research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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The 7-aza­norbornane nucleus of epibatidine: 7-aza­bi­cyclo­[2.2.1]heptan-7-ium chloride

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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

Edited by M. Zeller, Purdue University, USA (Received 18 August 2017; accepted 22 August 2017; online 30 August 2017)

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.

1. Chemical context

Since the discovery of the quinuclidine and tropane nuclei (Hamama et al., 2006[Hamama, W. S., Abd El-Magid, O. M. & Zoorob, H. H. (2006). Heterocycl. Chem. 43, 1397-1420.]; Pollini et al., 2006[Pollini, G. P., Benetti, S., De Risi, C. & Zanirato, V. (2006). Chem. Rev. 106, 2434-2454.]), 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[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.]; Gerzanich et al., 1995[Gerzanich, V., Peng, X., Wang, F., Wells, G., Anand, R., Fletcher, S. & Lindstrom, J. (1995). Mol. Pharmacol. 48, 774-782.]; Sullivan & Bannon, 1996[Sullivan, J. F. & Bannon, A. W. (1996). CNS Drug Rev. 2, 21-39.]; Dukat & Glennon, 2003[Dukat, M. & Glennon, R. A. (2003). Cell. Mol. Neurobiol. 23, 365-378.]). 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[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.]). 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[Carroll, I. F. (2004). Bioorg. Med. Chem. Lett. 14, 1889-1896.]; Daly et al., 2005[Daly, J. W., Spande, T. F. & Garraffo, H. M. (2005). J. Nat. Prod. 68, 1556-1575.]; Yogeeswari et al., 2006[Yogeeswari, P., Sriram, D., Bal, T. R. & Thirumurugan, R. (2006). Nat. Prod. Res. 20, 497-505.]; Garraffo et al., 2009[Garraffo, H. M., Spande, T. F. & Williams, M. (2009). Heterocycles, 79, 207-217.]). We are not aware, however, that an X-ray structure determination of the alkaloid itself has ever been reported, in spite of numerous publications related to its

[Scheme 1]
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[Braun, J. & Schwarz, K. (1930). Justus Liebigs Ann. Chem. 481, 56-68.]; Fraser & Swingle, 1970[Fraser, R. R. & Swingle, R. B. (1970). Can. J. Chem. 48, 2065-2074.]). In continuation of our studies related to bridged aza-heterocyclic systems (Britvin et al., 2015[Britvin, S. N. & Lotnyk, A. (2015). J. Am. Chem. Soc. 137, 5526-5535.], 2016[Britvin, S. N., Rumyantsev, A. M., Zobnina, A. E. & Padkina, M. V. (2016). Chem. Eur. J. pp. 14227-14235.], 2017[Britvin, S. N., Rumyantsev, A. M., Zobnina, A. E. & Padkina, M. V. (2017). J. Mol. Struct. 1130, 395-399.]), 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[link]) resembling the molecular geometry of its nearest carbocyclic counterpart, bicyclo[2.2.1]heptane (norbornane), 2 (Fitch & Jobic, 1993[Fitch, A. N. & Jobic, H. (1993). J. Chem. Soc. Chem. Commun. pp. 1516-1517.]). 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[Doms, L., Van Hemelrijk, D., Van de Mieroop, W., Lenstra, A. T. H. & Geise, H. J. (1985). Acta Cryst. B41, 270-274.]). 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 = [1 \over 2]. 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[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]). Further details of the inter­atomic distances and angles of 1 can be found in the supporting information.

[Figure 1]
Figure 1
The molecular structure and systematic atomic numbering scheme of the 7-azabicyclo[2.2.1]heptane (7-azanorbornane) parent ring in 1. Displacement ellipsoids are drawn at the 50% probability level. H atoms on C atoms in view (a) and the chloride counter-ion have been omitted for clarity. The labelling in the Figures corresponds to IUPAC notation (see text). Atoms C4, C3 and C5 are generated from C1, C2 and C6, respectively, by the symmetry operation (1 − x, y, z).

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[link]). 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[link]). That type of inter­leaved zigzag packing is known among chloride salts of secondary amines, both for alkyl- and aryl­amines (Adams et al., 1997[Adams, C., Raithby, P. R. & Davies, J. E. (1997). Private communication (deposition number 100996). CCDC, Cambridge, England.]; Nancy et al., 2003[Nancy Ghosh, S., Singh, N., Nanda, G. K., Venugopalan, P., Bharatam, P. V. & Trehan, S. (2003). Chem. Commun. pp. 1420-1421.]; Muller et al., 2007[Muller, M., Lerner, H.-W. & Bolte, M. (2007). Private communication (deposition number 661061). CCDC, Cambridge, England.]) and heterocyclic systems (Gribkov et al., 2006[Gribkov, D. V., Hultzsch, K. C. & Hampel, F. (2006). J. Am. Chem. Soc. 128, 3748-3759.]; Wang et al., 2011[Wang, J., Ma, C., Wu, Y., Lamb, R. A., Pinto, L. H. & DeGrado, W. F. (2011). J. Am. Chem. Soc. 133, 13844-13847.]; Fun et al., 2011[Fun, H.-K., Asik, S. I. J., Chandrakantha, B., Isloor, A. M. & Shetty, P. (2011). Acta Cryst. E67, o3115.]).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
N7—H7B⋯Cl1i 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: (i) [-x+1, -y+1, z+{\script{1\over 2}}].
[Figure 2]
Figure 2
Hydrogen bonding in the crystal structure of 1. Protonated mol­ecules of 7-aza­norbornane are linked via N—H⋯Cl hydrogen bonds to form infinite zigzag chains propagated along the c axis. Displacement ellipsoids are drawn at the 50% probability level. H atoms not involved in hydrogen bonding have been omitted for clarity.

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[Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171-179.]), 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[Ohwada, T., Achiwa, T., Okamoto, I., Shudo, K. & Yamaguchi, K. (1998). Tetrahedron Lett. 39, 865-868.]; Cheng et al. 2002[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.]; Otani et al. 2003[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.]; Hori et al. 2008[Hori, T., Otani, Y., Kawahata, M., Yamaguchi, K. & Ohwada, T. (2008). J. Org. Chem. 73, 9102-9108.]; Longobardi et al. 2015[Longobardi, L. E., Mahdi, T. & Stephan, D. W. (2015). Dalton Trans. 44, 7114-7117.]).

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 external standard, 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[link]), 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 refinement details are summarized in Table 2[link].

Table 2
Experimental details

Crystal data
Chemical formula C6H12N+·Cl
Mr 133.62
Crystal system, space group Orthorhombic, Cmc21
Temperature (K) 100
a, b, c (Å) 9.1532 (6), 8.7029 (8), 8.7336 (5)
V3) 695.71 (9)
Z 4
Radiation type Mo Kα
μ (mm−1) 0.45
Crystal size (mm) 0.08 × 0.06 × 0.04
 
Data collection
Diffractometer Bruker APEXII CCD
Absorption correction Multi-scan (SADABS; Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.])
No. of measured, independent and observed [I > 2σ(I)] reflections 3239, 777, 769
Rint 0.017
(sin θ/λ)max−1) 0.638
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.017, 0.048, 1.15
No. of reflections 777
No. of parameters 47
No. of restraints 1
H-atom treatment H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3) 0.21, −0.12
Absolute structure Refined as an inversion twin
Absolute structure parameter 0.19 (9)
Computer programs: APEX2 (Bruker, 2015[Bruker (2015). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]), SAINT (Bruker, 2015[Bruker (2015). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]), SHELXT (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]a), OLEX2 (Dolomanov et al., 2009[Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339-341.]), SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]b), Mercury (Macrae et al., 2008[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.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).

Supporting information


Computing details top

Data collection: APEX2 (Bruker, 2015); cell refinement: 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).

7-Azabicyclo[2.2.1]heptan-7-ium chloride top
Crystal data top
C6H12N+·ClDx = 1.276 Mg m3
Mr = 133.62Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmc21Cell parameters from 2988 reflections
a = 9.1532 (6) Åθ = 3.2–30.7°
b = 8.7029 (8) ŵ = 0.45 mm1
c = 8.7336 (5) ÅT = 100 K
V = 695.71 (9) Å3Block, colourless
Z = 40.08 × 0.06 × 0.04 mm
F(000) = 288
Data collection top
Bruker APEX-II CCD
diffractometer
777 independent reflections
Radiation source: fine focus sealed tube769 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.017
φ and ω scansθmax = 27.0°, θmin = 3.2°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2015)
h = 1111
k = 411
3239 measured reflectionsl = 1110
Refinement top
Refinement on F2Hydrogen site location: mixed
Least-squares matrix: fullH 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 = 1.15Δρmax = 0.21 e Å3
777 reflectionsΔρmin = 0.12 e Å3
47 parametersAbsolute structure: Refined as an inversion twin
1 restraintAbsolute structure parameter: 0.19 (9)
Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C10.62232 (18)0.2599 (3)0.5813 (2)0.0174 (4)
H10.72050.29600.55470.021*
C20.5850 (2)0.2715 (3)0.75142 (19)0.0202 (4)
H2B0.62330.36550.79560.024*
H2A0.62330.18430.80780.024*
C60.5848 (2)0.10083 (19)0.5202 (2)0.0209 (4)
H6A0.62310.02100.58640.025*
H6B0.62310.08630.41760.025*
N70.50000.3539 (2)0.5134 (2)0.0139 (4)
H7B0.50000.449 (4)0.548 (3)0.014 (7)*
H7A0.50000.347 (4)0.414 (5)0.030 (9)*
Cl10.50000.31735 (5)0.15788 (7)0.01568 (15)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0107 (7)0.0188 (10)0.0226 (9)0.0012 (7)0.0009 (6)0.0003 (7)
C20.0227 (10)0.0240 (10)0.0140 (8)0.0018 (8)0.0063 (7)0.0029 (7)
C60.0239 (9)0.0157 (9)0.0232 (9)0.0046 (6)0.0001 (7)0.0022 (8)
N70.0199 (10)0.0119 (9)0.0100 (9)0.0000.0000.0007 (8)
Cl10.0213 (2)0.0142 (2)0.0116 (2)0.0000.0000.0005 (2)
Geometric parameters (Å, º) top
C1—H10.9800C6—C6i1.553 (4)
C1—C21.528 (2)C6—H6A0.9700
C1—C61.523 (3)C6—H6B0.9700
C1—N71.508 (2)N7—C1i1.508 (2)
C2—C2i1.556 (4)N7—H7B0.88 (3)
C2—H2B0.9700N7—H7A0.87 (4)
C2—H2A0.9700
C2—C1—H1114.5C1—C6—C6i103.03 (9)
C6—C1—H1114.5C1—C6—H6A111.2
C6—C1—C2110.50 (19)C1—C6—H6B111.2
N7—C1—H1114.5C6i—C6—H6A111.2
N7—C1—C2100.39 (16)C6i—C6—H6B111.2
N7—C1—C6100.82 (15)H6A—C6—H6B109.1
C1—C2—C2i102.93 (9)C1—N7—C1i95.91 (18)
C1—C2—H2B111.2C1—N7—H7B111.8 (11)
C1—C2—H2A111.2C1i—N7—H7B111.8 (11)
C2i—C2—H2B111.2C1—N7—H7A110.6 (14)
C2i—C2—H2A111.2C1i—N7—H7A110.6 (14)
H2B—C2—H2A109.1H7B—N7—H7A115 (3)
C2—C1—C6—C6i70.63 (12)C6—C1—N7—C1i56.31 (19)
C2—C1—N7—C1i57.1 (2)N7—C1—C2—C2i35.24 (15)
C6—C1—C2—C2i70.56 (14)N7—C1—C6—C6i34.89 (12)
Symmetry code: (i) x+1, y, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N7—H7B···Cl1ii0.88 (3)2.25 (3)3.127 (2)175 (2)
N7—H7A···Cl10.87 (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).

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