Structure of 2(5)-Alkyl-4-hydroxy-3,4-diphenylcyclopent-2-en-1-ones, cyclocondensation products of benzil with aliphatic ketones

Article abstract of DOI:10.1134/S1070428010080026

The reaction of benzil with methyl alkyl ketones gave three isomeric cyclopentenone derivatives, 2-substituted and cis- and trans-5-substituted 4-hydroxy-3,4-diphenylcyclopent-2-en-1-ones. cis- and trans-2,5-Disubstituted 4-hydroxy-3,4-diphenylcyclopent-2-en-1-ones were formed in analogous reaction of benzil with dialkyl ketones. The structure of the products was confirmed by 1H NMR spectroscopy and molecular- mechanics calculations. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070428010080026

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 8, pp. 1116–1120. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © I.V. Mikhura, A.A. Formanovskii, A.S. Shashkov, 2010, published in Zhurnal Organicheskoi Khimii, 2010, Vol. 46, No. 8,
pp. 1118–1122.
Structure of 2(5)-Alkyl-4-hydroxy-3,4-diphenylcyclopent-
2-en-1-ones, Cyclocondensation Products of Benzil
with Aliphatic Ketones
I. V. Mikhuraa, A. A. Formanovskiia, and A. S. Shashkovb
a Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences,
ul. Miklukho-Maklaya 16/10, Moscow, 117997 Russia
e-mail: synorg@mx.ibch.ru
b Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences,
Leninskii pr. 47, Moscow, 119991 Russia
Received September 7, 2009
Abstract—The reaction of benzil with methyl alkyl ketones gave three isomeric cyclopentenone derivatives,
2-substituted and cis- and trans-5-substituted 4-hydroxy-3,4-diphenylcyclopent-2-en-1-ones. cis- and trans-2,5-
Disubstituted 4-hydroxy-3,4-diphenylcyclopent-2-en-1-ones were formed in analogous reaction of benzil with
1
dialkyl ketones. The structure of the products was confirmed by H NMR spectroscopy and molecular-
mechanics calculations.
DOI: 10.1134/S1070428010080026
Studies on reactions of benzil with ketones [1–4]
showed that the product of its reaction with acetone is
4-hydroxy-3,4-diphenylcyclopent-2-en-1-one. Other
aliphatic ketones reacted with benzil to give cyclopen-
tenones with alkyl groups on C2 and C5. If the initial
ketone is unsymmetrical, the condensation could give
rise to four isomeric products [5]. If R ≠ H, three
isomers I–III are obtained. Symmetrical ketones react
with benzil to form two stereoisomers IV and V.
concentrated alkali led to formation of mixtures of
isomers I–III [2–4, 6] which can readily be distin-
1
guished by H NMR spectroscopy. When the reactions
of benzil with pentan-2-one and heptan-2-one were
carried out in dilute solution, only one isomer I (R =
Et, Bu) was obtained [5]. Under analogous conditions,
octan-2-one (R = C5H11) and 4-methylpentan-2-one
(R = i-Pr) gave rise to mixtures in which three isomers
I–III can be distinguished by 1H NMR. In the reaction
with pentan-3-one only two isomers IV and V (R =
Me) were formed, the major product being isomer IV
Reactions of benzil with excess pentan-2-one (R =
Et) and octan-2-one (R = C5H11) and a small amount of
Scheme 1.
O
Ph
R
Ph
Ph
Ph
HO
O
Ph
O
+
+
+
+
Ph
R
O
O
O
Me
HO
HO
Ph
Ph
O
R
IIa–IId
R
Ia–Id
IIIa–IIId
O
Me
O
Ph
R
Ph
R
O
Ph
HO
HO
Ph
Me
+
Ph
R
R
O
O
OH
Ph
Ph
R
R
Ph
VI
IVa, IVb
Va, Vb
I–III, R = Me (a), Et (b), Pr (c), i-Pr (d); IV, V, R = Me (a), Et (b).
1116

STRUCTURE OF 2(5)-ALKYL-4-HYDROXY-3,4-DIPHENYLCYCLOPENT-2-EN-1-ONES
1117
(70%). 1-Phenylpropan-2-one readily reacted with
benzil, yielding substituted 4-hydroxycyclopent-2-en-
1-one, whereas the condensation of benzil with
1-phenylbutan-2-one or 1-phenylpentan-2-one was
very slow or the resulting cyclic ketones underwent
fast decomposition, so that they could not be detected
[7]. The structure of the isomeric products was studied
by 1H NMR spectroscopy [8] using compounds I (R1 =
indicated cis orientation of the phenyl rings. This con-
clusion was confirmed by the absence of correlation
between the hydroxy proton and ortho-protons in the
3-Ph group. In contrast, distinct correlation between
the OH proton and o-H in the 4-Ph group was
observed. In the two-dimensional NOESY spectrum of
isomer IIb the key correlation was that between 5-H
and ortho-protons in the 4-Ph group (cis orientation of
4-Ph and 5-H). Isomer IIIb displayed correlation be-
tween 5-H and OH proton rather than ortho-protons in
the 4-Ph group. Likewise, the steric structure of
isomers IVb and Vb was assigned taking into account
the presence of a correlation peak between 5-H and
ortho-protons in the 4-Ph group in the NOESY spec-
trum of IVb; isomer Vb was characterized by correla-
tion between ortho-protons in the 4-Ph group and
protons in the CH3 and CH2 groups of the ethyl sub-
stituent on C5.
1
H) and VI as models. The H NMR spectra of these
compounds are fairly simple, and unambiguous assign-
ment of protons and identification of substituents on C2
and C5 in the cyclopentenone ring are possible.
The goal of the present work was to synthesize and
isolate individual isomers of 2(5)-alkyl-substituted
4-hydroxy-3,4-diphenylcyclopen-2-en-1-ones and un-
ambiguously determine their structure. These com-
pounds attracted interest as substrates (heterodienes) in
Diels–Alder reactions with various dienophiles.
The above data allowed us to formulate some
general relations useful for identification of particular
stereoisomers provided that they cannot be isolated as
individual substances. If the ethyl group on C5 (5-Et)
and 4-Ph are oriented cis with respect to each other
(as in IIIb and Vb), signals from the 5-CH2 protons
appear in a stronger field relative to the corresponding
signals of compounds IIb and IVb with trans orienta-
tion of the 5-Et and 4-Ph groups. In contrast, the OH
proton signal is located in a stronger field in the
1H NMR of compounds IIb and IVb with trans con-
figuration of the OH and 3-Ph substituents, as com-
pared to their stereoisomers IIIb and IVb with cis
arrangement of the corresponding groups. This pattern
results from magnetic anisotropy of the benzene ring
which exerts shielding and deshielding effects on
protons at the α-carbon atom in the neighboring sub-
stituent (4-Ph); likewise, the 3-Ph group is responsible
for shielding or deshielding of the OH proton.
The conditions of the condensation of 1,2-diphen-
ylethane-1,2-dione (benzil) with various ketones were
similar to those reported in [1, 8]. A hot solution of
benzil in alcohol containing concentrated potassium
hydroxide was treated with aliphatic ketone. The latter
was taken in excess (20–60%), for it was partly con-
sumed for side self-condensation process. All conden-
sations were complete in 1.5–2 h. The progress of
reactions was monitored by TLC, following disappear-
ance of initial benzil from the reaction mixture. We
detected formation of all possible regio- and stereo-
isomers whose ratio depended on the initial ketone.
The overall yield of isomeric products was no less than
90% (calculated on the initial benzil). The isomers
were separated by chromatography on silica gel.
In the reactions of 1,2-diphenylethane-1,2-dione
with alkyl methyl ketones (butan-2-one, pentan-2-one,
hexan-2-one, and 4-methylpentan-2-one) we isolated
in each case three individual isomers I–III. The reac-
tions of benzil with symmetrical ketones, pentan-3-one
and heptan-4-one, could produce only two stereo-
isomeric cyclopentenones IVa/Va and IVb/Vb
(Scheme 1).
Taking the above stated into account we succeeded
in assigning proton signals in the spectra of isomers
which were not separated and determined their ratios.
Insofar as the isomers are not converted into each other
in solution, we believe that they are formed as a result
of independent reactions.
The effect of stereochemical environment on the
1
chemical shifts of protons in the H NMR spectra was
studied in monoethyl (Ib–IIIb) and diethyl derivatives
(IVb, Vb) which were isolated as individual sub-
stances. Theeir steric structure was determined using
two-dimensional NOESY technique. In the NOESY
spectrum of Ib we observed correlation between the
ortho-protons in the phenyl ring on C3 (3-Ph) and
ortho-protons in the phenyl ring on C4 (4-Ph), which
The predominant formation of isomers with the
alkyl group R on C2 in isomer mixtures I–III may be
related to their higher thermodynamic stability. Also,
the effect of a bulky cluster formed by water molecules
surrounding the hydroxy group cannot be ruled out [8].
However, the contribution of the latter factor is in-
significant when R = i-Pr. In this case, the major
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 8 2010

MIKHURA et al.
1118
isomer (47.5%) is IIId in which the isopropyl group
on C5 is oriented trans with respect to the hydroxy
group, while the two other isomers are formed in ap-
proximately equal amounts: Id, 25%, and IId (cis
orientation of the i-Pr and OH groups), 27.5%. In the
reaction of pentan-3-one (R = Me) with benzil, stereo-
isomer IVa with cis orientation of the hydroxy and
5-Me groups was formed almost exclusively (95%),
while the reaction of benzil with heptan-4-one (R = Et)
gave almost equal amounts of isomers IVb and Vb.
significant difference was also observed in the torsion
angles determining the molecular conformation of
methyl- and ethyl-substituted derivatives IIa/IIIa and
IIb/IIIb. However, appreciable difference was found
between the methyl- and ethyl-substituted compounds
Ia and Ib: increase in the size of the substituent R is
accompanied by increase of the torsion angle φ3, so
that the phenyl ring at the double bond turns away
from the plane of the five-membered ring, and con-
jugation between the aromatic ring and the double
bond weakens. The torsion angle φ3 = 0° corresponds
to coplanar orientation of the phenyl and cyclopente-
none rings, and their orthogonal orientation is char-
acterized by the angle φ3 = 90°. The torsion angle φ3 is
equal to 44° in isomer Ia and 61° in Ib. For com-
parison, the φ3 angle (experimental and calculated) in
unsubstituted molecule I (R = H) is ~20°. Obviously,
the torsion angle φ3 should further increase in parallel
with the size of the R substituent (in going to propyl
and isopropyl derivatives). Presumably, just weakening
of conjugation between the aromatic ring and the
endocyclic double bond is responsible for the reduced
fraction of isomer I (2-Alk) relative to isomers II and
III (5-Alk).
In order to confirm the structure of the compounds
under study, we performed molecular-mechanics cal-
culations of compounds Ia–IIIa and Ib–IIIb. The
calculations involved a force field based on modified
CHARMM parameterization [9]. The geometry of
molecules Ia–IIIa (R = Me) and Ib–IIIb (R = Et) was
characterized by the key torsion angles φ1, φ2, and φ3.
No more than 32 conformers were generated for each
molecule, and the most favorable conformer (i.e., that
having the lowest energy) was found. The applicability
of the calculation method, the set of initial parameters,
and the approach on the whole was preliminarily
checked by calculating the structure of the parent
compound in this series (I, R = H), whose molecule
possesses only one chiral center, while the energies of
enantiomers calculated by the molecular-mechanics
method are similar. The structure of compound I was
examined experimentally by X-ray analysis (CSD
reference code WIYBIP) [10]. It turned out that the
calculated (for the most energetically favorable con-
former) and experimental structures of I (R = H) are
almost identical. The mean-square deviation of coor-
dinates of heavy atoms is 0.06 Å. The torsion angles
are as follows, calculated/experimental, deg: φ1 =
22.1/28.9, φ2 = –59.9/–56.1, φ3 = 20.2/17.8. These
findings suggest that the selected calculation procedure
is appropriate.
EXPERIMENTAL
1
The H NMR spectra were recorded on a Varian
Unity INOVA spectrometer at 400 MHz using tetra-
methylsilane as internal reference and CDCl3 as sol-
vent. Two-dimensional NOESY experiments were per-
formed on a Bruker DRX-500 spectrometer according
to standard procedure (mixing time 500 ms). The
progress of reactions was monitored by TLC on Silica
gel 60 F254 plates; spots were detected by treatment
with iodine vapor or by UV irradiation. The products
were isolated by column chromatography on silica gel
Merck G 60. The melting points were determined on
a Boetius melting point apparatus and were not cor-
rected. Commercially available reagents were pur-
chased from Fluka.
φ1
φ3
φ2
HO
2(5)-(Di)alkyl-4-hydroxy-3,4-diphenylcyclopent-
2-en-1-ones (general procedure). A solution of 31.5 g
(0.15 mol) of 1,2-diphenylethane-1,2-dione and 0.244–
0.315 mol of aliphatic ketone in 100 ml of ethanol was
heated to 60°C, 10 ml of a 10% solution of potassium
hydroxide in ethanol was added, the mixture was
stirred until the initial diketone disappeared (TLC) and
cooled to room temperature, 3 ml of acetic acid was
added, and the mixture was poured into 600 ml of
O
According to the results of calculations, cis and
trans isomers (diastereoisomers) of both methyl (IIa,
IIIa) and ethyl-substituted derivatives (IIb, IIIb) have
almost similar energies. This is consistent with the
experimental data indicating insignificant difference in
the populations of cis and trans diastereoisomers. No
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 8 2010

STRUCTURE OF 2(5)-ALKYL-4-HYDROXY-3,4-DIPHENYLCYCLOPENT-2-EN-1-ONES
1119
water under vigorous stirring. The precipitate was
filtered off, dried under reduced pressure, ground with
100 ml of hexane, filtered off, washed with cold
hexane, and dried in air.
38.8 g (89%), ratio 50:25:25. TLC (2% of MeOH in
CHCl3): two spots, Rf 0.43, 0.59. 1H NMR spectrum, δ,
ppm: 4-hydroxy-3,4-diphenyl-2-propylcyclopent-2-en-
1-one (Ic) (1D TOCSY): 0.89 t (3H, Me, J = 7.5 Hz),
1.51 q (1H, CH2CH2CH3, J = 2.5 Hz), 1.54 q (1H,
CH2CH2CH3, J = 2.5 Hz), 2.27 t (2H, CH2CH2CH3,
J = 7.0 Hz), 2.37 s (1H, OH), 2.93 and 2.97 (1H each,
5-H, AB system, J = 18 Hz), 6.92–6.95 m (4H, o-H),
7.22–7.33 m (6H, m-H, p-H); (4S,5S)-4-hydroxy-3,4-
diphenyl-5-propylcyclopent-2-en-1-one (IIc) (1D
TOCSY): 0.85 t (3H, Me, J = 7.5 Hz), 1.49 m (2H,
CH2), 1.74 m (2H, CH2), 2.39 s (1H, OH), 2.62 d.d
(1H, 5-H, J = 6.6, 7.8 Hz), 6.66 s (1H, 2-H), 7.22–
7.33 m (8H, Harom), 7.39–7.41 m (2H, o-H); (4S,5R)-4-
hydroxy-3,4-diphenyl-5-propylcyclopent-2-en-1-one
(IIIc) (1D TOCSY): 0.75 t (3H, Me, J = 7.0 Hz),
0.80 m (1H, CH2CH2CH3), 1.35 m (1H, CH2CH2CH3),
1.45 m (2H, CH2CH2CH3), 2.58 s (1H, OH), 2.81 d.d
(1H, 5-H, J = 4.8, 8.1 Hz), 6.68 s (1H, 2-H), 7.22–
7.33 m (8H, Harom), 7.49–7.52 m (2H, o-H).
Reaction of 1,2-diphenylethane-1,2-dione with
butan-2-one. Overall yield of three isomeric com-
pounds Ia–IIIa 37.0 g (93%), ratio 75:10:15. TLC
(toluene–EtOAc, 4 : 1): two spots, Rf 0.42, 0.50.
1H NMR spectrum, δ, ppm: 4-hydroxy-2-methyl-3,4-
diphenylcyclopent-2-en-1-one (Ia): 1.91 s (3H, Me),
2.48 s (1H, OH), 2.92 and 2.97 (1H each, 5-H,
AB system, J = 16 Hz), 7.05–7.08 m (4H, o-H), 7.20–
7.37 m (6H, m-H, p-H); (4S,5S)-4-hydroxy-5-methyl-
3,4-diphenylcyclopent-2-en-1-one (IIa): 1.21 d (3H,
Me, J = 7.8 Hz), 2.42 s (1H, OH), 2.65 q (1H, 5-H, J =
7.34 Hz), 6.72 s (1H, 2-H), 7.43–7.45 m (2H, o-H),
7.20–7.37 m (8H, Harom); (4S,5R)-4-hydroxy-5-methyl-
3,4-diphenylcyclopent-2-en-1-one (IIIa) (according to
1
the H–1H NOESY and 1D TOCSY data): 0.69 d (3H,
Me, J = 7.8 Hz), 2.70 s (1H, OH), 2.90 q (1H, 5-H, J =
7.8 Hz), 6.68 s (1H, 2-H), 7.20–7.37 m (8H, Harom),
7.52–7.55 m (2H, o-H).
Reaction of 1,2-diphenylethane-1,2-dione with
4-methylpentan-2-one. Overall yield of three isomers
Id–IIId 39.2 g (90%), ratio 25:27.5:47.5. TLC (1% of
MeOH in CHCl3): three spots, Rf 0.15, 0.27, 0.42.
1H NMR spectrum, δ, ppm: 4-hydroxy-2-(1-methyl-
ethyl)-3,4-diphenylcyclopent-2-en-1-one (Id): 1.17 d
and 1.19 d (3H each, Me, J = 5.6 Hz), 1.67 m (1H,
CH), 2.89 and 2.95 (1H each, 5-H, AB system, J =
18.4 Hz), 3.19 s (1H, OH), 6.86–6.87 m (4H, o-H),
7.20–7.32 m (6H, m-H, p-H); (4S,5S)-4-hydroxy-5-(1-
methylethyl)-3,4-diphenylcyclopent-2-en-1-one (IId):
0.83 d (3H, Me, J = 6.2 Hz), 0.85 d (3H, Me, J =
6.2 Hz), 1.73 br.s (1H, OH), 2.67 m (1H, CH), 2.69 d
(1H, 5-H, J = 8.2 Hz), 6.68 s (1H, 2-H), 7.20–7.32 m
(8H, Harom), 7.39–7.49 m (2H, o-H); (4S,5R)-4-hy-
droxy-5-(1-methylethyl)-3,4-diphenylcyclopent-2-en-
1-one (IIId): 1.03 d (3H, Me, J = 6.6 Hz), 1.12 d (3H,
Me, J = 6.6 Hz), 2.35 m (1H, CH), 2.50 d (1H, 5-H,
J = 4.4 Hz), 2.74 s (1H, OH), 6.65 s (1H, 2-H), 7.20–
7.32 m (8H, Harom), 7.39–7.49 m (2H, o-H).
Reaction of 1,2-diphenylethane-1,2-dione with
pentan-2-one. Overall yield of three isomers Ib–IIIb
37.03 g (89%), ratio 85:5:10. The isomers were sepa-
rated by column chromatography on silica gel using
toluene–ethyl acetate as eluent (gradient elution, 15:1
to 4:1).
2-Ethyl-4-hydroxy-3,4-diphenylcyclopent-2-en-
1-one (Ib). Rf 0.45 (toluene–ethyl acetate, 4:1),
1
mp 102°C. H NMR spectrum (1H–1H NOESY), δ,
ppm: 1.08 t (3H, Me, J = 7.5 Hz), 2.30 m (2H, CH2),
2.48 s (1H, OH), 2.91 and 2.96 (1H each, 5-H,
AB system, J = 20 Hz), 7.00 m (2H, o-H in 3-Ph),
7.25–7.34 m (6H, Harom), 7.35 m (2H, o-H in 4-Ph).
(4S,5S)-5-Ethyl-4-hydroxy-3,4-diphenylcyclo-
1
pent-2-en-1-one (IIb). Rf 0.57, mp 154°C. H NMR
spectrum (1H–1H NOESY), δ, ppm: 1.04 s (3H, Me),
1.80 m (2H, CH2), 2.42 s (1H, OH), 2.52 q (1H, 5-H,
J = 6.2 Hz), 6.68 s (1H, 2-H), 7.21–7.34 m (8H, Harom),
7.45 m (1H, o-H in 4-Ph), 7.54 m (1H, o-H in 3-Ph).
Reaction of 1,2-diphenylethane1,2-dione with
pentan-3-one. Overall yield of two isomers IVa and
Va 38.6 g (93%), ratio 95:5. TLC (hexane–EtOAc,
(4S,5R)-5-Ethyl-4-hydroxy-3,4-diphenylcyclo-
1
1
pent-2-en-1-one (IIIb). Rf 0.52, mp 119°C. H NMR
4:1): two spots, Rf 0.26, 0.29. H NMR spectrum, δ,
spectrum (1H–1H NOESY), δ, ppm: 0.90 m (3H, Me),
1.50 m (2H, CH2), 2.64 s (1H, OH), 2.70 q (1H, 5-H,
J = 7.8 Hz), 6.70 s (1H, 2-H), 7.28–7.36 m (8H, Harom),
7.36 m (1H, o-H in 3-Ph), 7.44 m (1H, o-H in 4-Ph).
ppm, (4S,5S)-4-hydroxy-2,5-dimethyl-3,4-diphenyl-
cyclopent-2-en-1-one (IVa) (1H–1H NOESY): 1.20 d
(3H, 5-Me, J = 7.7 Hz), 1.60 s (1H, OH), 1.95 s (3H,
2-Me), 2.70 q (1H, 5-H, J = 7.7 Hz), 7.16–7.25 m (4H,
o-H), 7.24–7.37 m (6H, m-H, p-H); (4S,5R)-4-hy-
droxy-2,5-dimethyl-3,4-diphenylcyclopent-2-en-1-one
Reaction of 1,2-diphenylethane-1,2-dione with
hexan-2-one. Overall yield of three isomers Ic–IIIc
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 8 2010

MIKHURA et al.
1120
(Va) (1H–1H NOESY): 0.70 d (3H, 5-Me, J = 7.7 Hz),
2.00 s (3H, 2-Me), 2.05 s (1H, OH), 2.90 q (1H, 5-H,
J = 7.7 Hz), 7.16–7.25 m (4H, o-H), 7.24–7.37 m
(6H, m-H, p-H).
Reaction of 1,2-diphenylethane-1,2-dione with
heptan-4-one. Overall yield of two isomers IVb and
Vb 38.0 g (83%), ratio 50:50. The isomers were sepa-
rated by column chromatography on silica gel using
hexane–ethyl acetate (10:1) as eluent.
The authors thank Prof. I.V. Pletnev (Faculty of
Chemistry, Lomonosov Moscow State University) for
performing molecular-mechanics calculations.
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