Gold-catalyzed domino cycloisomerization/pictet-spengler reaction of 2-(4-aminobut-1-yn-1-yl)anilines with aldehydes: Synthesis of tetrahydropyrido[4,3-b ]indole scaffolds

Article abstract of DOI:10.1021/jo302068e

A domino cycloisomerization/Pictet-Spengler reaction of 2-(4-aminobut-1-yn-1-yl)aniline with aldehydes has been achieved using a AuIPrCl (5 mol %)/AgSbF₆ (10 mol %) catalytic system to produce the corresponding 1-aryl-N-tosyl-2,3,4,5-tetrahydropyrido[4,3-b] indole derivatives in good yields. This is the first report on the synthesis of tetrahydro pyrido[4,3-b]indole scaffolds through as tandem 5-endo-dig cyclization and Pictect-Spengler reaction.

Full text of DOI:10.1021/jo302068e

Note
pubs.acs.org/joc
Gold-Catalyzed Domino Cycloisomerization/Pictet−Spengler
Reaction of 2‑(4-Aminobut-1-yn-1-yl)anilines with Aldehydes:
Synthesis of Tetrahydropyrido[4,3‑b]indole Scaffolds
B. V. Subba Reddy,*,† Manisha Swain,† S.Madhusudana Reddy,† J. S. Yadav,*,† and B. Sridhar‡
‡
†Natural Product Chemistry and Laboratory of X-ray Crystallography, CSIR-Indian Institute of Chemical Technology, Hyderabad
500 007, India
S
* Supporting Information
ABSTRACT: A domino cycloisomerization/Pictet−Spengler reaction of 2-(4-aminobut-1-yn-1-yl)aniline with aldehydes has
been achieved using a AuIPrCl (5 mol %)/AgSbF6 (10 mol %) catalytic system to produce the corresponding 1-aryl-N-tosyl-
2,3,4,5-tetrahydropyrido[4,3-b] indole derivatives in good yields. This is the first report on the synthesis of tetrahydro pyrido[4,3-
b]indole scaffolds through as tandem 5-endo-dig cyclization and Pictect−Spengler reaction.
etrahydropyrido[4,3-b]indole core is frequently found in
electrophilic aromatic substitution at C3 position. A variety of
metal complexes have been investigated for the hydroamination
of 2-ethynylanilines,6 but the scope of this reaction has been
significantly broadened with gold catalysts. The alkynophilicity
of Au(I) and Au(III) complexes provides the new opportunities
for the cycloisomerization of a wide range of alkynes tethered
with nucleophiles.7 These catalysts can also act as Lewis acids
for the activation of electrophiles8a−d to facilitate C−C or C−X
bond formation. Inspired by recent advancement in gold
catalysis,9 we herein report a novel strategy for the one-pot
synthesis of 1-aryl-N-tosyl-2,3,4,5-tetrahydropyrido[4,3-b]-
indole scaffolds from 2-(4-aminobut-1-yn-1-yl)aniline via an
intramolecular hydroamination followed by Pictet−Spengler
cyclization.
T
biologically active molecules such as latrepirdine and
pyridoindolobenzodiazepine (Figure 1).1 The latrepirdine, also
To investigate the feasibility of the reaction, the starting
material, 2-(4-aminobut-1-yn-1-yl)aniline (1), was prepared by
Sonogashira coupling of 2-iodoaniline with 2-(4-aminobut-1-
yn-1-yl)aniline10 and was chosen as a model substrate (Scheme
1).
Initially, we intended to attempt the coupling of 2-(4-
aminobut-1-yn-1-yl)aniline (1) with 4-bromobenzaldehyde (2)
in the presence of 10 mol % of NaAuCl4·2H2O in ethanol. The
corresponding tetrahydro-γ-carboline 3a was isolated only in
44% yield after 12 h. Though NaAuCl4·2H2O promotes the
annulation of 2-alkynylanilines effectively,12 it did not show
much catalytic influence in the present reaction (Table 1, entry
f), and moreover, more than 40% of the starting material (1)
was recovered. Encouraged by these initial findings, we began a
Figure 1. Tetrahydropyrido[4,3-b]indole-containing biologically active
molecules.
known as dimebolin, is an orally active, antihistamine drug sold
as Dimebon. It has also been shown to inhibit brain cell death
in animal models of Alzheimer’s disease and Huntington’s
disease.2 Generally, tetrahydro-γ-carbolines are prepared by
means of Pictet−Spengler reaction. However, a few methods
have been reported for the direct synthesis of these tetrahydro-
γ-carbolines via tandem processes.3 On the other hand, the
indole ring has been a privileged core structural motif in
numerous naturally occurring alkaloids and biologically active
molecules.4 Of various syntheses of indoles, particular attention
has been paid to cyclization of 2-alkynylaniline, which is one of
the most efficient methods for the preparation of 2-substituted
indole derivatives,5 which can easily be functionalized by
Received: October 8, 2012
© XXXX American Chemical Society
A
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Scheme 1. Preparation of 2-(4-Aminobut-1-yn-1-yl)aniline11
(1)
fact, next we investigated the catalytic efficiency of Au(I)
complexes such as AuCl, AuIPrCl, Ph3PAuCl, and their
combination with silver salts like AgOTf and AgSbF6. To our
surprise, 10 mol % of AuCl in dry DCE at 60 °C afforded the
isotryptamine (1c) in very low yield (<20%) which upon
addition of aldehyde gave the undesired bis-indole derivative
(Table 1, entry i) instead of 3a. After screening several gold(I)
catalysts, the best conversions were achieved with NHC−gold
complex (AuIPrCl) in combination with AgSbF6 (Table 1,
entry m). The results are summarized in Table 1. The role of
cocatalyst Ag(I) salt was expected to increase the reaction rate
by generating Au(I) cationic species by the liberation of AgCl.
As depicted in Table 1, the combination of 5 mol % of AuIPrCl
and 10 mol % of AgSbF6 gave the best results (Table 1, entry
m, Scheme 2). In the above reaction, we found that more than
systematic screening of various Lewis acids in order to improve
the conversion. Preliminary experiments were carried out using
InCl3 and PtCl2. Surprisingly, no desired product 3a was
obtained with 10 mol % of InCl3 in toluene at 80 °C (Table 1,
entry a). Only a trace amount of 3a was obtained with 10 mol
% PtCl2 (Table 1, entry b). Furthermore, metal triflates such as
Cu(OTf)2 and AgOTf also afforded the product 3a with low
yield (Table 1, entries c−e).
Scheme 2. Condensation of p-Bromobenzaldehyde with 2-
(4-Aminobut-1-yn-1-yl)aniline
Though Cu(OTf)2 facilitates the intramolecular hydro-
amination of N-protected ethynylaniline,13 it did not show
much catalytic effect in subsequent Pictet−Spengler cycliza-
tions (Table 1, entry c). In order to realize the catalytic efficacy
of gold(III) complexes for this reaction, further investigations
were carried out using 10 mol % of AuCl3 and AuCl3/AgOTf.
No improvement in yields was obtained with Au(III)
complexes (Table 1, entries g and h). Therefore, to our
disappointment, neither Au(III) salt nor its combination with
Ag(I) salt has proved be to an optimal catalyst for this reaction.
In several instances of gold catalysis, Au(I) complexes are found
to be superior to the Au(III) in the cycloisomerization/C3-
functionalization of 2-ethnylanilines with a wide range of
aldehydes, nitrostyrenes, and isatin derivatives.14 Based on this
70% of 2-(4-aminobut-1-yn-1-yl)aniline (1) was converted into
isotryptamine 1c in the presence of 5 mol % of Au(I)/10 mol %
of Ag(I) in 1,2-dichloroethane within a short reaction time (2−
Table 1. Screening of the Catalysts for the Cyclization of 2-(4-Aminobut-1-yn-1-yl)aniline (1) with p-Bromobenzaldehyde (2)
a
b
entry
catalyst (mol %)
InCl3 (10)
solvent
temp (°C)
time (h)
% conversion (1c)
yield (%) (3a)
c
a
b
c
d
e
f
toluene
toluene
toluene
DCE
reflux
80
80
25
70
25
60
60
60
25
25
25
25
25
25
24
12
24
12
20
12
24
12
12
7
NR
trace
<10
∼30
∼35
∼50
<10
∼30
<20
>50
∼60
>50
>70
>40
∼55
NR
c
PtCl2 (10)
trace
Cu(OTf)2 (10)
AgOTf (10)
9
15
37
44
15
27
AgOTf (10)
DCE
NaAuCl4·2H2O (10)
AuCl3 (10)
EtOH
DCE
g
h
i
AuCl3 + AgOTf (10)
AuCl (10)
DCE
d
DCE
<5
j
AuIPrCl + AgOTf (10)
AuIPrCl + AgSbF6 (10)
AuIPrCl + AgSbF6 (10)
AuIPrCl (5) + AgSbF6 (10)
PPh3AuCl+AgOTf (10)
PPh3AuCl + AgSbF6 (10)
DCE
55
58
50
67
55
62
k
l
DCE
4
CH3CN
DCE
4
m
n
o
4
DCE
5
DCE
5
a
b
Unless specified, the reaction was carried out with 1 (0.33 mmol) and 2 (0.5 mmol) in 2 mL of solvent. Isolated yield after column
c
d
chromatography. Starting material was recovered. The major product was bisindole (80%).
B
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a
Table 2. Gold(I)-Catalyzed Synthesis of 1-Aryl-/Alkyl-N-tosyl-2,3,4,5-tetrahydropyrido[4,3-b]indole Scaffolds
a
b
The reaction was carried out with 1a (0.33 mmol) and 2 (0.5 mmol) in 2 mL of DCE. All of the products were characterized by 1H and 13C NMR,
c
IR, and HRMS spectroscopy . Yield refers to pure products after column chromatography.
3 h). A subsequent cyclization of 1c with 2 gave the expected
product 3a in 67% yield at 25 °C in 4 h (Table 1, entry m).
This reaction procceds by means of an intramolecular
hydroamination followed by Pictet−Spengler reaction. There-
fore, an intramolecular hydromination of substrate (1) likely
generates N-tosylisotryptamine 1c, which simultaneously
undergoes cyclization with aldehyde to give the tetrahydro-γ-
carboline via a highly reactive N-sulfonyliminium intermediate.
Next, we examined the scope of the reaction using various
aldehydes. As shown in Table 2, a wide range of aldehydes
reacted smoothly with 2-(4-aminobut-1-yn-1-yl)aniline (1)
under optimized conditions. Both aromatic and aliphatic
aldehydes participated smoothly in this reaction. However,
the electronic properties of the substituents on the aromatic
ring had shown some effect on the outcome of the reaction.
Indeed, electron-rich aldehydes afforded the tetrahydro-γ-
carbolines in good yields in shorter time compared to
electron-withdrawing counterparts. It is noteworthy to mention
that several functionalities such as aryl alkyl ethers, phenol,
olefin, and halide groups are well-tolerated under the reaction
C
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conditions. By using this procedure, a variety of 1-aryl-N-tosyl-
2,3,4,5-tetrahydropyrido[4,3-b]indole derivatives were prepared
in fairly good yields (Table 2).
In conclusion, we have successfully demonstrated a novel
one-pot strategy for the synthesis of 1-substituted 2-tosyl-
2,3,4,5-tetrahydropyrido[4,3-b]indole scaffolds through a se-
quential hydroamination/Pictet−Spengler cyclization. This
method allows C−C bond formation with excellent scope
and high functional group tolerance under relatively mild and
neutral conditions which makes this protocol a valuable
alternative to previously described procedures. This reaction
further demonstrates the ability of gold complexes to
participate in concurrent tandem catalytic processes.
The structure of compound 3g was confirmed by single-
crystal X-ray diffraction analysis.15 Further experiment was
carried out using 2-(4-aminobut-1-yn-1-yl)-5-methoxyaniline to
prepare substituted tetrahydrocarboline. Thus, treatment of 2-
(4-aminobut-1-yn-1-yl)-5-methoxyaniline with p-chlorobenzal-
dehyde under the optimized reaction conditions for 6 h
afforded the methoxy-substituted tetrahydro-γ-carboline 3s in
60% yield.
Mechanistically, the reaction is expected to proceed via two
steps with two intermediates. The first step involves the
gold(I)-catalyzed intramolecular hydroamination of 2-(4-
aminobut-1-yn-1-yl)aniline to give the isotryptamine 1c
(Scheme 3). It proceeds via an initial π-coordination of Lewis
EXPERIMENTAL SECTION
■
General Methods. All the solvents were dried according to
standard procedures. Reactions were performed in oven-dried round-
bottom flask. The flasks were fitted with rubber septa and the reactions
were conducted under nitrogen atmosphere. Glass syringes were used
to transfer the solvent. Crude products were purified by column
chromatography on silica gel of 60−120 or 100−200 mesh. TLC
plates were visualized by exposure to ultraviolet light and/or by
exposure to iodine vapors and/or by exposure to acidic methanolic
solution of p-anisaldehyde followed by heating (<1 min) on a hot plate
(∼250 °C). Organic solutions were concentrated on rotary evaporator
Scheme 3. Plausible Reaction Pathway for
Cycloisomerization/Pictet−Spengler Reaction
1
at 35−40 °C. IR spectra were recorded on FT-IR spectrometer. H
NMR and 13C NMR (proton-decoupled) spectra were recorded on
200, 300, 400, or 500 MHz NMR spectrometers in CDCl3 solvent.
Chemical shifts (δ) were reported in parts per million (ppm) with
respect to TMS as an internal standard. Coupling constants (J) are
quoted in hertz (Hz). Mass spectra were recorded on a mass
spectrometer by electrospray ionization (ESI) or an atmospheric
pressure chemical ionization (APCI) technique.
Preparation of Starting Materials. N-(But-3-yn-1-yl)-4-methyl-
benzenesulfonamide. Obtained as a white crystalline solid: yield 75%;
1H NMR (300 MHz, CDCl3) δ 7.76 (d, J = 7.7 Hz, 2H), 7.30 (d, J =
7.7 Hz, 2H), 4.98 (brs, 1H), 3.10 (dd, J = 6.6, 13.3 Hz, 2H), 2.42 (s,
3H), 2.33 (dt, J = 2.2, 6.6, 13.3 Hz, 2H), 2.00−1.97 (m, 1H); 13C
NMR (75 MHz, CDCl3) δ 143.3, 136.6, 129.5, 126.8, 129.3, 126.9,
117.7, 114.3, 107.7, 91.1, 79.3, 42.0, 21.4, 21.0; IR (KBr) ν 3271, 2923,
2876, 1593, 1416, 1318, 1157, 1066, 926, 677, 581 cm−1; MS (ESI) m/
z 246 [M + Na]+; HRMS (Orbitrap ESI) exact mass calcd for
C11H13O2NNaS [M + Na]+ 246.05592, found 246.05592.
acidity of cationic Au(I) species with the alkyne moiety of 2-(4-
aminobut-1-yn-1-yl)aniline (1) to form a π-complex 1a, and
subsequent nucleophilic attack of the tethered amino group
leads to ring closure to furnish the cyclic intermediate 1b which
on protodemetalation affords the isotryptamine 1c. The second
step involves the activation of aldehyde with regenerated Au(I)
species followed by an intramolecular nucleophilic addition of
indole moiety of 1c to a highly reactive N-sulfonyliminium
intermediate 1d (Scheme 3) to provide the tetrahydropyr-
idoindole derivative 3 with regeneration of active cationic gold
species (Scheme 3).
To realize the role of excess of AgSbF6, the intermediate
isotryptamine 1c was isolated and then subjected to the Pictet−
Spengler cyclization. Interestingly, isotryptamine 1c underwent
smooth cyclization with p-chlorobenzaldehyde in the presence
of 5 mol % of AgSbF6 in dichloroethane at 25 °C affording the
tetrahydro-γ-carboline 3j in 90% yield. This result clearly
reveals that Ag(I) facilitates the second step (Pictet−Spengler
reaction) (Scheme 4).
Typical Procedure for the Synthesis of N-(4-(2-
Aminophenyl)but-3-yn-1-yl)-4-methylbenzenesulfonamide
(1). To a stirred solution of 2-iodoaniline (1 mmol) in Et3N was added
Pd(PPh3)2Cl2 (0.02 mmol) and the mixture stirred for 5−10 min. To
this mixture were added N-(but-3-ynyl)-4-methylbenzenesulfonamide
(1.5 mmol) followed by CuI (0.01 mmol), and the resulting mixture
was stirred at room temperature until complete consumption of the
starting material. Removal of the solvent followed by purification on
silica gel using (ethyl acetate/n-hexane = 2:1) gradients afforded the
1
pure product 1 as orange solid: yield 70%; mp 92−95 °C; H NMR
(300 MHz, CDCl3): δ 7.77 (d, J = 7.7 Hz, 2H), 7.28 (d, J = 7.7 Hz,
2H), 7.19 (d, J = 6.6 Hz, 1H), 7.09 (t, J = 7.7 Hz, 1H), 6.77−6.63 (m,
2H), 5.19−5.11 (m, 1H), 3.96 (brs, 2H), 3.19 (dd, J = 6.6, 13.3 Hz,
2H), 2.65 (t, J = 6.6 Hz, 2H), 2.41 (s, 3H); 13C NMR (75 MHz,
CDCl3): δ 147.9, 143.4, 136.9, 132.0, 129.7, 129.3, 126.9, 117.7, 114.3,
107.7, 91.1, 79.3, 42.0, 21.4, 21.0; IR (KBr): υν 3473, 3373, 3254,
2925, 1616, 1488, 1449, 1311, 1155, 1072, 745, 549 cm−1; MS (ESI)
m/z 315 [M + H]+; HRMS (Orbitrap ESI) exact mass calcd for
C17H19O2N2S [M + H]+ 315.1167, found 315.1157.
Scheme 4. Ag(I)-Catalyzed Pictet−Spengler Reaction of
Isotryptamine 1c
N-(4-(2-Amino-5-methoxyphenyl)but-3-yn-1-yl)-4-methyl-
benzenesulfonamide: orange solid; yield 65%; mp 82−84 °C; 1H
NMR (300 MHz, CDCl3) δ 7.80−7.71 (m, 3H), 7.34−7.25 (m, 2H),
7.09 (d, J = 8.3 Hz, 1H), 6.27−6.18 (m, 2H), 3.75 (s, 3H), 3.17 (dd, J
= 6.0, 12.8 Hz, 2H), 2.62 (t, J = 6.0 Hz, 2H), 2.41 (s, 3H); 13C NMR
(75 MHz, CDCl3 + DMSO-d6) δ 159.8, 149.2, 142.1, 136.9, 132.2,
128.9, 126.1, 102.9, 98.3, 89.5, 78.0, 54.3, 41.5, 20.7, 20.3; IR (KBr) ν
3463, 3353, 3251, 2921, 1620, 1484, 1445, 1311, 1144, 1071, 745, 544
cm−1; MS (ESI) m/z 345 [M + H]+; HRMS (Orbitrap ESI) exact
mass calcd for C18H20O3N2S [M + H]+ 345.12674, found 345.12650.
D
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N-(2-(1H-Indol-2-yl)ethyl)-4-methylbenzenesulfonamide(1c). To
a stirred solution of 2-(4-aminobut-1-yn-1-yl)aniline (1, 0.33 mmol)
in dry DCE (1 mL) under N2 was added AuIPrCl (5 mol %)/AgSbF6
(10 mol %). The resulting mixture was allowed to stir at room
temperature for 2 h. After completion of the reaction as indicated by
TLC, the mixture was diluted with ethyl acetate and filtered through a
plug of silica gel. The filtrate was concentrated, and the residue was
purified by column chromatography on silica gel (60−120 mesh) using
(ethyl acetate/hexane = 2:1) to afford the isotrypatamine intermediate
(1c) as an off-white solid: yield 60%; mp 100−102 °C; 1H NMR (300
MHz, CDCl3) δ 8.24 (brs, NH, 1H) 7.70−7.62 (m, 2H), 7.49 (d, J =
7.5 Hz, 1H), 7.32−7.21 (m, 4H), 7.18−7.02 (m, 2H), 6.16 (s, 1H),
4.84 (t, J = 6.0 Hz, 1H), 3.24 (dd, J = 6.4, 12.8 Hz, 2H), 2.91 (t, J = 6.4
Hz, 2H), 2.40 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 143.4, 136.0,
135.3, 129.7, 129.4, 128.1, 127.0, 121.4, 119.8, 110.7, 100.5, 42.8, 28.5,
21.5 ; IR (KBr) ν 3472, 3371, 3252, 2924, 1620, 1488, 1454, 1310,
1155, 1071, 745, 549 cm−1; MS (ESI) m/z 315 [M + H]+; HRMS
(Orbitrap ESI) exact mass calcd for C17H19O2N2S [M + H]+ 315.1167,
found 315.1162.
General Procedure for Au(I)/Ag(I)-Catalyzed Cyclization. To
a stirred solution of 2-(4-aminobut-1-yn-1-yl)aniline (1)/N-(4-(2-
amino-5-methoxy phenyl)but-3-yn-1-yl)-4-methylbenzenesulfonamide
(4) (0.33 mmol) in dry DCE (1 mL) under N2 was added AuIPrCl (5
mol %)/AgSbF6 (10 mol %), and the resulting mixture was allowed to
stir at room temperature. The progress of the reaction was monitored
by TLC. To this mixture was added aldehyde (0.5 mmol), and stirring
was continued for the specified time (Table 2). After completion of
the reaction as indicated by TLC, the mixture was diluted with ethyl
acetate and filtered through a plug of silica gel. The filtrate was
concentrated and the residue was purified by column chromatography
on silica gel (60−120 mesh) using (ethylacetate/hexane = 4:1−2:1) to
afford the pure tetrahydro-γ-carboline.
NMR (300 MHz, CDCl3) δ 7.71 (brs, 1H, NH), 7.55 (d, J = 8.3 Hz,
2H), 7.27−7.15 (m, 4H), 7.12 (d, J = 7.5 Hz, 2H), 7.08−6.94 (m,
4H), 6.40 (s, 1H), 3.95 (dd, J = 6.0, 14.3 Hz, 1H), 3.32−3.18 (m, 1H),
2.72−2.40 (m, 2H), 2.31 (s, 6H); 13C NMR (75 MHz, CDCl3) δ
142.9, 140.5, 138.7, 137.2, 135.6, 129.7, 129.6, 129.3, 129.0, 128.5,
128.0, 127.6, 127.0, 126.9, 121.8, 119.8, 118.6, 115.4 110.6, 55.5, 38.5,
21.8, 21.4, 21.1; IR (KBr) ν 3341, 2923, 2855, 1649, 1604, 1459, 1318,
1151, 1088, 747 cm−1; MS-ESI m/z 417 [M + H]+; HRMS (MS-
TOF) exact mass calcd for C25H24O2N2NaS [M + Na]+ 439.1456,
found 439.1442.
1-(4-Methoxyphenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-
b]indole (3e; Table 2, entry e): off-white solid; yield 70%; mp 196−
198 °C; 1H NMR (300 MHz, CDCl3) δ 7.71 (brs, 1H, NH), 7.56 (d, J
= 8.5 Hz, 2H), 7.27−7.21 (m, 3H), 7.14−7.07 (m, 2H), 7.03 (d, J =
8.5 Hz, 2H), 6.99 (dd, J = 6.8, 14.4 Hz, 1H), 6.76 (d, J = 8.5 Hz, 2H),
6.40 (s, 1H), 3.99−3.92 (m, 1H), 3.77 (s, 3H), 3.30−3.22 (m, 1H),
2.64−2.54 (m, 1H), 2.41−2.36 (m, 2H), 2.31 (s, 3H); 13C NMR (75
MHz, CDCl3) δ 159.2, 142.9, 138.4, 135.7, 132.4, 132.0, 129.8, 129.3,
126.9, 126.1, 121.1, 121.8, 119.8, 118.6, 110.6, 108.9, 55.2, 53.4, 38.4,
21.9, 21.4; IR (KBr) ν 3354, 2920, 2864, 1610, 1509, 1459, 1314,
1151, 1091, 746 cm−1; MS (ESI) m/z 433 [M + H]+; HRMS (MS-
TOF) exact mass calcd for C25H24O3N2NaS [M + Na]+ 455.1405,
found 455.1403.
1-Phenyl-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole (3f;
1
Table 2, entry f): off-white solid; yield 52%; mp 240−242 °C; H
NMR (500 MHz, CDCl3+DMSO-d6) δ 8.57 (brs, 1H, NH), 7.57 (d, J
= 9.0 Hz, 2H), 7.38−7.33 (m, 2H), 7.31−7.27 (m, 3H), 7.27−7.23 (m,
3H), 7.15−7.09 (m, 1H), 7.05 (d, J = 8.0 Hz, 2H), 6.96−7.01 (m,
1H), 6.89 (t, J = 7.6 Hz 1H), 6.36 (s, 1H), 3.98−3.92 (m, 1H), 3.33−
3.24 (m, 1H), 2.65−2.55 (m, 2H), 2.32 (s, 3H); 13C NMR (125 MHz,
CDCl3 + DMSO) δ 142.8, 140.8, 137.9, 132.4, 135.6, 132.7, 129.5,
128.7, 128.2, 127.6, 127.1, 126.5, 125.3, 120.7, 118.6, 117.7, 114.3,
110.9, 55.3, 38.6, 21.3, 20.8; IR (KBr) ν 3379, 2923, 2853, 1711, 1599,
1464, 1331, 1159, 1090, 756 cm−1; MS-ESI: m/z 403 [M + H]+;
HRMS (Orbitrap ESI) exact mass calcd for C24H23O2N2S [M + H]+
403.1480, found 403.1463.
1-(4-Bromophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3a; Table 2, entry a): yellow solid; yield 67%; mp 191−192
1
°C; H NMR (300 MHz, CDCl3) δ 7.82 (brs, 1H, NH), 7.57 (d, J =
8.1 Hz, 2H), 7.38 (d, J = 8.5 Hz, 2H), 7.33−7.20 (m, 1H), 7.19−6.98
(m, 4H), 6.41 (s, 1H), 4.00 (dd, J = 5.1, 14.5 Hz, 1H), 3.30−3.17 (m,
1H), 2.62−2.37 (m, 2H), 2.31 (s, 3H); 13C NMR (75 MHz, CDCl3) δ
143.2, 139.3, 137.9, 135.7, 132.1, 131.3, 130.3, 129.7, 128.9, 129.4,
127.0, 126.8, 125.8, 121.9, 119.9, 118.3, 110.7, 107.8, 55.1, 38.5, 21.7,
21.4; IR (KBr) ν 3364, 2923, 2854, 1647, 1460, 1287, 1319, 1153,
1084, 747, 658 cm−1; MS-ESI m/z 481 [M + H]+, 483 [(M + 2) +
H]+; HRMS (Orbitrap ESI) exact mass calcd for C24H22O2N2BrS [M
+ H]+ 483.05364, found 483.05594.
1-(4-Fluorophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3g; Table 2, entry g): off-white solid; yield 78%; mp 204−206
1
°C; H NMR (300 MHz, CDCl3) δ 7.77 (brs, 1H, NH), 7.58 (d, J =
8.0 Hz, 2H), 7.42−7.26 (m, 4H), 7.20−7.00 (m, 2H), 7.00−6.92 (m,
2H), 6.46 (s, 1H), 3.98 (dd, J = 6.0, 15.0 Hz, 1H), 3.33−3.25 (m, 1H),
2.67−2.57 (m, 1H), 2.52 (dd, J = 5.0, 16.0 Hz, 1H), 2.30 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ 160.7, 143.1, 138.1, 136.0, 135.7, 135.6,
132.0, 130.2, 130.3, 129.6, 129.3, 127.0, 126.8, 121.9, 119.8, 118.4,
115.8, 115.2, 114.9 110.6, 55.0, 38.4, 21.7, 21.3; IR (KBr) ν 3365,
3336, 2924, 2858, 1650, 1504, 1464, 1325, 1151, 1087, 746 cm−1; MS-
ESI m/z 421 [M + H]+; HRMS (Orbitrap ESI) exact mass calcd for
C24H22O2N2FS [M + H]+ 421.1380, found 421.1381.
1-(2-Bromophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3b; Table 2; entry b): cream solid; yield 62%; mp 189−191
1
°C; H NMR (300 MHz, CDCl3) δ 7.79 (brs, 1H, NH), 7.57 (d, J =
8.3 Hz, 2H), 7.38−7.23 (m, 4H), 7.18−6.99 (m, 4H), 6.94 (dt, J = 2.3,
8.3 Hz, 2H), 6.45 (s, 1H), 4.00 (dd, J = 5.3, 14.4 Hz, 1H), 3.32−3.18
(m, 1H), 2.63−2.37 (m, 2H), 2.31 (s, 3H); 13C NMR (75 MHz,
CDCl3) δ 143.1, 138.1, 136.0, 135.6, 132.0, 131.3, 130.3, 130.2, 129.6,
129.3, 127.0, 126.8, 125.8, 121.9, 119.8, 118.4, 115.2, 114.9 110.7,
108.2, 55.0, 38.4, 21.7, 21.4 ; IR (KBr) ν 3364, 2923, 2854, 1647, 1460,
1287, 1319, 1153, 1084, 747, 658 cm−1; MS-ESI m/z 481 [M + H]+;
HRMS (Orbitrap ESI) exact mass calcd for C24H22O2N2BrNaS
505.05558, found 505.05324.
1-(2-Fluorophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3h; Table 2, entry h): off-white solid; yield 55%; mp 200−202
1
°C; H NMR (300 MHz, CDCl3) δ 8.47 (brs, 1H, NH), 7.48 (d, J =
8.3 Hz, 2H), 7.35−7.13 (m, 6H), 7.12−6.89 (m, 2H), 6.85−6.73 (m,
2H), 6.12 (s, 1H), 4.77−4.62 (m, 1H), 3.14−3.03 (m, 1H), 2.79−2.61
(m, 2H), 2.37 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 159.5, 143.4,
136.0, 135.3, 132.8, 129.8, 129.6, 128.2, 128.1, 127.7, 126.9, 121.9,
119.2, 118.9, 113.2, 111.0, 114.9 110.6, 42.2, 32.7, 21.4, 21.0; IR (KBr)
ν 3374, 2923, 1599, 1487, 1453, 1324, 1156, 1088, 750, 664 cm−1; MS
(ESI) m/z 421 [M + H]+; HRMS (Orbitrap ESI) exact mass calcd for
C24H22O2N2FS [M + H]+ 421.13805, found 421.13835.
1-(o-Tolyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole (3c;
1
Table 2, entry c): off-white solid; yield 64%; mp 145−146 °C; H
NMR (300 MHz, CDCl3) δ 7.67 (brs, 1H, NH), 7.46 (d, J = 7.7 Hz,
2H), 7.25 (m, 3H), 7.15−7.05 (m, 2H), 7.01−6.89 (m, 3H), 6.83 (d, J
= 8.6 Hz, 2H), 6.68 (s, 1H), 3.89 (dd, J = 5.7, 14.4 Hz, 1H), 3.50−3.39
(m, 1H), 2.78 (s, 3H), 2.62−2.47 (m, 2H), 2.28 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 142.9, 137.8, 135.6, 131.5, 131.1, 129.5, 129.2,
128.9, 127.9, 127.1, 127.0, 125.6, 125.4, 121.8, 119.7, 118.1, 110.5,
53.5, 38.4, 21.4, 21.1, 20.4; IR (KBr) ν 3341, 2923, 2855, 1649, 1604,
1459, 1318, 1151, 1088, 745 cm−1; MS-ESI: m/z 417 [M + H]+;
HRMS (Orbitrap ESI) exact mass calcd for C25H24O2N2NaS [M +
Na]+ 439.1456, found 439.1440.
1-(4-Nitrophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3i; Table 2, entry i): off-white solid; yield 51%; mp 105−107
1
°C; H NMR (300 MHz, CDCl3) δ 8.47 (brs, 1H, NH), 7.48 (d, J =
8.3 Hz, 2H), 7.35−7.13 (m, 6H), 7.12−6.89 (m, 2H), 6.85−6.73 (m,
2H), 6.12 (s, 1H), 4.77−4.62 (m, 1H), 3.14−3.03 (m, 1H), 2.79−2.61
(m, 2H), 2.37 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 147.6, 147.5,
143.5, 137.8, 135.7, 132.3 129.5, 127.0, 126.8, 125.7, 123.6, 122.3,
120.2, 118.2, 110.9, 107.2, 55.0, 38.8, 21.6, 21.5; IR (KBr) ν 3375,
2921, 2851, 1598, 1485, 1462, 1326, 1150, 1090, 750 cm−1; MS (ESI)
m/z 448[M + H]+; HRMS (Orbitrap ESI) exact mass calcd for
C24H21O4N3S [M + H]+ 448.13255, found 448.13280.
1-(p-Tolyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole (3d;
1
Table 2, entry d): off-white solid; yield 79%; mp 146−148 °C; H
E
dx.doi.org/10.1021/jo302068e | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
1-(4-Chlorophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3j; Table 2, entry j): white solid; yield 72%; mp 182−184 °C;
1H NMR (300 MHz, CDCl3) δ 7.71 (brs, 1H, NH), 7.55 (d, J = 8.3
1-(Naphthalen-1-yl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]-
indole (3p; Table 2, entry p): white solid; yield 35%; mp 82−84 °C;
1H NMR (300 MHz, CDCl3) δ 9.12 (d, J = 7.7 Hz, 1H), 7.96−7.83
Hz, 2H), 7.31−7.18 (m, 5H), 7.16−6.94 (m, 5H), 6.39 (s, 1H), 3.98
(dd, J = 5.3, 14.4 Hz, 1H), 3.27−3.14 (m, 1H), 2.64−2.37 (m, 2H),
2.32 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 143.1, 138.7, 133.6,
132.0, 129.9, 129.7, 129.3, 128.3, 126.9, 126.8, 125.8, 122.0, 119.9,
118.4, 114.2 110.7, 108.0, 55.0, 38.5, 21.7, 21.4; IR (KBr) ν 3357,
2924, 2854, 1597, 1461, 1324, 1149, 1088, 806, 652 cm−1; MS (ESI)
m/z 437 [M + H]+; HRMS (MS-TOF) exact mass calcd for
C24H21O2N2ClNaS [M + Na]+ 459.0909, found 459.0892.
1-(3,4-Dimethoxyphenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido-
[4,3-b]indole (3k; Table 2, entry k): orange solid; yield 71%; mp 210−
212 °C; 1H NMR (300 MHz, CDCl3) δ 7.80 (brs, 1H, NH), 7.59 (d, J
= 8.3 Hz, 2H), 7.34−7.21 (m, 2H), 7.18−7.10 (t, J = 7.6 Hz, 2H),
7.09−6.99 (m, 3H), 6,67 (s, 2H), 6.41 (s, 1H), 4.01 (dd, J = 5.3, 14.4
Hz, 1H), 3.86−3.67 (m, 6H), 3.39−3.24 (m, 1H), 2.69−2.35 (m, 2H),
2.31 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 148.8, 148.6, 142.9,
138.3, 135.6, 132.7, 132.1, 129.7, 129.3, 126.8, 126.1, 121.7, 120.7,
119.7, 118.5, 110.6, 110.2, 108.5, 55.8, 55.5, 38.5, 21.9, 21.4; IR (KBr)
ν 3359, 2924, 2852, 1661, 1598, 1512, 1459, 1327, 1259, 1155, 1088,
1022, 747, 548 cm−1; MS-ESI m/z 463 [M + H]+; HRMS (Orbitrap
ESI) exact mass calcd for C26H27O4N2S [M + H]+ 463.16860, found
463.16727.
(m, 1H), 7.86 (brs, NH, 1H), 7.80−7.67 (m, 2H), 7.64−7.50 (m, 4H),
7.35 (s, 1H), 7.21−7.05 (m, 3H), 7.03−6.90 (m, 4H), 3.75 (dd, J =
6.6, 14.3 Hz, 1H), 3.43−3.33 (m, 1H), 2.86−2.71 (m, 1H), 2.58−2.51
(m, 1H), 2.27 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 143.0, 137.4,
135.9, 135.1, 134.2, 129.6, 129.1, 128.9, 128.5, 127.9, 127.4, 127.0,
126.8, 125.9, 125.4, 124.6, 121.9, 119.8, 118.3, 115.3, 110.5, 109.6,
52.6, 38.3, 21.4; IR (KBr) ν 3377, 2922, 2852, 1658, 1605, 1459, 1323,
1155, 1087, 739 cm−1; MS-ESI m/z 453 [M + H]+; HRMS (Orbitrap
ESI) exact mass calcd for C28H23O2N2S [M − H]− 451.14748, found
451.14703.
1-(Naphthalen-2-yl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4, 3-b]-
indole (3q; Table 2, entry q): white solid; yield 48%; mp 99−101 °C;
1H NMR (300 MHz, CDCl3) δ 9.10 (d, J = 9.1 Hz, 1H), 8.15 (brs,
1H), 7.94−7.81 (m, 2H), 7.78−7.65 (m, 2H), 7.63−7.40 (m, 4H),
7.32 (s, 1H), 7.20−7.03 (m, 3H), 7.01−6.87 (m, 3H), 3.71 (dd, J =
6.0, 14.4 Hz, 1H), 3.41−3.28 (m, 1H), 2.89−2.69 (m, 1H), 2.51 (dd, J
= 8.3, 16.6 Hz, 1H), 2.27 (s, 3H); 13C NMR (125 MHz, CDCl3) δ
143.6, 136.9, 136.5, 136.1, 135.7, 132.2, 129.7, 129.5, 129.5, 129.1,
128.4, 128.0, 127.9, 127.4, 126.9, 126.6, 124.5, 121.5, 119.7, 118.3,
115.5, 110.7, 50.0, 42.8, 21.5, 21.4; IR (KBr) ν 3355, 2924, 2850, 1718,
1614, 1458, 1324, 1154, 1088, 754 cm−1; MS-ESI m/z 453 [M + H]+;
HRMS (Orbitrap ESI) exact mass calcd for C28H24O2N2NaS [M +
Na]+ 475.1456, found 475.1454.
1-(3,5-Dimethylphenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-
b]indole (3l; Table 2; entry l): white solid; yield 80%; mp 218−220
1
°C; H NMR (300 MHz, CDCl3) δ 7.71 (brs, 1H, NH), 7.55 (d, J =
8.9 Hz, 2H), 7.18−7.10 (m, 2H), 7.05−6.95 (m, 3H), 6.90 (s, 1H),
6,88 (s, 1H), 6.36 (s, 1H), 4.00 (dd, J = 6.7, 14.5 Hz, 1H), 3.37−3.29
(m, 1H), 2.70−2.58 (m, 1H), 2.50 (dd, J = 4.4, 15.5 Hz, 1H), 2.29 (s,
3H), 2.20 (s, 6H); 13C NMR (75 MHz, CDCl3) δ 142.8, 140.0, 138.3,
137.6, 135.7, 131.9, 129.6, 129.4, 129.1, 126.9, 126.3, 125.4, 121.7,
119.7, 118.6, 110.5, 108.9, 55.8, 38.6, 22.0, 21.4, 21.3; IR (KBr) ν
3334, 2920, 1604, 1461, 1324, 1151, 1088, 753 cm−1; MS-ESI m/z 431
[M + H]+; HRMS (Orbitrap ESI) exact mass calcd for C26H27O2N2S
[M + H]+ 431.17878, found 431.17805.
2-Bromo-4-(2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indol-1-
yl)phenol (3m; Table 2; entry m): light yellowish solid; yield 75%; mp
189−191 °C; 1H NMR (500 MHz, CDCl3 + DMSO-d6) δ 10.04 (brs,
1H), 9.48 (1br, 1H), 7.55 (d, J = 8.3 Hz, 2H), 7.35−7.27 (m, 2H),
7.13−7.01 (m, 5H), 6.99−6.89 (m, 1H), 6.84 (d, J = 8.5 Hz, 1H), 6.28
(s, 1H), 4.01−3.91 (m, 1H), 3.35−3.21 (m, 1H), 2.63−2.53 (m, 2H),
2.33 (s, 3H); 13C NMR (125 MHz, CDCl3 + DMSO) δ 152.9, 142.2,
137.4, 135.3, 132.2, 132.0 (d, J = 4.9 Hz), 128.6, 127.9, 124.9, 120.4,
118.3, 115.2, 110.2, 108.8, 106.2, 54.2, 37.8, 21.2, 20.6; IR (KBr) ν
3379, 2923, 2853, 1606, 1519, 1464, 1325, 1151, 1087, 746 cm−1; MS
(ESI) m/z 497[M + H]+; HRMS (Orbitrap ESI) exact mass calcd for
C24H22O3N2BrS [M + H]+ 497.05340, found 497.05322.
1-Cyclohexyl-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole
(3r; Table 2, entry r): pale yellow solid; yield 68%; mp 82−84 °C; 1H
NMR (300 MHz, CDCl3) δ 7.51 (brs, 1H, NH), 7.43 (d, J = 8.1 Hz,
2H), 7.23−7.17 (m, 1H), 7.15−7.06 (m, 2H), 6.86 (d, J = 7.9 Hz,
2H), 4.99 (d, J = 8.3 Hz, 1H), 4.12 (dd, J = 6.8, 15.3 Hz, 1H), 3.67−
3.51 (m, 1H), 2.48−2.25 (m, 2H), 2.21 (s, 3H), 2.06−1.55 (m,7H),
1.44−1.04 (m, 4H); 13C NMR (125 MHz, CDCl3) δ 142.6, 138.1,
135.4, 130.4, 128.9, 126.7, 121.5, 119.5, 119.1, 110.8, 110.3, 58.3,
44.1,38.6, 31.2, 30.3, 29.7, 21.3, 21.0 ; IR (KBr) ν 3385, 2928, 2853,
1738, 1619, 1458, 1324, 1153, 1091, 750 cm−1; MS-ESI m/z 409 [M +
H]+; HRMS (Orbitrap ESI) exact mass calcd for C24H29O2N2S [M +
H]+ 409.19443, found 409.19420.
1-(p-Chlorophenyl)-8-methoxy-2-tosyl-2,3,4,5-tetrahydro-1H-
pyrido[4,3-b]indole (3s): off-white solid; yield 55%; mp 186−188 °C;
1H NMR (300 MHz, CDCl3) δ 7.70 (brs, 1H, NH), 7.56 (d, J = 8.5
Hz, 2H), 7.28−7.21 (m, 3H), 7.12−7.08 (m, 2H), 7.03- 6.98 (m, 2H),
6.76 (d, J = 8.4 Hz, 2H), 6.40 (s, 1H), 3.98−3.92 (m, 1H), 3.77 (s,
3H), 3.30−3.22 (m, 1H), 2.46−2.36 (m, 2H), 2.31 (s, 3H); 13C NMR
(75 MHz, CDCl3+DMSO-d6) δ 159.2, 142.9, 138.4, 132.4, 132.0,
129.7, 129.2, 126.9, 126.0, 121.8, 119.8, 118.6, 113.5, 110.5, 55.2, 53.4,
38.3, 21.9, 21.4; IR (KBr) ν 3352, 2921, 2865, 1622, 1504, 1451, 1313,
1159, 1092, 745 cm−1; MS (ESI) m/z 467 [M + H]+; HRMS
(Orbitrap-ESI) exact mass calcd for C25H24O3N2ClS [M + H]+
467.11911, found 467.11906.
1-(2,4-Dichlorophenyl)-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-
b]indole (3n; Table 2, entry n): white solid; yield 59%; mp 178−180
1
°C; H NMR (300 MHz, CDCl3) δ 7.80 (brs, 1H, NH), 7.58 (d, J =
8.3 Hz, 2H), 7.39−7.28 (m, 2H), 7.25−7.01 (m, 6H), 6.38 (s, 1H),
4.10−3.99 (m, 1H), 3.27−3.19 (m, 1H), 2.67−2.41 (m, 2H), 2.32 (s,
3H); 13C NMR (75 MHz, CDCl3) δ 142.9, 140.5, 138.7, 137.3, 135.7,
129.7, 129.6, 129.3, 129.0, 128.5, 128.0, 127.6, 127.0, 126.9, 121.8,
119.8, 118.7, 115.4 110.6, 55.5, 38.5, 21.8, 21.4; IR (KBr) ν 3355,
2920, 2851, 1600, 1462, 1328, 1159, 1092, 753 cm−1; MS-ESI m/z 437
[M + H]+; HRMS (Orbitrap ESI) exact mass calcd for C24H20O2N2
NaCl2S [M + Na]+ 493.0520, found 493.0516.
ASSOCIATED CONTENT
■
S
* Supporting Information
Copies of 1H and 13C NMR spectra of products (3a−r),
preparation of starting materials, and X-ray data of compound
3g (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(E)-1-Styryl-2-tosyl-2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indole
1
(3o; Table 2, entry o): white solid; yield 45%; mp 128−130 °C; H
NMR (300 MHz, CDCl3) δ 7.71 (brs, 1H, NH), 7.55 (d, J = 8.3 Hz,
2H), 7.31−7.18 (m, 5H), 7.16−6.94 (m, 5H), 6.39 (s, 1H), 3.98 (dd, J
= 5.3, 14.4 Hz, 1H), 3.27−3.14 (m, 1H), 2.64−2.37 (m, 2H), 2.32 (s,
3H); 13C NMR (75 MHz, CDCl3) δ 150.1, 147.0, 143.2, 137.0, 135.9,
134.2, 130.7, 129.7, 129.3, 129.0, 128.8, 128.3, 127.8, 127.7, 127.0,
126.7, 118.3, 117.2, 115.7, 60.1, 41.4, 26.8, 21.5; IR (KBr) ν 3355,
2920, 2851, 1600, 1462, 1328, 1159, 1092, 754 cm−1; MS-ESI m/z 429
[M + H]+; HRMS (Orbitrap ESI) exact mass calcd for C26H23O2N2S
[M − H]− 427.14748, found 427.14735.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: 0091-40-27160512. E-mail: basireddy@iict.res.in,
yadavpub@iict.res.in.
Notes
The authors declare no competing financial interest.
F
dx.doi.org/10.1021/jo302068e | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
(11) (a) Shen, Z.; Lu, X. Adv. Synth. Catal. 2009, 351, 3107.
(b) Wetzel, A.; Gagosz, F. Angew. Chem., Int. Ed. 2011, 50, 7354;
Angew. Chem. 2011, 123, 749.
(12) Arcadi, A.; Bianchi, G.; Marinelli, F. Synthesis 2004, 610.
(13) Copper-promoted reaction of ethynylaniline derivative resulted
in the formation of indole derivative; see: Dai, W. M.; Guo, D. S.; Sun,
L. P. Tetrahedron Lett. 2001, 42, 5275.
(14) (a) Praveen, C.; Karthikeyan, K.; Perumal, P. T. Tetrahedron
2009, 65, 9244. (b) Praveen, C.; Sagayaraj, Y. W.; Perumal, P. T.
Tetrahedron Lett. 2009, 50, 644.
(15) Supplementary crystallographic data (CIF) and respective
ORTEP diagram (Figure 1) for compound 3g have been provided in
the Supporting Information. Also, CCDC-900791 contains the
supplementary crystallographic data for the compound. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ACKNOWLEDGMENTS
■
M.S. and S.M.R. thank CSIR, New Delhi, for the award of
fellowships.
REFERENCES
■
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