Gold(III)-Catalyzed Intermolecular Oxidation-Cyclization of Ynones: Access to 4-Substituted Chroman-3-ones

Article abstract of DOI:10.1002/adsc.201900260

A synthesis of 4-substituted chroman-3-one derivatives has been developed through a gold(III) catalyzed oxidation-cyclization of ynones in good to excellent yield using easily prepared substrates. A broad range of synthetically useful functional groups (halide, alkene, alkyne, phenolic hydroxyl) were tolerated. Further application of this method paves a new way to prepare the skeleton of oblarotenoids. A cascade oxidative cyclization for construction of pyrano[2,3-c]chromen-1(5H)-one derivatives is also presented. (Figure presented.).

Full text of DOI:10.1002/adsc.201900260

Title: Gold(III)-Catalyzed Intermolecular Oxidation-Cyclization of
Ynones: Access to 4-Substituted Chroman-3-ones
Authors: Jian Li, Fang Yang, Yang-Ting Ma, and Kegong Ji
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900260
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10.1002/adsc.201900260
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Gold(III)-Catalyzed Intermolecular Oxidation-Cyclization of
Ynones: Access to 4-Substituted Chroman-3-ones
Jian Li, a Fang Yang, a * Yang-Ting Ma, a and Kegong Ji a *
a
College of Chemistry and Pharmacy, Northwest A&F University; Shaanxi Key Laboratory of Natural Products &
Chemical Biology, 3 Taicheng Road, Yangling, 712100, Shaanxi (P.R. China).
[E-mail addresses: yf10@nwsuaf.edu.cn; jikegong@nwsuaf.edu.cn]
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A synthesis of 4-substituted chroman-3-one
derivatives has been developed through gold(III)
drawbacks associated with applying diazo-
compounds of the reported approaches, it is still
highly desirable to pave a novel and efficient route to
access the highly substituted and functionalized 4-
acyl-chroman-3-one derivatives.
a
catalyzed oxidation-cyclization of ynones in good to
excellent yield using easily prepared substrates. A broad
range of synthetically useful functional groups (halide,
alkene, alkyne, phenolic hydroxyl) were tolerated. Further
application of this method paves a new way to prepare the
skeleton of oblarotenoids. A cascade oxidative cyclization
for construction of pyrano[2,3-c]chromen-1(5H)-one
derivatives is also presented.
Keywords: Gold(III) catalyzed; Oxidation; 4-substituted
chroman-3-ones; Pyrano[2,3-c]chromen-1(5H)-ones
Figure 1. Natural products containing chromanones.
In recent years, homogeneous gold catalyzed
reactions have been recognized as one of the most
useful and powerful tools in organic synthesis.[5] Due
to the propensity of gold to act as a soft and
The chroman-one motif is a common structure
pattern found at the skeleton of numerous natural
products of varying complexity.[1] Furthermore, 4-
acyl-3-hydroxychromans as well as its tautomers are
also important structural units that occur in many
natural products such as oblarotenoids A and C,
hydroxymunduserone and munduserone.[2a] These
compounds possess diverse biological activities
carbophilic Lewis acid in the activation of carbon-
6]
carbon π-bonds,[5l,
it has been used to catalyze
various carbene transfer reactions such as X-H (X =
C, N, O) insertion,[7] cyclopropanation,[8] and
cycloaddition[9] reactions. On the other hand, gold(I)-
catalyzed oxidation reactions of ynones via the α-oxo
gold carbene intermediates also gained much
attention due to their flexible ability to participate in a
wide range of transformations.[10] However, in the
oxidation of alkynes using N-oxide as oxidant,
gold(III) is a less efficient catalyst, because of its
strong oxophilic nature. For a more useful approach
to oxidation of alkynes and the development of a gold
catalyst, as shown in Scheme 1C, our design
anticipated that with phenoxyl-ynone substrates 1 and
π-philicity/oxophilicity nature of gold(III) as catalyst,
a novel non-carbene model of allenyloxygold(III)-
species was proposed and subsequently the
allenyloxygold(III)-species would be attacked by an
appropriately tethered internal phenoxyl group via
including insecticidal, piscicidal, icthyocidal and anti-
[2b]
cancer activities (Figure 1).
The most frequent
methods to synthesize the 4-acyl-chroman-3-one
units rely on the reported procedures through
multistep routes from basic chemicals. In brief, the
typical
process
was
started
from
2-
benzyloxybenzaldehyde, with 6 steps, to afford 4-
acyl-chroman-3-one motifs in less than 20% yield
(Scheme 1A),[3] which is very inefficient. Recently, a
transition-metal-catalyzed aromatic C-H insertion of
α-diazo β-ketoesters was reported providing an
alternative route to 4-carbonyl chroman ester
derivatives (Scheme 1B). Although the route is
accessible, the α-diazo compounds are toxic,
explosive and difficult to obtain.[4] Considering the
multistep process as well as limited scope and the
concerted SN2’ type addition to affords 4-substituted
1
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10.1002/adsc.201900260
Advanced Synthesis & Catalysis
g2e as oxidant.
chroman-3-one 3, which is an important motif in
natural products. Herein, we report an efficient access
to obtian 4-substituted chroman-3-ones through
gold(III)-catalyzed oxidation of readily available
phenoxyl-ynones.
hRecovery of 1a.
Initially, reactions of 5-phenoxypent-3-yn-2-one
1a with pyridine N-oxide 2 in the presence of various
gold catalysts were investigated to achieve the
optimal reaction conditions. Interestingly, when the
reaction was conducted in fluorobenzene (PhF) for 24
h, in the presence of gold(I) catalysts derived from
typical ligands such as Ph3P, Phosphite, IPr, and
Scheme 1. Accesses to chroman-3-one motifs
t
Me4 BuXPhos, no desired product was achieved, only
leading to an 82-95% recovery of starting material 1a
(Table 1, entries 1 - 3). On the other hand, the use of
5 mol % PicAuCl2 as the catalyst and the 2,6-
dichloropyridine N-oxide 2a as the oxidant in PhF
resulted
in
the
formation
of
4-(1-
hydroxyethylidene)chroman-3-one 3a in 47% yield
after 24 h at room temperature (Table 1, entry 4).
That revealed that the gold(III) catalyst was more
efficient than the gold(I) catalyst, this mainly due to
the carbophilicity and oxophilicity of gold(III)
catalyst. In addition to the alkynyl activation, the
gold(III) catalyst shows stronger Lewis acidity to
activate the carbonyl group of the ynone to trigger
this cyclization. When increasing the temperature to
40, 50 or 60 oC, a slight improvement in the yield was
observed (Table 1, entries 5-7). When the reaction
Table 1 Optimization of Reaction Conditionsa
o
temperature was further increased to 70 or 80 C, a
Entry
1
Catalyst (5 mol %)
Ph3PAuCl/AgNTf2
Conditions
Yield (%)b
0(95)h
slight decrease in the yield was observed (Table 1,
entries 8-9). These results indicated that the optimal
PhF, rt, 24 h
o
reaction temperature was 60 C. A solvent study
0(93)h
2
3
IPrAuCl/AgNTf2
PhF, rt, 24 h
showed that changing the solvents to DCE or CHCl3,
resulted in lower yields (Table 1, entries 10 and 11).
When toluene and chlorobenzene (PhCl) were used as
the solvents, the yields of 3a were obtained in 74%
and 82%, respectively (Table 1, entries 12 and 13),
with PhCl turning out to be superior to other solvents.
Subsequent lowering of catalyst loading to 3 mol %
afforded 3a in 61% yield, even after a longer time.
(Table 1, entry 14). Moreover, Pyridine N-oxides, 2b-
2e, were less efficient than oxidant 2a (Table 1,
entries 15 - 18). Finally, Lewis acids such as
Cu(OTf)2, Zn(OTf)2 and AgNTf2, which showed
strong activation toward carbonyl group, but did not
catalyze this reaction (for details, see Supporting
Information (SI), table 1).
With the optimal conditions established (Table 1,
entry 10), a variety of functionalized ynones were
first examined in the oxidation-cyclization (Scheme
2). As expected, the reaction was proved to be
compatible with various R1 groups on the ynone
moieties. When R1 groups were aliphatic groups
(propyl and i-propyl) the desired chroman-ones 3b
and 3c were obtained in moderate to good yields.
Varying the R1 group to a phenyl group, 3d was
obtained in an excellent yield. Switching substituents
installed on the aromatic groups of the ynones
resulted a considerable change in the yields of desired
t
11(80)h
Me4 BuXPhosAuCl/ PhF, rt, 24 h
AgNTf2
4
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
PhF, rt, 24 h
47
60
66
72
70
68
55
49
76
82
61
55
52
44
20
5
PhF, 40 oC, 16 h
PhF, 50 oC , 8 h
PhF, 60 oC , 5 h
PhF, 70 oC, 2 h
PhF, 80 oC, 2 h
DCE, 60 oC, 5 h
CHCl3, 60 oC, 5 h
PhCH3, 60 oC, 5 h
PhCl, 60 oC, 1.5 h
PhCl, 60 oC, 4 h
PhCl, 60 oC, 6 h
PhCl, 60 oC, 6 h
PhCl, 60 oC, 6 h
PhCl, 60 oC, 6 h
6
7
8
9
10
11
12
13
14 c
15 d
16 e
17 f
18 g
aReaction conditions: All reactions were run in vials in the
presence of 1a (0.2 mmol), 2 (0.3 mmol); [1a] = 0.5 M.
bIsolated yields.
c3 mol % catalyst was used.
d2b as oxidant.
e2c as oxidant.
f2d as oxidant.
2
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10.1002/adsc.201900260
Advanced Synthesis & Catalysis
products. Both electron-deficient (o-F, p-Cl, p-Br and
p-NO2) and rich (p-OAc, p-OTBS and p-OMe)
ynones underwent the reaction smoothly providing
the corresponding products 3e - 3k in excellent yields
of 70 - 94%, irrespective of electronic effects.
Notably, the substrate 1l bearing a free phenolic
hydroxyl group also smoothly participated in this
reaction and the expected product 3l was delivered in
a 72% yield. In addition, the heteroaryl substituents
such as 2-furyl, 2-thienyl and N-Boc protected 3-
indolyl groups were well tolerated under the reaction
conditions, thus giving 3n - 3p in excellent yields
ranging from 86% to 95%. Interestingly, the styryl
derived substrate 1m yielded 3m in a moderate yield
indicating that the conjugated-ene was also suitable
for this annulation reaction. The stereochemistry of 3f
was established by X-ray crystallographic analysis
(CCDC 1816739)[12] and other 4-substituted
chroman-3-one derivatives were assigned by analogy.
(p-Me, m-Me, o-OMe, and p-OTBS) substituents on
the phenyl rings were well compatible for this
reaction, furnishing 3q - 3w in good to high yields.
The substrates with electron-rich substituent 3,4,5-
trimethoxyphneyl and naphthyl groups proceeded
smoothly under the standard conditions, and the
corresponding products 3x and 3y were obtained in
37% and 49% yields separately, along with 26% and
14% yields of 1-(2H-chromen-4-yl)ethanones 4x and
4y, which were formed directly through a competitive
gold catalyzed 6-endo-dig cyclization. Ynone 1z
equipped with O-allyl substituent on the phenyl ring
could also undergo the desired cyclization giving 3z
in a 79% yield without interfering the π bond. A
sterically demanding o-phenyl group on the aryl ring
was well suited, affording 3aa in a 75% yield. In the
case of 1ab, the gold catalyst activated the ynone
selectively rather than the TBS protected propargyl-ol
motif, providing the target molecule 3ab in a
moderate yield, which could offer opportunities for
further structural diversification via various reported
methods. Specially, the substrate, 5-phenoxyhex-3-
yn-2-one, gave a high yield of the desired product
3ac.
Scheme 2. Substrate Scope of Substituted Ynonesa
Scheme 3. Substrate Scope of Aromatic Ethersa
aAll reactions were carried out in 4 mL PhCl in the
presence of 1 (0.2 mmol), 2a (0.3 mmol); isolated yields
are reported.
aAll reactions were carried out in 4 mL PhCl in the
presence of 1 (0.2 mmol), 2a (0.3 mmol); isolated yields
are reported.
benol : diketo = 25 : 1.
cenol : diketo = 33 : 1.
benol : diketo = 2.3 : 1.
denol : diketo = 6.3 : 1.
eenol : diketo = 7.1 : 1.
Moreover, we attempted to expand the chemistry
to the all-carbon counterpart of 1a, i.e., 6-phenylhex-
3-yn-2-one 1ad. As illustrated in eq (1), the desired
(Z)-1-(1-hydroxyethylidene)-3,4-dihydronaphthalen-
2(1H)-one 3ad was also obtained in a 56% yield,
which is a useful motif in natural products.
fenol : diketo = 9 : 1.
Next, we anticipated that the use of substrates
derived from the functional phenoxyl ethers would
enable the incorporation of substituents R2 on the
various positions of the phenyl rings (Scheme 3).
Generally, substrates bearing both electron-
withdrawing (p-F, p-Cl, p-Br) and electron-donating
3
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10.1002/adsc.201900260
Advanced Synthesis & Catalysis
ones, several experiments were investigated. We
carried out the reaction in the presence of H2O (1.5
eq.) without adding N-oxide 2a; however, no reaction
was observed. The reaction was also attempted in the
Inspired by the scaffolds of natural products[2],
Scheme 5. Mechanistic Studies
and the reported literature, which indicates that
diynones could participate in diversified cascade
cyclization and rearrangement reactions,[11] we
explored the reactions of diynones (Scheme 4). Under
the standard conditions, the diynones did not convert
into the corresponding products efficiently. Various
reactions were tested to screen out optimal reaction
conditions (for details, see SI, Table 2). In brief,
employing Brønsted acids as additives gave better
results than other additives such as bases, Lewis acids
and some other coordinating reagents; in particular,
tartaric acid turned out to be the best additive, which
could activate the carbonyl group of the ynone
moiety to help to trigger the 6-endo-dig cyclization.
Treating the substrate 5a in the presence of PicAuCl2
(10 mol %) and tartaric acid (0.5 equiv), the oxidation
proceeded selectively at the C-5 atom of the tethered
diynones resulting 6a in a 41% yield. Moreover,
switching substituents of the diynones resulted a
slight change in the yields of desired products. The
phenyl, 4-tolyl and 4-chlorophenyl substituted
diynones were also tolerated.
presence of H2O18, and no O18 labeled product 3a′
was detected from HRMS, indicating that the new
oxygen of 3a was from the N-oxidant 2a (Scheme 5,
Eq(4)), (for control experiment details, see SI).
Based on these observations, a postulated
mechanism of these reactions is depicted in scheme 6.
Gold(III) first coordinates with C-C triple bonds and
carbonyl group, where the N-oxide subsequently
attacks the distal end of ynone to give allenyloxy-
gold(III)-species B. Then, the aryl group as a
nucleophile attacks at the C-2 position of the
allenyloxy-gold(III) intermediate B to drive the 2,6-
Scheme 4. Substrate Scope of Diynonesa
dichloropyridine away via SN2’-type addition,
resulting in the formation of intermediate C, which
aromatizes to give chromanone scaffold 3 and
releases the gold complex. When an alkynyl R’ group
is installed on an ynone moiety, the acid activates the
carbonyl group, and the gold(III) coordinates with the
alkynyl in the ynone system, which is attacked by the
internal ketone group via a 6-endo-dig cyclization to
give the oxonium species E. Finally, the oxonium
species E is rapidly isomerized to give product 6 and
release the gold complex.
aAll reactions were carried out in 4 mL PhCl in the
presence of 5 (0.2 mmol), 2a (0.3 mmol); isolated yields.
In order to gather additional experimental
evidence for the mechanism, we investigated the
gold(III)-catalyzed the authentic diazo carbonyl
species 7 in PhCl at 60 or 120 oC for 12 h as shown in
Scheme 5, where the desired product 3d was not
observed.[13] We also carried out 1a with an excess of
N-oxide 2a (2.5 equiv), where the double oxidation
by-product 13 was not observed and the desired
product 3a was isolated in 81% yield. 5-
(Cinnamyloxy)pent-3-yn-2-one 8[10g] was also tested
under standard conditions, but no desired gold
Scheme 6. Proposed Reaction Mechanism
carbene
cyclopropanation
product
9
and
cycloisomerization product 10 were observed. These
control experiments implied that the pure α-oxo gold
carbene might not be the true reactive intermediate in
this reaction (for details, see SI).
Additionally, in order to seek out the source of
the new oxygen atom of the 4-substituted chroman-3-
4
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10.1002/adsc.201900260
Advanced Synthesis & Catalysis
screened for the construction of pyrano[2,3-
c]chromen-1(5H)-one derivatives. These methods
provide concise and efficient access to synthesize the
main skeleton of some natural products.
During the derivatization of 4-substituted
chroman-3-ones, the esterification of 3l with 4-
bromobenzoyl chloride afforded the corresponding
diester derivative 11 in a good yield (Scheme 7).
Interestingly, the structure of the diester derivative
was not in accordance with its precursor, but was
similar to the tautomer
Experimental Section
Scheme 7. Esterification of 3l
A 2, 6-dichloropyridine N-oxide (1.5 eq, 0.3 mmol,
48.9 mg), PicAuCl2 (5 mol %, 0.01 mmol, 3.9 mg), PhCl
(3 mL) were added successively to a flame-dried vial at
room temperature and then a ynone 1a (0.2 mmol)
dissolved in PhCl (1 mL) was added to the above mixture.
Then the vial was capped with a Teflon-coated cap, and the
resulting solution was stirred at room temperature for 5
o
min, and then was heated at 60 C until the reaction was
complete, as monitored by thin layer chromatography.
Upon completion, the solvent was removed under vacuo,
the residue was purified by chromatography to get the
desired compound 3a as a colorless oil.
Figure 2. Solid-state molecular structure of 11
Acknowledgements
(3-hydroxyl-2H-chromene-4-carbalone), and the
relative stereochemistry of 11 was determined by X-
ray crystallographic analysis (CCDC 1860836)[12]
(Figure 2). Additionally, chroman-one 3e, which
could be prepared easily from phenol, was subjected
to further transformations to access more complex
scaffolds. The main skeleton of oblarotenoids 12
were obtained by heating 3e and K2CO3 in DMF at 90
oC in an acceptable yield (Scheme 8).
This work was supported by the National Natural Science
Foundation of China (NSF-21702170, 21502150), Natural
Science Foundation of Shaanxi (2018JQ2072) and the Youth
Training Programme of Northwest A&F University (Nos.
2452016006, 2452017034).
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unprecedented
gold(III)-catalyzed
oxidation/
cycloaddition of phenoxyl ynones to yield
biologically important and highly impactful 4-
substituted chroman-3-one derivatives. Easily
prepared starting material, good to excellent yield and
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10.1002/adsc.201900260
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