Highly enantioselective carbonyl-ene reactions of 2,3-diketoesters: Efficient and atom-economical process to functionalized chiral α-hydroxy-β-ketoesters

Article abstract of DOI:10.1002/anie.201402233

Carbonyl-ene reactions of 2,3-diketoesters catalyzed by [Cu{(S,S)-tBu-box}](SbF₆)₂ [box=bis(oxazoline)] generate chiral α-functionalized α-hydroxy-β-ketoesters in up to 94% yield and 97% ee. The 2,3-diketoesters are conveniently accessed from the corresponding α-diazo-β-ketoester, and a catalyst loading as low as 1.0 mol% can be achieved.

Full text of DOI:10.1002/anie.201402233

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DOI: 10.1002/anie.201402233
Asymmetric Catalysis
Highly Enantioselective Carbonyl–Ene Reactions of 2,3-Diketoesters:
Efficient and Atom-Economical Process to Functionalized Chiral
a-Hydroxy-b-Ketoesters**
Phong M. Truong, Peter Y. Zavalij, and Michael P. Doyle*
Abstract: Carbonyl–ene reactions of 2,3-diketoesters catalyzed
by [Cu{(S,S)-tBu-box}](SbF6)2 [box = bis(oxazoline)] gener-
ate chiral a-functionalized a-hydroxy-b-ketoesters in up to
94% yield and 97% ee. The 2,3-diketoesters are conveniently
accessed from the corresponding a-diazo-b-ketoester, and
a catalyst loading as low as 1.0 mol% can be achieved.
process which extends the complexity and functionality of the
product is a challenging task.
Recently our group has used 2,3-diketoesters in catalytic
approaches to the highly selective syntheses of functionalized
furans and cyclopentanones.[9] Because of its multiple coor-
dinating sites and the highly reactive electrophilic character of
the central carbonyl group, we envisioned that 2,3-diketoest-
ers would be excellent candidates for carbonyl–ene reactions.
Herein, we report the broadly applicable catalytic asymmetric
carbonyl–ene reactions, of 2,3-diketoester derivatives, which
occur with high yield and enantiocontrol (Scheme 1b). This
strategy allows the formation of functionalized chiral a-
hydroxy-b-ketoesters, an important structural motif found in
biological molecules, drug candidates, and key intermediates
in natural product synthesis.[10] Although the 2,3-diketoester
functional group has been used for many years in the
synthesis of numerous examples of carbocyclic compounds,
heterocycles, and natural products,[11] this is the first demon-
stration of its viability in a catalytic asymmetric transforma-
tion.
T
he carbonyl–ene reaction is a versatile, atom-economical
process for carbon–carbon bond formation.[1] Since the first
asymmetric examples of this reaction reported by Yamamoto
and co-workers,[2] numerous chiral Lewis[3–7] or Brønsted[8]
acid catalytic systems have been successfully developed. With
few exceptions,[4,5] glyoxylate systems have been the sole
carbonyl substrates in the development of asymmetric
carbonyl–ene reactions because they are highly activated,
allow bidentate coordination to the catalyst, and provide
access to functionalized chiral a-hydroxyacetates. Trifluoro-
acetyl ketone analogues have recently received considerable
attention (Scheme 1a, R2 = CF3),[4,8a–c] but there are only two
The preparation of 2,3-diketoesters (3) is achieved by
a variety of known methods.[11b] In our investigations, these
derivatives were readily obtained as hydrates in high yield
through diazo transfer to 1,3-dicarbonyl compounds with
subsequent dinitrogen replacement by oxygen using tert-butyl
hypochlorite (Scheme 2).[11b] Although oxidation of a-diazo-
Scheme 1. Asymmetric carbonyl–ene reactions.
reports in which an alkyl substituent has been installed at the
a-position.[5] Because of very long reaction times, which are
required with pyruvates,[5a] or the need to use a highly
activated enolsilyl counterpart to decrease reaction times,[5b]
the impression given by these reports is that effective
asymmetric carbonyl–ene reactions are restricted to glyox-
ylates,[1] and that the development of a general, asymmetric
Scheme 2. Preparation of 2,3-diketoester derivatives.
b-ketoesters to 2,3-diketoesters was achieved in excellent to
quantitative yield using dimethyldioxirane,[9] for large-scale
reactions the commercially available tert-butyl hypochlorite
oxidant is more practical. The 2,3-diketoesters 3 exist
predominantly as the hydrate in equilibrium with the keto
form, but the keto form is easily obtained (see the Supporting
Information) by heating (90–1008C) the hydrate under
vacuum for 10 minutes (Scheme 2).
[*] P. M. Truong, Dr. P. Y. Zavalij, Prof. M. P. Doyle
Department of Chemistry and Biochemistry
University of Maryland, College Park, MD 20742 (USA)
E-mail: mdoyle3@umd.edu
[**] Support for this research from the National Science Foundation
(CHE 1212446) is gratefully acknowledged.
Evans and Wu previously reported that the chiral
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402233.
scandium(III) bis(oxazolinyl)pyridine (pybox)[7e] complex
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Table 2: Exploring the substituent effects on yield and selectivity with
2,3-diketoesters.[a]
was an effective catalyst for highly enantioselective carbonyl–
ene reactions. As the success of this process was proposed to
result from the rigidity of the coordinated substrate as a result
of two-point binding, we thought this catalyst system would
also be suitable for high enantiocontrol in reactions with 2,3-
diketoesters. However, reaction of 3a with a-methylstyrene
(4a), catalyzed by scandium(III)[7e] triflate ligated with pybox
(L1), generated the product 5a with only 18% ee (Table 1,
Entry
3
5
mol%
R
R1
Yield
[%][c]
ee
Catalyst[b]
[%][d]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3 f
3g
3g
3g
3g
3g
3g
5a
5b
5c
5d
5e
5 f
5g
5g
5g
5g
5g
5g
10
10
10
10
10
10
10
5
1
5
5
5
Me
Me
Me
Me
Me
Me
Et
Et
Et
Et
Et
4-MeC6H4
4-ClC6H4
Ph
Bn
Me
tBu
Ph
Ph
Ph
92
91
90
90
64
58
91
91
90
91
88
87
80
83
82
82
85
86
92
92
92
94
94
91
Table 1: Catalyst screening and optimization of reaction conditions.[a]
9[e]
10[f]
11[f,g]
12[h]
Ph
Ph
Ph
Et
[a] Reactions were carried out on a 0.25 mmol scale of 3 (keto form) with
3 equiv of a-methylstyrene (4a) in 2.0 mL of solvent, unless noted
otherwise. [b] X mol% as listed; Y=1.2 times X. [c] Yield of product after
chromatographic purification. [d] Determined by chiral-stationary-phase
HPLC analysis. [e] Reaction was stirred for 3 h. [f] Reaction was
performed at À788C then slowly warmed to 08C. [g] Reaction was carried
out on a 5.0 mmol scale of 3g. [h] The hydrate form of 3g was used, and
the reaction was run for 24 h.
Entry
Lewis acid
Ligand
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
Sc(OTf)3
Cu(OTf)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
L1
L5
L5
L2
L3
L4
L5
L5
L5
L5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
CH3CN
DCE
toluene
91
90
92
68
84
18
70
75
58
65
28
–
–
74
65
89
trace
trace
87
entries 1–6). However, product yields were significantly
impacted with R1 = Me and tBu. However, modification of
the 3-keto unit from acetyl (3c) to propanoyl (3 f) increased
the enantioselectivity from 82% to 92% (entry 7) without
diminishing the product yields. Catalyst loading could be
decreased to 1 mol% using longer reaction times, without
effecting either the yield or enantioselectivity (entries 8 and
9). Enantioselectivities were further improved to 94% when
the reaction was performed at À788C and then slowly
warmed to 08C (entry 10). To test the reproducibility of this
process, a gram-scale reaction was performed with 3g, and the
a-hydroxy-b-ketoester 5g was obtained in 88% yield with
94% ee (entry 11). Remarkably, the readily accessible
hydrate form of 3g could also employed in the catalytic
asymmetric carbonyl–ene process, thus producing 5g with
similar yield and ee value upon isolation after a 24 h reaction
time, as compared to a 1 hour reaction time needed for the
keto form 3g (entry 12 versus entry 10). Use of the hydrate
rather than the keto form is obviously only a limitation in
reaction time.
Reactions between the structurally diverse 2,3-diketoest-
ers 3 and various alkenes 4 were examined under optimized
reaction conditions (Table 3). The alkyl (R) substituents of 3,
including methyl, ethyl, isopropyl, benzyl, and cyclohexyl,
reacted smoothly with a-methylstyrene to generate 5c and
5g–j in high yield and enantioselectivity. However, enantio-
meric excess fell to 68% when R = phenyl (5k). To further
expand the reaction scope of the 2,3-diketoesters 3, additional
functionalities (R) were explored. Reactions of 3 (R = styryl
derivatives) with a-methylstyrene provided 5l and 5m,
respectively, in excellent yield and enantiomeric access. The
84
[a] Reactions were carried out on a 0.25 mmol scale of 3a (keto form)
with 3.0 equiv of a-methylstyrene (4a) in 2.0 mL of solvent at room
temperature. [b] Yield of product isolated after after column chroma-
tography. [c] Determined by chiral-stationary-phase HPLC analysis.
DCE=1,2-dichloroethane, M.S.=molecular sieves, THF=tetrahydro-
furan, Tf=trifluoromethanesulfonyl.
entry 1). However, this outcome was surprising in view of
prior reports of the slow conversions of keto analogues of
glyoxylates,[5a] as the reaction was complete within 1 hour at
room temperature (entry 1). Encouraged by this result, we
examined reactions with other Lewis acid/chiral ligand
catalysts and discovered that bidentate chiral copper(II)
bis(oxazoline) (box) complexes, which had been previously
employed in asymmetric carbonyl–ene reactions[3d,5a] with
glyoxylates, were optimum. The best results were obtained
using Cu(OTf)2 or Cu(SbF6)2 ligated with L5 (entries 2
and 3). With the chiral box ligands L2–L4 and Cu(SbF6)2,
comparable activity was found, but the carbonyl–ene product
was formed with lower ee values (entries 4–6). Changing the
solvent to THF and CH3CN resulted in only trace amounts of
product, whereas DCE and toluene provided similar results to
those obtained with CH2Cl2 (entries 7–10). Since Cu(SbF6)2
ligated with L5 provided the highest yield and ee value, this
catalytic system was selected for further optimization.
[3d]
[5a]
A variety of aryl and alkyl ester derivatives (3) were
examined to determine the influence of structure on enantio-
selectivity, but they had only minimal impact (Table 2,
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Table 3: Substrate scope of carbonyl–ene reactions with 2,3-diketoest-
ers.
donating, electron-withdrawing, and halogen substituents
(5p–u), although the o-Me substituent caused a decrease in
product yield. However, as seen in the outcome for 5v, the
methoxy substituent dramatically decreased both the yield
and ee value.[3j] Acyclic alkenes and methylenecycloalkanes
are also suitable, thus forming 5w and 5x in high yield and ee
value, but there was a slightly lower ee value with methyl-
enecyclopentane 5y compared to methylenecyclohexane.
To demonstrate their utility, the keto group was reduced
by sodium borohydride in the presence of ZnCl2 at À408C in
high yield with complete diastereoselectivity, thus producing
the enantiomerically pure (2S,3R)-vicinal diol 6 bearing
a tertiary carbinol (Scheme 3). This structural motif has
played an essential role in natural products synthesis.[12]
Scheme 3. Stereoselective reduction of the a-hydroxy-b-ketoester 5g.
The absolute configuration of 5 was determined to be S
through single-crystal X-ray analysis of 5n (Figure 1a). The
relative stereochemistry of 6 was determined by 1H NMR
nOe experiments with the corresponding protected diol 7
(Figure 1b).
Figure 1. a) X-ray crystal structure of 5n from asymmetric carbonyl–
ene reaction. b) NOE experiment of the acetonide 7. CSA=camphor-
10-sulfonic acid, DMP=2,2-dimethoxypropane.
successes achieved with these substrates further demonstrate
the generality of the ene reactions with 2,3-diketoesters and
their applicability in forming products containing the a,b-
unsaturated carbonyl functionality, which is suitable for
further chemical transformations. The carbonyl–ene reaction
is also compatible with 3 where the R substituent is an alkyl
chain containing a keto functional group; the product from
this reaction (5o) was generated in 90% yield and 95% ee. In
contrast to the high ee value obtained with a-methylstyrene,
however, reaction of 3 (R = styryl) with the naphthyl ana-
logue 2-isopropenylnaphthalene produced 5n in only 73% ee.
Examination of the reactions of the 2,3-diketoester 3 with
various alkenes 4 further demonstrated the broad applicabil-
ity of this methodology. High yields and excellent ee values
were obtained for aromatic alkenes containing weak electron-
We have previously proposed Lewis acid activation of the
central carbonyl of 2,3-diketoesters through a bidentate
coordination with the more basic keto group rather than
with the carboxylate group,[9b] and our current data provides
further evidence for this claim. If the [Cu{(S,S)-tBu-box}]-
(SbF6)2 catalyst undergoes bidentate coordination with the
2,3-diketoester 3 in a square-planar complex,[13] so that the
central carbonyl oxygen atom and the adjacent keto carbonyl
are bound (8), the approach through the Si face is sterically
hindered by the tert-butyl substituent of the box ligand, thus
allowing olefin approach from only the Re face, and thus
generates the S enantiomer (Figure 2). In contrast, if biden-
tate coordination of the catalyst occurred with the central
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access to chiral a-substituted a-hydroxy-b-ketoesters beyond
that currently available.[10f–h] The placement of diverse func-
tional groups, at the a-position, which are suitable for
subsequent transformations, is of particular value for the
construction of enantiomerically enriched synthetic inter-
mediates for natural products.[10e,i] Furthermore, the demon-
stration of their high stereoselectivities in asymmetric ene
reactions suggest that 2,3-diketoesters should also be suscep-
tible to other nucleophiles in an asymmetric fashion. Further
studies with other nucleophiles are in progress.
Experimental Section
General procedure for the synthesis of functionalized a-hydroxy-b-
ketoesters (5): A solution of the 2,3-diketoester 3 (0.25 mmol,
1.0 equiv), after dehydration of the hydrate and 120 mg of 4 ꢀ
molecular sieves in 1.75 mL of CH2Cl2 under a nitrogen atmosphere,
was cooled to À788C. Then 0.25 mL (0.050m) of the chiral catalyst
solution and alkene 4 (0.75 mmol, 3.0 equiv) were added sequentially.
The reaction was stirred for 10 min at À788C, then transferred to an
ice bath and stirred for additional 50 min. The reaction mixture was
directly subjected to flash column chromatography (SiO2) eluting
with hexanes and ethyl acetate to provide the pure product 5.
Figure 2. Activation of phenyl 2,3-diketopropionate by [Cu{(S,S)-tBu-
box}](SbF6)2.
carbonyl oxygen atom and the ester carbonyl (9), olefin attack
would come from the less sterically hindered Si face to
generate the R enantiomer. Since, the S enantiomers are
formed in this carbonyl–ene reaction, the ligated copper
catalyst most likely forms bidentate complexes with the two
keto carbonyls of the 2,3-diketoesters system (8). Complex-
ation of the 2,3-diketoester 3g with [Cu{(S,S)-tBu-box}]-
(SbF6)2 was further verified by the spectral shift in the visible
region of the electromagnetic spectrum from the titration
experiment of the chiral copper complex with 3g (Figure 3).
Although the six-membered ring bidentate chelation of
oxazolidinones to copper(II)/(box) has been well studied,[13]
observation of the complexation of 2,3-diketoesters which
form five-membered ring chelates was not previously con-
firmed.
Received: February 9, 2014
Revised: March 12, 2014
Published online: May 20, 2014
Keywords: asymmetric catalysis · carbonyl–ene reaction ·
.
copper · ligand effects · synthetic methods
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