Highly enantioselective synthesis of multifunctionalized dihydrofurans by copper-catalyzed asymmetric [4 + 1] cycloadditions of α-benzylidene- β-ketoester with diazo compound

Article abstract of DOI:10.1021/cs400019u

Highly efficient synthesis of chiral tetrasubstituted 2,3-dihydrofuran derivatives has been realized by Cu-catalyzed asymmetric [4 + 1] cycloadditions of α-benzylidene-β-ketoester with a diazo compound. Following this methodology, a series of optically active multifunctionalized dihydrofurans were prepared in high yield with up to 96% ee and 99/1 dr.

Full text of DOI:10.1021/cs400019u

Letter
pubs.acs.org/acscatalysis
Highly Enantioselective Synthesis of Multifunctionalized
Dihydrofurans by Copper-Catalyzed Asymmetric [4 + 1]
Cycloadditions of α‑Benzylidene-β-ketoester with Diazo Compound
Jiao-Long Zhou, Li-Jia Wang, Hao Xu, Xiu-Li Sun, and Yong Tang*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Highly efficient synthesis of chiral tetrasubsti-
tuted 2,3-dihydrofuran derivatives has been realized by Cu-
catalyzed asymmetric [4 + 1] cycloadditions of α-benzylidene-
β-ketoester with a diazo compound. Following this method-
ology, a series of optically active multifunctionalized
dihydrofurans were prepared in high yield with up to 96% ee
and 99/1 dr.
KEYWORDS: asymmetric, copper carbene, [4 + 1] cycloadditions, dihydrofurans
Scheme 1. Synthesis of 2,3,4,5-Tetresubstituted 2,3-
dihydrofurans
ynthesis of oxygenated heterocycles via transition-metal-
Scatalyzed decomposition of diazo compounds generated by
transient carbonyl ylides has proved to be one of the most
efficient and practical protocols and has drawn considerable
interest.1,2 Of those studies, the carbonyl ylides-involved 1,3-
dipolar cycloaddition reactions have been well developed.3,4
However, [4 + 1] cycloadditions of α,β-unsaturated carbonyl
compounds with diazo compounds, which provide a facile
access to multifunctionalized 2,3-dihydrofurans, has been less
explored. In 1967, Spencer et al. pioneered CuSO4-catalyzed [4
+ 1] cycloadditions of β-methoxy-α,β-unsaturated ketones with
ethyl diazoacetate.5 Since then, great efforts have been devoted
to the exploitation of such a cycloaddition,6 but none of these
investigations has realized the enantioselective version until
Fu’s work in 2007,7a leading to optically active 2,3,5-
trisubstituted 2,3-dihydrofurans7 with good efficiency and
stereoselectivity under the catalysis of the Cu(I)/[(−)-bpy*]
complex. In view of dihydrofurans containing molecular
skeletons being key subunits of various biologically active
compounds as well as versatile intermediates for the
construction of complex molecules,8,9 they are of considerable
synthetic value, and efficient methodologies for the catalytic
enantioselective synthesis of chiral multifunctionalized 2,3-
dihydrofurans are still highly demanded.
In 2011, we developed a tunable carbonyl ylide reaction: in
the presence of side-arm-modified rac-iPrBox Lrac/Cu (I), the
reaction of a α-benzylidene-β-dicarbonyl compound with a
diazo 2,6-diisopropylphenyl ester can furnish 2,3,4,5-tetrasub-
stituted 2,3-dihydrofurans with high chemoselectivity in
excellent yields and with high diastereocontrol (Scheme 1).10
Recently, our attempts on the asymmetric version achieved
promising enantioselectivity to afford optically active 2,3,4,5-
tetrasubstituted 2,3-dihydrofuran products. In this letter, we
present our preliminary results on this subject.
Initially, a control experiment was carried out with non-side
i
i
arm PrBox L1 and side arm-modified PrBox L2, which
indicated that installing an aryl side arm at the bridge of the
bisoxazoline ligand could increase the enantioselectivity of this
reaction obviously (Table 1, entry 1 vs 2). In the case of L2 as a
ligand, we examined the cycloadditions of α-benzylidene-β-
ketoester 1a with diazo compound 2 in the presence of a
variety of commonly used copper salts. As shown in Table 1,
the reaction was carried out smoothly in good yield with 88%
ee for the major diastereoisomer when CuPF6(CH3CN)4 was
1
used, but 14% yield of 4a was also detected by H NMR
analysis (entry 2).10 Both CuBArF (CuCl + NaBArF) and
CuSbF6 (CuCl + AgSbF6) could give the desired 2,3-
dihydrofuran 3a in excellent yields with high enantioselectivities
(92% yield with 90% ee and 94% yield with 91% ee,
Received: January 9, 2013
Revised: March 2, 2013
Published: March 4, 2013
© 2013 American Chemical Society
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Letter
a
a
Table 1. Optimization of Reaction Conditions
Table 2. Survey of Ligands
b
c
entry
copper salt
R1
iPr
iPr
iPr
Me
L
yield of 3a (%)
ee (%)
b
c
entry
ligand
yield (%)
ee (%)
1
2
3
4
5
6
CuPF6(MeCN)4
CuPF6(MeCN)4
CuCl + NaBArF
CuCl + AgSbF6
CuCl + AgSbF6
CuCl + AgSbF6
L1
L2
L2
L2
L2
L2
90
75
88
90
89
89
91
d
d
1
2
3
4
5
6
L2
L3
L4
L5
L6
L7
94 (94)
91
99
96
95
84
94
85
e
f
e
45
94 (92)
d
93 (82)
88
90
94
g
tBu
iPr
e
88
94 (94)
f
86
a
Reaction conditions: Copper salt (5 mol %), L (5.2 mol %), MS 4 Å
a
b
Reaction conditions: CuCl (2.5 mg, 0.025 mmol, 5 mol %), AgSbF6
(100 mg/mL), DCM, r.t., 1a (0.5 mmol), 2 (2.0 mmol). Determined
c
1
(8.7 mg, 0.025 mmol, 5 mol %), ligand (0.026 mmol, 5.2 mol %), MS
by H NMR. For major diastereomer; determined by chiral HPLC.
d
e
1
4 Å (100 mg/mL), DCM, r.t., 1a (0.5 mmol), 2 (2.0 mmol).
4a was observed from H NMR in 14% yield with 53% ee. Isolated
b
c
1
f
g
yield. BARF− = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. Dia-
zo 2,6-di-tBu-4-Me-Phenyl ester and (E)-ethyl 2-benzoyl-3-(4-
chlorophenyl)acrylate used.
Determined by H NMR. For major diasteromer; determined by
d
e
1
chiral HPLC. Isolated yield. 4a was observed by H NMR in 55%
yield with 24% ee. 4a was observed by H NMR in 13% yield with
f
1
38% ee.
a
respectively, entries 3 and 6). The impact of the diazo ester was
next investigated, and it was found that the nature of the diazo
compounds influences the yield and stereoselectivity.11 Further
study showed that the diazo 2,6-diisopropylphenyl ester
provided the best combination of both yield and ee (94%
yield, 91% ee, entry 6).
Table 3. Substrate Scope
Further investigation was focused on the ligand effects of
modified bisoxazolines on the enantioselectivity.12,13 As
b
c
d
substrate (R, R1, R2)
yield
dr
ee (%)
i
t
entry
outlined in Table 2, comparing with the PrBox L2 and Bu-
Box L3, ligands L4−L7 with indane skeletons gave a better
result. For example, the enantioselectivity could be increased to
96% ee with a slight decrease in the isolated yield when InBox
L4 was employed (entry 3). By the use of chiral InBox L5 with
a 3,5-ditrifluoromethylbenzyl group as a side arm, 95% ee could
also be afforded (entry 4). In contrast, chiral ligand L6 with 3,5-
dimethoxybenzyl, which contains an electron-rich side arm, led
to an obvious decline in the ee value (84% ee, entry 5). In
addition, good enantioselectivity could also be achieved with
the chiral In-TOX ligand L7 (94% ee, entry 6).
1
2
3
1a (Ph, Et, Ph)
3a, 82%
3b, 71%
3c, 89%
3d, 85%
3e, 82%
3f, 93%
3g, 81%
3h, 79%
3i, 65%
3j, 68%
3k, 74%
99/1
99/1
99/1
99/1
99/1
99/1
99/1
99/1
99/1
90/10
92/8
96
95
95
90
90
96
96
89
90
81
78
1b (o-Me-C6H4, Et, Ph)
1c (p-Me- C6H4, Et, Ph)
1d (p-MeO- C6H4, Et, Ph)
1e (p-CF3- C6H4, Et, Ph)
1f (p-Cl- C6H4, Et, Ph)
1g (p-Br- C6H4, Et, Ph)
1h (Ph, Me, iPr)
e
4
5
6
7
8
9
1i (Ph, Et, Me)
f
10
1j (n-C8H17, Et, Ph)
1k (n-C12H25, Et, Ph)
f
11
Under the optimized conditions, we evaluated the generality
of the current asymmetric reaction. As shown in Table 3, a
variety of substrates were examined. When R and R2 were a
phenyl or substituted phenyl group, the reaction displayed
excellent stereoselectivity and chemoselectivity. All substrates
completed the reaction smoothly to afford optically active 2,3-
dihydrofuran derivatives in up to 96% yield with extremely high
diastereoselectivities (>99/1) as well as excellent enantiose-
lectivities (90−96%), regardless of the electronic nature and the
position of the substituent on phenyl group R (entries 1−7).
Further studies show that alkyl-substituted substrates are
a
Reaction conditions: CuCl (2.5 mg, 0.025 mmol, 5 mol %), AgSbF6
(8.7 mg, 0.025 mmol, 5 mol %), ligand (0.026 mmol, 5.2 mol %), MS
4 Å (100 mg/mL) and DCM, r.t., 1 (0.5 mmol), 2 (2.0 mmol).
b
c
d
Isolated yield. Determined by 1H NMR. Determined by chiral
e
f
HPLC. Ligand = L3. Ligand = L7, 10 mol % catalyst.
(entry 9). When R was n-C8H17 and n-C12H25, good
stereoselectivities and isolated yields were obtained, giving
68% yield with 90/10 dr and 81% ee and 74% yield with 92/8
dr and 78% ee, respectively (entry 10 and 11). The absolute
configuration of 3g was established as 2S, 3S by X-ray
crystallography (Figure 1).14
i
suitable to this catalysis. For instance, when R2 was Pr, the
reaction could furnish the desired 3h with 79% yield in up to
99/1 dr and 89% ee (entry 8). Using the Me group instead of
the Pr group, the high diastereoselectivity and enantioselectiv-
ity were maintained, albeit with a slight decrease in the yield
Previous study of the structure of side-armed BOX−metal
complexes showed that the pendant aryl groups of BOX−metal
complexes always bend toward the metal center.15 In
i
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Letter
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 13C NMR spectroscopic data of new compounds
and analytic data of the compounds 3 are included. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tangy@mail.sioc.ac.cn.
■
Notes
The authors declare no competing financial interest.
Figure 1. X-ray crystal structure of 3g.
ACKNOWLEDGMENTS
combination with our previous computational study10 and the
aforementioned experimental results, the stereoinductive model
shown in Scheme 2 was proposed to account for the observed
■
We are grateful for financial support from the National Natural
Sciences Foundation of China (Nos. 21121062, 20932008, and
21272250), the Major State Basic Research Development
Program (Grant No. 2009CB825300), and the Chinese
Academy of Sciences. We thank Dr. Xue-bing Leng (SIOC)
and Mr. Jie Sun (SIOC) for X-ray crystal analysis.
Scheme 2. Proposed Stereochemical Model
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arm-modified bisoxazolines, see: (a) Zhou, J.; Tang, Y. J. Am. Chem.
Soc. 2002, 124, 9030. (b) Ye, M. C.; Zhou, J.; Huang, Z. Z.; Tang, Y.
Chem. Commun. 2003, 2554. (c) Huang, Z. Z.; Kang, Y. B.; Zhou, J.;
Ye, M. C.; Tang, Y. Org. Lett. 2004, 6, 1677. (d) Foltz, C.; Stecker, B.;
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688
dx.doi.org/10.1021/cs400019u | ACS Catal. 2013, 3, 685−688