research communications
The 7-azanorbornane nucleus of epibatidine: 7-azabicyclo[2.2.1]heptan-7-ium chloride
aDepartment of Crystallography, Saint-Petersburg State University, Universitetskaya Nab. 7/9, 199034 St Petersburg, Russian Federation, and bDepartment of Genetics and Biotechnology, Saint-Petersburg State University, Universitetskaya Nab. 7/9, 199034 St Petersburg, Russian Federation
*Correspondence e-mail: sergei.britvin@spbu.ru
7-Azabicyclo[2.2.1]heptane (7-azanorbornane) is a bridged heterocyclic nucleus found in epibatidine, the alkaloid isolated from the skin of the tropical poison frog Epipedobates tricolor. Since epibatidine is known as one of the most potent acetylcholine nicotinic receptor agonists, a plethora of literature has been devoted to this alkaloid. However, there are no structural data on the unsubstituted 7-azanorbornane, the parent bicyclic ring of epibatidine and its derivatives. We herein present the structural characterization of the 7-azabicyclo[2.2.1]heptane parent ring as its hydrochloride salt, namely 7-azabicyclo[2.2.1]heptan-7-ium chloride, C6H12N+·Cl−. The compete cation is generated by a crystallographic mirror plane with the N atom lying on the mirror, as does the chloride anion. In the crystal, the cations are linked to the anions by N—H⋯Cl hydrogen bonds, which generate [001] chains.
Keywords: crystal structure; cage compounds; nitrogen heterocycles; amine; alkaloid; epibatidine.
CCDC reference: 1511233
1. Chemical context
Since the discovery of the quinuclidine and tropane nuclei (Hamama et al., 2006; Pollini et al., 2006), elegant frameworks of bridged aza-heterocycles have been the focus of chemists exploring biologically active substances. One famous example in this series is epibatidine, (−)-2-(6-chloropyridin-3-yl)-7-azabicyclo[2.2.1]heptane, an active component of the skin poison extracted from the small tropical frog Epipedobates tricolor (Spande et al., 1992; Gerzanich et al., 1995; Sullivan & Bannon, 1996; Dukat & Glennon, 2003). Epibatidine comprises the first natural example of a compound incorporating an 7-azabicyclo[2.2.1]heptane (7-azanorbornane) ring system (Fletcher et al., 1994). Due to the extreme binding affinity of the exo isomer of epibatidine towards nicotinic acetylcholine receptors, thousands of articles have been devoted to different aspects of its chemistry and biochemistry (see Carroll, 2004; Daly et al., 2005; Yogeeswari et al., 2006; Garraffo et al., 2009). We are not aware, however, that an X-ray of the alkaloid itself has ever been reported, in spite of numerous publications related to its
synthesis. Moreover, the molecular structure of 7-azanorbornane, the functional core of epibatidine, has also not been explored, in spite of the fact that 7-azanorbornane has been known since 1930 (Braun & Schwarz, 1930; Fraser & Swingle, 1970). In continuation of our studies related to bridged aza-heterocyclic systems (Britvin et al., 2015, 2016, 2017), we herein report on the structure of the unsubstituted 7-azabicyclo[2.2.1]heptane parent ring as its hydrochloride salt, namely 7-azabicyclo[2.2.1]heptan-7-ium chloride, 1.2. Structural commentary
The parent ring of 7-azabicyclo[2.2.1]heptane in 1 adopts a boat conformation (Fig. 1) resembling the molecular geometry of its nearest carbocyclic counterpart, bicyclo[2.2.1]heptane (norbornane), 2 (Fitch & Jobic, 1993). In order to achieve consistency of atomic labelling between the bicyclic cages of 1 and 2, we herein apply the numbering scheme according to IUPAC nomenclature (Fig. 1) (Doms et al., 1985). There are three unique C atoms (C1, C2 and C6) in the cation of 1, with their clones C1i [= C4 by IUPAC; symmetry code: (i) 1 − x, y, z], C2i (= C3 by IUPAC) and C6i (= C5 by IUPAC) generated by the mirror at x = . Interatomic distances between the respective framework sites of 1 are shorter compared with the corresponding values of 2. The distances (Å) in 1 and 2 are: C1—C2 = 1.528 (2) and 1.551 (3), C1—C6 = 1.523 (3) and 1.578 (1), and C1—N7(C7) = 1.508 (2) and 1.551 (3). The C2i—C2—C1—C6 torsion angle determining the boat-like conformation is 109.4 (1)° in 1 and 108.7 (2)° in 2. The s.u. values for 2 were generated using PLATON (Spek, 2009). Further details of the interatomic distances and angles of 1 can be found in the supporting information.
3. Supramolecular features
The structural integrity of 1 is maintained via intermolecular hydrogen bonding between the protonated secondary site N7 and the chloride counter-ion Cl1 (Table 1). Each chloride ion is linked to the two adjacent amine centres via N—H⋯Cl hydrogen bonds so that the 7-azanorbornane cages are arranged into zigzag chains flattened on (010) and propagating along the c-axis direction (Fig. 2). That type of interleaved zigzag packing is known among chloride salts of secondary both for alkyl- and arylamines (Adams et al., 1997; Nancy et al., 2003; Muller et al., 2007) and heterocyclic systems (Gribkov et al., 2006; Wang et al., 2011; Fun et al., 2011).
4. Database survey
Of more than 120 structures containing the 7-azanorbornane ring system in the Cambridge Structural Database (CSD, Version 5.38, latest update May 2017; Groom et al., 2016), 17 entries represent the 7-azabicyclo[2.2.1]heptane parent ring unsubstituted at the carbon sites. All these compounds belong to N-substituted derivatives of 7-azanorbornane (Ohwada et al. 1998; Cheng et al. 2002; Otani et al. 2003; Hori et al. 2008; Longobardi et al. 2015).
5. Synthesis and crystallization
7-Azabicyclo[2.2.1]heptane hydrochloride, 1, was obtained from Sigma Aldrich. The purity of the substance has been proven by elemental analysis (analysis calculated for C6H12ClN: C 53.93, H 9.05, N 10.48%; found: C 53.89, H 9.08, N 10.44%). 1H NMR (400 MHz) spectrum (Bruker Avance 400, SiMe4 D2O solution): δ 4.21–4.19 (m, 2H, 2 × CH at C1 and C4; the atom-numbering scheme is according to IUPAC nomenclature, see Fig. 1), 1.92–1.84 (m, 4H, 4 × endo-HCH at C2, C3, C5, C6), 1.78–1.71 (m, 4H, 4 × exo-HCH at C2, C3, C5, C6). 13C{1H} NMR (101 MHz): δ 58.9 (s, C1 and C4), 26.7 (s, C2, C3, C5, C6). Crystals of 1 suitable for structural studies were obtained by slow evaporation of its aqueous solution.
6. Refinement
H atoms at the protonated N7 atom were refined freely, whereas H atoms on C atoms were refined based on a riding model. Crystal data, data collection and structure .
details are summarized in Table 2Supporting information
CCDC reference: 1511233
https://doi.org/10.1107/S2056989017012105/zl2713sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989017012105/zl2713Isup2.hkl
Supporting information file. DOI: https://doi.org/10.1107/S2056989017012105/zl2713Isup3.mol
Supporting information file. DOI: https://doi.org/10.1107/S2056989017012105/zl2713Isup4.cml
Data collection: APEX2 (Bruker, 2015); cell
SAINT (Bruker, 2015); program(s) used to solve structure: SHELXT (Sheldrick, 2015a) and OLEX2 (Dolomanov et al., 2009); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).C6H12N+·Cl− | Dx = 1.276 Mg m−3 |
Mr = 133.62 | Mo Kα radiation, λ = 0.71073 Å |
Orthorhombic, Cmc21 | Cell parameters from 2988 reflections |
a = 9.1532 (6) Å | θ = 3.2–30.7° |
b = 8.7029 (8) Å | µ = 0.45 mm−1 |
c = 8.7336 (5) Å | T = 100 K |
V = 695.71 (9) Å3 | Block, colourless |
Z = 4 | 0.08 × 0.06 × 0.04 mm |
F(000) = 288 |
Bruker APEX-II CCD diffractometer | 777 independent reflections |
Radiation source: fine focus sealed tube | 769 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.017 |
φ and ω scans | θmax = 27.0°, θmin = 3.2° |
Absorption correction: multi-scan (SADABS; Sheldrick, 2015) | h = −11→11 |
k = −4→11 | |
3239 measured reflections | l = −11→10 |
Refinement on F2 | Hydrogen site location: mixed |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.017 | w = 1/[σ2(Fo2) + (0.0282P)2 + 0.1322P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.048 | (Δ/σ)max < 0.001 |
S = 1.15 | Δρmax = 0.21 e Å−3 |
777 reflections | Δρmin = −0.12 e Å−3 |
47 parameters | Absolute structure: Refined as an inversion twin |
1 restraint | Absolute structure parameter: 0.19 (9) |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
Refinement. Single-crystal data collection was performed using a Bruker Kappa APEX II DUO diffractometer equipped with microfocus optics. Refinement of lattice parameters and subsequent data reduction was carried out with the Bruker SAINT software. The crystal structure of 1 was solved and refined using SHELXT and SHELXL-2014 (Sheldrick, 2015) via the OLEX2 v.1.2 graphical user interface (Dolomanov et al., 2009). Refined as a 2-component inversion twin. |
x | y | z | Uiso*/Ueq | ||
C1 | 0.62232 (18) | 0.2599 (3) | 0.5813 (2) | 0.0174 (4) | |
H1 | 0.7205 | 0.2960 | 0.5547 | 0.021* | |
C2 | 0.5850 (2) | 0.2715 (3) | 0.75142 (19) | 0.0202 (4) | |
H2B | 0.6233 | 0.3655 | 0.7956 | 0.024* | |
H2A | 0.6233 | 0.1843 | 0.8078 | 0.024* | |
C6 | 0.5848 (2) | 0.10083 (19) | 0.5202 (2) | 0.0209 (4) | |
H6A | 0.6231 | 0.0210 | 0.5864 | 0.025* | |
H6B | 0.6231 | 0.0863 | 0.4176 | 0.025* | |
N7 | 0.5000 | 0.3539 (2) | 0.5134 (2) | 0.0139 (4) | |
H7B | 0.5000 | 0.449 (4) | 0.548 (3) | 0.014 (7)* | |
H7A | 0.5000 | 0.347 (4) | 0.414 (5) | 0.030 (9)* | |
Cl1 | 0.5000 | 0.31735 (5) | 0.15788 (7) | 0.01568 (15) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C1 | 0.0107 (7) | 0.0188 (10) | 0.0226 (9) | 0.0012 (7) | −0.0009 (6) | 0.0003 (7) |
C2 | 0.0227 (10) | 0.0240 (10) | 0.0140 (8) | 0.0018 (8) | −0.0063 (7) | 0.0029 (7) |
C6 | 0.0239 (9) | 0.0157 (9) | 0.0232 (9) | 0.0046 (6) | −0.0001 (7) | −0.0022 (8) |
N7 | 0.0199 (10) | 0.0119 (9) | 0.0100 (9) | 0.000 | 0.000 | 0.0007 (8) |
Cl1 | 0.0213 (2) | 0.0142 (2) | 0.0116 (2) | 0.000 | 0.000 | 0.0005 (2) |
C1—H1 | 0.9800 | C6—C6i | 1.553 (4) |
C1—C2 | 1.528 (2) | C6—H6A | 0.9700 |
C1—C6 | 1.523 (3) | C6—H6B | 0.9700 |
C1—N7 | 1.508 (2) | N7—C1i | 1.508 (2) |
C2—C2i | 1.556 (4) | N7—H7B | 0.88 (3) |
C2—H2B | 0.9700 | N7—H7A | 0.87 (4) |
C2—H2A | 0.9700 | ||
C2—C1—H1 | 114.5 | C1—C6—C6i | 103.03 (9) |
C6—C1—H1 | 114.5 | C1—C6—H6A | 111.2 |
C6—C1—C2 | 110.50 (19) | C1—C6—H6B | 111.2 |
N7—C1—H1 | 114.5 | C6i—C6—H6A | 111.2 |
N7—C1—C2 | 100.39 (16) | C6i—C6—H6B | 111.2 |
N7—C1—C6 | 100.82 (15) | H6A—C6—H6B | 109.1 |
C1—C2—C2i | 102.93 (9) | C1—N7—C1i | 95.91 (18) |
C1—C2—H2B | 111.2 | C1—N7—H7B | 111.8 (11) |
C1—C2—H2A | 111.2 | C1i—N7—H7B | 111.8 (11) |
C2i—C2—H2B | 111.2 | C1—N7—H7A | 110.6 (14) |
C2i—C2—H2A | 111.2 | C1i—N7—H7A | 110.6 (14) |
H2B—C2—H2A | 109.1 | H7B—N7—H7A | 115 (3) |
C2—C1—C6—C6i | 70.63 (12) | C6—C1—N7—C1i | 56.31 (19) |
C2—C1—N7—C1i | −57.1 (2) | N7—C1—C2—C2i | 35.24 (15) |
C6—C1—C2—C2i | −70.56 (14) | N7—C1—C6—C6i | −34.89 (12) |
Symmetry code: (i) −x+1, y, z. |
D—H···A | D—H | H···A | D···A | D—H···A |
N7—H7B···Cl1ii | 0.88 (3) | 2.25 (3) | 3.127 (2) | 175 (2) |
N7—H7A···Cl1 | 0.87 (4) | 2.25 (4) | 3.122 (2) | 178 (3) |
Symmetry code: (ii) −x+1, −y+1, z+1/2. |
Acknowledgements
The authors thank the X-ray Diffraction Center, Center for Magnetic Resonance and Computer Resource Center of Saint-Petersburg State University for instrumental and computational resources.
Funding information
Funding for this research was provided by: Saint-Petersburg State University (grant Nos. 0.37.235.2015 and 3.37.222.2015).
References
Adams, C., Raithby, P. R. & Davies, J. E. (1997). Private communication (deposition number 100996). CCDC, Cambridge, England. Google Scholar
Braun, J. & Schwarz, K. (1930). Justus Liebigs Ann. Chem. 481, 56–68. CAS Google Scholar
Britvin, S. N. & Lotnyk, A. (2015). J. Am. Chem. Soc. 137, 5526–5535. CrossRef CAS PubMed Google Scholar
Britvin, S. N., Rumyantsev, A. M., Zobnina, A. E. & Padkina, M. V. (2016). Chem. Eur. J. pp. 14227–14235. CrossRef Google Scholar
Britvin, S. N., Rumyantsev, A. M., Zobnina, A. E. & Padkina, M. V. (2017). J. Mol. Struct. 1130, 395–399. CrossRef CAS Google Scholar
Bruker (2015). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Carroll, I. F. (2004). Bioorg. Med. Chem. Lett. 14, 1889–1896. CrossRef PubMed CAS Google Scholar
Cheng, J., Zhang, C., Stevens, E. D., Izenwasser, S., Wade, D., Chen, S., Paul, D. & Trudell, M. L. (2002). J. Med. Chem. 45, 3041–3047. CrossRef PubMed CAS Google Scholar
Daly, J. W., Spande, T. F. & Garraffo, H. M. (2005). J. Nat. Prod. 68, 1556–1575. CrossRef PubMed CAS Google Scholar
Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339–341. Web of Science CrossRef CAS IUCr Journals Google Scholar
Doms, L., Van Hemelrijk, D., Van de Mieroop, W., Lenstra, A. T. H. & Geise, H. J. (1985). Acta Cryst. B41, 270–274. CrossRef CAS IUCr Journals Google Scholar
Dukat, M. & Glennon, R. A. (2003). Cell. Mol. Neurobiol. 23, 365–378. CrossRef PubMed CAS Google Scholar
Fitch, A. N. & Jobic, H. (1993). J. Chem. Soc. Chem. Commun. pp. 1516–1517. CSD CrossRef Web of Science Google Scholar
Fletcher, S. R., Baker, R., Chambers, M. S., Herbert, R. H., Hobbs, S. C., Thomas, S. R., Verrier, H. M., Watt, A. P. & Ball, R. G. (1994). J. Org. Chem. 59, 1771–1778. CrossRef CAS Google Scholar
Fraser, R. R. & Swingle, R. B. (1970). Can. J. Chem. 48, 2065–2074. CrossRef CAS Google Scholar
Fun, H.-K., Asik, S. I. J., Chandrakantha, B., Isloor, A. M. & Shetty, P. (2011). Acta Cryst. E67, o3115. Web of Science CSD CrossRef IUCr Journals Google Scholar
Garraffo, H. M., Spande, T. F. & Williams, M. (2009). Heterocycles, 79, 207–217. CAS Google Scholar
Gerzanich, V., Peng, X., Wang, F., Wells, G., Anand, R., Fletcher, S. & Lindstrom, J. (1995). Mol. Pharmacol. 48, 774–782. CAS PubMed Google Scholar
Gribkov, D. V., Hultzsch, K. C. & Hampel, F. (2006). J. Am. Chem. Soc. 128, 3748–3759. CrossRef PubMed CAS Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CSD CrossRef IUCr Journals Google Scholar
Hamama, W. S., Abd El-Magid, O. M. & Zoorob, H. H. (2006). Heterocycl. Chem. 43, 1397–1420. CrossRef CAS Google Scholar
Hori, T., Otani, Y., Kawahata, M., Yamaguchi, K. & Ohwada, T. (2008). J. Org. Chem. 73, 9102–9108. CrossRef PubMed CAS Google Scholar
Longobardi, L. E., Mahdi, T. & Stephan, D. W. (2015). Dalton Trans. 44, 7114–7117. CrossRef CAS PubMed Google Scholar
Macrae, C. F., Bruno, I. J., Chisholm, J. A., Edgington, P. R., McCabe, P., Pidcock, E., Rodriguez-Monge, L., Taylor, R., van de Streek, J. & Wood, P. A. (2008). J. Appl. Cryst. 41, 466–470. Web of Science CSD CrossRef CAS IUCr Journals Google Scholar
Muller, M., Lerner, H.-W. & Bolte, M. (2007). Private communication (deposition number 661061). CCDC, Cambridge, England. Google Scholar
Nancy Ghosh, S., Singh, N., Nanda, G. K., Venugopalan, P., Bharatam, P. V. & Trehan, S. (2003). Chem. Commun. pp. 1420–1421. Google Scholar
Ohwada, T., Achiwa, T., Okamoto, I., Shudo, K. & Yamaguchi, K. (1998). Tetrahedron Lett. 39, 865–868. CrossRef CAS Google Scholar
Otani, Y., Nagae, O., Naruse, Y., Inagaki, S., Ohno, M., Yamaguchi, K., Yamamoto, G., Uchiyama, M. & Ohwada, T. (2003). J. Am. Chem. Soc. 125, 15191–15199. CrossRef PubMed CAS Google Scholar
Pollini, G. P., Benetti, S., De Risi, C. & Zanirato, V. (2006). Chem. Rev. 106, 2434–2454. Web of Science CrossRef PubMed CAS Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Spande, T. F., Garraffo, H. M., Edwards, M. W., Yeh, H. J. C., Pannell, L. & Daly, J. W. (1992). J. Am. Chem. Soc. 114, 3475–3478. CrossRef CAS Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sullivan, J. F. & Bannon, A. W. (1996). CNS Drug Rev. 2, 21–39. CrossRef CAS Google Scholar
Wang, J., Ma, C., Wu, Y., Lamb, R. A., Pinto, L. H. & DeGrado, W. F. (2011). J. Am. Chem. Soc. 133, 13844–13847. CrossRef CAS PubMed Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
Yogeeswari, P., Sriram, D., Bal, T. R. & Thirumurugan, R. (2006). Nat. Prod. Res. 20, 497–505. CrossRef PubMed CAS Google Scholar
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