Cocrystals of 5-fluorocytosine. II. Coformers with variable hydrogen-bonding sites.

Two flexible molecules, biuret and 6-acetamidouracil, were cocrystallized with 5-fluorocytosine to study their conformational preferences. In the cocrystal with 5-fluorocytosine (I), biuret exhibits the same conformation as in its hydrate. In contrast, 6-acetamidouracil can adopt two main conformations depending on its crystal environment: in crystal (II) the trans form characterized by an intramolecular hydrogen bond is observed, while in the cocrystal with 5-fluorocytosine (III), the complementary binding induces the cis form. Three cocrystals of 6-methylisocytosine demonstrate that complementary binding enables the crystallization of a specific tautomer. In the cocrystals with 5-fluorocytosine, (IVa) and (IVb), only the 3H tautomer of 6-methylisocytosine is present, whereas in the cocrystal with 6-aminoisocytosine, (V), the 1H tautomeric form is adopted. The complexes observed in the cocrystals are stabilized by three hydrogen bonds similar to those constituting the Watson-Crick C·G base pair.

Cocrystals of 5-fluorocytosine. I. Coformers with fixed hydrogen-bonding sites.

The antifungal drug 5-fluorocytosine (4-amino-5-fluoro-1,2-dihydropyrimidin-2-one) was cocrystallized with five complementary compounds in order to better understand its drug-receptor interaction. The first two compounds, 2-aminopyrimidine (2-amino-1,3-diazine) and N-acetylcreatinine (N-acetyl-2-amino-1-methyl-5H-imidazol-4-one), exhibit donor-acceptor sites for R(2)(2)(8) heterodimer formation with 5-fluorocytosine. Such a heterodimer is observed in the cocrystal with 2-aminopyrimidine (I); in contrast, 5-fluorocytosine and N-acetylcreatinine [which forms homodimers in its crystal structure (II)] are connected only by a single hydrogen bond in (III). The other three compounds 6-aminouracil (6-amino-2,4-pyrimidinediol), 6-aminoisocytosine (2,6-diamino-3H-pyrimidin-4-one) and acyclovir [acycloguanosine or 2-amino-9-[(2-hydroxyethoxy)methyl]-1,9-dihydro-6H-purin-6-one] possess donor-donor-acceptor sites; therefore, they can interact with 5-fluorocytosine to form a heterodimer linked by three hydrogen bonds. In the cocrystals with 6-aminoisocytosine (Va)-(Vd), as well as in the cocrystal with the antiviral drug acyclovir (VII), the desired heterodimers are observed. However, they are not formed in the cocrystal with 6-aminouracil (IV), where the components are connected by two hydrogen bonds. In addition, a solvent-free structure of acyclovir (VI) was obtained. A comparison of the calculated energies released during dimer formation helped to rationalize the preference for hydrogen-bonding interactions in the various cocrystal structures.

Conformational studies of hydantoin-5-acetic acid and orotic acid.

Hydantoin-5-acetic acid [2-(2,5-dioxoimidazolidin-4-yl)acetic acid] and orotic acid (2,6-dioxo-1,2,3,6-tetrahydropyrimidine-4-carboxylic acid) each contain one rigid acceptor-donor-acceptor hydrogen-bonding site and a flexible side chain, which can adopt different conformations. Since both compounds may be used as coformers for supramolecular complexes, they have been crystallized in order to examine their conformational preferences, giving solvent-free hydantoin-5-acetic acid, C(5)H(6)N(2)O(4), (I), and three crystals containing orotic acid, namely, orotic acid dimethyl sulfoxide monosolvate, C(5)H(4)N(2)O(4)·C(2)H(6)OS, (IIa), dimethylammonium orotate-orotic acid (1/1), C(2)H(8)N(+)·C(5)H(3)N(2)O(4)(-)·C(5)H(4)N(2)O(4), (IIb), and dimethylammonium orotate-orotic acid (3/1), 3C(2)H(8)N(+)·3C(5)H(3)N(2)O(4)(-)·C(5)H(4)N(2)O(4), (IIc). The crystal structure of (I) shows a three-dimensional network, with the acid function located perpendicular to the ring. Interestingly, the hydroxy O atom acts as an acceptor, even though the carbonyl O atom is not involved in any hydrogen bonds. However, in (IIa), (IIb) and (IIc), the acid functions are only slightly twisted out of the ring planes. All H atoms of the acidic functions are directed away from the rings and, with respect to the carbonyl O atoms, they show an antiperiplanar conformation in (I) and synperiplanar conformations in (IIa), (IIb) and (IIc). Furthermore, in (IIa), (IIb) and (IIc), different conformations of the acid O=C-C-N torsion angle are observed, leading to different hydrogen-bonding arrangements depending on their conformation and composition.

Pseudopolymorphs of chelidamic acid and its dimethyl ester.

Different tautomeric and zwitterionic forms of chelidamic acid (4-hydroxypyridine-2,6-dicarboxylic acid) are present in the crystal structures of chelidamic acid methanol monosolvate, C(7)H(5)NO(5)·CH(4)O, (Ia), dimethylammonium chelidamate (dimethylammonium 6-carboxy-4-hydroxypyridine-2-carboxylate), C(2)H(8)N(+)·C(7)H(4)NO(5)(-), (Ib), and chelidamic acid dimethyl sulfoxide monosolvate, C(7)H(5)NO(5)·C(2)H(6)OS, (Ic). While the zwitterionic pyridinium carboxylate in (Ia) can be explained from the pK(a) values, a (partially) deprotonated hydroxy group in the presence of a neutral carboxy group, as observed in (Ib) and (Ic), is unexpected. In (Ib), there are two formula units in the asymmetric unit with the chelidamic acid entities connected by a symmetric O-H···O hydrogen bond. Also, crystals of chelidamic acid dimethyl ester (dimethyl 4-hydroxypyridine-2,6-dicarboxylate) were obtained as a monohydrate, C(9)H(9)NO(5)·H(2)O, (IIa), and as a solvent-free modification, in which both ester molecules adopt the hydroxypyridine form. In (IIa), the solvent water molecule stabilizes the synperiplanar conformation of both carbonyl O atoms with respect to the pyridine N atom by two O-H···O hydrogen bonds, whereas an antiperiplanar arrangement is observed in the water-free structure. A database study and ab initio energy calculations help to compare the stabilities of the various ester conformations.

A new polymorph and two pseudopolymorphs of pyrimethamine.

Due to its donor-acceptor-donor site, the antimalarial drug pyrimethamine [systematic name: 5-(4-chlorophenyl)-6-ethylpyrimidine-2,4-diamine] is a potential component of a supramolecular synthon. During a cocrystallization screen, one new polymorph of solvent-free pyrimethamine, C(12)H(13)ClN(4), (I), and two pseudopolymorphs, pyrimethamine dimethyl sulfoxide monosolvate, C(12)H(13)ClN(4)·C(2)H(6)OS, (Ia), and pyrimethamine N-methylpyrrolidin-2-one monosolvate, C(12)H(13)ClN(4)·C(5)H(9)NO, (Ib), were obtained. In (I), (Ia), (Ib) and the previously reported polymorph, the pyrimethamine molecules exhibit similar conformations and form R(2)(2)(8) dimers stabilized by a pair of N-H···N hydrogen bonds. However, the packing arrangements are completely different. In (I), the dimers are connected by two additional N-H···N hydrogen bonds to form ribbons and further connected into a two-dimensional network parallel to (100), while layers containing N-H···Cl hydrogen-bonded pyrimethamine ribbons are observed in the packing of the known polymorph. In the two pseudopolymorphs, two pyrimethamine molecules are linked to form R(2)(2)(8) dimers and the solvent molecules are connected to the dimers by R(2)(3)(8) interactions involving two N-H···O hydrogen bonds. These arrangements are connected to form zigzag chains by N-H···Cl interactions in (Ia) and to form ribbons by N-H...N interactions in (Ib). Unexpectedly, a reaction between pyrimethamine and N-methylpyrrolidin-2-one occurred during another cocrystallization experiment from a solvent mixture of N-methylpyrrolidin-2-one and dimethyl sulfoxide, yielding solvent-free 5,5'-{[5-(4-chlorophenyl)-6-ethylpyrimidine-2,4-diyl]bis(azanediyl)}bis(1-methylpyrrolidin-2-one), C(22)H(27)ClN(6)O(2), (II). In the packing of (II), the pyrimethamine derivatives are N-H···O hydrogen bonded to form ribbons. A database study was carried out to compare the molecular conformations and hydrogen-bonding interactions of pyrimethamine.

Cocrystals of 6-propyl-2-thiouracil: N-H···O versus N-H···S hydrogen bonds.

In order to investigate the relative stability of N-H···O and N-H···S hydrogen bonds, we cocrystallized the antithyroid drug 6-propyl-2-thiouracil with two complementary heterocycles. In the cocrystal pyrimidin-2-amine-6-propyl-2-thiouracil (1/2), C(4)H(5)N(3)·2C(7)H(10)N(2)OS, (I), the `base pair' is connected by one N-H···S and one N-H···N hydrogen bond. Homodimers of 6-propyl-2-thiouracil linked by two N-H···S hydrogen bonds are observed in the cocrystal N-(6-acetamidopyridin-2-yl)acetamide-6-propyl-2-thiouracil (1/2), C(9)H(11)N(3)O(2)·2C(7)H(10)N(2)OS, (II). The crystal structure of 6-propyl-2-thiouracil itself, C(7)H(10)N(2)OS, (III), is stabilized by pairwise N-H···O and N-H···S hydrogen bonds. In all three structures, N-H···S hydrogen bonds occur only within R(2)(2)(8) patterns, whereas N-H···O hydrogen bonds tend to connect the homo- and heterodimers into extended networks. In agreement with related structures, the hydrogen-bonding capability of C=O and C=S groups seems to be comparable.

Five pseudopolymorphs and a cocrystal of nitrofurantoin.

The antibiotic nitrofurantoin {systematic name: (E)-1-[(5-nitro-2-furyl)methylideneamino]imidazolidine-2,4-dione} is not only used for the treatment of urinary tract infections, but also illegally applied as an animal food additive. Since derivatives of 2,6-diaminopyridine might serve as artificial receptors for its recognition, we crystallized one potential drug-receptor complex, nitrofurantoin-2,6-diacetamidopyridine (1/1), C(8)H(6)N(4)O(5)·C(9)H(11)N(3)O(2), (I·II). It is characterized by one N-H···N and two N-H···O hydrogen bonds and confirms a previous NMR study. During the crystallization screening, several new pseudopolymorphs of both components were obtained, namely a nitrofurantoin dimethyl sulfoxide monosolvate, C(8)H(6)N(4)O(5)·C(2)H(6)OS, (Ia), a nitrofurantoin dimethyl sulfoxide hemisolvate, C(8)H(6)N(4)O(5)·0.5C(2)H(6)OS, (Ib), two nitrofurantoin dimethylacetamide monosolvates, C(8)H(6)N(4)O(5)·C(4)H(9)NO, (Ic) and (Id), and a nitrofurantoin dimethylacetamide disolvate, C(8)H(6)N(4)O(5)·2C(4)H(9)NO, (Ie), as well as a 2,6-diacetamidopyridine dimethylformamide monosolvate, C(9)H(11)N(3)O(2)·C(3)H(7)NO, (IIa). Of these, (Ia), (Ic) and (Id) were formed during cocrystallization attempts with 1-(4-fluorophenyl)biguanide hydrochloride. Obviously nitrofurantoin prefers the higher-energy conformation in the crystal structures, which all exhibit N-H···O and C-H···O hydrogen-bond interactions. The latter are especially important for the crystal packing. 2,6-Diacetamidopyridine shows some conformational flexibility depending on the hydrogen-bond pattern.

Pseudopolymorphs of 2,6-diaminopyrimidin-4-one and 2-amino-6-methylpyrimidin-4-one: one or two tautomers present in the same crystal.

The derivatives of pyrimidin-4-one can adopt either a 1H- or a 3H-tautomeric form, which affects the hydrogen-bonding interactions in cocrystals with compounds containing complementary functional groups. In order to study their tautomeric preferences, we crystallized 2,6-diaminopyrimidin-4-one and 2-amino-6-methylpyrimidin-4-one. During various crystallization attempts, four structures of 2,6-diaminopyrimidin-4-one were obtained, namely solvent-free 2,6-diaminopyrimidin-4-one, C(4)H(6)N(4)O, (I), 2,6-diaminopyrimidin-4-one-dimethylformamide-water (3/4/1), C(4)H(6)N(4)O·1.33C(3)H(7)NO·0.33H(2)O, (Ia), 2,6-diaminopyrimidin-4-one dimethylacetamide monosolvate, C(4)H(6)N(4)O·C(4)H(9)NO, (Ib), and 2,6-diaminopyrimidin-4-one-N-methylpyrrolidin-2-one (3/2), C(4)H(6)N(4)O·1.5C(5)H(9)NO, (Ic). The 2,6-diaminopyrimidin-4-one molecules exist only as 3H-tautomers. They form ribbons characterized by R(2)(2)(8) hydrogen-bonding interactions, which are further connected to form three-dimensional networks. An intermolecular N-H···N interaction between amine groups is observed only in (I). This might be the reason for the pyramidalization of the amine group. Crystallization experiments on 2-amino-6-methylpyrimidin-4-one yielded two isostructural pseudopolymorphs, namely 2-amino-6-methylpyrimidin-4(3H)-one-2-amino-6-methylpyrimidin-4(1H)-one-dimethylacetamide (1/1/1), C(5)H(7)N(3)O·C(5)H(7)N(3)O·C(4)H(9)NO, (IIa), and 2-amino-6-methylpyrimidin-4(3H)-one-2-amino-6-methylpyrimidin-4(1H)-one-N-methylpyrrolidin-2-one (1/1/1), C(5)H(7)N(3)O·C(5)H(7)N(3)O·C(5)H(9)NO, (IIb). In both structures, a 1:1 mixture of 1H- and 3H-tautomers is present, which are linked by three hydrogen bonds similar to a Watson-Crick C-G base pair.

1-(4-Fluoro-phen-yl)biguanid-1-ium chloride.

The title compound, C(8)H(11)FN(5) (+)·Cl(-), crystallized with a monoprotonated 1-(4-fluoro-phen-yl)biguanidinium cation and a chloride anion in the asymmetric unit. The biguanidium group is not planar [dihedral angle between the two CN(3) groups = 52.0 (1)°] and is rotated with respect to the phenyl group [τ = 54.3 (3)°]. In the crystal, N-H⋯N hydrogen-bonded centrosymmetric dimers are connected into ribbons, which are further stabilized by N-H⋯Cl interactions, forming a three-dimensional hydrogen-bonded network.

2-Amino-1-methyl-1H-imidazol-4(5H)-one dimethyl sulfoxide monosolvate.

In the title compound, C(4)H(7)N(3)O·C(2)H(6)OS, creatinine [2-amino-1-methyl-1H-imidazol-4(5H)one] exists in the amine form. The ring is planar (r.m.s. deviation for all non-H atoms = 0.017 Å). In the crystal, two creatinine mol-ecules form centrosymmetric hydrogen-bonded dimers linked by pairs of N-H⋯N hydrogen bonds. In addition, creatinine is linked to a dimethyl sulfoxide mol-ecule by an N-H⋯O inter-action. The packing shows layers parallel to (120).

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.