Palladium/Norbornene-Catalyzed Indenone Synthesis from Simple Aryl Iodides: Concise Syntheses of Pauciflorol F and Acredinone A

Article abstract of DOI:10.1002/anie.201813699

To show the synthetic utility of palladium/norbornene (Pd/NBE) cooperative catalysis, here we report concise syntheses of indenone-based natural products, pauciflorol F and acredinone A, which are enabled by direct annulation between aryl iodides and unsa

Full text of DOI:10.1002/anie.201813699

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Accepted Article
Title: Palladium/Norbornene-Catalyzed Indenone Synthesis from
Simple Aryl Iodides: Concise Syntheses of Pauciflorol F and
Acredinone A
Authors: Feipeng Liu, Zhe Dong, Jianchun Wang, and Guangbin Dong
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813699
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Palladium/Norbornene-Catalyzed Indenone Synthesis from
Simple Aryl Iodides: Concise Syntheses of Pauciflorol F and
Acredinone A
Feipeng Liu, Zhe Dong, Jianchun Wang* and Guangbin Dong*
Abstract: To show the synthetic utility of palladium/norbornene
(
Pd/NBE) cooperative catalysis, here we report concise syntheses of
indenone-based natural products, pauciflorol F and acredinone A,
which are enabled by direct annulation between aryl iodides and
unsaturated carboxylic acid anhydrides. Compared to the previous
indenone-preparation approaches, this method allows simple aryl
iodides to be used as substrates with complete control of the
regioselectivity. The total synthesis of acredinone A features two
different Pd/NBE-catalyzed ortho acylation reactions for constructing
penta-substituted arene cores, including the development of a new
ortho acylation/ipso borylation.
Indenones are commonly found in pharmaceuticals and
[
1]
bioactive natural products (Scheme 1a), and often serve as
[2]
versatile intermediates to access polysubstituted indanones.
While a number of methods have been developed for preparing
indenones, conventional approaches usually take multiple steps
[3]
and/or have limited scopes. Recently, the transition metal-
catalyzed cyclization with alkynes as coupling partners offers a
broad and useful strategy to form various indenones (Scheme
[
4,5]
1b).
However, it is not trivial to control regioselectivity when
[
2]
unsymmetrical alkynes are used. In addition, vicinal
difunctionalized aryl substrates or those containing a directing
group (DG) are needed. Thus, it remains attractive to realize a
general and regioselective indenone synthesis from simple
readily available starting materials.
[
4]
[5]
Scheme 1. Indenone/indanone containing natural products and indenone
synthesis
On the other hand, palladium/norbornene (Pd/NBE)
[6]
[7]
cooperative catalysis, originally discovered by Catellani, has
become an increasingly useful approach for preparing
polysubstituted arenes via simultaneous functionalization of both
In 2015, we reported an initial study on the ortho
[8i]
acylation/ipso hydrogenation using
a
mixed anhydride.
groups independently
[8k]
[8j]
Concurrently, the Liang
and Gu
[
8]
ortho and ipso positions of aryl halides. Notably, Lautens has
pioneered in developing bifunctional reagents, which enabled
forming various condensed rings via ipso-annulation
developed the ortho acylation/ipso Heck reaction. Interestingly,
when anhydrides derived from α,β-unsaturated carboxylic acids
were employed, indenone products could be obtained (Scheme
c). The reaction was likely initiated by oxidative addition of
Pd(0) into the Ar−X bond, subsequent NBE migratory insertion
[
8a,9]
reactions.
While numerous Pd/NBE catalysis methods have
1
been developed, only a few have been successfully applied to
[8p,10]
the total synthesis of natural products.
Herein, we describe
and C−H metalation to generate an aryl-NBE-palladacycle
concise syntheses of pauciflorol F(1) and acredinone A (2),
enabled by the development of a straightforward method to
construct indenones from simple aryl iodides and unsaturated
carboxylic anhydrides via Pd/NBE catalysis (Scheme 1c).
[11]
(
ANP) I, which would then react with the anhydride to afford
intermediate II. After NBE extrusion, the resulting intermediate III
could further undergo intramolecular cyclization to give the
indenone product (vide infra, Scheme 3).
The reaction was further optimized using 2-iodotoluene (3a)
as the substrate (see Supporting Information, Table S1).
[Pd(allyl)Cl] and tri(2-furyl)phosphine still proved to be the best
2
metal/ligand combination, and the amide-substituted NBE (N1)
was also found more efficient than simple NBE. Under the
optimal conditions, the scope of the reaction was examined
(Table 1). Aryl iodides contain both electron-rich and deficient
substituents were suitable substrates, and different substitution
patterns on arenes were tolerated. Note that sterically hindered
aryl iodides, such as 2,5-disubstituted ones (5q and 5r), were
[
*]
F. Liu, Z. Dong, J. Wang, Prof. Dr. G. Dong
Department of Chemistry, University of Chicago
Chicago, IL, 60637 (USA)
E-mail: jcwang1992@uchicago.edu (J.W.) gbdong@uchicago.edu
(
G.D.)
F. Liu
Department of Applied Chemistry, China Agricultural University
Beijing 100193, China
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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also competent substrates, which was the key requirement for
the synthesis of acredinone A. Other unsaturated anhydrides
were explored next as coupling partners. Anhydrides derived
from alkyl (5t) and aryl-substituted acrylic acids (5u-5ac) could
N1 or simple NBE under the standard conditions due to the
[14]
“ortho constraint”.
bridgehead-substituted norbornene (N2),
indenone product 5ad was successfully isolated in 56% yield.
However, with the recently developed
[
14b]
the desired
[12]
be successfully employed.
disubstituted acrylic anhydrides were used, 2-substituted and
,3-disubstituted indenones (5aa-5ac) were still afforded in good
In particular, when α- or α,β-
2
yields. Finally, a number of functional groups were found
compatible, such as tertiary amine (5c, 5j and 5k), silyl ether
(5g), tertiary alcohol (5i), MOM-protected alcohol (5e), Weinreb
amide (5n), ester (5f, 5m and 5p), Boc-protected amine (5l),
trifluoromethyl (5w), aryl bromide (5v) and thiophene (5z),
making this method attractive for complex molecule synthesis.
[
a]
Table 1. Substrate scope of Pd/NBE-catalyzed indenone synthesis.
Scheme 2. Further reaction exploration
The mechanism regarding the conversion of intermediate III
into indenone product was explored (Scheme 3). Two pathways
2
are possible: path a involves activation of the vinyl β-C(sp )−H
bond of the enone intermediate followed by reductive elimination,
while path b is through a Heck-like sequence: aryl migratory
insertion into the olefin followed by β-H elimination. To
differentiate these two pathways, both (Z) and (E)-anhydrides
(4aa) were allowed to react with 3aa in parallel under the
standard conditions. Interestingly, they gave almost identical
yields of 5aa with a similar kinetic profile (see Supporting
Information). Additionally, Z/E isomerization of the olefin moiety
in 4aa was not observed during the reaction. Taken together, the
observations are consistent with the Heck-like pathway (path b),
though other pathways including a Nazarov-type cyclization
cannot be excluded at the current stage (see Supporting
Information).
R2
C
H
reductive
elimination
Pd R2
metalation
FG
FG
FG
R1
path a
R1
Pd R2
O
O
-
H
FG
elimination
R1
R2
R2
isomerization
O
path b
R1
FG
Pd
R1
migratory
insertion
Pd
O
O
[
Pd(allyl)Cl]2 (5 mol%)
Me
Me
Ph
P(2-furyl)3 (20 mol%)
Ph
I
O
O
N1, Cs2CO3
i
Ph
O
OPr
dioxane/THF=3:1, 85 o
C
R
R
Ph
O
3aa
R = morpholino
(
Z)-4aa
5
aa, from (Z), 79%
from (E), 78%
(
E)-4aa
Scheme 3. Mechanistic exploration
[
a] Unless otherwise noted, the reaction was run with 3 (0.3 mmol), 4 (0.6
mmol), [Pd(allyl)Cl] (0.015 mmol), P(2-furyl) (0.06 mmol), N1 (0.15 mmol),
Cs CO (1.2 mmol) and NMe Cl (1.2 mmol) in dioxane (2.25 mL) and THF
0.75 mL) at 85 °C for 14 h. [b] 7 (0.6 mmol) was used instead of 4 and the
reaction was run at 95 °C without NMe Cl and THF. [c] run for 3 h. [d] run for 4
h. [e] 4ac’ (0.6 mmol) was used instead of 4 and the reaction was run for 2 h.
2
3
2
3
4
The utility of the method was first demonstrated in the formal
synthesis of pauciflorol F (1) (Scheme 4). As a polyphenolic
natural product derived from resveratrol, pauciflorol F has been
(
4
[
15]
an attractive synthetic target since its discovery in 2004.
Several elegant routes have been developed to date.
[
13]
[2,16]
Preliminary study showed that aryl bromides
could also
Our
be used as the substrate (eq. 1, Scheme 2) albeit in a lower
approach started with a two-step preparation of mixed anhydride
10 from commercially available aldehyde 8. As expected, the
[13a]
[17]
efficiency; the use of large bite-angle (DPEPhos) ligand
and
additional CsI was important. Not surprisingly, ortho-
unsubstituted aryl iodide 3ad was not a suitable substrate using
subsequent Pd/N1-catalyzed annulation between anhydride 10
and aryl iodide 3t provided indenone 11 in 82% yield, whose
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Angewandte Chemie International Edition
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COMMUNICATION
[
18]
structure was confirmed by X-ray crystallography.
Note that
the prior synthesis of indenone 11 via an alkyne insertion
[2]
strategy generated
a
1:1 mixture of regioisomers.
Demonstrated by Jeffery and Sarpong, indenone 11 can be
[2]
converted to 1 in two steps; thus, the synthesis of pauciflorol F
can now be accomplished in 5 total steps and a high overall
yield (~50%).
Scheme 5. Retrosynthetic analysis of acredinone A.
In
a forward manner, aryl iodides 14 and 16 were
synthesized from the same commercially available 3-methyl-4-
anisaldehyde 18 in 4 steps (Scheme 6). The Pd/NBE-catalyzed
indenone synthesis proceeded in a good yield on gram scales
despite the bulkiness of the substrates. Bromination of indenone
19 with NBS afforded fragment 12. Meanwhile, fragment 13 was
proposed to be prepared via Pd/NBE-catalyzed ortho
acylation/ipso borylation, which has been an unknown
transformation. Encouraged by Ritter’s ortho amination/ipso
[
21]
borylation,
anhydride 17 and B
delight, the desired product 13a was obtained in 21% yield in the
first attempt using Pd(OAc) as the catalyst, and the major side-
reaction was the ipso-hydrogenation (13a’) (entry 1). A quick
condition screening revealed that the use of Pd(dba) could
the three-component coupling of aryl iodide 16,
2
pin was carefully studied (Table 2). To our
2
Scheme 4. Formal synthesis of pauciflorol F.
2
We then focused on the total synthesis of acredinone A (2).
2
25
Acredinone A was recently isolated in <5 mg (with [α]
CHCl from marine-sponge-associated acremonium sp.
Fungus, which represents the first nonpeptidic natural product
D
: 0 in
suppress the formation of side-product 13a’ (entries 1-4), though
the exact reason is unclear. Further tuning the solvent and
reaction temperature finally afforded the desired borylation
product 13a in 67% isolation yield (entries 5-7).
3
)
a
[
19]
that can inhibit the voltage-gated potassium channel. Such an
activity holds potential for developing inhibitors for treating type-
[
a]
[20]
Table 2. Optimization for ortho acylation/ipso borylation.
II diabetes. In addition, total synthesis of acredinone A has not
been reported to date, and the structure-activity relationship
(
SAR) remains unknown. Given the pseudo-symmetry in 2, it
might be possible to establish an efficient biomimetic strategy.
However, to enable rapid access to structurally diversified
analogues for future SAR studies, it could be more valuable to
develop a modular and convergent approach. The challenges
are how to construct the highly congested indenone core and
how to prepare the two penta-substituted arenes efficiently.
From a retrosynthetic viewpoint (Scheme 5), we envisioned that
acredinone
A
could be constructed via
a
late-stage
Suzuki−Miyaura coupling of two fragments (12 and 13) with
similar complexity. Each fragment would be synthesized via a
unique Pd/NBE catalysis. Ultimately, the natural product would
be assembled using four arene building blocks (14-17).
[a] Conditions: 16 (0.1 mmol), 18 (0.12 mmol), B
2
pin
2
(0.105 mmol), [Pd] (0.01
(0.4 mmol), solvent (2
mmol), P(2-furyl) (0.02 mmol), N1 (0.05 mmol), Cs
3
2
CO
3
1
mL) at 100 °C for 24 h. Yields were determined by H NMR using 1,3,5-
trimethoxybenzene as the internal standard. [b] run at 120 C. [c] isolated yield.
o
The Suzuki−Miyaura coupling between aryl bromide 12 and
aryl boronic ester 13a turned out to be extremely challenging
due to the large steric hindrance in both coupling partners. The
commonly employed monodentate or bidentate ligands, such as
t
PPh
3
, P Bu
3
, Buchwald’s biarylphosphines, Hartwig’s Q-Phos
and NHC ligands, gave trace or no desired product. After
extensive studies, gratifyingly, the bulky and electron-rich
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bidentate dtbpf ligand afforded the desired coupling product 20
prepared. Indeed, the coupling between 12 and 13b proceeded
smoothly to afford product 20 in 68% yield, which was
[22]
albeit in a low yield.
The major side-reaction was the
protonation of boronic ester 13a, suggesting low stability of this
compound under the reaction conditions. Considering that
Molander salts generally exhibit enhanced stability than the
subsequently deprotected to give acredinone A (2). The
spectroscopic data of synthetic 2 matched those of the reported
natural sample in all respects. Hence, the first total synthesis of
acredinone A was accomplished in eight steps in the longest
linear sequence (LLS) from commercial chemicals.
[23]
corresponding boronic esters in cross couplings,
the
corresponding potassium aryltrifluoroborate 13b was then
Scheme 6. Total synthesis of acredinone A.
X.-H. Zhang, B.-L. Hu, X.-G. Zhang, J. Org. Chem. 2016, 81, 5710; g) B.
Suchand, G. Satyanarayana, J. Org. Chem. 2017, 82, 372.
In summary, a Pd/NBE-catalyzed annulation between aryl
iodides and unsaturated carboxylic acid anhydrides has been
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a
simple and straightforward
regio- and
a
chemoselective manner. Using this method, concise syntheses
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future SAR studies are ongoing.
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Keywords: acredinone
A • pauciflorol F • indenone •
norbornene • palladium
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Angewandte Chemie International Edition
10.1002/anie.201813699
COMMUNICATION
Entry for the Table of Contents
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COMMUNICATION
Feipeng Liu, Zhe Dong, Jianchun Wang*
and Guangbin Dong*
Page No. – Page No.
Palladium/Norbornene-Catalyzed
Indenone Synthesis from Simple Aryl
Iodides: Concise Syntheses of
Pauciflorol F and Acredinone A
A straightforward method for indenone synthesis was achieved via Pd/NBE-
catalyzed annulation between simple aryl iodides and unsaturated carboxylic acid
anhydrides. This method enabled streamlined syntheses of pauciflorol F and
acredinone A.
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