Side-arm-promoted highly enantioselective ring-opening reactions and kinetic resolution of donor-acceptor cyclopropanes with amines

Article abstract of DOI:10.1021/ja302691r

A Ni-catalyzed asymmetric ring-opening reaction of 2-substituted cyclopropane-1,1-dicarboxylates with aliphatic amines has been accomplished using the chiral indane-trisoxazoline (In-TOX) ligand. This highly enantioselective reaction provides an efficient approach to a variety of chiral γ-substituted γ-amino acid derivatives, which are readily transformed into multifunctionalized piperidines and γ-lactams. The single-crystal X-ray structure of the TOX-Ni complex is provided, and the role of the side arm in the chiral ligand is discussed.

Full text of DOI:10.1021/ja302691r

Communication
pubs.acs.org/JACS
Side-Arm-Promoted Highly Enantioselective Ring-Opening Reactions
and Kinetic Resolution of Donor−Acceptor Cyclopropanes with
Amines
You-Yun Zhou, Li-Jia Wang, Jun Li, Xiu-Li Sun,* and Yong Tang*
The State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, P. R. China
S
* Supporting Information
a
Table 1. Effects of the Ligand (L*) and Lewis Acid
ABSTRACT: A Ni-catalyzed asymmetric ring-opening
reaction of 2-substituted cyclopropane-1,1-dicarboxylates
with aliphatic amines has been accomplished using the
chiral indane−trisoxazoline (In-TOX) ligand. This highly
enantioselective reaction provides an efficient approach to
a variety of chiral γ-substituted γ-amino acid derivatives,
which are readily transformed into multifunctionalized
piperidines and γ-lactams. The single-crystal X-ray
structure of the TOX−Ni complex is provided, and the
role of the side arm in the chiral ligand is discussed.
he ring-opening reaction of activated donor−acceptor
T
(D−A) cyclopropanes with nucleophiles provides versa-
tile access to various functionalized carbon skeletons.1−7 Of the
strategies developed, Lewis acids have been shown to promote
such reactions under mild conditions for most nucleophiles.
Amine-initiated nucleophilic ring opening represents a very
useful transformation, affording γ-substituted γ-amino acid
derivatives.7 For example, Magolan and Kerr7d reported the
Yb(OTf)3-catalyzed racemic ring opening of cyclopropane-1,1-
dicarboxylates with indoline in the synthesis of a tetracyclic
tronocarpine subunit. Later on, the Ni(ClO4)2·6H2O- and
La(OTf)3-catalyzed ring openings with amines were reported
by Lifchits and Charette7e and Kotsuki and co-workers,7f
respectively. Although reactions of D−A cyclopropanes with
amines have been developed, most of them invariably require
vigorous conditions such as elevated temperature, even in the
presence of Lewis acids. This is due to the fact that the
complexation of the amine, particularly for aliphatic amines,
with the Lewis acid slows the reaction greatly.7e Furthermore,
to the best of our knowledge, no catalytic asymmetric version of
the ring openings with amines has been developed. Herein we
describe our efforts on this subject.
a
Unless otherwise noted, reactions were carried out under a N2
atmosphere with 1a (0.20 mmol), 2a (0.44 mmol), Lewis acid (0.02
mmol), L* (0.024 mmol), and 4 Å molecular sieves (MS) (200 mg) in
b
dimethoxyethane (DME) (2 mL, [1a]0 = 0.10 M) at rt. Isolated
c
d
yields. Determined by chiral HPLC. The enantioselectivity was
e
reversed. The reaction was run at 40 °C.
amine with the Lewis acid is supposed to retard the reaction,7e
we envisioned that a pendant coordination group in the BOX
ligand8 might modulate the steric and electronic nature of the
Ni(II) reactive site (Scheme 1), promoting the complexation
and activation of the cyclopropane by interfering with the
coordination of the amine.
On the basis of this concept, we synthesized ligands L3b and
L3c for comparison. When a benzyl group was introduced as
the side arm, the yield was 61% after 45 h of reaction (entry 4).
Noticeably, the indane−trisoxazoline (In-TOX) ligand L3c
bearing a chiral oxazoline group as a coordinating side arm
significantly speeded up the reaction, providing 3aa in 72%
yield with 91% ee in 25 h (entry 5).9 This result suggested a
strong effect of the ligand side arm on the reactivity and
As (S)-Ph-DBFOX (L1) and (S)-4-Cl-tBu-PYBOX (L2)
have shown excellent behavior in asymmetric annulations of
D−A cyclopropanes with nitrones5a and aldehydes,5b respec-
tively, using 1,1-cyclopropane diester 2a as a model substrate,
we initially employed these two ligands to investigate the
asymmetric reaction of 2a with aliphatic amine 1a (Table 1,
entries 1 and 2). Unfortunately, they both gave unsatisfactory
results. We next tested the indane-derived bisoxazoline (BOX)
ligand L3a with Ni(ClO4)2, and the product 3aa was also
obtained in poor yield (22%) with poor enantioselectivity (8%
ee) after almost 2 days (entry 3). As the complexation of the
Received: March 24, 2012
Published: May 11, 2012
© 2012 American Chemical Society
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Communication
temperature (rt) (entry 9). The reaction could be extended to
2-thienylcyclopropane 2k, which provided an excellent yield
(95%) and enantioselectivity (94% ee) at rt (entry 12).
Meanwhile, 2-styryl- and 2-vinyl-substituted cyclopropanes 2l
and 2m proved to be suitable substrates, delivering yields of 97
and 93% with 84 and 80% ee, respectively (entries 13 and 14).
The generality of various aliphatic amines was then explored
(Table 3), and a variety of multifunctionalized compounds that
Scheme 1. Strategy for the Reaction of D−A Cyclopropanes
with Amines
Table 3. Expansion of the Secondary Amine in the L3c−
a
enantiocontrol in the catalytic asymmetric reaction of D−A
cyclopropanes with amines. Zn(OTf)2, Fe(ClO4)2·6H2O, and
Cu(OTf)2 in combination with L3c gave poor results, and the
reactions were sluggish (entries 6−8). The Mg complex of L3c
gave 3aa in 28% yield with poor enantioselectivity (entry 9).
Increasing the reaction temperature from 25 to 40 °C in the
case of L3c−Ni(ClO4)2 as the catalyst improved the yield to
90% while almost maintaining the ee after 15 h (entry 10).
The substrate scope was investigated under the optimized
conditions (Table 1, entry 10). As shown in Table 2, a series of
Ni(II)-Catalyzed Ring Opening of D−A Cyclopropanes
Table 2. Reactions of D−A Cyclopropane Diesters with
a
Secondary Amines Catalyzed by L3c−Ni(II)
a
Unless otherwise noted, the reactions were carried out under a N2
atmosphere with 1a (0.20 mmol), 2a (0.44 mmol), Ni(ClO4)2·6H2O
(0.02 mmol), L3c (0.024 mmol), and 4 Å MS (200 mg) in DME (2
b
c
mL, [1]0 = 0.10 M) at rt. Isolated yields. Determined by chiral
d
HPLC. Catalyst loading = 2 mol %, [1]0 = 0.25 M in DME (4 mL).
Catalyst loading = 2 mol %, [1b]0 = 0.10 M in DME (10 mL).
Catalyst loading = 5 mol %, 40 °C, 1b/2a = 1/3, [1b]0 = 0.25 M in
e
f
DME (4 mL).
could be easily transformed into useful building blocks were
obtained. The ring-opening reaction proceeded smoothly with
dibenzylamine (1b), giving the target products 3bh and 3bn in
98 and 95% yield with 94 and 90% ee, respectively (entries 1
and 3). Functionalized benzylamines 1c−f derived from 2-
aminoethanol, 2,2-dimethoxyethanamine, bromoacetate, and
allyl bromide were well-tolerated in the current system,
affording the corresponding versatile γ-substituted γ-amino
acid derivatives in 82−99% yield with 88−94% ee (entries 6−
11). In particular, the ring opening with aniline-type
nucleophile 1g proceeded very well, providing the desired
product 3gh in 95% yield with 91% ee (entry 12). Remarkably,
when the catalyst loading was reduced to 2 mol %, both the
yield and the enantioselectivity were almost maintained (entries
1 vs 2, 3 vs 4, and 6 vs 7).
In the current reaction, an excess of the racemic cyclo-
propane was employed. Further study showed that the excess
cyclopropane could be recovered with good ee, suggesting that
the present catalysts might be highly efficient for both the
asymmetric ring-opening reaction and the kinetic resolution of
D−A cyclopropanes. Under optimal conditions, with 1.6 equiv
a
Unless otherwise noted, the reactions were carried out under a N2
atmosphere with 1a (0.20 mmol), 2 (0.44 mmol), Ni(ClO4)2·6H2O
(0.02 mmol), L3c (0.024 mmol), and 4 Å MS (200 mg) in DME (2
mL, [1a]0 = 0.10 M) at 40 °C. Isolated yields. Determined by chiral
HPLC. The reaction was carried out at rt. Catalyst loading = 2 mol
b
c
d
e
f
%, [1a]0 = 0.20 M in DME (2 mL). 1a/2m = 1/3.
2-substituted cyclopropanes 2a−m reacted smoothly with
aliphatic amine 1a, affording the corresponding products in
up to 97% yield with high enantioselectivity (up to 98% ee).
The substituents on the benzene ring of 2-arylcyclopropane
diesters had a slight effect on enantioselectivity, and all of them
gave >90% ee (entries 1−11). However, the position of the
substituent on the aryl group influenced the yield. For example,
2-p-MeOC6H4- and 2-o-MeOC6H4-substituted cyclopropanes
2h and 2i both gave excellent ees, but the former showed much
better reactivity, probably because of the smaller steric effect
relative to the latter (entry 8 vs 10). Notably, for 2h, when the
catalyst loading was reduced to 2 mol %, the corresponding
product 3ah was obtained in 92% yield with 93% ee at room
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Communication
of 2-substituted cyclopropane-1,1-dicarboxylate 2 and 1.0 equiv
of amine 1a, the ring-opening products 3 were obtained in 39−
46% yield with 90−97% ee (Table 4). Meanwhile, the
L3e with a side-arm oxazoline having the opposite config-
uration relative to L3c (Scheme 2). As expected, in the control
Scheme 2. Control Experiments Using L3d and L3e
Table 4. Kinetic Resolution of 2-Substituted Cyclopropane
a
Diesters with Secondary Amines Catalyzed by L3c−Ni(II)
a
experiment with L3d, a dramatic drop in the enantioselectivity
was observed (90% ee in Table 1 vs 13% ee in Scheme 2).
Notably, ligand L3e9d gave a 54% yield with 50% ee but with
reversed enantioselectivity, suggesting that the configuration of
the additional chiral centers in the pendant oxazoline is crucial
for the enantioselectivity.
X-ray crystallographic analysis showed that the Ni center in
the cationic complex with L3d is octahedrally coordinated with
the three N atoms of L3d and three O atoms from two
perchlorate ions (Figure 2). Interestingly, the oxazoline side
arm of L3d lies on the top of the Ni−bisoxazoline plane, which
is completely consistent with the proposed model.
Unless otherwise noted, the reactions were carried out under a N2
atmosphere with 1a (0.25 mmol), 2 (0.4 mmol), Ni(ClO4)2·6H2O
(0.025 mmol), L3c (0.03 mmol), and 4 Å MS (250 mg) in DME (2.5
b
1
mL, [2]0 = 0.16 M) at 40 °C. Determined by H NMR analysis.
c
d
e
Isolated yields based on 2. Determined by chiral HPLC. S = ln[(1
− C)(1 − eeSM)]/ln[(1 − C)(1 + eeSM)] (C = conversion; eeSM = ee
of the recovered substrate). 0.225 mmol of 1a was used.
f
cyclopropanes (R)-2, which are also important intermediates
because of their wide application in organic synthesis,6 were
recovered in 41−49% yield with 88−95% ee. Thus, this method
provides easy access to both synthetically useful γ-amino acid
derivatives and cyclopropanes with high enantioselectivity.
As observed in the above reaction, the side-arm oxazolinyl
group in L3c proved to be crucial for regulating the reaction
rate and asymmetric induction (Table 1). Though the exact
role of the side arm in this reaction is not clear, we developed
the model as shown in Figure 1 to explain the effect of the side
Figure 2. X-ray structure of [Ni(L3d)](ClO4)2 (H atoms omitted for
clarity).
The present reaction is potentially synthetically useful. For
example, the product 3ah was readily transformed into
multifunctional piperidine 4 in quantitative yield without loss
of ee under mild conditions (eq 1 in Scheme 3). 3bh was easily
Figure 1. Proposed model for asymmetric induction.
Scheme 3. Transformations of the Ring-Opening Products
arm on the stereochemical control. In this model, the pendant
oxazoline shields the top side, and the catalyst selectively
accommodates the S enantiomer of the racemic cyclopropane
because of the steric repulsion between the phenyl group in the
R enantiomer and the indanyl moiety of the In-TOX ligand; the
bent-forward indane−oxazoline side arm could enhance this
preference with the extended aryl moiety. Consequently, the S
enantiomer is preferentially activated, and subsequent attack of
the amine gives the R product and leaves the (R)-cyclopropane,
in complete agreement with the observed results. According to
this model, reducing the steric hindrance of the pendant
oxazoline group should decrease the enantioselectivity. We thus
synthesized L3d with a nonsubstituted oxazoline side arm and
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Journal of the American Chemical Society
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D.; Satheesh, G. Synthesis 2009, 3443. (n) Chagarovskiy, A. O.;
Budynina, E. M.; Ivanova, O. A.; Grishin, Y. K.; Trushkov, I. V.;
Verteletskii, P. V. Tetrahedron 2009, 65, 5385. (o) Qu, J.-P.; Deng, C.;
Zhou, J.; Sun, X.-L.; Tang, Y. J. Org. Chem. 2009, 74, 7684.
(p) Campbell, M. J.; Johnson, J. S.; Parsons, A. T.; Pohlhaus, P. D.;
Sanders, S. D. J. Org. Chem. 2010, 75, 6317. (q) Xing, S.-Y.; Pan, W.-
Y.; Liu, C.; Ren, J.; Wang, Z.-W. Angew. Chem., Int. Ed. 2010, 49, 3215.
(3) [3 + 3] annulations: (a) Young, I. S.; Kerr, M. A. Angew. Chem.,
Int. Ed. 2003, 42, 3023. (b) Young, I. S.; Kerr, M. A. Org. Lett. 2004, 6,
139. (c) Ganton, M. D.; Kerr, M. A. J. Org. Chem. 2004, 69, 8554.
(d) Young, I. S.; Williams, J. L.; Kerr, M. A. Org. Lett. 2005, 7, 953.
(e) Wanapun, D.; Van Gorp, K. A.; Mosey, N. J.; Kerr, M. A.; Woo, T.
K. Can. J. Chem. 2005, 83, 1752. (f) Lebold, T. P.; Carson, C. A.; Kerr,
M. A. Synlett 2006, 364. (g) Sapeta, K.; Kerr, M. A. J. Org. Chem. 2007,
72, 8597. (h) Karadeolian, A.; Kerr, M. A. J. Org. Chem. 2007, 72,
10251. (i) Carson, C. A.; Young, I. S.; Kerr, M. A. Synthesis 2008, 485.
(j) Perreault, C.; Goudreau, S.; Zimmer, L.; Charette, A. B. Org. Lett.
2008, 10, 689. (k) Dias, D. A.; Kerr, M. A. Org. Lett. 2009, 11, 3694.
(4) [3 + 4] annulations: (a) Ivanova, O. A.; Budynina, E. M.;
Grishin, Y. K.; Trushkov, I. V.; Verteletskii, P. V. Angew. Chem., Int. Ed.
2008, 47, 1107. (b) Ivanova, O. A.; Budynina, E. M.; Grishin, Y. K.;
Trushkov, I. V.; Verteletskii, P. V. Eur. J. Org. Chem. 2008, 532.
(c) Chagarovskiy, A. O.; Budynina, E. M.; Ivanova, O. A.; Grishin, Y.
K.; Trushkov, I. V.; Verteletskii, P. V. Tetrahedron 2009, 65, 5385.
(5) Asymmetric catalysis: (a) Sibi, M. P.; Ma, Z.; Jasperse, C. P. J.
Am. Chem. Soc. 2005, 127, 5764. (b) Kang, Y.-B.; Sun, X.-L.; Tang, Y.
Angew. Chem., Int. Ed. 2007, 46, 3918. (c) Parsons, A. T.; Johnson, J. S.
J. Am. Chem. Soc. 2009, 131, 3122. (d) Parsons, A. T.; Smith, A. G.;
Neel, A. J.; Johnson, J. S. J. Am. Chem. Soc. 2010, 132, 9688. (e) Trost,
B. M.; Morris, P. J. Angew. Chem., Int. Ed. 2011, 50, 6167.
(6) Total synthesis from activated cyclopropanes: (a) Carson, C. A.;
Kerr, M. A. Chem. Soc. Rev. 2009, 38, 3051. (b) Lerchner, A.; Carreira,
E. M. J. Am. Chem. Soc. 2002, 124, 14826. (c) Lerchner, A.; Carreira, E.
M. Chem.Eur. J. 2006, 12, 8208. (d) Carson, C. A.; Kerr, M. A.
Angew. Chem., Int. Ed. 2006, 45, 6560. (e) Young, I. S.; Kerr, M. A. J.
Am. Chem. Soc. 2007, 129, 1465. (f) Leduc, A. B.; Kerr, M. A. Angew.
Chem., Int. Ed. 2008, 47, 7945. (g) Carson, C. A.; Kerr, M. A. Org. Lett.
2009, 11, 777. (h) Campell, M. J.; Johnson, J. S. J. Am. Chem. Soc.
2009, 131, 10370. (i) Sanders, S. D.; Ruiz-Olalla, A.; Johnson, J. S.
Chem. Commun. 2009, 5135. (j) Jung, M. E.; Chang, J. J. Org. Lett.
2010, 12, 2962.
(7) (a) Stewart, J. M.; Westberg, H. H. J. Org. Chem. 1965, 30, 1951.
(b) Danishefsky, S.; Dynak, J. J. Org. Chem. 1974, 39, 1979.
(c) Blanchard, L. A.; Schneider, J. A. J. Org. Chem. 1986, 51, 1372.
(d) Magolan, J.; Kerr, M. A. Org. Lett. 2006, 8, 4561. (e) Lifchits, O.;
Charette, A. B. Org. Lett. 2008, 10, 2809. (f) Uddin, M. I.; Mimoto, A.;
Nakano, K.; Ichikawa, Y.; Kotsuki, H. Tetrahedron Lett. 2008, 49, 5867.
(g) Lebold, T. P.; Leduc, A. B.; Kerr, M. A. Org. Lett. 2009, 11, 3770.
(8) Selected reviews of BOX ligands in asymmetric catalysis:
(a) Ghosh, A. K.; Mathivanan, P.; Cappiello, J. Tetrahedron: Asymmetry
1998, 9, 1. (b) Jørgensen, K. A.; Johannsen, M.; Yao, S.; Audrain, H.;
Thorhauge, J. Acc. Chem. Res. 1999, 32, 605. (c) Johnson, J. S.; Evans,
D. A. Acc. Chem. Res. 2000, 33, 325. (d) McManus, H. A.; Guiry, P. J.
Chem. Rev. 2004, 104, 4151. (e) Desimoni, G.; Faita, G.; Jørgensen, K.
A. Chem. Rev. 2006, 106, 3561. (f) Hargaden, G. C.; Guiry, P. J. Chem.
Rev. 2009, 109, 2505.
decarboxylated, and the product was deprotected by Pd-
catalyzed hydrogenation and cyclized to give optically active γ-
lactam 7 (eq 2 in Scheme 3). These products contain important
structural motifs found in biologically active compounds.11
Thus, this reaction provides a facile way to prepare potentially
useful building blocks in organic synthesis.
In summary, we have developed the first catalytic
enantioselective ring-opening reaction of 2-substituted cyclo-
propane-1,1-dicarboxylates with multifunctional secondary
amines. With the Ni(II) complex of an indene-derived
trisoxazoline (In-TOX) ligand, the reaction performed well
over a series of substrates, giving the desired products in
excellent yield (up to 99%) and enantioselectivity (up to 98%
ee) under mild conditions. In this reaction, a strong effect of
the ligand side arm was observed, which will be helpful for the
modification of BOX ligands in asymmetric catalysis. The
single-crystal X-ray structure of the TOX−Ni complex was also
obtained, and an asymmetric induction model was developed.
This protocol provides a promising method for the synthesis of
versatile chiral γ-substituted γ-amino acid derivatives as well as
effective kinetic resolution of 2-substituted cyclopropane-1,1-
dicarboxylates. Further investigation of the applications of the
current reaction is underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
xlsun@mail.sioc.ac.cn; tangy@mail.sioc.ac.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21121062, 20932008, and 21072207), the 973 Program (
2009CB825300), and the CAS for financial support.
REFERENCES
■
(1) Reviews: (a) Danishefsky, S. Acc. Chem. Res. 1979, 12, 66.
(b) Wong, H. N.; Hon, M.-Y.; Tse, C.-W.; Yip, Y.-C.; Tanko, J.;
Hudlicky, T. Chem. Rev. 1989, 89, 165. (c) Reissig, H. U.; Zimmer, R.
Chem. Rev. 2003, 103, 1151. (d) Yu, M.; Pagenkopf, B. L. Tetrahedron
2005, 61, 321. (e) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. Rev.
2007, 107, 3117. (f) Simone, F. D.; Waser, J. Synthesis 2009, 3353.
(g) Lebold, T. P.; Kerr, M. Pure Appl. Chem. 2010, 82, 1797.
(2) [3 + 2] annulations: (a) Shimizu, I.; Ohashi, Y.; Tsuji, J.
Tetrahedron Lett. 1985, 26, 3825. (b) Sugita, Y.; Kawai, K.; Yokoe, I.
Heterocycles 2000, 53, 657. (c) Pohlhaus, P. D.; Johnson, J. S. J. Org.
Chem. 2005, 70, 1057. (d) Carson, C. A.; Kerr, M. A. J. Org. Chem.
2005, 70, 8242. (e) Kang, Y.-B.; Tang, Y.; Sun, X.-L. Org. Biomol.
Chem. 2006, 299. (f) Christie, S. D. R.; Davoile, R. J.; Jones, R. C. F.
Org. Bioorg. Chem. 2006, 4, 2683. (g) Gupta, A.; Yadav, V. K.
Tetrahedron Lett. 2006, 47, 8043. (h) Sliwinska, A.; Czardybon, W.;
Warkentin, J. Org. Lett. 2007, 9, 695. (i) Zhang, J.-H.; Shen, W.; Li, M.
Eur. J. Org. Chem. 2007, 4855. (j) Korotkov, V. S.; Larionov, O. V.;
Hofmeister, A.; Magull, J.; de Meijere, A. J. Org. Chem. 2007, 72, 7504.
(k) Jackson, S. K.; Karadeolian, A.; Driega, A. B.; Kerr, M. A. J. Am.
Chem. Soc. 2008, 130, 4196. (l) Pohlhaus, P. D.; Sanders, S. D.;
Parsons, A. T.; Li, W.; Johnson, J. S. J. Am. Chem. Soc. 2008, 130, 8642.
(m) Yadav, J. S.; Subba Reddy, B. V.; Narasimhulu, G.; Chandrakanth,
(9) Side-arm effects in oxazoline/Lewis acid-catalyzed asymmetric
reactions: (a) Zhou, J.; Tang, Y. Chem. Soc. Rev. 2005, 34, 664.
(b) Zhou, J.; Tang, Y. J. Am. Chem. Soc. 2002, 124, 9030. (c) Huang,
Z.-Z.; Kang, Y.-B.; Zhou, J.; Ye, M.-C.; Tang, Y. Org. Lett. 2004, 6,
1677. (d) Cao, P.; Deng, C.; Zhou, Y.-Y.; Sun, X.-L.; Zheng, J.-C.; Xie,
Z.-W.; Tang, Y. Angew. Chem., Int. Ed. 2010, 49, 4463. (e) Rendina, V.
L.; Moebius, D. C.; Kingsbury, J. S. Org. Lett. 2011, 13, 2004.
(10) The determination of the absolute configuration of 3ba see the
Supporting Information.
(11) (a) Werner, M.; Joachim, N.; Lucien, R. B. WO 2005/118535
A1, 2005. (b) Bostrom, J.; Cheng, L.-F.; Fex, T.; Karle, M.; Pettersen,
D.; Schell, P. U.S. Pat. Appl. 2010/0261755 A1, 2010.
9069
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