The proposed structures of phenolic compounds isolated from Piper betle L. differ from those of the compounds obtained by total synthesis

Article abstract of DOI:10.1080/14786419.2020.1739038

We describe the syntheses of phenolic compounds, 4-[(1E, 3E, 5E)?6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]benzene-1,2-diol (1) and 3-(n-dodecyloxy) phenol (2), isolated from Piper betle. The triene moiety of 4-[(1E, 3E, 5E)?6-(4-octyloxyphenyl)hexa-1,3,5

Full text of DOI:10.1080/14786419.2020.1739038

Natural Product Research
Formerly Natural Product Letters
ISSN: 1478-6419 (Print) 1478-6427 (Online) Journal homepage: https://www.tandfonline.com/loi/gnpl20
The proposed structures of phenolic compounds
isolated from Piper betle L. differ from those of the
compounds obtained by total synthesis
Toshiro Noshita, Tatsuya Sato, Takahito Iwayama, Yohei Yamada & Hidekazu
Ouchi
To cite this article: Toshiro Noshita, Tatsuya Sato, Takahito Iwayama, Yohei Yamada & Hidekazu
Ouchi (2020): The proposed structures of phenolic compounds isolated from Piperꢀbetle L.
differ from those of the compounds obtained by total synthesis, Natural Product Research, DOI:
10.1080/14786419.2020.1739038
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2020.1739038
The proposed structures of phenolic compounds isolated
from Piper betle L. differ from those of the compounds
obtained by total synthesis
a
a
a
a
Toshiro Noshita , Tatsuya Sato , Takahito Iwayama , Yohei Yamada
b
and Hidekazu Ouchi
a
Faculty of Life and Environmental Sciences, Department of Life Sciences, Prefectural University
b
of Hiroshima, Shobara, Japan; Faculty of Pharmaceutical Education, Department of Pharmacy,
Kindai University, Osaka, Japan
ABSTRACT
ARTICLE HISTORY
We describe the syntheses of phenolic compounds, 4-[(1E, 3E,
Received 1 December 2019
Accepted 25 February 2020
5
E)ꢀ6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]benzene-1,2-diol (1)
and 3-(n-dodecyloxy) phenol (2), isolated from Piper betle. The tri-
KEYWORDS
Piper betle;
Horner–Wadsworth–
Emmons reaction; McMurry
coupling; total synthesis
ene moiety of 4-[(1E, 3E, 5E)ꢀ6-(4-octyloxyphenyl)hexa-1,3,5-trien-
1-yl]benzene-1,2-diol was formed via two different methods, the
Horner–Wadsworth–Emmons reaction and the McMurry coupling
reaction. The spectral data of synthesized compounds show
differences with those of reported as the naturally occurring
compounds.
1. Introduction
Phenolic compounds (1 and 2, Figure 1) have been isolated from Piper betle L. by Atiya
et al. (2018 and 2020); these compounds have been reported to be secondary metabo-
lites that exhibit excellent DPPH free radical scavenging activities and moderate cyto-
toxic activities (Atiya et al., 2018 and 2020). The structure of 1 was proposed as 4-[(1E,
3
E, 5E)ꢀ6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]benzene-1,2-diol on the basis of spec-
troscopic analyses. We were interested in the unique structure of compound 1, which
CONTACT Toshiro Noshita
noshita@pu-hiroshima.ac.jp
Supplemental data for this article can be accessed at https://doi.org/10.1080/14786419.2020.1739038.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
T. NOSHITA ET AL.
Figure 1. Proposed structures for phenolic compounds 1 and 2 isolated from Piper betle and those
obtained by synthetic routes to produce triene 1. Reagents and conditions: (a) t-BuOK, THF, room
ꢁ
temperature, 1 h; (b) zinc powder, TiCl , pyridine, THF, 60 C (MW), 5 min; (c) TBAF, THF, room tem-
4
perature, 4 h, 41% in two steps via route A, 29% in two steps via route B.
has a very rare structure as the natural products containing two C6 units that are con-
nected by a triene. As such, confirmation of the structure of the natural product is of
particular importance from the viewpoint of natural products chemistry. On the other
hand, compound 2 is a very simple and common compound in contrast to 1, but it is
interesting to note that antioxidant activity comparable to ascorbic acid (vitamin C) has
been reported.
Thus, we herein report the syntheses of compounds 1 and 2, however, the NMR
data of synthesised compounds were not consistent with the reported natural prod-
ucts data. These results thereby indicate that the structures of the phenolic com-
pounds 1 and 2 isolated from P. betle require reinvestigation.
2. Results and discussion
The first synthetic route (route A) for the synthesis of 4-[(1E, 3E, 5E)ꢀ6-(4-octyloxyphe-
nyl)hexa-1,3,5-trien-1-yl]benzene-1,2-diol (1) is shown in Figure 1. In this synthetic route,
the formation of the triene moiety was performed via the Horner–Wadsworth–Emmons
(HWE) reaction, which is based on the method used by Ha et al. (2013) for syntheses of
1,6-bis(substituted phenyl)hexa-1,3,5-trienes. The starting aldehydes, 4-(octyloxy)benzal-
dehyde and 3,4-bis[(1,1-dimethylethyl)dimethylsilyloxy]benzaldehyde were prepared in a
conventional manner from 4-hydroxybenzaldehyde and 3,4-dihydroxybenzaldehyde,
respectively, and diphosphonate was prepared as described by Ha et al. (2013). The

NATURAL PRODUCT RESEARCH
3
1
Figure 2. Comparison of the H NMR spectral data for synthesised, reported as natural compounds
and reported similar compounds.
HWE olefination reaction of two aldehydes and diphosphonate produced an inseparable
triene mixture. The resulting mixture was treated with tetra-n-butylammonium fluoride
(TBAF) to produce highly polar compounds including the desired 1. Purification by silica
gel column chromatography yielded pure compound, which is considered to be triene
1
1
. In the H NMR spectra of synthetic 1, the presence of a broad signal of phenolic
hydroxy protons at 8.84-9.26 ppm and signals derived from the alkyl group (0.86,
.20–1.32, 1.40 and 1.69 ppm) indicate the success of deprotection following the
1
coupling reaction. In addition, the detection of the pseudomolecular ion at m/z
ꢀ
3
91.2263 [M–H] is consistent with the molecular formula of C H O , which confirmed
2
6 32 3
the total synthesis of compound with the desired structure.
1
The reported H NMR data for natural 1 is shown in Figure 2. The comparison of
the data for synthetic 1 with those reported for natural 1 reveals a distinct difference.
In addition, synthetic 1 is poorly soluble in methanol and chloroform; therefore,
DMSO-d was used to obtain NMR spectra, whereas Atiya et al. (2020) measured the
6
spectra of natural 1 in methanol-d and chloroform-d. This suggests that the physical
4
properties are different, which implies that either the structure of the natural product
determined by Atiya et al. (2020) or the synthesis performed by us is wrong.
Therefore, we decided to synthesise compound 1 with reported structure by another
route and compare the data with that reported for natural product 1.
The second synthetic route (route B) is shown in Figure 1. In this synthetic route,
the construction of the triene moiety was performed by the microwave-assisted
McMurry coupling reaction, which is based on the method used by Ramama et al.

4
T. NOSHITA ET AL.
(2004). Specifically, two aldehydes (i.e., coupling substrates) were prepared in a usual
manner (Demin et al. 2004; Kumar et al. 2006). The McMurry coupling of two a, b-unsat-
urated aldehydes produced inseparable crude trienes that contained the desired triene
as major product (ca. 62% by HPLC analysis) and minor dimeric trienes. The McMurry
reaction product was subjected to the next deprotection without purification, and the
obtained compound was the same triene made by the previous another method.
The same compound was obtained by a different synthetic route, which suggests a
possibility of an error in the structural analysis of the natural product. Therefore, it
was decided to solve this problem by analysing the spectra of similar compounds, as
shown in Figure 2. First, the chemical shift of methylene protons adjacent to oxygen
(
–CH –O–) was examined. The synthesised compound indicated a peak at 3.95 ppm;
2
however, the natural compound indicated a peak at 2.51 ppm. The chemical shift of
the –CH –O– proton in (n-octyloxy)benzene was observed at 3.92 ppm (AIST 2019),
2
which strongly suggests that assignments for the natural product may be wrong.
Next, triene protons were checked. The shift for these protons were observed with
those of benzene at 6.40–7.40 ppm as inseparable signals for synthetic compound 1
and at 4.09–5.41 ppm for natural compounds. For example, the chemical shifts of tri-
0
ene protons of 4,4 -[(1E, 3E, 5E)-hexa-1,3,5-triene-1,6-diyl]diphenol were observed at
6.41, 6.46 and 6.73 ppm (Ha et al. 2013). This result indicated a mistake in the structure
determination of the natural compound. In addition, the results of spectral calculations
from online NMR prediction services (nmrdb.org 2020) reinforce our conclusions.
Next, we synthesised 3-(n-dodecyloxy)phenol by microwave-assisted half alkylation
1
of the hydroxy group of catechol (Paul and Gupta 2004). The H NMR spectral data of
the obtained compound and that of the natural compound are shown in Figure 2.
The reported NMR data for natural 3-(n-dodecyloxy)phenol (2) and synthesised 2 was
compared. However, differences exist between the data even though they were
obtained using the same solvent. Therefore, in comparison with the spectral data of
this previously synthesised compound (Wang et al. 2006), the data in this study are in
complete agreement, as shown in Figure 2. Therefore, it is clear that there was an
error in the structure determination by Atiya; however, no revised structure can be
proposed. The synthesised 3-(n-dodecyloxy)phenol exhibited a moderate radical scav-
enging activity (data not shown), which was expected because compound 2 is a
mono-ether of resorcinol.
At this stage, it is unclear whether the correct structure was determined for natur-
ally occurring 1 and 2. However, given the synthetic route that was used and the
NMR and HRMS data, the determined structures for synthesised 1 and 2 are likely cor-
rect. The differences in the NMR data between synthesised and naturally occurring 1
and 2 suggest the possibility of an error in the interpretation of the data used for the
structural determination of naturally occurring 1 and 2.
3. Experimental section
3.1. General
All solvents were of a reagent grade. All commercial reagents were of the highest pur-
ity available and were purchased from FUJIFILM Wako pure chemical (Osaka, Japan),

NATURAL PRODUCT RESEARCH
5
Nacalai tesque (Kyoto, Japan), Tokyo chemical industry (Tokyo, Japan) and Kanto
Chemical (Tokyo, Japan). Infrared (IR) spectra were obtained using a Nicolet iS10 FT-IR
spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a diamond
horizontal attenuated total reflectance (ATR) accessory; the final spectra were the
1
13
result of co-addition of 16 interferograms. H and C NMR were obtained with an
Agilent 400-MR (Agilent, Santa Clara CA, USA) spectrometer. HRMS was carried out in
an electrospray ionisation mode using an Exactive Plus Orbitrap mass spectrometer
(Thermo Fisher Scientific, Waltham, MA, USA). Analytical TLC was performed on Merck
Silica gel 60 F254. Crude products were purified by column chromatography on a Silica
Gel 60 N [Kanto chemical, particle size (spherical, neutral) of 100–210 lm].
3.1.1. Preparation of 4-[(1E, 3E, 5E)-6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]
benzene-1,2-diol (1) via the horner–wadsworth–emmons (HWE) reaction
A solution of (E)-tetraethyl but-2-ene-1,4-diyldiphosphonate (400 mg, 1.2 mmol) in tetra-
hydrofuran (THF) and a solution of 4-(octyloxy)benzaldehyde (240 mg, 1.0mmol) and
3,4-bis[(1,1-dimethylethyl)dimethylsilyloxy]benzaldehyde (360 mg, 1.0 mmol) in THF were
added to a stirred suspension of potassium tert-butoxide (340mg, 3.0 mmol) in THF at
ice-cooling temperature; then, the reaction mixture was stirred at room temperature for
1
h. The reaction mixture was poured into a dilute HCl solution (ca. 3%). The mixture
was extracted with CHCl three times, and the combined organic layer was washed with
3
brine and dried with CaCl . The solvent was removed under reduced pressure. The resi-
2
due was purified by silica gel column chromatography (benzene-AcOEt ¼ 20/1) to yield
crude trienes. A 1 M TBAF solution of THF (2mL) at ice-cooling temperature was added
to a solution of crude trienes in THF, and the reaction mixture was stirred at room tem-
perature for 4 h. The reaction mixture was poured into a dilute HCl solution (ca. 3%).
The mixture was extracted with CHCl three times, and the combined organic layer was
3
washed with brine and dried with CaCl . The solvent was removed under reduced pres-
2
sure. The residue was purified by silica gel column chromatography (benzene-AcOEt ¼
8
1
/1) to yield 4-[(1E, 3E, 5E)ꢀ6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]benzene-1,2-diol (1,
60 mg, 0.4 mmol, 41% in two steps). 1: light brown amorphous powder. IR (diamond-
ꢀ
1 1
ATR)mmax 3381, 2921, 1602, 1508, 1249, 984, 860 and 798cm ; H NMR (DMSO-d ) d
6
0
.85 (3H, t, J ¼ 8.6Hz), 1.20–1.45 (10H, m), 1.69 (2H, m), 3.95 (2H, t, J ¼ 6.3 Hz), 6.40–6.58
13
(3H, m), 6.63–6.93 (8H, m), 7.40 (2H, d, J ¼ 8.3 Hz), 8.84–9.26 (2H, br). C NMR (DMSO-d )
6
d 14.0, 22.1, 25.5, 28.7, 28.8, 31.3, 34.4, 67.5, 113.1, 114.7, 115.8, 118.5, 124.9, 126.2,
1
3
27.3, 127.5, 128.1, 128.9, 129.8, 131.2, 132.3, 132.4, 133.1, 145.4, 145.6, 158.3. HRMS m/z
ꢀ
91.2263 [M–H] (calcd. for C H O 391.2268).
26 31 3
3.1.2. Preparation of 4-[(1E, 3E, 5E)-6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]
benzene-1,2-diol (1) via the McMurry coupling reaction
Under an N atmosphere, the suspension of zinc powder (130 mg, 2.0 mmol) in THF
2
was cooled to ice-cooling temperature, and TiCl (0.2 mL, 2.0 mmol) was slowly added
4
dropwise. The suspension mixture was warmed to room temperature, and two drops
of pyridine were added and stirred for 30 min. The solution of two aldehydes, 3,4-
bis[(1,1-dimethylethyl)dimethylsilyloxy]cinnamaldehyde (390 mg, 1.0 mmol) and 4-(octy-
loxy)cinnamaldehyde (260 mg, 1.0 mmol), in THF was added dropwise. After the

6
T. NOSHITA ET AL.
addition, the reaction mixture was heated at reflux for 5 min using the microwave
reactor (lReactor Ex, Shikoku Instrumentation Co., Ltd, Kagawa, Japan). Then, the reac-
tion mixture was poured into a 10% K CO3 aqueous solution and extracted with
2
CH Cl . The organic layer was combined and concentrated in vacuo. The residue was
2
2
purified by silica gel column chromatography (benzene–AcOEt ¼ 20/1) to yield crude
trienes. A 1 M TBAF solution of THF (2 mL) at ice-cooling temperature was added to
the solution of the residue including trienes, and the reaction mixture was stirred at
room temperature for 4 h. The reaction mixture was poured into a dilute HCl solution
(
ca. 3%). It was extracted with CHCl three times, and the combined organic layer was
3
washed with brine and dried with CaCl . The solvent was removed under reduced
2
pressure. The residue was purified by silica gel column chromatography
(
1
(
benzene–AcOEt ¼ 8/1) and preparative thin-layer chromatography (benzene–AcOEt ¼
0/1) to yield 4-[(1E, 3E, 5E)ꢀ6-(4-octyloxyphenyl)hexa-1,3,5-trien-1-yl]benzene-1,2-diol
1, 113 mg, 0.29 mmol, 29% in two steps). All spectral data were in complete agree-
ment with those of 1 synthesised via the HWE reaction.
3.1.3. Preparation of 3-(n-dodecyloxy)phenol (2)
A mixture of resorcinol (550 mg, 5.0 mmol), Zinc powder (65 mg), 1-bromododecane
(
1250 mg, 5.0 mmol) and N,N-dimethylacetamide (8 mL) in microwave vial was irradi-
ꢁ
ated at 160 C for 45 min. The reaction mixture was filtrated. After the addition of
diluted HCl, the aqueous solution was extracted with AcOEt five times, and the com-
bined organic layer was washed with brine and dried with MgSO . The solvent was
4
removed under reduced pressure, and the residue was purified by silica gel column
chromatography (CHCl -MeOH ¼ 30/1) to yield 3-(n-dodecyloxy)phenol (2, 500 mg,
3
1
1
.8 mmol, 36%). IR (diamond-ATR)m
029 cm ; The H-NMR spectral data (see Figure 2) was in complete agreement with
3400, 2919, 1614, 1586, 1492, 1279, 1148 and
max
ꢀ
1
1
previously synthesised 2 (Wang et al. 2006). negative-ion HRESIMS m/z 277.2173
ꢀ
[
M-H] (Calcd for C H O , 277.2162).
1
8 29 2
4. Conclusion
In this paper, we described the syntheses of 4-[(1E, 3E, 5E)ꢀ6-(4-octyloxyphenyl)hexa-
,3,5-trien-1-yl]benzene-1,2-diol and 3-(n-dodecyloxy)phenol (i.e., the proposed struc-
1
tures for the naturally occurring DPPH free radical scavengers isolated from Piper
betle). However, the spectral data of the reported and the synthesised compounds
considerably differed. These results suggest that the structures of DPPH free radical
scavengers isolated from Piper betle require reinvestigation.
Acknowledgement
The authors would like to thank Enago (www.enago.jp) for the English language review.
Disclosure statement
No potential conflict of interest was reported by the authors.

NATURAL PRODUCT RESEARCH
7
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