Synthesis of an enantiopure 2-arylcyclohexanols from prochiral enol acetates by an enantioselective protonation/diastereoselective reduction sequence

Article abstract of DOI:10.1016/j.tetasy.2003.09.002

The enantioselective protonation with 2-sulfinyl alcohols of lithium enolates of 2-arylcyclohexanones with different substituents on the phenyl group takes place with excellent enantioselectivities (89-99%). Chiral 2-phenylcyclohexanone and 2-arylcyclohexanones carrying electron donor substituents on the aromatic ring are converted into the corresponding trans-2-arylcyclohexanols by diastereoselective reduction with sodium naphthalenide in the presence of acetamide. The stereochemical integrity of the tertiary stereocenter is fully preserved using this reduction procedure. Interestingly, the chiral proton source is not consumed in the synthesis.

Full text of DOI:10.1016/j.tetasy.2003.09.002

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 3851–3855
Synthesis of an enantiopure 2-arylcyclohexanols from prochiral
enol acetates by an enantioselective protonation/diastereoselective
reduction sequence
Gregorio Asensio,* Ana Cuenca, Nuria Rodriguez and Mercedes Medio-Simo´n
Departamento de Quimica Organica, Universidad de Valencia, Avda Vicent Andres Estelles 46100, Burjassot, Spain
Received 15 August 2003; accepted 1 September 2003
Abstract—The enantioselective protonation with 2-sulfinyl alcohols of lithium enolates of 2-arylcyclohexanones with different
substituents on the phenyl group takes place with excellent enantioselectivities (89–99%). Chiral 2-phenylcyclohexanone and
2-arylcyclohexanones carrying electron donor substituents on the aromatic ring are converted into the corresponding trans-2-aryl-
cyclohexanols by diastereoselective reduction with sodium naphthalenide in the presence of acetamide. The stereochemical
integrity of the tertiary stereocenter is fully preserved using this reduction procedure. Interestingly, the chiral proton source is not
consumed in the synthesis.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
Herein we report now the synthesis of chiral 2-arylcy-
clohexanones by enantioselective protonation of lithium
enolates with chiral sulfinyl alcohols. Our procedure
has the advantage of high enantioselectivity and the
easy preparation and accessibility of the reagents if
compared with those required for the synthesis of the
same compounds by methods previously reported. In
addition, we describe the efficient conversion of some
chiral 2-arylketones to the corresponding chiral trans-2-
arylcyclohexanols. Previously, we have reported an
efficient new approach for the preparation of chiral
trans-2-phenylcyclohexanol, based on the diastereose-
lective reduction of chiral 2-phenylcylohexanone with a
sodium naphthalene/acetamide mixture.8 Cyclohexanols
occupy among the chiral auxiliaries a special position
because of the versatility and high levels of stereocon-
trol they offer.9 Owing to their importance, several
different methods for the preparation of this kind of
compounds, including resolution of racemic material
(enzymatic10 and nonenzymatic11) and asymmetric reac-
tions, have been reported.12,13 Resolution methods
allow trans-2-arylcyclohexanols including the parent
compound trans-2-phenylcyclohexanol to be obtained
in practically enantiopure form. Concerning asymmet-
ric synthesis methods, several procedures are suitable to
Chiral ketones are important building blocks in asym-
metric synthesis. The preparation of chiral 2-alkyl
ketones with a tertiary stereogenic center at the a
position can be accomplished by enantioselective proto-
nation (formation of a CꢀH bond)1,2 or alkylation
(formation of CꢀC bond)3 of the corresponding lithium
enolates. However, the preparation of 2-aryl ketones
through the coupling of enolates with aryl halides cata-
lyzed by palladium (CꢀC bond formation) is more
difficult to conduct.4 Although the asymmetric version
of this reaction is known, it is restricted to the genera-
tion of a quaternary center.5 Thus, the reported meth-
ods
for
arylation
of
carbonyl
compounds
(carbonꢀarene bond formation) do not allow homochi-
ral carbonyl compounds with a tertiary stereogenic
center at the a position to be obtained. Then the
creation of a carbon-hydrogen bond by enantioselective
protonation seems to be the simplest alternative
methodology to achieve this kind of compound.
Mikami6 and Yamamoto7 reported by this approach
the preparation of 2-arylketones with similar good
enantioselectivities (82–94% ee), by mixing samarium
enolates/chiral diols and silyl enol eters/BINOL/SnCl4
respectively.
obtain
trans-2-phenylcyclohexanol
with
high
stereoselectivity12 but that is not the case for trans-2-
* Corresponding author. E-mail: gregorio.asensio@uv.es
arylcyclohexanols.13 Thus development of new asym-
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.09.002

3852
G. Asensio et al. / Tetrahedron: Asymmetry 14 (2003) 3851–3855
metric reactions leading to these products remains as an
interesting topic.
The results of the enantioselective protonation of
lithium enolates 3a–f are collected in Table 1. In a first
series of runs enolates 3a–c (Table 1, entries 1–3) were
submitted to protonation with sulfinyl alcohol 1a.
While ketone 4a was obtained with excellent enantiose-
lectivity, ketones 4b and 4c gave poor results. The
differences observed can be interpreted by considering
that the basicity of the three enolates is not the same
since it is related to the type of substituent present in
the phenyl group. The basicity is expected to increase in
the order 3a<3c<3b according to the electron donor
ability of the substituents (H<Me<OMe) at the para
position of the phenyl group. Then, in the protonation
of enolates 3a–c with sulfinyl alcohol 1a the enantiose-
lectivity decreases as the basicity of the enolate
increases (Table 1, entries 1, 2 and 3).
2. Results and discussion
In previous papers we have demonstrated that under
defined reaction conditions 2-sulfinyl alcohols 1 (R1:
CF3, CHF2, CH2F, Me, i-Pr, t-Bu) are efficient proton
sources for the enantioselective protonation of lithium
enolates. Sulfinyl alcohols 1a (R1: CF3) and 1b (R1:
i-Pr) provide the higher enantioselectivities.14,15 There-
fore we decided to explore the enantioselective protona-
tion of enolates of 2-aryl ketones with sulfinyl alcohols
1a and 1b (Scheme 1).
Table 1. Enantioselective protonation of enolates 4a–f
with 2-sulfinylalcohols 1a and 1b
Enol acetates 2a–f were used as precursors of the
corresponding lithium enolates 3a–f because their
advantages over the alternative silyl enol ether precur-
sors such as 2g. The main advantage is related with the
facility to obtain the desired thermodynamic enol
acetates 2a–f16,17 in practically regioisomerically pure
form (98–99%) from the corresponding racemic ketones
4a–f. On the contrary, in the preparation of silyl ethers
as 2g, a mixture of the kinetic and thermodynamic
regioisomers is formed and an additional isomerization
step is required to obtain the thermodynamic isomer in
pure form.7
Entry
l
4
Ar
Eea
Yield
Ketone
1
2
3
4
5
6
7
8
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1a
1b
1b
4a
4b
4c
4a
4b
4c
4d
4b
4e
4e
4f
Ph
99
14
43
99
89
99
99
85
52
80b
6
85
70
81
85
90
81
75
91
60
62
78
72
65
(S)-4a
(S)-4b
(S)-4c
(S)-4a
(S)-4b
(S)-4c
(S)-4a
(S)-4b
(S)-4f
(S)-4f
(S)-4e
(S)-4e
(S)-4e
4-OCH3-C6H4
4-CH3-C6H4
Ph
4-OCH3-C6H4
4-CH3-C6H4
4-Cl-C6H4
4-OCH3-C6H4
2-Naphthyl
2-Naphthyl
1-Naphthyl
1-Naphthyl
1-Naphthyl
9
10
11
12
13
Racemic ketones 4a–f were prepared in two steps by
reaction of cyclohexene oxide with the corresponding
aryl organometallic derivative according to procedures
described in the literature12,13 followed by oxidation of
the resulting racemic trans-2-aryl cyclohexanols [( )-5]
to the corresponding ketone.6
4f
4f
43
31c
a Reaction temperature −78°C otherwise noted.
b Reaction temperature −100°C.
c Reaction temperature −50°C.
Enolates 3a–f were generated by treatment of enol
acetates 2a–f with 2.2 equiv. of methyllithium as its
complex with lithium bromide. Subsequent enantiose-
lective protonation with sulfinylacohols 1a or 1b
afforded the corresponding non-racemic chiral ketones
4a–f (Scheme 1).
Next we attempted the enantioselective protonation of
3 using a less acidic sulfinyl alcohol such as 1b. In this
way we were able to obtain ketone 4b with a very good
ee (89%) (Table 1, entry 5), and ketones 4a, 4c and 4d
with the maximum enantiomeric excess (99% ee) (Table
1, entries 4, 6 and 7). Since enolate 3b is the most basic
in the series, we attempted, unsuccessfully, to improve
further the enantioselectivity in the case of ketone 4b by
conducting the protonation reaction at low temperature
(−100°C) (Table 1, entry 8). On the contrary, the proto-
nation of enolate 3e with sulfinyl alcohol 1b took place
with moderate enantiomeric excess (52%, Table 1, entry
9) when the reaction was performed at −78°C but the
enantioselectivity could be enhanced (80% ee, Table 1,
entry 10) by increasing the protonation temperature to
−50°C. These data along with that previously reported
by our group15 shows that sulfinyl alcohol 1b is an
efficient proton source for a wide range of lithium
ketone enolates. Unfortunately, the protonation of the
crowded enolate 3f containing the 1-naphthyl group
with sulfinyl alcohols 1a and 1b at different tempera-
tures took place with lower enantioselectivity in all the
cases most probably due to steric reasons (Table 1,
entries 11–13). It seems reasonable that a planar con-
formation of the enolate in the proton transfer step
would contribute to minimize the steric interactions
Scheme 1.

G. Asensio et al. / Tetrahedron: Asymmetry 14 (2003) 3851–3855
3853
Table 2. Reduction with sodium naphthalenide/acetamide
of ketones 4
between the enolate and the sulfinyl alcohol moieties.
Then, the presence of the bulky 1-naphthyl group in the
enolate is expected to have a negative influence on the
enantioselectivity of the protonation reaction. An
examination of the geometry of the enolate 3f reveals
that this anion would suffer severe steric strain in the
planar conformation, therefore a twist in the dihedral
angle between the enolate double bond and the plane of
the aryl ring decreases the steric hindrance in the
molecule at the expense of the planarity. Thus the low
enantioselectivity achieved in the preparation of ketone
4f can be explained in these terms.
Run
(−)-4
(−)-5
Yield (%)
Ee (%)
1
2
3
4
5
6
a
b
c
d
e
f
a
b
c
d
e
f
87
62
71
0a
0a
0a
99
90
97
–
–
–
a Only formation of radical coupling products could be observed.
Once the ketones 4a–e were prepared in practically
homochiral form, we planned their conversion in the
corresponding chiral trans-2-arylcyclohexanols. Usual
methods for the reduction of a-substituted ketones with
dialkylboranes or hydrides fail to afford stereoselec-
tively the desired trans-cyclohexanols without the con-
comitant formation of the cis-isomer. So, common
hydrides such as lithium aluminium hydride and
sodium borohydride reduce a-substituted cycloalka-
nones to give predominantly (90%), but not exclusively,
the more stable thermodynamically trans-cycloalkanol.
Hindered dialkylboranes give preferentially the less sta-
ble cis-isomer.18 On the other hand, the reduction of
a-substituted ketones by dissolving metals is quite
stereoselective leading to the formation of the corre-
sponding trans-alcohols.19 However, the strongly basic
medium in which these reactions occur precludes the
application of this simple methodology to the reduction
of chiral ketones. Recently other procedures have been
described in the literature to obtain the thermodynamic
alcohols by reduction of cyclic ketones. However,
higher temperatures (25°C or reflux) and prolonged
reaction times (5 days) are required.20 These methods
are based on the reduction of the ketone to give in a
first instance both isomeric alcohols and then, the less
stable alcohol is converted under thermodynamic con-
trol in the most stable isomer by equilibration through
a Meerwein–Pondorf–Verley type reduction. Obviously
these procedures are not suitable to the streocontrolled
reduction of enolizable chiral ketones. We have
described previously8 a selective reduction procedure
that allows the conversion of chiral 2-phenyl cyclohex-
anone 4a upon treatment with sodium naphthalenide in
the presence of acetamide as proton source into the
corresponding trans-alcohol 5a without racemization.
By our method, the diastereoselective reduction takes
place quickly (1 h) at low temperature (−78°C), without
racemization of the starting chiral ketone. Then, we
tried the reduction of the chiral ketones above synthe-
sized by this method.
the electron transfer process is not selective in these
ketones and it occurs at not only the carbonyl moiety
but also involving the CꢀCl bond (case of 4d) and the
aryl group. The coupling of the different radicals so
formed would give rise to the complex mixture of
products observed in the reduction of ketones 4d–f
(Table 2, entries 4–6, Scheme 2).21
Scheme 2.
3. Conclusions
The protonation of lithium enolates of 2-arylcyclohex-
anones with 2-sulfinyl alcohol 1b allows to obtain the
corresponding ketones with excellent (enolates 3a–d) to
good (enolate 3e) enantioselectivities. Thus, the enan-
tioselective protonation reaction is among the reported
procedures, the best method to obtain chiral 2-aryl
ketones with a a-tertiary stereogenic center.
The chiral cyclohexanones can be reduced diastereose-
lectively with the mixture sodium naphthalenide/acet-
amide to afford exclusively the corresponding
trans-arylcyclohexanols following the procedure previ-
ously described by us. The reduction procedure is
restricted to aryl ketones in which the aryl group is
phenyl or an activated aryl ring to avoid undesired
radical side reactions.
4. Experimental
4.1. General
Treatment of the cyclic chiral ketones 4a–c with sodium
naphthalenide/acetamide at −78°C, gave the thermody-
namic reduction products trans-5a–c without contami-
nation with the corresponding cis isomers and with
total retention of the configuration at the adjacent
tertiary stereogenic center (Scheme 2, Table 2, entries
1–3). However, this method could not be extended to
the reduction of ketones 4d–f. With these substrates, we
obtained complex mixtures of products. Apparently,
1H and 13C NMR spectra were recorded on a Bruker
AC-300 with tetramethylsilane as an internal reference
and CDCl3 as a solvent. Optical rotation measurements
were determined on a Perkin–Elmer 241 polarimeter at
room temperature. CG analysis was done with a Fisons
series 9000 using a capillary column BPX5 (0.25 mm×30
m). All experiments were carried out under an atmo-
sphere of dry argon.

3854
G. Asensio et al. / Tetrahedron: Asymmetry 14 (2003) 3851–3855
4.2. Materials
1.91 (m, 4H), 1.94 (s, 3H), 2.21–2.32 (m, 2H), 2.34–2.47
(m, 2H), 3.80 (s, 3H), 6.84 (dd, 2H, J1=8.8 Hz, J2=2.5
Hz), 7.16 (dd, 2H, J1=8.8 Hz, J2=2.5 Hz). 13C NMR
l 20.9 (q), 22.6 (t), 22.8 (t), 27.5 (t), 30.1 (t), 55.1 (q),
113.4 (d), 124.7 (s), 128.5 (d), 129.7 (s), 143.0 (s), 158.2
(s), 169.5 (s).
Methyllithium (1.5 M solution in diethyl ether, d=
0.852; 1.0 M in LiBr) was purchased from Aldrich. All
solvents were dried before use. Diethyl ether was dis-
tilled under argon from sodium-benzophenone and
dichloromethane from calcium hydride. Enol acetates
2a–f were obtained following described procedures.16,17
Racemic ketone 4a was purchased by Aldrich Chemical
Co. Ketones 4b–f6,7were prepared following described
procedures. Spectroscopic data for ketones 4b and 4c
were coincident with literature data.6 The absolute
configuration of the chiral ketones 4a–f6,7 and the alco-
hols 5a–c10 were determined by comparison of specific
rotations values with literature data. Enantiomeric
excesses were determined by 1H NMR of the corre-
sponding MTPA esters of the trans-cyclohexanols.
4.8. Enol acetate 2c
Yield: 82%. Regioisomeric purity 98%. 1H NMR l
1.69–1.91 (m, 4H), 1.95 (s, 3H), 2.21–2.33 (m, 2H), 2.34
(s, 3H), 2.37–2.48 (m, 2H), 7.12 (s, 4H). 13C NMR l
21.3 (q), 21.6 (t), 23.2 (t), 27.9 (t), 30.5 (t), 125.5 (s),
127.7 (d), 128.5 (d), 129.2 (d), 136.7 (s), 143.5 (s), 169.9
(s).
4.9. Enol acetate 2d
4.3. Generation of enolates 3a–f from enol acetates 2a–f
Yield 86%. Regioisomeric purity 99%. 1H NMR l
1.76–1.80 (m, 4H), 1.85 (s, 3H), 2.18–2.31 (m, 2H),
2.32–2.50 (m, 2H), 7.15 (d, 2H, J=8.0 Hz), 7.25 (d, 2H,
J=8.0 Hz). 13C NMR l 20.6 (q), 22.4 (t), 22.5 (t), 27.3
(t), 29.7 (t), 124.2 (s), 128.1 (d), 128.8 (d), 132.3 (s),
137.6 (s), 143.8 (s), 169.0 (s).
To a stirred solution of 2 (1.0 mmol) in diethyl ether (9
ml) at 0°C was added an ether solution of methyl-
lithium as complex with lithium bromide 1.5 M (2.2
mmol). The mixture was stirred at room temperature
for 30 min.
4.4. General procedure for enantioselective protonation
4.10. Enol acetate 2e
The corresponding lithium enolate solution (10 ml)
cooled at −75°C was slowly added in 7 min to a
solution of the appropriate sulfinyl alcohol (3mmol) in
dichloromethane (30 ml) at −78°C. The mixture was
stirred (1.5 h) at the same temperature and then gradu-
ally warmed up to −35°C (temperature increase approx-
imately 1.2°C/min). The reaction mixture was quenched
with NH4Cl and extracted with hexane. The residue
was purified by column chromatography to give the
corresponding chiral ketone (90–94% yield).
Yield 95%. Regioisomeric purity 99%. 1H NMR l
1.60–1.80 (m, 4H), 1.85 (s, 3H), 2.40–2.50 (m, 4H),
7.20–7.40 (m, 3H), 7.55 (s, 1H), 7.65–7.80 (m, 3H); 13C
NMR l 21.3 (q), 23.1 (t), 23.2 (t), 28.0 (t), 30.5 (t),
125.8 (s), 126.1 (d), 126.3 (d), 126.5 (d), 126.6 (d), 127.9
(d), 128.0 (d), 128.3 (d) 132.7 (s), 133.7 (s), 137.3
(s),.144.2 (s), 169.9 (s).
4.11. Enol acetate 2f
1
4.5. General procedure for diastereoselective reduction
Yield 95%. Regioisomeric purity 99% H NMR l 1.61
(s, 3H), 1.84–1.95 (m, 4H), 2.36–2.50 (m, 4H), 7.23 (dd,
1H, J1=6.5 Hz, J2=1.2 Hz), 7.39 (dd, 1H, J1=6.5 Hz,
J2=1.2 Hz), 7.44–7.48 (m, 2H), 7.73 (d, 1H, J=8.1
Hz), 7.82–7.90 (m, 2H). 13C NMR l 20.3 (q), 22.6 (t),
22.9 (t), 27.2 (t), 31.0 (t), 124.6 (d), 125.0 (d), 125.3 (d),
125.5 (d), 125.6 (d), 127.0 (d), 128.1 (d), 130.8 (s), 133.5
(s), 137.4 (s), 144.6 (s), 169.0 (s).
Small pieces of sodium (4.25×10−3 atm-gr) were added
to a solution of naphthalene (3.78 mmol) in THF (10
ml). After sonication for 3 h in an ice bath, the result-
ing dark green solution was diluted with THF (10 ml)
and cooled to −78°C, and then acetamide (3.02 mmol)
was added. A solution of (−)-4a–c (0.378 mmol) in
THF (7 ml) was finally added dropwise over a period of
90 min. The reaction was quenched with methanol (5
ml) and poured into phosphate buffer. Usual workup
gave trans-2-arylcyclohexanols 5.
4.12. Ketone ( )-4d
1
Yield 50%. Mp: 68–69°C. H NMR l 1.66–2.53 (m,
8H), 3.60 (dd, 1H, J1=11.5 Hz, J2=5 Hz), 7.07 (d, 2H,
J=7 Hz), 7.31 (d, 2H, J=7 Hz). 13C NMR l 25.2 (t),
27.6 (t), 35.1 (t), 42.0 (t), 56.6 (t), 128.3 (d), 129.8 (d),
132.4 (s), 137.1 (s), 209.7 (s).
4.6. Enol acetate 2a
Yield 88%. Regioisomeric purity 98%. 1H NMR l
1.77–1.88 (m, 7H), 2.21–2.32 (m, 2H), 2.34–2.44 (m,
2H), 7.15–7.73 (m, 5H). 13C NMR l 20.4 (q), 23.1 (t),
23.2 (t), 27.3 (t), 30.5 (t), 125.7 (s), 127.5 (d), 128.2 (d),
128.5 (d), 139.8 (s), 143.4 (s), 169.5 (s).
4.13. Ketone ( )-4f
1
Yield.: 72%. Mp: 86–87°C. H NMR l 1.63–1.87 (m,
4.7. Enol acetate 2b
2H), 1.88–2.01 (m, 1H), 2.06–2.32 (m, 3H), 2.39–2.60
(m, 2H), 4.20 (dd, 1H, J1=12.2 Hz, J2=5.25 Hz), 7.22
(d, 1H, J=7.2 Hz), 7.31–7.34 (m, 3H), 7.59–7.62 (m,
Yield: 80%. Regioisomeric purity 99%.1H NMR l 1.69–

G. Asensio et al. / Tetrahedron: Asymmetry 14 (2003) 3851–3855
3855
1H), 7.65 (d, 1H, J=8 Hz), 7.72–7.75 (m, 1H).13C
NMR l 25.7 (t), 27.8 (t), 34.1 (t), 42.5 (t), 53.2 (d),
123.2 (d), 125.1 (d), 125.2 (d), 125.3 (d), 125.7 (d), 127.5
(d), 128.9 (d), 131.7 (q), 133.7 (q), 135.2 (q), 209.9 (s).
3. Hagihara, A.; Kawasaki, H.; Manabe, K.; Koga, K. J.
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Financial support by the Spanish Direccio´n General de
Investigacio´n Cient´ıfica y Te´cnica (BQU2000-1426) and
the Generalitat Valenciana (GV CTIDIB 2002/239) is
gratefully acknowledged. N.R. thanks the Spanish Min-
isterio de Educacio´n y Ciencia and A.C. the Generalitat
Valenciana for fellowships. We gratefully acknowledge
the SCSIE (Universidad de Valencia) for the access to
the instrumental facilities.
8. Asensio, G.; Cuenca, A.; Gavin˜a, P.; Medio-Simo´n, M.
Tetrahedron Lett. 1999, 40, 3939.
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