Facile synthesis of 5-alkylidene-1,5-dihydropyrrol-2-ones from Morita-Baylis-Hillman adducts

Full text of DOI:10.5012/bkcs.2012.33.4.1337

Notes
Bull. Korean Chem. Soc. 2012, Vol. 33, No. 4 1337
http://dx.doi.org/10.5012/bkcs.2012.33.4.1337
Facile Synthesis of 5-Alkylidene-1,5-dihydropyrrol-2-ones from
Morita-Baylis-Hillman Adducts
*
Bo Ram Park, Cheol Hee Lim, Jin Woo Lim, and Jae Nyoung Kim
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Korea
*
E-mail: kimjn@chonnam.ac.kr
Received December 5, 2011, Accepted December 27, 2011
Key Words : 5-Alkylidene-1,5-dihydropyrrol-2-ones, 5-Hydroxypyrrol-2(5H)-ones, Morita-Baylis-Hillman
adducts, γ-Alkylidenebutenolides
5-Alkylidene-1,5-dihydropyrrol-2-ones have received a
special attention due to their abundance in biologically
At the outset of our study, we examined the reaction of
7
γ-propylidenebutenolide 1a with ammonia in ethanol at
1a
important substances including pukeleimide, ampullicin,
1b,c
8
room temperature, as shown in Scheme 2. As expected, 5-
1b,c
1d-f
1b
pandamarine, and
isoampullicin,
pulchellalactam,
hydroxypyrrol-2(5H)-one 2a was obtained in good yield
1a
holomycin, as shown in Figure 1. Thus numerous methods
(88%) by following the mechanism shown below in Scheme
8,9b
2. Subsequent acid-catalyzed dehydration of 2a produced
for the synthesis of 5-alkylidene-1,5-dihydropyrrol-2-ones
1,2
have been reported; however, a facile synthetic pathway to
5-propylidene-1,5-dihydropyrrol-2-one (3a) in good yield
this class of compounds is highly required until now. In
addition, various 5-hydroxypyrrol-2(5H)-one derivatives,
the precursors of 5-alkylidene-1,5-dihydropyrrol-2-ones in
this paper, have been known to possess interesting biological
(97%) in a highly stereoselective manner. As observed in a
8,9b
7
similar case and in our previous paper, the Z-isomer was
formed exclusively due to the unfavorable steric hindrance
between the ethyl group of a propylidene moiety and the
phenyl group at the 4-position. However, the reaction of 1a
and benzylamine afforded 4a in moderate yield (70%)
although excess amounts (10 equiv) of benzylamine were
3
activites.
Recently various cyclic compounds have been synthesized
4
from Morita-Baylis-Hillman (MBH) adducts. Among the
9
used. Moreover, an acid-catalyzed dehydration of 4a pro-
cyclic compounds, syntheses of various lactam derivatives
5
6
have received much attention by us and other groups. Very
recently, we reported a facile synthesis of γ-alkylidene-
butenolides (such as 1a) from MBH bromides, as shown in
duced a mixture of E/Z isomers of N-benzyl-5-propylidene-
1,5-dihydropyrrol-2-one (5a). The Z-isomer was formed as a
major product (51%) along with appreciable amounts of E-
isomer (37%). The stereochemistry of 5a-Z was confirmed
by NOE experiments. The ratio of E/Z stated that the
unfavorable steric hindrance between the ethyl group of a
propylidene moiety and the phenyl group at the 4-position is
larger than that of the N-benzyl moiety. The Z-form of this
compound could be synthesized more easily from 3a and
benzyl bromide in good yield (90%) in the presence of
K2CO3. In the reaction we did not observe the formation of
5a-E in any trace amount.
7
Scheme 1. As a continuous work, we decided to examine
the synthesis of 5-alkylidene-1,5-dihydropyrrol-2-ones (such
7
as 3a) from the readily available γ-alkylidenebutenolides.
Encouraged by the successful results, we carried out the
synthesis of 5-hydroxypyrrol-2(5H)-ones 2b-d from γ-
7
alkylidenebutenolides 1b-d, and the results are summarized
in Table 1. The yields of 2b-d were moderate to good (62-
80%). For the synthesis of compound 2d, we used THF as a
co-solvent due to the limited solubility of 1d in EtOH. These
compounds were converted to 5-alkylidene-1,5-dihydro-
Figure 1
Scheme 1

1338 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 4
Notes
Scheme 2
Table 1. Conversion of γ-alkylidenebutenolides to 5-alkylidene-
1,5-dihydropyrrol-2-ones 3a-d
of the corresponding E-form was not observed in any trace
amount due to a similar steric reason.
γ-Alkylidene- 5-Hydroxypyrrol-2(5H)-
γ-Alkylidene
In summary, an expedient synthetic procedure of 5-alkyl-
idene-1,5-dihydropyrrol-2-ones was disclosed from the
corresponding γ-alkylidenebutenolides which were prepared
from the Morita-Baylis-Hillman bromides. The reaction of
γ-alkylidenebutenolides and ammonia and subsequent de-
hydration produced 5-alkylidene-1,5-dihydropyrrol-2-ones
stereoselectively, and the stereochemistry was not changed
during the benzylation to furnish a stereoselective synthetic
protocol of N-benzyl-5-alkylidene-1,5-dihydropyrrol-2-ones.
Enrty
a
b
c
butenolide
one
lactam
Experimental Section
Typical Procedure for the Synthesis of 2a. To a stirred
7
solution of γ-alkylidenebutenolide 1a (214 mg, 1.0 mmol)
in EtOH (2.0 mL) was added NH4OH (28%, 2.0 mL), and
the reaction mixture was stirred at room temperature for 24
h. After the aqueous extractive workup and column chromato-
graphic purification process (hexanes/EtOAc/CH2Cl2, 3:2:1),
compound 2a was obtained as a white solid, 203 mg (88%).
Other compounds were synthesized similarly, and the spectro-
scopic data of 2a-d are as follows.
a
b
Prepared according to Ref. 7. Conditions: substrate 1 (1.0 mmol),
c
NH4OH/EtOH (1:1, 4.0 mL), rt, 24 h. Conditions: substrate 2 (0.5
o d
mmol), p-TsOH (0.3 equiv), 1,2-dichloroethane, 60 C, 1 h. THF was
added as a co-solvent.
o
Compound 2a: 88%; white solid, mp 164-166 C; IR
−1
1
pyrrol-2-ones 3b-d in excellent yields (91-96%) in the
(KBr) 3375, 3214, 1708, 1666 cm ; H NMR (CDCl3, 300
MHz) δ 0.79 (t, J = 7.5 Hz, 3H), 1.05-1.17 (m, 1H), 1.25-
1.41 (m, 1H), 1.59-1.71 (m, 1H), 1.79-1.89 (m, 1H), 1.81 (s,
3H), 4.66 (br s, OH), 7.34-7.44 (m, 3H), 7.61-7.65 (m, 3H);
9
presence of p-TsOH. In all entries, the corresponding Z-
isomers were formed exclusively, as in the case of 3a (vide
supra).
13
As for the synthesis of 5a-Z (vide supra, Scheme 2), N-
benzylation reactions of 3c-Z and 3d-Z were carried out
similarly, and compounds 5c-Z and 5d-Z were synthesized in
good yields (89-92%), as shown in Scheme 3. The formation
C NMR (CDCl3, 75 MHz) δ 9.47, 14.00, 16.63, 38.95,
90.29, 128.39, 128.54, 128.63, 128.78, 132.44, 154.19,
+
173.65; ESIMS m/z 232 (M +H). Anal. Calcd for
C14H17NO2: C, 72.70; H, 7.41; N, 6.06. Found: C, 72.89; H,
Scheme 3

Notes
Bull. Korean Chem. Soc. 2012, Vol. 33, No. 4 1339
7.53; N, 5.97.
Found: C, 82.57; H, 6.03; N, 4.97.
Compound 3c-Z: 96%; white solid, mp 125-127 C; IR
o
Compound 2b: 62%; white solid, mp 140-142 C; IR
o
−1
1
−1
1
(KBr) 3335, 3263, 1702 cm ; H NMR (CDCl3, 300 MHz)
δ 1.83 (s, 3H), 1.93-2.04 (m, 1H), 2.15-2.25 (m, 1H), 2.45-
2.55 (m, 1H), 2.67-2.77 (m, 1H), 4.71 (br s, OH), 6.99-7.02
(m, 2H), 7.09-7.21 (m, 3H), 7.34-7.43 (m, 3H), 7.62-7.65
(KBr) 3183, 2957, 1690, 1361 cm ; H NMR (CDCl3, 300
MHz) δ 0.91 (t, J = 7.2 Hz, 3H), 1.31-1.50 (m, 4H), 1.95 (s,
3H), 2.33 (dt, J = 7.8 and 7.2 Hz, 2H), 5.14 (t, J = 7.8 Hz,
1H), 7.26-7.31 (m, 2H), 7.36-7.48 (m, 3H), 9.17 (br s, NH);
13
(m, 2H), 7.79 (br s, NH); C NMR (CDCl3, 75 MHz) δ
13
C NMR (CDCl3, 75 MHz) δ 9.06, 13.84, 22.37, 27.47,
9.54, 29.75, 38.62, 90.08, 125.87, 128.30, 128.32, 128.48,
128.55, 128.80, 128.94, 132.22, 141.15, 154.21, 173.79;
31.50, 115.35, 128.24, 128.31, 128.68, 129.44, 132.16,
+
137.69, 144.30, 172.29; ESIMS m/z 242 (M +H). Anal.
+
ESIMS m/z 294 (M +H). Anal. Calcd for C19H19NO2: C,
Calcd for C16H19NO: C, 79.63; H, 7.94; N, 5.80. Found: C,
79.91; H, 8.08; N, 5.83.
77.79; H, 6.53; N, 4.77. Found: C, 77.71; H, 6.42; N, 4.56.
o
Compound 2c: 80%; white solid, mp 114-116 C; IR
o
Compound 3d-Z: 93%; white solid, mp 207-209 C; IR
−1
1
−1
1
(KBr) 3401, 3293, 2953, 1700, 1649 cm ; H NMR (CDCl3,
300 MHz) δ 0.77 (t, J = 6.6 Hz, 3H), 1.05-1.20 (m, 5H),
1.25-1.34 (m, 1H), 1.62-1.72 (m, 1H), 1.80-1.90 (m, 1H),
1.82 (s, 3H), 4.80 (br s, OH), 7.33-7.44 (m, 3H), 7.58-7.64
(KBr) 3175, 3028, 1680 cm ; H NMR (CDCl3, 300 MHz)
δ 1.95 (s, 3H), 5.24 (d, J = 9.9 Hz, 1H), 5.62 (d, J = 9.9 Hz,
1H), 7.16-7.33 (m, 12H), 7.39-7.48 (m, 3H), 9.22 (br s, NH);
13
C NMR (CDCl3, 75 MHz) δ 9.15, 49.13, 115.95, 126.69,
13
(m, 3H); C NMR (CDCl3, 75 MHz) δ 9.46, 13.82, 22.31,
128.25, 128.42, 128.46, 128.68, 129.38, 129.42, 131.86,
+
137.82, 143.15, 144.72, 172.39; ESIMS m/z 352 (M +H).
22.81, 31.53, 36.65, 90.35, 128.36, 128.54, 128.64, 128.75,
+
132.44, 154.15, 173.68; ESIMS m/z 260 (M +H). Anal.
Anal. Calcd for C25H21NO: C, 85.44; H, 6.02; N, 3.99.
Found: C, 85.78; H, 6.32; N, 3.85.
Calcd for C16H21NO2: C, 74.10; H, 8.16; N, 5.40. Found: C,
74.44; H, 8.37; N, 5.19.
Typical Procedure for the Synthesis of 5a-Z. A stirred
solution of compound 3a (85 mg, 0.4 mmol), benzyl bromide
(103 mg, 0.6 mmol), and K2CO3 (83 mg, 0.6 mmol) in
CH3CN (1.0 mL) was heated to reflux for 15 h. After the
aqueous extractive workup and column chromatographic
purification process (hexanes/EtOAc/CH2Cl2, 6:1:1), com-
pound 5a-Z was obtained as colorless oil, 109 mg (90%).
Other compounds 5c-Z and 5d-Z were synthesized similarly,
and the spectroscopic data including 4a are as follows.
Compound 5a-Z: 90%; colorless oil; IR (film) 2964,
o
Compound 2d: 75%; white solid, mp 106-108 C; IR
−1
1
(KBr) 3313, 3301, 1699, 1493 cm ; H NMR (CDCl3, 300
MHz) δ 1.76 (s, 3H), 2.50 (dd, J = 14.4 and 8.7 Hz, 1H),
2.62 (dd, J = 14.4 and 5.7 Hz, 1H), 4.04 (br s, OH), 4.15 (dd,
J = 8.7 and 5.7 Hz, 1H), 5.90 (br s, NH), 7.04-7.24 (m, 10H),
13
7.32-7.38 (m, 3H), 7.45-7.51 (m, 2H); C NMR (CDCl3, 75
MHz) δ 9.60, 42.23, 46.27, 89.31, 126.28, 126.57, 127.41,
127.93, 128.35, 128.56, 128.58, 128.65, 128.76, 128.92,
132.10, 143.90, 144.64, 153.74, 172.36; ESIMS m/z 370
+
(M +H). Anal. Calcd for C25H23NO2: C, 81.27; H, 6.27; N,
−1
1
1691, 1439 cm ; H NMR (CDCl3, 300 MHz) δ 0.78 (t, J =
7.5 Hz, 3H), 1.96 (s, 3H), 2.15-2.25 (m, 2H), 4.96 (t, J = 8.1
Hz, 1H), 5.13 (s, 2H), 7.13-7.17 (m, 2H), 7.20-7.35 (m, 5H),
3.79. Found: C, 81.06; H, 6.44; N, 3.68.
Typical Procedure for the Synthesis of 3a-Z. A stirred
solution of compound 2a (116 mg, 0.5 mmol) and p-TsOH
(27 mg, 0.15 mmol) in 1,2-dichloroethane (1.0 mL) was
13
7.36-7.48 (m, 3H); C NMR (CDCl3, 75 MHz) δ 9.53,
14,47, 20.40, 44.74, 118.18, 125.82, 126.81, 126.88, 128.22,
128.24, 128.59, 129.72, 132.41, 137.78, 138.31, 145.62,
o
heated to 60 C for 1 h. After the aqueous extractive workup
+
171.43; ESIMS m/z 304 (M +H). Anal. Calcd for C21H21NO:
and column chromatographic purification process (CH2Cl2/
Et2O, 20:1), compound 3a-Z was obtained as a white solid,
103 mg (97%). Other compounds were synthesized similar-
ly, and the spectroscopic data of 3a-d are as follows.
C, 83.13; H, 6.98; N, 4.62. Found: C, 83.23; H, 7.12; N,
4.46.
Compound 5c-Z: 89%; colorless oil; IR (film) 2957,
o
Compound 3a-Z: 97%; white solid, mp 102-104 C; IR
−1
1
2925, 1694, 1441 cm ; H NMR (CDCl3, 300 MHz) δ 0.71
(t, J = 7.2 Hz, 3H), 1.04-1.14 (m, 4H), 1.96 (s, 3H), 2.17 (dt,
J = 7.8 and 7.2 Hz, 2H), 4.98 (t, J = 7.8 Hz, 1H), 5.14 (s,
2H), 7.12-7.17 (m, 2H), 7.19-7.37 (m, 5H), 7.38-7.47 (m,
−1
1
(KBr) 3183, 1686 cm ; H NMR (CDCl3, 300 MHz) δ 1.09
(t, J = 7.5 Hz, 3H), 1.95 (s, 3H), 2.29-2.39 (m, 2H), 5.12 (t, J
= 7.8 Hz, 1H), 7.26-7.31 (m, 2H), 7.37-7.48 (m, 3H), 8.91
13
(br s, NH); C NMR (CDCl3, 75 MHz) δ 9.10, 13.94, 21.13,
13
3H); C NMR (CDCl3, 75 MHz) δ 9.47, 13.61, 22.16,
116.62, 128.29, 128.33, 128.73, 129.44, 132.10, 137.12,
26.71, 32.14, 44.71, 116.89, 125.76, 126.62, 126.82, 128.16,
128.18, 128.53, 129.65, 132.39, 138.03, 138.26, 145.56,
+
144.45, 172.20; ESIMS m/z 214 (M +H). Anal. Calcd for
+
171.39; ESIMS m/z 332 (M +H). Anal. Calcd for
C14H15NO: C, 78.84; H, 7.09; N, 6.57. Found: C, 78.79; H,
7.35; N, 6.41.
C23H25NO: C, 83.34; H, 7.60; N, 4.23. Found: C, 83.39; H,
7.85; N, 4.32.
o
Compound 3b-Z: 91%; white solid, mp 160-162 C; IR
−1 1
(KBr) 3190, 1682 cm ; H NMR (CDCl3, 300 MHz) δ 1.94
o
Compound 5d-Z: 92%; yellow solid, mp 96-98 C; IR
−1
1
(s, 3H), 3.72 (d, J = 8.1 Hz, 2H), 5.33 (t, J = 8.1 Hz, 1H),
(KBr) 3027, 1695, 1444 cm ; H NMR (CDCl3, 300 MHz)
δ 1.99 (s, 3H), 5.03 (s, 2H), 5.05 (d, J = 10.8 Hz, 1H), 5.44
(d, J = 10.8 Hz, 1H), 6.75-6.79 (m, 4H), 7.08-7.24 (m, 9H),
13
7.16-7.31 (m, 7H), 7.33-7.45 (m, 3H), 9.79 (br s, NH);
C
NMR (CDCl3, 75 MHz) δ 9.12, 33.91, 113.25, 126.34,
13
7.26-7.46 (m, 7H); C NMR (CDCl3, 75 MHz) δ 9.64,
128.32, 128.35, 128.55, 128.61, 129.22, 129.42, 131.92,
+
138.18, 139.46, 144.55, 172.76; ESIMS m/z 276 (M +H).
44.64, 47.60, 117.12, 125.58, 126.50, 127.23, 127.68,
127.76, 128.37, 128.39, 128.59, 129.02, 129.65, 132.06,
Anal. Calcd for C19H17NO: C, 82.88; H, 6.22; N, 5.09.

1340 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 4
Notes
3431. (b) Haase, C.; Langer, P. Tetrahedron 2009, 65, 4530-4539.
(c) Mykhaylychenko, S.; Harakat, D.; Dupas, G.; Shermolovich,
Y. G.; Bouillon, J.-P. J. Fluorine Chem. 2009, 130, 418-427. (d)
Ngwe, H.; Kinoshita, H.; Inomata, K. Bull. Chem. Soc. Jpn. 1994,
67, 3320-3326. (e) Kinoshita, H.; Ngwe, H.; Kobori, K.; Inomata,
K. Chem. Lett. 1993, 1441-1442.
138.18, 138.41, 143.85, 145.78, 171.50; ESIMS m/z 442
+
(M +H). Anal. Calcd for C32H27NO: C, 87.04; H, 6.16; N,
3.17. Found: C, 86.89; H, 6.47; N, 3.04.
o
Compound 4a: 70%; white solid, mp 144-146 C; IR
−1
1
(KBr) 3324, 2959, 1675, 1436 cm ; H NMR (CDCl3, 300
MHz) δ 0.25 (t, J = 7.2 Hz, 3H), 0.37-0.49 (m, 1H), 0.55-
0.66 (m, 1H), 1.53-1.70 (m, 2H), 1.90 (s, 3H), 3.55 (br s,
OH), 4.42 (d, J = 15.0 Hz, 1H), 4.76 (d, J = 15.0 Hz, 1H),
3. For the synthesis and biological activity of 5-hydroxypyrrol-
2(5H)-ones, see: (a) Ma, S.; Xie, H. Org. Lett. 2000, 2, 3801-
3803. (b) Boukouvalas, J; Loach, R. P.; Ouellet, E. Tetrahedron
Lett. 2011, 52, 5047-5050 and further references cited therein. (c)
Watson, A. F.; Liu, J.; Bennaceur, K.; Drummond, C. J.; Endicott,
J. A.; Golding, B. T.; Griffin, R. J.; Haggerty, K.; Lu, X.;
McDonnell, J. M.; Newell, D. R.; Noble, M. E. M.; Revill, C. H.;
Riedinger, C.; Xu, Q.; Zhao, Y.; Lunec, J.; Hardcastle, I. R.
Bioorg. Med. Chem. Lett. 2011, 21, 5916-5919.
13
7.20-7.46 (m, 8H), 7.57-7.62 (m, 2H); C NMR (CDCl3, 75
MHz) δ 9.91, 13.10, 16.03, 36.51, 42.05, 93.46, 127.33,
128.39, 128.41, 128.55, 128.73, 128.75, 129.71, 132.29,
+
138.35, 151.52, 171.36; ESIMS m/z 322 (M +H). Anal.
Calcd for C21H23NO2: C, 78.47; H, 7.21; N, 4.36. Found: C,
78.75; H, 7.39; N, 4.18.
4. For the general review on Baylis-Hillman reaction, see: (a)
Basavaiah, D.; Rao, A. J.; Satyanarayana, T. Chem. Rev. 2003,
103, 811-891. (b) Basavaiah, D.; Reddy, B. S.; Badsara, S. S.
Chem. Rev. 2010, 110, 5447-5674. (c) Singh, V.; Batra, S.
Tetrahedron 2008, 64, 4511-4574. (d) Declerck, V.; Martinez, J.;
Lamaty, F. Chem. Rev. 2009, 109, 1-48. (e) Ciganek, E. In
Organic Reactions; Paquette, L. A., Ed.; John Wiley & Sons: New
York, 1997; Vol. 51, pp 201-350. (f) Kim, J. N.; Lee, K. Y. Curr.
Org. Chem. 2002, 6, 627-645. (g) Lee, K. Y.; Gowrisankar, S.;
Kim, J. N. Bull. Korean Chem. Soc. 2005, 26, 1481-1490. (h)
Radha Krishna, P.; Sachwani, R.; Reddy, P. S. Synlett 2008, 2897-
2912. (i) Gowrisankar, S.; Lee, H. S.; Kim, S. H.; Lee, K. Y.; Kim,
J. N. Tetrahedron 2009, 65, 8769-8780.
Typical Procedure for the Synthesis of 5a-E. A stirred
solution of compound 4a (96 mg, 0.3 mmol) and p-TsOH
(16 mg, 30 mol %) in 1,2-dichloroethane (1.0 mL) was
o
heated to 60 C for 1 h. After the aqueous extractive workup
and column chromatographic purification process (CH2Cl2),
compound 5a-E was obtained as colorless oil, 34 mg (37%),
along with 5a-Z (46 mg, 51%). The spectroscopic data of
5a-E are as follows.
Compound 5a-E: 37%; colorless oil; IR (film) 2964,
-1
1
1688, 1411 cm ; H NMR (CDCl3, 300 MHz) δ 0.71 (t, J =
7.5 Hz, 3H), 1.63-1.73 (m, 2H), 1.85 (s, 3H), 4.90 (s, 2H),
5.27 (t, J = 8.4 Hz, 1H), 7.21-7.27 (m, 5H), 7.29-7.35 (m,
5. For the synthesis of various lactams from MBH adducts in our
group, see: (a) Lee, K. Y.; Lee, Y. J.; Kim, J. N. Bull. Korean
Chem. Soc. 2007, 28, 143-146. (b) Kim, S. C.; Gowrisankar, S.;
Kim, J. N. Tetrahedron Lett. 2006, 47, 3463-3466. (c) Kim, S. C.;
Lee, K. Y.; Gowrisankar, S.; Kim, J. N. Bull. Korean Chem. Soc.
2006, 27, 1133-1139. (d) Lee, H. S.; Kim, S. J.; Kim, J. N. Bull.
Korean Chem. Soc. 2006, 27, 1063-1066. (e) Lee, M. J.; Lee, K.
Y.; Park, D. Y.; Kim, J. N. Bull. Korean Chem. Soc. 2005, 26,
1281-1285.
13
2H), 7.36-7.44 (m, 3H); C NMR (CDCl3, 75 MHz) δ 9.14,
14.66, 20.37, 42.74, 118.12, 126.95, 127.05, 128.10, 128.38,
128.53, 128.61, 131.45, 134.67, 137.05, 137.79, 142.70,
+
169.40; ESIMS m/z 304 (M +H). Anal. Calcd for C21H21NO:
C, 83.13; H, 6.98; N, 4.62. Found: C, 83.01; H, 7.17; N,
4.45.
6. For the synthesis of various lactams from MBH adducts in other
groups, see: (a) Amri, H.; El Gaied, M. M.; Ben Ayed, T.;
Villieras, J. Tetrahedron Lett. 1992, 33, 6159-6160. (b) Buchholz,
R.; Hoffmann, H. M. R. Helv. Chim. Acta 1991, 74, 1213-1220.
(c) Singh, V.; Saxena, R.; Batra, S. J. Org. Chem. 2005, 70, 353-
356. (d) Singh, V.; Yadav, G. P.; Maulik, P. R.; Batra, S. Tetra-
hedron 2008, 64, 2979-2991. (e) Singh, V.; Yadav, G. P.; Maulik,
P. R.; Batra, S. Tetrahedron 2006, 62, 8731-8739. (f) Coelho, F.;
Diaz, G.; Abella, C. A. M.; Almeida, W. P. Synlett 2006, 435-439.
(g) Basavaiah, D.; Rao, J. S. Tetrahedron Lett. 2004, 45, 1621-
1625. (h) Ramachandran, P. V.; Madhi, S.; Bland-Berry, L.; Reddy,
M. V. R.; O'Donnell, M. J. J. Am. Chem. Soc. 2005, 127, 13450-
13451. (i) Kawahara, S.; Nakano, A.; Esumi, T.; Iwabuchi, Y.;
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Acknowledgments. This work was supported by the
National Research Foundation of Korea Grant funded by the
Korean Government (2011-0002570). Spectroscopic data
were obtained from the Korea Basic Science Institute,
Gwangju branch.
References and Notes
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