Crystal structures of two 6-(2-hydroxybenzoyl)-5H-thiazolo[3,2-a]pyrimidin-5-ones

The title fused heterocycles arose from an unexpected intramolecular cyclization reaction. Each molecule features an intramolecular O—H⋯O hydrogen bond. In the crystal, chains mediated by C—H⋯O interactions arise.


Chemical context
Although heterocycles, namely those bearing thiazole or pyrimidine motifs, are reported to show a broad spectrum of pharmacological properties such as antimicrobial, anticancer and anti-inflammatory activities (Jiang et al., 2013;Mishra et al., 2015;Perrone et al., 2012), only a few compounds enclosing the thiazolo [3,2a]pyrimidine framework have been explored and screened towards the above-mentioned pharmacological activities. Even though some derivatives tested up to now have shown interesting anti-inflammatory (Bekhit et al., 2003), antiviral (Abd El-Galil et al., 2010) and antibacterial activities (Mulwad et al., 2010) and as calcium agonists (Balkan et al., 1992), the data acquired so far are insufficient to indicate the importance of the thiazolo [3,2a]pyrimidine motif as a positive contributor to the biological profile mentioned above. The same reflection is valid in relation to the data acquired for some thiazolo [3,2a]pyrimidine-5-one derivatives as 5-HT2a receptor antagonists, a putative therapeutic target for the treatment of depression, although they have structural similarity to ritanserin, a serotonin antagonist (Awadallah, 2008). In this last case, the pharmacological activity appears to be enhanced by the nature of the planar aromatic or heterocyclic ring systems, the type of spacer as well as the presence of a basic nitrogen atom.

Structural commentary
The molecules of (1) and (2) are shown in Figs. 1 and 2. The structural characterization reveals that the molecules have two cyclic units, viz. the hydroxybenzyl and the heterocyclic 5Hthiazolo[3,2-a]pyrimidin-5-one ring separated by a carbonyl spacer, as expected. In both compounds, the carbonyl O atoms are trans oriented with respect to each other, contributing to the establishment of an intramolecular O-HÁ Á ÁO hydrogen bond between the o-hydroxyl group of the benzene ring and the carbonyl group of the spacer (Tables 1 and 2), which generates an S(6) ring. Taken together, the benzene ring and hydrogen-bonded pseudo ring are roughly planar, the carbonyl oxygen atom deviates by 0.391 (3) and 0.055 (4) Å in (1) and (2), respectively from the least-square plane formed by the benzene ring atoms. The heterocyclic rings of both compounds are also almost planar, as expected; the maximum deviation from the best plane formed by the ten atoms of the thiazolopyrimidine moiety is 0.103 (1) Å for the carbonyl oxygen atom, O5, in (1) and 0.129 (1) Å for the same atom in (2). Thus, both molecules are twisted around the C6-C67 bond that links the ring systems, which are inclined to one another by 55.22 (5) and 46.83 (6) for (1) and (2), respectively.

Supramolecular features
As noted above, the hydroxyl group is involved in intramolecular hydrogen bonding, which leaves it unavailable for participation in intermolecular hydrogen bonding. Thus, the molecules are linked via weak C-HÁ Á ÁO interactions: in both compounds the oxygen acceptor atom is the oxo atom O5, being in (1) the hydrogen-bond donor atom is C2 (of the heterocyclic group) and in (2) the hydrogen-bond donor atom is C64 (located in the exocyclic benzene ring).
In (1) the molecules are linked by the C2-H2Á Á ÁO5 (x + 1 2 , Ày + 1 2 , z + 1 2 ) hydrogen bond, forming a C(6) chain, which runs parallel to [101] and results from the action of a c-glide at (0, 1 4 , 0) ( Table 1 and Fig. 3). The presence of the methyl group on atom C2 of the heterocyclic ring precludes the formation of a similar bond in (2). Thus in the supramolecular structure of this compound, the molecules are linked by a C64-H64Á Á ÁO5(Àx + 2, y + 1 2 , Àz + 1) hydrogen bond, forming a C(9) chain, which runs parallel to the b-axis direction and results from the action of a 2 1 screw axis at (1, y, 1 2 ) (  A view of the asymmetric unit of (2) with displacement ellipsoids drawn at the 70% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (1).

Figure 1
A view of the asymmetric unit of (1) with displacement ellipsoids drawn at the 70% probability level.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3 Representations of the compounds referred to in this work (the scaffold indicates the adopted numbering scheme for the 5H-thiazolo[3,2a]-pyrimidine-5-one residue).