Copper-catalyzed desymmetrization of prochiral 4,4-disubstituted cyclopentenes: Via a site-selective allylic oxidation: A concise total synthesis of untenone A

Article abstract of DOI:10.1039/c9cc01743g

The desymmetrization of prochiral 4,4-disubstituted cyclopentenes via a site-selective copper-catalyzed allylic oxidation is described. This study provides a direct comparison of a series of known methods for allylic oxidation, and thus identifies ligand-free copper(i) iodide as the optimal catalyst for this particular process. Notably, this work offers a convenient approach to the preparation of γ-quaternary α,β-unsaturated cyclopentenones, which permits an efficient three-step total synthesis of (±)-untenone A.

Full text of DOI:10.1039/c9cc01743g

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DOI: 10.1039/C9CC01743G
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Copper-Catalyzed Desymmetrization of Prochiral 4,4-
Disubstituted Cyclopentenes via a Site-Selective Allylic Oxidation
Reaction: A Concise Total Synthesis of Untenone A
Received 00th January 20xx,
Accepted 00th January 20xx
Qingwen Gui,a,b JuanJuan Wang,a,c Sean Ng,d Anja Dancevicd Timothy B. Wright,a and P. Andrew
Evans*,a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The desymmetrization of prochiral 4,4-disubstituted
cyclopentenes via site-selective copper-catalyzed allylic
corresponding α,β-unsaturated ketones in excellent yield
and selectivity. Nevertheless, a critical feature with these
transformations is the fact that the oxidation does not
introduce any stereochemical information. The problem
a
oxidation is described. This study provides a direct comparison
to a series of known methods for allylic oxidation, and thus
identifies ligand-free copper(I) iodide as the optimal catalyst for
this particular process. Notably, this work offers a convenient
approach to the preparation of -quaternary α,β-unsaturated
cyclopentenones, which permits an efficient three-step total
synthesis of (±)-untenone A.
A. Transition Metal-Catalyzed Allylic Oxidation Reactions – Previous Studies
R
R
R
cat. MLn
tBuOOH
•
n
n
n
O
• M = Rh, Pd, Mn • R = H, Alk, Ar, COR, NO2 • n = 1, 2, 3
The allylic oxidation of alkenes to α,β-unsaturated ketones
represents a fundamentally important process in target
directed synthesis.1 Nevertheless, the synthetic utility of this
process is often beset with problems associated with poor
B. Rhodium-Catalyzed Allylic Oxidation of (+)-Carvomenthene – Baldwin
Me Me Me
O
cat. Rh(I)
efficiency and selectivity.
Furthermore, the classical
[O]
methods for allylic oxidation tend to employ toxic,
stoichiometric reagents, which seriously detracts from their
use in industrial scale applications.1,2 Consequently, the
development of more environmentally friendly, metal-
catalyzed reactions that employ a stoichiometric terminal
oxidant has become a subject of increased focus. Although
Collman3a reported the first late transition-metal-catalyzed
oxidation of cyclohexene to cyclohexenone, the challenges
associated with garnering site-selectivity in unsymmetrical
systems were delineated by Solomon.3b In light of this
challenge, much of the work in this area has focused on
symmetrical and/or electronically biased alkenes that
furnish excellent selectivity (Scheme 1A). For example, the
metal-catalyzed allylic oxidation of cycloalkenes has been
reported with rhodium(I/II),3 palladium(II),4 manganese(III),5
copper(I/II),6 cobalt(II)7 and bismuth(III),8 which afford the
Me
Me
Me
Me
Me
Me
(+)
Prochiral
(±)
C. Copper-Catalyzed Allylic Oxidation of Prochiral Cyclopentenes – This Work
R1 R2
R1 R2
R1 R2
R1 R2
cat. Cu(I)
vs.
O
b
[O]
a
O
1
2
3
HO C16H33
R1 R2
Ar CH2OH
Me
Me
One-Step
CO2Me
O
O
2
Ar = 2-HO-5-MeC6H3
Untenone A (4)
Key Intermediate
1,13-Herbertenediol (5)
Scheme 1 Background for the development of the site-selective copper-catalyzed
allylic oxidation of prochiral 4,4-disubstituted cyclopentenes.
associated with attaining selective allylic hydrogen
abstraction is further manifested by the formation of a
fluxional allylic radical intermediate, which can afford
constitutional isomers and in certain cases leads to the
racemization of the substrate. For example, Baldwin and
Swallow provided compelling evidence for the intermediacy
a. Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6,
Canada.
b. College of Science, Hunan Agricultural University, Changsha, Hunan, 410128, P. R. China.
c. College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo,
Henan, 45400, P. R. China.
d. Department of Chemistry, University of Liverpool, Crown St., Liverpool, L69 7ZD, United
Kingdom.
Electronic Supplementary Information (ESI) available: Experimental procedures,
characterization data, copies of spectra and crystallographic data. CCDC1503541 (7). For
ESI and crystallographic data in CIF and other format, see DOI: 10.1039/x0xx00000x.
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of an allylic radical in the oxidation of (+)-carvomenthene to
(±)-carvotanacetone, which undergoes complete
prochiral 4,4-disubstituted cyclopentane 1a using known
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procedures that have been employed DfoOrI:th10e.1a0l3ly9/liCc9oCxCid01a7t4io3Gn
of cycloalkenes. Although the treatment of methyl 1-
phenylcyclopent-3-enecarboxylate (1a) with Wilkinson's
catalyst in benzene at 70 °C afforded the cyclopentenone 2a
in 46% yield with excellent selectivity (entry 1), Rh2(cap)4 was
both less efficient and selective (entry 1 vs 2). Additional
studies probed the utility of Pd(OAc)2-BINAP and Pd(OH)2/C,
which again furnished 2a in low yield, but with good
selectivity (entry 3 and 4). Similarly, manganese(III) acetate,
which had proven to be an excellent catalyst for simple
cycloalkenes, provided poor yield albeit excellent selectivity
(entry 5). In each case there are significant quantities of
unreacted starting material despite the extended reaction
times (entry 1-5). In light of the relatively poor efficiency
with each of these complexes, we elected to examine cupric
and cuprous salts, which had been demonstrated to
facilitate the allylic oxidation of alkenes in steroids.
Gratifyingly, the allylic oxidation of 1a with catalytic
copper(I) iodide furnished 2a in a significantly improved 59%
yield and with excellent selectivity (entry 10 vs. 6-9). Further
studies probed the impact of the reaction parameters to
determine the
racemization via a prochiral intermediate (Scheme 1B).9 We
envisioned an alternative scenario in which the allylic
oxidation of a prochiral 4,4-disubstituted cyclopentene 1
would permit the construction of -quaternary α,β-
unsaturated cyclopentenones 2 (Scheme 1C). A key and
striking feature of this approach is that the initial C-H
abstraction is inconsequential, whereas the site-selective
trapping of the incipient allylic radical is critical for attaining
selectivity. We anticipated that the selective formation of 2a
would be possible given the steric bias imparted by the
adjacent quaternary in the recombination of the incipient
allylic radical. Herein, we now describe the allylic oxidation
of prochiral 4,4-disubstituted cyclopentenes 1 with catalytic
copper(I) iodide using tert-butyl hydroperoxide as a terminal
oxidant to afford the corresponding α,β-unsaturated
cyclopentenones
2 in excellent yield (Scheme 1C).
Furthermore, we illustrate the synthetic utility with a three-
step synthesis of untenone A (4)10,11 along with access to an
intermediate that was employed in the synthesis of 1,13-
herbertenediol (5).12
Table 1 summarizes the preliminary studies on the
feasibility of the site-selective allylic oxidation of the
Table 1 Optimization of the site-selective copper-catalyzed allylic oxidation of the prochiral 4,4-disubstituted cyclopentene 1a
Ph CO2Me
CO2Me
CO2Me
O
Ph
Ph
cat. MLn, tBuOOH (TBHP)
+
base, solvent, temp.
1a
2a
O
3a
ratio of
2:3b
yield
of 2c
entrya
metal complex
mol%
base
mol%
solvent
temp (°C)
time (h)
1d
2
3e
Rh(PPh3)3Cl
Rh2(cap)4
Pd(OAc)2
Pd(OH)2/C
Mn(OAc)3•2H2O
CuCl2
1
1
-
-
50
25
50
-
PhH
70
23
23
”
48
24
24
24
48
16
”
99:1
24:1
30:1
26:1
40:1
26:1
28:1
27:1
28:1
34:1
50:1
52:1
46
29
K2CO3
CH2Cl2
10
5
”
”
22
4
”
”
32
5f
6
10
5
EtOAc
”
18

NaHCO3
100
”
CH3CN
”
32
7
CuCl
”
”
”
”
41
8
CuBr
”
”
”
”
”
”
25
9
CuCN
”
”
”
”
”
”
”
47
10
11
12
CuI
”
”
”
CH3CN
”
”
”
59
82(80)g
CuI
5
NaHCO3
100
”
40
60
36
”
”
”
57
a All reactions were carried out on a 0.25 mmol reaction scale using 5 equiv. of TBHP (5.5 M in decane) unless otherwise stated. b Regioselectivity was determined by
HPLC analysis of the crude reaction mixtures; 2a and 3a are inseparable by column chromatography and 3a was synthesized de novo by the desaturating of the
f
corresponding cyclopentanone. c HPLC yields relative to the internal standard m-cresol. d 4.0 equiv. TBHP (5.5 M in decane) was used. e10 mol% BINAP was added.
100 mg of 3Å molecular sieves were added. g Isolated yields.
optimal conditions for this transformation. For instance,
various inorganic bases were evaluated, which resulted in
lower yields of 2a (NaOAc, K2CO3, KHCO3, Na2CO3; SI, Table
S1). Furthermore, reducing the amount of base and
increasing the catalyst loading also did not impact the overall
efficiency (SI, Table S1). In addition, the examination of
various organic solvents, namely, N,N-dimethylformamide,
ethyl acetate, 1,2-dichloroethane, toluene and 1,4-dioxane
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also proved less effective for this transformation relative to
acetonitrile (SI, Table S1). Finally, increasing the reaction
temperature to 40 °C and extending the reaction time to 36
hours, furnished 2a in a significantly improved 80% isolated
yield and with excellent selectivity (entry 11).VieIwnAcrtoicnletOranlsinte,
further increasing the temperature proved detrimental to
DOI: 10.1039/C9CC01743G
the efficiency of this process (entry 12). Hence, the ability to
employ commercially available copper(I) iodide prompted
the examination of the substrate scope for this process.
Scheme 2 outlines the application of the optimized
reaction conditions (Table 1, entry 11) to a series of prochiral
4,4-disubstituted cyclopentenes 1.
Consequently,
cyclopentenes with 4-methyl ester (CO2Me), tert-
a
butyldimethylsilyl (TBS) protected alcohol or hydroxymethyl
pivalate (CH2OPiv) group in conjunction with either aryl or
alkyl substituents were examined, given the potential
application of these substituents in target directed synthesis
(vide infra). Gratifyingly, the reaction proceeds in a highly
efficient and selective manner with both electron-rich and
electron-deficient aryl groups (entries 1-9). Additional
studies examined the impact of replacing the aryl
substituent with alkyl groups, which also results in highly
selective oxidation, but with marginally lower efficiency
(entries 10-18). Nevertheless, the reaction is highly
chemoselective as illustrated by the allyl containing
derivatives (entry 16-18), wherein the cyclic olefin is
functionalized in preference to the acyclic terminal olefin.
Overall, this approach demonstrates excellent scope for the
selective oxidation of prochiral 4,4-disubstituted substituted
cyclopentenes.
A. Functional Group Manipulations - Iodination and Epoxidation
CO2Me
CO2Me
CO2Me
Me
I
Ar
R
I2, DMAP, K2CO3
H2O2, NaOH
O
THF/H2O, RT
92%
MeOH, RT
82%
O
O
O
7
6
2j R = Me
from 2j
from 2d
ds ≥19:1
2d R = 4-MeOC6H4
X-Ray
B. Applications to Total Synthesis - (±)-Untenone A (4)
• Natural Cyclopentenone
Prostaglandin
HO C16H33
CO2Me
Acylation
• Antiproliferative Activity
Allylic
IC50 = 0.4 g/mL
O
Oxidation
(±)-Untenone A (4)
• 3-Step Total Synthesis
HO C16H33
TMSO C16H33
TMSO C16H33
c
b
a
(±)-Untenone A
(4)
Gram-Scale
O
10
8
9
Reagents and conditions:(a) Hoveyda-Grubbs II (2.5 mol%), CH2Cl2, RT, 2
h, then DIPEA, TMSOTf, 0°C, 90%; (b) CuI (5 mol%), NaHCO3 (1.0 equiv.),
TBHP (5.0 equiv.), CH3CN, 40 °C, 36 h, 70%; (c) LDA,NCCO2Me (2.5 equiv),
HMPA/THF (4:1), –78°C to RT, 3 h, then HCl, MeOH, RT, 1 h, 65%.
aAll reactions were carried out on a 0.25 mmol scale using CuI (5 mol%), 1.0
equiv. NaHCO3 and 5.0 equiv. of TBHP (5.5 M in decane) in 1.25 mL CH3CN at
40 °C under air for ca. 36 hours. b Regioselectivity was determined by 500 MHz
1H NMR analysis. c Isolated yields.
Scheme 3. Functional group manipulations and total synthesis of (±)-untenone
A (4)
Scheme 2. Scope of the copper-catalyzed allylic oxidation of prochiral 4,4-
disubstituted cyclopentenes 1.a,b,c
Scheme 3 outlines the synthetic utility of the -
quaternary α,β-unsaturated cyclopentenones 2 for target
directed synthesis.
For instance, treatment of
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P21/c(No. 14), a = 7.5934(2)Å, b = 6.2414(2)Å, c = 26.6906(7)Å, α
cyclopentenone 2j with iodine in the presence of DMAP and
potassium carbonate, furnished the -iodo cyclopentenone
6 in 92% yield (Scheme 3A).13a Alternatively, the -bromo
cyclopentenone variant can be readily accessed in analogous
yield using a slightly modified protocol (see SI).13b The
construction of the -halo cyclopentenones is particularly
important, owing to the fact that they readily participate in
metal-mediated cross-coupling reactions.14 In another
useful demonstration of synthetic utility, the
cyclopentenone 2d was oxidized using basic hydrogen
peroxide to the epoxy ketone 7 in 82% yield and with ≥19:1
diastereocontrol.15 The relative configuration of the epoxy
ketone 7 was determined via the X-ray crystallographic
analysis, which provides an interesting building block for
further elaboration.
We envisaged that the copper-catalyzed allylic oxidation
could facilitate an expeditious synthesis of the key
cyclopentenone core of ()-untenone A (4), which is a
natural product that was isolated from the Okinawan marine
sponge, plakortis sp. that exhibits modest cytotoxicity
against leukemia L1210 cell lines.10 Ring closing metathesis
of the known bis-allyl tertiary alcohol 8,16 prepared via allyl
Grignard addition to methyl heptadecanoate, with Hoveyda-
Grubbs catalyst followed by in situ trimethylsilyl protection
of the tertiary alcohol, furnished the prochiral cyclopentene
9 in 90% overall yield (Scheme 3B). Site-selective copper-
catalyzed allylic oxidation of 9 on gram-scale afforded the
cyclopentenone 10 in 70% yield with excellent site-
selectivity (≥19:1). The synthesis was completed by treating
the enolate derived from cyclopentenone 10 with Mander’s
reagent,17 followed by acid-mediated desilylation to furnish
(±)-untenone A (4) in 41% overall yield for the three step
sequence to illustrates the utility of this methodology.18
In summary, we have developed a copper-catalyzed
allylic oxidation of prochiral 4,4-disubstituted cyclopentenes
for the construction of -quaternary α,β-unsaturated
cyclopentenones. This study provides a direct comparison of
the leading transition-metal-catalyzed allylic oxidation
methods and demonstrates a rare example of a highly site-
selective allylic oxidation. Finally, the synthetic utility of this
method was demonstrated in a three-step total synthesis of
()-untenone A (4). Overall, this process represents a mild
and efficient method for the construction of -quaternary
α,β-unsaturated cyclopentenones.
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= γ = 90 º, = 94.3100(10)º, V = 1261.38(6) Å3, Z = 4, Dcalc = 1.381
g cm-3, MoKα radiation, λ = 0.71073 Å, T = 180(2) K, θmax
DOI: 10.1039/C9=C3C00.1570443G°,
13271 reflections collected, 3582 unique (Rint = 0.0205). Final
GooF= 1.026, R1 = 0.0408 (I> 2σ(I)),wR2 = 0.1064 (all data, 3582
reflections), refinement on F2, 174 parameters and 0 restraints.
1
For a general review on allylic oxidation reactions, see: P.
C. Bulman Page, T. J. McCarthy, In Comprehensive Organic
Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, 1991, vol 7,
p. 83. (b) V. Weidmann, W. Maison Synthesis, 2013, 45,
2201.
(c) M. B. Andrus In Science of Synthesis:
Stereoselective Synthesis, ed.’s J. G. de Vries, P. A. Evans
and G. A. Molander, Thieme, Stuttgart, Germany, 2011, vol
3, p. 469.
2
3
For selected reviews on allylic oxidation with chromium
and selenium, see: (a) J. Muzart, Chem Rev. 1992, 92, 113.
(b) D. Liotta, R. Monahan, III, Science, 1986, 231, 356.
Rh: (a) J. P. Collman, M. Kubota, J. W. Masking, J. Am.
Chem. Soc., 1967, 89,4809. (b) J. M. Reuters, A. Sinha, R.
G. Salomon, J. Org. Chem. 1978, 43, 2438. (c) S. Bien, Y.
Segal, J. Org. Chem., 1977, 42, 1685. (d) A. J. Catino, R. E.
Forslund, M. P. Doyle, J. Am. Chem. Soc. 2004, 126, 13622.
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J. W. Blunt, M. H. G. Munro Org. Biomol. Chem. 2008, 6,
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P. Doyle J. Org. Chem. 2009, 74, 730. (g) Y. Yu, R. Humeidi,
J. R. Alleyn, M. P. Doyle J. Org. Chem. 2017, 82, 8506.
Pd: (a) E. J. Corey, J.-Q. Yu. Org. Lett. 2002, 4, 2727. (b) E.
J. Corey, J.-Q. Yu, J. Am. Chem. Soc. 2003, 125, 3232.
Mn: T. K. M. Shing, Y. –Y. Yeung, P. L. Su, Org. Lett. 2006,
8, 3149.
Cu: (a) J. A. L. Salvador, M. L. Sáe Melo, A. S. Campos Neves,
Tetrahedron Lett. 1997, 38, 119. For a recent review on
copper-catalyzed allylic oxidation, see: (b) A. L. Garcia-
Cabeza, F. J. Moreno-Dorado, M. J. Ortefa, F. M. Guerra,
Synthesis, 2016, 48, A
Co: (a) A. R. S. Jorge, H. C. James, Chem. Commun., 2001,
33. (b) J. G. Magdalena, C. S. Alice, R. H. W. John,
Tetrahedron Lett. 2003, 44, 4283.
4
5
6
7
8
9
Bi: J. A. R. Salvador, S. M. Silvestre, Tetrahedron Lett. 2005,
46, 2581.
J. E. Baldwin, J. C. Swallow. Angew. Chem. Int. Ed. 1969, 8,
601.
10 For the isolation of untenone A (4), see: M. Ishibashi, S.
Takeuchi, J. Kobayashi, Tetrahedron Lett. 1993, 34, 3749.
11 Untenone A (4) represents a rare example of a natural
product that was isolated as a racemate.
12 For the synthesis of 1,13-herbertenediol from a -
quaternary -unsaturated ketone intermediate, see: F.
Hu, Q. Zhou, F. Cao, W.-D. Chu, L. He, Q.-Z. Liu J. Org. Chem.
2018, 83, 12806.
13 (a) M. E. Kraff, J. W. Cran. Synlett 2005, 1263. (b) F. Geng,
J. Liu, L. A. Paquette. Org. Lett., 2002, 4, 71.
We sincerely thank NSERC for a Discovery Grant and a
Tier 1 Canada Research Chair (PAE). We also acknowledge
the National Natural Science Foundation of China (21502044
– JJW and 201506130048 - QWG) for financial support. Dr.
Gabriele Schatte is thanked for the X-ray crystallographic
analysis of the epoxy ketone 7.
14 For reviews on α-haloenones in metal-mediated cross-
coupling reactions, see: (a) N. Miyara, A. Suzuki, Chem. Rev.
1995, 95, 2457. (b) E. Negishi J Organomet Chem. 1999;
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C. P. Johnson, J. P. Adams, M. P. Braun, C. B. W.
Senanayake. Tetrahedron Lett. 1992, 33, 919. (d) P. A.
Evans, J. D. Nelson, T. Manangan, Synlett, 1997, 968.
15 (a) S. Takano, K. Inomata, T. Sato, M. Takahashi, K.
Ogasawara. J. Chem. Soc., Chem. Commun., 1990, 290. (b)
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Taite, E. Rohrbacher J. Med. Chem. 1985, 28, 597.
Conflicts of interest
There are no conflicts to declare.
Notes and references
17 L. N. Mander, S. P. Sethi. Tetrahedron Lett., 1983, 24, 5425.
‡Crystal structure data for 7: C14H14O5, M = 262.25, colourless,
plate-like, 0.199 0.181 0.120 mm3, monoclinic, space group
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18 For total syntheses of untenone A, see: (a) K. Takeda, I.
Nakayama, E. Yoshii, Synlett 1994, 178. (b) H. Miyaoka, T.
Watanuki, Y. Saka, Y. Yamada, Tetrahedron 1995, 51, 8749.
(c) M. Asami, T. Ishizaki, S. Inoue, Tetrahedron Lett. 1995,
36, 1893. (d) C. Kuhn, L. Skaltounis, C. Monneret, J.-C.
Florent, Eur. J. Org. Chem. 2003, 2585.
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