Remote ester groups switch selectivity: Diastereodivergent synthesis of tetracyclic spiroindolines

Article abstract of DOI:10.1021/ja503117q

Stereocontrol in the synthesis of structurally complex molecules, especially those with all-carbon quaternary stereocenters, remains a challenge. Here, we reported the preparation of a class of tetracyclic cyclopenta-fused spiroindoline skeletons through Cu(II)-catalyzed intramolecular [3 + 2] annulation reactions of donor-acceptor cyclopropanes with indoles. Both cis-and trans-diastereomers of tetracyclic spiroindolines are accessed with high selectivities by altering the remote ester groups of cyclopropanes. The origins of this stereocontrol are identified using DFT calculations: attractive interactions between the ester group and arene favor the generation of the trans isomer, while the formation of the cis isomer is preferred when steric repulsions become predominant.

Full text of DOI:10.1021/ja503117q

Communication
pubs.acs.org/JACS
Remote Ester Groups Switch Selectivity: Diastereodivergent
Synthesis of Tetracyclic Spiroindolines
Jun Zhu,† Yong Liang,‡ Lijia Wang,† Zhong-Bo Zheng,† K. N. Houk,*,‡ and Yong Tang*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Lu, Shanghai 200032, China
‡Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
S
* Supporting Information
of this stereochemical control are also determined by density
functional theory (DFT) calculations.
Initial attempts began with the intramolecular [3 + 2]
annulation of newly designed cyclopropane 1. In the presence
of 10 mol % of Sc(OTf)3, the reaction proceeded very well at 70
°C in 1,2-dichloroethane (DCE), giving the tetracyclic
spiroindoline in 80% yield as shown in Table 1 (entry1).
ABSTRACT: Stereocontrol in the synthesis of structur-
ally complex molecules, especially those with all-carbon
quaternary stereocenters, remains a challenge. Here, we
reported the preparation of a class of tetracyclic cyclo-
penta-fused spiroindoline skeletons through Cu(II)-
catalyzed intramolecular [3 + 2] annulation reactions of
donor−acceptor cyclopropanes with indoles. Both cis- and
trans-diastereomers of tetracyclic spiroindolines are
accessed with high selectivities by altering the remote
ester groups of cyclopropanes. The origins of this
stereocontrol are identified using DFT calculations:
attractive interactions between the ester group and arene
favor the generation of the trans isomer, while the
formation of the cis isomer is preferred when steric
repulsions become predominant.
Table 1. Reaction Optimization
a
b
cd
,
entry
Lewis acid
L
solvent
yield (%)
dr (2/3)
olycyclic spiroindolines are constituents of many alkaloid
1
2
3
4
5
6
Sc(OTf)3
−
DCE
80
50/50
66/34
75/25
83/17
55/45
54/46
83/17
81/19
76/24
−
P
natural products1 as well as some biologically active
molecules.2 Considerable efforts have been devoted to the
synthesis of this motif.3 Of those synthetic methods developed,
indole based cycloaddition3b,g and cascade reactions3e,f,h,i have
shown extraordinary elegance and efficiency. As chemical
transformations of donor−acceptor (D−A) cyclopropanes
were elucidated to be useful in organic synthesis,4−7 we
conceived that an intramolecular reaction might offer a new
strategy to prepare a class of tetracyclic cyclopenta-fused
spiroindoline skeletons by joining the indole and cyclopropane
with a linker (Scheme 1). Here, we describe the development of
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
L1
L2
L3
L4
L5
L3
L3
L3
L3
L3
DCE
57
DCE
55
DCE
61
DCE
87
DCE
26
e
7
DCE
83
e
8
e
toluene
THF
73
9
trace
NR
NR
e
10
CH3CN
DMSO
e
11
−
a
Conducted on 0.25 mmol scale, solvent (4 mL), reaction time: 4−48
b
c
1
h. Isolated yields of products 2 and 3. Determined by H NMR
d
Scheme 1. Newly-Designed Intramolecular [3+2] Annulation
Approach to Tetracyclic Spiroindolines
spectroscopy of the crude products. The cis isomer is defined by H-
atoms on the tertiary carbon which are on the same side of the
cyclopentyl ring. The trans is defined by H-atoms on the tertiary
carbon which are on opposite sides of the cyclopentyl ring. At 30 °C.
e
Unfortunately, the diastereoselectivity is very poor. Then we
explored the common strategies8 such as by optimizing the Lewis
acids,8a reaction conditions,8b−f and ligands8g to improve the
stereoselectivity. When In(OTf)3 and Cu(ClO4)2·6H2O were
employed, poor diastereoselectivities were obtained (see
Supporting Information (SI), Table S1, entries 2 and 11). By
using Zn(OTf)2, the annulation was sluggish (Table S1, entry 8).
this approach to tetracyclic spiroindolines with three continuous
stereocenters including an all-carbon quaternary stereocenters in
a five-membered ring. In particular, we observed that the
diastereoselectivity of this new reaction can be switched by
changing the remote ester groups of cyclopropanes: the
isopropyl ester is highly favorable for the trans diastereomer,
while the 2-adamantyl ester is the best for the cis one. The origins
Received: March 27, 2014
Published: April 25, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
When the reaction was catalyzed by Cu(SbF6)2 (10 mol %) in the
presence of diimine ligand L1, we were pleased to find that the
reaction speeded up significantly and finished in 4 h with 66/34
dr in 57% yield (entry 2). More hindered diimine ligands provide
better stereoselectivities (entries 2−4). To our delight, when N-
2,6-diisopropylphenyl diimine L3 was employed as the ligand,
the yield and diastereoselectivity increased to 61% and 83/17,
respectively (entry 4). Other ligands such as bisoxazoline L4 and
bipyridine L5 were also examined, but resulted in poor
diastereoselectivities (55/45 and 54/46 dr, entries 5 and 6).
Lowering the reaction temperature to 30 °C afforded an 83%
yield without erosion of diastereoselectivity (entry 7). Solvent
screening showed that toluene could lead to a 73% yield and 81/
19 dr, but THF, acetonitrile, and DMSO proved to be unsuitable
in this reaction (entries 8−11).
To investigate the generality of this reaction, we synthesized a
variety of cyclopropanes 1 with 2-propyl and 2-adamantyl ester
groups. Under the optimal conditions, we first evaluated with
isopropyl esters (iPr-1) for the synthesis of trans product 2. As
shown in Table 3, substituents such as F, Cl, Br, and alkyl on the
Table 3. Reaction Scope with Isopropyl Esters
a
b
c
entry
R
product
yield (%)
dr (trans/cis)
d
1
H
2a
77
71
77
84
83
60
73
80
84
50
71
68
81
90/10
94/6
2
5-Me
5-F
2b
2c
2d
2e
2f
After hundreds of trials failed to improve the diastereose-
lectivity to a satisfactory result (for details, see SI), we focused on
changing the ester R group of the cyclopropane. As shown in
Table 2, when the R group of the ester was changed from ethyl to
3
93/7
4
5-Cl
5-Br
6-F
93/7
5
93/7
6
88/12
89/11
91/9
7
6-Cl
7-Me
7-Br
4-Me
4-F
2g
2h
2i
Table 2. Effect of Ester Groups
8
9
94/6
10
11
12
13
2j
>99/1
95/5
2k
2l
4-Cl
4-Br
>99/1
>99/1
2m
a
b
a
b
c
Conducted on 0.25 mmol scale, DCE (4 mL). Isolated yields.
entry
R′
R
yield (%)
dr (2/3)
c
Determined by 1H NMR spectroscopy of the crude products.
1
H
Et
83
83/17
74/26
78/22
90/10
88/12
89/11
−
d
Confirmed by X-ray diffraction analysis.
2
H
Me
87
3
H
nHex
iPr
77
indole, as well as different substitution patterns (4, 5, 6, 7-), are
well tolerated in the reaction, giving the desired products in good
yields with high diastereoselectivities (from 88/12 to >99/1 dr,
entries 2−11). With electron-donating and -withdrawing groups
at different positions of the aromatic ring of indole, in cases of 2-
adamantyl ester substrates (Ad-1), the reactions showed good
reactivity and excellent diastereoselectivities affording cis
tetracyclic spiroindolines 3 (72−95% yield, 93/7−96/4 dr;
Table 4, entries 2−10). In addition, the reactions in Tables 3 and
4
H
77
5
H
cHex
3-Pent
tBu
83
6
H
45
7
H
trace
trace
64
8
H
1-Ad
−
9
H
CH2tBu
CH2-1-Ad
2-Ad
15/85
9/91
6/94
84/16
5/95
10
11
12
13
H
52
H
61
CH3
CH3
iPr
85
2-Ad
79
a
b
Table 4. Reaction Scope with 2-Adamantyl Esters
Conducted on 0.25 mmol scale, DCE (4 mL). Isolated yields of
c
products 2 and 3. Determined by 1H NMR spectroscopy of the crude
products. Pent = pentyl; Ad = adamantyl.
methyl, the diastereomeric ratio was decreased to 74/26 (entry
2), suggesting that the ester groups on the cyclopropane ring may
have a significant influence on the selectivity. Therefore, a variety
of substrates 1 with different ester groups were examined in this
intramolecular [3 + 2] annulation. It was found that changing the
ester R group from a primary to secondary alkyl group can
improve the diastereoselectivity greatly (entries 2−6).9 The
isopropyl group proved to be optimal with respect to the
selective synthesis of trans diastereomer 2 (2/3: 90/10, entry 4).
Continuing to increase the size of esters by introducing tertiary
alkyl groups, such as tert-butyl and 1-adamantyl, destroyed the
reactivity (entries 7−8). To our surprise, a sudden inversion of
selectivity to cis diastereomer 3 (2/3: 15/85, entry 9) was
observed when neo-pentyl ester was used, and the cis/trans ratio
can be further elevated to 94/6 by employing the 2-adamantyl
group (entry 11). This diastereo-divergence switched by the
ester group was also observed in the reactions using the
corresponding N-methylindole substrates (entries 12−13).
a
b
c
entry
R
product
yield (%)
dr (cis/trans)
d
1
H
3a
79
95
90
92
93
84
93
91
92
72
95/5
95/5
96/4
95/5
94/6
95/5
93/7
94/6
96/4
95/5
e
2
5-OMe
5-Me
5-F
3b
3c
3d
3e
3f
3
4
5
6
7
8
9
5-Cl
5-Br
6-F
3g
3h
3i
6-Cl
7-Me
4-OMe
e
10
3j
a
b
Conducted on 0.25 mmol scale, DCE (4 mL). Isolated yields.
c
Determined by 1H NMR spectroscopy of the crude products.
d
e
Confirmed by X-ray diffraction analysis. With 20 mol % catalyst. Ad
= 2-adamantyl.
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Journal of the American Chemical Society
Communication
4 were carried out under very mild conditions and finished within
24 h. The stereochemistries of 2a and 3a were identified by X-ray
analysis.10
The current method is suitable to prepare chiral tetracyclic
spiroindolines. As shown in Scheme 2, when the optically pure
Scheme 2. Synthesis of Chiral Tetracyclic Spiroindolines from
Optically Pure Substrates
isopropyl or 2-adamantyl substrate was employed, the chirality
from the cyclopropane is completely transferred to the product,
similar to the cycloaddition11 of aldehydes and D−A cyclo-
propanes.
The aforementioned results showed excellent generality of the
current reaction and that 2-propyl esters iPr-1 gave trans-isomers
2 while 2-adamantyl esters Ad-1 afforded cis-isomers 3. As the 2-
propyl and 2-adamantyl groups are remote to the first reactive
site and both are secondary alkyl groups, it is hard to explain the
effect of the ester group on the stereochemistry by only its steric
hindrance and electronic properties. To further understand the
mechanism of the intramolecular annulation of indole with a D−
A cyclopropane, especially the origins of the unique diaster-
eoselectivity control by the remote ester groups, DFT
calculations were performed on four reactions with different R
groups shown in Scheme 3.12 Once cyclopropane-1,1-dicarbox-
Figure 1. DFT-optimized structures of transition states TS1-trans-R
and TS1-cis-R and DFT-computed relative activation Gibbs free
energies (R = Me, iPr, CH2tBu, 2-Ad; carbon, gray; nitrogen, blue;
oxygen, red; sulfur, yellow; copper, brown; H-atoms are not shown for
clarity; distances are given in angstroms).
trans configuration via TS2-trans-R or the cis one via TS2-cis-R
(Scheme 3).
Scheme 3. DFT Study on Intramolecular [3 + 2] Reactions
Involving Different Ester Groups
As shown in Figure 1, for the reaction with the methyl ester,
DFT calculations using the ωB97XD13 and B3LYP14 methods
indicate that formation of the trans product via TS1-trans-Me is
slightly favored, in agreement with the experimental result
(Table 2, entry 2). In the case of the isopropyl ester, a better trans
selectivity is obtained experimentally (Table 2, entry 4). This
trend is only reproduced by the ωB97XD method, which yields
better results when describing medium- to long-range electron
correlation and dispersion effects than those from the traditional
B3LYP method.13,15 As the size of the R group from methyl to
isopropyl increases, there is no significant steric repulsion
generated between the ester and arene in the transition state
TS1-trans-iPr, but the attractive interactions,16 which include
dispersion forces,17become stronger. In the Wilcox molecular
torsion balance experiment, it was reported that the noncovalent
interactions between isopropyl and arene are ∼0.5 kcal mol−1
larger than the methyl-arene interactions in the organic solvent.18
This value is very close to the ωB97XD energy difference for the
enhanced trans selectivity (Me: −0.4 kcal mol−1, iPr: −0.7 kcal
mol−1, Figure 1). When the R group becomes much bulkier (R =
CH2tBu, 2-Ad), the steric repulsions increase dramatically in the
transition state TS1-trans-R as evidenced by the elongated
forming C−C bond (2.76 vs 2.60−2.62 Å, to avoid steric clashes,
Figure 1). Therefore, the formation of cis products via TS1-cis-
CH2tBu(2-Ad) is preferred.
ylate is activated by a Cu(II) catalyst, the intramolecular indole
C3-nucleophilic attack to the cyclopropane C2 carbon generates
two diastereomeric intermediates through transition states TS1-
trans-R and TS1-cis-R, respectively (Figure 1). Computational
results show that regardless of the size of the R group, this step is
irreversible (for details, see the SI), determining the stereo-
chemistry of final products. The subsequent ring-closure process
leads to the cycloadduct with two methine hydrogen atoms in a
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(5) For selected leading references of intermolecular asymmetric
anulation of D−A cyclopropane, see: (a) Sibi, M. P.; Ma, Z. H.; 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. (f) Xiong, H.; Xu, H.;
Liao, S.; Xie, Z.; Tang, Y. J. Am. Chem. Soc. 2013, 135, 7851. (g) Xu, H.;
Qu, J. P.; Liao, S.; Xiong, H.; Tang, Y. Angew. Chem., Int. Ed. 2013, 52,
4004. (h) Zhou, Y. Y.; Li, J.; Ling, L.; Liao, S. H.; Sun, X. L.; Li, Y. X.;
Wang, L. J.; Tang, Y. Angew. Chem., Int. Ed. 2013, 52, 1452.
(6) For selected leading references of intramolecular anulation of
cyclopropane, see: (a) Danishefsky, S. Acc. Chem. Res. 1979, 12, 66.
(b) Jackson, S. K.; Karadeolian, A.; Driega, A. B.; Kerr, M. A. J. Am.
Chem. Soc. 2008, 130, 4196. (c) Xing, S.; Pan, W.; Liu, C.; Ren, J.; Wang,
Z. Angew. Chem., Int. Ed. 2010, 49, 3215. (d) Xing, S.; Li, Y.; Li, Z.; Liu,
C.; Ren, J.; Wang, Z. Angew. Chem., Int. Ed. 2011, 50, 12605. (e) Bai, Y.;
Tao, W.; Ren, J.; Wang, Z. Angew. Chem., Int. Ed. 2012, 51, 4112. (f) Zhu,
W.; Fang, J.; Liu, Y.; Ren, J.; Wang, Z. Angew. Chem., Int. Ed. 2013, 52,
2032. (g) Cavitt, M. A.; Phun, L. H.; France, S. Chem. Soc. Rev. 2014, 43,
804.
In summary, we have succeeded in developing a simple,
efficient method for the highly diastereoselective construction of
tetracyclic spiroindoline skeletons through Cu(II)-catalyzed
intramolecular [3 + 2] reactions of cyclopropane-1,1-dicarbox-
ylates with indoles. Unprecedentedly, the diastereoselectivities of
the present reaction could be readily switched by altering the
remote ester groups of cyclopropanes: the isopropyl ester is
highly favorable for the trans diastereomer, while the 2-
adamantyl ester is the best for the cis one. Thus, it provides
not only a new approach for the remote control of
diastereoselection but also a one-step and facile access to both
cis- and trans-diastereomers of tetracyclic spiroindolines under
the same reaction conditions. DFT calculations show that
attractive interactions between the isopropyl ester and arene
enhance the trans selectivity, and that the steric repulsions caused
by the bulky 2-adamantyl group make the cis product
predominant.
ASSOCIATED CONTENT
* Supporting Information
■
(7) (a) Harrington, P.; Kerr, M. A. Tetrahedron Lett. 1997, 38, 5949.
(b) Kerr, M. A.; Keddy, R. G. Tetrahedron Lett. 1999, 40, 5671.
(c) England, D. B.; Kuss, T. D.; Keddy, R. G.; Kerr, M. A. J. Org. Chem.
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Chem. Soc. 2007, 129, 9631. (e) Larquetoux, L.; Ouhamou, N.; Chiaroni,
A.; Six, Y. Eur. J. Org. Chem. 2005, 2005, 4654.
S
Experimental procedures, characterizations and analytical data of
products, spectra of NMR and HPLC, and computational details.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(8) For selected leading references, see: (a) Lu, G.; Yoshino, T.;
Morimoto, H.; Matsunaga, S.; Shibasaki, M. Angew. Chem., Int. Ed. 2011,
50, 4382. (b) Moteki, S. A.; Han, J.; Arimitsu, S.; Akakura, M.;
Nakayama, K.; Maruoka, K. Angew. Chem., Int. Ed. 2012, 51, 1187.
(c) Krautwald, S.; Sarlah, D.; Schafroth, M. A.; Carreira, E. M. Science
2013, 340, 1065. (d) Oliveira, M. T.; Luparia, M.; Audisio, D.; Maulide,
N. Angew. Chem., Int. Ed. 2013, 52, 13149. (e) Sun, X.-L.; Tang, Y. Acc.
Chem. Res. 2008, 41, 937. (f) Deng, X.-M.; Cai, P.; Ye, S.; Sun, X.-L.;
Liao, W.-W.; Li, K.; Tang, Y.; Wu, Y.-D.; Da, L.-X. J. Am. Chem. Soc.
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(9) When the benzyl ester was examined, a 75% yield with 55/45
(trans/cis) was obtained.
(10) CCDC 981305 (2a) and 981312 (3a) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
AUTHOR INFORMATION
■
Corresponding Authors
tangy@mail.sioc.ac.cn
houk@chem.ucla.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Sciences Foundation of China (Nos. 21121062,
20932008, and 21272250), the Chinese Academy of Sciences,
and the U.S. National Science Foundation (CHE-1059084). We
thank Dr. Xue-bing Leng (SIOC) and Mr. Jie Sun (SIOC) for X-
ray crystal analysis.
(11) Pohlhaus, P. D.; Johnson, J. S. J. Am. Chem. Soc. 2005, 127, 16014.
(12) (a) Frisch, M. J. et al. Gaussian 09, Revision C.01; Gaussian, Inc.:
Wallingford, CT, 2010. Complete reference is in the SI. (b) Geometry
optimizations and frequency calculations were performed at the
B3LYP/6-31G(d)-LANL2DZ (for Cu) level. Single-point energy
calculations in 1,2-dichloroethane (DCE) using the CPCM model
were performed at the B3LYP/6-311G(d,p)-LANL2DZ or ωB97XD/6-
311G(d,p)-LANL2DZ level. For details, see the SI.
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