Organocatalytic enantioselective sulfenylation of β-keto phosphonates: A convenient approach to construct hetero-quaternary stereocenters

Article abstract of DOI:10.1002/adsc.201000679

The highly effective and enantioselective sulfenylation of β-keto phosphonates catalyzed by α,α-diaryl-L-prolinols has been developed. The optically active α-sulfenylated β-keto phosphonates could be obtained under mild reaction conditions in good yields

Full text of DOI:10.1002/adsc.201000679

COMMUNICATIONS
DOI: 10.1002/adsc.201000679
Organocatalytic Enantioselective Sulfenylation of b-Keto
Phosphonates: A Convenient Approach to Construct Hetero-
Quaternary Stereocenters
Aijun Lin,a Ling Fang,a Xi Zhu,a Chengjian Zhu,a,b,* and Yixiang Chengc,*
a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing University,
Nanjing 210093, Peopleꢀs Republic of China
Fax : (+86)-25-8359-4886; e-mail: cjzhu@nju.edu.cn
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, Shanghai 200032, Peopleꢀs Republic of China
School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of MOE, Nanjing University,
c
Nanjing 210093, Peopleꢀs Republic of China
Fax : (+86)-25-8368-6508; e-mail: yxcheng@nju.edu.cn
Received: August 31, 2010; Revised: December 5, 2010; Published online: March 2, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000679.
Abstract: The highly effective and enantioselective
sulfenylation of b-keto phosphonates catalyzed by
a,a-diaryl-l-prolinols has been developed. The opti-
cally active a-sulfenylated b-keto phosphonates
could be obtained under mild reaction conditions in
good yields (up to 92%) and with excellent enantio-
selectivities (up to 92% ee).
plementary procedure, several groups have successful-
ly reported the asymmetric sulfenylation of alde-
hydes,[8] ketones,[9] and substituted piperazine-2,5-
diones[10] with different electrophilic sulfur reagents.
Jørgensen[4a] described the first enantioselective a-sul-
fenylation of b-keto esters using the Cinchona alka-
loid derivatives as the catalysts, which gave the prod-
ucts in up to 91% ee. Moreover, Togni also successful-
ly reported the analogous transformation with chiral
Ti complexes.[4b–d]
Keywords: hetero-quaternary stereocenters; b-keto
phosphonates; organocatalysis; sulfenylation
As a result of the increasing applications of chiral
phosphonic acids and derivatives in peptide, pharma-
ceutical and medicinal chemistry, the development of
efficient methods for the synthesis of these com-
A quaternary stereocenter is an essential structural pounds is of particular significance.[11] On the basis of
motif in numerous biologically active natural mole- the feature of bond-forming reactions, a few reports
À
cules and pharmaceuticals, and the construction of on enantioselective a-amination (C N bond forma-
such a framework in an asymmetric catalytic manner tion, path a),[12,13b] a-halogenation (C Cl or C F bond
À
À
is one of the most challenging tasks for organic chem- formation, path b),[13] and Mannich-type reaction (C
À
ists. In the past few years, a lot of effort has been C bond formation, paths c and d)[14] of b-keto phos-
made on it and many powerful catalysts were devel- phonates catalyzed by chiral metal complexes have
oped.[1] The introduction of an electrophilic hetero been reported (Scheme 1).
atom to b-dicarbonyl compounds, such as b-keto
Due to their inexpensive nature and ready availa-
esters has been successfully established as a simple bility, a,a-diarylprolinols as organocatalysts have at-
and powerful method to construct hetero-quaternary tracted considerable interest and proved to be effec-
stereocenters[2–5] due to the activated C H bond in b- tive in many asymmetric reactions.[15] Recently, our
À
keto esters.
group successfully reported the sulfenylation of b-
Optically active sulfur-containing compounds con- keto esters under the catalysis of a,a-diaryl-l-proli-
stitute an important class of chiral ligands, auxiliaries nols with excellent enantioselectivities and satisfacto-
and synthetic intermediates in organic chemistry.[6] ry yields (92–97% ee with 70–82% yield).[4e] In this
Benzenethiol and thioacetic acid as nucleophiles used paper, we present the asymmetric a-sulfenylation of
in the direct asymmetric sulfenylation reaction have b-keto phosphonates based on a,a-diaryl-l-prolinols
been reported in the literature.[7] Recently, as a com- catalysis [Eq. (1)].
Adv. Synth. Catal. 2011, 353, 545 – 549
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
545

Aijun Lin et al.
COMMUNICATIONS
Table 1. Screening of the reaction conditions for the sulfeny-
lation of b-keto phosphonate 2a.[a]
Entry Cat. Solvent
T [8C] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
hexane
hexane
hexane
hexane
hexane
hexane
hexane
hexane
hexane
hexane
CH2Cl2
DCE
CH3CN
THF
Et2O
toluene
cyclohexane
hexane
hexane
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
78
84
50
76
73
77
38
37
78
60
80
79
58
74
70
73
75
67
58
75
82
35
74
81
78
5
32
90
89
72
76
60
70
83
84
86
88
85
Scheme 1. The system of a-functionalization of b-keto phos-
phonates.
9
10
11
12
13
14
15
16
17
18[d]
19[e]
À20
0
0
0
0
0
0
0
0
0
[a]
Reaction conditions: 0.12 mmol of 3a was added to the
solution of b-keto phosphonate 2a (0.1 mmol) and cata-
lyst (0.02 mmol, 20 mol%) in solvent (1 mL), and the
mixture was stirred for 48 h.
Yield of isolated product.
The ee values were determined by chiral HPLC analysis.
10 mol% of catalyst 1b was used.
[b]
[c]
[d]
[e]
5 mol% of catalyst 1b was used.
Initially, the air-stable and non-stinking N-(phenyl-
thio)phthalimide 3a was chosen as the sulfur source
and the b-keto phosphonate 2a as the model substrate 1h was chosen as the catalyst (Table 1, entries 7 and
for the optimization of the catalysts and the reaction 8), which suggested that the free OH and NH groups
conditions. Based on entries 1–6 of Table 1, the results of the catalyst are necessary in the procedure.
indicated that the electronic properties and the sub-
The enantioselectivity was also found to be influ-
stituents of the aromatic ring of the chosen catalysts enced by the temperature. A lower temperature
(1a–1f) had an important impact on its performance. (08C) can lead to an increase in the ee value (from
The sulfenylated product 4a could be isolated in 78% 82% to 90%), but an even lower temperature
yield with 75% ee in the presence of catalyst 1a (À208C) can cause an obvious loss of yield from 84%
(Table 1, entry 1). And an increase in enantioselectivi- to 60% (Table 1, entries 2, 9 and 10). A survey of sol-
ty (82% ee) was observed with 84% yield (Table 1, vents demonstrated that hexane is the best reaction
entry 2) when the catalyst 1b was used. A slight de- medium for this reaction (Table 1, entries 2 and 11–
crease in enantioselectivity (74% ee) was observed 17). Decreasing the catalyst loading to 0.1 or
when 1d bearing an electron-donating group on the 0.05 mol% had a negative impact on the yield and
aromatic ring was tested (Table 1, entry 4). Lower slightly decreased the enantioselectivity (Table 1, en-
yield and enantioselectivity were obtained when 1g or tries 9, 18 and 19).
546
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 545 – 549

Organocatalytic Enantioselective Sulfenylation of b-Keto Phosphonates
Table 2. Catalytic enantioselective sulfenylation of b-keto
nates ACTHNURGTNE(NUG 2a–2g) and different sulfur reagents (3a–3f) in
phosphonates.[a]
hexane (1 mL) at 08C in the presence of the catalyst
1b (20 mol%). The results are summarized in Table 2.
When b-keto phosphonate 2a was chosen as the sub-
strate, sulfur reagents (3a–3f) with an electron-rich or
an electron-deficient group on the aromatic ring can
afford the optically active a-sulfenylated b-keto phos-
phonates (4a–4f) with excellent enantioselectivities in
up to 92% ee (Table 2, entries 1–6). As is evident
from entries 7–9 of Table 2, the enantioselectivity is
slightly dependent on the substituted ester groups of
the given phosphonic acid ester (2b and 2c). More-
over, the b-keto phosphonate 2d containing an elec-
tron-donating group on the aromatic ring could afford
4j with 85% ee (Table 2, entry 10), and 2e with an
electron-withdrawing group could give 4n with 88%
ee and satisfactory yield (Table 2, entry 14). When the
aliphatic five-membered ring b-keto phosphonate 2f
was chosen as the substrate, the asymmetric sulfenyla-
tion reaction could be completed in 83% yield with
moderate enantioselectivity (Table 2, entry 16), which
revealed that the sterically demanding b-keto phos-
phonates can play a key role in enantioselectivity
under the catalysis of diarylprolinols. We have also
tested the acyclic b-keto phosphonate 2g with sulfur
reagent 3b under the optimal protocol, however, the
result was not as positive as presented above (Table 2,
entry 17).
Entry b-Keto phosphonates 2
3
Yield [%][b] ee [%][c]
1
2
3
4
5
6
3a 78 (4a)
3b 84 (4b)
3c 76 (4c)
3d 92 (4d)
3e 70 (4e)
3f 72 (4f)
90 (R)
92 (R)
86 (R)
75 (R)
80 (R)
80 (R)
7
8
3a 85 (4g)
3b 88 (4h)
90 (R)
90 (R)
9
3b 79 (4i)
84 (R)
On the basis of the results described above, the ac-
tivation mode of the sulfenylation of b-keto phospho-
nates may be attributed to a molecular interaction
rather than a covalent bond[4e,15e–k,16] (Scheme 2).The
catalysts, a,a-diarylprolinols could activate the b-keto
phosphonates (probably in the enol form) via the H-
bonds, which can increase the nucleophilic ability of
the reacting carbon center, and the electrophilic
sulfur reagents could attack the Re-face of the sub-
strate-catalyst complex to give the (R)-configured
product 4.
10
11
12
3a 68 (4j)
3b 79 (4k)
3f 65 (4l)
85 (R)
83 (R)
82 (R)
13
14
15
3a 84 (4m)
3b 85 (4n)
3f 67 (4o)
87 (R)
88 (R)
84 (R)
16
3b 83 (4p)
3b <5%
59
In summary, we have successfully developed the
enantioselective sulfenylation of b-keto phosphonates
À
17
ND
and constructed the chiral C S quaternary stereocen-
ter under the catalysis of a,a-diaryl-l-prolinols. The
reaction proceeds in good yields and with excellent
[a]
Reaction conditions: 0.12 mmol of compound 3 was
added to the solution of b-keto phosphonate
2
(0.1 mmol) and catalyst 1b (0.02 mmol, 20 mol%) in
hexane (1 mL), the mixture was stirred for 48–60 h.
Yield of isolated product.
The ee values were determined by chiral HPLC analysis
and the determination of absolute configuration was as-
signed on the comparison of CD spectroscopy of the
product with that of sulfenylated b-keto esters with es-
tablished absolute configuration.[4e]
[b]
[c]
After having established the optimal protocol for
the asymmetric sulfenylation reaction, we further ex-
Scheme 2. The proposed activation mode between b-keto
tended the reaction with a series of b-keto phospho- phosphonates and a,a-diarylprolinols.
Adv. Synth. Catal. 2011, 353, 545 – 549
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
547

Aijun Lin et al.
COMMUNICATIONS
Felouat, S. Woodward, A. Alexakis, Angew. Chem.
2010, 122, 7935–7938; Angew. Chem. Int. Ed. 2010, 49,
7769–7772; n) F. Gao, Y. Lee, K. Mandai, A. H. Hov-
eyda, Angew. Chem. 2010, 122, 8548–8552; Angew.
Chem. Int. Ed. 2010, 49, 8370–8374; o) X.-F. Wang, Q.-
L. Hua, Y. Cheng, X.-L. An, Q.-Q. Yang, J.-R. Chen,
W.-J. Xiao, Angew. Chem. 2010, 122, 8557–8561;
Angew. Chem. Int. Ed. 2010, 49, 8379–8383; p) F. Pes-
ciaioli, X. Tian, G. Bencivenni, G. Bartoli, P. Mel-
chiorre, Synlett 2010, 11, 1704–1708.
enantioselectivities (up to 92%) for various b-keto
phosphonates with N-(arylthio)-phthalimides as sufur
reagents. Further exploration of other a-functionaliza-
tions of b-keto phosphonates using chiral organocata-
lyst is now in progress in our laboratory.
Experimental Section
Typical Procedure for the Organocatalytic
Enantioselective Sulfenylation of b-Keto
Phosphonates
À
[2] For C O bonds, see: a) M. R. Acocella, O. G. Manche-
no, M. Bella, K. A. Jørgensen, J. Org. Chem. 2004, 69,
8165–8167; b) A. M. R. Smith, D. Billen, K. K. Hii,
Chem. Commun. 2009, 3925–3927; c) M. Lu, D. Zhu,
Y.-P. Lu, X.-F. Zeng, B. Tan, Z.-J. Xu, G.-F. Zhong, J.
Am. Chem. Soc. 2009, 131, 4562–4563.
A solution of catalyst 1b (6.2 mg, 0.02 mmol) and b-keto
phosphonate 2a (28.2 mg, 0.1 mmol) in hexane (1 mL) was
stirred for 10 min at 08C before the sulfur reagent 3a
(30.6 mg, 0.12 mmol) was added, and then the resulting mix-
ture was stirred for 48 h. The crude reaction mixture was di-
luted with ethyl acetate and then directly purified by flash
chromatography on silica gel to afford the corresponding
product 4a, the ee value was determined by chiral HPLC
analysis, 1H and 13C NMR spectroscopy, respectively see
Supporting Information for details.
À
[3] For C N bonds, see: a) M. Marigo, K. Juhl, K. A. Jør-
gensen, Angew. Chem. 2003, 115, 1405–1407; Angew.
Chem. Int. Ed. 2003, 42, 1367–1369; b) S.-M. Ma, N.
Jiao, Z.-L. Zheng, Z.-C. Ma, Z. Lu, L.-W. Ye, Y.-Q.
Deng, G.-F. Chen, Org. Lett. 2004, 6, 2193–2196; c) C.
Foltz, B. Stecker, G. Marconi, S. Bellemin-Laponnaz,
H. Wadepohl, L. H. Gade, Chem. Commun. 2005,
5115–5117; d) M. Terada, M. Nakano, H. Ube, J. Am.
Chem. Soc. 2006, 128, 16044–16045; e) Y. K. Kang,
D. Y. Kim, Tetrahedron Lett. 2006, 47, 4565–4568; f) C.
Foltz, B. Stecker, G. Marconi, S. Bellemin-Laponnaz,
H. Wadepohl, L. H. Gade, Chem. Eur. J. 2007, 13,
9912–9923; g) J. Comelles, ꢃ. Pericas, M. Moreno-
MaÇas, A. Vallribera, G. Drudis-Solꢄ, A. Lledos, T. Par-
ella, A. Roglans, S. Garcꢅa-Granda, L. Roces-Fernꢆn-
dez, J .Org. Chem. 2007, 72, 2077–2087; h) S. H. Jung,
D. Y. Kim, Tetrahedron Lett. 2008, 49, 5527–5530; i) M.
Gaab, S. Bellemin-Laponnaz, L. H. Gade, Chem. Eur.
J. 2009, 15, 5450–5462; j) M. V. Nandakumar, S.
Ghosh, C. Schneider, Eur. J. Org. Chem. 2009, 6393–
6398; k) J. Y. Mang, D. G. Kwon, D. Y. Kim, J. Fluorine
Chem. 2009, 130, 259–262; l) X.-F. Liu, B.-F. Sun, L.
Deng, Synlett 2009, 10, 1685–1189.
Acknowledgements
We gratefully acknowledge the National Natural Science
Foundation of China (20832001, 20972065, 21074054) and
the National Basic Research Program of China
(2007CB925103, 2010CB923303) for their financial support.
The Major Scientific and Technological Special Project
(2009ZX09103-081) is also acknowledged.
References
À
[4] For C S bonds, see: a) S. Sobhani, D. Fielenbach, M.
[1] For selected reviews on catalytic asymmetric synthesis
of quaternary stereocenters, see: a) E. J. Corey, A.
Guzman-Perez, Angew. Chem. 1998, 110, 402–415;
Angew. Chem. Int. Ed. 1998, 37, 388–401; b) J. Chris-
toffers, A. Mann, Angew. Chem. 2001, 113, 4725–4732;
Angew. Chem. Int. Ed. 2001, 40, 4591–4597; c) I. De-
nissova, L. Barriault, Tetrahedron 2003, 59, 10105–
10146; d) D. J. Ramon, M. Yus, Curr. Org. Chem. 2004,
8, 149–183; e) J. Christoffers, A. Baro, Adv. Synth.
Catal. 2005, 347, 1473–1482; f) B. M. Trost, C. Jiang,
Synthesis 2006, 3, 369–396; g) P. G. Cozzi, R. Hilgraf,
N. Zimmermann, Eur. J. Org. Chem. 2007, 5969–5994;
h) M. Bella, T. Gasperi, Synthesis 2009, 10, 1583–1614.
For some topical research papers on catalytic asymmet-
ric synthesis of quaternary stereocenter, see: i) T. Fuji-
moto, K. Endo, H. Tsuji, M. Nakamura, E. Nakamura,
J. Am. Chem. Soc. 2008, 130, 4492–4496; j) C. Baude-
quin, A. Zamfir, S. B. Tsogoeva, Chem. Commun. 2008,
4637–4639; k) K. Shibatomi, K. Futatsugi, F. Kobaya-
shi, S, Iwasa, H, Yamamoto, J. Am. Chem. Soc. 2010,
132, 5625–5627; l) Q. Zhu, Y. X. Lu, Angew. Chem.
2010, 122, 7919–7922; Angew. Chem. Int. Ed. 2010, 49,
7753–7756; m) C. Hawner, D. Mꢂller, L. Gremaud, A.
Marigo, T. C. Wabnitz, K. A. Jørgensen, Chem. Eur. J.
2005, 11, 5689–5694; b) M. Jereb, A. Togni, Org. Lett.
2005, 7, 4041–4043; c) M. Jereb, A. Togni, Chem. Eur.
J. 2007, 13, 9384–9392; d) S. K. Sirisailam, A. Togni,
Tetrahedron: Asymmetry 2006, 17, 2603–2607; e) L.
Fang, A.-J. Lin, H.-W. Hu, C.-J. Zhu, Chem. Eur. J.
2009, 15, 7039–7043.
À
[5] For C halogen bonds, see: a) D. Cahard, C. Audouard,
J. C. Plaquevent, N. Roques, Org. Lett. 2000, 2, 3699–
3701; b) N. Shibata, E. Suzuki, Y. Takeuchi, J. Am.
Chem. Soc. 2000, 122, 10728–10729; c) L. Hintermann,
A. Togni, Angew. Chem. 2000, 112, 4530–4533; Angew.
Chem. Int. Ed. 2000, 39, 4359–4362; d) D. Y. Kim, E. J.
Park, Org. Lett. 2002, 4, 545–547; e) M. Marigo, N. Ku-
maragurubaran, K. A. Jørgensen, Chem. Eur. J. 2004,
10, 2133–2137; f) G. Bartoli, M. Bosco, A. Carlone, M.
Locatelli, P. Melchiorre, L. Sambri, Angew. Chem.
2005, 117, 6375–6378; Angew. Chem. Int. Ed. 2005, 44,
6219–6222; g) S. Suzuki, H. Furuno, Y. Yokoyama, J.
Inanaga, Tetrahedron: Asymmetry 2006, 17, 504–507;
h) N. Shibata, H. Yasui, S. Nakamura, T. Toru, Synlett
2007, 7, 1153–1157; i) M. Althaus, C. Becker, A. Togni,
548
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 545 – 549

Organocatalytic Enantioselective Sulfenylation of b-Keto Phosphonates
A. Mezzetti, Organometallics 2007, 26, 5902–5911;
j) M. Frings, C. Bolm, Eur. J. Org. Chem. 2009, 4085–
4087.
c) H. Seto, T. Kuzuyama, Nat. Prod. Rep. 1999, 16,
589–596; d) V. P. Kukhar, H. R. Hudson, Aminophos-
phonic and Aminophosphinic Acids, Chemistry and
Biological Activity, John Wiley & Sons, New York,
2000, p 634.
[6] a) D. A. Evans, K. R. Campos, J. S. Tedrow, F. E. Mi-
chael, M. R. Gagnꢄ, J. Am. Chem. Soc. 2000, 122,
7905–7920; b) A. M. MasdeuBultꢇ, M. Diꢄguez, E.
Martin, M. Gꢇmez, Coord. Chem. Rev. 2003, 242, 159–
201.
[12] L. Bernardi, W. Zhuang, K. A. Jørgensen, J. Am.
Chem. Soc. 2005, 127, 5772–5773.
[13] a) L. Bernardi, K. A. Jørgensen, Chem. Commun. 2005,
1324–1326; b) S. M. Kim, H. R. Kim, D. Y. Kim, Org.
Lett. 2005, 7, 2309–2311; c) Y. Hamashima, T. Suzuki,
H. Takano, Y. Shimura, Y. Tsuchiya, K. Moriya, T.
Gotoa, M. Sodeoka, Tetrahedron 2006, 62, 7168–7179;
d) Y. Hamashima, T. Suzuki, Y. Shimura, T. Shimizu, N.
Umebayashi, T. Tamura, N. Sasamotoa , M. Sodeoka,
Tetrahedron Lett. 2005, 46, 1447–1450.
[14] a) A. Kjærsgaard, K. A. Jørgensen, Org. Biomol.
Chem. 2005, 3, 804–808; b) Z.-H. Chen, K. Yakura, S.
Matsunaga, M. Shibasaki, Org. Lett. 2008, 10, 3239–
3242.
[15] The catalysts activate substrate by a covalent bond,
see: a) J.-W. Xie, L. Yue, D. Xue, X.-L. Ma, Y.-C Chen,
Y. Wu, J. Zhu, J.-G. Deng, Chem. Commun. 2006,
1563–1565; b) J. L. Vicario, S. Reboredo, D. Bad, L.
Carrillo, Angew. Chem. 2007, 119, 5260–5262; Angew.
Chem. Int. Ed. 2007, 46, 5168–5170; c) C. Zhong, Y.-F.
Chen, J. L. Petersen, N. G. Akhmedov, X.-D. Shi,
Angew. Chem. 2009, 121, 1305–1308; Angew. Chem.
Int. Ed. 2009, 48, 1279–1282; d) W.-B. Chen, X.-L. Du,
L.-F. Cun, X.-M. Zhang, W.-C. Yuan, Tetrahedron 2010,
66, 1441–1446; the catalysts activate substrate by mo-
lecular interaction, see: e) A. Lattanzi, Org. Lett. 2005,
7, 2579–2582; f) A. Lattanzi, Tetrahedron: Asymmetry
2006, 17, 837–841; g) A. Lattanzi, Adv. Synth. Catal.
2006, 348, 339–346; h) A. Lattanzi, A. Russo, Tetrahe-
dron 2006, 62, 12264–12269; i) Y. Li, X. Liu, Y. Yang,
G. Zhao, J. Org. Chem. 2007, 72, 288–291; j) A. Russo,
A. Lattanzi, Eur. J. Org. Chem. 2008, 2767–2773; k) A.
Russo, A. Lattanzi, Adv. Synth. Catal. 2008, 350, 1991–
1995; l) C.-W. Zheng, Y.-W. Li, Y.-Q. Yang, H.-F.
Wang, H.-F. Cui, J.-K. Zhang, G. Zhao, Adv. Synth.
Catal, 2009, 351, 1685–1691; m) A. Russo, A. Lattanzi,
Org. Biomol. Chem. 2010, 8, 2633–2638.
[7] For an excellent review, see: a) D. Enders, K. Luttgen,
A. Narine, Synthesis 2007, 7, 959–980. For selected ex-
amples, see: b) J. Wu, X.-L. Hou, L.-X. Dai, L.-J. Xia,
M.-H. Tang, Tetrahedron: Asymmetry 1998, 9, 3431–
3436; c) P. McDaid, Y. Chen, L. Deng, Angew. Chem.
2002, 114, 348–350; Angew. Chem. Int. Ed. 2002, 41,
338–340; d) M. Marigo, T. Schulte, J. Franzꢄn, K. A.
Jørgensen, J. Am. Chem. Soc. 2005, 127, 15710–15711;
e) L. Zu, J. Wang, H. Li, H. Xie, W. Jiang, W. Wang, J.
Am. Chem. Soc. 2007, 129, 1036–1037; f) P. Ricci, A.
Carlone, G. Bartoli, M. Bosco, L. Sambri, P. Mel-
chiorre, Adv. Synth. Catal. 2008, 350, 49–53; g) D.
Leow, S. Lin, S. K. Chittimlla, X. Fu, C.-H. Tan, Angew.
Chem. 2008, 120, 5723–5727; Angew. Chem. Int. Ed.
2008, 47, 5641–5645; h) Z. Wang, X. Sun, S. Ye, W.
Wang, B. Wang, J. Wu, Tetrahedron: Asymmetry 2008,
19, 964–969; i) D. Enders, K. Hoffman, Eur. J. Org.
Chem. 2009, 1665–1668; j) Y. Liu, B. Sun, B. Wang, M.
Wakem, L. Deng, J. Am. Chem. Soc. 2009, 131, 418–
419; k) K. L. Kimmel, M. T. Robak, J. A. Ellman, J.
Am. Chem. Soc. 2009, 131, 8754–8755; l) G. D. Sala, A.
Lattanzi, Org. Lett. 2009, 11, 3330–3333; m) A. Lattan-
zi, G. D. Sala, Eur. J. Org. Chem. 2009, 1845–1848;
n) H.-H. Lu, F.-G. Zhang, X.-G. Meng, S.-W. Duan, W.-
J. Xiao, Org. Lett. 2009, 11, 3946–3949.
[8] a) M. Marigo, T. C. Wabnitz, D. Fielenbach, K. A. Jør-
gensen, Angew. Chem. 2005, 117, 804–807; Angew.
Chem. Int. Ed. 2005, 44, 794–797; b) G.-L. Zhao, R.
Rios, J. Vesely, L. Eriksson, A. Cꢇrdova, Angew. Chem.
2008, 120, 8596–8600; Angew. Chem. Int. Ed. 2008, 47,
8468–8472.
[9] a) W. Wang, H. Li, J. Wang, L. Liao, Tetrahedron Lett.
2004, 45, 8229–8231; b) J. S. Yadav, B. V. S. Reddy, R.
Jain, G. Baishya, Tetrahedron Lett. 2008, 49, 3015–
3019.
[16] For an enol mechanism in a primary amine-thiourea
catalyzed transformation, see: D. A. Yalalov, S. B. Tso-
goeva, T. E. Shubina, I. M. Martynova, T. Clark,
Angew. Chem. 2008, 120, 6726–6730; Angew. Chem.
Int. Ed. 2008, 47, 6624–6628.
[10] R. Dubey, N. W. Polaske, G. S. Nichol, B. Olenyuk, Tet-
rahedron Lett. 2009, 50, 4310–4313.
[11] a) P. Kafarski, B. Lejczak, Phosphorus Sulfur Silicon
Relat. Elem. 1991, 63, 193–215; b) V. Gouverneur,
M. N. Lalloz, Tetrahedron Lett. 1996, 37, 6331–6334;
Adv. Synth. Catal. 2011, 353, 545 – 549
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
549