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

7-Aza­bicyclo­[2.2.1]heptane (7-aza­norbornane) is a bridged heterocyclic nucleus found in epibatidine. 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, has been carried out.

Abstract

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.

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 structure determination 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.

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.

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).

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 amines, 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 ▸).

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	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) .

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.

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 ▸).

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 ▸), 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.

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 ▸.

Table 2. Experimental details.

Crystal data
Chemical formula	C6H12N+·Cl−
M r	133.62
Crystal system, space group	Orthorhombic, C m c21
Temperature (K)	100
a, b, c (Å)	9.1532 (6), 8.7029 (8), 8.7336 (5)
V (Å3)	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 ▸)
No. of measured, independent and observed [I > 2σ(I)] reflections	3239, 777, 769
R int	0.017
(sin θ/λ)max (Å−1)	0.638
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), 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 ▸), SAINT (Bruker, 2015 ▸), SHELXT (Sheldrick, 2015 ▸a), OLEX2 (Dolomanov et al., 2009 ▸), SHELXL2014 (Sheldrick, 2015 ▸b), Mercury (Macrae et al., 2008 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

CCDC reference: 1511233

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Crystal data

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

Data collection

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

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)

Special details

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 (Ų)

	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)

Atomic displacement parameters (Ų)

	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)

Geometric parameters (Å, º)

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.

Hydrogen-bond geometry (Å, º)

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.

Funding Statement

This work was funded by Saint-Petersburg State University grants 0.37.235.2015 and 3.37.222.2015.