Organocatalytic enantioselective direct aldol reaction in aqueous media catalyzed by a bifunctional diamine catalyst

Article abstract of DOI:10.1055/s-0030-1259524

Organocatalytic direct asymmetric anti-aldol reaction was developed in aqueous medium using a BINOL-derived diamine/protic acid bifunctional catalyst. The catalytic protocol could offer the opportunity to access anti-aldol products with high level of enan

Full text of DOI:10.1055/s-0030-1259524

CLUSTER
481
Organocatalytic Enantioselective Direct Aldol Reaction in Aqueous Media
Catalyzed by a Bifunctional Diamine Catalyst
a
a,b
O
V
rganocatalytic En
i
antios
s
D
h
irect
A
ldol
R
n
eactioninA
u
queous Med
m
ia aya Bisai, Vinod K. Singh*
a
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
b
Department of Chemistry, Indian Institute of Science Education and Research Bhopal,
ITI (Gas Rahat) Building, Govindpura, Bhopal, Madhya Pradesh 462023, India
Fax +91(512)2597436; E-mail: vinodks@iitk.ac.in
Received 26 October 2010
Barbas, Takabe and co-workers in 2006 developed an el-
egant highly enantioselective catalytic anti-aldol reaction
that could be performed in water using diamine 1 as a cat-
Abstract: Organocatalytic direct asymmetric anti-aldol reaction
was developed in aqueous medium using a BINOL-derived di-
amine/protic acid bifunctional catalyst. The catalytic protocol could
offer the opportunity to access anti-aldol products with high level of alyst having appropriate hydrophobic groups (Figure 1).
enantioselectivities with moderate diastereoselectivities.
According to them, the diamine 1 with nonpolar hydro-
phobic groups along with hydrophobic reactant molecules
keep themselves away and assemble in small volume
when water is the medium carrying out the reaction via a
compact transition state.11
Key words: aldol reaction, aqueous medium, organocatalyst, di-
amine, protic acid, enantioselectivity
Stereoselective construction of C–C bond through orga-
nocatalytic direct aldol reaction is currently one of the
most promising areas of research which has drawn a great
N
NH
1
deal of attention from synthetic standpoint. Since the pi-
oneering discovery of L-proline-catalyzed intermolecular2
aldol reactions involving enamine intermediates in 2000,
L-proline and its derivatives have been found to serve as
excellent catalytic systems for a direct asymmetric aldol
reaction.
(S)-1
O
R
Ph
N
N
H
OH
NH
NH
Ph
Among all the three types of asymmetric ways to carry out
3
aldol reactions viz biocatalysis, metal-catalyzed proce-
(S,S)-2a R = Ph
(S,S)-2b R = i-Bu
(S,S,S)-3
4
dures, and the asymmetric organocatalytic direct meth-
5
ods, the third approach has become the most vital and Figure 1 Selected organocatalysts in asymmetric aldol reactions
fancy. Recent literature has witnessed many highly effi-
6
cient small organic molecules as organocatalysts, which
In the year 2007, we disclosed the organocatalytic direct
have successfully been applied for asymmetric aldol reac-
asymmetric Michael reaction of unmasked ketones with
tion. The advantages of organocatalysis have been dem-
b-nitrostyrenes in the presence of a BINOL-derived di-
onstrated by the facile preparation of the catalysts, the
mild reaction conditions, and by their environmentally be-
12
amine/protic acid bifunctional organocatalyst. We were
especially interested in BINOL-derived diamine 3
7
nign aspects as compared to metal catalysis. From a
(
Figure 1), based on our belief, that the presence of substi-
green chemistry perspective, performing these organocat-
alytic reactions in aqueous media gains further impor-
tance as it is safe to use and environmentally friendly.8
tuted nonpolar dibenzylic-type hydrophobic groups in the
tertiary amine part, would facilitate the reaction in aque-
ous medium and should also work in organic medium due
1
3
To perform asymmetric transformations in aqueous me- to the aromatic nature of the binaphthyl part. We found
dia, a special design of catalyst is always required to have that our catalytic protocol was compatible with water as
sufficient hydrophobic effect to facilitate the reaction. well as with various other conventional organic solvents
Water imposes strong ionic interactions through its hydro- providing high enantio- and diastereoselectivities in the
gen bonding abilities which can often alter the enantio- asymmetric Michael reaction of ketones with both aryl
selectivity and thus it is not easy to access high level of and alkyl nitroolefins.
enantioselection in most of such asymmetric catalytic pro-
For the last few years, our group has been engaged in the
9
cesses. Inspired by the principle of the action of aldolase
development of L-prolinamide-type organocatalysts (2,
1
0
antibodies which contain a hydrophobic active site,
Figure 1), which have successfully been applied to highly
1
4
enantioselective direct asymmetric aldol reactions by us
15
and others. Encouraged by Barbas and Takabe’s work,
herein, we wish to report an efficient enamine-based orga-
nocatalytic direct asymmetric aldol reaction using our
BINOL-derived diamine 3 in conjunction with 2,4-dini-
SYNLETT 2011, No. 4, pp 0481–0484
Advanced online publication: 02.02.2011
DOI: 10.1055/s-0030-1259524; Art ID: Y02410ST
Georg Thieme Verlag Stuttgart · New York
x
x.
x
x.
2
0
1
1
©

4
82
V. Bisai, V. K. Singh
CLUSTER
trobenzenesulfonic acid (DNBSA) as a bifunctional cata- hydes having an electron-withdrawing group at the para
lyst in the presence of water. These reactions afforded the position were proved to be good substrates yielding aldol
desired anti-aldol products in high enantioselectivities products in high enantioselectivities (entries 2 and 4,
with reasonably good level of diastereoselectivities and Table 2) but with moderate diastereoselectivities. In addi-
1
6
high yields. Optimized conditions revealed that the or- tion to electron-withdrawing substrates, furfural and b-
ganocatalyst 3 in the presence of DNBSA or TFA is the naphthaldehyde were also found to be good substrates in
best catalytic system to carry out asymmetric aldol reac- terms of achieving very high level of enantioselectivities
tions in analogy with our earlier work.12
(entries 6 and 7, Table 2). However, aromatic aldehydes
having halogen and electron-donating groups gave enan-
tioselectivity in the range of 86–87% (entries 3 and 5,
Table 2). Unfortunately, except benzaldehyde, in all the
cases diastereoselectivities were found to be moderate.17
At the outset, the preliminary experiments were conduct-
ed by taking cyclohexanone as a donor and benzaldehyde
as an acceptor using 10 mol% of the diamine 3 in the pres-
ence of 10 mol% of DNBSA in different solvents
(
Table 1). It was found that almost all organic solvents af-
Table 2 Aldehyde Scope in Asymmetric Direct Aldol Reaction
forded high level of enantioselection (74–84% ee, entries
1
9
selectivities of the aldol reaction were equally good on
switching to water as medium. Further, we found that, us-
ing brine instead of water led to accelerated reactions with
comparatively high enantioselectivity (83% ee) and dia-
stereoselectivity (95:5), in good yield (85%) under opti-
mized conditions (entries 8 and 9, Table 1).
O
–6, Table 1) and very high diastereoselectivities (dr up to
8:2, entries 3 and 5, Table 1) in good yields. The enantio-
O
OH
O
3 (10 mol%), DNBSA (10 mol%)
r.t., brine
Ar
a–g
+
Ar
H
1
2–24 h
4
Entry Ar
Product Yield (%)a Ratio anti/synb % eec
1
2
3
4
5
6
7
Ph
4-O NC H
4
4a
4b
4c
4d
4e
4f
85
89
83
82
91
80
88
95:5
83
94
87
95
86
96
96
78:22
75:25
69:31
71:29
88:12
88:12
2
6
Table 1 Solvent Study for Organocatalytic Direct Aldol Reaction
4-MeOC H
6
4
4-F CC H
4
3
6
N
4-ClC H
N
H
6
4
2-furyl
(
S,S,S)-3
O
O
OH
O
2-naphthyl
4g
3
(10 mol%), DNBSA (10 mol%)
r.t., solvent
Ph
+
a
Ph
H
Yields are reported after column chromatography.
Diastereoselectivities were determined by H NMR analysis of the
b
1
4
a
products.
Time (h) Yield (%)a Ratio syn/antib ee (%)c
c
Entry Solvent
The ee values were determined by HPLC using chiral columns.
1
2
3
4
5
6
7
8
9
DMSO
DMF
16
18
15
18
18
20
18
15
12
77
65
87
48
80
77
48
80
85
94:6
94:6
98:2
97:3
98:2
96:4
97:3
98:2
95:5
78
79
74
76
78
84
75
77
83
The reaction was extended to other donors and the results
are summarized in Scheme 1. Only 36% ee was obtained
when acetone was used as a donor (5a, Scheme 1). The
asymmetric aldol reaction was extended to other six-
membered ketones as well (5b and 5c, Scheme 1) where
excellent enantioselectivities were observed with moder-
ate level of diastereoselection.
MeCN
CHCl3
DMEd
MeOH
THF
O
O
O
OH
3
(10 mol%), DNBSA (10 mol%),
r.t., brine
+
H O
2
Ph
H
Ph
1
2–24 h
brine
5
a–c
a
Yields are reported after column chromatography.
Diastereoselectivities were determined by H NMR analysis of the
b
1
O
OH
O
OH
products.
O
OH
c
Ph
The ee values were determined by HPLC using chiral columns.
Ph
d
Ph
DME = dimethoxyethane.
O
S
5
c
5
a
5b
With the optimized conditions in hand for asymmetric al-
dol reaction, we looked forward to examine the possible
substrate scope (Table 2). It was found that aromatic alde-
77%, 95% ee
69:31 dr
6
7%
76%, 98% ee
70:30 dr
3
6% ee
Scheme 1 Scope of ketones as donors in asymmetric direct aldol
reaction
Synlett 2011, No. 4, 481–484 © Thieme Stuttgart · New York

CLUSTER
Organocatalytic Enantioselective Direct Aldol Reaction in Aqueous Media
483
The faster reaction rates and the excellent level of enanti-
oselectivity in brine could be explained by the hydropho-
bic environment created by the binaphthyl group and
Synth. Catal. 2007, 349, 1041. (d) Evans, D. A.; Downey,
C. W.; Hubbs, J. L. J. Am. Chem. Soc. 2003, 125, 8706.
e) Trost, B. M.; Silcoff, E. R.; Ito, H. Org. Lett. 2001, 3,
497. (f) Kumagai, N.; Matsunaga, S.; Yoshikawa, N.;
(
2
1
8a
salting out effect which leads to volume constriction
bringing the reactant molecules in close vicinity thus fa-
cilitating the reaction via a compact transition state.18b,6d
The stereochemical outcome could be explained on the basis
of similar type of transition state as given by Yamamoto
Ohshima, T.; Shibasaki, M. Org. Lett. 2001, 3, 1539.
5) For reviews on asymmetric organocatalytic aldol reactions,
see: (a) Guillena, G.; Najera, C.; Ramon, D. J. Tetrahedron:
Asymmetry 2007, 18, 2249. (b) Tanaka, F.; Barbas, C. F. III.
In Enantioselective Organocatalysis; Dalko, P. I., Ed.;
Wiley-VCH: Weinheim, 2007, 19. (c) Pellissier, H.
Tetrahedron 2007, 63, 9267. (d) Mukherjee, S.; Yang, J.
W.; Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471.
6) (a) Duarte, F. J. S.; Cabrita, E. J.; Frenking, G.; Santos, A. G.
J. Org. Chem. 2010, 75, 2546; and references cited therein.
(
(
1
9
et al. in which attack from Re-face of aldehyde gives rise
to anti-aldol product (Figure 2).
O
OH
(
b) Luo, S.; Qiao, Y.; Zhang, L.; Li, J.; Li, X.; Cheng, J.-P.
2
N
R
J. Org. Chem. 2009, 74, 9521. (c) Xiong, Y.; Wang, F.;
Dong, S.; Liu, X.; Feng, X. Synlett 2008, 73. (d) Luo, S.;
Xu, H.; Zhang, L.; Li, J.; Cheng, J.-P. Org. Lett. 2008, 10,
Ar
O
H
Ar
R2
N+
R1
X–
H
R1
anti-aldol product
653. (e) Hayashi, Y.; Itoh, T.; Aratake, S.; Ishikawa, H.
Angew. Chem. Int. Ed. 2008, 47, 2082. (f) Xu, X.-Y.;
Wang, Y.-Z.; Gong, L.-Z. Org. Lett. 2007, 9, 4247. For
reviews, see: (g) Raj, M.; Singh, V. K. Chem. Commun.
Figure 2 Proposed transition state for asymmetric induction in
asymmetric aldol reaction
2
009, 6687. (h) List, B. Acc. Chem. Res. 2004, 37, 548.
In conclusion, we have developed an efficient catalytic
protocol using the diamine 3 derived from BINOL, in con-
junction with DNBSA as a bifunctional catalyst, which
has successfully been applied to the asymmetric anti-
selective aldol reaction of ketones with aromatic alde-
hydes. The sense of asymmetric induction was ex-
plained by invoking a similar kind of transition state
proposed earlier in the literature.
(i) Notz, W.; Tanaka, F.; Barbas, C. F. III. Acc. Chem. Res.
2
6
5
004, 37, 580. (j) Miller, S. J. Acc. Chem. Res. 2004, 37,
01. (k) Saito, S.; Yamamoto, H. Acc. Chem. Res. 2004, 37,
70.
(
7) (a) Dondoni, A.; Massi, A. Angew. Chem. Int. Ed. 2008, 47,
638. For recent reports on organocatalytic aldol reactions,
4
2
0
see: (b) Luo, S.; Xu, H.; Li, J.; Zhang, L.; Cheng, J.-P.
J. Am. Chem. Soc. 2007, 129, 3074. (c) Ramasastry, S. S.
V.; Zhang, H.; Tanaka, F.; Barbas, C. F. III. J. Am. Chem.
Soc. 2007, 129, 288. (d) Wang, F.; Xiong, Y.; Liu, X.; Feng,
X. Adv. Synth. Catal. 2007, 349, 2665. (e) Rodriguez, B.;
Rantanen, T.; Bolm, C. Angew. Chem. Int. Ed. 2006, 45,
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
6924. (f) D’Elia, V.; Zwicknagl, H.; Reiser, O. Org. Lett.
2008, 73, 3262. (g) Kano, T.; Takai, J.; Tokuda, O.;
Maruoka, K. Angew. Chem. Int. Ed. 2005, 44, 3055.
8) (a) Breslow, R.; Rizzo, C. J. J. Am. Chem. Soc. 1991, 113,
(
Acknowledgement
4340. (b) Herrmann, W. A.; Kohlpaintner, C. W. Angew.
V.K.S. thanks the Department of Science and Technology (DST),
India for a research grant through J. C. Bose Fellowship. V.B.
thanks the Council of Scientific and Industrial Research (CSIR) for
a Senior Research Fellowship (SRF).
Chem., Int. Ed. Engl. 1997, 36, 1049. For reviews, see:
(
c) Lindstrom, U. M. Chem. Rev. 2002, 102, 2751.
(
d) Kobayashi, S.; Manabe, K. Acc. Chem. Res. 2002, 35,
209.
(
9) (a) Sakthivel, K.; Notz, W.; Bui, T.; Barbas, C. F. III. J. Am.
Chem. Soc. 2001, 123, 5260. (b) Torii, H.; Nakadai, M.;
Ishihara, K.; Saito, S.; Yamamoto, H. Angew. Chem. Int. Ed.
References and Notes
2
004, 43, 1983. (c) Nyberg, A. I.; Usanp, A.; Pihko, P. M.
(
1) (a) Modern Aldol Reactions, Vol. 1; Mahrwald, R., Ed.;
Wiley-VCH: Weinheim, 2004. (b) For metal-catalyzed
reactions, see: Modern Aldol Reactions, Vol. 2; Mahrwald,
R., Ed.; Wiley-VCH: Weinheim, 2004. (c) For a review,
see: Casiraghi, G.; Zanardi, F.; Appendino, G.; Rassu, G.
Chem. Rev. 2000, 100, 1929.
Synlett 2004, 1891. (d) Cordova, A.; Notz, W.; Barbas, C. F.
III. Chem. Commun. 2002, 3024. (e) Darbre, T.;
Machuqueiro, M. Chem. Commun. 2003, 1090. (f) Tang,
Z.; Yamg, Z.-H.; Cun, L.-F.; Gong, L.-Z.; Mi, A.-Q.; Jiang,
Y.-Z. Org. Lett. 2004, 6, 2285. (g) Chimni, S. S.; Mahajan,
D.; Suresh Babu, V. V. Tetrahedron Lett. 2005, 46, 5617.
(
2) List, B.; Lerner, R. A.; Barbas, C. F. III. J. Am. Chem. Soc.
(
(
10) (a) Heine, A.; Desantis, G.; Luz, J. G.; Mitchell, M.; Wong,
C.-H.; Wilson, I. A. Science 2001, 294, 369. (b) Zhu, X.;
Tanaka, F.; Hu, Y.; Heine, A.; Fuller, R.; Zhing, G.; Olson,
A. J.; Lerner, R. A.; Barbas, C. F. III.; Wilson, I. A. J. Mol.
Biol. 2004, 343, 1269.
11) Mase, N.; Nakai, Y.; Ohara, N.; Yoda, H.; Takabe, K.;
Tanaka, F.; Barbas, C. F. III. J. Am. Chem. Soc. 2006, 128,
2
000, 122, 2395.
3) (a) Machajewski, T. D.; Wong, C.-H. Angew. Chem. Int. Ed.
000, 39, 1352. (b) Gijsen, H. J. M.; Qiao, L.; Fitz, W.;
(
2
Wong, C.-H. Chem. Rev. 1996, 96, 443. (c) Wagner, J.;
Lerner, R. A.; Barbas, C. F. III. Science 1995, 270, 1797.
(
d) Dean, S. M.; Greenberg, W. A.; Wong, C.-H. Adv. Synth.
Catal. 2007, 349, 1308. (e) Li, C.; Feng, X.-W.; Wang, N.;
Zhou, Y.-J.; Yu, X.-Q. Green Chem. 2008, 10, 616.
734.
(
(
12) Maya, V.; Singh, V. K. Org. Lett. 2009, 9, 1117.
13) For asymmetric aldol reaction using BINOL-derived
primary amine, see: Liu, Q.-Z.; Wang, X.-L.; Luo, S.-W.;
Zheng, B.-L.; Qin, D.-B. Tetrahedron Lett. 2008, 49, 7434.
(
4) (a) Li, H.; Da C, S.; Xiao, Y.-H.; Li, X.; Su, Y.-N. J. Org.
Chem. 2008, 73, 7398. (b) Kantam, M. L.; Ramani, T.;
Chakrapani, L.; Kumar, K. V. Tetrahedron Lett. 2008, 49,
1498. (c) Paradowska, J.; Stodulski, M.; Mlynarski, J. Adv.
Synlett 2011, No. 4, 481–484 © Thieme Stuttgart · New York

4
84
V. Bisai, V. K. Singh
CLUSTER
(
14) (a) Raj, M.; Maya, V.; Ginotra, S. K.; Singh, V. K. Org. Lett.
(20) Compound characterization data for selected compounds:
(2S,1¢R)-2-[Furan-2-yl(hydroxy)methyl]cyclohexan-1-
one (4f): It was obtained in a maximum of 80% yield and
96% ee. The optical purity was determined by HPLC on
chiralpak AD-H column (hexane–2-propanol, 90:10); flow
2006, 8, 4097. (b) Maya, V.; Raj, M.; Singh, V. K. Org. Lett.
2007, 9, 2593. (c) Gandhi, S.; Singh, V. K. J. Org. Chem.
2008, 73, 9411. (d) Raj, M.; Maya, V.; Singh, V. K. J. Org.
Chem. 2009, 74, 4289.
(
(
15) (a) Song, L.; Chen, X.; Zhang, S.; Zhang, H.; Li, P.; Luo, G.;
rate 0.5 mL/min, 220 nm; t
= 26.8 min, t
= 31.2
R(major)
R(minor)
2
5
1
Liu, W.; Duan, W.; Wang, W. Org. Lett. 2008, 10, 5489.
min; [a] +21 (c = 1.0, CHCl ). H NMR (500 MHz,
D
3
(
b) Zhang, H.; Zhang, S.; Liu, L.; Luo, G.; Duan, W.; Wang,
CDCl ): d = 1.23–1.35 (m, 1 H), 1.61–1.71 (m, 3 H), 1.83–
3
W. J. Org. Chem. 2010, 75, 368.
1.85 (m, 1 H), 2.10–2.37 (m, 1 H), 2.38–2.49 (m, 2 H), 2.89–
2.95 (m, 1 H), 3.89 (br s, 1 H), 4.83 (d, J = 8.6 Hz, 1 H),
6.27–6.34 (m, 2 H), 7.36–7.38 (m, 1 H). Anal. Calcd for
C H O : C, 68.02; H, 7.27. Found: C, 68.09; H, 7.25.
16) For a diamine derived from trans-1,2-diaminocyclohexane
and its application to direct asymmetric aldol reaction from
our group, see: (a) Raj, M.; Parashari, G. S.; Singh, V. K.
Adv. Synth. Catal. 2009, 351, 1284. (b) Raj, M.;
1
1
14
3
(2S,1¢R)-2-[Hydroxy(naphthalene-2-yl)methyl]-
Veerasamy, N.; Singh, V. K. Tetrahedron Lett. 2010, 51,
cyclohexan-1-one (4g): It was obtained in a maximum of
88% yield and 96% ee. The optical purity was determined by
HPLC on chiralpak AS-H column (hexane–2-propanol,
2157.
(
17) General Procedure for the Direct Aldol Reaction with
the Catalyst 3 in Brine: An aldehyde (0.5 mmol) was added
to a mixture of ketone (2 mmol) and an organocatalyst 3 (10
mol%) with DNBSA (10 mol%) in brine (0.5 mL) at r.t. The
reaction mixture was stirred and the progress of the reaction
was monitored by TLC. After reaction was over (as
90:10); flow rate 0.5 mL/min; t
= 26.6 min, t
=
R(major)
R(minor)
2
5
1
30.8 min; [a] +5.8 (c = 1.3, CHCl ). H NMR (400 MHz,
D
3
CDCl ): d = 1.25–1.35 (m, 2 H), 1.48–1.76 (m, 3 H), 2.04–
3
2.09 (m, 1 H), 2.36–2.51 (m, 2 H), 2.69–2.75 (m, 1 H), 4.08
(br s, 1 H), 4.96 (d, J = 8.8 Hz, 1 H), 7.45–7.49 (m, 2 H),
7.71–7.89 (m, 5 H). Anal. Calcd for C H O : C, 80.28; H,
indicated by TLC), the reaction mixture was diluted with
EtOAc (2 mL). The organic layer was separated and dried
over anhyd Na SO It was purified over silica gel by column
1
7
18
2
7.13. Found: C, 80.18; H, 7.11.
(2S,1¢R) 3-[Hydroxy(phenyl)methyl]tetrahydrothio-
pyran-4-one (5b): It was obtained in a maximum of 77%
yield and 98% ee. The optical purity was determined by
HPLC on chiralpak OD-H column (hexane–2-propanol,
2
4.
chromatography. The enantiomeric excess(ee) of the aldol
product was determined by chiral HPLC analysis. The
relative and absolute configurations of the products were
1
determined by comparison with the known H NMR, chiral
98:2); flow rate 0.5 mL/min, t
= 60.6 min, t
=
R(major)
R(minor)
2
5
1
HPLC analysis, and optical rotation values.
87.1 min; [a] +17.1 (c = 1.4, CHCl ). H NMR (400 MHz,
D
3
(
(
18) (a) Kumar, A.; Pawar, S. S. Tetrahedron 2003, 59, 5019.
CDCl ): d = 2.48–2.60 (m, 2 H), 2.75–2.81 (m, 1 H), 2.83–
3
(
b) Kleiner, C. M.; Schreiner, P. R. Chem. Commun. 2006,
315.
19) (a) Nakadai, M.; Saito, S.; Yamamoto, H. Tetrahedron 2002,
8, 8167. (b) Ishii, T.; Fiujioka, S.; Sekiguchi, Y.; Kotsuki,
2.88 (m, 1 H), 2.92–3.04 (m, 3 H), 3.42 (br s, 1 H), 4.97
(d, J = 8.8 Hz, 1 H), 7.26–7.39 (m, 5 H). Anal. Calcd for
C H O S: C, 64.83; H, 6.35. Found: C, 64.89; H, 6.33.
4
1
2
14
2
5
H. J. Am. Chem. Soc. 2004, 126, 9558. (c) Seebach, D.;
Golinski, J. Helv. Chim. Acta 1981, 64, 1413.
Synlett 2011, No. 4, 481–484 © Thieme Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not be copied or
emailed to multiple sites or posted to a listserv without the copyright holder's express written permission.
However, users may print, download, or email articles for individual use.