Direct organocatalytic asymmetric approach to baylis-hillman-type products through a push-pull dienamine platform

Article abstract of DOI:10.1002/ejoc.201100075

A general process for the asymmetric synthesis of highly substituted 3-alkyl-Hagemann's esters was achieved for the first time through organocatalytic Michael or Baylis-Hillman-type (BH-type) reaction of Hagemann's esters with β-nitrostyrenes in the presence of a catalytic amount of L-(3,5-Me₂)₂DPP/thiourea. We have discovered, for the first time, the chiral BH-type products from Hagemann's esters with β-nitrostyrenes by utilizing the push-pull dienamine platform. A general process for the asymmetric synthesis of 3-alkyl-Hagemann's esters was achieved for the first time through organocatalytic Baylis-Hillman-type (BH-type) reaction of Hagemann's esters with β-nitrostyrenes in the presence of a catalyticamount of L-(3,5-Me₂)₂DPP/thiourea. The chiral BH-type products were discovered by utilizing the push-pull dienamine platform.

Full text of DOI:10.1002/ejoc.201100075

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201100075
Direct Organocatalytic Asymmetric Approach to Baylis–Hillman-Type
Products Through a Push–Pull Dienamine Platform
Dhevalapally B. Ramachary*[a] and Kinthada Ramakumar[a]
Keywords: Amines / Baylis–Hillman reactions / Asymmetric synthesis / Organocatalysis
A general process for the asymmetric synthesis of highly sub-
stituted 3-alkyl-Hagemann’s esters was achieved for the first
time through organocatalytic Michael or Baylis–Hillman-type
(BH-type) reaction of Hagemann’s esters with β-nitrostyrenes
in the presence of a catalytic amount of L-(3,5-Me2)2DPP/
thiourea. We have discovered, for the first time, the chiral
BH-type products from Hagemann’s esters with β-nitro-
styrenes by utilizing the push–pull dienamine platform.
ketone and benzaldehydes or α,β-unsaturated aldehydes
and imines or β-nitrostyrenes was reported.[3] Interestingly,
to the best of our knowledge, there is no asymmetric cou-
pling between α,β-unsaturated ketones and β-nitrostyrenes
at the α-position of enone through dienamine catalysis or
zwitterionic catalysis [Equation (1)]. With these objectives,
herein we have designed an asymmetric approach to the
Baylis–Hillman-type (BH-type) products from commer-
cially available Hagemann’s esters (enones) and nitroolefins
through push–pull dienamine catalysis as shown in Equa-
tion (1).[4]
Introduction
Asymmetric amino catalysis has become one of the most
important and widespread areas of research through imin-
ium or enamine activation of carbonyl compounds.[1] The
direct Michael addition of saturated carbonyl compounds
to β-nitrostyrenes through enamine activation by amino ca-
talysis provides an expedient access for the development of
functionalized molecules.[2] Recently Barbas[2a] and his co-
workers discovered the novel technology for organocatalytic
asymmetric Michael addition of aldehydes/ketones with ni-
troolefins that provides a general route to a variety of
However, the amine-catalyzed Michael reaction of enones
Michael adducts in good yields with high enantioselectivity, 1 with nitroolefins 2 is not known, and the resulting products
which is known as the “Barbas–Michael” reaction.[2] The 5/6 will have a wide range of uses in synthetic chemistry
advent of this enamine-based Barbas–Michael technology [Equation (1)]. Herein, we report a metal-free and novel tech-
triggered a burst of activity towards the synthesis of a huge nology for the asymmetric synthesis of substituted alkyl 2-
chiral pool of Michael adducts through biomimetic en- alkyl-3-(2-nitro-1-arylethyl)-4-oxo-cyclohex-2-enecarboxyl-
amine chemistry.
ates (BH-type products) 5/6 by using organocatalytic
Recently a very interesting dual catalytic system based Michael or BH-type reactions from easily available Hagem-
on the combination of enamine and Lewis base catalysis ann’s esters 1, nitroolefins 2 and amines 3 through push–
for the Baylis–Hillman-type reaction between methyl vinyl pull dienamine catalysis [Equation (1) and Figure 1].[4]
(1)
[a] School of Chemistry, University of Hyderabad
Results and Discussion
We initiated our preliminary studies of the BH-type reac-
tions by screening a number of known and novel organo-
Hyderabad 500046, India
Fax: +91-40-23012460
E-mail: ramsc@uohyd.ernet.in
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100075.
catalysts for the Michael addition of Hagemann’s ester 1a
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D. B. Ramachary, K. Ramakumar
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Figure 1. Library of catalysts and co-catalysts screened for the direct asymmetric BH-type reactions through a push–pull dienamine
platform.
with 1.5 equiv. β-nitrostyrene 2a. Some representative re- l-2-benzhydryl-pyrrolidine (3f), l-DPPOTMS (3g), l-[3,5-
sults are shown in Table 1. Interestingly, reaction of 1a with (CF3)2]2DPP (3j), Q-NH2 (3m) and CD-NH2 (3n) as cata-
1.5 equiv. β-nitrostyrene 2a in DMSO without a catalyst lysts for the BH-type reaction of 1a with 2a in toluene, al-
furnished a 1:1 diastereomeric ratio of BH-type products though they did not furnish Michael products 5aa/6aa
5aa/6aa in 61% yield (Table 1, Entry 1). Further, we tested (Table 1, Entries 8–10, 13, 16, 17). Interestingly, the reaction
the catalyst-free, solvent-induced reaction in other solvents of 1a and 2a with d-DPP (3h) as catalyst in toluene at 25 °C
such as C6H5CH3, DCM, THF, CHCl3, CH3CN, EtOH, for 24 h furnishes the Michael products 5aa/6aa in 35%
MeOH, CH3COCH3, H2O, 1,4-dioxane and iPrOH, but we yield with 1.1:1 dr and –49/–8% ee, respectively (Table 1,
did not observe the formation of BH-type products 5aa/6aa Entry 11). The reaction with d-(3,5-Me2)2DPP (3i) in tolu-
(results not presented in Table 1). We then screened various ene at 25 °C for 72 h furnishes the Michael products 5aa/
natural and unnatural amino acids [(S)-indoline-2-carbox- 6aa in 45% yield with 1.2:1 dr and –55/–11% ee, respec-
ylic acid, hydroxy l-proline, l-phenylalanine, l-tyrosine, l- tively (Table 1, Entry 12). Quinine 3k catalyzed the BH-type
serine, l-threonine, l-leucine, o-tert-butyl-l-threonine and reaction to furnish the products in 66% yield with 1:1.2 dr
l-proline (3a)] as catalysts for the Michael addition of and 42/40% ee, respectively (Table 1, Entry 14).
Hagemann’s ester 1a with β-nitrostyrene 2a in toluene, but
To further improve the asymmetric BH-type reaction,
these amino acids did not furnish the products 5aa/6aa (re- with regard to decreasing the reaction time and increasing
sults not presented in Table 1). Reaction of 1a with the ee/de and yields, we tested the Michael reaction of 1a
1.5 equiv. 2a with diamine 3b as catalyst in toluene fur- and 2a catalyzed by 3h and 3i with different amines/acids
nishes a 1.3:1dr of products 5aa/6aa in 30% yield with 2/ 3e and 4a–f as co-catalysts in toluene at 25 °C (Table 2, En-
0% ee, respectively (Table 1, Entry 3). (S)-Prolinol 3c also tries 1–12). After many experiments, we were happy to find
catalyzes the reaction of 1a with 2a in CHCl3 at 25 °C and that the BH-type reaction of 1a with 2a in toluene under
0 °C to furnish a 1.5:1 and 1.8:1 ratio of products 5aa/6aa catalysis by 30 mol-% l-(3,5-Me2)2DPP/thiourea (3i/4b) at
in 75 and 40% yields with 0/3% ee and 31/0% ee, respec- 25 °C for 72 h furnished the products 5aa/6aa in 70% yield
tively (Table 1, Entries 4,5). The same reaction under cataly- with 70/7% ee and 13% de, respectively (Table 2, Entry 8).
sis by 3c in toluene furnishes a 1:1.6 ratio of products 5aa/ After successful results with thiourea 4b as catalyst, we scre-
6aa in 60% yield with 11/2% ee. The reaction with (S)-2- ened d-(3,5-Me2)2DPP 3i as catalyst for BH-type reaction
(methoxymethyl)pyrrolidine (3d) furnishes the Michael to monitor the outcome of selectivity (Table 2, Entry 10).
products 5aa/6aa with improved yield and moderate de/ee The BH-type reaction of 1a with 2a in toluene with 30 mol-
values (Table 1, Entry 7). We also tested a number of pri- % d-(3,5-Me2)2DPP/thiourea (3i/4b) at 25 °C for 72 h fur-
mary and secondary amines such as chiral thiourea (3e), nished the products 5aa/6aa in 80% yield with 69/15% ee
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Organocatalytic Asymmetric Approach to Baylis–Hillman-Type Products
Table 1. Effect of solvent and catalyst on the direct asymmetric BH-type reaction of 1a with 2a.[a]
[a] Reactions were carried out in 0.1 m solvent with 1.5 equiv. nitrostyrene (2a) relative to Hagemann’s ester 1a in the presence of 30 mol-
% of catalyst 3. [b] Yield refers to the column purified product. [c] dr and ee were determined by HPLC analysis. [d] Reaction performed
at 0 °C. [e] Reaction was carried out in 0.15 m solvent.
Table 2. Effect of co-catalyst on the direct asymmetric BH-type reaction of 1a with 2a.[a]
[a] Reactions were carried out in solvent (0.1 m) with 1.5 equiv. β-nitrostyrene 2a relative to Hagemann’s ester 1a in the presence of
30 mol-% catalyst 3 and 30 mol-% co-catalyst. [b] Yield refers to the column purified product. [c] dr and ee were determined by HPLC
analysis. [d] 10 mol-% co-catalyst was used. [e] Reaction performed at 50 °C.
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and 5% de, respectively (Table 2, Entry 10). Unfortunately NMR analysis and also by correlation with amine-catalyzed
we were not so successful in achieving high yields and ee/de Barbas–Michael reactions.[2]
with chiral thiourea-based catalysts 3e/4f as co-catalysts rel-
After successful demonstration and understanding of the
ative to thiourea 4b as shown in Table 2, Entries 4, 9, and BH-type reaction of 1a with 2a under amine catalysis, the
11–12. Finally, The optimized conditions were found to be scope and generality of the BH-type reaction was investi-
25 °C in toluene with 30 mol-% l-(3,5-Me2)2DPP/thiourea gated with functionalized Hagemann’s esters 1a–d and β-
(3i/4b) as catalysts, which furnished the highly substituted nitrostyrenes 2a–i. Although there is no methodology avail-
products 5aa/6aa in 70% yield with 70/7% ee and 13% de able to prepare achiral Michael products 5/6, herein we have
(Table 2, Entry 8). The structure and absolute stereochemis- been able to prepare a library of achiral Michael products
try of the BH-type products 5aa/6aa were confirmed by 5/6 in good yields under pyrrolidine catalysis through a
Table 3. Synthesis of chiral BH-type products 5/6.[a,b]
[a] Reactions were carried out in toluene (0.1 m) with 1.5 equiv. 2 relative to Hagemann’s ester 1 in the presence of 30 mol-% catalyst (S)-
3i and 30 mol-% co-catalyst 4b. [b] Yield refers to the column purified product and dr/ee was determined by HPLC analysis. [c] Reaction
time is 96 h. [d] 30 mol-% (R)-3i used as catalyst.
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Organocatalytic Asymmetric Approach to Baylis–Hillman-Type Products
push–pull dienamine platform (Table S1, see Supporting In- styrenes 2b–i in toluene at 25 °C to study the asymmetric
formation). By BH-type reaction of Hagemann’s esters 1a– induction in the products 5/6. A series of β-nitrostyrenes
d with a structurally diverse group of electron-donating, 2 containing different functional groups were treated with
electron-withdrawing, halogenated and heterocyclic-substi- Hagemann’s esters catalyzed by 30 mol-% l-(3,5-Me2)2DPP
tuted β-nitrostyrenes 2a–i, the expected products 5/6 were with 30 mol-% thiourea 4b as co-catalyst at 25 °C for 72 h
generated in good to excellent yields with 1:1 to 6.6:1 dr in toluene to furnish asymmetric BH-type products 5/6 in
as shown in Table S1. The structure and regiochemistry of good yields with good to moderate ee/de values as shown
achiral Michael products 5/6 were confirmed by NMR in Table 3.
spectroscopy and mass analysis.
Treatment of methyl keto-ester 1b with β-nitrostyrene 2a
With the optimized reaction conditions in hand, we de- in toluene at 25 °C for 72 h with 30 mol-% l-(3,5-Me2)2-
cided to investigate the asymmetric BH-type reaction be- DPP/thiourea (l-3i/4b) furnished a 1:1.6 dr of products 5ba/
tween functionalized Hagemann’s esters 1b–d and β-nitro- 6ba in 60% yield with 61% and 11% ee, respectively
Scheme 1. Proposed reaction mechanism.
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D. B. Ramachary, K. Ramakumar
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(Table 3, Entry 2). Interestingly, asymmetric BH-type reac- β-nitrostyrene 2a catalyzed by l-3i/4b is illustrated in
tion of β-nitrostyrene 2a with tert-butyl keto-ester 1c cata- Scheme 1. First, reaction of l-3i with 1c generates the imine
lyzed by l-3i/4b in toluene at 25 °C for 3 d furnished a 2.1:1 cation 7, which transforms into chiral push–pull dienamine
ratio of 5ca/6ca in 61% yield with 84% and 40% ee, respec- 8 through proton elimination. Formation of the Michael
tively (Table 3, Entry 3). In a similar manner, high asym- product (R)-9 over (S)-9 through re-face attack of β-nitro-
metric induction was observed with tert-butyl keto-ester 1c styrene 2a with chiral push–pull dienamine 8 will be faster
as substrate with two more β-nitrostyrenes 2b,c catalyzed than that with si-face attack as shown in TS-1 and TS-2,
by l-3i/4b in toluene at 25 °C for 3–4 d as shown in Table 3, on the basis of hydrogen-bonding interactions with OH. In
Entries 4,5. Reaction of higher alkyl-substituted ethyl keto- situ hydrolysis followed by keto-enol tautomerism of
ester 1d with 2a also furnished the expected BH-type prod- Michael products (R)-9 and (S)-9 leads to highly substi-
ucts 5da/6da in 36% yield with 77/3% ee and 2:1 dr, respec- tuted (S)-11 and (R)-11. Formation of the thermodynami-
tively (Table 3, Entry 6). Asymmetric BH-type reaction of cally stable (S, S)-5ca occurs faster than that of the kinet-
Hagemann’s ester 1a with functionalized β-nitrostyrenes ically stable (S, R)-6ca product from (S)-11. The calculated
2d–i catalyzed by l-3i/4b gave the products 5ad/6ad–5ai/6ai heat of formation (ΔE) for the (S, S)-5ca product is 1.3 kcal/
in 45–74% yields with moderate to good ee/de values, mol more than that of the (S, R)-6ca product as revealed
respectively as shown in Table 3, Entries 7–14. These results by DFT calculations (see Supporting Information). Inter-
clearly suggest that the electronic nature of the functional estingly, the reverse trend in case of (R)-11, as shown in
groups on β-nitrostyrenes 2d–i may control the asymmetric Scheme 1, may be because of the weak CH–π interactions
induction in BH-type reactions by decreasing the nitro between the phenyl ring and olefinic methyl group.
group intereactions with thiourea (see, for example, 2e/2g).
Strong support for our proposed dienamine catalysis was
Further, to demonstrate the broad scope of this novel meth- obtained through performing the BH-type reaction on sim-
odology for generating an opposite enantiomer of BH-type ple cyclohexenone 1g with 2a catalyzed by 3a, 3b, 3c and
products 5/6, we used d-3i/4b as catalyst to furnish products 3i/4b (results not shown in Table 3). Interestingly, as we ex-
(+)-5aa/6aa in 73% yield with –70/–14% ee and (–)-5ca/ pected, we did not observe the formation of the BH-type
6ca in 52% yield with –85/–8% ee (Table 3, Entries 15,16). product from the above reactions, and this gives support to
Interestingly, we did not observe the formation of BH-type the formation of dienamines as intermediates from 1a–d
products from ethyl keto-ester 1e,f [R1 = H and Ph, see with 3/4.
Equation (1)] with 2a under the optimized conditions, may
The proposed transition states and also the ratio of
be because of electronic and steric factors (results not enantiomers were confirmed by converting the products
shown in Table 3). The structures and regiochemistry of the (+)-5aa/6aa and (+)-5ca/6ca into (R)-(+)-14aa and (S)-(–)-
BH-type products were confirmed by NMR spectroscopy 14aa, respectively, by using hydrolysis and the decarboxyl-
and mass analysis. The absolute configuration of products ation sequence, as shown in Scheme 2. Reaction of BH-type
were established by correlation with Barbas–Michael reac- product (+)-5aa/6aa [dr 1.2:1 and ee 70 and 14%] with
tions.[2]
LiOH·H2O in MeOH/THF/H2O at 25 °C for 0.75 h fur-
Although further studies are needed to firmly elucidate nished the keto acid (+)-12aa/13aa in 50% yield, which
the mechanism of asymmetric BH-type reactions through upon treatment with acid in THF gave the decarboxylated
push–pull dienamine catalysis, the possible reaction mecha- product (R)-(+)-14aa in 60% yield with 33% ee, as shown in
nism for stereoselective synthesis of products 5ca/6ca Scheme 2. In a similar manner, (S)-(–)-14aa was synthesized
through the reaction of tert-butyl Hagemann’s ester 1c and from (+)-5ca/6ca [dr 2.1:1 and ee 84 and 40%] in 50% yield
Scheme 2. Synthesis of functionalized chiral cyclohexenones 14aa.
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Organocatalytic Asymmetric Approach to Baylis–Hillman-Type Products
[2] For selected recent papers on enamine-based Michael reaction
with –44% ee, as shown in Scheme 2. Obtaining lower ee
values for (R)-/(S)-14aa through the hydrolysis–decarboxyl-
ation sequence on (+)-5aa/6aa and (+)-5ca/6ca can be ex-
plained by the conversion of diastereomers into enantio-
mers instead of epimerization, as shown in Schemes 1 and
2 (see HPLC data).
of carbonyl compounds with β-nitrostyrenes, see: a) K. Sakthi-
vel, W. Notz, T. Bui, C. F. Barbas III, J. Am. Chem. Soc. 2001,
123, 5260–5267; b) J. M. Betancort, K. Sakthivel, R. Thayum-
anavan, C. F. Barbas III, Tetrahedron Lett. 2001, 42, 4441–
4444; c) J. M. Betancort, C. F. Barbas III, Org. Lett. 2001, 3,
3737–3740; d) B. List, P. Pojarliev, H. J. Martin, Org. Lett.
2001, 3, 2423–2425; e) N. Mase, R. Thayumanavan, F. Tanaka,
C. F. Barbas III, Org. Lett. 2004, 6, 2527–2530; f) J. M. Betan-
cort, K. Sakthivel, R. Thayumanavan, F. Tanaka, C. F.
Barbas III, Synthesis 2004, 1509–1521; g) A. J. A. Cobb, D. A.
Longbottom, D. M. Shaw, S. V. Ley, Chem. Commun. 2004,
1808–1809; h) Y. Hayashi, H. Gotoh, T. Hayashi, M. Shoji,
Angew. Chem. 2005, 117, 4284; Angew. Chem. Int. Ed. 2005,
44, 4212–4215; i) D. Enders, M. R. M. Hüttl, C. Grondal, G.
Raabe, Nature 2006, 441, 861–863; j) P. García-García, A.
Ladépêche, R. Halder, B. List, Angew. Chem. 2008, 120, 4797;
Angew. Chem. Int. Ed. 2008, 47, 4719–4721; k) P. Dinér, A.
Kjærsgaard, M. A. Lie, K. A. Jørgensen, Chem. Eur. J. 2008,
14, 122–127; l) X. Zhu, F. Tanaka, R. A. Lerner, C. F.
Barbas III, I. A. Wilson, J. Am. Chem. Soc. 2009, 131, 18206–
18207; m) H. Uehara, C. F. Barbas III, Angew. Chem. 2009,
121, 10032; Angew. Chem. Int. Ed. 2009, 48, 9848–9852; n)
D. B. Ramachary, R. Sakthidevi, Org. Biomol. Chem. 2010, 8,
4259–4265.
Conclusions
In summary, we have described a chemo-, regio- and
enantioselective process for the synthesis of highly substi-
tuted alkyl 2-alkyl-3-(2-nitro-1-arylethyl)-4-oxocyclohex-2-
enecarboxylates 5/6 by amine catalysis. Herein, we de-
scribed the l-(3,5-Me2)2DPP/thiourea-catalyzed asymmet-
ric BH-type reactions from Hagemann’s esters 1 with ni-
troolefins 2 at ambient conditions. This novel asymmetric
BH-type reaction proceeds in good yields with high
enantioselectivity through a push–pull dienamine platform.
Furthermore, we demonstrated the application of chiral
BH-type products in the synthesis of highly functionalized
cyclohexenones.
[3]
For selected reviews on BH reactions, see: a) D. Basavaiah,
P. D. Rao, R. S. Hayama, Tetrahedron 1996, 52, 8001–8062; b)
D. Basavaiah, A. J. Rao, T. Satyanarayana, Chem. Rev. 2003,
103, 811–892; c) D. Basavaiah, B. S. Reddy, B. S. Singh, Chem.
Rev. 2010, 110, 5447–5674; For selected papers on nitroalkenes
as donors in BH reactions, see: d) M. Dadwal, R. Mohan, D.
Panda, S. M. Mobin, I. N. N. Namboothiri, Chem. Commun.
2006, 338–340; e) I. Deb, M. Dadwal, S. M. Mobin, I. N. N.
Namboothiri, Org. Lett. 2006, 8, 1201–1204; f) M. Dadwal,
S. M. Mobin, I. N. N. Namboothiri, Org. Biomol. Chem. 2006,
4, 2525–2528; g) N. Rastogi, R. Mohan, D. Panda, S. M. Mo-
bin, I. N. N. Namboothiri, Org. Biomol. Chem. 2006, 4, 3211–
3214; For selected recent papers on organocatalytic BH-type
reactions, see: h) M. Shi, J.-K. Jiang, C.-Q. Li, Tetrahedron
Lett. 2002, 43, 127–130; i) J. E. Imbriglio, M. M. Vasbinder,
S. J. Miller, Org. Lett. 2003, 5, 3741–3743; j) C. E. Aroyan,
M. M. Vasbinder, S. J. Miller, Org. Lett. 2005, 7, 3849–3851; k)
M. M. Vasbinder, J. E. Imbriglio, S. J. Miller, Tetrahedron 2006,
62, 11450–11459; l) S.-H. Chen, B.-C. Hong, C.-F. Su, S. Sar-
shar, Tetrahedron Lett. 2005, 46, 8899–8903; m) N. Utsumi, H.
Zhang, F. Tanaka, C. F. Barbas III, Angew. Chem. 2007, 119,
1910; Angew. Chem. Int. Ed. 2007, 46, 1878–1880; n) J. Vesely,
P. Dziedzic, A. Córdova, Tetrahedron Lett. 2007, 48, 6900–
6904.
Experimental Section
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data for new prod-
ucts, and complete details about the synthesis of new products are
available.
Acknowledgments
This work was made possible by a grant from the Department of
Science and Technology (DST), New Delhi [Grant No.: DST/SR/
S1/OC-65/2008]. K. R. thanks the Council of Scientific and Indus-
trial Research (CSIR) (New Delhi) for a research fellowship.
[1] For selected reviews on asymmetric amino catalysis, see: a) B.
List, Synlett 2001, 11, 1675–1686; b) W. Notz, F. Tanaka, C. F.
Barbas III, Acc. Chem. Res. 2004, 37, 580–591; c) B. List, Acc.
Chem. Res. 2004, 37, 548–557; d) B. List, Chem. Commun.
2006, 819–824; e) S. Sulzer-Mossé, A. Alexakis, Chem. Com-
mun. 2007, 3123–3135; f) A. Erkkila, I. Majander, P. M. Pihko,
Chem. Rev. 2007, 107, 5416–5470; g) S. Mukherjee, J. W. Yang,
S. Hoffmann, B. List, Chem. Rev. 2007, 107, 5471–5569; h)
C. F. Barbas III, Angew. Chem. 2008, 120, 44; Angew. Chem.
Int. Ed. 2008, 47, 42–47; i) P. Melchiorre, M. Marigo, A. Car-
lone, G. Bartoli, Angew. Chem. 2008, 120, 6232; Angew. Chem.
Int. Ed. 2008, 47, 6138–6171; j) P. Melchiorre, Angew. Chem.
2009, 121, 1386; Angew. Chem. Int. Ed. 2009, 48, 1360–1363;
k) S. Bertelsen, K. A. Jørgensen, Chem. Soc. Rev. 2009, 38,
2178–2189.
[4] For selected recent papers on push–pull dienamines, see: a)
D. B. Ramachary, K. Ramakumar, V. V. Narayana, J. Org.
Chem. 2007, 72, 1458–1463; b) D. B. Ramachary, V. V. Naray-
ana, K. Ramakumar, Eur. J. Org. Chem. 2008, 3907–3911; c)
D. B. Ramachary, K. Ramakumar, V. V. Narayana, Chem. Eur.
J. 2008, 14, 9143–9147; d) D. B. Ramachary, V. V. Narayana,
M. S. Prasad, K. Ramakumar, Org. Biomol. Chem. 2009, 7,
3372–3378; e) D. B. Ramachary, K. Ramakumar, A. Bharani-
shashank, V. V. Narayana, J. Comb. Chem. 2010, 12, 855–876.
Received: January 18, 2011
Published Online: March 9, 2011
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