Rh-catalyzed highly enantioselective formation of functionalized cyclopentanes and cyclopentanones

Article abstract of DOI:10.1039/b712396e

A catalyst system, [Rh(COD)Cl]₂-BINAP-AgSbF₆, has been developed as a second-generation catalyst for the cycloisomerization of 1,6-enynes tethered by carbon chains. Cyclopentanes and cyclopentanones, which can contain functional grou

Full text of DOI:10.1039/b712396e

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Rh-catalyzed highly enantioselective formation of functionalized
cyclopentanes and cyclopentanones†
Fei Liu,a Qiang Liu,a Minsheng He,b Xumu Zhang*a,b and Aiwen Lei*a,b
Received 13th August 2007, Accepted 17th September 2007
First published as an Advance Article on the web 27th September 2007
DOI: 10.1039/b712396e
A catalyst system, [Rh(COD)Cl]2–BINAP–AgSbF6, has been developed as a second-generation catalyst
for the cycloisomerization of 1,6-enynes tethered by carbon chains. Cyclopentanes and
cyclopentanones, which can contain functional groups, such as the 1,4-dienes, vinyl ether, aldehyde etc.,
were obtained from readily available starting materials in high yields and selectivities. Both
regioselectivities and enantioselectivities are excellent: only 1,4-dienes were observed and in over 99% ee.
BINAP–AgSbF6 was proven to be more efficient and conve-
Introduction
nient for the cycloisomerization to form lactones, lactams, and
tetrahydrofurans.31–35 Herein, we communicate our results on the
highly enantioselective formation of functionalized cyclopentanes
and cyclopentanones by employing a second-generation catalyst
in the cycloisomerization of 1,6-enynes with a carbon tether.
Cyclopentanes and cyclopentanones are amongst the most basic
skeletons of organic molecules. Because numerous natural prod-
ucts and biologically active molecules contain these motifs, func-
tionalized cyclopentanes and cyclopentanones are obviously valu-
able building blocks for the construction of complex molecules.
The syntheses of natural products such as prostaglandins and
jasmonates illustrate the significance of developing a highly
efficient methodology for the formation of cyclopentanes and
cyclopentanones.1
Without doubt, the transition metal-catalyzed Alder-ene type
cycloisomerization of enynes is an elegant method to form these
skeletons.2–8 Since pioneering work was done with Pd-catalysts,
which lead to 1,3- and 1,4-dienes,9,10 highly regioselective cycloi-
somerization of enynes forming 1,4-dienes, has been carried out
using Ti-,11 Ru-,12 and Rh-catalysts.13–15 The asymmetric version
of this transformation has remained relatively unexplored and the
development of efficient catalysts for this purpose still remains a
challenge.7,16–20 Efforts by the Trost lab in 19899 and 199410 were
carried-out using a Pd-catalyst, chiral ligands, and chiral auxiliary
groups. Recently, Mikami et al. reported the Pd-mediated cycloiso-
merization of enynes in excellent enantioselectivities.21–26 However,
its application in forming cyclopentanes and cyclopentanones in
high enantioselectivities is still not satisfactory.9,10 Fu¨rstner et al.
developed a beautiful cycloisomerization of enynes promoted
by an Fe-catalyst to form carbobicyclic compounds,27 while
producing the enantiomerically pure carbobicyclic compounds
remains a challenge. Very recently, Nicolaou et al. showed the
potential of the Rh-catalyzed enyne cycloisomerization in the
total syntheses of platensimycin, which includes racemic and
asymmetric versions.28,29
Results and discussion
The reaction of enyne 1a in the presence of the second-generation
catalyst was examined first. It was carried out in the presence of 5
mol% [Rh(COD)Cl]2, 11 mol% (S)-BINAP, and 20 mol% AgSbF6.
Although the corresponding desired product can be identified
via NMR and GC, a complicated mixture was produced, from
which it was difficult to isolate the product. However, the rapid
consumption of the starting material 1a (100% conversion after
2 min reaction time) shows the high reactivity with this catalyst. We
were then pleased to find that when substrate 1b was employed in
the cycloisomerization with the second-generation catalyst using
(S)-BINAP as the ligand, the desired product 2b was obtained
exclusively in over 99% ee (Scheme 1). No 1,3-diene regioisomer
was detected in this reaction.
Scheme
1
Rh(I)-catalyzed intramolecular Alder-ene reaction of
Previously, the [Rh(diphos)Cl]2–AgSbF6 system for the in-
tramolecular Alder-ene reaction of 1,6-enynes was reported.15,30
Two years later, the second-generation catalyst, [Rh(COD)Cl]2–
1,6-enynes.
Encouraged by the excellent regioselectivity and enantioselec-
tivity with the reaction of 1b shown in Scheme 1, we examined
the reaction with the substrate, allylic ether 3 (Scheme 2). A
versatile alkenyl ether will be the product if this reaction remains
regiospecific. Our result is shown in Scheme 2. The corresponding
alkenyl ether 4 was obtained in high yield and excellent ee value.
Our success in constructing cyclopentanes encouraged us to
explore the formation of cyclopentanones using a similar strategy.
aCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan,
430072, P. R. China. E-mail: aiwenlei@whu.edu.cn; Fax: +86 27 6875 4067;
Tel: +86 27 6875 4672
bDepartment of Department of Chemistry, The Pennsylvania State Univer-
sity, University Park, Pennsylvania, 16802, USA
† Electronic supplementary information (ESI) available: Chromatograms
and spectra of products. See DOI: 10.1039/b712396e
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Scheme 2 Formation of functionalized cyclopentanes.
First, enyne 5 was examined as a substrate, and the results are
shown in Scheme 3. The desired product, cyclopentanone 6, was
obtained in high yield with excellent enantioselectivity in the
presence of (S)-BINAP(Scheme 3).
Scheme 6 Synthesis of dihydrojasmonate (+)-(1S,2S)-13.
substrate in the Rh-catalyzed intramolecular Alder-ene reaction.
The aldehyde-substituted cyclopentanone 12 was formed in high
yield and over 99% ee.
Scheme 3 Formation of functionalized cyclopentanones.
The results detailed above promoted us to further explore the
formation of functionalized cyclopentanones. The cycloisomer-
ization of 7a and 7b yielded the corresponding vinyl methyl ethers
and benzyl ether substituted cyclopentanones 9a and 9b in high
yields with over 99% ee (Scheme 4).
To demonstrate the synthetic utility of the cycloisomerization
of enynes and identify the absolute configuration of the products,
methyl dihydrojasmonate 13 was chosen as one of our target
molecules. The synthesis started from (+)-12. After mild oxidation,
methylation with (CH3)3SiCHN2 [(trimethylsilyl)diazomethane,
2.0 M in hexane)] and reduction using [(PPh3)HCu]6 in benzene,
trans-(+)-(1S,2S)-13 was obtained in over 99% ee and high
diastereoselectivity (trans : cis >97 : 3) from (+)-12 in 62% overall
yield. The results also revealed the absolute configuration of (+)-
12: the (S)-configuration.36
Conclusion
In summary, a highly efficient Rh(I)-catalyzed intramolecular
Alder-ene type cycloisomerization reaction of 1,6-enynes tethered
by carbon chains was developed. Polyfunctionalized cyclopen-
tanes and cyclopentanones were obtained in good yields. The
regioselectivity is specific, and only 1,4-dienes were observed as the
products. Excellent enantioselectivities were obtained in employ-
ing the commercially available BINAP as the ligand. Syntheses
of more complex biologically active molecules and mechanistic
details are currently under investigation in our laboratory and the
results will be published in due course.
Scheme 4 Formation of polyfunctionalized cyclopentanones.
The results shown in Scheme 5 demonstrate our efforts in
introducing more functional groups at the alkyne site in order
to obtain a more flexible synthetic motif. The cyclopentanones,
10a and 10b, in which both side chains are functionalized, were
formed in high yields and over 99% ee (Scheme 5).
Experimental
General methods
All reactions were carried out in an inert atmosphere using
standard Schlenk techniques. 1H NMR and 13C NMR spectra were
recorded on Bruker DPX-300, DRX-400 and AMX-360 spec-
trophotometers. 1,2-Dichloroethane was purchased from com-
mercial sources (Aldrich) and used without further purification.
All the substrates were synthesized according to the literature.37
Scheme 5 Formation of polyfunctionalized cyclopentanones.
The results of further experimentation are shown in Scheme 6.
The unprotected allylic alcohol, 11, also served as a good
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General procedure for the asymmetric Alder-ene reaction catalyzed
by rhodium complexes
39.20, 31.87, 29.80, 27.81, 22.82, 14.33; MS m/z: 209.2 [M+ + 1];
HRMS (APCI) Calcd for C13H21O2 (M+ + 1): 209.1542; found:
209.1545.
In a dried Schlenk tube, [Rh(COD)Cl]2 (2.5 mg, 0.005 mmol)
and S-BINAP (6.9 mg, 0.011 mmol) were dissolved in freshly dis-
tilled 1,2-dichloroethane (1 mL), then freshly prepared substrate
(0.1 mmol) was added into the solution at room temperature
under nitrogen. After stirring for 1 min, AgSbF6 (0.02 mmol)
was added into the mixture. The reaction was run at room
temperature and followed by GC or TLC. After the reaction was
complete, the reaction mixture was directly subjected to column
chromatography.
(Z)-3-((E)-2-(Benzyloxy)vinyl)-2-pentylidenecyclopentanone (8b)
>99.5% ee, HPLC, OJ-H, Hex : Iso = 90 : 10, 254 nm, 1 ml min−1,
t1 = 7.48, t2 = 9.69; S-BINAP, [a]2D5 = +55.3, c = 1, CHCl3.
1H NMR (300 MHz, CDCl3) d 7.36–7.24 (m, 5H), 6.38 (d, J =
12.6 Hz, 1H), 5.78 (dt, J = 7.6, 2.5 Hz, 1H), 4.76 (s, 2H), 4.69 (dd,
J = 8.8, 12.6 Hz, 1H), 3.16–3.13 (m, 1H), 2.67–2.63 (m, 2H), 2.31–
2.01 (m, 3H), 1.59–1.51 (m, 1H), 1.35–1.28 (m, 4H), 0.86 (t, J =
6.3 Hz, 3H); 13C NMR (75 MHz, CDCl3) d 208.37, 147.80, 142.87,
139.15, 137.30, 128.92, 128.37, 127.93, 107.79, 71.75, 42.83, 39.22,
31.87, 29.67, 27.82, 22.82, 14.36; MS m/z: 285.2 [M+ + 1]; HRMS
(APCI) Calcd for C19H25O2 (M+ + 1): 285.1855; found: 285.1857.
(E)-Dimethyl 3-benzylidene-4-vinylcyclopentane-1,
1-dicarboxylate (2b)
>99.0% ee, HPLC, OJ, Hex (hexane) : Iso (isopropyl alcohol) =
97 : 3, 0.5 ml min−1, t1 = 25.2, t2 = 28.3; S-BINAP, [a]2D5 = +33.53,
c = 0.5, CHCl3.
(Z)-2-(2-(Methoxymethoxy)ethylidene)-3-((E)-2-
methoxyvinyl)cyclopentanone (10b)
1H NMR (360 MHz, CDCl3) d 7.38–7.23 (m, 5H), 6.24 (s, 1H),
5.73–5.68 (m, 1H), 5.21–5.17 (m, 2H), 3.77 (s, 6H), 3.46–3.21 (m,
3H), 2.68–2.62 (m, 1H), 2.04 (dd, J = 6.7, 13.3 Hz, 1H); 13C NMR
(90 MHz, CDCl3) d 172.46, 172.36, 143.60, 139.65, 138.08, 128.68,
126.82, 124.45, 117.25, 59.78, 53.33, 50.14, 40.14, 39.22; MS m/z:
301.1 [M+ + 1]; HRMS (APCI) Calcd for C18H21O4 (M+ + 1):
301.1440; found: 301.1434.
◦
>99% ee, GC, Chiralselect 1000, 180 C, 1.5 mL min−1, t1 =
28.402, t2 = 28.949; S-BINAP, [a]2D5 = +45.60, c = 1, CHCl3.
1H NMR (300 MHz, CDCl3) d 6.38 (d, J = 12.6 Hz, 1H), 5.98–
5.94 (m, 1H), 4.75–4.53 (m, 5H), 3.54 (s, 3H), 3.46 (s, 3H), 3.23–
3.11 (m, 1H), 2.33–2.18 (m, 3H), 1.65–1.58 (m, 1H); 13C NMR
(75 MHz, CDCl3) d 208.03, 149.59, 140.26, 138.14, 104.57, 96.80,
65.73, 56.51, 55.74, 42.32, 38.65, 30.03; MS m/z: 195.1 [M+
−
OCH3]; HRMS (APCI) Calcd for C11H15O3 (M+ + 1): 195.1021;
found: 195.1022.
(E)-Dimethyl 3-benzylidene-4-((E)-2-methoxyvinyl)-
cyclopentane-1,1-dicarboxylate (4)
>99% ee, HPLC, OJ-H, Hex : Iso = 90 : 10, 254 nm, 1 ml min−1,
t1 = 11.47, t2 = 13.58; S-BINAP, [a]2D5 = −15.13, c = 1, CHCl3.
1H NMR (400 MHz, CDCl3) d 7.28–7.12 (m, 5H), 6.34 (d, J =
12.6 Hz, 1H), 6.17–6.15 (m, 1H), 4.54 (dd, J = 12.6, 9.0 Hz,
1H), 3.67 (s, 6H), 3.51 (s, 3H), 3.33 (d, J = 18.6 Hz, 1H), 3.18–
3.12 (m, 2H), 2.54–2.49 (m, 1H), 1.87 (t, J = 6.3 Hz, 1H); 13C
NMR (100 MHz, CDCl3) d 172.57, 172.47, 149.52, 144.99, 138.15,
128.71, 128.64, 126.73, 123.93, 104.53, 59.43, 56.57, 53.31, 44.95,
41.60, 39.14; MS m/z: 331.2 [M+ + 1]; HRMS (APCI) Calcd for
C19H23O5 (M+ + 1): 331.1546; found: 331.1546.
(Z)-2-(2-Methoxyethylidene)-3-((E)-2-
methoxyvinyl)cyclopentanone (10a)
◦
>99% ee, GC, Chiralselect 1000, 150 C, 1.5 mL min−1, t1 =
16.728, t2 = 17.495; S-BINAP, [a]2D5 = +36.39, c = 1, CHCl3.
1H NMR (300 MHz, CDCl3) d 6.38 (d, J = 12.7 Hz, 1H), 5.97–
5.93 (m, 1H), 4.58–4.47 (m, 3H), 3.53 (s, 3H), 3.35 (s, 3H), 3.21–
3.14 (m, 1H), 2.33–2.14 (m, 3H), 1.64–1.57 (m, 1H); 13C NMR
(75 MHz, CDCl3) d 207.98, 149.36, 139.92, 138.38, 104.28, 70.23,
58.74, 56.24, 42.07, 38.46, 29.82; MS m/z: 197.1 [M+ + 1]; HRMS
(APCI) Calcd for C11H17O3 (M+ + 1): 197.1178; found: 197.1184.
(Z)-2-Benzylidene-3-((E)-prop-1-enyl)cyclopentanone (6)
(Z)-2-(3-Oxo-2-pentylidenecyclopentyl)acetaldehyde (12)
>99.9% ee, GC with Chiralselect 1000, 150 ◦C, 1.5 ml min−1, t1 =
98.83, t2 = 101.16; S-BINAP, [a]2D5 = −15.4, c = 0.5, CHCl3.
1H NMR (400 MHz, CDCl3) d 7.84–7.80 (m, 2H), 7.37–7.29
(m, 3H), 6.56 (s, 1H), 5.65–5.61 (m, 1H), 5.43–5.37 (m, 1H),
3.44–3.40 (m, 1H), 2.42–2.16 (m, 3H), 1.77–1.71 (m, 4H); 13C
NMR (100 MHz, CDCl3) d 208.96, 142.43, 140.86, 137.92, 136.23,
134.03, 133.82, 132.54, 131.16, 51.62, 42.22, 31.13, 21.20.
>99% ee, GC, gama 225, 1.5 ml min−1, 170 ◦C, t1 = 12.916, t2 =
13.226; S-BINAP, [a]D25 = +4.20, c = 1, CHCl3, R-BINAP, [a]2D5
−4.19, c = 1, CHCl3.
=
1H NMR (360 MHz, CDCl3) d 9.83 (s, 1H), 5.54 (dt, J = 2.3,
7.4 Hz, 1H), 3.22–3.20 (m, 1H), 2.70–2.49 (m, 4H), 2.34–2.16 (m,
3H), 1.57–1.47 (m, 1H), 1.39–1.20 (m, 4H), 0.87 (t, J = 7.1 Hz,
3H); 13C NMR (90 MHz, CDCl3) d 207.61, 138.05, 49.28, 38.74,
36.85, 31.85, 27.83,27.31, 22.76, 14.27; MS m/z: 195.1 [M+ + 1];
HRMS (APCI) Calcd for C12H19O2 (M+ + 1): 195.1385; found:
195.1395.
(Z)-3-((E)-2-Methoxyvinyl)-2-pentylidenecyclopentanone (8a)
>99% ee, GC, Chiralselect 1000, 150 ◦C, 1.5 ml min−1, t1 = 28.646,
t2 = 29.549; S-BINAP, [a]2D5 = +47.60, c = 1, CHCl3, R-BINAP,
[a]2D5 = −46.70, c = 1, CHCl3
(Z)-Methyl 2-(3-oxo-2-pentylidenecyclopentyl)acetate
1H NMR (400 MHz, CDCl3) d 6.33 (d, J = 12.6 Hz, 1H), 5.84
(dt, J = 1.3, 6.2 Hz, 1H), 4.57–4.51 (m, 1H), 3.52 (s, 3H), 3.16–3.10
(m, 1H), 2.70–2.60 (m, 2H), 2.34–2.06 (m, 3H), 1.59–1.48 (m, 1H),
1.44–1.22 (m, 4H), 0.86 (t, J = 7.1 Hz, 3H); 13C NMR (100 MHz,
CDCl3) d 208.41, 149.13, 142.74, 139.29, 105.66, 56.58, 42.73,
A solution of KH2PO4 (20 mg, 0.15 mmol) and H2O2 (30%;
0.01 mL, 0.1 mmol) in 0.5 mL of water was added to a
solution of (3-oxo-2-pentylidenecyclopentyl)acetaldehyde (10 mg,
0.05 mmol) in 1 mL of acetonitrile. A solution of NaClO2 (80%;
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17 mg, 0.15 mmol) in 1 mL of water was then added dropwise
with ice water cooling, and the mixture was stirred at room
temperature for 24 h. After addition of a small amount of sodium
sulfite, the mixture was subject to standard work-up to provide
crude acid. The subsequent esterification of the crude acid with
(trimethylsilyl)diazomethane (2 M; 0.1 mL, 0.2 mmol) in THF–
MeOH (1 : 1, 4 mL) was allowed to stir for 1 h at room temperature.
Purification of the crude product by flash column yielded (4.5 mg,
40%) of product as a colorless residue.
1H NMR (360 MHz, CDCl3) d 5.90 (dt, J = 7.5, 2.2 Hz, 1H),
3.71 (s, 3H), 3.08–3.18 (m, 1H), 2.67–2.73 (m, 1H), 2.61 (dd, J =
15.4, 5.9 Hz, 1H), 2.24–2.42 (m, 2H), 2.17–2.23 (m, 1H), 1.26–1.40
(m, 6H), 0.90 (t, J = 7.2 Hz, 3H); 13C NMR (90 MHz, CDCl3)
d 207.77, 172.76, 138.05, 51.86, 39.63, 38.94, 38.46, 31.70, 27.67,
26.88, 22.58, 14.13.
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Methyl 2-((1S,2S)-3-oxo-2-pentylcyclopentyl)acetate (13)
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1H NMR (300 MHz, CDCl3) d 3.68 (s, 3H), 2.66–2.58 (m, 1H),
2.37–2.11 (m, 4H), 1.84–1.75 (m, 1H), 1.60–1.19 (m, 10H), 0.90 (t,
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We acknowledge the start-up fund of the Green Catalysis Institute
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