Synthesis of Benzo[a]carbazoles and an Indolo[2,3-a]carbazole from 3-Aryltetramic Acids

Article abstract of DOI:10.1021/acs.joc.6b01072

A simple and flexible approach to 3-pyrrolin-2-one fused carbazoles is disclosed. The key step involves the BF₃-mediated electrophilic substitution of indoles with N-alkyl-substituted 3-aryltetramic acids, which provides access to indole-substituted 3-pyrrolin-2-ones. Scholl-type oxidative cyclizations of these materials led to the formation of the corresponding 3-pyrrolin-2-one-fused benzo[a]carbazoles and indolo[2,3-a]carbazoles. This work represents the first synthesis of the benzo[a]pyrrolo[3,4-c]carbazol-3(8H)-one ring system, while the indolo[2,3-a]pyrrolo[3,4-c]carbazol-5-one ring system is found in a number of biologically active compounds including the protein kinase C (PKC) inhibitor, staurosporine.

Full text of DOI:10.1021/acs.joc.6b01072

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Note
Synthesis of Benzo[a]carbazoles and an
Indolo[2,3-a]carbazole from 3-Aryltetramic Acids
Nathanyal J. Truax, Fernando Banales Mejia, Deborah O.
Kwansare, Megan M. Lafferty, Maeve H. Kean, and Erin T. Pelkey
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01072 • Publication Date (Web): 08 Jul 2016
Downloaded from http://pubs.acs.org on July 10, 2016
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Synthesis of Benzo[a]carbazoles and an Indolo[2,3-a]carbazole from
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3-Aryltetramic Acids
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Nathanyal J. Truax, Fernando Banales Mejia, Deborah O. Kwansare, Megan M. Lafferty, Maeve
H. Kean, and Erin T. Pelkey*
Department of Chemistry, Hobart and William Smith Colleges, Geneva, NY 14456 USA
*pelkey@hws.edu
R1
benzene
or
indole
N
R1
N
R1
N
BF3•Et2O,
PhCl, 100 °C, 3Å MS
PIFA, BF3•Et2O
CH2Cl2, -40 °C
HO
O
O
O
N
N
N
R2
R2
R2
ABSTRACT
A simple and flexible approach to 3-pyrrolin-2-one fused carbazoles is disclosed. The key step
involves the BF3-mediated electrophilic substitution of indoles with N-alkyl-substituted 3-
aryltetramic acids, which provides access to indole-substituted 3-pyrrolin-2-ones. Scholl-type
oxidative cyclizations of these materials led to the formation of the corresponding 3-pyrrolin-2-
one-fused benzo[a]carbazoles and indolo[2,3-a]carbazoles. This work represents the first
synthesis of the benzo[a]pyrrolo[3,4-c]carbazol-3(8H)-one ring system, while the indolo[2,3-
a]pyrrolo[3,4-c]carbazol-5-one ring system is found in a number of biologically active
compounds including the protein kinase C (PKC) inhibitor, staurosporine.
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Indolo[2,3-a]carbazoles comprise an important class of biologically active heterocycles
(Figure 1).1,2 For example, staurosporine (1)3 is a potent inhibitor of protein kinase C (PKC).4
Hudkins and co-workers investigated structure-activity relationships by preparing carbocyclic5
and heterocyclic6 fused variants of the indolo[2,3-a]carbazole ring system. One of these analogs,
an indazolo[5,4-a]carbazole named CEP-11981 (2), was found to be a vascular endothelial
growth factor (VEGF) inhibitor7 and advanced to Phase I clinical trials.8 Interestingly, the parent
benzo[a]carbazole 3 has not previously been reported.9,10
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indolo[2,3-a]carbazole
indazolo[5,4-a]carbazole
benzo[a]carbazole
NHMe
MeO
Me
O
N
H
N
N
N
N
N
N
N
N
H
Me
O
O
N
H
O
N
H
N
H
1
2
3
staurosporine
CEP-11981
benzo[a]pyrrolo[3,4-c]carbazole-3(8H)-one
(unreported ring system)
Figure 1. Structures of fused carbazole ring systems.
We are interested in developing synthetic strategies that can be used to prepare
indolo[2,3-a]carbazoles and new heterocyclic analogs such as 3. Toward this end, we have
explored the use of tetramic acid derivatives to prepare 3,4-diaryl-3-pyrrolin-2-ones (4) (Scheme
1).11-13 Palladium-catalyzed cross-coupling reactions of tetramic acid triflates with arylboronic
acids gives 4. Rather than using costly indolylboronic acids to prepare indole-substituted 3-
pyrrolin-2-ones 5, we became interested in developing a new strategy that takes advantage of the
inherent reactivity of the electron-rich indole ring system.14 We were inspired by a report by
Prabhakar and co-workers;15 they found that treatment of N-benzoyltetramic acid with 2,2’-
biindole in the presence of BF3•Et2O gave a biindole-substituted 3-pyrrolin-2-one with loss of
the benzoyl group. To preclude the possibility of N-deprotection, we decided to systematically
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study the synthesis of 5 using N-alkyltetramic acids. Realization of this strategy would offer
significant advantages over the cross-coupling strategy as it does not require functionalization of
the enolic moiety into a triflate nor does it require the use of palladium catalysts or arylboronic
acids.
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Scheme 1. Arylation Reactions of Tetramic Acids
Previous Work (Ref 11)
Cross-coupling of tetramic acid triflates
R2
R3
R2
TfO
R3
Pd cat.
+
O
O
N
N
B(OH)2
R1
R1
indole-3-boronic acids
4
tetramic acid triflates
This Work
Direct arylation of tetramic acids
R3
N
R3
N
R2
R2
HO
R4
Lewis acid
+
R4
O
O
N
N
H
R1
R1
5
indoles
tetramic acids
The requisite N-alkyl-3-aryltetramic acids 8 were prepared using the one-pot tandem
amidation-Dieckmann cyclization reported by Le Gall and co-workers (Scheme 2).16,17 The
cyclocondensation of ethyl N-alkylglycinates 618 and arylacetates 719 by treatment with t-BuOK
gave tetramic acids 8 in mostly good yields. The latter were conveniently isolated as crystalline
materials directly from the work-up of these reactions. We chose to make N-tert-butyl tetramic
acid 8f in order to explore the possibility of making the free N-H lactams later by removal of the
tert-butyl groups.20 Interestingly, this tandem reaction failed to give indole-substituted tetramic
acid 8g.
Scheme 2. Synthesis of 3-Aryltetramic acids
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HO
Ar
O
4
5
t-BuOK, THF, Δ
O
H
N
+
O
Ar
N
R
8
EtO
R
MeO
6
7
6
7
8
9
R
MeO
OMe
MeO
OMe
Me
N
10
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R
HO
HO
HO
HO
O
O
O
O
N
N
N
N
8a R = H (79%)
8b R = F (65%)
8c R = OMe (53%)
8d R = H (54%)
8e R = OMe (42%)
8f (53%)
8g (0%)
We next explored Lewis-acid mediated arylation reactions using N-propyltetramic acid
8c as the electrophilic partner and N-methylindole (9a) as the nucleophilic partner (Table 1, entry
1). Following similar reaction conditions to that reported by Prabhakar,15 we treated 8c and 9a
with BF3•Et2O in CH2Cl2 and 3Å molecular sieves at rt for 20 h and obtained an 11% yield of
indole product 5c.
To improve the yield of 5c, we screened different reaction conditions (Table 1). The
1
reactions were analyzed by evaluating the crude H NMR spectra and determining the ratio of
product 5c to starting material 8c based on the relative respective methylene protons (δ3.90 for
8c and δ4.45 for 5c). The best conditions obtained included PhMe, 100 °C, and a reaction time
of 1 h (Table 1, Entry 14). Longer reaction times gave lower relative amounts of product (Table
1, Entry 12 vs Entry 13) and this result was rationalized by control experiments, which indicated
that the product was being degraded over time.21 We tried a few alternate Lewis acids (AlCl3,
FeCl3, InCl3, Cu(OTf)2) to mediate the transformation, but we did not find a better Lewis acid
than BF3•Et2O.
Table 1. Screening reaction conditionsa
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OMe
Me
N
OMe
5
6
7
Me
N
HO
reaction conditions
3Å molecular sieves
+
O
O
N
N
8
9
5c
8c
9a
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Lewis
acid
temp
(°C)
time
(h)
Entry
solvent
conversionb
1
2
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
BF3•Et2O
AlCl3
CH2Cl2
THF
40
20
1
13c
0
65
65
3
CH3CN
DCE
PhCl
PhCl
PhCl
PhMe
PhCl
PhCl
PhCl
PhCl
PhCl
PhMe
PhCl
PhCl
PhCl
PhCl
1
0
4
65
1
7
5
65
1
25
40
54
37
53
70
65
64
45
80
0
6
65
4
7
65
24
1
8
65
9
100
100
100
100
100
100
100
100
100
100
0.25
0.5
0.75
1
10
11
12
13
14
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17
18
4
1
0.5
0.5
0.5
0.5
FeCl3
---d
27
---d
InCl3
Cu(OTf)2
a0.40 mmol of 8c, 0.48 mmol of N-methylindole (9a), and 0.60 mmol of Lewis acid used; bEstimated by using ratio of ring methylene signals (1H
NMR) of 5c and 8c averaged from at least two trials; cUsing a 2.0 mmol scale, 11% isolated yield of 5c was obtained; dNMR showed neither 8c
nor 5c.
Using the best conditions from our screen, we next explored substrate scope (Scheme 3).
We quickly found that PhMe was not suitable for larger scale reactions (perhaps due to solubility
issues), so we used PhCl as the solvent moving forward (Table 1, Entry 10). Treatment of
indoles 9 with tetramic acids 8 gave 3,4-diaryl-3-pyrrolin-2-ones 5. The indolylation reaction
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worked well with N-methylindole 9a; on the other hand, a much lower yield was obtained with
parent indole (9b) during the preparation of N-unsubstituted indole 5h.
7
We also tried N-(phenylsulfonyl)indole 9c22 (R1 = SO2Ph) in the indolylation reaction
and we did not observe any arylation product 5i (Scheme 3); this result illustrates that electron-
withdrawing groups on indole inhibit the reaction.
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Scheme 3. Substrate Scope
R
N
BF3•Et2O
PhCl, 100 °C
3Å molecular sieves
R
N
HO
Ar
Ar
+
O
O
N
N
9
8
5
MeO
OMe
R
MeO
OMe
R
R
N
Me
N
Me
N
O
O
O
N
N
N
5a R = H (84%)
5b R = F (86%)
5c R = OMe (76%)
5d R = H (81%)
5e R = OMe (66%)
5h R = H (29%)
5i R = SO2Ph (0%)
A possible mechanism for this transformation is proposed (Scheme 4). Association of
BF3 with the lactam carbonyl promotes the condensation reaction between the tetramic acid and
the indole. Loss of water and disassociation of the Lewis acid gives the product. Similar
reactions involving indoles and cyclic ketones have been reported by others leading to 3-
vinylindoles.14
Scheme 4. Possible Mechanism
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Me
N
5
6
7
8
HO
Ar3
Ar3
H
O
Ar3
HO
O
O
O
N
N
N
BF3
BF3
BF3
9
10
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Me
N
Me
N
Ar3
3
Ar
OH
N
H
O
O
N
- H2O
- BF3
BF3
We examined the possibility of preparing N-unsubstituted 3-pyrrolin-2-ones. We
hypothesized that tert-butyl substrate 8f would undergo the indolylation reaction in a similar
fashion to the propyl substrate 8e, and then subsequently, the tert-butyl group of the
corresponding indole product 5f could be removed under acidic conditions (e.g., treatment with
TFA). In one of the early runs (Table 2, entry 1: PhCl, 100 °C, 0.5h), the indolylation of 8f gave
a mixture of products that included deprotected product 5j (31% yield) and deprotected tetramic
acid 10 (40% yield). We were pleased to see that the tert-butyl group was indeed removable.
Subjecting tetramic acid 10 to the PhCl/100 °C reaction conditions did not give product 5j
suggesting that N-substitution of the tetramic acid is required for the indolylation to proceed.
Under milder conditions (Table 2, entry 2: DCE, 65°C, 12h), the protected product 5f was
obtained in just 17% yield.
To improve upon these results, we next assessed the inherent stability of the tert-butyl
group of 8f in the presence of BF3•Et2O in PhCl at a range of different temperatures (70°C,
80°C, 90°C, 100°C, 110°C, 120°C). We found that the tert-butyl group of 8f was relatively
stable at 70°C and 80°C (only traces of 10 detected after 1h), whereas at higher temperatures,
significant amounts of 10 could be detected after 1h in the crude reaction mixtures. Using this
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information, we ran the indolylation of 8f at 80°C followed by heating to 120°C to remove the
tert-butyl group of the presumed intermediate product 5f. In addition, to help the indolylation
reaction compete with the deprotection, we used 5.0 equiv of 9a. Under these conditions (Table
2, entry 3), we were able to obtain a 55% yield of 5j. This result demonstrates that N-
unprotected 3-pyrrolin-2-ones can be prepared from tert-butyl protected tetramic acid
substrates.23
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Table 2. tert-Butyl Substrate
Me
N
MeO
OMe MeO
HO
OMe
Me
N
MeO
OMe
MeO
OMe
Me
N
9a
BF3•Et2O
3Å molecular sieves
HO
+
+
O
O
O
O
N
H
N
H
N
N
5f
5j
10
8f
temp
(°C)
time
(h)
5f
5j
10
Entry solvent
(%)a
0
(%)a
(%)a
40
1
2
PhCl
DCE
PhCl
100°C
0.5
12
31
0
65°C
17
---b
---b
3c
80°C/120°Cd
3/1
---b
55
aIsolated yield; bNot determined; c5.0 equiv of 9a used; dAfter heating at 80°C for 3h, reaction mixture was heated to 120°C for 1h
We next turned our attention to preparing fused carbazoles. We recently reported the use
of Scholl-type oxidative cyclization reactions to transform 3,4-diaryl-3-pyrrolin-2-ones into the
corresponding dibenzo[e,g]isoindol-1-ones using the hypervalent iodine reagent,
phenyliodine(III) bis(trifluoroacetate) (PIFA).11c,24 Treatment of 4-indolylpyrrolones 5d, 5h, and
5j with PIFA and BF3•Et2O gave the corresponding fused carbazoles 11d, 11h, and 11j (Scheme
5). To our knowledge, these benzo[a]carbazoles 11 represent the first reported examples of
simple benzo[a]pyrrolo[3,4-c]carbazol-3-ones. As we observed earlier,11c 3,4-dimethoxyphenyl
groups are superior to 4-methoxyphenyl groups in promoting the oxidative cyclization. Neither
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the 4-methoxyphenyl substrate 5c nor the 4-fluorophenyl substrate 5b yielded any of the
corresponding benzo[a]carbazole products 11c and 11b.
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Scheme 5. Synthesis of Benzo[a]carbazoles
R3
N
R3
N
Ar2
O
Ar2
O
PIFA, BF3•Et2O
CH2Cl2, -40 °C
N
R1
11
N
R1
5
Me
N
R
N
OMe
OMe
Me
OMe
OMe
R
N
O
O
O
N
N
H
N
11b R = F (0%)
11c R = OMe (0%)
11d R = Me (55%)
11h R = H (43%)
11j (83%)
Lastly, we applied our methodology to the preparation of the indolo[2,3-a]carbazole ring
system (Scheme 6).25 As mentioned earlier, we had tried to prepare tetramic acid 8g using the
same chemistry depicted in Scheme 2 but this failed. We managed to successfully prepare 8g
using an alternate two-step strategy starting from N-methylindole-3-acetic acid (see Supporting
Information). Treatment of 8g with 9a and BF3•Et2O in PhCl gave bisindole 5g in 43% yield.
Subsequent oxidative cyclization of 5g mediated by PIFA and BF3•Et2O gave indolo[2,3-
a]carbazole 11g in 69% yield. Overall, this result represents a relatively short and potentially
flexible strategy to indolocarbazoles.26
Scheme 6. Synthesis of Indolo[2,3-a]carbazole
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Me
N
Me
N
Me
N
Me
N
Me
N
Me
N
9a
BF3•Et2O
PhCl, 100 °C
3Å MS
HO
PIFA, BF3•Et2O
CH2Cl2, -40 °C
7
O
O
O
N
N
N
(43%)
(69%)
8
9
8g
5g
11g
10
11
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In conclusion, we have prepared indole-substituted 3-pyrrolin-2-ones using a direct
arylation reaction between tetramic acids and simple indole substrates. The indole-substituted 3-
pyrrolin-2-ones proved to be useful building blocks in the preparation of indolo[2,3-
a]pyrrolo[3,4-c]carbazoles and benzo[a]pyrrolo[3,4-c]carbazoles via Scholl-type oxidative
cyclization reactions. The entire synthetic sequence involves only one chromatographic
purification per final product and no precious metals or other costly reagents are required.
EXPERIMENTAL SECTION
General Methods.11c
General Method A for the Preparation of Ethyl N-Alkylglycinates 6.
A modification of a literature procedure was followed.18b To a rt stirred solution of primary
amine (400. mmol) in ether (200 mL) was added ethyl bromoacetate (5.5 mL, 50. mmol)
dropwise via syringe. The reaction mixture was stirred at rt for 24-48 h (as noted) during which
time a white precipitate forms. The reaction mixture was filtered to remove precipitate. The
organic layer was washed with an saturated solution of sodium bicarbonate (100 mL) and brine
(200 mL) and dried over sodium sulfate. Removal of the solvent in vacuo gave the ethyl N-
alkylglycinates 6 as colorless oils, which were used directly without further purification.
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Ethyl N-Propylglycinate (6a).18a
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Using General Method A, propylamine (23.6 g, 32.9 mL, 400. mmol), and a reaction time of 24
h gave the title compound 6a as a colorless oil (6.99 g, 48.2 mmol, 96% yield): IR (ATR, neat)
3332, 1736 cm-1; 1H NMR (400 MHz, CDCl3) δ 4.19 (q, J = 7.2 Hz, 2H), 3.40 (s, 2H), 2.57 (t, J
= 7.2 Hz, 2H), 1.73 (br s, 1H), 1.52 (sext, J = 7.2 Hz, 2H), 1.28 (t, J = 7.2 Hz, 3H), 0.93 (t, J =
7.6 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ 172.9, 61.0, 51.8, 51.3, 23.5, 14.5, 12.0 ppm.
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Ethyl N-(tert-Butyl)glycinate (6b).18c
Using General Method A, tert-butylamine (46.8 g, 67.3 mL, 640. mmol), and a reaction time of
48 h gave the title compound 6b as a colorless oil (11.53 g, 72.41 mmol, 91% yield): IR (ATR,
neat) 3330, 1737 cm-1; 1H NMR (400 MHz, CDCl3) δ 4.19 (q, J = 7.2 Hz, 2H), 3.39 (s, 2H), 1.59
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(br s, 1H), 1.27 (t, J = 7.2 Hz, 3H), 1.10 (s, 9H) ppm; C NMR (100 MHz, CDCl3) δ 173.3,
61.1, 50.5, 45.2, 29.1, 14.5 ppm.
General Method B for the Preparation of Methyl Arylacetates 7.
To a rt stirred solution of an arylacetic acid (100. mmol) in MeOH (200 mL) was added
concentrated H2SO4 (5.30 mL, 100. mmol) dropwise. The reaction mixture was heated to reflux
for 1-24 h (as noted) and then cooled to 0 °C with the aid of an external ice bath. An aqueous
solution of saturated NaHCO3 (100 mL) was added to the reaction mixture dropwise via an
addition funnel. The bulk of the MeOH was then removed in vacuo and the residue was
extracted with ether (2 x 200 mL). The combined organics were dried over Na2SO4. Removal of
the solvent in vacuo gave the methyl arylacetates 7, which were used directly without further
purification.
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Methyl 2-Phenylacetate (7a).19c
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Using General Method B, phenylacetic acid (10.00 g, 73.45 mmol), and a reaction time of 16 h
gave the title compound 7a as a colorless oil (10.40 g, 69.25 mmol, 94% yield): Rf = 0.80 (1:2
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EtOAc/petroleum ether); IR (ATR, neat) 1733 cm-1; H NMR (400 MHz, CDCl3) δ 7.25-7.36
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(m, 5H), 3.70 (s, 3H), 3.63 (s, 2H) ppm; C NMR (100 MHz, CDCl3) δ 172.3, 134.2, 129.5,
128.9, 127.4, 52.4, 41.5 ppm.
Methyl 2-(4’-Fluorophenyl)acetate (7b).19e
Using General Method B, 2-(4’-fluorophenyl)acetic acid (10.0 g, 64.9 mmol), and a reaction
time of 1.5 h gave the title compound 7b as a colorless oil (8.77 g, 52.1 mmol, 80% yield): Rf =
0.84 (1:1 EtOAc/petroleum ether); IR (ATR, neat) 1734 cm-1; H NMR (400 MHz, CDCl3) δ
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7.22-7.26 (m, 2H), 6.98-7.04 (m, 2H), 3.70 (s, 3H), 3.60 (s, 2H) ppm; C NMR (100 MHz,
CDCl3) δ 172.2 (d, J = 1.3 Hz), 162.3 (d, J = 244 Hz), 131.1 (d, J = 7.8 Hz), 129.9 (d, J = 3.4
Hz), 115.7 (d, J = 21 Hz), 52.4, 40.6 ppm.
Methyl 2-(3’,4’-Dimethoxyphenyl)acetate (7d).19
Using General Method B, 2-(3’,4’-dimethoxyphenyl)acetic acid (5.00 g, 25.5 mmol), and a
reaction time of 23 h gave the title compound 7d as a light yellow oil (4.91 g, 23.4 mmol, 92%
yield): Rf = 0.44 (1:2 EtOAc/petroleum ether); IR (ATR, neat) 1732 cm-1; H NMR (400 MHz,
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CDCl3) δ 6.82 (s, 3H), 3.88 (s, 3H), 3.87 (s, 3H), 3.70 (s, 3H), 3.57 (s, 2H) ppm; 13C NMR (100
MHz, CDCl3) δ 172.6, 149.2, 148.4, 126.7, 121.7, 112.6, 111.5, 56.2, 56.1, 52.3, 41.0 ppm.
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Using General Method B, 2-(2’,3’,4’-dimethoxyphenyl)acetic acid (5.00 g, 22.1 mmol), and a
reaction time of 23 h gave the title compound 7d as a light yellow amorphous solid (4.362 g,
18.16 mmol, 82% yield): mp 35-38 °C (lit.19a mp 40.5-41.5 °C); Rf = 0.38 (1:2 EtOAc/petroleum
ether); IR (ATR, neat) 1734 cm-1; 1H NMR (400 MHz, CDCl3) δ 6.50 (s, 2H), 3.86 (s, 6H), 3.83
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(s, 3H), 3.71 (s, 3H), 3.56 (s, 2H) ppm; C NMR (100 MHz, CDCl3) δ 172.3, 153.5, 137.3,
129.8, 106.5, 61.1, 56.4, 52.4, 41.7 ppm.
Methyl 1-Methylindole-3-acetate (7g).19b
Using a modification of General Method B (3 equivalents of H2SO4 and room temperature), 1-
methylindole-3-acetic acid (3.00 g, 15.9 mmol), and a reaction time of 1 h gave the title
compound 7g as a reddish-brown oil (2.50 g, 12.3 mmol, 77% yield): Rf = 0.60 (1:2
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EtOAc/petroleum ether); IR (ATR, neat) 1731, 1616 cm-1; H NMR (400 MHz, CDCl3) δ 7.59-
7.61 (m, 1H), 7.29-7.32 (m, 1H), 7.22-7.26 (m, 1H), 7.11-7.15 (m, 1H), 7.05 (s, 1H), 3.78 (d, J =
0.8 Hz, 2H), 3.77 (s, 3H), 3.70 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ 172.8, 137.1, 128.0,
127.9, 122.0, 119.4, 119.2, 109.5, 107.0, 52.2, 32.9, 31.3 ppm.
General Method C for the Preparation of 3-Aryltetramic Acids 8.
A modification of a literature procedure by Le Gall and co-workers was followed.16 To a rt
stirred mixture of ethyl N-alkylglycinates 6 (30.0 mmol) and methyl arylacetates 7 (30.0 mmol)
in THF (150 mL) was added solid t-BuOK (4.04 g, 36.0 mmol). The reaction mixture was
heated to reflux for 2-24 h as noted. The reaction mixture was then cooled to 0 °C (with the aid
of an external ice bath) and treated with an aqueous solution of KHSO4 (1.0 M, 120 mL)
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dropwise via addition funnel. After stirring the biphasic mixture for 15 min, the bulk of the THF
was removed in vacuo. The resulting residue was extracted with EtOAc (4 x 100 mL).* The
combined EtOAc layers were washed with brine (400 mL) and dried over sodium sulfate. The
EtOAc layer was concentrated to approximately half the original volume and then placed in a
refrigerator. The precipitate that formed was collected by filtration. The concentration/filtration
sequence was repeated with the mother liquor 2-3 times and additional precipitate collected. The
combined precipitated solids were dried in vacuo giving 3-aryltetramic acids as white (or close to
white) powders, which were used without further purification. *In some cases, treatment of the
aqueous residue with EtOAc gave a precipitate which turned out to be the desired product;
filtration before the drying step then gave an additional crop of product.
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4-Hydroxy-3-phenyl-1-propyl-1H-pyrrol-2(5H)-one (8a).
Using General Method C, amine 6a (4.36 g, 30.0 mmol), ester 7a (4.51 g, 30.0 mmol), and a
reaction time of 11 h gave the title compound 8a as a white powder (5.18 g, 23.8 mmol, 79%
yield): mp 217-222 °C; Rf = 0.38 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1661, 1584 cm-1;
1H NMR (400 MHz, D6-DMSO) δ 11.61 (br s, 1H), 7.97 (dd, J = 1.0, 8.4 Hz, 2H), 7.27-7.32 (m,
2H), 7.12-7.16 (m, 1H), 3.93 (s, 2H), 3.28 (t, J = 7.2 Hz, 2H), 1.51 (sext, J = 7.2 Hz, 2H), 0.85 (t,
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J = 7.2 Hz, 3H) ppm; C NMR (100 MHz, D6-DMSO) δ 170.8, 166.6, 132.5, 127.7, 126.7,
125.4, 102.9, 48.9, 42.5, 21.3, 11.3 ppm; HRMS (ESI-FTICR) calcd for C13H15NO2•Na
240.0995, found 240.0996.
3-(4’-Fluorophenyl)-4-Hydroxy-1-propyl-1H-pyrrol-2(5H)-one (8b).
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Using General Method C, amine 6a (2.90 g, 20.0 mmol), ester 7b (3.36 g, 20.0 mmol), and a
reaction time of 18 h gave the title compound 8b as a white powder (2.95 g, 12.5 mmol, 65%
yield): mp 220-225 °C; Rf = 0.31 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1663, 1581, 1560
cm-1; 1H NMR (400 MHz, CDCl3) δ 11.71 (br s, 1H), 8.02-8.06 (m, 2H), 7.12-7.17 (m, 2H), 3.93
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(s, 2H), 3.28 (t, J = 7.2 Hz, 2H), 1.51 (sext, J = 7.2 Hz, 2H), 0.84 (t, J = 7.2 Hz, 3H) ppm; C
NMR (100 MHz, CDCl3) δ 170.7, 166.4 (d, J = 1.4 Hz), 160.0 (d, J = 241 Hz), 129.0 (d, J = 3.1
Hz), 128.4 (d, J = 7.6 Hz), 114.5 (d, J = 21.6 Hz), 102.0, 48.9, 42.5, 21.2, 11.3 ppm; HRMS
(ESI-FTICR) calcd for C13H14FNO2•Na 258.0901, found 258.0902.
4-Hydroxy-3-(4’-methoxyphenyl)-1-propyl-1H-pyrrol-2(5H)-one (8c).
Using General Method C, amine 6a (2.18 g, 15.0 mmol), commercially available methyl 2-(4’-
methoxyphenyl)acetate (7c) (2.70 g, 15.0 mmol), and a reaction time of 6 h gave the title
compound 8c as a white powder (1.97 g, 7.97 mmol, 53% yield): mp 212-220 °C; Rf = 0.35 (2:1
EtOAc/petroleum ether); IR (ATR, neat) 1663, 1582 cm-1; 1H NMR (400 MHz, CDCl3) δ 11.40
(br s, 1H), 7.92 (d, J = 9.2 Hz, 2H), 6.89 (d, J = 9.2 Hz, 2H), 3.90 (s, 2H), 3.74 (s, 3H), 3.27 (t, J
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= 7.2 Hz, 2H), 1.50 (sext, J = 7.2 Hz, 2H), 0.84 (t, J = 7.2 Hz, 3H) ppm; C NMR (100 MHz,
CDCl3) δ 171.1, 165.0, 157.0, 127.8, 125.0, 113.1, 102.7, 54.9, 48.9, 42.5, 21.3, 11.3 ppm;
HRMS (ESI-FTICR) calcd for C14H17NO3•Na 270.1101, found 270.1102.
4-Hydroxy-3-(3’,4’-dimethoxyphenyl)-1-propyl-1H-pyrrol-2(5H)-one (8d).
Using General Method C, amine 6a (2.90 g, 20.0 mmol), ester 7d (4.20 g, 20.0 mmol), and a
reaction time of 18 h gave the title compound 8d as a light yellow powder (3.00 g, 10.8 mmol,
54% yield): mp 204-210 °C; Rf = 0.18 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1660, 1582
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cm-1; 1H NMR (400 MHz, CDCl3) δ 11.42 (br s, 1H), 7.72 (d, J = 2.0 Hz, 1H), 7.55 (dd, J = 2.0,
8.4 Hz, 1H), 6.91 (d, J = 8.4 Hz, 1H), 3.90 (s, 2H), 3.74 (s, 3H), 3.28 (t, J = 7.2 Hz, 2H), 1.51
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(sext, J = 7.2 Hz, 2H), 0.85 (t, J = 7.2 Hz, 3H) ppm; C NMR (100 MHz, CDCl3) δ 171.0,
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165.1, 147.9, 146.7, 125.4, 119.4, 111.4, 110.6, 102.7, 55.4, 55.3, 48.8, 42.5, 21.3, 11.3 ppm;
HRMS (ESI-FTICR) calcd for C15H19NO4•Na 300.1206, found 300.1207.
4-Hydroxy-3-(3’,4’,5’-trimethoxyphenyl)-1-propyl-1H-pyrrol-2(5H)-one (8e).
Using General Method C, amine 6a (2.91 g, 20.0 mmol), ester 7e (4.81 g, 10.0 mmol), and a
reaction time of 24 h gave the title compound 8e as an off-white powder (2.58 g, 8.39 mmol,
42% yield): mp 190-196 °C; Rf = 0.15 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1662, 1590
cm-1; 1H NMR (400 MHz, D6-DMSO) δ 11.67 (br s, 1H), 7.42 (s, 2H), 3.92 (s, 2H), 3.74 (s, 6H),
3.65 (s, 3H), 3.28 (t, J = 7.2 Hz, 2H), 1.51 (sext, J = 7.2 Hz, 2H), 0.84 (t, J = 7.2 Hz, 3H) ppm;
13C NMR (100 MHz, CDCl3) δ 170.9, 166.4, 152.2, 135.5, 128.2, 104.2, 102.5, 60.0, 55.6, 48.9,
42.5, 21.3, 11.3 ppm; HRMS (ESI-FTICR) calcd for C16H21NO5•Na 330.1312, found 330.1312.
1-(tert-Butyl)-4-hydroxy-3-(3’,4’-dimethoxyphenyl)-1H-pyrrol-2(5H)-one (8f).
Using General Method C, amine 6b (2.99 g, 18.8 mmol), ester 7d (3.95 g, 18.8 mmol), and a
reaction time of 22 h gave the title compound 8f as a white fluffy powder (2.90 g, 9.95 mmol,
53% yield): mp 218-223 °C; Rf = 0.37 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1677, 1591
cm-1; 1H NMR (400 MHz, CDCl3) δ 11.25 (br s, 1H), 7.68 (d, J = 2.0 Hz, 1H), 7.50 (dd, J = 2.0,
8.8 Hz, 1H), 6.89 (d, J = 8.8 Hz, 1H), 3.95 (s, 2H), 3.73 (s, 3H), 3.72 (s, 3H), 1.39 (s, 9H) ppm;
13C NMR (100 MHz, CDCl3) δ 171.4, 164.5, 147.8, 146.7, 125.4, 119.6, 111.3, 110.7, 103.9,
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55.4, 55.3, 52.7, 47.2, 27.7 ppm; HRMS (ESI-FTICR) calcd for C16H21NO4•Na 314.1363, found
314.1364.
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General Method D for the Preparation of 3-Aryl-4-(indol-3’-yl)-3-pyrrolin-2-ones 5.
To a rt stirred mixture of tetramic acid 8 (2.00 mmol) and N-methylindole 9a or indole 9b (2.40
mmol) in PhCl (20 mL) was added 3Å molecular sieves (1.0 g) followed by BF3•Et2O (0.43 g,
0.37 mL, 3.0 mmol). The reaction mixture was heated to 100 °C for 0.5 h to 1.5 h (as noted) and
then allowed to cool to rt and treated with MeOH (20 mL). The reaction mixture was decanted
to remove the molecular sieves and the solvent was removed in vacuo. The residue was treated
with CH2Cl2 (20 mL) and silica gel (2.0 g) and the solvent was removed in vacuo (dry load).
Purification by flash chromatography (EtOAc/petroleum ether gradient) gave the desired
products as lightly colored amorphous solids.
4-(1”-Methylindol-3”-yl)- 3-phenyl-1-propyl-1H-pyrrol-2(5H)-one (5a).
Using General Method D, tetramic acid 8a (0.500 g, 2.30 mmol), N-methylindole (9a) (0.429 g,
3.27 mmol), and a reaction time of 1 h gave the title compound 5a as a light yellow amorphous
solid (0.640 g, 1.94 mmol, 84% yield): mp 43-47 °C; Rf = 0.44 (1:1 EtOAc/petroleum ether); IR
(ATR, neat) 1660, 1597, 1571 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.49-7.52 (m, 2H), 7.29-7.36
(m, 4H), 7.16-7.24 (m, 2H), 7.09 (s, 1H), 7.00-7.04 (m, 1H), 4.46 (s, 2H), 3.73 (s, 3H), 3.57 (t, J
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= 7.2 Hz, 2H), 1.74 (sext, J = 7.2 Hz, 2H), 1.01 (t, J = 7.2 Hz, 3H) ppm; C NMR (100 MHz,
CDCl3) δ 171.6, 143.2, 137.3, 133.9, 129.8, 129.6, 128.6, 128.5, 127.8, 125.7, 122.6, 121.3,
120.7, 110.0, 109.1, 53.4, 44.4, 33.4, 22.3, 11.8 ppm; HRMS (ESI-FTICR) calcd for
C22H22N2O•Na 353.1624, found 353.1624.
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3-(4’-Fluorophenyl)-4-(1”-methylindol-3”-yl)-1-propyl-1H-pyrrol-2(5H)-one (5b).
Using General Method D, tetramic acid 8b (0.500 g, 2.13 mmol), N-methylindole (9a) (0.335 g,
2.55 mmol), and a reaction time of 1 h gave the title compound 5b as a light yellow amorphous
solid (0.638 g, 1.83 mmol, 86% yield): mp 60-63 °C; Rf = 0.44 (1:1 EtOAc/petroleum ether); IR
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(ATR, neat) 1735, 1661, 1599, 1571 cm-1; H NMR (400 MHz, CDCl3) δ 7.47-7.52 (m, 2H),
7.31-7.33 (m, 1H), 7.21-7.25 (m, 1H), 7.11-7.13 (m, 1H), 7.10 (s, 1H), 6.98-7.04 (m, 3H), 4.43
(s, 2H), 3.76 (s, 3H), 3.56 (t, J = 7.2 Hz, 2H), 1.73 (sext, J = 7.2 Hz, 2H), 1.0 (t, J = 7.2 Hz, 3H)
ppm; 13C NMR (100 MHz, CDCl3) δ 171.5, 162.5 (d, J = 246 Hz), 143.2, 137.4, 131.6 (d, J = 7.9
Hz), 129.8 (d, J = 3.4 Hz), 129.4, 127.5, 125.5, 122.7, 121.3, 120.8, 115.6 (d, J = 21.3 Hz),
110.0, 109.0, 53.4, 44.4, 33.5, 22.3, 11.7 ppm; HRMS (ESI-FTICR) calcd for C22H21FN2O•Na
371.1530, found 371.1530.
3-(4’-Methoxyphenyl)-4-(1”-Methylindol-3”-yl)-1-propyl-1H-pyrrol-2(5H)-one (5c).
Using General Method D, tetramic acid 8c (1.00 g, 4.04 mmol), N-methylindole (9a) (0.636 g,
4.85 mmol), and a reaction time of 1 h gave the title compound 5c as a yellow powder (1.11 g,
3.08 mmol, 76% yield): mp 132-133 °C; Rf = 0.36 (2:1 EtOAc/petroleum ether); IR (ATR, neat)
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7.20-7.24 (m, 2H), 7.12 (s, 1H), 7.01-7.05 (m, 1H), 6.87 (d, J = 8.8 Hz, 2H), 4.43 (s, 2H), 3.82
(s, 3H), 3.74 (s, 3H), 3.56 (t, J = 7.2 Hz, 2H), 1.72 (sext, J = 7.2 Hz, 2H), 1.00 (t, J = 7.2 Hz, 3H)
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ppm; C NMR (100 MHz, CDCl3) δ 171.8, 159.3, 142.1, 137.3, 131.0, 129.4, 128.1, 126.3,
125.8, 122.5, 121.4, 120.7, 114.1, 110.0, 109.3, 55.5, 53.4, 44.4, 33.4, 22.3, 11.8 ppm; HRMS
(ESI-FTICR) calcd for C23H24N2O2•Na 383.1730, found 383.1728.
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3-(3’,4’-Dimethoxyphenyl)-4-(1”-methylindol-3”-yl)-1-propyl-1H-pyrrol-2(5H)-one (5d).
Using General Method D, tetramic acid 8d (0.150 g, 0.541 mmol), N-methylindole (9a) (0.085
g, 0.65 mmol), and a reaction time of 1 h gave the title compound 5d as a yellow amorphous
solid (0.172 g, 0.440 mmol, 81% yield): mp 66-69 °C; Rf = 0.35 (2:1 EtOAc/petroleum ether);
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IR (ATR, neat) 1654, 1604 cm-1; H NMR (400 MHz, CDCl3) δ 7.30-7.33 (m, 1H), 7.18-7.25
(m, 2H), 7.08-7.14 (m, 3H), 7.02-7.06 (m, 1H), 6.83 (d, J = 9.2 Hz, 1H), 4.43 (s, 2H), 3.88 (s,
3H), 3.75 (s, 3H), 3.70 (s, 3H), 3.56 (t, J = 7.2 Hz, 2H), 1.73 (sext, J = 7.2 Hz, 2H), 1.00 (t, J =
13
7.2 Hz, 3H) ppm; C NMR (100 MHz, CDCl3) δ 171.7, 148.9, 148.8, 143.0, 137.3, 129.5,
128.0, 126.5, 125.7, 122.6, 122.5, 121.5, 120.8, 112.9, 111.4, 110.0, 109.3, 56.1, 56.0, 55.4, 44.4,
33.4, 22.3, 11.8 ppm; HRMS (ESI-FTICR) calcd for C24H26N2O3•Na 413.1836, found 413.1834.
3-(3’,4’,5’-Trimethoxyphenyl)-4-(1”-methylindol-3”-yl)-1-propyl-1H-pyrrol-2(5H)-one (5e).
Using General Method D, tetramic acid 8e (0.250 g, 0.813 mmol), N-methylindole (9a) (0.128
g, 0.976 mmol), and a reaction time of 1 h gave the title compound 5e as a white film (0.224 g,
0.533 mmol, 66% yield): mp 45-50 °C; Rf = 0.26 (2:1 EtOAc/petroleum ether); IR (ATR, neat)
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1660, 1620, 1578 cm-1; H NMR (400 MHz, CDCl3) δ 7.30-7.33 (m, 1H), 7.17-7.25 (m, 2H),
7.15 (s, 1H), 7.03-7.07 (m, 1H), 6.78 (s, 2H), 4.46 (s, 2H), 3.85 (s, 3H), 3.76 (s, 3H), 3.66 (s,
6H), 3.57 (t, J = 7.2 Hz, 2H), 1.73 (sext, J = 7.2 Hz, 2H), 1.01 (t, J = 7.2 Hz, 3H) ppm; 13C NMR
(100 MHz, CDCl3) δ 171.5, 153.4, 143.0, 137.8, 137.3, 129.7, 129.2, 127.8, 125.5, 122.7, 121.5,
120.9, 110.0, 109.0, 107.0, 61.1, 56.2, 53.3, 44.4, 33.5, 22.3, 11.7 ppm; HRMS (ESI-FTICR)
calcd for C25H28N2O4•Na 443.1941, found 443.1938.
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3-(3’,4’-Dimethoxyphenyl)-4-(indol-3”-yl)-1-propyl-1H-pyrrol-2(5H)-one (5h).
Using General Method D, tetramic acid 8d (0.500 g, 1.80 mmol), indole (9b) (0.634 g, 5.41
mmol), and a reaction time of 1.5 h gave the title compound 5h as a light yellow amorphous
solid (0.195 g, 0.518 mmol, 29% yield): mp 184-187 °C; Rf = 0.27 (2:1 EtOAc/petroleum ether);
IR (ATR, neat) 3351, 3230, 1731, 1647, 1603 cm-1; 1H NMR (400 MHz, D6-DMSO) δ 11.54 (br
s, 1H), 7.59 (d, J = 2.8 Hz, 1H), 7.39 (d, J = 8.4 Hz, 1H), 7.04-7.09 (m, 1H), 7.01 (dd, J = 2.0,
8.4 Hz, 1H), 6.81-6.93 (m, 4H), 4.49 (s, 2H), 3.74 (s, 3H), 3.43 (m, 2H), 3.33 (s, 3H), 1.64 (sext,
J = 7.2 Hz, 2H), 0.91 (t, J = 7.2 Hz, 3H) ppm; 13C NMR (100 MHz, D6-DMSO) δ 170.4, 148.02,
148.01, 143.2, 136.4, 126.4, 126.2, 126.0, 124.1, 121.8, 121.7, 120.8, 119.6, 113.0, 111.9, 111.4,
109.0, 55.4, 55.0, 52.5, 43.4, 21.4, 11.4 ppm; HRMS (ESI-FTICR) calcd for C23H24N2O3•Na
399.1679, found 399.1677.
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1-(tert-Butyl)-3-(3’,4’-dimethoxyphenyl)-4-(1”-methylindol-3”-yl)--1H-pyrrol-2(5H)-one
(5f).
Using a modification of General Method D (solvent = DCE; reaction temperature = 65 °C),
tetramic acid 8f (0.200 g, 0.686 mmol), N-methylindole (9a) (0.108 g, 0.824 mmol), and a
reaction time of 12 h gave the title compound as an off-white amorphous solid (46 mg, 0.11
mmol, 17% yield): mp 141-145 °C; Rf = 0.50 (2:1 EtOAc/petroleum ether); IR (ATR, neat)
1654, 1602 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.28-7.32 (m, 1H), 7.19-7.24 (m, 1H), 7.15-7.17
(m, 1H), 7.13 (s, 1H), 7.06-7.11 (m, 2H), 7.00-7.04 (m, 1H), 6.82 (d, J = 8.4 Hz, 1H), 4.47 (s,
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2H), 3.87 (s, 3H), 3.75 (s, 3H), 3.68 (s, 3H), 1.58 (s, 9H) ppm; C NMR (100 MHz, CDCl3) δ
171.9, 148.9, 148.7, 141.6, 137.3, 129.35, 129.30, 126.4, 125.6, 122.7, 122.5, 121.6, 120.7,
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113.0, 111.4, 109.9, 109.3, 56.14, 56.08, 54.3, 51.7, 33.4, 28.4 ppm; HRMS (ESI-FTICR) calcd
for C25H28N2O3•Na 427.1992, found 427.1989.
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3-(3’,4’-Dimethoxyphenyl)-4-(1”-methylindol-3”-yl)-1H-pyrrol-2(5H)-one (5j).
Using General Method D, tetramic acid 8f (0.500 g, 1.72 mmol), N-methylindole (9a) (0.270 g,
2.06 mmol), and a reaction time of 0.5 h gave the title compound 5j as a light yellow amorphous
solid (0.814 g, 0.528 mmol, 31% yield). Trituration (CH2Cl2/pentane) gave an analytical sample
of 5j as an off-white powder: mp 83-86 °C; Rf = 0.30 (1:9 MeOH/EtOAc); IR (ATR, neat) 3214,
1731, 1660, 1600 cm-1; 1H NMR (400 MHz, D6-DMSO) δ 8.23 (br s, 1H), 7.65 (s, 1H), 7.45 (d,
J = 8.4 Hz, 1H), 7.10-7.14 (m, 1H), 6.99 (dd, J = 2.0, 8.4 Hz, 1H), 6.93 (d, J = 2.0 Hz, 1H), 6.88
(d, J = 8.4 Hz, 1H), 6.82-6.86 (m, 1H), 6.77 (d, J = 8.0 Hz, 1H), 4.35 (d, J = 1.2 Hz, 2H), 3.79 (s,
3H), 3.74 (s, 3H), 3.42 (s, 3H) D ppm; 13C NMR (100 MHz, D6-DMSO) δ 173.3, 148.01, 147.95,
144.8, 137.0, 130.3, 126.3, 126.0, 124.4, 121.9, 121.7, 121.0, 119.7, 113.1, 111.4, 110.1, 108.5,
55.4, 55.1, 47.9, 32.8 ppm; HRMS (ESI-FTICR) calcd for C21H20N2O3•Na 371.1366, found
371.1366. In a subsequent experiment using General Method D, tetramic acid 8f (57 mg, 0.20
mmol), N-methylindole (9a) (131 mg, 1.00 mmol), in PhCl (5 mL) with heating at 80°C for 3h
followed by heating at 120°C for 1h, the title compound 5j (39. mg, 0.11 mmol ) was obtained in
55% yield.
3-(3,4-Dimethoxyphenyl)-4-hydroxy-1H-pyrrol-2(5H)-one (10).
Using General Method D, tetramic acid 8f (0.500 g, 1.72 mmol), N-methylindole (9a) (0.270 g,
2.06 mmol), and a reaction time of 0.5 h gave the title compound 10 (after elution of 5j) as a
white amorphous solid (0.164 g, 0.697 mmol, 40% yield): mp 234-237 (dec) °C; Rf = 0.25 (1:9
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MeOH/EtOAc); IR (ATR, neat) 3349, 1666, 1613, 1602, 1582 cm-1; H NMR (400 MHz,
CDCl3) δ 11.38 (br s, 1H), 7.70 (d, J = 2.0 Hz, 1H), 7.55 (dd, J = 2.0, 8.6 Hz, 1H), 7.41 (br s,
1H), 6.90 (d, J = 8.6 Hz, 1H), 3.83 (d, J = 0.8 Hz, 2H), 3.73 (s, 3H), 3.72 (s, 3H) ppm; 13C NMR
(100 MHz, CDCl3) δ 174.0, 167.4, 147.9, 146.7, 125.4, 119.5, 111.3, 110.7, 102.9, 55.4, 55.3,
44.7 ppm; HRMS (ESI-FTICR) calcd for C12H13NO4•Na 258.0737, found 258.0739.
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General Method E for the Oxidative Cyclization to Fused Carbazoles 11.
A modification of a literature procedure was followed.11c A mixture of 3-pyrrolin-2-one 5 (0.20
mmol) and phenyliodine(III) bis(trifluoroacetate) (PIFA) (95 mg, 0.22 mmol) in CH2Cl2 (10 mL)
in 20 mL vial with a septum-style cap was cooled to -40 °C using an external cooling bath
(acetonitrile/dry ice). To the cooled reaction mixture was added BF3•Et2O dropwise via syringe.
The reaction mixture was stirred at -40 °C for 3.5-4 h (as noted). The bulk of the solvent was
then removed and the crude residue was treated with EtOH (10 mL) and the solution was
transferred to a centrifuge tube and placed in a freezer (-20 °C) until a product precipitated.
Centrifugation, decantation of the solvent, and drying in vacuo gave the desired products as
colored powders.
1,2-Dihydro-5,6-dimethoxy-8-methyl-2-propylbenzo[a]pyrrolo[3,4-c]carbazol-3(8H)-one
(11d).
Using General Method E, 3-pyrrolin-2-one 5d (0.100 g, 0.256 mmol), and a reaction time of
3.5 h gave title compound 11d as a pink powder (54.6 mg, 0.141 mmol, 55% yield): mp 205-210
°C; Rf = 0.63 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1661, 1629, 1578 cm-1; H NMR
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(400 MHz, CDCl3) δ 8.88 (s, 1H), 8.15 (s, 1H), 8.00 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 8.4 Hz,
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1H), 7.51-7.55 (m, 1H), 7.32-7.36 (m, 1H), 4.87 (s, 2H), 4.43 (s, 3H), 4.04 (s, 3H), 3.95 (s, 3H),
3.59 (t, J = 7.2 Hz, 2H), 1.75 (sext, J = 7.2 Hz, 2H), 0.94 (t, J = 7.2 Hz 3H) ppm; 13C NMR (100
MHz, CDCl3) δ 169.3, 148.9, 147.9, 140.6, 136.7, 136.2, 124.7, 124.4, 121.1, 120.7, 120.1,
116.7, 116.3, 111.7, 110.0, 103.5, 103.4, 55.3, 55.2, 48.5, 43.3, 33.6, 21.4, 11.4 ppm; HRMS
(ESI-FTICR) calcd for C24H24N2O3•Na 411.1679, found 411.1679.
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1,2-Dihydro-5,6-dimethoxy-2-propylbenzo[a]pyrrolo[3,4-c]carbazol-3(8H)-one (11h).
Using General Method E, 3-pyrrolin-2-one 5h (0.116 g, 0.310 mmol), and a reaction time of 4
h gave title compound 11h as a light brown powder (49.9 mg, 0.133 mmol, 43% yield): mp 179-
184 °C; Rf = 0.52 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 3302, 1630, 1584 cm-1; 1H NMR
(400 MHz, CDCl3) δ 12.36 (br s, 1H), 8.74 (s, 1H), 8.07 (s, 1H), 8.03 (d, J = 7.6 Hz, 1H), 7.69
(d, J = 8.0 Hz, 1H), 7.43-7.48 (m, 1H), 7.27-7.32 (m, 1H), 4.96 (s, 2H), 4.01 (s, 3H), 3.94 (s,
3H), 3.61 (t, J = 7.2 Hz, 2H), 1.75 (sext, J = 7.2 Hz, 2H), 0.94 (t, J = 7.2 Hz, 3H) ppm; 13C NMR
(100 MHz, CDCl3) δ 169.5, 149.5, 148.6, 138.9, 137.4, 136.5, 124.7, 123.2, 122.3, 120.9, 119.9,
116.5, 115.5, 111.4, 111.0, 103.5, 102.6, 55.6, 55.3, 48.6, 43.3, 21.4, 11.4 ppm; HRMS (ESI-
FTICR) calcd for C23H22N2O3•Na 397.1523, found 397.1523.
1,2-Dihydro-5,6-dimethoxy-8-methylbenzo[a]pyrrolo[3,4-c]carbazol-3(8H)-one (11j).
Using General Method E, 3-pyrrolin-2-one 5j (59 mg, 0.17 mmol), and a reaction time of 3.5 h
gave title compound 11j as a brown powder (49 mg, 0.14 mmol, 83% yield): mp >300 °C; Rf =
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0.35 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 3382, 1663, 1630, 1579 cm-1; H NMR (400
MHz, CDCl3) δ 8.88 (s, 1H), 8.53 (br s, 1H), 8.20 (s, 1H), 7.99 (d, J = 7.6 Hz, 1H), 7.85 (d, J =
8.4 Hz, 1H), 7.50-7.55 (m, 1H), 7.31-7.35 (m, 1H), 4.83 (s, 2H), 4.47 (s, 3H), 4.05 (s, 3H), 3.94
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(s, 3H) ppm; 13C NMR (100 MHz, D6-DMSO) δ 172.3, 148.8, 147.8, 140.5, 138.6, 136.8, 124.6,
121.2, 120.6, 120.0, 116.6, 116.2, 111.8, 109.9, 103.5, 103.2, 55.24, 55.21, 43.9, 33.5 (missing
one peak) ppm; HRMS (ESI-FTICR) calcd for C21H18N2O3•Na 369.1210, found 369.1210.
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Ethyl 2-(2-(1-Methyl-1H-indol-3-yl)-N-propylacetamido)acetate (i).
To a rt stirred mixture of commercially available N-methylindole-3-acetic acid (3.00 g, 15.9
mmol) and DCC (3.94 g, 19.1 mmol) in CH2Cl2 was added amine 6a (2.53 g, 17.4 mmol) and
DMAP (0.194 g, 1.59 mmol). The reaction mixture was stirred at rt for 2.5 h and then the
precipitate which formed was removed by filtration (twice). The solution was treated with an
aqueous solution of KHSO4 (1.0 M, 100 mL) and the organic layer was separated. The aqueous
layer was re-extracted with CH2Cl2 (100 mL). The combined organic layers were washed with
brine (200 mL) and dried over Na2SO4. Removal of the solvent in vacuo gave a crude brown oil
(5.4 g) which was purified by flash chromatography (EtOAc/petroleum ether gradient).
Fractions containing product were filtered (traces of DCU) and the solvent was removed in
vacuo to give the title compound i as a pink oil (2.99 g, 9.45 mmol, 60% yield). Upon standing
at rt for a week, the oil solidified into a light pink powder: mp 47-49 °C; Rf = 0.47 (1:1
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EtOAc/petroleum ether); IR (ATR, neat) 1745, 1636 cm-1; H NMR (400 MHz, CDCl3) δ
(mixture of rotamers) 7.55-7.58 (m, 1H), 7.27-7.31 (m, 1H), 7.21-7.25 (m, 1H), 7.09-7.14 (m,
1H), 6.97 and 7.14 (s, 1H), 4.10-4.26 (m, 2H), 4.03 and 4.09 (s, 2H), 3.86 (d, J = 0.8 Hz, 2H),
3.75 and 3.77 (s, 3H), 3.31-3.35 (m, 2H), 1.51-1.56 (m, 2H), 1.20-1.30 (m, 3H), 0.84-0.89 (m,
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3H) ppm; C NMR (100 MHz, CDCl3) δ (major rotamer) 172.3, 169.80, 137.1, 127.9, 127.7,
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121.9, 119.2, 118.8, 109.5, 107.7, 61.3, 51.5, 47.9, 33.00, 30.8, 22.3, 14.4, 11.4 ppm; C NMR
(100 MHz, CDCl3) δ (minor rotamer) 172.0, 169.83, 127.8, 127.6, 122.0, 119.4, 119.0, 110.3,
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109.4, 107.4, 61.7, 50.4, 49.5, 32.97, 31.7, 20.9, 14.3, 11.6 ppm HRMS (ESI-FTICR) calcd for
C18H24N2O3•Na 339.1679, found 339.1678.
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1-(tert-Butyl)-4-hydroxy-3-(1’-methylindol-3’-yl)-1H-pyrrol-2(5H)-one (8g).
Using General Method C, amide i (3.46 g, 10.9 mmol), and a reaction time of 2 h gave the title
compound 8g as a white powder (2.445 g, 9.045 mmol, 83% yield): mp 220-228 °C; Rf = 0.31
(2:1 EtOAc/petroleum ether); IR (ATR, neat) 1682, 1595 cm-1; 1H NMR (400 MHz, D6-DMSO)
δ 10.94 (br s, 1H), 7.95-7.98 (m, 1H), 7.62 (s, 1H), 7.36-7.39 (m, 1H), 7.10-7.14 (m, 1H), 6.97-
7.01 (m, 1H), 3.95 (s, 2H), 3.78 (s, 3H), 3.31 (t, J = 7.2 Hz, 2H), 1.53 (sext, J = 7.2 Hz, 2H), 0.87
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(t, J = 7.2 Hz, 3H) ppm; C NMR (100 MHz, D6-DMSO) δ 171.5, 162.1, 136.1, 128.0, 126.4,
122.3, 120.9, 118.2, 109.2, 105.5, 100.7, 49.2, 42.8, 32.4, 21.4, 11.3 ppm; HRMS (ESI-FTICR)
calcd for C16H18N2O2•Na 293.1266, found 293.1261.
3,4-Bis(1’-methylindol-3’-yl)-1-propyl-1H-pyrrol-2(5H)-one (5g).
Using General Method D, tetramic acid 8g (0.500 g, 1.85 mmol), N-methylindole (9a) (0.291 g,
2.22 mmol), and a reaction time of 0.5 h gave the title compound 5g as an off-white film (0.307
g, 0.801 mmol, 43% yield). Trituration (CH2Cl2/pentane) gave an analytical sample of 5g as an
off-white powder: mp 195-199 °C; Rf = 0.44 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1670,
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1630, 1608 cm-1; H NMR (400 MHz, CDCl3) δ 7.62-7.64 (m, 1H), 7.62 (s, 1H), 7.31-7.34 (m,
2H), 7.23-7.28 (m, 1H), 7.12-7.17 (m, 2H), 7.00 (s, 1H), 6.96-6.98 (m, 1H), 6.82-6.86 (m, 1H),
4.63 (s, 2H), 3.87 (s, 3H), 3.61 (t, J = 7.2 Hz, 2H), 3.57 (s, 3H), 1.76 (sext, J = 7.2 Hz, 2H), 1.01
(t, J = 7.2 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ 172.4, 140.2, 137.3, 137.0, 131.2, 130.9,
126.5, 125.4, 122.4, 122.3, 121.5, 120.9, 120.7, 119.1, 110.1, 109.53, 109.50, 107.4, 53.5, 44.6,
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406.1890, found 406.1889.
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12,13-Dimethyl-6,7,12,13-tetrahydro-6-propyl-5H-indolo[2,3-a]pyrrolo[3,4-c]carbazol-5-
one (11g).
Using General Method E, 3-pyrrolin-2-one 5g (50. mg, 0.13 mmol), and a reaction time of 4 h
gave title compound 11g as a light brown powder (34 mg, 0.89 mmol, 69% yield): mp 258-260
1
°C; Rf = 0.73 (2:1 EtOAc/petroleum ether); IR (ATR, neat) 1661, 1618, 1589 cm-1; H NMR
(400 MHz, D6-DMSO) δ 9.43 (d, J = 7.6 Hz, 1H), 8.10 (d, J = 7.6 Hz, 1H), 7.79 (d, J = 8.4 Hz,
1H), 7.71 (d, J = 8.4 Hz, 1H), 7.57-7.61 (m, 1H), 7.52-7.56 (m, 1H), 7.37-7.41 (m, 1H), 7.27-
7.31 (m, 1H), 5.06 (s, 2H), 4.29 (s, 3H), 4.25 (s, 3H), 3.65 (t, J = 7.2 Hz, 2H), 1.79 (sext, J = 7.2
13
Hz, 2H), 0.96 (t, J = 7.2 Hz, 3H) ppm; C NMR (100 MHz, CDCl3) δ 170.3, 144.3, 144.0,
131.7, 130.6, 129.8, 126.7, 126.3, 125.9, 124.2, 123.8, 121.1, 121.0, 120.8, 120.4, 119.9, 116.9,
110.7, 109.9, 50.3, 44.7, 37.1, 36.9, 22.4, 11.8 ppm; HRMS (ESI-FTICR) calcd for
C25H23N3O•Na 404.1733, found 404.1733.
ASSOCIATED CONTENT
Supporting Information
1H NMR spectra and 13C NMR spectra for all synthesized compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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*E-mail: pelkey@hws.edu
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
Financial support of this research from the NSF (RUI: 1362813), Hobart and William Smith
Colleges, Drs. Carey and Cohen (MHK), Dr. Edward Franks, and the Patchett Family is
gratefully acknowledged.
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