Syntheses of four enantiomers of 2,3-diendo- and 3-endo-aminobicyclo[2.2.2] oct-5-ene-2-exo-carboxylic acid and their saturated analogues

Article abstract of DOI:10.3390/molecules181215080

Ethyl 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylate ((±)-1) was resolved with O,O′-dibenzoyltartaric acid via diastereomeric salt formation. The efficient synthesis of the enantiomers of 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylic aci

Full text of DOI:10.3390/molecules181215080

Molecules 2013, 18, 15080-15093; doi:10.3390/molecules181215080
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Syntheses of Four Enantiomers of 2,3-Diendo- and
3-Endo-aminobicyclo[2.2.2]oct-5-ene-2-exo-carboxylic
Acid and Their Saturated Analogues
Márta Palkó 1, Mikko M. Hänninen 2, Reijo Sillanpää 2 and Ferenc Fülöp 1,3,
*
1
Institute of Pharmaceutical Chemistry, University of Szeged, Szeged H-6720, Eötvös utca 6,
Hungary; E-Mail: palko@pharm.u-szeged.hu
2
3
Department of Chemistry, University of Jyväskylä, Turku FIN-40014, Finland;
E-Mails: mikko.m.hanninen@jyu.fi (M.M.H.); resillan@jyu.fi (R.S.)
Stereochemistry Research Group of the Hungarian Academy of Sciences, University of Szeged,
Szeged H-6720, Eötvös utca 6, Hungary
* Author to whom correspondence should be addressed; E-Mail: fulop@pharm.u-szeged.hu;
Tel.: +36-62-545-562; Fax: +36-62-545-705.
Received: 11 November 2013; in revised form: 2 December 2013 / Accepted: 3 December 2013 /
Published: 6 December 2013
Abstract: Ethyl 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylate ((±)-1) was
resolved with O,O'-dibenzoyltartaric acid via diastereomeric salt formation. The efficient
synthesis of the enantiomers of 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylic
acid ((+)-7 and (–)-7), 3-endo-aminobicyclo[2.2.2]oct-5-ene-2-exo-carboxylic acid ((+)-5
and (–)-5), cis- and trans-3-aminobicyclo[2.2.2]octane-2-carboxylic acid ((+)-6, (–)-6,
(+)-8 and (–)-8) was achieved via isomerization, hydrogenation and hydrolysis of the
corresponding esters (–)-1 and (+)-1. The stereochemistry and relative configurations of the
3
synthesized compounds were determined by NMR spectroscopy (based on the J(H,H)
coupling constants) and X-ray crystallography.
Keywords: bicyclic β-amino acid derivatives; constrained chiral β-amino acids; resolution;
continuous flow hydrogenations

Molecules 2013, 18
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1. Introduction
In the past decade, the number of investigations on β-amino acids, in both racemic and optically
active forms, has risen exponentially as a consequence of their increasing chemical and biological
importance. β-Amino acids and their derivatives possess noteworthy pharmacological effects. For
example, the first natural alicyclic β-amino acid, (1R,2S)-2-aminocyclopentanecarboxylic acid
(cispentacin) exhibits antifungal activity [1–4]. Alicyclic β-amino acids can also be used as building
blocks of modified analogues of pharmacologically active peptides [5–10].
While a range of stereoselective syntheses of enantiomerically enriched small (three- or four-
membered: [11–13] and medium (five- to eight-membered) cyclic [14–17] and sterically hindered
bicyclic norbornane and norbornene β-amino acids [18,19] or their derivatives have been reported,
relatively few examples cocerning the synthesis and applications of enantiomerically enriched
bicyclo[2.2.2]octane β-amino acid derivatives have been described [20–24].
Many attractive techniques are available for the production of enantiomerically pure amino acids.
Resolution via diastereomeric salt formation, for instance, is still useful for the production of
enantiomerically pure compounds on a laboratory scale. Resolution with an equivalent of the resolving
agent (Pasteur’s method) is the most convenient in work with small amounts of the racemate (DL).
In resolution with one equivalent of the resolving agent (R), diastereomers are separated
(DL+2R → DR+LR), while when half an equivalent of resolving agent and half an equivalent of
achiral acid or base (I) are used (Pope-Peacheay’s method [25–28], a diastereomeric salt is separated
from a salt with an achiral agent (DL+R+I → DR+LI). The role of the achiral acid or base is to form a
highly soluble salt with the enantiomer remaining in the solution. If there is a large difference in
stability of the resulting diastereomeric salt and the enantiomer remaining in solution, the resolutions
may be performed with half an equivalent of resolving agent and the achiral auxiliary material is omitted
(DL+R → DR+L). A synthesis of racemic 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylic
acid has been published [29], but its C-2 epimer and their enantiomers have not yet been examined.
Our present aim was to synthetize of the title alicyclic β-amino acid enantiomers. This report focuses
on the resolution of racemic 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylic acid derivatives
by diastereomeric salt formation.
2. Results and Discussion
To find the most suitable resolving agent for (±)-1, three kinds of commercially available acidic
resolving agents were examined. The solvent in each resolution experiment was EtOH. We first used
half an equivalent of L-tartaric acid, and S-mandelic acid as the resolving agent for (±)-1, but
surprisingly poor results were obtained. We next focused our attention on O,O'-dibenzoyltartaric acid
(DBTA) as a resolving agent which is often used in gram-scale productions. In the first experiments of
the Pope-Peacheay resolutions, 0.7 equivalent of aqueous HCl (achiral acid), 0.5 equivalent of DBTA
as resolving agent and 1 equivalent of (±)-1 were mixed with the solvent, and the mixture was heated
at 70 °C. The solution was subsequently stirred and further cooled to room temperature. After stirring
1
for 2 h, the solids were collected. Finally, this protocol was repeated without HCl. As the H-NMR
spectra of the salt-pairs were identical, the diastereomeric purity of the salt obtained in each resolution

Molecules 2013, 18
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experiment was determined as the diastereomeric excess (de%), based on the enantiomeric excess
(ee%) of 1 in the salt. The experimental results are listed in Table 1.
Table 1. Resolutions of (±)-1 with chiral acids.
Entry
Resolving agent
L-tartaric acid
S-mandelic acid
L-DBTA a
Yield (%)
de b (%)
S factor c
0.22
1
2
3
4
23
30
47
45
48
45
70
84
0.27
0.66
L-DBTA
0.76
a In the presence of 0.7 equivalent of aq. HCl; b The de of the salt was determined via the ee
of ester 1 liberated from the salt; c The efficiency factor S is used to indicate how effective a
resolution is: S-factor = 2 × yield × de [30].
Recrystallization of the L-DBTA salt containing (+)-1 with 84% de in EtOH furnished enantiomer
(+)-1 with 96% de. Similarly, enantiomer (–)-1 with D-DBTA was obtained from the mother liquor in
98% de (Scheme 1).
Scheme 1. Resolution of ester 1 with DBTA.
O
O
COOH
O
COOH
O
(S)
(R)
(S)
i
(R)
ii
COOEt
NH2
EtOOC
H2N
EtOOC
H2N
(±)-1
O
HOOC
O
HOOC
O
O
(+)-(1) (+)-DBTA
(-)-(1) (-)-DBTA
Reagents and Conditions: (i) (+)-DBTA, EtOH, 70 °C; (ii) (‒)-DBTA, EtOH, 70 °C.
The C-2 isomerization of (–)-1 with NaOEt in EtOH at 70 °C resulted in amino ester (+)-2.
Compounds (–)-1 and (+)-2 were also transformed into amino ester enantiomers (–)-3 and (–)-4 with
H2 in the presence of Pd/C. Continuous flow hydrogenations were carried out in a ThalesNano
H-cube™ system. For each run, 150 mg of catalyst was added to in a tubular catalyst cartridge with an
inner diameter of 4 mm and a length of 30 mm. The catalyst was rinsed for 0.5 h with a flow of EtOH
1 mL min−1, followed by pretreatment with H2 for 0.5 h in the same solvent. The reactant was
dissolved in EtOH and this solution was delivered to the hydrogenation system via a conventional
HPLC pump, through the mixer of the apparatus, where H2 was mixed into the liquid flow under a
pressure of 1 MPa. The catalyst cartridge holder was equipped with an externally controlled cooling
jacket. The mixture was pumped through the catalyst bed so as to obtain an ascending flow of the
reaction components. After one reaction cycle, samples of 1 mL were taken from the product flow and
analysed by GC analysis: complete conversion was observed.
When subjected to microwave irradiation in H2O at 150 °C for 1 h, ester (–)-1, (+)-2, (–)-3, and
(–)-4 gave amino acids (+)-5, (–)-6, (–)-7 and (–)-8 (Scheme 2). All the reactions were first optimized

Molecules 2013, 18
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for the racemic compounds. The whole syntheses were repeated, starting from racemic amino ester
(±)-1 and enantiomer (+)-1.
Scheme 2. Synthesis of amino acid enantiomers (+)-5, (–)-6, (–)-7 and (–)-8.
EtOOC
EtOOC
ii
ii
i
EtOOC
H2N
(-)-1
EtOOC
H2N
H2N
(+)-2
H2N
(-)-4
(-)-3
iii
iii
iii
iii
HOOC
HOOC
HOOC
H2N
(-)-7
HOOC
H2N
H2N
(-)-6
H2N
(+)-5
(-)-8
Reagents and Conditions: (i) NaOEt, EtOH, 70 °C; (ii) Pd/C, H2; (iii) MW, H2O, 100 °C.
X-ray investigations revealed the absolute configurations of (–)-1 and (–)-2. Amino esters (–)-1 and
(–)-2 were transformed into ureas (+)-9 and (–)-10 upon reaction with (S)-(–)-α-methylbenzyl
isocyanate (Scheme 3). The X-ray structures clearly showed the 1S,2R,3S,4R configuration of (–)-1
(Figure 1) and the 1R,2R,3R,4S configuration of (–)-2 (Figure 2).
Scheme 3. Synthesis of ureas (+)-9 and (–)-10.
8
1
7
COOEt
2
6
5
4
EtOOC
HN
3
COOEt
NH
i
i
O
O
EtOOC
H2N
NH
(S)
HN
(S)
NH2
Me
Me
(-)-2
(-)-1
(-)-10
(+)-9
Reagents and Conditions: (i) (S)-(‒)--methylbenzyl isocyanate, Et2O, rt.
Figure 1. Molecular structure of carbamate (+)-9. Thermal ellipsoids have been drawn at
the 20% probability level.

Molecules 2013, 18
15084
Figure 2. Molecular structure of carbamate (–)-10. Thermal ellipsoids have been drawn at
the 20% probability level.
3. Experimental
3.1. General
The chemicals were purchased from Aldrich (Budapest, Hungary) or Fluka (Budapest, Hungary)
1
13
The H-NMR spectra were recorded at 400 MHz and the C-NMR spectra at 100 MHz on a Bruker
AM 400 spectrometer at ambient temperature (27 °C) in DMSO-d6. Chemical shifts are given in δ
(ppm) relative to TMS as internal standard. Elemental analyses were performed with a Perkin-Elmer
CHNS-2400 Ser II Elemental Analyzer. Melting points were measured with a Hinotek
X-4 melting point apparatus and are uncorrected. Optical rotations were measured with a Perkin-Elmer
341 polarimeter. Microwave reactions were performed in a CEM Discover LabMate MW reactor.
Continuous hydrogenations were carried out in an H-Cube high-pressure continuous-flow system
purchased from Thales Nanotechnology Inc. Racemic ethyl 2,3-diendo-3-aminobicyclo[2.2.2]oct-5-ene-
2-carboxylate (±)-1 was prepared by a literature method [29]. The ee values for the esters (–)-1, (+)-1,
(‒)-4 and (+)-4 were determined after acylation with Ac2O in the presence of base by using Perkin-Elmer
GC instrumentation equipped with CP-Chirasil L-Val columns [31]. All analyses for derivatized amino
esters were performed with N2 as carrier gas; the column temperature and inlet pressure were adjusted
to an optimized program. The injector and detector temperature were set at 250 °C and 270 °C,
respectively. The ee values of 2 and 3 were determined by HPLC, using a Daicel Chiralcel ODH
column. Samples were prepared for ee determination by HPLC analysis as follows: (+)-2·HCl,
(–)-2·HCl, (+)-3·HCl or (–)-3·HCl (10 mg) was placed in a vial, to which 1 M aqueous NaOH (1 mL),
toluene (1 mL) and benzoyl chloride (15 μL) were added. The mixture was allowed to stand for 15 min
at room temperature, during which N-benzoylation proceeded to yield N-benzoyl-2 or N-benzoyl-3.
After separation, drying and evaporation of the toluene layer, the residue was dissolved in 2 mL a
mixture of n-hexane and IPA (8:2) and the insolubles were removed by filtration. The filtrate (2 μL)
was injected into the HPLC apparatus. The analytical conditions were as follows: (a) eluent: a mixture
of n-hexane and IPA (8:2), flow rate: 0.25 mL min−1, detection at 233 nm, retention times: (–)-2: 26.1 min,
(+)-2: 34.3 min; (b) a mixture of n-hexane and IPA (95:5), flow rate: 0,25 mL min−1, detection at 233 nm,
retention times: (–)-3: 33.48 min, (+)-3: 35.2 min.

Molecules 2013, 18
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3.2. Determination of de Values of O,O'-dibenzoyltartaric Acid (DBTA) Salts of 1
The de values of the salts were determined from the ee values of esters 1. Samples were
prepared for ee determination by GC analysis as follows: diastereomeric salt (5 mg) was dissolved
in CH2Cl2 (50 μL). The sample was acylated with Ac2O (15 μL) and a mixture of
4-dimethylaminopyridine (DMAP) and pyridine (1:9) (15 μL) was added. After shaking for 2–3 s, the
derivatized samples were analysed on GC CP-Chirasil L-Val columns (30 m) with the temperature
held at 140 °C for 5 min and then raised to 160 °C at 10 °C min−1, flow rate: 2 mL min−1, retention
times: (–)-1: 6.96 min, (+)-1: 7.11 min.
3.3. A Typical Resolution Procedure
A mixture of racemic ester base 1 (10.98 g, 5 mmol), (–)-DBTA (0.94 g, 2.5 mmol) and EtOH (9 mL)
in a 50 mL flask was heated under stirring at 70 °C to obtain a clear solution. The solution was
gradually cooled to 20 °C during about 30 min, and the precipitated diastereomeric salt was filtered off
and washed with Et2O (10 mL) to yield white crystals, which were dried at room temperature to afford
the (1S,2R,3S,4R)-(–)-1.(–)-DBTA salt (0.92 g, de 84%). The product was recrystallized from EtOH
(yield 0.7 g, de 96%). The mother liquor was evaporated down and, 2 N NaOH solution (10 mL) was
added to the residue. The mixture was extracted with CH2Cl2 (3 × 20 mL). The combined organic
phase was dried (Na2SO4) and the solvent was evaporated off. (+)-DBTA (0.48 g, 1.3 mmol) and
EtOH (6 mL) were added to the ester residue (+)-1 (0.5 g, 1.6 mmol, ee 57%) followed by heating
under stirring at 70 °C to give a clear solution. The solution was gradually cooled to 20 °C during
about 30 min, and the precipitated diastereomeric salt was filtered off and washed with Et2O (10 mL)
to yield white crystals, which were dried at room temperature to afford the (1R,2S,3R,4S)-(+)-1.
(+)-DBTA salt (0.7 g, de 92.5%). The crystals were recrystallized from EtOH (yield 0.5 g, de 98%)
Ethyl (1S,2R,3S,4R)-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylate (–)-1. (–)-1·(–)-DBTA salt, mp
20
20
186–188 °C,

D = –59.9 (c, 0.5, EtOH), de = 96%.; (–)-1.HCl salt mp 192–194 °C,

D = –14.8
1
13
(c, 0.6, EtOH), ee = 96%. The H- and C-NMR spectroscopic and elemental analysis data on the
enantiomeric derivative were in accordance with those for racemic (±)-1·HCl [26].
Ethyl (1R,2S,3R,4S)-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylate (+)-1. (+)-1·(+)DBTA salt, mp
20
20
189–190 °C,


D = +63.5 (c, 0.5, EtOH), de = 98%.; (+)-1.HCl salt, mp 192–194 °C,


D = +15.3
1
13
(c, 0.6, EtOH), ee = 98%. The H- and C-NMR spectroscopic and elemental analysis data on the
enantiomeric derivative were in accordance with those for racemic (±)-1·HCl [26].
3.4. Isomerization of Amino Esters (–)-1 and (+)-1
Freshly prepared NaOEt (280 mg, 4 mmol) was added to a solution of the free base of (–)-1 or (+)-1
(394 mg, 2 mmol) in dry EtOH (12 mL), and the mixture was heated for 2 h at 70 °C. It was then
concentrated under reduced pressure, taken up in CH2Cl2 and washed with H2O (2 × 20 mL). The
combined organic phase was dried (Na2SO4), the solvent was evaporated off, the residue was dissolved
in EtOH containing 22% HCl (2 mL), and the solution was stirred for 10 min at room temperature.

Molecules 2013, 18
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After removal of the solvent, amino ester hydrochloride 2 was obtained, which was recrystallized from
EtOH/Et2O.
Ethyl (1S,2S,3S,4R)-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylate hydrochloride (+)-2. White crystals
20
1
(256 mg, 55%), mp 220 °C (with sublimation),
 D = +47.9 (c, 0.5, EtOH), ee = 98%. H-NMR: δ =

0.98–1.09 (m, 1H, H-8), 1.18–1.26 (t, J = 7.1 Hz, 3H, CH3), 1.18–1.38 (m, 3H, H-7, H-7, H-8,),
2.282.32 (m, 1H, H-2), 2.82–2.91 (m, 2H, H-1, H-4), 3.64 (t, J = 3.2 Hz, 1H, H-3), 4.8–4.23 (m, 2H,
+
OCH2), 6.18 (t, J = 7.1Hz, 1H, H-5) 6.53 (t, J = 7.5 Hz 1H, H-6) 7.95 (bs, 3H, NH3 ) ppm. 13C-NMR:
δ = 14.9, 19.9, 23.0, 33.3, 33.4, 49.3, 51.7, 61.6, 130.9, 136.6, 172.4 ppm. Anal. Calcd for
C11H18ClNO2 (231,74): C, 57.02; H, 7.83; Cl 15,30; N, 6,04. Found: C, 56.81; H, 8.04; N, 6.05.
Ethyl (1R,2R,3R,4S)-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylate hydrochloride (–)-2. White
20
crystals (263 mg, 56%), mp 178–180 °C,

D = –43 (c, 0.4, EtOH), ee = 98%. The 1H- and 13C-NMR
spectroscopic data on the enantiomeric derivative were consistent with the data on the antipode (+)-2.
Anal. Calcd for C11H18ClNO2 (231.74): C, 57.02; H, 7.83; Cl 15.30; N, 6.04. Found: C, 56.94; H, 8.11;
N, 6.12.
3.5. General Procedure for Hydrogenation of Amino Esters (–)-1 (+)-1, (–)-2 and (+)-2 over Pd/C
Catalyst in a Fixed-Bed Reactor
Free amino ester (–)-1, (+)-1, (–)-2 or (+)-2 (98 mg, 5 mmol) was dissolved in dry EtOH (100 mL)
to form a 0.05 M solution. This solution was hydrogenated in the ThalesNano H-cube™ system, using
a 10% Pd/C catalyst at a flow rate of 1 mL min−1 at 50 °C and a H2 pressure of 50 bar. The EtOH was
removed under vacuum. The residue was dissolved in EtOH containing 22% HCl (2 mL) and the
solution was stirred for 10 min at room temperature. After removal of the solvent, the amino ester
hydrochloride was obtained.
Ethyl (2S,3S)-3-aminobicyclo[2.2.2]octane-2-carboxylate Hydrochloride (–)-3. An oil (90 mg, 91%),
20
1


D = –17.5 (c, 1.0, MeOH), ee = 99%. H-NMR: δ = 1.21 (t, J = 7.2, 3H, CH3) 1.25–1.86 (m, 9H,
H-5, H-5, H-6, H-6, H-7, H-7, H-8, H-8, H-2), 1.90–1.95 (m, 1H, H-4), 2.54–2.58 (m, 1H, H-4),
+
13
3.54–3,64 (m, 1H, H-3), 4.06–4.21 (m, 2H, OCH2), 8.16 (bs, 3H, NH3 ) ppm. C-NMR: δ = 14.7,
18.6, 21.0, 24.5, 25.3, 27.8, 28.8, 47.9, 50.5, 61.58, 166.4 ppm. Anal. Calcd for C11H20ClNO2
(233.74): C, 56.52; H, 8.62; Cl 15.17; N, 5.99. Found: C, 56.21; H, 8.39; N, 5.93.
Ethyl (2R,3R)-3-aminobicyclo[2.2.2]octane-2-carboxylate Hydrochloride (+)-3. An oil (90 mg, 91%),
20
1

D = +15,4 (c, 0.9, MeOH), ee = 99%. The H- and 13C-NMR spectroscopic data on the
enantiomeric derivative were consistent with the data on the antipode (–)-3. Anal. Calcd for
C11H20ClNO2 (233.74): C, 56.52; H, 8.62; Cl 15.17; N, 5.99. Found: C, 56.29; H, 8.88; N, 6.14.
Ethyl (2R,3S)-3-aminobicyclo[2.2.2]octane-2-carboxylate Hydrochloride (–)-4. White crystals (86 mg,
20
1
86%,), mp 195–196 °C,
 D = –31.5 (c, 0.6, EtOH), ee = 99%. H-NMR: δ = 1.22 (t, J = 7.1, 3H,

CH3) 1.29–1.98 (m, 10H, H-5, H-5, H-6, H-6, H-7, H-7, H-8, H-8, H-1, H-4), 3.00 (dd, J = 5.1, J = 1.1
+
13
H-2), 3.47 (d, J = 4.6 1H, H-3), 4.03–4.23 (m, 2H, OCH2), 8.00 (bs, 3H, NH3 ) ppm. C-NMR: δ =

Molecules 2013, 18
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14.9, 18.7, 21.2, 24.5, 25.2, 28.0, 28.7, 43.4, 48.8, 61.4, 172.8 Anal. Calcd for C11H20ClNO2 (233.74):
C, 56.52; H, 8.62; Cl 15.17; N, 5.99. Found: C, 56.76; H, 8.49; N, 6.17. The ee values for esters (–)-4
and (+)-4 were determined after derivatization with Ac2O in the presence of DMAP and pyridine (1:9)
by using Perkin-Elmer GC instrumentation equipped with CP-Chirasil L-Val columns (30 m) with the
temperature held at 140 °C for 5 min and then raised to 160 °C at 10 °C min−1, flow rate: 1 mL min−1,
retention times: (–)-4: 9.62 min; (+)-4: 9.97 min.
Ethyl (2S,3R)-3-aminobicyclo[2.2.2]octane-2-carboxylate Hydrochloride (+)-4. White crystals
20
1
(81 mg, 81%,), mp 193–194°C,


D = +30.1 (c, 0.4, EtOH) ee = 99%. The H- and 13C-NMR
spectroscopic data on the enantiomeric derivative were consistent with the data on the antipode (–)-4.
Anal. Calcd for C11H20ClNO2 (233.74): C, 56.52; H, 8.62; Cl 15.17; N, 5.99. Found: C, 56.81; H, 8.75;
N, 6.21.
3.6. General Procedure for Hydrolyses of Amino Esters 1–4
The individual amino ester enantiomers 1–4 (198 mg, 1 mmol) were dissolved in water (4 mL) in a
10 mL pressurized reaction vial, and the solution was stirred at 100 °C for 60 min at max. 150 W
microwave irradiation. The solvent was evaporated off, and the crude amino acid was recrystallized
from H2O/acetone to afford a white crystalline product.
(1S,2S,3S,4R)-3-Aminobicyclo[2.2.2]oct-5-ene-2-carboxylic Acid (+)-5. White crystals (80 mg, 46%),
20
mp 245–260 °C (dec.),


D = +51 (c, 0.3, H2O). 1H-NMR: δ = 0.95–1.55 (m, 4H, H-7, H-7, H-8, H-8),
2.13–2.21 (m, 1H, H-2), 2.75–2.85 (m, 1H, H-1) 2.85–2.95 (m, 1H, H-4), 3.63 (t, J = 3.0 Hz, 1H, H-3),
+
13
3.76 (bs, 3H, NH3 ) 6.17 (t, J = 7.1 Hz, 1H, H-5) 6.52 (t, J = 7.4 Hz 1H, H-6) ppm. C-NMR: δ =
20.0, 23.2, 33.3, 33,6, 49.9, 52.0, 130.9, 136.8, 173.9 ppm. Anal. Calcd for C9H13NO2 (167.09): C,
64.65; H, 7.84; N, 8.38. Found: C, 64.71; H, 7.98; N, 8.45.
(1R,2R,3R,4S)-3-Aminobicyclo[2.2.2]oct-5-ene-2-carboxylic Acid (–)-5. White crystals (96 mg, 56%),
20
1
13
mp 270–277 °C (dec.),
 D = –64 (c, 0.5, H2O). The H- and C-NMR spectroscopic data on the

enantiomeric derivative are consistent with the data of the antipode (+)-5. Anal. Calcd for C9H13NO2
(167.09): C, 64.65; H, 7.84; N, 8.38. Found: C, 64.83; H, 7.99; N, 8.61.
(2S,3S)-3-Aminobicyclo[2.2.2]octane-2-carboxylic Acid (–)-6. White crystals (116 mg, 67%), mp
20
1
255–260 °C (dec.),
 D = –35.4 (c, 0.5, H2O). H-NMR: δ = 1.39–1.75 (m, 8H, H-5, H-5, H-6, H-6,

H-7, H-7, H-8, H-8), 1.87–1.91 (m, 1H, H-4), 2.07–2.11 (m, 1H, H-1) 2.34 (d, J = 6.1, 1H, H-2), 3.90
(d, J = 6.1 Hz, 1H, H-3). 13C-NMR: δ = 18.4, 20.9, 24.2, 25.5, 28.1, 29.2, 51.0, 52.9, 180.9 ppm. Anal.
Calcd for C9H15NO2 (169.11): C, 63.88; H, 8.93; N, 8.28. Found: C, 64.07; H, 9.11; N, 8.08.
(2R,3R)-3-Aminobicyclo[2.2.2]octane-2-carboxylic Acid (+)-6. White crystals (100 mg, 58%), mp
20
1
13
255–260 °C (dec.),
 D = +35.1 (c, 0.4, H2O). The H- and C-NMR spectroscopic data on the

enantiomeric derivative were consistent with the data on the antipode (–)-6. Anal. Calcd for C9H15NO2
(169.11): C, 63.88; H, 8.93; N, 8.28. Found: C, 64.12; H, 9.05; N, 8.41.

Molecules 2013, 18
15088
(1S,2R,3S,4R)-3-Aminobicyclo[2.2.2]oct-5-ene-2-carboxylic Acid (–)-7. White crystals (96 mg, 56%),
20
1
13
mp 215–219 °C (dec.),
 D = –3 (c, 0.4, H2O). The H- and C-NMR spectroscopic and elemental

analysis data on the enantiomeric derivative were in accordance with those for racemic (±)-7 [29].
(1R,2S,3R,4S)-3-Aminobicyclo[2.2.2]oct-5-ene-2-carboxylic Acid (+)-7. White crystals (88 mg, 51%),
20
1
13
mp 220–221 °C (dec.),
 D = +3.2 (c, 0.4, H2O) The H- and C-NMR spectroscopic and elemental

analysis data on the enantiomeric derivative were in accordance with those for racemic (±)-7 [29].
(2R,3S)-3-Aminobicyclo[2.2.2]octane-2-carboxylic Acid (–)-8. White crystals (92 mg, 54%),
20
1
13
mp 223–230 °C (dec.),
 D = –49 (c, 0.3, H2O). The H- and C-NMR spectroscopic and elemental

analysis data on the enantiomeric derivative were in accordance with those for racemic (±)-8 [29].
(2S,3R)-3-Aminobicyclo[2.2.2]octane-2-carboxylic Acid (+)-8. White crystals (83 mg, 48%), mp
20
1
13
236–240 °C (dec.),
 D = +46.5 (c, 0.3, H2O). The H- and C-NMR spectroscopic and elemental

analysis data on the enantiomeric derivative were in accordance with those for racemic (±)-8 [29].
3.7. Syntheses of Ureas (+)-9 and (–)-10
Amino ester base (–)-1 or (–)-2 (100 mg, 0.33 mmol) was dissolved in dry Et2O (20 mL) and a 10%
excess of (S)-(–)-1-phenylethyl isocyanate (50 mg, 0.34 mmol) was added. The mixture was allowed to
stand for 24 h at room temperature. After evaporation, the resulting crystalline urea adducts were
recrystallized from iPr2O.
Ethyl (1S,2R,3S,4R)-3-[3-(1-(S)-phenylethyl)ureido]bicyclo[2.2.2]oct-5-ene-2-carboxylate (+)-9.
20
White crystals (92 mg, 82%), mp 195–200 °C,


D = +8 (c, 0.3, EtOH). 1H-NMR: δ = 0.99–1.07 (m
1H, H-7) 1.11–1.20 (m, 1H, H-7) 1.16 (t, J = 7.2, 3H, CH3) 1.23 (d, J = 7.1 (3H, CH3) 1.43–1.53 (m,
2H, H-8), 2.44 (m, 1H, H-1) 2.66 (m, 1H, H-4) 2.96 (d, J = 9.8, 1H, H-2) 3.90–4.06 (m, 2H, OCH2)
4.3 (td, J = 10.1, J = 1.5, 1H, H-3) 4.68 (m, 1H, CH) 5.34 (d, J = 10.4 H, NH) 6.12 (t, J = 7.2, 1H, H-5,
13
6.47 (t, J = 7.4, 1H, H-6) 6.54 (d, J = 8.1 1H, NH) 7.16–7.32 (m, 5H, Ar) C-NMR: δ = 14.8, 22.7,
24.4, 25.3, 32.6, 36.9, 49.3, 51.3, 51.4, 60.4, 126.5, 127.2, 129.0, 130.5, 137.1, 146.6, 157.0, 173.0.
Anal. Calcd for C20H26N2O3 (342.43): C 70.15, H 7.65; N 8.18. Found C 70.22, H 7.92, N 8.12.
Ethyl (1R,2R,3R,4S)-3-[3-(1-(S)-phenylethyl)ureido]bicyclo[2.2.2]oct-5-ene-2-carboxylate (–)-10.
20
1
White crystals (108 mg, 96%), a white solid, mp. 130–135 °C,
 D = –101 (c, 0.2, EtOH) H-NMR:

δ = 0.92–1.06 (m 1H, H-7) 1.19–1.23 (m, 1H, H-7) 1.16 (t, J = 7.1, 3H, CH3) 1.28 (d, J = 6.9 (3H,
CH3) 1.36–1.52 (m, 2H, H-8), 1.95 (m, 1H, H-2) 2.58 (m, 1H, H-1) 2.70 (m, 1H, H-4) 4,08–4.23 (m,
3H, OCH2, H-3) 4.64 (m, 1H, CH) 5.54 (d, J = 7.9 H, NH) 6.12–6,21 (m, 2H, H-5, NH), 6.44 (t, J =
7.3, 1H, H-6) 7.14–7.35 (m, 5H, Ar) 13C-NMR: δ = 14.9, 20.3, 23.3, 24.3, 33.6, 36.0, 49.3, 51.5, 53.1,
60.8, 126.6, 127.3, 129.1, 132.8, 135.7, 146.5, 157.3, 173.8, Anal. Calcd for C20H26N2O3 (342.43): C
70.15, H 7.65, N 8.18. Found C 70.31, H 7.79, N 8.27.

Molecules 2013, 18
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3.8. Racemic Compounds
1
All the reactions were first optimized for the racemic compounds. The H- and 13C-NMR
spectroscopic and elemental analysis data on the racemic derivatives were in accordance with those
for enantiomers.
Data on racemates ()-2–()-8
Ethyl 3-endo-aminobicyclo[2.2.2]oct-5-ene-2-exo-carboxylate hydrochloride ()-2: White crystals, mp
227–229 °C.
Ethyl trans-3-aminobicyclo[2.2.2]octane-2-carboxylate hydrochloride ()-3: White crystals, mp 130–133 °C.
Ethyl cis-3-aminobicyclo[2.2.2]octane-2-carboxylate hydrochloride ()-4: White crystals, mp 189–191 °C.
3-endo-Aminobicyclo[2.2.2]oct-5-ene-2-exo-carboxylic acid ()-5: White crystals, mp 250–255 °C.
trans-3-Aminobicyclo[2.2.2]octane-2-carboxylic acid ()-6: White crystals, mp 255–260 °C.
2,3-Diendo-3-aminobicyclo[2.2.2]oct-5-ene-2-carboxylic acid ()-7: White crystals, mp 228–231 °C,
Lit. mp 204–208 °C [29].
cis-3-Aminobicyclo[2.2.2]octane-2-carboxylic acid ()-8: White crystals, mp 208–210, Lit. mp
232–235 °C [32].
3.9. X-Ray Crystallographic Studies
The crystallographic data on compounds (+)-9 and (–)-10 were collected at 123 K with an Agilent
SuperNova dual wavelength diffractometer equipped with an Atlas CCD area detector with the use of
Cu-Kα radiation and the CrysAlisPro program package [33]. The empirical absorption correction was
performed with the SCALE3 ABSPACK scaling algorithm as implemented in the CrysAlisPro
program [33]. The crystal data along with selected refinement details for compounds (+)-9 and (–)-10
are presented in Table 2.
The structures were solved by direct methods with the SHELXS-97 [34] program or the SIR-97 [35]
program, and the full-matrix least squares refinements on F2 were performed with the SHELXL-9728
program. Figures were drawn with Diamond 3 [36]. For all compounds, the heavy atoms were refined
anisotropically. The CH hydrogen atoms were included at the calculated distances with fixed
displacement parameters from their host atoms (1.2 or 1.5 times the host atom). The NH hydrogens
were located from the electron density map and refined isotropically. The absolute configurations of
the compounds were determined from the Flack parameters.
Detailed crystallographic data for compound (+)-9 (CCDC 918507) and (–)-10 (CCDC 918508) is
deposited at the Cambridge Crystallographic Data Centre. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).

Molecules 2013, 18
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Table 2. Crystallographic data for compounds (+)-9 and (–)-10.
Compound
Formula
(+)-9
C20H26N2O3
342.43
(–)-10
C20H26N2O3
342.43
Mr
Crystal system
orthorhombic
P212121(19)
12.0973(2)
17.4189(2)
17.5471(2)
90
orthorhombic
P212121 (19)
8.9562(3)
18.4189(6)
22.9915(3)
90
Space group (no.)
a (Å)
b (Å)
c (Å)
α ()
β ()
90
90
γ ()
90
90
V (Å)
3697.55(9)
8
3792.75(18)
8
Z
Dc (g cm−3)
T (K)
1.230
1.199
123
123
μ(CuKα)
Observed reflections
Rint
0.665
0.648
6269
6726
0.0279
0.0213
Parameters
Flack’s parameter
467
477
0.05(17)
0.0406 (0.0386) b
0.0991 (0.0969)
0.0(2)
0.0521 (0.0480)b
a
R1
c
wR2
0.1266 (0.1230)
a R1 = Fo − Fc/Fo; b Values in parentheses for reflections with I > 2(I); c wR2 = {[w(F2o − F2c)2]/
[w(F2o)2]}½ and w = 1/[2(F2o) + (aP)2 + (bP)], where P = (2F2c + F2o)/3.
4. Conclusions
In conclusion, we have successfully synthetized all four enantiomers of 2,3-diendo-3-aminobicyclo
[2.2.2]oct-5-ene-2-carboxylic acid and 3-endo-aminobicyclo[2.2.2]oct-5-ene-2-exo-carboxylic acid
and their saturated analogues in a simple scalable protocol. The prepared compounds are highly likely
to be useful and well applicable in synthetic chemistry and drug research, with the creation of diverse
libraries of conformationally constrained compounds.
Acknowledgments
The authors thank István Mándity for his kind assistance in the H-Cube experiments. Their thanks
are also due to the Hungarian Research Foundation (OTKA grants TS 04888 and T 034901) and
TAMOP-4.2.2.A-II/1/KONV-2012-0035 for financial support.
Conflicts of Interest
The authors declare no conflict of interest.

Molecules 2013, 18
15091
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Sample Availability: Samples of the compounds 1–10 are available from the authors.
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distributed under the terms and conditions of the Creative Commons Attribution license
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