Synthesis and structure characterization of unsymmetrical oxacalix[2]benzene[2]pyrazines

Article abstract of DOI:10.1016/j.tet.2009.04.045

The synthesis of two unsymmetrically linked oxacalix[2]benzene[2]pyrazines (1 and 2) is described. X-ray single crystal structure analysis revealed a highly distorted 1,3-alternate conformation of compound 1 (containing ortho- and meta-diphenol components

Full text of DOI:10.1016/j.tet.2009.04.045

Tetrahedron 65 (2009) 4639–4643
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Synthesis and structure characterization of unsymmetrical
oxacalix[2]benzene[2]pyrazines
*
Mei Li, Ming-Liang Ma, Xiao-Yan Li, Ke Wen
Division of Supramolecular Chemistry and Medicinal Chemistry, Key Laboratory of Brain Functional Genomics, MOE & STCSM, Shanghai Institute
of Brain Functional Genomics, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of two unsymmetrically linked oxacalix[2]benzene[2]pyrazines (1 and 2) is described.
X-ray single crystal structure analysis revealed a highly distorted 1,3-alternate conformation of com-
pound 1 (containing ortho- and meta-diphenol components) and a distorted boat conformation of
compound 2 (containing meta- and para-diphenol components). Oxacalix[2]benzene[2]pyrazine con-
taining both ortho- and para-diphenol components was not obtained via similar synthetic strategy.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 16 January 2009
Received in revised form 8 April 2009
Accepted 10 April 2009
Available online 17 April 2009
1. Introduction
characterization of unsymmetrical oxacalix[2]benzene[2]hetar-
enes. During our literature search, we found that calixarenes and
Calix[4]arenes are an interesting class of macrocyclic com-
pounds, which can adopt four different conformations. Among
them, cone and 1,3-alternate conformations present high level of
molecular symmetry.1 Due to their interesting conformational and
cavity structures, they have been widely used as molecular plat-
forms and hosts in supramolecular chemistry.2 Hetero-
calixaromatics,3 in which the methylene linkages between the
aromatic units in calixarenes are replaced by heteroatoms, such as
sulfur,4 oxygen,5 and nitrogen,6 have attracted much attention re-
cently because of their synthetic availability, tunable cavities, and
potential applications in supramolecular chemistry. Katz et al.7 and
Wang and Yang8 have successfully incorporated nitrogen contain-
ing heterocycles into the oxacalix[4]arene systems via nucleophilic
aromatic substitution (SNAr) reactions, conformationally, these
compounds are all oxacalix[4]arene analogues with cage-like
structures and high molecular symmetry. Post-synthetic function-
alization of oxacalix[2]- arene[2]hetarenes have been demon-
strated by Dehaen9 and Wang10 et al. Oxacalix[n]arene[n]hetarenes
have already found their applications in recognition of neutral and
ionic species.7b,10b,11
heteracalixarenes with their aromatic units being linked via ortho-,
or para-, or unsymmetrical fashion, rather than the regular sym-
metrical meta-linkage were much less studied,13 possibly due to
their synthetic difficulties. We envisioned that the introduction of
unsymmetrical linkage between the aromatic units of the calixar-
enes or heteracalixarenes could increase not only the molecular
diversities and functionalities of this class of macrocycles, but also
new conformations with unsymmetrical cavities. Host molecules of
such would potentially offer us new opportunity to study molecular
recognition toward specific unsymmetrical guests. Calixarenes are
usually synthesized under hash reaction conditions, and it is still
a challenge to introduce unsymmetrical linkage into calixarene
systems. However, oxacalix[n]arene[n]hetarenes were mostly
synthesized under milder reaction conditions, and they could also
be constructed via fragment coupling strategies. The synthetic
convenience of oxacalix[n]arene[n]hetarenes would thus allow us
to introduce unsymmetrical linkage into this class of macrocycles
and study their structural and conformational properties. Herein
we wish to report the synthesis and X-ray single crystal structures
of unsymmetrical oxacalix[2]benzene[2]pyrazines 1 and 2 (see
Schemes 1 and 2). To our knowledge, this is the first attempt to
address the synthesis as well as structures and conformations of
unsymmetrical oxacalix[2]benzene[2]hetarenes.
Recently, we became interested in the synthesis, structures, and
conformations of oxacalix[n]benzene[n]hetarene analogues with
novel cavity scaffolds, which resulted in the development of some
new macrocyclic structures.12 In extension of our work on the
synthesis and structures of oxacalix[n]benzene[n]hetarene ana-
logues, we are now focusing on the synthesis and structure
2. Results and discussion
The synthesis of the unsymmetrical oxacalix[2]benzene[2]pyr-
azines (1 and 2) involves the fragment coupling of 2,6-dichlo-
ropyrazine with two different diphenols (ortho-, meta-, and
para-diphenols) in two-step reaction sequences (Schemes 1 and 2).
* Corresponding author. Tel.: þ86 21 62237102; fax: þ86 21 62601953.
E-mail address: kwen@brain.ecnu.edu.cn (K. Wen).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.04.045

4640
M. Li et al. / Tetrahedron 65 (2009) 4639–4643
O
O
O
N
O
O
N
N
N
O
N
O
N
+
N
N
O
b
c
O
N
O
N
HO
OH
30%
4
8%
1
a
+
N
N
N
Cl Cl
Cl
N
Cl
3
O
O
N
N
N
N
O
O
Scheme 1. Synthesis of compound 1. (a) K2CO3, DMF, rt; (b) m-diphenol, Cs2CO3, DMSO, 80 ꢀC; (c) p-diphenol, Cs2CO3, DMSO, 80 ꢀC.
As shown in Scheme 1, compound 312 was first prepared in 80%
yield by the reaction of ortho-diphenol with 2,6-dichloropyrazine
(1:2 ratio) in the presence of K2CO3 in DMF at room temperature.
After screening the bases (Na2CO3, K2CO3, Cs2CO3, NaH) and sol-
vents (DMSO, DMF, THF, and CH3CN), the optimized reaction con-
dition for cyclocondensation of meta-diphenol with 3 was set in
DMSO at 80 ꢀC for 24 h with Cs2CO3 as a base. The yield for the
desired product 1 was 30%. In the reaction mixture, symmetrical
product, ortho-linked oxacalix[2]benzene[2]pyrazine (4),12 was
obtained in 8% yield, and other unknown species were also present
as by-products due to the bridging C–O bond breaking and refor-
mation in the reaction process.7a Prolonged reaction time or high
temperature resulted in diminished yield of the desired product 1
and increased yield of by-product 4. Compound 1 could also be
obtained in 21% yield by reaction of compound 5 with ortho-
diphenol under similar reaction conditions. Attempts to synthesize
unsymmetrical oxacalix[2]benzene[2]pyrazine with ortho- and
para-diphenol components via similar strategy by employing
compound 3 as intermediate were unsuccessful (Scheme 1). We
believe that the high molecular strain of the proposed structure is
responsible for the failure. Symmetrical compound 4 was the
only isolated product (23%) in this reaction. Compound 2 was
prepared in 18% yield (Scheme 2). In this protocol, symmetrical by-
products 6 and 7 were obtained as the result of the bridging C–O
bond breaking and reformation in the reaction process. Structure of
the compound similar to 6 (oxacalix[2]arene[2]pyrazine) has been
described by Katz et al.,7a while the structure of compound 7 has
not been precedented.
Oxacalix[4]arenes and related oxacyclophanes with four oxygen
atom bridges appeared in the literature all adopt 1,3-alternate
conformations.3 In our previous report, ortho-linked oxacalix[2]-
benzene[2]pyrazine (4) was found to adopt an 1,3-alternate con-
formation, and 1,3,5-alternate, core conformations were found for
ortho-linked oxacalix[3]benzene[3]pyrazine and oxacalix[3]-
benzene[3]pyrimidine, respectively.12 In order to determine the
conformational characters of the unsymmetrical oxacalix[2]-
benzene[2]pyrazines (1 and 2) in the solid state, single crystals of
compounds 1 and 2 with X-ray diffraction quality were grown by
slow evaporation from sample solution in ethyl acetate. As shown
in Figure 1 (top), compound 1 adopts a highly distorted 1,3-alter-
nate conformation, the pair of the opposite pyrazine rings are
arranged in an unparalleled face-to-face orientation with a dihedral
angle of 96.8ꢀ, and a centroid–centroid distance of 5.498 Å. The
transannular N/N distances of two upper rim nitrogen atoms are
6.810 Å and 3.732 Å for the two lower rim nitrogen atoms. The two
benzene planes (ortho-linked and meta-linked) are also arranged in
a distorted face-to-face orientation with a dihedral angle of 48.4ꢀ,
and a centroid–centroid distance of 5.638 Å. The bridging oxygen
atoms are partially conjugated to the pyrazine rings (average C–O
length 1.36 Å), while no conjugation exists between the bridging
O
O
N
N
N
N
N
Cl
N
O
O
O
OH
Cl
HO
Cl
2
18%
b
a
+
N
O
O
N
O
O
O
N
N
Cl
N
N
N
N
N
N
N
N
O
O
5
O
O
6
7
Scheme 2. Synthesis of compound 2. (a) K2CO3, DMF, rt; (b) p-diphenol, Cs2CO3, DMSO, 80 ꢀC.

M. Li et al. / Tetrahedron 65 (2009) 4639–4643
4641
Figure 1. X-ray crystal structures of compound 1 (top left, top view; top right, side view), 2 (middle left, top view; middle right, side view), and 7 (bottom left, top view; bottom
right, side view). Color codes: oxygen¼red, nitrogen¼purple, carbon¼gray, hydrogen¼black.
oxygen atoms and the benzene planes (average C–O length
1.395 Å). In the solid state, the benzene rings are in close contact
solvents (even in high temperature), which could be caused by the
strong intermolecular interaction ( stacking interaction) that
p–p
with the concave surface of another molecule via weak
p–
p
prevents the liberation of compound 7 from its crystal lattice.
stacking interaction. Stacking interactions exist between the
p–p
pyrazine rings in the solid state (Supplementary data, Fig. S1). Also
shown in Figure 1 (middle), compound 2 adopts a distorted ‘boat’
conformation with the two pyrazine rings approach coplanarity
(154.5ꢀ between the ring planes). The two benzene planes are po-
sitioned in opposite side of the ‘boat’ with a dihedral angle of 33.5ꢀ
and a centroid–centroid distance of 4.610 Å. Partial conjugation
between the bridged oxygen atoms and pyrazine rings were ob-
served (average C–O length 1.364 Å) while no conjugation exists
between the bridging oxygen atoms and the benzene planes (av-
3. Conclusions
In conclusion, unsymmetrical oxacalix[2]benzene[2]pyrazines (1
and 2) have been synthesized by nucleophilic aromatic substitution
of 2,6-dichloropyrazine with two different diphenols (ortho- and
meta-, or meta- and para-) via fragment coupling strategies. The
hydroxyl substitution pattern of the two diphenols employed in the
assembly of the cyclotetramers is responsible for the conformations
of the unsymmetrical products. As a result, compound 1 was found to
adopt a distorted 1,3-alternate conformation in the solid state, and
compound 2, a distorted boat conformation. Our continuous efforts
on the synthesis, structures, properties, as well as searching specific
applications of these unsymmetrical oxacalix[n]benzene[n]hetar-
enes will be reported in due course.
erage C–O length 1.408 Å).
p–p Stacking interactions were also
observed between the pyrazine rings in the solid state (Supple-
mentary data, Fig. S2). By-product 7, para-linked oxaca-
lix[2]benzene[2]pyrazine (Fig. 1, bottom) adopts
a
boat
conformation with very high molecular symmetry. The two pyr-
azine rings are arranged in coplanarity (179.7ꢀ), while the two
benzene planes are arranged in a face-to-face orientation with
a dihedral angle of 57.4ꢀ and a centroid-to-centroid distance of
4.645 Å. Similarly, the bridging oxygen atoms in compound 7 only
conjugate with the pyrazine rings (average C–O length 1.359 Å) but
not the benzene planes (average C–O length 1.409 Å). In the solid
state, compound 7 packs in a layered fashion with all the pyrazine
4. Experimental section
4.1. General
All chemicals were used as received without further purifica-
tion. Chemical reactions were performed in oven-dried glassware
under an atmosphere of nitrogen. Classic column chromatography
rings participated in the strong
p–p stacking interactions (Sup-
was performed using Merck 60 (70–230 mesh) silica gel. 1H and 13
C
plementary data, Fig. S3). Crystalline material of 7 has poor solu-
bility in any of organic (such as DMF and DMSO) or inorganic
NMR spectra were recorded at Bruker Avance 500 spectrometer in

4642
M. Li et al. / Tetrahedron 65 (2009) 4639–4643
CDCl3 or DMSO-d6. Chemical shifts are reported in parts per million
versus tetramethylsilane. Mass spectra were recorded on a Bruker
micrOTOF-Q spectrometer (LC/MS). Single crystal X-ray diffraction
data were collected on a Bruker SMART APEX 2 X-ray diffracto-
g
¼90ꢀ; V¼3425.78(16) Å3; rcalcd¼1.444 g cmꢁ3; T¼296(2) K; 19,405
independent measured reflections; F2 refinement; R1¼0.0292;
wR2¼0.0731. This data was deposited in the Cambridge Crystallo-
graphic data centre, CCDC 709285.
meter equipped with
¼0.71073 Å).
a normal focus Mo-target X-ray tube
(l
4.6. Crystallographic data for compound 7
4.2. Synthesis of compound 1
[C20H12N4O4]; Mr¼372.34; orthorhombic; space group Cmca;
a¼17.3725(4); b¼13.5070(3); c¼14.1108(4) Å; rcalcd¼1.494 g cmꢁ3
;
T¼296(2) K; 18,344 independent measured reflections; F2 re-
finement; R1¼0.0297; wR2¼0.0876. This data was deposited in the
Cambridge Crystallographic data centre, CCDC 709286.
To a stirred solution of cesium carbonate (3.01 g, 9.2 mmol) in
DMSO (100 mL) was added dropwise a solution of resorcinol
(0.46 g, 4.2 mmol) in 100 mL DMSO and a solution of compound 312
(1.41 g, 4.2 mmol) in 100 mL DMSO at the same rate during 6 h
(achieving high dilution). The reaction mixture was stirred for an-
other 24 h at 80 ꢀC and then poured into 500 mL of water, extracted
with ethyl acetate for three times. The combined organic layers
were washed with water, brine, and dried over Na2SO4. The solvent
was evaporated and the crude product was purified by chromato-
graph on a silica gel column (petroleum ether/ethyl acetate 4:1).
Compound 1 (0.47 g, 30%) was obtained as white solid. 1H NMR
Acknowledgements
The authors thank Shanghai Commission for Science and
Technology (06Pj14034, 06DZ19002), Ministry of Education
(106078), and Ministry of Science and Technology of China (973
Project, 2003CB716600) for financial support. We think Dr. Xiao-Li
Zhao of Shanghai Key Laboratory of Green Chemistry and Chemical
Processes and Department of Chemistry of East China Normal
University for performing the X-ray crystallographic study.
(500 MHz, DMSO-d6):
1H), 7.13 (m, 4H), 6.86 (dd, J1¼2.3, J2¼8.15, 2H), 6.78 (t, J¼2.25, 1H);
13C NMR (500 MHz, DMSO-d6):
d
¼8.37 (s, 2H), 8.21 (s, 2H), 7.26 (t, J¼8.15,
d
¼157.52, 156.57, 153.31, 145.01,
129.94, 128.84, 128.24, 126.49, 124.28, 114.89, 112.49; HRMS (ESI):
m/z calcd for C20H13N4Oþ4 [MþHþ]: 373.0931; found: 373.0929.
Compound 412 was obtained in 8% yield.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.04.045.
4.3. Synthesis of compound 2
References and notes
By replacing catechol with resorcinol, compound 5 was obtained
(82.7%) as white solid by using a similar protocol as in the synthesis
of compound 3.12 1H NMR (500 MHz, DMSO-d6):
d
¼8.58 (s, 2H),
1. (a) Gutsche, C. D. Calixarenes; Royal Society of Chemistry: Cambridge, UK, 1989;
(b) Gutsche, C. D. Calixarenes Revisited; Royal Society of Chemistry: London,
2000.
8.55 (s, 2H), 7.57 (t, J¼8.25, 1H), 7.31 (t, J¼2.2, 1H), 7.23 (dd, J1¼8.2,
J2¼2.25, 2H); 13C NMR (500 MHz, DMSO-d6):
d
¼157.92, 153.31,
2. (a) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713–745; (b) Ikeda, A.;
Shinkai, S. Chem. Rev. 1997, 97, 1713–1734; (c) Rebek, J., Jr. Acc. Chem. Res. 1999,
32, 278–286; (d) Hof, F.; Craig, S. L.; Nuckolls, C.; Rebek, J., Jr. Angew. Chem., Int.
Ed. 2002, 41, 1488–1508; (e) Cram, D. J.; Cram, J. M. Container Molecules and
their Guests; Royal Society of Chemistry: Cambridge, 1994; (f) Biros, S. M.;
Ullrich, E. C.; Hof, F.; Trembleau, L.; Rebek, J., Jr. J. Am. Chem. Soc. 2004, 126,
2870–2876; (g) Colasson, B.; Reinaud, O. J. Am. Chem. Soc. 2008, 130,
15226–15227; (h) Nishimura, N.; Kobayashi, K. Angew. Chem., Int. Ed. 2008, 47,
6255–6258; (i) Srinivasan, K.; Gibb, B. C. Chem. Commun. 2008, 4640–4642; (j)
Liu, S.; Gibb, B. C. Chem. Commun. 2008, 3709–3716.
144.37, 137.88, 133.72, 130.98, 118.29, 114.39; HRMS (ESI): m/z calcd
for C14H9Cl2N4Oþ2 [MþHþ]: 335.0097; found 335.0092. In the sec-
ond step, replacing 3 with 5 as in the synthesis of 1, compound 2
was isolated as white solid in 18% yield. 1H NMR (500 MHz, CDCl3):
d
¼8.16 (s, 2H), 8.13 (s, 2H) 7.29 (m, 1H), 6.80 (s, 4H), 6.70 (m, 2H),
6.67 (d, J¼7.0, 1H); 13C NMR (500 MHz, CDCl3):
¼159.1, 158.6,
d
154.5, 150.2, 129.9, 126.8, 126.7, 123.5, 119.5, 118.0; HRMS (ESI): m/z
calcd for C20H13N4Oþ4 [MþHþ]: 373.0931; found: 373.0939. Com-
pounds 6 and 7 were also isolated during the column chromato-
graphy in less than 5% yields. 1H NMR (500 MHz, CDCl3) for
3. Recent reviews see: (a) Wang, M.-X. Chem. Commun. 2008, 4541–4551; (b)
Maes, W.; Dehaen, W. Chem. Soc. Rev. 2008, 37, 2393–2402.
4. Lhotak, P. Eur. J. Org. Chem. 2004, 1675–1692.
5. (a) Sommer, N.; Staab, H. A. Tetrahedron Lett. 1966, 7, 2837–2841; (b) Chambers,
R. D.; Hoskin, P. R.; Khalil, A.; Richmond, P.; Sandford, G.; Yufit, D. S.; Howard, J.
A. K. J. Fluorine Chem. 2002, 116, 19–22; (c) Li, X.-H.; Upton, T. G.; Gibb, C. L. D.;
Gibb, B. C. J. Am. Chem. Soc. 2003, 125, 650–651; (d) Yang, F.; Yan, L.; Ma, K.;
Yang, L.; Li, J.; Chen, L.; You, J. Eur. J. Org. Chem. 2006, 1109–1112; (e) Katz, J. L.;
Feldman, M. B.; Conry, R. R. Org. Lett. 2005, 7, 91–94; (f) Katz, J. L.; Selby, K. J.;
Conry, R. R. Org. Lett. 2005, 7, 3505–3507; (g) Konishi, H.; Tanaka, K.; Teshima,
Y.; Mita, T.; Morikawa, O.; Kobayashi, K. Tetrahedron Lett. 2006, 47, 4041–4044;
(h) Konishi, H.; Mita, T.; Morikawa, O.; Kobayashi, K. Tetrahedron Lett. 2007, 48,
3029–3032; (i) Hao, E.; Fronczek, F. R.; Vicente, M. G. H. J. Org. Chem. 2006, 71,
1233–1236; (j) Konishi, H.; Mita, T.; Yasukawa, Y.; Morikawa, O.; Kobayashi, K.
Tetrahedron Lett. 2008, 49, 6831–6834.
compound 6:
d
¼8.12 (s, 4H), 7.25 (t, J¼8.0, 2H), 6.82 (q, J1¼2.5,
J2¼6.0, 4H), 6.60 (t, J¼2.0, 2H); 13C NMR (500 MHz, CDCl3):
d
¼157.93, 153.14, 130.24, 127.19, 118.76, 115.94. HRMS (ESI): m/z
calcd for C20H13N4Oþ4 [MþHþ]: 373.0931; found: 373.0926. 1H NMR
(500 MHz, DMSO-d6) for compound 7:
d
¼8.22 (s, 4H), 7.00 (s, 8H);
13C NMR (500 MHz, DMSO-d6):
d
¼157.79, 149.08, 126.14, 123.07.
HRMS (ESI): m/z calcd for C20H13N4Oþ4 [MþHþ]: 373.0931; found:
373.0936.
6. (a) Ito, A.; Ono, Y.; Tanaka, K. J. Org. Chem. 1999, 64, 8236–8241; (b) Miyazaki, Y.;
Kanbara, T.; Yamamoto, T. Tetrahedron Lett. 2002, 43, 7945–7948; (c) Wang,
M.-X.; Zhang, X.-H.; Zheng, Q.-Y. Angew. Chem., Int. Ed. 2004, 43, 838–842; (d)
Wang, Q.-Q.; Wang, D.-X.; Ma, H.-W.; Wang, M.-X. Org. Lett. 2006, 8,
5967–5970; (e) Liu, S.-Q.; Wang, D.-X.; Zheng, Q.-Y.; Wang, M.-X. Chem. Com-
mun. 2007, 3856–3858; (f) Gong, H.-Y.; Zhang, X.-H.; Wang, D.-X.; Ma, H.-W.;
Zheng, Q.-Y.; Wang, M.-X. Chem.dEur. J. 2006, 12, 9262–9275; (g) Gong, H.-Y.;
Wang, D.-X.; Xiang, J.-F.; Zheng, Q.-Y.; Wang, M.-X. Chem.dEur. J. 2007, 13,
7791–7802; (h) Vale, M.; Pink, M.; Rajca, S.; Rajca, A. J. Org. Chem. 2008, 73, 27–
35; (i) Tsue, H.; Matsui, K.; Ishibashi, K.; Takahashi, H.; Tokita, S.; Ono, K.; Ta-
mura, R. J. Org. Chem. 2008, 73, 7748–7755; (j) Touil, M.; Lachkar, M.; Siri, O.
Tetrahedron Lett. 2008, 49, 7250–7252.
4.4. Crystallographic data for compound 1
[C20H12N4O4]; Mr¼372.34; monoclinic; space group P21/c;
a¼13.8804(7); b¼12.8192(7); c¼19.5735(10) Å;
a
¼90ꢀ;
b
¼105.
8030(10)ꢀ;
g
¼90ꢀ; V¼3351.2(3) Å3; rcalcd¼1.476 g cmꢁ3
;
T¼296
(2) K; 27,609 independent measured reflections; F2 refinement;
R1¼0.0333; wR2¼0.0737. This data was deposited in the Cambridge
Crystallographic data centre, CCDC 709284.
7. (a) Katz, J. L.; Geller, B. J.; Conry, R. R. Org. Lett. 2006, 8, 2755–2758; (b) Katz, J.
L.; Geller, B. J.; Foster, P. D. Chem. Commun. 2007, 1026–1028.
8. Wang, M.-X.; Yang, H.-B. J. Am. Chem. Soc. 2004, 126, 15412–15422.
9. (a) Maes, W.; Van Rossom, W.; Van Hecke, K.; Van Meervelt, L.; Debaen, W. Org.
Lett. 2006, 8, 4161–4164; (b) Van Rossom, W.; Maes, W.; Kishore, L.; Ovaere, M.;
Van Meervelt, L.; Dehaen, W. Org. Lett. 2008, 10, 585–588.
4.5. Crystallographic data for compound 2
[C20H12N4O4]; Mr¼372.34; monoclinic; space group C2/c;
a¼18.8314(5); b¼10.7633(3); c¼18.2294(5) Å;
a¼90; ¼112.0020(10);
b

M. Li et al. / Tetrahedron 65 (2009) 4639–4643
4643
10. (a) Wang, Q.-Q.; Wang, D.-X.; Zheng, Q.-Y.; Wang, M.-X. Org. Lett. 2007, 9,
2847–2850; (b) Yang, H.-B.; Wang, D.-X.; Wang, Q.-Q.; Wang, M.-X. J. Org. Chem.
2007, 72, 3757–3763; (c) Hou, B.-Y.; Zheng, Q.-Y.; Wang, D.-X.; Wang, M.-X.
Tetrahedron 2007, 63, 10801–10808; (d) Hou, B.-Y.; Zheng, Q.-Y.; Wang, D.-X.;
Huang, Z.-T.; Wang, M.-X. Chem. Commun. 2008, 3864–3866.
11. (a) Hou, B.-Y.; Wang, D.-X.; Yang, H.-B.; Zheng, Q.-Y.; Wang, M.-X. J. Org. Chem.
2007, 72, 5218–5226; (b) Wang, D.-X.; Zheng, Q.-Y.; Wang, Q.-Q.; Wang, M.-X.
Angew. Chem., Int. Ed. 2008, 47, 7485–7488.
12. Ma, M.-L.; Wang, H.-X.; Li, X.-Y.; Liu, L.-Q.; Jin, H.-S.; Wen, K. Tetrahedron 2009,
65, 300–304.
13. (a) Lehmann, F. P. A. Tetrahedron 1974, 30, 727--733; (b) Gilbert, E. E. J. Hetero-
cycl. Chem. 1974, 11, 899–904; (c) Bottino, F.; Foti, S.; Pappalardo, S. Tetrahedron
1976, 32, 2567–2570; (d) Kime, D. E.; Norymberski, J. K. J. Chem. Soc., Perkin
Trans. 1 1977, 1048–1052; (e) Boros, E. E.; Andrews, C. W.; Davis, A. O. J. Org.
Chem. 1996, 61, 2553–2555; (f) Ogoshi, T.; Kanai, S.; Fujinami, S.; Yamagishi, T.;
Nakamoto, Y. J. Am. Chem. Soc. 2008, 130, 5022–5023.