NMR and computational studies on tautomerism of 3-hydroxy-2-(2- thienylcarbonyl)cyclohex-2-en-1-one

Article abstract of DOI:10.1016/j.molstruc.2013.11.057

The tautomeric system of 3-hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one 1 has been investigated by NMR spectroscopy between 224 and 298 K. At all temperatures an endocyclic enol tautomer was the major isomer; however, at low temperatures two other eno

Full text of DOI:10.1016/j.molstruc.2013.11.057

Journal of Molecular Structure 1059 (2014) 176–184
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
NMR and computational studies on tautomerism of
3-hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one
⇑
Guillermo M. Chans, E. Laura Moyano, María Teresa Baumgartner
INFIQC – Department of Organic Chemistry, Faculty of Chemical Sciences, Universidad Nacional de Córdoba, Córdoba, Argentina
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
We report the first experimental
study of the tautomerism of a 2-
acylcyclohexane-1,3-dione.
ꢀ
ꢀ
In theoretical calculations, monoenols
showed the lowest energy.
The endocyclic enol is the isomer in
higher proportion.
a r t i c l e i n f o
a b s t r a c t
Article history:
The tautomeric system of 3-hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one 1 has been investigated
by NMR spectroscopy between 224 and 298 K. At all temperatures an endocyclic enol tautomer was the
major isomer; however, at low temperatures two other enol isomers were found. Conformational search
of the potential energy surfaces of all tautomers of cyclohexenone 1 was also carried out. Extensive cal-
culations were performed for two triketones and four cis-endocyclic double bond enol tautomers with the
lowest energies. Syntheses of 3-methoxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one 2 and 2-benzoyl-3-
hydroxycyclohex-2-en-1-one 3 were carried out to analyze the features of thienyl group rotation and
structural differences with a symmetrical substituent, respectively.
Received 19 September 2013
Received in revised form 28 November 2013
Accepted 28 November 2013
Available online 3 December 2013
Keywords:
Acylcyclohexanediones
Tautomerism
Ó 2013 Elsevier B.V. All rights reserved.
NMR
Computational chemistry
1. Introduction
hereditary diseases connected with the tyrosine catabolism is tyro-
sinemia type I [5], which is a fatal autosomal recessive disorder
The chemistry of 2-acylcycloalkane-1,3-diones has acquired in-
creased importance due to the creation and wide application of a
number of modern plant growth regulators based on 2-acylcyclo-
hexane-1,3-dione derivatives [1–3]. The mode of action of those
compounds is the inhibition of (4-hydroxyphenyl)pyruvate dioxy-
genase (HPPD), the enzyme that catalyzes the conversion of
(4-hydroxyphenyl)pyruvate to homogentisate [4], which is a key
step in the tyrosine catabolism pathway. One of the severe human
that causes hepatic failure, liver cirrhosis, and an early onset of pri-
mary liver cancer [6]. The biological activity of a molecule gener-
ally depends only of a particular isomer, among all possible
isomers [7,8]. This bioactive conformation is not necessarily the
most stable; nevertheless, it cannot be very high in energy since
it would be excluded from the population of conformers in solu-
tion. In a great number of compounds, the biological activity is re-
lated to their tautomeric structure [9–11].
The keto-enol tautomerization is one of the most well studied
topics both from experimental and theoretical viewpoints
[12,13]. An important number of publications was focused on
⇑
Corresponding author. Tel.: +54 515353876.
E-mail address: tere@fcq.unc.edu.ar (M.T. Baumgartner).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.11.057

G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
177
tautomerism of 1,3-diones [14–21]. Nevertheless, 2-acylcyclohex-
ane-1,3-diones are triketones that present several tautomeric
forms and have been barely studied [22–24].
7''
O
H
2
3''
O
1'
S
3
2'
4
5
7
5'
4'
One of the ways of modifying the keto-enol tautomerism equi-
librium is with the variation of solvent polarity. In solvents like
D2O and CHCl3, the monoenol of b,b0-triketones is the most stable
form (Fig. 1), whereas triketone form could not be detected [25].
On the other hand, theoretical calculations performed by Huang
et al. on benzoylcyclohexane-1,3-dione and similar triketones [23]
suggested two enol forms in solution, the exocyclic and endocyclic
enols, but these have not been experimentally determined. It has
been reported that substitution in position 2 of benzoyl group af-
fects the interconversion of endocyclic and exocyclic enol forms
[23]. The tautomeric equilibrium tends to favor the enol tautomer
with endocyclic double bond, despite the substituent character.
This trend is more obvious when solvent is considered. In addition,
it has been found that the inhibitory activity of HPPD in animals in-
creases when position 2 of benzoyl group is substituted, being the
most active compound the one with a nitro group in such position
[23,26,27]. The NTBC ((2-nitro-4-trifluoromethylbenzoyl)cyclo-
hexane-1,3-dione) is a strong competitive inhibitor of HPPD.
Crystallographic data of an analog of NTBC, 2-benzoyl-5,5-dimeth-
ylcyclohexane-1,3-dione [28], and other synthesized derivatives
[24,27], like 2-(2-nitro-benzoyl)cyclohexane-1,3-dione, demon-
strated that this kind of compounds exists predominantly as endo-
cyclic enol.
3'
O
1
6
1''
1a
Fig. 2. 3-Hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one 1a.
Table 1
Representative isomers of 1 with their respective energies (kJ molꢁ1, HF/6-31G(d)).
Isomer Isomer
EA
D
DE
Monoenol
0.0
Triketone
1.3
10.6
12.7
16.2
44.8
10.9
Dienol
100.8
112.9
220.7
Trienol
To extend our knowledge of the tautomeric equilibrium of b-
triketones, we have studied the tautomerism of a thienyl-carbonyl
derivative of cyclohexane-1,3-dione. NMR measurements, sup-
ported by computational calculations, have provided new informa-
tion on its solution structure. The present article documents the
first experimental variable temperature-NMR determination of 2-
acylcycloalkane-1,3-dione isomers, as well as the first computa-
tional study of energy barriers in such compounds.
48.1
222.4
A
D
E = Energy difference respect to 1a.
2. Results and discussion
ones that do not have. In addition, trans-triketones were more sta-
bles than the cis ones (see Fig. S3 and Table S1).
2.1. Conformational analysis
Trans-triketones (1e–f) and 4 monoenols (1a–d), which can
have an intramolecular hydrogen bond, had the lowest energies.
These compounds were analyzed by different methods (HF, DFT)
and bases set (6-31G(d), 6-31+G(d,p)) (Table S2). Endocyclic
monoenol 1a was the most stable in all used methods, indicating
that this isomer could be the predominant tautomeric form of 1.
In all cases, rotamers that have the sulfur atom of the thienyl group
3-Hydroxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one (Fig. 2,
enol form 1a) has 8 possible tautomeric forms (one triketo, six
keto-enol and one trienol). Considering the orientation of the OH
00
hydrogen (in the same or the opposite side of carbonyl C7@O7 ),
the number of isomers ascends to 28, and taking into account
the thienyl group rotation, to 56 (see Fig. S1).
00
opposite to carbonyl C7@O7 were less stables than the ones in the
All geometries of isomers were submitted to a complete optimi-
zation using HF/6-31G(d), where 44 of the total of 56 isomers
evolved to stable structures (see Fig. S2 and Table S1). Table 1
shows a summary of some representative calculated isomers,
grouped by tautomeric form. The most stable isomer was taken
as reference to obtain the energy of stabilization relative to the
other isomers. Monoenol and triketone forms were the most stable
ones, whereas all dienols and trienols were 100–167 and 205–
246 kJ molꢁ1 higher than monoenols, respectively.
same direction.
On the other hand, comparing optimized geometries obtained
with different methodologies of calculation, the results were anal-
ogous. For ulterior analysis, we decided to work with HF/6-
31+G(d,p), same as literature [23], for comparative purposes.
According to the calculated dipoles of endo and exo forms
(Table 2), it is probable that the endo form (higher dipolar moment)
increase in water. Energy calculations of these isomers in solution
were in agreement with their dipolar moment.
The energy of the same enol form varied with the orientations
of hydroxyl groups. In general, monoenols and dienols with an
intramolecular hydrogen bond showed lower energies than the
Table 2
Relative energies (
respective dipolar moments (D).
D
E) of the most stable tautomers in solution (kJ molꢁ1), with their
O
O
O
OH
R2
Tautomer
DD
G
D
E (CHCl3)
D
E (H2O)
[D]
R1
R2
R1
1a
1b
1c
1d
0.0
0.0
5.2
15.6
19.8
0.0
4.6
18.4
22.8
3.98
2.88
2.06
1.07
7.0
11.8
16.0
R3
O
R3
O
Fig. 1. b,b0-Triketone structure and their monoenol form.

178
G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
Table 3
2.2. Tautomerism and rotation energy barriers
Gibbs free energies of interconversion (DG) and activation energies of tautomeriza-
tion and rotation(
D
Gà) in different solvents. Values are given in kJ molꢁ1
.
Scheme 1 shows the proposed tautomeric interconversion be-
tween endocyclic enol (1a, 1b) and exocyclic enol forms (1c, 1d),
and the rotation process (1a ? 1b and 1c ? 1d).
Tautomerization
1a M 1c
Rotation
1b M d
1a M 1b
1c M 1d
Tautomers with endocyclic double bond (1a and 1b) are the
most stable in gas phase, and both processes are endergonic
(Fig. 3).
In the analysis of the reaction coordinate of thienyl group rota-
tion for both endocyclic and exocyclic enols (1a and 1c) (Scheme 1),
two maxima were found, which correspond to transitional states
(TS3 and TS4, respectively). Dipolar moments of these transitional
states were 3.81 D and 2.42 D, respectively.
D
G
D
Gà
D
G
D
Gà
D
G
D
Gà
D
G
D
Gà
Gas
CHCl3
H2O
9.3
13.1
15.9
12.8
17.5
21.1
7.7
13.4
17.0
13.0
18.4
22.0
5.5
3.7
3.1
24.0
19.8
17.2
4.0
4.0
4.2
18.3
13.0
9.6
energies, solvation effect was more important in tautomerism than
in rotation.
In summary, isomer 1a was the most stable of all calculated iso-
mers, among rotamers and tautomers, both in gas phase as in
solution.
Since the energy barriers of rotation and tautomerism processes
showed a small energy difference between them, both processes
should be carried out at room temperature. Conformers’ relation-
ship was calculated using
pected to be found in
By comparing with the rotation process (Fig. 3), the endo–exo
tautomerization is more feasible to occur.
In order to determine the influence of solvation in tautomeriza-
tion and rotation barriers of 1, computational calculations with
CHCl3 and H2O were carried out (Table 3). In both solvents the
rotation barriers decreased, whereas the opposite effect was ob-
served in tautomerism. Nevertheless, according to the reaction free
D
a
G. Thus, the endocyclic isomer is ex-
high excess, with relation of
a
1a:1c ꢂ 98:2 and 1a:1b ꢂ 90:10.
2.3. NMR analysis
With the aim to determine experimentally the predominant
tautomeric form of 1 in solution, different 1D and 2D NMR exper-
iments were carried out. In its 1H NMR spectrum at 298 K (Fig. 4),
the highly deshielded peak at 17.28 ppm corresponds to an OH
proton, characteristic for the enolic form of tricarbonylic systems
with an intramolecular hydrogen bond [22,24,27]. A methine pro-
ton absence (d ffi 6 ppm) in 1H NMR spectra rules out triketone
form in solution. These results confirmed that 1 is fully enolized
in solution at room temperature.
A number of factors can affect the enolization, e.g., nature of sol-
vent, sample concentration and temperature. Additionally, the
relation of the enol form is inversely proportional to the solvent
polarity [17]. Hence, 1H and 13C experiments were performed at
room temperature in CDCl3, (CD3)2CO, CD3CN and (CD3)2SO
(DMSO-d6) (Table 4). Our results indicated that 1 is completely
enolized in all the solutions studied, no matter the polarity of the
solvent, because the signal corresponding to the keto form was
Scheme 1.
Fig. 3. Plot of energy barriers of tautomerization (a) and rotation (b) in gas phase.

G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
179
7'O'
O3''
H
1'
S
3
2'
2
4
5
7
H-4
H-6
5'
3' O
1
4'
6
1''
1a
2.8
2.7
2.6
2.5
ppm
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
ppm
Fig. 4. 1H NMR spectrum of 1 at 298 K in CDCl3.
Table 4
Proton chemical shifts (ppm) of the enolic hydrogen of 1 in different solvents at room temperature.
Hydrogen
CDCl3 (4.8)A
17.28
(CD3)2CO (21.0)A
16.10
CD3CN (36.6)A
15.79
DMSO-d6 (47.2)A
11.76
300 (s, 1H (OH))
A
Dielectric constant (e). Values taken from literature [29].
not observed. In the spectra, the shifts were changed due to solvent
exchange.
Assignment of all carbons and hydrogens was accomplished by
HSQC and HMBC (data recently published) [30]. Exocyclic carbon
chemical shift (196.44 ppm) is expected to be assigned to enol
C3, according to the information found in literature [16,24,27].
NOESY experiment at room temperature showed no correlation be-
tween the hydroxylic hydrogen with any proton, which made
impossible to confirm the presence of the endocyclic enol as the
major isomer through this experiment.
0
C7 and hydrogen H3 of thienyl group were correctly assigned due
to their mutual long-range correlation C–H (HMBC). A considerable
difference of ca. 10 ppm can be noticed between the more
deshielded carbons C1 and C3 respect to the exocyclic carbon C7
[24] as shown in Table 5.
As can be seen in Fig. 4, both separated shielded triplet signals
at ꢂ2.6 and 2.7 ppm correspond to methylene hydrogens (H4 and
H6), confirming that each of these hydrogens have a different envi-
ronment. In 13C NMR spectrum, the carbons C1 and C3 appear as
separated peaks, as well as C4 and C6. Unfortunately, it was unable
to establish an accurate assignment of H4, H6 and quaternary car-
bons C1 and C3 at room temperature. Nevertheless, the higher
A series of 1H NMR experiments was performed, decreasing the
temperature from 298 K to 224 K (Fig. 5). As the temperature went
down, two new highly deshielded signals appeared in the range
between 15 and 18 ppm, corresponding to hydroxylic hydrogens
of other isomeric species (enol-I and enol-II). Proton signals corre-
sponding to 17.16 and 17.44 ppm coalesced at 270 K, whereas the
major peak corresponding to the OH at 15.75 ppm kept practically
constant at the same chemical shift by varying the temperature.
Contrary to the room temperature spectrum, a NOE effect was
observed between OH at 17.16 ppm and one of the methylene
groups (2.72 ppm) in the NOESY spectrum at low temperature
(Fig. 6). This correlation demonstrated that the major enol is endo-
cyclic (1a, Scheme 1) and thus confirms the assignment of carbons
C1 and C3 as well as hydrogens H4 and H6 (Table 5).
Table 5
1H and 13C chemical shifts (ppm) of 1 relative to TMS at 298 K.
According to the calculated geometry of 1a, hydrogens H4 and
H5 were 2.45 Å between each other, whereas the distance between
Atom
dH (ppm), mult. (J (Hz))
dC (ppm)
COSY
HMBC
1
2
3
4
5
6
7
20
30
40
50
194.48
112.78
196.44
32.97
19.02
38.45
187.37
141.07
136.38
127.63
135.49
00
hydrogens H4 and H3 was 3.36 Å. The relationship between the
À
Á
calculated NOE NOE4—5=NOE4—3 ¼ rꢁ4—65=r4ꢁ—6300 with these dis-
00
2.72, t (6.2)
2.05, quint. (6.2)
2.57, t (6.2)
5, 6
4, 6
4, 5
2, 3, 5, 6
1, 3, 4, 5
1, 4, 5
tances was of approx. 7:1. Thus, the NOE effect between H4 and
00
H5 is 7 times higher than for H4 and H3 . A series of NOESY spectra
was performed at low temperature (218.5 K, Fig. 6). In this case,
00
the NOE induced by hydrogens H4 and H3 (comparing with H4
8.08, dd (4.0, 1.1)
7.11, dd (5.0, 4.0)
7.70, dd (5.0, 1.1)
40, 50
30, 50
30, 40
7, 20, 40, 50
20, 30, 50
20, 30, 40
and H5) was proportional to the calculated distances, confirming
the structure 1a, where the endocyclic enol can form an intramo-
lecular hydrogen bond. In the isomer where OH is not forming an

180
G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
T(K)
275
270
266
260
256
250
246
241
235
1a
Enol-II
228
Enol-I
18.0 17.8 17.6 17.4 17.2 17.0 16.8 16.6 16.4 16.2 16.0 15.8 15.6 15.4
ppm
Fig. 5. 1H NMR spectra of 1 in CDCl3 at different temperatures.
(a)
(b)
(c)
H3
H5
ppm
17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
Fig. 6. NOESY spectra of 1 at 218.5 K using different mixing times: (a) 50, (b) 250 and (c) 900 ms [31].

G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
181
intramolecular hydrogen bond (see Fig. S2, isomer 1K2,3-
2.4. Synthesis of derivatives 2 and 3
00
E(t)7KTh(c)), the calculated distance of H3 –H4 is 2.21 Å long;
therefore, a NOE effect of similar intensity is expected to occur just
as in H4-H5.
With the aim to block the keto-enol tautomerism of 1, this com-
pound was methylated with dimethylsulfate (DMS) according to a
methodology based on the Khlebnicova procedure [32], generating
3-methoxy-2-(thienylcarbonyl)cyclohex-2-en-1-one (2) (Fig. 8),
which could allow to analyze the features of the thienyl group
rotation.
The main differences between spectra of 1 and 2 are the disap-
pearance of a signal corresponding to an OH and the presence of a
singlet signal corresponding to a methyl group at 3.78 ppm
(Table 6). A NOE effect was also observed between methoxyl group
hydrogens and methylene hydrogens H4 in the NOESY experiment
of 2 at room temperature, confirming the methoxyl group in C3.
In the next step, an exhaustive analysis of the 2D spectra was
carried out, with the aim to determine the structure of both minor
isomers (enol-I and enol-II, Fig. 5) found at low temperature. Four
of the calculated most stable isomers of 1 are shown in Scheme 1.
A HMBC spectrum at low temperature (224 K) is shown in
Fig. 7. Interactions between carbons at 33.0, 112.8 and
196.4 ppm and hydroxyl of major species 1a (17.16 ppm) and the
most deshielded minor isomer (17.44 ppm, enol-I) are very similar.
Hence, it can be implied that the carbon skeleton in the proximity
of the OH hydrogen for both isomers is the same. Therefore, the
structure assigned for enol-I was rotamer 1b.
0
On the other hand, the hydrogen signal H3 of thienyl group in 2
is shifted 0.53 ppm to higher fields than in 1 (Fig. 9). This result
could be explained by a ring current effect produced by the C1 car-
bonyl group in 1, whereas this effect is not observed in 2.
Computational calculations indicated that thiophene-2-car-
bonyl substituent is orthogonal to the cyclohexenyl system in 2,
and its rotamers (2a and 2b, Fig. S4) can coexist at room tempera-
ture. These structures could be very well correlated with the NMR
experimental results. According to its relationship with 1, this
study allowed us to corroborate the coplanarity between the thio-
phene-2-carbonyl substituent and the endocyclic carbonyl of 1
1a
Enol-I
ppm
20
40
60
00
0
(C1@O1 ) and the interaction of this carbonyl group with H3 .
In order to simplify the study of rotamers and tautomers of 1, 2-
benzoyl-3-hydroxycyclohex-2-en-1-one (3) was also synthesized
[30]. NMR analysis of this compound could give clear information
about the species present in solution, excluding the possibility of
rotamers’ signals due to the symmetry of phenyl substituent. Sig-
nals assignments of 3 are shown in Table 6. The signal at
16.96 ppm corresponded to a hydroxyl group, indicating that the
major species is an enol with an intramolecular hydrogen bond.
80
100
120
140
160
180
200
00
A NOE effect was also observed between hydroxyl H3 and methy-
lene hydrogens H4 in the NOESY experiment at 228 K, indicating
that the enol is endocyclic. These results were comparable to the
obtained for 1.
From the 1H NMR analysis of hydrogens H2 and H6 of phenyl
group, we can observe that these protons are more shielded
(7.51 ppm) than in acetophenone (7.94 ppm). Consequently, it
can be inferred that the phenyl substituent is displaced from the
plane of the molecule and does not have the influence of the car-
bonyl group.
0
0
ppm
18.0
17.5
17.0
16.5
Fig. 7. HMBC spectrum of 1 at 224 K in CDCl3.
8
7''
3''
H
3''
7''
Computational calculations of 3 were carried out in solution
(CHCl3) for a better comparison with NMR experiments. From the
analysis similarly performed for 1, we could determine that the
most stable structure was 3a (endocyclic enol), being
11.97 kJ molꢁ1 more stable than the exocyclic enol. In this confor-
mation, the phenyl group is out of the plane. These results are in
agreement with the experimental NMR measurements (Fig. 10).
In the 1H NMR spectra of 3 at 224 K, a minor peak was observed
at higher fields (16.11 ppm) than the major OH signal (16.96 ppm)
(relation of 0.3% between each other), which corresponds to a hy-
droxyl group of an isomer of 3. This peak remains at all the studied
O
OCH3
O
O
1'
2'
5'
3
3
2'
2
1
S
2
1'
4
3'
4'
4
5
7
7
5'
3'
6'
1''
5
6
O
O
1
4'
1''
6
2
3
Fig. 8. 3-Methoxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-one (2) and 2-benzoyl-3-
hydroxycyclohex-2-en-1-one (3).
Table 6
1H and 13C chemical shifts (ppm) comparison of 1 with 2 and 3 at room temperature.
1
–
2
–
3 (OH)
17.28, s, 1H 2.72, t, 2H 2.05, quint, 2H 2.57, t, 2H
32.97 19.02 38.45
2.68, t, 2H 2.12, quint, 2H 2.45, t, 2H
4
5
6
7
8
20
–
30
40
50
1
2
dH
dC
dH
dC
–
–
–
8.08, dd, 1H 7.11, dd, 1H 7.70, dd, 1H
194.48 112.78 196.44
–
196.01 120.31 175.08
1
–
187.37
–
187.08 56.66
7
–
141.07 136.38
–
145.13 133.84
1’
–
127.63
135.49
–
–
3.78, s, 3H
7.55, dd,1H
7.08, d, 1H
128.18
30/50
7.39, m, 2H
127.84
7.63, dd, 1H
134.31
40
7.49, dd, 1H
132.04
25.89
4
20.61
5
36.46
6
2
–
3 (OH)
–
–
–
20/60
7.51, m, 2H
3
dH
dC
16.96, s, 1H 2.75, t, 2H 2.07, quint, 2H 2.50, t, 2H
32.57 19.27 38.27
194.32 113.39 196.38
199.14
138.30 128.40

182
G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
H3´
H5´
H4´
8.0
7.9
7.8
7.7
7.6
7.5
7.4
7.3
7.2
7.1
7.0
ppm
H4´
H5´
H3´
8.0
7.9
7.8
7.7
7.6
7.5
7.4
7.3
7.2
7.1
7.0
ppm
Fig. 9. 1H NMR of 1 and 2 (224 K), amplified between 6.9 and 8.1 ppm.
T(K)
293
282
271
250
233
224
17.8
17.6
17.4
17.2
17.0
16.8
16.6
16.4
16.2
16.0
15.8
ppm
Fig. 10. 1H NMR spectra of 3 in CDCl3 at different temperatures.
temperatures (224–293 K, Fig. 10) and it does not coalesce. Thus,
for this compound, it is possible the formation of the exocyclic enol
3c (Fig. 10). Taking into account these findings, by analogy, the
enol-II of 1 could be proposed to be the exocyclic enol 1c.
respective rotamer. It was shown that the endocyclic enol 1a is
the isomer in higher proportion (>95%) from thermodynamic anal-
ysis, rotation study and tautomerism barriers. Nevertheless, the in-
volved energies in both processes are quite small.
Analyzing NMR spectra, it is possible to establish that the pre-
dominant isomer is the endocyclic enol with the sulfur atom of thi-
enyl ring in the same direction of the exocyclic carbonyl group.
At low temperatures, other two isomers were observed. The
most deshielded signal corresponds to rotamer 1b, whereas enol-
II was assigned to the exo tautomer 1c.
Thus, the first analysis of 2-acylcyclohexanediones involving
other enol isomers is presented. The important tautomeric
information gathered from the study here disclosed could be a
3. Conclusions
This work represents the first experimental study of the tau-
tomerism of a 2-acylcyclohexane-1,3-dione, using NMR and com-
putational studies concurrently.
In theoretical calculations, monoenols showed the lowest en-
ergy, being the most stable the endocyclic tautomer with their

G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
183
significant contribution for the investigation of interaction mecha-
nisms comprising these compounds with the HPPD enzyme.
The following parameters were used to measure 1H NMR spec-
tra: spectral width, 8.220 Hz (20 ppm); pulse P1 = 7.5 s, digital
resolution, 0.39 Hz/point; number of scans, 16. For 13C NMR spec-
l
tra: spectral width, 20,500 Hz; pulse P1 = 10.6 ls, digital resolution,
4. Experimental
0.63 Hz/point; number of scans >2.000. In the case of 13C NMR, par-
tially saturated spectra were recorded with the application of
WALTZ 16 proton decoupling. FIDs were multiplied by an exponen-
tial weight (lb = 1 Hz for 1H and lb = 1.2 Hz for 13C) before Fourier
transformation.
4.1. Synthesis
Triketones 1 and 3 were synthesized by treating the appropriate
acyl chloride with dried potassium cyanide in the presence of
anhydrous acetonitrile under ultrasound irradiation at 50 °C to
form the corresponding acylcyanide. Then, triethylamine and
cyclohexane-1,3-dione were added in situ and the mixture was
stirred overnight at room temperature, according to the conditions
previously described [30].
Accessory publication
General procedure for the synthesis of these new compounds
with their corresponding 1H, 13C and 2D NMR spectra and the com-
putational calculations are available on the Journal’s Website.
4.2. Synthesis of 3-methoxy-2-(2-thienylcarbonyl)cyclohex-2-en-1-
one 2
Acknowledgments
Calcined K2CO3 (0.379 g; 0.274 mmol) and dimethyl sulfate
(0.065 g; 0.051 mmol) were added to a solution of 3-hydroxy-2-
(2-thienylcarbonyl)cyclohex-2-en-1-one (1) (0.102 g; 0.045 mmol)
in absolute toluene (10 mL). The reaction mixture was boiled for
10 h, the solid was filtered off, and washed with toluene. After
removing the toluene on the rotary evaporator, the residue was
dissolved in ethyl acetate and then purified by chromatographic
column using ethyl acetate:hexane [9:1] as eluent. White solid.
1H NMR (CDCl3): d = 2.12 (quint., J = 6.5, 2H, H-5), 2.46 (t, J = 6.5,
2H, H-6), 2.68 (t, J = 6.5, 2H, H-4), 3.78 (s, 3H, H-8) 7.08 (dd,
J = 4.9, 3.9, 1H, H-40), 7.55 (dd, J = 3.9, 0.9, 1H, H-30), 7.63 (dd,
J = 4.9, 0.9, 1H, H-50) ppm. 13C NMR (CDCl3): d = 20.61 (C-5),
25.89 (C-4), 36.46 (C-6), 56.66 (C-8), 120.31 (C-2), 128.18 (C-40),
133.84 (C-30), 134.31 (C-50), 145.13 (C-20), 175.08 (C-3), 187.08
(C-7), 196.01 (C-1) ppm. MS (EI): m/z (%) = 236 [M]+ (28), 203
(21), 180 (9), 151 (21), 139 (14), 123 (9), 111 (100), 97 (27), 83
(16), 55 (15).
This work was financially supported by the National Research
Council of Argentina (CONICET) and the Secretary of Science and
Technology – Universidad Nacional de Córdoba (SECyT-UNC).
G. M. C. thanks CONICET for a doctoral fellowship. The authors
are grateful for the contribution of Ph. D. Walter J. Peláez and the
technical assistance provided by Ph. D. Gloria Bonetto.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.
11.057.
References
[1] R. Beaudegnies, A.J.F. Edmunds, T.E.M. Fraser, R.G. Hall, T.R. Hawkes, G.
Mitchell, J. Schaetzer, S. Wendeborn, J. Wibley, Biorg. Med. Chem. 17 (2009)
4134, http://dx.doi.org/10.1016/j.bmc.2009.03.015.
4.3. Computational calculations
[2] F.E. Dayan, N. Singh, C.R. McCurdy, C.A. Godfrey, L. Larsen, R.T. Weavers, J.W.
Van Klink, N.B. Perry, J. Agric. Food Chem. 57 (2009) 5194, http://dx.doi.org/
10.1021/jf9005593.
All calculations of the investigated species were performed
using the Gaussian03 program package [33], employing HF and
DFT (B3LYP) levels of theory, with 6-31+G(d,p) basis set for ther-
modynamic calculations. The solvent effect was taken into account
using the PCM of Tomasi et al. [34]. The enol hydrogen was treated
explicitly (radii taken from the UFF force field) [35]. Total energies
were corrected for the zero-point energies and for thermal and
entropic contributions at 298.15 K calculated at the same levels
of theory as were used for optimization.
AM1 method [36] was used to obtain the initial geometries of
the 8 tautomeric forms of 1. Subsequently, a conformational anal-
ysis was carried out, considering all the possible torsion angles (see
Fig. S1 in Supporting Information).
[3] G. Meazza, B.E. Scheffler, M.R. Tellez, A.M. Rimando, J.G. Romagni, S.O. Duke, D.
Nanayakkara, I.A. Khan, E.A. Abourashed, F.E. Dayan, Phytochemistry 60 (2002)
281, http://dx.doi.org/10.1016/s0031-9422(02)00121-8.
[4] N.P. Crouch, R.M. Adlington, J.E. Baldwin, M.H. Lee, C.H. MacKinnon,
Tetrahedron
53
(1997)
6993,
http://dx.doi.org/10.1016/s0040-
4020(97)00398-0.
[5] S. Lindstedt, E. Holme, E.A. Lock, O. Hjalmarson, B. Strandvik, Lancet 340 (1992)
813, http://dx.doi.org/10.1016/0140-6736(92)92685-9.
[6] E. Holme, S. Lindstedt, J. Inherit. Metab. Dis. 21 (1998) 507, http://dx.doi.org/
10.1023/a:1005410820201.
[7] W.P. Walters, J. Green, J.R. Weiss, M.A. Murcko, J. Med. Chem. 54 (2011) 6405,
http://dx.doi.org/10.1021/jm200504p.
[8] C. Bissantz, B. Kuhn, M. Stahl, J. Med. Chem. 53 (2010) 5061, http://dx.doi.org/
10.1021/jm100112j.
[9] A. Toropova, A. Toropov, E. Benfenati, G. Gini, D. Leszczynska, J. Leszczynski,
Cent. Eur. J. Chem. 9 (2011) 846, http://dx.doi.org/10.2478/s11532-011-0064-
0.
[10] Y.C. Martin, J. Comput.-Aided Mol. Des. 24 (2010) 613, http://dx.doi.org/
10.1007/s10822-010-9325-9.
[11] F. Caruso, C. Pettinari, F. Marchetti, M. Rossi, C. Opazo, S. Kumar, S. Balwani, B.
Ghosh, Biorg. Med. Chem. 17 (2009) 6166, http://dx.doi.org/10.1016/
j.bmc.2009.07.064.
[12] B. Kuhn, P. Mohr, M. Stahl, J. Med. Chem. 53 (2010) 2601, http://dx.doi.org/
10.1021/jm100087s.
[13] T. Steiner, Angew. Chem. Int. Ed. 41 (2002) 48. http://dx.doi.org/10.1002/1521-
3773(20020104)41:1<48::aid-anie48>3.0.co;2-u.
[14] T. Ishida, F. Hirata, S. Kato, J. Chem. Phys. 110 (1999) 3938, http://dx.doi.org/
10.1063/1.478249.
[15] A.I. Koltsov, J. Mol. Struct. 444 (1998) 1, http://dx.doi.org/10.1016/s0022-
2860(97)00261-5.
[16] C.F.G.C. Geraldes, M.T. Barros, C.D. Maycock, M.I. Silva, J. Mol. Struct. 238
(1990) 335, http://dx.doi.org/10.1016/0022-2860(90)85025-e.
[17] J. Emsley, Struct. Bond. 57 (1984) 147, http://dx.doi.org/10.1007/BFb0111456.
[18] M. Gorodetsky, Z. Luz, Y. Mazur, J. Am. Chem. Soc. 89 (1967) 1183, http://
dx.doi.org/10.1021/ja00981a026.
4.4. NMR
All spectra were taken with a Bruker Avance II spectrometer
with a BVT3000 temperature controller (solution, 9 T, 400.16 and
100.56 MHz for 1H and 13C NMR, respectively), equipped with an
inverse detection probe H-X (BBI) of 5 mm and a z-gradient coil
for 1H and 13C. Chemical shifts (d) are reported in ppm and cou-
pling constants in Hz. Chemical shifts are given by internal sol-
vents, CDCl3 (7.26, 77.16), CD3COCD3 (2.05, 29.84), and DMSO-d6
(2.50, 39.52) ppm for 1H and 13C, respectively. The internal stan-
dard was TMS for CDCl3 and CD3COCD3, whereas in the other sol-
vents, the signal corresponding to the undeuterated residual peak
was used as standard.

184
G.M. Chans et al. / Journal of Molecular Structure 1059 (2014) 176–184
[19] J. Palomar, J.L.G. De Paz, J. Catalán, J. Phys. Chem. A 104 (2000) 6453, http://
dx.doi.org/10.1021/jp994067o.
[20] G.A. Echeverría, J.L. Jios, J.C. Autino, G. Punte, Struct. Chem. 11 (2000) 367,
http://dx.doi.org/10.1023/a:1026522105977.
[21] L. Turi, J.J. Dannenberg, J. Phys. Chem. 96 (1992) 5819, http://dx.doi.org/
10.1021/j100193a033.
[22] M.Z. Chalaça, J.D. Figueroa-Villar, J. Mol. Struct. 554 (2000) 225, http://
dx.doi.org/10.1016/s0022-2860(00)00674-8.
[31] All spectra are acquired and processed under the same experimental
conditions. The larger the mixing time, the more NOEs’ effect will be
observed, but the less difference in intensity by distance.
[32] T. Khlebnicova, V.G. Isakova, F.A. Lakhvich, P.V. Kurman, Chem. Heterocyc.
Compd. 44 (2008) 301, http://dx.doi.org/10.1007/s10593-008-0044-2.
[33] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E.R. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, J.T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.
Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth,
P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C.
Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V.
Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham,
C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen,
M.W. Wong, C. Gonzalez, J.A. Pople Gaussian 03, Gaussian 03, Revision C.02;
Gaussian, Inc.: Wallingford, CT, 2004
[23] M.-L. Huang, J.-W. Zou, D.-Y. Yang, B.-Z. Ning, Z.-C. Shang, Q.-S. Yu, J. Mol.
Struct. THEOCHEM 321 (2002) 589–590, http://dx.doi.org/10.1016/s0166-
1280(02)00300-7.
´
[24] P. Szczecinski, A. Gryff-Keller, S. Molchanov, J. Org. Chem. 71 (2006) 4636,
http://dx.doi.org/10.1021/jo060583g.
[25] A. Rojas, A. Perez-Encabo, I. Herraiz-Sierra, C.A. Blanco, Can. J. Chem. 79 (2001)
448, http://dx.doi.org/10.1139/v01-061.
[26] Y.-L. Lin, C.-S. Wu, S.-W. Lin, D.-Y. Yang, Bioorg. Med. Chem. Lett. 10 (2000)
843, http://dx.doi.org/10.1016/s0960-894x(00)00115-3.
[27] C.-S. Wu, J.-L. Huang, Y.-S. Sun, D.-Y. Yang, J. Med. Chem. 45 (2002) 2222,
http://dx.doi.org/10.1021/jm010568y.
[28] E. Borisov, A. Verenich, A. Govorova, A. Lyakhov, T. Khlebnikova, Russ. Chem.
Bull. 49 (2000) 1068, http://dx.doi.org/10.1007/bf02494895.
[29] D.R Lide, In CRC Handbook of Chemistry and Physics, vol. 6, 87th ed., Taylor
and Francis, Florida, USA, 2007. pp. 132.
[34] J. Tomasi, B. Mennucci, R. Cammi, Chem. Rev. 105 (2005) 2999, http://
dx.doi.org/10.1021/cr9904009.
[35] A.K. Rappé, C.J. Casewit, K.S. Colwell, W.A. Goddard, W.M. Skiff, J. Am. Chem.
Soc. 114 (1992) 10024, http://dx.doi.org/10.1021/ja00051a040.
[30] G.M. Chans, E.L. Moyano, G.I. Yranzo, Aust. J. Chem. 64 (2011) 638, http://
dx.doi.org/10.1071/CH11015.
[36] M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107
(1985) 3902, http://dx.doi.org/10.1021/ja00299a024.