Synthesis of substituted phenanthrenes via intramolecular condensation based on temperature-dependent deprotonation using a weak carbonate base

Article abstract of DOI:10.1055/s-2006-956451

Construction of substituted phenanthrenes via intramolecular condensation of 2′-methylbiphenyl-2-carbaldehydes using a mild base at 200 °C is described. The required high temperature can be quickly reached and easily maintained using microwave flash heating. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-956451

LETTER
3225
Synthesis of Substituted Phenanthrenes via Intramolecular Condensation
Based on Temperature-Dependent Deprotonation Using a Weak Carbonate
Base
a
a
b
b
a
S
K
ynthesis of Subst
a
itute
d
Phen
t
anthren
r
es via In
i
tramol
e
ecular Cond
n
ensation Monsieurs, Geert Rombouts, Pál Tapolcsányi, Péter Mátyus,* Bert U. W. Maes*
a
Department of Chemistry, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium
Fax +32(3)2653233; E-mail: bert.maes@ua.ac.be
b
Department of Organic Chemistry, Semmelweis University, Högyes E. u. 7., 1092 Budapest, Hungary
Fax +36(1)217085; E-mail: matypet@szerves.sote.hu
Received 28 June 2006
This paper is dedicated to Professor Miha Tišler on the occasion of his 80 birthday.
th
was achieved by Directed remote Metalation (DreM) in
which the deprotonation of the 2¢-methyl hydrogen by
lithium diisopropylamide initiates the mechanism.2
Abstract: Construction of substituted phenanthrenes via intramo-
lecular condensation of 2¢-methylbiphenyl-2-carbaldehydes using a
mild base at 200 °C is described. The required high temperature can
0,21
be quickly reached and easily maintained using microwave flash The resulting 9-phenanthrols could be easily converted
heating.
into the corresponding phenanthrenes. Secondly, de
Koning and co-workers adopted a comparable strategy,
also starting with a Suzuki cross-coupling reaction for the
synthesis of the starting material, i.e. a 2¢-methylbiphenyl-
Key words: phenanthrene, Suzuki reaction, condensation, carbon-
ate base, microwave irradiation, 11H-benzo[a]carbazole
2
-carbaldehyde. This substrate type undergoes a ring-clo-
sure reaction when treated with potassium tert-butoxide
Within the chemical class of polycyclic aromatic com-
pounds phenanthrene is an important core structure. The
phenanthrene moiety can not only be found in several
natural products, of which many exhibit interesting bio-
under simultaneous irradiation with a 400 watt high-pres-
2
2
sure mercury lamp. The suggested mechanisms for this
cyclization reaction include a photochemical pathway
based on photoenolization and a simultaneous anionic
mechanism starting with deprotonation of the methyl sub-
stituent. Finally, Kraus and coworkers recently used a
similar ring-closure reaction for the synthesis of the natu-
1
logical activity, but more recently phenanthrenes have
also been shown as interesting ligands for novel catalyst
2
systems. Therefore, the development of new short syn-
thetic methods for this important class of organic com-
pounds has regained importance. Thanks to the progress
made during the past two decades in the field of biphenyl
synthesis by palladium-catalyzed carbon–carbon bond-
23
ral product Denbinobin. As in the case of de Koning, a
2
¢-methylbiphenyl-2-carbaldehyde served as the starting
compound, but in this case P -t-Bu was used as a base, and
4
3
,4
irradiation with light was omitted.
formation reactions, the synthesis of phenanthrenes
starting from biphenyls became a straightforward Clearly, the deprotonation of the methyl group is the cru-
approach. Formation of the C9–C10 bond can be carried cial step in the abovementioned reaction sequences. Not
out in multiple ways, depending on the substitution unexpectedly, all three research groups used a strong base
pattern of the biphenyl compound. Recently reported to accomplish this deprotonation since the pK value of
a
5,6
synthetic procedures include ring-closing metathesis,
the benzylic methyl protons is very high. However, the
or transition-metal-catalyzed use of strong (and often nucleophilic) bases imposes a se-
of 2-alkynylbiphenyl compounds, oxi- rious limitation on the functional group compatibility. We
7
–9
electrophile-induced
1
0,11
cyclization
dative coupling of in situ generated ylides, ring closure reasoned that the acidity should be greatly influenced by
1
2
1
3–15
24,25
via chromium–carbene complexes
catalyzed annulation reactions.
and palladium- the chosen solvent and reaction temperature.
The work of three over, even if the deprotonation equilibrium is still very
More-
1
6–18
research groups came into our specific attention due to the unfavorable, the occurring aromatization in the cycliza-
parallels with our ongoing research concerning the tion might still drive the reaction to completion.
synthesis of azapolycyclic aromatic compounds via a
Based on this argument, we decided to examine the
combination of a Suzuki and a subsequent ring-closure
possibility of using only a weak, non-nucleophilic base
1
9
reaction. Snieckus described a combination of his
Directed ortho-Metalation (DoM) methodology (aryl-
boronic acid preparation) with a Suzuki cross-coupling
reaction for the synthesis of a N,N-diethyl 2¢-methylbi-
phenyl-2-carboxamide substrate. Subsequent ring closure
for the ring closure of 2¢-methylbiphenyl-2-carbalde-
hydes. Since the need of applying a high temperature
24
could be foreseen (pK is temperature-dependent), the
a
use of microwave heating in a closed vessel was an ob-
vious choice. After all, microwave irradiation is known to
be highly efficient for rapid and safe heating of reaction
mixtures and temperatures far above the boiling point of
the solvent can be easily reached and maintained.2
Interestingly, to the best of our knowledge temperature
SYNLETT 2006, No. 19, pp 3225–3230
0
1
.1
2
.2
0
0
6
6,27
Advanced online publication: 23.11.2006
DOI: 10.1055/s-2006-956451; Art ID: G19406ST
©
Georg Thieme Verlag Stuttgart · New York

3
226
K. Monsieurs et al.
LETTER
dependence of pK , involving high-temperature chemis- Table 2 Effect of the Temperature on the Ring Closure of 2a29
a
try, has not yet been exploited synthetically.
CHO
For the first test and the subsequent optimization of the
reaction conditions unsubstituted 2¢-methylbiphenyl-2-
carbaldehyde was chosen as the substrate and cesium
carbonate was selected as the weak non-nucleophilic base.
The starting compound 2a could be easily prepared in ex-
cellent yield via Suzuki coupling of 2-bromotoluene with
4 equiv Cs CO
2
3
Solvent
µw (Mars)
Me
2a
3a
Solvent
Temp (°C)
150
Time (min) RP/SM
2
-formylphenylboronic acid under standard Gronowitz
DMFa
1
2
3
4
30
30
30
30
30
Only SM
Only SM
Only SM
0.35
2
8
reaction conditions. The subsequent ring-closure reac-
tion was carried out in an industrial multimode microwave
oven (Mars) with four equivalents of base. The crude re-
action mixtures were worked up by extraction and subse-
DMFa
160
DMFa
170
DMFa
180
1
quently analyzed by TLC and H NMR. In the cases where
the reaction did not reach completion, the ratio of reaction
product (RP) to starting material (SM) was deduced from
DMFa
5
200
3.19
a
1
Anhyd DMF was used
the H NMR spectrum.
In the first part of the optimization study different solvents
were tested. The results are summarized in Table 1. Tet- tion to a considerable extent, as can be seen from the
rahydrofuran, dioxane and 1,2-dimethoxyethane (Table 1, significantly improved reaction product/starting material
entries 1–3) did not give any conversion, while in dimeth- ratio (Table 2, entry 5).
yl sulfoxide or N-methyl-2-pyrrolidinone (Table 1, entries
Finally, the effect of some other weak bases on the ring-
4
and 5) the starting compound decomposed. Only when
closure reaction was tested. The results are summarized in
Table 3. The importance of the cation was investigated by
substituting cesium carbonate by potassium carbonate
N,N-dimethylformamide was chosen as the solvent, the
starting material was successfully converted into the de-
sired phenanthrene. A reaction time of 30 minutes turned
out to be insufficient for complete conversion (Table 1,
entry 6), but after 90 minutes all the starting compound
was consumed, and the product could be isolated in 54%
yield after column chromatography (Table 1, entry 7).
(
Table 3, entry 2). As could be expected from the HSAB
principle, the harder potassium counterion has a stronger
interaction with the hard carbonate base, making the latter
less ‘available’ for the deprotonation of the substrate.
Likewise, after deprotonation of the substrate, the potassi-
um counterion will be less dissociated from the formed
carbanion than the softer cesium ion. Therefore the nu-
cleophilic attack of the carbanion on the carbonyl group is
hampered when potassium carbonate is used instead of ce-
sium carbonate. This characteristic is reflected in the de-
creasing reaction product/starting material ratio. In line
with this observation, when sodium carbonate was used,
no conversion to the reaction product was observed
Table 1 Effect of the Solvent on the Ring Closure of 2a29
CHO
4
equiv Cs2CO3
Solvent
µw (Mars)
Me
2a
3a
Solvent
Temp Time RP/ Yield Comment
(
Table 3, entry 3).
(
°C)
(min) SM
(%)
1
2
3
4
5
6
THF
130
200
90
90
Only SM
Table 3 Effect of the Base on the Ring Closure of 2a29
CHO
Dioxane
DME
Only SM
4
equiv base
19330 30
Only SM
anhyd DMF
µw (Mars)
DMSO
200
200
200
200
30
30
30
90
Decomposition
Decomposition
Me
2a
3a
NMP
DMFa
Base
Cs CO
Temp (°C)
Time (min) RP/SM
3.19
DMFa
5431
1
2
3
4
5
200
200
200
200
30
30
30
30
30
3.19
7
SM was consumed
2
3
a
Anhyd DMF was used.
K
2
CO
3
0.05
Na CO
Only SM
Only SM
0.05
2
3
Secondly, the role of the reaction temperature in the ring-
closure reaction was examined (Table 2). The results
clearly show that the temperature must be raised to around
CsOAc
K CO + 18-crown-6 200
2
3
1
80 °C to get the reaction started (Table 2, entry 4). More-
over, heating the mixture to 200 °C accelerated the reac-
Synlett 2006, No. 19, 3225–3230 © Thieme Stuttgart · New York

LETTER
Synthesis of Substituted Phenanthrenes via Intramolecular Condensation
3227
3
2
Table 4 Synthesis of 2¢-Methylbiphenyl-2-carbaldehydes and 1-(2¢-Methylbiphenyl-2-yl)ethanone
OH
Ar2
B
OH
Ar1 Br
a–g
Ar1 Ar2
5
mol% Pd(PPh3)4
Na2CO3
1
2a–g
DME, H2O
reflux
Phenylbromide 1a–g
Boronic acid
Product 2a–g
Yield (%)
90%
1
Me
CHO
OHC
2
a
Br
B(OH)2
Me
2
3
4
5
6
7
Me
CHO
B(OH)2
OHC
Me
49%
2
b
MeO
Br
MeO
Me
CHO
B(OH)2
OHC
70%
2
c
NC
Et
Br
NC
Me
Et
Me
CHO
B(OH)2
OHC
80%
2d
N
O
O
N
Br
Et
Et
Me
OHC
Me
CHO
B(OH)2
72%
2
e
O2N
Br
O2N
Me
Me
OHC
CHO
Me
93%
2
f
Br
B(OH)2
Me
Me
COMe
Br
Me
MeOC
95%
2
g
B(OH)2
Me
The possibility of using an even weaker base was also ‘availability’ (basicity) of the carbonate ion or carbanion
examined. Therefore, the experiment was repeated with any further, thus emphasizing the importance of the sol-
four equivalents of cesium acetate (Table 3, entry 4), but vent. From the above described experiments it is clear that
no reaction product was formed after 30 minutes at 200 the best results for the ring-closure reaction were obtained
°C. As a final experiment in this set we tested if the basic with cesium carbonate as base in N,N-dimethylformamide
character of the anion could be enhanced by complexation at 200 °C.
of the counterion with a crown ether. 18-Crown-6 was
In the next part of the investigation the functional group
compatibility of the new reaction conditions was tested.
selected due to its efficiency in capturing potassium ions.
On comparing the ring-closure reaction using four equiv-
alents of potassium carbonate and 0.5 equivalents of 18-
crown-6 (Table 3, entry 5) with the reaction in which the
crown ether was omitted (Table 3, entry 2), the conver-
sion remained unchanged. This observation can be ex-
plained by taking into account that the potassium ion is
already solvated by the N,N-dimethylformamide and its
complexation with the crown ether does not improve the
Therefore a diverse set of substituted 2¢-methylbiphenyl-
2
-carbaldehydes was synthesized via Suzuki cross-cou-
pling reaction of the properly substituted phenylbromide
and phenylboronic acid under Gronowitz conditions.
Table 4 shows that the corresponding biphenyl com-
pounds were isolated in good to excellent yields. The re-
sulting substrates were then subjected to the optimized
Synlett 2006, No. 19, 3225–3230 © Thieme Stuttgart · New York

3
228
K. Monsieurs et al.
LETTER
conditions for ring closure (Table 5). A methoxy group Table 5 Synthesis of Substituted Phenanthrenes via Ring Closure of
29
(
Table 5, entry 1) was the first challenging substituent in- 2b–g
vestigated since its electron-releasing property makes the
methyl group less acidic. Nevertheless, the ring-closure
reaction was completed after 90 minutes, and the phenan-
threne 3b could be isolated in a very good yield. Subse-
quently, some functional groups that are sensitive to
strong nucleophilic bases were screened, namely, a cyano
group, an amide group and a nitro group (Table 5, entries
R2
R2
2
10
1
O
R1
R1
9
Cs CO
2
3
3
8
4
anhyd DMF
Me
7
5
µw (Mars)
6
200 °C
2b–g
3b–g
R1
H
H
H
H
H
R2
Time (min) Yield (%) Product
2
–4). Interestingly, the biphenyl compounds 2c–2e could
be converted into the corresponding phenanthrenes 3c–3e
under our optimized reaction conditions without any
problem. To enable the comparison of our weak base/high
temperature approach with the classical procedures based
on the use of a strong base, the ring-closure reactions of
the biphenyl compounds 2c–2e were repeated with potas-
sium tert-butoxide (4 equiv) as the base at a reaction tem-
perature of 80 °C with N,N-dimethylformamide as the
solvent. TLC analysis of the reaction mixtures revealed
that in all three cases the starting compound was com-
pletely consumed after ten minutes. For the cyano and the
amide functionalities, the presence of the strong base re-
sulted in a more complex reaction mixture and the chro-
matographic purification was consequently more
problematic and time-consuming. In accordance with this
observation, the isolated yields of 3c (60%) and 3d (70%)
were lower than those obtained with our weak base/high
temperature approach. Interestingly, when the nitro-sub-
stituted compound 2e was subjected to potassium tert-bu-
toxide in N,N-dimethylformamide at 80 °C, no expected
1
2
3
4
5
6
2-MeO
2-CN
90
90
84
3b
3c
3d
3e
3f
70
2-CON(Et)2
2-NO2
3-Me
45
82
45
91
90
4031
89
Me
H
420
3g
In order to test the further scope of this new ring-closure
strategy, we decided to apply the method for the synthesis
a heterocyclic compound. 11-Methyl-11H-ben-
zo[a]carbazole (6) was selected as the target since it
of
permits a comparison with the method of de Koning et al.
34
(
Scheme 1). Starting compound 5 could easily be pre-
pared via Suzuki cross-coupling reaction of 2-bromo-1-
methyl-1H-indole-3-carbaldehyde (4) with 2-methyl-
phenylboronic acid. Subsequently, 5 could be smoothly
converted into 11-methyl-11H-benzo[a]carbazole (6),
which was isolated in excellent yield. On comparing our
condensation reaction with the result obtained by de
Koning, it is clear that although with cesium carbonate as
the base a longer reaction time is needed, the yield is sig-
nificantly higher.
2
-nitrophenanthrene 3e was formed. Instead, the main re-
action product was identified as 1-hydroxy-2-nitro-
3
3
phenanthrene (33%). This result demonstrates that in
some cases the choice of the base can even influence the
outcome of the reaction.
Finally, two substrates bearing an extra methyl group (2f,
In summary, the obtained results reveal that a careful
choice of solvent in combination with the application of a
high temperature opens the possibility of using a weak
base for a deprotonation reaction that normally would re-
quire addition of a strong base. This new strategy was il-
lustrated by the conversion of 2¢-methylbiphenyl-2-
carbaldehyde into phenanthrene by using the weak non-
nucleophilic base cesium carbonate in N,N-dimethylform-
amide as the solvent at a reaction temperature of 200 °C.
2
g) were subjected to the new ring-closure reaction con-
ditions. The resulting phenanthrenes 3f and 3g could be
isolated in moderate to good yields. For 1-(2¢-methyl-
biphenyl-2-yl)ethanone (2g) a longer reaction time was
required, probably due to competitive deprotonation of
the extra, substantially more acidic, methyl group of the
acetyl group that slowed down the desired cyclization
reaction.
4
equiv KOt-Bu
hν
DMF
8
0 °C, 10 min
O
O
Me
77%
H
H
B(OH)2
Br
3 4
Pd(PPh )
N
N
N
DME–EtOH
Na CO (aq)
Me
MeMe
4 equiv Cs CO
µw (Mars)
DMF
2
3
Me
2
3
reflux
94%
4
5
6
200 °C, 90 min
94%
Scheme 1
Synlett 2006, No. 19, 3225–3230 © Thieme Stuttgart · New York

LETTER
Synthesis of Substituted Phenanthrenes via Intramolecular Condensation
3229
The reaction conditions were successfully applied on sub-
strates bearing different types of challenging substituents,
and additionally as an example of the applicability of our
new protocol in heterocyclic chemistry a 11H-ben-
zo[a]carbazole was synthesized.
Acknowledgment
We are grateful to the ‘Fund for Scientific Research Flanders’ (re-
search grant 1.5.088.04), the University of Antwerp and the Hunga-
rian National Office for Research and Technology for financial
support.
Figure 1
References and Notes
microwave heating. For an experimental set-up to compare
reactions under conventional (oil bath) and microwave
heating, see: Hostyn, S.; Maes, B. U. W.; Van Baelen, G.;
Gulevskaya, A.; Meyers, C.; Smits, K. Tetrahedron 2006,
(
1) de Koning, C. B.; Michael, J. P.; Rousseau, A. L.
Tetrahedron Lett. 1998, 39, 8725.
(
2) Pathak, R.; Vandayar, K.; van Otterlo, W. A. L.; Michael, J.
P.; Fernandes, M. A.; de Koning, C. B. Org. Biomol. Chem.
6
2, 4676.
28) Gronowitz, S.; Bobosik, V.; Lawitz, K. Chem. Scr. 1984, 23,
20.
29) General Procedure for the Synthesis of Phenanthrenes
(
(
2004, 2, 3504.
1
(
(
3) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. Rev. 2002, 102, 1359.
4) Li, J. J.; Gribble, G. W. Palladium in Heterocyclic
Chemistry; Pergamon: Amsterdam, 2000.
3a–g and 11-Methyl-11H-benzo[a]carbazole (6): An
80-mL Greenchem vessel was charged with biphenyl
compound 2a–g (0.75 mmol), Cs CO (3 mmol) and anhyd
2
3
(
(
5) Iuliano, A.; Piccioli, P.; Fabbri, D. Org. Lett. 2004, 6, 3711.
6) Walker, E. R.; Leung, S. Y.; Barrett, A. G. M. Tetrahedron
Lett. 2005, 46, 6537.
DMF (5 mL). The vessel was flushed with argon under
magnetic stirring for a few minutes. Subsequently, the vessel
was sealed and heated to 200 °C in a Mars multi-mode
microwave oven (CEM). The set power was 300 W. The
total irradiation time (including the ramp time to the set
temperature) was 90 min, unless indicated otherwise. After
(
(
(
7) Tovar, J. D.; Swager, T. M. J. Organomet. Chem. 2002, 653,
2
15.
8) Yao, T. L.; Campo, M. A.; Larock, R. C. Org. Lett. 2004, 6,
677.
2
cooling the reaction mixture was poured into H O (100 mL)
2
9) Goldfinger, M. B.; Crawford, K. B.; Swager, T. M. J. Org.
Chem. 1998, 63, 1676.
and was extracted with EtOAc (3 × 100 mL). The combined
organic fractions were dried over MgSO , evaporated to
4
(
(
(
(
(
10) Fürstner, A.; Mamane, V. Chem. Commun. 2003, 2112.
11) Fürstner, A.; Mamane, V. J. Org. Chem. 2002, 67, 6264.
12) Shi, M.; Xu, B. J. Org. Chem. 2002, 67, 294.
13) Jackson, T. J.; Herndon, J. W. Tetrahedron 2001, 57, 3859.
14) Merlic, C. A.; Xu, D. Q.; Gladstone, B. G. J. Org. Chem.
dryness and purified via flash column chromatography on
silica gel.
(
30) It was not possible to reach 200 °C when DME was used as
the solvent. At a temperature of 193 °C the autogenic
pressure rose to the maximum allowed value of 200 psi. The
safety settings of the microwave apparatus stopped
microwave irradiation of the vessel. The reaction mixture
was subsequently held at 193 °C not to exceed 200 psi.
Figure 2 shows the heating profile.
1
993, 58, 538.
15) Merlic, C. A.; Roberts, W. M. Tetrahedron Lett. 1993, 34,
379.
16) Campo, M. A.; Larock, R. C. J. Am. Chem. Soc. 2002, 124,
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17) Mandal, A. B.; Lee, G. H.; Liu, Y. H.; Peng, S. M.; Leung,
M. K. J. Org. Chem. 2000, 65, 332.
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Chem. 1997, 62, 7536.
19) Mátyus, P.; Maes, B. U. W.; Riedl, Z.; Hajós, G.; Lemière,
G. L. F.; Tapolcsányi, P.; Monsieurs, K.; Éliás, O.;
Dommisse, R. A.; Krajsovszky, G. Synlett 2004, 1123.
20) Fu, J. M.; Snieckus, V. Can. J. Chem. 2000, 78, 905.
21) Cai, X. W.; Brown, S.; Hodson, P.; Snieckus, V. Can. J.
Chem. 2004, 82, 195.
(
(
(
(
(
7
1
(
(
(22) de Koning, C. B.; Michael, J. P.; Rousseau, A. L. J. Chem.
Soc., Perkin Trans. 1 2000, 787.
(23) Kraus, G. A.; Zhang, N. Tetrahedron Lett. 2002, 43, 9597.
(24) Perrin, D. D. Aust. J. Chem. 1964, 17, 484.
(25) March, J. Advanced Organic Chemistry, 4th ed.; Wiley-
Interscience: New York, 1992, 269.
Figure 2
(
(
31) The isolation of unsubstituted phenanthrene and 3-methyl-
(
(
26) Kappe, C. O. Angew. Chem. Int. Ed. 2004, 43, 6250.
27) Figure 1 shows the heating profiles of a mixture of DMF (5
mL) and Cs CO (3 mmol) in an 80 mL vessel, when heated
phenanthrene was hampered by their volatility. It is possible
that this also contributes to the lower yields obtained for
these compounds.
2
3
by microwave irradiation on one hand and conventional
heating in a preheated oil bath (oil-bath temperature: 220 °C)
on the other hand. It is clear that the mixture reaches the set
temperature of 200 °C considerably faster in the case of
32) General Procedure for the Synthesis of Biphenyls 2a–g:
A two-necked flask was charged with phenylbromide 1a–g,
arylboronic acid (1.5 equiv), Pd(PPh ) (5 mol%), and DME
3 4
Synlett 2006, No. 19, 3225–3230 © Thieme Stuttgart · New York

3
230
K. Monsieurs et al.
LETTER
3
5
(
6 mL/mmol 1). The flask was connected to a reflux
(33) 1-Hydroxy-2-nitrophenanthrene: mp 160 °C (Lit. 161–162
1
condenser and flushed with nitrogen (via the second neck)
for 2 min under magnetic stirring. Subsequently, an aq
solution of 10% Na CO (1 mL/mmol 1) was added and the
reaction mixture was stirred and refluxed overnight in an oil
bath under a N atmosphere. After cooling, the reaction
°C). H NMR: (acetone-d ): d = 8.82–8.88 (m, 1 H), 8.44 (d,
6
J = 9.5 Hz, 1 H), 8.38 (d, J = 9.2 Hz, 1 H), 8.27 (d, J = 9.5
Hz, 1 H), 8.07–8.12 (m, 1 H), 8.03 (d, J = 9.1 Hz, 1 H), 7.77–
7.83 (m, 2 H). A derivatization of this compound was carried
out to provide an extra confirmation of its structure.
1-Methoxy-2-nitrophenanthrene was synthesized by
methylation of 1-hydroxy-2-nitrophenanthrene; mp 120–
2
3
2
mixture was poured into H O and extracted with CH Cl .
2
2
2
The combined organic layers were dried over MgSO4,
evaporated to dryness and purified by flash column
chromatography on silica gel.
3
5
121 °C (Lit. 122–123 °C).
(34) de Koning, C. B.; Michael, J. P.; Rousseau, A. L. J. Chem.
Soc., Perkin Trans. 1 2000, 1705.
(
35) Ota, E.; Iso, F. Nippon Kagaku Kaishi 1975, 5, 938.
Synlett 2006, No. 19, 3225–3230 © Thieme Stuttgart · New York