Rearrangements of the [2+2]-cycloadducts of DDQ and 2-ethynylpyrroles

Article abstract of DOI:10.1016/j.tetlet.2010.07.100

The [2+2]-cycloadducts of DDQ and 2-ethynylpyrroles, upon ethanolysis (reflux, 15 min or room temperature, 4 h), rearrange from bicyclo[4.2.0] octadienediones to bicyclo[3.2.0]heptadienone- and cyclobutenyl- dihydrofuranone moieties in 55-83% yields, the former rearrangement being the major direction. Benzoquinone ring cleavage is regioselective to afford mostly bicyclo[3.2.0]heptadienone-pyrrole ensembles (85-90% selectivity) in 39-78% yields. The only exception is when the starting compounds contain an ethoxycarbonyl substituent and the pyrrole counterpart is a 4,5,6,7- tetrahydroindole fragment. In this case, the ratio of the rearrangement products is 1:1.2 in a total yield of 83%. An important feature of the dihydrofuranone pathway rearrangement is its 100% diastereoselectivity.

Full text of DOI:10.1016/j.tetlet.2010.07.100

Tetrahedron Letters 51 (2010) 5028–5031
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Tetrahedron Letters
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Rearrangements of the [2+2]-cycloadducts of DDQ and 2-ethynylpyrroles
a
a
a
a
Boris A. Trofimov a, , Lyubov N. Sobenina , Zinaida V. Stepanova , Igor A. Ushakov , Albina I. Mikhaleva ,
*
Denis N. Tomilin a, Olga N. Kazheva b, Grigorii G. Alexandrov c, Anatolii N. Chekhlov b, Oleg A. Dyachenko b
a A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Str., 664033 Irkutsk, Russian Federation
b Institute of Problems of Chemical Physics, Russian Academy of Sciences, 1 Academician N. N. Semenov Str., 142432 Chernogolovka, Russian Federation
c N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninskii Prosp., 119991, Moscow, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
The [2+2]-cycloadducts of DDQ and 2-ethynylpyrroles, upon ethanolysis (reflux, 15 min or room temper-
ature, 24 h), rearrange from bicyclo[4.2.0]octadienediones to bicyclo[3.2.0]heptadienone- and cyclobute-
nyl-dihydrofuranone moieties in 55–83% yields, the former rearrangement being the major direction.
Benzoquinone ring cleavage is regioselective to afford mostly bicyclo[3.2.0]heptadienone-pyrrole ensem-
bles (85–90% selectivity) in 39–78% yields. The only exception is when the starting compounds contain an
ethoxycarbonyl substituent and the pyrrole counterpart is a 4,5,6,7-tetrahydroindole fragment. In this
case, the ratio of the rearrangement products is 1:1.2 in a total yield of 83%. An important feature of
the dihydrofuranone pathway rearrangement is its 100% diastereoselectivity.
Received 20 May 2010
Revised 24 June 2010
Accepted 16 July 2010
Available online 22 July 2010
Keywords:
Bicyclo[4.2.0]octadienedione
Ethanolysis
Ó 2010 Elsevier Ltd. All rights reserved.
Rearrangement
Bicyclo[3.2.0]heptadienone
Cyclobutene
Dihydrofuranone
Recently, we revealed1 an extremely facile [2+2]-cycloaddition
of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to the read-
ily available2 2-ethynylpyrroles 1a–e, bearing acyl- or alkoxycar-
bonyl groups at the triple bond, which makes the cycloadducts
2a–e easily accessible for further investigations (Scheme 1).
Herein, we report on the skeletal rearrangements of cycload-
ducts 2a–e which occur in ethanol (reflux or room temperature)
to afford pyrrolylbicyclo[3.2.0]heptadienones 3a–e and pyrrolylcy-
clobutenyl-dihydrofuranones 4a–e (ratio 44:56–90:10, depending
on the structure of the starting compound 2a–e), in total yields
ranging from 55% to 83% (Scheme 2, Table 1).3
As follows from Table 1, the reaction was regioselective pro-
ceeding with preferable formation of bicycloheptadienones 3a–d
(selectivity 85–90%), while dihydrofuranones 4a–d were the minor
products (the latter products were identified by 1H NMR). The only
exception was cycloadduct 2e, which gave products 3e and 4e in a
ratio of 44:56 (isolated yields, 39% and 44%).
Also, the 1H and 13C NMR spectra entirely supported the struc-
tures of rearrangement products 3a–e and 4e. The signals of the
Cl
Cl
R2
R1
O
NC
Cl
R2
R1
O
O
Cl
N
H
O
NC CN
N
H
O
CN
acetone
R3
O
R3
r.t., 15 min
2a-e
1a-e
Cycloadduct
R1
Ph
R2
H
H
H
R3
Ph
Ph
2a
2b
2c
2d
2e
4-ClC6H4
4-MeOC6H4
4-O2NC6H4
Ph
(CH2)4
(CH2)4
OEt
Scheme 1.
The structures of the rearrangement products were proved
unambiguously by X-ray crystallographic analysis as seen in the
example of diethyl 3-chloro-1,5-dicyano-4-oxo-7-(4,5,6,7-tetrahy-
dro-1H-indol-2-yl)bicyclo[3.2.0]hepta-2,6-diene-2,6-dicarboxylate
(3e) (Fig. 1) and ethyl 3,4-dicyano-4-(3,4-dichloro-2-ethoxy-5-oxo-
2,5-dihydro-2-furanyl)-2-(4,5,6,7-tetrahydro-1H-indol-2-yl)-1-
cyclobutene-1-carboxylate (4e) (Fig. 2).4
OEt
Cl
R2
O
O
R2
R1
Cl
Cl
CN
O
NC
R1
+
EtOH
2a-e
N
H
N
H
reflux,15 min
or r.t., 24 h
O
OEt
CN
R3
CN
R3
3a-e
O
O
4a-e
* Corresponding author. Tel.: +7 (3952)461411/511431; fax: +7 (3952)419346.
E-mail address: boris_trofimov@irioch.irk.ru (B.A. Trofimov).
Scheme 2.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.100

B. A. Trofimov et al. / Tetrahedron Letters 51 (2010) 5028–5031
5029
Table 1
Rearrangement products 3a–e and 4e were synthesized according to Scheme 2
Entry
Cycloadduct
Rearrangement product
OEt
Cl
Yield (%)
O
NC
Cl
O
NC
Cl
N
H
N
H
O
O
CN
O
CN
1
55
O
2a
3a
O
OEt
Cl
Cl
O
NC
NC
Cl
N
H
Cl
N
H
Cl
O
O
CN
O
CN
2
3
4
57
O
2b
3b
O
NC
OEt
Cl
O
NC
Cl
Cl
N
H
MeO
N
H
MeO
O
O
CN
O
CN
O
78
NO2
NO2
2c
3c
OEt
O
Cl
O
NC
NC
Cl
Cl
N
H
N
H
O
O
CN
O
CN
71
O
2d
3d
O
OEt
Cl
Cl
O
NC
NC
Cl
N
H
N
H
O
O
CN
OEt
39
O
CN
OEt
O
3e
5
Cl
NC
EtO
Cl
N
H
O
CN
OEt
44
O
O
4e
quaternary carbon atoms C1 and C5 in the cyclobutene ring (53.1–
48.1 ppm) and those of the three carbonyl carbons in the region
187.8–160.5 ppm were important for the structural assignment
of the 13C NMR spectra of compounds 3a–e. The signals at 162.2,
160.9, 50.6, and 34.6 ppm in the 13C spectrum of dihydrofuranone
4e were attributable to two carbonyl carbon atoms and carbons C3
and C4 of cyclobutene ring bonded to the CN-groups, respectively.
The carbon atom C2 in the dihydrofuranone ring of this compound
resonated at 104.9 ppm. In the 1H NMR spectrum of compound 4e,
the proton of the cyclobutene ring appeared at 4.36 ppm and dis-
played long range correlations (2D HMBC) with the carbon atoms
at 50.6, 104.9, 106.0, 114.0 (CN), and 144.8 (C2 of pyrrole ring).
The NOEs between this proton and H3 of the pyrrole ring
(6.60 ppm) and the protons of the OCH2 group (3.86 ppm) were ob-
served in the 2D NOESY spectrum.
The rearrangement of bicyclo[4.2.0]octadienediones 2a–e into
bicyclo[3.2.0]heptadienones 3a–e is likely triggered by the addi-
tion of ethanol to the quinone carbonyl group (at C2). The interme-
diate hemiacetal A then undergoes ring opening to form the ester
group with simultaneous cleavage of the C1–C2 bond. This leads to
the formation of the carbanion center at the C1 which attacks the
C3 carbon atom in an intramolecular nucleophilic substitution

5030
B. A. Trofimov et al. / Tetrahedron Letters 51 (2010) 5028–5031
R2
R1
Cl
R2
R1
O
O
Cl
2
3
5
EtOH
2
3
5
NC
NC
1
1
4
Cl
OEt
OH
4
Cl
*
6
6
N
H
N
H
*
O
CN
R3
CN
O
O
R3
2a-e
B
R2
R1
O
R2
R1
Cl
O
Cl
Cl
NC
3
NC
O
*
O
Cl
*
N
H
*
4
2
H
N
H
OEt
CN
R3
4a-e
OEt
O
CN
R3
O
Scheme 4.
Figure 1. X-ray crystal structure of 3e.
pound 2e and formation of the C3–H bond occur from the same
side of the four-membered ring.
Similar bicyclo[4.2.0]octadienediones, prepared by photoaddi-
tion of diphenylacetylene to benzo- and naphthoquinones, upon
alcoholysis, rearranged via the cleavage of the cyclobutene ring.5
This differs completely from the rearrangement we observed here.
The rearrangement products 3a–e and 4a–e are interesting com-
pounds due to the combination of densely functionalized cyclobu-
tene and pyrrole scaffolds. Only a few representatives containing
exhaustively substituted cyclobutene rings are known6,7 and their
syntheses are multistep and laborious.8 In addition, some are em-
ployed as key intermediates in Corey’s brefeldin A synthesis.9
The rearrangements observed herein enable easy entry to hith-
erto unknown densely functionalized heterocyclic ensembles with
rare combinations of heterocycles and functional groups.
Figure 2. X-ray crystal structure of 4e.
Acknowledgments
H
This work was carried out under the financial support of the
leading scientific schools by the President of the Russian Federa-
tion (Grant NSh-3230.2010.3), the Russian Foundation of Basic Re-
search (Grant No. 09-03-00064a).
O
R2
R1
R2
R1
EtO
NC
Cl
O
Cl
EtOH
NC
2
-H+
3
4
5
1
Cl
Cl
6
N
H
N
H
O
O
CN
CN
R3
2a-e
O
O
R3
A
References and notes
OEt
Cl
O
R2
R1
OEt
R2
1. Trofimov, B. A.; Sobenina, L. N.; Stepanova, Z. V.; Ushakov, I. A.; Sinegovskaya, L.
M.; Vakul’skaya, T. I.; Mikhaleva, A. I. Synthesis 2010, 470–477.
2. (a) Trofimov, B. A.; Stepanova, Z. V.; Sobenina, L. N.; Mikhaleva, A. I.; Ushakov, I.
A. Tetrahedron Lett. 2004, 45, 6513–6516; (b) Trofimov, B. A.; Sobenina, L. N.;
Stepanova, Z. V.; Petrova, O. V.; Ushakov, I. A.; Mikhaleva, A. I. Tetrahedron Lett.
2008, 49, 3946–3949.
NC
O
Cl
-Cl-
NC
1
5
R1
N
Cl
N
H
O
H
CN
R3
3a-e
O
O
CN
R3
O
3. Typical procedure. Rearrangement of cycloadducts 2a–d. Cycloadduct 2a–d
(0.1 g) was refluxed for 15 min in EtOH (20 mL). Bright red crystals begin to
precipitate from the hot solution after 5–10 min. After cooling to room
temperature the crystals were filtered and dried to give bicyclo[3.2.0]
heptadienones 3a–c. The red crystals of compound 3d were filtered from the
solution after 24 h. In the case of cycloadduct 2e, formed after 24 h the mixture of
yellow and red crystals was filtered and fractionated by silica gel column
chromatography (CH2Cl2) to afford compounds 3e and 4e. All new compounds
exhibited spectral data consistent with their structures. Selected spectral data:
Ethyl 7-benzoyl-3-chloro-6-[5-phenyl-1H-pyrrol-2-yl]-1,5-dicyano-4-oxobicyclo
[3.2.0]hepta-2,6-diene-2-carboxylate (3a). Red crystals; mp: 234–236 °C; IR
(KBr, cmÀ1) 3441 (NH), 2250 (CN), 1749, 1717 (C@O), 1627 (C@C); 1H NMR
(400.13 MHz, CDCl3) d 12.98 (br s, 1H, NH), 8.06 (m, 2H, CH-2,6 COPh), 7.76 (m,
2H, CH-2,6 Ph), 7.65 (m, 1H, CH-4 COPh), 7.58 (m, 2H, CH-3,5 COPh), 7.50 (m, 3H,
H-3, CH-3,5 Ph), 7.42 (m, 1H, CH-4 Ph), 6.90 (dd, 3J = 4.4 Hz, 4J = 2.5 Hz, 1H, H-4),
4.51 (q, J = 7.2 Hz, 2H, OCH2), 1.44 (t, J = 7.2 Hz, 3H, CH3); 13C NMR (100.6 MHz,
CDCl3) d 187.8, 183.7, 160.5, 148.7, 144.7, 143.0, 141.8, 136.4, 133.6, 129.9, 129.6,
129.5, 129.0, 128.5, 125.6, 125.3, 123.8, 115.2, 113.0, 112.0, 111.9, 64.0, 53.2,
48.3, 14.1. Anal. Calcd. for C29H18ClN3O4: C, 68.58; H, 3.57; Cl, 6.98; N, 8.27.
Found: C, 68.79; H, 3.46; Cl, 7.20; N, 8.68.
Scheme 3.
with elimination of hydrogen chloride. Finally, the ring contraction
occurs to yield the products 3a–e (Scheme 3).
The formation of furanones 4a–e occurs by similar addition of
ethanol to the other quinone carbonyl (at C5). In the intermediate
hemiacetal B, the hydroxy group attacks the C2 carbonyl group
with ring opening via cleavage of the same C1–C2 bond. This is
accompanied by a concerted proton transfer to the carbanion-like
C1 center (Scheme 4).
As compared to the starting cycloadduct 2e, in furanone 4e, the
configuration of the C3 atom in the cyclobutene ring may change.
Moreover, another asymmetric center is present at the C2 atom in
the dihydrofuranone ring. Therefore, two diastereomeric pairs of
4e may exist. However, in the 1H and 13C NMR spectra, the signals
of only one diastereomer were detected (no signal doublings are
discernible). The relative configuration of this diastereomer shown
in Figure 2 suggests that the cleavage of the C1–C2 bond in com-
Ethyl 7-benzoyl-3-chloro-6-[5-(4-chlorophenyl)-1H-pyrrol-2-yl]-1,5-dicyano-4-
oxobicyclo[3.2.0]hepta-2,6-diene-2-carboxylate (3b). Red crystals; mp: 233–
234 °C; IR (KBr, cmÀ1) 3436 (NH), 2254 (CN), 1749, 1716 (C@O), 1631 (C@C);
1H NMR (400.13 MHz, CDCl3) d 12.99 (br s, 1H, NH), 8.08 (m, 2H, CH-2,6 COPh),
7.68 (m, 2H, CH-2,6 Ph), 7.64 (m, 1H, CH-4 COPh), 7.58 (m, 2H, CH-3,5 COPh), 7.52
(dd, 3J = 4.1 Hz, 4J = 1.6 Hz, 1H, H-3), 7.46 (m, 2H, CH-3,5 Ph), 6.90 (dd, 3J = 4.1 Hz,

B. A. Trofimov et al. / Tetrahedron Letters 51 (2010) 5028–5031
5031
4J = 2.4 Hz, 1H, H-4), 4.54 (q, J = 7.0 Hz, 2H, OCH2), 1.44 (t, J = 7.0 Hz, 3H, Me); 13
C
(or from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223 336
033; or deposit@ccdc.cam.ac.uk). Any request to the CCDC for data should
quote the full literature citation and CCDC reference numbers 776907 and
776908 for 3e and 4e, respectively.
NMR (100.6 MHz, CDCl3) d 187.8, 183.7, 160.5, 148.8, 144.6, 141.9, 141.4, 136.2,
135.8, 133.7, 129.8, 129.1, 128.6, 128.1, 126.7, 125.5, 123.8, 115.7, 113.0, 112.0,
111.8, 64.0, 53.2, 48.3, 14.1. Anal. Calcd. for C29H17Cl2N3O4: C, 64.22; H, 3.16; Cl,
13.07; N, 7.75. Found: C, 64.29; H, 3.30; Cl, 13.38; N, 7.58.
ꢀ
Crystal and experimental data for 3e: C23H20ClN3O5, M = 453.87, triclinic, P1,
Ethyl
7-(4-nitrobenzoyl-3-chloro-6-[5-(4-methoxyphenyl)-1H-pyrrol-2-yl]-1,5-
a = 8.211(1) Å, b = 11.909(1) Å, c = 12.942(1) Å,
= 106.17(2)°, V = 1123.5(2) Å3, Z = 2, Dcalcd = 1.34 g cmÀ3
reflections observed/independent 8229/4225, 290 parameters refined,
R = 0.060 for 1731 reflections with [F0 >4 (F0)].
a
= 108.80(2)°, b = 96.10(2)°,
dicyano-4-oxobicyclo[3.2.0]hepta-2,6-diene-2-carboxylate (3c). Cherry red
crystals; mp: 279–280 °C; IR (KBr, cmÀ1) 3449 (NH), 2248 (CN), 1746, 1716
(C@O), 1626 (C@C); 1H NMR (400.13 MHz, CDCl3) d 12.94 (br s, 1H, NH), 8.41 (m,
2H, CH-2,6 4-NO2C6H4), 8.15 (m, 2H, CH-2,6 4-MeOC6H4), 7.74 (m, 2H, CH-3,4 4-
NO2C6H4), 7.63 (dd, 3J = 4.1 Hz, 4J = 1.6 Hz, 1H, H-3), 7.03 (m, 2H, CH-3,4 4-
MeOC6H4), 6.91 (dd, 3J = 4.1 Hz, 4J = 2.4 Hz, 1H, H-4), 4.51 (q, J = 7.1 Hz, 2H,
OCH2), 3.88 (s, 3H, MeO), 1.44 (t, J = 7.1 Hz, 3H, Me); 13C NMR (100.6 MHz, CDCl6)
d 185.7, 183.7, 161.6, 159.9, 153.3, 149.9, 144.8, 142.4, 141.9, 129.1, 127.4, 123.8,
121.8, 121.5, 121.3, 120.5, 115.3, 115.1, 113.2, 112.8, 111.7, 63.9, 55.5, 53.4, 48.7,
14.2. Anal. Calcd. for C30H19ClN4O7: C, 61.81; H, 3.29; Cl, 6.08; N, 9.61. Found: C,
61.59; H, 3.30; Cl, 5.88; N, 9.48.
Ethyl 6-benzoyl-3-chloro-1,5-dicyano-4-oxo-7-(4,5,6,7-tetrahydro-1H-indol-2-yl)
bicyclo[3.2.0]-hepta-2,6-diene-2-carboxylate (3d). Red crystals; mp: 233–235 °C;
IR (KBr, cmÀ1): 3459 (NH), 2243 (CN), 1743, 1719 (C@O), 1628 (C@C); 1H NMR
(400.13 MHz, CDCl3) d 12.17 (br s, 1H, NH), 8.04 (m, 2H, CH-2,6 Ph), 7.64 (m, 1H,
CH-4 Ph), 7.58 (m, 2H, CH-3,5 Ph), 7.20 (d, 3J = 1.9 Hz, 1H, H-3), 4.53 (q, 2H,
J = 7.2 Hz, OCH2), 2.81 (m, 2H, CH2-7), 2.65 (m, 2H, CH2-4), 1.90 (m, 2H, CH2-5),
1.83 (m, 2H, CH2-6), 1.48 (t, 3H, J = 7.2 Hz, CH3); 13C NMR (100.6 MHz, CDCl3) d
187.3, 183.8, 160.5, 148.2, 144.7, 143.4, 141.7, 136.6, 133.2, 128.9, 128.3, 126.5,
123.6, 120.7, 112.5, 112.3, 112.2, 63.8, 53.1, 48.2, 24.1, 23.1, 23.0, 22.4, 14.0. Anal.
Calcd. for C27H20ClN3O4: C, 66.74; H, 4.15; Cl, 7.30; N, 8.65. Found: C, 66.63; H,
4.50; Cl, 7.59; N, 8.79.
Diethyl 3-chloro-1,5-dicyano-4-oxo-7-(4,5,6,7-tetrahydro-1H-indol-2-yl)bicyclo
[3.2.0]-hepta-2,6-diene-2,6-dicarboxylate (3e). Red crystals, mp 230–231 °C. IR
(KBr, cmÀ1): 3300 (NH), 2248 (CN), 1755, 1716, 1685 (CO), 1615 (C@C); 1H NMR
(400.13 MHz, CDCl3) d 10.87 (br s, 1H, NH), 7.00 (d, 3J = 1.9 Hz, 1H, H-3), 4.48 (q,
2H, J = 7.1 Hz, CH2OC(O)C-6), 4.32 (m, 2H, CH2OC(O)C-2), 2.68 (m, 2H, CH2-7),
2.55 (m, 2H, CH2-4), 1.82 (m, 2H, CH2-5), 1.75 (m, 2H, CH2-6), 1.43 (t, J = 7.1 Hz,
3H, CH3), 1.38 (t, J = 7.1 Hz, 3H, CH3); 13C NMR (100.6 MHz, CDCl3) d 182.9, 162.8,
160.5, 148.2, 144.2, 142.0, 141.8, 123.9, 121.5, 119.2, 111.9, 111.2, 105.7, 63.8,
62.2, 52.0, 47.4, 23.8, 23.2, 22.8, 22.6, 14.2, 14.0. Anal. Calcd. for C23H20ClN3O5: C,
60.86; H, 4.44; Cl, 7.81; N, 9.26. Found: C, 60.53; H, 4.55; Cl, 8.00; N, 9.12.
c
,
l
= 0.209 mmÀ1
,
r
Crystal and experimental data for 4e: C23H21Cl2N3O5, M = 490.33, monoclinic, P21,
a = 8.213(2) Å, b = 7.710(2) Å, c = 18.425(5) Å, b = 91.22(3)°, V = 1166.4(5) Å3,
Z = 2, Dcalcd = 1.40 g cmÀ3
,
l
= 0.318 mmÀ1
,
reflections observed/independent
2353/2184, 300 parameters refined, R = 0.079 for 1181 reflections with
[F0 >4 (F0)].
r
Crystals of bicyclo[3.2.0]heptadienone 3e form with one crystallographically
independent molecule C23H20ClN3O5 (Fig. 1) taking a general position. The
dihedral angle between the planes of the cyclobutene and cyclopentene rings is
115.6°. The cyclobutene fragment is almost planar, maximum atom deviation
from the average plane does not exceed 0.02 Å. The C–C bond length in the
cyclobutene ring is 1.58(8) Å. The dihedral angles formed by the cyclobutene and
cyano substituents are 126.8° and 109.0°, respectively. The angle between the
cyclobutene moiety and the average plane of the ester unit is 174.5°. The pyrrole
fragmenthasanalmostplanarstructure, maximumatomdeviationfromtheplane
being 0.005 Å. The dihedral angle between the cyclobutene and pyrrole planes is
170.8°.
The crystal structure of dihydrofuranone 4e forms with one crystallographically
independent molecule C23H21Cl2N3O5 (Fig. 2), taking a general position. The
cyclobutene fragment is almost planar, maximum atom deviation from the
averaged plane did not exceed 0.002 Å. In the fragment, one of a longest ordinary
C–C bond 1.62(1) Å was detected. The dihedral angles formed by the cyclobutene
plane and cyano substituents are 53.4° and 71.1°, respectively. The angle between
the cyclobutene moiety and the average plane of the ester unit is 172.0°. The
pyrrolefragmenthasanalmostplanar structure, the maximumdeviationof atoms
from its plane being 0.01 Å. The dihedral angle between the cyclobutene and
pyrrole planes equals 173.4°. The dihydrofuranone substituent is also almost
planar, the maximum deviation of atoms from its plane being 0.01 Å. The dihedral
angle formed by this fragment with the cyclobutene plane is 63.1°. The angle
between the ethoxy group and the furan fragment is 82.5°.
Ethyl
3,4-dicyano-4-(3,4-dichloro-2-ethoxy-5-oxo-2,5-dihydro-2-furanyl)-2-
5. Pappas, S. P.; Pappas, B. S.; Portnoy, N. A. J. Org. Chem. 1969, 34, 520–525.
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Bagryanskaya, I. Y.; Gatilov, Y. V.; Osadchii, S. A. Russ. Zh. Strukt. Khim. 1984,
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3405; (d) Yeh, W.-Y.; Liu, Y.-C.; Peng, S.-M.; Lee, G.-H. Organometallics 2003, 22,
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(4,5,6,7-tetrahydro-1H-indol-2-yl)-1-cyclobutene-1-carboxylate (4e). Yellow
crystals; mp: 211–212 °C; IR (KBr, cmÀ1): 3285 (NH), 2248 (CN), 1807 (CO),
1686 (CO(O)), 1619 (C@C). 1H NMR (400.13 MHz, CDCl3) d 10.71 (br s, 1H, NH),
6.60 (d, J = 2.0 Hz, 1H, H-3), 4.36 (s, 1H, CH-CN), 4.31 (m, 1H, CH2C@O), 4.21 (m,
1H, CH2C@O), 3.58 (m, 2H, OCH2), 2.66 (m, 2H, CH2-7), 2.53 (m, 2H, CH2-4), 1.82
(m, 2H, CH2-5), 1.75 (m, 2H, CH2-6), 1.35 (t, J = 7.1 Hz, 3H, CH3), 1.33 (t, J = 7.1 Hz,
3H, CH3); 13C NMR (100.6 MHz, CDCl3) d 162.2, 160.9, 148.5, 144.8, 140.2, 125.0,
123.2, 121.9, 117.4, 114.2, 114.0, 106.0, 104.9, 62.0, 61.9, 50.6, 34.6, 23.6, 23.2,
22.7, 22.6, 14.8, 14.4. Anal. Calcd. for C23H21Cl2N3O5: C, 56.34; H, 4.32; Cl, 14.46;
N, 8.57. Found: C, 56.74; H, 4.34; Cl, 14.53; N, 8.35.
4. The X-Ray diffraction study of 3e was carried out using a Bruker SMART APEX2
CCD diffractometer at room temperature (Mo K
of 4e was carried out with an Enraf-Nonius CAD-4 diffractometer at room
temperature /2h-scanning, Mo radiation, graphite monochromator).
a radiation). X-Ray diffraction
(
x
Ka
Crystalline structures of 3e and 4a were solved by direct methods followed
by Fourier synthesis using SHELXS-97.10 All non-hydrogen atoms were refined
using anisotropic full-matrix approximation using SHELXL-97.10 The coordinates
of the hydrogen atoms were calculated. Atom coordinates, bond lengths, and
angle values were deposited at Cambridge Crystallographic Data Centra
(CCDC). These data are available via www.ccdc.cam.uk/conts/retrieving.html
8. (a) Baudouy, R.; Crabbe, P.; Greene, A. E.; Le Drain, C.; Orr, A. F. Tetrahedron Lett.
1977, 34, 2973–2976; (b) Gibson, S. E.; Mainolfi, N.; Kalindijan, S. B.; Wright, P.
T. Angew. Chem., Int. Ed. 2004, 43, 5680–5682.
9. Pandya, B. A. Boston College Dissertations and Theses. Paper AAI3283895,
2007.
10. Sheldrick, G. M. SHELXS 97, SHELXL 97; University of Gottingen: Germany, 1997.