Intramolecular Biaryl Oxidative Coupling of Stilbenes by Vanadium Oxytrichloride (VOCl3): Facile Synthesis of Substituted Phenanthrene Derivatives

Article abstract of DOI:10.1081/SCC-120027245

Vanadium oxytrichloride (VOCl₃) has proven to be a highly efficient reagent for intramolecular biaryl oxidative coupling reaction of electron-rich stilbenes. Accordingly, a mild and efficient route to phenanthrene derivatives from stilbenes oxy

Full text of DOI:10.1081/SCC-120027245

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Intramolecular Biaryl Oxidative Coupling of Stilbenes
by Vanadium Oxytrichloride (VOCl3): Facile Synthesis
of Substituted Phenanthrene Derivatives
Zhong Jin a , Qingmin Wang a & Runqiu Huang a
a Institute and State Key Laboratory of Elemento‐organic Chemistry, Nankai University,
Weijin Road 94#, Tianjin, 300071, P.R. China
Version of record first published: 16 Aug 2006.
To cite this article: Zhong Jin , Qingmin Wang & Runqiu Huang (2004): Intramolecular Biaryl Oxidative Coupling of
Stilbenes by Vanadium Oxytrichloride (VOCl3): Facile Synthesis of Substituted Phenanthrene Derivatives, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:1, 119-128
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 1, pp. 119–128, 2004
Intramolecular Biaryl Oxidative Coupling
of Stilbenes by Vanadium Oxytrichloride
(VOCl3): Facile Synthesis of Substituted
Phenanthrene Derivatives
*
Zhong Jin, Qingmin Wang, and Runqiu Huang
Institute and State Key Laboratory of Elemento-organic Chemistry,
Nankai University, Tianjin, P.R. China
ABSTRACT
Vanadium oxytrichloride (VOCl3) has proven to be a highly efficient
reagent for intramolecular biaryl oxidative coupling reaction of electron-
rich stilbenes. Accordingly, a mild and efficient route to phenanthrene
derivatives from stilbenes oxygenated is developed.
Key Words: Vanadium oxytrichloride; Biaryl oxidative coupling;
Stilbenes; Phenanthrene derivatives.
*
Correspondence: Runqiu Huang, Institute and State Key Laboratory of Elemento-
organic Chemistry, Nankai University, Weijin Road 94#, Tianjin 300071, P.R. China;
Fax: þ86 022 2350 3438; E-mail: jinzhong2000@263.net.
119
DOI: 10.1081/SCC-120027245
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

120
Jin, Wang, and Huang
The biaryl unit is extensively presented in many classes of natural
products, such as polyketides, terpenes, lignanes, coumarins, flavonoids,
tannins, and many alkaloids.[1] It has long been recognized that an
intramolecular oxidative phenolic or nonphenolic coupling reaction serves
as the key step in the biosynthesis of these natural products, and the non-
enzymic analogue of this transformation can lead the elegantly simple
syntheses of these compounds.[2] During the last decade, a large number of
oxidative coupling reagents, such as ferric chloride (FeCl3),[3–5] phenyl-
iodine(III)
bis(trifluoroacetate)
(PIFA),[6–8]
lead(IV)
tetraacetate
(Pb(OAc)4),[9] thallium(III) triflouroacetate (TTFA),[10,11] as well as
vanadium oxytrifluoride (VOF3),[12–15] have been developed for this target.
However, extensive application of this synthetic potential has been limited by
low yields and unexpected side reactions usually encountered.
As part of the total syntheses of Asclepiadaceae alkaloids, several
polymethoxyphenanthrene derivatives were required, but they were found to
be tediously accessible by conventional Ullman’s method. After comprehen-
sive investigation, vanadium oxytrichloride (VOCl3) is found to be very
efficient reagent for this type of oxidative coupling reaction due to its liquid
property and solubility in most reaction media. Herein, we report a concise
and efficient pathway for substituted phenanthrene by intramolecular biaryl
oxidative coupling utilizing VOCl3 as an oxidant.
Condensation of substituted phenylacetic acids 1 and aromatic aldehydes
2 was carried out in refluxing acetic anhydride in the presence of triethylamine
with excellent yields.[16] Although the product thus obtained is invariably a
mixture of Z- and E-a-(arylmethylene)phenylacetic acids 3, both of them can
be used for subsequent intramolecular oxidative coupling. On treatment with
ethereal diazomethane, acids 3 were smoothly transformed into corresponding
methyl esters 4 in almost quantitative yields. Oxidation of 4 with VOCl3 in
dichloromethane at 2788C afforded the methyl phenanthrene-9-carboxylates
5 in moderate to excellent yields (Table 1, entry 1–5). The coupling occuring
from C6 and C60 of 4a to give 5a is evident from the appearance of four single
peaks in the phenanthrene ring in the 1H NMR spectrum of compound 5a. In
addition, it is noteworthy that both (Z)- and (E)-4 gave the same products
under this condition (Sch. 1).
The above result shows that two methoxy groups are necessary in at least
one of the aromatic rings. However, the fact that esters 4, which bear m-
dimethoxy substituents, do not cyclize implies that the two groups should be
ortho-disposed. Furthermore, 2,3-dimethoxy derivative 4 fail to deliver
corresponding 5, what implies that aromatic rings of substrates with oxygen
functionality at the 3- and 4- and/or 30- and 40-positions are essential to afford
oxidative cyclization.

Intramolecular Biaryl Oxidative Coupling
121
Table 1. Intramolecular oxidative coupling of stilbene substrates with VOCl3.
Stilbene substratea
Entry
No.
R1
R2
R3
R4
R
Product Yieldb
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
OCH3 OCH3 OCH3 OCH3 CO2CH3
5a
5b
5c
5d
5e
5f
98
82
76
72
54
97
58
96
61
OCH3 OCH3 OCH3
OCH3 OCH3
H
H
CO2CH3
CO2CH3
H
OCH3
H
H
H
OCH3 OCH3 CO2CH3
OCH3 OCH3 CO2CH3
OCH3 OCH3 OCH3 OCH3
OCH3 OCH3
OCH3 OCH3 OCH3 OCH3
OCH3 OCH3
CN
CN
4g
4h
4i
H
H
5g
5h
5i
NO2
NO2
H
H
aA mixture of Z- and E-isomer in entry 1–5, only Z-isomer in entry 6–9.
bIsolated yields.
Similarly, when ester group of 4 on the double bond was replaced by a
nitro or cyano group (Sch. 2), the (a-nitro/cyano)stilbenes 4f–i prepared from
(3,4-dimethoxy)nitromethylbenzene[17] 6 or (3,4-dimethoxyphenyl)acetoni-
triles 7 with aromatic aldehydes 2 were readily for oxidative cyclization to the
corresponding phenananthrene derivatives 5 in moderate to excellent yields
(Table 1, entry 6–9).
In order to examine the generality of phenanthrene derivatives formation
by this means, effects of substituents on the double bond of stilbene substrates
were studied in detail. When ester group on the double bond was replaced by
an electron-donating group such as a hydroxyl or methoxyl group, no coupling
product 8 was formed. Further investigation shows that stilbene also fail to
give phenanthrene derivative 9. However, compounds 8 and 9 may be easily
obtained from corresponding nitro compound 5h. Reduction of 5h with
ferrous sulfate afforded corresponding 2,3,6,7-tetramethoxy-9-aminophenan-
threne 10 in a yield of 86%, further treatment with sodium nitrite in
hydrochloric acid to yield corresponding diazo compound, which was
subsequently transformed to the desired phenanthrene derivatives 8 and 9 in a
yield of 66% and 74%, respectively (Sch. 3).
From above results we believed that, in addition to substitution effect on
two aryl groups, intramolecular biaryl coupling of stilbene substrates was also
sensitive to substituents on the double bond. Particularly, this reaction fails in
absence of electron-donating substituents. Up now, to the best of our
knowledge, no mechanism has yet been formulated for coupling reactions
employing the vanadium reagents, but it is believed to involve electron

122
Jin, Wang, and Huang

Intramolecular Biaryl Oxidative Coupling
123

124
Jin, Wang, and Huang
Scheme 3.
transfer and form radical cations on electron rich substrates.[18,19] If this is the
case, it therefore seems likely that electron-donating group increases the
electron-density on the double bond of stilbene substrates. As a result, double
bond is oxidized prior to aromatic rings. Whereas, with an electron-
withdrawing group on double bond, aryl rings of substrates will be oxidized
prior to double bond and form aromatic radical cations.
In summary, a mild and efficient route to phenanthene derivatives from
stilbenes oxygenated by intramolecular biaryl oxidative coupling with VOCl3
has been described. Effect of substitution on stilbene substrates has been
examined and an electron-withdrawing group on the double bond has proven
to be essential for the oxidative coupling. Application this oxidative coupling
method for the total synthesis of Asclepiadaceae and Amaryllidaceous
alkaloids is under investigation.
EXPERIMENTAL
All solvents were distilled from sodium/benzophenone prior to use. All
melting points were uncorrected. 1H NMR were recorded on a BRUKER AC-
P 200 using tetramethylsilane as the internal standard. Elemental analyses
were performed on a Yanaco CHN CORDER MT-3 instrument.
Representative Procedure for Intramolecular
Oxidative Coupling
A solution of 10.8 g (30 mmol) of methyl (Z,E)-a-(arylmethylene)pheny-
lacetate 4 in 500 mL of dichloromethane was cooled to 2788C with well
stirred under N2. Then, 5.7 mL (33 mmol) of VOCl3 was added in one portion
at this point. After being stirred for 16 hours at 2508C, the reaction mixture
was quenched with cold water, and the organic phase was well washed with

Intramolecular Biaryl Oxidative Coupling
125
saturated brine (3 ꢀ 150 mL). Then the organic phase was dried over MgSO4,
filtered through a short silica gel column, and evaporated to give desired
products, methyl phenanthrene-9-carboxylate 5 in 55–98% yields. The
following compounds were obtained:
Methyl 2,3,6,7-tetramethoxyphenanthrene-9-carboxylate (5a). 98%
yield, white solid, m.p. 202–2048C (lit.[20] 202–2048C). 1H NMR (200 MHz,
CDCl3) d (ppm): 8.64 (s, 1H), 8.41 (s, 1H), 7.78 (s, 1H), 7.74 (s, 1H), 7.25 (s,
1H), 4.14 (s, 3H), 4.13 (s, 3H), 4.08 (s, 3H), 4.04 (s, 3H), 4.02 (s, 3H). EI-MS
m/z: 356 (Mþ, 100). Anal. calcd. for C20H20O6: C, 67.41, H, 5.66; Found: C,
67.45, H, 5.71.
Methyl 3,6,7-trimethoxyphenanthrene-9-carboxylate (5b). 82%
1
yield, white solid, m.p. 155–1568C, H NMR (200 MHz, CDCl3) d (ppm):
8.42 (s, 1H), 8.20 (s, 1H), 8.10 (s, 1H), 7.75 (s, 1H), 7.44 (m, 2H), 4.15 (s, 3H),
4.06 (s, 3H), 4.05 (s, 3H), 3.97 (s, 3H). Anal. calcd. for C19H18O5: C, 69.93, H,
5.56; Found: C, 69.73, H, 5.60.
Methyl 6,7-dimethoxyphenanthrene-9-carboxylate (5c). 76% yield,
white solid, m.p. 130–1318C, 1H NMR (200 MHz, CDCl3) d (ppm): 8.40 (s,
1H), 8.28 (s, 1H), 8.18 (s, 1H), 7.8–7.4 (m, 4H), 4.00 (s, 3H), 3.96 (s, 3H),
3.92 (s, 3H). Anal. calcd. for C18H16O4: C, 72.96, H, 5.44; Found: C, 72.78, H,
5.62.
Methyl 2,3,7-trimethoxyphenanthrene-9-carboxylate (5d). 72%
1
yield, white solid, m.p. 148–1498C, H NMR (200 MHz, CDCl3) d (ppm):
8.48 (s, 1H), 8.26 (s, 1H), 8.15 (s, 1H), 7.8–7.6 (m, 2H), 7.50 (s, 1H), 4.10 (s,
3H), 3.99 (s, 3H), 3.97 (s, 3H), 3.92 (s, 3H). Anal. calcd. for C19H18O5: C,
69.93, H, 5.56; Found: C, 69.92, H, 5.83.
Methyl 2,3-dimethoxyphenanthrene-9-carboxylate (5e). 54% yield,
white solid, m.p. 142–1448C, 1H NMR (200 MHz, CDCl3) d (ppm): 8.78 (s,
1H), 8.24 (s, 1H), 8.05 (s, 1H), 7.6–7.4 (m, 3H), 6.99 (s, 1H), 4.10 (s, 3H),
3.99 (s, 3H), 3.97 (s, 3H), 3.92 (s, 3H). Anal. calcd. for C18H16O4: C, 72.96, H,
5.44; Found: C, 72.92, H, 5.26.
9-Cyano-2,3,6,7-tetramethoxyphenanthrene (5f). 97% yield, white
1
solid, m.p. 133–1348C, H NMR (200 MHz, CDCl3) d (ppm): 8.98 (s, 1H),
8.45 (s, 1H), 8.25 (s, 1H), 7.6 (s, 1H), 7.4 (s, 1H), 4.15 (s, 3H), 4.09 (s, 3H),
4.07 (s, 3H), 4.02 (s, 3H). Anal. calcd. for C18H17NO6: C, 62.97, H, 4.99, N,
4.08; Found: C, 62.89, H, 5.07, N, 4.10.
9-Cyano-6,7-dimethoxyphenanthrene (5g). 58% yield, light yellow
1
solid, m.p. 125–1278C, H NMR (200 MHz, CDCl3) d (ppm): 8.83 (s, 1H),
8.39 (s, 1H), 8.00 (s, 1H), 7.9–7.6 (m, 4H), 4.11 (s, 3H), 4.02 (s, 3H). Anal.
calcd. for C16H13NO4: C, 67.84, H, 4.63, N, 4.94; Found: C, 67.91, H, 4.66, N,
4.80.
9-Nitro-2,3,6,7-tetramethoxyphenanthrene (5h). 96% yield, yellow
1
solid, m.p. 156–1578C, H NMR (200 MHz, CDCl3) d (ppm): 8.71 (s, 1H),

126
Jin, Wang, and Huang
8.34 (s, 1H), 8.18 (s, 1H), 7.60 (s, 1H), 7.49 (s, 1H), 4.10 (s, 3H), 4.00 (s, 3H),
3.99 (s, 3H), 3.95 (s, 3H). Anal. calcd. for C19H17NO4: C, 70.58, H, 5.30, N,
4.33; Found: C, 70.52, H, 5.66, N, 4.50.
9-Nitro-6,7-dimethoxyphenanthrene (5i). 61% yield, yellow solid,
m.p. 139–1428C, 1H NMR (200 MHz, CDCl3) d (ppm): 8.78 (s, 1H), 8.39 (s,
1H), 7.99 (s, 1H), 7.7–7.5 (m, 4H), 4.05 (s, 3H), 3.95 (s, 3H). Anal. calcd. for
C17H13NO2: C, 77.55, H, 4.98, N, 5.32; Found: C, 77.72, H, 5.01, N, 5.40.
9-Amino-2,3,6,7-tetramethoxyphenanthrene (10). To a solution of the
9-nitro-2,3,6,7-tetramethoxyphenanthrene 5h (3.43 g, 10 mmol) in 95%
aqueous EtOH (100 mL) was added ferrous sulfate heptahydrate (20 g)
dissolved in distilled water (100 mL) and concentrated aqueous NH4OH
(100 mL). The reaction mixture was refluxed for 2 h, cooled to room
temperature, filtered on Celite and acidified with acetic acid. The solid was
collected by filtration and recrystallization from EtOH yielded the 9-amino-
phenanthrene 10 in 86% yield, brown solid, m.p. 225–2268C. 1H NMR
(200 MHz, CDCl3) d (ppm): 8.02 (s, 1H), 7.74 (s, 1H), 7.08 (s, 1H), 7.04 (s,
1H), 6.95 (s, 1H), 4.11 (s, 3H), 4.09 (s, 3H), 4.07 (s, 3H), 4.04 (s, 3H), 2.90–
2.60 (br, 2H). Anal. calcd. for C18H19NO4: C, 68.99, H, 6.11, N, 4.47; Found:
C, 68.85, H, 6.18, N, 4.66.
9-Hydroxy-2,3,6,7-tetramethoxyphenanthrene (8). To a cooled solu-
tion of concentrated H2SO4 (10 mL) in distilled water (60 mL) was added
9-amino-phenanthrene 10 (3.13 g, 10 mmol). The mixture was cooled to 08C
in an ice bath with stirring. To the reaction mixture was added a solution of
sodium nitrite (2.1 g, 30 mmol) in water (15 mL). After stirred at the room
temperature for 2 h, 50 mL distilled water was added. The mixture was
refluxed for 1 h, cooled, filtered. The filtrate was extracted with CHCl3
(3 ꢀ 50 mL). The organic layer was dried over anhydrous MgSO4, filtered and
evaporated to yield the corresponding 9-hydroxy-phenanthrene 8 in 66%
yield, pale solid, m.p. 267–2688C, 1H NMR (200 MHz, CDCl3) d (ppm): 8.98
(br s, 1H), 8.69 (s, 1H), 7.88 (s, 1H), 7.7 (s, 1H), 7.68 (s, 1H), 6.95 (s, 1H), 4.05
(s, 3H), 3.95 (s, 3H), 3.92 (s, 3H), 3.88 (s, 3H). Anal. calcd. for C18H18O5: C,
68.78, H, 5.77; Found: C, 68.72, H, 5.71.
2,3,6,7-Tetramethoxyphenanthrene (9). To a cooled solution of
concentrated HCl (10 mL) in distilled water (15 mL) was added 9-amino-
phenanthrene 10 (3.13 g, 10 mmol). The mixture was cooled to 08C in an ice
bath with stirring. To the reaction mixture was added a solution of sodium
nitrous (2.1 g, 30 mmol) in water (15 mL) at 08C. After stirred at 08C for 2 h,
the mixture was filtered. The filtrate was added into aqueous H3PO2 (50%,
80 mL). The mixture was stirred at 08 for 2 h and extracted with CHCl3
(3 ꢀ 50 mL). The organic layer was dried over anhydrous MgSO4, filtered and
evaporated to yield the corresponding 2,3,6,7-tetramethoxyphenanthrene 9 in
74% yield, m.p. 1818C, (lit.[21] m.p. 180–1818C). 1H NMR (200 MHz,

Intramolecular Biaryl Oxidative Coupling
127
CDCl3) d (ppm): 7.77 (s, 2H), 7.54 (s, 2H), 4.10 (s, 6H), 4.02 (s, 6H). Anal.
calcd. for C18H18O4: C, 72.47, H, 6.08; Found: C, 72.74, H, 5.99.
ACKNOWLEDGMENT
We gratefully acknowledge the National Natural Science Foundation of
China (NNSFC) (No. 20002003) for financial support.
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Received in Poland April 8, 2003