Phenylboronic acid catalyzed-cyanide promoted, one-pot synthesis of 2-(2-hydroxyphenyl)benzoxazole derivatives

Article abstract of DOI:10.1016/j.tetlet.2011.06.039

A novel, one-pot, phenylboronic acid catalyzed, cyanide promoted synthesis of 2-(2-hydroxyphenyl)benzoxazoles from salicylaldehydes and o-aminophenols is described. The synthesis is characterized by mild conditions, short reaction times, and simple workup of crystalline, high purity products.

Full text of DOI:10.1016/j.tetlet.2011.06.039

Tetrahedron Letters 52 (2011) 4308–4312
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Phenylboronic acid catalyzed-cyanide promoted, one-pot synthesis
of 2-(2-hydroxyphenyl)benzoxazole derivatives
Heraclio López-Ruiz a, , Horacio Briseño-Ortega a, Susana Rojas-Lima a, Rosa Santillan b, Norberto Farfán c
⇑
a Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, Carretera Pachuca-Tulancingo Km 4.5, Ciudad Universitaria, 42184 Mineral de la Reforma, Mexico
b Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional Apto., Postal 14-740, 07000 México D.F., Mexico
c Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, Circuito Escolar, Ciudad Universitaria, 04510 México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
A
novel, one-pot, phenylboronic acid catalyzed, cyanide promoted synthesis of 2-(2-hydroxy-
Received 18 March 2011
Revised 6 June 2011
Accepted 9 June 2011
Available online 17 June 2011
phenyl)benzoxazoles from salicylaldehydes and o-aminophenols is described. The synthesis is character-
ized by mild conditions, short reaction times, and simple workup of crystalline, high purity products.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
One-pot synthesis
Schiff bases
Boronates
Potassium cyanide
2-Arylbenzoxazoles
Introduction
reported that heterobicyclic[4.3.0] boron compounds, containing
a dative N–B bond, were converted into 2-(2-hydroxyphenyl)benz-
There has been a recent surge in the development of new benz-
oxazole syntheses because of their occurrence in a number of
natural products,1 and their potential use as cytotoxic agents,2
cathepsin S inhibitors,3 HIV reverse transcriptase inhibitors,4 estro-
gen receptor agonists,5 selective peroxisome proliferator activated
receptor antagonists,6 anticancer agents,7 and orexin-1 receptor
antagonists.8 They have also found application as herbicides and
as fluorescent whitening agent dyes.9 The methods which have
been used to provide access to benzoxazoles include: (a) conven-
tional thermal or microwave accelerated condensation of 2-amino-
phenols with carboxylic acid derivatives under strongly acidic
conditions10 (b) metal catalyzed cyclization of 2-halo N-acylani-
lines,11 and (c) oxidative cyclization of the Schiff bases derived
from 2-aminophenols and aldehydes using oxidants such as 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),12 PhI(OAc)2,13
NiO2,14 Ba(MnO4)2,15 pyridinium chlorochromate (PCC),16 Mn-
(OAc)3,17 PbO2,18 Pb(OAc)4,19 ThClO4,20 and Pd(OAc)2.21 In addition,
oxidative cyclization conditions considerably milder than those
referred above, have been described for the generation of benzox-
azoles from the Schiff bases derived from 2-aminophenols.22 Other
significant one-pot syntheses of benzoxazoles have recently been
developed by Chakraborti et al.23 and Kim et al.24 Recently, we
oxazoles in the presence of potassium cyanide.25 During the
preparation of this manuscript, Reyes reported the synthesis of
2-phenylbenzoxazoles by potassium cyanide assisted reaction of
o-aminophenols and benzaldehydes.26 Herein we describe an
exceptionally mild, one-pot, phenylboronic acid catalyzed synthe-
sis of such 2-arylbenzoxazoles directly from salicylaldehydes and
2-aminophenols in the presence of potassium cyanide.
Results and discussions
Salicylaldehyde (1a) and 2-amino-m-cresol (2b) were chosen as
prototypical substrates for reaction parameter optimization. Both
the quantities of phenylboronic acid and potassium cyanide were
varied, with 10 mol % being optimal for the former (Scheme 1),
and 100 mol % for the latter (Table 1, entries 1 and 2). Methanol,
O
HO
O
N
1. PhB(OH)2
2. KCN
+
H
H2N
OH
OH
1a
2b
3b
PhB(OH)2, 10 % mol, 50 % yield
PhB(OH)2, 20 % mol, 38 % yield
⇑
Corresponding author.
E-mail addresses: heracliol@yahoo.com.mx, heraclio@uaeh.edu.mx (H. López-
Ruiz).
Scheme 1.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.039

H. López-Ruiz et al. / Tetrahedron Letters 52 (2011) 4308–4312
4309
Table 1
ethanol, THF, and acetonitrile all could be used as solvents for the
process (Table 1), but the reactions occurred most rapidly, and usu-
ally in superior yields, in methanol.
Effect of amount of KCN and solvent on the synthesis of 2-phenylbenzoxazolea
Entry
KCN (mol %)
Solvent
Time (h)
Yieldb (%)
With the optimized reaction conditions in hand, the generality
of the 2-(2-hydroxyphenyl)benzoxazole synthesis was examined.
The reaction showed considerable tolerance for substituents in
the o-aminophenol component with both salicylaldehyde 1a and
5-bromosalicyladehyde 1b (Table 2). Electron donating and
attracting substituents in this partner led to the desired products
with a tendency for higher product yields and shorter reaction
times for donor substituents. In contrast, a strong donor substitu-
1
2
3
4
5
100
200
100
100
100
Methanol
Methanol
Ethanol
THF
Acetonitrile
4
4
12
18
12
50
45
42
44
47
a
Reaction conditions: 1.0 equiv of salicylaldehyde, 1.1 equiv of 2-amino-m-
cresol, 10 mol % of PhB(OH)2, room temperature (25 °C), open flash.
b
Isolated and unoptimized.
Table 2
Synthesis of 2-phenylbenzoxazoles
7
6'
5'
3'
O
6
3
O
N
6 R2
HO
1
2
6
5
4
1'
2'
1 eq. KCN, MeOH, r.t.
10 % mol of PhB(OH)2
4'
R1
1
5
H
R2
+
5
R1
2
4
H2N
4
OH
2
OH
3
1
2
3
Entry
1
R1
H
R2
H
Time (h)
Benzoxazole
Yield (%)a [Ref.]
48 [25,31,32]
O
4
4
4
N
3a
3b
OH
OH
OH
O
N
2
3
4
5
H
H
H
H
3-CH3
4-CH3
5-CH3
4-Cl
50 [25,32]
57 [25,31,32]
45 [32]
O
N
3c
O
N
4
14
4
OH
OH
3d
O
N
78 [25,31,32]
Cl
Cl
3e
O
N
6
7
H
H
5-Cl
55 [32]
61 [21]
OH
OH
3f
O
N
4-NO2
4
NO2
3g
NO2
O
N
8
9
H
H
5-NO2
5
4
45 [25]
32
OH
OH
3h
NO2
O
N
4-Cl, 5-NO2
Cl
3i
Br
Br
O
10
11
5-Br
5-Br
H
4
4
56 [21]
N
3j
OH
O
N
3-CH3
35
3k
OH
(continued on next page)

4310
H. López-Ruiz et al. / Tetrahedron Letters 52 (2011) 4308–4312
Table 2 (continued)
Entry
R1
R2
Time (h)
4
Benzoxazole
Yield (%)a [Ref.]
Br
O
N
12
13
14
15
5-Br
4-CH3
68
OH
Br
3l
O
N
5-Br
5-Br
5-Br
5-CH3
4-Cl
4
4
4
50
46
49
OH
Br
3m
O
N
Cl
Cl
OH
Br
3n
O
N
5-Cl
OH
Br
3o
O
N
16
5-Br
5-Br
4-NO2
5-NO2
6
6
2721
NO2
3p
OH
Br
NO2
O
N
17
18
19
OH
Br
3q
NO2
O
N
5-Br
4-Cl, 5-NO2
4
48
30
Cl
3r
OH
N
O
N
19
4-N(CH2CH3)2
4-Cl
24
Cl
OH
3s
a
Isolated and unoptimized.
ent in the salicyladehyde component, as represented by 4-dieth-
ylaminosalicyladehyde 1c, had a negative effect both on the prod-
uct yield and the reaction time, at least with 2-amino-4-
chloropenol 2e as the o-aminophenol partner (Table 2, entry 19).
The structure of most of the benzoxazoles was supported by the
usual spectroscopic data,27 and that of 3s was established unequiv-
ocally by single crystal X-ray spectroscopy (Fig. 1),28 which inter-
estingly showed an intramolecular H-bond between the phenolic
hydroxyl group and the benzoxazole nitrogen atom (Fig. 1). The
role of potassium cyanide is through benzoin condensation trig-
gered by phenylboronic acid, thus it requires a stoichiometric
amount of potassium cyanide.29
The generality of the process was further illustrated by using
the methylene bis-salicylaldehyde 4, prepared by the acid cata-
lyzed reaction of salicylaldehyde with trioxane,30 as the aldehyde
component. The methylene bis-benzoxazoles 3t, 3u, and 3v were
thus obtained from 2-aminophenol 2a, 2-amino-4-methylphenol
2c, and 2-amino-4-chlorophenol 2e in 63%, 63%, and 54% yields,
respectively (Scheme 2).
Conclusions
In conclusion, we have developed a simple, room temperature,
one-pot synthesis of 2-(2-hydroxyphenyl)benzoxazoles from
Figure 1. X-ray structure of 3s.

H. López-Ruiz et al. / Tetrahedron Letters 52 (2011) 4308–4312
4311
R
R
O
O
O
O
6
3
HO
1
2
5
PhB(OH)2
KCN
N
N
H
H
+
4
HO
OH
H2N
R
HO
OH
3t,
R = -H
3u, R = -CH3
3v,
4
2a,
R = -H
2c, R = -CH3
2e, R = -Cl
R = -Cl
Scheme 2.
19. Stephens, F. F.; Bower, J. D. J. Chem. Soc. 1949, 2971.
20. Park, K. H.; Jun, K.; Shin, S. R.; Oh, S. W. Tetrahedron Lett. 1996, 37, 8869.
21. Chen, W.-H.; Pang, Y. Tetrahedron Lett. 2009, 50, 6680.
22. (a) Shoar, R. H.; Heidary, M.; Farzaneh, M.; Malakouti, R. Synth. Commun. 2009,
39, 1742; (b) Chen, Y.-X.; Qian, L. F.; Zhang, W.; Han, B. Angew. Chem. Int. Ed.
2008, 47, 9330; (c) Baltork, I. M.; Moghadam, M.; Tangestaninejad, S.;
Mirkhani, V.; Zolfigol, M. A.; Hojati, S. F. J. Iran. Chem. Soc. 2008, 5, S65; (d)
Ponnala, S.; Sahu, D. P. Synth. Commun. 2006, 36, 2189; (e) Kawashita, Y.;
Nakamishi, N.; Kawabata, H.; Hayashi, M. Org. Lett. 2003, 5, 3713.
2-aminobenzaldeyhdes and salicylaldehydes. This process shows
good generality with regard to substituents in both reaction part-
ners, and is thus amenable to the generation of a large library of
2-(2-hydroxyphenyl)benzoxazoles.
Acknowledgments
23. Kumar, D.; Rudrawar, S.; Chakraborti, A. K. Aust. J. Chem. 2008, 61, 881.
24. Lee, J. J.; Kim, J.; Jun, Y. M.; Lee, B. M.; Kim, B. H. Tetrahedron 2009, 65, 8821.
25. López-Ruiz, H.; Briseño-Ortega, H.; Rojas-Lima, S.; Santillán, R.; Farfán, N.
Tetrahedron Lett. 2010, 51, 2633.
We are indebted to Dr. J. M. Muchowski for friendly, helpful dis-
cussions and to CONACyT for financial support (Grant J49336-Q).
26. Reyes, H.; Beltran, H. I.; Rivera-Becerril, E. Tetrahedron Lett. 2011, 52, 308.
27. General procedure: In a 100 mL round-bottomed flask containing a magnetic
stirring bar was placed 1 equiv of 2-hydroxybenzaldehyde (1a–c) in 50 mL of
methanol, 1 equiv of 2-aminophenol (2a–i), 10 mol % of phenylboronic acid
and 1 equiv of potassium cyanide. The resulting solution was stirred at room
temperature for 4 h and the solvent was removed in vacuum. The crude
product was purified by crystallization from methanol at 0 °C. Compound 3i:
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.06.039.
mp 187–188 °C; IR (KBr): m ;
max 2955, 1770, 1375, 1246, 1056, and 750 cmÀ1 1H
References and notes
NMR (CDCl3, 400 MHz) d 10.84 (s, 1H), 8.17 (s, 1H), 7.99 (dd, 1H, J = 8.0 Hz,
J = 1.8 Hz),7.84 (s, 1H), 7.51 (td, 1H, J = 8.4 Hz, J = 1.4 Hz), 7.12 (d, 1H,
J = 8.4 Hz), 7.03 (td, 1H, J = 8.0 Hz, J = 0.9 Hz). 13C NMR (CDCl3, 100 MHz) d
167.6, 159.5, 146.6, 144.5, 143.8, 135.5, 127.7, 124.5, 121.5, 120.2, 118.0, 109.1,
108.8. Anal. Calcd for C13H7ClN2O4: C, 53.72; H, 2.43; N, 9.64. Found: C, 53.70;
1. (a) Deluca, M. R.; Kervin, S. M. Tetrahedron Lett. 1997, 38, 199; (b) Sato, Y.;
Yamada, M.; Yoshida, S.; Soneda, T.; Ishikawa, M.; Nizato, T.; Suzuli, K.; Konno,
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Ucarturk, N. Farmaco 1998, 53, 337; (d) Wang, C.; Widon, J.; Petronijevic, F.;
Burnett, J. C.; Nuss, J. E.; Bavari, S.; Gussio, R.; Wipf, P. Heterocycles 2009, 79,
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Williams, J. A.; Nguyen, K. T.; Woodmansee, D. H.; Tumanut, C.; Li, J.; Spraggon,
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H, 2.38; N, 9.22. For 3l: mp 166–167 °C; IR (KBr):
mmax 3064, 2919, 1628, 1579,
1478, 1250, 1225, 818, 798, and 708 cmÀ1 1H NMR (CDCl3, 400 MHz) d 11.47
;
(s, 1H), 8.06 (d, 1H, J = 2.6 Hz), 7.47 (s, 1H), 7.46 (d, 1H, J = 8.8 Hz), 7.44 (d, 1H,
J = 8.0 Hz), 7.17 (d,1H, J = 7.6 Hz), 6.97 (d,1H, J = 8.8 Hz), 2.46 (s, 3H). 13C NMR
(CDCl3, 100 MHz) d 161.6, 157.6, 147.4, 139.9, 136.0, 135.2, 129.3, 126.9, 119.3,
119.3, 112.3, 111.2, 110.1, 21.5. Anal. Calcd for C14H10BrNO2: C, 55.29; H, 3.31;
N, 4.61. Found: C, 55.13; H, 3.06; N, 4.45. For 3m: mp 139–140 °C; IR (KBr):
mmax 3446, 2921, 1629, 1582, 1479, 1249, 1233, 816, 808, and 706 cmÀ1 1H
;
NMR (CDCl3, 400 MHz) d 11.43 (s, 1H), 8.03 (d, 1H, J = 2.5 Hz), 7.53 (d. 1H,
J = 8.0 Hz), 7.44 (dd, 1H, J = 8.7 Hz, J = 2.5 Hz), 7.34 (s, 1H), 7.15 (d, 1H,
J = 8.0 Hz), 6.96 (d, 1H, J = 8.8 Hz), 2.48 (s, 3H). 13C NMR (CDCl3, 100 MHz) d
160.9, 157.4, 149.3, 137.4, 136.5, 135.8, 139.1, 126.4, 119.2, 118.7, 112.2, 111.2,
110.8, 21.8. Anal. Calcd for C14H10BrNO2: C, 55.29; H, 3.31; N, 4.61. Found: C,
55.39; H, 3.09; N, 4.43. For 3n: mp 175-176 °C; IR (KBr):
m
max 3067, 2918, 1739,
;
1600, 1580, 1481, 1244, 1225, 1053, 822, 804, and 704 cmÀ1
1H NMR (THF-d8,
400 MHz) d 11.08 (s, 1H), 8.08 (d, 1H, J = 2.2 Hz), 7.78 (d, 1H, J = 1.8 Hz), 7.68 (d,
1H, J = 8.8 Hz), 7.56 (dd, 1H, J = 9.1 Hz, 2.5 Hz), 7.43 (dd,1H, J = 8.8 Hz, 2.2 Hz),
7.00 (d,1H, J = 8.7 Hz). 13C NMR (THF-d8, 100 MHz) d 163.0, 158.0, 147.9, 141.0,
136.6, 130.6, 129.2, 126.1, 119.4, 119.1, 111.7, 111.0. Anal. Calcd for
5. Leventhal, L.; Brandt, M. R.; Cummonts, T. A.; Piesla, M. J.; Rogers, K. E.; Harris,
H. A. Eur. J. Pharmacol 2006, 553, 146.
6. Nishiu, J.; Ito, M.; Ishida, Y.; Kakutani, M.; Shibata, T.; Matsushita, M.; Shindo,
M. Diabetes, Obes. Metab. 2006, 8, 508.
7. Easmon, J.; Pürstinger, G.; Thies, K.-S.; Heinisch, G.; Hofmann, J. J. Med. Chem.
2006, 49, 6343.
8. Rasmussen, K.; Hsu, M.-A.; Yang, Y. Neuropsychopharmacology 2007, 32, 786.
9. Leaver, I. H.; Milligam, B. Dyes Pigm. 1984, 5, 109.
C13H7BrClNO2: C, 48.11; H, 2.17; N, 4.32. Found: C, 47.60; H, 1.87; N, 4.00.
For 3o: mp 198–199 °C; IR (KBr): mmax 3061, 1628, 1578, 1478, 1338, 1229,
1048, 822, 813, 703, and 661 cmÀ1; Anal. Calcd for C13H7BrClNO2: C, 48.11; H,
2.17; N, 4.32. Found: C, 48.19; H, 2.02; N, 4.06. For 3q: mp 271–272 °C; IR
(KBr): m ;
max 3110, 1740, 1631, 1579, 1479, 1239, 1057, 838, 769, and 732 cmÀ1
10. For a general overview of synthetic methods for benzoxazoles, see: (a) Boyd, G.
V. In Science of Synthesis: Houben-Weyl Methods of Molecular Transformations;
Schaumann, E., Ed.; Thieme: Stuttgart, 2001; Vol. 11, pp 481–492; (b) Lin, A.;
Yang, L. Tetrahedron Lett. 2005, 46, 4315; (c) Kawashita, Y.; Nakamichi, N.;
Kawabata, H.; Hayashi, M. Org. Lett. 2003, 5, 3713; (d) Kidwai, M.; Bansal, V.;
Saxena, A.; Aerry, S.; Mozudar, S. Tetrahedron Lett. 2006, 47, 8049; (e) Kumar,
D.; Rudrawar, S.; Chakaborti, A. K. Aust. J. Chem. 2008, 61, 881; (f) Johnson, S.
M.; Connelly, S.; Wilson, I. A.; Kelly, J. W. J. Med. Chem. 2008, 51, 260; (g)
Orlando, C. M.; Wirth, J. G.; Heath, D. R. J. Org. Chem. 1970, 9, 3147.
11. (a) Evindar, G.; Batey, R. A. J. Org. Chem. 2006, 71, 1802; (b) Viirre, R. D.;
Evindar, G.; Batey, R. A. J. Org. Chem. 2008, 73, 3452; (c) Bonnamour, J.; Bolm, C.
Org. Lett. 2008, 10, 2665; (d) Saha, P.; Ramana, T.; Purkait, N.; Ali, M. A.; Paul, R.;
Punniyamurthy, T. J. Org. Chem. 2009, 74, 8719; (e) Kantam, M. L.; Venkanna, G.
T.; Kumar, K. B. S.; Balasubrahmanyam, V.; Bhargava, S. Synlett 2009, 1753.
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14. Nakagawa, K.; Onoue, H.; Sugita, J. Chem. Pharma Bull. 1961, 12, 1135.
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16. Praveen, C.; Kumar, K. H.; Muralidharan, D.; Perumal, P. T. Tetrahedron 2008,
64, 2369.
Anal. Calcd for C13H7BrN2O4: C, 46.59; H, 2.11; N, 8.36. Found: C, 46.51; H,
1.90; N, 7.99. For 3r: mp 220–221 °C; IR (KBr): mmax 3103, 2923, 1620, 1579,
1542, 1444, 1338, 1246, 880, 836, and 759 cmÀ1 1H NMR (CDCl3, 400 MHz) d
;
10.95 (s, 1H), 8.67 (s, 1H), 8.24 (s, 1H), 8.07 (d, 1H, J = 2.5 Hz), 7.66 (dd, 1H,
J = 9.1 Hz, J = 2.5 Hz), 7.09 (d, 1H, J = 8.8 Hz). 13C NMR (CDCl3, 100 MHz) d
165.9, 157.7, 147.8, 145.3, 144.3, 137.6, 131.6, 122.3, 122.0, 120.4, 113.2, 111.1,
109.8. Anal. Calcd for C13H6BrClN2O4: C, 42.25; H, 1.64; N, 7.68. Found: C,
42.30; H, 1.53; N, 7.35. For 3s: mp 150–151 °C; IR (KBr):
mmax 2969, 1637, 1565,
1498, 1451, 1353, 1142, 914, and 753 cmÀ1 1H NMR (CDCl3, 400 MHz) d 10.92
;
(b, 1H), 7.68 (d, 1H, J = 8.8 Hz), 7.52 (d, 1H, J = 1.8 Hz), 7.36 (d, 1H, J = 8.8 Hz),
7.17 (dd, 1H, J = 8.4 Hz, J = 1.8 Hz), 6.27 (dd,1H, J = 8.8 Hz, J = 1.8 Hz), 6.24
(d,1H, J = 2.6 Hz), 3.36 (c, 2H, J = 7.2 Hz), 1.18 (t, 3H, J = 7.2 Hz). 13C NMR (CDCl3,
100 MHz) d 165.2, 160.6, 152.3, 147.6, 141.9, 129.8, 128.4, 123.8, 118.0, 110.6,
104.4, 98.0, 97.7, 44.6, 12.7. Anal. Calcd for C11H17ClN2O2: C, 64.46; H, 5.41; N,
8.84. Found: C, 64.48; H, 5.11; N, 8.42. For 3t: mp 237–239 °C; IR (KBr): mmax
3119, 2918, 2317, 1634, 1592, 1494, 1454, 1246, 1050, 801, and 763 cmÀ1
;
Anal. Calcd for C27H18N2O4: C, 74.64; H, 4.18; N, 6.45. Found: C, 74.77; H, 4.56;
N, 5.87. For 3u: mp 259–261 °C; IR (KBr): mmax 2916, 2329, 1758, 1634, 1596,
1547, 1496, 1264, 1183, 1052, and 803 cmÀ1; Anal. Calcd for C29H22N2O4: C,
75.31; H, 4.79; N, 6.06. Found: C, 74.94; H, 4.94; N, 4.57. For 3v: mp 250–
252 °C; IR (KBr): mmax 3178, 2346, 1740, 1634, 1592, 1546, 1495, 1240, 1179,
17. Varma, R. S.; Kumar, D. J. Heterocycl. Chem. 1998, 35, 1539.
18. Nakawawa, K.; Onoue, H.; Sugita, J. Chem Pharm. Bull. 1964, 12, 1135.

4312
H. López-Ruiz et al. / Tetrahedron Letters 52 (2011) 4308–4312
1053, and 804 cmÀ1; Anal. Calcd for C27H16N2O4: C, 63.43; H, 3.20; N, 5.57.
Found: C, 64.44; H, 3.54; N, 5.01.
29. (a) Johnson, J. S. Angew. Chem. Int. Ed. 2004, 43, 398; (b) Reich, B. J. E.;
Justice, A. K.; Beckstead, B. T.; Reibenspies, J. H.; Miller, S. A. J. Org. Chem.
2004, 69, 1357.
30. Marvel, C. S.; Tarköy, N. J. Am. Chem. Soc. 1957, 79, 6000.
31. Belyaev, E.-Y.; Kondrat’eva, L. E.; Shakhov, N. A. Chem. Heter. Comp. 1970, 8, 8.
32. Brewster, K.; Chittenden, R. A.; Harrison, J. M.; Inch, T. D.; Brown, C. J. Chem.
Soc., Perkin Trans. 1 1976, 12, 1291.
28. Crystallographic data (excluding structure factors) for the structures in this
paper has been deposited with the Cambridge Crystallographic Data Centre as a
Supplementary Publication Numbers, CCDC 809646 No. for 3s Copy of the data
can be obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [fax: +44 0 1223 336033 or e-mail: deposit@ccdc.
cam.ac.uk].