Total Synthesis of Antiproliferative Parvifloron F

Article abstract of DOI:10.1021/acs.orglett.7b03763

The first total synthesis of parvifloron F, a bioactive highly oxidized abietane diterpene, was achieved. The abietane skeleton was constructed by Lewis acid promoted cyclization. Preliminary structure-activity relationship correlations were established f

Full text of DOI:10.1021/acs.orglett.7b03763

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis of Antiproliferative Parvifloron F
Yohei Saito,† Masuo Goto,‡ and Kyoko Nakagawa-Goto*,†,‡
†School of Pharmaceutical Sciences, College of Medical, Pharmaceutical and Health Science, Kanazawa University, Kanazawa,
920-1192, Japan
‡Chemical Biology and Medicinal Chemistry, UNC Eshelman School of Pharmacy, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-7568, United States
S
* Supporting Information
ABSTRACT: The first total synthesis of parvifloron F, a
bioactive highly oxidized abietane diterpene, was achieved. The
abietane skeleton was constructed by Lewis acid promoted
cyclization. Preliminary structure−activity relationship corre-
lations were established for the synthetic intermediates against
human tumor cell lines. Certain compounds showed unique
selective antiproliferative activity against triple-negative breast
cancer. The oxidation level of the abietane ring affected the
selectivity.
Plectranthus, a genus of family Lamiaceae, contains about 300
species. It has been widely used as traditional medicine in
Tropical Africa, Asia, and Australia for digestive, skin, and
respiratory diseases.1 Phytochemical studies on Plectranthus have
discovered various types of abietane diterpenes, such as
parviflorons (Pfs).2 Among them, Pfs-D, -F (1), and -G contain
a different benzoate group at the C-2 position (Figure 1). Pf-F
As shown in Scheme 1, a retrosynthetic analysis suggested that
esterification of quinone methide 2 with protocatechuic acid
Scheme 1. Retrosynthetic Analysis of Parvifloron F
Figure 1. Structure of parvifloron D, F, and G.
would provide 1. Intermediate 2 could be generated through the
repeated oxidation and isomerization of catechol 3, prepared by
biomimetic cyclization of epoxide 4. Epoxide 4 could be
synthesized by an SN2 reaction of a dithiane intermediate
produced by reacting benzylalcohol 5 with a chiral epoxygeranyl
bromide.
Accordingly, benzylalcohol 5 was synthesized via isopropyla-
tion7 of veratrole followed successively by a Friedel−Crafts
acylation,8 haloform reaction,8 and reduction.9 IBX oxidation of
5 gave benzaldehyde 6, which was converted to the desired
dithioacetal 710 (Scheme 2). Lithiation of 7 with t-BuLi followed
by addition of chiral epoxygeranyl bromide 811 provided epoxide
9 in 76% yield. Reductive desulfurization of the dithiane moiety12
on 9 with AIBN and Bu3SnH afforded the cyclization precursor 4
in 86% yield.
(1) was first isolated from P. parviflorus together with Pf-D and
Pf-G was separated from P. strigosus by Eugster et al. in 19783 and
1984,4 respectively. In 2001, 1 was also isolated from P.
nummularius by Narukawa et al.5 Compound 1 exhibits multiple
biological effects, including DPPH radical scavenging activity
(EC50 0.13 mM),5 antibacterial activity against Listeria
monocytogenes (MIC 31.2 μg/mL) and Mycobacterium tuber-
culosis (MIC 95 μg/mL), antiproliferative activity against the
Vero cell line (IC50 1.6 μg/mL), and tyrosinase inhibitory
activity.6 In addition, our original study revealed potent
antiproliferative activity against several human tumor cell lines
(HTCLs). However, the total synthesis and structure−activity
relationship (SAR) correlations of 1 have not been reported
despite its attractive biological activities. Herein, we describe the
first total synthesis of 1 and the antiproliferative activity of
synthetic intermediates against HTCLs.
Received: December 3, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03763
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Olefination of 10 with phosphoryl chloride15 proceeded well to
generate 13. Hydroboration−oxidation of 13 with 9-BBN gave
14 with a hydroxyl at C-2 in 53% yield and its regioisomer 15
with a hydroxyl at C-3 in 29% yield. Deprotection of the methoxy
groups using sodium hydride and ethanethiol with heating16
provided catechol 3.
Scheme 2. Synthesis of Epoxide 4
Oxidation of 3 with Ag2O17 provided quinone 16, and
subsequent isomerization under reflux afforded o-hydroxy-p-
quinone methide 17 in good yield (Scheme 4). Unexpectedly,
compound 17 did not isomerize to the related catechol 18 under
the reflux conditions.17 However, when the reaction was carried
out under neat conditions, further isomerization did occur to
produce 18 in 58% yield (77%, based on recovered starting
material; brsm). The same reaction sequence from 3 to 17 was
then applied to 18 to obtain the ester precursor 2. The
subsequent esterification step was more difficult than expected,
likely because the compound skeleton was unstable under acidic
conditions and migration/decomposition could occur readily
through a C-10 carbonium cation.18 In fact, the esterification of 2
with protocatechuic acid protected as its orthoester proceeded in
moderate yield, but the subsequent deprotection of the
protecting group with p-toluenesulfonic acid monohydrate or
Amberlyst 15 resulted in low yield and reproducibility. When the
esterification reaction was attempted with unprotected proto-
catechuic acid, only the starting material (2) was recovered.
Finally, because phenolic benzoates can be removed under mild
basic conditions,19 compound 2 was esterified successfully via a
Shiina reaction under basic conditions20 with protocatechuic acid
protected as its dibenzoate (20).21 The condensation produced
the desired 21 in 24% yield along with the unexpected
unsubstituted benzoate 22 in 31% yield. The latter compound
was produced when the hydroxyl group of 2 attacked the
protected benzoate moiety rather than the activated carboxylic
anhydride. The selective removal of the phenolic benzoate
groups in 21 using n-butyl amine22 proceeded smoothly to give 1
in 68% yield. It should be noted that the reaction proceeded well
in spite of the presence of a nucleophile. The spectroscopic data
of the synthetic sample were identical with those of the natural
product.
Cyclization of 4 was successfully accomplished by the use of
tin(IV) chloride as a Lewis acid13 at room temperature to provide
the desired product 10 in good yield (Scheme 3). Other Lewis
Scheme 3. Synthesis of Catechol 3
We next focused on the antiproliferative activity of the
synthetic compounds against HTCLs.23 Compounds 10, 13, and
14 containing methoxy groups exhibited moderate activity.
Interestingly, catechol 3, o-quinone 16, and o-hydroxy-p-quinone
methide 17 showed 4-fold selectivity against triple-negative
breast cancer (TNBC) cell line MDA-MB-231 compared with
A549 (Table 2).
acids, such as boron trifluoride diethyl etherate and titanium(IV)
chloride, were tested under various conditions for the cyclization
of ( )-4, which was prepared separately. However, the desired
( )-10 was obtained in poor yield due to the formation of
( )-11, which was formed by cyclization at an undesired
position, and oxabicyclo[2.2.1]heptane14 [( )-12] (Table 1).
Table 1. Cyclization Conditions of Racemic 4 Using Several Lewis Acids
entry
conditions
temp
workup
products
1
2
3
4
5
6
7
BF3·OEt2 (1.2 equiv), 1.5 h
BF3·OEt2 (1.2 equiv), 1 h
SnCl4 (1.2 equiv), 1 h
BF3·OEt2 (1.2 equiv), 1 h
SnCl4 (1.2 equiv), 2 h
TiCl4 (1.2 equiv), 2 h
SnCl4 (1.2 equiv), 1 h
−78 °C
−78 °C
−78 °C
−20 to 0 °C
−20 to 0 °C
−20 to 0 °C
room temp
H2O
10 (19%), 12 (33%)
TEA (1.2 equiv), NaHCO3 aq
10:11 = 1:1 (40%), 12 (16%)
10:11 = 5:4 (36%), 12 (23%)
10 (27%), 12 (49%)
TEA (1.2 equiv), NaHCO3 aq
H2O
H2O
H2O
H2O
10 (47%), 12 (30%)
10 (35%), 12 (8%)
10 (60%), 12 (5%)
B
DOI: 10.1021/acs.orglett.7b03763
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Synthesis of Parvifloron F
ative activity against several HTCLs. The SAR study indicated
that the oxidation level of the abietane ring affected
antiproliferative selectivity against HTCLs. Compounds 3, 16,
and 17 showed selective activity against TNBC MDA-MB-231.
Table 2. Antiproliferative Activity of Synthetic Compounds
against Human Tumor Cell Lines
a
b
cell lines /IC50 (μM)
compd
A549
KB KB-VIN MDA-MB-231 MCF-7
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03763.
compounds containing methoxy group
■
10
13
14
>40
>40
>40
>40
>40
>40
>40
>40
>40
>40
39.70
S
25.38
22.34
20.50
30.73
compounds showing selectivity against triple-negative breast cancer
Experimental procedures and spectroscopic data for
synthetic compounds (PDF)
3
19.45
20.44
20.12
15.72
18.49
19.36
19.89
21.43
21.42
5.13
5.14
5.18
13.53
15.17
16.19
16
17
AUTHOR INFORMATION
compounds showing broad spectrum antiproliferative activity
■
19
8.36
4.91
4.64
4.92
4.67
6.97
8.04
5.47
4.49
4.74
5.03
6.63
19.59
5.62
4.75
4.70
4.97
5.16
5.01
4.99
4.91
5.15
8.96
10.11
5.02
4.67
4.74
5.36
10.96
Corresponding Author
*E-mail: kngoto@p.kanazawa-u.ac.jp.
ORCID
2
1
c
1 (NP)
( )-1
Masuo Goto: 0000-0002-9659-1460
Kyoko Nakagawa-Goto: 0000-0002-1642-6538
Notes
PXL (nM)
1969
a
Lung carcinoma (A549), originally isolated from epidermoid
carcinoma of the nasopharynx (KB), MDR subline of KB over-
expressing P-glycoprotein (KB-VIN), triple-negative breast cancer
(MDA-MB-231) and breast cancer (MCF-7) were used for
antiproliferative activity evaluation of synthetic compounds. Antipro-
liferative activity as IC50 values for each cell line; the concentration of
compound that caused 50% reduction of cell growth relative to
The authors declare no competing financial interest.
b
ACKNOWLEDGMENTS
■
We thank Dr. Yuji Narukawa at Keio University for providing the
1
sample and H NMR data for parvifloron F. We appreciate
c
untreated cells determined by the sulforhodamine B assay. Natural
Product (NP).
critical comments, suggestions, and editing on the manuscript by
Dr. Susan L. Morris-Natschke (UNC−CH). This work was
supported by JSPS KAKENHI Grant Number 25293024
awarded to K.N.G. and 26750368 awarded to Y.S. as well as
the Eshelman Institute for Innovation, Chapel Hill, North
Carolina, awarded to M.G.
The results suggested that the oxidation level of the abietane
ring might be important for the selective TNBC antiproliferative
activity. Compound 2 with a free hydroxyl at C-2 was equipotent
to the natural product 1. Thus, the ester moiety of 1 did not affect
the antiproliferative activity against the tested HTCLs and
presumably is hydrolyzed by cellular esterase. In addition, the
stereochemistry did not influence the activity, since ( )-1,
prepared by the same reaction sequence using ( )-8, and 1 were
equipotent.
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DOI: 10.1021/acs.orglett.7b03763
Org. Lett. XXXX, XXX, XXX−XXX