Diastereoselective synthesis of enantiopure homopropargylic n-tert-butylsulfinylamines

Article abstract of DOI:10.1055/s-0031-1289531

The diastereoselective synthesis of enantiopure homopropargylic amines by propargylation of various N-tert-butylsulfinylimines (tBS-imines) with 1-trimethylsilyl allenylzinc bromide is presented. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1289531

LETTER
2681
Diastereoselective Synthesis of Enantiopure Homopropargylic
N-tert-Butylsulfinylamines
Enantiopure
H
omopr
o
a
pargylic
N
-
t
tert-Bu
h
tylsulfinylam
i
ines eu Cyklinsky, Candice Botuha, Fabrice Chemla,* Franck Ferreira, Alejandro Pérez-Luna
UPMC-Univ Paris 06, Institut Parisien de Chimie Moléculaire (UMR CNRS 7201), Institut de Chimie Moléculaire (FR 2769),
case 183, 4 place Jussieu, 75005 Paris Cedex 05, France
Fax +33(1)44277567; E-mail: fabrice.chemla@upmc.fr
Received 21 June 2011
We first studied the reaction of lithium species Li-1 with
enantiopure phenyl tBS-imine 2a (Scheme 1). Li-1, gen-
erated by treating 1-trimethylsilylpropyne with tert-butyl-
lithium (1 equiv) at –78 °C in diethyl ether, reacted with
2a almost completely within 30 minutes at –78 °C to af-
ford adducts 3a and 4a. Under these conditions, both
regio- and diastereoselectivities were modest; an 88:12
mixture of homopropargylic and a-allenic amines 3a and
4a was obtained with a diastereomeric ratio (dr) of 72:28
for amine 3a (Table 1, entry 1). Performing the reaction in
the presence of HMPA (4 equiv) gave homopropargylic
amine 3a exclusively, albeit with an incomplete conver-
sion of 76% and a still modest dr of 76:24 (Table 1, entry
2).
Abstract: The diastereoselective synthesis of enantiopure ho-
mopropargylic amines by propargylation of various N-tert-butyl-
sulfinylimines (tBS-imines) with 1-trimethylsilyl allenylzinc
bromide is presented.
Key words: amines, asymmetric synthesis, chiral auxiliaries, im-
ines, zinc
Propargylic and homopropargylic compounds, including
homopropargylic amines, are of great interest in organic
chemistry owing to their multiple possibilities of transfor-
mation and because of the wide range of conditions toler-
ated by their carbon–carbon triple bond.1 In spite of the
important place they occupy as nitrogen-containing build-
ing blocks in asymmetric synthesis, a general and practi-
cal stereoselective synthesis of homopropargylic amines
is still lacking. Reported strategies2 include Barbier-type
reactions between propargylic bromides and aldimines,
addition of pre-formed allenyl/propargyl metals to imines
and imine derivatives,3 and iminium-ion activation
through organocatalysis.4
Similar regio- and diastereoselectivities were attained
when 2a reacted in diethyl ether at –78 °C with zinc spe-
H
H
[M]
metalation
(see text)
•
TMS
TMS
[M]-1
[M] = Li, ZnBr
t-Bu
S
t-Bu
S
t-Bu
S
Nucleophilic addition and, more specifically, propargyla-
tion of chiral N-tert-butylsulfinylimines (tBS-imines) has
proved to be a powerful way to obtain enantioenriched
functionalized amine derivatives.5 In this field, we have
reported an efficient method for the stereoselective syn-
thesis of enantioenriched acetylenic aziridines,6 1,2-ami-
no alcohols,7 and 2-amino-1,3-diols8 by the reaction of
racemic 3-chloro- and 3-methoxymethoxyallenyl zinc
compounds with enantiopure tBS-imines. Thus, the syn-
theses of a number of natural products have been success-
fully achieved.9
[M]-1
HN
O
•
N
O
HN
O
TMS
+
–78 °C, 30 min
Ph
Ph
H
Ph
2a
3a
4a
TMS
Scheme 1 Preparation of [M]-1 and its subsequent reaction with
tBS-imine 2a (see Table 1)
Table 1 Optimization of the Reaction of [M]-1 with tBS-Imine 2a
Entry [M]-1
Solvent
Conv
(%)a
Ratiob
3a/4a
drc of
3a
(equiv)
1
2
3
4
5
Li-1 (1)
Et2O
98
76
30
31
88:12
>98:02
>98:02
72:28
76:24
80:20
As part of an ongoing project devoted to the synthesis of
natural alkaloids, we required easy access to enantiopure
a-branched homopropargylamines. Building on our expe-
rience on the synthetic use of pre-formed allenyl/propar-
gyl zinc reagents and on two literature precedents
describing the diastereoselective propargylation of enan-
tiopure tBS-imines,10,11 we reasoned that metalation of
readily available 1-trimethylsilylpropyne should generate
efficient tBS-imine propargylation reagents.
Li-1 (1)
Et2O–HMPA
Et2O
ZnBr-1 (1)
ZnBr-1 (1)
ZnBr-1 (2)
THF
>98:02 >98:02
THF
100 (79)[d] >98:02 >98:02
a Conversion observed in the propargylation reaction based on unre-
acted 2a determined by 1H NMR analysis of the crude reaction mix-
ture.
b Regioselectivity determined by 1H NMR analysis of the crude reac-
tion mixture.
c Diastereoselectivity determined by 1H NMR analysis of the crude
reaction mixture
SYNLETT 2011, No. 18, pp 2681–2684
x
x
.x
x
.2
0
1
1
Advanced online publication: 19.10.2011
DOI: 10.1055/s-0031-1289531; Art ID: D19511ST
d Isolated yield after silica gel chromatography given in parentheses.
© Georg Thieme Verlag Stuttgart · New York

2682
M. Cyklinsky et al.
LETTER
cies ZnBr-1 (generated by the transmetalation of Li-1 with diastereomeric ratio was achieved that was higher than
zinc bromide at –35 °C) although, in this case, a poor con- 98:2 for 3b (Table 2, entry 3). In contrast, a better result
version of 30% was observed (Table 1, entry 3).
was obtained when the propargylation reaction was per-
formed at room temperature. Homopropargylic amine 3b
was exclusively formed in 98% yield and dr >98:2
(Table 2, entry 4). The same conditions allowed alkyl and
alkenyl homopropargylic amines 3c–f to be isolated in
high yields (70–98%) and dr >98:2 from the correspond-
ing tBS-imines, meaning that steric hindrance of the R
substituent of the imine had little influence on the level of
the stereoselectivity (Table 2, entries 5–7 and 9). It is wor-
thy of note that, in the case of aromatic imine 2a, a lower
dr of 84:16 was observed when the reaction was per-
formed at room temperature (Table 2, entry 2). More un-
expectedly, when the reaction was conducted at –78 °C
with tBS-imine 2f bearing a silyloxy substituent a to the
imino group, a-allenylic amine 4f was formed with a high
regioselectivity of 95:5 as a single stereoisomer (Table 2,
entry 8).
Conversely, we found that 3a could be obtained as the sin-
gle product in dr >98:2 when the reaction was carried out
between 2a and ZnBr-1 in tetrahydrofuran at –78 °C. In
this solvent, the organozinc species was generated by met-
alation of 1-trimethylsiylylpropyne with sec-butyllithium
(1 equiv) at –20 °C and subsequent transmetalation at
–35 °C. However, although both regio- and stereoselec-
tivities were excellent under these conditions, using one
equivalent of ZnBr-1 led to a poor conversion of 31%
(Table 1, entry 4). Finally, full conversion of imine 2a
was observed when two equivalents of ZnBr-1 were used,
giving homopropargylic amine 3a as a single isomer in
79% isolated yield (Table 1, entry 5).
The optimized propargylation reaction conditions (i.e.,
two equivalents of ZnBr-1 in THF) were next applied to a
range of tBS-imines (Scheme 2).12
The relative configuration of the two stereocenters of 3a–
c was determined to be anti by transformation into known
homoallylamines 5a–c13 and comparison of their NMR
data, which were found to be identical to the reported val-
ues (Scheme 3). The anti-configuration of 3d–e was in-
ferred from these results. In contrast, the product obtained
from 3f following the same sequence had different NMR
and physical data to those reported for the expected (S,SS)
stereoisomer,13 suggesting that the relative configuration
of the two stereocenters is opposite in this case. The R
configuration of the newly created center in 3f was further
confirmed by its transformation into b-amino alcohol 6,
Surprisingly, at –78 °C, the propargylation with alkyl-
substituted imine 2b was not completely regioselective,
since a 82:18 mixture of homopropargylic and a-allenylic
amines 3b and 4b was obtained, nevertheless, an excellent
t-Bu
S
t-Bu
S
t-Bu
S
HN
O
•
N
O
HN
O
TMS
ZnBr-1
+
THF, T (°C), 30 min
R
R
H
R
R
R
2a–e
t-Bu
3a–e
4a–e
TMS
t-Bu
t-Bu
S
S
S
25
N
O
HN
O
•
HN
O
TMS
ZnBr-1
for which the specific rotation {[a]D +6.4 (c 1.4,
+
THF, T (°C), 30 min
MeOH)} was found to be opposite to that reported for the
H
R
25
antipode of 6 {[a]D –6.0 (c 1.0, MeOH)} (Scheme 3).14
2f
3f
4f
TMS
Thus, the sense of stereoinduction is reversed between the
Scheme 2 Scope of the propargylation reaction of tBS-imines 2a–f
(see Table 2)
Table 2 Diastereoselective Addition of Allenylzinc ZnBr-1 to tBS-Imines 2a–f
Entry
R
tBS-Iminea
Temp (°C)
–78
r.t.
Ratiob 3/4
>98:02
>98:02
82:18
Yieldc (%)
Major product drd
1
2
3
4
5
6
7
8
9
Ph
2a
2a
2b
2b
2c
2d
2e
2f
79
79
90
98
70
86
98
91[e]
81
3a
3a
3b
3b
3c
3d
3e
4f
>98:02
Ph
84:16
>98:02
>98:02
96:04
n-Pr
–78
r.t.
n-Pr
>98:02
>98:02
>98:02
>98:02
05:95
i-Pr
r.t.
c-Hex
(E)-PhCH=CH
TBSOCH2
TBSOCH2
r.t.
>98:02
>98:02
>98:02
>98:02
r.t.
–78
r.t.
2f
>98:02
3f
a Propargylation reaction performed with two equivalents of ZnBr-1 and one equivalent of imine 2a–f.
b Regioselectivity determined by 1H NMR analysis of the crude reaction mixture.
c Combined isolated yield after silica gel chromatography.
d Diastereoselectivity of major product determined by 1H NMR analysis (400 MHz) on the crude reaction mixture.
e Conversion determined by 1H NMR analysis of the crude reaction mixture.
Synlett 2011, No. 18, 2681–2684 © Thieme Stuttgart · New York

LETTER
Enantiopure Homopropargylic N-tert-Butylsulfinylamines
2683
addition to alkyl- or aryl-substituted tBS-imines and the in favor of propargylic amines 3g and 3h. However, while
addition to a-silyloxy-substituted imine 2f.
the formation of anti-3g with excellent diastereoselectivi-
ty (anti/syn >98:2) was observed from imine 2g, the pro-
pargylation of diastereoisomeric imine 2h led to syn-3h
with very low levels of selectivity (anti/syn = 42:58). The
anti-configuration of 3g and the syn-configuration of the
Furthermore, pre-formed allenylzinc ZnBr-1 reacts with
tBS-imines 2a–e with a stereocontrol opposite to that ob-
served by Fandrick et al. in their zinc-catalyzed propargy-
lation of tBS-imines starting from propargylborolanes,
wherein the reactive species is likely to be ZnEt-1.11 This
interesting reversal of selectivity represents another ex-
ample in which the stereoselectivity of nucleophilic addi-
tion to tBS-imines is controlled by the reaction
conditions.13a,15
3
major isomer 3h were deduced from the J(Ha–Hb) cou-
pling constants measured for the corresponding cis (J =
13.4 Hz) and trans (J = 6.2 Hz) oxazolidinones cis-7 and
trans-7 (Scheme 4). This was corroborated by NOE en-
hancements of 4 and 0% for cis-7 and trans-7, respective-
ly.
t-Bu
S
t-Bu
S
t-Bu
S
t-Bu
S
O
1. K2CO3, MeOH
2. H2, Lindlar Pd
HN
O
TMS
HN
O
N
O
HN
O
TMS
ZnBr-1
O
NH
Hb
R
R
THF, r.t.
H
3a–c
5a–c, 48–91%
Ha
OTBS
OTBS
t-Bu
S
cis-7
2g
3g, 70%, anti/syn >98:02
1. TBAF
2. HCl, MeOH
3J(Ha–Hb) = 13.4 Hz
NOE (Ha–Hb) = 4%
NH2
HN
O
TMS
HO
t-Bu
t-Bu
S
TBSO
3. H2, Lindlar Pd
O
S
N
O
HN
O
TMS
3f
6, 18%
ZnBr-1
O
NH
Hb
H
Scheme 3 Structural assignment of 3a–f
THF, r.t.
OTBS
Ha
OTBS
2h
3h, 90%, anti/syn = 42:58
The results obtained by Fandrick and coworkers have
been rationalized by the six-membered chelated transi-
tion-state model TS2 (Figure 1), which is similar to the
model we previously proposed for the addition of a 3-
chloroallenylzinc bromide to tBS-imines.6a,b The stereo-
selectivity observed in the reactions with ZnBr-1 can be
explained by the six-membered transition-state model
TS1, analogous to that invoked for the addition of 3-meth-
oxymethoxyallenylzinc bromide on tBS-imines,7b,c
wherein the zinc atom is coordinated only to the nitrogen
atom of the imine that adopts its lowest energy conforma-
tion.6c
trans-7 (from syn-3h)
3J(Ha–Hb) = 6.2 Hz
NOE (Ha–Hb) = 0%
Scheme 4 Propargylation of a-silyloxy tBS-imines 2g and 2h
In the case of imines 2g and 2h, both the stereocenter a to
the imino group and the N-tert-butylsulfinyl auxiliary can
direct the facial selectivity of the propargylation. The
anti-selectivity observed for the addition of ZnBr-1 to 2g
is consistent with a Felkin–Anh model TS3 (similar to
TS1) as previously reported for nucleophilic additions to
N-tert-butylsulfinyl-a-silyloxyacetaldimines17
(Scheme 5). The poor selectivity observed for 2h can be
explained by an opposition of the inherent selectivity pro-
vided by the sulfinyl group and that provided by the a-ste-
reocenter through Felkin–Anh control. The major product
was thus obtained via the anti-Felkin–Anh model TS4, as
previously reported by Ellman on enolate additions to N-
tert-butylsulfinyl-a-silyloxy aldimines.17 The stereo-
chemical outcome of these reactions is worthy of mention
since, generally, in additions to tBS-imines, the influence
of the sulfinyl auxiliary overrides that of the a-stereo-
O
t-Bu
S
EtZn
TMS
BrZn
TMS
t-Bu
S
N
H
O
N
H
•
H
•
H
H
R
H
R
mono-coordinated TS1
chelated TS2
(Frandrick's work)
(our work)
Figure 1 Transition-state model TS1 for additions to 2a–e
The case of the imine 2f, bearing an a-silyloxy group, is center.6d,8,16,18,19
intriguing. Reversal of facial selectivity of additions to
alkyl-substituted tBS-imines and additions to tBS-imines
t-Bu
t-Bu
S
BrZn
BrZn
TMS
S
bearing an a-coordinating group has a precedent, although
O
O
N
N
H
TMS
no explanation has been provided.16 Similarly, in our case,
we cannot put forward a convincing explanation as to why
addition on 2f cannot be rationalized by model TS1.
•
•
H
H
H
H
H
TBSO
H
TBSO
H
Felkin–Anh TS3
anti-Felkin–Anh TS4
syn-3h (major)
The reactivity of tBS-imines 2g and 2h, bearing an a-
oxygenated stereocenter, was also examined (Scheme 4).
In both cases, propargylation in THF at room temperature
proceeded with good yield and complete regioselectivity
3g
Scheme 5 Proposed transition-state models TS3 and TS4
Synlett 2011, No. 18, 2681–2684 © Thieme Stuttgart · New York

2684
M. Cyklinsky et al.
LETTER
Chem. Soc. Rev. 2009, 38, 1162. (g) Robak, M. T.; Herbage,
M. A.; Ellman, J. A. Chem. Rev. 2010, 110, 3600.
(6) (a) Chemla, F.; Ferreira, F. J. Org. Chem. 2004, 69, 8244.
(b) Chemla, F.; Ferreira, F. Synlett 2004, 983. (c) Ferreira,
F.; Audouin, M.; Chemla, F. Chem. Eur. J. 2005, 11, 5269.
(d) Botuha, C.; Chemla, F.; Ferreira, F.; Pérez-Luna, A.;
Roy, B. New J. Chem. 2007, 31, 1552.
(7) (a) Palais, L.; Chemla, F.; Ferreira, F. Synlett 2006, 1039.
(b) Chemla, F.; Ferreira, F. Synlett 2006, 2613. (c) Chemla,
F.; Ferreira, F.; Gaucher, X.; Palais, L. Synthesis 2007,
1235. (d) Roy, B.; Pérez-Luna, A.; Ferreira, F.; Botuha, C.;
Chemla, F. Tetrahedron Lett. 2008, 49, 1534.
In conclusion, we have reported an efficient and highly
diastereoselective propargylation of tBS-imines with the
allenylzinc bromide derived from 1-trimethylsilyl prop-
yne. This methodology provides a practical and competi-
tive access to enantioenriched homopropargylic amines.
Besides this, a remarkable difference of diastereofacial se-
lectivity is observed between alkyl, aryl, and a-silyloxy
tBS-imines. We are currently examining the rationale be-
hind the postulated transition-state models by using theo-
retical calculations; the results will be reported in due
course.
(8) Voituriez, A.; Pérez-Luna, A.; Ferreira, F.; Botuha, C.;
Chemla, F. Org. Lett. 2009, 11, 931.
(9) (a) Voituriez, A.; Ferreira, F.; Chemla, F. Org. Lett. 2007, 9,
4705. (b) Voituriez, A.; Ferreira, F.; Chemla, F. J. Org.
Chem. 2007, 72, 5358. (c) Ferreira, F.; Botuha, C.; Chemla,
F.; Perez-Luna, A. J. Org. Chem. 2009, 74, 2238.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
(d) Séguin, C.; Ferreira, F.; Botuha, C.; Chemla, F.; Perez-
Luna, A. J. Org. Chem. 2009, 74, 6986. (e) Hélal, B.;
Ferreira, F.; Botuha, C.; Chemla, F.; Pérez-Luna, A. Synlett
2009, 3115. (f) Louvel, J.; Botuha, C.; Chemla, F.; Demont,
E.; Ferreira, F.; Pérez-Luna, A. Eur. J. Org. Chem. 2010,
2921. (g) Botuha, C.; Chemla, F.; Ferreira, F.; Louvel, J.;
Pérez-Luna, A. Tetrahedron: Asymmetry 2010, 21, 1147.
(10) Hashmi, A. S. K.; Schäfer, S.; Bats, J. W.; Frey, W.;
Rominger, F. Eur. J. Org. Chem. 2008, 4891.
References and Notes
(1) (a) Acetylene Chemistry, Chemistry, Biology and Material
Science; Diederich, F.; Stang, P. J.; Tykwinski, R. R., Eds.;
Wiley-VCH: Weinheim, 2005. (b) Hegedus, L. S. In
Transition Metals in the Synthesis of Complex Organic
Molecules, 2nd ed.; University Science Books: Mill Valley
(USA), 1999. (c) Parrodi, C. A.; Walsh, P. J. Angew. Chem.
Int. Ed. 2009, 48, 4679.
(11) Fandrick, D. R.; Johnson, C. S.; Fandrick, K. R.; Reeves,
J. T.; Tan, Z.; Lee, H.; Song, J. J.; Yee, N. K.; Senanayake,
C. H. Org. Lett. 2010, 12, 748.
(2) Ding, C.-H.; Hou, X.-L. Chem. Rev. 2011, 111, 1914.
(3) For recent regioselective propargylation of imines, see:
(a) Saidi, M. R.; Azizi, N. Tetrahedron: Asymmetry 2002,
13, 2523. (b) Kagoshima, H.; Uzawa, T.; Akiyama, T.
Chem. Lett. 2002, 298. (c) Song, Y.; Okamoto, S.; Sato, F.
Tetrahedron Lett. 2002, 43, 8635. (d) Prajapati, D.; Laskar,
D. D.; Gogoi, B. J.; Devi, G. Tetrahedron Lett. 2003, 44,
6755. (e) Alcaide, B.; Almendros, P.; Aragoncillo, C.;
Rodriguez-Acebes, R. Synthesis 2003, 1163. (f) Miyabe,
H.; Yamaoka, Y.; Naito, T.; Takemoto, Y. J. Org. Chem.
2004, 69, 1415. (g) Lee, P. H.; Kim, H.; Lee, K.; Kim, M.;
Noh, K.; Kim, H.; Seomoon, D. Angew. Chem. Int. Ed. 2005,
44, 1840. (h) Schneider, U.; Sugiura, M.; Kobayashi, S. Adv.
Synth. Catal. 2006, 348, 323. (i) Jiang, B.; Tian, H.
Tetrahedron Lett. 2007, 48, 7942. (j) Yanagisawa, A.;
Suzuki, T.; Koide, T.; Okitsu, S.; Arai, T. Chem. Asian J.
2008, 3, 1793. (k) Kouznetsov, V. V.; Méndez, L. Y. V.
Synthesis 2008, 491. (l) For recent examples of asymmetric
propargylation of imines, see: Fuchibe, K.; Hatemata, R.;
Akiyama, T. Tetrahedron 2006, 62, 11304. (m) For
allenylboration, see: Gonzalez, A. Z.; Soderquist, J. A. Org.
Lett. 2007, 9, 1081. For oxazolidine synthesis of
(12) Typical procedure for the preparation of 3a–h (Table 2): To
a solution of 1-trimethylsilylpropyne (296 mL, 2.00 mmol)
in THF (7 mL) was added s-BuLi (1.3 M in pentane, 1.54
mL, 2.00 mmol) at such a rate that the internal temperature
did not exceed –45 °C. When the addition was complete, the
reaction mixture was warmed to –20 °C over a period of 20
min. After 45 min of stirring at –20 °C, the reaction mixture
turned bright yellow and was cooled to –35 °C prior to the
addition of ZnBr2 (1 M in THF, 2.00 mL, 2.00 mmol) over a
period of 5 min. After 30 min stirring at –35 °C, the resulting
colorless mixture was cooled to –78 °C (for 2a) or warmed
to 20 °C (for 2b–h). Sulfinylimine 2a–h (1.00 mmol) in THF
(1 mL) was then added dropwise. The reaction mixture was
stirred at r.t. until the reaction was complete (reaction
monitored by TLC; ~30 min) then quenched by addition of
NH4Cl/NH3 (2:1), extracted with Et2O, dried (MgSO4), and
concentrated to give 3a–h, which were purified by flash
chromatography over silica gel
(13) For the correlation with reported homoallylic amines, see:
(a) Sun, X.-W.; Xu, M.-H.; Lin, G.-Q. Org. Lett. 2006, 8,
4979; and references cited therein. (b) Rech, J. C.; Yato, M.;
Duckett, D.; Ember, B.; LoGrasso, P. V.; Bergman, R. G.;
Ellman, J. A. J. Am. Chem. Soc. 2007, 129, 490.
(14) For the correlation with the reported antipode of amino
alcohol 6, see: Farmer, J. J.; Schroeder, F.; Meinwald, J.
Helv. Chim. Acta 2000, 83, 2594.
homopropargylic amines, see: (n) Guesne, S.; Comesse, S.;
Kadouri-Puchot, C. Lett. Org. Chem. 2006, 3, 315.
(o) Comesse, S.; Bertin, B.; Kadouri-Puchot, C. Tetrahedron
Lett. 2004, 45, 3807. (p) Agami, C.; Comesse, S.; Kadouri-
Puchot, C. J. Org. Chem. 2002, 67, 1496.
(4) Jiang, H.; Holub, N.; Paixão, M. W.; Tiberi, C.; Falcicchio,
A.; Jørgensen, K. A. Chem. Eur. J. 2009, 15, 9638.
(5) For leading references on tBS-imines, see: (a)Ellman, J. A.;
Owens, T. D.; Tang, T. P. Acc. Chem. Res. 2002, 356, 984.
(b) Ellman, J. A. Pure Appl. Chem. 2003, 75, 39. (c) Zhou,
P.; Chen, B.-C.; Davies, F. A. Tetrahedron 2004, 60, 8003.
(d) Senanayake, C. H.; Krishnamurthy, D.; Lu, Z.-H.; Han,
Z.; Gallou, I. Aldrichimica Acta 2005, 38, 93. (e) Morton,
D.; Stockman, R. A. Tetrahedron 2006, 62, 8869.
(15) Denolf, B.; Mangelinckx, S.; Tornoos, K. W.; De Kimpe, N.
Org. Lett. 2006, 8, 3129.
(16) Tang, T. P.; Volkman, S. K.; Ellman, J. A. J. Org. Chem.
2001, 66, 8772.
(17) Evans, J. W.; Ellman, J. A. J. Org. Chem. 2003, 68, 9948.
(18) Weix, D. J.; Ellman, J. A. Org. Lett. 2003, 5, 1317.
(19) Barrow, J. C.; Ngo, P. L.; Pellicore, J. M.; Selnick, H. G.;
Natermet, P. G. Tetrahedron Lett. 2001, 42, 2051.
(f) Ferreira, F.; Botuha, C.; Chemla, C.; Perez-Luna, A.
Synlett 2011, No. 18, 2681–2684 © Thieme Stuttgart · New York

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