Hydroamination of 2-ethynyl-4,5,6,7-tetrahydroindoles: Toward 2-substituted amino derivatives of indole

Article abstract of DOI:10.1055/s-0029-1218768

Hydroamination of-phenyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-yn-1- ones with secondary dialkylamines proceeds under mild conditions (room temperature, aqueous ethanol, 1 h) to afford the corresponding(2E)-3- dialkylamino-3-(4,5,6,7-tetrahydro-1H-in

Full text of DOI:10.1055/s-0029-1218768

2468
PAPER
Hydroamination of 2-Ethynyl-4,5,6,7-tetrahydroindoles:
Toward 2-Substituted Amino Derivatives of Indole
H
ydroamination
y
of 2-Ethyny
u
l-4,5,6,7-tet
b
rahydroindol
o
es v’ N. Sobenina, Denis N. Tomilin, Ol’ga V. Petrova, Igor’ A. Ushakov, Al’bina I. Mikhaleva,
Boris A. Trofimov*
A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky St., Irkutsk 664033,
Russian Federation
Fax +7(3952)419346; E-mail: boris_trofimov@irioch.irk.ru
Received 19 March 2010; revised 24 March 2010
applications of enaminone-functionalized indoles as high-
Abstract: Hydroamination of 1-phenyl-3-(4,5,6,7-tetrahydro-1H-
ly potent building blocks for organic and medicinal chem-
istry. These products, which combine the practically
inexhaustible synthetic potentials of the indole nucleus,
indol-2-yl)prop-2-yn-1-ones with secondary dialkylamines pro-
ceeds under mild conditions (room temperature, aqueous ethanol, 1
h) to afford the corresponding (2E)-3-dialkylamino-3-(4,5,6,7-tet-
rahydro-1H-indol-2-yl)prop-2-en-1-ones in 72–92% stereoselectiv- functional aminoethenes, and carbonyl compounds, show
ity and 64–88% yield. Under the same conditions, ethyl 3-(4,5,6,7-
tetrahydro-1H-indol-2-yl)prop-2-ynoates react with dimethylamine
and diethylamine in different ways; dimethylamine converts the es-
ter function into an amide, giving the corresponding N,N-dimethyl-
extraordinary flexibility as starting materials for the de-
sign of complex heterocyclic ensembles, including new
alkaloids.
Recently, thanks to the discovery of transition-metal-free
and solvent-free techniques for the ethynylation of the
pyrrole nucleus with electrophilic haloalkynes on
alumina6 or other active surfaces,7 2-ethynylpyrroles, 2-
ethynyl-4,5,6,7-tetrahydroindoles, and 3-ethynylindoles
bearing electron-withdrawing substituents (acyl or alkoxy-
carbonyl groups) at the triple bond have become readily
available. The new methodology has made 3-(4,5,6,7-tet-
rahydro-1H-indol-2-yl)prop-2-yn-1-ones and 3-(4,5,6,7-
tetrahydro-1H-indol-2-yl)prop-2-ynoates particularly ac-
cessible on a large-scale by using 4,5,6,7-tetrahydro-1H-
indole8 as the starting material. The hydroamination of
these compounds with secondary amines can be consid-
ered as a shortcut to indole derivatives containing enami-
none moieties, which, because of their ease of
aromatization, may be suitable as intermediates for the
synthesis of 2-functionalized indoles containing an amino
function, e.g. a-analogues of tryptophan, tryptamine, se-
rotonin, or melatonin.
3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-ynamides in 70–86%
yield, whereas diethylamine adds to the triple bond to give the cor-
responding ethyl 3-(diethylamino)-3-(4,5,6,7-tetrahydro-1H-indol-
2-yl)prop-2-enoates with ~100% stereoselectivity and up to 85%
yield. The difference in the reactivity of the two amines can be ra-
tionalized in terms of steric hindrance.
Key words: indoles, alkynes, aminations, nucleophilic additions,
amides
The addition of an N–H moiety to a C–C triple bond, a
process known as hydroamination, offers an attractive
route for the synthesis of substituted nitrogen-containing
organic molecules without the formation of any side prod-
ucts (atom-economic methodology).1 Hydroamination of
an alkyne can provide either an enamine or an imine; these
products can undergo further transformations,2 the nature
of which depends on the types of N–H and acetylenic spe-
cies.
Enamines containing a carbonyl group in the b-position
are important compounds in synthetic organic chemistry
because of their use as 1,3-bielectrophilic or 1,3-binucleo-
philic synthons.3,4 There are many reports on the function-
alization of enaminones by means of various
transformations at the nitrogen atom, the a-carbon atom,
or the carbonyl group. These derivatives are used exten-
sively in the preparation of a variety of heterocyclic sys-
tems, including some natural products and their
analogues.5
In the present work, we have developed a convenient
method for the synthesis of indole derivatives substituted
with an enaminone moiety. The method is based on the
nucleophilic addition of a secondary amine to the C–C tri-
ple bond of the 2-ethynyl-4,5,6,7-tetrahydro-1H-indoles
1a–c and 2a–c. Dimethylamine and diethylamine were
chosen as secondary amines, and benzoyl and ethoxycar-
bonyl functions were selected as substituents for the eth-
ynyl moiety.
The hydroamination of 1-phenyl-3-(4,5,6,7-tetrahydro-
1H-indol-2-yl)prop-2-ynones 1a–c with dimethylamine
or diethylamine proceeded smoothly and stereoselectively
at room temperature in ethanol for one hour (dimethyl-
amine was used as a 24% aqueous solution). The adducts
(64–88% yield) were E-isomers of the corresponding 3-
(dialkylamino)-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)-1-
phenylprop-2-en-1-ones 3a–c and 4a–c; these were ob-
tained either exclusively (in the case of indole 1a) or pre-
dominantly (in the case of indoles 1b and 1c) (Table 1).
The combination of an enaminone moiety and an indole
nucleus in a single molecule can result in synergism be-
tween the properties of these important chemical struc-
tures, and could substantially expand the range of
SYNTHESIS 2010, No. 14, pp 2468–2474
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x.
x
x
.
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Advanced online publication: 05.05.2010
DOI: 10.1055/s-0029-1218768; Art ID: Z07310SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Hydroamination of 2-Ethynyl-4,5,6,7-tetrahydroindoles
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Table 1 The Hydroamination of 1-Phenyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-yn-1-ones 1a–c
R2
R2
R2
R2
N
N
(R2)2NH
r.t., 1 h
N
R1
N
R1
N
R1
+
O
O
Ph
O
Ph
Ph
1a–c
(Z)-3b,c, 4b,c
(E)-3a–c, 4a–c
Product
R1
H
R2
Me
Me
Me
Et
Yield (%)
3a
3b
3c
4a
4b
4c
88
87
80
64
83
81
Me
Bn
H
Me
Bn
Et
Et
R2
In the case of N-substituted indoles 1b and 1c, the stereo-
selectivity for the addition of diethylamine was always
higher (92%) than that for dimethylamine (72–78%). The
stereochemistry of the hydroamination of alkynes is
known to be controlled by simultaneous intramolecular
proton transfer to the emerging carbanion centre in the in-
termediate zwitterion A. This allows the Z-configuration
of the carbanion to be fixed (Scheme 1).9 In this case,
however, the E-isomers were obtained not only as the ki-
netic, but also as the thermodynamic products, although
minor amounts of the Z-isomer were obtained, probably
as a result of the kinetically controlled reactions. Indeed,
when a 78:22 mixture of the E- and Z-isomers of 3c was
boiled in benzene for one hour, only the E-isomer of 3c re-
mained in solution (1H NMR), showing that the Z-isomer
is readily and completely converted into the E-isomer.
R2
N
N
H
.
.
O
Ph
Figure 1 Intramolecular hydrogen bonding in compounds 3a and
4a
The hydroamination of indole 1a with diethylamine (1:1
aqueous ethanol, triethylamine, 50 °C, 1 h) was accompa-
nied by formation of minor amounts of enol 5a and its
keto tautomer 5b (4:1 ratio; yield 6%), as a result of hy-
drolysis of adduct 4a (Scheme 2). This shows that water
does not add across the triple bond of indole 1a, but that
the adduct 4a is almost completely converted into the
enol–ketone equilibrium mixture of 5a and 5b (4:1).
R2
R2
N
H
OH
N
R1
O
3a–c, 4a–c
NEt2
–
H2O
NEt2
O
N
H
N
H
– Et2NH
Ph
A
O
Ph
Ph
Scheme 1 The stereochemistry of the alkyne hydroamination reac-
4a
tion
O
OH
The higher stability of the E-isomers of adducts 3a–c, 4a–
c is surprising because they are sterically more strained
than the corresponding Z-isomers. Whereas the NH-ad-
ducts 3a and 4a can be stabilized by intramolecular hydro-
gen bonding between the NH and the benzoyl function
(Figure 1), this cannot take place for the N-substituted ad-
ducts 3b, 3c, 4b, and 4c. It is therefore likely that most of
the stabilization of the E-isomers is provided by the stron-
ger conjugation between the dialkylamino and benzoyl
substituents when these have a trans-orientation, in agree-
ment with reports in the literature.9
N
H
N
H
O
O
4:1
Ph
Ph
5a
5b
Scheme 2 The formation of the enol–ketone mixture of 5a and 5b
In contrast to the hydroamination of indoles 1a–c by di-
methylamine, this amine failed to add to the triple bond of
ethyl 3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-ynoates
2a–c. Surprisingly, under analogous conditions (aqueous
24% dimethylamine, room temperature, 1 h), the esters
Synthesis 2010, No. 14, 2468–2474 © Thieme Stuttgart · New York

2470
L. N. Sobenina et al.
PAPER
Table 2 Reactions of Ethyl 3-(4,5,6,7-Tetrahydro-1H-indol-2-yl)prop-2-ynoates 2a–c with Dimethylamine
NMe2
Me2NH
N
R1
N
R1
N
H
+
O
O
O
OEt
NMe2
OEt
2a–c
7a
6a–c
Pyrrole
R1
H
Yield (%)
6a
6b
6c
70a
77
Me
Bn
86
a Minor amounts of the expected product 7a were also obtained (NMR).
2a–c were readily converted into the corresponding Propynamides are generally considered to be key interme-
N,N-dimethyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop- diates in heterocyclic chemistry,10 so that the synthesized
ynamides 6a–c in 70–86% yields, the triple bond remain- N,N-dimethyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-
ing intact despite the presence of a large excess of ynamides 6a–c are likely to be useful building blocks, par-
dimethylamine (Table 2).
ticularly for the preparation of previously inaccessible
functional indole derivatives.
Minor amounts (6–8%) of the expected adduct (7a) were
discernible (1H NMR) only in the case of the reaction of Unlike dimethylamine, diethylamine reacted with prop-
indole 2a. When the reaction of indole 2a with dimethyl- ynoates 2a–c (aqueous ethanol, 50 °C, 1 h) by hydroami-
amine was performed at a higher temperature (50 °C, 1 h), nation to give the E-isomers of adducts 10a–c
N,N-dimethyl-3-oxo-3-(4,5,6,7-tetrahydro-1H-indol-2-
yl)propanamide (8) was obtained as the major product, to-
gether with the propynamide 6a, in a ratio of 4:1.
stereospecifically in up to 85% yield (Table 3).
The reaction of propynoate 2a with diethylamine was ac-
companied by partial intramolecular cyclization of the
Ketone 8 was also formed selectively in 64% yield by primary adduct 10a to form the pyrroloindole 11; the ratio
heating propynamide 6a in aqueous dimethylamine at of 10a to 11 was 1:1. Attempts to separate this mixture by
50 °C for one hour, probably as a result of hydrolysis of column chromatography on alumina resulted in the isola-
adduct 9 (Scheme 3). The straightforward hydrolysis of tion of pyrroloindole 11 exclusively as a result of the com-
amide 6a with water in the presence of triethylamine plete cyclization of adduct 10a.
(50 °C, 1 h) did not occur, and amide 6a was recovered.
NMe2
O
O
Me2NH
N
H
H2O
O
N
H
N
H
O
NMe2
6a
NMe2
NMe2
8
9
Scheme 3 Synthesis of N,N-dimethyl-3-oxo-(4,5,6,7-tetrahydro-1H-indol-2-yl)propanamide (8)
Table 3 Reactions of Ethyl 3-(4,5,6,7-Tetrahydro-1H-indol-2-yl)prop-2-ynoates 2a–c with Diethylamine
NEt2
Et2NH
NEt2
N
R1
N
R1
O
N
+
O
50 °C, 1 h
O
OEt
OEt
2a–c
10a–c
11
Pyrrole
R1
H
Yield (%)
10a
11
37
34
85
84
10b
10c
Me
Bn
Synthesis 2010, No. 14, 2468–2474 © Thieme Stuttgart · New York

PAPER
Hydroamination of 2-Ethynyl-4,5,6,7-tetrahydroindoles
2471
and dried (K2CO3). The residue after removal of Et2O was purified
by flash chromatography (Al2O3, hexane then Et2O) to give corre-
sponding adducts 3a–c.
Thus, dimethylamine and diethylamine reacted with in-
doles 2a–c in different ways. With dimethylamine, the
propynamides 6a–c were formed, whereas amidation did
not occur in the case of diethylamine. When aqueous di-
ethylamine of the same concentration as dimethylamine
(24%) was used, the indole 2a also failed to give the prop-
ynamide either at room temperature or on heating (50 °C,
1 h). In the latter case, as with neat diethylamine, the ad-
ducts 10a–c were formed exclusively.
(2E)-3-(Dimethylamino)-1-phenyl-3-(4,5,6,7-tetrahydro-1H-in-
dol-2-yl)prop-2-en-1-one (3a)
Light yellow crystals; yield: 88%; mp 144–145 °C.
IR (KBr): 3435, 3030, 2921, 2850, 2804, 1656, 1618, 1590, 1559,
1500, 1484, 1432, 1383, 1276, 1238, 1218, 1171, 1153, 1141, 1051,
1024, 991, 967, 922, 832, 758, 746, 699, 621 cm–1.
1H NMR (400.13 MHz, CDCl3): d = 1.80 (m, 4 H, CH2-5,6), 2.57
(m, 2 H, CH2-4), 2.70 (m, 2 H, CH2-7), 3.14 (s, 6 H, NMe2), 5.67 (s,
1 H, HC=), 6.31 (s, 1 H, H-3), 7.36 (m, 3 H, Hm,p), 7.85 (m, 2 H, Ho),
10.00 (br s, 1 H, NH).
13C NMR (101.6 MHz, CDCl3): d = 23.0, 23.3, 23.5, 23.7 (CH2-
4,5,6,7), 43.6 (NMe2), 93.1 (HC=), 114.4 (C-3), 119.9 (C-4), 124.1
(C-2), 127.6 (Co), 128.0 (Cm), 130.2 (Cp), 132.1 (C-5), 142.6 (Ci),
158.3 (=CN), 185.6 (CO).
This unexpected and pronounced difference in the reactiv-
ity of dimethylamine and diethylamine towards prop-
ynoates 2a–c can probably be attributed to differences in
the steric characteristics of these two amines, because
their basicities and their nucleophilicities are similar.11
There is at least one other reported case in which the rate
of reaction of diethylamine was found to be two orders of
magnitude slower than that of dimethylamine as a result
of their different steric requirements.12
Anal. Calcd for C19H22N2O: C, 77.52; H, 7.53; N, 9.52. Found:
77.28; H, 7.39; N, 9.73.
To summarize, a new method has been developed for the
synthesis of 2-substituted amino derivatives of 4,5,6,7-tet-
rahydro-1H-indoles. The method is based on the hy-
droamination of available 2-ethynyl-4,5,6,7-tetrahydro-
1H-indoles bearing electron-withdrawing substituents at
the triple bond. The addition of a secondary dialkylamine
across the triple bond of a 1-phenyl-3-(4,5,6,7-tetrahydro-
1H-indol-2-yl)prop-2-yn-1-one gives predominantly the
corresponding E-adduct in 72–92% stereoselectivity and
64–88% yield. Dimethylamine and diethylamine showed
different behaviors in their reactions with ethyl 3-(4,5,6,7-
tetrahydro-1H-indol-2-yl)prop-2-ynoates. Whereas the
former chemoselectively converted the ester function into
an amide function, the latter added chemo-, regio-, and
stereoselectively to the triple bond. In the case of the N–H
derivative of 4,5,6,7-tetrahydro-1H-indole, partial in-
tramolecular cyclization occurred to give pyrroloindole in
addition to the corresponding adduct. This new methodol-
ogy opens a route to previously inaccessible 2-substituted
amino derivates of indole and to analogues of tryptophan,
tryptamine, serotonin, melatonin, and other important
physiologically significant indole compounds.
(2E/Z)-3-(Dimethylamino)-3-(1-methyl-4,5,6,7-tetrahydro-1H-
indol-2-yl)-1-phenylprop-2-en-1-one (3b)
Yellow oil; Yield: 87%; E/Z = 72:28.
IR (KBr): 3437, 3055, 2924, 2852, 1631, 1596, 1574, 1514, 1461,
1423, 1384, 1296, 1219, 1195, 1168, 1132, 1046, 1016, 936, 918,
764, 699 cm–1.
1H NMR (400.13 MHz, CDCl3): d (E-isomer) = 1.70 (m, 4 H, CH2-
5,6), 2.48 (m, 2 H, CH2-4), 2.52 (m, 2 H, CH2-7), 3.00 (s, 6 H,
NMe2), 3.30 (s, 3 H, NMe), 5.83 (s, 1 H, HC=), 5.85 (s, 1 H, H-3),
7.31 (m, 2 H, Hm), 7.34 (m, 1 H, Hp), 7.72 (m, 2 H, Ho).
1H NMR (400.13 MHz, CDCl3): d (Z-isomer) = 1.80 (m, 4 H, CH2-
5,6), 2.48 (m, 2 H, CH2-4), 2.53 (m, 2 H, CH2-7), 3.10 (s, 6 H,
NMe2), 3.44 (s, 3 H, NMe), 5.68 (s, 1 H, HC=), 6.18 (s, 1 H, H-3),
7.38 (m, 3 H, Hm,p), 7.92 (m, 2 H, Ho).
13C NMR (101.6 MHz, CDCl3): d (E-isomer) = 22.2, 23.0, 23.2,
23.5 (CH2-4,5,6,7), 30.0 (NMe), 41.0 (NMe2), 97.4 (HC=), 110.3
(C-3), 117.3 (C-4), 125.1 (C-2), 127.5 (Cm), 127.6 (Co), 130.1 (Cp),
131.1 (C-5), 142.4 (Ci), 156.4 (=CN), 189.0 (CO).
13C NMR (101.6 MHz, CDCl3): d (Z-isomer) = 22.2, 23.0, 23.2,
23.5 (CH2-4,5,6,7), 31.2 (NMe), 43.5 (NMe2), 96.5 (HC=), 113.6
(C-3), 118.2 (C-4), 127.9 (Co), 128.0 (Cm), 128.7 (C-2), 130.4 (Cp),
133.2 (C-5), 142.3 (Ci), 155.9 (=CN), 184.1 (CO).
Anal. Calcd for C20H24N2O: C, 77.89; H, 7.84; N, 9.08. Found: C,
77.58; H, 7.95; N, 9.24.
IR spectra were recorded on a Bruker Vertex 70 spectrometer (400–
4000 cm–1, KBr pellets). The 1H (400.13 MHz) and 13C NMR (101.6
MHz) spectra were recorded on Bruker DPX-400 instrument. In all
cases, the 1H and 13C resonances were assigned by concerted appli-
cation of COSY and NOESY 1H–1H 2D homonuclear experiments
(2E/Z)-3-(1-Benzyl-4,5,6,7-tetrahydro-1H-indol-2-yl)-3-(di-
methylamino)-1-phenylprop-2-en-1-one (3c)
Yellow oil; Yield: 80%; E/Z = 78:22.
1
and by HSQC and HMBC H–13C 2D heteronuclear experiments,
IR (KBr): 3435, 3058, 3027, 2923, 2850, 1630, 1575, 1513, 1426,
1390, 1294, 1218, 1133, 1050, 1022, 918, 767, 696 cm–1.
respectively. 1-Phenyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-
yn-1-ones 1a–c and ethyl 3-(4,5,6,7-tetrahydro-1H-indoles-2-
yl)prop-2-ynoates 2a–c were prepared from the corresponding pyr-
roles and 3-bromo-1-phenylprop-2-yn-1-one or ethyl 3-bromoprop-
2-ynoate, respectively, according to the literature methods.6,7
1H NMR (400.13 MHz, CDCl3): d (E-isomer) = 1.79 (m, 4 H, CH2-
5,6), 2.48 (m, 4 H, CH2-4,7), 2.73 (br s, 6 H, NMe2), 4.80 (d,
J = 16.0 Hz, 1 H, CH2), 4.94 (d, J = 16.0 Hz, 1 H, CH2), 5.78 (s, 1
H, HC=), 5.91 (s, 1 H, H-3), 6.98 (m, 2 H, Ho CH2Ph), 7.19–7.33
(m, 6 H, Hm,p CH2Ph, COPh), 7.81 (m, 2 H, Ho COPh).
1H NMR (400.13 MHz, CDCl3): d (Z-isomer) = 1.79 (m, 4 H, CH2-
5,6), 2.48 (m, 4 H, CH2-4,7), 2.98 (s, 6 H, NMe2), 5.06 (br s, 2 H,
CH2), 5.58 (s, 1 H, HC=), 6.20 (s, 1 H, H-3), 6.84 (m, 2 H, Ho
CH2Ph), 7.19–7.33 (m, 6 H, Hm,p CH2Ph, COPh), 7.70 (m, 2 H, Ho
COPh).
Hydroamination of 1-Phenyl-3-(4,5,6,7-tetrahydro-1H-indoles-
2-yl)prop-2-yn-1-ones 1a–c with Dimethylamine; General Pro-
cedure
A 24% aq soln of Me2NH (5 mL) was added to a soln of indole 1a–
c (1 mmol) in EtOH (2 mL), and the mixture was stirred at r.t. for 1
h. The mixture was then diluted with H2O (1:2) and extracted with
Et2O (5 × 3 mL). The extracts were washed with H2O (3 × 3 mL)
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L. N. Sobenina et al.
PAPER
13C NMR (101.6 MHz, CDCl3): d (E-isomer) = 22.4, 23.1, 23.2,
23.6 (CH2-4,5,6,7), 29.7 (NMe2), 46.8 (CH2), 95.6 (HC=), 109.9 (C-
3), 117.6 (C-4), 124.7 (C-2), 127.0 (Cp CH2Ph), 127.1 (Cm CH2Ph),
127.5 (Co CH2Ph), 127.9 (Cm COPh), 128.3 (Co COPh), 130.2 (Cp
COPh), 130.8 (C-5), 139.3 (Ci CH2Ph), 142.3 (Ci COPh), 156.6
(=CN), 187.5 (CO).
13C NMR (101.6 MHz, CDCl3): d (Z-isomer) = 22.4, 23.1, 23.2,
23.6 (CH2-4,5,6,7), 30.6 (NMe2), 47.2 (CH2), 95.8 (HC=), 113.8 (C-
3), 118.3 (C-4), 126.0 (C-2), 127.1 (Cp CH2Ph), 127.4 (Cm, CH2Ph),
127.8 (Co CH2Ph), 128.4 (Cm COPh), 128.5 (Co COPh), 130.5 (Cp
COPh), 133.1 (C-5), 138.3 (Ci CH2Ph), 141.4 (Ci COPh), 155.7
(=CN), 183.8 (CO).
13C NMR (101.6 MHz, CDCl3): d (enol 5a) = 22.8, 22.9, 23.1, 23.5
(CH2-4,5,6,7), 92.9 (HC=), 114.5 (C-3), 121.1 (C-4), 126.4 (C-2),
128.6 (Cm), 128.9 (Co), 131.4 (Cp), 134.7 (C-5), 136.3 (Ci), 175.2
(CO), 181.5 (COH).
(2E/Z)-3-(Diethylamino)-3-(1-methyl-4,5,6,7-tetrahydro-1H-in-
dol-2-yl)-1-phenylprop-2-en-1-one (4b)
Light yellow crystals; yield: 83%; E/Z = 92:8; mp 94–96 °C.
IR (KBr): 3449, 3057, 2976, 2926, 2845, 1614, 1596, 1575, 1532,
1508, 1479, 1433, 1354, 1276, 1215, 1198, 1155, 1072, 1052, 1026,
992, 897, 812, 777, 705, 656 cm–1.
1H NMR (400.13 MHz, CDCl3): d (E-isomer) = 1.18 (br s, 6 H,
Me), 1.72 (m, 4 H, CH2-5,6), 2.45 (m, 4 H, CH2-4,7), 3.24 (s, 3 H,
NMe), 3.34 (br m, 4 H, NCH2), 5.82 (s, 1 H, HC=), 5.92 (s, 1 H, H-
3), 7.28 (m, 3 H, Hm,p), 7.68 (m, 2 H, Ho).
Anal. Calcd for C26H28N2O: C, 81.21; H, 7.34; N, 7.29. Found: C,
80.89; H, 7.15; N, 7.13.
Hydroamination of 1-Phenyl-3-(4,5,6,7-tetrahydro-1H-indol-2-
yl)prop-2-yn-1-one (1a) with Diethylamine; Typical Procedure
Et2NH (1 mL) was added to a soln of indole 1a (0.200 g, 0.8 mmol)
in EtOH (2 mL), and the mixture was stirred at r.t. for 1 h. The mix-
ture was then diluted with H2O (1:3) and extracted with Et2O (5 × 3
mL). The extracts were washed with H2O (3 × 3 mL) and dried
(K2CO3). The residue (0.194 g) after removal of Et2O was washed
with cold Et2O (3 mL) to give enol 5a together with its tautomer 5b
in a 4:1 ratio; yield: 0.013 g (6%). Fractionated of the Et2O soln by
column chromatography [Al2O3, hexane–Et2O (1:1)] gave the ad-
duct 4a; yield: 0.166 g (64%).
13C NMR (101.6 MHz, CDCl3): d (E-isomer) = 13.2 (2 Me), 21.9,
23.0, 23.1, 23.5 (CH2-4,5,6,7), 29.9 (NMe), 46.2 (2 NCH2), 96.6
(HC=), 108.3 (C-3), 116.9 (C-4), 124.9 (C-2), 127.2 (Cm), 127.5
(Co), 129.7 (Cp), 130.2 (C-5), 142.4 (Ci), 155.0 (=CN), 188.6 (CO).
Anal. Calcd for C22H28N2O: C, 78.53; H, 8.39; N, 8.33. Found: C,
78.13; H, 8.19; N, 8.01.
(2E/Z)-3-(1-Benzyl-4,5,6,7-tetrahydro-1H-indol-2-yl)-3-(dieth-
ylamino)-1-phenylprop-2-en-1-one (4c)
Light yellow solid; yield: 81%; E/Z = 92:8; mp 106–108 °C.
IR (KBr): 3436, 3059, 3030, 2964, 2927, 2849, 1626, 1572, 1524,
1479, 1457, 1434, 1378, 1355, 1345, 1218, 1197, 1158, 1074, 1055,
999, 898, 782, 730, 698 cm–1.
1H NMR (400.13 MHz, CDCl3): d (E-isomer) = 0.99 (br s, 6 H,
Me), 1.70 (m, 4 H, CH2-5,6), 2.45 (m, 2 H, CH2-4), 2.54 (m, 2 H,
CH2-7), 3.24 (br m, 4 H, NCH2), 4.78 (d, J = 16.0 Hz, 1 H, CH2),
4.90 (d, J = 16.0 Hz, 1 H, CH2), 5.86 (s, 1 H, HC=), 5.91 (s, 1 H, H-
3), 6.98 (m, 2 H, Ho CH2Ph), 7.20 (m, 3 H, Hm,p CH2Ph), 7.32 (m, 3
H, Hm,p COPh), 7.78 (m, 2 H, Ho COPh).
13C NMR (101.6 MHz, CDCl3): d (E-isomer) = 14.1 (2 Me), 22.6,
23.2, 23.4, 23.7 (CH2-4,5,6,7), 41.1 (2 NCH2), 46.9 (CH2), 94.6
(HC=), 109.2 (C-3), 117.3 (C-4), 124.8 (C-2), 127.0 (Cp CH2Ph),
127.2 (Cm CH2Ph), 127.4 (Co CH2Ph), 127.9 (Cm COPh), 128.4 (Co
COPh), 130.1 (Cp COPh), 130.9 (C-5), 139.4 (Ci CH2Ph), 142.7 (Ci
COPh), 155.5 (=CN), 187.6 (CO).
Adducts 4b and 4c were obtained analogously.
(2E)-3-(Diethylamino)-1-phenyl-3-(4,5,6,7-tetrahydro-1H-in-
dol-2-yl)prop-2-en-1-one (4a)
Yellow crystals; yield: 64%; mp 128–129 °C.
IR (KBr): 3207, 3004, 2927, 2850, 2838, 1599, 1582, 1541, 1496,
1456, 1436, 1356, 1340, 1321, 1308, 1293, 1263, 1250, 1150, 1140,
1093, 1067, 998, 817, 787, 772, 739, 702, 639, 515 cm–1.
1H NMR (400.13 MHz, CDCl3): d = 1.23 (t, J = 7.0 Hz, 6 H, Me),
1.75 (m, 4 H, CH2-5,6), 2.50 (m, 2 H, CH2-4), 2.61 (m, 2 H, CH2-
7), 3.52 (q, J = 7.0 Hz, 4 H, NCH2), 5.79 (s, 1 H, HC=), 6.25 (s, 1
H, H-3), 7.34 (m, 3 H, Hm,p), 7.79 (m, 2 H, Ho), 10.65 (br s, 1 H,
NH).
13C NMR (101.6 MHz, CDCl3): d = 13.0 (2 Me), 23.0, 23.1, 23.2,
23.7 (CH2-4,5,6,7), 45.8 (2 NCH2), 95.3 (HC=), 112.6 (C-3), 119.4
(C-4), 124.6 (C-2), 127.5 (Co), 127.9 (Cm), 130.1 (Cp), 131.6 (C-5),
142.7 (Ci), 157.3 (=CN), 186.4 (CO).
Anal. Calcd for C28H32N2O: C, 81.51; H, 7.82; N, 6.79. Found: C,
81.13; H, 7.52; N, 6.57.
Anal. Calcd for C21H26N2O: C, 78.22; H, 8.13; N, 8.69. Found: C,
77.94; H, 8.25; N, 8.47.
N,N-Dimethyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-yn-
amide (6a); Typical Procedure
A 24% aq soln of Me2NH (5 mL) was added to a soln of indole 2a
(0.217 g, 1 mmol) in EtOH (2 mL), and the mixture was stirred at
r.t. for 1 h. The mixture was then diluted with H2O (1:2) and filtered
to give yellow crystals: yield: 0.152 g (70%); mp 220–221 °C [aq
acetone (1:1)]. Minor amounts of the expected product 7a were de-
tected by 1H NMR.
(2Z)-3-Hydroxy-1-phenyl-3-(4,5,6,7-tetrahydro-1H-indol-2-
yl)prop-2-en-1-one (5a) and 1-Phenyl-3-(4,5,6,7-tetrahydro-1H-
indol-2-yl)propane-1,3-dione (5b)
Yellow crystals; yield: 6%; mp 200–202 °C.
IR (KBr): 3414, 3368, 3276, 3063, 3030, 2925, 2851, 1730, 1626,
1598, 1573, 1529, 1507, 1473, 1447, 1447, 1389, 1368, 1389, 1364,
1327, 1271, 1225, 1190, 1145, 1068, 972, 833, 815, 802, 771, 691,
614, 561 cm–1.
1H NMR (400.13 MHz, CDCl3): d (enol 5a) = 1.78 (m, 4 H, CH2-
5,6), 2.52 (m, 2 H, CH2-4), 2.63 (m, 2 H, CH2-7), 6.47 (s, 1 H, HC=),
6.73 (s, 1 H, H-3), 7.43 (m, 3 H, Hm,p), 7.87 (m, 2 H, Ho), 9.17 (br s,
1 H, NH), 16.04 (s, 1 H, OH).
1H NMR (400.13 MHz, CDCl3): d (diketone 5b) = 1.78 (m, 4 H,
CH2-5,6), 2.52 (m, 2 H, CH2-4), 2.58 (m, 2 H, CH2-7), 4.34 (s, 2 H,
CH2), 6.77 (s, 1 H, H-3), 7.52 (m, 3 H, Hm,p), 8.03 (m, 2 H, Ho), 9.04
(br s, 1 H, NH).
IR (KBr): 3225, 3157, 3072, 2936, 2853, 2183, 1616, 1583, 1434,
1397, 1359, 1302, 1264, 1182, 1160, 1114, 827, 814, 719 cm–1.
1H NMR (400.13 MHz, CDCl3): d = 1.74 (m, 4 H, CH2-5,6), 2.45
(m, 2 H, CH2-4), 2.55 (m, 2 H, CH2-7), 2.98 (s, 3 H, NMe2), 3.20 (s,
3 H, NMe2), 6.38 (s, 1 H, H-3), 8.39 (br s, 1 H, NH).
13C NMR (101.6 MHz, CDCl3): d = 22.7, 23.0, 23.1, 23.5 (CH2-
4,5,6,7), 34.1 (NMe2), 38.3 (NMe2), 84.9 (C≡), 86.2 (≡C), 108.0 (C-
2), 117.3 (C-3), 118.9 (C-4), 132.5 (C-5), 155.6 (CO).
Anal. Calcd for C13H16N2O: C, 72.19; H, 7.46; N, 12.95. Found: C,
71.86; H, 7.25; N, 12.59.
Synthesis 2010, No. 14, 2468–2474 © Thieme Stuttgart · New York

PAPER
Hydroamination of 2-Ethynyl-4,5,6,7-tetrahydroindoles
2473
Ethyl 3-(Dimethylamino)-3-(4,5,6,7-tetrahydro-1H-indol-2-
yl)acrylate (7a)
Reaction of Ethyl 3-(4,5,6,7-Tetrahydro-1H-indol-2-yl)prop-2-
ynoates 2a–c with Diethylamine; General Procedure
Et2NH (1 mL) was added to a soln of indole 2a–c (1 mmol) in EtOH
(2 mL), and the mixture was stirred at r.t. for 1 h. The mixture was
then diluted with H2O (1:3) and extracted with Et2O (5 × 3 mL). The
extracts were washed with H2O (3 × 3 mL) and dried (K2CO3). The
residue after removing Et2O was fractionated by column chroma-
tography [Al2O3, hexane–Et2O (1:1)] to afford the corresponding
adduct 10b, 10c, or 11.
1H NMR (400.13 MHz, CDCl3): d = 1.23 (t, J = 7.1 Hz, 3 H, Me),
1.75 (m, 4 H, CH2-5,6), 2.48 (m, 2 H, CH2-4), 2.61 (m, 2 H, CH2-
7), 2.90 (s, 6 H, NMe2), 4.03 (q, J = 7.1 Hz, 2 H, OCH2), 4.71 (s, 1
H, HC=), 6.17 (s, 1 H, H-3), 9.80 (br s, 1 H, NH).
The N,N-dimethyl-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-2-yn-
amides 6b and 6c were prepared similarly.
N,N-Dimethyl-3-(1-methyl-4,5,6,7-tetrahydro-1H-indol-2-
yl)prop-2-ynamide (6b)
Yellow crystals; yield: 77%; mp 107–108 °C.
Ethyl (2E)-3-(Diethylamino)-3-(4,5,6,7-tetrahydro-1H-indol-2-
yl)acrylate (10a)
1H NMR (400.13 MHz, CDCl3): d = 1.07 (m, 6 H, Me), 1.23 (t,
J = 7.1 Hz, 3 H, Me), 1.75 (m, 4 H, CH2-5,6), 2.48 (m, 2 H, CH2-4),
2.61 (m, 2 H, CH2-7), 3.24 (q, J = 7.0 Hz, 4 H, NCH2), 4.00 (q,
J = 7.1, 2 H, OCH2), 4.78 (s, 1 H, HC=), 6.05 (s, 1 H, H-3), 8.90 (br
s, 1 H, NH).
IR (KBr): 3437, 2934, 2849, 2185, 1617, 1503, 1465, 1388, 1264,
1202, 1116, 1054, 837, 816, 722, 545 cm–1.
1H NMR (400.13 MHz, CDCl3): d = 1.70 (m, 2 H, CH2-5), 1.80 (m,
2 H, CH2-6), 2.45 (m, 2 H, CH2-4), 2.50 (m, 2 H, CH2-7), 2.99 (s, 3
H, NMe2), 3.22 (s, 3 H, NMe2), 3.51 (s, 3 H, NMe), 6.39 (s, 1 H, H-
3).
13C NMR (101.6 MHz, CDCl3): d = 22.4, 22.8, 22.9, 23.3 (CH2-
4,5,6,7), 31.1 (NMe), 34.1 (NMe2), 38.2 (NMe2), 84.9 (C≡), 89.9
(≡C), 111.0 (C-2), 116.6 (C-3), 118.6 (C-4), 133.6 (C-5), 155.4
(CO).
Ethyl (2E)-3-(Diethylamino)-3-(1-methyl-4,5,6,7-tetrahydro-
1H-indol-2-yl)acrylate (10b)
Light brown oil; yield: 85%.
IR (KBr): 3494, 3095, 2975, 2930, 2851, 2184, 1699, 1666, 1590,
1564, 1506, 1445, 1362, 1273, 1253, 1191, 1171, 1136, 1083, 1049,
1008, 927, 799, 713 cm–1.
1H NMR (400.13 MHz, CDCl3): d = 1.04 (m, 9 H, Me), 1.66 (m, 2
H, CH2-6), 1.77 (m, 2 H, CH2-5), 2.47 (m, 4 H, CH2-4,7), 3.13 (m,
4 H, NCH2), 3.19 (s, 3 H, NMe), 3.90 (m, 2 H, OCH2), 4.83 (s, 1 H,
HC=), 5.72 (s, 1 H, H-3).
Anal. Calcd for C14H18N2O: C, 73.01; H, 7.88; N, 12.16. Found: C,
72.95; H, 7.62; N, 12.43.
3-(1-Benzyl-4,5,6,7-tetrahydro-1H-indol-2-yl)-N,N-dimethyl-
prop-2-ynamide (6c)
13C NMR (101.6 MHz, CDCl3): d = 14.6 (3 Me), 22.1, 23.2, 23.4,
23.8 (CH2-4,5,6,7), 29.9 (NMe), 43.7 (2 NCH2), 58.4 (OCH2), 87.9
(HC=), 106.7 (C-2), 116.7 (C-3), 125.0 (C-4), 128.8 (C-5), 154.0
(=CN), 168.0 (CO).
Greyish crystals; yield: 86%; mp 94–95 °C.
IR (KBr): 3456, 3064, 2930, 2854, 2182, 1622, 1495, 1436, 1389,
1377, 1361, 1268, 1237, 1148, 824, 805, 730, 697 cm–1.
1H NMR (400.13 MHz, CDCl3): d = 1.68 (m, 2 H, CH2-6), 1.73 (m,
2 H, CH2-5), 2.49 (m, 2 H, CH2-4), 2.59 (m, 2 H, CH2-7), 2.93 (s, 3
H, NMe2), 3.02 (s, 3 H, NMe2), 5.11 (s, 2 H, CH2), 6.49 (s, 1 H, H-
3), 7.01 (m, 2 H, Ho), 7.26 (m, 3 H, Hm,p).
13C NMR (101.6 MHz, CDCl3): d = 22.6, 22.9, 23.0, 23.3 (CH2-
4,5,6,7), 34.1 (NMe2), 38.1 (NMe2), 48.2 (CH2), 84.9 (C≡), 87.9
(≡C), 111.3 (C-2), 117.4 (C-3), 119.3 (C-4), 126.5 (Co), 127.4 (Cp),
128.8 (Cm), 133.6 (C-5), 137.7 (Ci), 155.3 (CO).
Anal. Calcd for C18H28N2O2: C, 71.02; H, 9.27; N, 9.20. Found: C,
70.75; H, 9.00; N, 8.95.
Ethyl (2E)-3-(1-Benzyl-4,5,6,7-tetrahydro-1H-indol-2-yl)-3-(di-
ethylamino)acrylate (10c)
Light yellow oil; yield: 84%.
IR (KBr): 3481, 3063, 3030, 2975, 2931, 2852, 2242, 2183, 1954,
1695, 1662, 1588, 1557, 1505, 1455, 1419, 1380, 1291, 1273, 1254,
1197, 1189, 1167, 1132, 1116, 1088, 1049, 1049, 925, 799, 698
cm–1.
Anal. Calcd for C20H22N2O: C, 78.40; H, 7.24; N, 9.14. Found: C,
78.21; H, 7.02; N, 8.89.
1H NMR (400.13 MHz, CDCl3): d = 0.91 (m, 6 H, Me), 1.12 (t,
J = 7.1 Hz, 3 H, Me), 1.78 (m, 4 H, CH2-5,6), 2.49 (m, 2 H, CH2-4),
2.56 (m, 2 H, CH2-7), 2.99 (br s, 4 H, NCH2), 3.95 (q, J = 7.1 Hz, 2
H, OCH2), 4.82 (s, 1 H, HC=), 4.80 (d, J = 7.6 Hz, 1 H, CH2Ph),
4.85 (d, J = 7.6 Hz, 1 H, CH2Ph), 5.84 (s, 1 H, H-3), 7.01 (m, 2 H,
Ho), 7.19 (m, 3 H, Hm,p).
13C NMR (101.6 MHz, CDCl3): d = 14.3 (3 Me), 22.3, 22.9, 23.2,
23.5 (CH2-4,5,6,7), 43.1 (2 NCH2), 46.6 (CH2), 58.0 (OCH2), 86.5
(HC=), 107.7 (C-2), 116.7 (C-3), 124.5 (C-4), 126.6 (Cp), 126.9
(Co), 128.0 (Cm), 128.9 (C-5), 138.6 (Ci), 153.8 (=CN), 167.6 (CO).
N,N-Dimethyl-3-oxo-3-(4,5,6,7-tetrahydro-1H-indol-2-yl)prop-
anamide (8)
A 24% aq soln of Me2NH (5 mL) was added to a soln of propyn-
amide 6a (0.216 g, 1 mmol) in EtOH (2 mL), and the mixture was
stirred at 50 °C for 1.5 h. The mixture was diluted with H2O (1:3)
and extracted with Et2O (5 × 3 mL). The extracts were washed with
H2O (3 × 3 mL) and dried (K2CO3). The residue after removal of
Et2O was washed with cold Et2O and dried to give grey crystals;
yield: 0.150 g (64%); mp 148–149 °C.
IR (KBr): 3429, 3263, 2929, 2857, 1649, 1621, 1516, 1499, 1463,
1398, 1364, 1334, 1261, 1228, 1182, 1144, 1124, 1053, 956, 856,
614 cm–1.
Anal. Calcd for C24H32N2O2: C, 75.75; H, 8.48; N, 7.36. Found: C,
75.52; H, 8.39; N, 7.48.
1H NMR (400.13 MHz, CDCl3): d = 1.75 (m, 4 H, CH2-5,6), 2.49
(m, 2 H, CH2-4), 2.59 (m, 2 H, CH2-7), 2.94 (s, 3 H, NMe2), 3.01 (s,
3 H, NMe2), 3.80 (s, 2 H, CH2), 6.80 (s, 1 H, H-3), 9.13 (br s, 1 H,
NH).
1-(Diethylamino)-5,6,7,8-tetrahydro-3H-pyrrolo[1,2-a]indol-3-
one (11)
Orange crystals; yield: 34%; mp 100–101 °C.
13C NMR (101.6 MHz, CDCl3): d = 22.7, 22.9, 23.1, 23.5 (CH2-
4,5,6,7), 35.7 (NMe2), 38.3 (NMe2), 45.7 (CH2), 117.4 (C-3), 129.0
(C-4), 129.7 (C-2), 137.5 (C-5), 167.6 (NCO), 181.9 (CO).
IR (KBr): 3439, 3105, 2987, 2969, 2930, 2867, 2847, 1694, 1598,
1505, 1468, 1451, 1427, 1395, 1378, 1362, 1348, 1325, 1303, 1257,
1210, 1154, 1111, 1077, 991, 958, 802, 779, 750, 710, 648, 620, 607
cm–1.
Anal. Calcd for C13H18N2O2: C, 66.64; H, 7.74; N, 11.96. Found: C,
66.28; H, 7.56; N, 11.59.
Synthesis 2010, No. 14, 2468–2474 © Thieme Stuttgart · New York

2474
L. N. Sobenina et al.
PAPER
(5) (a) Baraldi, P. G.; Barco, A.; Benetti, S.; Pollini, G. P.;
1H NMR (400.13 MHz, CDCl3): d = 1.23 (t, J = 6.6 Hz, 6 H, Me),
1.72 (m, 4 H, CH2-5,6), 2.39 (m, 2 H, CH2-4), 2.74 (m, 2 H, CH2-
7), 3.28 (br s, 2 H, NCH2), 3.46 (br s, 2 H, NCH2), 4.23 (s, 1 H,
HC=), 5.90 (s, 1 H, H-3).
13C NMR (101.6 MHz, CDCl3): d = 13.7 (2 Me), 22.4, 22.5, 22.9,
23.3 (CH2-4,5,6,7), 46.2 (2 NCH2), 80.6 (HC=), 110.9 (C-3), 122.1
(C-4), 126.8 (C-5), 131.5 (C-2), 157.8 (=CN), 167.7 (CO).
Simoni, D. Synthesis 1987, 857. (b) Lue, P.; Greenhill, J. V.
Adv. Heterocycl. Chem. 1997, 67, 207.
(6) (a) Trofimov, B. A.; Stepanova, Z. V.; Sobenina, L. N.;
Mikhaleva, A. I.; Ushakov, I. A. Tetrahedron Lett. 2004, 45,
6513. (b) Stepanova, Z. V.; Sobenina, L. N.; Mikhaleva, A.
I.; Ushakov, I. A.; Elokhina, V. N.; Voronov, V. K.;
Trofimov, B. A. Synthesis 2004, 2736. (c) Sobenina, L. N.;
Demenev, A. P.; Mikhaleva, A. I.; Ushakov, I. A.;
Vasil’tsov, A. M.; Ivanov, A. V.; Trofimov, B. A.
Tetrahedron Lett. 2006, 47, 7139. (d) Trofimov, B. A.;
Sobenina, L. N.; Stepanova, Z. V.; Demenev, A. P.;
Mikhaleva, A. I.; Ushakov, I. A.; Vakul’skaya, T. I.;
Petrova, O. V. Russ. J. Org. Chem. 2006, 42, 1348.
(e) Trofimov, B. A.; Sobenina, L. N.; Stepanova, Z. V.;
Ushakov, I. A.; Petrova, O. V.; Tarasova, O. A.; Volkova, K.
A.; Mikhaleva, A. I. Synthesis 2007, 447. (f) Trofimov, B.
A.; Sobenina, L. N.; Demenev, A. P.; Stepanova, Z. V.;
Petrova, O. V.; Ushakov, I. A.; Mikhaleva, A. I. Tetrahedron
Lett. 2007, 48, 4661.
MS: m/z = 244 [M+].
Anal. Calcd for C15H20N2O: C, 73.74; H, 8.25; N, 11.47. Found: C,
73.69; H, 8.48; N, 11.36.
Acknowledgment
The work was carried with financial support of leading scientific
schools by the President of the Russian Federation (Grant NSh-
3230.2010.3), the Russian Foundation of Basic Research (Grant
No. 09-03-00064a), and Project of RAN No. 5.9.
(7) (a) Trofimov, B. A.; Sobenina, L. N.; Stepanova, Z. V.;
Vakul’skaya, T. I.; Kazheva, O. N.; Aleksandrov, G. G.;
Dyachenko, O. A.; Mikhaleva, A. I. Tetrahedron 2008, 64,
5541. (b) Trofimov, B. A.; Sobenina, L. N.; Stepanova, Z.
V.; Petrova, O. V.; Ushakov, I. A.; Mikhaleva, A. I.
Tetrahedron Lett. 2008, 49, 3946.
(8) (a) Trofimov, B. A.; Mikhaleva, A. I. N-Vinylpyrroles;
Nauka: Novosibirsk, 1984. (b) Trofimov, B. A. Adv.
Heterocyclic Chem. 1990, 51, 177. (c) Trofimov, B. A.;
Mikhaleva, A. I.; Schmidt, E. Yu.; Ryapolov, O. A.;
Platonov, V. B. RU 2297410, 2006; Chem. Abstr. 2007, 146,
462130.
(9) Yakimovich, S. I.; Golodova, K. G. In Reactivity and
Organic Reaction Mechanisms, Vol. 2; Temnikova, T. I.,
Ed.; Leningrad State University Publishing House:
Leningrad, 1974, 186–213.
(10) (a) Patra, R.; Maiti, S. B.; Chatterjee, A.; Chakravarty, A. K.
Tetrahedron Let. 1991, 32, 1363. (b) Jacobi, P. A.; Guo, J.;
Rajeswari, S.; Zheng, W. J. Org. Chem. 1997, 62, 2907.
(c) Katritzky, A. R.; Zhang, Y.; Singh, S. K. J. Org. Chem.
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Monatsh. Chem. 2002, 133, 1067. (e) Reisch, J.; Usifoh, C.
O.; Oluwadiya, J. O. J. Heterocycl. Chem. 2009, 27, 1953.
(f) Burling, S.; Field, L. D.; Li, H. L.; Messerle, B. A.;
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