Asymmetric hetero-diels-alder reaction of danishefsky's dienes with α-carbonyl esters catalyzed by an indium(III)-PyBox complex

Article abstract of DOI:10.1021/ol400841s

An efficient catalytic enantioselective hetero-Diels-Alder reaction of Danishefsky's dienes with α-carbonyl esters using a chiral In(III)-pybox complex has been demonstrated. This protocol offers several advantages, including mild reaction conditions, relatively low catalyst loading, and good to excellent enantioselectivities. Furthermore, the absolute configurations of the new alkynyl-containing products were determined by CD spectra in combination with TD-DFT calculations.

Full text of DOI:10.1021/ol400841s

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Asymmetric Hetero-DielsꢀAlder Reaction of
Danishefsky’s Dienes with r‑Carbonyl Esters
Catalyzed by an Indium(III)ꢀPyBox Complex
Bei Zhao† and Teck-Peng Loh*,‡
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s
Republic of China, and Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
teckpeng@ntu.edu.sg
Received March 27, 2013
ABSTRACT
An efficient catalytic enantioselective hetero-DielsꢀAlder reaction of Danishefsky’s dienes with R-carbonyl esters using a chiral In(III)ꢀpybox
complex has been demonstrated. This protocol offers several advantages, including mild reaction conditions, relatively low catalyst loading, and
good to excellent enantioselectivities. Furthermore, the absolute configurations of the new alkynyl-containing products were determined by CD
spectra in combination with TD-DFT calculations.
The asymmetric hetero-DielsꢀAlder (HDA) cycloaddi-
tion of Danishefsky type dienes with aldehydes is an attrac-
tive and well-investigated reaction in organic synthesis.1 Since
the enantioenriched oxygen-containing heterocycles gener-
ated by this reaction are versatile building blocks for the
syntheses of numerous biologically active compounds,2 the
development of chiral catalysts for this asymmetrical HDA
reaction is of great synthetic importance. Danishefsky et al.
initiated the use of chiral Lewis acid catalysts to accelerate the
highly enantioselective preparation of six-membered hetero-
cycles.3 Since then, several efficient metal-assisted4 and metal-
free5 chiral Lewis acids have been developed for this powerful
reaction. To our surprise, while many catalytic systems give
good yields and enantioselectivities in the asymmetric HDA
reactions of Danishefsky type dienes with aldehydes, only
several efficient methods dealing with the [4 þ 2] cycloaddi-
tion of Danishefsky’s dienes with bidentate dienophiles, such
as glyoxylate esters6 and R-keto esters,7 have been developed
so far. In this context, the catalysts developed by Jørgensen’s
group,7b Ghosh’s group,7c and Inanaga’s group7d,e deserve
special mention, as enantioselectivities of up to 99% have
been obtained for the cycloaddition of R-keto esters to
Danishefsky’s diene.
† Soochow University.
‡ Nanyang Technological University.
(1) (a) Danishefsky, S. J.; Deninno, M. P. Angew. Chem., Int. Ed. 1987,
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r
10.1021/ol400841s
XXXX American Chemical Society

Our group and others have successfully employed Inand
In(III) salts in asymmetric synthesis for several years.4t,u,8
Inthecourseof ourefforttoapplychiralindium complexes
in asymmetric synthesis, we discovered that In(III)ꢀpybox
complexes are efficient catalysts for asymmetric carbonyl-
ene reactions of polymeric glyoxylates and trifluoro-
pyruvates.9 Recently, we successfully employed an In(III)ꢀ
pybox complex to catalyze enantioselective Mukaiyama
Table 1. Screening of Indium Salts, Chiral Ligands, and Dienesa
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yield
(%)b
ee
entry R1
R
InX3
ligand (h)
(%)c
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
Et
TMS In(OTf)3
TMS InF3
1
1
1
1
1
1
1
2
3
4
5
1
1
1
24
36
18
24
18
40
18
24
24
20
20
42
42
24
26
48
33
30
60
28
80
51
63
68
76
52
60
48
12
7
TMS InCl3
45
52
63
7
TMS InBr3
TMS InI3
TMS In(CF3COO)3
2004, 6, 2185. (p) Lin, L.; Fan, Q.; Qin, B.; Feng, X. J. Org. Chem. 2006, 71,
ꢀ
4141. (q) Ruano, J. L. G.; Fernandez-Ibanez, M. A.; Maestro, M. C. J. Org.
ꢀ
ꢀ~
n-Bu TMS InI3
n-Bu TMS InI3
n-Bu TMS InI3
61
31
5
€
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10 n-Bu TMS InI3
11 n-Bu TMS InI3
12 n-Bu TIPS InI3
13 n-Bu TBS InI3
14 n-Bu TES InI3
21
29
60
50
63
a Reactions were carried out on a 0.5 mmol scale with 1.5 equiv of
Danishefsky’s diene in 5.0 mL of anhydrous DCM at room temperature,
unless noted otherwise. TFA = trifluoroacetic acid. b Isolated yield.
c The ee values were determined by chiral-phase HPLC analysis.
aldol reactions between polymeric or hydrated glyoxylates
and enolsilanes, in which excellent enantioselectivities and
yields were obtained with a broad substrate scope.10 We
hypothesized that this strategy could also be applied to
the more challenging asymmetric HDA reaction between
R-carbonyl esters and Danishefsky’s diene, which had been
shown to undergo both Mukaiyama aldol and hetero-
DielsꢀAlder reactions,4f,6e,7c and the promising products
could be used in MukaiyamaeꢀMichael addition to afford
the corresponding 2,6-anti-tetrahydropyran adducts.11
Unfortunately, when we used chiral In(OTf)3-(þ)-1 in the
reaction of commercially available ethyl glyoxylate (50% in
˚
toluene) and Danishefsky’s diene in the presence of 4 A mo-
lecular sieves, the desired product was obtained in very low ee.
After six different commercially available In(III) Lewis
acids, five chiral pybox compounds, and four different
siloxy-substituted Danishefsky dienes were screened,
InI3-(þ)-1 gave the best result with good yield and satis-
factory enantioselectivity (Table 1, entry 7).
Using this catalytic system, we investigated the steric
bulk effect of glyoxylate esters, with the hope of achieving
better enantioselectivity. A series of glyoxylate esters with
(9) (a) Zhao, J. F.; Tsui, H. Y.; Wu, P. J.; Lu, J.; Loh, T. P. J. Am.
Chem. Soc. 2008, 130, 16492. (b) Zhao, J. F.; Tjan, T. B. W.; Tan, B. H.;
Loh, T. P. Org. Lett. 2009, 11, 5714. (c) Zhao, J. F.; Tan, B. H.; Zhu,
M. K.; Tjan, T. B. W.; Loh, T. P. Adv. Synth. Catal. 2010, 352, 2085.
(10) Zhao, J. F.; Tan, B. H.; Loh, T. P. Chem. Sci. 2011, 2, 349.
(11) Chua, S.-S.; Alni, A.; Chan, L.-T. J.; Yamane, M.; Loh, T. P.
Tetrahedron 2011, 67, 5079.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 2. Screening of the Glyoxylate Estersa
Table 3. In(III)ꢀpybox Complex Catalyzed Asymmetric HDA
Reactions of Danishefsky Type Dienesa
a Reactions were carried out on a 0.5 mmol scale with 1.5 equiv of
Danishefsky’s diene in 5.0 mL of anhydrous DCM at room temperature,
unless noted otherwise. TFA = trifluoroacetic acid. b Isolated yield.
c The ee values were determined by chiral-phase HPLC analysis, and the
absolute configuration of the major products is S, assigned by compar-
ing HPLC data with literature data. d The starting diene was (E)-1-
methoxy-2-methyl-3-trimethylsiloxy-1,3-butadiene.
different ester groups were prepared according to the
reported method.12
Substantial improvement in the enantioselectivity was rea-
lized with increasing steric bias of the ester groups, as shown in
Table 2. The highest ee (92%) could be achieved when tert-
butyl glyoxylate was used as the dienophile. We then evalu-
ated the effect of solvent, temperature, and catalyst loading.
As expected, this reaction is sensitive to the solvent, where
dichloromethane (DCM) proved to be the optimum choice of
solvent and the catalyst loading could be reduced to 10 mol %.
It is noteworthy that the enantiomeric excess is considered
high at room temperature for this HDA reaction.13
With this promising result in hand, we turned to the
more challenging cycloaddition between R-carbonyl esters
and Danishefsky’s diene. First, some synthetically useful
methods for a reasonable number of R-keto esters were
modified on consulting the published processes.14 The
a Reactions were carried out on a 0.5 mmol scale with 1.5 equiv of
Danishefsky’s diene in 5.0 mL of anhydrous DCM at room temperature,
unless noted otherwise. TFA = trifluoroacetic acid. nd = not detected.
b Isolated yield. c The ee values were determined by chiral-phase HPLC
analysis, and the absolute configurations of the major products were
assigned by comparing HPLC data with literature data.7c
catalytic results employing the preprepared chiral catalyst
InI3-(þ)-1 are summarizedin Table 3. It was found that the
alkyl groups directly attachedtothe R-positionofcarbonyl
groups had great effects on the reaction efficiency. For
example, the best result, up to 94% ee value and 84% yield,
could be obtained when isopropyl pyruvate was used as the
(12) Hu, X. H.; Liu, F.; Loh, T. P. Org. Lett. 2009, 11, 1741.
(13) See the Supporting Information (SI).
(14) (a) Kijima, M.; Miyamori, K.; Sato, T. J. Org. Chem. 1988, 53, 1719.
(b) Zhao, Y.; Wang, G.; Li, Y. Q.; Wang, S. H.; Li, Z. M. Chin. J. Chem.
2010, 28, 475. (c) Chiu, C. C.; Jordan, F. J. Org. Chem. 1994, 59, 5763. (d)
ꢀ
Guo, M.; Li, D.; Zhang, Z. J. Org. Chem. 2003, 68, 10172. (e) Soule, J. F.;
Mathieu, A.; Norsikian, S.; Beau, J. M. Org. Lett. 2010, 12, 5322.
Org. Lett., Vol. XX, No. XX, XXXX
C

dienophile (Table 3, entry 1). The identity of the alkoxy
groups had only slight effects on the reaction, with isopropyl,
tert-butyl, and cyclopentyl pyruvates giving moderate yields
and high ee values (Table 3, entries 1ꢀ3). However, the yield
and enantioselectivity were significantly decreased when
the substituents at the R-position of carbonyl groups were
bromomethyl and isopropyl (Table 3, entries 4 and 5). There
was no reaction observed when ethyl 2-oxo-2-phenylacetate
or methyl 3,3,3-trifluoro-2-oxopropanoate was used as
the substrate. Presumably steric hindrance in the transition
state influences the reaction efficiency substantially. This
hypothesis was proven when we used linear trimethylsilyl
acetyl to replace the bulky aliphatic or aromatic substituents
at the R-position of the carbonyl group. The yields were
moderate and enantioselectivities were excellent as expected
(Table 3, entries 6 and 7). High ee values can be also achieved
using the Danishefsky type diene (E)-1-methoxy-2-methyl-3-
trimethylsiloxy-1,3-butadiene as the substrate (Table 2, entry
6, and Table 3, entries 8ꢀ11).
Figure 1. Experimental ECD spectrum (full trace) and simu-
lated spectrum (dashed trace), proving the (R)-11f absolute
configuration.
(CuTC). As expected, the desired 1,2,3-triazole (S)-13 was
obtained in good yield and excellent ee.
Furthermore, the absolute configuration (AC) of the
novel alkynyl-containing products was determined by
means of chiroptical methods.15 For further discussion,
ECD spectra were also calculated by the TD-DFT method,
which has been proven to be useful in predicting ECD spectra
and assigning the AC of organic molecules.16 Calculations of
the ECD spectra of (R)- and (S)-11f were carried out using
the TD-DFT-B3LYP/6-31G(d) level with Gaussian 03.13,17
All conformations were calculated at the same level to
confirm their stability (no imaginary frequencies). Electronic
excitation energies (nm) and rotational strengths (Δε) were
calculated for 11f. In order to cover the 180ꢀ400 nm
range, 30 transitions were calculated. In Figure 1, the
simulated spectrum is in good agreement with the
experimental spectral data, and the R configuration
could be reliably assigned to 11f.
In conclusion, we have presented an efficient catalytic
enantioselective HDA reaction of Danishefsky type dienes
with R-carbonyl esters using a chiral In(III)ꢀpybox com-
plex, which is easily prepared from commercially available
InI3 and pybox 1. The absolute configurations of novel
alkynyl-containing products were determined by CD
spectra in combination with TD-DFT calculations.
This protocol offers several advantages, including mild
reaction conditions, relatively low catalyst loading, and
good to excellent enantioselectivities. In addition, the meth-
od grants important enantioselective access to the chiral
center bearing an alkynyl group, which allows for further
functional transformations, including click chemistry,
metathesis, cyclization, and so on.
Finally, to get an insight into the importance of the
alkynyl functionality, a classical click reaction was per-
formed in the presence of copper(I) thiophene-2-carboxylate
Acknowledgment. We thank the Jiangsu Government
Scholarship for Oversea Studies for assistance. We grate-
fully thank Dr. Yong Wang (Soochow University) for
TD-DFT calculations. We also gratefully acknowledge
the Singapore Ministry of Education Academic Research
Funding (MOE2010-T2-2-067 and MOE2011-T2-1-013),
the National Natural Science Foundation of China (Grant
21172165), and the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions for finan-
cial support.
(15) For a perspective on chiroptical methods, see a dedicated issue:-
Polavarapu, P. L.; Nafie, L. A., Beova, N. Eds. Chirality, 2009, 21, E1.
(16) For recent examples of this method to assign the absolute
configurations of organic molecules, see: (a) Diedrich, C.; Grimme, S.
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2009, 131, 9614. (f) Jacquemin, D.; Perpete, E. A.; Ciofini, I.; Adamo, C.;
Valero, R.; Zhao, Y.; Truhlar, D. G. J. Chem. Theory Comput. 2010, 6,
2071. (g) Pescitelli, G.; Di Pietro, S.; Cardellicchio, C.; Capozzi, M. A.
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E.; Zanasi, R.; Berova, N.; Rosini, C. J. Org. Chem. 2010, 75, 4600. (i)
Zea, A.; Alba, A.-N. R.; Mazzanti, A.; Moyano, A.; Rios, R. Org.
Biomol. Chem. 2011, 9, 6519. (j) Huang, X.-F.; Liu, Z.-M.; Geng, Z.-C.;
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Supporting Information Available. Text, figures, and
a table giving additional experimental procedures and
full spectroscopic data for all of the products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(17) Frisch, M. J. et al. Gaussian 03 (Revision B.04); Gaussian, Inc.,
Wallingford, CT, 2003. The full reference is given in the SI.
(18) Raushel, J.; Fokin, V. V. Org. Lett. 2010, 12, 4952.
The authors declare no competing financial interest.
D
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