Utilization of a copper-catalyzed diaryl ether synthesis for the preparation of verbenachalcone

Article abstract of DOI:10.1016/S0040-4020(02)00915-8

2-Hydroxy-2′-methoxydiphenyl ethers were efficiently assembled utilizing a catalytic copper mediated coupling of 2-benzyloxybromobenzene derivatives and 2-methoxyphenol derivatives in the presence of cesium carbonate in pyridine followed by debenzylation.

Full text of DOI:10.1016/S0040-4020(02)00915-8

Tetrahedron 58 (2002) 7903–7910
Utilization of a copper-catalyzed diaryl ether synthesis for the
preparation of verbenachalcone
*
Xuechao Xing, Deepa Padmanaban, Li-An Yeh and Gregory D. Cuny
Laboratory for Drug Discovery in Neurodegeneration, Harvard Center for Neurodegeneration and Repair, 65 Landsdowne Street,
Cambridge, MA 02139, USA
Received 16 May 2002; accepted 24 July 2002
Abstract—2-Hydroxy-20-methoxydiphenyl ethers were efficiently assembled utilizing a catalytic copper mediated coupling of 2-benzyl-
oxybromobenzene derivatives and 2-methoxyphenol derivatives in the presence of cesium carbonate in pyridine followed by debenzylation.
Utilization of this method allowed for a concise synthesis of verbenachalcone, a compound reported to enhance nerve growth factor’s ability
to stimulate neurite outgrowths in PC12D cells. Initial SAR data is also presented. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
the isoquinoline alkaloid (þ)-vateamine (5)4 all contain this
substructure.
Compounds containing diaryl ethers are abundant in nature.
In many instances, the diaryl ether is part of a complex
acyclic or macrocyclic structure. A significant proportion of
these compounds contain 2-hydroxy-20-methoxydiphenyl
ethers 1. For example, the recently isolated dimeric
dihydrochalcone verbenachalcone (2),1 the cyclic alkaloid
isocodonocarpine (3),2 the cyclic ketone galleon (4),3 and
Many compounds comprising substructure 1 have interest-
ing biological properties. For example, 2 has been reported
to enhance nerve growth factor’s (NGF’s) ability to
stimulate neurite outgrowths from PC12D cells.1 Com-
pounds that possess this property may be useful in the
treatment of both acute (e.g. trauma or stroke) and chronic
Keywords: copper; catalysis; diaryl ether; verbenachalcone; nerve growth factor.
*
Corresponding author. Tel.: þ1-617-768-8640; fax: þ1-617-768-8606; e-mail: gcuny@rics.bwh.harvard.edu
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00915-8

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X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
Scheme 1.
neurological disorders, such as Parkinson’s disease (PD),
Alzheimer’s disease (AD), Huntington’s disease (HD),
amyotrophic lateral sclerosis (ALS), and human immuno-
deficiency virus associated dementia (HAD).
Identifying and preparing low molecular weight molecules,
such as 2, that enhance the activity of NGF can assist in
elucidating the role of neurotrophins in the patho-
physiology of neurological diseases and can provide lead
compounds for therapeutic development. In the present
study, we report methodology for the assembly of
2-hydroxy-20-methoxydiphenyl ethers and its application
to the synthesis of 2. The methodology described herein will
be applicable for the preparation of derivatives of 2 for
structure–activity relationship (SAR) studies related to
enhancing NGF activity on neurite outgrowths and for the
synthesis of other na0tural and non-natural substances
containing 2-hydroxy-2 -methoxydiphenyl ethers.
NGF belongs to a class of proteins called neurotrophins.5
These proteins are a subclass of growth factors whose
effects are almost exclusively on the central and peripheral
nervous system. Other members of this family include
brain-derived neurotrophic factor (BDNF), NT-3, NT-4/5,
NT-6 and NT-7. Several related classes of growth factors
also specific to the nervous system have been identified,
such as glial cell-derived neurotrophic factor (GDNF) and
ciliary neurotrophic factor (CNTF). These proteins serve a
vital role in the development and homeostasis of the nervous
system. Recently, numerous studies have demonstrated the
potential benefits of enhancing, augmenting or mimicking
the actions of neurotrophins in both in vitro and in vivo
models of neurodegenerative diseases.6,7 However, direct in
vivo application of neurotrophins is limited by several
factors, including the inherent problems frequently asso-
ciated with peptide therapeutics (e.g. lack of both blood–
brain-barrier penetration and stability).
2. Results and discussion
The interesting biological activity reported for 2 prompted
an investigation into its synthesis. A retrosynthetic analysis
of 2 began with disconnecting the resorcinol groups to give
6. Further disconnection of the protected 2-hydroxy-20-
methoxydiphenyl ether gave phenol derivative 7 and
2-bromoanisole derivative 8 (Scheme 1, pathway a).
Scheme 2.

X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
7905
Scheme 3. (i) Br2, AcOH, 85%; (ii) BnBr, EtOH, K2CO3, D, 85%; (iii) 10, 5 mol% (CuOTf)2PhMe, Cs2CO3, py, 1108C, 78%; (iv) 8N NaOH, THF/MeOH
(2:1), room temperature, 95%; (v) MeNHOMe, HBTU, DIEA, DCM, 94%; (vi) 1-Li-2,4-(OBn)2Ph, THF, 2788C then 1N HCl, 70%; (vii) 1 atm H2, 5% Pd/C,
AcOH, 2 h, 88%; (viii) Ac2O, py, room temperature, 90%; (ix) 1 atm H2, 5% Pd/C, MeOH/THF, 16 h, 74%.
Alternatively, disconnection of 6 gave protected 2-hydroxy-
bromobenzene derivative 9 and 2-methoxyphenol deriva-
tive 10 (Scheme 1, pathway b). Initially, a model system
was used to assess the feasibility of both approaches and to
determine an appropriate hydroxyl protecting group.
for 24 h, the 2-benzyloxyphenol derivative 11 failed to react
with the 2-bromoanisole derivative 12.11 However, utilizing
identical reaction conditions, the 2-benzyloxybromo-
benzene derivative 14 reacted with the 2-methoxyphenol
derivative 15 (0.83 equiv.) to give the diaryl ether 13 in
moderate yield (51%).9f The reaction produced one product
that could be easily purified from remaining starting
materials by flash chromatography on basic aluminum
oxide. Afterward, the benzyl group was readily removed by
catalytic hydrogenation (1 atm H2, 5% Pd/C, MeOH, 1.5 h)
to give 16 in excellent yield (94%). These results
demonstrated that disconnection b in Scheme 1 would be
best for the synthesis of 2 and that a benzyl protecting group
would be tolerated in the coupling reaction and readily
removed in a subsequent transformation.
Due to the prevalence of diaryl ethers in many natural
products and medicinally interesting compounds, various
strategies have been devised for their construction. Most
prominent among these methods are the classic Ullmann
ether synthesis and nucleophilic aromatic substitution
reactions (SNAr).8 Typically, the SNAr reaction is
applicable for coupling electron-deficient aryl halides to
phenols under basic conditions. Recently, catalytic metal
mediated (copper9a – e and palladium10) couplings of
phenols with aryl halides have been reported. Successful
implementation of these reactions can be less dependent of
the electronic nature of the aryl halide. Since the present
work envisioned coupling electron-rich aryl halides, a
modified copper-catalyzed reaction was employed.
Having established a convenient means of assembling
2-hydroxy-20-methoxydiphenyl ethers, the methodology
was applied to the synthesis of verbenachalcone (2). The
synthesis began by first converting ethyl 3-(4-hydroxy-
phenyl)propionate (17) to the aryl bromide 18 by treatment
with bromine in acetic acid (Scheme 3).12 Benzylation of
the phenol was accomplished with benzyl bromide in
refluxing ethanol in the presence of potassium carbonate to
give 19.13 Next, the aryl bromide 19 was coupled to ethyl
The model systems used for accessing methods of
constructing 2-hydroxy-20-methoxydiphenyl ethers are
illustrated in Scheme 2. In the presence of 5 mol%
(CuOTf)2PhMe and cesium carbonate in pyridine at 1108C

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X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
Figure 1. Enhancement of NGF’s effects for stimulating neurite outgrowths in PC12 cells with 2 and 24. Cells were incubated in the presence of NGF (2 or
30 ng/mL) alone and in the presence of 2 (15 or 30 mM) or 24 (15 or 30 mM) plus NGF (2 ng/mL) for 48 h before being fixed with 2% glutaraldehyde (378C,
1 h). Neurite outgrowth was accessed under a phase-contrast microscope. Neurite processes with a length equal to or greater than the diameter of the neuron cell
body were scored as a neurite-bearing cell. The ratio of neurite-bearing cells to total cells was determined and expressed as a percentage^standard deviation
(N¼12). The data was analyzed by Student unpaired t-test. * Indicates no significant difference verses 2 ng/mL NGF; ** indicates a significant difference
(p,0.007) verses 2 ng/mL NGF; *** indicates a significant difference (p,0.0002) verses 2 ng/mL NGF.
3-(4-hydroxy-3-methoxyphenyl)propionate (10), which
was readily prepared from commercially available
3-(4-hydroxy-3-methoxyphenyl)propionic acid, to give 20
in 78% yield (based on recovered 19; 66% conversion).
The diaryl ether coupling was accomplished utilizing the
method previously developed (5 mol% (CuOTf)2PhMe,
Cs2CO3, pyridine, 1108C, 24 h) with the model system.
The esters were hydrolyzed with sodium hydroxide (8N) in
a mixture of THF and methanol (2:1) to give 21.14 This
material was converted to its corresponding Weinreb amide
22 utilizing O-benzotriazol-1-yl-N,N,N0,N0-tetramethyl-
uronium tetrafluoroborate (HBTU) in dichloromethane
(DCM) in the presence of diisopropyl ethylamine
(DIEA).15 Treatment of the amide with 4 equiv. of
1-lithio-2,4-dibenzyloxybenzene (generated in situ from
1-bromo-2,4-dibenzyloxybenzene and n-BuLi in THF at
2788C for 1.5 h) followed by the addition of hydrochloric
acid (1N) yielded 23.16,17 Finally, debenzylation was
accomplished by hydrogenation (1 atm, 2 h) in the presence
of 5% Pd/C in acetic acid to give the natural product 2.18
The 1H and 13C NMR spectra of the synthetic product were
identical to those reported for the natural product.
were similar to those observed with the 30 ng/mL NGF
exposure. Again, this effect was not observed at 30 mM due
to toxicity. The increase in activity of 24 compared to 2
maybe due to an increase in intrinsic activity at the
molecular target or an increase in cell permeability. Further
experiments will be necessary to elucidate the reason.
Neither compound was active in the absence of NGF.
Compound 25 did not enhance NGF’s effects for stimulating
neurite outgrowth at either 15 or 30 mM. This result
illustrated the importance of the ketones for activity.
3. Conclusions
2-Hydroxy-20-methoxydiphenyl ethers were efficiently
assembled utilizing a catalytic copper mediated coupling
of 2-benzyloxybromobenzene derivatives and 2-methoxy-
phenol derivatives in the presence of cesium carbonate in
pyridine followed by debenzylation. This method allowed
for a concise synthesis of verbenachalcone (2), a compound
reported to significantly enhance NGF’s effects for
stimulating neurite outgrowths in PC12D cells.1 A prelimi-
nary SAR study demonstrated that verbenachalcone penta-
acetate (24) was more active compared to the natural
product. However, a derivative of verbenachalcone that
lacked the ketone functional groups was devoid of activity.
The methodology employed for the synthesis of verbena-
chalcone should be amenable to the preparation of
additional derivatives for SAR studies to enhance NGF’s
effects on neurite outgrowths and for the synthesis of other
natural and non-natural substances containing 2-hydroxy-20-
methoxydiphenyl ethers.
Two derivatives were also prepared for a preliminary SAR
study. Treatment of 2 with acetic anhydride in pyridine gave
verbenachalcone pentaacetate (24). This compound was
designed to probe the relevance of the hydroxyl substituents
for activity. Similarly, a derivative was made to explore the
importance of the ketone functional groups. Hydrogenation
(1 atm H2, 5% Pd/C, MeOH/THF) of 23 for an extended
period of time (16 h) resulted in reduction of the ketones to
give 25 in 74% yield.
The ability of 2, 24, and 25 to enhance NGF’s effects for
stimulating neurite outgrowths was accessed utilizing
methodology previous reported.7d In control experiments,
the percentage of neurite-bearing cells was 2.5% following
incubation with 2 ng/mL NGF and 7.5% with 30 ng/mL
NGF after 48 h exposure in agreement with a previous
report (Fig. 1).19 A significant enhancement of NGF’s
effects was demonstrated with 2 (15 mM) in agreement with
Li et al.1 However, this effect was not observed at 30 mM.
The failure to see enhancement at the higher dose was most
likely due to cell toxicity, which was apparent during
microscopic examination of the cells. Likewise, 24 at
15 mM significantly enhanced NGF’s effects. These results
4. Experimental
4.1. General experimental procedures
Unless otherwise noted, all reagents and solvents were
purchased from commercial sources and used without
further purification. The NMR spectra were obtained using
a Bruker 400 MHz, Varian 400 MHz, or Varian 500 MHz
1
spectrometer. All H NMR spectra are reported in d units
ppm and referenced to tetramethylsilane (TMS). All 13C
NMR spectra are reported in d units ppm and referenced to
the central line of the triplet at 77.23 ppm. Column

X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
7907
chromatography was performed on silica gel (Merck, grade
60, 230–400 mesh) or utilizing a CombiFlash Sg 100c
separation system (ISCO) with RediSep disposable silica
gel columns (ISCO), or activated basic aluminum oxide
CDCl3): d 1.24 (t, 3H, J¼7.0 Hz), 2.57 (t, 2H, J¼8.0 Hz),
2.86 (t, 2H, J¼8.0 Hz), 4.13 (q, 2H, J¼7.0 Hz), 5.47 (s, 1H),
6.93 (d, 1H, J¼8.5 Hz), 7.05 (dd, 1H, J¼2.0, 8.5 Hz), 7.30
(d, 1H, J¼2.0 Hz); 13C NMR (100.5 MHz, CDCl3): d 14.56
(CH3), 30.08 (CH2), 36.26 (CH2), 60.77 (CH2), 110.13 (C),
116.11 (CH), 129.19 (CH), 131.70 (CH), 134.29 (C), 150.72
(C), 172.66 (CvO).
˚
(Brockmann I, standard grade, ,150 mesh, 58A).
High-resolution mass spectra were obtained by using an
SX-102A mass spectrometer (JEOL USA, Inc., Peabody,
MA) or an LCT mass spectrometer (Micromass Inc.,
Beverly, MA).
4.1.4. Ethyl 3-(4-benzyloxy-3-bromophenyl)propionate
(19). A mixture of 18 (546 mg, 2 mmol), benzyl bromide
(0.36 mL, 3 mmol), K2CO3 (833 mg, 6 mmol) and ethanol
(18 mL) was refluxed for 2 h, then allowed to cool to room
temperature and concentrated. The residue was partitioned
between 30 mL of water and 80 mL of ethyl acetate. The
organic solution was washed with sat. NaHCO3 (20 mL) and
brine (15 mL), dried over anhydrous MgSO4, filtered, and
concentrated. The residue was purified by chromatography
on silica gel using hexane/ethyl acetate (95:5) as eluant
to give 19 as a colorless oil (620 mg, 85%). 1H NMR
(500 MHz, CDCl3): d 1.23 (t, 3H, J¼7.0 Hz), 2.57 (t, 2H,
J¼8.0 Hz), 2.86 (t, 2H, J¼8.0 Hz), 4.12 (q, 2H, J¼7.0 Hz),
5.12 (s, 2H), 6.85 (d, 1H, J¼8.5 Hz), 7.06 (dd, 1H, J¼2.0,
8.5 Hz), 7.31 (t, 1H, J¼7.5 Hz), 7.38 (t, 2H, J¼7.5 Hz), 7.41
(d, 1H, J¼2.0 Hz), 7.46 (d, 2H, J¼7.5 Hz); 13C NMR
(100.5 MHz, CDCl3): d 14.57 (CH3), 30.08 (CH2), 36.22
(CH2), 60.73 (CH2), 71.14 (CH2), 112.51 (C), 114.05 (CH),
127.09 (2£CH), 127.98 (CH), 128.30 (CH), 128.64 (2£CH),
133.27 (CH), 134.69 (C), 136.70 (C), 153.52 (C), 172.61
(CvO); FT-IR (film, nmax, cm21): 3032w, 2980m, 2934m,
2868w, 1732s, 1604w, 1496s, 1282s, 1254s, 1183s, 1051s,
809w, 737m, 696w.
4.1.1. 2-Benzyloxy-20-methoxy-40,5-dimethyldiphenyl-
ether (13). A Schlenk flask was charged with 2-benzyl-
oxy-5-methylbromobenzene (14, 166 mg, 0.6 mmol),
pyridine (2 mL), 2-methoxy-4-methylphenol (15, 63.3 mL,
0.5 mmol), cesium carbonate (179 mg, 0.55 mmol), and
copper(I) trifluoromethanesulfonate toluene complex
(6.5 mg, 0.0125 mmol). The flask was purged with argon,
sealed, and heated at 1108C for 24 h. The reaction was
poured into 2N hydrochloric acid (25 mL). The mixture was
extracted with ethyl acetate (50 mL). The organic extract
was stirred with 10% sodium hydroxide (40 mL) for 15 min
and then washed with brine (25 mL), dried over anhydrous
magnesium sulfate, filtered, and concentrated to give a
brown oil. The oil was purified by column chromatography
on silica gel using hexane/ethyl acetate (95:5) as eluant to
give 95 mg of 13 as a colorless oil (57%). 1H NMR
(400 MHz, CDCl3): d 2.25 (s, 3H), 2.38 (s, 3H), 3.86 (s,
3H), 5.15 (s, 2H), 6.70–6.84 (m, 5H), 6.90 (d, 1H, J¼
8.4 Hz), 7.30–7.34 (m, 5H); 13C NMR (100 MHz, CDCl3):
d 20.78, 21.32, 56.03, 71.26, 113.54, 115.53, 118.44,
120.04, 120.97, 123.78, 127.04 (2C), 127.36, 128.10 (2C),
131.22, 133.17, 137.27, 144.08, 146.57, 147.06, 149.94;
HREIMS [MþH]þ: 335.1642 (calcd for [C22H22O3þH]þ,
335.1647).
4.1.5. Ethyl 3-{4-[2-benzyloxy-5-(2-ethoxycarbonyl-
ethyl)phenoxy]-3-methoxyphenyl}propionate (20).
A
mixture of 19 (175 mg, 0.5 mmol), ethyl 3-(4-hydroxy-3-
methoxyphenyl)propionate (10, 168 mg, 0.75 mmol),
(CF3SO3Cu)2·C6H5CH3 (14 mg, 0.026 mmol), Cs2CO3
(179 mg, 0.55 mmol) and pyridine (2 mL) was stirred at
1108C under argon for 24 h. The reaction mixture was
allowed to cool before being diluted with 30 mL of ethyl
acetate and washed with 1N HCl (3£10 mL). The organic
layer was dried over anhydrous MgSO4, filtered and
concentrated. The residue was purified by column chroma-
tography on basic aluminum oxide using hexane/ethyl
acetate (85:15) as eluant to give 60 mg of recovered 19 and
130 mg of the coupling product 20 (66% conversion) as a
colorless oil (78%). 1H NMR (500 MHz, CDCl3): d 1.21 (t,
3H, J¼7.5 Hz), 1.24 (t, 3H, J¼7.5 Hz), 2.52 (t, 2H, J¼
8.0 Hz), 2.63 (t, 2H, J¼8.0 Hz), 2.82 (t, 2H, J¼8.0 Hz), 2.93
(t, 2H, J¼8.0 Hz), 3.82 (s, 3H), 4.09 (q, 2H, J¼7.5 Hz), 4.14
(q, 2H, J¼7.5 Hz), 5.08 (s, 2H), 6.68 (dd, 1H, J¼2.0,
8.5 Hz), 6.70 (d, 1H, J¼8.5 Hz), 6.76 (d, 1H, J¼2.0 Hz),
6.82 (d, 1H, J¼2.0 Hz), 6.83 (dd, 1H, J¼2.0, 8.5 Hz), 6.89
(d, 1H, J¼8.5 Hz), 7.20–7.31 (br m, 5H); 13C NMR
(100.5 MHz, CDCl3): d 14.56 (CH3), 14.61 (CH3), 30.55
(CH2), 31.08 (CH2), 36.33 (CH2), 36.47 (CH2), 56.30
(CH3), 60.63 (CH2), 60.70 (CH2), 71.31 (CH2), 112.98
(CH), 115.60 (CH), 118.22 (CH), 120.05 (CH), 120.43
(CH), 123.70 (CH), 127.22 (2£CH), 127.67 (CH), 128.36
(2£CH), 134.17 (C), 135.97 (C), 137.29 (C), 145.12 (C),
146.44 (C), 148.12 (C), 150.11 (C), 172.78 (CvO), 172.90
(CvO); FT-IR (film, nmax, cm21): 3062w, 3033w, 2980m,
2835m, 2871w, 1732s, 1594m, 1510s, 1421m, 1263s,
4.1.2. 2-Hydroxy-20-methoxy-40,5-dimethyldiphenyl-
ether (16). A solution of 13 (42 mg, 0.171 mmol) and 5%
Pd/C (5 mg) in methanol (5 mL) was stirred at room
temperature for 1.5 h under 1 atm H2. The reaction mixture
was filtered and the filtrate was concentrated to give a
colorless oil. The oil was purified by column chroma-
tography on silica gel using hexane/ethyl acetate (90:10) as
eluant to give 25 mg of 16 as a colorless oil (94%). 1H NMR
(400 MHz, CDCl3): d 2.16 (s, 3H), 2.32 (s, 3H), 3.81 (s,
3H), 6.00 (bs, 1H), 6.62 (d, 1H, J¼1.6 Hz), 6.68–6.77 (m,
3H), 6.85 (d, 1H, J¼8.8 Hz), 6.92 (d, 1H, J¼8.8 Hz); 13C
NMR (100 MHz, CDCl3): d 20.84, 21.44, 55.98, 113.44,
115.42, 118.29, 120.66, 121.37, 124.37, 129.60, 134.93,
143.00, 144.72 (2C), 150.44; HREIMS [MþH]þ: 245.1167
(calcd for [C15H16O3þH]þ, 245.1178).
4.1.3. Ethyl 3-(3-bromo-4-hydroxyphenyl)propionate
(18). To a stirred solution of ethyl 3-(4-hydroxyphenyl)-
propionate (17, 971 mg, 5 mmol) in acetic acid (10 mL) was
slowly added a solution of bromine (0.13 mL, 2.5 mmol) in
acetic acid (10 mL) at room temperature. After the addition,
stirring was continued for 30 min. The reaction mixture was
then diluted with 80 mL of ethyl acetate and washed with
brine (2£30 mL). The organic layer was dried over
anhydrous MgSO4, filtered and concentrated. The crude
product was purified by column chromatography on silica
gel using hexane/ethyl acetate (90:10) as eluant to afford
1
1.16 g of 18 as a white solid (85%). H NMR (500 MHz,

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X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
1215s, 1157s, 1037s, 886m, 809m, 739m, 697m; HRESMS
[MþH]þ: 507.2399 (calcd for [C30H34O7þH]þ, 507.2383).
(236 mg, 0.64 mmol) in THF (2 mL) at 2788C under
argon. The mixture was stirred at 2788C for 1.5 h before
adding a solution of 22 (86 mg, 0.16 mmol) in THF (2 mL).
After the addition, stirring was continued at 2788C for
another 2 h, then the reaction was quenched with 1N HCl
(10 mL) before allowing the reaction mixture to warm to
room temperature. The mixture was extracted with ethyl
acetate (40 mL). The extracts were washed with 1N HCl
(10 mL) and brine (10 mL). The organic solution was dried
over anhydrous MgSO4, filtered, and concentrated. The
product was isolated by column chromatography on silica
gel using hexane/ethyl acetate (85:15) as eluant to afford 23
4.1.6. 3-{4-[2-Benzyloxy-5-(2-carboxyethyl)phenoxy]-3-
methoxyphenyl}propionic acid (21). A mixture of 20
(133 mg, 0.26 mmol), NaOH (1.28 g, 32 mmol), and 7 mL
of H2O/THF/MeOH (4:2:1) was stirred at room temperature
for 2 h and then acidified (pH 1) with concentrated HCl. The
mixture was extracted with ethyl acetate (80 mL). The
organic layer was dried over anhydrous MgSO4, filtered
and concentrated to give 110 mg of 21 as a pale yellow
1
solid (94%). H NMR (500 MHz, CDCl3): d 2.53 (t, 2H,
1
J¼6.5 Hz), 2.74 (t, 2H, J¼7.5 Hz), 2.82 (t, 2H, J¼6.5 Hz),
2.99 (t, 2H, J¼7.5 Hz), 3.79 (s, 3H), 5.18 (s, 2H), 6.52 (d,
1H, J¼2.0 Hz), 6.72 (dd, 1H, J¼2.0, 8.5 Hz), 6.76 (dd, 1H,
J¼2.0, 8.5 Hz), 6.83 (d, 1H, J¼2.0 Hz), 6.86 (d, 1H, J¼
8.5 Hz), 6.87 (d, 1H, J¼8.5 Hz), 7.25–7.29 (br m, 5H); 13C
NMR (100.5 MHz, CDCl3): d 30.07 (CH2), 30.91 (CH2),
35.94 (2£CH2), 56.21 (CH3), 71.50 (CH2), 113.56 (CH),
115.71 (CH), 118.12 (CH), 120.14 (CH), 120.93 (CH),
123.10 (CH), 127.30 (2£CH), 127.74 (CH), 128.45 (2£CH),
134.03 (C), 135.89 (C), 137.41 (C), 144.44 (C), 147.67
(C), 147.68 (C), 150.67 (C), 178.95 (CvO), 178.96 (CvO);
as a pale yellow oil (111 mg, 70%). H NMR (500 MHz,
CDCl3): d 2.82 (t, 2H, J¼8.0 Hz), 2.92 (t, 2H, J¼8.0 Hz),
3.18 (t, 2H, J¼8.0 Hz), 3.28 (t, 2H, J¼8.0 Hz), 3.72 (s, 3H),
5.04 (s, 2H), 5.07 (s, 4H), 5.08 (s, 2H), 5.08 (s, 2H), 6.47
(dd, 1H, J¼2.0, 8.5 Hz), 6.57–6.65 (br m, 6H), 6.68 (d, 1H,
J¼2.0 Hz), 6.71 (d, 1H, J¼2.0 Hz), 6.80 (d, 1H, J¼8.5 Hz),
7.24–7.41 (br m, 25H), 7.77 (d, 1H, J¼8.5 Hz), 7.82 (d, 1H,
J¼9.0 Hz); 13C NMR (100.5 MHz, CDCl3): d 30.02 (CH2),
30.49 (CH2), 45.65 (CH2), 45.90 (CH2), 56.19 (CH3), 70.47
(2£CH2), 70.96 (CH2), 70.99 (CH2), 71.40 (CH2), 100.45
(CH), 100.50 (CH), 106.51 (2£CH), 113.09 (CH), 115.66
(CH), 118.10 (CH), 120.11 (CH), 120.29 (CH), 121.63 (C),
121.72 (C), 123.44 (CH), 127.21 (2£CH), 127.60 (6£CH),
127.68 (2£CH), 127.71 (2£CH), 128.35 (6£CH), 128.76
(7£CH), 132.80 (2£CH), 135.37 (C), 135.85 (2£C), 136.22
(2£C), 137.12 (C), 137.59 (C), 144.74 (C), 146.41 (C),
147.73 (C), 149.91 (C), 159.74 (C), 159.78 (C), 163.32 (C),
163.35 (C), 199.27 (CvO), 199.39 (CvO); FT-IR (film,
HRESMS
(C26H26O7þNH4]þ, 468.2022).
[MþNH4]þ:
468.2031
(calcd
for
4.1.7. 3-(4-{2-Benzyloxy-5-[2-(methoxymethylcarba-
moyl)ethyl]phenoxy}-3-methoxyphenyl)-N-methoxy-N-
methylpropionamide (22). A mixture of 21 (45 mg,
0.1 mmol), HBTU (76 mg, 0.2 mmol), DIEA (0.12 mL,
0.7 mmol), and N,O-dimethylhydroxylamine hydrochloride
(29 mg, 0.3 mmol) in 2 mL of CH2Cl2 was stirred at room
temperature for 1 h before removal of the solvent. The
residue was partitioned between ethyl acetate (40 mL) and
1N HCl (20 mL). The organic layer was washed sequen-
tially with 1N HCl (10 mL), saturated NaHCO3 (10 mL) and
brine (10 mL). The organic solution was then dried over
anhydrous Na2SO4, filtered and concentrated. The product
was purified by column chromatography on silica gel using
hexane/ethyl acetate (50:50 to 30:70) as eluant to give
n
max, cm21): 3032w, 2926m, 2869w, 1663s, 1598s, 1508s,
1454m, 1257s, 1177s, 1124s, 1026s, 832m, 736m, 697m;
HRESMS [MþH]þ: 995.4174 (calcd for [C66H58O9þH]þ,
995.4159).
4.1.9. Verbenachalcone (2). A mixture of 23 (25 mg,
0.025 mmol), 5% Pd/C (25 mg) and acetic acid (2 mL) was
stirred at room temperature under hydrogen (1 atm) for 2 h,
before being diluted with 5 mL of methanol. After removal
of the catalyst by filtration through anhydrous Na2SO4, the
solution was concentrated and the product purified by
column chromatography on silica gel using CHCl3/MeOH
(100:2) as eluant to afford 2 as a pale yellow oil (12 mg,
88%). 1H NMR (500 MHz, CDCl3): d 2.88 (t, 2H, J¼
7.5 Hz), 3.05 (t, 2H, J¼7.5 Hz), 3.12 (t, 2H, J¼7.5 Hz), 3.22
(t, 2H, J¼7.5 Hz), 3.80 (s, 3H), 5.76 (br s, 1H), 5.98 (s, 1H),
6.13 (br s, 1H), 6.34 (dd, 1H, J¼2.5, 8.5 Hz), 6.34 (dd, 1H,
J¼2.5, 8.5 Hz), 6.58 (d, 1H, J¼2.0 Hz), 6.75 (dd, 1H, J¼
2.0, 8.5 Hz), 6.81 (d, 1H, J¼2.0 Hz), 6.84 (dd, 1H, J¼2.0,
8.5 Hz), 6.90 (d, 1H, J¼8.0 Hz), 6.92 (d, 1H, J¼8.0 Hz),
7.56 (d, 1H, J¼8.5 Hz), 7.58 (d, 1H, J¼8.5 Hz), 12.62 (s,
1H), 12.76 (s, 1H); 13C NMR (100.5 MHz, CDCl3): d 30.37
(CH2), 31.16 (CH2), 39.68 (CH2), 40.10 (CH2), 56.26
(CH3), 103.75 (CH), 103.78 (CH), 107.87 (CH), 107.99
(CH), 113.25, 113.92 (C), 114.19 (C), 116.01 (CH), 117.54
(CH), 121.01 (2£CH), 123.77 (CH), 132.49 (CH), 132.51
(CH), 132.71 (C), 138.08 (C), 143.46 (C), 144.97 (C),
145.47 (C), 150.82 (C), 162.69 (C), 162.79 (C), 165.28 (C),
165.46 (C), 203.85 (CvO), 204.07 (CvO); HRESMS
[MþH]þ: 545.1815 (calcd for [C31H28O9þH]þ, 545.1811).
1
51 mg of 22 as a colorless oil (95%). H NMR (500 MHz,
CDCl3): d 2.65 (t, 2H, J¼8.0 Hz), 2.75 (t, 2H, J¼8.0 Hz),
2.83 (t, 2H, J¼8.5 Hz), 2.95 (t, 2H, J¼8.5 Hz), 3.15 (s, 3H),
3.18 (s, 3H), 3.58 (s, 3H), 3.61 (s, 3H), 3.84 (s, 3H), 5.08 (s,
2H), 6.70 (d, 2H, J¼1.0 Hz), 6.80 (d, 1H, J¼2.0 Hz), 6.85–
6.90 (br m, 3H), 7.22–7.30 (br m, 5H); 13C NMR
(100.5 MHz, CDCl3): d 30.25 (CH2), 30.82 (CH2), 32.49
(2£CH3), 34.05 (CH2), 34.25 (CH2), 56.32 (CH3), 61.46
(2£CH3), 71.30 (CH2), 113.17 (CH), 115.60 (CH), 118.11
(CH), 120.21 (CH), 120.50 (CH), 123.85 (CH), 127.21
(2£CH), 127.65 (CH), 128.24 (2£CH), 134.92 (C), 136.79
(C), 137.32 (C), 145.05 (C), 146.39 (C), 148.02 (C), 150.04
(C), 173.61 (2£CvO); FT-IR (film, nmax, cm21): 3032w,
2936m, 2872w, 1661s, 1594w, 1509s, 1420s, 1266s, 1215s,
1131m, 1034m, 990m, 854w, 808w, 740w, 698w; HRESMS
[MþH]þ: 537.2596 (calcd for [C30H36N2O7þH]þ,
537.2601).
4.1.8. 3-(4-{2-Benzyloxy-5-[3-(2,4-bis-benzyloxyphenyl)-
3-oxopropyl]phenoxy}-3-methoxyphenyl)-1-(2,4-bis-
benzyloxyphenyl)propan-1-one (23). A solution of n-BuLi
(2.5 M in hexane, 0.26 mL, 0.64 mmol) was added to
a stirred solution of 1-bromo-2,4-dibenzyloxybenzene
4.1.10. Verbenachalcone pentaacetate (24). A mixture of
2 (8 mg, 0.015 mmol), acetic anhydride (0.5 mL) and

X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
7909
anhydrous pyridine (2 mL) was stirred at room temperature
under argon for 3 h, then diluted with 30 mL of ethyl
acetate. The organic solution was washed with 1N HCl
(3£10 mL), dried over anhydrous MgSO4, filtered and
concentrated. The product was purified by column chroma-
tography on silica gel using hexane/ethyl acetate (50:50) as
solutions were prepared at 10 mM in DMSO. The DMSO
concentration in the assays was 0.3%. After 48 h the cells
were fixed with 2% glutaraldehyde at 378C for 1 h. The
neurite outgrowth was accessed under a phase-contrast
microscope. Neurite processes with a length equal to or
greater than the diameter of the neuron cell body were
scored as a neurite-bearing cell. The ratio of neurite-bearing
cells to total cells (with at least 100 cells examined/viewing
area; 3 viewing areas/well; 4 wells/sample) was determined
and expressed as a percentage. The data was analyzed by
Student unpaired t-test.
1
eluant to give 24 as a pale yellow oil (10 mg, 90%). H
NMR (500 MHz, CDCl3): d 2.23 (s, 3H), 2.28 (s, 3H), 2.31
(s, 3H), 2.32 (s, 3H), 2.32 (s, 3H), 2.89 (t, 2H, J¼7.5 Hz),
3.00 (t, 2H, J¼7.5 Hz), 3.11 (t, 2H, J¼7.5 Hz), 3.22 (t, 2H,
J¼7.5 Hz), 3.81 (s, 3H), 6.65 (d, 1H, J¼2.0 Hz), 6.73 (dd,
1H, J¼2.0, 8.0 Hz), 6.84 (d, 1H, J¼2.0 Hz), 6.87 (d, 1H, J¼
8.0 Hz), 6.89 (dd, 1H, J¼2.0, 8.5 Hz), 6.95 (d, 1H, J¼
2.0 Hz), 6.97 (d, 1H, J¼2.0 Hz), 7.03 (d, 1H, J¼8.0 Hz),
7.08 (dd, 1H, J¼8.5 Hz), 7.10 (dd, 1H, J¼8.5 Hz), 7.74
(d, 1H, J¼8.5 Hz), 7.81 (d, 1H, J¼8.5 Hz). HRESMS
[MþNH4]þ: 772.2607 (calcd for [C31H28O9þNH4]þ,
772.2605).
Acknowledgements
We thank April Case for performing the initial biological
assays.
4.1.11. 1-(2,4-Dihydroxyphenyl)-3-(4-{5-[3-(2,4-dihy-
droxyphenyl)propyl]-2-hydroxyphenoxy}-3-methoxy-
References
phenyl)propane (25).
A
mixture of 23 (25 mg,
0.025 mmol), 5% Pd/C (20 mg) and THF/MeOH (2:3,
2.5 mL) was stirred at room temperature under hydrogen
(1 atm) for 16 h, before being diluted with 5 mL of
methanol. After removal of the catalyst by filtration through
anhydrous Na2SO4, the solution was concentrated and the
residue purified by column chromatography on silica gel
using CHCl3/MeOH (100:7) as eluant to afford 25 as a pale
1. Li, Y.-S.; Matsunaga, K.; Kato, R.; Ohizumi, Y. J. Nat. Prod.
2001, 64, 806–808.
2. Ahmad, V. U.; Ismail, N.; Amber, A. Phytochemistry 1989,
28(9), 2493–2495.
˚
3. Malterud, K. E.; Anthonsen, T.; Hjortas, J. Tetrahedron Lett.
1976, 17, 3069–3072.
4. Bruneton, J.; Shamma, M.; Minard, R. D.; Freyer, A. J. J. Org.
Chem. 1983, 48, 3957–3960.
1
yellow solid (10 mg, 74%). H NMR (500 MHz, CDCl3/
CD3OD (3:1)): d 1.78 (quintet, 2H, J¼7.5 Hz), 1.91
(quintet, 2H, J¼7.5 Hz), 2.48 (t, 2H, J¼7.5 Hz), 2.50 (t,
2H, J¼7.5 Hz), 2.57 (t, 2H, J¼7.5 Hz), 2.64 (t, 2H, J¼
7.5 Hz), 3.85 (s, 3H), 6.24 (d, 1H, J¼2.5 Hz), 6.26 (dd, 1H,
J¼2.5, 7.5 Hz), 6.28 (d, 1H, J¼2.5 Hz), 6.30 (dd, 1H, J¼
2.5, 8.0 Hz), 6.67 (d, 1H, J¼2.0 Hz), 6.74 (dd, 1H, J¼2.0,
8.5 Hz), 6.80 (dd, 1H, J¼2.0, 8.5 Hz), 6.83 (d, 1H, J¼
2.0 Hz), 6.83 (d, 1H, J¼8.0 Hz), 6.89 (d, 1H, J¼8.5 Hz),
6.90 (d, 1H, J¼8.5 Hz), 6.91 (d, 1H, J¼8.5 Hz); 13C NMR
(100.5 MHz, CDCl3/CD3OD (3:1)): d 29.04 (CH2), 29.27
(CH2), 31.85 (2£CH2), 34.99 (CH2), 35.57 (CH2), 56.22
(CH3), 102.53 (CH), 102.59 (CH), 106.78 (CH), 106.83
(CH), 113.06 (CH), 115.92 (CH), 118.04 (CH), 119.89
(CH), 120.23 (C), 120.26 (C), 121.07 (CH), 124.00 (CH),
130.57 (CH), 130.80 (CH), 134.70 (C), 139.51 (C), 143.37
(C), 144.49 (C), 145.00 (C), 150.20 (C), 155.00 (C), 155.06
(C), 155.21 (C), 155.36 (C); HRESMS [MþH]þ: 517.2222
(calcd for [C31H32O7þH]þ, 517.2226).
5. (a) Landreth, G. E. In Basic Neurochemistry: Molecular,
Cellular and Medical Aspects. 6th ed; Siegel, G. J., Agranoff,
B. W., Albers, R. W., Fisher, S. K., Uhler, M. D., Eds.;
Lippincott/Williams & Wilkins: New York, 1998; pp
383–398, Chapter 19. (b) Kaplan, D. R.; Miller, F. D. Curr.
Opin. Neurobiol. 2000, 10, 381–391.
6. For review of neurotrophic factors in neurodegenerative
diseases see: (a) Siegel, G. J.; Chauhan, N. B. Brain Res.
Rev. 2000, 33, 199–227. (b) Connor, B.; Dragunow, M. Brain
Res. Rev. 1998, 27, 1–39. (c) Hefti, F. J. Neurobiol. 1994, 25,
1418–1435.
7. For examples of small molecules that enhance NGF’s ability
to stimulate neurite outgrowths see: (a) Fukuyama, Y.;
Nakade, K.; Minoshima, Y.; Yokoyama, R.; Zhai, H.;
Mitsumoto, Y. Bioorg. Med. Chem. Lett. 2002, 12,
1163–1166. (b) Dagø, L.; Bonde, C.; Peters, D.; Møller, A.;
Bomholt, S. F.; Hartz, B. P.; Meyer, M.; Drejer, J.; Grønborg,
M. J. Neurochem. 2002, 81, 17–24. (c) Dagø, L.; Peters, D.;
Meyer, M.; Hartz, B.; Kruse, V.; Drejer, J.; Grønborg, M.
Neurochem. Res. 2002, 27(1/2), 107–111. (d) Li, P.;
Matsunaga, K.; Yamamoto, K.; Yoshikawa, R.; Kawashima,
K.; Ohizumi, Y. Neurosci. Lett. 1999, 273, 53–56.
(e) Pradines, A.; Magazin, M.; Schiltz, P.; Le Fur, G.;
Caput, D.; Ferrara, P. J. Neurochem. 1995, 64, 1954–1964.
(f) Middlemiss, P. J.; Glasky, A. J.; Rathbone, M. P.; Werstuik,
E.; Hindley, S.; Gysbers, J. Neurosci. Lett. 1995, 199,
131–134. (g) Angus, W. G. R.; Contreras, M. L. Neurosci
Lett. 1995, 191, 23–26.
4.2. Neurite outgrowth assay
The ability of compounds to enhance NGF’s effects for
stimulating neurite outgrowths was accessed utilizing the
methodology previous reported.7d PC12 cells were dis-
sociated by incubation with 1 mM ethylene glycol-bis-
(2-aminoethylether)-N,N,N0N0-tetraacetic acid (EGTA) in
phosphate-buffered saline (PBS) for 1 h and then were
seeded in 24-well culture plates (2£104 cells/well) coated
with collagen. After 24 h, the medium was changed to test
medium containing various concentrations of NGF (30 ng/
mL for control and 2 ng/mL for test samples), 1% fetal calf
serum, 2% horse serum, and various concentrations of test
compounds (15 and 30 mM). All test compound stock
8. Sawyer, J. S.; Schmittling, E. A.; Palkowitz, J. A.; Smith, III.,
W. J. J. Org. Chem. 1998, 63, 6338–6342, and references
cited therein.
9. (a) Buck, E.; Song, Z. J.; Tschaen, D.; Dormer, P. G.; Volante,
R. P.; Reider, P. J. Org. Lett. 2002, 4(9), 1623–1626.

7910
X. Xing et al. / Tetrahedron 58 (2002) 7903–7910
(b) Gujadhur, R. K.; Bates, C. G.; Venkataraman, D. Org. Lett.
was prepared from 2-bromo-5-methylphenol; see: Vyas, G. N.;
Shah, N. M. Organic Syntheses; 1963; Collect. Vol. IV; p 836.
1H NMR (500 MHz, CDCl3): d 2.38 (s, 3H), 3.77 (s, 3H), 6.61
(dd, 1H, J¼3.5, 9.0 Hz), 6.78 (d, 1H, J¼3.5 Hz), 7.40 (d, 1H,
J¼9.0 Hz.
2001, 3(26), 4315–4317. (c) Fagan, P. J.; Hauptman, E.;
Shapiro, R.; Casalnuovo, A. J. Am. Chem. Soc. 2000, 122,
5043–5051. (d) Kalinin, A. V.; Bower, J. F.; Riebel, P.;
Snieckus, V. J. Org. Chem. 1999, 64, 2986–2987.
(e) Marcoux, J.-F.; Doye, S.; Buchwald, S. L. J. Am. Chem.
Soc. 1997, 119, 10539–10540. (f) Utilizing conditions (14
(1 equiv.), 15 (1.4 equiv.), Cs2CO3 (1.4 equiv.), (CuOTf)2·
PhCH3 or (CuOTf)2·Ph (0.025 equiv.), EtOAc (0.05 equiv.),
12. Kita, Y.; Egi, M.; Takada, T.; Tohma, H. Synthesis 1999(5),
885–897.
´
13. Kotecha, N. R.; Ley, S. V.; Montegani, S. Synlett 1992,
395–399.
˚
1-naphthoic acid (1 equiv.), 4 A activated molecular sieves
14. Ralph, J.; Quideau, S.; Grabber, J. H.; Hatfield, R. D. J. Chem.
Soc., Perkin Trans. 1 1994, 23, 3485–3498.
15. Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D.
Tetrahedron Lett. 1989, 30(15), 1927–1930.
16. Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22(39),
3815–3818.
(250 mg/mmol 14), toluene (5 mL/mmol 14)) similar to those
reported in Ref. 9e failed to yield the desired product.
10. (a) Aranyos, A.; Old, D. W.; Kiyomori, A.; Wolfe, J. P.;
Sadighi, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121,
4369–4378. (b) Mann, G.; Incarvito, C.; Rheingold, A. L.;
Hartwig, J. F. J. Am. Chem. Soc. 1999, 121, 3224–3225.
11. Compound 11 was prepared from 3-bromo-4-benzyloxy-
toluene; see: (a) Cambie, R. C.; Hayward, R. C.; Palmer,
B. D. Aust. J. Chem. 1982, 35, 1679–1697, 1H NMR
(500 MHz, CDCl3): d 2.26 (s, 3H), 5.07 (s, 2H), 5.58 (s,
1H), 6.62 (dd, 1H, J¼1.5, 8.5 Hz), 6.77 (d, 1H, J¼1.5 Hz),
6.81 (d, 1H, J¼8.5 Hz), 7.38–7.40 (m, 5H). (b) Compound 12
17. Treating the lithium salt of 21 with 1-lithio-2,4-dibenzyl-
oxybenzene resulted in very low yield (,5%) of 23.
18. Kametani, T.; Fukumoto, K.; Fujihara, M. Chem. Pharm. Bull.
1972, 20, 1800–1804.
19. Jacovina, A. T.; Zhong, F.; Khazanova, E.; Lev, E.; Deora,
A. B.; Hajjar, K. A. J. Biol. Chem. 2001, 276(52),
49350–49358.