Synthesis of a Masked 2,3-Diaminoindole

Article abstract of DOI:10.1021/acs.joc.6b02318

A three-step synthesis of masked 2,3-diaminoindole 1 from 2-iodo-3-nitro-1-(phenylsulfonyl)indole (2) has been developed. Treatment of 1 with trifluoroacetic acid generates the unstable 2,3-diamino-1-(phenylsulfonyl)indole (3), which can be trapped with α-dicarbonyl compounds to afford 5H-pyrazino[2,3-b]indoles 7-10.

Full text of DOI:10.1021/acs.joc.6b02318

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Synthesis of a Masked 2,3-Diaminoindole
Philip Z Mannes, Evans O Onyango, and Gordon Wayne Gribble
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02318 • Publication Date (Web): 07 Nov 2016
Downloaded from http://pubs.acs.org on November 11, 2016
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Synthesis of a Masked 2,3-Diaminoindole
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Philip Z. Mannes, Evans O. Onyango, and Gordon W. Gribble*
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Department of Chemistry, Dartmouth College, Hanover, NH 03755-3564, USA
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ABSTRACT
O
R
R
R
O
O
NO2
NH2
NH2
HN
N
N
O
R
TFA
3 steps
N
I
NH2
N
N
4 examples
(27-58%)
N
1
SO2Ph
SO2Ph
SO2Ph
SO2Ph
7-10
2
3
Abstract:
A three-step synthesis of masked 2,3-diaminoindole 1 from 2-iodo-3-nitro-1-
(phenylsulfonyl)indole (2) has been developed. Treatment of 1 with trifluoroacetic acid generates the
unstable 2,3-diamino-1-(phenylsulfonyl)indole (3), which can be trapped with α-dicarbonyl compounds to
afford 5H-pyrazino[2,3-b]indoles 7–10.
Although functionalized 3- and 2-aminoindoles are well established in synthesis (e.g., 3-
acylaminoindoles, 2-amino-3-cyanoindoles) and several synthetic routes are known,1,2 the parent 3- and 2-
aminoindoles are labile and only rarely have been isolated.3 In contrast to the situation with 3- and 2-
aminoindoles, 2,3-diaminoindoles, functionalized or not, are virtually unknown compounds.4 In an
extension of our study of the synthesis and chemistry of 3- and 2-nitroindoles1d,2e,5, we now describe the
synthesis of a masked N-protected 2,3-diaminoindole that can serve, for example, as a precursor to 5H-
pyrazino[2,3-b]indoles, imidazo[4,5-b]indoles, 6H-indolo[2,3-b]quinoxalines, and similarly fused indoles
that exhibit cytotoxicity, anti-viral, antibacterial, and other biological activities (Figure 1).6
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N
4
5
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7
N
N
N
N
N
H
H
8
9
6H-Indolo[2,3-b]quinoxaline
5H-pyrazino[2,3-b]indole
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H
S
N
N
N
NH
NH
N
H
N
H
[1,2,3]triazolo[4,5-b]indole
Imidazo[4,5-b]indole
Figure 1. Representative compounds bearing the 2,3–diaminoindole scaffold
We envisioned a route to a 2,3-diaminoindole, such as 2,3-diamino-1-(phenylsulfonyl)indole (3),
from indole via 2-amino-3-nitro-1-(phenylsulfonyl)indole (4) as depicted in Scheme 1.
Scheme 1. Synthesis of 4 via CuI catalyzed amination of 2-iodo-3-nitroindole 2
NH3 (aq)
10% mol CuI
20% mol L
NO2
NO2
NH2
SO2Ph
3 steps (34%)
ref 2m
I
N
N
N
K2CO3 , DMSO
12 - 25%
H
SO2Ph
4
2
L = L-proline or 4-hydroxy-L-proline
As we described earlier,2e indole was converted to 2-iodo-3-nitro-1-(phenylsulfonyl)indole (2) by a
sequence of N-phenylsulfonylation, C2-iodination, and C-3 nitration (34% overall yield) (Scheme 1). We
initially attempted to aminate 2 with ammonium salts7 and aqueous ammonia8 as the nitrogen source and
with CuI as the catalyst. However, under various conditions only 25% of the desired 2-amino-3-nitroindole
4 was obtained. We also attempted a direct amination of 2 using a palladium-catalyzed coupling of
ammonia (Pd[(P(o-tol)3)]2, CyPF-t-Bu, NaO-t-Bu, 0.5M ammonia solution in 1,4-dioxane, 50–80 C).9
o
Under these conditions, we isolated a mixture of unidentified products, without any evidence of 4. In
contrast, treatment of 2 with tert-butylamine under our standard conditions2e gave 5, which was further
exposed to HCl to induce cleavage of the tert-butyl group (Scheme 2).10 This gave 4 in 47% yield for the
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two steps. The attempted amination of 2 with p-methoxybenzylamine only resulted in removal of the
5
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phenylsulfonyl group.
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Scheme 2. Synthesis of 4 via SNAr displacement of 2-iodo-3-nitroindole 2
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NO2
NO2
NO2
NH2
SO2Ph
tert-butylamine,
MeOH, 45 oC
HCl, EtOH
H
I
N
N
N
N
70 oC
SO2Ph
SO2Ph
4
5
47% (2 steps)
2
With the desired 2-amino-3-nitroindole 4 in hand, the stage was set for the reduction of the nitro
group. Classical metal based methods 11 and metal catalytic hydrogenation 12 methods only returned the
starting material and in some cases formation of intractable mixtures (Table 1). We ultimately achieved the
reduction of the nitro group by employing a combination of indium/Boc2O/AcOH in MeOH, which we
used previously to reduce 3-nitroindoles.1d Given the known instability of 2-and 3-aminoindoles, and the
inherent advantage of indium as a reducing agent,13 the success of this reaction depended on the in situ
trapping of the amino group as it formed.1d, 14 No reduction product was isolated in the absence of the Boc
anhydride.
Although we initially assigned the crude product of this reaction as the desired 2,3-
diaminoindole 3 based on HRMS analysis, 15 we overlooked a broader and less pronounced tert-butyl H
1
NMR peak. 16-19 Thus, after further spectral analysis and high temperature H NMR experiments,20 the
1
structure of the product resulting from treatment of the 4 with indium/Boc2O/AcOH was assigned as 1
isolated in 82% yield (Scheme 3). Despite using excess Boc anhydride, both H and 13C NMR strongly
1
supported the presence of a single Boc group (no 6 was formed). The regioselectivity of this reaction can
be attributed to two factors: (i) the C-3 amine is probably inherently more reactive than the C-2 amine due
to the inductive effect of the SO2Ph group, and (ii) steric interaction from the SO2Ph group makes the C-2
amine less accessible. A NOESY NMR analysis of 1 showed correlations between the C-3 tert-
butylcarboxamide proton and that of the C-4 indole ring, as well as between the C-2 amino group and the
aromatic protons of the N-phenylsulfonyl group. We also attempted the indium reduction in the presence
of acetic anhydride, but an intractable mixture of polar compounds was isolated.
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Scheme 3. Synthesis of 1 via indium mediated reduction of 4
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9
O
NO2
NH2
SO2Ph
NHBoc
NHBoc
SO2Ph
(not formed)
NH
O
In, Boc2O (excess)
NH2
SO2Ph
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AcOH, MeOH, 45 oC
82%
N
N
N
6
4
1
Table 1. Reaction conditions for the reduction of the C-3 nitro group of 4.
Conditions
Boc2O
Time
Yield of 1 (%)
H2/Pd-C/EtOH/25 oC
H2/Pd-C/MeOH/60 oC
Fe/EtOH/AcOH/70 oC
Fe/EtOH/AcOH/45 oC
Sa/NH4Cl/I2(cat)/THF/25 oC
Sa/NH4Cl/I2(cat)/THF/45 oC
SnCl2/EtOH/reflux
no
overnight
overnight
overnight
overnight
48 hours
48 hours
overnight
5 hours
0a
yes
no
16
0a,b
0a
yes
no
0a
yes
no
0a
0a,b
0a
SnCl2EtOH/45 oC
yes
no
NH2NH2/Pd-C/MeOH/80 oC
In/AcOH/MeOH/45 oC
In/AcOH/MeOH/45 oC
5 mins
0a,b
82
0b
yes
no
10 hours
4 hours
astarting material recovered
bunidentified compound isolated
When the Boc protected aminoindole 1 was treated with trifluoroacetic acid (TFA), TLC analysis
showed complete consumption of the starting material and the formation of a new polar compound.
However, work-up gave a yellow residue that quickly turned into black tar (1H NMR analysis showed an
intractable mixture). In a separate experiment, when 1 was treated with TFA and 2,3–butanedione was
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added after 15 min (when TLC analysis showed no remaining 1), the pyrazino[2,3-b]indole 7 was formed
in 34% yield (Scheme 4). Similarly, pyrazino[2,3-b]indoles 8 and 9 were obtained by trapping
diaminoindole 3 with glyoxal and benzil, respectively. Condensation of 3 with 1-phenyl-1,2-propanedione
gave separable pyrazino[2,3-b]indoles 10a and 10b in 35 and 23% yields, respectively. Based on the
product ratio, the more nucleophilic C-3 indole amino group reacts with the more reactive ketone carbonyl
of 1-phenyl-1,2-propanedione. The isomers were fully characterized and distinguished by X-ray
crystallography of the major isomer 10a (cf. Supporting Information). Attempted condensation of in situ
generated 3 with 2-oxopropanal and 2-oxophenylacetaldehyde gave low yields of inseparable isomeric
mixtures.
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Scheme 4. Condensation of diamine 3 with various α-dicarbonyl compounds
R'
O
O
R'
NHBoc
NH2
SO2Ph
NH2
NH2
SO2Ph
N
TFA
R
R
N
N
N
CH2Cl2
N
SO2Ph
1
3
7 (34%), R =Me, R' = Me
8 (52%), R = H, R' = H
9 (27%), R = Ph, R' = Ph
10a (35%), R = Ph, R' =Me
10b (23%), R = Me, R' = Ph
Cleavage of the SO2Ph group in pyrazino[2,3-b]indole 8 with K2CO3 in MeOH/H2O afforded the
known pyrazino[2,3-b]indole (11) in 91% yield (Scheme 5). 21d
Scheme 5. Cleavage of the N–phenylsulfonyl protecting group to afford 11
N
N
K2CO3
N
N
N
MeOH/H2O, reflux
91%
N
H
SO2Ph
11
8
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In conclusion, we have synthesized the masked 2,3-diaminoindole 1 and demonstrated that it can
be converted in situ to 2,3-diaminoindole 3 with TFA and subsequently trapped with α-dicarbonyl
compounds to generate 5H-pyrazino[2,3-b]indoles. Our general method complements the well-known
syntheses of 5H-pyrazino[2,3-b]indoles and related fused indoles via the condensation of isatins with
phenylene diamines.21
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Experimental Section
General Experimental Methods
All reactions were performed in the appropriate oven–dried glass apparatus under a balloon of nitrogen gas
(N2). Solvents were reagent grade and in most cases rigorously dried before use. Methylene chloride and
THF were dried and stored over molecular sieves.
Furan was distilled and dried from potassium
hydroxide. All reagents were obtained commercially as reagent grade and, unless otherwise noted, used
without further purification. The organolithium reagents were titrated with N–benzylamide prior to use.
All glassware utilized for Diels–Alder reactions was flame–dried. Reactions were monitored by TLC silica
gel plates (0.25 mm) and visualized by UV light or p–anisaldehyde. Column chromatography was
performed using silica gel (60, particle size 40–60 mm). Additionally, flash column chromatography was
performed on Biotage® Automated Liquid Chromatography System Isolera One® using Biotage® SNAP
Ultra 25 um HP-Sphere 10 g silica gel cartridges. The organic extracts were dried over anhydrous MgSO4.
Proton (1H), carbon (13C), and fluorine (19F) nuclear magnetic resonance spectra were recorded at 500 or
600, 150, or 565 MHz, respectively. Chemical shifts are reported in parts per million (ppm, δ) with the
residual deuterated solvent as an internal standard (7.26 ppm for chloroform, 5.32 ppm for methylene
chloride, 2.50 for dimethylsulfoxide). Splitting patterns are designated as follows: s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet. High–resolution mass spectra (HRMS) were obtained employing electron
ionization (EI) or electrospray (ES), with TOF as the mass analyzer. Infrared (IR) spectroscopy was
conducted using Fourier Transform Infrared spectrometers using the cast film procedure.
Procedures:
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N–(tert–Butyl)–3–nitro–1–(phenylsulfonyl)–1H–indol–2–amine (5). A stirred suspension of 2 (284 mg,
0.67 mmol) in MeOH (4.5 mL) at 0 oC was treated dropwise with tert–butylamine (0.210 mL, 2.01 mmol),
and subsequently warmed to 38 oC for 7 h. The reaction was then loaded directly onto silica gel (SiO2, ~0.5
g). Dry–pack purification (10:1 hexanes/ethyl acetate) afforded 5 as a bright yellow solid (124 mg, 50%).
Recrystallization from hexanes/ethyl acetate gave 5 as a yellow crystals, Rf = 0.54 (3:1 hexanes/ethyl
acetate); mp dec 148 oC; 1H NMR (600 MHz, CD2Cl2) δ 9.20 (s br, 1H), 7.99–7.98 (m, 1H), 7.84–7.83 (m,
1H), 7.48–7.47 (m, 3H), 7.30–7.27 (m, 4H), 1.58 (s, 9H); 13C NMR (150 MHz, CD2Cl2) δ 151.8, 134.8,
134.6, 131.9, 129.3, 128.8, 127.2, 126.8, 124.9, 124.3, 119.5, 117.7, 62.2, 30.80; UV (EtOH) λmax 201, 203,
206, 215, 217, 262, 268, 273 nm; HRMS (ES) calcd for C18H20N3O4S 374.1175, [M+1]+ found 374.1175.
3–Nitro–1–(phenylsulfonyl)–1H–indol–2–amine (4). Concentrated HCl (2 mL) was added dropwise to a
stirred suspension of 5 (108 mg, 0.29 mmol) in EtOH (3 mL) and heated to 70 oC overnight. After cooling
to rt, the reaction was diluted with ethyl acetate and then the pH was adjusted to 7 with a saturated solution
of NaHCO3. The organic layer was separated, washed with brine, and dried over MgSO4. Concentration of
the solvent in vacuo gave 4 as a dark–yellow solid (86 mg, 94%). Recrystallization of 4 from ethanol/ethyl
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o
1
acetate gave 4 as a yellow crystals, Rf = 0.59 (1:1 hexanes/ethyl acetate); mp 198–199 C; H NMR (600
MHz, CD2Cl2) δ 8.02 (d, J = 7.8 Hz, 1H), 7.97 (d, J = 8.5, 2H), 7.90 (d, J = 8.2 Hz, 1H), 7.69 (t, J = 7.5 Hz,
1H), 7.55 (t, J = 7.8 Hz, 2H), 7.33 (t, J = 7.8 Hz, 1H), 7.26 (t, J = 8.4 Hz, 1H); 13C NMR (150 MHz,
CD2Cl2) δ 148.9, 136.4, 135.6, 129.9, 129.4, 127.1, 126.0, 124.6, 122.1, 119.3, 113.2; IR (KBr) 3442,
3311, 1630 cm-1; UV (EtOH) λmax 202, 204, 219, 238, 267, 275 nm; HRMS (ES) calcd for C14H12N3O4S
318.0549, [M+1]+ found 318.0550.
tert–Butyl (2–amino–1–(phenylsulfonyl)–1H–indol–3–yl)carbamate (1). To a stirred suspension of 4
(889 mg, 2.8 mmol), AcOH (1.6 mL, 28 mmol), and di–tert–butyl dicarbonate (3.05 g, 14 mmol) in MeOH
o
(17 mL) at rt was added indium metal (1.6 g, 14 mmol). The reaction was stirred for 10 h at 45 C. After
cooling to rt, the opaque solution was filtered through Celite and the filtrate was concentrated in vacuo. The
resulting residue was with diluted with ethyl acetate and washed saturated NaHCO3 to remove the acetic
acid. The organic layer was washed with brine, dried over MgSO4, and concentrated in vacuo. Column
purification (5:1 hexanes/ethyl acetate) gave 1 as a light brown/orange solid (885 mg, 82%). The
1
compound was resistant to crystallization, Rf = 0.26 (3:1 hexanes/ethyl acetate); H NMR (600 MHz,
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CDCl3) δ 7.94 (d, J = 8.2 Hz, 1H), 7.87 (d, J = 7.8 Hz, 2H), 7.54 (t, J = 7.2 Hz, 1H), 7.41 (t, J = 7.9 Hz,
2H), 7.18–7.11 (m, 1H), 7.11–7.03 (m, 2H), 1.51 (s, 9H); 13C NMR (150 MHz, CDCl3) δ 153.7, 137.7,
136.6, 134.0, 130.6, 129.2, 127.8, 126.8, 124.2, 121.5, 114.5, 114.1, 97.9, 80.9, 28.3; IR (KBr) 3372, 2977,
1699, 1651, 1459, 1365 cm-1; HRMS (ES) calcd for C19H21N3O4S 387.1253, [M]+ found 387.1260.
2,3–Dimethyl–5–(phenylsulfonyl)–5H–pyrazino[2,3–b]indole (7). Trifluoroacetic acetic acid (0.15mL)
was added dropwise to a stirred solution of 1 (20 mg, 0.052 mmol) in methylene chloride (1 mL) under
nitrogen. The reaction proceeded for 15 min and then 2,3–butanedione (0.005 mL, 0.052 mmol) was added
dropwise. After 30 min, the reaction was diluted with ethyl acetate and poured onto a saturated solution of
NaHCO3 (~10 mL). The organic layer was then washed with brine, dried over MgSO4, and concentrated in
vacuo. Column purification (5:1 hexanes/ethyl acetate) gave 7 as an off–white solid (6 mg, 34%); 1H NMR
(600 MHz, CDCl3) δ 8.48 (d, J = 8.5 Hz, 1H), 8.19 (d, J = 7.7 Hz, 1H), 8.14 (d, J = 7.6 Hz, 2H), 7.63 (t, J =
7.2 Hz, 1H), 7.53 (t, J = 7.5 Hz, 1H), 1H), 7.47–7.37, (m, 3H), 2.70 (s, 3H), 2.65 (s, 3H); 13C NMR (150
MHz, CDCl3) δ 149.1, 148.5, 143.3, 138.6, 138.5, 134.8, 134.1, 129.5, 128.9, 127.7, 124.3, 122.6, 120.8,
114.9, 22.9, 22.1; HRMS (EI) calcd for C18H16N3O2S 338.0964, [M+1]+ found 338.0963.
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5-(Phenylsulfonyl)-5H-pyrazino[2,3-b]indole (8). Trifluoroacetic acid (0.30 mL) was added dropwise to
a stirred solution of 1 (40 mg, 0.10 mmol) in methylene chloride (2 mL) under nitrogen. The reaction
proceeded for 45 min and then glyoxal (40% in water, 0.010 mL) was added dropwise. After 20 min, the
reaction was diluted with ethyl acetate and poured onto a saturated solution of NaHCO3 (~10 mL). The
organic layer was then washed with brine, dried over MgSO4, and concentrated in vacuo. Column
1
purification (3:1 hexanes/ethyl acetate) gave 8 a light yellow solid (16 mg, 52%); H NMR (600 MHz,
CDCl3) δ 8.58 (d, J = 2.4 Hz, 1H), 8.52 (d, J = 8.4 Hz, 1H), 8.49 (d, J = 2.4 Hz, 1H), 8.26 (d, J = 7.8 Hz,
1H), 8.14 (d, J = 7.8 Hz, 2H), 7.72–7.69 (m, 1H), 7.55 (t, J = 7.8 Hz, 1H), 7.51 (t, J = 7.2 Hz, 1H), 7.44 (t,
J = 7.2 Hz, 2H); 13C NMR (150 MHz, CDCl3) δ 145.2, 140.5, 140.3, 139.1, 138.4, 134.4, 130.8, 129.2,
127.5, 124.7, 122.2, 121.6, 114.9; HRMS (EI) calcd for C16H11N3O2S 309.0572, [M]+ found 309.0573.
2,3-Diphenyl-5-(phenylsulfonyl)-5H-pyrazino[2,3-b]indole (9). Trifluoroacetic acid (0.018 mL, 0.23
mmol) was added slowly to a stirred solution of 1 (30 mg, 0.078 mmol) and benzil (18 mg, 0.086 mmol) in
methylene chloride (2 mL) under nitrogen. After 16 h, the reaction was diluted with ethyl acetate and
poured onto a saturated solution of NaHCO3 (~5 mL). The organic layer was then washed with brine,
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dried over MgSO4, and concentrated in vacuo. Purification by flash column chromatography on a Biotage®
SNAP HP Sphere 10 g cartridge with 94:6 to 50:50 (hexanes-EtOAc) gradient elution afforded 9 as a white
solid (9 mg, 27%); 1H NMR (600 MHz, CDCl3) δ 8.55 (d, J = 8.5 Hz, 1H), 8.31 (d, J = 7.5 Hz, 1H), 8.25–
8.21 (m, 2H), 7.71-7.68 (m, 1H), 7.56 (t, J = 7.5 Hz, 1H), 7.52–7.42 (m, 7H), 7.36–7.30 (m, 6H); 13C NMR
(150 MHz, CDCl3) δ 149.4, 148.8, 143.4, 139.7, 139.3, 138.8, 138.6, 135.7, 134.3, 130.4, 130.3, 130.0,
129.0, 128.6, 128.4, 128.3, 128.1, 124.4, 122.3, 121.7, 114.9; HRMS (EI) calcd for C28H19N3O2S 461.1198,
[M]+ found 461.1203.
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2-Methyl-3-phenyl-5-(phenylsulfonyl)-5H-pyrazino[2,3-b]indole (10a) and 3-Methyl-2-phenyl-5-
(phenylsulfonyl)-5H-pyrazino[2,3-b]indole (10b). Trifluoroacetic acid (0.036 mL) was dropwise added
to a stirring solution of 1 (60 mg, 0.156 mmol) and 1-phenyl-1,2-propanedione (46 mg, 0.312 mmol) in
methylene chloride (6 mL) under nitrogen. After 16 h, the reaction was diluted with ethyl acetate and
poured onto a saturated solution of NaHCO3 (~5 mL). The organic layer was then washed with brine, dried
over MgSO4, and concentrated in vacuo. Purification by column chromatography (15:1 hexanes/ethyl
acetate) afforded 10a as a white solid (22 mg, 35%) and 10b as a white solid (14 mg, 23%).
1
Recrystallization from methylene chloride gave 10a as white crystals, mp 193–194 oC. 10a: H NMR (600
MHz, CDCl3) δ 8.46 (dt, J = 8.5 Hz, 0.9 Hz, 1H), 8.21 (d, J = 7.5 Hz, 1H), 8.12–8.07 (m, 2H), 7.63-7.55
(m, 3H), 7.54-7.38 (m, 5H), 7.38–7.32 (m, 2H), 2.70 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 150.0, 147.6,
143.2, 139.4, 138.9, 138.8, 138.6, 135.5, 134.2, 130.1, 129.6, 128.9, 128.7, 128.3, 128.0, 124.4, 121.3,
1
114.9, 23.5; HRMS (EI) calcd for C23H18N3O2S 400.1120, [M+1]+ found 400.1116. (10b): H NMR (600
MHz, CDCl3) δ 8.44 (dt, J = 8.5 Hz, 0.8 Hz, 1H), 8.36-8.04 (m, 3H), 8.12–8.07 (m, 2H), 7.63-7.30 (m, 8H),
2.69 (s, 3H); 13C NMR (150 MHz, CDCl3) δ 150.2, 148.4, 143.5, 139.0, 138.6, 135.2, 134.2, 130.0, 134.2,
129.9, 128.9, 129.9, 129.0, 128.9, 128.6, 128.5, 128.3, 128.0, 124.4, 121.4, 114.9, 24.0; HRMS (EI) calcd
for C23H18N3O2S 400.1120, [M+1]+ found 400.1115.
5H-Pyrazino[2,3-b]indole (11). To a stirring solution of 8 (6 mg, 0.019 mmol) in MeOH/H2O (1 mL :
0.300 mL) was added K2CO3 (36 mg, 0.26 mmol). The reaction was then heated at reflux for 30 min.
After cooling to rt, the reaction was diluted with ethyl acetate and poured onto brine. The organic layer
was separated and dried over MgSO4 and subsequently concentrated in vacuo to give 11 as a yellow solid
(3 mg, 91%); 1H NMR (600 MHz, DMSO) δ 12.14 (br s, 1H), 8.50 (d, J = 2.7 Hz, 1H), 8.44 (d, J = 2.7 Hz,
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1H), 8.23 (d, J = 7.9 Hz, 1H), 7.63–7.58 (m, 2H), 7.36–7.32 (m, 1H); 13C NMR (150 MHz, DMSO) δ
146.0, 140.7, 140.4, 136.9, 135.7, 129.5, 121.4, 121.0, 119.8, 112.6. These spectral data matched those
reported.21d
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ASSOCIATED CONTENT
Supporting Information
1H NMR, 13C NMR spectra for new compounds and X-ray data (CIF file) for 10a. This information is
available free of charge at http://pubsacs.org
AUTHOR INFORMATION
Corresponding Author
*Email: gordon.w.gribble@dartmouth.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was supported in part by the Donors of the Petroleum Research Fund administered by the
American Chemical Society. P.Z.M. acknowledges support from the Zabriskie Fellowship from Dartmouth
College. We are grateful to Dr. Jerry Jasinski of Keene State University for X-ray analyses.
References
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(15) Analysis of both the HRMS and MS showed a fragmentation pattern reminiscent of loss of CO2 and t-
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338.0963.
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o
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