ANOMALOUS DIRECTION OF THE ENE REACTION OF PULEGONE
1431
described in [4], yields one more isomer, 1-{1-methyl-
methylene groups requires surmounting energy bar-
riers of 26 and 47 kcal/mol, respectively.
1
4
-[(4R)-4-methyl-6-oxocyclohex-1-en-1-yl]ethyl}-
-phenyl-1,2,4-triazolidine-3,5-dione (3). The fraction
3
The stage of proton transfer from C to the nitrogen
atom is characterized by not only high energy barrier
but also higher energy of the isomer thus formed
of isomer 3 in the product mixture was 20%. The other
products were epimeric 1‑[(1S,4R)-4-methyl-2-oxo-1-
(
prop-1-en-2-yl)cyclohexyl]-4-phenyl-1,2,4-triazoli-
dine-3,5-dione (4) and 1‑[(1R,4R)-4-methyl-2-oxo-1-
prop-1-en-2-yl)cyclohexyl]-4-phenyl-1,2,4-triazoli-
(
endothermic process), which is responsible for the
lower thermodynamic stability of isomer 3. Since
isomer 3 is an anti-Markovnikov adduct, we call
isomer 3 and the corresponding direction of ene reac-
tion anomalous.
(
dine-3,5-dione (5) which constituted 75–80% of the
product mixture.
The structure of the isomeric products was deter-
mined by NMR spectroscopy. The largest differences
were observed for the position of the olefinic proton
A solution of 0.2 g (1.3 mmol) of pulegone (1) in
5
mL of chloroform was added at room temperature to
a solution of 0.228 g (1.3 mmol) of compound 2 in
mL of chloroform, and the mixture was stirred for
2
signals. The HC =C signal of 3 appeared as a doublet
5
of doublets at δ 6.91 ppm (J = 5.8, 2.7 Hz) due to
3
5 min (until it became colorless). The solvent was
removed under reduced pressure, and the residue was
analyzed by NMR.
couplings with two nonequivalent protons on C .
Isomer 3 was characterized by a relatively low sta-
bility. According to the NMR data, it underwent
complete oxidation in solution within 48 h. The vinylic
The NMR spectra were recorded at the Joint Ana-
lytical Center (Razuvaev Institute of Organometallic
Chemistry, Russian Academy of Sciences) on a Bruker
Avance III spectrometer at 400 MHz; the chemical
shifts were referenced to tetramethylsilane.
1
protons of epimers 4 and 5 resonated in the H NMR
spectrum at δ 4.97 (s) and 5.26 ppm (q, J = 0.7 Hz) (4)
and δ 5.08 (s) and 5.38 ppm (q, J = 1.0 Hz) (5).
Quantum chemical simulation of the ene reaction
1
+ 2 (GAUSSIAN 09) revealed two possible path-
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 16-53-00110-Bel-a).
ways. The first pathway (energy barrier 22 kcal/mol)
involves attack of the C atom of pulegone by one
2
nitrogen atom of diketone 2 with simultaneous migra-
tion of one proton of the geminal methyl groups of
pulegone to the other nitrogen atom of the heterocyclic
fragment and migration of the C=C double bond to the
terminal position. This pathway leads to the formation
of epimers 4 and 5.
REFERENCES
1
2
. Syrgiannis, Z., Koucyanopoulos, F., Muir, K.W., and
Elemes, Y., Tetrahedron Lett., 2009, vol. 50, p. 277.
. Vougioukalakis, G.C. and Orfanopoulos, M., Synlett,
Following the other pathway, attack of the nitrogen
atom of 2 is directed at the double-bonded C atom of
2005, p. 713.
7
3. Bozovic, M. and Rango, R., Molecules, 2017, vol. 22,
pulegone, so that a cyclic carbocationic intermediate is
formed through an energy barrier of 21 kcal/mol.
Proton migration in that cation to the anionic center on
the nitrogen atom of 2 is possible from both geminal
methyl groups to give epimers 4 and 5 and endocyclic
methylene group to produce anomalous ene reaction
product 3. Proton migration from the methyl and
p. 290.
4. Shiloff, J.D. and Hunter, N.R., Tetrahedron Lett., 1976,
vol. 17, no. 42, p. 3773.
5. Hunter, N.R., Krawchuk, B.P., and Shiloff, J.D., Can. J.
Chem., 1982, vol. 60, p. 835.
6. Hoye, T.R., Bottorff, K.J., Caruso, A.J., and Dellaria, J.F.,
J. Org. Chem., 1980, vol. 45, no. 22, p. 4287.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 54 No. 9 2018
Bodrikov
