Asymmetric synthesis of 2,3,4-trisubstituted functionalised tetrahydrofurans via an organocatalytic michael addition as key step

Article abstract of DOI:10.1002/adsc.200900879

The organocatalytic Michael addition of various aldehydes to (2E,4E)-ethyl 5-nitropenta-2,4dienoate has been achieved under the catalysis of diphenylprolinol trimethylsilyl ether furnishing the products in good to excellent yields (61-94%) and high stereo

Full text of DOI:10.1002/adsc.200900879

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DOI: 10.1002/adsc.200900879
Asymmetric Synthesis of 2,3,4-Trisubstituted Functionalised
Tetrahydrofurans via an Organocatalytic Michael Addition as
Key Step
Dieter Enders,a,* Chuan Wang,a and Andreas Greba
a
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax : (+49)-241-809-2127; e-mail: enders@rwth-aachen.de
Received: December 18, 2009; Published online: March 23, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900879.
Abstract: The organocatalytic Michael addition of
various aldehydes to (2E,4E)-ethyl 5-nitropenta-2,4-
dienoate has been achieved under the catalysis of
diphenylprolinol trimethylsilyl ether furnishing the
products in good to excellent yields (61–94%) and
high stereoselectivities (dr up to >98:2, ee=97 to
>99%). Starting from these Michael adducts, 2,3,4-
trisubstituted functionalized tetrahydrofurans are
available in two steps by reduction of the aldehyde
followed by an intramolecular oxa-Michael addition
in good yields (54–76%) and stereoselectivities (dr
up to >95:5, ee=97 to >99%).
Scheme 1. Asymmetric synthesis of 2,3,4-trisubstituted tetra-
Keywords: asymmetric synthesis; Michael addition;
hydrofurans – retrosynthetic analysis.
nitroalkenes; organocatalysis; tetrahydrofurans
tuted tetrahydrofurans are frequently found as sub-
AHCTUNGTREGUNuNN nits in many natural products such as Annonaceous
acetogenins,[6] lignans,[7] polyether ionophores[8] and
After intensive investigations and tremendous prog- macrodiolides,[9] which possess various biological ac-
ress in the last decade, organocatalysis has been de- tivities including antitumour, antihelmic, antimalarial,
veloped to one of the main branches of asymmetric antimicrobial and antiprotozoal activity. Due to these
synthesis.[1] Within the field of organocatalysis, the important biological activities, many efforts have
amine-catalysed Michael addition of aldehydes and been made for the stereoselective synthesis of poly-
ketones to nitroalkenes via an enamine intermediate, substituted tetrahydrofurans.[10,11]
which was independently discovered by List et al.,
Initially we performed the reaction with (2E,4E)-
Barbas et al. and our group,[2] has attracted great in- ethyl 5-nitropenta-2,4-dienoate (2) and 3 equivalents
terest in recent years, since the resulting g-nitroalde- of propanal (1a) in dichloromethane at room temper-
hydes or ketones are precursors of many important ature. The catalyst diphenylprolinol silyl ether 4[12,13]
compounds such as g-amino acids. Many efforts have (5 mol%) was used, which shows good catalytic activi-
been made to develop new efficient organocatalysts ty and excellent asymmetric induction in enamine-
to expand the substrate spectrum of this Michael re- mediated Michael additions. Encouragingly, the reac-
action or to involve it as a key step in domino reac- tion was complete within an hour affording the nitro-
tions.[3–5] Our attention was drawn to (2E,4E)-ethyl 5- aldehyde 3a in excellent yield (89%) and enantiose-
nitropenta-2,4-dienoate (B) as a Michael acceptor, be- lectivities (98% ee syn, 94% ee anti). However, only a
cause the 1,4-adducts (C) may be converted into moderate diastereoselectivity (syn:anti=77:23) was
2,3,4-trisubstituted tetrahydrofurans (E) in two steps achieved (Table 1, entry 1). A brief solvent screen was
by reduction of the aldehyde followed by an intramo- undertaken and it was found that excellent enantio-
lecular oxa-Michael addition (Scheme 1). Polysubsti- meric excesses were obtained in every solvent used
Adv. Synth. Catal. 2010, 352, 987 – 992
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987

Dieter Enders et al.
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Table 1. Catalyst and solvent screening for the organocatalytic Michael addition.[a]
Entry
Catalyst
Solvent
t [h]
Yield [%][b]
dr[c] syn:anti
ee [%][d] syn:anti
1
2
3
4
5
6
7
8
4
4
4
4
5
6
7
8
4
4
CH2Cl2
DMF
1
2
2
2
2
2
2
2
9
89
60
90
87
71
70
75
20
95
94
77:23
83:17
83:17
84:16
56:44
60:40
84:16
84:16
86:14
95:5
98, 94
95, 70
>99, 86
>99, 77
54, 54
MeOH
toluene
toluene
toluene
toluene
toluene
toluene
toluene
À70, À71
89, 77
n.d.[e]
9[f]
10 g]
>99, 94
>99,>99
24
[a]
Unless otherwise specified, reactions were performed on a 1 mmol scale of (2E,4E)-ethyl 5-nitropenta-2,4-dienoate (2)
using 3.0 equiv. of propanal (1a), 5 mol% catalyst 4, 5, 6, 7 or 8 at room temperature in 2.0 mL solvent.
Yield of the isolated product after flash chromatography.
Determined by H NMR spectroscopy on the isolated product.
Determined by HPLC analysis on a chiral stationary phase of the corresponding a,ß-unsaturated ester.
[b]
[c]
[d]
[e]
[f]
1
Not determined.
Carried out at 08C.
Carried out at À108C.
[g]
(Table 1, entries 2–4). The best outcome with respect temperatures (Table 1, entries 9 and 10). To our de-
to the diastereoselectivity (84:16) was acquired when light, the diastereomeric ratio increased to an excel-
the reaction was carried out in toluene (Table 1, lent level (95:5) when the reaction was carried out at
entry 4). Next, four additional secondary amines were À108C. Importantly, the yield of this process re-
evaluated for this Michael addition with toluene as mained high (94%) with a perfect enantioselectivity
solvent. In the cases of the diamine 5 and (R)-2-me- (>99% ee syn, >99% ee anti, Table 1, entry 10).
thoxymethylpyrrolidine (RMP, 6),[14] the reaction oc-
Based on these results, we investigated the scope of
curred with good yields but poor stereoselectivities the reaction by variation of the structure of the alde-
(Table 1, entries 5 and 6). Under the catalysis of (S)- hydes (Table 2). In the cases of butanal (1b), pentanal
2-(1-methoxy-1-ethylpropyl)-pyrrolidine (7),[15] no im- (1c), 3-phenylpropanal (1f) and 4-methoxyphenylpro-
proved results with regard to both yield and stereose- panal (1h), the reactions were carried out at 08C and
lectivity were obtained in comparison to catalyst 4 warmed up to room temperature yielding the prod-
(Table 1, entry 7). Employing diarylprolinol silyl ether ucts in good to high yields (61–87%) and stereoselec-
8 [Ar=3,5-(CF3)2C6H3] as catalyst, very low conver- tivities (dr: 87:13 to 95:5, ee: >97%). Hexanal (1e)
sion of the starting material was observed after two reacted well at room temperature, providing the Mi-
hours (Table 1, entry 8). In order to improve the dia- chael adduct 3e in a good yield (76%) and excellent
stereoselectivity, the reaction was performed at lower stereoselectivity (dr: 94:6, ee: 99%). When 3-methyl-
988
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 987 – 992

Asymmetric Synthesis of 2,3,4-Trisubstituted Functionalised Tetrahydrofurans
Table 2. Organocatalytic Michael additions of aldehydes 1 to (2E,4E)-ethyl 5-nitropenta-2,4-dienoate (2).[a]
3
R
T [8C][b]
Yield [%][c]
dr[d] syn:anti
ee [%][e]
a
b
c
d
e
f
Me
Et
n-Pr
i-Pr
n-Bu
Bn
c-Hex
4-MeOC6H4CH2
À10
94
87
81
78
76
74
74
61[g]
95:5
93:7
95:5
92:8
>99
>99
98
0 to r.t.
0 to r.t.
r.t. to 45
r.t.
98
94:6
À99[f]
À>99[f]
99
0
r.t.
0 to r.t.
90:10
>98:2
87:13
g
h
97
[a]
Unless otherwise specified, reactions were performed on a 1 mmol scale of (2E,4E)-ethyl 5-nitropenta-2,4-dienoate (2)
using 3.0 equiv. of aldehyde (1), 5 mol% catalyst 4 in 2.0 mL solvent.
For the reaction time, see Supporting Information.
[b]
[c]
[d]
[e]
[f]
Yield of the isolated product after flash chromatography.
1
Determined by H NMR spectroscopy on the isolated product.
Determined by HPLC analysis on a chiral stationary phase of the corresponding a,ß-unsaturated ester.
(R)-4 was used as the catalyst.
Yield after reduction of the aldehyde to the corresponding alcohol (BH3·THF, CHCl3, À138C).
[g]
butanal (1d) was utilised as the donor, the reaction
mixture was heated to 458C to achieve a good yield
(78%), whereas the stereoselctivity (dr: 92:8, ee:
98%) remained high. Notably, the reaction employing
the sterically hindered cyclohexylacetaldehyde (1g)
also proceeded well at room temperature, furnishing
the corresponding g-nitroaldehyde 3g in a virtually
enantiopure from (dr: >98:2, ee: 99%) and in a good
yield (74%).
Table 3. Asymmetric synthesis of 2,3,4-trisubstituted tetrahy-
drofurans 10.[a]
In order to reach the title functionalised 2,3,4-tri-
substituted tetrahydrofurans and starting from the Mi-
chael adducts 3 only two further steps are necessary.
Firstly, we performed the chemoselective reduction of
the aldehyde group in the presence of the a,ß-unsatu-
rated ester moiety, which was accomplished by utilis-
ing borane as the reducing reagent. Subsequently, the
resulting alcohols 9 were treated with KO-t-Bu in
THF at À558C furnishing the 2,3,4-trisubstituted tet-
rahydrofurans 10 in good yields (54–76%) via an in-
tramolecular oxa-Michael addition (Table 3). In most
cases high diastereoselectivities (dr: >95:5) were ob-
tained. A relatively low diastereomeric ratio of dr
>90:10 was observed when the alcohol 9f bearing a
benzyl group was used as substrate.
10
R
Yield [%][b]
dr[c]
ee [%][d]
b
c
d
e
f
Et
54
71
76
63
58
62
>95:5
>95:5
>95:5
>95:5
>90:10
>95:5
>99
98
98
À99
À>99
99
n-Pr
i-Pr
n-Bu
Bn
g
c-Hex
The relative configuration of the main diastereo-
mers of the tetrahydrofurans 10 was determined by
NOE measurements on compound 10b. It turned out
that the ethyl and the nitromethylene groups of 10b
have a cis-relationship, whereas the ester-bearing sub-
stituent is trans to the other two groups (Figure 1).
[a]
[b]
For the reaction conditions, see Supporting Information.
Yield of the isolated product after flash chromatography
after two steps.
[c]
[d]
Determined by 1H NMR spectroscopy on the isolated
product.
Based on the results of nitroaldehyde 3.
Adv. Synth. Catal. 2010, 352, 987 – 992
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Dieter Enders et al.
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(1.0 mmol, 1 equiv.) in toluene (2 mL) was added aldehyde 1
(3.0 mmol, 3 equiv.) at the temperature displayed in Table 2.
The reaction mixture was then stirred for the time given in
the Supporting Information. The crude product was purified
by flash chromatography on silica gel (pentane:ether mix-
ture) affording the corresponding g-nitroaldehyde 3 as a
yellow oil or solid.
Figure 1. Results of the NOE mesurements of 10b.
General Procedure of the Borane Reduction
To a solution of g-nitroaldehyde 3 (1.0 mmol, 1 equiv.) in
chloroform (4 mL) was added borane (1M in THF,
1.7 mmol, 1.7 equiv.) at À138C. After stirring for 2 h, the re-
action was quenched with acetic acid (5 mmol, 5 equiv.) and
the solvent was evaporated. The crude product was purified
by flash chromatography on silica gel (pentane:ether mix-
ture) affording the corresponding d-nitro alcohol 9 as a col-
ourless oil.
General Procedure of the Intramolecular Oxa-
Michael Addition
Figure 2. Proposed transition state explaining the (R,R)-con-
figuration of the Michael products 3.
To a solution of the d-nitro alcohol 9 (1.0 mmol, 1 equiv.) in
THF (10 mL) was added KO-t-Bu (1.4 mmol, 1.4 equiv-) at
À558C. After stirring for 2.5 h, the reaction was quenched
with saturated NH4Cl (10 mL) and the mixture was extract-
ed three times with ether (40 mL). The combined organic
phases were dried over MgSO4. After evaporation of the
solvent, the crude product was purified by flash chromatog-
raphy on silica gel (pentane:ether mixture) affording the
corresponding tetrahydrofurans 10 as a colourless oil.
Based on these results, the Michael adducts 3 must
have the syn-configuration.
According to the obtained results in other (S)-di-
phenylprolinol TMS ether [S-(4)] catalysed Michael
additions to nitroalkenes,3–5 the absolute configuration
of the a-C stereogenic centre of the aldehyde was
always assigned to be R, if the a-substituent has the
minor priority according to the CIP rules. A proposed
transition state is illustrated in Figure 2, in which
(2E,4E)-ethyl 5-nitropenta-2,4-dienoate (2) ap-
proaches the Si-face of the enamine, while its Re-face
is effectively shielded by the bulky side chain of the
catalyst. In accordance with all previous results the
absolute configuration of the g-nitroaldehydes 3 is as-
signed as R,R.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft (priority program Organocatalysis) and the Fonds der
Chemischen Industrie. We thank the former Degussa AG and
BASF AG for the donation of chemicals.
In summary, we have developed an organocatalytic
Michael addition of various aldehydes to ethyl 5-ni-
tropenta-2,4-dienoate under diphenyl-prolinol silyl
ether catalysis providing the functionalized nitroalde-
hydes in good to excellent yields (61–94%) and high
stereoselectivities (dr: up to >98:2, ee: >97%). Fur-
thermore, the Michael adducts have been successfully
converted to 2,3,4-trisubstituted tetrahydrofurans by
means of an aldehyde reduction and subsequent intra-
molecular oxa-Michael addition in good yields (54–
76%) and stereoselectivities (dr: up to >95:5, ee=97
to >99%).
References
[1] For recent reviews on organocatalysis, see: a) A. Ber-
kessel, H. Grçger, Asymmetric Organocatalysis, Wiley-
VCH, Weinheim, 2005; b) P. I. Dalko, Enantioselective
Organocatalysis: Reactions and Experimental Proce-
dures, Wiley-VCH, Weinheim, 2007; c) H. Pellissier,
Tetrahedron 2007, 63, 9267–9331; d) Special issue orga-
nocatalysis, Chem. Rev. 2007, 107, 5413–5883, guest
editor B. List; e) R. M. De Figueiredo, M. Christmann,
Eur. J. Org. Chem. 2007, 2575–2600; f) D. Enders, C.
Grondal, M. R. M. Hꢁttl, Angew. Chem. 2007, 119,
1590–1601; Angew. Chem. Int. Ed. 2007, 46, 1570–
1581; g) A. Dondoni, A. Massi, Angew. Chem. 2008,
120, 4716–4739.; Angew. Chem. Int. Ed. 2008, 47,
4638–4660; h) D. Enders, A. A. Narine, J. Org. Chem.
2008, 73, 7857–7870; i) H. Kotsuki, H. Ikishima, A.
Okuyama, Heterocycles 2008, 75, 757–797; j) H. Kotsu-
ki, H. Ikishima, A. Okuyama, Heterocycles 2008, 75,
493–529; k) P. Melchiorre, M. Marigo, A. Carlone, G.
Bartoli, Angew. Chem. 2008, 120, 6232–6265; Angew.
Experimental Section
General Procedure of the Organocatalytic Michael
Addition
To a solution of diphenylprolinol TMS ether (4) (0.05 mmol,
5 mol%) and (2E,4E)-ethyl 5-nitropenta-2,4-dienoate (2)
990
asc.wiley-vch.de
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 987 – 992

Asymmetric Synthesis of 2,3,4-Trisubstituted Functionalised Tetrahydrofurans
Chem. Int. Ed. 2008, 47, 6138–6171; l) K. A. Jørgensen,
S. Bertelsen, Chem. Soc. Rev. 2009, 38, 2178–2189;
m) M. Bella, T. Gasperi, Synthesis 2009, 1583–1614;
n) C. Grondal, M. Jeanty, D. Enders, Nature Chemistry
2010, 2, 167–178.
kenes via enamine intermediates, see: a) D. Enders,
M. R. M. Hꢁttl, C. Grondal, G. Raabe, Nature 2006,
441, 861–863; b) D. Enders, M. R. M. Hꢁttl, J. Runsink,
G. Raabe, B. Wendt, Angew. Chem. 2007, 119, 471–
473; Angew. Chem. Int. Ed. 2007, 46, 467–469; c) Y.
Hayashi, T. Okano, S. Aratake, D. Hazelard, Angew.
Chem. 2007, 119, 5010–5013; Angew. Chem. Int. Ed.
2007, 46, 4922–4925; d) D. Enders, M. R. M. Hꢁttl, G.
Raabe, J. W. Bats, Adv. Synth. Catal. 2008, 350, 267–
279; e) D. Enders, C. Wang, J. W. Bats, Angew. Chem.
2008, 120, 7649–7653; Angew. Chem. Int. Ed. 2008, 47,
7539–7542; f) F.-L. Zhang, A.-W. Xu, Y.-F. Gong, M.-
H. Wei, X.-L. Zhang, Chem. Eur. J. 2009, 15, 6815–
6818; g) C.-L. Cao, Y.-Y. Zhou, J. Zhou, X.-L. Sun, Y.
Tang, Y.-X. Li, G.-Y. Li, J. Sun, Chem. Eur. J. 2009, 15,
11384–11389; h) D. Zhu, M. Lu, L. Dai, G. Zhong,
Angew. Chem. 2009, 121, 6205–6208; Angew. Chem.
Int. Ed. 2009, 48, 6089–6092; i) L.-Y. Wu, G. Benciven-
ni, M. Mancinelli, A. Mazzanti, G. Bartoli, P. Mel-
chiorre, Angew. Chem. 2009, 121, 7332–7335; Angew.
Chem. Int. Ed. 2009, 48, 7196–7199; j) S. Belot, K. A.
Vogt, C. Besnard, N. Krause, A. Alexakis, Angew.
Chem. 2009, 121, 9085–9088; Angew. Chem. Int. Ed.
2009, 48, 8923–8926; k) S. T. Scroggins, Y. Chi, J. M.
Frꢂchet, Angew. Chem. 2009, 121, DOI: 10.1002/
ange.200902945; l) D. Enders, R. Krꢁll, W. Bettray,
Synthesis 2010, 567–572.
[2] a) B. List, P. Pojarliev, H. J. Martin, Org. Lett. 2001, 3,
2423–2425; b) J. M. Betancort, C. F. Barbas III, Org.
Lett. 2001, 3, 3737–3740; c) J. M. Betancort, K. Sakthi-
vel, R. Thayumanavan, C. F. Barbas III, Tetrahedron
Lett. 2001, 42, 4441–4444; d) D. Enders, A. Seki, Syn-
lett 2002, 26–28;.
[3] For recent reviews on organocatalyzed 1,4-additions,
see: a) S. B. Tsogoeva, Eur. J. Org. Chem. 2007, 1701–
1716; b) S. Sulzer-Mossꢂ, A. Alexakis, Chem. Commun.
2007, 3123–3135; c) J. L. Vicario, D. Badꢃa, L. Carrillo,
Synthesis 2007, 2065–2092; d) D. Almas¸i, D. A. Alonso,
C. Nꢄjera, Tetrahedron: Asymmetry 2007, 18, 299–365;
e) D. Enders, C. Wang, J. X. Liebich, Chem. Eur. J.
2009, 15, 11058–11076.
[4] For selected examples of organocatalytic Michael addi-
tions to nitroalkenes via enamine intermediates, see:
a) N. Mase, R. Thayumanavan, F. Tanaka, C. F. Barbas
III, Org. Lett. 2004, 6, 2527–2530; b) A. J. A. Cobb,
D. A. Longbottom, D. M. Shaw, S. V. Ley, Chem.
Commun. 2004, 1808–1809; c) W. Wang, J. Wang, H.
Li, Angew. Chem. 2005, 117, 1393–1395; Angew. Chem.
Int. Ed. 2005, 44, 1369–1371; d) Y. Hayashi, H. Gotoh,
T. Hayashi, M. Shoji, Angew. Chem. 2005, 117, 4284–
4287; Angew. Chem. Int. Ed. 2005, 44, 4212–4215;
e) S. H. McCooey, S. J. Connon, Org. Lett. 2007, 9,
599–602; f) D. Enders, S. Chow, Eur. J. Org. Chem.
2006, 4578–4584; g) M. P. Lalonde, Y. Chen, E. N. Ja-
cobsen, Angew. Chem. 2006, 118, 6514–6518; Angew.
Chem. Int. Ed. 2006, 45, 6366–6370; h) S. Zhu, S. Yu,
D. Ma, Angew. Chem. 2008, 120, 555–558; Angew.
Chem. Int. Ed. 2008, 47, 545–548; i) M. Wiesner, J. D.
Revell, H. Wennemers, Angew. Chem. 2008, 120, 1897–
1900; Angew. Chem. Int. Ed. 2008, 47, 1871–1874; j) Y.
Chi, L. Guo, N. A. Kopf, S. H. Gellman, J. Am. Chem.
Soc. 2008, 130, 5608–5609; k) M. Wiesner, J. D. Revell,
S. Tonazzi, H. Wennemers, J. Am. Chem. Soc. 2008,
130, 5610–5611; l) P. Garcꢃa-Garcꢃa, A. LadꢂpÞche, R.
Hadler, B. List, Angew. Chem. 2008, 120, 4797–4799;
Angew. Chem. Int. Ed. 2008, 47, 4719–4721; m) Y.
Hayashi, T. Itoh, M. Ohkubo, H. Ishikawa, Angew.
Chem. 2008, 120, 4800–4802; Angew. Chem. Int. Ed.
2008, 47, 4722–4724; n) N. Ruiz, E. Reyes, J. L. Vicar-
io, D. Badꢃa, L. Carrillo, U. Uria, Chem. Eur. J. 2008,
14, 9357–9367; o) T. Mandal, C.-G. Zhao, Angew.
Chem. 2008, 120, 7828–7831; Angew. Chem. Int. Ed.
2008, 47, 7714–7717; p) S. Belot, A. Massaro, A. Tenti,
A. Mordoni, A. Alexakis, Org. Lett. 2008, 10, 4557–
4560; q) B. Han, Y.-C. Xiao, Z.-Q. He, Y.-C. Chen,
Org. Lett. 2009, 11, 4660–4663; r) L. Guo, Y. Chi,
A. M. Almeida, I. A. Guzei, B. K. Parker, S. H. Gell-
man, J. Am. Chem. Soc. 2009, 131, 16018–16020; s) T.
He, J.-Y. Qian, H.-L. Song, X.-Y. Wu, Synlett 2009,
3195–3197; t) H. Uehara, C. F. Barbas III, Angew.
Chem. 2009, 121, 10032–10036; Angew. Chem. Int. Ed.
2009, 48, 9848–9851.
[6] A. Bermejo, B. Figadere, M.-C. Zafra-Polo, I. Barra-
china, E. Estornell, D. Cortes, Nat. Prod. Rep. 2005, 22,
269–303.
[7] M. Saleem, H. J. Kim, M.-S. Ali, Y. S. Lee, Nat. Prod.
Rep. 2005, 22, 696–716.
[8] M. M. Faul, B. E. Huff, Chem. Rev. 2000, 100, 2407–
2474.
[9] E. J. Kang, E. Lee, Chem. Rev. 2005, 105, 4348–4378.
[10] For recent reviews on the stereoselective synthesis of
tetrahydrofurans, see: a) J. P. Wolfe, M. B. Hay, Tetrahe-
dron 2007, 63, 261–290; b) G. Jalce, X. Franck, B. Figa-
dꢅre, Tetrahedron: Asymmetry 2009, 20, 2537–2581.
[11] For recent selected examples of stereoselective synthe-
ses of tetrahydrofurans, see ref.[5j] and: a) J. P. Wolfe,
M. A. Rossi, J. Am. Chem. Soc. 2004, 126, 1620–1621;
b) T. K. Sarkar, S. A. Haque, A. Basak, Angew. Chem.
2004, 116, 1441–1443; Angew. Chem. Int. Ed. 2004, 43,
1417–1419; c) H. Y. Jang, F. W. Hughes, H. Gong, J.
Zhang, J. S. Brodbelt, M. J. Kirsche, J. Am. Chem. Soc.
2005, 127, 6174–6175; d) P. D. Pohlhaus, J. S. Johnson,
J. Am. Chem. Soc. 2005, 127, 16014–16015; e) T. J. Do-
nohoe, S. Butterworth, Angew. Chem. 2005, 117, 4844–
4846; Angew. Chem. Int. Ed. 2005, 44, 4766–4768;
f) J. S. Nakhla, J. W. Kampf, J. P. Wolfe, J. Am. Chem.
Soc. 2006, 128, 2893–2901; g) C. G. Nasveschuk, N. T.
Jui, T. Rovis, Chem. Commun. 2006, 3119–3121;
h) S. N. Chavre, H. Choo, J. H. Cha, A. N. Pae, K. I.
Choi, Y. S. Cho, Org. Lett. 2006, 8, 3617–3619; i) Z.
Zhang, A. S. Widenhoefer, Angew. Chem. 2007, 119,
287–289; Angew. Chem. Int. Ed. 2007, 46, 283–285;
j) A. T. Parsons, J. S. Johnson, J. Am. Chem. Soc. 2009,
131, 3122–2123.
[12] For first applications of diphenylprolinol TMS ether
[5] For selected examples of organocatalytic domino or
catalysis, see ref.[4d] and M. Marigo, T. C. Wabnitz, D.
tandem reactions involving Michael addition to nitroal-
Adv. Synth. Catal. 2010, 352, 987 – 992
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
991

Dieter Enders et al.
COMMUNICATIONS
Fielenbach, K. A. Jørgensen, Angew. Chem. 2005, 117,
804–807; Angew. Chem. Int. Ed. 2005, 44, 794–797.
[13] For reviews on diphenylprolinol TMS ether catalysis,
see: a) C. Palomo, A. Mielgo, Angew. Chem. 2006, 118,
8042–8046; Angew. Chem. Int. Ed. 2006, 45, 7876–
7880; b) A. Mielgo, C. Palomo, Chem. Asian J. 2008, 3,
922–948.
[14] Review: D. Enders, M. Klatt, Synthesis 1996, 1403–
1418.
[15] D. Enders, H. Kipphardt, P. Gerdes, L. J. BreÇa-Valle,
V. Bhushan, Bull. Soc. Chim. Belg. 1988, 97, 691–704.
992
asc.wiley-vch.de
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 987 – 992