Synthesis of the Strychnos Alkaloid (-)-Strychnopivotine and Confirmation of its Absolute Configuration

Article abstract of DOI:10.1002/chem.201601319

The first enantioselective synthesis of (-)-strychnopivotine from a known and inexpensive phenol has been achieved in 15 steps. The strategy is based on a new diastereoselective aza-Michael-enol-ether cascade desymmetrization of a dienone, guided by a removable lactic acid-derived chiral auxiliary. Synthesis involves a phenol dearomatization, a conjugated silicon addition, a stereoselective double reductive amination, and two Heck-type carbopalladations as key steps. The absolute configuration of the natural compound, which, to date, has been uncertain, was confirmed by using circular dichroism (CD) spectroscopy and X-ray analyses.

Full text of DOI:10.1002/chem.201601319

DOI: 10.1002/chem.201601319
Communication
&
Total Synthesis
Synthesis of the Strychnos Alkaloid (À)-Strychnopivotine and
Confirmation of its Absolute Configuration
[a]
Gatan Maertens and Sylvain Canesi*
cascade to produce the ABCE substructure of the natural com-
pound. However, an enantioselective synthetic route to strych-
nopivotine is still required to validate its absolute configura-
tion and enable further research on the biological activity of
this compound and its analogs. Our interest in the total syn-
Abstract: The first enantioselective synthesis of (À)-strych-
nopivotine from a known and inexpensive phenol has
been achieved in 15 steps. The strategy is based on a new
diastereoselective aza-Michael-enol-ether cascade desym-
metrization of a dienone, guided by a removable lactic
acid-derived chiral auxiliary. Synthesis involves a phenol
dearomatization, a conjugated silicon addition, a stereo-
selective double reductive amination, and two Heck-type
carbopalladations as key steps. The absolute configuration
of the natural compound, which, to date, has been
uncertain, was confirmed by using circular dichroism (CD)
spectroscopy and X-ray analyses.
[
7]
thesis of natural products through oxidative dearomatization
led us to develop an asymmetric pathway to strychnopivotine,
which is reported in this communication.
Our retrosynthetic analysis began with intramolecular carbo-
palladation to close the D ring from iodoalkene 3 (Scheme 1).
Strychnopivotine 1 (see Figure 1), a pentacyclic Strychnos alka-
[1]
loid from the curan family, was first isolated by the group of
Figure 1. Strychnopivotine and strychnine, two Strychnos alkaloids.
[
2]
Angenot in 1980 from the root bark of Strychnos variabilis.
However, to date, the absolute configuration of strychnopivo-
tine has been uncertain, due to the absence of X-ray crystal
structure analyses of the natural compound. Despite its unusu-
al structure and, unlike others alkaloids from this family, such
Scheme 1. Retrosynthesis of (À)-strychnopivotine.
[
3,4]
as strychnine 2,
strychnopivotine has received little atten-
tion from the scientific community. Indeed, its biological activi-
ty has not been evaluated, and although some efforts towards
The latter can be obtained by acetylation of indoline 4, which
can be produced by hydrolysis of the chiral auxiliary of 5. The
C ring can be created by stereoselective double reductive ami-
[
5]
the construction of its core structure have been reported,
[
8]
only one total synthesis of (Æ)-strychnopivotine has been ach-
nation using amine 6 from 7. The B ring can be constructed
by diastereoselective desymmetrization of dienone 8 via an
aza-Michael-enol-ether tandem process followed by Heck-type
coupling. The latter can be generated by amidification of ester
9 and phenol dearomatization.
[6]
ieved, by the leading group of Padwa in 2008. Their strategy
employed a noteworthy [4+2]-cycloaddition/rearrangement
[
a] G. Maertens, Prof. S. Canesi
Laboratoire de MØthodologie et Synth›se de
Produit Naturels, UniversitØ du QuØbec à MontrØal
C.P.8888, Succ. Centre-Ville, MontrØal, H3C 3P8 QuØbec (Canada)
E-mail: canesi.sylvain@uqam.ca
Synthesis began with the treatment of the known ester 9
with the aluminum salt of 2-iodoaniline to produce amide 10
[
9]
(
87%; see Scheme 2). The latter was used in oxidative
[
10]
dearomatization mediated by a hypervalent iodine reagent,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601319.
[
11]
as demonstrated by Kita and co-workers, to generate dien-
Chem. Eur. J. 2016, 22, 7090 – 7093
7090
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Scheme 3. Proposed mechanism for formation of 11. Reaction conditions:
a) TBSOTf, Et N, CH Cl , 08C to RT, 12 h, 70%. TBSOTf=tert-butyldimethylsilyl
3 2 2
trifluoromethanesulfonate.
gated addition onto enone 7. This addition might force the
molecule to further adopt a favorable transition state, in which
the hydride must approach on the required (Re)-face, thereby
directing a reductive amination and leading to the formation
of the desired diastereoisomer 5. Therefore, Michael acceptor 7
was reacted with a silicon-based cuprate developed by Flem-
Scheme 2. Synthesis of the key tetracyclic intermediate 12 through an aza-
Michael-enol-ether tandem process, and ORTEP representation of 7. Reaction
conditions: a) 2-iodoaniline, DIBAL-H, THF, 08C to RT, 12 h, 87%; b) DIB,
3 2 2
MeOH, 08C, 5 min, 89%; c) TBSOTf, Et N, CH Cl , 08C to RT, 12 h;
[
13]
ing and co-workers to generate the silylated compound 14
in 63% as the sole diastereoisomer (Scheme 4). The cis rela-
d) Pd(PPh , Et N, CH CN, reflux, 12 h, 70% over two steps, 1:19 d.r.;
3
)
4
3
3
e) KHMDS, allylbromide, THF, À788C to 08C, 1 h, 75%. DIBAL-H=diisobutyl-
aluminum hydride, THF=tetrahydrofuran, DIB=(diacetoxyiodo)benzene,
TBSOTf=tert-butyldimethylsilyl trifluoromethanesulfonate, KHMDS=potassi-
um bis(trimethylsilyl)amide.
one 8 (89%). Subsequently, 8 was treated by using a novel dia-
stereoselective aza-Michael-enol-ether tandem process to pro-
duce 11. This method completes other noteworthy processes
that have been developed in the search for asymmetric path-
[12]
ways leading to chiral cyclohexenones. This transformation is
our key step in desymmetrizing dienone 8 and developing
asymmetric synthesis of this target. The lactic acid-derived
moiety present on the lateral chain is used as a chiral auxiliary
that will later be removed. TBS-activation produces an enol-
ether that is essential for this process, being an ideal Heck-
type acceptor to yield the indoline ring of strychnopivotine by
a carbopalladation strategy. This step delivered tetracycle 12 in
Scheme 4. Installation of the C ring of strychnopivotine. Reaction conditions:
a) (PhMe
PPh , MeOH, RT, 1 h; c) 6, NaBH
over two steps.
2
Si)
2
CuLi, THF, À788C, 1 h, 63%; b) O
3
, MeOH, À788C, 10 min, then
7
0% yield over two steps and at a minimum of 1:19 d.r. within
3
3
CN, cat. AcOH, MeOH, 08C to RT, 1 h, 63%
the limits of NMR at 300 MHz. Finally, regio- and stereoselec-
tive allylation of 12 under classical conditions yielded the key
intermediate 7 (75%), which contains the quaternary carbon
center of strychnopivotine. Crystallographic analysis of 7 con-
firmed its absolute configuration, depicted in Scheme 2.
To explain the stereoselectivity of the desymmetrization, we
hypothesized that the transformation involved the formation
of imino-ether 13 (Scheme 3). The nitrogen atom then at-
tacked the activated Michael acceptor on the face it was
tethered through a half-chair transition state, in which the
methyl group of the chiral auxiliary was located equatorially
to minimize steric interactions.
tionship between the methoxy and silylated groups was deter-
mined by NOE NMR. A subsequent ozonolysis converted the
olefin into the corresponding aldehyde 15, which was directly
submitted to the reductive amination conditions involving pri-
mary amine 6 in the presence of NaBH CN as a reductant and
3
acetic acid as a catalyst. Under these conditions, compound 5
exhibiting the desired cis CE ring junction was generated as
a single diastereoisomer (confirmed by NMR at 300 MHz) in
63% over two steps.
To close the final ring of the target and complete the
synthesis, the chiral auxiliary and silylated group had to be
removed. To this end, we first considered an acidic aqueous
hydrolysis. However, although simple treatment of 5 with
At this stage, it was necessary to prepare the molecule for
construction of the C ring. To this end, we envisaged the intro-
duction of a bulky and removable functional group by a conju-
Chem. Eur. J. 2016, 22, 7090 – 7093
www.chemeurj.org
7091
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
concentrated aqueous HCl allowed us to hydrolyze the ketal
function and remove the silylated functional group (Scheme
5
), no traces of 17 were observed. Modification of the experi-
Scheme 6. Completion of synthesis of (À)-strychnopivotine. Reaction condi-
tions : a) TMSOTf, Et
to RT, 30 min; b) TsOH, MeOH, RT, 30 min; then TPAP, NMO, CH
3
N, CH
2
Cl
2
, 08C, 10 min; then AcCl, pyridine, CH
2
Cl
CN, RT, 1 h,
, MeOH, 708C, 10 min,
0%. TMSOTf=trimethylsilyl trifluoromethanesulfonate, TPAP=tetrapropyl-
2
, 08C
3
5
2
2 2 2 2 3
2% over four steps; c) PdCl (dppf)·CH Cl , K CO
ammonium perruthenate, NMO=N-methylmorpholine-N-oxide, dppf=1,1’-
bis(diphenylphosphino)ferrocene.
[
2]
2
8.8 and De(298) =À8.8) to allow a high degree of confidence
that our synthetic (À)-strychnopivotine is the same enantiomer
as the extracted strychnopivotine. Moreover, crystallographic
analysis of 7 confirmed that the absolute configuration of the
natural strychnopivotine was the one proposed by Angenot
Scheme 5. Removal of the chiral auxiliary. Reaction conditions: a) Conc. HCl,
:1 AcOH/H O, 508C, 1 h; b) NaBH , EtOH, 08C to RT, 12 h, 85% over two
steps.
1
2
4
[
2]
and co-workers.
mental conditions, such as pH (including basic conditions),
choice of acid, reaction time or temperature, mostly resulted in
degradation of the molecule. Further experiments, such as nu-
cleophilic attacks on the acetamide, also failed to produce the
desired intermediate 17. On this basis, we hypothesized that
the auxiliary was trapped in a hindered hemi-ketal-lactam ar-
rangement, and was thus resistant to removal. To open the
blocked form and enable amide hydrolysis, we proposed to
reduce the hemi-ketal 16 selectively to the corresponding alco-
In summary, we achieved the enantioselective synthesis of
(À)-strychnopivotine in 15 steps from a simple and inexpensive
phenol. The most innovative step of the synthesis was a novel
diastereoselective aza-Michael-enol-ether cascade desymmetri-
zation of a dienone system, guided by a removable lactic acid-
derived chiral auxiliary. The synthesis included dearomatization
of a phenol mediated by a hypervalent iodine reagent,
stereoselective conjugated Si addition, stereoselective double
reductive amination, and two Heck-type carbopalladations. CD
spectroscopy and X-ray crystal structure analyses confirmed
the absolute configuration of the natural extracted compound.
hol. Interestingly, when 16 was treated with NaBH in ethanol,
4
we observed complete and facile removal of the auxiliary
during the reduction—probably due to the presence of
sodium ethoxide—leading to an epimeric mixture of alcohols
4
in 85% yield over two steps.
Acknowledgements
Alcohol mixture 4 was protected with a TMS group under
classical conditions, and acetamide 19 was produced using
We are very grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canada Foundation
for Innovation (CFI), and the provincial government of Quebec
acetyl chloride and pyridine (Scheme 6). Deprotection under
[
14]
mild acidic conditions followed by Ley–Griffith oxidation of
the mixture of alcohols converged on the intermediate 3 in
(
FQRNT and CCVC) for their financial support of this research.
5
2% yield over four steps, including one-pot procedures. With
this direct precursor in hand, the D ring was finally closed by
a Heck-type coupling involving PdCl (dppf)·CH Cl in basic con-
We also thank Prof. Steve Bourgault (UQAM) for the CD experi-
ments and Prof. Xavier Ottenwaelder (Concordia University,
MontrØal) for the X-ray experiments.
2
2
2
[
15]
ditions. A large amount of methanol solvent is required to
avoid the formation of an enolate on the acetamide moiety,
which could trigger undesired side reactions (e.g., migration of
the acetyl group), as judiciously described by Padwa and co-
Keywords: alkaloid · chiral auxiliary · desymmetrization ·
hypervalent iodine · total synthesis
[
6]
workers. Those conditions allowed us to complete the first
asymmetric synthesis of Strychnopivotine in 20% yield.
Circular dichroism (CD) spectroscopy of our synthetic (À)-
[1] a) J. Bosch, J. Bonjoch, M. Amat in The Alkaloids, Vol. 48 (Ed.: G. A. Cor-
dell), Academic Press, New York, 1996, pp. 75–189; b) J. Sapi, G. Mas-
siot in Monoterpenoid Indole Alkaloids (Ed.: J. E. Saxton) in The Chemistry
of Heterocyclic Compounds (Ed. E. C. Taylor), Wiley, New York, 1994,
Supp. to Vol. 25, part 4, pp. 5213–5216.
1
displayed a positive Cotton effect at 253 nm (De ꢀ +28.5)
(253)
[16]
and a negative Cotton effect at 298 nm (De(298) ꢀÀ10). These
values are sufficiently close to the literature values (De = +
[2] M. Tits, D. Tavernier, L. Angenot, Phytochemistry 1980, 19, 1531.
(
252)
Chem. Eur. J. 2016, 22, 7090 – 7093
www.chemeurj.org
7092
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[
[
3] For a review about syntheses of Strychnine, see: J. S. Cannon, L. E. Over-
man, Angew. Chem. Int. Ed. 2012, 51, 4288; Angew. Chem. 2012, 124,
sØgu, T. Sylla, T. Garnier, L. B. Rojas, J. Charris, D. Deffieux, S. Quideau,
Tetrahedron 2010, 66, 5908; e) T. Jen, B. A. Mendelsohn, M. A. Ciufolini,
J. Org. Chem. 2011, 76, 728; f) L. PouysØgu, D. Deffieux, S. Quideau, Tet-
rahedron 2010, 66, 2235; g) M. TraorØ, S. Ahmed-Ali, M. Peuchmaur, Y. S.
Wong, J. Org. Chem. 2011, 76, 1409; h) L. PouysØgu, M. Marguerit, J.
Gagnepain, G. Lyvinec, A. J. Eatherton, S. Quideau, Org. Lett. 2008, 10,
5211; i) A. Ozanne-Beaudenon, S. Quideau, Angew. Chem. Int. Ed. 2005,
44, 7065; Angew. Chem. 2005, 117, 7227; j) J. Singh, F. V. Rehbein, T.
Wirth, ChemistryOpen 2012, 1, 245; k) U. Farid, F. Malmedy, R. Claveau,
L. Albers, T. Wirth, Angew. Chem. Int. Ed. 2013, 52, 7018; Angew. Chem.
2013, 125, 7156; l) V. V. Zhdankin, Hypervalent Iodine Chemistry: Prepara-
tion, Structure, and Synthetic Applications of Polyvalent Iodine Com-
pounds, Wiley, Chichester, 2013; m) P. Mizar, A. Laverny, M. El-Sherbini,
U. Farid, M. Brown, F. Malmedy, T. Wirth, Chem. Eur. J. 2014, 20, 9910;
n) V. Singh, T. Wirth, Chem. Asian J. 2014, 9, 950; o) V. V. Zhdankin, P. J.
Stang, Chem. Rev. 2008, 108, 5299; p) A. Yoshimura, K. R. Middleton,
A. D. Todora, B. J. Kastern, S. R. Koski, A. V. Maskaev, V. V. Zhdankin, Org.
Lett. 2013, 15, 4010; q) N. T. Vo, R. D. M. Pace, F. O’Hara, M. J. Gaunt, J.
Am. Chem. Soc. 2008, 130, 404; r) R. Leon, A. Jawalekar, T. Redert, M. J.
Gaunt, Chem. Sci. 2011, 2, 1487.
4
362.
4] a) P. Magnus, M. Giles, R. Bonnert, C. S. Kim, L. McQuire, A. Merritt, N.
Vicker, J. Am. Chem. Soc. 1992, 114, 4403; b) S. D. Knight, L. E. Overman,
G. Pairaudeau, J. Am. Chem. Soc. 1993, 115, 9293; c) M. E. Kuehne, F. Xu,
J. Org. Chem. 1993, 58, 7490; d) V. H. Rawal, S. Iwasa, J. Org. Chem.
1994, 59, 2685; e) M. E. Kuehne, F. Xu, J. Org. Chem. 1998, 63, 9427; f) D.
SolØ, J. Bonjoch, S. García-Rubio, E. Peidró, J. Bosch, Angew. Chem. Int.
Ed. 1999, 38, 395; Angew. Chem. 1999, 111, 408; g) M. Ito, C. W. Clark, M.
Mortimore, J. B. Goh, S. F. Martin, J. Am. Chem. Soc. 2001, 123, 8003;
h) M. J. Eichberg, R. L. Dorta, K. Lamottke, K. P. C. Vollhardt, Org. Lett.
2000, 2, 2479; i) M. Nakanishi, M. Mori, Angew. Chem. Int. Ed. 2002, 41,
1
934; Angew. Chem. 2002, 114, 2014; j) G. J. Bodwell, J. Li, Angew. Chem.
Int. Ed. 2002, 41, 3261; Angew. Chem. 2002, 114, 3395; k) T. Ohshima, Y.
Xu, R. Takita, S. Shimuzu, D. Zhong, M. Shibasaki, J. Am. Chem. Soc.
2002, 124, 14546; l) Y. Kaburagi, H. Tokuyama, T. Fukuyama, J. Am.
Chem. Soc. 2004, 126, 10246; m) H. Zhang, J. Boonsombat, A. Padwa,
Org. Lett. 2007, 9, 279; n) G. Sirasani, T. Paul, W. , Jr. Dougherty, S. Kassel,
R. B. Andrade, J. Org. Chem. 2010, 75, 3529; o) C. Beemelmanns, H. U.
Reissig, Angew. Chem. Int. Ed. 2010, 49, 8021; Angew. Chem. 2010, 122,
[11] a) Y. Tamura, T. Yakura, J. Haruta, Y. Kita, J. Org. Chem. 1987, 52, 3927;
b) Y. Kita, H. Tohma, K. Hatanaka, T. Takada, S. Fujita, S. Mitoh, H. Sakurai,
S. Oka, J. Am. Chem. Soc. 1994, 116, 3684; c) T. Dohi, A. Maruyama, N.
Takenaga, K. Senami, Y. Minamitsuji, H. Fujioka, S. B. Caemmerer, Y. Kita,
Angew. Chem. Int. Ed. 2008, 47, 3787; Angew. Chem. 2008, 120, 3847;
d) T. Dohi, Y. Kita, Chem. Commun. 2009, 2073; e) T. Dohi, M. Itob, N. Ya-
maokaa, K. Morimotoa, H. Fujiokab, Y. Kita, Tetrahedron 2009, 65, 10797;
f) T. Dohi, N. Yamaoka, Y. Kita, Tetrahedron 2010, 66, 5775; g) T. Dohi, D.
Kato, R. Hyodo, D. Yamashita, M. Shiro, Y. Kita, Angew. Chem. Int. Ed.
2011, 50, 3784; Angew. Chem. 2011, 123, 3868; h) K. Yasuyuki, D. Toshi-
fumi, M. Eisuke, Y. Daisuke, M. Keitaro, Heterocycles 2014, 88, 245.
[12] a) P. Wipf, Y. Kim, Tetrahedron Lett. 1992, 33, 5477; b) P. Wipf, Y. Kim, D.
Goldstein, J. Am. Chem. Soc. 1995, 117, 11106; c) P. Wipf, W. Li, J. Org.
Chem. 1999, 64, 4576; d) P. Wipf, D. A. Mareska, Tetrahedron Lett. 2000,
41, 4723; e) P. Wipf, J. L. Methot, Org. Lett. 2000, 2, 4213; f) T. A. Wender-
ski, C. Hoarau, L. Mejorado, T. R. R. Pettus, Tetrahedron 2010, 66, 5873;
g) G. Maertens, M. A. MØnard, S. Canesi, Synthesis 2014, 46, 1573.
[13] D. J. Ager, I. Fleming, S. K. Patel, J. Chem. Soc. Perkin Trans. 1 1981, 2520.
[14] a) W. P. Griffith, S. V. Ley, G. P. Whitcombe, A. D. White, J. Chem. Soc.
Chem. Commun. 1987, 1625; b) S. V. Ley, J. Norman, W. P. Griffith, S. P.
Marsden, Synthesis 1994, 639.
8
195; p) D. B. C. Martin, C. D. Vanderwal, Chem. Sci. 2011, 2, 649; q) S. B.
Jones, B. Simmons, A. Mastracchio, D. W. C. MacMillan, Nature 2011,
75, 183; r) J. Bonjoch, D. SolØ, Chem. Rev. 2000, 100, 3455; s) M. Mori,
Heterocycles 2010, 81, 259; t) L. Furst, C. R. Stephenson, Nat. Chem. Biol.
011, 7, 582; u) C. Beemelmanns, H. U. Reissig, Chem. Soc. Rev. 2011, 40,
199; v) D. B. C. Martin, L. Q. C. Nguyen, D. Vanderwal, J. Org. Chem.
012, 77, 17; w) C. Beemelmanns, H. U. Reissig, Chem. Rec. 2015, 15,
72.
4
2
2
2
8
[
5] H. J. Teuber, C. Tsaklakidis, J. W. Bats, Liebigs Ann. Chem. 1992, 461; D.
SolØ, X. Urbaneja, A. Cordero-Vargas, J. Bonjoch, Tetrahedron 2007, 63,
1
6] J. Boonsombat, H. Zhang, M. Chughtai, J. Hartung, A. Padwa, J. Org.
Chem. 2008, 73, 3539.
7] a) K. C. GuØrard, V. Hamel, A, GuØrinot, C. Bouchard-Aubin, S. Canesi,
Chem. Eur. J. 2015, 21, 18068; b) G. Jacquemot, G. Maertens, S. Canesi,
Chem. Eur. J. 2015, 21, 7713; c) G. Maertens, C. L’Homme, S. Canesi,
Front. Chem. 2015, 2, 115; d) S. Desjardins, G. Maertens, S. Canesi, Org.
Lett. 2014, 16, 4928; e) C. L’Homme, M.-A. MØnard, S. Canesi, J. Org.
Chem. 2014, 79, 8481; M. A. Beaulieu, X. Ottenwaelder, S. Canesi, Chem.
Eur. J. 2014, 20, 7581; f) K. C. GuØrard, A. GuØrinot, C. Bouchard-Aubin,
M. A. MØnard, M. Lepage, M. A. Beaulieu, S. Canesi, J. Org. Chem. 2012,
0177.
[
[
[15] a) A. B. Dounay, P. G. Humphreys, L. E. Overman, A. D. Wrobleski, J. Am.
Chem. Soc. 2008, 130, 5368; b) D. SolØ, E. Peidró, J. Bonjoch, Org. Lett.
2000, 2, 2225; c) D. SolØ, J. Diaba, J. Bonjoch, J. Org. Chem. 2003, 68,
5746; d) J. Bonjoch, F. Diaba, G. Puigbó, E. Peidró, D. SolØ, Tetrahedron
Lett. 2003, 44, 8387; e) D. SolØ, X. Urbaneja, J. Bonjoch, Adv. Synth.
Catal. 2004, 346, 1646; f) D. SolØ, X. Urbaneja, J. Bonjoch, Org. Lett.
2005, 7, 5461.
7
7, 2121; g) M. A. Beaulieu, K. C. GuØrard, G. Maertens, C. Sabot, S.
Canesi, J. Org. Chem. 2011, 76, 9460; h) M. A. Beaulieu, C. Sabot, N. Ach-
ache, K. C. GuØrard, S. Canesi, Chem. Eur. J. 2010, 16, 11224; i) C. Sabot,
K. C. GuØrard, S. Canesi, Chem. Commun. 2009, 2941; j) K. C. GuØrard, C.
Sabot, L. Racicot, S. Canesi, J. Org. Chem. 2009, 74, 2039; k) C. Sabot, D.
BØrard, S. Canesi, Org. Lett. 2008, 10, 4629.
[
[
8] a) P. Liu, J. Wuang, J. Zhang, F. G. Qiu, Org. Lett. 2011, 13, 6426; b) M. E.
Krafft, J. W. Cran, Synlett 2005, 1263.
9] P. Q. Huang, X. Zheng, X.-M. Deng, Tetrahedron Lett. 2001, 42, 9039.
[16] The CD spectrum is given in the Supporting Information.
[
10] a) S. Quideau, M. A. Looney, L. PouysØgu, Org. Lett. 1999, 1, 1651; b) M.
TraorØ, S. Ahmed-Ali, M. Peuchmaur, Y. S. Wong, Tetrahedron 2010, 66,
Received: March 19, 2016
5
863; c) H. Liang, M. A. Ciufolini, Tetrahedron 2010, 66, 5884; d) L. Pouy-
Published online on April 13, 2016
Chem. Eur. J. 2016, 22, 7090 – 7093
www.chemeurj.org
7093
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim