Synthesis, 17O NMR spectroscopy and structure of 2-trifluoroacetyl-1-methoxycycloalkenes

Article abstract of DOI:10.1016/j.jfluchem.2005.08.003

Among the synthesis of a series of five well-known 2-trifluoroacetyl-1- methoxycycloalkenes derived from cyclopentanone and substituted cyclohexanones, this paper describes the synthesis of three new 2-trifluoroacetyl-1- methoxycycloalkenes derived from cycloheptanone, cyclooctanone and cyclododecanone in 60-68% yield. Subsequently, the ¹⁷O NMR chemical shift analysis of the carbonyl and the methoxy groups for these cyclic molecules clearly showed the electron push-pull phenomenon and revealed large and irregular variations of ¹⁷O NMR chemical shifts with the ring size. Finally, a more stable conformation of these trifluoroacetyl-containing cycloalkenes was determined by energy minimization calculations using Austin Model 1 (AM1) semi-empirical method and correlations between ¹⁷O NMR data and torsion angles or oxygen net charge calculated by AM1 semi-empirical method were performed.

Full text of DOI:10.1016/j.jfluchem.2005.08.003

Journal of Fluorine Chemistry 126 (2005) 1396–1402
www.elsevier.com/locate/fluor
17
Synthesis, O NMR spectroscopy and structure of
2-trifluoroacetyl-1-methoxycycloalkenes
*
Helio G. Bonacorso , Michelle B. Costa, Sidnei Moura, Lucas Pizzuti,
Marcos A.P. Martins, Nilo Zanatta, Alex F.C. Flores
´ ´ ´
Departamento de Quımica, Universidade Federal de Santa Maria, Nucleo de Quımica de Heterociclos,
NUQUIMHE, 97105-900 Santa Maria, RS, Brazil
Received 2 August 2005; accepted 4 August 2005
Available online 8 September 2005
Abstract
Among the synthesis of a series of five well-known 2-trifluoroacetyl-1-methoxycycloalkenes derived from cyclopentanone and substituted
cyclohexanones, this paper describes the synthesis of three new 2-trifluoroacetyl-1-methoxycycloalkenes derived from cycloheptanone,
cyclooctanone and cyclododecanone in 60–68% yield. Subsequently, the 17O NMR chemical shift analysis of the carbonyl and the methoxy
groups for these cyclic molecules clearly showed the electron push–pull phenomenon and revealed large and irregular variations of 17O NMR
chemical shifts with the ring size. Finally, a more stable conformation of these trifluoroacetyl-containing cycloalkenes was determined by
energy minimization calculations using Austin Model 1 (AM1) semi-empirical method and correlations between 17O NMR data and torsion
angles or oxygen net charge calculated by AM1 semi-empirical method were performed.
# 2005 Elsevier B.V. All rights reserved.
Keywords: 17O NMR; NMR; Trifluoroacetylcycloalkenes; Cycloalkanones; Methoxycycloalkenes; Enones
1. Introduction
problems [8] and it provides insights into the understanding
of chemical reactivity [9].
Historically, b-alkoxyvinyl trihalomethyl ketones deri-
vated from cyclopentanone and cyclohexanone have been
developed by our research group [1a]. The results obtained
from the reaction of these carbocyclic trihaloacetylated
enolethers with some dinucleophiles are very interesting
[1–5]. Parallel to the synthetic works, some publications
have reported studies about the molecular structure of some
heterocyclic compounds derived from 2-trihaloacetyl-1-
Recently, it was reported that the cyclization reactions
between acyclic b-alkoxyvinyl trifluoromethyl ketone,
derived from phenones and urea, to obtain phenyl substituted
2(1H)-pyrimidinones [10], strongly depended on the
conformation of the enone precursors. The MO calculations
carried out by the Austin Model 1 (AM1) semi-empirical
method demonstrated that the conformation of these acyclic
b-alkoxyvinyl trifluoromethyl ketones is the main factor to
explain the yields of these reactions. In fact, when electron
push–pull structures are available, the electronic density of
each atom of this system is directly related to conformation.
In b-alkoxyvinyl trifluoromethyl ketones, due to the
possibility of p-p electronic conjugation, the 17O NMR
chemical shifts of the carbonyl and methoxy oxygen’s are
related to the electronic density of both oxygens; moreover,
the value of the chemical shift depends essentially on the
efficiency of this conjugative interaction. Considering the
1
methoxycyclohexen using H and 13C NMR spectroscopy,
X-ray diffraction and semi-empirical MO calculation tools
[6,7].
On the other hand, since the 60s, 17O NMR spectroscopy
is becoming an increasing important method in organic
chemistry for examining a wide variety of structural
* Corresponding author.
E-mail address: heliogb@base.ufsm.br (H.G. Bonacorso).
0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.08.003

H.G. Bonacorso et al. / Journal of Fluorine Chemistry 126 (2005) 1396–1402
1397
Scheme 1. Synthetic route for trifluoroacetylcycloalkenes.
electron push–pull structure of these substrates, where there
2. Results and discussion
is a donor group (OMe) and an acceptor group (CO), they
can be used as good models to study the relationships
between the 17O NMR chemical shifts and the conforma-
tions with the advantage of allowing direct observation of
the two oxygen sites involved.
According to Scheme 1, the cycloalkanone dimethyl
acetals 2a–h were synthesized from the reaction of the
respective cycloalkanones 1a–h with trimethyl orthoformate
in molar ratio 1:1.5 and in the presence of p-toluenesulfonic
acid as catalyst and using anhydrous methanol as solvent by
similar procedure described in the literature [1a–d] (Scheme
1). The acylation of 2a–h with trifluoroacetic anhydride in
pyridine, in molar ratio 1:2:2, was carried out in anhydrous
chloroform as solvent. Two equivalents of the acylating
reagent per dimethyl acetal were necessary to obtain the
cyclic b-methoxyvinyl trifluoromethyl ketones 3a–h since
one molecule of the acylating promotes the acylation and a
second traps the methoxy group liberated by the ketone
dimethyl acetal and to give methyl trifluoroacetate. A mixture
of distilled ketone dimethyl acetals, pyridine and anhydrous
chloroform is added to pure trifluoroacetic anhydride at 0 8C
with cooling in an ice bath. After the addition, the optimal
reactiontimeandreactiontemperaturefortrifluoroacetylation
were found to be 16 h at 40–45 8C. Under the conditions
employed, ketone dimethyl acetals reacted with trifluoroa-
cetic anhydride to give the cyclic trifluoroacetyl vinyl enones
3a–h in good yields. In all reactions, we have not found
polymerization products and all crude products were purified
by distillation. More satisfactory results of these reactions,
selected physical (b.p.), NMR and mass spectral data of 3a–h
are presented in the experimental part.
On the other hand, the effect of the ring size and its
conformations on 17O NMR of the carbonyl and methoxy
chemical shifts for 2-trifluoroacetyl-1-methoxycycloalkenes
(cyclic b-alkoxyvinyl trifluoromethyl ketones) have not
been systematically studied and in due to the importance of
such systems to synthetic chemistry, the additional
information which may be obtained on the electronic
distribution of these compounds, by employing 17O NMR
spectroscopy methodology, seems likely to be valuable.
The influence of electronic and steric effects, torsion
angles, electron density on 17O NMR chemical shifts of
anthraquinones and flavones [11], methyleneindanones [12],
aromatic carbonyl [13] and nitro compounds [14] has also
recently been well documented and various correlation
analyses have produced quantitative relationship between
carbonyl 17O NMR chemical shifts and the substituent
constants for aryl carbonyl derivatives including substituted
aromatic ketones [15], benzoic esters [16] and aldehydes
[17], and chalcones [18]. However, despite the interest in the
molecular structure of cyclic b-alkoxyvinyl trifluoromethyl
ketones, no systematic effort to correlate these molecular
structures with 17O chemical shifts has appeared.
Thus, considering the importance of b-alkoxyvinyl
trihalomethyl ketones, as 1,3-dielectrophile precursor and,
despite our extensive studies on the NMR spectroscopy of
acyclic b-alkoxyvinyl trihalomethyl ketones, we now wish
to report the synthesis, 17O NMR study and conformational
analysis of a series of eight 4(6)-alkyl-2-trifluoroacetyl-1-
methoxycycloalkenes derived from one cyclopentanone,
four substituted cyclohexanones and three new larger
cycloalkanones.
The present compounds 3a–h can be considered not just as
a,b-unsaturated olefinic ethers but also as a,b-unsaturated
ketones and, therefore, they can be characterized as possible
model for p-p conjugation in the b-methoxyvinyl ketone
system (Scheme 2).
In 1994, Taskinen [19] reported the 1H, 13C and 17O NMR
data in chloroform-d1 solution, force field and MNDO
calculations of 4- to 9-membered 1-methoxycycloalkenes.
The 17O NMR chemical shifts revealed unexpected large and
Scheme 2. The p-p conjugation in the b-methoxyvinyl trifluoromethyl ketone system 3a–h.

1398
H.G. Bonacorso et al. / Journal of Fluorine Chemistry 126 (2005) 1396–1402
Table 1
17O NMR data for 4(6)-alkyl-2-trifluoroacetyl-1-methoxycycloalkenes (3a–
h)
Table 2
13C NMR chemical shifts of 5- to 8-membered 1-methoxycycloalkenes and
2-trifluoroacetyl derivatives (3)
1-Methoxycycloalkenesa
2-Trifluoroacetyl-1-methoxycycloalkenes (3)
Ring size C-1 C-2
Ring size C-1 C-2
5
6
161.1 93.3
155.4 93.1
162.0 96.0
158.3 94.0
5
6
177.8 108.1
170.0 110.4
175.4 116.7
185.5 118.3
178.8 109.5
7
7
No.
n
No. of
R
C
O
W 1/2 OCH3
(Hz)a
W 1/2
(Hz)a
8
8
ring atoms
(ppm)
(ppm)
b
b
12
12
3a
3b
3c
3d
3e
3f
1
2
2
2
2
3
4
8
5
6
H
508.45 (550)
92.45
80.0
(410)
(430)
(470)
(560)
(820)
(590)
(520)
(1700)
a
Data from reference [19].
No 13C NMR data.
H
532.0
545.0
(500)
(590)
b
6
6-Me
4-Me
64.5
6
534.03 (710)
79.09
79.08
76.89
114.53
55.0
5- to 8-membered 1-methoxycycloalkenes [19] in Table 2.
One can observe that the trifluoroacetyl group at the position
2 induces a remarkable deshielding effect on C-1 (ꢀ18 ppm)
and C-2 (ꢀ19.3 ppm). Considering the series 3a–b and 3f–g,
the deshielding effect is highest at C-1 (27.2 ppm) and C-2
(24.3 ppm) for the 8-membered ring 3g.
6
4-t-Bu 536.77 (900)
7
H
H
H
535.60 (595)
486.77 (630)
3g
3h
8
12
567.0
(900)
a
Values in parentheses are for peak widths at half-height.
irregular variations with the ring size (14 ppm). According to
the author, since the oxygen atom is exocyclic to the
cycloalkene ring, the irregularities in the 17O NMR chemical
shiftsofthesecompoundswasnotexplainablebyvariationsin
the inductive and shielding effects of the different ring sizes.
These variations were attributed due to the differences in the
relative amounts of planar and non-planar conformers about
the CH3–O–C C bonds. Taskinen concluded that in addition
to the s-cis and possibly the planar s-trans conformers,
varying amounts of non-planar conformers (gauche) exist for
the methoxy group in the cyclic 1-methoxycycloalkenes [19].
Table 1 contains 17O NMR spectroscopy data for
4(6)-alkyl-2-trifluoroacetyl-1-methoxycycloalkenes (3a–h)
obtained, at natural abundance, also in chloroform-d1
solution at 27 8C. The 17O NMR signals for the carbonyl
group of these compounds appear between 487 and
567 ppm, a range of 80 ppm and for the methoxy group
between 55 and 114 ppm, a range of 59 ppm. In comparison
with non-substituted 1-methoxycycloalkenes [19], one can
observe that the introduction of a trifluoroacetyl group at the
2-position increased the range of the 17O NMR chemical
shifts of the methoxy group of the 5- to 8-membered 1-
methoxycycloalkenes from 11 to 22 ppm. According to
Table 1, the electron push–pull phenomenon can also clearly
observed. It is observed for each compound that when the
17O NMR chemical shifts of the carbonyl oxygen increase,
the chemical shifts of the methoxy oxygen decrease.
The intensity of this phenomenon is variable and may be
attributed to different conformations of the CH3–O–C C–
C O system resulting in planar and non-planar structures for
the Me–O–C C and C C–C O moieties. Accordingly, the
variations on the 17O NMR data of 3a–h may be indicative of
varying strengths of p-p conjugation in these compounds. In
this case, the highest contribution of p-p interaction is found
for compound 3g (8-membered ring size) and the lowest for
3h (12-membered ring size). This fact can be proved by the
13C NMR data analysis when the 13C NMR chemical shifts
of 3a–b and 3f–g are compared with those of the respective
According to Table 1, it is also observed that the 17O
NMR data for the methoxy group of 3b–e (cyclohexen
derivatives) show a deshielding effect only for 3c. We noted
that the chemical shift of 6-methyl-2-trifluoroacetyl-1-
methoxycyclohexene (3c) was downfield from the non-
substituted 2-trifluoroacetyl-1-methoxycyclohexene (3b) by
15.5 ppm. This shift is presumably, mostly, a result of steric
inhibition of p-p electronic conjugation of the methoxy
group with the carbonyl group. As expected, we do not
observe a deshielding effect when a methyl- or t-butyl-
substituent is attached at the 4-position for 2-trifluoroacetyl-
1-methoxycyclohexenes (3d, 3e). Similar deshielding effect
due to the steric effect of a vicinal methyl group has been
reported in rigid planar molecules, as nitrotoluene [14] and
methylanthraquinone [11] derivatives.
Considering also the work reported by Taskinen [19], we
agree that in addition to the s-cis and possibly s-trans
conformers on the Me–O–C C moiety, varying amounts of
non-planar conformers may exist for compounds 3a–h,
when in solution. Moreover, from our point of view, the
planarity or non-planarity of the C C–C O moiety and the
strong electron withdrawing effect of the trifluoromethyl
group are also responsible for the variations on both 17O
NMR chemical shift values.
The 17O NMR data for compounds 3a–h presented in
Table 1 suggested quantitative relationships between the
chemical shifts of the carbonyl oxygen and the oxygen of the
methoxy group. A good relationship with r = 0.9856 and
95% confidence limits for error in slope was obtained
(Eq. (1)). When substituted cyclohexen derivatives (3c–e)
are not evaluated, a better relationship with r = 0.9940
involving non-substituted 5-, 6-, 7-, 8- and 12-membered
carbocycles (3a, 3b, 3f–h) was found (Eq. (2)).
dC¼O ¼ ðꢁ1:32 ꢂ 7:61ÞdOMe þ 636:98
dC¼O ¼ ðꢁ1:37 ꢂ 7:51ÞdOMe þ 641:15
ð3aꢁhÞ
(1)
ð3aꢁb; 3fꢁhÞ
(2)

H.G. Bonacorso et al. / Journal of Fluorine Chemistry 126 (2005) 1396–1402
1399
Table 3
Selected structural parameters calculated by AM1 for 4(6)-alkyl-2-trifluoroacetyl-1-methoxycycloalkenes (3a–h)
No.
n
No. of ring atoms
R
C
C–O (8)
C
C–C O (8)
C
C–O–C (8)
Oxygen net charges
O/OCH3
Energy
(kcal molꢁ1
C
)
3a
3b
3c
3d
3e
3f
1
2
2
2
2
3
4
8
5
6
H
134.73
116.84
127.66
116.90
117.04
128.69
127.80
127.03
24.82
35.45
73.89
37.64
38.23
72.81
76.27
75.00
1.42
6.68
1.16
7.37
6.54
0.15
0.03
1.45
ꢁ0.222/ꢁ0.172
ꢁ0.222/ꢁ0.187
ꢁ0.219/ꢁ0.208
ꢁ0.229/ꢁ0.213
ꢁ0.218/ꢁ0240
ꢁ0.215/ꢁ0.245
ꢁ0.212/ꢁ0.223
ꢁ0.220/ꢁ0.225
ꢁ74832.26
ꢁ78428.15
ꢁ82018.24
ꢁ82.020.16
ꢁ92791.69
ꢁ82015.94
ꢁ85607.20
ꢁ99980.33
H
6
6-Me
4-Me
4-t-Bu
H
6
6
7
3g
3h
8
H
12
H
The predominant conformational structures of compounds
The solvent effect on the conformations of 3a–h could be
suggested as an important factor for disagreement between
the 17O NMR data obtained in chloroform solution and the
parameters obtained via AM1 calculations in gas phase.
However, in a recent publication [20], we have reported a
multi-linear regression analysis using the Kamlet-Abboud-
Taft (KAT) solvatochromic parameters in order to elucidate
and quantify the solvent effect on the 17O NMR chemical
shifts of two acyclic b-methoxyvinyl trichloromethyl
ketones, which presented the resonance in the conjugated
system equal and different of 100% (planar and non-planar
structures). The chemical shifts of carbonyl group of these
molecules showed similar and very low dependencies on the
solvent polarity-polarizability, solvent hydrogen-bond-
donor acidities (HBD) and hydrogen-bond-acceptor basi-
cities (HBA) and showed a negligible effect on the chemical
shifts of the methoxy group. It was observed that the
influence of the solvent is weakly considerable on the 17O
NMR chemical shifts when an acyclic b-methoxyvinyl
trichloromethyl ketone has dihedral angles C C–C O and
C C–O–Me equal zero when the resonance in the
conjugated system is ꢀ100% (planar) and slightly for the
non-planar structure. In the case related to the cyclic 3a–h,
the dihedral angles C C–C O and C C–O–Me are
different of zero and the resonance in the conjugated
system is not 100%. Thus, due to the low polarity-
polarizability and hydrogen-bond-donor acidity of the
chloroform-d1 (solvent of the NMR experiments), we
suggest that the solvent effect is not the main factor for
the disagreement between the 17O NMR data and parameters
obtained via AM1 calculations in gas phase.
3a–h were determined by energy minimization calculations
using AM1 semi-empirical method. The calculated AM1 data
listed in Table 3 show dihedral angles C C–C O and C C–
O–Me different of zero and because of this fact, the resonance
in the all conjugated system for 3a–h is not 100%. With the
exception of compound 3c (748), which present a vicinal
methyl substituent to the methoxy group, the compounds
derived from cyclopentanone or other substituted cyclohex-
anone show the carbonyl group between 25 and 388 out of
C C–OCH3 plane and the best resonance in this conjugated
system. For compound 3c and cyclic b-alkoxyvinyl
trifluoromethyl ketones derived from larger cycloalkanones
(3f–h), the carbonyl group is between 73 and 778 out of the
C C–OCH3 plane. TheAM1calculationsrevealsalsothatthe
dihedral angle of the C C–O–Me moiety varies from 0.038
for 3g and 7.378 for 3d, as well, that the s-cis-Z-s-cis is the
main conformation for compounds 3a, 3c–h while that the s-
trans-Z-s-cis is the preferred conformation for 3b, in gas
phase. Thus, in this case, the p-p conjugation is more efficient
for C C–O–Me moiety than for C C–C O moiety in the
molecules 3a–h (Scheme 3).
Unfortunately, quantitative relationships between the 17
O
NMR chemical shifts of the carbonyl oxygen and the dihedral
angles and the oxygen net charge (Table 1) of compounds 3a–
h show unsatisfactory correlations (r ꢃ 0.2000). However,
from our point of view, some other factor(s) must be involved
to explain the irregularities in the 17O NMR shift values and
the disagreement between the 17O NMR data and some
parameters deduced via AM1 calculations.
3. Conclusion
In summary, this work demonstrated a single and useful
method for the synthesis of new 2-trifluoroacetyl-1-
methoxycycloalkenes derived from large cycloalkanones
under mild conditions and good yields, as well as the 17O
Scheme 3. Conformers for the b-methoxyvinyl trifluoromethyl ketone
system 3a–h.

1400
H.G. Bonacorso et al. / Journal of Fluorine Chemistry 126 (2005) 1396–1402
NMR data for a series of eight new 4(6)-alkyl-2-
trifluoroacetyl-1-methoxycycloalkenes. The electron push–
pull phenomenon by the conjugation of the system O C–C–
C–O–Me could also be clearly observed and shows that there
is an extensive p-p conjugation for these new non-planar
molecules. Finally, possibly in additionto the s-cis ands-trans
conformers on the Me–O–C C moiety, varying amounts of
non-planar conformers (s-gauche) exist for compounds 3a–h
when in solution. Moreover, from our point of view, the
planarity or non-planarity of the C C–C O moiety, as well
20 Hz. The spectra were recorded with a RIDE (Ring Down
Eliminate) sequence for suppression of acoustic ringing [23–
25]. The general reproducibility of chemical shift data is
estimated to be better than ꢂ1.0 ppm (ꢂ0.2 within the same
series). The half-height widths were in the range 400–
900 Hz. All spectra were acquires in a 10 mm tube, at natural
abundance in chloroform-d1 as solvent. The concentration of
the compounds used in these experiments was 0.5 M and the
signal was referenced to external water (in a capillary
coaxial tube).
theCF3 groupisalsoresponsible forthevariationsonboth 17
O
NMR chemical shift values. Until the present moment, we
cannot manage to qualify or quantify some other factor(s),
which furnished unsatisfactory correlations involving 17O
NMR data and dihedral angles (conformational analysis) or
the oxygen net charge calculated by AM1 semi-empirical
method.
4.3. Compounds
All ketone dimethyl acetals were synthesized by similar
procedure described in the literature [1] and purified by
distillation under reduced pressure.
The b-methoxyvinyl trifluoromethyl ketones 3a–e
derived from cyclohexanones (1a–d) and cyclopentanone
(1e) were prepared previously [1a,1d]. Herein, the new b-
methoxyvinyl trifluoromethyl ketones 3f–h derived from
cycloheptanone (1f), cyclooctanone (1g) and cyclododeca-
none (1h) were synthesized from the reaction of the
respective 1,1-dimethylcycloalkanone acetals with trifluor-
oacetic anhydride by similar procedure described in the
literature [1d] furnishing acceptable yields.
4. Experimental
Unless otherwise indicated, all common reagents and
solvents were used as obtained from commercial suppliers
without further purification. Mass spectra were registered in
a HP 6890 GC connected to a HP 5973 MSD and interfaced
by a Pentium PC. The GC was equipped with a split–
splitless injector, autosampler, cross-linked HP-5 capillary
column (30 m, 0.32 mm of internal diameter), and helium
was used as the carrier gas.
4.4. 2-Trifluoroacetyl-1-methoxycyclopentene (3a)
Yellow oil; yield 65%; bp 49–50 8C/3.2 mbar. [Ref. 1(a)]:
yield 70%; bp 97–99 8C/10 mbar.
4.1. Semi-empirical molecular orbital calculations
13C NMR (CDCl3) d = 177.8 (C-1); 175.8 (C O,
2JCF = 36.0 Hz); 116.8 (CF3, JCF = 290 Hz); 108.1 (C-2);
1
The MO calculations were carried out by the Austin
Model 1 (AM1) semi-empirical method [21], implemented
in the Hyperchem 4.5 Package (1999) [22]. Geometries were
completely optimized without fixing any parameter, thus
bringing all geometric variables to their equilibrium values.
The energy minimization protocol employs the Polak–
Ribiere algorithm, a conjugated gradient method [22].
Convergence to a local minimum is achieved when the
energy gradient is <0.01 kcal molꢁ1. The calculations were
performed on a PC Pentium-IV 2.4 GHz Dell.
58.6 (OCH3); 31.8 (CH2); 28.5 (CH2); 19.4 (CH2). MS [m/z
(%)] for C8H9F3O2 (194.15): 194 (M+,17), 125 (100), 95 (7),
67 (22).
4.5. 2-Trifluoroacetyl-1-methoxycyclohexene (3b)
Yellow oil; yield 65%; bp 44–46 8C/3.0 mbar. [Ref. 1(a)]:
yield 65%; bp 106–109 /10 mbar.
13C NMR (CDCl3) d = 181.4 (C O, 2JCF = 35 Hz); 170.0
1
(C-1); 116.8 (CF3, JCF = 289 Hz); 110.4 (C-2); 54.8
(OCH3); 26.0 (CH2); 24.0 (CH2); 22.0 (CH2); 21.6 (CH2).
MS [m/z (%)] for C9H11F3O2 (208.18): 208 (M+, 42), 139
(100), 79 (66), 69 (31).
4.2. NMR spectroscopy
1H and 13C NMR spectra were acquired on a Bruker
DPX 400 spectrometer (1H at 400.13 MHz and 13C at
100.62 MHz), 5 mm sample tubes, 298 K, digital resolu-
tion ꢂ0.01 ppm, in CDCl3 and using TMS as internal
reference.
17O NMR spectra were acquired on a Bruker DPX 400
spectrometer at 54.24 MHz. The sample temperature was set
at 300 ꢂ 1 K. The instrumental settings were as follows:
spectral width 38 kHz (705 ppm), 8 K data points, pulse
width 12 ms, acquisition time 54 ms, peak acquisition delay
10 ms, 16,000–90,000 scans. LB of 100 Hz, sample spinning
4.6. 6-Methyl-2-trifluoroacetyl-1-methoxycyclohexene
(3c)
Yellow oil; yield 58%; bp 80–82 8C/4.0 mbar. [Ref. 1(d)]:
yield 60%; bp 79–81 8C/3.5 mbar. 13C NMR (CDCl3)
2
d = 183.0 (C O, JCF = 37 Hz); 172.7 (C-1); 116.5 (CF3,
1JCF = 289 Hz); 112.0 (C-2); 55.4 (OCH3); 30.0 (CH2); 29.0
(CH2); 24.4 (CH2); 18.0 (CH2); 17.5 (CH3). MS [m/z (%)]
for C10H13F3O2 (222.20): 222 (M+, 24), 153 (100), 93 (33),
69 (15).

H.G. Bonacorso et al. / Journal of Fluorine Chemistry 126 (2005) 1396–1402
1401
1
4.7. 4-Methyl-2-trifluoroacetyl-1-methoxycyclohexene
(3d)
(C-2); 116.7 (CF3, JCF = 292 Hz); 56.3 (OCH3); 33.8
(CH2); 31.4 (CH2); 28.9 (CH2); 26.8 (CH2); 22.4 (CH2); 13.8
(CH2) MS [m/z (%)] for C11H15F3O2 (236.23): 236 (M+, 19),
167 (100), 79 (86), 69 (35).
Yellow oil; yield 61%; bp 87–89 8C/3.0 mbar. [Ref. 1(d)]:
yield 60%; bp 89–92 8C/3.4 mbar.
13C NMR (CDCl3) d = 181.5 (C O, 2JCF = 35 Hz); 170.0
4.12. 2-Trifluoroacetyl-1-methoxycyclododecene (3h)
1
(C-1); 116.9 (CF3, JCF = 290 Hz); 110.0 (C-2); 54.8
(OCH3); 32.0 (CH2); 30.0 (CH2); 27.9 (CH2); 25.9 (CH2);
20.8 (CH3). MS [m/z (%)] for C10H13F3O2 (222.20): 222
(M+, 8), 153 (100), 69 (9).
Yellow oil; yield 60%; bp 118–120 8C/2.4 mbar.
1H NMR (CDCl3) d = 3.98 (s, 3H, OCH3); 2.50–2.44 (m,
2H, CH2); 1.87–1.72 (m, 4H, CH2); 1.3 (m, 14H, CH2). 13
C
NMR (CDCl3) d = 187.8 (C O, 2JCF = 36 Hz); 178.8 (C-1);
1
109.5 (C-2); 119.2 (CF3, JCF = 276 Hz); 60.0 (OCH3);
4.8. 4-(1,1-Dimethyethyl)-2-trifluoroacetyl-1-
methoxycyclohexene (3e)
32.4–23.0 (CH2). MS [m/z (%)] for C15H23F3O2 (292.34):
292 (M+,28), 223 (42), 111 (96), 69 (48), 55 (100).
Yellow oil; yield 57%; bp 90–92 8C/2.4 mbar. [Ref. 1(d)]:
yield 60%; bp 84–86 8C/1.5 mbar.
Acknowledgments
13C NMR (CDCl3) d = 181.8 (C O, 2JCF = 35 Hz); 170.3
1
(C-1); 117.0 (CF3, JCF = 290 Hz); 110.6 (C-2); 55.0
The authors are thankful for the financial support from
(OCH3); 43.8 (CH2); 32.3 (CH2); 27.5 (3CH3), 27.3 (Cquat);
25.8 (CH2); 23.6 (CH2). MS [m/z (%)] for C13H19F3O2
(264.28): 264 (M+, 21), 249 (66), 111 (50), 57 (100).
´
Conselho Nacional de Desenvolvimento Cientıfico e Tecno-
logico-CNPq. Fellowships from Coordenac¸a˜o de Aper-
´
´
feic¸oamento de Pessoal de Nıvel Superior-CAPES (M.B.
Costa, S.M. Silva and L. Pizzuti) are also acknowledged.
4.9. Synthesis of 2-trifluoroacetyl-1-
methoxycycloalkenes (3f–h)
References
To a stirred solution of dimethoxy acetals derived from
cycloalkanones (30 mmol) and pyridine (60 mmol, 4.8 g) in
chloroform (30 mL) kept at 0 8C (ice bath), trifluoroacetic
anhydride (60 mmol) was added dropwise. The mixture was
stirred for 16 h at 45 8C. The mixture was quenched and
extracted with 0.1 M hydrochloric acid solution
(3 mL ꢄ 15 mL) and after with water (1 mL ꢄ 15 mL).
The organic layer was dried with magnesium sulfate and
filtered. The solvent was evaporated and the products were
obtained in high purity by distillation under reduced
pressure.
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