Palladium-Catalyzed C-S Bond Cleavage with Allenoates: Synthesis of Tetrasubstituted 2-Alkenylfuran Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b02253

Palladium-catalyzed C-S cleavage of tetrasubstituted internal alkene α-oxo ketene dithioacetals was realized with allenoates as the coupling partners, efficiently affording tetrasubstituted 2-alkenylfuran derivatives with excellent regioselectivity under mild conditions. Allenoates acted as C1 synthons in the desulfurative [4 + 1] annulation.

Full text of DOI:10.1021/acs.orglett.8b02253

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium-Catalyzed C−S Bond Cleavage with Allenoates: Synthesis
of Tetrasubstituted 2‑Alkenylfuran Derivatives
Quannan Wang,†,‡ Zhuqing Liu,†,‡ Jiang Lou,†,‡ and Zhengkun Yu*,†,§
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354
Fenglin Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Palladium-catalyzed C−S cleavage of tetrasub-
stituted internal alkene α-oxo ketene dithioacetals was realized
with allenoates as the coupling partners, efficiently affording
tetrasubstituted 2-alkenylfuran derivatives with excellent
regioselectivity under mild conditions. Allenoates acted as
C1 synthons in the desulfurative [4 + 1] annulation.
Scheme 1. Strategy for Furan Synthesis
he furan motif is a key structural unit in many bioactive
1
Tmolecules, natural products, and pharmaceuticals, and
furan compounds can be used as the important building blocks
in organic synthesis.2 To access multisubstituted furan
derivatives, two strategies can be applied: (a) functionalization
of an existing furan ring and (b) direct construction of a
substituted furan ring by ring closure of suitable precursor
compounds. Although direct C−H functionalization of furans
seems to be an attractive method to prepare multisubstituted
furans,3 this method remains a great challenge in the reaction
scope, efficiency, and chemoselectivity. As for the ring closure
method, much progress has recently been made by means of
the cyclization reactions of allenes. In this regard, cyclo-
isomerization of allenyl ketones has been applied for the
construction of multisubstituted furans by Marshall,4a
Hashmi,4b Ma,4c−f Gevorgyan,4g,h and others.4i−m Palladium-
catalyzed cycloisomerization of homoallenyl amides was
developed to furnish 2-aminofurans.5 Intramolecular cycliza-
tion of the allene precursors, that is, propargylic alcohols6 and
γ-acyloxybutynoates,7 was also used to synthesize multi-
substituted furans. Owing to their intrinsic reactivity,8 allenes
usually act as C2 and C3 synthons in the synthesis of
heterocyclic compounds through cycloaddition reactions.9
However, such cycloaddition reactions have rarely been
applied for furan synthesis from allenes.10 So far, only a few
examples have been documented for allenes serving as the C1
synthons in the synthesis of vinyl-substituted heterocycles,11
and transition-metal-catalyzed cycloaddition by means of
allenes as the C1 synthons has not yet been achieved for the
construction of furan derivatives.
arylation,15 and other C−H functionalization reactions16 of
trisubstituted α-oxo ketene dithioacetals have been achieved,
and transition-metal-catalyzed construction of heterocyclic
compounds was also reported.17 In comparison to trisub-
stituted α-oxo ketene dithioacetals, their tetrasubstituted
analogues have been paid much less attention, and only a
few reports have been documented for the desulfurative
synthesis of tetrasubstituted alkenes,18 carbocycles,19 and N-20
and O-containing heterocycles.21 We recently reported iron-
catalyzed oxidative C−H/C−H cross-coupling of trisubstituted
α-oxo ketene dithioacetals with carbonyl methylenes, affording
tetrasubstituted furans (Scheme 1b).17a Thus, we reasonably
envisioned that tetrasubstituted 2-alkenylfurans might be
accessed through the cyclization of tetrasubstituted α-oxo
ketene dithioacetals with allenes as the C1 synthons. Herein,
we disclose palladium-catalyzed [4 + 1] annulation of
α-Oxo ketene dithioacetals have recently demonstrated their
diversity in organic synthesis.12 However, they have usually
been used as substrates in the trisubstituted form which is
obtained by hydrolysis of the corresponding tetrasubstituted
diketone ketene dithioacetals under strong acidic conditions
(Scheme 1a). Direct C−H alkylation,13 alkenylation,14
Received: July 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02253
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Letter
tetrasubstituted α-oxo ketene dithioacetals with allenoates for
the synthesis of tetrasubstituted 2-alkenylfurans (Scheme 1c).
Initially, the reaction of α,α-diacetyl ketene di(methylthio)-
acetal (1a) with benzyl buta-2,3-dienoate (2a) was conducted
to screen the reaction conditions (eq 1) (see the Supporting
Scheme 2. Scope of Tetrasubstituted α-Oxo Ketene
Dithioacetals (1)
a
Information for details). Under a nitrogen atmosphere, in the
presence of 10 mol % of Pd(PPh3)4 catalyst, 2 equiv of
copper(I) thiophene-2-carboxylate (CuTC) as the sacrifacial
additive, and 2 equiv of Cs2CO3 base, the reaction of 1a with
2a in a 1:3 molar ratio was undergone in diethyl ether at 25 °C
for 12 h, affording tetrasubstituted 2-alkenylfuran 3a in 71%
yield. Elevating the temperature to 30 °C led to more efficient
formation of 3a. It is noteworthy that a base is not necessary
for the reaction. Lowering the amount of CuTC diminished
the reaction efficiency. The control experiments revealed that
the reaction could not efficiently proceed in the absence of
Pd(PPh3)4 or CuTC. On a 0.3 mmol scale the target product
1
3a was isolated in 74% yield. The H NMR analysis of the
reaction mixture of 1a and 2a revealed exclusive formation of
the stereospecific product 3a.
Next, the scope of tetrasubstituted α-oxo ketene dithioace-
tals 1 was investigated under the optimized conditions
(Scheme 2). The ethylthio-featuring α,α-diacetyl ketene
di(ethylthio)acetal (1b) reacted less efficiently than 1a to
form the target product 3b (59%). Although two different
carbonyl groups, i.e., acetyl and benzoyl, were present in
ketene dithioacetals 1c, only the acetyl group was involved in
the reaction with 2a, and the reaction exclusively gave product
3c (67%). 2-(Bis(methylthio)methylene)cyclohexane-1,3-
dione (1d) exhibited no reactivity to 2a. The α-acetyl-α-
ester ketene dithioacetals 1e−i efficiently underwent the
reactions with 2a, affording products 3e−i in 71−81% yields,
while α-amide and α-cyano-functionalized α-acetyl ketene
dithioacetals 1j and 1k could not efficiently react to yield 3j
(25%) and 3k (34%), respectively. α-Phenyl-α-acetyl ketene
dithioacetals 1l also reacted efficiently with 2a to form 3l
(77%). It is noteworthy that α-ester-functionalized α-alkanoyl
ketene di(methylthio)acetals 1m−q reacted well with 2a to
produce products 3m−q in 73−83% yields. Cyclopropyl and
cyclobutyl moieties facilitated the formation of 3p (83%) and
3q (80%), respectively, whereas the tert-butyl group in α-ester-
α-pivaloyl ketene dithioacetal (1r) completely inhibited the
reaction with 2a, exhibiting a remarkable steric effect.
Then, the protocol generality was explored by extending the
substrate scope to α-aroyl ketene dithioacetals of type 1.
Notably, α-ester-functionalized α-benzoyl ketene dithioacetal
1s efficiently underwent the reaction to generate 3s (84%).
The analogues of 1s, that is, substituted α-benzoyl ketene
dithioacetals 1t−z2, exhibited various reactivities to form the
target furan products 3t−z2 in 69−81% yields. Bulky α-(2-
naphthoyl) ketene dithioacetal 1z3 reacted well with 2a to give
3z3 (74%), showing no obvious steric effect from the 2-
naphthyl group. α-Heteroaroyl (2-furoyl and 2-thienoyl)
ketene dithioacetals 1z4 and 1z5 also reacted efficiently with
2a to form 3z4 (76%) and 3z5 (74%), respectively. It should
be noted that reacting α,α-dibenzoyl ketene dithioacetal 1z6
with allenoate 2a led to the target product 3z6 in 77% yield.
a
Conditions: 1 (0.3 mmol), 2a (0.9 mmol), Pd(PPh3)4 (0.03 mmol),
CuTC (0.6 mmol), ether (3 mL), 0.1 MPa N2, 30 °C, 12 h. Yields
b
refer to the isolated products. Pd(PPh3)4 (0.06 mmol).
The protocol generality was further explored by using
various allenoates 2 as the coupling partners (Scheme 3). α-
Benzoyl-α-ester ketene dithioacetal 1s reacted with ethyl buta-
2,3-dienoate (2b) to afford the target product 4a in 73% yield.
Compound 1s also efficiently reacted with allenoates 2b−f to
give tetrasubstituted 2-alkenylfurans 4b−e (70−76%), and
variation of the alkyl ester groups in the allenoates had no
obvious impact on the reaction efficiency. Phenyl buta-2,3-
dienoate (2g) exhibited a moderate reactivity, and its reaction
with 1s gave product 4f in 50% yield. Both ethyl 2-methylbuta-
2,3-dienoate (2h) and 2-benzylbuta-2,3-dienoate (2i) showed
an obvious steric effect on the formation of the target products
1
4g (50%) and 4h (44%). The H NMR analysis of the crude
product 4g or 4h before separation was made, but the proton
NMR signals were too complicated to distinguish the product
stereoselectivity. We only obtained the stereospecific products
4g and 4h by silica gel column chromatography. Both the
B
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Letter
a
corresponding α,α-diacetyl ketene di(methylthio)acetal (1a)
reacted with 2a to give the target product 3a in 74% yield
(Scheme 2). These results have unambiguously revealed that
such a di(alkylthio) functionality is crucial for α-oxo ketene
dithioacetals of type 1 to undergo the palladium-catalyzed [4 +
1] annulation with allenoates. Allene derivatives 9a and 9b
were used to replace 2a in the annulation reaction, but they
could not undergo the same annulation reactions, implicating
the crucial role of an ester group at the terminus of the allene
backbone (eq 3).
Scheme 3. Scope of Allenoates (2)
To demonstrate the utility of the synthetic protocol, a gram-
scale reaction of 1s with 2a was performed, and the target
product 3s was obtained in 72% yield (eq 5). A two-step
a
Conditions: 1 (0.3 mmol), 2 (0.9 mmol), Pd(PPh3)4 (0.03 mmol),
CuTC (0.6 mmol), ether (3 mL), 0.1 MPa N2, 30 °C, 12 h. Yields
b
refer to the isolated products. 60 °C, in a 25 mL sealed tube.
reactions of α-acetyl ketene dithioacetals 1a with 2e and 1f
with 2b proceeded smoothly to form products 4i (70%) and 4j
(67%), respectively. The molecular structures of compounds
3e and 4g were further confirmed by the X-ray single-crystal
crystallographic determinations (see the SI for details).
A comparative evaluation of the reactivities was made
between the tetra- and trisubstituted α-oxo ketene dithioacetals
1 and 5. It was found that trisubstituted α-benzoyl ketene
di(methylthio)acetals 5a−c exhibited a reactivity lower than
their tetrasubstituted analogues 1s, 1w, and 1z, and their
reactions with allenoate 2a formed the corresponding
trisubstituted 2-alkenylfurans 6a−c in 50−58% yields (eq 2).
procedure was developed to transform 2-alkenylfuran 3v to
lactone 1122 (59%), which has been proved to be a key
structural motif in some bioactive molecules, natural products,
and pharmaceuticals (eq 6).23 With meta-chloroperoxybenzoic
acid (m-CPBA) as the oxidant, 2-alkenylfuran 3s was readily
oxidized to the corresponding sulfone 12 in 89% yield (eq 7).
A plausible mechanism is proposed in Scheme 4. The
copper(I) additive initially activates one of the C−S bonds in
Scheme 4. Proposed Reaction Mechanism
In order to verify the role of the dialkylthio functionality in
1, 3-((methylthio)methylene)pentane-2,4-dione (7a) and 3-
benzylidenepentane-2,4-dione (7b) were reacted with 2a
under the standard conditions. No reaction was observed to
form the desired products 8a and 8b (eq 3), while the
1a through coordination with the sulfur atom by assistance of
the directing α-acetyl group, forming species A. Pd(0) species
is then inserted into the activated C−S bond to yield Pd(II)
species B in which both Pd−C and Pd−S bonds are formed.
Interaction of species B with allenoate 2a generates Pd(II)
species C through insertion of the terminal alkenyl of the
C
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02253.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zkyu@dicp.ac.cn.
ORCID
Quannan Wang: 0000-0002-4136-0016
Zhuqing Liu: 0000-0002-5225-1771
Jiang Lou: 0000-0002-5653-1359
Zhengkun Yu: 0000-0002-9908-0017
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Basic Research Program of
China (2015CB856600) and the National Natural Science
Foundation of China (21472185 and 21871253) for support of
this research.
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