Stereoselective access to trisubstituted cyclopentanols from chiral unsaturated oxo esters by ketyl radical cyclization

Article abstract of DOI:10.1002/ejoc.201301767

Substituted cyclopentanols with three contiguous stereocenters were prepared with impressive diastereocontrol through a samarium diiodide induced 5-exo-trig radical process. The chiral unsaturated oxo ester precursors were prepared with ee values up to 90 % from propionaldehyde through a two-step organocatalyzed aldolization and Wittig-Horner olefination reaction sequence. Copyright

Full text of DOI:10.1002/ejoc.201301767

FULL PAPER
DOI: 10.1002/ejoc.201301767
Stereoselective Access to Trisubstituted Cyclopentanols from Chiral
Unsaturated Oxo Esters by Ketyl Radical Cyclization
Sophie Feuillastre,[a] Béatrice Pelotier,[a] and Olivier Piva*[a]
Keywords: Radicals / Samarium / Organocatalysis / Michael addition / Cyclization
Substituted cyclopentanols with three contiguous stereo-
centers were prepared with impressive diastereocontrol
pared with ee values up to 90% from propionaldehyde
through a two-step organocatalyzed aldolization and Wittig–
through a samarium diiodide induced 5-exo-trig radical pro- Horner olefination reaction sequence.
cess. The chiral unsaturated oxo ester precursors were pre-
Introduction
Five-membered structures that contain one or more
hydroxy groups are common carbocyclic structures in na-
ture. A large number of methods have been devised to ac-
Scheme 1. SmI2-induced reductive cyclization of oxo esters (THF
cess them, some of which allow relative as well as absolute
control of the contiguous stereogenic centers.[1,2] Cycliza-
tions that involve ketyl radicals are especially appealing to
prepare cycloalkanols that are small to medium in size (see
Scheme 1). Among the different reagents that could be
used, samarium diiodide has been extensively considered
for this purpose,[3] as it avoids the use of stannyl derivatives,
which are especially problematic in terms of purification
and toxicity.[4] Additionally, a significant level of selectivity
is usually observed to control the relative configuration of
the two stereogenic centers that are created during this pro-
cess. For example, Enholm et al. achieved a highly ste-
reocontrolled cyclization of formyl α,β-unsaturated esters,
which are readily available from the chiral pool, into cyclo-
pentanols.[5] More recently, this strategy has been efficiently
extended to the total synthesis of different heterocycles such
as secosyrin 1,[6] marine prostanoids,[7] and spirane deriva-
tives.[8] Procter et al. also developed an elegant cascade rad-
ical process that was promoted by SmI2 for the synthesis
of (+)-pleuromutilin, which possesses significant antibiotic
activities.[9]
= tetrahydrofuran).
anticipated that the new stereogenic center created during
the first process could induce the stereocontrol of the two
additional centers that are formed during the cyclization
into the five-membered ring. The ester moiety, an effective
electron-withdrawing group, was deliberately chosen to
control the regioselectivity of the radical cyclization (5-exo-
trig vs. 6-endo-trig).[10] As previously noted, this chelating
group could play a major role in the control of the relative
configuration of the two newly created stereogenic cen-
ters.[3]
With the goal to develop a more general process toward
enantioenriched cyclopentanols and to avoid the use of the
chiral pool, we have investigated the combination of an or-
ganocatalyzed Michael addition and a ketyl radical cycliza-
tion to prepare chiral cyclopentanols (see Scheme 2). It was
Scheme 2. Our strategy toward the formation of chiral trisubsti-
tuted cyclopentanols.
Over the past decade, organocatalysis has reemerged as
a powerful method to create new stereogenic centers.[11,12]
Initially developed for intramolecular aldol condensations,
this domain has rapidly grown with numerous applications
to natural products synthesis. Organocatalyzed Michael ad-
ditions have also been widely used to prepare chiral 1,4-
dicarbonyl compounds, and a large number of organocata-
lysts have been designed for this purpose (see
Scheme 3).[13,14] Presently, (S)-proline (1) has been sur-
[a] Université Lyon 1 – Institut de Chimie et de Biochimie
Moléculaires et Supramoléculaires,
(ICBMS) – UMR CNRS 5246
43, Bd du 11 novembre 1918, 69622 Villeurbanne cedex, France
E-mail: piva@univ-lyon1.fr
http://surcoof.univ-lyon1.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301767.
Eur. J. Org. Chem. 2014, 1753–1759
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S. Feuillastre, B. Pelotier, O. Piva
FULL PAPER
passed by more efficient catalysts such as 2–6, which allow cess at the newly created stereocenter. The absolute configu-
for rapid conversion and afford higher enantioselectivities. ration of this new center was assigned on the basis of the
In particular, the organocatalyzed Michael addition of pro- chemical shifts in the NMR spectra of the two dia-
pionaldehyde to methyl vinyl ketone (MVK) has been stereomers, which were formed from the derivatization step,
studied in detail,[15] and the resulting 1,4-diketone has been and by comparison with the literature.
directly involved in the total synthesis of different chiral
Table 1. Enantioselective Michael addition of propionaldehyde to
enone 7a.
natural products.[16] Curiously, this process has been not ex-
tensively investigated with aryl vinyl ketones (R1 = Ar).
L* (mol-%)
T [°C]
Conditions
Conversion [%]
er[a]
3 (5)
4 (5)
4 (5)
5 (5)
6 (20)
0
0
40
0
neat
neat
EtOH (2 m)
neat
100
20
100
100
100
78:22
87:13
69:31
89:11
87:13
25
neat
1
[a] The enantiomeric ratio (er) was determined by H NMR spec-
troscopic analysis after derivatization, according to a procedure by
Gellman.[21]
Scheme 3. Organocatalyzed Michael addition in the presence of
proline derivatives (TMS = trimethylsilyl).
Organocatalyst 5 appeared to be the most efficient, and
we further explored the reaction with different Michael ac-
ceptors. As noted in Table 2, the reaction proceeded to give
reasonable yields with all the aryl ketones tested, with the
exception of the p-methoxy derivative. The presence of the
strong electron-donating group could easily explain the
poor reactivity of this substrate with regard to the Michael
addition. By using the NMR method described above, the
ee values were easily determined. In contrast to the highly
efficient condensation of propionaldehyde with MVK,
which provided enantioselectivities of up to 95%, the ee val-
ues observed from the reactions with the aryl ketones were
not higher than 78%.
Results and Discussion
1-p-Tolyl-2-propen-1-one (7a) was chosen as the model
substrate, and its reaction with propionaldehyde was
studied in presence of the various organocatalysts 3–6.
Among the proline derivatives at our disposal, we selected
those that have the OH group protected as a methyl or a
trimethylsilyl ether[17] to avoid the formation of a hemiketal
structure as a side product.[18] As previously reported,[14a,19]
catechol 8 was also added to the media to activate the
Michael acceptor and enhance the rate of the reaction (see
Table 1). The first attempt was performed under neat condi-
tions with 3 (5 mol-%) at 0 °C and was completed in 24 h.
Unfortunately, this resulted in moderate selectivity. When
the reaction was conducted with 4 as the catalyst, the ee
value notably increased to 74%, albeit the extent of conver-
sion decreased. The reaction was then conducted in the
presence of 4 in EtOH at 40 °C, and – although the reaction
went to complete conversion – the ee value decreased to
only 38%. Finally, the best results were obtained at 0 °C
with catalyst 5, for which both the conversion and ee value
reached 100 and 78%, respectively. Similar selectivities were
achieved by performing the Michael addition in the pres-
ence of chiral imidazolidinone 6,[20] of which, however, a
larger amount was introduced. For all these reactions, the
selectivities were determined by 1H NMR spectroscopic
analysis after derivatization of the aldehyde group as a chi-
ral imine, according to the method initially developed by
Gellman et al.[21] The formation of the imine was directly
Table 2. Enantioselective Michael addition of propionaldehyde to
vinyl ketones 7a–7g.
7
R1
9
Yield [5]
ee [%][a]
a
b
c
d
e
f
p-MeC6H4
Ph
p-BrC6H4
p-MeOC6H4
m-CF3C6H4
Me
a
b
c
d
e
f
94
73
78
68
74
72
70
90
78
95
48[b]
86
57[b,c]
67[b,c]
g
cyclohexyl
g
[a] The ee value was determined by 1H NMR spectroscopic analysis
(see ref.[16a]). [b] Incomplete conversion. [c] On the basis of reco-
vered starting material.
performed by the addition of (S)-methoxy-2-propylamine
to the NMR tube just before recording the H NMR spec- group was suspected to have a significant effect on the ee
The size of the substituent directly linked to the oxo
1
trum. These precautions minimized the epimerization pro- value. This hypothesis was confirmed when cyclohexyl de-
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Stereoselective Access to Trisubstituted Cyclopentanols
rivative 7g was treated with propionaldehyde in the presence and another diastereomer and product, which resulted from
of catalytic amounts of 5. After the same reaction time, the an elimination reaction, were also detected. It was antici-
conversion was not totally complete, and the selectivity was pated that tertiary benzylic alcohol 11d could easily deliver
only 78%, a ee value close to those observed from the reac- a highly stabilized carbocation that could be re-attacked on
tions with the aryl derivatives.
the opposite face by water.
Oxo aldehydes 9a–9f were submitted to a Wittig–Horner
To rationalize the formation of the major diastereomer,
reaction, according to the well-established Roush–Masa- we considered the different transition states for the cycliza-
mune conditions, to prevent epimerization of the new ste- tion of the ketyl radical that is generated by the selective
reogenic center.[22] To our delight, the condensation fur- reduction of the aryl ketone moiety of 10b (see Figure 1).
nished the corresponding α,β-unsaturated esters 10 with Moreover, it was anticipated that the configuration of the
complete (E) selectivity (see Table 3). Moreover, the absence stereogenic center at C-4 was fully maintained during the
of the epimerization of 10c was clearly demonstrated after entire process. Compared to other radical processes, the
performing a highly stereoselective reduction of the oxo high diastereocontrol in favor of anti,anti-11 could result
group under Corey–Bakshi–Shibata (CBS) conditions[23] from the chelation of the samarium atom to the two distal
and after derivatization of the alcohol group with (S)-2- oxygen atoms present in the substrate. This argument has
phenylacetic acid. The configurational stability of this all- been extensively considered in numerous radical cycliza-
ylic center was assumed for the other substrates synthetized tions that are promoted by SmI2. We assumed that the mo-
by using the same sequence.
lecule also adopts a chair-like six-membered ring as its pre-
dominant conformation, in which the configuration of the
double bond is maintained. On the basis of these hypothe-
ses, transition state I (see Figure 1) appears as the more
favorable among the four plausible ones. It is noteworthy
that I effectively leads to the stereoisomer that is mainly
observed.
Table 3. Formation of ω-oxo esters 10a–10f followed by radical cy-
clization.
10
R1
Yield [%][a]
11
Yield [%][a]
dr[b]
a
b
c
d
e
f
p-MeC6H4
Ph
p-BrC6H4
p-MeOC6H4
m-CF3C6H4
Me
81
78
74
62
76
73
a
b
c
d
e
f
61
80
82
58
67
50
85:15
Ͼ95:5
Ͼ95:5
65:35
Ͼ95:5
Ͼ95:5
[a] Yield of isolated compound. [b] The diastereomeric ratio (dr)
1
was determined by H NMR spectroscopic analysis.
Having different oxo esters at hand, we performed the
key diastereoselective radical cyclization. By analogy to re-
lated processes, the SmI2-induced cyclization was carried
out in THF at room temperature in the presence of tert-
butyl alcohol as the hydrogen donor.[24] The formation of
the five-membered rings nicely occurred regardless of the
nature of the alkyl or aryl group. Noteworthy, the reactions
mainly occurred with a high level of stereochemical control
with selectivities better than 90%, with the exception of the
reaction of 10d. The relative anti/anti configuration of the
three contiguous stereogenic centers for major stereoisomer
11 was determined in each case by NMR analysis, which
Figure 1. Four possible transition states for the diastereoselective
cyclization of esters 10.
Conclusions
The combination of an organocatalyzed Michael ad-
included NOESY experiments. In contrast to the other sub- dition and a regioselective and stereoselective Wittig–
strates, the selectivity measured for 11d was disappointing, Horner olefination followed by a radical cyclization pro-
Eur. J. Org. Chem. 2014, 1753–1759
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S. Feuillastre, B. Pelotier, O. Piva
FULL PAPER
35.3, 24.9, 13.7 ppm. HRMS (ESI): calcd. for C13H16NaO3 [M +
Na]+ 243.0992; found 243.0981.
vides rapid access to cyclopentanol derivatives with three
contiguous stereogenic centers, moderate to high enantio-
selectivities, and very high diastereocontrol. Further studies
are in progress to apply this reaction sequence to a total
synthesis.
(R)-2-Methyl-5-oxo-5-[3-(trifluoromethyl)phenyl]pentanal (9e): The
reaction was carried out with propanal (151 mg). Flash column
chromatography (PE/EtOAc, 8:2) gave compound 9e (576 mg,
1
86%, 85:15 er) as a yellow oil. H NMR (300 MHz, CDCl3): δ =
9.66 (d, J = 1.7 Hz, 1 H), 8.19 (s, 1 H), 8.13 (d, J = 7.8 Hz, 1 H),
7.81 (d, J = 7.8 Hz, 1 H), 7.61 (t, J = 7.8 Hz, 1 H), 3.06 (ddd, J =
8.0, 6.5, 4.8 Hz, 2 H), 2.50 (m, 1 H), 2.15 (dtd, J = 14.3, 7.5, 6.5 Hz,
1 H), 1.86 (m, 1 H), 1.19 (d, J = 7.2 Hz, 3 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 204.5, 198.1, 137.3, 131.7, 131.3, 129.7,
129.5, 125.0, 122.0, 45.7, 35.8, 24.5, 13.8 ppm. 19F NMR
(282 MHz, CDCl3): δ = –63.3 ppm. HRMS (ESI): calcd. for
C13H13F3NaO2 [M + Na]+ 281.0760; found 281.0755.
Experimental Section
General Procedure for the Organocatalyzed Michael Addition:
Propanal (1 equiv.) and the corresponding enone derivative
(1.5 equiv.) were added at 0 °C to a mixture of (S)-2-[diphenyl(tri-
methylsilyloxy)methyl]pyrrolidine (5; 5 mol-%) and ethyl 3,4-di-
hydroxybenzoate (8; 20 mol-%) under nitrogen. The reaction mix-
ture was stirred at 0 °C for 24 h. The crude mixture was then puri-
fied by silica gel flash chromatography [petroleum ether (PE)/ethyl
acetate (EtOAc)]. The pure product was directly employed in the
next step. The enantiomeric excess value of the pure product was
(R)-2-Methyl-5-oxohexanal (9f): The reaction was carried out with
propanal (400 mg). Flash column chromatography (PE/EtOAc, 8:2)
gave compound 9f (297 mg, 57%, conversion 60%, 95:5 er) as a
colorless oil. 1H NMR (300 MHz, CDCl3): δ = 9.57 (d, J = 1.8 Hz,
1 H), 2.56–2.39 (m, 2 H), 2.35 (qdapp, J = 7.2 Hz, 1 H), 2.12 (s, 3
H), 1.93 (hexapp, J = 7.2 Hz, 1 H), 1.72–1.56 (m, 1 H), 1.09 (d, J
= 7.2 Hz, 3 H) ppm. The spectroscopic data are in agreement with
the literature.
1
determined by H NMR spectroscopic analysis using the Gellman
method with (S)-1-methoxy-2-propylamine as the chiral amine.[21]
(R)-2-Methyl-5-oxo-5-(p-tolyl)pentanal (9a): The reaction was car-
ried out with propanal (215 mg). Flash column chromatography
(PE/EtOAc, 7:3) gave compound 9a (710 mg, 94%, 89:11 er) as a
yellow oil. [α]2D0 = –1.6 (c = 1.0, CHCl3). 1H NMR (300 MHz,
CDCl3): δ = 9.64 (d, J = 1.8 Hz, 1 H), 7.83 (d, J = 8.0 Hz, 2 H),
7.23 (d, J = 8.0 Hz, 2 H), 2.98 (ddd, J = 8.0, 6.8, 2.8 Hz, 2 H), 2.46
(m, 1 H), 2.38 (s, 3 H), 2.12 (ddt, J = 14.3, 7.9, 6.8 Hz, 1 H), 1.81
(m, 1 H), 1.15 (d, J = 7.2 Hz, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 204.6, 199.0, 144.0, 134.3, 129.4, 128.2, 45.7, 35.5,
24.7, 21.7, 13.6 ppm. HRMS (ESI): calcd. for C13H16NaO2 [M +
Na]+ 227.1043; found 227.1038.
(R)-5-Cyclohexyl-2-methyl-5-oxopentanal (9g): The reaction was
carried out with propanal (110 mg). Flash column chromatography
(PE/EtOAc, 8:2) gave compound 9g (38 mg, 67%, conversion 15%,
89:11 er) as a yellow oil. 1H NMR (500 MHz, CDCl3): δ = 9.58 (d,
J = 1.9 Hz, 1 H), 2.51–2.44 (m, 2 H), 2.37–2.25 (m, 2 H), 1.93 (td,
J = 14.4, 7.5 Hz, 1 H), 1.85–1.72 (m, 4 H), 1.70–1.60 (m, 2 H),
1.35–1.14 (m, 5 H), 1.09 (d, J = 7.0 Hz, 3 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 213.4, 204.7, 51.0, 45.8, 37.5, 28.6, 25.9,
25.8, 24.2, 13.6 ppm. HRMS [chemical ionization (CI)]: calcd. for
C12H20NaO2 [M + Na]+ 197.1536; found 197.1535.
(R)-2-Methyl-5-oxo-5-phenylpentanal (9b): The reaction was carried
with propanal (115 mg). Flash column chromatography (PE/
EtOAc, 8:2) gave compound 9b [276 mg, 73%, 83:17 er (measured
at 5 °C at 500 MHz)] as a yellow oil. 1H NMR (300 MHz, CDCl3):
δ = 9.64 (d, J = 1.8 Hz, 1 H), 7.96–7.88 (m, 2 H), 7.54 (t, J =
7.3 Hz, 1 H), 7.48–7.38 (m, 2 H), 3.02 (ddd, J = 8.0, 6.8, 3.1 Hz, 2
H), 2.47 (m, 1 H), 2.13 (dtd, J = 14.3, 7.6, 6.8 Hz, 1 H), 1.82 (ddt,
J = 14.3, 8.0, 6.2 Hz, 1 H), 1.15 (d, J = 7.1 Hz, 3 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 204.7, 197.4, 139.4, 133.3, 128.8,
128.2, 45.8, 35.7, 24.8, 13.7 ppm. HRMS (ESI): calcd. for
C12H14NaO2 [M + Na]+ 213.0886; found 213.0881.
General Procedure for the Wittig–Horner Reaction: To a suspension
of dry LiCl (1.2 equiv.) in dry acetonitrile (0.1 m) at room tempera-
ture under nitrogen were added triethyl phosphonoacetate
(1.2 equiv.) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 1 equiv.).
The mixture was stirred for 15 min, and the aldehyde derivative
(1 equiv.) was then added. Upon completion of the reaction (ap-
proximately 4 h, monitored by TLC), the reaction was quenched
by the addition of a saturated solution of NH4Cl. The aqueous
layer was extracted with CH2Cl2 (5ϫ). The combined organic lay-
ers were dried with MgSO4 and then filtered, and the solvent was
removed in vacuo. The residue was purified by silica gel flash
chromatography (PE/EtOAc).
(R)-5-(4-Bromophenyl)-2-methyl-5-oxopentanal (9c): The reaction
was carried with propanal (105 mg). Flash column chromatography
(PE/EtOAc, 8:2) gave compound 9c (463 mg, 95%, 87:13 er) as a
Ethyl (2E,4R)-4-Methyl-7-oxo-7-(p-tolyl)hept-2-enoate (10a): The
reaction was carried out with compound 9a (710 mg). Flash col-
umn chromatography (PE/EtOAc, 75:25) gave compound 10a
(778 mg, 81%, 89:11 er) as a yellow oil. [α]2D0 = –4.7 (c = 1.0,
1
yellow oil. H NMR (300 MHz, CDCl3): δ = 9.63 (d, J = 1.7 Hz,
1 H), 7.79 (d, J = 8.6 Hz, 2 H), 7.57 (d, J = 8.6 Hz, 2 H), 2.97
(ddd, J = 8.1, 6.6, 4.5 Hz, 2 H), 2.45 (m, 1 H), 2.10 (dtd, J = 14.3,
7.8, 6.6 Hz, 1 H), 1.81 (m, 1 H), 1.15 (d, J = 7.2 Hz, 3 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 204.5, 198.4, 135.5, 132.0, 129.6,
128.4, 45.7, 35.6, 24.6, 13.7 ppm. HRMS (ESI): calcd. for
C12H13BrNaO2 [M + Na]+ 290.9991; found 290.9996.
1
CHCl3). H NMR (300 MHz, CDCl3): δ = 7.79 (d, J = 8.1 Hz, 2
H), 7.20 (d, J = 8.1 Hz, 2 H), 6.83 (dd, J = 15.7, 8.0 Hz, 1 H), 5.77
(dd, J = 15.7, 1.1 Hz, 1 H), 4.14 (q, J = 7.1 Hz, 2 H), 2.88 (dd, J
= 8.0, 7.1 Hz, 2 H), 2.40 (m, 1 H), 2.36 (s, 3 H), 1.90–1.66 (m, 2
H), 1.24 (t, J = 7.1 Hz, 3 H), 1.08 (d, J = 6.8 Hz, 3 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 199.3, 166.6, 153.4, 143.8, 134.4,
129.2, 128.1, 120.5, 60.2, 36.1, 35.9, 30.1, 21.6, 19.6, 14.3 ppm.
HRMS (ESI): calcd. for C17H22NaO3 [M + Na]+ 297.1461; found
297.1458.
(R)-5-(4-Methoxyphenyl)-2-methyl-5-oxopentanal (9d): The reaction
was carried out with propanal (76 mg). Flash column chromatog-
raphy (PE/EtOAc, 7:3) gave compound 9d (138 mg, 48%, 86:14 er)
as a yellow oil. 1H NMR (300 MHz, CDCl3): δ = 9.65 (d, J =
1.7 Hz, 1 H), 7.93 (d, J = 8.9 Hz, 2 H), 6.92 (d, J = 8.9 Hz, 2 H),
3.86 (s, 3 H), 2.97 (ddd, J = 8.0, 6.7, 3.1 Hz, 2 H), 2.48 (m, 1 H),
2.13 (dtd, J = 14.2, 7.5, 6.7 Hz, 1 H), 1.83 (ddt, J = 14.2, 8.0,
6.2 Hz, 1 H), 1.16 (d, J = 7.1 Hz, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 204.8, 198.1, 163.7, 130.4, 129.9, 113.9, 55.6, 45.9,
Ethyl (2E,4R)-4-Methyl-7-oxo-7-phenylhept-2-enoate (10b): The re-
action was carried out with compound 9b (193 mg). Flash column
chromatography (PE/EtOAc, 8:2) gave compound 10b (204 mg,
78%, 87:13 er) as a colorless oil. [α]2D0 = –20.3 (c = 0.8, CHCl3). 1H
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Stereoselective Access to Trisubstituted Cyclopentanols
NMR (300 MHz, CDCl3): δ = 7.93–7.90 (m, 2 H), 7.60–7.51 (m, 1
H), 7.47–7.40 (m, 2 H), 6.85 (dd, J = 8.1, 15.7 Hz, 1 H), 5.80 (dd,
J = 1.2, 15.7 Hz, 1 H), 4.16 (q, J = 7.1 Hz, 2 H), 2.93 (t, J = 7.4 Hz,
a purge-and-refill technique with vacuum and argon. SmI2 (0.1 m
in THF, 4 equiv.) was added at room temperature, and the reaction
mixture was stirred until completion (approximately 4 h, monitored
2 H), 2.41 (m, 1 H), 1.90–1.74 (m, 2 H), 1.27 (t, J = 7.1 Hz, 3 H), by TLC). The solvent was removed in vacuo, and a saturated solu-
1.10 (d, J = 6.8 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = tion of NH4Cl was added. The aqueous layer was extracted with
199.8, 166.8, 153.5, 137.0, 133.1, 128.7, 128.1, 120.7, 60.4, 36.3,
36.1, 30.2, 19.8, 14.4 ppm. HRMS (ESI): calcd. for C16H20NaO3: and then filtered, and the solvent was removed in vacuo. The resi-
[M + Na]+ 283.1305; found 283.1295.
due was purified by silica gel flash chromatography (PE/EtOAc).
Et2O (4ϫ). The combined organic layers were dried with MgSO4
Ethyl
(2E,4R)-7-(4-Bromophenyl)-4-methyl-7-oxohept-2-enoate
Ethyl 2-[(1S,2S,5R)-2-Hydroxy-5-methyl-2-(p-tolyl)cyclopentyl]-
(10c): The reaction was carried out with compound 9c (463 mg).
Flash column chromatography (PE/EtOAc, 7:3) gave compound
10c (437 mg, 74%, 87:13 er) as a yellow oil. [α]2D0 = –26.1 (c = 1.0,
acetate (11a): The reaction was carried out with compound 10a
(100 mg). Flash column chromatography (PE/EtOAc, 75:25) gave
compound 11a (61 mg, 61%, 85:15 dr) as a yellow oil. [α]2D0 = –10.2
1
(c = 0.6, CHCl ). IR: ν
= 3472, 2952, 2869, 1710, 1206 cm–1.
max
CHCl3). H NMR (300 MHz, CDCl3): δ = 7.78 (d, J = 8.5 Hz, 2
˜
3
1H NMR (500 MHz, CDCl3): δ = 7.21 (d, J = 8.2 Hz, 2 H), 7.11
(d, J = 8.2 Hz, 2 H), 3.98 (q, J = 7.1 Hz, 2 H), 2.32 (s, 3 H), 2.24–
2.09 (m, 4 H), 2.01 (dddd, J = 12.5, 8.1, 6.8, 3.0 Hz, 1 H), 1.83
(dd, J = 17.1, 10.3 Hz, 1 H), 1.77 (dt, J = 10.0, 6.4 Hz, 1 H), 1.67
(dq, J = 12.5, 10.0 Hz, 1 H), 1.13 (t, J = 7.1 Hz, 3 H), 1.06 (d, J =
6.4 Hz, 3 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 175.0, 143.4,
136.2, 126.6, 125.9, 84.6, 60.8, 56.6, 41.8, 40.0, 34.5, 32.8, 21.1,
19.3, 14.2 ppm. HRMS (ESI): calcd. for C17H24NaO3 [M + Na]+
299.1618; found 299.1615.
H), 7.58 (d, J = 8.5 Hz, 2 H), 6.83 (dd, J = 15.7, 8.1 Hz, 1 H), 5.80
(dd, J = 15.7, 1.2 Hz, 1 H), 4.17 (q, J = 7.1 Hz, 2 H), 2.90 (ddd, J
= 7.9, 7.0, 1.0 Hz, 2 H), 2.40 (m, 1 H), 1.94–1.70 (m, 2 H), 1.28 (t,
J = 7.1 Hz, 3 H), 1.11 (d, J = 6.8 Hz, 3 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 198.7, 166.7, 153.3, 135.7, 132.0, 129.6,
128.3, 120.8, 60.4, 36.2, 36.1, 30.0, 19.8, 14.4 ppm. HRMS (ESI):
calcd. for C16H19BrNaO3 [M + Na]+ 361.0410; found 361.0422.
Ethyl
(2E,4R)-7-(4-Methoxyphenyl)-4-methyl-7-oxohept-2-enoate
(10d): The reaction was carried out with compound 9d (138 mg).
Flash column chromatography (PE/EtOAc, 75:25) gave compound
10d (112 mg, 62%, 86:14 er) as a yellow oil. [α]2D0 = –15.9 (c = 1.1,
Ethyl 2-[(1S,2S,5R)-2-Hydroxy-5-methyl-2-phenylcyclopentyl]acet-
ate (11b): The reaction was carried out with compound 10b
(80 mg). Flash column chromatography (PE/EtOAc, 8:2) gave com-
1
CHCl3). H NMR (300 MHz, CDCl3): δ = 7.91 (d, J = 8.9 Hz, 2
pound 11b (65 mg, 80%, Ͼ95:5 dr) as a yellow oil. [α]2D0 = –49.2 (c
H), 6.92 (d, J = 8.9 Hz, 2 H), 6.86 (dd, J = 15.7, 8.0 Hz, 1 H), 5.80
(dd, J = 15.7, 1.2 Hz, 1 H), 4.18 (q, J = 7.1 Hz, 2 H), 3.86 (s, 3 H),
2.89 (ddd, J = 7.9, 7.1, 1.0 Hz, 2 H), 2.41 (m, 1 H), 1.93–1.70 (m,
2 H), 1.28 (t, J = 7.1 Hz, 3 H), 1.11 (d, J = 6.8 Hz, 3 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 198.4, 166.8, 163.6, 153.6, 130.4,
130.1, 120.6, 113.8, 60.4, 55.6, 36.3, 35.8, 30.4, 19.8, 14.4 ppm.
HRMS (ESI): calcd. for C17H22NaO4 [M + Na]+ 313.1410; found
313.1404.
1
= 0.4, CHCl ). IR: ν
= 3470, 2953, 1709, 1206 cm–1. H NMR
˜
3
max
(500 MHz, CDCl3): δ = 7.36–7.28 (m, 5 H), 3.97 (qd, J = 7.1,
1.7 Hz, 2 H), 2.27–2.11 (m, 4 H), 2.02 (dtd, J = 12.1, 7.4, 3.0 Hz,
1 H), 1.84–1.79 (m, 2 H), 1.69 (dq, J = 12.1, 10.7 Hz, 1 H), 1.12
(t, J = 7.1 Hz, 3 H), 1.06 (d, J = 6.4 Hz, 3 H) ppm. 13C NMR
(125 MHz, CDCl3): δ = 175.0, 146.5, 127.9, 126.7, 126.0, 84.6, 60.8,
56.8, 41.9, 40.1, 34.7, 32.9, 19.2, 14.2 ppm. HRMS (ESI): calcd. for
C16H22NaO3 [M + Na]+ 285.1461; found 285.1459.
Ethyl (2E,4R)-4-Methyl-7-oxo-7-[3-(trifluoromethyl)phenyl]hept-2-
enoate (10e): The reaction was carried out with compound 9e
(559 mg). Flash column chromatography (PE/EtOAc, 75:25) gave
compound 10e (538 mg, 76%, 85:15 er) as a yellow oil. [α]2D0 = –2.5
Ethyl 2-[(1S,2S,5R)-2-(4-Bromophenyl)-2-hydroxy-5-methylcyclo-
pentyl]acetate (11c): The reaction was carried out with compound
10c (100 mg). Flash column chromatography (PE/EtOAc, 8:2) gave
compound 11c (83 mg, 82%, Ͼ95:5 dr) as a yellow oil. [α]2D0 = –65.3
1
(c = 1.1, CHCl3). H NMR (300 MHz, CDCl3): δ = 8.17 (s, 1 H),
(c = 0.6, CHCl ). IR: νmax = 3457, 2954, 1708, 1207 cm–1. 1H NMR
8.10 (d, J = 7.8 Hz, 1 H), 7.80 (d, J = 7.8 Hz, 1 H), 7.59 (t, J =
7.8 Hz, 1 H), 6.84 (dd, J = 15.7, 8.2 Hz, 1 H), 5.81 (dd, J = 15.7,
1.2 Hz, 1 H), 4.17 (q, J = 7.1 Hz, 2 H), 2.97 (t, J = 7.4 Hz, 2 H),
2.42 (m, 1 H), 1.96–1.72 (m, 2 H), 1.27 (t, J = 7.1 Hz, 3 H), 1.12
(d, J = 6.8 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 198.3,
166.7, 153.2, 137.5, 131.6, 131.2, 129.6, 129.4, 124.9, 123.6, 120.9,
60.4, 36.20, 36.16, 29.9, 19.8, 14.3 ppm. 19F NMR (282 MHz,
CDCl3): δ = –63.3 ppm. HRMS (ESI): calcd. for C17H19F3NaO3
[M + Na]+ 351.1179; found 351.1183.
˜
3
(500 MHz, CDCl3): δ = 7.42 (d, J = 8.7 Hz, 2 H), 7.21 (d, J =
8.7 Hz, 2 H), 4.20 (br. s, 1 H), 3.98 (q, J = 7.1 Hz, 2 H), 2.28 (dd,
J = 17.6, 4.2 Hz, 1 H), 2.18–2.10 (m, 3 H), 2.00 (tdd, J = 12.4, 7.4,
3.3 Hz, 1 H), 1.79 (dd, J = 17.6, 11.0 Hz, 1 H), 1.79–1.71 (m, 1 H),
1.67 (dq, J = 12.4, 9.6 Hz, 1 H), 1.14 (t, J = 7.1 Hz, 3 H), 1.05 (d,
J = 6.3 Hz, 3 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 175.0,
145.8, 130.9, 127.9, 120.6, 84.4, 61.0, 56.8, 42.1, 40.2, 34.5, 32.8,
19.1, 14.1 ppm. HRMS (ESI): calcd. for C16H21BrNaO3 [M +
Na]+ 363.0564; found 363.0566.
Ethyl (2E,4R)-4-Methyl-7-oxooct-2-enoate (10f): The reaction was
carried out with compound 9f (290 mg). Flash column chromatog-
raphy (PE/EtOAc, 8:2) gave compound 10f (327 mg, 73%, 95:5 er)
as a yellow oil. [α]2D0 = –31.5 (c = 1.0, CHCl3). 1H NMR (300 MHz,
CDCl3): δ = 6.78 (dd, J = 15.7, 8.2 Hz, 1 H), 5.77 (dd, J = 15.7,
1.0 Hz, 1 H), 4.18 (q, J = 7.1 Hz, 2 H), 2.40 (t, J = 7.4 Hz, 2 H),
2.30 (dt, J = 13.8, 6.9 Hz, 1 H), 2.12 (s, 3 H), 1.77–1.51 (m, 2 H),
1.28 (t, J = 7.1 Hz, 3 H), 1.05 (d, J = 6.9 Hz, 3 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 208.4, 166.8, 153.4, 120.6, 60.4, 41.2, 36.1,
30.2, 29.6, 19.7, 14.4 ppm. HRMS (ESI): calcd. for C11H18NaO3
[M + Na]+ 221.1148; found 221.1147.
Ethyl
2-[(1S,2S,5R)-2-Hydroxy-2-(4-methoxyphenyl)-5-methylcy-
clopentyl]acetate (11d): The reaction was carried with compound
10d (105 mg). Flash column chromatography (PE/EtOAc, 8:2) gave
compound 11d (62 mg, 58%, 65:35 dr) as a yellow oil. [α]2D0 = –32.1
(c = 0.2, CHCl ). IR: ν
= 2491, 2954, 1729, 1511, 1246 cm–1.
max
˜
3
1H NMR (500 MHz, CDCl3): δ = 7.17 (d, J = 8.9 Hz, 2 H), 6.76
(d, J = 8.9 Hz, 2 H), 3.91 (qd, J = 7.1, 1.3 Hz, 2 H), 3.72 (s, 3 H),
2.15–2.02 (m, 4 H), 1.92 (dddd, J = 12.4, 10.3, 7.4, 2.9 Hz, 1 H),
1.75 (dd, J = 16.9, 10.0 Hz, 1 H), 1.71–1.64 (m, 1 H), 1.58 (dq, J
= 12.4, 9.8 Hz, 1 H), 1.07 (t, J = 7.1 Hz, 3 H), 0.98 (d, J = 6.4 Hz,
3 H) ppm. 13C NMR (125 MHz, CDCl3): δ = 175.0, 158.3, 138.4,
127.1, 113.2, 84.4, 60.8, 56.5, 55.3, 41.6, 39.9, 34.9, 32.6, 19.3,
14.1 ppm. HRMS (ESI): calcd. for C17H24NaO4 [M + Na]+
315.1567; found 315.1556.
General Procedure for the Radical Cyclization: To a solution of the
corresponding Wittig derivative (1 equiv.) in dry THF (0.01 m) was
added tert-butyl alcohol (5 equiv.). Argon was bubbled through the
solution, and then the reaction mixture was carefully degassed by
Eur. J. Org. Chem. 2014, 1753–1759
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1757

S. Feuillastre, B. Pelotier, O. Piva
FULL PAPER
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2-{(1S,2S,5R)-2-Hydroxy-5-methyl-2-[3-(trifluoromethyl)-
phenyl]cyclopentyl}acetate (11e): The reaction was carried out with
compound 10e (100 mg). Flash column chromatography (PE/
EtOAc, 8:2) gave compound 11e (67 mg, 67%, Ͼ95:5 dr) as a pink
oil. [α]2D0 = –4.2 (c = 0.6, CHCl ). IR: ν
= 3456, 2957, 1708,
˜
3
max
1327, 1121 cm–1. H NMR (500 MHz, CDCl3): δ = 7.63 (s, 1 H),
7.49 (m, 2 H), 7.42 (t, J = 7.7 Hz, 1 H), 3.95 (q, J = 7.1 Hz, 2 H),
2.33 (dd, J = 17.8, 4.0 Hz, 1 H), 2.27–2.13 (m, 3 H), 2.04 (ddd, J
= 18.7, 6.4, 3.1 Hz, 1 H), 1.80 (tt, J = 11.0, 6.3 Hz, 1 H), 1.77–1.65
(m, 2 H), 1.09 (t, J = 7.1 Hz, 3 H), 1.12 (d, J = 6.3 Hz, 3 H) ppm.
13C NMR (125 MHz, CDCl3): δ = 174.9, 147.9, 130.2, 129.5, 128.3,
124.4, 123.5, 122.8, 84.2, 61.1, 57.0, 42.4, 40.3, 34.4, 32.9, 18.9,
14.0 ppm. 19F NMR (470 MHz, CDCl3): δ = –62.6 ppm. HRMS
(ESI): calcd. for C17H21F3NaO3 [M + Na]+ 353.1335; found
353.1337.
1
Ethyl
2-[(1S,2S,5R)-2-Hydroxy-2,5-dimethylcyclopentyl]acetate
[3]
(11f): The reaction was carried out with compound 10f (100 mg).
Flash column chromatography (PE/EtOAc, 7:3) gave compound
11f (50 mg, 50%, conversion 63%, Ͼ95:5 dr) as an orange oil.
[α]2D0 = –65.5 (c = 0.2, CHCl ). IR: ν
= 3443, 2953, 1714, 1205,
˜
3
max
1155 cm–1. 1H NMR (500 MHz, CDCl3): δ = 4.14 (qd, J = 7.1,
1.0 Hz, 2 H), 3.02 (br. s, 1 H), 2.45 (dd, J = 16.8, 4.5 Hz, 1 H),
2.30 (dd, J = 16.8, 10.5 Hz, 1 H), 1.86 (ddd, J = 13.0, 10.0, 5.6 Hz,
1 H), 1.81–1.70 (m, 2 H), 1.63 (ddd, J = 13.0, 8.2, 6.9 Hz, 1 H),
1.53–1.45 (m, 1 H), 1.40–1.31 (m, 1 H), 1.26 (t, J = 7.1 Hz, 3 H),
1.13 (s, 3 H), 0.99 (d, J = 6.6 Hz, 3 H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 175.1, 79.8, 61.0, 54.2, 40.3, 38.8, 33.9, 30.8, 24.7,
19.7, 14.3 ppm. HRMS (ESI): calcd. for C11H20NaO3 [M + Na]+
223.1305; found 223.1306.
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www.eurjoc.org
1759