organic compounds
rac-3-exo-Ammonio-7-anti-carboxytricyclo[2.2.1.0.2,6]heptane-3-endo-carboxylate monohydrate
aScience and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia, bSchool of Biomolecular and Physical Sciences, Griffith University, Nathan, Queensland, 4111, Australia, and cEskitis Institute, Griffith University, Nathan, Queensland, 4111, Australia
*Correspondence e-mail: g.smith@qut.edu.au
The racemic title compound, C9H11NO4·H2O, a tricyclic rearranged aminonorbornane dicarboxylic acid, is a conformationally rigid analogue of glutamic acid and exists as an ammonium-carboxylate zwitterion, with the bridghead carboxylic acid group anti-related. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds involving the ammonium, carboxylic acid and water donor groups with both water and carboxyl O-atom acceptors give a three-dimensional framework structure.
Related literature
For background to G-protein receptors, see: Liu & Doller (2011). For the Strecher and Bucherer–Bergs reactions, see: Strecher (1850); Bucherer & Steiner (1934). For the synthesis of aminonorbornane carboxylic acids, see: Apgar & Ludwig (1972); Tager & Christensen (1972); Wermuth (1995). For the chemistry of hydantoins, see: Avendaño López & González Trigo (1985). For the structure of a similar monocarboxylic acid tricyclic see: Fortier et al. (1979). For graph-set analysis, see: Etter et al. (1990).
Experimental
Crystal data
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Refinement
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Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) within WinGX (Farrugia, 1999); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON.
Supporting information
10.1107/S1600536812016236/lh5454sup1.cif
contains datablocks global, I. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536812016236/lh5454Isup2.hkl
Supporting information file. DOI: 10.1107/S1600536812016236/lh5454Isup3.cml
The title compound (I) was synthesized (Wermuth, 1995) by the hydrolysis with Ba(OH)2 of the diasteroisomeric hydantoin mixture (II), which was obtained by a Read synthesis (Avendaño López & González Trigo, 1985) performed on the nortricyclic keto-ester (IV) (Fig. 3). Briefly, a Strecker aminonitrile (Streker, 1850) is formed in the usual manner (50% yield) and this was converted to a hydrochloride (III) and reacted with KOCN in an acetic acid–water mixture at 273K for 1 h followed by the addition of conc. HCl and heating for a further 15 min at 273K. The product was a diastereomeric mixture of hydantoins in 49% yield after recrystallization from 50% aqueous ethanol. The stereochemistry of the amino acid moiety is the inverse (carboxylic acid group exo) of that normally formed in the Bucherer-Bergs reaction (Bucherer & Steiner, 1934). The colourless product obtained gave an elemental analysis consistent with a 0.25 hydrate but recrystallization from various solvents gave no crystals suitable for X-ray analysis. However, colourless plates of a monohydrate (I) were obtained from the attempted reaction of this partial hydrate with picrylsulfonic acid in 80% propan-2-ol-water and a specimen suitable for the X-ray analysis was cleaved from a larger crystal.
Ammonium and water H atoms were located in a difference Fourier map and both positional and isotropic displacenment parameters were refined. Other H atoms were included in the
at calculated positions [C—H = 0.97–0.98 Å] with Uiso(H) = 1.2Ueq(C), using a riding-model approximation. The of the molecule described for (I) is C1(R), C2(S), C3(R), C4(R), C6(S), C7(R).Data collection: CrysAlis PRO (Oxford Diffraction, 2010); cell
CrysAlis PRO (Oxford Diffraction, 2010); data reduction: CrysAlis PRO (Oxford Diffraction, 2010); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008) within WinGX (Farrugia, 1999); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON (Spek, 2009).Fig. 1. Molecular configuration and atom naming scheme of the zwitterionic title compound (I). The inter-species hydrogen bond is shown as a dashed line and displacement ellipsoids are drawn at the 40% probability level. | |
Fig. 2. Hydrogen-bonding (shown as dashed lines) in the three-dimensionlal structure of the title compound, viewed approximately along the c axis. The symmetry codes are as in Table 1. | |
Fig. 3. The reaction scheme for the synthesis of the title compound. |
C9H11NO4·H2O | F(000) = 456 |
Mr = 215.20 | Dx = 1.569 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 3396 reflections |
a = 7.7565 (2) Å | θ = 3.2–28.6° |
b = 11.4103 (2) Å | µ = 0.13 mm−1 |
c = 10.3339 (3) Å | T = 223 K |
β = 94.888 (2)° | Plate, colourless |
V = 911.27 (4) Å3 | 0.30 × 0.30 × 0.15 mm |
Z = 4 |
Oxford Diffraction Gemini-S Ultra CCD-detector diffractometer | 2128 independent reflections |
Radiation source: Enhance (Mo) X-ray source | 1589 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.026 |
Detector resolution: 16.077 pixels mm-1 | θmax = 28.6°, θmin = 3.2° |
ω scans | h = −9→10 |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2010) | k = −15→15 |
Tmin = 0.990, Tmax = 1.000 | l = −13→13 |
7572 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.038 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.091 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.97 | w = 1/[σ2(Fo2) + (0.0525P)2 + 0.0562P] where P = (Fo2 + 2Fc2)/3 |
2128 reflections | (Δ/σ)max = 0.001 |
160 parameters | Δρmax = 0.31 e Å−3 |
0 restraints | Δρmin = −0.25 e Å−3 |
C9H11NO4·H2O | V = 911.27 (4) Å3 |
Mr = 215.20 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 7.7565 (2) Å | µ = 0.13 mm−1 |
b = 11.4103 (2) Å | T = 223 K |
c = 10.3339 (3) Å | 0.30 × 0.30 × 0.15 mm |
β = 94.888 (2)° |
Oxford Diffraction Gemini-S Ultra CCD-detector diffractometer | 2128 independent reflections |
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2010) | 1589 reflections with I > 2σ(I) |
Tmin = 0.990, Tmax = 1.000 | Rint = 0.026 |
7572 measured reflections |
R[F2 > 2σ(F2)] = 0.038 | 0 restraints |
wR(F2) = 0.091 | H atoms treated by a mixture of independent and constrained refinement |
S = 0.97 | Δρmax = 0.31 e Å−3 |
2128 reflections | Δρmin = −0.25 e Å−3 |
160 parameters |
Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | ||
O31 | 0.60834 (14) | 0.12616 (8) | 0.03042 (10) | 0.0217 (3) | |
O32 | 0.57555 (15) | 0.27409 (8) | 0.16495 (11) | 0.0291 (4) | |
O71 | −0.10727 (14) | 0.05736 (9) | 0.32326 (12) | 0.0311 (4) | |
O72 | −0.08102 (14) | −0.06274 (9) | 0.16170 (11) | 0.0273 (3) | |
N31 | 0.53370 (17) | −0.03193 (10) | 0.20183 (13) | 0.0166 (3) | |
C1 | 0.26343 (19) | 0.07189 (12) | 0.39029 (14) | 0.0190 (4) | |
C2 | 0.43453 (18) | 0.12186 (11) | 0.35275 (13) | 0.0165 (4) | |
C3 | 0.45247 (17) | 0.08675 (11) | 0.21340 (13) | 0.0142 (4) | |
C4 | 0.25626 (18) | 0.08423 (11) | 0.16738 (13) | 0.0152 (4) | |
C5 | 0.19989 (19) | 0.20682 (11) | 0.20992 (13) | 0.0176 (4) | |
C6 | 0.27559 (19) | 0.19863 (12) | 0.35020 (14) | 0.0184 (4) | |
C7 | 0.18333 (18) | 0.00565 (12) | 0.27376 (13) | 0.0171 (4) | |
C31 | 0.55486 (18) | 0.17107 (11) | 0.13073 (14) | 0.0163 (4) | |
C71 | −0.01187 (19) | 0.00291 (12) | 0.25744 (15) | 0.0210 (4) | |
O1W | 0.10015 (16) | −0.21234 (11) | 0.05407 (12) | 0.0312 (4) | |
H1 | 0.24590 | 0.04870 | 0.47950 | 0.0230* | |
H2 | 0.53470 | 0.13290 | 0.41600 | 0.0200* | |
H4 | 0.22510 | 0.06230 | 0.07670 | 0.0180* | |
H5A | 0.25210 | 0.26910 | 0.16270 | 0.0210* | |
H5B | 0.07510 | 0.21590 | 0.20280 | 0.0210* | |
H6 | 0.26610 | 0.26310 | 0.41190 | 0.0220* | |
H7 | 0.23080 | −0.07380 | 0.27130 | 0.0210* | |
H31A | 0.509 (3) | −0.0598 (15) | 0.117 (2) | 0.033 (5)* | |
H31B | 0.650 (3) | −0.0273 (14) | 0.2222 (17) | 0.028 (5)* | |
H31C | 0.501 (2) | −0.0843 (15) | 0.2564 (18) | 0.024 (4)* | |
H72 | −0.007 (4) | −0.115 (2) | 0.124 (2) | 0.063 (7)* | |
H11W | 0.108 (3) | −0.283 (2) | 0.092 (2) | 0.063 (7)* | |
H12W | 0.207 (3) | −0.1929 (19) | 0.026 (2) | 0.054 (6)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O31 | 0.0263 (6) | 0.0193 (5) | 0.0209 (5) | −0.0048 (4) | 0.0095 (4) | −0.0035 (4) |
O32 | 0.0346 (7) | 0.0157 (5) | 0.0393 (7) | −0.0075 (5) | 0.0158 (5) | −0.0080 (5) |
O71 | 0.0185 (6) | 0.0331 (6) | 0.0433 (7) | 0.0021 (5) | 0.0119 (5) | −0.0028 (5) |
O72 | 0.0153 (5) | 0.0251 (5) | 0.0410 (7) | −0.0032 (5) | 0.0002 (5) | −0.0035 (5) |
N31 | 0.0143 (6) | 0.0135 (5) | 0.0223 (7) | 0.0010 (5) | 0.0038 (5) | 0.0020 (5) |
C1 | 0.0167 (7) | 0.0230 (7) | 0.0175 (7) | 0.0017 (6) | 0.0032 (6) | 0.0026 (5) |
C2 | 0.0156 (7) | 0.0182 (6) | 0.0157 (7) | −0.0008 (6) | 0.0008 (5) | −0.0003 (5) |
C3 | 0.0126 (7) | 0.0126 (6) | 0.0176 (7) | 0.0006 (5) | 0.0020 (5) | −0.0021 (5) |
C4 | 0.0135 (7) | 0.0164 (6) | 0.0156 (7) | −0.0001 (5) | 0.0008 (5) | −0.0002 (5) |
C5 | 0.0167 (7) | 0.0160 (6) | 0.0203 (7) | 0.0025 (6) | 0.0030 (6) | 0.0009 (5) |
C6 | 0.0187 (7) | 0.0182 (6) | 0.0187 (7) | 0.0015 (6) | 0.0034 (6) | −0.0031 (6) |
C7 | 0.0140 (7) | 0.0158 (6) | 0.0220 (7) | 0.0021 (6) | 0.0038 (6) | 0.0026 (5) |
C31 | 0.0127 (7) | 0.0167 (6) | 0.0193 (7) | −0.0005 (5) | 0.0008 (5) | 0.0002 (5) |
C71 | 0.0172 (7) | 0.0168 (6) | 0.0294 (8) | −0.0010 (6) | 0.0047 (6) | 0.0055 (6) |
O1W | 0.0263 (7) | 0.0294 (6) | 0.0394 (7) | −0.0032 (5) | 0.0118 (5) | 0.0012 (5) |
O31—C31 | 1.2580 (17) | C2—C6 | 1.511 (2) |
O32—C31 | 1.2341 (16) | C3—C4 | 1.5556 (19) |
O71—C71 | 1.2174 (19) | C3—C31 | 1.5498 (19) |
O72—C71 | 1.3180 (18) | C4—C7 | 1.5615 (19) |
O72—H72 | 0.94 (3) | C4—C5 | 1.5407 (18) |
O1W—H11W | 0.90 (2) | C5—C6 | 1.520 (2) |
O1W—H12W | 0.93 (2) | C7—C71 | 1.509 (2) |
N31—C3 | 1.5026 (17) | C1—H1 | 0.9800 |
N31—H31A | 0.94 (2) | C2—H2 | 0.9800 |
N31—H31B | 0.91 (2) | C4—H4 | 0.9800 |
N31—H31C | 0.874 (18) | C5—H5A | 0.9700 |
C1—C7 | 1.510 (2) | C5—H5B | 0.9700 |
C1—C6 | 1.5095 (19) | C6—H6 | 0.9800 |
C1—C2 | 1.525 (2) | C7—H7 | 0.9800 |
C2—C3 | 1.5125 (19) | ||
C71—O72—H72 | 116.7 (16) | C1—C7—C4 | 97.18 (11) |
H11W—O1W—H12W | 109 (2) | C4—C7—C71 | 110.69 (11) |
H31A—N31—H31B | 110.8 (18) | O31—C31—O32 | 125.46 (13) |
C3—N31—H31C | 114.7 (11) | O31—C31—C3 | 114.95 (11) |
C3—N31—H31A | 109.1 (12) | O32—C31—C3 | 119.58 (12) |
C3—N31—H31B | 110.1 (10) | O71—C71—C7 | 125.36 (13) |
H31B—N31—H31C | 103.1 (15) | O72—C71—C7 | 115.84 (12) |
H31A—N31—H31C | 108.9 (16) | O71—C71—O72 | 118.79 (14) |
C6—C1—C7 | 106.96 (12) | C2—C1—H1 | 122.00 |
C2—C1—C7 | 106.99 (11) | C6—C1—H1 | 122.00 |
C2—C1—C6 | 59.72 (9) | C7—C1—H1 | 122.00 |
C3—C2—C6 | 106.15 (11) | C1—C2—H2 | 122.00 |
C1—C2—C6 | 59.64 (9) | C3—C2—H2 | 122.00 |
C1—C2—C3 | 107.21 (11) | C6—C2—H2 | 122.00 |
N31—C3—C4 | 111.41 (10) | C5—C4—H4 | 117.00 |
N31—C3—C31 | 106.12 (11) | C7—C4—H4 | 117.00 |
N31—C3—C2 | 112.81 (11) | C3—C4—H4 | 117.00 |
C2—C3—C31 | 116.96 (11) | C4—C5—H5A | 112.00 |
C4—C3—C31 | 112.14 (11) | C6—C5—H5A | 112.00 |
C2—C3—C4 | 97.41 (10) | C6—C5—H5B | 112.00 |
C3—C4—C5 | 100.95 (10) | C4—C5—H5B | 112.00 |
C5—C4—C7 | 101.06 (11) | H5A—C5—H5B | 110.00 |
C3—C4—C7 | 101.48 (10) | C2—C6—H6 | 122.00 |
C4—C5—C6 | 96.91 (10) | C5—C6—H6 | 122.00 |
C2—C6—C5 | 107.50 (11) | C1—C6—H6 | 122.00 |
C1—C6—C5 | 107.01 (11) | C4—C7—H7 | 111.00 |
C1—C6—C2 | 60.64 (9) | C71—C7—H7 | 111.00 |
C1—C7—C71 | 116.11 (12) | C1—C7—H7 | 111.00 |
C6—C1—C2—C3 | −98.97 (12) | C2—C3—C4—C7 | −50.96 (11) |
C7—C1—C2—C3 | 1.10 (14) | C31—C3—C4—C5 | −70.29 (13) |
C7—C1—C2—C6 | 100.07 (12) | C31—C3—C4—C7 | −174.09 (10) |
C2—C1—C6—C5 | 100.87 (12) | N31—C3—C31—O31 | 34.29 (16) |
C7—C1—C6—C2 | −100.12 (12) | N31—C3—C31—O32 | −146.79 (13) |
C7—C1—C6—C5 | 0.76 (15) | C2—C3—C31—O31 | 161.14 (12) |
C2—C1—C7—C4 | −31.88 (13) | C2—C3—C31—O32 | −19.94 (19) |
C2—C1—C7—C71 | −149.16 (12) | C4—C3—C31—O31 | −87.56 (14) |
C6—C1—C7—C4 | 30.86 (13) | C4—C3—C31—O32 | 91.36 (15) |
C6—C1—C7—C71 | −86.42 (14) | C3—C4—C5—C6 | −51.88 (12) |
C1—C2—C3—N31 | −86.65 (13) | C7—C4—C5—C6 | 52.27 (12) |
C1—C2—C3—C4 | 30.35 (12) | C3—C4—C7—C1 | 51.65 (12) |
C1—C2—C3—C31 | 149.86 (11) | C3—C4—C7—C71 | 173.10 (11) |
C6—C2—C3—N31 | −149.18 (11) | C5—C4—C7—C1 | −52.08 (12) |
C6—C2—C3—C4 | −32.19 (12) | C5—C4—C7—C71 | 69.37 (13) |
C6—C2—C3—C31 | 87.33 (14) | C4—C5—C6—C1 | −32.48 (14) |
C1—C2—C6—C5 | −100.06 (12) | C4—C5—C6—C2 | 31.34 (13) |
C3—C2—C6—C1 | 100.80 (12) | C1—C7—C71—O71 | 6.7 (2) |
C3—C2—C6—C5 | 0.74 (14) | C1—C7—C71—O72 | −174.76 (12) |
N31—C3—C4—C5 | 170.94 (11) | C4—C7—C71—O71 | −102.86 (16) |
N31—C3—C4—C7 | 67.13 (13) | C4—C7—C71—O72 | 75.74 (15) |
C2—C3—C4—C5 | 52.85 (11) |
D—H···A | D—H | H···A | D···A | D—H···A |
O1W—H11W···O71i | 0.90 (2) | 2.02 (2) | 2.9161 (16) | 176 (2) |
O1W—H12W···O31ii | 0.93 (2) | 1.77 (2) | 2.6792 (16) | 168 (2) |
N31—H31A···O31ii | 0.94 (2) | 1.87 (2) | 2.7712 (17) | 161 (2) |
N31—H31B···O71iii | 0.91 (2) | 2.29 (2) | 3.1261 (17) | 153.0 (15) |
N31—H31B···O72iii | 0.91 (2) | 2.27 (2) | 3.0720 (17) | 147.6 (15) |
N31—H31C···O32iv | 0.874 (18) | 1.925 (17) | 2.7769 (16) | 164.4 (17) |
O72—H72···O1W | 0.94 (3) | 1.60 (3) | 2.5282 (16) | 174 (3) |
C5—H5A···O32 | 0.97 | 2.51 | 3.0865 (19) | 118 |
C6—H6···O72v | 0.98 | 2.53 | 3.1105 (17) | 118 |
C7—H7···N31 | 0.98 | 2.56 | 2.9097 (19) | 101 |
C7—H7···O32iv | 0.98 | 2.35 | 3.2678 (17) | 155 |
Symmetry codes: (i) −x, y−1/2, −z+1/2; (ii) −x+1, −y, −z; (iii) x+1, y, z; (iv) −x+1, y−1/2, −z+1/2; (v) −x, y+1/2, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | C9H11NO4·H2O |
Mr | 215.20 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 223 |
a, b, c (Å) | 7.7565 (2), 11.4103 (2), 10.3339 (3) |
β (°) | 94.888 (2) |
V (Å3) | 911.27 (4) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.13 |
Crystal size (mm) | 0.30 × 0.30 × 0.15 |
Data collection | |
Diffractometer | Oxford Diffraction Gemini-S Ultra CCD-detector diffractometer |
Absorption correction | Multi-scan (CrysAlis PRO; Oxford Diffraction, 2010) |
Tmin, Tmax | 0.990, 1.000 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 7572, 2128, 1589 |
Rint | 0.026 |
(sin θ/λ)max (Å−1) | 0.673 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.038, 0.091, 0.97 |
No. of reflections | 2128 |
No. of parameters | 160 |
H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
Δρmax, Δρmin (e Å−3) | 0.31, −0.25 |
Computer programs: CrysAlis PRO (Oxford Diffraction, 2010), SIR92 (Altomare et al., 1994), SHELXL97 (Sheldrick, 2008) within WinGX (Farrugia, 1999), PLATON (Spek, 2009).
D—H···A | D—H | H···A | D···A | D—H···A |
O1W—H11W···O71i | 0.90 (2) | 2.02 (2) | 2.9161 (16) | 176 (2) |
O1W—H12W···O31ii | 0.93 (2) | 1.77 (2) | 2.6792 (16) | 168 (2) |
N31—H31A···O31ii | 0.94 (2) | 1.87 (2) | 2.7712 (17) | 161 (2) |
N31—H31B···O71iii | 0.91 (2) | 2.29 (2) | 3.1261 (17) | 153.0 (15) |
N31—H31B···O72iii | 0.91 (2) | 2.27 (2) | 3.0720 (17) | 147.6 (15) |
N31—H31C···O32iv | 0.874 (18) | 1.925 (17) | 2.7769 (16) | 164.4 (17) |
O72—H72···O1W | 0.94 (3) | 1.60 (3) | 2.5282 (16) | 174 (3) |
Symmetry codes: (i) −x, y−1/2, −z+1/2; (ii) −x+1, −y, −z; (iii) x+1, y, z; (iv) −x+1, y−1/2, −z+1/2. |
Acknowledgements
The authors acknowledge financial support from the Australian Research Council, the Science and Engineering Faculty and the University Library, Queensland University of Technology and Griffith University.
References
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G-protein-coupled receptors (GPCRs) constitute a superfamily of proteins whose main function is to convert extracellular stimuli into intracellular signals (Liu & Doller, 2011). Metabotropic glutamate (mGu) receptors belong to the class C GPCR group and are activated by L-glutamate. The title compound, C9H11NO4. H2O (I), is a hydrated tricyclic rearranged aminonorbornane dicarboxylic acid cage compound which is a conformationally rigid analogue of glutamic acid, and was synthesized as a potential ligand for metabotropic glutamate receptors in order to explore the requirements for activity at these receptors (Wermuth, 1995). For the synthesis of amino-substituted norbornane carboxylic acids, see also Tager & Christensen (1972) and Apgar & Ludwig (1972).
The title compound exists as an ammonium carboxylate zwitterion with the C3 carboxylate group endo-oriented (Fig. 1). Note that the stereochemical assignment of exo and endo on such nortricyclic systems is somewhat arbitrary and depends on how the system is drawn. The carboxylic acid group at C7 in (I) is exo and has the acid H-atom (H72) anti-located, forming a hydrogen bond with the water molecule of solvation (Table 1). This water molecule gives intermolecular hydrogen-bonding associations with carboxyl O-atom acceptors, while the ammonium group also forms four hydrogen bonds with carboxyl O-atom acceptors. These include a symmetric cyclic N—H···O,O' head-to-tail association [graph set R21(4) (Etter et al., 1990)] which links the molecules along (100). Overall, a three-dimensional framework structure is formed (Fig. 2).
The structures of similar tricyclic norbornane compounds are rare in the crystallographic literature. The nortricyclic keto acid which served as the precursor to (IV) in the synthesis of (I) (Fig. 3) is known (Fortier et al., 1979).