Highly chemo-, enantio-, and regioselective synthesis of α,α-disubstituted furanones by Cu-catalyzed conjugate addition

Article abstract of DOI:10.1002/chem.201403446

A highly chemo-, enantio-, and regioselective synthesis of furanones bearing an α,α-disubstituted quaternary stereogenic center is reported. The Cu-catalyzed enantioselective conjugate addition of organoaluminum reagents to unsaturated ketoesters at room temperature and subsequent lactonization took place. Synthetic transformations of furanones represent facile approaches to various cyclic or acyclic compounds bearing a quaternary stereogenic center. Selective chemistry: A highly chemo-, enantio-, and regioselective synthesis of furanones bearing an α,α-disubstituted quaternary stereogenic center is reported (see scheme). Cu-catalyzed enantioselective conjugate addition of organoaluminum reagents to unsaturated ketoesters at room temperature and subsequent lactonization took place.

Full text of DOI:10.1002/chem.201403446

DOI: 10.1002/chem.201403446
Communication
&
Organic Synthesis
Highly Chemo-, Enantio-, and Regioselective Synthesis of a,a-
Disubstituted Furanones by Cu-Catalyzed Conjugate Addition
Kohei Endo,*[a, b] Sayuri Yakeishi,[c] Ryotaro Takayama,[c] and Takanori Shibata*[c]
Abstract: A highly chemo-, enantio-, and regioselective
synthesis of furanones bearing an a,a-disubstituted qua-
ternary stereogenic center is reported. The Cu-catalyzed
enantioselective conjugate addition of organoaluminum
reagents to unsaturated ketoesters at room temperature
and subsequent lactonization took place. Synthetic trans-
formations of furanones represent facile approaches to
Figure 1. All-carbon chiral quaternary stereogenic centers in butenolides
(R1, R2 ¼H).
various cyclic or acyclic compounds bearing a quaternary
stereogenic center.
Carbonyl compounds bearing an a-chiral carbon atom are
ubiquitous building blocks for the synthesis of various biologi-
cally active compounds.[1]There have been many reports on
asymmetric alkylation reactions of carbonyl compounds; espe-
cially, butenolides, which have been recognized as attractive
synthetic targets.[2] While enantioselective alkylation of the g-
Scheme 1. Present work.
position of butenolides has been extensively studied, there
have been few reports of the enantioselective and regioselec-
tive alkylation at the a-position. Since a- and g-alkylation reac-
tions via metal enolate intermediates derived from furanones
are typically competitive, regio- and enantioselective alkylation
should be difficult to achieve (Figure 1). The use of benzofura-
nones naturally gives a-alkylation or acylation products; how-
ever, the benzofused products are structurally limited.[3,4] We
report here a novel approach to the synthesis of furanones by
a Cu-catalyzed chemo-, enantio-, and regioselective conjugate
addition reaction (Scheme 1).
would be obtained via an enolate intermediate. However, the
chemoselectivity for the creation of a chiral quaternary carbon
rather than a tertiary carbon atom is difficult to control.[5,6] Fur-
thermore, 1,2-addition reactions of hard Lewis acidic organo-
aluminum reagents to ester and ketone moieties might take
place. The precedent chemo- and enantioselective conjugate
addition showed the use of cyclic ketoesters and organozinc
reagents at low temperature (Scheme 2a).[7a] During our origi-
nal studies, Hoveyda et al. reported the reaction using acyclic
enones including two examples of ketoesters and organoalu-
minum reagents to give simple acyclic ketoesters in moderate
yields with good enantioselectivities; unexpectedly, furanones
were not described (Scheme 2b).[7b,c] Fillion et al. reported the
synthesis of lactones by the enantioselective conjugate addi-
tion of organozinc reagents to Meldrum’s acid derivatives, but
a few steps for the transformations were required and fura-
nones could not be synthesized (Scheme 2c).[7d–f] We hypothe-
sized the tandem reaction through the enantioselective addi-
tion to acyclic ketoesters for the synthesis of furanones. Typi-
cally, the synthesis of furanones bearing an a,a-alkyl,alkyl qua-
ternary stereogenic center has been developed via enolate an-
alogues. Enantioselective decarboxylative allylation via
a carbonate derivative is a useful approach, but the regio- and
Since the chemoselective asymmetric conjugate addition
takes place at the b-position of the ketone moiety, furanones
[a] Prof. Dr. K. Endo
Department of Chemistry, Graduate School of Natural Science
and Technology, Kanazawa University
Kakuma, Kanazawa, Ishikawa, 920-1192 (Japan)
E-mail: kendo@se.kanazawa-u.ac.jp
[b] Prof. Dr. K. Endo
PRESTO (Japan) Science and Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan)
[c] S. Yakeishi,+ R. Takayama,+ Prof. Dr. T. Shibata
Department of Chemistry and Biochemistry
Graduate School of Science and Technology
Waseda University, Ohkubo, Shinjuku
Tokyo, 169-8555 (Japan)
enantioselectivity
needs
to
be
further
improved
E-mail: tshibata@waseda.ac.jp
[+] S.Y. and R.T. contributed equally to this work.
(Scheme 2d).[8,9] A chiral organocatalyst for asymmetric migra-
tion of carbonate derivatives gave chiral b-dicarbonyl com-
pounds and d-dicarbonyl compounds as mixtures; the severe
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403446.
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difficulty in the present reaction is the chemoselective conju-
gate addition to create a quaternary stereogenic center. Gener-
ally, while conjugate addition to create a tertiary stereocenter
and use of cyclic enones has been comprehensively studied,
conjugate alkylation to acyclic enones to create an all-carbon
quaternary stereogenic center is still a challenging topic.
We examined the use of our original Cu complexes in the
chemoselective conjugate addition of organoaluminum re-
agents to unsaturated ketoesters.[12] The present reaction pro-
ceeds by the asymmetric conjugate addition of organoalumi-
num reagents, followed by the intramolecular trapping of an
aluminum enolate with an ester moiety to give the desired fu-
ranones exclusively without the generation of a regioisomer.
Our original ligands, BmP, BP, and SP (Figure 2), were exam-
ined for the present reaction (Table 1). Optimization of the re-
action conditions revealed that the use of benzyl ester 1a in
the presence of CuCl2·2H2O (5 mol%) and SP (5 mol%) in THF
at 08C to RT gave the desired product 2a in high yield with
excellent enantioselectivity, although Hoveyda et al. reported
the reaction using acyclic unsaturated ketoesters gave acyclic
Figure 2. Representative ligands.
Table 1. Conditions for Me3Al.
Scheme 2. Addition to unsaturated ketoesters and previous approaches to
furanones.
Entry
R
Ligand (X)
Cu salt
t [h]
Yield, ee [%]
structural limitations and insufficient enantioselectivity are still
problematic (Scheme 2e).[10]
1
2
3
4
5
6
7
8
9
10
11
12
13
14[a]
Me (1a-Me)
Me (1a-Me)
Me (1a-Me)
Et (1a-Et)
Et (1a-Et)
Et (1a-Et)
Bn (1a)
Bn (1a)
Bn (1a)
Bn (1a)
Bn (1a)
BmP (10)
BP (5)
SP (5)
BmP (10)
BP (5)
SP (5)
BmP (10)
BP (5)
SP (5)
SP (5)
SP (5)
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
CuCl2·2H2O
Cu(OTf)2
2
4
1
1
1
1
2
2
1
2
2
2
16
1
80, 52 (+)
58, 36 (+)
82, 83 (ꢀ)
56, 69 (+)
93, 70 (+)
74, 90 (ꢀ)
90, 70 (+)
81, 36 (+)
80, 96 (ꢀ)
69, 97 (ꢀ)
74, 98 (ꢀ)
81, 98 (ꢀ)
77, 98 (ꢀ)
88, 97 (ꢀ)
We envisaged the conjugate addition/intramolecular lactoni-
zation as a practical and regioselective synthesis of furanones
bearing a quaternary stereogenic center. To achieve our goal,
acyclic a,b-unsaturated ketoesters were designed as acceptors
for organometallic reagents. Our laboratory recently developed
an original multinuclear system for Cu- or Pd-catalyzed asym-
metric addition reactions of organometallic reagents.[11] The
original Cu catalyst achieved the highly enantioselective conju-
gate addition of organoaluminum reagents to b,b-disubstitut-
ed enones for the creation of a quaternary stereogenic cen-
ter.[11d] We envisaged that the present approach could be ap-
plied to unsaturated ketoesters to give chiral furanones. The
[b]
Cu(acac)2
Bn (1a)
Bn (1a)
Bn (1a)
SP (5)
SP (5)
SP (5)
Cu(OAc)2
CuBr·SMe2
Cu(OAc)2
[a] Me3Al (1.5 equiv) was used. [b] acac=acetylacetone.
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g-ketoesters (Table 1, entries 1–9). BINOL derivatives BmP and
BP or the SPINOL derivative SP gave opposite major enantio-
mers, respectively.[11c,d] The screening of Cu salt showed that
the use of Cu(OAc)2 gave the product in better yield with ex-
cellent ee values (entries 10–14). The optimized reaction condi-
Table 2. Conditions for Et3Al.
tions were used for
a
variety of benzyl ketoesters
(Scheme 3).[13] Benzoyl moieties bearing an electron-withdraw-
ing group, such as F, Cl, or Br, or electron-donating group were
compatible; the products 2a–f were obtained in high to excel-
Entry
Cu salt
t [h]
Yield, ee [%][a]
1
2
3
4
5[b]
6[c]
7[c]
Cu(OAc)2
CuCl2·2H2O
Cu(OTf)2
Cu(acac)2
Cu(OAc)2
Cu(OAc)2
Cu(acac)2
6
24
21
21
6
60, 80
71, 77
62, 74
66, 82
52, 74
79, 84
74, 85[d]
6
4
[a] NMR spectroscopic yield. [b] Me2AlCl (20 mol%) was added.
[c] Me2AlCl (20 mol%) and AgOAc (20 mol%) were added. [d] Isolated
yield.
the catalytic performance (Figure 3).[16] Furthermore, conjugate
addition of phenylaluminum to obtain a-arylfuranones was ex-
amined; the intermolecular a-arylation of carbonyl compounds
for the creation of an all-carbon quaternary stereogenic center
is important, but has not yet been well established for further
transformations.[17] The reaction using Ph3Al prepared in situ
from PhLi or PhMgBr and AlCl3 did not give the product
(Table 3, entry 1). In contrast, the reaction using PhEt2Al or
PhMe2Al gave the desired product 2o (entries 2 and 3). Optimi-
zation of the reaction conditions showed that the use of
PhMe2Al and Cu(OTf)2 as a Cu salt could give the product 2o
Scheme 3. List of products for Me3Al. [a] For conditions, see Table 1,
entry 14. [b] BmP (10 mol%) and Me3Al (2 equiv) were used. [c] The corre-
sponding ethyl ester, Cu(OAc)2 (5 mol%), BmP (10 mol%), Me3Al (6 equiv),
and ether as a solvent were used.
Figure 3. Proposed effect of additives.[15]
Table 3. Conditions for PhR2Al.
lent yields with high ee values. The absolute configuration of
2d (S) was deduced by X-ray single-crystallographic analysis.[14]
Electron-rich substituents, such as 2-naphthyl, 4-biphenyl, and
thiophen-2-yl, were also compatible and the products 2g–
i were obtained. The reaction using a ketoester bearing
a longer alkyl group gave the product 2j in 92% yield with
more than 98% ee. The aryl-substituted ketoesters gave the
products 2k–n with high ee values when BmP was used as
a ligand; the present reaction provides an alternative approach
to the a-arylation of carbonyl compounds.
Entry
R
Cu salt
t [h]
Yield, ee [%][a]
1
2
3
4
5
6
7[b]
Ph
Et
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OTf)2
CuCl2·2H2O
Cu(acac)2
Cu(OTf)2
24
3
6
6
6
no reaction
49, 72
67, 85
42, 95
38, 83
Me
Me
Me
Me
Me
Conjugate addition of ethyl- and phenylaluminums was ex-
amined as representative for alkyl- and aryl-addition reactions
(Table 2). The addition of Et3Al required Me2AlCl and AgOAc as
additives to give 2o in high yield with high ee (Table 2, en-
tries 1–7).[15] The typical transition-state model according to the
literature suggested that Lewis acid additives could change
2
1
55, 84
82, 96
[a] NMR spectroscopic yield. [b] Cu(OTf)2 (10 mol%) and SP (10 mol%)
were used.
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in high yield with excellent enantioselectivity (entry 7); the
present reaction generated the Et or Me adducts as byprod-
ucts, respectively (entries 2–7).[15] Ketoesters bearing a methyl,
butyl, or phenyl group participated in the addition of Et3Al to
give 2a, 2o, and 2p with high ee values (Scheme 4). The addi-
tion of PhMe2Al to ketoesters bearing a methyl, ethyl, or butyl
group gave the products 2k, 2o, and 2q with high to excel-
lent ee values, respectively.
Several transformations of the products were preliminary ex-
amined (Scheme 5). A Friedel–Crafts-type reaction of 2a unex-
pectedly gave the sterically congested product 4,4-disubstitut-
Scheme 4. List of products for Et3Al and PhMe2Al. [a] For conditions, see
Table 2, entry 7. [b] Cu(OAc)2 (10 mol%) and SP (10 mol%) were used.
[c] For conditions, see Table 2, entry 7.
Scheme 5. a) AlCl3 (3 equiv), benzene, RT, 4 h; b) InCl3 (20 mol%), benzyl-
amine, reflux, 24 h; c) 6n aq. NaOH, reflux, 2 h; d) Pd(OAc)2 (5 mol%), HCO2K
(3 equiv), DMF, 608C, 24 h; e) Pd(OAc)2 (10 mol%), HCO2K (5 equiv), DMF,
608C, 24 h; f) Pd/C (10 mol%), H2 (1 atm.), EtOH, 608C, 12 h; g) DIBAL-H
(1.5 equiv), THF, ꢀ788C, 2 h; h) KOH, THF/EtOH, RT, 1 h.
ed g-lactone 3a in 78% yield without addition to a carbonyl
moiety (Scheme 5a). The amidation of 2a by using benzyl-
amine in the presence of InCl3 catalyst under reflux conditions
gave the g-lactam 3b in 82% yield (Scheme 5b). The treatment
of 2a under basic conditions gave the ring-opening product
3c in quantitative yield; the present approach would give
a wide variety of g-ketoacids bearing a quaternary stereogenic
center (Scheme 5c). We found that the formal reduction of
a ketone to a methylene group took place; the Pd-catalyzed
hydrogenation and subsequent debenzylation of 2a gave the
simple carboxylic acid 3d in 72% yield (Scheme 5d). The hy-
drogenation of 2n in the presence of Pd(OAc)2 gave the corre-
sponding lactone 3e as a single diastereomer (Scheme 5e). In
addition, hydrogenation in the presence of Pd/C gave the de-
sired product 3e and 3e’ quantitatively as a diastereomeric
mixture (Scheme 5f). The reduction of 2n by using DIBAL-H
and the intramolecular cyclization gave cyclopentenone 3g,
which is the synthetic intermediate for cuparenone, as a repre-
sentative small natural product (Scheme 5g,h).[18,19] Many natu-
ral products bear a methylated quaternary stereogenic center;
thus, the present facile synthesis of furanones should be note-
worthy as a “synthetic platform” in organic chemistry.
enantioselectivities. The synthetic transformations of furanones
reflect their versatility as synthetic intermediates. The present
inexpensive and easily available Cu catalyst is useful for prepar-
ing densely functionalized scaffolds bearing a quaternary ste-
reogenic center. We opened up novel synthetic targets, a,a-
disubstituted furanones, via various asymmetric conjugate ad-
dition reactions to unsaturated ketoesters; further studies
using our original catalysts with a Lewis acid controlled system
are underway, and the use of unsaturated ketoesters as
Michael acceptors for the synthesis of biologically active com-
pounds will also be reported in the future.
Experimental Section
Representative procedure for Me3Al
Cu(OAc)2 (2.3 mg, 0.0125 mmol, 5 mol%) and SP (7.8 mg,
0.0125 mmol, 5 mol%) were dissolved in THF (2.5 mL) and the mix-
ture was stirred at room temperature for 30 min and then cooled
to 08C. A solution of Me3Al in heptane (0.38 mmol, 0.19 mL, 2m)
was added to the mixture. Ketoester 1a (73.6 mg, 0.25 mmol) was
added to the clear colorless solution at once. The reaction was car-
ried out at RT and monitored by TLC analysis. After the completion
of the reaction, a minimal amount of sat. aq. NH4Cl was added at
08C. After stirring at 08C for 30 min, the mixture was passed
through a pad of silica gel with ether (50 mL). Concentration and
In conclusion, we have developed the first chemo-, regio-,
and enantioselective synthesis of furanones bearing an a-chiral
quaternary stereogenic center via the asymmetric Cu-catalyzed
conjugate addition of organoaluminum reagents to unsaturat-
ed ketoesters under ambient conditions. Me-, Et-, and Ph-addi-
tion were described along with a demonstration of Lewis acid-
controlled catalytic performance. Thus, a wide variety of nu-
cleophiles could take part in the present reaction with high
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Acknowledgements
This work was supported by the JST PRESTO program and
Grant-in-Aid for Scientific Research (B).
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[14] CCDC-999623 (for 2d) contains the supplementary crystallographic
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The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Keywords: addition reactions
synthesis · copper · heterocyclic compounds
· aluminum · asymmetric
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Received: May 7, 2014
Published online on June 17, 2014
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