Efficient synthesis of rottlerin and its two subunits

Article abstract of DOI:10.1016/j.tetlet.2016.03.049

Rottlerin, a natural product isolated from Mallotus philippensis, is associated with a range of biological activities. Its chemical structure is featured by two different substituted phloroglucinol units linked by a methylene group. In this study, we accomplished a total synthesis of rottlerin using phenol-aldehyde condensation as the key reaction. By our method, gram-scale preparation of the two structural subunits was achieved, and rottlerin was obtained in a longest eight linear step with 20% overall yield. Our study provides a practical solution for obtaining the sample of rottlerin in an efficient way.

Full text of DOI:10.1016/j.tetlet.2016.03.049

Accepted Manuscript
Efficient synthesis of rottlerin and its two subunits
Yangfeng Li, Biao Yu, Renxiao Wang
PII:
S0040-4039(16)30271-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.03.049
TETL 47437
Reference:
Tetrahedron Letters
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10 March 2016
15 March 2016
Please cite this article as: Li, Y., Yu, B., Wang, R., Efficient synthesis of rottlerin and its two subunits, Tetrahedron
Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.03.049
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Efficient synthesis of rottlerin and its two subunits
Yangfeng Li , Biao Yu, and Renxiao Wang *

1
Tetrahedron Letters
Efficient synthesis of rottlerin and its two subunits
Yangfeng Lia , Biao Yua, and Renxiao Wanga,b
a State Key Laboratory of Bioorganic and Natural Products Chemistry, Collaborative Innovation Center of Chemistry for Life Sciences, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, Shanghai, People’s Republic of China
bState Key Laboratory of Quality Research in Chinese Medicine, Macau Institute for Applied Research in Medicine and Health, Macau University of Science and
Technology, Macau, People’s Republic of China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Rottlerin, a natural product isolated from Mallotus philippensis, is associated with a range of
biological activities. Its chemical structure is featured by two different substituted
phloroglucinol units linked by a methylene group. In this study, we accomplished a total
synthesis of rottlerin using phenol-aldehyde condensation as the key reaction. By our
method, gram-scale preparation of the two structural subunits was achieved, and rottlerin was
obtained in a longest eight linear step with 20% overall yield. Our study provides a practical
solution for obtaining the sample of rottlerin in an efficient way.
Available online
Keywords:
rottlerin
total synthesis
2009 Elsevier Ltd. All rights reserved.
phenol-aldehyde condensation
ortho‑ quinone methide
gram-scale
Introduction
change”,18 discovered during trials of synthesis of rottlerin
analogues, reveals its sensitivity in both acidic and basic
Mallotus philippensis, also known as “Kamala tree”, is
distributed widely in the rainforests of Southeast Asia.1-3 It has
long been valued as the source of dyeing material known as
Kamala, which is also a rich source of biologically active
conditions (Figure 1). Therefore, rottlerin is often sold in small
quantities at high price. Difficulty in obtaining available samples
has certainly put a limitation on the research related to rottlerin.
compounds.4-8 Rottlerin (1), also called mallotoxin, is
a
chromenochalcone isolated from Kamala along with other
structurally related molecules, such as kamalachalcones and
acylphloroglucinol derivatives.9 Rottlerin began to attract the
public attention as a useful compound with pharmaceutical
potential in 1994, when Gschwendt et. al10 showed it to be a
specific inhibitor of protein kinase C delta (PKCδ). Since then,
rottlerin has been used as a PKC-δ inhibitor in a number of
biological assays related to PKC-δ.11 Other bioactivities of
rottlerin have also been discovered in recent years, such as
cytotoxicity against four human cancer cell lines,1 autophagy-
inducer in chronic lymphocytic leukemia cells and human
pancreatic cancer stem cell,12 inhibition of human T cell
responses,13 abrogation of reactive oxygen species production in
hepatic stellate cells,14 neuroprotective agent in cell culture and
Figure 1. Chemical structure of rottlerin (1) and the “rottlerone
change” reaction.
The first total synthesis of rottlerin was reported by Kumar et
al in late 2015 (Scheme 1).19 They used phloroacetophenone as
the common starting material. Acetophenone (4) was obtained
through formylation and Clemmensen reduction in 63% yield.
Chromene (6) was prepared in three steps with known methods20
and then aldol condensation and deprotection in five steps with
yield up to 63% (computed by assuming that the first three steps
were fully complete). Then, 6 was converted into a tertiary amine
(7) with Eschenmoser’s salt in 88% yield. Finally, 7 was used as
animal models,15 activator of
potassium channel,16 and
prevention of histamine-induced H1-receptor gene expression in
HeLa cells.17
Isolation of rottlerin from the kamala fruit extract is difficult.
It typically results in different levels of purity as many
structurally similar natural products also exist in the source. In
addition, rottlerin is found to be somewhat unstable. “Rottlerone
———
 Corresponding author. Tel.: +86-21-54925128; E-mail: wangrx@sioc.ac.cn

2
Tetrahedron
the precursor to prepare an o-quinone methide, which then went
through Michael addition with the corresponding acetophenone
(4) to obtain rottlerin in 63% yield. The overall yield by their
synthetic route was up to 4.6%.
Scheme 2. Retro-synthetic analysis of rottlerin.
The chromene scaffold exists in the structures of many natural
products, but no efficient preparation of it has been reported.
Here, we developed an efficient route for obtaining this useful
scaffold (Scheme 3). Our method began with protection of
phloro-acetophenone (2) with MOM groups. It then underwent
condensation with benzaldehyde to give 11. After that, the two
MOMs were removed using 3N hydrochloric acid to obtain 12 in
92% yield. Treatment of 12 with TBSCl provided a fully
protected product, and without purification, it was deprotected
selectively by iodine in methanol at room temperature to produce
13 in two-step 71% yield. EDDA-catalyzed chromene cyclization
of 13 produced 14.23 Here, EDDA (ethylenediamine diacetic acid)
was prepared by acid-base neutralization of 1 equiv of
ethylenediamine and 2 equiv of acetic acid. Then, removal of the
TBS group in 14 with TBAF in THF produced 6. Through this
route, compound 6 could be prepared in seven steps with 47%
overall yield. This route could be accomplished at the gram scale
(see the Supporting Information).
Scheme 1. The first total synthesis of rottlerin reported by Kumar et
al.19
By an independent effort, we have developed an efficient
synthetic route for obtaining rottlerin, which is somewhat
different from Kumar’s method. In particular, our route is
featured by high-yield synthesis of the two main structural
subunits of rottlerin, i.e. 4 and 6, where 6 was prepared in seven
steps with 47% overall yield and 4 was prepared in three steps
with 67% overall yield. Both subunits could be prepared at the
gram scale. Finally, the classic phenol-aldehyde condensation
was adopted to produce rottlerin in 42% yield, which provides an
alternative approach to the linkage of two subunits through an
ortho-quinone methide (oQM) mediated reaction.
Results and Discussion
The structure of rottlerin (1) contains a chalcone skeleton and
phloroglucinol and chromene rings (Figure 1). Unlike other
symmetrical phloroglucinol dimers,21 rottlerin has
a
pseudodimeric structure that two phloroacetophenone derived
aryl cores are linked by a methylene bridge. If the chromene and
acetophenone scaffolds are available, connection of them to yield
rottlerin can be accomplished via an ortho-quinone methide
(oQM) mediated reaction (the first two retrosynthetic routes in
Scheme 2) or a direct phenol-formaldehyde condensation (the
last route in Scheme 2). In either way, obtaining the two subunits
in an efficient way is the primary task. The oQM intermediate
strategy has long been applied to the total synthesis of many
natural products.22 As mentioned in the Introduction section,
Kumar et al. has accomplished total synthesis of rottlerin through
the second route in Scheme 2. In this paper, we will mainly
describe our efforts on exploring the other two routes.
Scheme 3. Synthesis of chromene 6
Our synthesis of acetophenone started with phloro-
acetophenone (15). Vilsmeier−Haack formylation and subsequent
reduction with sodium cyanoborohydride produced 17. Then, it
was acetylated into 4 using acetic anhydride and boron trifluoride
diethyl etherate in 65% yield. Another alternative route was as

3
follows: 2, 4, 6-trimethoxybenzoaldehyde (18) was reduced into
19, followed by acetylation to form 20 in 92% yield. Finally,
removal of three methyl groups by boron tribromide solution at
room temperature produced 4 in 74% yield (Scheme 4), and
acetophenone 4 was obtained in three steps with an overall yield
as high as 67%. Both routes could be accomplished at the gram
scale (see the Supporting Information).
The third synthetic route shown in Scheme 2 seems to be
feasible since the key factors in phenol-aldehyde condensation29
are methylene source and acid. We conducted the condensation
of 4 and 6 using four different methylene reagents (Table 1). In
most cases, no reaction between the reactants and
paraformaldehyde, dimethoxymethane, or chloromethyl methyl
ether was observed, indicating the low reactivity of releasing the
active methylene intermediate. To tackle this problem, a variety
of Brønsted acids were examined. Finally, we found that by using
formalin (30 equiv) and concentrated sulphuric acid in ethanol at
the room temperature, rottlerin was obtained in 42% yield along
with two homogeneous dimers as the result of the “rottlerone
change” reaction (Figure 1). Nevertheless, all three products
could be easily isolated by regular chromatography.
Conclusions
We accomplished a total synthesis of rottlerin in a modular
manner by starting from readily available materials. Our
synthetic route requires a longest eight linear step in a total of 11
steps, which results in an overall yield of 20%. Compared to the
method reported by other researchers, our synthetic route is more
efficient, and the two major structural subunits of rottlerin can be
prepared in the gram scale. Although condensation of the two
subunits also yields two homogeneous dimers as by-products,
they can be separated from rottlerin easily by chromatography.
Efficient production of the sample of rottlerin will hopefully
facilitate the studies on its biological or pharmaceutical
indications.
Scheme 4. Synthesis of acetophenone 4.
After obtaining 4, we first attempted to obtain the oQM
precursor 8-OH. A few hydroxymethylation reactions24 were
attempted, but unfortunately no desired product was observed.
Besides, our attempts of acetoxymethylation,25 chloro-
methylation,26 and amine methylation27 of 4 all failed. We
assumed that this may be explained by the interference of the
naked hydroxyl groups to the desired reaction. Thus, hydroxyl
protected 20 was tested as reactants under the same conditions.
However, no reaction was observed either.
Acknowledgement
The authors thank Dr. Marc J. Adler at the Northern Illinois
University and Dr. Sandip B. Bharate at the Indian Institute of
Integrative Medicine for sharing their experience in the synthesis
of rottlerin. The authors are grateful for the financial supports
from the National Natural Science Foundation of China (grant
#81430083, #81172984, #21472227 to Prof. R. Wang) and the
Science and Technology Development Foundation of Macao
SAR (Grant #055/2013/A2 to Prof. R. Wang).
Supplementary Data
Experimental procedures, characterization Data, 1H and 13C NMR
spectra of rottlerin, two homogeneous dimers of the rottlerone
change, as well as allbn bn of the key intermediates mentioned in
this paper.
Scheme 5. Attempts on acetylation of 22 and 23.
Next, we turned to another possible synthetic route of 8-OH
by introducing hydroxymethyl group before acetylation (Scheme
5). Compound 19 was formylated using Vilsmeier−Haack
reagent to produce aldehyde 21 in 94% yield. Reduction of 21
with sodium borohydride produced alcohol 22, which was then
protected with TBS group to give 23. Acetylation of both 22 and
23 were unsuccessful with
a few attempted conditions
(Ac2OBF3OEt2; AcClSnCl4; AcClAlCl3; Ac2OTMSOTf).
Only the dimer product 25 was isolated, which proved the high
activity of phenol ring. Such a reaction probably takes place via a
retro Friedel−Crafts mechanism prompted by cation-π
interactions to generate formaldehyde. Similar phenomenon was
reported by Chiba et al.28 These failed attempts demonstrate that
the acetophenone-derived oQM precursor is not suitable for the
synthesis of rottlerin because it is difficult to obtain.

4
Tetrahedron
Table 1. Phenol-aldehyde condensation and the results.
Entry
Conditions
Result
13%
37% HCHO (30 equiv), conc HCl (10 equiv), RT
37% HCHO (30 equiv), conc H2SO4 (15 equiv), RT
37% HCHO (30 equiv), AcOH (30 equiv), RT
MOMCl (10 equiv), conc HCl (10 equiv), RT
(CH2O)n (30 equiv), conc H2SO4 (15 equiv), RT
CH2(OMe)2 (30 equiv), conc H2SO4 (15 equiv), RT
37% HCHO (30 equiv), TsOH (10 equiv), RT
1
2
3
4
5
6
7
42%
< 5%
No Product
No Product
No Product
<10%
Deshpande, M. V.; Joshi, S. P. Nat. Prod. Res. 2014, 28,
245-250.
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