Studies on the Dimerization of 2-Benzylidene-1-indanone

Article abstract of DOI:10.1021/jo970402y

It has been observed that 2-(-E)-benzylidene-1-indanone (1) undergoes dimerization under basic conditions. The reaction is highly stereoselective and provides almost exclusively dimer 2b using NaHCO₃/DMF, guanidine carbonate/DMF, or Cs₂CO₃/CH₃CN. The structure and the relative stereochemistry of compound 2b were initially established on the basis of COSY, HMQC, HMBC, and NOESY NMR correlation techniques. The structure and the stereochemistry were then confirmed by X-ray crystallographic analysis. Two other stereoisomers were obtained, in minor proportions, by varying the experimental conditions. A fourth isomer was also produced using 2-(Z)-benzylidene-1-indanone as the starting material.

Full text of DOI:10.1021/jo970402y

J . Org. Chem. 1997, 62, 4339-4342
4339
Stu d ies on th e Dim er iza tion of 2-Ben zylid en e-1-in d a n on e
Carl Berthelette, Chris McCooye, Yves Leblanc,* and Laird A. Trimble*
Merck Frosst Centre for Therapeutic Research, P.O. Box 1005,
Pointe Claire-Dorval, Quebec, Canada H9R 4P8
Nancy N. Tsou
Merck Research Laboratories, Department of Molecular Design and Diversity,
P.O. Box 2000 (RY50-105), Rahway, New J ersey 07065-0900
Received March 4, 1997X
It has been observed that 2-(E)-benzylidene-1-indanone (1) undergoes dimerization under basic
conditions. The reaction is highly stereoselective and provides almost exclusively dimer 2b using
NaHCO /DMF, guanidine carbonate/DMF, or Cs CO /CH CN. The structure and the relative
3 2 3 3
stereochemistry of compound 2b were initially established on the basis of COSY, HMQC, HMBC,
and NOESY NMR correlation techniques. The structure and the stereochemistry were then
confirmed by X-ray crystallographic analysis. Two other stereoisomers were obtained, in minor
proportions, by varying the experimental conditions. A fourth isomer was also produced using
2
-(Z)-benzylidene-1-indanone as the starting material.
In tr od u ction
Sch em e 1
Vinylic substitution reactions on an enone functionality
by an aryl group catalyzed by palladium are well docu-
1
mented in the literature. Generally, these reactions are
performed in DMF in the presence of a base and an
ammonium salt such as nBu NCl. While attempting to
4
perform such a substitution on 2-(E)-benzylidene-1-
indanone (1), using the preceding protocol, we observed
the exclusive formation of an unexpected dimer (2)
(Scheme 1). Fascinated by this dimerization in which
five contiguous stereogenic centers are created from an
achiral molecule, we decided to explore the stereochem-
ical outcome of this reaction.
Ta ble 1. Rea ction Con d ition s a n d Isola ted Yield s for th e
Rea ction of 1 to Give th e Va r iou s Isom er s of Dim er 2
yield, %a
Resu lts a n d Discu ssion
entry
conditions
2a
2b
2c
Initial studies showed that the palladium salt is not
necessary for this dimerization to occur since the reaction
takes place efficiently in the absence of palladium acetate
as has been previously demonstrated.2 Thus, 2-(E)-
benzylidene-1-indanone (1) was subjected to varying basic
conditions (Table 1) in order to evaluate the isomeric
distribution of dimer 2 (Scheme 2). In this study, only
three of the sixteen possible diastereoisomers of dimer 2
were observed, and each was isolated and fully charac-
terized. Their relative stereochemistry was established
by NMR techniques. As exemplified in Table 1, dimer 2
was isolated in moderate (entries 1, 2, and 3) to high
yields (entry 5) as experimental conditions were changed.
However, in all cases, isomer 2b was predominantly
formed in contrast with a previous report.2 Using
b
1
2
DMF/NaHCO3/100 °C/2 days
DMF/guanidine carbonate/
5
0
69
64
0
0
c
d
1
00 °C/18 h
3
4
5
PrOH/NaOPr/urea/100 °C/5 h
PrOH/NaOPr/urea/100 °C/2 h
CH3CN/Cs2CO3/room
12
-
0
56
-
93
0
10
4
e
e
f
temperature/18 h
a
Isolated yields. b Crude NMR analysis shows traces of isomer
c
d
2
3
c = 2%. Crude NMR shows 2% of isomer 2a . Crude NMR after
h shows 2% of isomer 2c. e Not isolated. Experimental and
purification conditions optimized in order to isolate isomer 2c.
f
Isomer 2c was isolated as a mixture with isomer 2b.
Furthermore, 2-(Z)-benzylidene-1-indanone (3) was
prepared using a modified procedure of George and
subjected to guanidine/DMF conditions. A new diaste-
reoisomer (2d ) was thus obtained in 15% yield presum-
ably by the dimerization of two Z isomer molecules
4
3
NaHCO /DMF as a base (Table 1, entry 1) it was possible
to isolate a second stereoisomer (2a ) in 5% yield. A third
3
isomer (2c) was also produced in 10% yield using PrOH/
(
Scheme 3). Isomer 2b was also produced in 26% yield
2
NaOPr/urea conditions (Table 1, entry 4). When cesium
in this reaction. Since 2-(Z)-benzylidene-1-indanone (3)
is converted to the E isomer (1) under these experimental
conditions, the new isomer 2d could also be derived from
the dimerization of one Z monomer with one E monomer.
It is likely that dimer 2b was produced via the condensa-
tion of two molecules having the E stereochemistry.
carbonate was used as the base (Table 1, entry 5) dimer
2
b was formed in high yield.
X
Abstract published in Advance ACS Abstracts, J une 15, 1997.
(
1) See: Amoress, A.; Arcadi, A.; Bernocchi, E.; Cacchi, S.; Cerrini,
S.; Fedeli, W.; Ortar, G. Tetrahedron 1988, 45, 813 and cited references.
2) Wendelin, W.; Schermanz, K.; Britmaier, E. Monatsh. Chem.
988, 119, 355.
3) The isomer 2c was found to be unstable under the NaOPr/urea/
PrOH conditions. After 5 h at 100 °C isomer 2c was not detected.
(
1
(
(4) George, H.; Roth, H. J . Tetrahedron Lett. 1971, 43, 4057.
(5) Derome, A.; Williamson, M. J . Magn. Reson. 1990, 88, 177.
S0022-3263(97)00402-7 CCC: $14.00 © 1997 American Chemical Society

4
340 J . Org. Chem., Vol. 62, No. 13, 1997
Berthelette et al.
Sch em e 2
NOESY spectrum containing the aromatic signals, strong
NOE cross peaks between H-3 and H-12′ and between
H-2 and H-12 are found. Only the stereochemistry
shown for isomer 2b in Scheme 2 satisfies all of the
observed NOEs. Thus, in this particular isomer, protons
H-2, H-3, and H-3′ and the two phenyl rings are on the
same side of the fused ring system.
Dimer 2b was then submitted for X-ray crystal-
lographic analysis in order to confirm that the relative
stereochemistry derived by NMR was correct. The solu-
8
tion to the X-ray crystal structure of 2b is shown in
Figure 1 and is consistent with the derived NMR struc-
ture.9 As a result, the configuration of the other isomers
of 2 were determined by NMR only.
The structures and relative stereochemistries of iso-
mers 2a and 2d were determined by NMR in a fashion
similar to that described for dimer 2b. Analysis of dimer
2
c proved to be problematic due to extensive line broad-
ening of several of the key proton and carbon signals as
a result of chemical exchange. Cooling the sample in
acetone-d to -53 °C caused the lines to sharpen suf-
6
ficiently to determine the structure and relative stereo-
chemistry of 2c to be that shown in Scheme 2. Interest-
ingly, it is clear that the chemical exchange phenomenon
results from restricted rotation about the C-10′, C-11′
bond. At -53 °C the rate of rotation about this bond is
slow enough to destroy the chemical equivalency of the
two H-12′ protons and the two H-13′ protons. As a result,
unique signals are observed for each of the five protons
on this aromatic ring.
Sch em e 3
A potential mechanism for the formation of dimer 2 is
shown in Scheme 4. The first step of the reaction is the
abstraction of one of the methylene protons of 1 to give
a cross-conjugated enolate. This enolate can then per-
form an intermolecular Michael addition on a second
molecule of monomer 1. The newly generated enolate is
then free to undergo an intramolecular Michael addition
to generate the desired ring system which when proto-
nated produces dimer 2.
In conclusion, we have shown that the base-catalyzed
dimerization of 2-(E)-benzylidene-1-indanone (1) provides
mainly dimer 2b. Modification of the reaction conditions
can produce two other dimers, 2a and 2c, in minor
amounts. A fourth dimer is formed in low yield when
The structure of dimer product 2 was determined by
5
NMR using a combination of DQF-COSY, NOESY,
6
7
HMQC, and HMBC techniques. From the 1D proton
and carbon spectra, a doubling of the expected number
of peaks was observed, indicating that a dimer was the
likely product. The protons were assigned using DQF-
COSY and NOESY data, and protonated carbons were
assigned with the HMQC experiment. All quaternery
carbons were then identified from the HMBC data.
Careful examination of the HMBC spectrum showed
several extra cross peaks in addition to those expected
for the monomers. Long-range correlations from H-3 to
C-10′, from H-10 to C-1′, C-2′, and C-3′, from H-10′ to
C-3 and C-4, and from H-3′ to C-10 are only consistent
with the spiro-fused ring system shown in the schemes.
2
-(Z)-benzylidene-1-indanone (3) is used as a starting
material. The NMR techniques used were sufficient for
the determination of the relative stereochemistry of the
dimers as was demonstrated with dimer 2b where the
X-ray structure is in complete agreement with that
obtained from the NMR analysis. The relative stereo-
chemistry of the major dimer 2b under all reaction
conditions tried was different than that obtained in the
The relative stereochemistry of each dimer could then
be determined from the NOESY experiment. For ex-
ample, in the case of isomer 2b, the relative stereochem-
istry of the H-2 and H-3 protons is likely cis due to the
difficulty in forming a trans-fused 5,5 ring system. This
stereochemistry is confirmed by the observation of an
intense NOE cross peak between H-2 and H-3 in the
NOESY spectrum. Cross peaks are also observed be-
tween H-3 and one of the diastereotopic methylene
protons at H-3′ and between H-2 and the other diaste-
reotopic methylene proton at H-3′. Therefore the H-3′
methylene group must be on the same side of the
molecule as are protons H-2 and H-3. The observation
of a cross peak between H-10 and H-10′ indicates that
these two protons are on the same side of the five-
membered ring. Upon examination of the region of the
2,10
two previous studies.
Exp er im en ta l Section
Gen er a l. Melting points (uncorrected) were determined in
an open capillary tube. All reactions were performed in the
dark to avoid olefin isomerization. Proton and carbon NMR
1
spectra were recorded at 500 and 125 MHz, respectively.
H
(
(
8) Manuscript in preparation.
9) During the period of peer review of the manuscript, a report
10
appeared which also describes the dimerization of 1. However, the
reported dimer which is spectroscopically identical to dimer 2b and
has the same melting point as 2b has been assigned a relative
stereochemistry that disagrees with that described in this manuscript.
(10) Houlihan, W. J .; Shapiro, M. J .; Chin, J . A. J . Org. Chem. 1997,
62, 1529.
(
(
6) Bax, A.; Subramanian, S. J . Magn. Reson. 1986, 67, 565.
7) Bax, A.; Summers, M. F. J . Am. Chem. Soc. 1986, 108, 2093.

Dimerization of 2-Benzylidene-1-indanone
J . Org. Chem., Vol. 62, No. 13, 1997 4341
F igu r e 1. Stereoview of the ORTEP representation of 2b. Non-hydrogen atoms are represented by ellipsoids corresponding to
2
0% probability. Hydrogen atoms are omitted for clarity.
Sch em e 4
(d, J ) 9.7 Hz, 1H), 2.79 (d, J ) 17.6 Hz, 1H), 2.71 (d, J )
1
2
1
1
7.6 Hz, 1H); 13C NMR (125 MHz, CDCl
3
, 295 K) δ 209.0,
07.0, 154,7, 152.1, 139.8, 137.6, 137.2, 136.2, 134.7, 134.2,
30.2, 128.7, 128.2, 128.1, 128.0, 127.4, 127.2, 127.2, 126.6,
25.7, 123.7, 123.7, 69.4, 56.9, 56.6, 53.6, 49.1, 36.6; HRMS
+
calcd for C32
Calcd for C32
.35. Dimer 2b: mp 225-226 °C (EtOH); H NMR (500 MHz,
CDCl , 295 K) δ 7.75 (d, J ) 7.5 Hz, 1H), 7.52 (d, J ) 7.8 Hz,
H), 7.43 (td, J ) 7.3, 1.4 Hz, 1H), 7.38 (t, J ) 7.3 Hz, 1H),
H
H
24
O
24
2
[M + H] 441.1855, found 441.1856. Anal.
O : C, 87.25; H, 5.49. Found: C, 87.00; H,
2
1
5
3
1
7
8
4
.28 (d, J ) 7.5 Hz, 2H), 7.25-7.19 (m, 3H), 7.19-7.02 (m,
H), 6.93 (d, J ) 7.8 Hz, 1H), 4.56 (dd, J ) 10.5, 8.7 Hz, 1H),
.08 (d, J ) 10.7 Hz, 1H), 3.92 (dd, J ) 10.7, 8.5 Hz, 1H), 3.82
(d, J ) 10.7 Hz, 1H), 3.05 (d, J ) 17.1 Hz, 1H), 2.98 (d, J )
1
2
1
1
7.1 Hz, 1H); 13C NMR (125 MHz, CDCl
3
, 295 K) δ 207.7,
05.6, 155.8, 152.8, 137.2, 136.7, 136.5, 135.8, 135.2, 134.7,
28.5, 128.3, 128.3, 128.2, 128.2, 127.2, 127.0, 127.0, 125.7,
25.3, 124.5, 123.4, 70.2, 59.2, 54.1, 53.0, 46.0, 29.6; HRMS
+
calcd for C32
Calcd for C32
.29.
Ta ble 1, En tr y 2 Rea ction Con d ition s. To a solution of
H
24
O
2
[M + H] 441.1855, found 441.1856. Anal.
H
24
2
O : C, 87.25; H, 5.49. Found: C, 87.70; H,
5
the monomer 1 (0.500 g, 2.27 mmol) in DMF (7 mL) was added
guanidine carbonate (0.205 g , 1.2 mmol). The resulting
mixture was heated at 100 °C for a period of 18 h. After the
2
solution was cooled to rt, H O (4 mL) was added followed by
concentrated HCl until the solution reached a pH of 6. The
solid was collected by vacuum filtration to yield isomer 2b
(0.339 g, 68%).
chemical shifts (δ) were reported in ppm relative to the
residual solvent lines of CHCl and acetone-d at 7.24 and 2.04
ppm, respectively, and C chemical shifts were reported
3
5
1
3
relative to the solvent lines of CDCl
and 29.8 ppm, respectively.
3
and acetone-d
6
at 77.0
Ta ble 1, En tr y 3 Rea ction Con d ition s. Sodium (0.250
g, 1.09 mmol) was dissolved in n-propanol (7.0 mL). Thiourea
(0.173 g, 2.27 mmol) and monomer 1 (0.500 g, 2.27 mmol) were
then added, and the solution was heated at 100 °C. After a
period of 5 h, the solution was filtered and the filtrate was
allowed to cool to rt and then to 0 °C. The resulting precipitate
was collected by filtration and then purified by flash chroma-
tography (15% EtOAc in hexane) to provide 60 mg (12%) of
isomer 2a and 280 mg (56%) of isomer 2b.
2
-(E)-Ben zylid en e-1-in d a n on e (1). To a mixture of 1-in-
danone (2.00 g, 15.1 mmol) and benzaldehyde (1.72 g, 16.2
mmol) in EtOH (20 mL) was added concentrated HCl (100 µL).
The resulting mixture was heated at 80 °C for a period of 18
h. The reaction mixture was cooled to rt to give a yellow solid.
The solid was filtered and washed with EtOH to provide 2.15
1
g (65%) of the title compound: H NMR (500 MHz, acetone-d
6
,
2
2
95 K) δ 7.82-7.77 (m, 3H), 7.71-7.66 (m, 2H), 7.57 (t, J )
Ta ble 1, En tr y 4 Rea ction Con d ition s. The conditions
for Table 1, entry 3 were used except the reaction mixture was
heated at 100 °C for 2 h. The reaction mixture was then cooled
.2 Hz, 1H), 7.51 (t, J ) 7.4 Hz, 2H), 7.50-7.46 (m, 1H), 7.44
1
3
(
t, J ) 7.3 Hz, 1H), 4.13 (d, J ) 2.2 Hz, 2H); C NMR (125
MHz, acetone-d , 295 K) δ 193.5, 150.4, 138.2, 135.9, 135.7,
35.0, 133.1, 131.1, 130.0, 129.3, 128.0, 127.0, 124.0, 32.4;
6
to rt and partitioned between H
phase was dried over Na SO , filtered, and evaporated. The
2 2 2
O and CH Cl . The organic
1
2
4
+
HRMS calcd for C16
H
13O [M + H] 221.0966, found 221.0966.
resulting residue was purified by flash chromatography (6%
EtOAc in toluene) to provide 103 mg of impure isomer 2c from
0.480 g of monomer 1. The dimer 2c was then recrystallized
Ta ble 1, En tr y 1 Rea ction Con d ition s. To a solution of
-(E)-benzylidene-1-indanone (1) (1.00 g, 4.54 mmol) in DMF
2
(
62 mL) was added an excess of NaHCO
resulting mixture was heated at 100 °C. After a period of 2
days, the reaction mixture was partitioned between CH Cl
and H O. The organic phase was dried over Na SO , filtered,
3
(1.00 g). The
in EtOH to provide 46 mg (10%) of pure material: mp 248-
1
250 °C (EtOH); H NMR (500 MHz, acetone-d
6
, 220 K) δ 7.78
2
2
(d, J ) 7.3 Hz, 1H), 7.51 (d, J ) 7.5 Hz, 2H), 7.47-7.39 (m,
4H), 7.39-7.30 (m, 3H), 7.29-7.21 (m, 3H), 7.16 (td, J ) 7.3,
0.9 Hz, 1H), 7.02 (t, J ) 7.4 Hz, 1H), 6.77 (td, J ) 7.8, 1.1 Hz,
1H), 6.52 (d, J ) 7.6 Hz, 1H), 6.23 (d, J ) 8.2 Hz, 1H), 4.74
(dd, J ) 10.1, 8.4 Hz, 1H), 4.15 (d, J ) 10.5 Hz, 1H), 3.96 (d,
J ) 3.4 Hz, 1H), 3.82 (dd, J ) 8.1, 3.4 Hz, 1H), 2.91 (d, J )
2
2
4
and evaporated under reduced pressure. The residue was
purified by flash chromatography (15% EtOAc in hexane) to
yield 690 mg (69%) of dimer 2b and 50 mg (5%) of dimer 2a .
1
Dimer 2a : mp 246-248 °C (EtOH); H NMR (500 MHz, CDCl
3
,
1
3
2
7
7
4
95 K) δ 7.69 (m, 1H), 7.52 (d, J ) 7.8 Hz, 1H), 7.34 (td, J )
17.8 Hz, 1H), 2.68 (d, J ) 17.8 Hz, 1H); C NMR (125 MHz,
acetone-d , 220 K) δ 208.1, 206.0, 153.9, 152.2, 143.0, 139.3,
.5, 1.2 Hz, 1H), 7.27-7.21 (m, 4H), 7.16 (t, J ) 7.6 Hz, 1H),
.14-7.01 (m, 7H), 6.78-6.70 (m, 3H), 5.11 (t, J ) 9.2 Hz, 1H),
.17 (dd, J ) 9.5, 8.5 Hz, 1H), 4.06 (d, J ) 10.0 Hz, 1H), 3.72
6
136.3, 136.2, 135.0, 132.9, 132.0, 130.4, 129.1, 128.9, 128.8,
128.1, 127.6, 127.5, 127.1, 127.1, 126.8, 126.3, 123.3, 123.0,

4
342 J . Org. Chem., Vol. 62, No. 13, 1997
Berthelette et al.
6
+
6.2, 58.9, 57.7, 71.8, 48.4, 36.6; HRMS calcd for C32
H] 441.1855, found 441.1856.
H
24
O
2
[M
The reaction was cooled to rt and quenched with the addition
of water. The reaction mixture was extracted with ethyl
acetate, and the organic phase was separated, dried over
Na SO , filtered, and evaporated under reduced pressure. The
2 4
residue was then purified by flash chromatography (15%
+
Ta ble 1, En tr y 5 Rea ction Con d ition s. To a solution of
monomer 1 (0.200 g, 0.908 mmol) in CH CN (3 mL) was added
Cs CO (0.775 g, 2.40 mmol). The solution was stirred at rt
for 18 h. Crude H NMR analysis of a sample extracted with
O-EtOAc showed a 5/1 ratio of isomer 2b and 2c. The
reaction mixture was partitioned between H O and CH Cl
The organic phase was separated, dried over Na SO , filtered,
3
2
3
1
EtOAc in hexane) to provide 70 mg (26%) of isomer 2b and 40
1
H
2
mg (15%) of isomer 2d : mp 225-227 °C (EtOH); H NMR (500
2
2
2
.
MHz, CDCl , 295 K) δ 7.62 (d, J ) 7.5 Hz, 1H), 7.55 (d, J )
3
2
4
7.7 Hz, 1H), 7.44 (td, J ) 7.5, 1.3 Hz, 1H), 7.36 (t, J ) 7.3 Hz,
1H), 7.32-7.26 (m, 3H), 7.23-7.11 (m, 6H), 7.08 (t, J ) 7.1
Hz, 1H), 6.97 (d, J ) 7.5 Hz, 1H), 6.92 (d, J ) 7.7 Hz, 1H),
and evaporated under reduced pressure. The residue was
recrystallized from EtOH/hexane (2/1) to yield 166 mg (84%)
of isomer 2b. The filtrate was recrystallized to provide 27 mg
of a 5/1 mixture of isomers 2c and 2b.
6
)
1
1
1
1
1
.89-6.84 (m, 2H), 4.88 (dd, J ) 10.8, 8.6 Hz, 1H), 4.47 (dd, J
10.8, 8.4 Hz, 1H), 3.91 (d, J ) 10.8 Hz, 1H), 3.59 (d, J )
0.8 Hz, 1H), 2.67 (d, J ) 17.8 Hz, 1H), 2.60 (d, J ) 17.8 Hz,
2
-(Z)-Ben zylid en e-1-in d a n on e (3). 2-(E)-Benzylidene-1-
indanone (0.500 g, 2.27 mmol) was dissolved in CH CN (300
3
13
H); C NMR (125 MHz, CDCl , 295 K) δ 209.6, 206.3, 155.4,
3
mL) and irradiated with a UV mercury lamp for 4 h while
stirring at 10 °C. The solvent was removed under reduced
pressure, and the resulting solid was purified by flash chro-
matography (10% EtOAc in hexane) to provide 336 mg (67%)
of the title compound: mp 82-86 °C (EtOAc-hexane); lit. mp
8
7
52.2, 140.4, 137.9, 136.8, 136.2, 134.9, 134.7, 129.6, 129.0,
28.5, 128.4, 128.0, 127.5, 127.2, 127.1, 125.6, 125.5, 123.7,
23.6, 69.8, 59.3, 57.0, 53.0, 48.9, 35.3; HRMS calcd for
+
C
32
H
24
O
2
[M + H] 441.1855, found 441.1856.
4
1
8 °C; H NMR (500 MHz, acetone-d
6
, 295 K) δ 8.19 (dd, J )
Ack n ow led gm en t. The authors thank Ms. Chun Li
for providing mass spectral data.
.7, 2.2 Hz, 2H), 7.75 (d, J ) 7.5 Hz, 1H), 7.66 (td, J ) 7.5, 1.3
Hz, 1H), 7.59 (dt, J ) 7.7, 0.9 Hz, 1H), 7.45 (td, J ) 7.3, 0.9
Hz, 1H), 7.44-7.37 (m, 3H), 7.13 (t, J ) 1.9 Hz, 1H), 3.96 (t,
1
3
J ) 1.5 Hz, 2H); C NMR (125 MHz, acetone-d
6
, 295 K) δ
Su p p or tin g In for m a tion Ava ila ble: NMR spectra (10
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
1
1
+
92.2, 149.8, 141.1, 139.5, 136.0, 135.9, 135.1, 131.9, 130.3,
28.7, 128.2, 127.0, 124.4, 35.7; HRMS calcd for C16
H
13O [M
+
H] 221.0966, found 221.0966.
Isom er 2d . To a solution of monomer 3 (0.270 g, 1.23
mmol) in DMF (1 mL) was added guanidine carbonate (0.110
g, 0.61 mmol). The solution was stirred at 100 °C for 1.5 h.
J O970402Y