One-pot synthesis of 2-imino-1,3-selenazolidines by reaction of isoselenocyanates with propargylamine

Article abstract of DOI:10.3987/COM-06-10766

One-pot synthesis of 2-imino-5-methylene-1,3-selenazolidines has been achieved by reactions of alkylisoselenocyanates with propargylamines in high yields. ¹H NMR and NOESY experiments were used to explain the selenium coupling with hydrogen of

Full text of DOI:10.3987/COM-06-10766

HETEROCYCLES, Vol. 68, No. 8, 2006
1607
HETEROCYCLES, Vol. 68, No. 8, 2006, pp. 1607 - 1615. © The Japan Institute of Heterocyclic Chemistry
Received, 13th April, 2006, Accepted, 8th June, 2006, Published online, 8th June, 2006. COM-06-10766
ONE-POT SYNTHESIS OF 2-IMINO-1,3-SELENAZOLIDINES BY
REACTION OF ISOSELENOCYANATES WITH PROPARGYLAMINE
Mamoru Koketsu,a,* Tsutomu Sakai,b Takashi Kiyokuni,b Dinesh R.
Garud,b Hiromune Ando,a and Hideharu Ishiharab,*
aDivision of Instrumental Analysis, Life Science Research Center, Gifu
University, Gifu 501-1193, Japan
bDepartment of Chemistry, Faculty of Engineering, Gifu University, Gifu
501-1193, Japan; koketsu@cc.gifu-u.ac.jp
Abstract – One-pot synthesis of 2-imino-5-methylene-1,3-selenazolidines has
been achieved by reactions of alkylisoselenocyanates with propargylamines in
high yields. 1H NMR and NOESY experiments were used to explain the selenium
coupling with hydrogen of carbon-carbon double bond.
INTRODUCTION
1,3-Selenazole derivatives, selenium-nitrogen heterocycles, have enthusiastically been studied in the
fields of chemistry and pharmaceutical science, because they biologically act as antioxidants, can protect
oxidant-induced DNA damage and inhibit inducible nitric oxide production. 1 Therefore, several
researchers have put in efforts to develop synthetic methods of 1,3-selenazole derivatives. For the
synthesis of the 1,3-selenazole, several selenating reagents have been used such as selenoamides,2
selenoureas3 and selenazadienes.4 In contrast, the chemistry of heterocycles using isoselenocyanates has
been limited. Recently, heterocycles such as 1,3-selenazolidines, perhydro-1,3-selenazines and
5
1,3-selenazoles have been synthesized by reactions using alkyl isoselenocyanates.
Use of
isoselenocyanates are one of the efficient methods for the synthesis of 1,3-selenazole derivatives. Herein,
we describe one-pot synthesis of 2-imino-1,3-selenazolidines and their characterization.
RESULTS AND DISCUSSION
The alkyl and aryl isoselenocyanates (1) were prepared by reactions of N-substituted formamides with an
excess of triphosgene, selenium and triethylamine (Scheme 1).6

1608
HETEROCYCLES, Vol. 68, No. 8, 2006
triphosgene, Et3N, Se
reflux, CH2Cl2
R NHCHO
R N C Se
1
R
Yield / %
94% (1a)
p-CH3C6H4
C6H5
96% (1b)
72% (1c)
p-ClC6H4
92% (1d)
89% (1e)
30% (1f)
Naphthyl
Benzyl
cyclo-C6H11
Scheme 1
Reaction of isoselenocyanate (1) with propargylamine was carried out at room temperature for 1.5 h to
afford 2-imino-5-methylidene-1,3-selenazolidine (2) (Scheme 2).
1
13
77
The structure of 2a was elucidated by studies of IR, H-, C-, Se-NMR, COSY, HMQC, HMBC and
NOESY, MS, HRMS, elemental analysis and X-ray analysis. Under the similar reaction conditions, the
reactions of six kinds of isoselenocyanates (1) with propargylamine gave the
2-imino-5-methylidene-1,3-selenazolidines (2) in excellent yields (Table 1). Both alkyl and aryl
isoselenocyanates provided the corresponding 2-imino-5-methylidene-1,3-selenazolidines (2) in excellent
yields. The structures of products (2b-2f) were determined by comparing the spectral data with those of
2a.
1
In the H NMR spectra of 2a in CDCl3, an interesting spectral feature was observed. This was the
3
selenium coupling with the H5b proton J(77Se-1H) = 23.9 Hz, but the same coupling was not observed
with H5a proton. The H5b proton is more downfield as compared to the H5a proton (Figure 1). In order to
determine the complete assignment of protons, 1D NOE and NOESY experiments were carried out. The
Se
THF
R
R NCSe
1
+
H2N
rt / 1.5 h
N
N
H
2
Scheme 2

HETEROCYCLES, Vol. 68, No. 8, 2006
1609
Table 1. Reaction of alkyl isoselenocyanates with propargylamine
77Se NMR
Yield
77Se NMR
Yield
(%)a
Product (2)
Product (2)
(%)a
(δ)
(δ)
Se
Se
89
435.8
88
85
93
434.9
481.4
479.8
N
N
N
N
H
H
2a
2d
Se
Se
N
96
97
435.6
429.0
N
N
N
H
H
2e
2b
Cl
Se
Se
N
N
N
N
H
H
2f
2c
a: Isolated yield.
Selenium Coupling
H5b
H4
H5a
Figure 1. 1H NMR of 2a
H3C
(26%)
NOE
H5a
Se
N
H5b
N
H4
(4.5%)
NOE
H4
H
Figure 2

1610
HETEROCYCLES, Vol. 68, No. 8, 2006
NOE experiment of compound (2a) showed, the NOE of H5a proton with H5b (26%) and NOE of H5b
proton with H4 protons (4.5%) (Figure 2). From the above results it is clear that selenium shows coupling
with the trans-proton. We feel that these regularities, if found to be general, may be an important aid for
determining structures and conformations of organoselenium compounds for which such NMR
77
77
information is not available. In the Se NMR spectra of the 1,3-selenazolidines (2), Se signals were
observed in the range of δ 449.4±24.3, which are at a higher field as compared with 77Se signals of
selenocarbonyl compounds (δ 1420-2131).7 The values are typical for a C-Se single bond with an sp3
selenium atom not for a C=Se double bond with an sp2 selenium atom.8 Fujiwara et al. reported the
synthesis of 5-alkylideneselenazolin-2-one by the reaction of 3-aminoalkynes with carbon monoxide and
selenium, in which the rate of the reaction is accelerated by the addition of a proton source.9 But in the
present reaction the additional proton source is infact not needed.
In order to confirm the structure of 2a, we carried out the X-Ray analysis of this compound. An ORTEP
drawing, depicted in Figure 3, shows the molecular structure of the 2a.10 The bond angle the selenium
atom C1-Se1-C3 was 87.4(2)o, consistent with the previous reported value.11 The bond lengths of C1-N2
and C3-C11in 2a are 1.291(7) Å and 1.312(10) Å respectively and clearly shows that these are the double
bonds. The two C-N bond lengths of both N1-C1 (1.345(7) Å) and N1-C2 (1.428(8) Å) in 2a are shorter
than the usual single bond length of 1.47 Å.12 There are no significant geometric differences between the
molecules, except the orientation of the phenyl rings. Each of the independent molecules forms a
centrosymmetric hydrogen-bonded dimer. Hydrogen-bond distances are similar in both independent
dimers. Compound (2a) possesses an exocyclic carbon-nitrogen double bond with Z-configuration. The
five-membered ring is planar.
Figure 3. X-Ray-crystal structure of 2a

HETEROCYCLES, Vol. 68, No. 8, 2006
1611
Next, the effect of substituent at the amine nitrogen was examined. Reaction of isoselenocyanate (1) with
N-methylpropargylamine was carried out to afford 2-imino-3-methyl-5-methylidene-1,3-selenazolidine
(3) (Scheme 4).
THF
Se
R NCSe
+
N
R
rt / 2 h
H
N
N
1
3
Scheme 4
Reactions of six kinds of alkyl isoselenocyanates (1) with N-methylpropargylamine were carried out at
room temperature for 2 h. Purification of the reaction mixture by silica gel column chromatography
afforded the 2-imino-3-methyl-5-methylidene-1,3-selenazolidines (3) in high yields (Table 2). In the 77Se
NMR spectra of the 3-methyl-1,3-selenazolidines (3), 77Se signals were observed in the range of δ
410.6±10.6. The reactions of primary amines gave higher yields as compared to the secondary amine.
Table 2. Reaction of alkyl isoselenocyanates with N-methylpropargylamine
77Se NMR
77Se NMR
Yield
(%)a
Yield
(%)a
Product (3)
Product (3)
(δ)
(δ)
Se
Se
N
N
65
76
71
414.9
415.6
417.7
78
72
73
419.6
403.6
392.1
N
N
3a
3d
Se
Se
N
N
N
N
3e
3b
Cl
Se
Se
N
N
N
N
3f
3c
a: Isolated yield.
In conclusion, we have demonstrated a one-pot synthesis of 2-imino-5-methylidene-1,3-selenazolidines
by reactions of the acyl isoselenocyanates (1) with propargylamine.

1612
HETEROCYCLES, Vol. 68, No. 8, 2006
EXPERIMENTAL
General
The chemical shifts of 77Se NMR were expressed in ppm deshielded with respect to neat Me2Se in CDCl3.
3J(77Se-1H) values are the satellites of the 1H NMR spectra.
Synthesis of 2-(4-methylphenyl)imino-5-methylene-1,3-selenazolidine (2a). To a solution of
4-Methylphenylisoselenocyanate (98.1 mg, 0.50 mmol) in dry THF (8.0 mL), propargylamine (48 μL,
0.75 mmol) was added. The mixture was stirred at ambient for 1.5 h. The reaction mixture was extracted
with diethylether and washed with water. The combined organic layer was dried over sodium sulfate and
evaporated to dryness. The residue was purified by flash chromatography on silica gel with ethyl
acetate:n-hexane (1:3) as the eluent to give 2a (112.4 mg, yield 89%). mp 82.0-83.0 °C; IR (KBr): 1629,
1549 cm-1; 1H NMR (500 MHz, CDCl3): δ 2.32 (3H, s, CH3), 4.51 (2H, t, J = 1.9 Hz, CH2), 5.19 (1H, dd,
J = 2.9, 3.8 Hz, =CH2), 5.53 (1H, dd, J = 1.9, 3.8 Hz, 3J(77Se-1H) = 23.9 Hz, =CH2), 6.98 (2H, d, J = 7.7
Hz, Ar), 7.08 (2H, d, J = 7.7 Hz, Ar), (9.29 (1H, br, NH), at -60°C); 13C NMR (125 MHz, CDCl3): δ 20.8,
59.3, 108.8, 121.2, 129.6, 133.4, 143.4, 145.0, 157.8; 77Se NMR (95 MHz, CDCl3): δ 435.8; MS (FAB):
m/z = 253 [M++1]; Anal. Calcd for C11H12N2Se: C, 52.60; H, 4.82; N, 11.15. Found: C, 52.55; H, 4.85; N,
10.79.
1
2-Phenylimino-5-methylene-1,3-selenazolidine (2b) mp 83.9-84.6 °C; IR (KBr): 1618, 1553 cm-1; H
NMR (500 MHz, CDCl3, -40°C): δ 4.46 (2H, s, CH2), 5.23 (1H, d, J = 2.3 Hz, =CH2), 5.58 (1H, d, J = 1.7
Hz, 3J(77Se-1H) = 24.6 Hz, =CH2), 7.08 (2H, d, J = 7.5 Hz, Ar), 7.13 (1H, t, J = 7.5 Hz, Ar), 7.33 (2H, d,
13
J = 7.5 Hz, Ar), 8.27 (1H, br, NH); C NMR (125 MHz, CDCl3): δ 58.7, 108.9, 121.2, 123.7, 129.1,
142.9, 147.8, 158.0; 77Se NMR (95 MHz, CDCl3): δ 435.6; MS (CI): m/z = 239 [M++1]; Anal. Calcd for
C10H10N2Se: C, 50.64; H, 4.25; N, 11.81. Found: C, 50.70; H, 4.34; N, 11.60.
2-(4-Chlorophenyl)imino-5-methylene-1,3-selenazolidine (2c) mp 125.7-126.2 °C; IR (KBr): 1646,
1583 cm-1; 1H NMR (500 MHz, CDCl3, -40℃): δ 4.38 (2H, s, CH2), 5.24 (1H, d, J = 1.7 Hz, =CH2), 5.58
3
(1H, d, J = 1.7 Hz, J(77Se-1H) = 24.6 Hz, =CH2), 6.96 (2H, d, J = 8.6 Hz, Ar), 7.26 (2H, d, J = 8.6 Hz,
13
Ar), 8.99 (1H, br, NH); C NMR (125 MHz, CDCl3): δ 57.2, 109.6, 122.5, 128.9, 129.1, 141.5, 147.3,
158.7; 77Se NMR (95 MHz, CDCl3): δ 429.0; MS (EI): m/z = 272 [M+]; Anal. Calcd for C10H9ClN2Se: C,
44.22; H, 3.34; N, 10.31. Found: C, 44.48; H, 3.32; N, 10.32.
2-(2-Naphthyl)imino-5-methylene-1,3-selenazolidine (2d) mp 147.9-148.2 °C; IR (KBr): 1623, 1558
cm-1; 1H NMR (500 MHz, CDCl3): δ 4.49 (2H, t, J = 2.3 Hz, CH2), 5.21 (1H, dd, J = 2.3, 4.6 Hz, =CH2),
5.56 (1H, dd, J = 2.3, 4.0 Hz, 3J(77Se-1H) = 25.0 Hz, =CH2), 7.21 (1H, dd, J = 2.3, 8.6 Hz, Ar), 7.33-7.47
13
(2H, m, Ar), 7.54 (1H, d, J = 1.1 Hz, Ar), 7.71-7.82 (3H, m, Ar); C NMR (125 MHz, CDCl3): δ 58.3,

HETEROCYCLES, Vol. 68, No. 8, 2006
1613
77
109.2, 117.0, 122.2, 124.6, 126.2, 127.3, 127.6, 129.0, 130.6, 134.1, 142.5, 145.9, 158.0; Se NMR (95
MHz, CDCl3): δ 434.9; MS (EI): m/z = 288 [M+]; Anal. Calcd for C14H12N2Se: C, 58.54; H, 4.21; N, 9.75.
Found: C, 58.57; H, 4.29; N, 9.25.
2-Benzylimino-5-methylene-1,3-selenazolidine (2e) mp 109.3-109.5 °C; IR (KBr): 1607, 1515 cm-1; 1H
NMR (500 MHz, CDCl3): δ 4.41 (2H, s, CH2), 4.64 (2H, t, J = 2.3 Hz, CH2), 5.16 (1H, dd, J = 2.3, 4.0
Hz, =CH2), 5.48 (1H, dd, J = 2.3, 4.0 Hz, 3J(77Se-1H) = 25.8 Hz, =CH2), 7.20-7.38 (5H, m, Ar); 13C NMR
77
(125 MHz, CDCl3): δ 49.1, 69.7, 107.1, 127.4, 127.6, 128.5, 138.2, 150.5, 155.1; Se NMR (95 MHz,
CDCl3): δ 481.4; MS (FAB): m/z = 253 [M+ +1]; Anal. Calcd for C11H12N2Se: C, 52.60; H, 4.82; N, 11.15.
Found: C, 53.02; H, 4.85; N, 11.13.
2-Cyclohexylimino-5-methylene-1,3-selenazolidine (2f) mp 160.1-161.0 °C; IR (KBr): 1624, 1538
cm-1; 1H NMR (500 MHz, CDCl3): δ 1.12-1.41 (5H, m, ringCH2), 1.55-1.75 (3H, m, ringCH2), 1.98-2.07
(2H, m, ringCH2), 3.42-3.51 (1H, m, ringCH), 4.49 (1H, br, NH), 4.69 (2H, t, J = 2.3 Hz, CH2), 5.17 (1H,
dd, J = 2.3, 4.0 Hz, =CH2), 5.49 (1H, dd, J = 2.3, 4.0 Hz, 3J(77Se-1H) = 25.0 Hz, =CH2); 13C NMR (125
MHz, CDCl3): δ 24.8, 25.5, 33.6, 53.9, 70.2, 106.9, 150.8, 153.9; 77Se NMR (95 MHz, CDCl3): δ 479.8;
MS (CI): m/z = 245 [M++1]; Anal. Calcd for C10H16N2Se: C, 49.38; H, 6.63; N, 11.52. Found: C, 49.05;
H, 6.52; N, 11.20.
2-(4-Methylphenyl)-3-methyl-5-methylene-1,3-selenazolidine (3a) IR (neat): 1614 cm-1; 1H NMR (500
MHz, CDCl3): δ 2.22 (3H, s, CH3), 3.00 (3H, s, CH3), 4.15 (2H, t, J = 2.3 Hz, CH2), 5.11 (1H, dd, J = 2.3,
4.0 Hz, =CH2), 5.48 (1H, dd, J = 1.7, 4.0 Hz, 3J(77Se-1H) = 23.5 Hz, =CH2), 6.74 (2H, d, J = 8.0 Hz, Ar),
6.99 (2H, d, J = 8.0 Hz, Ar); 13C NMR (125 MHz, CDCl3): δ 20.8, 33.7, 60.4, 109.6, 121.4, 129.5, 132.8,
136.5, 150.5, 155.5; 77Se NMR (95 MHz, CDCl3): δ 414.9; MS (EI): m/z = 266 [M+].
1
2-Phenylimino-3-methyl-5-methylene-1,3-selenazolidine (3b) IR (neat): 1620, 1586 cm-1; H NMR
(500 MHz, CDCl3): δ 3.09 (3H, s, CH3), 4.25 (2H, t, J = 2.3 Hz, CH2), 5.20 (1H, dd, J = 2.3, 4.0 Hz,
=CH2), 5.57 (1H, dd, J = 1.7, 4.0 Hz, 3J(77Se-1H) = 23.5 Hz, =CH2), 6.90-6.97 (2H, m, Ar), 7.02-7.10 (1H,
13
m, Ar), 7.23-7.31 (2H, m, Ar); C NMR (125 MHz, CDCl3): δ 33.7, 60.5, 109.7, 121.7, 123.5, 128.9,
77
136.4, 153.0, 155.5; Se NMR (95 MHz, CDCl3): δ 415.6; MS (EI): m/z = 252 [M+]; Anal. Calcd for
C11H12N2Se: C, 52.60; H, 4.82; N, 11.15. Found: C, 52.37; H, 4.77; N, 10.74.
2-(4-Chlorophenyl)imino-3-methyl-5-methylene-1,3-selenazolidine (3c) IR (neat): 1620, 1584 cm-1;
1H NMR (500 MHz, CDCl3): δ 3.08 (3H, s, CH3), 4.27 (2H, t, J = 2.3 Hz, CH2), 5.22 (1H, dd, J = 2.3, 4.0
3
Hz, =CH2), 5.59 (1H, dd, J = 1.7, 4.0 Hz, J(77Se-1H) = 23.5 Hz, =CH2), 6.85 (2H, d, J = 8.6 Hz, Ar),
7.22 (2H, d, J = 8.6 Hz, Ar); 13C NMR (125 MHz, CDCl3): δ 33.7, 60.5, 110.0, 123.1, 128.6, 129.0, 136.0,
151.6, 156.0; 77Se NMR (95 MHz, CDCl3): δ 417.7; MS (EI): m/z = 286 [M+]; Anal. Calcd for

1614
HETEROCYCLES, Vol. 68, No. 8, 2006
C11H11ClN2Se: C, 46.25; H, 3.88; N, 9.81. Found: C, 46.34; H, 3.90; N, 9.42.
2-(2-Naphthyl)imino-3-methyl-5-methylene-1,3-selenazolidine (3d) mp 66.3-66.6 °C; IR (KBr): 1614,
1586 cm-1; 1H NMR (500 MHz, CDCl3): δ 3.13 (3H, s, CH3), 4.25 (2H, t, J = 2.3 Hz, CH2), 5.18 (1H, dd,
J = 2.3, 4.0 Hz, =CH2), 5.56 (1H, dd, J = 1.7, 4.0 Hz, 3J(77Se-1H) = 24.1 Hz, =CH2), 7.15 (1H, dd, J = 1.7,
8.6 Hz, Ar), 7.30 (1H, s, Ar), 7.35 (1H, t, J = 6.9 Hz, Ar), 7.41 (1H, t, J = 6.9 Hz, Ar), 7.71-7.81 (3H, m,
13
Ar); C NMR (125 MHz, CDCl3): δ 33.8, 60.5, 109.8, 117.1, 123.2, 124.3, 125.9, 127.2, 127.6, 128.7,
130.6, 134.2, 136.4, 151.0, 155.8; 77Se NMR (95 MHz, CDCl3): δ 419.6; MS (EI): m/z = 302 [M+]; Anal.
Calcd for C15H14N2Se: C, 59.81; H, 4.68; N, 9.30. Found: C, 60.01; H, 5.00; N, 9.10.
1
2-Benzylimino-3-methyl-5-methylene-1,3-selenazolidine (3e) IR (neat): 1627, 1585 cm-1; H NMR
(500 MHz, CDCl3): δ 2.94 (3H, s, CH3), 4.07 (2H, t, J = 2.3 Hz, CH2), 4.26 (2H, s, CH2), 5.19 (1H, dd, J
= 2.3, 4.0 Hz, =CH2), 5.52 (1H, dd, J = 1.7, 4.0 Hz, 3J(77Se-1H) = 24.5 Hz, =CH2), 7.10-7.28 (5H, m, Ar);
77
13C NMR (125 MHz, CDCl3): δ 33.9, 60.3, 62.1, 109.9, 126.6, 127.4, 128.2, 136.4, 140.6, 155.6; Se
NMR (95 MHz, CDCl3): δ 403.6; MS (EI): m/z = 266 [M+]; HRMS: m/z =267.0322, calcd. for
C12H15N2Se, found 267.0409 [M++H].
2-Cyclohexylimino-3-methyl-5-methylene-1,3-selenazolidine (3f) IR (neat): 1645, 1587 cm-1; 1H NMR
(500 MHz, CDCl3): δ 1.12-1.48 (5H, m, ringCH2), 1.54-1.65 (1H, m, ringCH2), 1.70-1.82 (4H, m,
ringCH2), 2.35-2.46 (1H, m, ringCH2), 2.93 (3H, s, CH3), 4.06 (2H, t, J = 2.3 Hz, CH2), 5.26 (1H, dd, J =
3
13
2.3, 4.0 Hz, =CH2), 5.59 (1H, dd, J = 1.7, 4.0 Hz, J(77Se-1H) = 22.3 Hz, =CH2); C NMR (125 MHz,
77
CDCl3): δ 25.0, 25.7, 34.1, 34.8, 60.0, 69.1, 109.5, 137.0, 151.7; Se NMR (95 MHz, CDCl3): δ 392.1;
MS (EI): m/z = 272 [M+].
ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Science Research from the Ministry of Education, Culture,
Sports, Science and Technology of Japan (No. 15550030 and 17550099) to which we are grateful.
REFERENCES AND NOTES
1. (a) A. Sekiguchi, A. Nishina, H. Kimura, R. Fukumoto, K. Kanoh, H. Ishihara, and M. Koketsu,
Chem. Pharm. Bull., 2005, 53, 1439. (b) S. Ueda, H. Terauchi, K. Suzuki, A. Yano, M. Matsumoto,
T. Kubo, H. Minato, Y. Arai, J. Tsuji, and N. Watanabe, Bioorg. Med. Chem. Lett., 2005, 15, 1361.
(c) V. D. Silva, M. M. Woznichak, K. L. Burns, K. B. Grant, and S. W. May, J. Am. Chem. Soc.,
2004, 126, 2409. (d) Y. Park, M. Koketsu, J. Kim, J. Yeo, H. Ishihara, K. Lee, S. Y. Kim, and C.
Kim, Biol. Pharm. Bull., 2003, 26, 1657. (e) M. Koketsu, S. Y. Choi, H. Ishihara, B. O. Lim, H. Kim,
and S. Y. Kim, Chem. Pharm. Bull., 2002, 50, 1594.

HETEROCYCLES, Vol. 68, No. 8, 2006
1615
2. (a) K. Geisler, A. Kuenzler, H. Below, E. Bulka, W.-D. Pfeiffer, and P. Langer, Synthesis、2004, 97.
(b) M. Koketsu, Y. Takenaka, and H. Ishihara, Heteroatom Chem., 2003, 14, 106. (c) P.-F. Zhang
and Z.-C. Chen, Synthesis, 2000, 1219.
3. (a) K. Geisler, W.-D. Pfeiffer, A. Kuenzler, H. Below, E. Bulka, and P Langer, Synthesis, 2004, 875.
(b) R. M. Moriarty, B. K. Vaid, M. P. Duncan, S. G. Levy, O. Prakash, and S. Goyal, Synthesis, 1992,
845.
4. (a) M. Koketsu, M. Imagawa, T. Mio, and H. Ishihara, J. Heterocycl. Chem., 2005, 42, 831. (b) M.
Koketsu, T. Mio, and H. Ishihara, Synthesis, 2004, 233.
5. (a) G. L. Sommen, A. Linden, and H. Heimgartner, Eur. J. Org. Chem., 2005, 3128. (b) P. K.
Atanassov, A. Linden, and H. Heimgartner, Helv. Chim. Acta, 2004, 87, 1452. (c) K. Banert and C.
Toth, Angew. Chem., Int. Ed. Engl., 1995, 34, 1627.
6. J. G. Fernández-Bolaños, Ó. López, V. Ulgar, I. Maya, and J. Fuentes, Tetrahedron Lett., 2004, 45,
4081.
7. (a) T. Murai, K. Kakami, A. Hayashi, T. Komuro, H. Takada, M. Fujii, T. Kanda, and S. Kato, J. Am.
Chem. Soc., 1997, 119, 8592. (b) T. C. Wong, F. S. Jr., Guziec, and C. A. Moustakis, J. Chem. Soc.,
Perkin Trans. 2, 1983, 1471. (c) E. R. Cullen, F. S. Guziec, C. J. Murphy, T. C. Wong, and K. K.
Andersen, J. Am. Chem. Soc., 1981, 103, 7055.
8. (a) T. C. Wong and E. M. Engler, J. Mol. Struct., 1980, 67, 279. (b) J. R. Bartels-Keith, M. T.
Burgess, and J. M. Stevenson, J. Org. Chem., 1977, 42, 3725. (c) M. Garreau, G. J. Martin, M. L.
Martin, J. Morel, and C. Paulmier, Org. Magn. Reson., 1974, 6, 648.
9. Y. Asanuma, S. Fujiwara, T. Shin-ike, and N. Kambe, J. Org. Chem., 2004, 69, 4845.
10. CCDC 603852 for 2a contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
11. V. Barba, C. Hernandez, S. Rojas-Lima, N. Farfan, and R. Santillan, Can. J. Chem., 1999, 77, 2025.
12. (a) Tables of Interatomic Distances and Configuration in Molecules and Ions; The Chemical Society:
London, 1958. (b) Tables of Interatomic Distances and Configuration in Molecules and Ions; The
Chemical Society: London, 1965. (c) G. M. Li, R. A. Zingaro, M. Segi, J. H. Reibenspies, and T.
Nakajima, Organometallics, 1997, 16, 756.