Synthesis of bisphenol neolignans inspired by honokiol as antiproliferative agents

Article abstract of DOI:10.3390/molecules25030733

Honokiol (2) is a natural bisphenol neolignan showing a variety of biological properties, including antitumor activity. Some studies pointed out 2 as a potential anticancer agent in view of its antiproliferative and pro-apoptotic activity towards tumor cells. As a further contribution to these studies, we report here the synthesis of a small library of bisphenol neolignans inspired by honokiol and the evaluation of their antiproliferative activity. The natural lead was hence subjected to simple chemical modifications to obtain the derivatives 3–9; further neolignans (12a-c, 13a-c, 14a-c, and 15a) were synthesized employing the Suzuki–Miyaura reaction, thus obtaining bisphenols with a substitution pattern different from honokiol. These compounds and the natural lead were subjected to antiproliferative assay towards HCT-116, HT-29, and PC3 tumor cell lines. Six of the neolignans show GI₅₀ values lower than those of 2 towards all cell lines. Compounds 14a, 14c, and 15a are the most effective antiproliferative agents, with GI₅₀ in the range of 3.6–19.1 μM, in some cases it is lower than those of the anticancer drug 5-fluorouracil. Flow cytometry experiments performed on these neolignans showed that the inhibition of proliferation is mainly due to an apoptotic process. These results indicate that the structural modification of honokiol may open the way to obtaining antitumor neolignans more potent than the natural lead.

Full text of DOI:10.3390/molecules25030733

molecules
Article
Synthesis of Bisphenol Neolignans Inspired by
Honokiol as Antiproliferative Agents
Nunzio Cardullo 1,*, Vincenza Barresi 2, Vera Muccilli 1 , Giorgia Spampinato 2,
Morgana D’Amico 2, Daniele Filippo Condorelli 2 and Corrado Tringali 1,
*
1
Department of Chemical Sciences, University of Catania, Viale A. Doria 6, 95125 Catania, Italy;
v.muccilli@unict.it
Department of Biomedical and Biotechnological Sciences, Section of Medical Biochemistry,
University of Catania, Via Santa Sofia 97, 95123 Catania, Italy; vincenza.barresi@unict.it (V.B.);
giorgiaspampinato@unict.it (G.S.); morganadamico01@gmail.com (M.D.);
daniele.condorelli@unict.it (D.F.C.)
2
*
Correspondence: ctringali@unict.it (C.T.); ncardullo@unict.it (N.C.); Tel.: +39-095-7385025 (C.T.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: David Barker
Received: 15 January 2020; Accepted: 5 February 2020; Published: 7 February 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Honokiol (2) is a natural bisphenol neolignan showing a variety of biological properties,
including antitumor activity. Some studies pointed out 2 as a potential anticancer agent in view
of its antiproliferative and pro-apoptotic activity towards tumor cells. As a further contribution to
these studies, we report here the synthesis of a small library of bisphenol neolignans inspired by
honokiol and the evaluation of their antiproliferative activity. The natural lead was hence subjected
to simple chemical modifications to obtain the derivatives 3–9; further neolignans (12a-c, 13a-c, 14a-c,
and 15a) were synthesized employing the Suzuki–Miyaura reaction, thus obtaining bisphenols with a
substitution pattern different from honokiol. These compounds and the natural lead were subjected
to antiproliferative assay towards HCT-116, HT-29, and PC3 tumor cell lines. Six of the neolignans
show GI50 values lower than those of 2 towards all cell lines. Compounds 14a, 14c, and 15a are the
most effective antiproliferative agents, with GI50 in the range of 3.6–19.1 µM, in some cases it is lower
than those of the anticancer drug 5-fluorouracil. Flow cytometry experiments performed on these
neolignans showed that the inhibition of proliferation is mainly due to an apoptotic process. These
results indicate that the structural modification of honokiol may open the way to obtaining antitumor
neolignans more potent than the natural lead.
Keywords: honokiol; bisphenol neolignans; polyphenols; Suzuki–Miyaura cross-coupling; antitumor
activity; apoptosis
1. Introduction
The biaryl skeleton is relatively common among natural products and this structural feature is
distinctive of bisphenol neolignans, a group of polyphenols belonging to the neolignan family [
These compounds are biosynthesized through oxidative coupling of phenoxy radicals generated by
enzymes such as laccase, peroxidase, or a cytochrome P450 [ ]. The most representative bisphenol
1].
2
neolignans are magnolol (1, Figure 1) and honokiol (2, Figure 1), originally isolated from roots and stem
bark of Magnolia officinalis and Magnolia obovata. The extracts of Magnolia spp. (mainly M. officinalis)
have been employed for centuries in traditional Chinese and Japanese medicine to treat many diseases,
including anxiety, allergy, or gastrointestinal disorders [3,4]. These extracts have shown to possess
promising biological activities, including anti-inflammatory, antioxidant, antiviral, anti-depressant,
and anti-platelet activity [4,5].
Molecules 2020, 25, 733; doi:10.3390/molecules25030733
www.mdpi.com/journal/molecules

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Figure 1. Chemical structures of magnolol (1) and honokiol (2).
Magnolol and honokiol are the main bioactive ingredients of these extracts [
array of biological properties, including antioxidant [ ], anti-inflammatory [
and antitumor activity [10 11]. Specifically, 1 and 2 inhibit proliferation of tumor cells, inducing
differentiation and apoptosis, and suppressing angiogenesis [12 15]. Furthermore, their unique
6] and have shown an
6,7
8], neuroprotective [9],
,
–
pharmacophore structure, that is two phenolic rings linked through a C–C bond allows the interaction
with a variety of biological targets [16].
The above-cited properties have prompted many researchers to synthesize magnolol and
honokiol analogues and evaluate their biological properties to obtain new potential therapeutic
agents. These efforts have afforded new bisphenol neolignans with optimized properties, among
which antimicrobial [17,18], neuroprotective [19], anti-inflammatory [20], antitumor [18,21,22],
and antiangiogenic activity [23]. According to some studies, the antitumor activity of honokiol,
magnolol, and their analogues is related to the presence of free hydroxyl group and allylic chains on a
bisphenolic moiety [18,23].
Although several synthetic methods have been employed to obtain biaryl compounds,
the Pd-catalyzed Suzuki–Miyaura (S–M) cross-coupling reaction is one of the most efficient [24,25].
Moreover, with respect to other Pd-catalyzed reactions, S–M coupling has the advantage of requiring
mild conditions and employing commercially available boronic acids that are environmentally safer
than organometallic reagents [24]. On the other hand, oxidative coupling methods, based on the
use of enzymes such as horseradish peroxidase [26], allow the synthesis of bisphenol neolignans in
eco-friendly conditions but provide moderate or poor yield.
Thus, in continuation of our previous studies on the synthesis of natural-derived polyphenols
with antitumor [27
–31], antioxidant, hypoglycemic [32,33], antifungal [34], and anti-inflammatory
activity [35 36], we oriented our recent works toward the synthesis of magnolol analogues, which
,
were evaluated as potential antidiabetic [26], anticancer [37], and antioxidative [38] agents. As a
further contribution, in the present work we report the synthesis of bisphenols neolignans inspired
by honokiol (2). All the synthetic neolignans, in comparison with 2, have been evaluated for their
potential antiproliferative activity towards three tumor cell lines, (HCT-116, HT-29 and PC3).
2. Results and Discussion
2.1. Synthesis
On the basis of the above-cited biological properties of the natural lead 2, we planned to synthesize
a small library of honokiol-inspired bisphenol neolignans. A first group of honokiol analogues was
obtained through simple modifications of 2, as depicted in Scheme 1. By acetylation and methylation,
we obtained compounds 3 and 4, respectively. Peracetate derivatives usually undergo in vivo enzymatic
hydrolysis and are frequently prepared to overcome the low metabolic stability and poor bioavailability
of natural polyphenols [29
cases enhanced biological activity and high metabolic stability [39
,
39], whereas methylated analogues of polyphenols have shown in many
40]. The neolignans 2, 3, and 4 were
,
subjected to catalytic hydrogenation to give respectively 5, 6, and 7, as it is useful to establish the
possible role of the terminal double bond. The spectroscopic data of compounds 3–5, and 7 were in
agreement with those previously reported in the literature [41,42], whereas the new bisphenol neolignan

Molecules 2020, 25, 733
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6 was subjected to spectroscopic characterization and the analysis of HRMS, 1H and 13C-NMR spectra
confirmed the expected structure.
Scheme 1. Synthesis of honokiol derivatives 3–9.
According to the above cited report [18], the allylic chains on the bisphenolic core of honokiol
are important structural requirements for antiproliferative activity; thus, we planned to investigate
the effect of further allylic or O-allylic substituents. Namely, 2 was subjected to SN2 reaction with
allyl bromide to obtain the bis-O-allyl honokiol 8, whose structure was confirmed by analysis of
1
its H and 13C-NMR data, in agreement with those previously reported [20]. As a further step,
the Claisen rearrangement of 8 was planned to obtain the bis-C-allyl derivative 9. This reaction was
carried out in mild conditions, namely at room temperature and in the presence of Et2AlCl which
catalyzes the [
3
,
3
]-sigmatropic rearrangement via an ether–aluminum complex, avoiding the use
1
of high temperature. The analysis of H and 13C-NMR data, in agreement with those reported in
literature, confirmed the structure of 9 [43].
Another set of bisphenol neolignans has been synthesized starting from commercial phenolics
and employing the synthetic strategy reported in Schemes 2 and 3 based on the Suzuki–Miyaura
cross-coupling reaction; accordingly, bisphenols 12a-c, 13a-c, 14a-c, and 15a were obtained.

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Scheme 2. Synthesis of bisphenol neolignans 12a-c. (
a) These conditions were employed to obtain 11a
and 11b; (b) these conditions were employed to obtain 11c.
Scheme 3. Synthesis of bisphenol neolignans 13a-c, 14a-c, and 15a.
Schemes 2 and 3 summarize the final reaction conditions employed for each step, these were then
optimized through a series of preliminary reactions. More specifically, the substrate 10a was used to
optimize both the bromination and the S–M reactions. Preliminary experiments for the bromination
were carried out employing three different brominating agents (namely, Br2, N-bromosuccinimide,
and NaBr/oxone) with or without a catalyst (AlCl3 or I2) and testing different solvents (CH3CN, CHCl3,
and acetone). The reaction mixtures were analyzed by HPLC-UV on a C18 reversed-phase column
in order to quantify the yield of the product 11a. These experiments are reported in detail in the
experimental section and the results are summarized in Table 1.

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Table 1. Reactions for bromination of 10a.
Entry
Brominating Agent
Catalyst
Solvent1
% Yield (11a)2
1
2
3
4
5
NBS
NBS
NBS
NBS
Br2
I2
I2
AlCl3
AlCl3
CH3CN
CHCl3
CH3CN
CHCl3
CH3CN
CHCl3
CHCl3
10
12
15
25
18
47
63
5
AlCl3
6
Br2
Br2
NaBr/oxone
/
/
/
2
73
84
acetone/water
If it is not indicated, the reactions were carried out at rt. The yield was determined by HPLC-UV. 3 The reaction
1
was carried out at 0 ◦C. 4 The reaction was performed at −10 ◦C.
When the substrate 10a was treated with Br2 (entry 6) the expected monobromo derivative
was obtained with higher yield (47%) respect to when the reaction occurred in other conditions.
Furthermore, by working at 0 ◦C (entry 7) the yield grew up to 63%.
The same methodology was employed for the bromination of 10b, thus obtaining 11b with 67%
yield; 10c afforded 11c with a lower yield (36%), hence, we applied the procedure previously described
by Bovicelli et al. [44], namely by treating tyrosol (10c) with NaBr and oxone; in contrast with the low
yield obtained for 11a (entry 8), 11c was recovered with 78% yield.
Also for the S–M coupling step a careful analysis of different reaction conditions was performed;
the results for the coupling of 11a with 4-hydroxyphenyl boronic acid are summarized in Table 2,
reporting the yields for the product 12a, subsequently established as the expected bisphenol neolignan
(see below). The reaction was carried out varying solvent or solvent mixtures, the temperature or the
bromide concentration (entries 3 and 4); Pd(OAc)2 and the ligand 1,10-bis(diphenylphosphino)ferrocene
(dppf) were used to generate in situ the catalyst, and K2CO3 as base. The yield of neolignan 12a was
determined by HPLC-UV analysis of the reaction mixtures. The results clearly indicated the best
conditions for this step: a mixture THF/H2O as solvent system at 70 ◦C, with 0.05 M concentration of
bromide, affording 12a with 67% yield.
Table 2. Reactions for Suzuki–Miyaura cross coupling of 11a.
Entry
Solvent
Temperature
% Yield (12a)1
1
2
3
4
5
6
THF
THF
25 ◦C
70 ◦C
70 ◦C
70 ◦C
70 ◦C
180 ◦C
5
10
20
67
6
THF/H2O2
THF/H2O3
1,4-dioxane
1,4-dioxane
8
1
The yield was determined by HPLC-UV. 2 The concentration of bromide 11a was 0.10 M. 3 The concentration of
bromide 11a was 0.05 M.
On the basis of these encouraging results, the S–M coupling was carried out in a preparative
scale and, after purification, 12a was submitted to a complete characterization by means of HRMS,
1H and 13C-NMR spectra analysis, including two-dimensional methods (COSY, HSQC, and HMBC).
The HRMS spectrum confirmed the formation of a biphenyl structure. The NMR spectra showed the
signals of a typical AA0XX0 aromatic spin system assigned to ring B of 12a, namely two proton doublets
at
and 115.0 (C-3B/C-5B). Two sp2 quaternary carbon signals were assigned at C-4B (
(δ 134.4) on the basis of chemical shift and HMBC correlations with H-2B/H-6B and H-3B/H-5B. C-1B
also showed a correlation with the singlet at 6.78, assigned to H-2A; this signal was HMBC correlated
with the carbon at 134.6, assigned to C-1A on the basis of further HMBC correlations with H-5A and
δ
7.14 (H-2B/H-6B) and 6.85 (H-3B/H-5B), with corresponding carbon signals at
δ
130.7 (C-2B/C-6B)
δ
154.4) and C-1B
δ
δ

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H-2B/H-6B; C-1A was further correlated with the signal at
δ
2.47, evidently due to the H2-7A protons
of the propyl chain. Overall, these and other HMBC data unambiguously established structure 12a.
The new bisphenol neolignan 12a was reacted with allyl bromide and afforded the bis-O-allyl
derivative 13a, whose structure was confirmed by analysis of HRMS, 1H and 13C-NMR spectra. Namely,
the mass spectrum proved that a double substitution occurred. This was of course confirmed by the
NMR data clearly showing signals due to two allyl chains; these were distinguished on the basis of the
HMBC correlations of C-3A and C-4B with the pertinent methylene signals in the 1H-NMR spectrum.
As final step, the allyloxy neolignan 13a was used as substrate for a Claisen rearrangement
(carried out as above reported for the preparation of 9) affording two main products. The HRMS
and NMR data of the more polar product indicated that both allyl chains underwent the Claisen
rearrangement. COSY and HMBC experiments corroborated this assumption, thus establishing
structure 14a for this bisphenol neolignan. The less polar product showed 1H and 13C-NMR signals of
ring B and those of one O-allyl chain substantially superimposable with those of 13a; the other signals
indicated that the rearrangement occurred only for the ring A chain; thus, structure 15a was assigned
to this product.
With the same protocol (Schemes 2 and 3) the new bisphenols 12b and c, 13b and c, and 14b and c
were obtained and fully characterized.
2.2. Biochemical Assay
The honokiol derivatives 3–9 and the bioinspired bisphenol neolignans 12a-c, 13a-c, 14a-c, and 15a
were evaluated as potential antiproliferative agents towards three tumor cell lines: HCT-116, HT-29
(both human colorectal adenocarcinoma) and PC3 (human prostate adenocarcinoma) employing the
MTT colorimetric assay. The anticancer drug 5-fluorouracil (5-FU) was used as positive control, while
honokiol (2) was included in the study for comparison. The results are reported in Table 3 as GI50
values (
on one cell line, a higher activity than that of the lead compound honokiol (GI50 = 18.2, 40.6 and 52.1
M towards HCT-116, HT-29, and PC3 cells, respectively). In particular, six compounds (9, 12b, 14a-c,
15a) show GI50 values lower than those of 2 for all cell lines, with GI50 values in the range 3.6–47.9
M. Also 5, the hydrogenated analogue of 2, shows GI50 values lower than those of the natural lead
µM) and in Figure 2 for the sake of clarity. The majority of the tested compounds shows, at least
µ
µ
for both HT-29 and PC3 cell lines and a comparable value towards HCT-116. Finally, compound 3 is
comparable to 2 towards both HCT-116 and PC3 cell lines. These results indicate that the structural
modification of honokiol may open the way to obtaining antitumor neolignans more potent than the
lead compound.
Figure 2. GI50 values (µM) of bisphenol neolignans 2–9, 12a-c, 13a-c, 14a-c, and 15a and of the reference
compound 5-fluorouracil (5-FU) on HCT-116, HT-29, and PC3 cell lines after an incubation time of 72 h.
The results shown are means ± SD of four experiments.

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Table 3. Antiproliferative activity of bisphenol neolignans inspired by honokiol.
GI50 (µM) ± SD 1
Compound
HCT-116
HT-29
PC3
2
3
4
5
6
7
8
9
18.2 ± 2.1
22.5 ± 3.4
63.9 ± 5.9
21.2 ± 2.6
20.1 ± 2.9
64.7 ± 7.9
38.1 ± 0.6
11.1 ± 0.9
24.1 ± 3.0
14.7 ± 2.1
> 100
40.6 ± 3.9
52.1 ± 7.1
51.3 ± 7.2
> 100
10.5 ± 1.7
20.6 ± 3.2
> 100
> 100
> 100
9.9 ± 2.1
> 100
70.2 ± 8.7
40.7 ± 0.5
9.8 ± 1.4
65.9 ± 8.3
20.5 ± 3.1
> 100
> 100
19.8 ± 1.8
12a
12b
12c
13a
13b
13c
14a
14b
14c
15a
5-FU
> 100
47.9 ± 4.7
> 100
> 100
> 100
> 100
5.8 ± 1.9
17.2 ± 1.5
19.1 ± 2.6
8.9 ± 2.0
9.0 ± 0.9
> 100
> 100
69.7 ± 6.9
84.5 ± 8.0
5.3 ± 1.5
8.2 ± 1.1
3.7 ± 0.7
3.6 ± 0.6
6.2 ± 0.8
42.3 ± 4.7
42.0 ± 4.1
13.0 ± 2.0
12.3 ± 1.6
11.3 ± 2.2
12.7 ± 2.1
7.3 ± 0.7
1
GI50 value were calculated after 72 h of continuous exposure relative to untreated controls; values are the mean
SD) of four experiments. HCT-116 and HT-29: human colorectal adenocarcinoma cells. PC3: human prostate
cancer cells. 5-FU: 5-fluorouracil.
(±
The bisphenol neolignans 14a, 14c, and 15a gave the most promising results, in particular 14a
and 15a showed antiproliferative activity higher or comparable with that of the anticancer drug 5-FU
against both HCT-116 and PC3.
Although the data reported in Table 3 do not allow conclusive assessments about the
structural determinants required for an optimized antiproliferative activity of honokiol-inspired
neolignans analogues, some considerations about structure-activity relationships can be made and are
reported below.
The neolignans 9, 12b, 14a-c, and 15a, with higher antiproliferative activity than 2, possess one
allyl or propyl chain in ortho position to a free phenolic group. Among these, 14a, 14b and 14c
present the same structural motif of honokiol on ring B. In particular, the presence of free phenolic
groups seems to be a pivotal requirement: in fact, compound 4, the dimethyl ether of honokiol, is
practically inactive, and the majority of poorly or not active neolignans have no free phenolic groups,
with the exception of 12a and 12c, lacking of the allyl chains present in honokiol. Diacetates 3 and
6 show an activity slightly lower than that of 2 towards HCT-116 and PC3 cells, suggesting that
these compounds may act as prodrug and may release 2 or the hydrogenated honokiol in presence of
intracellular esterases. The above cited structural features are not present in compounds with very low
antiproliferative activity. The presence of one or more allyloxy groups does not seem to be, by itself,
an essential structural motif, being present in active compounds such as 15a, but also in poor or not
active neolignans, such as 8, 13a-c. Finally, it is worthy of note that the hydrogenation of honokiol
to give 5 causes an enhancement of activity toward HT-29 and PC3 cells, thus suggesting that these
terminal double bonds are not essential for the activity.
On the basis of the above data, we selected three of the most potent neolignans, namely 14a,
14c, and 15a for a flow cytometric analysis on HCT-116 and PC3 cells. This analysis showed that the
inhibition of proliferation is mainly due to an apoptotic process, with high values of apoptotic cells
in almost all assays (Table 4). Cells treated with 15a showed the highest values of apoptotic cells
in both lines: 53.3% of HCT-116 and 38.4% of PC3 were detected in early and late apoptosis status
(Figure 3, Table 4). On the contrary, necrotic cells detectable by Propidium Iodide (PI) staining alone

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were not revealed at significant levels for all tested compounds in both cell lines (Table 4 and Figure 3
for 15a). The results obtained by antiproliferative assay and flow cytometry suggest that selected
bioinspired bisphenol neolignans could be further examined in depth in order to define the underlying
antitumor mechanism.
Table 4. Apoptotic death of 14a, 14c, and 15a in HCT-116 and PC3 Cells1.
HTC-116 Cell Distribution (%)
PC3 Cell Distribution (%)
Entry
Ctrl 14a
14c
15a
5-FU
Ctrl
14a
14c
15a
5-FU
Live
82.1 74.8 70.9 46.3
17.0
55.0
0.2
92.7
0.9
1.7
79.9
12.4
0.1
86.5
5.3
0.2
61.3
26.0
0.4
24.0
50.0
0.3
Early apoptosis (Annexin+)
Necrotic cells (PI+)
1.5
1.6
9.5
4.8
6.0
4.4
17.4
0.4
Late apoptosis (Annexin+/PI+) 14.9 10.9 18.7 35.9
27.8
4.7
7.7
8.0
12.4
25.7
1
Determined by Alexa Fluor 488; annexin V/propidium iodide (annexin/PI) staining after treatment with 14a, 14c,
M) for 72 h. The analysis was performed on 10,000 events for each condition and expressed in
percentage of total number of events.
15a, and 5-FU (10
µ
Figure 3. Flow cytometry: (
Each cell (event) is visible in a bright field and stained by annexin-V positive, propidium iodide positive,
and double positive cells; Flow cytometric dot plot of specific cell populations in HCT-116 ( ) and PC3
) cell lines in the presence of 15a: live (double annexin/PI negative), necrosis (annexin negative and PI
a) Typical images of cells analyzed by flow cytometry (Amnis FlowsSigh).
b
(c
positive), early apoptosis (annexin positive), and late apoptosis (double annexin/PI positive).
3. Materials and Methods
3.1. General Information
All chemicals were of reagent grade, and were used without further purification. Honokiol,
4-hydroxyphenylboronic acid, 1,1 0-bis(diphenylphosphino)ferrocene (dppf), Pd(OAc)2 were purchased
from TCI Europe (Milan, Italy), eugenol, 2-allyl phenol, tyrosol were purchased from Sigma Aldrich
(Milan, Italy).
Preparative liquid chromatography was performed on silica gel (63-200 µm, Merck, Darmstadt,
Germany), or Sephadex-LH20 (Sigma-Aldrich, Milan, Italy) using different mixtures of solvents,
as reported for each compound. TLC was carried out using pre-coated silica gel F254 plates

Molecules 2020, 25, 733
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(Macherey-Nagel, Düren, Germany); visualization of reaction components was achieved under
UV light at wavelengths of 254 and 366 nm, and by staining with a solution of cerium sulfate followed
by heating.
HPLC-UV instrument (Agilent, Milan, Italy), equipped with an auto-sampler (G1313A), a pump
(G1354A) and a diode array detector (DAD; G1315D), was employed for quantitative analysis with
an analytical reversed-phase column (Luna C18, 5
BO, Italy) and eluting at 1 mL/min with the following gradient of CH3CN
µ
m; 4.6
×
250 mm; Phenomenex, Castel Maggiore,
HCOOH (99:1 v/v; A)
−
in H2O−HCOOH (99:1 v/v; B): t0 min A = 50%, t15 min A = 100%, t20 min A = 50%.
NMR spectra were run on a Varian Unity Inova spectrometer (Italy, Milan) operating at 499.86 (1H)
and 125.70 MHz (13C), and equipped with a gradient-enhanced, reverse-detection probe. Chemical
shifts (δ) are indirectly referred to TMS using residual solvent signals. All NMR experiments, including
2D spectra, i.e., g-COSY, g-HSQCAD, and g-HMBCAD, were performed using software supplied
by the manufacturer, and acquired at constant temperature (300 K). g-HMBCAD experiments were
optimized for a long-range 13C-1H coupling constant of 8.0 Hz. High-resolution mass spectra were
acquired with an Orbitrap Fusion Tribrid®(Q-OT-qIT) mass spectrometer (Thermo Fisher Scientific,
Bremen, Germany) equipped with an ESI ion source operating in positive or negative mode. Samples
were directly infused and converted to the gas phase using the following parameters: source voltage,
2.6 kV; sheath gas flow rate, 25 au; and auxiliary gas, 8 au. The ions were introduced into the mass
spectrometer through a heated ion transfer tube (300 ◦C). Survey scan was performed from m/z 150 to
1000 at 500k resolution (@ 200 m/z) using the following parameters: RF lens, 60%; auto gain control
target, 20,000.
3.2. Synthesis of Compound 3
Honokiol (214.2 mg, 0.76 mmol) was mixed with acetic anhydride (10 mL) and with K2CO3
(445.3 mg, 3.2 mmol) at rt for 30 min. The mixture was diluted with cold water (10 mL) and partitioned
with EtOAC (3 x 15 mL), the combined organic phase was dried over anhydrous Na2SO4, filtered
and taken to dryness. The recovered organic layer was purified by column chromatography on
silica gel (n-hexane
Spectroscopic data were in agreement with those previously reported [41].
→
n-hexane:acetone 60:40) to give acetyl honokiol (3) with 56% yield (150.4 mg).
3.3. Synthesis of Compound 4
Honokiol (200 mg, 0.75 mmol) was solubilized in dry acetone (15 mL) and the solution was mixed
with K2CO3 (630.3 mg, 4.5 mmol) for 10 min. Then, MeI was added (0.19 mL, 3.2 mmol) and the
mixture was refluxed for 24 h. The mixture was filtered out and the permethylated compound 4 was
recovered after column chromatography on silica gel (cyclohexane
84% yield (185.6 mg). Spectroscopic data were in agreement with those previously reported [42].
→
cyclohexane:acetone 80:20) with
3.4. Synthesis of Compounds 5–7, 10a and b
The hydrogenation of 2–4, eugenol, and 2-allyl phenol (0.5 mmol) was carried out employing
absolute EtOH (7 mL) and Pd/CaCO3 (10% w/w; 17.4 mg) as catalyst. The reaction flask was filled with H2
(1 atm) and stirred at rt for 24 h. The catalyst was removed by filtration on Celite 545. In these conditions
the expected products have been obtained quantitatively without further purification. The spectroscopic
data of 30,5-dipropyl-(1,10-biphenyl)-2,40-diol (5) [42] 2,40-dimethoxy-30,5-dipropyl-1,10-biphenyl
(7) [42] 2-methoxy-4-propylphenol (10a) [45] and 2-propylphenol (10b) [46] were in agreement with
literature data.
30,5-Dipropyl-(1,10-biphenyl)-2,40-diyl diacetate (6). Yield: 96% (169.7 mg). Rf 0.47 (n-hexane:acetone
1
70:30). H-NMR (CDCl3):
δ 7.29 (d, J = 2.0 Hz, 1 H, H-2B), 7.25 (dd, J = 8.5, 2.0 Hz, 1 H, H-6B), 7.20 (d,
J = 1.9 Hz, 1 H, H-6A), 7.17 (dd, J = 8.2, 1.9 Hz, 1 H, H-4A), 7.06 (d, J = 8.5 Hz, 1 H, H-5B), 7.03 (d,
J = 8.2 Hz, 1 H, H-3A), 2.62 (bdd, J = 7.5 Hz, 2 H, CH2-7A), 2.54 (bdd, J = 7.6 Hz, 2 H, CH2-7B), 2.34
(s, 3 H, OCOCH3), 2.09 (s, 3 H, OCOCH3), 1.66 (m, 4 H, CH2-8A and CH2-8B), 0.96 (m, 6 H, CH3-9A

Molecules 2020, 25, 733
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and CH3-9B). 13C-NMR (CDCl3):
δ
169.8 (C, OCOCH3), 169.6 (C, OCOCH3), 148.5 (C, C-4B), 145.8 (C,
C-2A), 140.9 (C, C-5A), 135.7 (C, C-3B), 134.1 (C, C-1B), 133.9 (C, C-1A), 131.0 (CH, C-4A), 130.9 (CH,
C-2B), 128.6 (CH, C-6A), 127.5 (CH, C-6B), 122.6 (CH, C-3A), 122.2 (CH, C-5B), 37.6 (CH2, C-7A), 32.4
(CH2, C-7B), 24.6 (CH2, C-8A), 23.3 (CH2, C-8B), 21.1 (CH3, OCOCH3), 21.0 (CH3, OCOCH3), 14.2
(CH3, C-9B), 14.0 (CH3, C-9A). HRMS (ESI+) m/z 377.1739 [M+Na] + (calcd for C22H26O4Na, 377.1728).
3.5. Synthesis of Bromophenols 11a-c
Preliminary experiments for bromination have been performed and the details of these experiments
have been reported in Supplementary Materials.
According to the experiment 7 reported in Supplementary Materials (entry 7 of Table 1), a solution
of each compound (10a-b; 5 mmol) in CHCl3 (17 mL) was kept in ice bath and a solution containing
Br2 (300 µL; 1.2 mmol in 10 mL CHCl3), was added dropwise. The reaction was monitored by TLC
and the Br2 was quenched by the addition of a saturated Na2S2O3 solution (15 mL). The mixture was
partitioned with CH2Cl2 (3 x 15 mL) and the organic layer was dried and taken to dryness.
5-Bromo-2-methoxy-4-propylphenol (11a). The expected compound was recovered by column
chromatography on silica gel (cyclohexane:EtOAc 98:2
(765.2 mg). Rf 0.48 (cyclohexane:EtOAc 75:25).
→
cyclohexane:EtOAc 96:4) with 63% yield
4-Bromo-2-propylphenol (11b). The organic layer was purified on silica gel column chromatography
(cyclohexane cyclohexane:EtOAc 95:5) obtaining 11b with 67% yield (720.1 mg). Rf 0.31
→
(n-hexane:acetone 70:30).
2-Bromo-4-(2-hydroxyethyl)phenol (11c). Compound 11c was obtained as previously described [44].
Briefly, a solution of tyroso◦l (10c; 340.3 mg; 2.5 mmol) in acetone (9.2 mL) was stirred with NaBr
(514.3 mg; 5 mmol) at
added. The mixture was stirred for 1 h at
mL). The combined organic layer was dried over anhydrous Na2SO4, filtered and taken to dryness.
−
10 C and a 0.33 M oxone solution (2 gr in 10 mL of H2O) was dropwise
◦
−
10 C and then it was partitioned with EtOAc (3 x 10
The column chromatography on silica gel (cyclohexane
with 78% yield (423.5 mg).
→
cyclohexane: acetone 65:35) afforded 11c
NMR data of the isolated compounds are in agreement with those previously reported for 11a [47],
11b [48], and 11c [44].
3.6. Suzuki–Miyaura Cross-Coupling Reaction: Synthesis of Bisphenols 12a-c
The experimental conditions for the preliminary experiments performed on 11a for S–M reaction
have been reported in Supplementary Materials.
According to entry 4 of these experiments (entry 4 in Table 2), each aryl bromide 11a-c (1.0 mmol)
was solubilized in THF (17 mL) and mixed with 4-hydroxyphenylboronic acid (207.2 mg, 1.5 mmol),
dppf (165.7 mg, 0.3 mmol), Pd(OAc)2 (22.5 mg, 0.1 mmol). Then, a 3 M K2CO3 solution (1.7 mL, 5.0
mmol) was added and the mixture was stirred at 70 ◦C for 6 h. The mixture was diluted with water
(20 mL) and partitioned with EtOAc (3 x 25 mL). The combined organic layer was washed, dried
over anhydrous Na2SO4, filtered and taken to dryness. The expected bisphenol was recovered after
column chromatography.
4-Methoxy-6-propyl-(1,10-biphenyl)-3,40-diol (12a). The silica gel column chromatography (petroleum
ether
→
petroleum ether:acetone 92:8) furnished the bisphenol neolignan 12a with 65% yield (167.7
1
mg). Rf 0.39 (cyclohexane:acetone 70:30). H-NMR (CDCl3):
δ 7.14 (d, J = 8.5 Hz, 2 H, H-2B/H-6B), 6.85
(d, J = 8.5 Hz, 2 H, H-3B/H-5B), 6.78 (s, 1 H, H-2A), 6.76 (s, 1 H, H-5A), 5.52 (bs, 1 H, 3A-OH), 5.07 (bs, 1
H, 4B-OH), 3.92 (s, 3 H, OCH3), 2.47 (m, 2 H, CH2-7A), 1.47 (m, 2 H, CH2-8A), 0.82 (t, J = 7.4 Hz, 3 H,
CH3-9A). 13C-NMR (CDCl3):
δ 154.4 (C, C-4B), 145.7 (C, COCH3), 143.1 (C, C-3A), 134.6 (C, C-1A),
134.4 (C, C-1B), 132.2 (CH, C-6A), 130.7 (CH, C-2B/C-6B), 116.4 (C, C-2A), 115.0 (CH, C-3B/C-5B), 111.7
(CH, C-5A), 56.1 (CH3, OCH3), 35.0 (CH2, C-7A), 25.0 (CH2, C-8A), 14.1 (CH3, C-9A). HRMS (ESI-) m/z
257.1169 [M-H]− (calcd for C16H17O3, 257.1178).

Molecules 2020, 25, 733
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3-Propyl-(1,10-biphenyl)-4,40-diol (12b). The silica gel column chromatography (n-hexane:acetone
88:12) afforded the bisphenol neolignan 12b with 70% yield (158.2 mg). Rf 0.35
(cyclohexane:acetone 70:30). 1H-NMR ((CD3)2CO):
7.40 (d, J = 8.4 Hz, 2 H, H-2B/H-6B), 7.31
95:5
→
δ
(d, J = 1.9 Hz, 1 H, H-2A), 7.22 (dd, J = 8.2, 1.9 Hz, 1 H, H-6A), 6.88 (d, J = 8.4 Hz, 2 H, H-3B/H-5B),
6.87 (d, J = 8.2 Hz, 1 H, H-5A), 2.64 (m, 2 H, CH2-7A), 1.67 (m, 2 H, CH2-8A), 0.98 (t, J = 7.4 Hz, 3 H,
CH3-9A). 13C-NMR ((CD3)2CO):
δ 157.1 (C, C-4B), 145.9 (C, C-4A), 133.6 (C, C-1B), 133.2 (C, C-1A),
129.7 (C, C-3A), 129.0 (CH, C-6A), 128.2 (CH, C-3B/C-5B), 125.5 (C, C-2A), 116.3 (CH, C-2B/C-6B), 116.0
(CH, C-5A), 33.1 (CH2, C-7A), 23.8 (CH2, C-8A), 14.3 (CH3, C-9A). HRMS (ESI-) m/z 227.1065 [M-H]−
(calcd for C15H15O2, 227.1072).
5-(2-Hydroxyethyl)-(1,10-biphenyl)-2,40-diol (12c).
The neolignan 12c was recovered after
70:30) with 68% yield (155.7 mg) Rf 0.2
8.30 (bs C-4B-OH), 7.91 (bs, C-2A-OH), 7.43 (d,
column chromatography (cyclohexane.acetone 98:2
→
1
(cyclohexane:acetone 60:40). H-NMR ((CD3)2CO):
δ
J = 8.6 Hz, 2 H, H-2B/H-6B), 7.12 (d, J = 2.2 Hz, 1 H, H-6A), 6.99 (dd, J = 8.2, 2.2 Hz, 1 H, H-4A), 6.87 (d,
J = 8.6 Hz, 2 H, H-3B/H-5B), 6.85 (d, J = 8.2 Hz, 1 H, H-3A), 3.75 (t, J = 7.1 Hz, 2 H, CH2-8A), 3.53 (bs, 1
H, OH-8A), 2.76 (t, J = 7.1 Hz, 2 H, CH2-7A). 13C-NMR ((CD3)2CO):
δ 157.2 (C, C-4B), 153.1 (C, C-2A),
131.7 (CH, C-6A), 131.6 (C, C-5A), 131.2 (CH, C-2B/C-6B), 131.0 (C, C-1B), 129.1 (CH, C-4A), 129.0 (C,
C-1A), 116.7 (CH, C-3A), 115.7 (CH, C-3B/C-5B), 64.2 (CH2, C-8A), 39.5 (CH2, C-7A). HRMS (ESI-) S
m/z 229.0874 [M-H]− (calcd for C14H13O3, 229.0865).
3.7. Synthesis of O-Allyloxy Neolignans 8 and 13a-c
A solution of the proper bisphenol neolignan (2, 12a-c; 0.5 mmol) in dry acetone (5 mL) was
mixed with K2CO3 (275.7 mg; 2.0 mmol) for 10 min, then, allyl bromide (130 µL; 1.5 mmol) was
added and the mixture was refluxed overnight. The mixture was filtered and where necessary it
was chromatographed.
30,5-Diallyl-2,40-bis(allyloxy)-1,10-biphenyl (8). The expected compound was recovered with 97%
yield (184.9 mg) after filtration of the mixture. Rf 0.79 (cyclohexane:acetone 70:30). Spectroscopic data
were in agreement with those reported in the literature [20].
40,5-Bis(allyloxy)-4-methoxy-2-propyl-1,10-biphenyl (13a).
The mixture was purified on
Sephadex-LH20 column chromatography eluting with CH2Cl2, to give 13a with 95% yield (160.5 mg).
1
Rf 0.69 (cyclohexane:acetone 70:30). H-NMR (CDCl3):
δ 7.19 (d, J = 8.6 Hz, 2 H, H-2B/H-6B), 6.94
(d, J = 8.6 Hz, 2 H, H-3B/H-5B), 6.78 (s, 1 H, H-5A), 6.73 (s, 1 H, H-2A), 6.11 (m, 1 H, H-11A), 6.07
(m, 1 H, H-8B), 5.45 (dd, J = 15.0, 5.0 Hz, 1 H, CHa-12A), 5.39 (dd, J = 15.0, 5.0 Hz, 1 H, CHa-9B), 5.31
(dd, J = 10.0, 5.0 Hz, 1 H, CHb-12A), 5.26 (dd, J = 10.0, 5.0 Hz, 1 H, CHb-9B), 4.59 (d, J = 5.3 Hz, 2 H,
CH2-7B), 4.58 (d, J = 5.4 Hz, 2 H, CH2-10A), 3.91 (s, 3 H, OCH3), 2.48 (m, 2 H, CH2-7A), 1.49 (m, 2 H,
CH2-8A), 0.32 (t, J = 7.3 Hz, 3 H, CH3-9A). 13C-NMR (CDCl3):
δ 157.7 (C, C-4B), 148.5 (C, COCH3),
145.7 (C, C-3A), 134.6 (C, C-1B), 133.8 (C, C-1A), 133.7 (CH, C-11A), 133.5 (CH, C-8B), 133.1 (C, C-6A),
130.5 (CH, C-2B/C-6B), 117.9 (CH2, C-9B), 117.8 (CH2, C-12A), 115.8 (CH, C-2A), 114.4 (CH, C-3B/C-5B),
112.9 (CH, C-5A), 70.1 (CH2, C-10A), 69.0 (CH2, C-7B), 56.2 (CH3, OCH3), 35.0 (CH2, C-7A), 24.9 (CH2,
C-8A), 14.2 (CH3, C-9A). HRMS (ESI+) m/z 361.1791 [M+Na]+ (calcd for C22H26O3Na, 361.1780).
4,40-Bis(allyloxy)-3-propyl-1,10-biphenyl (13b). The expected compound was recovered after filtration
1
without further purification with 96% yield (147.2 mg). Rf 0.79 (cyclohexane:acetone 75:25). H-NMR
(CDCl3):
δ 7.47 (d, J = 8.7 Hz, 2 H, H-2B/H-6B), 7.33 (d, J = 1.9 Hz, 1 H, H-2A), 7.31 (dd, J = 8.2, 1.9
Hz, 1 H, H-6A), 6.97 (d, J = 8.7 Hz, 2 H, H-3B/H-5B), 6.87 (d, J = 8.2 Hz, 1 H, H-5A), 6.09 (m, 2 H,
H-11A and H-8B), 5.45 (m, 2H, CH2-12A), 5.30 (m, 2 H, CH2-9B), 4.58 (d, J = 5.1 Hz, 4 H, CH2-10A
and CH2-7B), 2.68 (t, J = 7.5 Hz, 2 H, CH2-7A), 1.68 (sext, J = 7.5 Hz, 2 H, CH2-8A), 0.98 (t, J = 7.5, 3 H,
CH3-9A). 13C-NMR (CDCl3):
δ 157.8 (C, C-4B), 155.8 (C, C-4A), 134.1 (C, C-1B), 133.3 (C, C-1A), 133.8
(CH, C-11A), 133.5 (CH, C-8B), 131.8 (C, C-3A), 128.6 (CH, C-2A), 127.9 (CH, C-2B/C-6B), 125.0 (CH,
C-6A), 117.8 (CH2, C-9B), 116.9 (CH2, C-12A), 115.1 (CH, C-3B/C-5B), 112.0 (CH, C-5A), 69.1 (CH2,
C-10A), 68.1 (CH2, C-7B), 32.7 (CH2, C-7A), 23.3 (CH2, C-8A), 14.3 (CH3, C-9A). HRMS (ESI+) m/z
331.1683 [M+Na]+ (calcd for per C21H24O2Na, 331.1674).

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5-(2-Hydroxyethyl)-2,40-bis(allyloxy)-1,10-biphenyl (13c).
The Sephadex-LH20 column
chromatography (CH2Cl2) afforded the expected O-allyl derivative 13c with 75% yield (116.8 mg). Rf
1
0.49 (cyclohexane.acetone 65:35). H-NMR ((CD3)2CO):
δ 7.49 (d, J = 8.8 Hz, 2 H, H-2B/H-6B), 7.18 (d,
J = 1.9 Hz, 1 H, H-6A), 7.13 (dd, J = 8.3, 1.9 Hz, 1 H, H-4A), 6.97 (d, J = 8.7 Hz, 2 H, H-3B/H-5B), 6.96
(d, J = 8.3 Hz, 1 H, H-3A), 6.10 (m, 1 H, H-11A), 6.01 (m, 1 H, H-8B), 5.44 (dd, J = 17.3, 1.5 Hz, 1 H,
CHa-12A), 5.34 (dd, J = 17.3, 1.7 Hz, 1 H, CHa-9B), 5.25 (dd, J = 10.6, 1.5 Hz, 1 H, CHb-12A), 5.16 (dd,
J = 10.6, 1.7 Hz, 1 H, CHb-9B), 4.61(d, J = 5.2 Hz, 2 H, CH2-10A), 4.53 (d, J = 3.2 Hz, 2 H, CH2-7B), 3.75
(t, J = 6.6 Hz, 2 H, CH2-8A), 2.79 (t, J = 6.6 Hz, 2 H, CH2-7A). 13C-NMR ((CD3)2CO):
δ 158.6 (C, C-4B),
154.8 (C, C-2A), 134.88 (CH, C-11A), 134.86 (CH, C-8B), 133.2 (C, C-5A), 132.2 (C, C-1B), 131.9 (CH,
C-6A), 131.4 (CH, C-2B/C-6B), 131.1 (C, C-1A), 129.4 (CH, C-4A), 117.3 (CH2, C-9B), 116.7 (CH, C-3A),
114.9 (CH, C-3B/C-5B), 113.9 (CH2, C-12A), 69.8 (CH2, C-10A), 69.3 (CH2, C-7B), 64.1 (CH2, C-8A), 39.5
(CH2, C-7A). HRMS (ESI-) m/z 309.1501 [M-H]− (calcd for C20H21O3, 309.1491).
3.8. General Procedure for Claisen Rearrangement: Synthesis of Bisphenols 14a-c and 15a
A 1 M Et2AlCl solution (in n-hexane; 1.5 mL) was added dropwise to a solution of O-allyl
derivatives 8, 13a-c (0.35 mmol) in dry CH2Cl2 (2 mL). The mixture was stirred at rt for 2 h and then
the reaction was quenched adding 2 N HCl solution (5 mL). The mixture was partitioned with CH2Cl2
(2
×
5 mL); the combined organic phase was washed with water, dried oved anhydrous Na2SO4,
filtered and taken to dryness. Column chromatography on Sephadex-LH20 (CH2Cl2) afforded the
pure products.
3,30,5,50-Tetraallyl-(1,10-biphenyl)-2,40-diol (9). Yield: 20% (24.7 mg). Rf 0.55 (cyclohexane.acetone
70:30). Spectroscopic data were in agreement with those previously reported [43].
2,30-Diallyl-4-methoxy-6-propyl-(1,10-biphenyl)-3,40-diol (14a). Yield: 18% (21.5 mg). Rf 0.44
(cyclohexane:acetone 70:30). 1H-NMR (CDCl3):
δ 6.88 (m, 1 H, H-6B), 6.87 (bs, 1 H, H-2B), 6.81
(d, J = 7.9 Hz, 1 H, H-5B), 6.66 (s, 1 H, H-5A), 6.03 (m, 1 H, H-8B), 5.82 (m, 1 H, H-11A), 5.59 (bs, 1H,
3A-OH), 5.13 (m, 2 H, CH2-9B), 4.96 (bs, 1 H, 4B-OH), 4.87 (bd, J = 10.1 Hz, 1 H, CHa-12A), 4.75 (bd,
J = 17.1 Hz, 1 H, CHb-12A), 3.91 (s, 3 H, OCH3), 3.41 (d, J = 6.2 Hz, 2 H, CH2-7B), 3.12 (d, J = 6.1 Hz, 2
H, CH2-10A), 2.24 (bdd, J = 7.5 Hz, 2 H, CH2-7A), 1.40 (sext, J = 7.5 Hz, 2 H, CH2-8A), 0.77 (t, J = 7.5
Hz, 3 H, CH3-9A). 13C-NMR (CDCl3):
δ 152.7 (C, C-4B), 145.2 (C, COCH3), 141.4 (C, C-3A), 136.8 (CH,
C-11A), 136.4 (CH, C-8A), 134.5 (C, C-1A), 132.6 (C, C-6A), 132.5 (C, C-1B), 132.3 (CH, C-2B), 129,4
(CH, C-6B), 124.5 (C, C-2A), 124.4 (C,C-3B), 116.3 (CH, C-5B), 115.2 (CH2, C-9B), 114.4 (CH2, C-12A),
109.1 (CH, C-5A), 55.9 (CH3, OCH3), 35.8 (CH2, C-7A), 34.9 (CH2, C-7B), 32.1 (CH2, C-10A), 24.9 (CH2,
C-8A), 14.0 (CH3, C-9A). HRMS (ESI-) m/z 337.2123 [M-H]− (calcd for C22H25O3, 337.2117).
3,30-Diallyl-5-propyl-(1,10-biphenyl)-4,40-diol (14b). Yield: 40% (42.8 mg) Rf 0.48 (cyclohexane:acetone
1
70:30). H-NMR (CDCl3):
δ 7.31 (dd, J = 8.2, 2.1 Hz, 1 H, H-6B), 7.28 (d, J = 2.1 Hz, 1 H, H-2B), 7.19 (d,
J = 1.5 Hz, 1 H, H-2A), 7.13 (d, J = 1.5 Hz, 1 H, H-6A), 6.85 (d, J = 8.2 Hz, 1 H, H-5B), 6.06 (m, 2 H,
H-11A and H-8B), 5.21 (m, 4 H, CH2-12A and CH2-9B), 4.98 (bs, 1H, 4A-OH) 4.96 (bs, 1 H, 4B-OH), 3.47
(d, J = 6.3 Hz, 4 H, CH2-10A and CH2-7B), 2.73 (bt, J = 7.7 Hz, 2 H, CH2-7A), 1.68 (sext, J = 7.5 Hz, 2 H,
CH2-8A), 1.0 (t, J = 7.5 Hz, 3 H, CH3-9A). 13C-NMR (CDCl3):
δ 153.3 (C, C-4B), 151.7 (C, C-4A), 136.6
(CH, C-11A and C-6A), 136.5 (CH, C-8B), 134.3 (C, C-1B), 133.4 (C, C-1A), 129,4 (C, C-3A), 127.2 (CH,
C-2A), 126.4 (CH, C-6B), 125.5 (C, C-3B), 125.0 (C, C-5A), 117.0 (CH2, C-12A), 116.8 (CH2, C-9B), 116.3
(CH, C-5B), 36.1 (CH2, C-10A), 35.6 (CH2, C-7B), 32.5 (CH2, C-7A), 23.3 (CH2, CH2-8A), 14.3 (CH3,
C-9A). HRMS (ESI-) m/z 307.1707 [M-H]− (calcd for C21H23O2, 307.1698).
3,30-Diallyl-5-(2-hydroxyethyl)-(1,10-biphenyl)-2,40-diol (14c). Yield: 48% (51.1 mg). Rf 0.42
(cyclohexane:acetone 65:35). 1H-NMR ((CD3)2CO):
δ 7.15 (bs, 1 H, H-6A), 7.12 (bd, J = 8.2 Hz,
1 H, H-6B), 6.91, (bs, 1 H, H-4A), 6.90 (bs, 1 H, H-2B), 6.88 (d, J = 8.2 Hz, 1 H, H-5B), 6.00 (m, 2 H,
H-11A and H-8B), 5.09 (m, 2 H, CH2-12A), 5.04 (m, 2 H, CH2-9B), 3.80 (t, J = 6.6 Hz, 2 H, CH2-8A), 3.40
(bt, J = 7.1 Hz, 4 H, CH2-10A and CH2-7B), 2.77 (t, J = 6.6 Hz, 2 H, CH2-7A). 13C-NMR ((CD3)2CO):
δ
154.0 (C, C-4B), 148.8 (C, C-2A), 136.5 (CH, C-11A), 136.1 (CH, C-8B), 130.7 (CH, C-6A), 129.7 (C,
C-5A), 129.4 (CH, C-4A), 128.7 (C, C-3A), 128.5 (CH, C-2B), 128.0 (CH, C-6B), 127.9 (C, C-3B), 126.8 (C,

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C-1A), 126.1 (C, C-1B), 115.88 (CH2, C-12A), 115.86 (CH2, C-9B), 115.5 (CH, C-5B), 63.5 (CH2, C-8A),
38.1 (CH2, C-7A), 34.5 (CH2, C-10A), 34.3 (CH2, C-7B). HRMS (ESI-) m/z 309.1484 [M-H]− (calcd for
C20H21O3, 309.1491).
2-Allyl-40-(allyloxy)-4-methoxy-6-propyl-(1,10-biphenyl)-3-ol (15a). Yield: 15% (17.7 mg). Rf 0.52
1
(cyclohexane:acetone 70:30). H-NMR (CDCl3):
δ 7.03 (d, J = 7.5 Hz, 2 H, H-2B/H-6B), 6.92 (d, J = 7.5
Hz, 2 H, H-3B/H-5B), 6.66 (s, 1 H, H-5A), 6.11 (m, 1 H, H-8B), 5.82 (m, 1 H, H-11A), 5.59 (bs, 1 H, OH),
5.46 (dd, J = 17.2, 1.2 Hz, 1 H, CHa-9B), 5.31 (dd, J = 10.5, 1.2 Hz, 1 H, CHb-9B), 4.87 (dd, J = 10.1, 1.4 Hz,
1 H, CHa-12A), 4.76 (dd, J = 17.0, 1.4 Hz, 1 H, CHb-12A), 4.58 (d, J = 7.5 Hz, 2 H, CH2-7B), 3.91 (s, 3 H,
OCH3), 3.12 (d, J = 6.1 Hz, 2 H, CH2-10A), 2.24 (m, 2 H, CH2-7A), 1.40 (m, 2 H, CH2-8A), 0.76 (t, J = 7.5
Hz, 3 H, CH3-9A). 13C-NMR (CDCl3):
δ 157.5 (C, C-4B), 145.4 (C, COCH3), 141.5 (C, C-3A), 136.9 (CH,
C-11A), 134.6 (C, C-1A), 133.6 (CH, C-8B), 132.7 (C, C-6A), 132.6 (C, C-1B), 131.3 (CH, C-2B/C-6B), 124.6
(C, C-2A), 117.8 (CH2, C-9B), 114.6 (CH2, C-12A), 114.1 (CH, C-3B/C-5B), 109.3 (CH, C-5A), 69.0 (CH2,
C-7B), 56.1 (CH3, OCH3), 35.9 (CH2, C-7A), 32.1 (CH2, C-10A), 24.9 (CH2, C-8A), 14.2 (CH3, C-9A).
HRMS (ESI-) m/z 337.1812 [M-H]− (calcd for C22H25O3, 337.1804).
3.9. Human Cell Cultures
Two human colorectal adenocarcinoma cell lines (HCT-116 and HT-29) and one human prostate
adenocarcinoma cell line (PC3) have been used in the present work and been cultured as previously
reported [28
,49]. The HCT-116 (ATCC number: CCL-247) cell line was cultured in McCoy’s 5A
(Medium, GlutaMAX
™
Supplement, cod. 36600021 containing 3 g/L of d-glucose), HT-29 (ATCC
number: HTB-38) was cultured in in DMEM medium (Dulbecco’s Modified Eagle Medium 1X; GIBCO,
Cat No. 31965-023 containing 4.5g/L of d-glucose) while PC3 cell line (ATCC number: CRL-1435) was
cultured in DMEM/F-12, (Dulbecco’s Modified Eagle Medium/Nutrient Mixture F-12, GlutaMAX
™
Supplement, cod. 31331028). Each medium was supplemented with 10% Fetal Bo◦vine Serum (FBS)
and 100 U/ml of penicillin-streptomycin. The cell cultures were incubated at 37 C in humidified
atmosphere with 5% of CO2 and 95% of air. The medium was changed twice in the week.
3.10. Antiproliferative Assay
HCT-116 HT-29 and PC3 cell lines (1.8 and 2.5
×
103 cells/0.33 cm2 respectively) were plated
™ Microwell™” (Thermo Fisher Scientific, Bremen, Germany),
in 96 well plates from “Nunclon
◦
and were incubated at 37 C. After 24 h cells (60% confluence) were treated with compounds 2-9,
12a-c, 13a-c, 14a-c, and 15a at increasing concentration from 0.001 to 100 µM. The cellular vitality
and/or the cellular cytotoxicity has been evaluated by colorimetric assay with tetrazolium salt MTT
(3-(4,5-methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) as previously reported [27]. After 72
hours of treatment with the bisphenol neolignans, MTT salt was added and maintained for 3 hours.
The purple formazan dye, produced by the metabolic process of vital cells, was solubilized by adding
150
scanning spectrophotometer (Multiskan
a wavelength of 570 nm. Each value was the average of four–six wells. The percentage of cellular
growth was calculated according to NCI [50]: 100 (T T0) / (C T0) (where T is the optical density of
µL/well of DMSO (dimethyl sulfoxide) and the optical density (OD) values were read on a multiwell
™
reader, Thermo Fisher Scientific, Bremen, Germany), using
×
−
−
the test well after a 72 h period of exposure to test compound, T0 is the optical density at time zero,
and C is the optical density of the untreated control cell cultures). When the optical density of treated
cells was lower than the T0 value the following formula was used: 100
calculated by the software GraphPad Prism v.6.
×
(T
−
T0) /T0. GI50 values were
3.11. Apoptosis Analysis by Imaging Flow Cytometry
HCT116 and PC3 cell lines were cultured in six-well plates (about 3.5
treated with the following compounds: 14a and c, and 15a. Vehicle-treated cells and the anticancer
5-fluorouracil (5-FU, final concentration of 10 M) were used as negative and positive control,
respectively. Cells were harvested by trypsinization after 72 h of treatment, washed in PBS, and stained
×
105 cells/9.6 cm2) and
µ

Molecules 2020, 25, 733
14 of 17
with Alexa Fluor
®488 dye conjugated to annexin V and propidium iodide (PI) in according to
manufacturing protocol (The Alexa Fluor®488 annexin V /Dead Cell Apoptosis Kit by Invitrogen, Cat.
V13245; Thermo Fisher Scientific, Bremen, Germany). PI is impermeable to live cells and to cells in early
apoptosis, but stains dead cells and late apoptosis cells with red fluorescence, binding tightly to the
nucleic acids while Alexa Fluor®488 dye, conjugated to annexin V, stains in green the apoptotic cells.
After staining the cellular suspension was analyzed on a flow cytometer (Amnis FlowSight Millipore,
Merck KgaA, Darmstadt, Germany), and results were analyzed using Image Data Exploration and
Analysis (IDEAS) software (Amnis part of EMD Millipore, Seattle, WA, USA).
4. Conclusions
The present work reports the synthesis of a library of bisphenol neolignans inspired by honokiol.
These products were evaluated for their potential antiproliferative activity against three cancer cell
lines, namely HCT-116, HT-29 (both human colorectal adenocarcinoma) and PC3 (human prostate
adenocarcinoma). This study suggests some structural motifs important for the activity and overall
highlights that bisphenol neolignans 9, 12b, 14a-c, and 15a, structurally related to honokiol, show higher
antiproliferative activity than the natural lead. The most promising antiproliferative agents (14a, 14c,
and 15a) were selected for a flow cytometric analysis; this showed that the inhibition of proliferation
is mainly due to apoptotic process. In conclusion, some of the bisphenol neolignans reported in
the present study have shown promising properties for the development of new natural-derived
antitumor agents.
Supplementary Materials: A copy of HRMS and NMR spectra of the new synthesized bisphenol neolignans is
available online at http://www.mdpi.com/1420-3049/25/3/733/s1.
Author Contributions: Conceptualization, C.T. and D.F.C.; Methodology, N.C., V.M. and V.B.; Validation, V.M.,
N.C., V.B., and G.S.; Investigation, N.C., G.S., and M.D.; Resources, V.M., C.T. and D.F.C.; Data curation, N.C., V.M.
and V.B.; Writing—Original draft preparation, N.C., V.M. and V.B.; Writing—Review and editing, C.T., D.F.C. and
V.B.; Visualization, N.C., G.S., and V.B.; Supervision, N.C., V.M., and V.B.; Project administration, C.T.; Funding
acquisition, V.M. and V.B.
Funding: This research was funded by ‘Piano della Ricerca di Ateneo 2016-2018, Linea d’intervento 20 of Universit
à
degli Studi di Catania, by MIUR ITALY PRIN 2017 (Project No. 2017A95NCJ), and partially supported by project
“FIR 2014, University of Catania, Italy” (project No. 668A01).
Acknowledgments: The authors acknowledge the PON project Bio-nanotech Research and Innovation Tower
(BRIT), financed by the Italian Ministry for Education, University and Research (MIUR) (Grant no. PONa3_00136).
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
References
1.
2.
3.
4.
5.
6.
Zhang, J.; Chen, J.J.; Liang, Z.Z.; Zhao, C.Q. New lignans and their biological activities. Chem. Biodivers.
2014, 11, 1–54. [CrossRef] [PubMed]
Aldemir, H.; Richarz, R.; Gulder, T.A.M. The biocatalytic repertoire of natural biaryl formation. Ange. Chem.
Int. Ed. 2014, 53, 8286–8293. [CrossRef] [PubMed]
Lee, Y.J.; Lee, Y.M.; Lee, C.K.; Jung, J.K.; Han, S.B.; Hong, J.T. Therapeutic applications of compounds in the
Magnolia family. Pharmacol. Ther. 2011, 130, 157–176. [CrossRef] [PubMed]
Patocˇka, J.; Jakl, J.; Strunecká, A. Expectations of biologically active compounds of the genus Magnolia in
biomedicine. J. Appl. Biomed. 2006, 4, 171–178. [CrossRef]
Kelm, M.A.; Nair, M.G. A brief summary of biologically active compounds from Magnolia spp. Stud. Nat.
Prod. Chem. 2000, 24, 845–873.
Shen, J.L.; Man, K.M.; Huang, P.H.; Chen, W.C.; Chen, D.C.; Cheng, Y.W.; Liu, P.L.; Chou, M.C.; Chen, Y.H.
Honokiol and magnolol as multifunctional antioxidative molecules for dermatologic disorders. Molecules
2010, 15, 6452–6465. [CrossRef]

Molecules 2020, 25, 733
15 of 17
7.
Amorati, R.; Zotova, J.; Baschieri, A.; Valgimigli, L. Antioxidant activity of magnolol and honokiol: Kinetic
and mechanistic investigations of their reaction with peroxyl radicals. J. Org. Chem. 2015, 80, 10651–10659.
[CrossRef]
8.
9.
Lin, Y.R.; Chen, H.H.; Ko, C.H.; Chan, M.H. Effects of honokiol and magnolol on acute and inflammatory
pain models in mice. Life Sci. 2007, 81, 1071–1078. [CrossRef]
Lin, Y.R.; Chen, H.H.; Ko, C.H.; Chan, M.H. Neuroprotective activity of honokiol and magnolol in cerebellar
granule cell damage. Eur. J. Pharmacol. 2006, 537, 64–69. [CrossRef]
10. Fried, L.E.; Arbiser, J.L. Honokiol, a multifunctional antiangiogenic and antitumor agent. Antioxid. Redox
Signal. 2009, 11, 1139–1148. [CrossRef]
11. Ranaware, A.M.; Banik, K.; Deshpande, V.; Padmavathi, G.; Roy, N.K.; Sethi, G.; Fan, L.; Kumar, A.P.;
Kunnumakkara, A.B. Magnolol: A neolignan from the Magnolia family for the prevention and treatment of
cancer. Int. J. Mol. Sci. 2018, 19, 2362. [CrossRef]
12. Kim, G.D.; Oh, J.; Park, H.J.; Bae, K.; Lee, S.K. Magnolol inhibits angiogenesis by regulating ROS–Mediated
apoptosis and the PI3K/AKT/mTOR signaling pathway in mES/EB-derived endothelial-like cells. Int. J.
Oncol. 2013, 43, 600–610. [CrossRef] [PubMed]
13. Shen, J.; Ma, H.L.; Zhang, T.C.; Liu, H.; Yu, L.H.; Li, G.S.; Li, H.S.; Hu, M.C. Magnolol inhibits the growth
of non-small cell lung cancer via inhibiting microtubule polymerization. Cell. Physiol. Biochem. 2017, 42,
1789–1801. [CrossRef] [PubMed]
14. Shen, L.; Zhang, F.; Huang, R.M.; Yan, J.; Shen, B. Honokiol inhibits bladder cancer cell invasion through
repressing SRC-3 expression and epithelial-mesenchymal transition. Oncol. Lett. 2017, 14, 4294–4300.
[CrossRef] [PubMed]
15. Yeh, P.S.; Wang, W.; Chang, Y.A.; Lin, C.J.; Wang, J.J.; Chen, R.M. Honokiol induces autophagy of
neuroblastoma cells through activating the PI3K/Akt/mTOR and endoplasmic reticular stress/ERK1/2
signaling pathways and suppressing cell migration. Cancer Lett. 2016, 370, 66–77. [CrossRef]
16. Hajduk, P.J.; Bures, M.; Praestgaard, J.; Fesik, S.W. Privileged molecules for protein binding identified from
NMR-based screening. J. Med. Chem. 2000, 43, 3443–3447. [CrossRef]
17. Jada, S.; Doma, M.R.; Singh, P.P.; Kumar, S.; Malik, F.; Sharma, A.; Khan, I.A.; Qazi, G.N.; Kumar, H.M.S. Design
and synthesis of novel magnolol derivatives as potential antimicrobial and antiproliferative compounds.
Eur. J. Med. Chem. 2012, 51, 35–41. [CrossRef]
18. Amblard, F.; Govindarajan, B.; Lefkove, B.; Rapp, K.L.; Detorio, M.; Arbiser, J.L.; Schinazi, R.F. Synthesis,
cytotoxicity, and antiviral activities of new neolignans related to honokiol and magnolol. Bioorg. Med. Chem.
Lett. 2007, 17, 4428–4431. [CrossRef]
19. Tripathi, S.; Chan, M.H.; Chen, C.P. An expedient synthesis of honokiol and its analogues as potential
neuropreventive agents. Bioorg. Med. Chem. Lett. 2012, 22, 216–221. [CrossRef]
20. Lee, S.H.; Fei, X.; Lee, C.; Do, H.T.T.; Rhee, I.; Seo, S.Y. Synthesis of either C2-or C40-alkylated derivatives of
honokiol and their biological evaluation for anti-inflammatory activity. Chem. Pharm. Bull. 2019, 67, 966–976.
[CrossRef]
21. Lin, J.M.; Gowda, A.S.P.; Sharma, A.K.; Amin, S. In vitro growth inhibition of human cancer cells by novel
honokiol analogs. Bioorg. Med. Chem. 2012, 20, 3202–3211. [CrossRef] [PubMed]
22. Maioli, M.; Basoli, V.; Carta, P.; Fabbri, D.; Dettori, M.A.; Cruciani, S.; Serra, P.A.; Delogu, G. Synthesis
of magnolol and honokiol derivatives and their effect against hepatocarcinoma cells. PLoS ONE 2018, 13,
e0192178. [CrossRef] [PubMed]
23. Sanchez-Peris, M.; Murga, J.; Falomir, E.; Carda, M.; Marco, J.A. Synthesis of honokiol analogues and
evaluation of their modulating action on VEGF protein secretion and telomerase-related gene expressions.
Chem. Biol. Drug Des. 2017, 89, 577–584. [CrossRef] [PubMed]
24. Kotha, S.; Lahiri, K.; Kashinath, D. Recent applications of the Suzuki–Miyaura cross-coupling reaction in
organic synthesis. Tetrahedron 2002, 58, 9633–9695. [CrossRef]
25. Martin, R.; Buchwald, S.L. Palladium-catalyzed Suzuki–Miyaura cross-coupling reactions employing
dialkylbiaryl phosphine ligands. Acc. Chem. Res. 2008, 41, 1461–1473. [CrossRef]
26. Pulvirenti, L.; Muccilli, V.; Cardullo, N.; Spatafora, C.; Tringali, C. Chemoenzymatic synthesis and
alpha-glucosidase inhibitory activity of dimeric neolignans inspired by magnolol. J. Nat. Prod. 2017
80, 1648–1657. [CrossRef]
,

Molecules 2020, 25, 733
16 of 17
27. Cardullo, N.; Spatafora, C.; Musso, N.; Barresi, V.; Condorelli, D.; Tringali, C. Resveratrol-related
polymethoxystilbene glycosides: Synthesis, antiproliferative activity, and glycosidase inhibition. J. Nat. Prod.
2015, 78, 2675–2683. [CrossRef]
28. Cardullo, N.; Pulvirenti, L.; Spatafora, C.; Musso, N.; Barresi, V.; Condorelli, D.F.; Tringalii, C.
Dihydrobenzofuran neolignanamides: Laccase-mediated biomimetic synthesis and antiproliferative activity.
J. Nat. Prod. 2016, 79, 2122–2134. [CrossRef]
29. Chillemi, R.; Cardullo, N.; Greco, V.; Malfa, G.; Tomasello, B.; Sciuto, S. Synthesis of amphiphilic resveratrol
lipoconjugates and evaluation of their anticancer activity towards neuroblastoma SH-SY5Y cell line. Eur. J.
Med. Chem. 2015, 96, 467–481. [CrossRef]
30. Capolupo, A.; Tosco, A.; Mozzicafreddo, M.; Tringali, C.; Cardullo, N.; Monti, M.C.; Casapullo, A. Proteasome
as a new target for bio-inspired benzo[k,l]xanthene lignans. Chem. Eur. J. 2017, 23, 8371–8374. [CrossRef]
31. Spatafora, C.; Barresi, V.; Bhusainahalli, V.M.; Di Micco, S.; Musso, N.; Riccio, R.; Bifulco, G.; Condorelli, D.;
Tringali, C. Bio-inspired benzo[k,l]xanthene lignans: Synthesis, DNA-interaction and antiproliferative
properties. Org. Biomol. Chem. 2014, 12, 2686–2701. [CrossRef]
32. Cardullo, N.; Catinella, C.; Floresta, G.; Muccilli, V.; Rosselli, S.; Rescifina, A.; Bruno, M.; Tringali, C. Synthesis
of rosmarinic acid amides as antioxidative and hypoglycemic agents. J. Nat. Prod. 2019, 82, 573–582.
[CrossRef]
33. Cardullo, N.; Muccilli, V.; Pulvirenti, L.; Cornu, A.; Poiségu, L.; Deffieux, D.; Quideau, S.; Tringali, C.
C-glucosidic ellagitannins and galloylated glucoses as potential functional food ingredients with anti-diabetic
properties: A study of
[PubMed]
α-glucosidase and α-amylase inhibition. Food Chem. 2020, 313, 126099. [CrossRef]
34. Genovese, C.; Pulvirenti, L.; Cardullo, N.; Muccilli, V.; Tempera, G.; Nicolosi, D.; Tringali, C. Bioinspired
benzoxanthene lignans as a new class of antimycotic agents: Synthesis and Candida spp. growth inhibition.
Nat. Prod. Res. 2018. publicated ahead of print. [CrossRef] [PubMed]
35. Di Micco, S.; Spatafora, C.; Cardullo, N.; Riccio, R.; Fischer, K.; Pergola, C.; Koeberle, A.; Werz, O.;
Chalal, M.; Vervandier-Fasseur, D.; et al. 2,3-Dihydrobenzofuran privileged structures as new bioinspired
lead compounds for the design of mPGES-1 inhibitors. Bioorg. Med. Chem. 2016, 24, 820–826. [CrossRef]
[PubMed]
36. Gerstmeier, J.; Kretzer, C.; Di Micco, S.; Miek, L.; Butschek, H.; Cantone, V.; Bilancia, R.; Rizza, R.; Troisi, F.;
Cardullo, N.; et al. Novel benzoxanthene lignans that favorably modulate lipid mediator biosynthesis: A
promising pharmacological strategy for anti-inflammatory therapy. Biochem. Pharmacol. 2019, 165, 263–274.
[CrossRef] [PubMed]
37. Di Micco, S.; Pulvirenti, L.; Bruno, I.; Terracciano, S.; Russo, A.; Vaccaro, M.C.; Ruggiero, D.; Muccilli, V.;
Cardullo, N.; Tringali, C.; et al. Identification by inverse virtual screening of magnolol-based scaffold as new
tankyrase-2 inhibitors. Bioorg. Med. Chem. 2018, 26, 3953–3957. [CrossRef] [PubMed]
38. Baschieri, A.; Pulvirenti, L.; Muccilli, V.; Amorati, R.; Tringali, C. Chain-breaking antioxidant activity of
hydroxylated and methoxylated magnolol derivatives: The role of H-bonds. Org. Biomol. Chem. 2017, 15,
6177–6184. [CrossRef]
39. Cardile, V.; Lombardo, L.; Spatafora, C.; Tringali, C. Chemo-enzymatic synthesis and cell-growth inhibition
activity of resveratrol analogues. Bioorg. Chem. 2005, 33, 22–33. [CrossRef]
40. Ng, S.Y.; Cardullo, N.; Yeo, S.C.M.; Spatafora, C.; Tringali, C.; Ong, P.-S.; Lin, H.-S. Quantification of the
resveratrol analogs trans-2,3-dimethoxy-stilbene and trans-3,4-dimethoxystilbene in rat plasma: Application
to pre-clinical pharmacokinetic studies. Molecules 2014, 19, 9577–9590. [CrossRef]
41. Schuhly, W.; Hufner, A.; Pferschy-Wenzig, E.M.; Prettner, E.; Adams, M.; Bodensieck, A.; Kunert, O.;
Oluwemimo, A.; Haslinger, E.; Bauer, R. Design and synthesis of ten biphenyl-neolignan derivatives and their
in vitro inhibitory potency against cyclooxygenase-1/2 activity and 5-lipoxygenase-mediated LTB4-formation.
Bioorg. Med. Chem. 2009, 17, 4459–4465. [CrossRef] [PubMed]
42. Hu, Y.; Shen, Y.F.; Tu, X.; Wu, X.H.; Wang, G.X.; Ling, F. Isolation of anti-Saprolegnia lignans from Magnolia
officinalis and SAR evaluation of honokiol/magnolol analogs. Bioorg. Med. Chem. Lett. 2019, 29, 389–395.
[CrossRef] [PubMed]
43. Tanaka, T.; Sakurai, Y.; Okazaki, H.; Hasegawa, T.; Fukuyama, Y. Biphenyl Derivative Composition for Nerve
Cell Degeneration Repairing or Protective Agent and Process for Preparing a Phenyl Derivative Contained
in the Composition. U.S. Patent 5053548, 1 October 1991.

Molecules 2020, 25, 733
17 of 17
44. Bovicelli, P.; Antonioletti, R.; Mancini, S.; Causio, S.; Borioni, G.; Annnendola, S.; Barontini, M. Expedient
synthesis of hydroxytyrosol and its esters. Synt. Commun. 2007, 37, 4245–4252. [CrossRef]
45. Ichikawa, T.; Netsu, M.; Mizuno, M.; Mizusaki, T.; Takagi, Y.; Sawama, Y.; Monguchi, Y.; Sajiki, H. Development
of a unique heterogeneous palladium catalyst for the Suzuki–Miyaura reaction using (hetero)aryl chlorides
and chemoselective hydrogenation. Adv. Synth. Catal. 2017, 359, 2269–2279. [CrossRef]
46. Brunel, J.M. Scope, limitations and mechanistic aspects in the selective homogeneous palladium-catalyzed
reduction of alkenes under transfer hydrogen conditions. Tetrahedron 2007, 63, 3899–3906. [CrossRef]
47. Freudenberg, K.; Renner, K.C. Biphenyls and diaryl ethers among the precursors of lignin. Chem. Ber. 1965
98, 1879–1892. [CrossRef]
,
48. Zhang, J.; Sun, T.J.; Diao, H.P.; Li, M.P. Quantitative structure-activity relationship studies of phenol’s
biological activity. Shanxi Daxue Xuebao 2011, 34, 468–474.
49. Accardo, A.; Del Zoppo, L.; Morelli, G.; Condorelli, D.F.; Barresi, V.; Musso, N.; Spampinato, G.; Bellia, F.;
Tabbi, G.; Rizzarelli, E. Liposome antibody-ionophore conjugate antiproliferative activity increases by cellular
metallostasis alteration. Med. Chem. Comm. 2016, 7, 2364–2367. [CrossRef]
50. Scherf, U.; Ross, D.T.; Waltham, M.; Smith, L.H.; Lee, J.K.; Tanabe, L.; Kohn, K.W.; Reinhold, W.C.; Myers, T.G.;
Andrews, D.T.; et al. A gene expression database for the molecular pharmacology of cancer. Nat. Genet.
2000, 24, 236–244. [CrossRef]
Sample Availability: Samples of the compounds 3-9, 12a-c, 13a-c, 14a-c and 15a are available from the authors.
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).