Separation of racemic salycylaldehydes containing isobornyl substituent using (R)-1-phenylethylamine

Article abstract of DOI:10.1134/S1070428010050088

Racemic salicylaldehydes containing isobornyl substituent were separated by their conversion into diastereomers by the reaction with (R)-1- phenylethylamine. The absolute configuration of the intermediate Schiff bases and separated aldehyde eneatiomers wa

Full text of DOI:10.1134/S1070428010050088

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 5, pp. 649−654. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.V. Buravlev, I.Yu. Chukicheva, F.M. Dolgushin, A.V. Kuchin, 2010, published in Zhurnal Organicheskoi Khimii, 2010,
Vol. 46, No. 5, pp. 660−665.
Separation of Racemic Salycylaldehydes
Containing Isobornyl Substituent
Using (R)-1-Phenylethylamine
E. V. Buravleva, I. Yu. Chukichevaa, F. M. Dolgushinb, andA. V. Kuchina
aInstitute of Chemistry, Komi Scientific Center, Ural Division, Russian Academy of Sciences,
Syktyvkar167982, Russia
e-mail: buravlev-ev@chemi.komisc.ru
bNesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, Moscow
Received June 1, 2009
Abstract—Racemic salicylaldehydes containing isobornyl substituent were separated by their conversion into
diastereomers by the reaction with (R)-1-phenylethylamine. The absolute configuration of the intermediate Schiff
bases and separated aldehyde eneatiomers was established by XRD analysis.
DOI: 10.1134/S1070428010050088
Isobornyls rac-(I) were obtained by alkylation of
phenol and p-cresol in the presence of aluminum
phenolate and cresolate respectively [8]. Salicylaldehydes
rac-(II) were synthesized by modified procedure [9]
consisting in the formylation of isobornylphenols with
paraformaldehyde in the presence of montmorillonite KSF
and triethylamine. In the published procedure the
formylation was carried out at high pressure. We
performed the synthesis at the normal pressure by heating
the reaction mixture at 100°C. Aldehydes rac-(IIa, IIb)
were obtained in 60 and 71% yield, respectively.
Terpenophenol compounds are physiologically active
substances of a versatile activity possessing low toxicity
[1]. The antithrombogenic and antithrombocyte activity
has been found in phenols possessing isobornyl substituent;
they are employed as means of the local treatment of
infection sources [2, 3]. Inasmuch as the enentiomers
of a chiral biologically active substance sometimes
differently affect the living body [4] it is urgent for
revealing new qualities of terpenophenols to separate their
racemates into enentiomers.
The Schiff bases prepared from the enentiomerically
enriched formyl derivatives of isobornylphenol and (S)-
tert-leucinol are efficient tridentate ligands in the
vanadium-catalyzed asymmetrical oxidation of sulfides
[5]. Therefore the physiological activity and the possibility
of the application of chiral alkylphenols to the organic
synthesis call for the development of the procedures of
their preparation in optically active forms.
Racemates II were separated after their conversion
into diastereomeric imines by the reaction with the
enantiomerically pure (R)-1-phenylethylamine. The Schiff
bases were obtained in the quantitative yield as pairs of
diastereomers III and IV in the ratio 1:1 (according to
the data of 1H NMR spectrum and GLC). Both pairs of
diastereomers were separated by fractional crystallization
from pentane to obtain imines enriched with
diastereomers IIIa, IVa and IIIb, IVb. Diastereomeric
purity of the samples in the course of separation was
checked by GLC.
The separation of phenols containing a chiral fragment
is carried out using preparative HPLC that requires the
application of expensive columns packed with chiral
stationary phases [6, 7]. The other approach employs a
preliminary preparation of therpenophenol derivatives with
expensive chiral reagents, for instance, (R)-phenylglycinol
[6]. The obtained diastereomes mixture was separated
by the preparative HPLC.
The slow evaporation of solutions in pentane of imines
IVa and IIIb furnished single cryatls fit for the XRD
analysis (see the figure). Both compounds have the E-
configuration with respect to the C=N bond. In the
649

BURAVLEV et al.
650
(S)
OH
R
(S)
(R)
H+
Ph
N
(R)
(_)-IIа, IIb
OH
(R)-1-phenylethylamine
Fractional crystallization
O
IIIа, IIIb
(R)
R
OH
rac-IIа, IIb
(R)
(R)
H+
Ph
N
(S)
(+)-IIа, IIb
Clay, Et3N
HCHO
4
R
3
2
5
OH
7
8
9
IVа, IVb
12
11
6
13
14
1
10
15
R
rac-Iа, Ib
molecules an intramolecular hydrogen bond is present.
The hydrogen atoms were revealed objectively, in the
structure IIIb the position and the isotropic thermal
parameter of atom H1 were refined, in the structure IVa
the atom H1 was included into the refinement in the rider
model. The parameters of the hydrogen bond are listed
in Table 1, the crystallographic parameters, in Table 2.
imine. The enentiomeric purity of separated aldehydes
was established using analytic HPLC on columns packed
with chiral stationary phases ChiralpakAD and Chiralcel
OJ-H.
The absolute configuration of the chiral centers in the
enantiomerically-enriched salicylaldehydes (+)- and (–)-
(IIa, IIb) was established from the configuration of
imines prior to hydrolysis. The data on the absolute
configuration and signs of the angles of the optical rotation
of (+)- and (–)- (IIb) enantiomers are consistent with
those published in [6].
Both compounds crystallize in the chiral space group
P212121. The absolute configuration of compounds IVa
and IIIb was established from the data on the relative
configuration found by XRD analysis and starting from
the configuration of the initial (R)-1-phenylethylamine.
The chiral centers were assigned the configurations
(1S,2R,4R,18R) in compound IVa and (1R,2S,4S,19R) in
compound IIIb. Thus the configurations of the terpene
fragment in compounds IVa and IIIb are opposite. The
chiral centers in imines IVb and IIIa have configurations
(1R,2S,4S,18R) and (2R,4R,19R) respectively.
Hence we developed a procedure for separation of
racemic salicylaldehydes containing the isobornyl
substituents into enantiomers applying (R)-1-
phenylethylamine. The method did not require the use of
the preparative HPLC and expensive chiral reagents. The
configuration of the chiral centers of Schiff bases and
separated aldehydes was established by means of XRD
analysis.
Enantiomerically-enriched salicylaldehydes (+)- and
(–)-(IIa, IIb) were isolated by acid hydrolysis of each
EXPERIMENTAL
Table 1 Geometrical characteristics of hydrogen bonds in
structures IVa and IIIb
IR spectra were recorded on a Fourier spectrometer
Shimadzu IR Prestige 21 from pellets with KBr. 1H and
13C NMR spectra of compounds obtained were registered
on a spectrometer Bruker Avance II 300 at operating
frequencies 300.17 and 75.48 MHz respectively from
solutions in CDCl3 at room temperature. The chemical
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 5 2010

SEPARATION OF RACEMIC SALYCYLALDEHYDES
Table 2. Crystallographic data and parameters of XRD
651
experiment for compounds IVa and IIIb
,
max
Molecular structure of compounds IVa and IIIb.
The reaction progress was monitored by TLC on
Sorbfil plates. The spots visualization was performed by
treating with a solution of 15 g of KMnO4, 300 ml of
H2O, and 0.5 ml of conc. H2SO4. The purity of initial
terpenophenols I and aldehydes II was checked by GLC
on a chromatograph Shimadzu GC-2010AF equipped with
a flame-ionization detector (carrier gas helium), capillary
column SPB-35 (Supelco, 60 m × 0.25 mm × 0.5 μm,
ramp 135–255°C, heating rate 6 deg/min). Diastereomeric
purity of Schiff bases was determined on a capillary
column SPB-35 (Supelco, 30 m × 0.32 mm × 0.25 μm,
255°C).
a
R1 = Σ|Fo – |Fc||/Σ(Fo).
b wR2 = (Σ[w(Fo2 – Fc ) ]/Σ[w(Fo ) ])1/2
.
2 2
2 2
shifts were measured with respect to the signals of
deuterochloroform (δH 7.26, δC 77.00 ppm). The melting
points were measured on the Koeffler heating block. The
angles of optical rotation were registered with an
automatic digital polarimeter P3002RS Kruss Optronic
(λ 589 nm).
Enantiomeric purity of aldehydes was measured by
HPLC on a chromatograph Agilent 1100 (UV detector,
λ 224 nm, 20°C), columns Chiralcel OJ-H (Daicel,
25 cm × 4.6 mm, grains 5 μm, eluent hexane–i-PrOH,
200 : 1, flow rate 1.0 ml/min) for enentiomers of compound
IIa, ChiralpakAD (Daicel, 25 cm × 4.6 mm, grains 10 μm,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 5 2010

BURAVLEV et al.
652
eluent hexane –i-PrOH, 200:1, flow rate 0.5 ml/min) for
enentiomers of compound IIb.
2-Hydroxy-3-(1,7,7-trimethylbicyclo[2.2.1]-hept-
exo-2-yl)benzaldehyde rac-(IIa). Yield 60%. Light
yellow powder, mp 67–68°C (75–78°C [12]). Found, %:
C 79.06; H 8.56. C17H22O2. Calculated, %: C 79.03;
H 8.58.
The aldehydes were purified by column chromato-
graphy (“wet” filling) on silica gel Alfa Aesar 70/230μ.
Toluene was dried with anhydrous CaCl2 and distilled
from metal sodium. Petroleum ether used boiled within
65–70°C. Pentane was used just after distillation.
Molecular sieves (4 Å) used in the synthesis of Schiff
bases were activated by calcining at 140°C for 3 h; the
montmorillonite clay KSF (Acros Organics) prior to the
synthesis was calcined for 10 h at 130°C. Paraform-
aldehyde of the “chemically pure” grade was used without
additional purification.
2-Hydroxy-5-methyl-3-(1,7,7-trimethylbicyclo-
[2.2.1]hept-exo-2-yl)benzaldehyde rac-(IIb). Yellow
powder, mp 100–101°C (103°C [6]). Found, %: C 79.44;
H 8.93. C18H24O2. Calculated, %: C 79.36; H 8.87.
Schiff bases III, IV. Into a two-neck round-bottom
flask equipped with a reflux condenser was charged
7.74 mmol of racemic aldehyde II dissolved in 30 ml of
anhydrous toluene, 0.95 ml (7.74 mmol) of (R)-(+)-1-
phenylethylamine, and 8.5 g of molecular sieves. The
reaction mixture was heated at reflux in an argon flow
for 3.5 h. On completing the reaction the solution was
filtered through a glass frit, molecular sieves were washed
with Et2O, and the solution was evaporated at a reduced
pressure. The mixture of diastereomers was separated
by fractional crystallization from pentane. The course of
separation was checked by GLC.
In the syntheses were used without additional
purification triethylamine (Sigma-Aldrich) and (R)-(+)-
1-phenylethylamine (AlfaAesar, ChiPros®, enantiomeric
purity over 99%).
Crystallographic data and the main refinement
parameters for compounds IVa and IIIb obtained by XRD
analysis are compiled in Table 2. The set of experimental
data was obtained on a diffractometer Bruker Smart 1000
CCD [10] with a coordinate detector [graphite mono-
2-{(E)-[(R)-1-phenylethylimino]methyl}-6-
{(1R,2S,4S)-1,7,7-trimethylbicyclo[2.2.1]hept-exo-
2-yl}phenol (IIIa). Yield 58%, diastereomeric purity
98%. Retention time 20.7 min. Yellow powder, mp 92–
93°C. IR spectrum, ν, cm–1: 3437 (OH), 1628 (C=N).
1H NMR spectrum, δ, ppm: 0.84 s (3H, C10H3), 0.87 s
(3H, C9H3), 0.93 s (3H, C8H3), 1.34–1.43 m (1H, H5),
1.56–1.68 m (6H, H3, C6H2, C19H3), 1.79–1.92 m (2H,
H4,5), 2.11–2.18 m (1H, H3), 3.41 t (1H, H2, J 9.0 Hz),
4.56 q (1H, H18, J 6.6 Hz), 6.84 m (1H, H14), 7.08 d (1H,
H16, J 7.2 Hz), 7.25–7.40 m (6H, H15,21,21',22,22',23),
8.41 br.s (1H, H17), 13.87 br.s (1H, OH). 13C NMR
spectrum, δ, ppm: 12.34 (C10), 20.45 (C9), 21.44 (C8),
24.90 (C19), 27.52 (C5), 34.18 (C3), 39.77 (C6), 44.54 (C2),
45.74 (C4), 47.96 (C7), 49.86 (C1), 68.48 (C18), 118.05
(C13), 117.53, 126.44, 127.16, 128.61, 128.87
(C14,15,16,21,21',22,22'), 130.78 (C23), 131.93 (C11), 143.96
(C20), 160.49 (C12), 163.93 (C17). Found, %: C 83.02;
H 8.71; N 3.88. C25H31NO. Calculated, %: C 83.06;
H 8.64; N 3.87.
°
chromator, λ(MoKα) 0.71073A, ω-scanning]. The struc-
tures were solved by the direct method and refined in
a full-matrix least-mean-squares procedure for F2hkl with
anisotropic thermal parameters for all the nonhydrogen
atoms. The hydrogen atoms of the hydroxy groups were
revealed from the difference synthesis, the other hydrogen
atoms in the structures IVa and IIIb were placed in the
geometrically calculated locations and were refined in
the rider model. The solving and refining of the structures
was carried out using software SHELXTL [11].
Racemic salicylaldehydes with isobornyl sub-
stituent II. Into a three-neck flask equipped with
a thermometer and a reflux condenser was charged
10.0 mmol of phenol rac-(I) dissolved in 20 ml of toluene,
40.0 mmol of paraformaldehyde, 3.2 g of montmorillonite
KSF, and 10.0 mmol of Et3N. The reaction mixture was
heated at reflux in an argon flow for 15 h. The reaction
progress was monitored by TLC (eluent petroleum ether–
Et2O, 10 : 1) and GLC. On completing the reaction the
catalyst was filtered off, washed with Et2O, and the
solution was evaporated at a reduced pressure. The
reaction products were separated by column chromato-
graphy (graduent elution with petroleum ether–Et2O with
growing content of the latter). The spectral characteristics
of aldehydes obtained were in agreement with those
described in [6, 12].
4-Methyl-2-{(E)-[(R)-1-phenylethylimino]-
methyl}-6-{(1R,2S,4S)-1,7,7-trimethylbicyclo-
[2.2.1]hept-exo-2-yl}phenol (IIIb). Yield 76%,
diastereomeric purity 91%. Retention time 23.1 min. Light
yellow powder, mp 111–113°C (from pentane). IR
1
spectrum, ν, cm–1: 3422 (OH), 1632 (C=N). H NMR
spectrum, δ, ppm: 0.83 s (3H, C10H3), 0.87 s (3H, C9H3),
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 5 2010

SEPARATION OF RACEMIC SALYCYLALDEHYDES
653
0.93 s (3H, C8H3), 1.33–1.42 m (1H, H5), 1.56–1.66 m
(6H, H3, C6H2, C20H3), 1.81–1.93 m (2H, H4,5), 2.07–
2.19 m (1H, H3), 2.29 s (3H, C17H3), 3.38 t (1H, H2,
J 9.2 Hz), 4.55 q (1H, H19, J 6.6 Hz), 6.87 d, 7.19 d (1H
each, H14,16, J 1.8, J 1.8 Hz), 7.22–7.39 m (5H,
H22,22',23,23',24), 8.36 br.s (1H, H18), 13.61 br.s (1H, OH).
13C NMR spectrum, δ, ppm: 12.34 (C10), 20.45 (C9),
20.78 (C17), 21.46 (C8), 24.91 (C20), 27.51 (C5), 34.10
(C3), 39.73 (C6), 44.50 (C2), 45.73 (C4), 47.98 (C7), 49.85
(C1), 68.48 (C19), 117.62 (C13), 126.27 (C15), 126.45,
127.11 (C22,22',23,23'), 128.58, 128.84 (C14,16), 131.59
(C11), 131.82 (C24), 144.05 (C21), 158.22 (C12), 163.93
(C18). Found, %: C 83.11; H 8.90; N 3.69. C26H33NO.
Calculated, %: C 83.15; H 8.86; N 3.73.
(C20), 27.52 (C5), 34.15 (C3), 39.78 (C6), 44.59 (C2), 45.74
(C4), 47.96 (C7), 49.84 (C1), 68.45 (C19), 117.63 (C13),
126.28 (C15), 126.46, 127.13 (C22,22',23,23'), 128.83, 128.62
(C14,16), 131.66 (C11), 131.84 (C24), 144.12 (C21), 158.20
(C12), 163.86 (C18). Found, %: C 83.20; H 8.88; N 3.70.
C26H33NO. Calculated, %: C 83.15; H 8.86; N 3.73.
Enantiomerically enriched aldehydes IIa, IIb. To
a solution of 0.5 g of each of separated imines IIIa,
IIIb, IVa, IVb in 5 ml of EtOH was added 7 ml of 37%
HCl. The solution was stirred for 6 h at room temperature.
Excess solvent was removed at a reduced pressure, to
the residue 75 ml of dichloromethane was added, the
organic layer was washed with water from acid (3 ×
30 ml) till pH 7, dried with anhydrous Na2SO4, evaporat-
ed, the product was purified by column chromatography
(eluent petroleum ether–Et2O) and recrystallization from
pentane.
2-{(E)-[(R)-1-Phenylethylimino]methyl}-6-
{(1S,2R,4R)-1,7,7-trimethylbicyclo[2.2.1]hept-exo-
2-yl}phenol (IVa). Yield 60%, diastereomeric purity
74%. Retention time 21.6 min. Light yellow powder, mp
118–119°C. IR spectrum, ν, cm–1: 3439 (OH), 1626
(C=N). 1H NMR spectrum, δ, ppm: 0.79 s (3H, C10H3),
0.85 s (3H, C9H3), 0.92 s (3H, C8H3), 1.33–1.43 m (1H,
H5), 1.56–1.68 m (6H, H3, C6H2, C19H3), 1.81–1.93 m
(2H, H4,5), 2.10–2.19 m (1H, H3), 3.40 t (1H, H2,
J 9.0 Hz), 4.56 q (1H, H18, J 6.6 Hz), 6.84 m (1H, H14),
7.08 d (1H, H16, J 7.2 Hz), 7.25–7.42 m (6H,
H15,21,21',22,22',23), 8.42 br.s (1H, H17), 13.90 br.s (1H,
OH). 13C NMR spectrum, δ, ppm: 12.39 (C10), 20.49
(C9), 21.41 (C8), 25.03 (C19), 27.53 (C5), 34.25 (C3), 39.83
(C6), 44.63 (C2), 45.74 (C4), 47.94 (C7), 49.85 (C1), 68.43
(C18), 117.98 (C13), 117.55, 126.43, 127.17, 128.64, 128.87
(C14,15,16,21,21',22,22'), 130.78 (C23), 132.00 (C11), 144.02
(C20), 160.48 (C12), 163.86 (C17). Found, %: C 83.03;
H 8.66; N 3.90. C25H31NO. Calculated, %: C 83.06;
H 8.64; N 3.87.
2-Hydroxy-3-{(1S,2R,4R)-1,7,7-trimethylbicyclo-
[2.2.1]hept-exo-2-yl}benzaldehyde (+)-(IIa). Yield
96%, enantiomeric purity 72%. Retention time 6.08 min.
Light yellow powder, mp 84–87°C, [α]D22 +55.1° (c 0.5,
CHCl3).
2-Hydroxy-3-{(1R,2S,4S)-1,7,7-trimethylbicyclo-
[2.2.1]hept-exo-2-yl}benzaldehyde (–)-(IIa). Yield
97%, enantiomeric purity 98%. Retention time 4.54 min.
Light yellow powder, mp 89–90°C, [α]D22 –87.6° (c 0.6,
CHCl3).
2-Hydroxy-5-methyl-3-{(1S,2R,4R)-1,7,7-tri-
methylbicyclo[2.2.1]hept-exo-2-yl}benzaldehyde
(+)-(IIα). Yield 97%, enantiomeric purity 96%. Retention
time 9.02 min. Light yellow powder, mp 109–110°C
(109°C [6]), [α]D22 +41.0° (c 0.6, CHCl3) {[α]D20 +42.7°
(c 0.3, CHCl3), ee >99% [6]}.
4-Methyl-2-{(E)-[(R)-1-phenylethylimino]-
methyl}-6-{(1S,2R,4R)-1,7,7-trimethylbicyclo-
[2.2.1]hept-exo-2-yl}phenol (IVb). Yield 68%,
diastereomeric purity 98%. Retention time 22.1 min. Light
yellow powder, mp 110–112°C. IR spectrum, ν, cm–1:
2-Hydroxy-5-methyl-3-{(1R,2S,4S)-1,7,7-
trimethylbicyclo[2.2.1]hept-exo-2-yl}benzaldehyde
[(–)-IIα]. Yield 98%, enantiomeric purity 92%. Retention
time 8.22 min. Light yellow powder, mp 108–110°C (108–
109°C [6]), [α]D22 –40.9° (c 0.6, CHCl3) {[α]D20 –42.3°
(c 0.07, CHCl3), ee >99% [6]}.
1
3420 (OH), 1632 (C=N). H NMR spectrum, δ, ppm:
0.79 s (3H, C10H3), 0.86 s (3H, C9H3), 0.92 s (3H, C8H3),
1.27–1.42 m (1H, H5), 1.55–1.66 m (6H, H3, C6H2,
C20H3), 1.81–1.93 m (2H, H4,5), 2.11–2.20 m (1H, H3),
2.29 s (3H, C17H3), 3.38 t (1H, H2, J 9.0 Hz), 4.55 q (1H,
H19, J 6.6 Hz), 6.88 d, 7.19 d (1H each, H14,16, J 0.9,
J 1.2 Hz), 7.27–7.41 m (5H, H22,22',23,23',24), 8.37 br.s (1H,
H18), 13.65 br.s (1H, OH). 13C NMR spectrum, δ, ppm:
12.35 (C10), 20.48 (C9), 20.79 (C17), 21.43 (C8), 25.07
The study was carried out under a financial support
of the Russian Foundation for Basic Research (grant no.
10-03-01129-a) and a grant of the Ural Division of the
Russian Academy of Sciences (competition of the
scientific projects of young scientists and post-graduate
students of the Ural Division of the Russian Academy of
Sciences).
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BURAVLEV et al.
7. Saito, H., Uchiyama, T., Makino, M., Katase, T., Fuji-
654
REFERENCES
moto, Y., and Hashizume, D., J. Health Science, 2007,
vol. 53, no. 2, p. 177.
8. Chukicheva, I.Yu. and Kuchin,A.V., Ros. Khim. Zh., 2004,
vol. 48, no. 3, p. 21.
9. Bigi, F., Confori, M.L., Maggi, R., and Sartori, G.,
Tetrahedron, 2000, vol. 56, p. 2709.
10. SMART and SAINT, Release, 5.0. Area Detector Control
and Integration Software, Bruker, AXS, Analytical X-ray
instruments, Madison, Wisconsin, USA, 1998.
11. Sheldrick, G.M. SHELXL-97. Program for the Refinement
of Crystal Structures, University of Göttingen, Germany,
1997.
12. Kochnev, A.I., Oleinik, I.I., Oleinik, I.V., Ivanchev, S.S.,
and Tolstikov, G.A., Izv. Akad. Nauk, Ser. Khim., 2007,
p. 1084.
1. Kuzakov, E.V. and Shmidt, E.N., Khim. Polim. Soed., 2000,
no. 3, p. 198.
2. Cirri, M., Mura, P., Corvi, and Mora, P., Int. J. Pharm.,
2007, vol. 30, p. 84.
3. Plotnikov, M.B., Smol’yakova, V.I., Ivanov, I.S., Ku-
chin, A.V., Chukicheva, I.Yu., and Krasnov, E.A., Byull.
Eksp. Biol. Med., 2008, vol. 145, no. 3, p. 296.
4. Drug Stereochemistry. Analytical Methods and
Pharmacology. Wainer, I.W. and Drayer., D.E., New York:
Marcel Dekker, 1988, p. 246.
5. Vetter, A.H. and Berkessel, A., Tetrahedron Lett., 1998,
vol. 39, p. 1741.
6. Berkessel, A., Vennemann, M.R., and Lex, J., Eur. J. Org.
Chem., 2002, p. 2800.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 5 2010