Stereoselective synthesis of (3R,4S)-3-amino-4-methyl pyrrolidine

Article abstract of DOI:10.1016/j.tetasy.2008.10.005

A stereoselective approach for the synthesis of (3R,4S)-3-amino-4-methyl pyrrolidine, a part of the structure of a quinoline antibacterial compound and the naphthyridine antitumor agent, is described. The key reaction is the one-pot reduction and regioselective cyclization of azidoditosyl derivative 9 to pyrrolidine 10.

Full text of DOI:10.1016/j.tetasy.2008.10.005

Tetrahedron: Asymmetry 19 (2008) 2402–2405
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Stereoselective synthesis of (3R,4S)-3-amino-4-methyl pyrrolidine
*
G. Jayachitra, Ravi Kumar Anchoori, B. Venkateswara Rao
Organic Division III, Indian Institute of Chemical Technology, Hyderabad 500007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A stereoselective approach for the synthesis of (3R,4S)-3-amino-4-methyl pyrrolidine, a part of the struc-
ture of a quinoline antibacterial compound and the naphthyridine antitumor agent, is described. The key
reaction is the one-pot reduction and regioselective cyclization of azidoditosyl derivative 9 to pyrrolidine
10.
Received 12 September 2008
Accepted 9 October 2008
Available online 5 November 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
mers to obtain the enantiomerically pure form of 2, which is essen-
tial for biological activity. In continuation of our work on the
Chiral, non-racemic pyrrolidines are common building blocks
for many natural and unnatural compounds1 that possess impor-
tant biological activity. Amino substituted chiral pyrrolidines pro-
vided a noticeable improvement in the biological activity of
quinoline and naphthyridine compounds2 by attachment at the
C-7 position. Miyamoto et al.3 showed that a new series of quino-
line compounds, for example 1a, bearing (3R,4S)-3-amino-4-
methyl pyrrolidine 2 at the C-7 position exhibited highly potent
activity equal or superior to those of ciprofloxacin4 and ofloxacin5
against both gram positive and gram negative bacteria including
Pseudomonas aeruginosa. Tomita et al.6 reported that amino meth-
ylpyrrolidine 2 attached to the naphthyridine ring at C-7 of 1b pos-
sessed potency that was comparable to that of cisplatin with an
synthesis of polysubstituted pyrrolidines,8a–e piperidines,9 we re-
ported the synthesis of chiral 3-methoxy-4-methyl amino pyrroli-
dine.8a,b Herein, we report the stereoselective synthesis of (3R,4S)-
3-amino-4-methyl pyrrolidine 2 starting from D-glucose.
2. Results and discussion
Our approach to the synthesis of (3R,4S)-3-amino-4-methyl
-Glucose was converted into
pyrrolidine is outlined in Scheme 1.
D
compound 3 by the sequence of reactions reported earlier.10 The
primary alcohol was converted into azide 5 via the mesylate using
MsCl–Et3N and NaN3 in DMF. Compound 5, upon hydrolysis with
cat. HCl and 60% aq AcOH, furnished 1,2-diol 6. Oxidative cleavage
of diol 6 with NaIO4 in MeOH–H2O and subsequent reduction with
IC50 value of 0.011
lg/mL against Murine P388 Leukemia cells.
O
O
F
F
COOH
COOH
H2N
N
N
N
.2HCl
N
N
N
H
CH3
S
N
H2N
H2N
1a
2
1b
Therefore, the pyrrolidine moiety 2 is an essential structural
feature for the activity of compounds 1a and 1b; and this impor-
tant activity of 2 prompted us to undertake its synthesis. Although
synthetic approaches for compound 2 are known in the literature,7
they have the disadvantage of requiring the separation of the iso-
NaBH4 gave diol 8, which was converted into ditosyl derivative 9
using TsCl/Et3N. Reaction of compound 9 in TPP/MeOH at reflux re-
sulted in azide reduction and regioselective cyclization to give the
pyrrolidine derivative, which was subsequently treated with Boc2O
to give 10. Next, the tosyl group in compound 10 underwent SN2
displacement with NaN3 in DMF to furnish azide 11, which on fur-
ther reduction with Pd/C, H2, and protection with Boc2O afforded
compound 12; finally, deprotection of the Boc group by using
HCl–MeOH gave compound 2 as its HCl salt.
* Corresponding author. Tel./fax: +91 40 27193003.
E-mail addresses: venky@iict.res.in, drb.venky@gmail.com (B. V. Rao).
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.10.005

G. Jayachitra et al. / Tetrahedron: Asymmetry 19 (2008) 2402–2405
2403
D-glucose
10
ref
O
O
O
O
OH
OH
R
a,b
N3
O
c
HO
O
O
4
R = OMs
6
3
5 R = N3
OTs
OH
OCHO
f
e
d
g
N3
N3
N3
OTs
OH
O
9
8
7
BocHN
N3
H2N
TsO
j
h
i
.2HCl
N
N
N
N
Boc
Boc
Boc
H
2
11
12
10
Scheme 1. Reagents and conditions: (a) MsCl, Et3N, CH2Cl2, 0 °C–rt, 30 min, 86%; (b) NaN3, DMF, 90 °C, 12 h, 82%; (c) 60% aq AcOH, concd HCl, 12 h, 72%; (d) NaIO4, MeOH–
H2O, 30 min; (e) NaBH4, MeOH, 0 °C–rt, 30 min, 82%; (f) TsCl, Et3N, DMAP, CH2Cl2, 4 h, 81%; (g) (i) PPh3, MeOH, reflux, 12 h; (ii) Boc2O, Et3N, THF, 51%; (h) NaN3, DMF, 90 °C,
12 h, 78%; (i) (i) H2, Pd/C, MeOH, 2 h, (ii) Boc2O, Et3N, CH2Cl2, 14 h, 91% (j) MeOH, HCl, 0 °C–rt, 30 min, 91%.
2856, 1456, 1217, 1168, 948 cmꢁ1 1H NMR (300 MHz, CDCl3) d
;
3. Conclusion
1.10 (d, 3H, J = 6.8 Hz), 1.32 (s, 3H), 1.49 (s, 3H), 2.02 (m, 1H),
3.03 (s, 3H), 3.92 (m, 1H), 4.21 (dd, 1H J = 4.5, 11.3 Hz), 4.39 (dd,
1H J = 2.2, 11.3 Hz), 4.53 (t, 1H, J = 3.8 Hz), 5.74 (d, 1H, J = 3.0 Hz);
MS (LC) m/z 289 [M+Na]+.
In conclusion, a stereoselective synthesis of enantiomerically
pure pyrrolidine 2 was achieved via a simple reaction sequence
starting from D-glucose, involving the regioselective cyclization of
the azidoditosyl compound 9. This approach is helpful in obtaining
analogues of 2.
4.2. (3aR,5S,6R,6aR)-5-(Azidomethyl)-dihydro-2,2,6-trimethyl-
5H-furo[2,3-d][1,3]dioxole 5
4. Experimental
A mixture of compound 4 (3 g, 11.27 mmol) and sodium azide
(2.93 g, 45.11 mmol) in DMF (15 mL) was heated at 90 °C for
12 h. The reaction mixture was poured into water and extracted
with EtOAc. The extract was washed with saturated brine, dried
over Na2SO4, and concentrated in vacuo. The residue was purified
by column chromatography with hexane–EtOAc (24:1) to give
TLC was performed on Merck Kiesel Gel 60, F254 plates (layer
thickness 0.25 mm). Column chromatography was performed on
silica gel (60–120 mesh) using ethyl acetate and hexane mixture
as eluents. Melting points were determined on a Fisher John’s
melting point apparatus, and are uncorrected. IR spectra were re-
corded on a Perkin–Elmer RX-1 FT-IR system. 1H NMR and 13C
NMR spectra were recorded using Varian Gemini-200 MHz or Bru-
ker Avance-300 MHz spectrometers. Chemical shifts are expressed
in ppm (d) with tetramethylsilane as an internal standard, followed
by multiplicity (s-singlet; d-doublet; t-triplet; q-quartet; m-multi-
compound 5 as a colorless oil (1.96 g, 82%): ½a D30
¼ þ62:0 (c 0.34,
ꢀ
CHCl3); IR mmax 2984, 2933, 2101, 1457, 1378, 1113, 1014 cmꢁ1
;
1H NMR (200 MHz, CDCl3) d 1.05 (d, 3H, J = 7.5 Hz), 1.31 (s, 3H),
1.48 (s, 3H), 2.02 (m, 1H), 3.15 (dd, 1H, J = 4.1, 13.3 Hz), 3.58 (dd,
1H, J = 2.49, 13.3 Hz), 3.80–3.93 (m, 1H), 4.52 (t, 1H, J = 4.15 Hz),
5.77 (d, 1H, J = 3.3 Hz); MS (EI) m/z 213 [M]+.
plet), number of proton(s), and coupling constant(s)
J (Hz).
13C NMR chemical shifts are expressed in ppm. Optical rotations
were measured with Horiba-SEPA-300 digital polarimeter.
Accurate mass measurement was performed on a Q STAR mass
spectrometer.
4.3. (3R,4S,5S)-5-(Azidomethyl)-4-methyltetrahydrofuran-
2,3-diol 6
To compound 5 (1.8 g, 8.45 mmol) was added 60% aq AcOH
(20 mL) and catalytic amount of concd HCl after which the mixture
was stirred for 12 h at rt. The reaction mixture was then neutral-
ized with solid NaHCO3 until pH 7, and the reaction mixture was
filtered by washing with EtOAc. The organic layer was dried over
Na2SO4, concentrated in vacuo, and purified by column chromatog-
raphy using hexane–EtOAc (2:1) to give compound 6 as a colorless
4.1. (3aR,5S,6R,6aR)-2,2,6-Trimethylperhydrofuro[2,3-d]-
[1,3]dioxol-5-yl]methylmethanesulfonate 4
A solution of methanesulfonyl chloride (1.36 mL, 17.55 mmol)
in CH2Cl2 (5 mL) was added to a mixture of compound 3 (3 g,
15.95 mmol) and triethylamine (6.66 mL, 47.85 mmol) in CH2Cl2
(25 mL) under 0 °C. The reaction mixture was brought to rt in
30 min under stirring, then extracted with CH2Cl2, washed with
brine, dried over Na2SO4, and concentrated in vacuo. The residue
was purified by column chromatography on silica gel with hex-
ane–EtOAc (6:1) to give compound 4 as a white solid (3.65 g,
oil as a mixture of anomers (ꢂ1:1) (1.05 g, 72%). ½a D30
¼ þ88:0 (c
ꢀ
1.86, CHCl3); IR mmax 3304, 2929, 2857, 2104, 1690, 1643, 1531,
1072 cmꢁ1 1H NMR (300 MHz, CDCl3) d 1.07 (d, 3H, J = 4.7 Hz),
;
1.09 (d, 3H, J = 4.7 Hz), 2.15 (m, 1H), 2.36 (m, 1H), 3.18 (dd, 1H
J = 4.7, 13.2 Hz), 3.32 (dd, 1H J = 5.4, 13.0 Hz), 3.56 (dd, 2H,
J = 3.0, 13.0 Hz), 3.94–4.23 (m, 4H), 5.29 (s, 1H), 5.47 (br s, 1H);
MS (EI) m/z 155 [M+ꢁ18].
86%): ½a 3D0
¼ þ29:7 (c 1.32, CHCl3); mp 57–59 °C; IR mmax 2924,
ꢀ

2404
G. Jayachitra et al. / Tetrahedron: Asymmetry 19 (2008) 2402–2405
4.4. (2S,3S)-4-Azido-2-methylbutane-1,3-diol 8
The reaction mixture was poured into water and extracted with
EtOAc. The extract was washed with saturated brine, dried over
Na2SO4, and concentrated in vacuo. The residue was column chro-
matographed with hexane–EtOAc (24:1) to give compound 11 as a
To a solution of compound 6 (1 g, 5.78 mmol) in 80% aq MeOH
(10 mL) was added NaIO4 (2.47 g, 11.56 mmol). The reaction mix-
ture was stirred for 30 min. The solvent was then removed under
reduced pressure and the residue was extracted with EtOAc, dried
over Na2SO4, and concentrated to afford the crude product 7. A
solution of compound 7 in dry MeOH was added to NaBH4
(0.55 g, 14.45 mmol) at 0 °C. The reaction mixture was stirred at
0 °C to rt for 30 min. The methanol was then removed under vacuo
and the residue was quenched with NH4Cl, extracted with EtOAc,
washed with brine, dried over Na2SO4, and concentrated in vacuo.
The residue was column chromatographed with hexane–EtOAc
colorless oil (0.1 g, 78%): ½a D30
ꢀ
¼ þ34:2 (c 1.31, CHCl3); IR mmax
2924, 2854, 2105, 1701, 1402, 1173 cmꢁ1
;
1H NMR (200 MHz,
CDCl3) d 1.13 (br d, 3H, J = 5.2 Hz), 1.46 (s, 9H), 2.10–2.37 (m,
1H), 2.90–3.10 (m, 1H), 3.15–3.39 (m, 1H), 3.47–3.84 (m, 3H);
13C NMR (75 MHz, CDCl3) d 16.4, 28.4, 38.1 and 38.8*, 49.3 and
49.7*, 50.7 and 51.1*, 65.2 and 65.9*, 79.6, 154.2; *rotamers; MS
(ESI) m/z 227 [M+1]+.
4.8. (3R,4S)-1-tert-Butoxycarbonyl-3-tert-
butoxycabonylamino-4-methylpyrrolidine 12
(2:1) to give compound 8 as a colorless oil (0.68 g, 82%): ½a D30
¼
ꢀ
þ35:6 (c 1.73, CHCl3); IR mmax 3353, 2967, 2928, 2887, 2094,
1439, 1273, 1023 cmꢁ1 1H NMR (300 MHz, CDCl3) d 0.9 (d, 1H,
;
A solution of compound 11 (50 mg, 0.22 mmol) in MeOH (5 mL)
was hydrogenated over 10% Pd–C for 2 h at rt. The mixture was fil-
tered and (Boc)2O (50 mg, 0.23 mmol) was added under ice cool-
ing. The reaction mixture was stirred at rt for 14 h, and then
concentrated in vacuo. The residue was column chromatographed
with hexane–EtOAc (4:1) to give compound 12 as a colorless oil
J = 6.8 Hz), 1.9 (m, 1H), 3.31 (dd, 1H, J = 6.8, 12.0 Hz), 3.48 (dd,
1H, J = 3.0, 12.8 Hz), 3.60–3.72 (m, 2H), 3.77 (dd, 1H, J = 3.8,
10.5 Hz); 13C NMR (75 MHz, CDCl3) d 13.4, 37.5, 55.3, 67.0, 75.8;
MS (ESI) m/z 168 [M+Na]+.
(60 mg, 91%): ½a 3D0
ꢀ
¼ þ36:7 (c 1.86, CHCl3); IR mmax 3351, 2926,
4.5. (2S,3S)-4-Azido-2-methyl-3(4-methylphenylsulfonyloxy)-
butyl-4-methyl-1-benzene sulfonate 9
2857, 1686, 1411, 1169 cmꢁ1
;
1H NMR (200 MHz, CDCl3) d 1.07
(d, 3H, J = 6.6 Hz), 1.45 (s, 18H), 1.91–2.11 (m, 1H), 2.88–3.14 (m,
2H), 3.47–3.86 (m, 3H), 4.43–4.60 (br s, 1H); 13C NMR (75 MHz,
CDCl3) d 15.8, 28.3, 28.4, 38.5 and 39.6*, 50.3 and 50.8*, 51.1 and
A solution of tosyl chloride (2.3 g, 12.06 mmol) in CH2Cl2
(15 mL) was added to a mixture of compound 8 (0.5 g, 3.44 mmol),
triethylamine (3.35 mL, 24.13 mmol) in CH2Cl2 (10 mL) and a cata-
lytic amount of DMAP under 0 °C. The reaction mixture was stirred
at rt for 4 h, after which it was extracted with CH2Cl2, washed with
brine, dried over Na2SO4, and concentrated in vacuo. The residue
was column chromatographed with hexane–EtOAc (17:3) to give
*
51.4*, 55.6 and 56.2*, 79.4, 79.7, 154.4, 155.4; rotamers; MS (ES)
m/z 301 [M+1]+.
4.9. (3R,4S)-3-Amino-4-methylpyrrolidine dihydrochloride 2
compound 9 as a white solid (1.26 g, 81%): ½a D30
¼ ꢁ22:0 (c 1.44,
ꢀ
To a solution of compound 12 (30 mg, 0.1 mmol) in MeOH
(5 mL) was added 4 M HCl–MeOH (1 mL) at 0 °C. The reaction mix-
ture was stirred at 0 °C for 30 min. Then MeOH was removed under
reduced pressure, washed with dry ether, and dried to give com-
CHCl3); mp 87–90 °C; IR mmax 2923, 2853, 2104, 1360,
1174 cmꢁ1 1H NMR (200 MHz, CDCl3) d 0.92 (d, 3H, J = 6.7 Hz),
;
2.26 (m, 1H), 2.47 (s, 3H), 2.49 (s, 3H), 3.26 (dd, 1H, J = 4.17,
13.3 Hz), 3.67 (dd, 1H, J = 4.17, 13.3 Hz), 3.76–3.95 (m, 2H), 4.46–
4.57 (m, 1H), 7.31 (d, 2H, J = 3.3 Hz), 7.35 (d, 2H, J = 3.3 Hz), 7.72
(d, 2H, J = 5.8 Hz); 7.76 (d, 2H, J = 5.8 Hz); 13C NMR (75 MHz, CDCl3)
d 12.7, 21.6, 34.9, 51.5, 70.3, 80.3, 127.8, 127.9, 129.9, 130.0, 132.5,
133.2, 145.0, 145.4; MS (LC) m/z 476 [M+Na]+.
pound 2 as a white solid (15.7 mg, 91%): ½a D30
¼ þ15:3 (c 0.19,
ꢀ
MeOH); mp 250 °C dec; IR mmax 3376, 2971, 1613, 1515, 1459,
1045 cmꢁ1 1H NMR (300 MHz, D2O) d 1.11 (d, 3H, J = 6.8 Hz),
;
2.42 (m, 1H), 2.9 (dd, 1H, J = 9.8, 12.0 Hz), 3.32 (dd, 1H, J = 7.5,
12.8 Hz), 3.54–3.65 (m, 2H), 3.75 (dd, 1H, J = 8.3, 12.8 Hz); 13C
NMR (75 MHz, CDCl3) d 14.3, 37.5, 47.8, 51.0, 54.9; HRMS (ESI)
Calcd for C5H13N2 [M+1]+ 101.1078, found 101.1083.
4.6. (3S,4S)-1-tert-Butoxycarbonyl-3-(4-
methylphenylsulfonyloxy)-4-methyl pyrrolidine 10
Acknowledgments
A solution of compound 9 (1 g, 2.20 mmol) in methanol (20 mL)
was added to triphenylphosphene (0.39 g, 1.47 mmol) and refluxed
for 12 h. Methanol was then removed from the reaction mixture
under vacuo. The residue was dissolved in THF (15 mL) after which
were added triethylamine (0.92 mL, 6.62 mmol), and then Boc2O
(0.50 mL, 2.20 mmol) under ice cooling and stirred for 14 h at rt.
The reaction mixture was then taken in water and extracted with
EtOAc, washed with brine, dried over Na2SO4, and concentrated
in vacuo. The residue was purified by column chromatography
with hexane–EtOAc (9:1) to give compound 10 as a white solid
Authors thank late Dr. A. K. Singh, Dr. J. S. Yadav, Dr. T. K.
Chakraborty, and Dr. A. C. Kunwar for their support and
encouragement.
References
1. (a) Attygalle, A. B.; Morgan, D. E. Chem. Soc. Rev. 1984, 13, 245–278; (b) Pichon,
M.; Figade- re, B. Tetrahedron: Asymmetry 1996, 7, 927–964; (c) O’Hagan, D. Nat.
Prod. Rep. 1997, 14, 637–651.
2. (a) Hagen, S. E.; Domagala, J. M.; Heifetz, C. L.; Sanchez, J. P.; Solomon, M. J. Med.
Chem. 1990, 33, 849–854; (b) Okada, T.; Ezumi, K.; Yamakawa, M.; Sato, H.;
Tsuji, T.; Tsushima, T.; Motokawa, K.; Komatsu, Y. Chem. Pharm. Bull. 1993, 41,
126–131; (c) Tsuzuki, Y.; Tomita, K.; Shibamori, K.; Sato, Y.; Kashimoto, S.;
Chiba, K. J. Med. Chem. 2004, 47, 2097–2109.
3. Miyamoto, H.; Ueda, H.; Otsuka, T.; Aki, S.; Tamaoka, H.; Tominaga, M.;
Nakagawa, K. Chem. Pharm. Bull. 1990, 38, 2472–2475.
4. Wise, R.; Andrews, J. M.; Edwards, L. J. Antimicrob. Agents Chemother. 1983, 23,
559.
5. Hayakawa, T.; Hiramitsu, Y.; Tanaka, A. Chem. Pharm. Bull. 1984, 32, 4907.
6. Tomita, K.; Tsuzuki, Y.; Shibamori, K.; Tashima, M.; Kajikawa, F.; Sato, Y.;
Kashimoto, S.; Chiba, K.; Hino, K. J. Med. Chem. 2002, 45, 5564–5575.
7. (a) Matsumoto, J.; Nakamura, S.; Miyamoto, T.; Uno, H. European Patent
0132845, 1985; Chem. Abstr. 1985, 102, 220858a.; (b) Di Cesare, P.; Essiz, M.;
Jacquet, J. P.; Kiechel, J. R.; Remuzon, P.; Kessler, R. E.; Fung-Tomc, J.; Desiderio,
(0.4 g, 51%): ½a 3D0
ꢀ
¼ ꢁ7:4 (c 0.40, CHCl3); mp 103–106 °C; IR mmax
2975, 2928, 1696, 1408, 1175, 900, 766 cmꢁ1
;
1H NMR (200 MHz,
CDCl3) d *1.02 (2d, 3H, J = 6.6–18.4 Hz), 1.42 (s, 9H), 2.20–2.39
(m, 1H), 2.47 (s, 3H), 3.0 (t, 1H, J = 10.3 Hz) 3.32–3.64 (m, 3H),
4.87 (m, 1H), 7.33 (d, 2H, J = 8.0 Hz), 7.77 (d, 2H, J = 8.0 Hz), (*rota-
mers); MS (LC) m/z 378 [M+Na]+.
4.7. (3R,4S)-3-Azido-1-tert-butoxycarbonyl-4-methyl
pyrrolidine 11
A mixture of compound 10 (0.2 g, 0.56 mmol) and sodium azide
(0.15 g, 2.25 mmol) in DMF (5 mL) was heated at 90 °C for 12 h.

G. Jayachitra et al. / Tetrahedron: Asymmetry 19 (2008) 2402–2405
2405
J. J. Med. Chem. 1992, 35, 4205–4213; (c) Yoshida, T.; Takeshita, M.; Orita, H.;
Kado, N.; Yasuda, S.; Kato, H.; Itoh, Y. Chem. Pharm. Bull. 1996, 44, 1128–1131;
(d) Zhenkun, Ma.; Chu, T. W.; Cooper, C. S.; Li, Qun.; Fung, A. K. L.; Wang, S.;
Shen, L.; Flamm, R. K.; Nilius, A. M.; Alder, J. D.; Meulbroek, J. A.; Or, Y. S. J. Med.
Chem. 1999, 42, 4202–4213; (e) Einsiedel, J.; Weber, K.; Thomas, C.; Lehmann,
T.; Hubner, H.; Gmeiner, P. Bioorg. Med. Chem. Lett. 2003, 13, 3293–3296; (f)
Davies, S. G.; Garner, A. C.; Goddard, E. C.; Kruchinin, D.; Roberts, P. M.; Smith,
A. D.; Rodriguez-Solla, H.; Thomson, J. E.; Toms, S. M. Org. Biomol. Chem. 2007, 5,
1961–1969.
8. (a) Ravikumar, A.; Santosh Reddy, J.; Rao, B. V. Tetrahedron Lett. 2003, 44, 5687–
5689; (b) Madhan, A.; Rao, B. V. Tetrahedron Lett. 2005, 46, 323–324; (c) Reddy,
J. S.; Rao, B. V. J. Org. Chem. 2007, 72, 2224–2227; (d) Madhan, A.; Rao, B. V.
Tetrahedron Lett. 2003, 44, 5641–5643; (e) Chandrashekar, B.; Madhan, A.
Tetrahedron 2007, 63, 8746–8751.
9. (a) Bhaskar, G.; Rao, B. V. Tetrahedron Lett. 2003, 44, 915–917; (b) Kumar, A. S.;
Haritha, B.; Rao, B. V. Tetrahedron Lett. 2003, 44, 4261–4263.
10. Witty, D. R.; Fleet, G. W. J.; Chori, S.; Vogt, K.; Wilson, F.; Wang, Y.; Storer, R.;
Myers, P. L.; Wallis, C. J. Tetrahedron Lett. 1990, 47, 6927–6930.