Research Article | Open Access
Crystal Structures of Two Macrocyclic Bischalcones Possessing 26-Membered Rings
Single crystal X-ray diffraction of two macrocyclic bischalcones, namely, (2E,25E)-11,17,33,37-tetraoxapentacyclo[36.4.0.05,10.018,23.027,32]dotetraconta-1(42),2,5,7,9,18,20,22,25,27,29,31,38,40-tetradecaene-4,24-dione(1) and (2E,24E)-11,16,32,37-tetraoxapentacyclo[36.4.0.05,10.017,22.026,31]dotetraconta-1(42),2,5,7,9,17,19,21,24,26,28,30,38,40-tetradecaene-4,23-dione(2), each containing a 26-membered ring, has been studied. Compound 1 belongs to the monoclinic system, space group C2/c with a = 34.3615(9) Å, b = 12.7995(3) Å, c = 14.6231(3) Å, β = 96.912(2)°, V = 6,384.6(3) Å3, and Z = 8. Compound 2 is triclinic, space group P-1 with a = 10.066(2) Å, b = 10.670(3) Å, c = 16.590(3) Å, α = 85.95(2), β = 89.244(14), γ = 62.211(13), V = 1572.0(6) Å3, and Z = 2. Intermolecular C–H⋯O hydrogen bonding interactions are present in both compounds.
Compounds containing the chalcone (1,3-diphenyl-2-propen-1-one) moiety  are known to possess important biological activities such as antimicrobial , antiviral , antibacterial [4, 5], antihelminthic , antimalarial [6, 7], and antitumor  activities. Moreover, these compounds can be transformed in a number of ways into other biologically active heterocycles, for example, pyrimidines , isoxazole and pyrazole , and pyridines . Macrocycles incorporating more than one chalcone moiety have great potential in generating new compounds suitable for molecular recognition [12–15] and photophysical [16, 17] studies. Recently, we reported the synthesis of a new family of 22- to 28-membered macrocycles containing two chalcone moieties. For structural confirmation, an X-ray crystallographic study of one compound of this family has been done . The interesting aspects of that study arising out of the folded nature of the molecule encouraged us to record the X-ray crystallographic data of some other compounds of the family. In this paper, we report the X-ray crystallographic data of two other compounds, (2E,25E)-11,17,33,37-tetraoxapentacyclo[36.4.0.05,10.018,23.027,32]-dotetraconta-1(42),2,5,7,9,18,20,22,25,27,29,31,38,40-tetradecaene-4,24-dione (1) and (2E,24E)-11,16,32,37-tetraoxapentacyclo[36.4.0.05,10.017,22.026,31]dotetraconta-1(42),2,5,7,9,17,19,21,24,26,28,30,38,40-tetradecaene-4,23-dione (2), each having a 26-membered ring.
2.1. Synthesis of 1 and 2
Synthesis of the macrocyclic bischalcones (2E,25E)-11,17,33,37-tetraoxapentacyclo-[36.4.0.05,10.018,23.027,32]dotetraconta-1(42),2,5,7,9,18,20,22,25,27,29,31,38,40-tetradecaene-4,24-dione (1) and (2E,24E)-11,16,32,37-tetraoxapentacyclo[36.4.0.05,10.017,22.026,31]-dotetraconta-1(42),2,5,7,9,17,19,21,24,26,28,30,38,40-tetradecaene-4,23-dione (2) (Scheme 1) was completed using the Claisen-Schmidt reaction in highly diluted aqueous methanol solution. The methods for the synthesis of 1 and 2 and their spectral data have been presented in our previous paper . The light yellow single crystals of these compounds were obtained by slow evaporation of solvents from their solutions in n-hexane and acetone (1 : 3, v/v) (melting points: 1, 182–184°C; 2, 162–164°C).
2.2. Single Crystal X-Ray Crystallography
A summary of the crystallographic data and conditions is given in Table 1. The selected bond lengths and bond angles of 1 and 2 are listed in Table 2. The structures of 1 and 2 as well as the packing arrangement in a unit cell of these compounds are shown in Figures 1 and 2, respectively. X-ray single crystal data collection and cell refinement were carried out at room temperature using a “Bruker SMART CCD-detector” diffractometer equipped with a normal focus, sealed tube X-ray source with graphite monochromated Mo-Kα radiation ( = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares based using SHELXS 97 set of programs . ORTEP-3 for Windows  and PLATON  programs were used for molecular graphics. The structures were drawn with Mercury for Windows .
3. Results and Discussion
The molecular structures of 1 and 2 are shown in Figure 1. Hydrogen bonding in the molecules is summarized in Table 2. For compound 1, the distances C15–C16 (1.458(3) Å), C17–C18 (1.458(3) Å), and C18–C19 (1.359(13) Å) and, for compound 2, the distances C16–C17 (1.424(6) Å), C18–C19 (1.438(7) Å), and C19–C20 (1.484(7) Å) are shorter than the normal C–C single bond (1.53 Å). However, the C16–C17 (1.324(3) Å) distance for compound 1 is a bit shorter while C17–C18 (1.378(6) Å) distance for compound 2 is somewhat longer than the normal C–C double bond (1.34 Å). Again, in the case of compound 1, the C18–C19 bond length is very close to that of a C–C single bond. In both 1 and 2 each of the two phenyl rings attached to the >C=O is not coplanar with the α,β-unsaturated carbonyl moiety. All these may be due to accommodation of the polymethylene linkers in the molecules.
In case of compound 1, for the first chalcone moiety, the α,β-unsaturated carbonyl unit (C16–C17–C18) is almost coplanar with the phenyl ring C10–C11–C12–C13–C14–C15 (angle 3.69(5)°) but is not so with the phenyl ring C19–C20–C21–C22–C23–C24 (angle 69.69(12)°), while, for the second chalcone moiety, the α,β-unsaturated carbonyl unit (C36–C37–C38) is not coplanar with any of the two phenyl rings C30–C31–C32–C33–C34–C35 and C1–C2–C3–C4–C5–C6 (angles 16.82(9) and 22.90(11)°, resp.). On the other hand, in compound 2, arrangement is the same in both chalcone moieties. In each case the α,β-unsaturated carbonyl unit is almost coplanar with one phenyl ring C1–C2–C3–C4–C5–C6 or C11–C12–C13–C14–C15–C16 (angle 1.20(3) or 3.33(4)°) while it makes an angle of nearly 67° with the other phenyl rings C30–C31–C32–C33–C34–C35 or C20–C21–C22–C23–C24–C25. It is therefore apparent that the extent of electron delocalization between the phenyl ring attached to carbonyl and the remaining part of the chalcone moiety is very poor for both compounds 1 and 2. The aliphatic chains O1–C7–C8–C9–O2 and O4–C25–C26–C27–C28–C29–O5 in 1 and O1–C7–C8–C9–C10–O2 and O4–C26–C27–C28–C29–O5 in 2 are mostly in staggered arrangement with the dihedral angles ranging from 57 to 64°. Antiarrangements were observed for C25–C26–C27–C28 and C26–C27–C28–C29 in 1 and C7–C8–C9–C10 in 2 (dihedral angles 177.32(17), 176.19(17), and 175.10(9)°, resp.). It is interesting to note that the arrangements for the apparently similar units C7–C8–C9–C10 and C26–C27–C28–C29 in 2 are somewhat different in the crystalline state. For the first unit it is anti while for the second unit it is staggered. In the unit cell containing two molecules of 2 (Figure 2), two anti units are much closer than two staggered ones.
The salient feature of the crystal packing of 1 and 2 is that both of them form intermolecular hydrogen bonding networks (Figure 2). For the crystal of each compound there are two types of C–H⋯O intermolecular hydrogen bonds and one type of C–H⋯π supramolecular interactions (Table 3). In the crystal lattices, all the abovementioned hydrogen bonds and intermolecular interactions play a significant role in stabilizing the crystal structures. The intermolecular hydrogen bonds in 1 and 2 are summarized in Table 3.
Supplementary full crystallographic information for compounds 1 (CCDC 999278) and 2 (CCDC 999279) can be obtained via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, UK.
Several interesting structural aspects of two macrocyclic bischalcones 1 and 2, each containing a 26-membered ring, in the solid state were evident from their single crystal X-ray diffraction studies.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial assistance from the UGC-CAS and DST-PURSE programs and instrumental facilities from the DST-FIST program, Department of Chemistry, Jadavpur University, are gratefully acknowledged. Rina Mondal and Nayim Sepay are thankful to the UGC, New Delhi, for the award of research fellowships.
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