Microwave assisted synthesis, crystal structure and modelling of cytotoxic dehydroacetic acid enamine: a natural alkaloid from Fusarium incarnatum (HKI0504)

Abstract

A novel, fast and efficient method for the synthesis of (3E)-3-(1-aminoethylidene)-6-methyl-3,4-dihydro-2H-pyran-2,4-dione, a natural antiproliferative and cytotoxic product isolated from Fusarium incarnatum (HKI0504), was developed from dehydroacetic acid and urea under solvent-free microwave irradiation. The analysis of the co-crystal structure revealed an asymmetric unit formed by a pair of molecules. Each molecule is joined by two different hydrogen bonds to another two molecules, ordered as four-unit clusters linked by π-stacking, assembled in a brick like layered structure in a set of parallel walls. Besides, the preferred tautomer for crystal structure is the enamine form. This is corroborated by computational NBO analysis, outlining the contribution of enamine resonance and modelling the non-covalent interactions involved by means of Hirshfeld surfaces and G09 counterpoise calculations.

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Introduction

Fungal endophytes are receiving growing attention due to their diverse and structurally multifarious compounds which make them interesting candidates for drug discovery. Fusarium is one of the most important genera of fungi, causing an array of plant diseases, producing a wide range of toxins and adversely affecting human and animal health.¹

Recently, [(3E-3-(1-aminoethylidene)-6-methyl-3,4-dihydro-2H-pyran-2,4-dione)] (1) has been identified as a natural product which was isolated from the culture broth of Fusarium incarnatum (HKI0504), an endophytic fungus of the mangrove plant Aegiceras corniculatum.² It showed antiproliferative activity against human umbilical vein endothelial cells (HUVEC), K562 human chronic myeloid leukemia cells (DSM ACC 10) and cytotoxicity against HeLa human cervix carcinoma (DSM ACC 57) cell lines. This endophyte also has been identified as a novel producer of laccase with potential in bioremediation of bisphenol A.³

Compound 1 is an enamine derivative of dehydroacetic acid, (3-acetyl-4-hydroxy-6-methyl-2H-pyran-2-one, 2). Dehydroacetic acid acts as complexing ligand and possesses interesting biological properties such as fungicide and antibacterial activities.⁴ Its sodium salt is recognized by Food and Agriculture Organization (FAO) and the World Health Organization (WHO) as a safe food preservative, since it has a relatively broad spectrum of antibacterial activity against food-borne pathogens and spoilage organisms. Its enamine derivatives have also been object of wide studies, because of their different biological activities and ability to act as ligands in transition metal complexes.⁵

Results and discussion

The product of the reaction of dehydroacetic acid and ammonia is known since 1876,⁶ and several preparations were reported afterwards,⁷ although the first completely reported synthesis was that of Wang et al.⁸ where 1 was prepared by reaction of dehydroacetic acid with aq. NH₃ for 3 days at room temperature. Our experience in the enhancement of organic reactions by microwaves, led us to consider the possibility to improve the synthetic method using urea instead of NH₃, since it had proved to be a suitable source of ammonia under microwave irradiation for the synthesis of imides and enamines.⁹ Thus, a stoichiometric mixture of dehydroacetic acid and urea was irradiated, without adding any solvent, at 150 °C for 15 minutes in a monomode microwave oven (200 W power), yielding 85% of 1 after purification by column chromatography. As expected, this preparative method is a competitive alternative to non-assisted microwave synthesis.

The crystallization of the synthetized compound 1 from methanol, rendered a crystal whose structure was resolved resulting in a non-merohedral twin with the twin components related by a 180.0 degree rotation about the [1 0 0] axis. TWINABS was used to apply post-collection corrections. Both twin components were employed in corrections and overlaps in addition to the two components were included in the reflection file. An extra parameter was included on the refinement to properly calculate the twin ratio 0.58(2)/0.42(2). Asymmetric unit (AU, Fig. 2) has two components with slightly different geometries (Table 1).

Fig. 1 Dehydroacetic acid enamine (1), dehydroacetic acid (2), dehydroacetic acid imine (3).

Fig. 2 Molecules I and II in asymmetric unit of 1.

Table 1 Selected bond distances of 1

Molecule II	Å	Molecule I	Å [% II to I]
O1–C6	1.377(6)	O1B–C6B	1.360 (6) [101.3]
C3–C30	1.420 (7)	C3B–C30B	1.430 (7) [101.0]
C3–C4	1.448(6)	C3B–C4B	1.426 (7) [101.5]
C30–N31	1.308 (7)	C30B–N31B	1.312 (6) [100.3]
C30–C32	1.488 (7)	C30B–C32B	1.490 (7) [100.1]

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The analysis of the co-crystal¹⁰ confirms that the structure of both components in solid state correspond to enamine (1) rather than to imine (3) which agrees with previous studies carried out on this kind of compounds.¹¹ The two molecules in AU are bonded through π-stacking interaction. The value of this interaction (14.18 kcal mol⁻¹) was calculated with Gaussian 09 (MP2/6-311++G(2d,2p) method) with counterpoise correction,¹² using the coordinates determined from the co-crystal.

Each molecule of the AU has also π-stacking with a molecule (II′) in the upper face and with another (I′) in the bottom (Fig. 3). The calculated energies for these interactions were 15.01 and 14.93 kcal mol⁻¹ respectively.

Fig. 3 Molecules I and II are inside AU. Molecules I′ (−1 + x, y, z) and II' (1 + x, y, z) belong to different AU's.

In order to study the slight variation between the two components of the AU, the energy of these molecules was calculated separately (B3LYP/6-311++G(2d,2p)). For one component of the pair (Fig. 3, molecule I) the Hartree–Fock energy was −590.8085 a.u. with a dipole moment of 1.87 D. The other component of the asymmetric unit (Fig. 3, molecule II) rendered HF = −590.8079 a.u. and a dipole moment of 2.00 D. The difference of energy between both is negligible (0.348 kcal mol⁻¹).

Analysis of the co-crystal shows that each molecule is part of a four-molecule cluster joined by hydrogen bonds. Since enamine 1 contains one donor and two different acceptors of hydrogen (Fig. 4a), three types of hydrogen bonding are observed (Fig. 4b): (i) intramolecular bonds N_31B⋯O_40B and N₃₁⋯O₄₀ (corresponding to molecules II and I respectively), (ii) intermolecular between the amino and the ketone group N₃₁⋯O_40B and (iii) intermolecular between the amino group and the lactone N_31B⋯O₂₀. The values for intermolecular hydrogen bonds (Table 2), fall inside the category of moderate mostly electrostatic of hydrogen bonds with donor–acceptor as defined by Jeffrey.¹³

Fig. 4 (a) Acceptor and donor sites in compound 1, (b) (i) intra- and (ii–iii) inter-molecular hydrogen bonds present in four molecules-cluster.

Table 2 Hydrogen bonds of 1 (Å, °)

Hydrogen bond	D–H	H⋯A	D⋯A	D–H⋯A
a Symmetry code: x − 1, y − 1, z.b Symmetry code: −x + 1, −y + 2, −z.
N31–H31B⋯O40	0.89 (2)	1.81 (4)	2.58 (6)	143 (5)
N31–H31A⋯O40B^a	0.90 (5)	2.06 (5)	2.94 (6)	168 (6)
N31B–H31C⋯O40B	0.90 (2)	1.78 (3)	2.59 (6)	148 (5)
N31B–H31D⋯O20^b	0.90 (2)	2.08 (4)	2.85 (6)	143 (5)

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The calculated total interaction energy of this cluster (MP2/6-311++G(2d,2p)) was 35.49 kcal mol⁻¹ (i.e. 8.87 kcal mol⁻¹ per each pair of molecules). The different types of intermolecular hydrogen bond were studied separately. A couple of molecules joined by CO_lactone–hydrogen present an interaction of 7.997 kcal mol⁻¹, meanwhile for CO_ketone–hydrogen is 6.568 kcal mol⁻¹. The sum of these energies indicates an additional stabilization of 6.640 kcal mol⁻¹ when the four molecules cluster is considered.

Note that these four molecules have their rings in the same plane and these small clusters are arranged as bricks in a wall, besides all the clusters in a tier have their atoms in the same plane. Thus, two bricks in a tier are bounded by π-stacking (mortar) to one brick (four-molecule cluster) in the upper tier (Fig. 5a), building the wall (Fig. 5b). A plane is separated 3.321 Å from the next parallel plane in the same wall. Planes in vicinal walls are deviated 1.129 Å (Fig. 5c), and the tiers share planes each three walls, showed with a dotted line in Fig. 5d, where is also shown the orientation of the clusters walls inside BFDH predicted morphology of the co-crystal.

Fig. 5 (a) π-stacking interaction between several four-molecules clusters. (b) Wall built with clusters (bricks), each color represents a four-molecules cluster. (c) Distances between planes defined by the clusters (d) Tiers share planes (dotted line), each three walls, and predicted BFDH morphology of the co-crystal.

The Hirshfeld surface is defined in a crystal as that region around a molecule where the molecule contribution to the crystal electron density exceeds that from all other molecules in the crystal. It allows analyzing how molecules interact with their direct environment.¹⁴

When the cluster of four molecules is represented by their corresponding Hirshfeld surfaces with electrostatic potential mapped on it,¹⁵ the interactions among them are clearly shown as complimentary, those individual surfaces can be integrated in a four molecules common surface (Fig. 6a and b), being its arrangement in a wall as bricks (Fig. 6c). Fig. 6d shows the disposition of the electrostatic potential mapped on Hirshfeld surfaces of the π-stacked molecules represented in Fig. 2. The d_norm surfaces (Fig. 6e), reveal the close contacts of hydrogen bond donors and acceptors represented in Fig. 4b. The large circular depressions (cyan) are the indicators of hydrogen bonding contacts. The dominant O⋯H–N interactions are evident in the Hirshfeld surface and confirm the nature of the binding forces inside the cluster, π-stacking can also be observed with the upper row of molecules.

Fig. 6 Hirshfeld surfaces mapped with electrostatic potentials (a) individual surfaces in the four molecules cluster, (b) integrated surface for the cluster, (c) surfaces of the clusters in a wall like bricks, (d) individual molecular surfaces showing π-stacking (see Fig. 3), (e) d_norm surface displaying close contacts of hydrogen bonding and π-stacking.

Natural bond orbital (NBO) analysis of dehydroacetic acid derivatives has proved to be a useful way for understanding the structure of β-enaminones.¹¹ Now, the knowledge of the crystal structure of 1 will allow a better comparison between experimental and calculated data. Thus, the study of the properties in gas phase (b3lyp/6-311++G(2d,2p)) of the more stable component of the AU (molecule I, Fig. 1) was carried out. In this analysis (Table 3), stabilization energy E(2) related to the delocalization trend of electrons from donor to acceptor orbitals, is calculated via perturbation theory. Thus, NBO calculation using Gaussian 09 indicated that the highest interaction correspond to orbitals n_N31 → π_C3–C30* characteristic of the enamine structure, according to data from X-ray. The next higher energy interactions were π_C3–C30 → π_C2–O20* and π_C3–C30 → π_C4–O40* resulting from conjugation of the enamine double bond with both carbonyl groups. The lone pair of the oxygen in the ring is delocalized with the lactone carbonyl and the endocyclic alkene: n_O1 → π_C2–O20* and n_O1 → π_C5–C6*. Furthermore, the delocalization π_C5–C6 → π_C4–O40* supports the β-enaminone structure.

Table 3 Principal donor–acceptor interactions between NBO

Entry	Donor bond	Isovalue 95%	Acceptor bond	Isovalue 95%	(E2) kcal mol^−1
1	42. nN31		484. πC3–C30*		72.03
2	8. πC3–C30		481. πC2–O20*		39.33
3	8. πC3–C30		487. πC4–O40*		37.91
4	39. nO1		481. πC2–O20*		32.81
5	39. nO1		490. πC5–C6*		31.64
6	14. πC5–C6		487. πC4–O40*		24.90
7	44. nO40		494. σN31–H31B*		10.68
8	43. nO40		494. σN31–H31B*		2.12

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The principal donor–acceptor interactions between NBO orbitals are summarized in Table 3, showing the surfaces whose isovalue represents 95% of the orbital, so the effectiveness of the overlapping can be visualized.

The electronic conjugation in molecules was studied by Wiberg bond indexes,¹⁶ the results showed that C₃–C₃₀ and C₃₀–N₃₁ bonds have higher values than single bonds as C₃₀–C₃₂ (Fig. 7). So, this structure could be represented as a three centre delocalized bond in C₃–C₃₀–N₃₁.

Fig. 7 Wiberg bond indexes for molecules I and II in AU, dashed lines indicate three centre delocalized bonds.

As it is known, theoretical calculations of magnetic properties can be performed through different methods. The most widely used for this type of calculation is the GIAO method,¹⁷ where a source of potential vector of the external magnetic field for each atom is settled independently. Therefore, the coordinates of the structure derived from X-ray data were used to calculate NMR proton shifts (b3lyp/6-311++G(2d,2p)) by this method in the gas phase. The absolute shielding returned by the program was transformed in chemical shifts subtracting the absolute shielding of TMS from the absolute shielding of the molecule. However, the theoretical shifts obtained did not match with the experimental values. GIAO method may include the effect of solvent, and the polarizable continuum model (PCM) is generally used.¹⁸ Therefore, the presence of solvent was considered by placing the solute (coordinates from X-ray structure) in a cavity within the solvent reaction field (PCM). The results from this model implemented in Gaussian 09 also showed no accuracy; this could be due to the intermolecular hydrogen bonding present in the crystal. A refined molecular structure was calculated by minimizing the crystal structure with PCM model in chloroform; in this case the shifts (GIAO-PCM) were similar to experimental (Table 4).

Table 4 NMR proton shifts (δ, ppm)

Crystal coord. GIAO-(CDCl3)		Crystal coord. GIAO-(CDCl3)		Crystal coord. minimized (CDCl3)	δexp.
I	^1HNMR δ	II	^1HNMR δ	GIAO-(CDCl3) δ
H31B	9.51	H31C	7.99	12.95	12.56
H31A	3.79	H31D	3.46	6.22	6.96
H5	3.35	H5B	3.46	5.97	5.70
H32A	−1.71	H32D	−1.75	1.71	2.65
H32B	−0.39	H32E	−0.57	3.04	2.65
H32C	−0.75	H32F	−0.64	3.04	2.65
H60A	−1.65	H60D	−1.75	1.94	2.13
H60B	−1.47	H60E	−1.39	2.26	2.13
H60C	−1.47	H60F	−1.48	2.26	2.13

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The minimized structure presented a change in the relative positions of H_31A, H_31B, H_31C and H_31D (involved in hydrogen bonding). The position of nitrogen was also modified affecting to the length of the hydrogen bonds present (Table 5).

Table 5 Interatomic distances (Å)

Hydrogen bond	Crystal Xray I	Hydrogen bond	Crystal Xray II	Minimized PCM/CHCl3
O40B⋯H31C	1.78(3)	O40⋯H31B	1.81(4)	1.75
O40B⋯N31B	2.59(5)	O40⋯N31	2.58(5)	2.58
O40B⋯H31D	3.42(3)	O40⋯H31A	3.38(5)	3.49

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The interaction between O₄₀ and H_31B and O_40B and H_31C can be considered as an example of resonance-assisted hydrogen bond which is a model of synergistic interplay between π-delocalization and hydrogen-bond strengthening (RAHB).¹⁹ The interatomic distances (Table 5) from crystal structure and minimized are inside the values found by Gilli for substituted β-enaminones.²⁰ This heteronuclear RAHB is of great chemical and biochemical relevance because chains of H-bonded amide groups determine the secondary structure of proteins. NBO analysis reflects the stabilization gained by donation from two lone pairs of O₄₀ to the acceptor σ_N10–H20* being 10.68 and 2.12 kcal mol⁻¹ (Table 3, entries 7 and 8). Meanwhile, in the minimized structure the distance between N₁₀ and O₄₀ is shorter, leading to stabilization of 14.92 and 3.42 kcal mol⁻¹, respectively; higher than the observed for crystal, which agrees to a stronger hydrogen bond in the structure in solution.

Experimental

General experimental procedures

NMR spectra were recorded on a Varian Mercury 300 7.04 T (300.13 MHz for ¹H and 75.48 for ¹³C). Mass spectra were performed on a HP-Series 1100-MSD. IR spectra were performed on a ABM BOMEN MB102 on KBr pellets. For column chromatography was used 230–400 mesh silica gel. For microwave reaction a single-mode oven model CEM Discover was used.

(3E)-3-(1-Aminoethylidene)-6-methyl-3,4-dihydro-2H-pyran-2,4-dione (1). In a 10 mL tube, dehydroacetic acid (338 mg, 2.01 mmol) and urea (126 mg, 2.1 mmol) were well mixed. The mixture was irradiated in a monomode microwave oven with stirring at 150 °C for 15 min (200 W). The reaction crude was purified by column chromatography (eluent CH₂Cl₂/MeOH, 95 : 5), obtaining (3E)-3-(1-aminoethylidene)-6-methyl-3,4-dihydro-2H-pyran-2,4-dione (283 mg, 85%), as a light yellow solid. m. p. 210–212 °C (methanol) (lit. 210–213 °C).^{21 1}H-NMR (300 MHz, CDCl₃), δ: 12.56 (br s, 1H, NH₂), 6.96 (br s, 1H, NH₂), 5.70 (s, 1H, CH₃C=CH), 2.65 (s, 3H, CH₃C=CH), 2.13 (s, 3H, CH₃CN). ¹³C-NMR (75 MHz, DMSO), δ: 184.0 (COCH), 177.3 (CN), 163.7 (CH₃C=CH), 162.8 (COO), 108.0 (CH₃C=CH), 96.0 (CCOO), 24.7 (CH₃C=CH), 19.8 (CH₃CN). MS m/z (%): 167 (M⁺, 100), 152 (20), 126 (5), 124 (10), 97 (8), 83 (74), 68 (32), 55 (23). IR ν_max (KBr, film): 3281 (NH₂), 1715 (COO), 1684 (CO), 1668, 1585, 1571, 1468, 1355, 1068, 1037.

X-ray crystallographic analysis

For compound 1, single-crystal X-ray diffraction data were collected on an APPEX2 (BRUKER AXS, 2005); with Mo Kα radiation (λ = 0.7107 Å). The structure was solved by direct methods (SHELXS-86) and refined using SHELXL2012.

Crystal data for 1: C₁₆H₁₈N₂O₆, M = 334.32, a = 6.650(3) Å, b = 8.898(4) Å, c = 13.970(6) Å, α = 92.24(2)°, β = 98.98(2)°, γ = 111.168(19)°, V = 757.3(6) ų, T = 100(2) K, space group P1̄, Z = 2, μ(MoKα) = 0.113 mm⁻¹, 3506 reflections measured, 3506 independent reflections (R_int = 0.111). The final R₁ values were 0.0706 (I > 2σ(I)). The final wR(F²) values were 0.1431 (I > 2σ(I)). The final R₁ values were 0.1713 (all data). The final wR(F²) values were 0.1889 (all data). The goodness of fit on F² was 0.997. In the crystal structure coordinates for hydrogens H_31A, H_31B, H_31C and H_31D were positioned by electronic density and refined with a restraint to the length distance of 0.87 Å except H_31A which is completely free. For more details on geometry and refinement see ESI.†

Conclusions

This article presents an efficient solvent free microwave assisted synthesis for cytotoxic pyrandione alkaloid (3E)-3-(1-aminoethylidene)-6-methyl-3,4-dihydro-2H-pyran-2,4-dione, improving previously described synthesis. The study of the X-ray structure of this recently isolated natural product, shows a co-crystal with a basic four molecules cluster joined by two different types of intermolecular hydrogen bonds. These flat clusters are linked by π-stacking, adopting a brick like layered structure constituting a set of parallel walls. Besides, the preferred tautomer structure is the enamine form. This is corroborated by NBO analysis outlining the contribution of enamine resonance. The study of the Hirshfeld surfaces showed the influence of molecular electrostatic potential in the spatial disposition of the molecules. These results might be valuable for further structure–activity studies of substituted pyrandione, molecular design and synthesis of more potent and selective antiproliferative compounds.

Acknowledgements

XUNTA DE GALICIA for financial support: Grant INCITE09 262346PR. X.F. would also like to thank the Xunta de Galicia (Isidro Parga Pondal Program for young researchers, Grant no. IPP-020). Centro de Supercomputación de Galicia (CESGA) for providing computing facilities (Gaussian 09).²²

Notes and references

1. B. A. Summerell and J. F. Leslie, Fungal Divers., 2011, 50, 135; M. Nucci and E. Anaissie, Clin. Microbiol. Rev., 2007, 20, 695; R. Galimberti, A. C. Torre, M. C. Baztán and F. Rodriguez-Chiappetta, Clin. Dermatol., 2012, 30, 633; K. Kazan, D. M. Gardiner and J. M. Manners, Mol. Plant Pathol., 2012, 13, 399; L. V. Khoa, K. Hatai and T. Aoki, J. Fish Dis., 2004, 27, 507; S. Naiker and B. Odhav, Mycoses, 2004, 47, 50.
2. L. Ding, H.-M. Dahse and C. Hertweck, J. Nat. Prod., 2012, 75, 617.
3. U. Chhaya and A. Gupte, J. Hazard. Mater., 2013, 254–255, 149.
4. Y. Huang, M. Wilson, B. Chapman and A. D. Hocking, Food Microbiol., 2010, 27, 33; H. Zhang, H. Wei, Y. Cui, G. Zhao and F. Feng, J. Food Sci., 2009, 74, M418.
5. L. Duraković, A. Skelin, S. Sikora, F. Delaš, M. Mrkonjić-Fuka, K. Huić-Babić and M. Blažinkov, Afr. J. Biotechnol., 2011, 10, 10798; S. Kiryu, Acta Crystallogr., 1967, 23, 392; M. Laćan, I. Susnik-Rybarski, E. Mesić and H. Džanic, Liebigs Ann. Chem., 1980, 681; E. Budzisz, M. Malecka, I.-P. Lorenz, P. Mayer, R. A. Kwiecien, P. Paneth, U. Krajewska and M. Rozalski, Inorg. Chem., 2006, 45, 9688; S. Shahid, T. Mughal and H. U. Shah, Asian J. Chem., 2013, 25, 633; A. S. Munde, V. A. Shelke, S. M. Jadhav, A. S. Kirdant, S. R. Vaidya, S. G. Shankarwar and T. K. Chondhekar, Adv. Appl. Sci. Res., 2012, 3, 175; V. A. Shelke, S. M. Jadhav, S. G. Shankarwar, C. S. Munde and T. K. Chondhekar, J. Korean Chem. Soc., 2011, 55, 436; L. C. Dias, A. J. Demuner, V. M. M. Valente, L. C. A. Barbosa, F. T. Martins, A. C. Doriguetto and J. Ellena, J. Agric. Food Chem., 2009, 57, 1399.
6. A. Oppenheim, Ber. Dtsch. Chem. Ges., 1876, 9, 1099.
7. F. Feist, Ber. Dtsch. Chem. Ges., 1890, 26, 315; F. Feist, Justus Liebigs Ann. Chem., 1890, 257, 253; J. N. Collie and S. Myers, J. Chem. Soc. Trans., 1893, 63, 122; S. Iguchi and K. Hisatune, Yakugaku Zasshi, 1957, 77, 98; S. Kiryu, Acta Crystallogr., 1967, 23, 392; S. Iguchi, S. Goto and Y. Kodama, J. Pharm. Soc. Jpn., 1959, 79, 1100.
8. C. S. Wang, J. P. Easterly and N. E. Skelly, Tetrahedron, 1971, 27, 2581.
9. J. A. Seijas, M. P. Vázquez-Tato, C. González-Bande, M. M. Martínez and B. López-Pacios, Synthesis, 2001, 999; P. Ruault, J.-F. Pilard, B. Touaux, F. Texier-Boullet and J. Hamelin, Synlett, 1994, 935.
10. N. Upadhyay, T. P. Shukla, A. Mathur, Manmohan and S. K. Jha, Int. J. Pharm. Sci. Rev. Res., 2011, 8, 144.
11. A. Amar, H. Meghezzi, A. Boucekkine, R. Kaoua and B. Kolli, C. R. Chim., 2010, 13, 553; S. A. Hameed, S. K. Alrouby and R. Hilal, J. Mol. Model., 2013, 19, 559; P. E. Hansen, S. Bolvig and T. Kappe, J. Chem. Soc., Perkin Trans. 2, 1995, 1901.
12. N. Kobko and J. J. Dannenberg, J. Phys. Chem., 2001, 105, 1944.
13. G. A. Jeffrey, An introduction to hydrogen bonding, Oxford University Press, New York, 1997.
14. J. J. McKinnon, M. A. Spackman and A. S. Mitchell, Acta Crystallogr., Sect. B: Struct. Sci., 2004, 60, 627.
15. Hirshfeld surfaces were calculated using CrystalExplorer (Version 3.1, S. K. Wolff, D. J. Grimwood, J. J. McKinnon, M. J. Turner, D. Jayatilaka, M. A. Spackman, University of Western Australia, 2012) electrostatic potential was calculated using Tonto, D. Jayatilaka and D. J. Grimwood, Computational Science-ICCS 2003, 4, 142–151, in DFT calculation with a 6-311G(d,p) basis, Becke88 exchange potential and LYP correlation potential.
16. K. B. Wiberg, Tetrahedron, 1968, 24, 1083.
17. R. Ditchfield, J. Chem. Phys., 1972, 56, 5688.
18. S. Miertus, E. Scrocco and J. Tomasi, Chem. Phys., 1981, 55, 117.
19. G. Gilli, F. Bellucci, V. Ferretti and V. Bertolasi, J. Am. Chem. Soc., 1989, 111, 1023; V. Bertolasi, P. Gilli, V. Ferretti and G. Gilli, J. Am. Chem. Soc., 1991, 113, 4917; V. Bertolasi, L. Pretto, G. Gilli and P. Gilli, Acta Crystallogr., Sect. B: Struct. Sci., 2006, 62, 850; P. Gilli, L. Pretto, V. Bertolasi and G. Gilli, Acc. Chem. Res., 2009, 42, 33; V. Bertolasi, P. Gilli and G. Gilli, Cryst. Growth Des., 2012, 12, 4758; G. Gilli, V. Bertolasi and P. Gilli, Cryst. Growth Des., 2013, 13, 3308.
20. P. Gilli, V. Bertolasi, V. Ferretti and G. Gilli, J. Am. Chem. Soc., 2000, 122, 10405.
21. J. D. Edwards, J. E. Page and M. Pianka, J. Chem. Soc., 1964, 5200.
22. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN 09 (Revision A.02), Gaussian, Inc., Wallingford CT, 2010.

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Footnote

† Electronic supplementary information (ESI) available: cif file for compound 1, crystallographic tables, ¹H and ¹³C NMR spectra for compound 1. CCDC 945618. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00582a

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