Optical resolution of racemic 4-hydroxy-3-isobornyl-5-methylbenzaldehyde

Article abstract of DOI:10.1134/S1070428012010095

Racemic 4-hydroxy-3-isobornyl-5-methylbenzaldehyde was separated into particular enantiomers via transformation into diastereoisomeric Schiff bases by reaction with (R)-1-phenylethanamine. The absolute configuration of the products was determined on the basis of the X-ray diffraction data for camphanate derived from one enantiomer of 4-hydroxy-3-isobornyl-5- methylbenzaldehyde. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S1070428012010095

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2012, Vol. 48, No. 1, pp. 64–68. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.V. Buravlev, I.Yu. Chukicheva, A.V. Churakov, A.V. Kutchin, 2012, published in Zhurnal Organicheskoi Khimii, 2012, Vol. 48,
No. 1, pp. 70–74.
Optical Resolution of Racemic 4-Hydroxy-3-isobornyl-
5-methylbenzaldehyde
E. V. Buravleva, I. Yu. Chukichevaa, A. V. Churakovb, and A. V. Kutchina
a Institute of Chemistry, Komi Research Center, Ural Division, Russian Academy of Sciences,
ul. Pervomaiskaya 48, Syktyvkar, 167982 Russia
e-mail: buravlev-ev@chemi.komisc.ru
b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
Received May 30, 2011
Abstract—Racemic 4-hydroxy-3-isobornyl-5-methylbenzaldehyde was separated into particular enantiomers
via transformation into diastereoisomeric Schiff bases by reaction with (R)-1-phenylethanamine. The absolute
configuration of the products was determined on the basis of the X-ray diffraction data for camphanate derived
from one enantiomer of 4-hydroxy-3-isobornyl-5-methylbenzaldehyde.
DOI: 10.1134/S1070428012010095
The presence of an aldehyde group in the molecules
of 4-hydroxy-3,5-dialkylbenzaldehydes opens pros-
pects in the synthesis of porphyrins [1] and fused,
heterocyclic [2–4], and phosphorus-containing com-
pounds [5] functionalized by 2,4-dialkylphenol frag-
ments. These compounds attract interest from the
viewpoint of their physiological and antioxidant prop-
erties. Racemic phenols having an isobornyl substit-
uent are synthetically accessible, they possess anti-
thrombogenic and antithrombocytic properties [6] and
can be used as antioxidants [7]. It is desirable that both
racemic and optically active derivatives of isobornyl-
phenols be available to study their biological activity.
We previously reported on optical resolution of
racemic salicylaldehydes having an isobornyl substit-
uent with the use of (R)-1-phenylethanamine [8]. The
Scheme 1.
(R)
Me
Me
OH
(R)
Me
(S)
H+
Me
(–)-II
18
NH2
(R)
N
4
19
8
Me
fractional
crystallization
Ph
5Me
Me
Me
3
OH
OH
Me
Me
Ph
Urotropin
AcOH
17
Me
7
9
III
16
6Me
Me
15
2
(S)
1
11
12
10
Me
Me
OH
14
Me
Me
(S)
13
CHO
(R)
rac-I
rac-II
H+
Me
(+)-II
N
Me
Ph
IV
64

OPTICAL RESOLUTION OF RACEMIC 4-HYDROXY-3-ISOBORNYL-5-METHYL...
65
present article reports on the synthesis and optical
resolution of racemic isobornylphenol possessing
an aldehyde group in the para position with respect to
the hydroxy group. Isobornylphenol rac-I was synthe-
sized by alkylation of o-cresol with camphene in the
presence of aluminum methylphenoxide [7]. The cor-
responding aldehyde rac-II was prepared by treatment
of rac-I with hexamethylenetetramine (urotropin) in
acetic acid (Duff reaction) according to the procedure
described in [9]. Racemate II was separated into indi-
vidual enantiomers via transformation into diastereo-
isomeric Schiff bases by reaction with enantiomeri-
cally pure (R)-1-phenylethanamine (Scheme 1). The
Schiff bases were obtained in quantitative yield as
mixtures of diastereoisomers III and IV at a ratio of
~1:1 (GLC data). By fractional crystallization from
hexane we succeeded in isolating compounds III and
IV enriched in one diastereoisomer. The diastereo-
isomeric purity was estimated by GLC.
other isobornylphenol derivatives [10]. However, the
conformation of V differed considerably from that
typical of structurally related compounds. The torsion
angle C6C1C22C21 in molecule V is –149.2(2)° against
–79.8(3) to –82.6(3)° reported in [10] for analogous
compounds. Some selected geometric parameters of
molecule V in crystal are collected in table.
The absolute configuration of compound V was
determined on the basis of anomalous X-ray scattering;
it coincided with the relative configuration assumed
from the known configuration of the camphane substit-
uent. The chiral centers in the isobornyl fragment of
(+)-II, IV, and V have (1R,2S,4S) configuration. The
configuration of the terpene fragment in compounds
III and (–)-II is the opposite, (1S,2R,4R).
EXPERIMENTAL
The IR spectra were recorded in KBr on a Specord
M-80 spectrometer. The 1H and 13C NMR spectra were
measured on a Bruker Avance II 300 spectrometer
(300.17 and 75.48 MHz, respectively) from solutions
in CDCl3. The chemical shifts were determined rela-
tive to the solvent signals (δ 7.26 ppm, δC 77.00 ppm).
Signals were assigned on the basis of J-modulation 13C
NMR spectra and two-dimensional spectra (HSQC,
COSY, NOESY). The melting points were determined
on a Kofler hot stage. The optical rotations were
measured using a P3002RS Krüss Optronic automatic
digital polarimeter (λ 589 nm).
The NOESY spectra of III and IV revealed cou-
pling between the 18-H and 19-H protons,* indicating
that these protons are spatially close to each other.
These findings suggest that Schiff bases III and IV
have E configuration with respect to the double C=N
bond. Aldehydes (+)-II and (–)-II enriched in partic-
ular enantiomer were isolated by acid hydrolysis of
each Schiff base. Their optical purity was estimated by
analytical HPLC on a Chiralcel OD-H column.
Scheme 2.
10'
Me
4'
O
8'
C4
C13
C12
5' Me
3'
C19
7'
9'
6' Me
Me
C14
C3
C7
C18
C9
O
O
C5
1'
11'
Me
Me
Et3N
(+)-II +
Me
Me
O
O
C2
C16
O
O
C15
O4
Me
C20
C11
C1 O2
C22
C6
C10
Cl
C8
Me
C17 O3
O1
CHO
V
C21
C23
C24
C27
Ester V was synthesized by acylation of (+)-II at
the hydroxy group with (1S)-camphanic chloride
(Scheme 2). Slow evaporation of a solution of V in
hexane–diethyl ether gave single crystals suitable for
X-ray analysis (see figure). According to the X-ray
diffraction data, compound V crystallized in P21 chiral
space group. The bond lengths and bond angles in
molecule V were similar to those found previously for
C28
C26
C25
O5
Structure of the molecule of 4-formyl-2-methyl-6-
{(1R,2S,4S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl}phen-
yl (1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-
1-carboxylate (V) according to the X-ray diffraction data.
* For atom numbering, see Schemes 1 and 2.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 48 No. 1 2012

BURAVLEV et al.
66
Selected bond lengths and bond angles in the molecule of
4-formyl-2-methyl-6-{(1R,2S,4S)-1,7,7-trimethylbicyclo-
[2.2.1]heptan-2-yl}phenyl (1S,4R)-4,7,7-trimethyl-3-oxo-2-
oxabicyclo[2.2.1]heptane-1-carboxylate (V)
Proteum automatic diffractometer with a rotating
anode at 100.0(2) K (CuKα irradiation, λ 1.54178 Å,
graphite monochromator). Monoclinic crystals
(C28H36O5, M 452.57), space group P21, with the
following unit cell parameters: a = 7.9126(2), b =
7.0320(1), c = 21.4735(5) Å; β = 93.661(1)°; V =
1192.38(4) Å3; Z = 2; dcalc = 1.261 g/cm3; μ(CuKα) =
0.681 mm–1; F(000) = 488. Total of 7933 reflections
(3361 independent reflections with Rint = 0.0345) were
measured by φ- and ω-scanning through a step of 0.5°
in the range 2.06 < θ < 63.03° (–9 ≤ h ≤ 8, –7 ≤ k ≤ 8,
–22 ≤ l ≤ 24). Correction for absorption was intro-
duced by measuring the intensities of equivalent
reflections (transmission factors min/max =
0.905/ 0.935). The structure was solved by the direct
method and was refined by the full-matrix least-
squares procedure in anisotropic approximation with
respect to F2 for all non-hydrogen atoms (SHELXTL-
PLUS [11]). All hydrogen atoms were placed into posi-
tions calculated on the basis of geometry considera-
tions and were refined according to the riding model.
The final divergence factors were R1 = 0.0350, wR2 =
0.0922 for 3303 reflections with I > 2σ(I) and R1 =
0.0355, wR2 = 0.0927 (for all reflections); 305 refined
parameters; absolute structure parameter –0.12(17);
goodness of fit 1.046; Δρmin/max = –0.191/0.319.
Bond
d, Å
Angle
ω, deg
O1–C20
O1–C21
O2–C20
O3–C11
O3–C15
O4–C11
O5–C27
1.350(2)
1.412(2)
1.203(2)
1.372(2)
1.470(2)
1.200(2)
1.226(3)
C20O1C21
C11O3C15
O4C11O3
O4C11C12
O3C11C12
O2C20O1
O2C20C15
O1C20C15
O5C27C24
116.52(14)
105.92(14)
121.47(19)
131.22(18)
107.30(16)
124.40(19)
125.01(18)
110.55(16)
124.1(2)00
The progress of reactions was monitored by TLC
on Sorbfil plates. Aldehyde spots were detected by
treatment with a solution of 15 g of potassium perman-
ganate in 300 ml of water containing 0.5 ml of concen-
trated sulfuric acid. Ester V was detected by treatment
with a solution of Bromocresol Purple, followed by
heating to 100–120°C. The purity of phenol I and
aldehydes II was checked by GLC on a Shimadzu GC-
2010AF gas chromatograph equipped with a flame-
ionization detector (carrier gas helium; Agilent HP-1
capillary column, 60 m × 0.25 mm × 0.25 μm; oven
temperature programming from 100 to 240°C at a rate
of 6 deg/min); diastereoisomeric purity of Schiff bases
III and IV was determined at an oven temperature of
270°C. Enantiomeric purity of aldehydes II was
estimated by HPLC on an Agilent 1100 chromatograph
[UV detector, λ 224 nm; 20°C; Chiralcel OD-H
column (Daicel), 25 cm × 4.6 mm, 10 μm, eluent hex-
ane–i-PrOH (19:1), flow rate 1.0 ml/min].
4-Hydroxy-3-methyl-5-(1,7,7-trimethylbicyclo-
[2.2.1]heptan-exo-2-yl)benzaldehyde (rac-II). A mix-
ture of 3.2 g (13.1 mmol) of phenol rac-I, 1.47 g
(10.5 mmol) of urotropin, and 8 ml of 90% aqueous
acetic acid was heated for 3 h under reflux. The prog-
ress of the reaction was monitored by thin-layer chro-
matography using petroleum ether–diethyl ether (5:1)
as eluent. The precipitate was filtered off, dried, and
purified by column chromatography (gradient elution
with petroleum ether–diethyl ether). Yield 2.6 g (73%),
colorless powder, mp 155–157°C (from petroleum
ether). IR spectrum, ν, cm–1: 3232 (OH), 1674 (C=O).
1H NMR spectrum, δ, ppm: 0.78 s (3H, C10H3), 0.85 s
(3H, C9H3), 0.89 s (3H, C8H3), 1.45–1.49 m (1H, 5-H),
1.59–1.73 m (3H, 3-H, 6-H), 1.78–1.95 m (2H, 4-H,
5-H), 2.32–2.46 m (1H, 3-H), 2.32 s (3H, C17H3),
3.07 t (1H, 2-H, J = 8.8 Hz), 5.37 s (1H, OH), 7.53 br.s
and 7.73 br.s (1H each, 14-H, 16-H), 9.82 s (1H,
18-H). 13C NMR spectrum, δC, ppm: 12.42 (C10), 16.17
(C17), 20.33 (C9), 21.30 (C8), 27.46 (C5), 34.21 (C3),
40.01 (C6), 45.45 (C4), 45.65 (C2), 48.23 (C7), 49.83
(C1); 123.52, 128.80, 129.99 (C11, C13, C15); 128.68,
130.39 (C14, C16); 159.07 (C12), 191.66 (C18). Found,
%: C 79.12; H 8.97. C18H24O2. Calculated, %: C 79.37;
H 8.88.
Aldehydes II and ester V were purified by column
chromatography on silica gel Alfa Aesar 70/230μ (wet
packing). Toluene was dried over anhydrous CaCl2 and
was distilled over metallic sodium. Petroleum ether
with bp 65–70°C was used. Hexane was distilled just
before use. Molecular sieves (4 Å) were activated by
calcination at 140°C over a period of 3 h. (R)-(+)-1-
Phenylethanamine (Alfa Aesar, ChiPros®, enantio-
meric purity >99%), (1S)-camphanic acid chloride
(Acros Organics), triethylamine (Sigma–Aldrich),
4-dimethylaminopyridine, urotropin, acetic acid, and
diethyl ether of chemically pure grade were used
without additional purification.
X-Ray analysis was performed for a 0.15×0.15×
0.10-mm single crystal of ester V on a Bruker Smart
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 48 No. 1 2012

OPTICAL RESOLUTION OF RACEMIC 4-HYDROXY-3-ISOBORNYL-5-METHYL...
67
Enantiomerically enriched aldehydes II. A mix-
ture of 0.3 g (0.8 mmol) of Schiff base III or IV and
5 ml of 90% aqueous acetic acid was heated for 2.5 h
under reflux, 10 ml of diethyl ether was added, the
organic layer was separated, washed with water (2×
7 ml) to remove acid, dried over anhydrous sodium
sulfate, and evaporated. The residue was purified by
column chromatography using petroleum ether–diethyl
ether as eluent.
(C11), 145.64 (C21), 155.45 (C12), 159.59 (C18). Found,
%: C 83.31; H 8.93; N 3.87. C26H33NO. Calculated, %:
C 83.15; H 8.86; N 3.73.
2-Methyl-6-{(1R,2S,4S)-1,7,7-trimethylbicyclo-
[2.2.1]heptan-2-yl}-4-{(E)-[(R)-1-phenylethylimino]-
methyl}phenol (IV). Yield 0.71 g (34%), diastereo-
isomeric purity 86%. Retention time 20.2 min. Light
yellow powder, mp 127–129°C (from hexane). IR
1
spectrum, ν, cm–1: 3396 (OH), 1642 (C=N). H NMR
spectrum, δ, ppm: 0.79 s (3H, C10H3), 0.86 s (3H,
C9H3), 0.93 s (3H, C8H3), 1.33–1.49 m (2H, 5-H, 6-H),
1.59 d (3H, C20H3, J = 6.6 Hz), 1.62–1.72 m (2H, 3-H,
6-H), 1.81–1.96 m (2H, 4-H, 5-H), 2.20–2.32 m (1H,
3-H), 2.27 s (3H, C17H3), 3.09 t (1H, 2-H, J = 8.7 Hz),
4.51 q (1H, 19-H, J = 6.6 Hz), 4.98 br.s (1H, OH),
7.20–7.54 m (7H, 14-H, 16-H, 22-H, 22′-H, 23-H,
23′-H, 24-H), 8.26 br.s (1H, 18-H). 13C NMR spec-
trum, δC, ppm: 12.39 (C10), 16.08 (C17), 20.34 (C9),
21.43 (C8), 24.86 (C20), 27.56 (C5), 34.28 (C3), 40.13
(C6), 45.59 (C4), 45.81 (C2), 48.22 (C7), 49.83 (C1),
69.48 (C19), 123.17 (C13), 126.61 and 128.32 (C22, C22′,
C23, C23′, C24), 127.23 and 127.57 (C14, C16), 128.20
(C15), 128.81 (C11), 145.59 (C21), 155.45 (C12), 159.63
(C18). Found, %: C 83.34; H 9.01; N 3.61. C26H33NO.
Calculated, %: C 83.15; H 8.86; N 3.73.
4-Hydroxy-3-methyl-5-{(1S,2R,4R)-1,7,7-tri-
methylbicyclo[2.2.1]hept-2-yl}benzaldehyde (–)-(II).
Yield 0.18 g (82%), enantiomeric purity 60.8%.
Retention time 9.53 min. Colorless powder, mp 139–
140°C (from petroleum ether), [α]D23 = –28.3° (c = 0.3,
CHCl3).
4-Hydroxy-3-methyl-5-{(1R,2S,4S)-1,7,7-tri-
methylbicyclo[2.2.1]hept-2-yl}benzaldehyde (+)-(II).
Yield 0.19 g (86%), enantiomeric purity 85.4%.
Retention time 10.56 min. Colorless powder, mp 140–
142°C (from petroleum ether), [α]D23 = +49.4° (c = 0.3,
CHCl3).
Schiff bases III and IV. A mixture of 1.5 g
(5.5 mmol) of racemic aldehyde II dissolved in 25 ml
of toluene, 0.71 ml (5.5 mmol) of (R)-(+)-1-phenyl-
ethanamine and 6.5 g of molecular sieves was heated
for 3.5 h under reflux while stirring in a stream of
argon. The mixture was filtered through a glass filter,
the precipitate (molecular sieves) was washed with
CHCl3, and the filtrate was evaporated. The residue
was separated by fractional crystallization from hex-
ane, the separation process being monitored by GLC.
4-Formyl-2-methyl-6-{(1R,2S,4S)-1,7,7-trimeth-
ylbicyclo[2.2.1]heptan-2-yl}phenyl (1S,4R)-4,7,7-tri-
methyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carbox-
ylate (V). A mixture of 0.027 g (0.1 mmol) of
aldehyde (+)-II dissolved in 3 ml of toluene, 0.032 g
(0.15 mmol) of (1S)-camphanic acid chloride, 0.021 ml
(0.15 mmol) of triethylamine, and 0.0012 g
(0.01 mmol) of 4-dimethylaminopyridine was heated
for 4 h under reflux with stirring in a stream of argon.
The mixture was evaporated, and the residue was
purified by column chromatography using petroleum
ether–diethyl ether as eluent. Yield 0.039 g (87%),
colorless powder, mp 142–145°C (from hexane),
[α]D23 = +9.1° (c = 0.1, CHCl3). IR spectrum, ν, cm–1:
1798, 1764 (C=O, ester), 1706 (CH=O), 1260 (C–O).
1H NMR spectrum, δ, ppm: 0.76–1.00 m (9H, C8H3,
C9H3, C10H3), 1.17 s (9H, C8′H3, C9′H3, C10′H3), 1.23–
1.34 m (1H, 5-H), 1.35–1.49 m (1H, 6-H), 1.58–
1.91 m (5H, 3-H, 4-H, 5-H, 5′-H, 6-H), 1.96–2.05 m
(1H, 5′-H), 2.20–2.31 m (2H, 3-H, 6′-H), 2.24 s (1H,
C17H3), 2.50–2.60 m (1H, 6′-H), 2.82 t (1H, 2-H, J =
8.7 Hz), 7.60 br.s (1H, 14-H), 7.86 s (1H, 16-H), 9.94 s
(1H, 18-H). 13C NMR spectrum, δC, ppm: 9.67, 16.78,
16.95 (C8′, C9′, C10′); 17.42 (C17), 21.30 (C8, C9, C10),
27.30 (C3, C5), 28.83 and 28.99 (C5′, C6), 31.82 (C6′),
45.52 (C4), 46.74 (C2), 48.10 (C1, C7), 54.48 and 54.91
2-Methyl-6-{(1S,2R,4R)-1,7,7-trimethylbicyclo-
[2.2.1]hept-2-yl}-4-{(E)-[(R)-1-phenylethylimino]-
methyl}phenol (III). Yield 0.4 g (19%), diastereo-
isomeric purity 63%. Retention time 20.1 min. Brown
powder, mp 64–69°C (from hexane). IR spectrum, ν,
1
cm–1: 3408 (OH), 1640 (C=N). H NMR spectrum, δ,
ppm: 0.79 s (3H, C10H3), 0.86 s (3H, C9H3), 0.87 s
(3H, C8H3), 1.28–1.52 m (2H, 5-H, 6-H), 1.59 d (3H,
C20H3, J = 6.6 Hz), 1.60–1.73 m (2H, 3-H, 6-H), 1.82–
1.96 m (2H, 4-H, 5-H), 2.19–2.34 m (1H, 3-H), 2.26 s
(3H, C17H3), 3.09 t (1H, 2-H, J = 8.7 Hz), 4.50 q (1H,
19-H, J = 6.6 Hz), 5.06 br.s (1H, OH), 7.21–7.59 m
(7H, 14-H, 16-H, 22-H, 22′-H, 23-H, 23′-H, 24-H),
8.26 br.s (1H, 18-H). 13C NMR spectrum, δC, ppm:
12.38 (C10), 16.11 (C17), 20.37 (C9), 21.38 (C8), 24.80
(C20), 27.52 (C5), 34.20 (C3), 40.05 (C6), 45.52 (C4),
45.73 (C2), 48.17 (C7), 49.78 (C1), 69.52 (C19), 123.34
(C13), 126.63 and 128.32 (C22, C22′, C23, C23′, C24),
127.06 and 127.76 (C14, C16), 128.22 (C15), 128.81
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 48 No. 1 2012

BURAVLEV et al.
68
(C4′, C7′), 90.45 (C1′), 128.37 (C16), 130.05 (C14),
131.08 (C11), 133.82 (C13), 156.86 (C12), 165.50 and
177.74 (C3′, C11′), 191.54 (C18). Found, %: C 74.05;
H 8.24. C28H36O5. Calculated, %: C 74.31; H 8.02.
3. Hori, H., Nagasawa, H., Ishibashi, M., Uto, Y.,
Hirata, A., Saijo, K., Ohkura, K., Kork, K.L., and
Uehara, Y., Bioorg. Med. Chem., 2002, vol. 10, p. 3257.
4. Boldron, C., Özalp-Yaman, Ş., Gamez, P., Tooke, D.M.,
Spek, A.L., and Reedijk, J., J. Chem. Soc., Dalton
Trans., 2005, p. 3535.
5. Prishchenko, A.A., Livantsov, M.V., Novikova, O.P.,
Livantsova, L.I., Shpakovskii, D.B., and Milaeva, E.R.,
Russ. J. Gen. Chem., 2006, vol. 76, p. 1753.
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This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 10-03-01129-a) and by the Ural Division of the
Russian Academy of Sciences (competition of research
projects of young scientists and post-graduate students
of the Ural Division of the Russian Academy of
Sciences, project no. 10-3-NP-95). The authors thank
the Faculty of Chemistry, Durham University (United
Kingdom) for providing X-ray diffraction facilities.
7. Chukicheva, I.Yu., Buravlev, E.V., Fedorova, I.V.,
Borisenkov, M.F., and Kutchin, A.V., Russ. Chem. Bull.,
2010, vol. 59, p. 2276.
8. Buravlev, E.V., Chukicheva, I.Yu., Dolgushin, F.M., and
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