Conformational study of α-arylethylamides of (-)-camphanic acid

Article abstract of DOI:10.1016/S0957-4166(98)00162-1

The absolute conformation and configuration of diastereomeric amides (4A,B-6A,B) of (1S,3R)-camphanic acid (lactone of 1-hydroxy-2,2,3- trimethylcyclopentan-1,3-dicarboxylic acid, (-)-camphanic acid 9) with α- arylethylamines 1-3 are deduced from ¹H NMR data and MM2 calculations. The α-arylethyl group in diastereomers A and B adopt nearly opposite absolute conformations, stabilized by hydrogen bonding in the syn-oriented O-C(1)- C(6)-N-H unit, and repulsive interaction between the 1'C-Me group and the amide C=O group. The absolute configuration (1'S) is assigned to the 4A-6A diastereomers, and the (1'R)-configuration to the 4B-6B diastereomers; this assignment is confirmed by the preparation of 4A and 5A from enantiomerically pure (1'S)-α-arylethylamines 1 and 2, respectively. These results also enabled the assignment of pro-R (H(R)) and pro-S (H(S)) protons in the benzyl derivative 7.

Full text of DOI:10.1016/S0957-4166(98)00162-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 1891–1897
Conformational study of α-arylethylamides of (−)-camphanic
acid
Zdenko Hameršak, Ana Selestrin, Andreja Lesac and Vitomir Šunjic´ ∗
‘Ruder Boškovic´’ Institute, PO Box 1016, Bjenicˇka c. 54, HR-10000 Zagreb, Croatia
¯
Received 16 March 1998; accepted 21 April 1998
Abstract
The absolute conformation and configuration of diastereomeric amides (4A,B–6A,B) of (1S,3R)-camphanic
acid (lactone of 1-hydroxy-2,2,3-trimethylcyclopentan-1,3-dicarboxylic acid, (−)-camphanic acid 9) with α-
arylethylamines 1–3 are deduced from 1H NMR data and MM2 calculations. The α-arylethyl group in diastereo-
mers A and B adopt nearly opposite absolute conformations, stabilized by hydrogen bonding in the syn-oriented
O–C(1)–C(6)–N–H unit, and repulsive interaction between the 10C–Me group and the amide C_O group. The
absolute configuration (10S) is assigned to the 4A–6A diastereomers, and the (10R)-configuration to the 4B–6B
diastereomers; this assignment is confirmed by the preparation of 4A and 5A from enantiomerically pure (10S)-
α-arylethylamines 1 and 2, respectively. These results also enabled the assignment of pro-R (HR) and pro-S (HS)
protons in the benzyl derivative 7. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
(1S,3R)-Camphanic acid (lactone of 1-hydroxy-2,2,3-trimethylcyclopentan-1,3-dicarboxylic acid, 9) is
repeatedly used as a chiral auxiliary for the separation of racemates,1–3 in the preparation of chiral ligands
for organometallic catalysts,4–6 and in structural studies.2,7,8 Amides of camphanic acid are prepared for
the separation of racemic, cyclic sec amines.9 An unusual fragmentation process, observed during the
course of their cleavage by strong nucleophiles,9 can be circumvented by using NaH/CS2 under mild
conditions, a method recently used in the preparation of thiocamphanic acid from amides.10 Another
particular application of camphanic acid is Gerlach’s method for the determination of the enantiomeric
excess of α-deuterated primary alcohols.11
Within an ongoing project for the preparation of new chiral modifiers for heterogeneous catalysis, we
encountered the problem of effective resolution and configurational and conformational characterization
of diastereomeric camphanic acid amides obtained from racemic α-arylethylamines 1–3. Because of
∗
Corresponding author. E-mail: sunjic@rudjer.irb.hr
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00162-1

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their high π-electron surface density combined with strong steric perturbations by the camphanoyl
unit, pure diastereomeric amides 4A,B–6A,B, and their corresponding amines, are expected to be
effective surface modifiers in heterogeneous catalysis. Herewith we report on conformational properties
of chromatographically separated amides, as studied by NMR and computational methods, which allow
straightforward assignment of the absolute configuration at the stereogenic center of α-arylethylamino
units. This is the first report on the use of the anisotropic effect of the aromatic ring in racemic
compounds to be resolved, on the specific groups in the chiral auxiliary, as a tool for determination
of absolute configuration in the former. All so-far reported examples of the determination of absolute
configuration are based on the anisotropy of aromatic rings within the chiral auxiliary, such as O-methyl
mandelic acid derivatives,12 arylmethoxy acetic acid derivatives,13 O-aryllactic acid derivatives,14 and
diazacyclophosphamides derived from L- and D-prolylphenylamine.15
2. Results and discussion
Starting from racemic α-arylethylamines 1–3 diastereomeric pairs 4A,B–6A,B were prepared by
standard procedures. Effective separation of diastereomers was performed by chromatography on silica
gel, whereby diastereomers 6A and 6B exhibited large differences in Rf values (∆Rf ca. 0.2) in a single
solvent like dichloromethane. To separate diastereomers 4A,B and 5A,B binary solvent mixtures served
the purpose (see Experimental).
1
The most striking feature of the H NMR spectra of diastereomeric pairs 4A,B–6A,B represents a
uniform and significant difference in the chemical shifts of the three methyl groups in the camphanic
unit, and nearly superimposable peaks of diastereotopic C01–Me groups in the α-arylethylamine unit of
two diastereomers (Table 1). In diastereomeric mixtures 4A,B–6A,B, singlets for Me-8, Me-9 and Me-
10 appear as two signals in ca. 1:1 ratio; on chromatographic separation only three singlets remain. The
assignment of Me-8 to Me-10 is based upon our recent NMR spectroscopic study of camphanic acid 9
and its complexes.7 The correlation of 1H and 13C signals for diastereotopic α-arylethylamides with the
reference amides 7, 8 and parent acid 9 is evident from Table 1.
1
Taking prochiral benzylamide 7 as the standard for evaluation of the H NMR shifts, ∆δ in Table 1,
one can see that the three methyl groups of the camphanate unit in diastereomers 4B–6B exhibit shifts
to higher field, and those in 4A–6A to lower field relative to 7. A noticeable and opposite influence of
α-arylethyl units on magnetic nonequivalence of Me-8, Me-9 and Me-10 signals in the camphanic unit,
all distant from the aromatic ring, indicated different stable conformers for diastereomeric series A and
B. A tentative explanation can be offered from the inspection of models, which reveal that syn-oriented
O–C(1)–C(6)–N–H units minimize repulsive interactions, ensure hydrogen bonding, and maximize σ–π
interactions within the nearly coplanar, anti-related lactone group and amide carbonyl groups. This strong

Z. Hameršak et al. / Tetrahedron: Asymmetry 9 (1998) 1891–1897
1893
Table 1
Signals of the methyl groups in 1H and 13C NMR spectra of diastereomeric pairs 4A,B–6A,B, and of
7–9 in deuterochloroform
hydrogen bonding is clearly observed in the 1H NMR spectrum of 7, where diastereotopic protons of the
(10CH2) group from two well separated quartets are centered at 4.43 ppm, and at 4.53 ppm, respectively.
On N–H(D) exchange, they collapse into an AB system of two doublets. Inspection of models, confirmed
by calculations described hereafter, reveal that HR gives the signal at higher field and HS gives the signal
at lower field. Although the difference between signals for diastereotopic benzylic protons in 7 is ca. 10
times smaller than observed by Gerlach on incorporation of benzylamine into cyclic thioamide,16 it is
still significant in the determination of absolute configuration and optical purity of α-2H1-benzylamines.
In spite of the expected high conformational mobility (free rotation) of the α-arylethylamino unit in
both diastereomeric series A and B, two distinctly different spatial regions are available to the aromatic
ring. In diastereomers 4B–6B the aromatic ring can approach more closely the methyl groups of the
camphanic unit than in diastereomers 4A–6A. This conformational arrangement places methyl groups in
diastereomers 4B–6B, in particular Me-8 and Me-9, within the anisotropic cone of the aromatic ring,17

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and explains their regular higher shift in diastereomeric series A than in the series B, Table 1. Taking into
account this relationship, and the fact that the Me groups on (C0-1) in both series are approximately in the
same electronic surroundings, distant from the electron-rich amide C_O group, it is easy to determine
from the models that the α-arylethyl unit in the A series possesses the (10S) configuration and in the B
series the (10R) configuration. Surprisingly, the high conformational stability of camphanic amides 4–6
is reflected by minimal changes of the 1H NMR spectrum of 4B on heating in CDCl3 from 20°C to 55°C;
only the signals for Me-8, Me-9, and Me-10 were shifted downfield by 0.001, 0.005, and 0.013 ppm,
respectively.
It is interesting to note that Latypov et al.13 observed for (+)-bornylamides of α-arylmethoxyacetic
acid the lack of any strong conformational preference about the NH–CO bond because of the high
mobility about CO–Cα bond. This is reflected in much smaller ∆δRS values for the Me-8, Me-9 and
Me-10 groups in the (+)-bornyl unit, than for the same groups in (−)-camphanic acid, which comprises
a similar bicyclic system. In the average ∆δRS the values in the former are 2–5 times lower than we
observed for the signals of the analogous methyl groups in 4A,B–6A,B, where conformational mobility
seems rather restricted (Table 1). To confirm the above considerations, and to obtain more exact data on
the conformational population, we performed MM2 calculations for 4A, 4B and reference amide 7. The
results are summarized in Fig. 1. According to MM2 calculations, 4A and 4B adopt two energy minima
conformations, characterized by nearly opposite values for torsional angles ≤50 ,10, 20, 30; for 4A it is
−82°, and for 4B it is +83°. For the achiral benzylic unit in 7, three shallow energy minima are found,
characterized by torsional angles of ca. +80° (7a), −80° (7b) and +160° (7c). The most stable conformer
7b (MMX 33.34 kcal/mol), ca. 0.7 and 0.9 kcal/mol more stable than 7a and 7c, respectively, possess HR
in the shielding region of the amide carbonyl group, and HS distant from this group.
Thus, the results of MM2 calculations confirm qualitative conformational analysis based on the NMR
data and inspection of the models. For both diastereomers 4A and 4B C10–Me is placed nearly trans
to the amide C_O group and cis to the hydrogen bonded proton. This spatial arrangement moves the
phenyl group in 4a away from the methyl groups in the bicyclic unit, while in 4b, phenyl is found
close to this unit. This explains the stronger shielding of Me groups of the bicyclic system in 4B than
in 4A. Consequently, the absolute configuration at C10 in 4A should be (1S), and in 4B (1R). The
assignment was simply confirmed by independent preparation of 4A and 5A from the commercially
valuable (1S)-α-arylethylamines 1 and 2, respectively. The extension of this configurational assignment
to 6A is straightforward.
In conclusion, we have demonstrated that α-arylethylamides of camphanic acid, 4A,B–6A,B, potential
chiral modifiers in heterogeneous catalysis, possess well defined conformations, which can be deduced
by a combined study of 1H NMR spectra and MM2 calculations. These data also allow the assignment
of absolute configuration at (C10) in both diastereomeric series.
3. Experimental
1
IR: Perkin–Elmer 297 spectrometer for KBr pellets. H and 13C NMR: Varian Gemini XL 300
spectrometer for CDCl3 solutions, δ in ppm relative to TMS as internal reference, and J in hertz. HPLC:
HP 1050 chromatograph with Eurosphere C18 RP column; separation was monitored by an HP 1050
UV detector set up at 254 nm and connected to HP 3396A integrator. Mp: electrothermal apparatus, not
corrected. Optical rotations: Optical Activity AA-10 automatic polarimeter in a 1 dm cell; c in g/100 ml.

Z. Hameršak et al. / Tetrahedron: Asymmetry 9 (1998) 1891–1897
1895
Fig. 1. Force field calculated stable conformers of 4A, 4B and 7b
3.1. Acylation of α-arylethylamines. General procedure
Camphanic acid, thionylchloride, and a drop of DMF were heated for 6 h, then the thionylchloride
was evaporated to dryness. Evaporation was repeated after addition of toluene, then the solid residue was
dissolved in dichloromethane (5 ml). α-Arylethylamine, and triethylamine (100 mg, 1.0 mmol) were then
added. After 18 h stirring at ambient temperature, the reaction solution was washed first with 0.5 M aq.
bicarbonate, then dil. hydrochloric acid. The organic phase was dried (Na2SO4), the solvent evaporated
and the diastereomeric product mixture separated by chromatography.
Compounds 1, 2 and 9 were purchased from Aldrich, compounds 3 and 8 were prepared according to
known methods.18,19
3.2. (1S,3R)-N-[(10S)- and -(10R)-Phenylethyl]-1-carboxamido-1-hydroxy-2,2,3-trimethylcarboxylic
acid lactone (4A and 4B)
Starting from 200.0 mg (1.0 mmol) of camphanic acid and 250.0 mg (2.0 mmol) of (ꢀ)-1-
phenylethylamine was obtained 302 mg of crude 4A,B. Separation was performed on a silica gel column
(40 g) using cyclohexane:diisopropylether (1:1) as eluent. Obtained were 122 mg of 4A and 126 mg of
4B (total yield 82%), both with ≥97% purity according to 1H NMR.
Data for compound 4A: mp 84.9–86.6°C; [α]D=−44.4 (c 1.1, CH2Cl2); IR: 3360, 2960, 1790, 1670,
1530, 1450, 1170, 1060, 920, 700 cm−1; 1H NMR: 0.96 (s, 3H), 1.12 (s, 3H), 1.13 (s, 3H), 1.51 (d, 3H),
1.64–1.72 (m, 1H), 1.83–1.99 (m, 2H), 2.45–2.55 (m, 1H), 5.14–5.22 (m, 1H), 6.72 (d, NH), 7.27–7.38
(m, 5H); 13C NMR: 9.4, 16.3, 16.5, 21.8, 28.8, 30.0, 48.4, 53.7, 55.1, 92.3, 126.0, 127.5, 128.7, 142.4,

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166.0, 178.4. Anal. for C18H23NO3 (301.386), calcd: C 71.73, H 7.69, N 4.65; found: C 71.79, H 7.46,
N 4.68.
Data for compound 4B: mp 111.2–113.0°C; [α]D=+25.1 (c 1.9, CH2Cl2); IR: 3380, 2970, 1790, 1670,
1680, 1530, 1450, 1170, 1060, 920, 700 cm−1; 1H NMR: 0.80 (s, 3H), 1.06 (s, 3H), 1.09 (s, 3H), 1.51
(d, 3H), 1.66–1.73 (m, 1H), 1.89–2.00 (m, 2H), 2.50–2.57 (m, 1H), 5.10–5.20 (m, 1H), 6.73 (d, NH),
7.24–7.36 (m, 5H); 13C NMR: 9.4, 16.2, 16.5, 21.6, 28.9, 30.1, 48.4, 53.9, 55.1, 92.3, 126.0, 127.4,
128.6, 142.9, 166.1, 178.4. Anal. for C18H23NO3 (301.386), calcd: C 71.73, H 7.69, N 4.65; found: C
71.80, H 7.59, N 4.50.
3.3. (1S,3R)-N-[(10S)- and -(10R)-Naphth-10-yl-ethyl]-1-carboxamido-1-hydroxy-2,2,3-trimethylcarb-
oxylic acid lactone (5A and 5B)
Starting from 1.00 g (5.0 mmol) of camphanic acid and 1.60 g (9.3 mmol) of (ꢀ)-1-naphthylethylamine
was obtained 1.70 g (97%) of 5A,B, HPLC asssay 89%. Separation was performed on a silica gel column
(130 g) using cyclohexane:diisopropylether (1:1) as eluent. Obtained were 766 mg of 5A and 780 mg of
5B, both with ≥99% HPLC purity.
Data for compound 5A: mp 127.3–128.9°C; [α]D=+18.6 (c 1.8, CH2Cl2); IR: 3360, 2960, 1785, 1675,
1
1535, 1450, 1260, 1060, 920, 775 cm−1; H NMR: 0.98 (s, 3H), 1.10 (s, 3H), 1.16 (s, 3H), 1.68 (d,
3H), 1.77–1.97 (m, 3H), 2.47–2.57 (m, 1H), 5.96–6.01 (m, 1H), 6.74 (d, NH), 7.43–8.07 (m, 7H); 13C
NMR: 9.4, 16.3, 16.6, 20.9, 28.9, 30.1, 44.1, 53.8, 55.1, 92.3, 122.5, 123.0, 125.2, 125.8, 126.5, 128.5,
128.9, 130.9, 133.9, 137.5, 165.9, 178.3. Anal. for C22H25NO3 (351.446), calcd: C 75.19, H 7.17, N
3.98; found: C 75.06, H 7.19, N 4.20.
Data for compound 5B: mp 64.3–66.0°C (dec.); [α]D=−39.8 (c 2.0, CH2Cl2); IR: 3380, 2980, 1790,
1665, 1535, 1450, 1190, 1060, 920, 770 cm−1; 1H NMR: 0.72 (s, 3H), 1.06 (s, 3H), 1.10 (s, 3H), 1.69 (d,
3H), 1.90–1.98 (m, 3H), 2.53–2.59 (m, 1H), 5.93–6.03 (m, 1H), 6.73 (d, NH), 7.43–8.14 (m, 7H); 13C
NMR: 9.4, 16.3, 16.5, 20.6, 28.9, 30.2, 44.2, 53.9, 55.1, 92.4, 122.5, 123.2, 125.2, 125.8, 126.3, 128.5,
128.9, 130.8, 133.9, 137.8, 165.9, 178.3. Anal. for C22H25NO3 (351.446), calcd: C 75.19, H 7.17, N
3.98; found: C 75.11, H 7.09, N 4.09.
3.4. (1S,3R)-N-[(10S)- and -(10R)-Anthran-90-yl-ethyl-1-carboxamido-1-hydroxy-2,2,3-trimethylcarb-
oxylic acid lactone (6A and 6B)
Starting from 1.20 g (6.0 mmol) of camphanic acid and 2.40 g (10.8 mmol) of (ꢀ)-1-
anthracenylethylamine was obtained 2.40 g (99%) of 6A,B, HPLC asssay 89%. Separation was
performed on a silica gel column (140 g) using cyclohexane:diisopropylether (1:1) as eluent. Obtained
were 1.10 g of 6A and 1.00 g of 6B, both with ≥99% HPLC purity.
Data for compound 6A: mp 137.5–140.0°C; [α]D=−95.2 (c 1.4, CH2Cl2); IR: 3440, 2960, 1795, 1680,
1515, 1445, 1170, 1015, 920, 730 cm−1; 1H NMR: 1.12 (s, 3H), 1.14 (s, 3H), 1.17 (s, 3H), 1.56–1.92 (m,
2H), 1.98 (d, 3H), 2.34–2.43 (m, 1H), 6.62–6.71 (m, 1H), 7.44–8.46 (m, 9H); 13C NMR: 9.4, 16.4, 16.6,
21.5, 28.8, 29.9, 44.6, 53.8, 55.1, 92.4, 123.6, 124.8, 126.3, 128.3, 128.8, 129.8, 131.7, 132.9, 166.5,
178.3. Anal. for C26H27NO3 (401.506), calcd: C 77.78, H 6.78, N 3.49; found: C 77.61, H 6.57, N 3.41.
Data for compound 6B: mp 129.5–133.5°C; [α]D=+32.2 (c 1.5, CH2Cl2); IR: 3440, 2970, 1790, 1675,
1520, 1450, 1165, 1015, 920, 730 cm−1; 1H NMR: 0.47 (s, 3H), 0.91 (s, 3H), 1.03 (s, 3H), 1.66–2.05 (m,
3H), 1.96 (d, 3H), 2.52–2.59 (m, 1H), 6.57–6.66 (m, 1H), 7.39–8.45 (m, 9H); 13C NMR: 9.4, 16.1, 16.3,
21.2, 28.9, 30.1, 44.9, 53.7, 55.1, 92.5, 123.7, 124.8, 126.2, 128.2, 128.8, 129.7, 131.7, 133.4, 166.6,
178.4. Anal. for C26H27NO3 (401.506), calcd: C 77.78, H 6.78, N 3.49; found: C 77.53, H 6.76, N 3.57.

Z. Hameršak et al. / Tetrahedron: Asymmetry 9 (1998) 1891–1897
1897
3.5. (1S,3R)-N-Phenylmethyl-1-carboxamido-1-hydroxy-2,2,3-trimethylcarboxylic acid lactone (7)
Compound 7 was prepared as described above for diastereomeric mixtures. Obtained were 594 mg
(96%) of crude 7, dense oil that slowly crystallized: mp 110.6–112.2°C; [α]D=−6.3 (c 1.9, CH2Cl2); IR
3360, 2960, 1770, 1670, 1540, 1450, 1260, 1065, 920, 725 cm−1; 1H NMR: 0.91 (s, 3H), 1.10 (s, 3H),
1.13 (s, 3H), 1.63–1.72 (m, 1H), 1.88–2.00 (m, 2H), 2.51–2.61 (m, 1H), 4.43 (dd, 1H, J=14.4, 5.7 Hz),
4.53 (dd, 1H, J=14.4, 5.7 Hz), 6.79 (s, 1H), 7.27–7.36 (m, 5H); 13C NMR: 9.4, 16.3, 16.5, 28.8, 30.1,
42.9, 53.8, 55.1, 92.4, 127.6, 127.7, 128.7, 137.5, 166.9, 178.2. Anal. for C17H21NO3 (287.359), calcd:
C 71.06, H 7.37, N 4.87; found: C 71.16, H 7.20, N 5.03.
3.6. Calculations
The force field calculations were performed using a standard procedure by the SYBIL software
package of TRYPOS Assoc. (Munchen, Germany).
Acknowledgements
Financial support by the Croatian Ministry of Science and Technology (Project No. 980701) is
gratefully acknowledged. The authors are indebted to Dr. M. Žinic´ for his help in MM2 calculations.
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