7
58
KOVALENKO, PROKHOREVICH
racemization and rearrangement in a strongly alkaline
environment even at prolonged heating. At 100°С the
racemization of an ester of an optically active allylic
alcohol, trans-pent-3-en-2-yl 4-nitrobenzoate, occurred
significantly faster than the hydrolysis of the initial
ester [5]. The racemization rate fit to the kinetic law of
the first order thus confirming the intramolecular
mechanism of the formation of the ion-pair intermediate:
The ions constituting the latter recombined giving the
racemic 4-nitrobenzoate. In the medium of a protic
solvent of a high dielectric permittivity the formation
of 4-nitrobenzoate anion and allylic carbocation is
expectable. The racemization mechanism was proved
solution of potassium tert-butylate heated at 75°С at
vigorous stirring was added within 10 min a solution
of 54.0 g (0.160 mol) of acid phthalate (S)-2 [1] in 50 g
of anhydrous tert-butyl alcohol. After adding all the
substance the mixture was boiled for 5 min at stirring
and afterwards the reaction mixture was quickly
cooled to the room temperature (20°С). The flask was
cooled by an ice water bath, and at stirring into the
reaction mixture 500 mL of anhydrous methanol was
added. The reaction mixture was left standing at room
temperature for 60 h. The reaction mixture was
evaporated on a rotary evaporator at a reduced
pressure, the residue was diluted with 800 mL of
water, the reaction product was extracted with tert-
butyl methyl ether (4 × 150 mL). The ether extract was
1
8
using an ester labelled at the carbonyl group with O
[5]. We may conclude that the phthalate of ipsdienol
(
S)-1 not yet subjected to alcoholysis may similarly
washed with saturated solutions of NaHCO and NaCl,
3
suffer racemization in alcohol with a considerable rate,
especially at a high temperature.
dried with calcined Na SO , and evaporated on a
2
4
rotary evaporator at a reduced pressure. The residue
was purified by chromatography on a column charged
with 300 g of silica gel (70‒230 mesh), eluting first
with petroleum ether (bp 40–60°С), then with its
mixtures with ethyl acetate, from 50 : 1 to 10 : 1. Yield
The dehydrohalogenation under the action of potas-
sium tert-butylate occurred in a boiling solution
practically instantly after mixing the reagents, whereas
the alcoholysis of the phthalate group proceeded relatively
slowly, and the reaction decelerated even more in con-
centrated solutions. We presumed that the alcoholysis
of the ester may be accelerated by replacement of the
sterically hindered tert-butylate by a methylate anion,
and the reaction should be carried out at a deliberately
low temperature preventing the racemization of the
phthalate of ipsdienol (S)-1 along the ion-pair me-
chanism. After the end of the dehydro-chlorination
stage which was performed using slightly less con-
centrated 16.5% solution of potassium tert-butylate the
reaction mixture was at once treated with methanol.
The methanolysis of ipsdienol (S)-1 phthalate proceeded
without notable difficulties and completed after
maintaining the reaction mixture at room temperature
for 60 h in a yield exceeding 70%. The analysis of the
17.1 g (70%). Spectral characteristics and the angle of
optical rotation of compounds obtained coincided with
the previously measured data [1].
1
13
Н and С NMR of compounds solutions in CDCl
3
were registered on a spectrometer Bruker Avance 400
at operating frequencies 400 and 100 МHz respec-
tively. IR spectra were recorded on a spectrophoto-
meter Vertex 70. Optical rotation was measured on a
polarimeter CM-3 at 20±2°C. The solvents were dried
by standard methods and distilled.
ACKNOWLEDGMENTS
The study was carried out under a financial support
of Forestry Ministry of Belarus’ Republic. The authors
express their gratitude to Professor R.M. Kellogg
(“Syncom B.V.,” The Netherlands) for the participation
in the discussion of obtained results.
1
Н NMR spectrum of the Mosher ester of the obtained
ipsdienol (S)-1 confirmed the complete retention of the
optical purity.
REFERENCES
Thus we have developed and substantiated the
improved version of the formerly advanced
transformation providing a possibility to increase the
yield of ipsdienol (S)-1 and to prevent the racemization
that considerably reduces the preparative value of the
reslution of allylic alcohol by crystallization.
1
. Kovalenko, V.N. and Matyushenkov, E.A., Russ. J. Org.
Chem., 2012, vol. 48, p. 1168.
2. Kenyon, J., Partridge, S.M., and Philips, H., J. Chem.
Soc., 1937, p. 207.
3. Duveen, D.I. and Kenyon, J., J. Chem. Soc., 1939,
p. 1697.
(
4S)-2-Methyl-6-methylideneocta-2,7-dien-4-ol
(S)-ipsdienol] (S)-1. In 490 g of anhydrous tert-butyl
alcohol at vigorous stirring and boiling was dissolved
9.9 g (0.767 mol) of metal potassium. To the obtained
4. Balfe, M.P. and Kenyon, J., Trans. Faraday Soc., 1941,
vol. 37, p. 721.
5. Harlan, L.G. and Melvin, M.P., J. Am. Chem. Soc.,
[
1
960, vol. 82, p. 2515.
2
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 52 No. 5 2016