Facile addition of ketones to activated isoquinolines using N-methyl-2-pyrrolidinone

Article abstract of DOI:10.1055/s-2007-965961

Ketones undergo a smooth addition to N-acylisoquinolinium ions in N-methyl-2-pyrrolidinone, under mild conditions, to produce 1-alkylated isoquinoline derivatives in excellent yields with high regioselectivity. The use of commercially available N-methyl-2

Full text of DOI:10.1055/s-2007-965961

PAPER
1077
Facile Addition of Ketones to Activated Isoquinolines Using
N-Methyl-2-pyrrolidinone
A
ddition o
.
f
Ketone
s
to
S
A
ctivated
I
so
.
quinolines Yadav,* B. V. S. Reddy, Manoj K. Gupta, I. Prathap, Uttam Dash
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax +91(40)27160512; E-mail: yadavpub@iict.res.in
Received 9 October 2006; revised 12 December 2006
O
Abstract: Ketones undergo a smooth addition to N-acylisoquino-
NMP
r.t.
Cl–
O
+
N
OEt
+
linium ions in N-methyl-2-pyrrolidinone, under mild conditions, to
produce 1-alkylated isoquinoline derivatives in excellent yields
with high regioselectivity. The use of commercially available N-
methyl-2-pyrrolidinone makes this process quite simple, more con-
venient and practical.
N
OEt
Me
Ph
Ph
O
3a
96%
2
1a
O
Keywords: activated isoquinolines, ketones, 1-alkyl-1,2-dihydro-
Scheme 1
quinolines
NMP, resulted in the formation of ethyl 1-(2-oxo-2-phe-
nylethyl)-1,2-dihydro-2-isoquinolinecarboxylate (3a) in
96% yield (Scheme 1).
Quinoline and isoquinoline ring systems are important
structural units found in various naturally occurring alka-
loids and synthetic analogues with interesting biological
properties.1 Moreover, this class of compounds exhibit
good cytotoxic activity.2 Activated aza-aromatics are use-
ful building blocks in organic synthesis, especially for the
synthesis of various biologically active nitrogen-contain-
ing alkaloids.3 Knowing the importance of isoquinoline
derivatives, the preparation of new analogues of such
compounds is of prime importance for both synthetic and
medicinal chemistry. Generally, aza-aromatic compounds
can be activated by chloroformates or acyl chlorides.4
Therefore, it continues to be an important challenge in
synthetic organic chemistry to develop synthetic methods
based on activated quinolines and isoquinolines, since the
development of new analogues of isoquinoline would cer-
tainly extend their use in drug discovery. To the best of
our knowledge, there has been no study of addition reac-
tions of ketones to aza-aromatic systems in N-methyl-2-
pyrrolidinone (NMP).
Encouraged by the result obtained from acetophenone, we
turned our attention towards various other substituted ke-
tones. Satisfyingly, a range of ketones such as acetone, cy-
clohexanone and cyclopentanone underwent smooth
addition to the N-acylisoquinolinium system, under simi-
lar conditions, to afford 1-substituted-1,2-dihydroisoquin-
oline derivatives (Table 1; entries b–d). Similarly, the 5-
nitroisoquinolinium salt also reacted well with acetophe-
none, acetone, cyclohexanone and cyclopentanone to give
the corresponding adducts in good to excellent yields
(Table 1; entries e–h). However, unsymmetrical ketones
such as 2-butanone, 2-pentanone and 2-hexanone gave a
mixture of regioisomeric products 3 and 4 (Scheme 2 and
Table 1; entries i–k).
The adduct 3 was obtained as the major product and could
be easily separated from the minor product 4 by column
chromatography. Surprisingly, in contrast to the success-
ful results obtained with N-acylisoquinolinium salts,
when the addition of ketones to activated quinolines was
attempted under similar conditions, the N-acylquinolini-
um salt did not give the desired product.
As part of our studies on activated quinolines and isoquin-
olines,5 we herein report our results on the nucleophilic
addition of ketones to 2-(ethoxycarbonyl)isoquinolinium
salts using NMP. Accordingly, treatment of 2-(ethoxycar-
bonyl)isoquinolinium salt (1a) with acetophenone in
The effect of solvent on the reaction was studied through
the reaction of acetone with the N-acylisoquinolinium ion
in various solvents (Table 2).
O
NMP
Cl–
O
+
N
OEt
Me
+
+
N
OEt
N
R
OEt
r.t.
R
Me
O
O
R
O
O
R = Me, Et, n-Pr
3 (major)
4 (minor)
Scheme 2
SYNTHESIS 2007, No. 7, pp 1077–1081
x
x
.
x
x
.2
0
0
7
Advanced online publication: 28.02.2007
DOI: 10.1055/s-2007-965961; Art ID: Z19806SS
© Georg Thieme Verlag Stuttgart · New York

1078
J. S. Yadav et al.
PAPER
Table 1 Nucleophilic Addition of Ketones to Activated Isoquinolines
Entry
a
Activated isoquinolines Ketones
Producta
Time (h) Yield (%)b
1
2
3
4
1.0
95
O
O
O
+
N
N
Me
Ph
OEt
Ph
Cl–
OEt
O
N
b
c
2.5
1.5
82
93
O
O
Me
Me
OEt
Me
O
N
O
O
OEt
O
d
e
1.5
1.5
91
93
O
O
N
OEt
O
NO2
O
NO2
NO2
NO2
Me
Ph
O
O
+
N
N
O
OEt
OEt
Ph
Cl–
f
2.0
2.5
85
91
O
Me
Me
O
N
O
OEt
Me
g
O
O
N
OEt
O
h
2.0
3.0
89
O
NO2
O
N
OEt
O
i
85 (8:2)c
O
O
O
O
Me
+
N
Cl–
N
O
N
O
Me
OEt
OEt
Me
OEt
Me
Me
Synthesis 2007, No. 7, 1077–1081 © Thieme Stuttgart · New York

PAPER
Addition of Ketones to Activated Isoquinolines
1079
Table 1 Nucleophilic Addition of Ketones to Activated Isoquinolines (continued)
Entry
j
Activated isoquinolines Ketones
Producta
Time (h) Yield (%)b
1
2
3
4
3.5
86 (9:1)c
NO2
NO2
O
NO2
Me
O
Me
O
O
+
N
O
N
Cl–
N
O
OEt
Me
OEt
OEt
Me
Me
k
3.0
80 (8:2)c
NO2
O
NO2
Me
Me
O
O
N
O
N
O
OEt
OEt
Me
Me
Me
a All products were characterized by NMR, IR, ESI/MS and HRMS.
b Isolated yields after column chromatography.
c Ratio of 3 to 4.
Table 2 Reaction of 1a with Acetone in a Range of Solvents
General Procedure
To a stirred mixture of ketone (1.5 mmol) in NMP (5 mL) was add-
ed N-acylisoquinolinium ion (1 mmol), generated in situ from iso-
quinoline (1 mmol) and ethyl chloroformate (1.2 mmol). The
resulting mixture was stirred at r.t. for the appropriate time
(Table 1). After complete conversion (indicated by TLC), EtOAc
(10 mL) and H2O (10 mL) was added and the reaction mixture was
further stirred at r.t. for an additional 10 min. The organic layer was
separated and the aqueous phase was extracted with EtOAc (2 × 10
mL). The combined organic extracts were washed with brine (1 ×
10 mL), dried over anhydrous Na2SO4 and concentrated in vacuo.
The resulting crude product was purified by column chromatogra-
phy on silica gel (Merck, 60–120 mesh; EtOAc–hexane, 2:8) to af-
ford the pure product.
Entry
Solvent
NMP
Time (h)
2.5
Yield (%)a
1
2
3
4
5
82
65
25
15
70
DMF
7.5
DMSO
MeOH
PEG-400
7.0
6.0
4.0
a Isolated yields after column chromatography.
Ethyl 1-(2-Oxo-2-phenylethyl)-1,2-dihydro-2-isoquinolinecar-
boxylate (3a)
Colorless solid; mp 85–87 °C.
Among these solvents, NMP was found to give the best
results, with no reaction occurring in its absence, even af-
ter a long reaction time (12 h). This method is highly se-
lective, affording 1-alkyl substituted isoquinolines
without the formation of 4-alkyl substituted products.
IR (KBr): 2924, 2853, 1712, 1680, 1632, 1598, 1453, 1415, 1373,
1324, 1285, 1245, 1172, 1119, 1018, 932, 779, 692, 570 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.89 (d, J = 7.5 Hz, 1 H), 7.82 (d,
J = 7.5 Hz, 1 H), 7.53–7.38 (m, 3 H), 7.17–6.78 (m, 5 H), 5.97–5.88
(m, 1 H), 5.82 (d, J = 7.5 Hz, 1 H), 4.18 (q, J = 6.7 Hz, 2 H), 3.30
(dd, J = 7.5, 14.3 Hz, 1 H), 3.19 (dd, J = 5.2, 13.5 Hz, 1 H), 1.29 (t,
J = 6.7 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = (14.3 and 14.5), (43.5 and 43.9),
(52.5 and 52.9), 62.2, (108.3 and 108.6), (124.5 and 124.6), 125.3,
(126.4 and 126.7), 127.0, 127.9, 128.1, 128.3, 128.4 (2 × C), (129.9
and 130.1), (131.4 and 131.7), (132.7 and 132.9), 137.0, (152.2 and
152.6), 196.4.
In conclusion, we have developed a simple and efficient
protocol for the preparation of 1-alkyl substituted isoquin-
olines using readily available N-methyl-2-pyrrolidinone
as the promoter. In addition to its simplicity and mild re-
action conditions, this method provides high yields of
products in short reaction times. The products would be of
great use in the synthesis of a range of isoquinoline-based
natural products.
ESI-MS: m/z = 344.1 [M + Na]+, 253.2, 218, 202, 181.
Melting points were recorded on Buchi R-535 apparatus and are un-
corrected. IR spectra were recorded on a Perkin–Elmer FT-IR 240-
c spectrophotometer using KBr optics. H NMR spectra were re-
corded on Bruker Avance 300 and Varian Unity 200 spectrometers
in CDCl3, using TMS as internal standard. Peak pairs in the 13C
NMR data refer to diastereomers. Mass spectra were recorded on a
Finnigan MAT 1020 mass spectrometer operating at 70 eV.
Ethyl 1-(2-Oxopropyl)-1,2-dihydro-2-isoquinolinecarboxylate
(3b)
Viscous yellow liquid.
1
IR (KBr): 2929, 1704, 1420, 1322, 1237, 1106, 966, 851, 767, 618
cm–1.
Synthesis 2007, No. 7, 1077–1081 © Thieme Stuttgart · New York

1080
J. S. Yadav et al.
PAPER
1H NMR (300 MHz, CDCl3): d = 7.16–6.77 (m, 5 H), 5.90–5.71 (m,
2 H), 4.23 (q, J = 7.5 Hz, 2 H), 2.90–2.72 (m, 1 H), 2.61 (dd,
J = 14.3, 6.0 Hz, 1 H), 2.10 (s, 3 H), 1.31 (t, J = 7.5 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.1, (30.6 and 30.9), 48.6, (52.1
and 52.3), (63.1 and 63.2), (109.2 and 109.3), (124.8 and 124.9),
125.1, 126.6, 126.8, 127.7, 132.1, 143.1, 152.3, 205.8.
IR (KBr): 2923, 1716, 1621, 1524, 1466, 1397, 1374, 1343, 1266,
1233, 1113, 1034, 912, 771, 549 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.85 (d, J = 8.1 Hz, 1 H), 7.60–
7.52 (m, 1 H), 7.29–7.02 (m, 2 H), 6.64 (t, J = 7.3 Hz, 1 H), 5.89 (d,
J = 4.9 Hz, 1 H), 4.30 (q, J = 6.5 Hz, 2 H), 2.96 (dd, J = 15.5, 8.1
Hz, 1 H), 2.65 (dd, J = 15.5, 4.0 Hz, 1 H), 2.08 (s, 3 H), 1.35 (t,
J = 6.5 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.3, (29.5 and 30.5), (46.7 and
51.0), 49.2, (63.0 and 63.1), 102.4, (124.4 and 125.1), 126.6, 129.2,
129.4, 132.0, 133.7, 144.4, 152.0, 204.9.
ESI-MS: m/z = 282 [M + Na]+, 259.1, 246.1, 218, 202, 173.9, 153,
130.1.
HRMS: m/z calcd for C15H17NO3Na: 282.1106; found: 282.1099.
Ethyl 1-(2-Oxocyclohexyl)-1,2-dihydro-2-isoquinolinecarbox-
ESI-MS: m/z = 327 [M + Na]+, 304.1, 247, 216.9, 203, 175.
ylate (3c)
Viscous brown liquid.
HRMS: m/z calcd for C15H16N2O5Na: 327.0956; found: 327.0953.
IR (KBr): 2936, 2862, 1709, 1630, 1568, 1455, 1415, 1374, 1325,
1238, 1123, 1018, 936, 770, 539 cm–1.
Ethyl 5-Nitro-1-(2-oxocyclohexyl)-1,2-dihydro-2-isoquinoline-
carboxylate (3g)
Yellow solid, mp 125–128 °C.
1H NMR (300 MHz, CDCl3): d = 7.42–6.92 (m, 4 H), 6.80 (d,
J = 7.5 Hz, 1 H), 6.0–5.87 (m, 1 H), 5.79 (d, J = 7.5 Hz, 1 H), 4.20
(q, J = 6.7 Hz, 2 H), 2.83–2.56 (m, 2 H), 2.43–1.51 (m, 6 H), 1.41
(s, 1 H), 1.30 (t, J = 6.7 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = (14.4 and 14.5), 20.7, (26.6 and
27.4), (28.0 and 28.3), (40.3 and 41.2), (52.4 and 52.6), (53.1 and
54.5), (62.0 and 62.1), (108.3 and 108.5), (124.4 and 124.5), (125.3
and 126.3), (126.5 and 126.7), (127.3 and 127.5), (129.1 and 129.3),
(129.6 and 131.2), (130.7 and 132.6), (152.4 and 152.7), (208.5 and
209).
IR (KBr): 2925, 2855, 1713, 1615, 1522, 1458, 1403, 1373, 1338,
1264, 1178, 1119, 1022, 767, 672 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.84 (dd, J = 1.5, 8.3 Hz, 1 H),
7.80–7.69 (m, 1 H), 7.20–7.01 (m, 2 H), 6.79–6.71 (m, 1 H), 5.91–
5.85 (m, 1 H), 4.26 (q, J = 6.0 Hz, 2 H), 2.78 (s, 1 H), 2.35–2.31 (m,
1 H), 2.27–2.01 (m, 3 H), 1.95 (d, J = 4.5 Hz, 2 H), 1.62 (d, J = 7.5
Hz, 2 H), 1.33 (t, J = 6.0 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.4, 25.1, 28.4, (31.9 and 32.5),
43.1, (52.5 and 52.6), (53.5 and 53.6), 62.9, 104.7, 124.0, 124.9,
126.5, (129.1 and 130.0), (133.4 and 133.5), 134.6, 144.5, (153.1
and 153.4), 210.2.
ESI-MS: m/z = 322.1 [M + Na]+, 202, 102, 74.2, 59.3.
Ethyl 1-(2-Oxocyclopentyl)-1,2-dihydro-2-isoquinolinecarbox-
ylate (3d)
ESI-MS: m/z = 367 [M + Na]+, 337, 301, 246.9, 160, 59.3.
Viscous brownish liquid.
Ethyl 5-Nitro-1-(2-oxocyclopentyl)-1,2-dihydro-2-isoquinoline-
IR (KBr): 2963, 2924, 1734, 1710, 1632, 1569, 1456, 1414, 1373,
1316, 1234, 1120, 1014, 926, 892, 769, 590 cm–1.
carboxylate (3h)
Yellowish liquid.
1H NMR (300 MHz, CDCl3): d = 7.19–6.92 (m, 4 H), 6.90 (d,
J = 6.7 Hz, 1 H), 5.80–5.75 (m, 1 H), 5.63 (d, J = 6.7 Hz, 1 H), 4.24
(q, J = 6.7 Hz, 2 H), 2.54–2.36 (m, 1 H), 2.21–1.91 (m, 4 H), 1.84–
1.47 (m, 2 H), 1.33 (t, J = 7.5 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.1, 20.0, (24.4 and 25.3), (37.0
and 37.9), 51.9, (54.2 and 54.8), 61.7, (107.4 and 107.6), 124.0,
125.2, 126.0, (126.3 and 126.4), (127.1 and 127.4), (129.3 and
129.4), (130.7 and 130.9), (152.0 and 152.1), (215.3 and 215.4).
IR (KBr): 2924, 2854, 1720, 1619, 1523, 1460, 1401, 1373, 1339,
1270, 1231, 1155, 1112, 1017, 924, 769, 590 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.88 (d, J = 8.3 Hz, 1 H), 7.54 (d,
J = 7.5 Hz, 1 H), 7.29–7.06 (m, 2 H), 6.52 (d, J = 8.3 Hz, 1 H),
6.16–6.06 (m, 1 H), 4.26 (q, J = 6.7 Hz, 2 H), 2.67–2.60 (m, 2 H),
2.36–2.25 (m, 1 H), 1.93–1.48 (m, 4 H), 1.33 (t, J = 6.7 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = (14.2 and 14.4), (20.1 and 20.3),
(25.5 and 25.7), (37.9 and 38.1), 51.9, (54.2 and 54.4), 62.7, 102.0,
[124.1 and 124.4 (2 × C)], [126.2 and 126.4 (2 × C)], (130.1 and
130.5), 131.6, 144.3, (151.9 and 152.4), 215.0.
ESI-MS: m/z = 308.1 [M + Na]+, 202, 74.2, 59.3.
Ethyl 5-Nitro-1-(2-oxo-2-phenylethyl)-1,2-dihydro-2-isoquino-
linecarboxylate (3e)
Yellow solid, mp 126–128 °C.
ESI-MS: m/z = 353 [M + Na]+, 337, 319.1, 301, 284, 262, 247, 216,
59.3.
IR (KBr): 2923, 2853, 1719, 1679, 1619, 1522, 1453, 1400, 1372,
1338, 1269, 1225, 1181, 1113, 1019, 948, 883, 765, 692, 578 cm–1.
Ethyl 1-(2-Oxobutyl)-1,2-dihydro-2-isoquinolinecarboxylate
(3i)
Viscous yellow liquid.
1H NMR (300 MHz, CDCl3): d = 7.86 (m, 3 H), 7.55–7.38 (m, 4 H),
7.25–7.05 (m, 2 H), 6.76–6.66 (m, 1 H), 6.05 (s, 1 H), 4.24 (q,
J = 6.7 Hz, 2 H), 3.49 (dd, J = 8.3, 15.1 Hz, 1 H), 3.14 (dd, J = 4.5,
15.1 Hz, 1 H), 1.32 (t, J = 6.7 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 14.4, (42.7 and 42.9), (51.8 and
51.9), 62.8, 102.6, 124.4, 125.0, 126.4 (3 × C), 128.1, 128.5 (2 × C),
129.0, 132.1, 133.1, 136.6, 144.5, 152.1, 195.4.
IR (KBr): 2981, 1713, 1632, 1455, 1416, 1373, 1326, 1219, 1124,
935, 772 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.25–6.79 (m, 5 H), 5.98–5.45 (m,
2 H), 4.23 (q, J = 6.7 Hz, 2 H), 3.11–2.21 (m, 2 H), 1.90–1.70 (m,
2 H), 1.34 (t, J = 6.7 Hz, 3 H), 1.04 (t, J = 6.7 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 7.5, (14.1 and 14.4), (34.4 and
34.6), 47.1, (51.6 and 52.0), (62.3 and 62.4), (109.5 and 110.2),
(124.7 and 124.9), (125.0 and 125.3), (126.8 and 126.9), (127.8 and
127.9), 129.9, (130.1 and 131.8), 130.4, (153.3 and 15.7), (209.9
and 210.7).
ESI-MS: m/z = 389.1 [M + Na]+, 339, 319.1, 305.1, 289.1, 247,
229.1, 181, 141.1.
Ethyl 5-Nitro-1-(2-oxopropyl)-1,2-dihydro-2-isoquinolinecar-
boxylate (3f)
Viscous yellowish liquid.
ESI-MS: m/z = 296 [M + Na]+.
HRMS: m/z calcd for C16H19NO3Na: 296.1262; found: 296.1257.
Synthesis 2007, No. 7, 1077–1081 © Thieme Stuttgart · New York

PAPER
Addition of Ketones to Activated Isoquinolines
1081
Ethyl 5-Nitro-1-(2-oxobutyl)-1,2-dihydro-2-isoquinolinecar-
boxylate (3j)
References
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Viscous yellow liquid.
IR (KBr): 2927, 1713, 1622, 1524, 1342, 1271 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.89 (d, J = 7.1 Hz, 1 H), 7.48–
6.65 (m, 4 H), 5.87–5.60 (m, 1 H), 4.25 (q, J = 7.1 Hz, 2 H), 3.15–
2.15 (m, 2 H), 1.91–1.83 (m, 2 H), 1.34 (t, J = 7.1 Hz, 3 H), 1.09–
0.93 (m, 3 H).
13C NMR (75 MHz, CDCl3): d = 7.4, (14.1 and 14.4), (30.4 and
30.9), 49.6, (55.5 and 56.1), 63.0, (103.8 and 104.3), 124.4, 125.2,
126.7, (129.3 and 129.9), 132.3, (133.4 and 133.7), 144.5, (152.8
and 153.1), (209.2 and 209.9).
ESI-MS: m/z = 341.1 [M + Na]+, 318, 279.1, 247, 217.1, 175.
HRMS: m/z calcd for C16H18N2O5Na: 341.1113; found: 341.1108.
Ethyl 5-Nitro-1-(2-oxopentyl)-1,2-dihydro-2-isoquinolinecar-
(3) For reviews, see: (a) Stout, D. M.; Meyers, A. I. Chem. Rev.
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boxylate (3k)
Yellowish liquid.
IR (KBr): 2964, 1716, 1618, 1525, 1460, 1374, 1344, 1264, 1228,
1114, 769, 619, 476 cm–1.
1H NMR (200 MHz, CDCl3): d = 7.89 (d, J = 8.3 Hz, 1 H), 7.52–
7.45 (m, 1 H), 7.28–7.05 (m, 2 H), 6.72–6.57 (m, 1 H), 5.93 (d,
J = 7.5 Hz, 1 H), 4.28 (q, J = 7.5 Hz, 2 H), 3.04–2.93 (m, 1 H), 2.60
(dd, J = 15.8, 4.5 Hz, 1 H), 2.28–2.23 (m, 2 H), 1.55–1.45 (m, 2 H),
1.37 (t, J = 7.5 Hz, 3 H), 0.85 (t, J = 7.5 Hz, 3 H).
13C NMR (75 MHz, CDCl3): d = 13.5, 14.4, 17.0, 44.2, (45.7 and
45.9), 51.2, 63.1, 102.4, 124.5, 126.6, 129.8, 130.8, 132.5, 134.9,
145.1, (151.3 and 153.5), 209.1.
ESI-MS: m/z = 355.1 [M + Na]+, 332.9, 282.1, 267.2, 247, 204.
(4) Yamaguchi, R.; Nakayasu, T.; Hatano, B.; Nagura, T.;
Kozima, S.; Fujitha, K.-I. Tetrahedron 2001, 57, 109; and
references cited therein.
HRMS: m/z calcd for C17H20N2O5Na: 355.1269; found: 355.1270.
(5) (a) Yadav, J. S.; Reddy, B. V. S.; Sreenivas, M.; Sathaiah, K.
Tetrahedron Lett. 2005, 46, 8906. (b) Yadav, J. S.; Reddy,
B. V. S.; Sathaiah, K.; Vishnumurthy, P. Synlett 2005, 2811.
(c) Yadav, J. S.; Reddy, B. V. S.; Srinivas, M.; Sathaiah, K.
Tetrahedron Lett. 2005, 46, 3489. (d) Yadav, J. S.; Reddy,
B. V. S.; Gupta, M. K.; Prabhakar, A.; Jagadeesh, B. Chem.
Commun. 2004, 2124.
Acknowledgment
M.K.G., I.P. and U.D. thank CSIR, New Delhi for the research fel-
lowships.
Synthesis 2007, No. 7, 1077–1081 © Thieme Stuttgart · New York