Photochemistry of benzene and quinoxaline fused Δ2-1,2,3-triazolines and their trapping products

Article abstract of DOI:10.1016/j.tet.2009.11.004

The benzene and quinoxaline fused Δ²-1,2,3-triazolines 1a and 1b were synthesized in good yields using Knoevenagel condensation and intramolecular 1,3-dipolar cycloaddition as two of the key reactions. Photolysis (254 nm) of Δ²-1,2,3-triazoline 1a or 1b in acetonitrile led to the homolytic cleavage of nitrogen that generated diethyl diazomalonate 7, highly reactive intermediates aziridines 8a,b, and isoindoles B. The latter two species subsequently underwent rearrangement to give the nitrogen extrusion products 9a,b, and polymers. Furthermore, the reactive intermediates were trapped by dienophiles to give the corresponding cycloadducts. Subsequent rearrangement of the N-bridged cycloadducts gave N-substituted pyrrolo[3,4-b]quinoxalines 12b and 15b in 6% and 9% yields, respectively. Irradiation of 1a in the presence of fumaronitrile led to the isolation of cycloadduct 16a with retention of stereochemistry. Thermal reaction of 1b gave more nitrogen extruded product 9b (58-63% yield) than that by photolysis (5-23% yield), which implied that zwitterionic intermediate might be involved in the former.

Full text of DOI:10.1016/j.tet.2009.11.004

Tetrahedron 66 (2010) 176–182
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Photochemistry of benzene and quinoxaline fused
and their trapping products
D
2-1,2,3-triazolines
,
a,
*
Yu-Jen Chen a, Hao-Chih Hung a, Chin-Kang Sha b , Wen-Sheng Chung
*
a Department of Applied Chemistry, National Chiao Tung University, Hsinchu, 30050, Taiwan, Republic of China
b Department of Chemistry, National Tsing Hua University, Hsinchu, 300, Taiwan, Republic of China
a r t i c l e i n f o
a b s t r a c t
The benzene and quinoxaline fused D
2-1,2,3-triazolines 1a and 1b were synthesized in good yields using
Article history:
Received 9 August 2009
Received in revised form 31 October 2009
Accepted 3 November 2009
Available online 6 November 2009
Knoevenagel condensation and intramolecular 1,3-dipolar cycloaddition as two of the key reactions.
Photolysis (254 nm) of
D
2-1,2,3-triazoline 1a or 1b in acetonitrile led to the homolytic cleavage of ni-
trogen that generated diethyl diazomalonate 7, highly reactive intermediates aziridines 8a,b, and iso-
indoles B. The latter two species subsequently underwent rearrangement to give the nitrogen extrusion
products 9a,b, and polymers. Furthermore, the reactive intermediates were trapped by dienophiles to
give the corresponding cycloadducts. Subsequent rearrangement of the N-bridged cycloadducts gave N-
substituted pyrrolo[3,4-b]quinoxalines 12b and 15b in 6% and 9% yields, respectively. Irradiation of 1a in
the presence of fumaronitrile led to the isolation of cycloadduct 16a with retention of stereochemistry.
Thermal reaction of 1b gave more nitrogen extruded product 9b (58–63% yield) than that by photolysis
(5–23% yield), which implied that zwitterionic intermediate might be involved in the former.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
D
2-1,2,3-Triazoline
Photochemistry
Aziridine
Intramolecular 1,3-dipolar cycloaddition
Knoevenagel condensation
1. Introduction
biradicals,6 were involved in the thermolysis of 2-(allenyl)phenyl
azides. On the other hand, Chiba7a and one of us7b favored the
The annulation of heteroaromatic compounds or benzene to
isoindoles1 makes them reactive dienes that undergo Diels–Alder
reaction with dienophiles to form various cycloadducts. Accord-
ingly, iso-condensed heteroaromatic pyrrole derivatives have been
frequently used in the research fields of conducting polymers,2a
organic light emitting diodes,2b organic field effect transistors,2c and
metal ion sensors.2d The 1,3-dipolar cycloaddition reaction is one of
the most useful synthetic methods to construct five membered
heterocycles. The intramolecular 1,3-dipolar cycloaddition of an
aromatic azide with an alkene forms an aromatic fused triazolines,3
which can be used in the synthesis of various alkaloids through
thermolysis of the triazolines to extrude a nitrogen and formation of
the aziridine intermediates.4
Although numerous efforts have been devoted to the synthesis
and reaction of heteroaromatic fused 1,2,3-triazolines,5–8 the
mechanism in the thermolysis of these triazolines that leads to the
loss of nitrogen remains to be established. Based on experimental
results and density functional theory calculation, Feldman et al.5
proposed that azatrimethylenemethane (ATMM) diyls (Scheme
1a), a nitrogen version of the classical trimethylenemethane (TMM)
formation of zwitterionic intermediates in the thermolysis of some
triazolines that led to isoindoles or 1,2-dihydroisoquinolines
(Schemes 1b and 1c). Based on theoretical calculations, Wlad-
kowsk, Michejda,7c and Shea7d supported a heterolytic cleavage of
the N–N bond in the thermolysis of protonated triazolines or tor-
sionally strained triazolines. In the thermolysis of adducts from
azide anion to allenyl esters, Huang8a proposed both biradical and
zwitterionic intermediates to rationalize their experimental re-
sults; however, Smith and co-workers favored zwitterionic in-
termediates (Scheme 1d) based on sensitive product variation to
solvent polarity.8b
Compared to the popularity in the thermolysis of aromatic fused
triazolines, reports on the photochemistry of triazolines are still
rare.9 Based on stereoselective product distributions as well as high
quantum yields of the photoreactions of phenyl triazolines, the
mechanism of a one-step expulsion of nitrogen to give a singlet
1,3-diradical rather than a path involving a 1,5-diradical was
favored.9a–c,f Our results, however, showed that 1,5-diradical was
initially formed during the photolysis (vide infra), which was then
converted to 1,3-diradical or trapped by dienophiles to various
products. Results from the thermolysis of the 4,4-dicarbolylate
substituted triazolines seem to involve a zwitterionic intermediates.
Herein, we report the synthesis of quinoxaline fused triazoline 1b,
the preparation of benzo-fused triazoline 1a,10 and their photo-
chemistry and trapping of reactive intermediates.
* Corresponding authors. Tel.: þ886 3 513 1517; fax: þ886 3 572 3764 (W.-S.C.);
tel.: þ886 3 572 2427; fax: þ886 3 571 1082 (C.-K.S.).
E-mail addresses: cksha@mx.nthu.edu.tw (C.-K. Sha), wschung@cc.nctu.edu.tw
(W.-S. Chung).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.11.004

Y.-J. Chen et al. / Tetrahedron 66 (2010) 176–182
177
R1
R1
R1
(a)
R
N
R
R
R
-N2
5
N
R1
N
N
N
N
H
ATMM
R1
R1
H
R
R
N
R
N
R1
H
HN
R
R
R
R
R
R
(b)
R
H
R
N2
N
N
-N2
N
N
N
NH
CO2Et
CO2Et
CO2Et
CO2Et
EtO2C
EtO2C
(c)
CO2Et
N
R1
R2
R1
R2
N
Rh(OAc)x
N
N
CO2Et
NH
R1
R2
£G
N
NH
N
(d)
O
O
N
O
O
N
£G
+
H
R
N
N
N
CHR
N
CH2R
N2
CH2R
Scheme 1. Literature proposed reaction mechanisms in the thermolysis of
D
2-1,2,3-triazolines: (a) Ref. 5a, (b) Ref. 7a, (c) Ref. 7b, and (d) Ref. 8b.
2. Results and discussion
dibenzoyl peroxide gave 3 in 84% yield.12 Oxidation of compound
3 by the sodium salt of 2-nitropropane in methanol gave the al-
dehyde 4 in 86% yield.13 Knoevenagel condensation of compound
4 with diethyl malonate afforded the alkylidenemalonate 5 in 72%
yield.14 Subsequent bromination of 5 with NBS gave bromoalky-
lidenemalonate 6 in 60% yield. Treatment of 6 with sodium azide
led to the displacement of bromide by azide. Subsequent intra-
molecular 1,3-dipolar cycloaddition of the azide group to the
olefin gave triazoline 1b in 91% yield.
Triazolines 1a was prepared according to literature pro-
cedures.10 The synthetic pathway for triazoline 1b is depicted in
Scheme 2.11d Bromination of the readily available 2,3-dimethyl-
quinoxaline with N-bromosuccinimide (NBS) in the presence of
N
N
N
N
i
iii
ii
Br
H
N
N
The formation of 1b from 6 could be monitored by 1H NMR. The
disappearance of the methylene protons in 6 at 4.8 ppm and the
appearance of two doublets of 1b at 4.9 and 5.6 ppm (J¼17.4 Hz)
were observed. The signal of methylene carbon also downfield
shifted from 30.3 ppm in bromide 6 to 53.8 ppm in triazoline 1b. In
addition, the signal of alkylidine proton in 6 at 8.1 ppm disappeared
and a new signal of a methine proton appeared at 5.9 ppm in 1b. All
spectroscopic data supported that an intramolecular 1,3-dipolar
cycloaddition of azide and alkene occurred. However, the newly
created chiral center on triazoline 1b made the 1H NMR spectrum
a bit complex. HRMS also supported the formation of triazoline 1b.
The Uv–vis absorption of triazolines 1a and 1b in acetonitrile
O
2
3 (84%)
4 (86%)
N
Br
N
N
N
N
N
N
CO2Et
CO2Et
iv
CO2Et
CO2Et
v
N
EtO2C
N
CO2Et
1b (91%)
6 (60%)
5 (72%)
N
N
N
CO2Et
showed a lmax at 249 nm (
3
¼6.5ꢀ103 Mꢁ1cmꢁ1) and 239 nm
EtO2C
(3
¼2.9ꢀ104 Mꢁ1cmꢁ1), respectively. Triazoline 1a or 1b (0.025 M in
1a
CD3CN) was photolyzed in Rayonet photoreactor (254 nm) under
degassed condition at room temperature. To follow the reaction
closely, we took 1H NMR of the samples with different irradiation
periods.
Scheme 2. Synthesis of quinoxaline fused
D
2-1,2,3-triazoline 1b. Reagents and con-
ditions: (i) NBS, dibenzoyl peroxide, o-dichlorobenzene, 60 ꢂC, 2.5 h; (ii)
[(CH3)2CNO2]ꢁNaþ, MeOH, rt, 6 h; (iii) CH2(CO2Et)2, piperidine, AcOH, PhH, reflux, 12 h;
(iv) NBS, dibenzoyl peroxide, o-dichlorobenzene, 40 ꢂC, 12 h; (v) NaN3, EtOH, rt, 12 h.

178
Y.-J. Chen et al. / Tetrahedron 66 (2010) 176–182
After irradiation for ca. 15 min, some changes in the 1H NMR
CO2Et
CO2Et
1a (25%)
1b (39%)
polymer
N
N
+
+
spectra of both 1a and 1b were observed. A significant new triplet
signal appeared at ca. 0.7 ppm (see Figs. 1 and 2). These triplets
were likely due to the aziridine intermediates 8a,b9 that were
formed by the photo-extrusion of nitrogen from the triazolines.
Some other changes of signals also appeared, which were consis-
tent with the formation of 7 and 9a or 9b; however, parts of the
signals were severely overlapped with the starting materials. Sim-
ilar phenomenon was observed when d8-THF, CDCl3, and CD3CN/
CD3OD (v/v¼1/4) were used as the solvents in the photolysis
(Supplementary data). Even with repeated column chromatogra-
phy, we failed to isolate the putative aziridine intermediates9,15 8a
and 8b; however, the diethyl diazomalonate 7 and nitrogen ex-
trusion products 9a and 9b were isolated (Scheme 3). Compounds
9a and 9b were unstable in chloroform; however, they lived long
enough for spectral identification.
N
N
N
CO2Et
7 from 1a = 21%
1b = 16%
EtO2C
1a
hv
30 min
N
N
NH
Ar
N
Ar
N
N
CO2Et
CO2Et
N
CO2Et
CO2Et
EtO2C
EtO2C
9a (6%)
9b (17%)
8a, b
1b
N
=
a:
; b:
Ar
where
N
Scheme 3. Photochemical products from the photolysis of triazolines 1a and 1b.
To gain insight into the mechanism of the photo reaction, tri-
azoline 1a or 1b was photolyzed in the presence of dienophiles to
trap the intermediates involved. Using acetonitrile as the solvent, in
the presence of 3 equiv of N-phenylmaleimide (NPM) as dienophile,
the diethyl diazomalonate 7 and diethyl 2-(2,3-dihydropyrrolo[3,4-
b]quinoxalin-1-ylidene)malonates 9a,b were formed, along with
the Diels–Alder adducts 10a and 11a (for 1a) or 10b and 12b (for 1b)
and other unidentified side products. For example, when 1a was
photolyzed in the presence of NPM for 2 h, the reaction products
analyzed by HPLC included: diethyl diazomalonate 7 (32%), 9a
(14%), N-H-cycloadduct 10a (11%) and N-substituted-cycloadduct
11a (5%) (Scheme 4). Similar types of reaction products were found
when 1a or 1b was photolyzed in the presence of excess dimethyl
acetylenedicarboxylate (DMAD). Notably, photolysis of 1b in the
presence of dienophiles did not afford the cycloadducts 11b and
14b, which were expected from the trapping of N-substituted
pyrrolo[3,4-b]quinoxaline with dienophiles. Instead, compounds
12b and 15b were observed, which were associated with the
breaking of the N-bridge (Scheme 4). Moreover, products from the
irradiation of 1a with fumaronitrile (FN) gave a trapping product
16a with retention of stereochemistry. The stereochemistry of the
exo-10b and endo-11a was determined based on those established
on a thieno[2,3-c]pyrrole system.11b,c,e
(d)
(c)
(b)
*
(a)
ppm
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
ppm
1.5
1.0
ppm
Figure 1. 1H NMR spectra of (a) triazoline 1a (0.025 M in CD3CN), and (b) 1a after
irradiated (254 nm) for 15 min, (c) 9a (in CD3CN), and (d) 7 (in CD3CN); where * de-
notes an internal standard 1,4-dioxane, and the signals around 0.7 ppm were from
aziridines 8a.
Photolysis of
D
2-1,2,3-triazolines in general involves the ho-
molytic extrusion of nitrogen to form 1,3-diradical A, which then
recombines to form aziridine intermediates 8a,b.9 The 1,3-diradical
A was extremely short lived, which has been eluded for EPR de-
tection. Nevertheless, we were able to obtain EPR16 signals of
monoradicals when a glassy matrix of 1a or 1b in 2-methylte-
trahydrofuran (MTHF) at 77 K was irradiated (230ꢁ325 nm), the
corresponding signals appeared at 3423 G for 1a and at 3435 G for
1b, respectively. Thus, EPR experiments proved the involvement of
radical intermediates in the process. Based on the experimental
results, we proposed the mechanism as follows. Photolysis of 1a or
1b by route a (Scheme 5) led to the formation of aziridine 8a or 8b,
owing to the high strain energy of 8, it either underwent ring
opening reaction to form compound 9a or 9b (through pathway c)
or was first converted into the N-substituted iso-condensed het-
eroaromatic pyrroles (pathway d) and then trapped by dienophiles
to form cycloaddition adducts 11a, 12b, 14a, 15b and 16a. Alterna-
tively, photolysis of triazolines 1a and 1b may initially underwent
an homolysis of the N–N bond followed by nitrogen extrusion to
form diethyl diazomalonate 7 and isoindole/pyrrolo[3,4-b]-qui-
noxaline B. The reactive isoindoles or pyrrolo[3,4-b]-quinoxaline
intermediates could further react with dienophiles to give Diels–
Alder adducts (Scheme 5, route b) or it may undergo polymeriza-
tion as well.17
(d)
(c)
(b)
*
(a)
9.0
8.5
8.0
ppm
5.5
5.0
4.5
4.0
ppm
1.5
1.0 ppm
Figure 2. 1H NMR spectra of (a) triazoline 1b (0.025 M in CD3CN), and (b) 1b after
irradiated (254 nm) for 15 min, (c) 9b (in CD3CN), and (d) 7 (in CD3CN); where * de-
notes an internal standard 1,4-dioxane, and the signals around 0.7 ppm were from
aziridines 8b.
We also studied the thermal reaction of triazoline 1b and the
results are shown in Scheme 6. Heating triazoline 1b alone in
benzene at reflux for 24 h gave products 7 (18%) and 9b (63%).

Y.-J. Chen et al. / Tetrahedron 66 (2010) 176–182
179
CH(CO2Et)2
H
N
N
O
O
NPM
+
Ph
Ar
7 from 1a = 32% + 9a (14%) + polymer +
1b = 25% 9b (23%)
N
Ar
N
Ph
O
O
11a (5%)
11b
10a (11%)
10b (8%)
CH(CO2Et)2
HN
O
N
N
N
Ph
O
12b (6%)
CH(CO2Et)2
H
N
N
Ar
N
N
N
CO2Et
hv
CO2Me
CO2Me
CO2Me
CO2Me
DMAD
7 from 1a = 14% + 9a (3%) + polymer +
1b = 16% 9b (5%)
+
Ar
EtO2C
2 h
13a (17%)
14a (11%)
14b
1a, b
CH(CO2Et)2
HN
N
N
CO2Me
CO2Me
15b (9%)
CH(CO2Et)2
N
CN
CN
FN
7 (28%) + 9a (10%) + polymer +
16a (3%)
Scheme 4. Photochemical products from the photolysis of triazolines 1a and 1b in the presence of various dienophiles.
N
N
N
Ar
H
EtO2C
CO2Et
NH
CO2Et
Ar
N
Ar
N
c
Ar
Ar
N
c
route a
-N2
1a,b
CO2Et
EtO2C
A
CO2Et
d
CO2Et
EtO2C
CO2Et
d
EtO2C
8a, b
9a, b
N
Ar
b
a
CH(CO2Et)2
when
Ar =
N
N
N
N
CH(CO2Et)2
R1
EtO2C
HN
N
CO2Et
R1
N
N
CO2Et
CO2Et
dienophile
Ar
N
Ar
R2
C-N cleavage
R2
12b or 15b
11a, 14a or 16a
H
N
H
R1
R2
CO2Et
dienophile
route b
NH
+
Ar
N
N
Ar
Ar
N
CO2Et
7
B
10a,b or 13a
N
N
= a:
; b:
Ar
where
polymers
Scheme 5. Proposed mechanism for photochemistry of heteroaromatic contains triazolines 1a and 1b.
7 (18%) + 9b (63%)
Thermolysis of 1b with NPM in the same conditions also gave the
same four products as did the photolysis: diethyl diazomalonate 7
(14%), nitrogen extrusion compound 9b (58%), N-H-cycloadduct
10b (12%), and the N-substituted-cycloadduct 12b (5%). The results
were similar to those reported in literatures,7c,d,9 indicating the
involvement of intermediate 8 and the fragmentation to form
diethyl diazomalonate 7.7a,8 The thermolysis of triazoline 1b
N
PhH, reflux, 24 h
N
N
N
CO2Et
N
EtO2C
NPM
7 (14%) + 9b (58%) +
10b (12%) + 12 (5%)
PhH, reflux, 24 h
1b
Scheme 6. Thermolysis of triazoline 1b with and without NPM (3 equiv) in benzene.

180
Y.-J. Chen et al. / Tetrahedron 66 (2010) 176–182
perhaps went through zwittzerionic intermediate, which led to
higher yield of diethyl 2-(2,3-dihydropyrrolo[3,4-b]quinoxalin-1-
ylidene)-malonates 9b. Thus, 9b was obtained in 63% yield by
thermolysis but was only 17% by photolysis. Furthermore, ther-
molysis of triazoline 1b gave cleaner products with higher yields,
whereas, the photolysis which involved 1,3-diradical gave various
unidentified polymers as side products.
to room temperature, the reaction mixture was passed through
a short silica gel column and washed with hexane to remove 1,2-
dichlorobenzene. After concentration under reduced pressure, the
residue was column chromatographed using hexane/ethyl acetate
(7:1) as eluent to give 3 (0.50 g, 84%) as awhite solid; mp 120–122 ꢂC;
Rf¼0.30 (hexane/ethyl acetate¼6:1); 1H NMR (300 MHz, CDCl3)
d
2.89 (s, 3H), 4.76 (s, 2H), 7.69–7.78 (m, 2H), 8.01–8.05 (m, 2H); 13
C
NMR (75 MHz, CDCl3) d 22.4 (CH3), 31.8 (CH2),128.4 (CH),129.0 (CH),
3. Conclusion
129.4 (CH), 130.4 (CH), 140.8 (Cq), 141.9 (Cq), 150.8 (Cq), 153.1 (Cq);
EIMS m/z 328/326 (Mþ, 24/24), 157 (100); HR-EIMS m/z calcd for
C10H799BrN2 235.9949, found 235.9956.
In conclusion, D
2-1,2,3-triazoline 1b can be readily synthesized
in five steps. The photochemistry of triazoline derivatives 1a and 1b
in acetonitrile was studied. Although photolysis of 1a and 1b led to
complicated reaction products, the Diels–Alder adducts and their
rearranged products could be successfully isolated and character-
ized. Based on these results, we proposed that the photolysis of
triazolines 1a and 1b underwent homolysis of the N–N bond first to
form a 1,5-diradical, which was then converted into the 1,3-di-
radical A by the extrusion of nitrogen. The resultant 1,3-diradical A
instantly formed the aziridine intermediate 8, which was then
converted into the unstable iso-condensed heteroaromatic pyrrole
or immediately gave the rearranged product 9. Alternatively, the
incipient 1,5-diradical gave isoindole derivative B along with
diethyl diazomalonate 7. Thermal reaction of 1b presumably went
through zwitterionic intermediates,11 therefore, it gave cleaner
products with higher yields; in contrast, the photolysis which in-
volved 1,3-diradical, gave similar reaction products but with some
unidentified polymers as side products.
4.1.3. Preparation of 3-methylquinoxaline-2-carbaldehyde, 4. A
mixture of 2-nitropropane (0.11 g, 1.24 mmol) and sodium meth-
oxide (0.09 g,1.67 mmol) in 10 mL of methanol was heated to reflux
for 30 min and then cooled to room temperature. Compound 3
(0.21 g, 0.89 mmol) was added and the reaction mixture was stirred
at room temperature for 4 h. After removal of the solvent by rotary
evaporator, the residue was partitioned between water (45 mL) and
dichloromethane (3ꢀ15 mL). The organic layer was dried over
MgSO4 and concentrated under reduced pressure. The residue was
column chromatographed using hexane/ethyl acetate (8:1) as elu-
ent to give 4 (0.13 g, 86%) as a yellow solid; mp 128–130 ꢂC; Rf¼0.35
(hexane/ethyl acetate¼6:1); 1H NMR (300 MHz, CDCl3)
d 3.05 (s,
3H), 7.79–7.94 (m, 2H), 8.09–8.23 (m, 2H), 10.3 (s, 1H); 13C NMR
(75 MHz, CDCl3)
d 23.2 (CH3), 128.6 (CH), 130.0 (CH), 130.0 (CH),
133.0 (CH), 140.8 (Cq), 142.7 (Cq), 145.2 (Cq), 153.5 (Cq), 194.0 (Cq);
EIMS m/z 172 (Mþ, 90), 144 (79), 143 (100), 102 (62); HR-EIMS m/z
calcd for C10H8N2O 172.0637, found 172.0632.
4. Experimental
4.1. General
4.1.4. Preparation of diethyl 2-((3-methylquinoxalin-2-yl)methylene)-
malonate, 5. A stirred mixture of aldehyde 4 (1.51 g, 8.78 mmol),
diethyl malonate (1.90 g, 11.9 mmol), piperidine (172 mg,
2.02 mmol), and glacial acetic acid (89.0 mg, 1.48 mmol) in 40 mL of
dry benzene was heated to reflux for 12 h using a Dean–Stark trap.
The solvent was removed by rotary evaporator and the crude
products were column chromatographed using hexane/ethyl ace-
tate (3:1) as eluent to give 5 (1.98 g, 72%) as a yellow solid; mp 105–
107 ꢂC; Rf¼0.28 (hexane/ethyl acetate¼3:1); 1H NMR (300 MHz,
Flash column chromatography was performed using silica gel
(70–230 mesh). Melting points were determined on a Yanaco
MP500D apparatus and were uncorrected. 1H NMR and 13C NMR
spectra were recorded at 300 MHz and 75.4 MHz, respectively, with
CDCl3 (d 7.26) or TMS (d 0.00) as an internal standard. Mass spectra
were recorded at electron ionization or FAB mode. Compounds 1a10
and 710 were prepared according to literature procedures. Com-
pound 1b was first reported by Li, however, no spectral data were
available.11d All reported yields in this work were isolated yields.
CDCl3)
(q, J¼7.2 Hz, 2H), 4.45 (q, J¼7.2 Hz, 2H), 7.68–7.80 (m, 2H), 7.92–8.00
(m, 2H), 8.03 (s, 1H); 13C NMR (75 MHz, CDCl3)
14.1 (CH3), 22.6
d
1.32 (t, J¼7.2 Hz, 3H), 1.38 (t, J¼7.2 Hz, 3H), 2.89 (s, 3H), 4.38
d
(CH3), 61.4 (CH2), 62.1 (CH2), 128.4 (CH), 129.3 (CH), 129.6 (CH), 131.1
(CH), 132.4 (Cq), 134.8 (CH), 140.7 (Cq), 141.9 (Cq), 145.8 (Cq), 153.1
(Cq), 163.5 (Cq), 166.0 (Cq); EIMS m/z 314 (Mþ, 53), 285 (33), 269
(38), 241 (100); HR-EIMS m/z calcd for C17H87N2O4 314.1267, found
314.1265.
4.1.1. Diethyl 3aH-[1,2,3]triazolo[5,1-a]isoindole-3,3-(8H)-dicarboxy-
late, 1a. A mixture of diethyl 2-(2-(bromomethyl)benzylidene)-
malonate10 (0.45 g, 1.32 mmol) and sodium azide (0.26 g,
3.96 mmol) in 12 mL of methanol/water (v/v¼3/1) was stirred at
room temperature for 3 h. The reaction mixture was treated with
water (20 mL) and dichloromethane (3ꢀ10 mL). The organic layer
was dried over MgSO4 and concentrated under reduced pressure.
The residue was column chromatographed using hexane/ethyl ac-
etate (2:1) as eluent to give 1a (0.37 g, 92%) as a white solid; mp 99–
101 ꢂC; Rf¼0.58 (hexane/ethyl acetate¼2:1); 1H NMR (300 MHz,
4.1.5. Preparation of diethyl 2-((3-(bromomethyl)-quinoxalin-2-yl)-
methylene)malonate, 6. A mixture of 5 (0.63 g, 2.01 mmol), N-bro-
mosuccinimide (0.47 g, 2.49 mmol) and dibenzoyl peroxide
(78.0 mg, 0.03 mmol) was dissolved in 35 mL of 1,2-dichlorobenzene
under nitrogen. The reaction mixture was stirred at 40 ꢂC for 12 h
and then cooled to room temperature. The reaction mixture was
passed through a short silica gel column and washed with hexane to
remove 1,2-dichlorobenzene, then eluted with dichloromethane.
After removal of the solvent under reduced pressure, the residue was
column chromatographed using hexane/ethyl acetate (8:1) as eluent
to give 6 (0.47 g, 60%) as a black solid; mp 101–103 ꢂC; Rf¼0.23
CDCl3)
d
1.14 (t, J¼7.3 Hz, 3H), 1.37 (t, J¼7.3 Hz, 3H), 4.15–4.48 (m,
4H), 4.68 (d, J¼15.5 Hz, 1H), 5.34 (d, J¼15.5 Hz, 1H), 5.72 (s, 1H), 7.11
(d, J¼7.2 Hz,1H), 7.20–7.40 (m, 4H); 13C NMR (75 MHz, CDCl3)
d 13.7
(CH3), 13.9 (CH3), 55.0 (CH2), 62.4 (CH2), 63.3 (CH2), 68.1 (CH), 95.0
(Cq), 123.0 (CH), 123.3 (CH), 127.7 (CH), 129.1 (CH), 135.9 (Cq), 137.5
(Cq), 164.4 (Cq), 164.9 (Cq); FABMS m/z 304 (MþHþ); HR-FABMS
(MþHþ) m/z calcd for C15H18N3O4 304.1292, found 304.1301.
(hexane/ethyl acetate¼6:1); 1H NMR (300 MHz, CDCl3)
d 1.33 (t,
J¼6.9 Hz, 3H), 1.39 (t, J¼7.2 Hz, 3H), 4.39 (q, J¼7.2 Hz, 2H), 4.46 (q,
J¼7.2 Hz, 2H), 4.85 (s, 2H), 7.76–7.81 (m, 2H), 7.95–8.05 (m, 2H), 8.12
4.1.2. Preparation of 2-(bromomethyl)-3-methylquinoxaline, 3. 2,3-
Dimethylquinoxaline 2 (0.40 g, 2.53 mmol), N-bromosuccinimide
(0.60 g, 3.33 mmol), and dibenzoyl peroxide (0.01 g, 0.04 mmol) was
dissolved in 30 mL of 1,2-dichlorobenzene under nitrogen. The re-
action mixture was heated to 60 ꢂC and stirred for 2.5 h. After cooling
(s, 1H); 13C NMR (75 MHz, CDCl3)
d 14.0 (CH3),14.0 (CH3), 30.3 (CH2),
61.4 (CH2), 62.1 (CH2), 129.0 (CH), 129.2 (CH), 131.0 (CH), 131.5 (CH),
132.6 (Cq), 133.7 (CH), 141.3 (Cq), 141.4 (Cq), 145.4 (Cq), 150.7 (Cq),
163.3 (Cq), 165.8 (Cq); EIMS m/z 394/392 (Mþ, 19/19), 321 (100), 319

Y.-J. Chen et al. / Tetrahedron 66 (2010) 176–182
181
(100), 240 (45), 239 (48), 196 (31), 195 (61), 168 (52), 167 (54); HR-
173 (47), 131 (38), 130 (60); HR-EIMS m/z calcd for C25H24N2O6
448.1634, found 448.1631.
EIMS m/z calcd for C17H1779BrN2O4 392.0372, found 392.0381.
4.1.6. Preparation of 3,3-bis-(ethoxycarbonyl)-3,3a-dihydro-10H-
[1,2,3]triazolo[1’,5’:1,2]-pyrrolo[3,4-b]-quinoxaline, 1b. A mixture of
6 (0.41 g,1.05 mmol) and sodium azide (0.17 g, 2.61 mmol) in 15 mL
of 95% ethanol was stirred at room temperature for 12 h. The sol-
vent was removed and the residue was treated with water (25 mL)
and dichloromethane (3ꢀ10 mL). The organic layer was dried over
MgSO4 and concentrated under reduced pressure. The residue was
column chromatographed using hexane/ethyl acetate (1:1) as elu-
ent to give 1b (0.34 g, 91%). As a white solid; mp 125–126 ꢂC;
Rf¼0.65 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz, CDCl3)
4.2.4. Data for cycloadduct 12b. A yellow solid (6%); mp 172–
173 ꢂC; Rf¼0.49 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz,
CDCl3)
d
1.21 (t, J¼7.1 Hz, 3H), 1.26 (s, 1H), 1.40 (t, J¼7.2 Hz, 3H), 3.32
(d, J¼7.2 Hz, 1H), 3.75 (d, J¼7.2 Hz, 1H), 4.15 (t, J¼6.9 Hz, 2H), 4.41–
4.49 (m, 2H), 4.46 (s,1H), 5.07 (s,1H), 7.33–7.52 (m, 5H), 7.71–7.79
(m, 2H), 8.02–8.05 (m, 2H); 13C NMR (75 MHz, CDCl3)
d 13.9 (CH3),
14.1 (CH3), 47.8 (CH), 48.3 (CH), 51.3 (CH), 61.9 (CH2), 62.1 (CH2),
62.4 (CH), 70.9 (Cq), 126.5 (CH), 128.9 (CH), 129.0 (CH), 129.2 (CH),
129.6 (CH), 129.7 (CH), 129.9 (CH), 131.8 (Cq), 140.3 (Cq), 140.6 (Cq),
160.1 (Cq), 160.3 (Cq), 166.6 (Cq), 168.0 (Cq), 173.9 (Cq), 174.4 (Cq);
EIMS m/z 500 (Mþ, 2), 327 (100), 255 (60), 254 (80), 210 (51), 209
(40), 208 (87), 127 (94), 173 (87); HR-EIMS m/z calcd for C27H24N4O6
500.1696, found 500.1694.
d
1.23 (t, J¼7.2 Hz, 3H), 1.40 (t, J¼7.2 Hz, 3H), 4.29–4.38 (m, 3H),
4.46–4.54 (m, 1H), 4.93 (d, J¼17.4 Hz, 1H), 5.55 (d, J¼17.4 Hz, 1H),
5.86 (s,1H), 7.72–7.82 (m, 2H), 7.90–8.10 (m, 2H); 13C NMR (75 MHz,
CDCl3)
d 13.8 (CH3), 13.9 (CH3), 53.8 (CH2), 62.7 (CH2), 63.6 (CH2),
65.6 (CH), 95.9 (Cq), 129.2 (CH), 129.3 (CH), 130.2 (CH), 130.6 (CH),
142.0 (Cq), 142.4 (Cq), 152.4 (Cq), 153.3 (Cq), 163.6 (Cq), 164.2 (Cq);
EIMS m/z 355 (Mþ, 1), 255 (39), 210 (50), 209 (47), 183 (100), 181
(63), 169 (45), 142 (36), 102 (46); HR-EIMS m/z calcd for C17H17N5O4
355.1281, found 355.1282.
4.2.5. Data for cycloadduct 13a. A red liquid (17%); Rf¼0.73 (hex-
ane/ethyl acetate¼1:1); 1H NMR (300 MHz, CDCl3)
d 1.62 (s, 1H),
3.96 (s, 6H), 7.63 (dd, J¼6.3, 3.3 Hz, 2H), 7.93 (dd, J¼6.3, 3.3 Hz, 2H),
8.26 (s, 2H); 13C NMR (75 MHz, CDCl3)
d 52.7 (CH3),128.4 (Cq),128.6
(CH), 128.7 (CH), 130.1 (CH), 133.4 (Cq), 168.2 (Cq); EIMS m/z 259
(Mþ, 1), 244 (35), 213 (100); HR-EIMS m/z calcd for C14H13NO4
259.0845, found 259.0837.
4.2. Photolysis of 1a, 1b in CH3CN by UV light (254nm) at
room temperature
4.2.6. Data for cycloadduct 14a. A red solid (11%); mp 91–92 ꢂC;
Rf¼0.48 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz, CDCl3)
Samples 1a and 1b (25 mM) in CH3CN without dienophiles were
irradiated in a Rayonet photoreactor with 254 nm at room tem-
perature for 30 min using 3 mm sealed quartz tubes, respectively.
In the presence of various dienophiles (75 mM), the irradiation
period is 2 h. Yields were determined using 1,4-dioxane as an in-
ternal standard and analyzed by HPLC. Because of facing difficulty
in separation, we were unable to obtain a pure form of 14a and 16a;
nevertheless, the characteristic peaks of 14a and 16a can be dis-
cerned from their 1H and 13C NMR spectra.
d
1.28 (t, J¼7.2 Hz, 6H), 3.79 (s, 6H), 4.25 (q, J¼7.2 Hz, 4H), 5.26 (s,
2H), 7.08 (dd, J¼5.1, 3.0 Hz, 2H), 7.41 (dd, J¼5.1, 3.0 Hz, 2H); 13C NMR
(75 MHz, CDCl3) d 13.9 (CH3), 52.3 (CH3), 62.0 (CH2), 64.8 (CH), 72.3
(CH), 123.3 (CH), 126.2 (CH), 144.7 (Cq), 149.4 (Cq), 163.3 (Cq), 166.2
(Cq); EIMS m/z 417 (Mþ, 3), 344 (19), 312 (100), 275 (20), 224 (21),
213 (32); HR-EIMS m/z calcd for C21H23NO8 417.1424, found 417.1421.
4.2.7. Data for cycloadduct 15b. A red solid (9%); mp 157–158 ꢂC;
Rf¼0.53 (hexane/ethyl acetate¼1:1); 1H NMR (600 MHz, CDCl3)
d
1.17 (t, J¼7.2 Hz, 6H), 3.72 (d, J¼3.0 Hz, 1H), 3.90 (s, 3H), 3.96 (s,
4.2.1. Data for diethyl 2-(2,3-dihydropyrrolo[3,4-b]quinoxalin-1-yl-
idene)malonate, 9b. A yellow solid (5–23%); mp 182–183 ꢂC;
Rf¼0.35 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz, CDCl3)
3H), 4.18–4.25 (m, 4H), 7.81–7.86 (m, 4H), 8.12–8.14 (m, 1H), 8.18–
8.19 (m, 1H); 13C NMR (150 MHz, CDCl3)
d
14.1 (CH3), 52.0 (CH), 52.1
(CH3), 52.5 (CH3), 61.4 (CH2), 117.0 (Cq), 129.5 (CH), 130.8 (CH), 131.8
(CH), 134.8 (Cq), 136.8 (Cq), 141.0 (Cq), 142.8 (Cq), 143.8 (Cq), 150.2
(Cq), 167.3 (Cq), 168.3 (Cq), 169.0 (Cq); EIMS m/z 469 (Mþ, 100), 423
(85), 396 (49), 368 (46), 351 (32), 308 (95), 276 (45); HR-EIMS m/z
calcd for C23H23N3O8 469.1483, found 469.1485.
d
1.35 (t, J¼7.2 Hz, 3H), 1.38 (t, J¼7.2 Hz, 3H), 4.29 (q, J¼7.2 Hz, 2H),
4.51 (q, J¼7.2 Hz, 2H), 4.88 (s, 2H), 7.80–7.90 (m, 2H), 8.11–8.16 (m,
2H), 9.00 (s, 1H); 13C NMR (75 MHz, CDCl3)
d
14.1 (CH3), 14.4 (CH3),
29.7 (Cq), 49.3 (CH2), 60.3 (CH2), 61.3 (CH2), 129.0 (CH), 130.3 (CH),
130.4 (CH), 131.5 (CH), 142.3 (Cq), 142.5 (Cq), 147.3 (Cq), 154.7 (Cq),
155.0 (Cq), 166.9 (Cq), 167.9 (Cq); EIMS m/z 327 (Mþ, 26), 282 (33),
255 (43), 210 (56), 183 (100), 102 (20); HR-EIMS m/z calcd for
C17H17N3O4 327.1219, found 327.1210.
4.2.8. Data for cycloadduct 16a. A white solid (3%); mp 117–119 ꢂC;
Rf¼0.58 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz, CDCl3)
d
1.19–1.27 (m, 6H), 2.65 (d, J¼4.5 Hz, 1H), 3.63 (s, 1H), 3.70 (t,
J¼4.5 Hz, 1H), 4.05–4.20 (m, 4H), 4.93 (s, 1H), 5.05 (d, J¼4.2, 1H),
4.2.2. Data for cycloadduct exo-10b. A white solid (8%); mp 228–
229 ꢂC; Rf¼0.38 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz,
7.35–7.49 (m, 4H); 13C NMR (75 MHz, CDCl3)
d
13.9 (CH3), 35.4 (CH),
36.3 (CH), 62.0 (CH2), 62.1 (CH2), 63.4 (CH), 66.3 (CH), 68.4
(CH), 117.0 (Cq), 118.3 (Cq), 122.5 (CH), 124.4 (CH), 129.2 (CH), 129.2
(CH), 138.3 (Cq), 140.1 (Cq), 166.0 (Cq), 166.0 (Cq); EIMS m/z 353
(Mþ, 1), 275 (100), 131 (32), 130 (64); HR-EIMS m/z calcd for
C19H19N3O4 353.1736, found 353.1743.
CDCl3)
d
3.06 (s, 1H), 3.26 (s, 2H), 5.11 (s, 2H), 7.35 (d, J¼7.2 Hz, 2H),
7.42–7.52 (m, 3H), 7.76–7.80 (m, 2H), 8.04–8.07 (m, 2H); 13C NMR
(75 MHz, CDCl3) 46.9 (CH), 63.7 (CH), 126.5 (CH), 128.9 (CH), 129.2
d
(CH), 129.3 (CH), 130.0 (CH), 132.0 (Cq), 140.6 (Cq), 160.8 (Cq), 174.7
(Cq); EIMS m/z 342 (Mþ, 2), 169 (100); HR-EIMS m/z calcd for
C20H14N4O2 342.1117, found 342.1113.
Acknowledgements
4.2.3. Data for cycloadduct endo-11a. A red solid (5%); mp 125–
126 ꢂC; Rf¼0.43 (hexane/ethyl acetate¼1:1); 1H NMR (300 MHz,
We thank the National Science Council (NSC) and the MOE ATU
Program of the Ministry of Education, Taiwan, Republic of China, for
financial support.
CDCl3)
d
1.27 (t, J¼7.2 Hz, 6H), 3.58 (s, 1H), 4.03 (dd, J¼3.3, 1.8 Hz,
2H), 4.20–4.28 (m, 4H), 4.98 (dd, J¼3.5, 1.8 Hz, 2H), 6.38–6.41 (m,
2H), 7.24–7.26 (m, 3H), 7.33–7.37 (m, 4H); 13C NMR (75 MHz, CDCl3)
Supplementary data
d
13.9 (CH3), 46.9 (CH), 62.1 (CH2), 64.2 (CH), 66.6 (CH), 124.0 (CH),
126.2 (CH),126.3 (CH),128.5 (CH),128.6 (CH),128.9 (CH),130.9 (Cq),
1H and 13C NMR spectra of compounds reported in this work and
EPR spectra of the photolysis of 1a and 1b in MTHF at 77 K.
139.0 (Cq), 166.2 (Cq), 174.2 (Cq); EIMS m/z 448 (Mþ, 2), 275 (100),

182
Y.-J. Chen et al. / Tetrahedron 66 (2010) 176–182
8. (a) Huang, X.; Shen, R.; Zhang, T. J. Org. Chem. 2007, 72, 1534; (b) Smith, P. A. S.;
Chou, S.-S. P. J. Org. Chem. 1981, 46, 3970.
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.tet.2009.11.004.
9. (a) Scheiner, P. J. Am. Chem. Soc. 1968, 90, 988; (b) Edwards, O. E.; Elder, J. W.;
Lesage, M.; Rtallack, R. W. Can. J. Chem.1975, 53,1019; (c) Schultz, A. G.; Sha, C.-K.
J. Org. Chem. 1980, 45, 2040; (d) Perrocheau, J.; Carrie´, R.; Fleury, J.-P. Can. J.
Chem. 1994, 72, 2458; (e) Ciufolini, M. A.; Chen, M.; Lovett, D. P.; Deaton, M. V.
Tetrahedron Lett. 1997, 38, 4355; (f) Tukada, H. Chem. Commun. 2000, 63.
10. (a) Romming, C.; Kosaker, P. Acta Chem. Scand. 1978, B32, 679; (b) Kolsaker, P.;
Ellingsen, P. O.; WØien, G. Acta Chem. Scand. 1978, B32, 683.
11. (a) Sha, C.-K.; Tsou, C.-P.; Li, Y.-C.; Lee, R.-S.; Tsai, F.-Y.; Yeh, R.-H. J. Chen. Soc.,
Chem. Commun. 1988, 1081; (b) Sha, C.-K.; Tsou, C.-P. J. Org. Chem. 1990, 55,
2446; (c) Sha, C.-K.; Tsou, C.-P. J. Chem. Soc., Perkin Trans. I 1994, 3065; (d) Li,
Y.-C. M.S. Thesis, National Tsing Hua University, 1987; (e) Sha, C.-K.; Hsu, H.-Y.;
Cheng, S.-Y.; Kuo, Y.-L. Tetrahedron 2003, 59, 1477 and references therein; (f)
Sha, C.-K. Advances in nitrogen heterocycles. Synthesis and Cycloaddition Re-
actions of Iso-condensed Heteroaromatic Pyrroles; JAI: 1996; Vol. 2.
12. (a) Li, X.; Jain, N.; Russell, R. K.; Ma, R.; Branum, S.; Xu, J.; Sui, Z. Org. Process. Res.
Dev. 2006, 10, 354; (b) Wen, J.-F.; Hong, W.; Yuan, K.; Mak, T. C. W.; Wong, H. N.
C. J. Org. Chem. 2003, 68, 8918.
References and notes
1. (a) Remy, D. E.; Bissett, F. H. J. Org. Chem. 1978, 43, 4469; (b) Babichev, F. S.;
Kovtunenko, V. K.; Tyltin, A. K. Russ. Chem. Rev. 1981, 50, 1087; (c) Kovtunenko,
V. A.; Voitenko, Z. V. Russ. Chem. Rev. 1994, 63, 997; (d) Gabbutt, C. D.; Hep-
worth, J. D.; Heron, B. M.; Pugh, S. L. J. Chem. Soc., Perkin Trans. 1 2002, 2799.
´
´
´
2. (a) Gallego-Gomez, F.; Quintana, J. A.; Villalvilla, J. M.; Dıaz-Garcıa, M. A.;
Gomis, L. M.; Ferna´ndez-La´zaro, F.; Sastre-Santos, A. Chem. Mater. 2009, 21,
2417; (b) Mi, B.-X.; Wang, P.-F.; Liu, M.-W.; Kwong, H.-L.; Wong, N.-B.; Lee, C.-S.;
Lee, S. T. Chem. Mater. 2003, 15, 3148; (c) Chen, Z.; Mu1ller, P.; Swager, T. M. Org.
Lett. 2006, 8, 273; (d) Faridbod, F.; Ganjali, M. R.; Dinarvand, R.; Norouzi, P.
Sensors 2008, 8, 2331.
3. Bra¨se, S.; Friedrich, A.; Gartner, M.; Schro¨der, T. Top. Heterocycl. Chem. 2008,12, 45.
4. (a) Singh, G. S.; D’hooghe, M.; Kimpe, N. D. Chem. Rev. 2007, 107, 2080; (b) Kim,
S.; Lee, Y. M.; Lee, J.; Lee, T.; Fu, Y.; Song, Y.; Cho, T.; Kim, D. J. Org. Chem. 2007,
72, 4886; (c) Naira, V.; Suja, T. D. Tetrahedron 2007, 63, 12247.
5. (a) Feldman, K. S.; Iyer, M. R.; Lo´pez, C. S.; Faza, O. N. J. Org. Chem. 2008, 73,
5090; (b) Feldman, K. S.; Iyer, M. R.; Hester, D. K. Org. Lett. 2006, 8, 3113; (c)
Feldman, K. S.; Iyer, M. R. J. Am. Chem. Soc. 2005, 127, 4590.
13. (a) Has, H. B.; Bender, M. L. J. Am. Chem. Soc. 1949, 71, 1767; (b) Lieberman, S. V.
J. Am. Chem. Soc. 1954, 77, 1114; (c) Mohanakrishnan, A. K.; Balamurugan, R.;
Ramesh, N. Tetrahedron Lett. 2005, 46, 8189.
14. (a) Mowry, D. T. J. Am. Chem. Soc. 1945, 67, 1050; (b) Prout, F. S.; Abdel-Latif, A.
A.; Kamal, N. R. J. Chem. Eng. Data 1963, 8, 597.
15. Zhou, Y.; Murphy, P. V. Org. Lett. 2008, 10, 3777.
6. (a) Dowd, P. Acc. Chem. Res. 1972, 5, 242; (b) Little, R. D. Chem. Rev. 1988, 86, 875;
(c) Little, R. D. Chem. Rev. 1996, 96, 93; (d) Allan, A. K.; Carroll, G. L.; Little, R. D.
Eur. J. Org. Chem. 1998, 1.
7. (a) Hui, Q.-A. B.; Chiba, S. Org. Lett. 2009, 11, 729; (b) Liu, J.-M.; Young, J.-J.; Li, Y,
J.; Sha, C.-K. J. Org. Chem. 1986, 51, 1120; (c) Wladkowski, B. D.; Smith, R. H.;
Michejda, C. J. J. Am. Chem. Soc. 1991, 113, 7893; (d) Shea, K. J.; Kim, J. S. J. Am.
Chem. Soc. 1992, 114, 4846.
16. (a) Wasserman, E. W.; Hutton, R. S. Acc. Chem. Res. 1977, 10, 27; (b) Eaton, S. S.;
More, K. M.; Sawant, B. M.; Eaton, G. R. J. Am. Chem. Soc. 1983, 105, 6560; (c)
Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. J. Am. Chem. Soc. 1967, 89, 1112.
17. (a) Satoh, M.; Kaneto, K.; Yoshino, K. J. Chem. Soc., Chem. Commun. 1984, 1627;
(b) Lazzaroni, R.; Riga, J.; Verbist, J. J.; Christiaens, L.; Renson, M. J. Chem. Soc.,
Chem. Commun. 1985, 999.