Substituted 3(2H)-Furanones by a tandem michael addition/palladium- catalyzed ring-closing protocol

Article abstract of DOI:10.1002/ejoc.201201253

A novel palladium-catalyzed route for the synthesis of substituted 3(2H)-furanones from activated alkenes and 4-chloroacetoacetate was developed. The first step of the tandem reaction is the Michael addition of an acetoacetate to the alkene followed by palladium-catalyzed ring closure of the adduct to form the furanone. The reaction was extended to a number of substituted alkenes, and the corresponding substituted 3(2H)-furanones were obtained in good to excellent yields. Copyright

Full text of DOI:10.1002/ejoc.201201253

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201253
Substituted 3(2H)-Furanones by a Tandem Michael Addition/Palladium-
Catalyzed Ring-Closing Protocol
Jubi John[a] and Henning Hopf*[a]
Keywords: Homogeneous catalysis / Palladium / Domino reactions / Michael addition / Oxygen heterocycles
A novel palladium-catalyzed route for the synthesis of substi-
duct to form the furanone. The reaction was extended to a
tuted 3(2H)-furanones from activated alkenes and 4-chloro- number of substituted alkenes, and the corresponding substi-
acetoacetate was developed. The first step of the tandem re-
action is the Michael addition of an acetoacetate to the al-
kene followed by palladium-catalyzed ring closure of the ad-
tuted 3(2H)-furanones were obtained in good to excellent
yields.
Introduction
Catalytic tandem reactions are powerful tools for syn-
thetic organic chemists because multiple bonds can be
formed in one pot.[1] Michael addition is one of the most
important carbon–carbon bond-forming reactions, and it
has been widely explored over the last two decades.[2] It is
known that Pd0 can undergo oxidative addition to bonds α
to a carbonyl group, and this leads to the formation of oxa-
π-allylpalladium complexes, but only a few reports are
known for the oxidative addition to α-halo ketones.[3] In
this communication, we disclose a simple route to substi-
tuted 3(2H)-furanones by a tandem Michael addition/palla-
dium-catalyzed ring-closing process.
We recently developed a completely new framework for
liquid crystalline materials, namely, the perhydroazulene
(HAZ) unit.[4] The reported (linear) synthetic route to this
core was, though high yielding, too long and required too
many separation steps. We were therefore interested in de-
vising new synthetic routes towards the perhydroazulene-
dione core structure with a smaller number of steps and
overall higher efficiency. Our first aim was to find a new
method to fuse the cyclopentanone unit to the cyclohep-
tenone core. For the intended cyclopentanone annulation,
we chose 4-chloroacetoacetate, which can be viewed as a β-
diketone with a leaving group at the γ-carbon atom. We
were interested in its reactivity towards activated alkenes
under palladium catalysis, and we hoped that this reaction
would result in cyclopentanone annulation of the alkene
(Scheme 1).
Scheme 1. Our intended route for cyclopentanone annulation.
Results and Discussion
Our preliminary studies involved substituted styrene 4a
and ethyl 4-chloroacetoacetate (5a) as substrates. When
these were allowed to react in the presence of Pd(PPh3)4
(5 mol-%) and K2CO3 (1.0 equiv.) in THF at 60 °C, substi-
tuted 3(2H)-furanone 6 was obtained in 36% yield, con-
trary to our expectation of a substituted cyclopentanone
(Scheme 2). Diethyl 1,4-dihydroxycyclohexa-1,4-diene-2,5-
dicarboxylate (7) was obtained as a side product in 9%
yield.
Scheme 2. Pd-catalyzed synthesis of 3(2H)-furanone.
A number of synthetic routes to 3(2H)-furanones have
been reported in the literature, as these molecules form the
core structure of many natural products and pharmaceuti-
cally important compounds (Figure 1).[5] Some of these
strategies involve acid-catalyzed transformations of substi-
tuted α-hydroxy-1,3-diketones,[5g,5i] synthetic manipulations
of substituted furans,[6] cyclization of allenic hydroxy-
[a] Institute für Organische Chemie, Technische Universität
Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: +49-531-391-5388
E-mail: h.hopf@tu-bs.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201253.
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J. John, H. Hopf
SHORT COMMUNICATION
ketones under basic conditions,[7] and transition-metal cused on finding the best catalyst/ligand combination. The
(Au,[8] Pt,[9] Pd,[10] Hg[11])-catalyzed strategies. Recently, two catalysts tested were Pd(PPh3)4 and Pd2(dba)3·CHCl3, and
asymmetric routes to 3(2H)-furanones were reported by Lu the ligands tested were 1,2-bis(diphenylphosphanyl)ethane
et al.[12] and Yan et al.,[13] and these methods involved an (dppe), 1,1Ј-bis(diphenylphosphanyl)ferrocene (dppf), and
organocatalyzed reaction between 4-haloacetoacetate esters P(o-furyl)3. The combination of Pd(PPh3)4 and P(o-furyl)3
and nitrostyrene.
afforded the furanone in 76% yield (Table 1, entry 7). We
went on optimizing other parameters such as the solvent
and temperature. Among the solvents tested, THF, CH3CN,
dioxane, and DMF, the highest yield of 81% for 6 was ob-
tained in dioxane (Table 1, entries 7, 12–14). When a solu-
tion of substrates 4a and 5a was stirred in the presence of
Pd(PPh3)4/P(o-furyl)3/K2CO3 in dioxane at room tempera-
ture, 91% of 7 was obtained after 24 h. Thus, the addition
process can be switched between products 6 and 7 by
changing only the temperature.
Figure 1. Bioactive molecules with the 3(2H)-furanone moiety.
Two control experiments were carried out to establish
whether palladium was the active catalyst. The first reaction
Hence, the current interest in the 3(2H)-furanone ring was performed without the catalyst/ligand and only in the
system prompted us to investigate the scope of the transfor- presence of base (Table 1, entry 16). The second reaction
mation. Table 1 describes our efforts towards optimizing was carried out in the presence of Pd(PPh3)4/P(o-furyl)3
various reaction parameters with 4a and 5a as model sub- without base (Table 1, entry 17). The first reaction afforded
strates. The yield of furanone 6 almost doubled when almost equal amounts of 6 (46%) and 7 (38%). The second
2.0 equiv. of base was used (Table 1, entry 1). Next, screen- reaction yielded only trace amounts of the desired product.
ing of the base revealed that K2CO3 was more effective than Thus, we can confirm that palladium was catalyzing the
either Na2CO3 or Cs2CO3 (Table 1, entries 1–3). When reaction by comparing the first control experiment and the
Et3N was employed as the base, the side product was reaction depicted in entry 13 of Table 1. Analysis of the re-
formed in higher amounts (Table 1, entry 4). We then fo- sults reveals that under palladium catalysis the yield of
Table 1. Optimization studies.[a]
[a] Reaction conditions: 4a (1.0 equiv.), 5a (1.0 equiv.), catalyst (5 mol-%), ligand (10 mol-%), base (2.0 equiv.), solvent (2 mL), 60 °C,
12 h. [b] Isolated yield. [c] Reaction performed at r.t., 24 h.
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Substituted 3(2H)-Furanones by Tandem Reactions
3(2H)-furanone 6 was doubled. Under the influence of the 5a), the connectivity was additionally established qualita-
catalyst, the Michael addition thus occurs much faster and tively by X-ray structure determination, but severe disorder
thereby prevents the dimerization of 5a to form product 7. of the substituents prevented satisfactory refinement.
The scope of the new protocol was next investigated un-
Under the optimized conditions, the reactions of 5a and
der optimized conditions [alkene (1.0 equiv.), 4-chloroace- 5b were also attempted with trans-β-nitrostyrene (4f). Both
toacetate (1.0 equiv.), Pd(PPh3)4 (5 mol-%), P(o-furyl)3 reactions afforded corresponding 3(2H)-furanones 17 and
(10 mol-%) K2CO3 (2.0 equiv.), dioxane, 60 °C, 12 h]. In all 18 in excellent yields (Scheme 3).
cases, investigated styrenes 4a–e reacted with 4-chloroace-
toacetates 5a and 5b, and this led to the corresponding sub-
stituted 3(2H)-furanones in good to excellent yield
(Table 2). We also checked if electronic effects could influ-
ence the reaction by introducing both electron-donating
(i.e., 4c) and electron-withdrawing (i.e., 4d) substituents on
the phenyl ring. It was found that the 3(2H)-furanones were
formed in slightly better yields with styrene 4d bearing a
nitro group on the phenyl ring. Notably, in all the cases the
side product was formed in negligible amounts (Յ5%).
Scheme 3. Pd-catalyzed synthesis of 3(2H)-furanones from β-nitro-
styrene.
We were then interested to determine if the phenyl ring
Table 2. Generalization of the furanone-forming process.[a]
in styrene had any influence on the outcome of the reaction.
Thus, we prepared activated alkene 4g from the aliphatic
aldehyde propanal. The reactions of alkene 4g with 5a and
5b were attempted under the optimized conditions. From
both reactions, corresponding 3(2H)-furanones 19 and 20
were obtained in good yields (Scheme 4).
Scheme 4. Pd-catalyzed synthesis of 3(2H)-furanones from alkene
4g.
To determine if the first step of the tandem process, that
is, the Michael addition, was palladium catalyzed, we car-
ried out two reactions with 4f and methylacetoacetate. The
first reaction was done under basic conditions [K2CO3
(1.0 equiv.), dioxane, 60 °C, 12 h], and the second reaction
was performed under the optimized conditions. Both reac-
tions afforded the Michael addition product in similar
yields, and this proves that the Michael addition was uncat-
alyzed. Scheme 5 depicts a plausible mechanism for both
the catalyzed and the uncatalyzed version of our 3(2H)-fur-
anone synthesis from activated alkenes and 4-chloroace-
toacetate. Both the catalyzed and uncatalyzed pathways
commence with the Michael addition of the enolate to the
α-carbon of the activated double bond to form intermediate
22. In the next step of the catalyzed route, we believe that
Pd0Ln then undergoes oxidative addition to the C–Cl bond
in intermediate 22 to form 23. This species can subsequently
give rise to oxy-π-allyl palladium intermediate 24.[3] This
[a] Reaction conditions: 4 (1.0 equiv.), 5 (1.0 equiv.), Pd(PPh3)4
(5 mol-%), P(o-furyl)3 (10 mol-%), K2CO3 (2.0 equiv.), dioxane
(2 mL), 60 °C, 12 h. [b] Isolated yield.
The structure of furanones 6 and 8–16 follows from the step is followed by abstraction of the acidic proton by the
usual spectroscopic data (see the Supporting Information). base, which forces the ester enolate to attack the carbon end
In the case of 13 (product formed by the reaction of 4d with of 24 towards the ring closure to yield the 3(2H)-furanone.
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J. John, H. Hopf
SHORT COMMUNICATION
Scheme 5. Mechanism for the formation of 3(2H)-furanones.
Conclusions
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formation of either 3(2H)-furanone or diethyl 1,4-dihydrox-
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Experimental Section
General Experimental Procedure for the Preparation of Substituted
3(2H)-Furanones: Styrene (1.0 equiv.), Pd(PPh3)4 (5 mol-%), P(o-fu-
ryl)3 (10 mol-%), and K2CO3 (2.0 equiv.) were placed in a Schlenk
tube. The reaction flask was degassed and flushed with argon (3ϫ).
To this mixture, under an atmosphere of argon, was added dioxane
(2 mL) followed by 4-chloroacetoacetate (1.0 equiv.); the mixture
was stirred at 60 °C for 12 h. After completion of the reaction, the
mixture was diluted with dichloromethane (20 mL) and washed
with water (2ϫ20 mL) and saturated brine (20 mL) solution. The
organic layer was then dried with anhydrous magnesium sulfate,
and the solvent was evaporated in vacuo. The residue was chro-
matographed on flash silica gel (ethyl acetate/pentane) to afford the
product in good to excellent yields (see Tables 1 and 2).
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Supporting Information (see footnote on the first page of this arti-
cle): General methods, experimental procedures, and characteriza-
tion data for all new compounds.
Acknowledgments
J. J. thanks the Alexander von Humboldt foundation for a postdoc-
toral fellowship.
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Received: September 20, 2012
Published Online: January 4, 2013
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