Synthesis and antiproliferative activities of quebecol and its analogs

Article abstract of DOI:10.1016/j.bmcl.2013.07.058

Simple and efficient synthesis of quebecol and a number of its analogs was accomplished in five steps. The synthesized compounds were evaluated for antiproliferative activities against human cervix adenocarcinoma (HeLa), human ovarian carcinoma (SK-OV-3), human colon carcinoma (HT-29), and human breast adenocarcinoma (MCF-7) cancer cell lines. Among all the compounds, 7c, 7d, 7f, and 8f exhibited antiproliferative activities against four tested cell lines with inhibition over 80% at 75 μM after 72 h, whereas, compound 7b and 7g were more selective towards MCF-7 cell line. The IC₅₀ values for compounds 7c, 7d, and 7f were 85.1 μM, 78.7 μM, and 80.6 μM against MCF-7 cell line, respectively, showing slightly higher antiproliferative activtiy than the synthesized and isolated quebecol with an IC₅₀ value of 104.2 μM against MCF-7.

Full text of DOI:10.1016/j.bmcl.2013.07.058

Bioorganic & Medicinal Chemistry Letters 23 (2013) 5329–5331
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and antiproliferative activities of quebecol and its analogs
Kasiviswanadharaju Pericherla a, Amir Nasrolahi Shirazi b, V. Kameshwara Rao a, Rakesh K. Tiwari b,
Nicholas DaSilva b, Kellen T. McCaffrey b, Yousef A. Beni b, Antonio González-Sarrías b,
b,
a,
Navindra P. Seeram b, , Keykavous Parang , Anil Kumar
⇑
⇑
⇑
a Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan, India
b Department of Biomedical and Pharmaceutical Sciences, College of Pharmacy, University of Rhode Island, Kingston 02881, RI, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Simple and efficient synthesis of quebecol and a number of its analogs was accomplished in five steps.
The synthesized compounds were evaluated for antiproliferative activities against human cervix adeno-
carcinoma (HeLa), human ovarian carcinoma (SK-OV-3), human colon carcinoma (HT-29), and human
breast adenocarcinoma (MCF-7) cancer cell lines. Among all the compounds, 7c, 7d, 7f, and 8f exhibited
Received 26 May 2013
Revised 23 July 2013
Accepted 25 July 2013
Available online 2 August 2013
antiproliferative activities against four tested cell lines with inhibition over 80% at 75
whereas, compound 7b and 7g were more selective towards MCF-7 cell line. The IC50 values for com-
pounds 7c, 7d, and 7f were 85.1 M, 78.7 M, and 80.6 M against MCF-7 cell line, respectively, showing
slightly higher antiproliferative activtiy than the synthesized and isolated quebecol with an IC50 value of
104.2 M against MCF-7.
lM after 72 h,
Keywords:
Antiproliferative
Phenolic
Quebecol
Structure–activity relationship
Synthesis
l
l
l
l
Ó 2013 Elsevier Ltd. All rights reserved.
Maple syrup is obtained by thermal evaporation of sap collected
from certain maple (Acer) species including the sugar maple (A.
saccharum) tree, and contains a mixture of native phenolics and
compounds formed during the intensive heating process required
to transform maple sap into syrup.1 Quebecol [2,3,3-tri-(3-meth-
oxy-4-hyroxyphenyl)-1-propanol] (Fig. 1) was isolated by us from
a butanol extract of Canadian maple syrup,2,3 and it was named
after the province of Quebec in Canada the world’s largest producer
of maple syrup. Quebecol is a process-derived polyphenolic com-
pound with a unique chemical structure that has never before been
identified in nature nor is it present in sap.
Quebecol displays some similarity to the known drug, tamoxi-
fen that is a widely used chemotherapy agent for hormonally
dependent cancers such as breast cancer. However, tamoxifen
has severe side effects. Quebecol is a phytochemical derived com-
pound present in maple syrup, which has been consumed for cen-
turies without showing toxicity. Thus, based on structural
similarities to tamoxifen and in vitro biological assays4 on breast
and colon cancer cell lines in our laboratory, we hypothesized that
quebecol and analogs could exert greater anticancer effects than
tamoxifen without the adverse side effects. Moreover, it has been
reported that maple syrup extracts have antioxidant, antimuta-
genic, and human cancer cell antiproliferative properties.5
Unfortunately, sufficient quantity of the pure quebecol could
not be isolated to conduct detailed biological evaluation. Thus,
we focused our attention towards the synthesis of quebecol and
its analogs to evaluate their biological activities.6 Herein, we report
total synthesis of quebecol and its derivatives as mixed isomers
and evaluation of their antiproliferative activities against a panel
of cancer cell lines.
Synthesis of quebecol and its analogs was accomplished as de-
picted in Scheme 1. The key starting materials, substituted bromo-
benzenes (1a–d) as well as benzaldehydes (2a–d) were
synthesized by benzylation of commercially available phenolic
compounds with benzyl bromide in the presence of potassium car-
bonate in acetone. The synthesis of quebecol (8a) commenced with
Grignard reaction of 1-(benzyloxy)-4-bromo-2-methoxybenzene
(1a) and 4-(benzyloxy)-3-methoxybenzaldehyde (2a) to give
(bis(4-(benzyloxy)-3-methoxyphenyl)methanol (3a) in 46% yield.
Similarly, lithiation of bromo compound 1a using n-BuLi at
OCH3
OH
HO
HO
OH
OCH3
OCH3
⇑
Corresponding author.
E-mail addresses: nseeram@uri.edu (N.P. Seeram), kparang@uri.edu (K. Parang),
Figure 1. Structure of quebecol isolated from butanol extract of Canadian maple
syrup.
anilkumar@pilani.bits-pilani.ac.in (A. Kumar).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.07.058

5330
K. Pericherla et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5329–5331
OH
Br
Br
CHO
i
+
ii
R
R
R
R
R
R
R1
R1
R1
R1
R1
R1
1a-d
2a-d
3a-d
4a-d
a R = OBn, R1 = OCH3; b R = R1 = OCH3;
c R = OBn, R1 = H; d R = OCH3, R1 = H
R3
R2
OEt
a' R2 = OBn, R3 = OCH3; b' R2 = R3 = OCH3;
c' R2 = OBn, R3 = H; d' R2 = OCH3, R3 = H;
e' R2 = H, R3 = OCH3; f' R2 = CH3, R3 = H;
g' R2 = R3 = H;
iii
O
5a'-g'
R3
R3
R3
R2
R4
R2
O
HO
HO
EtO
v
iv
R
R
HO
OH
R
R
R1
7aa'-dg'
R1
R1
R1
R1
R1
6aa'-dg'
9a-g
v
R3
For 8 & 9
R4
a R1 = OCH3, R3 = OCH3, R4 = OH;
b R1 = R3 = H; R4 = OCH3;
c R1 = H, R3 = OCH3, R4 = H;
d
O
EtO
R
1 = R3 = H, R4 = CH3;
e
R
1 = R3 = H, R4 = OH;
f R1 = R3 = R4 = H
HO
OH
g R1 = H, R3 = OCH3, R4 = OH
R1
R1
8a-g
Scheme 1. Synthesis of quebecol and its derivatives. Reagents and conditions: (i) n-BuLi, THF, ꢀ78 °C, 2 h; (ii) CH3COBr, benzene, 25–30 °C, 5 h; (iii) N,N-diisopropylamine, n-
BuLi, THF, ꢀ78 °C, 3 h; (iv) LiAlH4, THF, 0 °C, 2 h; (v) Pd/C, HCO2NH4, MeOH: EtOAc, 25–30 °C, 16 h.
ꢀ78 °C followed by the addition of aldehyde (2a) offered good
yields of 3a (45–62%) along with minor impurity, 1-(4-(benzyl-
oxy)-3-methoxy phenyl)pentan-1-ol, produced by direct substitu-
tion of butyl ion on benzaldehyde. Although, Grignard reaction
furnished reasonable yield of 3a, n-BuLi reaction was preferred
because of the concern over reproducibility of Grignard results.
The reaction of biphenyl alcohol 3a with ethyl 2-(4-(benzyl-
oxy)-3-methoxyphenyl)acetate (5a0) in the presence of a catalytic
amount of p-toluenesulphonic acid in benzene and 1,2-dichloro-
ethane to obtain coupling product, ethyl 2,3,3-tris(4-(benzyloxy)-
3-methoxyphenyl)propanoate (6aa0) as reported by Harig et al.7
on similar substrates. Unfortunately, disproportionation products,
bis(4-(benzyloxy)-3-methoxyphenyl)methanone and bis(4-(ben-
zyloxy)-3-methoxyphenyl)methane resulted as major products,
which can also be probable because of electron rich aryl rings in
acidic media.8 Wittig reaction of bis(4-(benzyloxy)-3-methoxy-
phenyl)methanone and (1-(4-(benzyloxy)-3-methoxyphenyl)-
2-ethoxy-2-oxoethyl)triphenylphosphonium bromide failed to
give corresponding ethyl 3,3-bis(4-(benzyloxy)-3-methoxy-
phenyl)-2-phenylacrylate.
Bromination of diphenyl methanol derivative (3a) to give 4a
was considered as an alternative, which can subsequently react
with ester (5a0) in the presence of lithium diisopropylamide
(LDA) to give coupled product (6aa0). Several failed attempts were
made for the bromination of alcohol (3a) using brominating agents
such as PBr3, POBr3, and CBr4/PPh3 (Apple reaction). With PBr3, the
reaction proceeded smoothly, but isolation was found to be tricky
because of labile nature of bromo compound towards moisture
(work up and purifications trials led to massive decomposition to
starting material). Surprisingly, disproportionation was observed
when POBr3 used as brominating agent. No conversion was
observed in case of Apple reaction conditions. Even tosylation,
mesylation and chlorination (using SOCl2)9 of 3a also showed
negative results. Finally, acetyl bromide was found to be effective,
and the bromo compound was isolated with excellent yield (80%).
By-product, acetic acid, was removed by simple hexane washings
under N2 atmosphere and the reaction proceeded to the next step
without further purification.
LDA was in situ generated from N,N-diisopropylamine and
n-BuLi (1.6 M in hexanes) at 0 °C and then treated with ester
(5a0) at ꢀ78 °C. Addition of the bromo compound resulted in cou-
1
pled product (6aa0) in good yield (52%). The H NMR of 6aa0
showed characteristic peaks at d 4.38 (d, J = 12.0 Hz, 1H) and 4.09
(d, J = 12.0 Hz, 1H). Presence of peaks at d 57.08 and 54.33 in addi-
tion to all other carbons of 6aa0 in 13C NMR and peak at m/z 756.5
for [M+H2O]+ ion in ESI-MS spectrum confirmed the structure of
coupling product 6aa0.
Subsequent reduction of ethyl ester (6) to alcohol (7) was
achieved by lithium aluminum hydride in THF at 0 °C for 2 h with
excellent yields (67–87%). The final step for the synthesis of queb-
ecol and its derivatives involved the removal of benzyl protecting
group, which was effected by reaction of compound 7 with Pd/C
and ammonium formate as a hydrogen source at 25–30 °C for
16 h to provide quebecol and its analogs (9a–g) in excellent yields
(63–88%) as mixed optical isomers. Debenzylation of compound
(6) using the same method afforded compounds 8a–g (Scheme 1).
All compounds were characterized by their 1H and 13C NMR
spectra data (Supplementary data).
Synthesized quebecol and its derivatives 7aa0-dg0, 8a–g and
9a–f were assessed for their in vitro antiproliferative activity
against a panel of cancer cell lines; human cervix adenocarcinoma
(HeLa), human ovarian carcinoma (SK-OV-3), human colon carci-
noma (HT-29), and human breast adenocarcinoma (MCF-7). The
antiproliferative activities of 7aa0–dg0, 8a–g and 9a–f at 75
lM
concentration is shown in terms of percentage cell viability after
72 h (Fig. 2).

K. Pericherla et al. / Bioorg. Med. Chem. Lett. 23 (2013) 5329–5331
5331
250
200
150
100
50
HeLa
SK-OV-3
HT-29
MCF-7
0
Figure 2. Comparative antiproliferative activity of quebecol and its derivatives.
Ethyl ester derivatives 8c, 8d, 8f, and alcohol derivative 9f
inhibited proliferation of four tested cell lines over 80% at 75
after 72 h. Compounds 8b and 8g selectively inhibited over 75%
proliferation of MCF-7 cells at 75 M after 72 h.
Synthesized quebecol 9a was also compared with isolated
quebecol and showed a similar IC50 value of 103.2 M against
MCF-7 after 72 h treatment. Some of the quebecol derivatives
showed slightly higher antiproliferative activity compared to
quebecol itself. For example, the IC50 values for compounds 8c,
our laboratory to determine the mechanism of action of this class
of compounds.
lM
l
Acknowledgments
l
The authors thank UGC, New Delhi [Grant No. 39-733/2010
(SR)] for financial support. K.P. thanks UGC, New Delhi for junior
research fellowships. We also acknowledge National Center for Re-
search Resources, NIH, and Grant Number 8 P20 GM103430-12 for
sponsoring the core facility. This project was partially supported by
the Federation of Quebec Maple Syrup Producers (Quebec, Canada).
8d, and 8f were 85.1 lM, 78.7 lM and 80.6 lM, respectively,
against MCF-7 cell line. The quebecol derivatives with protected
phenolic groups on the phenyl ring (6 and 7) were not active
against most of the cell lines. Structure–activity relationship stud-
ies indicate that the presence of free phenolic group at para-posi-
tion of the phenyl ring is essential for antiproliferative activity
and smaller substitutents at meta-position or no substitient is
preferred.
The results showed that quebecol and some of its analogs exert
cytotoxic effect on cancer cell lines suggesting that they may have
potential as cancer chemopreventive agents. However, it should be
noted that some compounds such as polyphenols are known to be
poorly bioavailable, and extensively metabolized and converted by
colonic microbiota into other bioactive forms. Thus, the current
study provides initial data to support the antiproliferative poten-
tial of some quebecol derivatives.
In conclusion, we developed a simple and efficient method for
the synthesis of quebecol and its analogs. The method allowed
the introduction of different functional groups in the three aryl
rings and gave good overall yield in five steps. The synthesized
compounds were evaluated for antiproliferative activity against a
panel of cancer cell lines. Three analogs of quebecol viz. 8c, 8d
and 8f showed slightly higher antiproliferative activity compared
to quebecol itself. The structure–activity relationship data provide
insights for further optimization of the quebecol scaffold in discov-
ery of antiproliferative agents. Additional studies are ongoing in
Supplementary data
Supplementary data (including copies of the 1H and 13C NMR of
the synthesized compounds) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.
2013.07.058.
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