One-Pot Michael Addition/Radical Cyclization Reaction of N -Acryloyl Indoles

Article abstract of DOI:10.1055/s-0036-1589105

From N -acryloyl indoles, ten examples of 1,2-annulated indole products were generated in a one-pot procedure via a Michael addition and radical cyclization mediated by Mn(OAc) ₃.

Full text of DOI:10.1055/s-0036-1589105

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–F
letter
A
en
L. C. Irwin, M. A. Kerr
Letter
Synlett
One-Pot Michael Addition/Radical Cyclization Reaction of
N-Acryloyl Indoles
R3
Lauren C. Irwin
Michael A. Kerr*
O
O
R3
ONE-POT
NaH
THF
R4
R5
O
O
R4
R6
R2
+
R5
R6
N
Department of Chemistry, The University of Western Ontario,
London, ON, N6A 5B7, Canada
makerr@uwo.ca
then:
Mn(OAc)3
AcOH, reflux
N
R2
R5, R6 = Me, OMe
R1
O
R1
O
R1 = H, alkyl
R2 = H, phenyl
R3 = H, alkyl
10 examples, 25–65%
R
4 = H, Br, NO2, OMe
Received: 29.07.2017
Accepted after revision: 11.08.2017
Published online: 21.09.2017
doing this, we became interested in the use of electrophilic
radicals to forge carbon–carbon bonds to the indole moi-
ety.5–7 Some time ago we reported the radical cyclization of
indoles bearing a pendant β-dicarbonyl moiety tethered to
the indole nitrogen. Scheme 1 shows a selected example.
When 1 was treated with Mn(OAc)3 in refluxing MeOH, cy-
clization product 2 was formed in 82% yield. The reaction
presumably proceeds via the malonic radical 3 which un-
dergoes radical cyclization to yield benzylic radical 4. Sub-
sequent oxidation to cation 5 followed by deprotonation re-
sults in the formation of 2. This was demonstrated in the
synthesis of the tetracyclic core of tronocarpine and later in
the synthesis of mersicarpine (9).8,9
In our previous efforts, the cyclization substrates 12
were prepared via Michael addition of active methylene
compounds to N-acryloyl indoles 10 or indolines 11
(Scheme 2). The substrates were isolated and purified prior
to cyclization to products 13. It was our hope that we may
obviate the isolation of these substrates by generating the
cyclization substrate in situ in a tandem, or at the very least,
a one-pot protocol. Additionally, it was our hope to expand
the substrate scope to include substituents on the acryloyl
moiety (i.e., R3 in compound 14).10 This letter reports the
realization of a one-pot Michael addition/radical cyclization
protocol for the synthesis of functionalized indoles with
potential value in natural products synthesis.
DOI: 10.1055/s-0036-1589105; Art ID: st-2017-r0529-l
Abstract From N-acryloyl indoles, ten examples of 1,2-annulated
indole products were generated in a one-pot procedure via a Michael
addition and radical cyclization mediated by Mn(OAc)3.
Key words indoles, radical reaction, heterocycles, alkaloids, Michael
addition
Indoles have long held a position of prominence as a
substructure in biologically active molecules, including a
variety of anticancer, antibacterial, and antiviral drugs.1
Many bioactive indoles are naturally occurring and a subset
of these bear a six-membered ring fused to the 1,2-face of
the indole core. A significant variety of these 1,2 substituted
indoles have been found in the flowering Malaysian plant
Tabernaemontana corymbosa.2 These compounds, such as
tronocarpine, chippiine, and ervataine, have intriguing bio-
active properties and novel pentacyclic structures that
present a challenge to the organic chemist (Figure 1).2,3
MeO
NH
NH
MeO2C
O
O
N
N
N
N
Optimization of the one-pot procedure was performed
using N-methacryloyl skatole (14a) and dimethylmalonate
(Table 1). It was our hope that the two distinct processes
(Michael addition and radical cyclization) would occur un-
der one set of reaction conditions. To this end, 14a and di-
methyl malonate were subjected to typical radical-cycliza-
tion conditions in MeOH with the hope that the Michael ad-
dition would occur via a radical or Lewis acid promoted
process (Table 1, entries 1 and 2).11 Unfortunately, only
HO
HO
tronocarpine
chippiine
ervataine
Figure 1 Indole-containing natural products isolated from Tabernae-
montana corymbosa
Given the importance of indoles in both the milieu of
medicinal chemistry and natural products total synthesis,
the importance of functionalizing this heterocyclic motif is
of paramount interest.4 While there is a host of methods for
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

B
L. C. Irwin, M. A. Kerr
Letter
Synlett
Me
N
Me
CO2Me
CO2Me
Mn(OAc)3 (3 equiv)
CO2Me
CO2Me
N
MeOH, reflux
82%
O
O
1
2
[O]
Me
[O]
Me
Me
N
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
N
N
O
O
O
3
4
5
1. Mn(OAc)3 (3 equiv)
MeOH, reflux
CN
NH
2. H2, Raney Ni, EtOH, THF
O
63% overall
CO2Me
CO2Me
CO2Me
N
N
O
O
7: tronocarpine core structure
6
N
Me
OH
N
O
O
N
O
Me
O
8
9: mersicarpine
Scheme 1 Mechanism of Mn(OAc)3-mediated radical addition of malonyl tethers to the indole 2-position
(1) Previous work (ref 8):
solvent without isolation of the Michael adduct or even re-
moval of the initial solvent. Addition of Et3N, K2CO3, or DBU
to promote the Michael addition portion of the reaction
was ineffective (Table 1, entries 3–5). It became apparent
that during the Michael addition, deacylation of the N-acry-
loyl indole (14a) was problematic in basic methanol. Fur-
ther trials were therefore done in MeCN and THF with
promising results yielding 50% of 15a occurring in THF us-
ing NaH base (Table 1, entry 7). The Michael addition por-
tion of the reaction failed to proceed to completion in the
presence of 1.2 equivalents of dimethyl malonate (Table 1,
entry 10), but was successful in 1.5 equivalents providing
the optimized amount of malonate required. When avoid-
ing an aqueous workup after the oxidation reaction, and
opting for filtration of the reaction mixture, yields of the
1,2-annulated indole 15a were increased. Optimized results
(Table 1, entry 13) yielded 65% of product 15a in a two-sol-
vent system (THF, then AcOH) with NaH as the base and us-
R2
R2
EWG
EWG
R1
R1
N
CO2Me
CO2Me
N
K2CO3, THF or toluene
EWG = CO2Me, C(O)Me
O
O
or
10
12
Mn(OAc)3
R1
N
2 CO2Me
R
CO2Me
R1
O
N
11
O
13
(2) This work:
Michael addition/
radical cyclization
R2
2 EWG
R1
R
EWG
EWG
R1
EWG
"one-pot"
N
N
R3
R3
O
O
14
15
12
ing seven equivalents of Mn(OAc)3 (see general experi-
mental procedure in References and Notes or additional
Supporting Information).
Scheme 2 (1) Previous Kerr work using single-electron oxidant
Mn(OAc)3 to perform C–H insertion at the 2-position of various indoles
(2) this work
Using the optimized reaction conditions, a substrate
scope of the one-pot procedure was investigated on a vari-
ety of N-acryloyl indoles (Table 2). To begin, we tested dif-
ferent 1,3-dicarbonyl reagents. Acetylacetone (Table 2, en-
try 2) or methylacetoacetate (Table 2, entry 3) both yielded
the desired 1,2-annulated indoles products 15b and 15c in
small amounts of cyclized product 15a were isolated. At
this stage an adjustment to the strategy was made. We
aimed to promote the Michael addition with use of base.
Then, after a period of time, add Mn(OAc)3 and a second
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

C
L. C. Irwin, M. A. Kerr
Letter
Synlett
Table 1 Optimization of the One-Pot Michael Addition, Radical Cyclization of N-Acryloylindoles
CO2Me
CO2Me
One-Pot
base, solvent 1
MeO2C
CO2Me
Nuc.
+
N
N
then Mn(OAc)3,
solvent 2
temp (°C)
14a
15a
O
O
Entry
Nuc. (equiv)
Base
Solvent 1
Solvent 2
Mn(III)a (equiv)
Temp (°C)
Yield of 15a (%)
1
2
3
–
MeOH
AcOH
DCM
MeOH
MeOH
MeCN
THF
–
6
6
65
110
35
trace
5
3
–
–
3
1.5
2
NEt3
K2CO3
DBU
DBU
NaH
NaH
NaH
NaH
NaH
NaH
NaH
NaH
–
3
0
4
–
3
65
0b
5
2
–
3
65
0b
6
2
MeOH
AcOH
MeOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
3
65
43c
50c
36c
23c
42d
47d
57d
65d
58d
7
2
6
110
65
8
2
THF
6
9
2
THF
5
110
110
110
110
110
110
10
11
12
13
14
1.2
1.2
1.5
1.5
1.5
THF
4
THF
6
THF
6
THF
7
THF
10
a Mn(OAc)3.
b Deacylation occurred yielding 3-methylindole.
c Aqueous workup after completion of both reactions.
d Nonaqueous workup after completion of both reactions.
respective 59% and 54% yields. It quickly became clear that
a competitive aldol condensation of these 1,3-dicarbonyl
reagents in the presence of NaH was faster than the Michael
addition, and that the weaker K2CO3 was superior in gener-
ating annulated products 15b and 15c.13 When comparing
substitution off the acryloyl functionality (R1 and R2 of 14),
it was observed that a bulky phenyl substituent, when lo-
cated at the β-position, sterically hindered the Michael ad-
dition thereby leading to a lower overall yield of product
15f (Table 2, entry 6). Having a methyl substituent at the α-
position of the acryloyl moiety (14d, R1) compared to a hy-
drogen (14e), gave slightly better yields (15d and 15e, re-
spectively), possibly due to a lower tendency toward po-
lymerization (Table 2, compare entries 4 and 5).
Varying the electronics of the N-acryloyl indole by
changing the substituent at position 5 showed that elec-
tronically unbiased indoles, or those with an electron-do-
nating group, produced higher yields (Table 2, entries 1, 4,
and 7). When the N-acryloyl indole bore an electron-with-
drawing nitro group (14h), the yield diminished sharply to
36% (Table 2, entry 8). It is thought that because the reac-
tion proceeds through an electrophilic radical 4, an absence
of electron density would destabilize this intermediate
making it less likely to form.
This would inherently cause the lower yields observed.
Substitution at the indole position 3 gave improved yields
by stabilizing both the radical and the positive charge
formed at that position after further oxidation by an addi-
tional equivalent of Mn(OAc)3. Table 2, entry 10 represents
a more elaborate indole 14j with the potential to be used as
an intermediate 15j towards the total synthesis of natural
product tronocarpine (Figure 1). During the reaction of 14j
with dimethyl malonate, no explicit 6-exo radical cycliza-
tion to the terminal olefin of the molecule was observed.
This reaction, however, did generate an isolated decomposi-
tion product that was both an unclean mixture and uneluci-
dated. We suspect this mixture to be caused by the pres-
ence of excess dimethyl malonate which also forms radicals
under the reaction conditions. These radicals would possess
the potential to combine with each other and/or add into
either the terminal or acrylic alkene of product 14j, perhaps
accounting for a multitude of undesired products isolated
as the aforementioned decomposition mixture.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

D
L. C. Irwin, M. A. Kerr
Letter
Synlett
Table 2 One-Pot Michael Addition, C–H Insertion of Malonyl Radicals from N-Acryloyl Indoles
R5(O)C
C(O)R6
R3
R3
(1.5 equiv)
C(O)R5
C(O)R6
NaH (1.5 equiv)
THF
R4
R4
N
N
R2
R2
then:
O
Mn(OAc)3 (7 equiv)
AcOH, reflux
14
R1
15
R1
O
Entry
Substrate
1,3-Dicarbonyl
Product (yield, %)
CO2Me
CO2Me
N
MeO2C
CO2Me
N
1
O
O
15a 65
14a
14b
14c
C(O)Me
C(O)Me
N
Me(O)C
C(O)Me
N
2
3
O
O
15b 59a
CO2Me
C(O)Me
N
MeO2C
C(O)Me
N
O
O
15c 54a
15d 45
15e 38
CO2Me
CO2Me
N
N
MeO2C
MeO2C
MeO2C
MeO2C
CO2Me
CO2Me
CO2Me
CO2Me
4
5
6
7
O
O
14d
14e
CO2Me
CO2Me
N
N
O
O
CO2Me
CO2Me
N
N
Ph
Ph
O
O
14f
15f 27
Br
Br
CO2Me
CO2Me
N
N
O
O
15g 63
14g
O2N
O2N
CO2Me
CO2Me
N
N
MeO2C
CO2Me
8
O
O
15h 36
14h
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

E
L. C. Irwin, M. A. Kerr
Letter
Synlett
Table 2 (continued)
Entry
9
Substrate
1,3-Dicarbonyl
Product (yield, %)
MeO
MeO
CO2Me
CO2Me
N
O
N
MeO2C
CO2Me
O
15i 61
14i
BocHN
NHBoc
CO2Me
N
MeO2C
CO2Me
CO2Me
10
N
O
O
14j
a K2CO3 was used instead of NaH.
15j 30
We showed previously that Mn(OAc)3 additionally has
the ability to oxidize indolines to its respective indole en
route to radical cyclization products.8,9,14 Performing this
one-pot chemistry from indoline 16, where Mn(OAc)3 acts
as a single-electron oxidant to generate the 1,3-dicarbonyl
radical and additionally oxidize the indoline to the desired
1,2-annulated indole, worked with moderate success
(Scheme 3). Using ten equivalents of the Mn(OAc)3, product
15e was generated from the indoline, performing three
steps in one flask (Michael addition/indoline oxidation/rad-
ical cyclization).
Funding Information
We are grateful to the Natural Sciences and Engineering Research
Council of Canada (NSERC) for generous funding of this research. LCI
is a recipient of an OGS scholarship.
N()
a
utarl
Secinces
a
n
d
E
n
gn
i
e
enri
g
Reseacrh
C
o
u
n
lc
i
of
C
a
n
a
d
a
Acknowledgment
We would like to thank Mr. Doug Hairsine of the University of
Western Ontario for performing the mass spectrometry analyses.
Supporting Information
One-Pot
NaH (1.5 equiv), THF
CO2Me
CO2Me
CO2Me
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1589105.
+
CO2Me
N
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
N
then Mn(OAc) (9 equiv)
AcOH, 110 °C, 22 h
18%
O
O
15e
16
References and Notes
Scheme 3 Three steps, including oxidation of the indoline to the
(1) (a) Samala, S.; Arigela, R. K.; Kant, R.; Kundu, B. J. Org. Chem.
2014, 79, 2491. (b) Sessler, J. L.; Cho, D.-G.; Lynch, V. J. Am.
Chem. Soc. 2006, 128, 16518. (c) Zhang, M.-Z.; Chen, Q.; Yang,
G.-F. Eur. J. Med. Chem. 2015, 89, 421. (d) Hamann, M. T.;
Waseem, G. Life. Sci. 2005, 78, 442. (e) Gul, W.; Hamann, M. T.
Life Sci. 2005, 78, 442.
(2) Tronocarpine: (a) Sim, K.-M.; Lim, T.-M.; Kam, T.-S. Tetrahedron
Lett. 2000, 41, 2733. (b) Sapeta, K.; Kerr, M. A. Org. Lett. 2009,
11, 2081. (c) Torres-Ochoa, R. O.; Reyes-Gutierrez, P. E.;
Martinez, R. Eur. J. Org. Chem. 2014, 48.
(3) (a) Ervataine: Jin, Y.-S.; Du, J.-L.; Chen, H.-S.; Jin, L.; Liang, S.
Fitoterapia 2010, 81, 63. (b) Chippiinne: Van Beek, T. A.;
Verpoorte, R.; Baerheim Svendsen, A.; Fokkens, R. J. Nat. Prod.
1985, 48, 400.
(4) (a) Abubakar, I. B.; Lim, K.-H.; Kam, T. S.; Loh, H.-S. Phyto-
chemistry 2017, 30, 74. (b) Raja, V. J.; Lim, K.-H.; Leong, C.-O.;
Kam, T.-S.; Bradshaw, T. D. Invest. New Drugs 2014, 32, 838.
indole, in a one-pot procedure to generate the desired 1,2 annulated
indole product 15e
In conclusion, a one-pot procedure was used to prepare
highly substituted 1,2-annulated indoles generated in good
yields given the number of transformations involved. Start-
ing from acylated indoles, both a Michael addition and a
radical cyclization to the 2-position of the indoles were
possible by employing Mn(OAc)3 as a single-electron trans-
fer agent. We were able to construct a variety of annulated
indoles in an efficient step-cutting, one-pot procedure. The
use of this chemistry towards the total synthesis of indole
alkaloids is in progress.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F

F
L. C. Irwin, M. A. Kerr
Letter
Synlett
(c) Low, Y.-Y.; Lim, K.-H.; Choo, Y.-M.; Pang, H.-S.; Etoh, T.;
Hayashi, M.; Komiyama, K.; Kam, T.-S. Tetrahedron Lett. 2010,
51, 269.
Celite and then flushed with even more EtOAc. The solvent was
removed under reduced pressure with added toluene to aid in
the removal of AcOH. The obtained dried crude product was
purified with flash column chromatography (EtOAc in hexanes).
The desired fractions of the column were collected to a separa-
tory funnel and washed twice with 1 M NaOH solution and then
followed with a brine wash. The organic layer was collected,
dried with MgSO4, and concentrated in vacuo to yield product.
Product 15a
Following the general experimental procedure, 15a was synthe-
sized from acyl-indole 14a (0.30 g, 1.51 mmol), NaH (0.091g,
2.27 mmol), dimethyl malonate (0.30 g, 2.27 mmol, 0.26 mL) in
10 mL of THF then Mn(OAc)3 (2.80 g, 10.6 mmol) in AcOH
(13 mL). Compound 15a was isolated as a yellow solid (0.33 g,
65%); Rf = 0.40 (20% EtOAc in hexanes); mp 126–129 °C.
(5) For other examples of radical chemistry to functionalize
indoles, see: (a) Chuang, C.-P.; Wang, S.-F. Tetrahedron Lett.
1994, 35, 1283. (b) Bhat, V.; Mackay, J. A.; Rawal, V. H. Org. Lett.
2011, 13, 3214. (c) Tsai, A.-I.; Lin, C.-H.; Chuang, C.-P. Hetero-
cycles 2005, 65, 2381. (d) Lopchuk, J. M.; Montgomery, W. L.;
Jasinski, J. P.; Gorifard, S.; Gribble, G. W. Tetrahedron Lett. 2013,
54, 6142. (e) Baciocchi, E.; Muraglia, E. J. Org. Chem. 1993, 58,
7610.
(6) For reviews on Mn(OAc)3 chemistry, see: (a) Mondal, M.; Bora,
U. RCS Adv. 2013, 3, 18716. (b) Snider, B. B. Chem. Rev. 1996, 96,
339. (c) Snider, B. B.; Cole, B. M. J. Org. Chem. 1995, 60, 5376.
(d) Snider, B. B. Tetrahedron 2009, 65, 10735. (e) Mohan, R.;
Kates, S. A.; Domroski, M. A.; Snider, B. B. Tetrahedron Lett. 1987,
28, 854.
1H NMR (400 MHz, CDCl3): δ = 8.50 (d, J = 8.2 Hz, 1 H), 7.50 (d,
= 7.7 Hz, 1 H), 7.35 (t, J = 7.7 Hz, 1 H), 7.30 (t, J = 7.5 Hz, 1 H),
3.83 (s, 3 H), 3.79 (s, 3 H), 2.90 (ddq, J = 12.6, 6.5, 6.3 Hz, 1 H),
2.84 (dd, J = 13.0, 4.4 Hz, 1 H), 2.39 (t, J = 13.0 Hz, 1 H), 2.16 (s,
3 H), 1.42 (d, J = 6.8 Hz, 3 H). 13C NMR (101 MHz, CDCl3):
δ = 170.9, 170.4, 169.0, 134.5, 131.1, 128.2, 125.7, 124.0, 118.6,
117.9, 116.8, 55.7, 53.6, 37.5, 35.4, 15.7, 9.2 IR 3027, 2954, 1785,
1702, 1456, 1386, 1385, 1308, 1245, 751. HRMS: m/z calcd for
(7) (a) Artis, D. R.; Cho, I.-S.; Muchowski, J. M. Can. J. Chem. 1992,
70, 1838. (b) Artis, D. R.; Cho, I.-S.; Jaime-Figueroa, S.;
Muchowski, J. M. J. Org. Chem. 1994, 59, 2456.
(8) Magolan, J.; Kerr, M. A. Org. Lett. 2006, 8, 4561.
(9) Magolan, J.; Carson, C. A.; Kerr, M. A. Org. Lett. 2008, 10, 1437.
(10) Kandukuri, S. R.; Schiffner, J. A.; Oestreich, M. Angew. Chem. Int.
Ed. 2012, 51, 1265.
C
18H19NO5: 329.1263; found [M+]: 329.12682.
(11) (a) Michael, A. J. Prakt. Chem. 1887, 35, 349. (b) Liu, X.; Chen, X.;
Mohr, J. T. Chem. Eur. J. 2016, 22, 2274. (c) Alcaide, B.;
Almendros, P.; Aragoncillo, C. J. Org. Chem. 2001, 66, 1612.
(d) Wu, B.; Gao, X.; Yan, Z.; Chen, M.-W.; Zhou, Y.-G. Org. Lett.
2015, 17, 6134.
Product 15d
Following the general experimental procedure, 15d was synthe-
sized from acryloyl indole 14d (0.25 g, 1.35 mmol), dimethyl
malonate (0.27 g, 2.02 mmol, 0.23 mL), NaH (0.081 g, 2.02
mmol) in 9 mL of THF. Following completion of the Michael
addition was then added Mn(OAc)3 (2.53 g, 9.40 mmol) and
AcOH (11 mL). Compound 15d was isolated as a pale orange
solid (0.18 g, 45%); Rf = 0.27 (20% EtOAc in hexanes); mp 109–
113 °C.
(12) General Experimental Procedure: One-Pot Michael Addition,
Radical Cyclization (15a–j, 16)
To an argon-flushed round-bottom flask was added half of the
total volume of THF (0.15 M) required followed by NaH (60%
dispersed in mineral oil, 1.5 equiv). The 1,3-dicarbonyl species
(1.5 equiv) was added dropwise via syringe with stirring under
argon. The resultant mixture was stirred for 15 min at which
point the desired indole (1 equiv), dissolved in the other half-
volume of THF, was added via syringe or cannula. The Michael
addition was monitored by TLC. Once TLC confirmed complete
consumption of starting indole, Mn(OAc)3(7 equiv) was added
to the round-bottom flask followed by AcOH (0.12 M). The flask
was equipped with a reflux condenser and put back under an
argon atmosphere. The reaction was brought to 110 °C and
refluxed until the mixture changed color from a dark brown to
now containing obvious white solid in a yellow/orange solution.
At this point, TLC analysis always indicated complete consump-
tion of starting materials. The crude reaction mixture was
allowed to cool to r.t. and then diluted with a large excess of
EtOAc. The solution was vacuum filtered through a thick pad of
1H NMR (400 MHz, CDCl3): δ = 8.48 (d, J = 8.2 Hz, 1 H), 7.52 (d,
J = 8.4 Hz, 1 H), 7.39–7.31 (m, 1 H), 7.28 (m, 1 H), 6.68 (s, 1 H),
3.89 (s, 3 H), 3.77 (s, 3 H), 2.80 (ddd, J = 13.3, 6.7, 4.5 Hz, 1 H),
2.72 (dd, J = 13.6, 4.5 Hz, 1 H), 2.51 (t, J = 13.5 Hz, 1 H), 1.43 (d,
J = 6.8 Hz, 3 H). 13C NMR (101 MHz, CDCl3): δ = 171.0, 169.6,
168.8, 135.4, 133.0, 129.3, 125.5, 124.3, 120.8, 116.8, 109.2,
55.7, 53.7, 42.0, 36.4, 35.3, 15.9. IR: 2956, 2923, 2852, 1746,
1708, 1437, 1300, 1144, 1063, 836 cm–1. HRMS: m/z calcd for
C
17H17NO5: 315.11067; found [M+]: 315.11120.
(13) Cai, G.-X.; Wen, J.; Lai, T.-T.; Xie, D.; Zhou, C.-H. Org. Biomol.
Chem. 2016, 14, 2390.
(14) For other indoline to indole oxidations involving Mn, see:
(a) Ketcha, D. M. Tetrahedron Lett. 1988, 29, 2151.
(b) Gourdoupis, C. G.; Stamos, I. K. Synth. Commun. 1993, 23,
2241.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–F