Highly enantioselective synthesis of 3-substituted furanones by palladium-catalyzed kinetic resolution of unsymmetrical allyl acetates

Article abstract of DOI:10.1002/anie.201109075

Resolving the issue: A near-perfect Pd-catalyzed kinetic resolution of 1,3-disubstituted unsymmetrical allylic acetates uses silyl enol ethers as nucleophiles to access the important 3-substituted-furanone scaffold (see scheme; DACH=diaminocyclohexyl, dba=dibenzylideneacetone). The reaction proceeds under mild conditions and provides the desired products with excellent chemo-, regio-, and enantioselectivity. Copyright

Full text of DOI:10.1002/anie.201109075

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DOI: 10.1002/anie.201109075
Asymmetric Catalysis
Highly Enantioselective Synthesis of 3-Substituted g-Butenolides by
Palladium-Catalyzed Kinetic Resolution of Unsymmetrical Allyl
Acetates**
Bin Mao, Yining Ji, Martꢀn FaÇanꢁs-Mastral, Giuseppe Caroli, Auke Meetsma, and
Ben L. Feringa*
The Pd-catalyzed asymmetric allylic alkylation (AAA) holds
a prominent position among the most versatile methods for
Herein, we present the Pd-catalyzed kinetic resolution of
1,3-disubstituted unsymmetrical allylic substrates with non-
stabilized silyl enol ethers as nucleophiles, which provides
a highly regio- and enantioselective synthesis of 3-substituted
g-butenolides. The g-butyrolactone skeleton is present in
more than 13000 natural products, which have attracted
considerable attention because of the range of important
biological activities associated with this class of compounds.[11]
As part of our program to develop catalytic enantioselective
methods to access optically active g-butenolides and lac-
tones,[12] we envisioned the possibility of using 2-trimethyl-
silyloxyfuran (TMSOF)[13] as nucleophile in a Pd-catalyzed
AAA reaction.
À
C C bond formation that are widely applied in natural
product synthesis.[1] This transformation typically features
broad functional group tolerance and excellent regio- and
enantioselectivity.[2] In particular, Pd-catalyzed kinetic reso-
lutions[3,4] of symmetrical allylic substrates and dynamic
kinetic transformations[5] have been developed recently,
which extends their potential in synthetic chemistry. How-
ever, Pd-catalyzed kinetic resolutions of unsymmetrical
acyclic allylic substrates with high yield and enantioselectivity
are rare.[6] Furthermore, the enantioselective allylic alkylation
of silyl enol ethers is still a challenging reaction.[7] Graening
and Hartwig reported a highly enantioselective alkylation of
monosubstituted allylic substrates with silyl enol ethers
catalyzed by an Ir complex.[8] Although excellent results of
allylic alkylations have been reported with enolates that are
preformed or generated in situ,[9] to the best of our knowl-
edge, Pd-catalyzed AAA with a nonstabilized silyl enol ether
as nucleophile remains an elusive goal (Scheme 1).[10]
Initially, we examined the allylic substitution of carbonate
1a by utilizing TMSOF as nucleophile and a chiral Pd catalyst
based on Trost Ligand L1 (Table 1, entry 1). The 3-substituted
product 3 and 5-substituted product 4 were obtained in a 2:1
ratio and in 36% ee for product 3 (Table 1, entry 1). Using
a
lower temperature (08C), the ee value only slightly
increased (49% ee, Table 1, entry 2). With tert-butyl carbon-
ate as the leaving group, the regioselectivity shifted in favor of
the formation of the 5-substituted product (3/4 = 1:2, Table 1,
Table 1: Selection of reaction parameters.
Scheme 1. Pd-catalyzed asymmetric allylic alkylation of unsymmetrical
substrates with silyl enol ethers as nucleophiles.
Entry
1
T [8C]
t
Conv. Recovered 1 3/4[c]
3
[*] B. Mao, Dr. Y. Ji, Dr. M. FaÇanꢀs-Mastral, Dr. G. Caroli, A. Meetsma,
Prof. Dr. B. L. Feringa
[h] [%][c]
ee [%][d]
ee [%][d]
1[a]
2[a]
3[a]
4[a]
5[a]
6[b]
1a RT
1a
1b RT
21 100
25 100
22 100
24 80
–
–
–
2:1
2:1
1:2
5:1
>99:1 99
>99:1 99
36
49
60
78
Stratingh Institute for Chemistry, University of Groningen
Nijenborgh 4, 9747 AG, Groningen (The Netherlands)
E-mail: b.l.feringa@rug.nl
0
1c
1c
1c
RT
0
0
96(S)[e]
99(S)[e]
99(S)[e]
[**] Financial support from The Netherlands Organization for Scientific
Research (NWO-CW) is acknowledged. B.M. thanks the China
Scholarship Council for financial support (No. 2008618001). Y.J.
thanks the MEC for a fellowship. M.F.M. thanks the Spanish
Ministry of Science and Innovation (MICINN) for a postdoctoral
fellowship. We thank Prof. Dr. A. J. Minnaard for fruitful discussions.
7
7
53
52
[a] 2 equiv of 2a used. [b] 1 equiv of 2a used. [c] The conversion and ratio
of regioisomers were determined by GC analysis with n-dodecane as the
internal standard. [d] Determined by HPLC analysis on a chiral stationary
phase. [e] Absolute configuration was assigned by comparison of the
sign of optical rotation with published values.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109075.
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Table 2: Substrate scope for kinetic resolution.[a]
entry 3). Remarkably, when the simple allylic ace-
tate 1c was used, 80% conversion was reached at
room temperature and both the regio- and enantio-
selectivity were significantly enhanced (3/4 = 5:1,
78% ee, Table 1, entry 4).
Encouraged by these results, the reaction of allyl
acetate 1c was carried out at 08C instead of room
temperature. In this case, the reaction ceased at
53% conversion but provided enantiomerically
pure recovered (S)-1c through kinetic resolution
of the substrate. Furthermore, in this Pd-catalyzed
allylic alkylation, 3c is the only reaction product
that is obtained in near-perfect chemo-, regio- (3/4
> 99:1) and enantioselectivity (99% ee). Notably,
the reaction could also be carried out with one
equivalent of 2a to give the same results (Table 1,
entry 6).[14]
Entry Conv.[b]
[%]
1
Acetate 1
3/4[b]
3
Product 3
S[f]
ee [%][c] yield [%][d]
ee[%][c] yield%[d,e]
Remarkably, when the kinetic resolution of 1c
was performed under the optimized conditions
shown in Table 1, entry 6, but over a longer reaction
time, the transformation virtually ceased at 50%
conversion and 1c was recovered in 99% ee, which
illustrates a near-perfect selectivity in the kinetic
resolution as well (Figure 1). It is also remarkable
that the reaction proceeds with complete regiose-
lectivity towards the formation of 3c (Table 1,
entry 5) instead of 4c, which would be expected
based on the common reactivity pattern of
TMSOF.[13] Only very few examples are known in
which TMSOF reacts at the C3 position.[15]
To establish unequivocally the absolute stereo-
chemistry of 3c, it was converted into the corre-
sponding chromiumtricarbonyl complex by treat-
ment with [Cr(CO)3(CH3CN)3] in THF.[16] The
absolute configuration of 3c was determined as
the R configuration by X-ray diffraction analysis on
a single crystal of the resulting chromium complex
(see the Supporting Information).[17]
1
2
3
4
52
52
53
49
48
46
41
52
100
1c 99(S)
1d 99
1e 99
1 f 94
1g 94
1h 92
1i 91
1j 99
43
42
38
44
36
44
18
33
–
>99:1 3c 99(R)[g]
>99:1 3d 99
>99:1 3e 99
>99:1 3 f 98
47
47
46
41
39
36
25
35
55
116
116
80
>200
>200
87
25
116
–
5
22:1
3g 99
6[h]
7
>99:1 3h 99
14:1
37:1
3:1
3i 88
3j 99
3k 87
8
9
1k –
[a] Reaction conditions: 1/2a/(R,R)-L1/[Pd2(dba)3]·CHCl3 (100:100:15:5), 1 in
CH2Cl2 (0.175 m) at 08C, 0.5 h for addition of 1 by syringe pump. [b] The conversion
and the ratio of regioisomers were determined by GC analysis with n-dodecane as
the internal standard. [c] Determined by HPLC and GC analyses on chiral stationary
phases. [d] Yield of isolated product. [e] No trace of SN2’ product was detected.
[f] S=kfast/kslow =ln[(1ÀC/100) (1Àee/100)]/ln[(1ÀC/100)(1+ee/100)] (C=conver-
sion; ee=enantiomeric excess of recovered substrate). [g] The absolute config-
uration of 3c was assigned based on the X-ray analysis of a single crystal. [h] The
reaction was performed at room temperature with 2 equiv of 2a.
excellent enantioselectivity both in recovered 1 f and the
The scope of the reaction was examined under the
optimized conditions (Table 1, entry 6) for a range of racemic
allylic acetates (Table 2). Generally, most of the products
were obtained with excellent ee values and very high S fac-
tors. When unsymmetrical acetates 1d and 1e with electron-
withdrawing or -donating groups at the para position of
phenyl ring, respectively, were investigated, excellent regio-
and enantioselectivity was maintained (Table 2, entries 2 and
3). The ortho-methoxy-substituted substrate 1 f also led to
product 3 f (Table 2, entry 4).
We next turned our attention to dialkyl substituted allylic
acetates for the kinetic resolution. Although the recovered
allylic acetates had a slightly lower enantiomeric excess than
the aryl-substituted allylic acetates, the 3-substituted products
3g and 3h were obtained in 99% ee and with excellent
regioselectivity, both with respect to butenolide and unsym-
metrical disubstituted allyl fragments (Table 2, entries 5 and
6). It is important to note that the reaction proceeds very well
with both aromatic- and alkyl-substituted unsymmetrical
substrates and provides impressive selectivity in both cases.
The allylation of symmetrical, dimethyl-substituted sub-
strate 1i shows only a slight decrease in enantioselectivity,
both for recovered 1i and the product 3i (Table 2, entry 7).[18]
Furthermore, treatment of racemic cyclohexenyl acetate 1j
under the optimized conditions gave the corresponding
product 3j almost exclusively with an excellent ee value,
and 1j was also recovered in 99% ee (Table 2, entry 8). In
contrast, racemic 1,3-diphenylallyl acetate 1k provided the 3-
substituted product 3k with full conversion in 87% ee and
with poor regioselectivity (Table 2, entry 9). The reaction of
Figure 1. Kinetic resolution of 1c (see Table 1, entry 6).
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this common model substrate for allylic alkylation does not
follow the kinetic resolution pathway.[19]
To gain further insight into the key structural parameters
and mechanistic features of the reaction, different methyl-
substituted 2-silyloxyfurans were investigated (Schemes 2 and
3). When the 3 position of the furan ring was blocked by
a methyl group, the allylation reaction went exclusively
through attack at the 5-position, which afforded a 4:1 mixture
of diastereomers and the major isomer in 86% ee (Scheme 2).
Scheme 2. Allylic alkylation of substituted 2-silyloxyfuran 2b. TMS=tri-
methylsilyl.
Scheme 3. Allylic alkylation of 1c and 1l with 2a.
A moderate enantiomeric excess (46% ee) was obtained for
recovered 1c.
A remarkable shift in selectivity was obtained when we
carried out the reaction with allyl acetate 1l, that is, the
regioisomer of 1c (Scheme 3). Instead of exclusive formation
of 3c in 99% ee, surprisingly, the reaction only provided a 9:1
diastereomeric mixture of 4l, in which the 3-substituted
product was not detected. Furthermore, the recovered start-
ing material was obtained as a nearly racemic mixture.
To further understand the mechanism of the reaction, we
carried out model studies and DFT calculations.[20] Based on
the conformation of Pd/(R,R)-L1 reported by Lloyd-Jones
and co-workers,[21] we performed a conformational search on
the Pd–olefin complexes of both enantiomers of 1c (see the
Supporting Information). These results indicate that the
acetate carbonyl group of enantiomer (R)-1c forms a hydro-
gen bond with the amide hydrogen of ligand L1 on the
concave side of catalyst Pd/(R,R)-L1 (Figure 2a), whereas the
same stabilization for enantiomer (S)-1c is absent (Fig-
ure 2b). The energy difference of about 11 kcalmolÀ1
between these (h2-allyl)Pd complexes would explain the
result of the kinetic resolution in which the R substrate has
been completely consumed. A similar result was reported by
Lloyd-Jones and coworkers.[21] In the neutral pre-ionization of
h2-cyclic ester complexes, they found that the acetate carbonyl
group of the S enantiomer accepts a hydrogen bond from the
amide hydrogen on the concave side of the Pd complex,
whereas no corresponding stabilization is available for the
R enantiomer. This result suggests that only one enantiomer
of the acetate can be selectively ionized because of the
Figure 2. B3LYP structures of a) Pd/(R,R)-L1/(R)-1c, b) Pd/(R,R)-L1/
(S)-1c, and c) the 3-substituted product 3c coordinated with complex
Pd/(R,R)-L1 (arrow indicates hydrogen bond).
stabilization of the leaving acetate anion through hydrogen
bonding.
Furthermore, the DFT calculations showed that the most
stable product of the reaction is the one that is obtained from
allylic alkylation at the 3 position on the furanone ring (which
bears a conjugated double bond) with the R configuration.
We also determined that there is hydrogen bond between the
oxygen of the carbonyl group of the furanone product 3c and
the amide hydrogen on the concave side of the Pd complex
(Figure 2c).
It has been described, for the Pd-catalyzed allylic alkyl-
ation of malonate derivatives with this particular ligand
(R,R)-L1, that the hydrogen bonding interaction between the
enolate oxygen atom and the amide hydrogen atom can direct
the enolate carbon atom to the closest enantioface of the h3-
allyl complex.[21] We envision that hydrogen bonding (also
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lectivity. Further studies on the
unusual reactivity of TMSOF, the
mechanistic implications of our find-
ings, and extension of this highly
selective catalytic method are ongo-
ing.
Experimental Section
General procedures for Pd-catalyzed
kinetic resolution (Table 2): A solution
of the ligand (0.0525 mmol, 38 mg) in
CH2Cl2 (1 mL) was added to
a dry
Schlenk tube that contained [Pd2-
(dba)3]·CHCl3 (0.0175 mmol, 18 mg).
After stirring at room temperature for
15 min, a solution of TMSOF (0.35 mmol,
60 mL) in CH2Cl2 (0.5 mL) was added
dropwise. After the mixture had been
stirred for another 15 min, a solution of
the allylic acetate (0.35 mmol) and n-
dodecane (30 mL, internal standard) in
CH2Cl2 (0.5 mL) were added by syringe
pump over 0.5 h at 08C. The progress of
the reaction was monitored by GC and
GC-MS. After the completion of the
reaction, the reaction solvent was evapo-
rated under vacuum. The crude product
was purified by flash chromatography on
silica gel by using different mixtures of
pentane/Et2O as eluents. Toluene/Et2O
95:5 was used as the eluent for TLC to
distinguish between the regioisomers.
Scheme 4. Proposed catalytic cycle.
present in the final product, see Figure 2c) between the
oxygen anion of the incoming enolate and the amide hydro-
gen atom of the catalyst (with which the carbonyl oxygen
atom of the acetate leaving group interacts) controls the
stereoselectivity of the nucleophilic attack (Scheme 4). That
is, the removal of the acetate leaving group and the delivery of
the nucleophile proceeds by an identical enantioface selection
pathway. The key to the selectivity is a selective acetate–
enolate exchange, which suggests that the whole process takes
place with net retention of configuration, which is in perfect
agreement with the obtained experimental results. This result
is in full accordance with the proposal of Lloyd-Jones and co-
workers,[21] in which they identify that the hydrogen-bond
Received: December 22, 2011
Revised: January 17, 2012
Published online: February 16, 2012
Keywords: allyl acetates · g-butenolides · kinetic resolution ·
.
palladium · silyl enol ethers
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unit for ionization and also directs the orientation of
nucleophile delivery.[22] Most probably, a subsequent olefin
isomerization of the butenolide also takes place in the Pd
complex, which gives rise to the final product.[23]
In summary, we have developed a Pd-catalyzed kinetic
resolution of 1,3-disubstituted unsymmetrical allylic acetates
and a concomitant allylic alkylation by using silyl enol ethers
as nucleophiles, to access important 3-substituted g-buteno-
lides. The reaction proceeds under mild conditions and
provides the desired product with excellent chemo-, regio-,
and enantioselectivity. Preliminary studies indicate that
hydrogen-bonding interactions with the chiral ligand might
play a key role in the control of the regio- and enantiose-
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Angewandte
Chemie
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