Synthesis of spirocarbamate oxindoles via intramolecular trapping of a β-silyl carbocation by an N-Boc group

Article abstract of DOI:10.1021/ol4010867

We report the Lewis acid catalyzed additions of allylsilanes to N-Boc-iminooxindoles and the formation of novel silicon-containing spirocarbamates via intramolecular trapping of a β-silyl carbocation by an N-Boc group. Several transformations display the synthetic utility of these spirocarbamate oxindoles, including a reductive cyclization to access new silylated furoindoline derivatives.

Full text of DOI:10.1021/ol4010867

ORGANIC
LETTERS
2013
Vol. 15, No. 13
3218–3221
Synthesis of Spirocarbamate Oxindoles
via Intramolecular Trapping of a β‑Silyl
Carbocation by an N-Boc Group
Benjamin H. Shupe, Emily E. Allen, Jacob P. MacDonald, Sean O. Wilson, and
Annaliese K. Franz*
Department of Chemistry, University of California, One Shields Avenue, Davis,
California 95616, United States
akfranz@ucdavis.edu
Received April 19, 2013
ABSTRACT
We report the Lewis acid catalyzed additions of allylsilanes to N-Boc-iminooxindoles and the formation of novel silicon-containing spirocarbamates
via intramolecular trapping of a β-silyl carbocation by an N-Boc group. Several transformations display the synthetic utility of these spirocarbamate
oxindoles, including a reductive cyclization to access new silylated furoindoline derivatives.
Synthetic interest in spirooxindoles has increased over
the past decade due to the notable biological activity and
occurrence of this class of heterocycles in natural products
and pharmaceutical lead compounds.1 Recent synthetic
methods to access spirocyclic 3-aminooxindoles have uti-
lized preformed iminooxindoles2 and in situ generated
iminium ions3 in various spirocyclization strategies; how-
ever, allylsilanes have not previously been investigated for
annulations of iminooxindoles. Based on the steric and
electronic effects of different silyl groups, allylsilane re-
agents can exhibit either an allylation (i.e., elimination) or
annulation pathway.4 Both pathways proceed through a
transient β-silyl stabilized carbocation intermediate,5 which
can be directly intercepted (as a 1,2-dipole synthon),6 or a
1,2-silyl migration can occur with interception in a [3 þ 2]
annulation (as a 1,3-dipole synthon).7
Herein we describe the discovery and development of
a new Lewis acid catalyzed allylsilane annulation with
N-Boc-iminooxindoles (1) to form spirocyclic carbamates
such as 3 (Scheme 1). Previous work from our group has
shown that chiral scandium(III)-indapybox complexes
catalyze allylsilane additions to isatins to selectively afford
(1) For reviews, see: (a) Ball-Jones, N. R.; Badillo, J. J.; Franz, A. K.
Org. Biomol. Chem. 2012, 10, 5165. (b) Trost, B. M.; Brennan, M. K.
Synthesis 2009, 3003. (c) Galliford, C. V.; Scheidt, K. A. Angew. Chem.,
Int. Ed. 2007, 46, 8748. (d) Marti, C.; Carreira, E. M. Eur. J. Org. Chem.
2003, 2209. For selected examples of bioactive nitrogen-containing
spirooxindoles, see: (a) Yong, S. R.; Ung, A. T.; Pyne, S. G.; Skelton,
B. W.; White, A. H. Tetrahedron 2007, 63, 5579. (b) Vintonyak, V.;
(4) (a) Hosomi, A. Acc. Chem. Res. 1988, 21, 200. (b) Chabaud, L.;
James, P.; Landais, Y. Eur. J. Org. Chem. 2004, 15, 3173. (c) Panek, J. S.;
Yang, M. J. Am. Chem. Soc. 1991, 113, 9868. (d) Danheiser, R. L.; Dixon,
€
Warburg, K.; Kruse, H.; Grimme, S.; Hubel, K.; Rauh, D.; Waldmann,
H. Angew. Chem., Int. Ed. 2010, 49, 5902. (c) Bhaskar, G.; Arun, Y.;
Balachandran, C.; Saikumar, C.; Perumal, P. T. Eur. J. Med. Chem.
2012, 51, 79. (d) Ding, K.; Lu, Y.; Nikolovska-Coleska, Z.; Wang, G.;
Qiu, S.; Shangary, S.; Gao, W.; Qin, D.; Stuckey, J.; Krajewski, K.;
Roller, P. P.; Wang, S. J. Med. Chem. 2006, 49, 3432.
(2) For selected recent examples, see: (a) Lv, H.; Tiwari, B.; Mo, J.;
Xing, C.; Chi, Y. R. Org. Lett. 2012, 14, 5412. (b) Pei, C.; Jiang, Y.; Shi,
M. Eur. J. Org. Chem. 2012, 12, 4206. (c) Zhang, B.; Feng, P.; Sun, L.;
Cui, Y.; Ye, S.; Jiao, N. Chem.;Eur. J. 2012, 18, 9198.
(3) For selected recent examples, see: (a) Badillo, J. J.; Silva-Garcıa,
A.; Shupe, B. H.; Fettinger, J. C.; Franz, A. K. Tetrahedron Lett. 2011,
52, 5550. (b) Taghizadeh, M. J.; Arvinnezhad, H.; Samadi, S.; Jadidi, K.;
Javidan, A.; Notash, B. Tetrahedron Lett. 2012, 53, 5148. (c) Shi, F.;
Tao, Z.; Luo, S.; Tu, S.; Gong, L. Chem.;Eur. J. 2012, 18, 6885.
€
B. R.; Gleason, R. W. J. Org. Chem. 1992, 57, 6094. (e) Knolker,
H.-J.; Foitzik, N.; Graf, R.; Pannek, J.-B. Tetrahedron 1993, 49, 9955.
(5) (a) Lambert, J. B.; Zhao, Y. J. Am. Chem. Soc. 1996, 118, 7867. (b)
Creary, X.; Kochly, E. D. J. Org. Chem. 2009, 74, 2134. (c) Lambert,
J. B.; Zhao, Y.; Emblide, R. W.; Salvador, L. A.; Liu, X.; So, J.; Chelius,
E. C. Acc. Chem. Res. 1999, 32, 183.
(6) (a) Brocherieux-Lanoy, S.; Dhimane, H.; Poupon, J.-C.; Vanucci,
C.; Lhommet, G. J. Chem. Soc. Perkin Trans. 1997, 1, 2163. (b) Angle,
S. R.; El-Said, N. A. J. Am. Chem. Soc. 1999, 121, 10211. (c) Horino, Y.;
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48, 5421. (e) Romero, A.; Woerpel, K. A. Org. Lett. 2006, 8, 2127. (f)
Dressel, M.; Restorp, P.; Somfai, P. Chem.;Eur. J. 2008, 14, 3072.
r
10.1021/ol4010867
Published on Web 06/12/2013
2013 American Chemical Society

Scheme 1. Annulation Pathways with N-Boc-iminooxindoles
Scheme 2. Addition of Methallyltrimethylsilane to N-Aryl and
N-Boc-iminooxindoles
either allylation8 or annulation9 products in high yields
and enantioselectivities. The allylation of iminooxindoles
represents a synthetic challenge, and limited examples
have been reported.10,11 We also present methods to access
3-allyl-3-aminooxindoles and furoindolones upon trans-
formation of the spirocarbamate.
Our initial studies compared the reactivity of iminoox-
indoles 1 and 6 in Lewis acid catalyzed allylation reactions
with methallyltrimethylsilane 7 (Scheme 2, eqs 1 and 2).
Using the conditions we have previously optimized for
isatins (i.e., Sc(OTf)3 with NaSbF6 and TMSCl),8 the
allylation of ketimine 6 afforded the protected 3-aminoox-
indole 8 in 68% yield after 3 days.12 In comparison, the
electron-deficient N-Boc protected ketimines 113 exhibit a
significant increase in reactivity compared to N-aryl keti-
mines. The addition of methallyltrimethylsilane 7 to imino-
oxindoles 1proceeds rapidly (<3h) in the presence of catalytic
Sc(OTf)3. Under these reaction conditions, the Boc group was
deprotected to afford amine 9 in high yields (eq 2).14
When the addition of unsubstituted allyltrimethylsilane
2a was investigated, no allylation was observed after 2 days;
however, a new annulation product (3) was isolated as a
64:36 mixture of diastereomers, albeit in low yield.15 The low
yield was attributed to competing imine hydrolysis pro-
moted by Sc(OTf)3, leading to regeneration of isatin and
also subsequent allylation of the isatin. Although we initially
hypothesized that a [3 þ 2] cyclization could occur to afford
spiropyrrolidines such as 5,16 this product was not ob-
served. The structure of spirocarbamate 3 was assigned on
the basisof massspectrometry and13C NMRdataand then
further confirmed by X-ray crystallography.17
These initial results with Sc(OTf)3 prompted us to
investigate other Lewis acids to optimize the formation
of spirocarbamate 3b (Table 1).18 We turned our attention
tometal chloride salts ascatalysts because we hypothesized
that the metal triflates were promoting Boc deprotection
and imine hydrolysis.19 Although we observed the lower
conversion for the formation of 3b with various metal
chloride salts (entries 2ꢀ5), the formation of byproducts
was also reduced. A moderate diastereomeric ratio was
observed for all catalysts, and selectivity was relatively
unaffected by the choice of metal salt.
Continuing our investigation, we looked toward additives
that would enhance catalytic activity and increase the yield for
the spirocarbamate product. We initially investigated addi-
tives such as AgSbF6 and NaSbF6, which have been shown to
enhance the reactivity of metal chloride catalysts by formation
of a cationic complex;20,9 however, the addition of NaSbF6 in
this reaction did not improve the yield (entry 5 vs 6). Further
investigations revealed that the addition of TMSCl
(7) For selected examples, see references 4cꢀe, and also see: (a)
Akiyama, T.; Kirino, M. Chem. Lett. 1995, 24, 723. (b) Brengel, G. P.;
Meyers, A. I. J. Org. Chem. 1996, 61, 3230. (c) Akiyama, T.; Sugano, M.;
Kagoshima, H. Tetrahedron Lett. 2001, 42, 3889.
(8) (a) Hanhan, N. V.; Tang, Y. C.; Tran, N. T.; Franz, A. K. Org. Lett.
2012, 14, 2218. (b) Silverio, D. L.; Torker, S.; Pilyugina, T.; Vieira, E. M.;
Snapper, M. L.; Haeffner, F.; Hoveyda, A. H. Nature 2013, 494, 216.
(9) Hanhan, N. V.; Ball-Jones, N. R.; Tran, N. T.; Franz, A. K.
Angew. Chem., Int. Ed. 2012, 51, 989.
(10) (a) Alcaide, B.; Almendros, P.; Aragoncillo, C. Eur. J. Org.
Chem. 2010, 15, 2845. (b) Lesma, G.; Landoni, N.; Pilati, T.; Sacchetti,
A.; Silvani, A. J. Org. Chem. 2009, 74, 4537.
(11) An allylation of N-aryliminooxindoles with allylsilane reagents
has been reported previously using mercury(II) salts as catalysts, see:
Cao, Z. Y.; Zhang, Y.; Ji, C. B.; Zhou, J. Org. Lett. 2011, 13, 6398.
(12) See Supporting Information Table S1 for a comparison of
reaction conditions and additive effects.
(13) N-Boc-protected ketimine oxindoles were prepared using an
aza-Wittig reaction: Yan, W.; Wang, D.; Feng, J.; Li, P.; Zhao, D.;
Wang, R. Org. Lett. 2012, 14, 2512.
(14) The reaction was performed using rigorous anhydrous condi-
tions; in all cases the NHBoc product was not observed. When the
reaction was run in the presence of 2,6-di-tert-butyl-4-methylpyridine,
amine 9 was still obtained as the major product. Using CuCl2/NaBArF
conditions (vida infra) also provided amine 9 as the major product in
66% yield. See Table S2 in the Supporting Information for details.
(15) Rh(II)-catalyzed oxidative intramolecular spiroannulations of in-
doles have been reported for the synthesis of spirocarbamate oxindoles, see:
(a) Padwa, A.; Stengel, T. Org. Lett. 2002, 4, 2137. (b) Sato, S.; Shibuya, M.;
Kanoh, N.; Iwabuchi, Y. J. Org. Chem. 2009, 74, 7522. (c) A cyclic
carbamate has also been observed as a side product upon reaction of an
allylsilane with N-Boc-aldimines, see: Ollevier, T.; Li, Z. Adv. Synth. Catal.
2009, 351, 3251. (d) For the synthesis of cyclic carbamates through in situ
(16) Wu, J.; Zhu, K.; Yuan, P.; Panek, J. S. Org. Lett. 2012, 14, 3624.
(17) The X-ray structure of spirocarbamate 3i (CCDC 925542) was
solved with a disordered molecule of CH2Cl2.
(18) When additional catalysts (e.g., thioureas, phosphoric acids,
MgCl2, and PdCl2) were evaluated, no product formation was observed.
See Supporting Information Table S3 for a full summary of catalysts
and conditions investigated.
€
imine formation and alkene addition, see: Richter, H.; Frohlich, R.;
(19) Bose, D. S.; Kumar, K. K.; Reddy, A. V. N. Synth. Commun.
2003, 33, 445.
~
Daniliuc, C.; Mancheno, O. G. Angew. Chem., Int. Ed. 2012, 51, 8656.
Org. Lett., Vol. 15, No. 13, 2013
3219

Table 1. Optimization of Spirocyclization
Table 2. Scope of Imines and Allylsilanes
yield
(%)b
additive
conversionb
(% yield)c
entry
R1
X
SiR3 (2)
SiMe3 (2a)
3
dra
entry
Lewis acid
(mol %)
dra
1c
2
3d
Me
Me
Bn
Ac
H
3a
3b
3c
3d
3e
3f
60:40
65:35
60:40
66:34
66:34
60:40
65:35
55:45
66:34
70:30
63:37
78:22
75:25
56:44
80:20
69:31
75:25
91
82
87
60
74
76
93
66
76
78
91
83
64
51
36
37
76
1
Sc(OTf)3
ScCl3(THF)3
InCl3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
64:36
68:32
62:38
70:30
68:32
nd
21
Br
H
SiMe3 (2a)
2
3
SiMe3 (2a)
3
6
4
H
SiMe3 (2a)
4
SnCl2
CuCl2
CuCl2
CuCl2
Sc(OTf)3
CuCl2
ScCl3
10
5c
6c
7
Ph
Me
Me
Ac
H
SiMe3 (2a)
5
12
F
SiMe3 (2a)
6
NaSbF6 (30)
TMSCl (300)
NaBArF (11)
NaBArF (11)
NaBArF (11)
NaBArF (11)
NaBArF (11)
NaBArF (11)
<5
Br
H
Si(i-Pr)3 (2b)
3g
3h
3i
7
62:38
50:50
65:35
50:50
62:38
55:45
ꢀ
70 (62)
39
8
Si(i-Pr)3 (2b)
8
9
Me
Me
Me
Me
Me
Ac
F
Si(i-Pr)3 (2b)
9
99 (82)
93 (54)
78 (54)
99
10
11
12
13
14
15
16
17
Br
OMe
H
SiMe2CHPh2 (2c)
SiMe2CHPh2 (2c)
SiMe2CHPh2 (2c)
SiMe2CHPh2 (2c)
SiMe2CHPh2 (2c)
Si(i-Pr)2CHPh2 (2d)
Si(i-Pr)2CHPh2 (2d)
SiPh2t-Bu (2e)
3j
10
11
12
13
3k
3l
InCl3
SnCl2
ꢀ
F
3m
3n
3o
3p
3q
no rxn
H
a Diastereoselectivity was determined for the unpurified mixture
using 1H NMR spectroscopy. b Conversion was determined using 1H
NMR spectroscopy. c Isolated yield.
Me
Me
Me
Br
F
F
a Determined based on analysis of 1H NMR spectra of the unpurified
reaction mixture. b Isolated yield. c Reaction was run in the presence of
22 mol % NaBArF. d Reaction was run for 72 h.
enhanced the yield of spirocycle 3b (62%, entry 5 vs 7).21
Next, we evaluated the addition of sodium tetrakis[(3,5-
trifluoromethyl)phenyl]borate (NaBArF),22,23 where mod-
erate to high conversion to spirocarbamate 3b was observed
with all metal chlorides evaluated (entries 8ꢀ12). The com-
bination of CuCl2 with NaBArF resulted in the highest
isolated yield (82%, entry 9). A control experiment using
only NaBArF (entry 13) confirmed that the NaBArF alone is
not a catalyst for this reaction.
With the optimal catalyst system identified, we proceeded
to demonstrate the scope of iminooxindoles for this reaction
(Table 2). The N-acyl substrate (entry 4) exhibited lower
reactivity (60% yield) compared to N-alkyl substrates,
which we attribute to a competitive binding mode for the
Lewis acid compared to a 1,2-binding mode of the ketimine
oxindole. In other cases, yields of substrates with lower
reactivity could be improved by utilizing 22 mol % NaBArF.
To determine the influence ofthe silyl group on yield and
diastereoselectivity, we investigated several aryl and alkyl
groups on silicon. The addition of allyltri-isopropylsilane
(2b) to imines1b,c,e providedincreased yields(entries 7ꢀ9)
compared to allylsilane 2a while maintaining consistent
diastereoselectivity. Reactions with silane 2c also pro-
ceeded with high yields (entries 10ꢀ14). The bulkier aryl-
substituted allylsilane 2d proceeded with the highest dia-
stereoselectivity (80:20 dr) and also showed the lowest
reactivity (36% yield) (entries 15ꢀ16). Additionally, the
allyl(tert-butyl)-diphenylsilane 2e (entry 17) afforded the
spirocarbamate product in good yield and moderate dia-
stereoselectivity (75% yield, 75:25 dr).
The proposed mechanism of spirocyclization (Scheme 3)
is initiated upon chelation of the copper(II) catalyst to the
iminooxindole (intermediate I), which increases the electro-
philicity of the imine.24 Addition of the allylsilane affords
a β-silyl stabilized carbocation (intermediate II). Although
elimination of the silyl group would afford allylation pro-
duct 4, the carbocation can be intercepted by the nucleo-
philic oxygen of the Boc group to afford intermediate III.
Elimination of 2-methylpropene from intermediate III gives
rise to spirocarbamate 3. Two possible roles can be envi-
sioned for the NaBArF additive. First, NaBArF can serve
(20) For examples using AgSbF6 to prepare cationic metal com-
plexes, see: (a) Evans, D., A.; Barnes, D. M.; Johnson, J. S.; Lectka, T.;
Matt, P.; Miller, S. J.; Murry, J. A.; Norcross, R. D.; Shaughnessy, E. A.;
Campos, K. R. J. Am. Chem. Soc. 1999, 121, 7582. (b) Evans, D. A.;
Rovis, T.; Kozlowski, M. C.; Downey, C. W.; Tedrow, J. S. J. Am.
Chem. Soc. 2000, 122, 9134.
(21) For examples of TMSCl increasing the reactivity of Lewis acids, see:
(a)Onishi,Y.;Ito, T.;Yasuda, M.;Baba, A.Eur. J. Org. Chem. 2002, 9, 1578.
(b) Yamanaka, M.; Nishida, A.; Nakagawa, M. Org. Lett. 2000, 2, 159.
(22) For synthesis and preparation of NaBArF, see: Yakelis, N. A.;
Bergman, R. G. Organometallics 2005, 24, 3579.
(24) (a) France, S.; Shah, M. H.; Weatherwax, A.; Wack, H.; Roth, J.
P.; Lectka, T. J. Am. Chem. Soc. 2005, 127, 1206. (b) Evans, D. A.;
Johnson, D. S. Org. Lett. 1999, 4, 595. (c) Evans, D. A; Fandrick, K. R.;
Song, H. J. Am. Chem. Soc. 2005, 127, 8942.
(25) For examples of NaBArF creating cationic complexes, see:
(a) Schreiber, D. F.; Ortin, Y.; Muller-Bunz, H.; Phillips, A. D. Organome-
ꢀ
tallics 2011, 30, 5381. (b) Morales, D.; Perez, J.; Riera, L.; Riera, V.; Corzo-
ꢀ
Suarez, R.; Garcıa-Granda, S.; Miguel, D. Organometallics 2002, 21, 1540.
(23) For use of NaBArF with Lewis acid catalysis, see: (a) Onodera,
G.; Yamamoto, E.; Tonegawa, S.; Iezumi, M.; Takeuchi, R. Adv. Synth.
Catal. 2011, 353, 2013. (b) Nomura, N.; Jin, J.; Park, H.; RajanBabu,
T. V. J. Am. Chem. Soc. 1998, 120, 459.
(26) For examples of boron anions stabilizing carbocations, see:
(a) Cordoneanu, A.; Drewitt, M. J.; Bavarian, N.; Baird, M. C. New
J. Chem. 2008, 32, 1890. (b) Shaffer, T. D.; Ashbaugh, J. R. J. Polym.
Sci. 1997, 35, 329.
3220
Org. Lett., Vol. 15, No. 13, 2013

Scheme 3. Proposed Mechanism for Spirocyclization
Scheme 4. Reduction of Spirocarbamate 3 to Furoindoline 12
the silyl group was observed, followed by decarboxylation,
to give 3-allyl-3-amino-oxindole 4 in high yield. Elimina-
tion was also observed upon treatment of the TMS-
containing spirocycle 3f with TBAF conditions, giving amine
4b in 77% yield (entry 3). In contrast, 3f did not undergo
elimination using CsF conditions (entry 4), and the TIPS
group was resistant to elimination with TBAF (entry 5).
NaOH hydrolysis conditions to access an amino alcohol,15d
a substrate that may be more successful for CꢀSi oxida-
tion, also led to elimination of the silyl group and forma-
tion of 4a in high yield (entry 6).
The reduction of spirocarbamate 3 with LAH was in-
vestigated and determined to be an efficient route for access
to tetrahydrofuroindoles (Scheme 4). LAH reduction of
trans-3b afforded a single diastereomer of 12 in high yield.
The chemoselective reduction of the carbamate to access the
1,3-amino alcohol was not observed.15d The tetrahydrofur-
oindole product results from a tandem reduction of the
carbamate and the oxindole amide to produce an iminium
ion (11), which is poised for cyclization.28,29
Table 3. Acessing 3-Amino-3-allyloxindoles
yield
(%)
entry R1
SiR3
conditions
product
In conclusion, we have developed new methodology utiliz-
ing allylsilanes and iminooxindoles to access spirocarbamate
oxindoles30 via intramolecular trapping of a transient β-silyl
carbocation with the N-Boc group. The N-Boc-iminooxin-
dole displays significantly increased reactivity compared to
N-aryl substrates; however, NaBArF is still integral for
efficient reactivity. We hypothesize that the noncoordinating
borate anion plays a key role to stabilize the β-silyl carboca-
tion and increase the Lewis acidity of the CuCl2 upon
formation of a cationic complex. Transformation of the
spirocarbamate can be achieved using a fluoride-promoted
elimination to afford 3-amino-3-allyloxindole products or
upon reduction with LAH to access furoindoline products.
1
2
3
4
5
6
Br SiMe2CHPh2 TBAF, 0 °C, 15 min
4a
4b
4b
4b
4b
4a
89
89
77
0
F
F
F
F
SiMe2CHPh2 CsF, 24 °C, 16 h
SiMe3
TBAF, 24 °C, 2 h
CsF, 24 °C, 16 h
TBAF, 24 °C, 2 h
SiMe3
Si(i-Pr)3
0
Br SiMe2CHPh2 NaOH, reflux, 2 h
92
as a loosely coordinating anion to promote formation of a
cationic copper(II) species.25 Alternatively, NaBArF can
stabilize formation of the β-silyl carbocation.5,26 Although
it is likely that one role of the NaBArF is more dominant, a
dual mode of activation may also be occurring in the reaction.
Upon investigating TamaoꢀFleming oxidation condi-
tions to modify the silyl group (i.e., to access the corre-
sponding alcohol 10),27 we determined that various
conditions favored elimination of the silyl group and
formation of 3-allyl-3-amino-oxindole 4. We initially in-
vestigated dimethylbenzhydryl spirocycles 3j and 3m using
TBAF or CsF as the fluoride source (Table 3, entries 1 and
2); however, before addition of the oxidant, elimination of
Acknowledgment. We acknowledge the donors of the
American Chemical Society Petroleum Research Fund, NSF
(CHE-10004925) and NIH/NIGMS (P41-GM0089153) for
support of this research. S.O.W. acknowledges UC Davis
and U.S. Department of Education for a GAANN fellow-
ship. Special thanks to G. Gunbus (UCD) for contribution
of NaBArF, and N. Ball-Jones and J. Badillo (UCD) for
contributions of allylsilane reagents.
(27) (a) Jones, G. R.; Landias, Y. Tetrahedron 1996, 52, 7599. (b)
Roberson, C. W.; Woerpel, K. A. Org. Lett. 2000, 2, 621. (c) Peng, Z.;
Woerpel, K. Org. Lett. 2000, 2, 1379.
(28) Itoh, T.; Ishikawa, H.; Hayashi, Y. Org. Lett. 2009, 17, 3854.
(29) For an example of the furoindoline core in natural products, see:
Sunazuka, T.; Yoshida, K.; Kojima, N.; Shirahata, T.; Hirose, T.; Handa,
M.; Yamamoto, D.; Harigaya, Y.; Kuwajima, I.; Omura, S. Tetrahedron
Lett. 2005, 46, 1459.
Supporting Information Available. Experimental proce-
dures and spectral data for all compounds; X-ray crystal
structure coordinates and files in CIF format for 3i. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(30) Preliminary investigations for enantioselective catalysis af-
forded an enantiomeric ratio of 70:30 using 10 mol % of copper(II)
chloride and NaBArF with an isopropyl-bisoxazoline ligand.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 13, 2013
3221