate, C(4)H(4)FN(3)O.0.5C(4)H(9)NO, (II), and 5-fluorocytosine hemihydrate, C(4)H(4)FN(3)O.0.5H(2)O, (III). Similar hydrogen-bond patterns are observed in all three crystal structures. The 5-fluorocytosine molecules form ribbons with repeated R(2)(2)(8) dimer interactions. These dimers are stabilized by N-H...N and N-H...O hydrogen bonds. The solvent molecules adopt similar positions with respect to 5-fluorocytosine. Depending on the hydrogen bonds formed by the solvent, the 5-fluorocytosine ribbons form layers or tubes. A database study was carried out to compare the hydrogen-bond pattern of compounds (I)-(III) with those of other (pseudo)polymorphs of 5-fluorocytosine.