The efficient synthesis and biological evaluation of justicidin B

Article abstract of DOI:10.1080/14786419.2021.1948843

A facile new synthetic method for the preparation of a Type-A 1-arylnaphthalene lactone skeleton was developed and used to synthesise justicidin B and several derivatives. Key synthesis steps included Hauser–Kraus annulation of a phthalide intermediate and Suzuki–Miyaura cross coupling between a triflated naphthalene lactone intermediate and various potassium organotrifluoroborates. With two exceptions, the derivatives showed significant inhibitory effect on lipopolysaccharide (LPS)-induced nitric oxide (NO) production in mouse macrophages. Moreover, several compounds, including justicidin B, had marked cytotoxicity towards six human tumour cell lines.

Full text of DOI:10.1080/14786419.2021.1948843

Natural Product Research
Formerly Natural Product Letters
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gnpl20
The efficient synthesis and biological evaluation of
justicidin B
Taejung Kim, Young-Joo Kim, Kyu-Hyuk Jeong, Young-Tae Park, Hyukjoon
Kwon, Pilju Choi, Ha-Neul Ju, Cheol Hee Yoon, Ji-Yool Kim & Jungyeob Ham
To cite this article: Taejung Kim, Young-Joo Kim, Kyu-Hyuk Jeong, Young-Tae Park, Hyukjoon
Kwon, Pilju Choi, Ha-Neul Ju, Cheol Hee Yoon, Ji-Yool Kim & Jungyeob Ham (2021): The
efficient synthesis and biological evaluation of justicidin B, Natural Product Research, DOI:
10.1080/14786419.2021.1948843
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2021.1948843
The efficient synthesis and biological evaluation of
justicidin B
a
ꢀ
a
ꢀ
a
a
Taejung Kim
, Young-Joo Kim , Kyu-Hyuk Jeong , Young-Tae Park
,
a
a
a
a
a,b
Hyukjoon Kwon , Pilju Choi , Ha-Neul Ju , Cheol Hee Yoon , Ji-Yool Kim and
Jungyeob Ham
a,b
a
Korea Institute of Science and Technology (KIST), Natural Products Research Institute, Gangneung,
b
Republic of Korea; Division of Bio-Medical Science & Technology, University of Science and
Technology (UST), Daejeon, Republic of Korea
ABSTRACT
ARTICLE HISTORY
A facile new synthetic method for the preparation of a Type-A 1-
arylnaphthalene lactone skeleton was developed and used to syn-
thesise justicidin B and several derivatives. Key synthesis steps
included Hauser–Kraus annulation of a phthalide intermediate
and Suzuki–Miyaura cross coupling between a triflated naphtha-
lene lactone intermediate and various potassium organotrifluoro-
borates. With two exceptions, the derivatives showed significant
inhibitory effect on lipopolysaccharide (LPS)-induced nitric oxide
Received 22 March 2021
Accepted 20 June 2021
KEYWORDS
Justicidin B; total synthesis;
natural products;
annulation; cross coupling
(
NO) production in mouse macrophages. Moreover, several com-
pounds, including justicidin B, had marked cytotoxicity towards
six human tumour cell lines.
1. Introduction
Naturally occurring 1-arylnaphthalene lactones are a plant-derived subclass of lignans
Ward 1995; Man et al. 2012; Schmidt et al. 2012; Tadross and Stoltz 2012; Jullian-
(
Pawlicki et al. 2015; Hemmati and Seradj 2016; Teponno et al. 2016; Li et al. 2020).
They possess anti-tumour (Yu et al. 2010; Gui et al. 2011; Shi et al. 2012), anti-platelet
(Chen et al. 1996; Weng et al. 2004), and anti-bacterial (Kawazoe et al. 2001) properties
as well as phosphodiesterase (Ukita et al. 1999), 5-lipooxygenase (Delorme et al. 1996),
and HIV reverse transcriptase inhibitory functions (Chang et al. 1995; Lee et al. 1996).
The 1-arylnaphthalene lactone structure may be used as a template to develop lead
compounds for novel therapeutics derived from natural products (Nandy et al. 2009;
Kumar and Waldmann 2009a, 2009b; Zhao et al. 2021). 1-Arylnaphthalene lactone
lignans are classified as Type-A, Type-B, Type-C and Type-D (Scheme 1). The Type-A 1-
CONTACT Jungyeob Ham
These authors contributed equally to this work.
ham0606@kist.re.kr
ꢀ
Supplemental data for this article can be accessed online at https://doi.org/10.1080/14786419.2021.1948843.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2
T. KIM ET AL.
Scheme 1. Typical structure of 1-arylnaphthalene lactone lignans and the retrosynthesis of Type-A
justicidin B (1).
arylnaphthalene lactone, justicidin B (1), is the main active component of Phyllanthus
piscatorum, exhibiting anti-fungal and anti-proliferative properties (Gertsch et al. 2003;
Rao et al. 2006; Luo et al. 2014; Ai et al. 2020).
Since the chemical structure was first reported in the 1960s, justicidin B (1) has
been efficiently prepared using several synthetic strategies: naphthalene construction
from a seven membered ring system with ether linkage (Munakata et al. 1967), cyclisa-
tion of a cinnamyl phenylpropiolate ester (Stevenson and Block 1971), Michael initi-
ated ring closure (Kamal et al. 1994), conjugate addition–aldol reaction of cyanohydrin
(
(
Ogiku et al. 1995), regio-controlled benzannulation of gem-dichlorocyclopropane
Nishii et al. 2005) and continuous photoflow intramolecular Diels–Alder reaction of 3-
phenylprop-2-yn-1-yl 3-phenylpropiolate (Ge et al. 2021). Despite many synthetic stud-
ies on justicidin B (1), the synthetic strategies of the common intermediate 5 from
Hayat et al. (2015) and Mal and Jana (2016) are most suitable for Type-A derivative
synthesis. However, to the best of our knowledge, this is the first study to report the
chemical derivatisation of 5 for biological studies. Herein, we report a short and facile
new synthetic method for the preparation of justicidin B (1) derivatives and the bio-
logical evaluation of anti-inflammatory and cytotoxic activities.
Our synthesis and derivatisation strategy were based on the efficient construction
of the 1-arylnaphthalene lactone framework. The retrosynthetic analysis of 1-arylnaph-
thalene lactone structure predicted the formation of a triflated naphthalene lactone
intermediate and corresponding potassium organotrifluoroborates. Owing to the effi-
cient synthesis of the natural 1-arylnaphthalene lactone, this compound could contrib-
ute to structure–activity relationship (SAR) compound libraries to investigate the anti-
inflammatory and anti-tumour activities of these compounds.

NATURAL PRODUCT RESEARCH
3
Scheme 2. Synthesis of justicidin (1) from 2-bromo-3,4-dimethoxybenzaldehyde (7) or veratric
acid (10).
2. Research and discussion
2.1. Chemistry
Justicidin (1) synthesis is described in Scheme 2. First, the synthesis of phthalide 6.
N,N-Dimethylamide 9 was prepared from commercially available 2-bromo-3,4-dime-
thoxybenzaldehyde (7) using our previous report on the synthesis of Type-C and
Type-D 1-arylnaphthalene lactones (Kim et al. 2017). Then, sodium borohydride reduc-
tion of the formyl group was followed by acidic treatment with hydroxylmethyl, gener-
ating phthalide 6. An alternative approach to phthalide 6 via lactonisation (Singh and
Khade 2011) of veratric acid (10) using formaldehyde under acidic conditions was
also generated.
Next, a Hauser–Kraus annulation between phthalide 6 using ɤ-crotonolactone (11)
was performed in the presence of lithium bis(trimethylsilyl)amide (LHMDS) in tetra-
ꢂ
hydrofuran at ꢁ78 C, followed by aromatisation using camphorsulfonic acid in one
pot to generate an unstable naphthol intermediate. The subsequent formation of the
triflate group using trifluoromethanesulfonic anhydride (Tf O) and pyridine in CH Cl ,
2
2
2
yielded the key intermediate 5 for the synthesis of justicidin B (1) and derivatives.
Finally, the Suzuki–Miyaura cross-coupling reaction of 5 was successfully performed
using potassium 1,3-benzodioxol-5-yltrifluoroborate (12) in four steps to yield 36.4%
from the veratric acid (10). The other derivatives, i.e., 13–23 were similarly synthesised
(
Scheme 3) from the common intermediate triflate 5. The structures of the final com-
1
13
pounds 1 and 13–23 were identified from spectral data ( H & C nuclear magnetic
resonance and mass spectrometry). Analysis data of all the synthesized compounds
are available in Supplementary Material.
2.2. Bioassays
Before investigating the inhibitory effects of the synthesized compounds on lipopoly-
saccharide (LPS)-induced nitric oxide (NO) production, their cytotoxicity was evaluated
in RAW264.7 cells. The compounds, except for 1 (IC50 ¼ 10.1 lM), 15 (IC ¼ 33.3 lM)
50
and 23 (IC50 ¼ 25.4 lM), did not affect cell viability at concentrations up to 40 lM
(Table S1 in Supplemental Material). The following experiment was performed in the

4
T. KIM ET AL.
Scheme 3. Synthesis of justicidin (1) and derivatives.
concentration range of those without cytotoxicity. The derivatives 13, 14, 16–22 sig-
nificantly inhibited NO production in a concentration-dependent manner (Figure S33
in Supplemental Material).
Next, cytotoxicity of justicidin B (1) and derivatives (13–23) was evaluated in six
human cancer cell lines: AGS (gastric), A549 (lung), HCT116 (colon), PC-3 (prostate),
HepG2 (liver) and MDA-MB-231 (breast). Table S2 in Supplemental Material reveals
that the compounds 1, 15, 17 and 23 exerted cytotoxic effect on the cancer cells. In
particular, the derivatives 15, 17 and 23 were more cytotoxic against PC-3 and HepG2
cells than VP-16, which is used as a drug for cancer, although justicidin B (1) was the
most effective.
3. Conclusions
We demonstrated a facile new synthetic preparative approach for a naphthalene lac-
tone skeleton using Hauser–Kraus annulation of phthalide 6. Moreover, the
Suzuki–Miyaura cross-coupling reaction of triflated naphthalene lactone intermediate 5
with the corresponding potassium organotrifluoroborates generated justicidin B (1)
and derivative (13–23) formation. Additionally, compounds were screened for their
inhibitory effects on the NO production in LPS-induced RAW 264.7 and on cytotoxicity
in six human tumour cell lines. The derivatives 13, 14, 16 2 22 had the most potent
inhibitory activity towards NO production in LPS-stimulated RAW267.4 cells. In add-
ition, the compounds 1, 15, 17 and 23 exerted cytotoxic effect on various cancer cell
lines. Further studies on structure optimisation and biological activity mechanisms are
underway in our laboratory.

NATURAL PRODUCT RESEARCH
5
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the Korea Institute of Science and Technology (KIST) institutional
program (Project No. 2Z06481 and 2E31311). This research was also conducted under the indus-
trial infrastructure program for fundamental technologies, which is funded by the Ministry of
Trade, Industry & Energy (MOTIE, Korea) (20000105).
ORCID
Taejung Kim
Young-Tae Park
Jungyeob Ham
http://orcid.org/0000-0002-9449-4763
http://orcid.org/0000-0003-1787-1577
http://orcid.org/0000-0003-3046-9267
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