A tandem Mannich addition-palladium catalyzed ring-closing route toward 4-substituted-3(2H)-furanones

Article abstract of DOI:10.3762/bjoc.10.150

A facile route towards highly functionalized 3(2H)-furanones via a sequential Mannich addition-palladium catalyzed ring closing has been elaborated. The reaction of 4-chloroacetoacetate esters with imines derived from aliphatic and aromatic aldehydes unde

Full text of DOI:10.3762/bjoc.10.150

A tandem Mannich addition–palladium
catalyzed ring-closing route toward
4-substituted-3(2H)-furanones
Jubi John1, Eliza Târcoveanu1, Peter G. Jones2 and Henning Hopf*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1462–1470.
1Institute für Organische Chemie, Technische Universität
Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany, Fax:
+49 (0)531/391 and 2Institut für Anorganische und Analytische
Chemie, Technische Universität Braunschweig, Hagenring 30,
D-38106 Braunschweig, Germany
doi:10.3762/bjoc.10.150
Received: 24 March 2014
Accepted: 27 May 2014
Published: 27 June 2014
Email:
Associate Editor: K. Itami
Henning Hopf* - h.hopf@tu-bs.de
© 2014 John et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
aza-prostaglandin analogue; 3(2H)-furanone; Mannich addition;
palladium catalysis; tandem reaction
Abstract
A facile route towards highly functionalized 3(2H)-furanones via a sequential Mannich addition–palladium catalyzed ring closing
has been elaborated. The reaction of 4-chloroacetoacetate esters with imines derived from aliphatic and aromatic aldehydes under
palladium catalysis afforded 4-substituted furanones in good to excellent yields. 4-Hydrazino-3(2H)-furanones could also be
synthesized from diazo esters in excellent yields by utilising the developed strategy. We could also efficiently transform the substi-
tuted furanones to aza-prostaglandin analogues.
Introduction
Organic chemists welcome the introduction of facile tandem α-Halo ketones are known to react with zero-valent palladium
protocols because of the advantages of multiple bond formation like allyl halides to form oxa-π-allylpalladium complexes.
in one-pot processes, which in turn makes the process economic Despite this fact, only minor attention has been paid to the for-
and most importantly eco-friendly [1,2]. Conjugate addition is a mation of oxa-π-allylpalladium species from α-halo ketones and
very efficient tool used by synthetic chemists for the construc- to their reactivity [4-8]. In this report we describe the use of a
tion of carbon–carbon or carbon–heteroatom bonds [3]. Much tandem methodology involving a Mannich addition/palladium-
effort has been invested to develop different variants of conju- catalyzed ring-closing protocol for the facile synthesis of
gate additions, both in catalyzed and non-catalyzed pathways. 4-substituted furanones.
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Since 3(2H)-furanones form the main component of many thetic routes for the preparation of these compounds involve
natural products and pharmaceutically important compounds transformations of substituted α-hydroxy-1,3-diketones [15,17],
(Figure 1) a number of routes for their synthesis [9-18] to have substituted furans [27-29], cyclization of allenic hydroxyke-
been reported in the literature.
tones [30], transition metal-catalyzed strategies (Au [31-33], Pt
[34,35], Pd [36], Hg [37]), etc. Lately, organo-catalyzed asym-
metric routes towards 3(2H)-furanones from 4-haloaceto-
acetates and nitrostyrene were reported by Lu et al. [38] and
Yan et al. [39].
We have recently reported on a palladium-catalyzed tandem
methodology for the synthesis of 4-substituted-3(2H)-fura-
nones from activated alkenes and 4-chloroacetoacetates
(Scheme 1) [40].
The reaction was found to be general for a wide range of
alkenes derived from aromatic and aliphatic aldehydes. The
reaction proceeded via Michael addition of the acetoacetate to
the alkene with a subsequent palladium-catalyzed ring closure
of the primary adduct to form the furanone. In this paper, we
report our investigations that extend the reaction to heteroatom-
containing electrophiles such as imines and diazo esters.
Figure 1: Bioactive molecules I [19], II [26], III & IV [21,22] with 3(2H)-
furanone moiety.
Results and Discussion
Following the work on nitrostyrenes, Yan et al. also reported a
two-step asymmetric route towards 4-substituted-furanones
Substituted 3(2H)-furanones exhibit a wide range of biological from imines [41]. Recently Fructos et al. have shown that N-p-
activities such as antiallergic, antiulcer, antiinflammatory, and toluenesulfonyl-protected imines were better candidates for
antitumor activities or selective COX-2 inhibition [19-26]. Syn- gold catalyzed Mannich addition of acetoacetates when
Scheme 1: Pd-catalyzed synthesis of 3(2H)-furanones from activated alkenes [40].
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compared to N-Boc (tert-butoxycarbonyl) and N-PMP than either K2CO3 or KOt-Bu (Table 1, entries 1–3). When
(p-methoxyphenyl) imines [42]. Hence, we commenced our KOt-Bu was employed as base, byproduct 4 (formed by the
investigations with the reaction of tosylimine 1a and ethyl dimerisation of 2a) was formed in higher amounts (Table 1,
4-chloroacetoacetate (2a) in the presence of 5 mol % of entry 3). From the tested catalysts, Pd(PPh3)4 and
Pd(PPh3)4 and 2.0 equivalents of Na2CO3, in dioxane at 50 °C Pd2dba3·CHCl3, and ligands, dppe and P(o-furyl)3, the combi-
for 10 hours. The reaction afforded 4-substituted-3(2H)-fura- nation of Pd2dba3·CHCl3 and dppe afforded the furanone 3 in
none 3 in 57% yield (Scheme 2). Interestingly, we could not 88% yield (Table 1, entry 8). A solvent screen revealed that
detect the formation of diene-2,5-dicarboxylate 4 (formed by dioxane was best for the present transformation from which the
the dimerization of 2a) as a side product, although this was product 3 was obtained in 88% yield (Table 1, entries 8–10). It
observed previously in the reaction of activated alkenes [40] was found that lowering the temperature had a negative influ-
and 4-chloroacetoacetates.
ence on the yield as 3 was obtained only in 43% even after
24 hours when the substrates 1a and 2a were stirred in the pres-
Table 1 summarizes our efforts towards optimizing various ence of Pd2dba3·CHCl3/dppe/Na2CO3 in dioxane at room
reaction parameters with 1a and 2a as model substrates. temperature. When the catalyst loading was decreased to
Screening of bases revealed that Na2CO3 was more effective 1 mol %, the reaction afforded the furanone 3 in 78% yield after
Scheme 2: Pd-catalyzed synthesis of 3(2H)-furanone from tosylimine 1a.
Table 1: Optimisation studiesa.
Entry
Catalyst
Ligand
Base
Yield 3b(%)
1
2
Pd(PPh3)4
Pd(PPh3)4
–
–
Na2CO3
K2CO3
57
34
3
Pd(PPh3)4
–
KOt-Bu
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
–
19c
66
4
Pd(PPh3)4
P(o-furyl)3
dppe
–
5
Pd(PPh3)4
70
6
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
Pd2dba3·CHCl3
–
85
7
P(o-furyl)3
dppe
dppe
dppe
dppe
–
82
8
88
9
80d
84e
43f
65
10
11
12
13
14
Pd2dba3·CHCl3
Pd2dba3·CHCl3
dppe
dppe
trace
78g
Na2CO3
aReaction conditions: 1a (1.0 equiv), 2a (1.1 equiv), catalyst (5 mol %), ligand (10 mol %), base (2.0 equiv), dioxane (2 mL), 50 °C, 10 h. bIsolated
yield. cDimerisation product 4 was formed in 27% yield.dCH3CN instead of dioxane. eTHF instead of dioxane. frt, 24 h. g1 mol % of Pd2dba3·CHCl3,
5 mol % of dppe.
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10 hours (Table 1, entry 14). The catalytic role of palladium in In all the cases substituted 3(2H)-furanones were obtained in
the present transformation was proved by conducting two good to excellent yields (Figure 2). The reaction was also
control experiments. The first reaction was performed only in extended to tosylimines derived from aliphatic aldehydes,
the presence of base (Table 1, entry 12) and the second one only pentanal (for 1d) and 3-phenylpropionaldehyde (for 1e) and
in the presence of the Pd2dba3·CHCl3/dppe combination thus we could rule out any influence of the aromatic moiety on
(Table 1, entry 13). Furanone 3 was obtained in 65% yield from the outcome of the reaction. The corresponding 4-susbtituted-
the first reaction whereas the second reaction afforded only 3(2H)-furanones were obtained from these two substrates in
trace amounts of the desired product. Thus we can see that good yields (Figure 2). We also tried a reaction with N-Boc
under palladium catalysis there is an increase in the yield of protected imine under optimized conditions and the corres-
3(2H)-furanone 3 by 23% by comparing the first control experi- ponding furanone was obtained in good yield (Figure 2, com-
ment and the reaction depicted in entry 8 of Table 1, thereby pound 14). The structure elucidation of the furanone products 3,
proving that palladium is catalyzing the reaction.
and 5–14 was accomplished by the usual spectroscopic methods
(see Supporting Information File 1) and also by X-ray structure
With optimal conditions in hand (imine (1.0 equiv), determination [43] for furanones 7 (product formed by the reac-
4-chloroacetoacetate (1.1 equiv), Pd2dba3·CHCl3 (5 mol %), tion of 1b with 2b) and 10 (product formed by the reaction of
dppe (10 mol %) Na2CO3 (2.0 equiv) in dioxane at 50 °C for 1d with 2a) (Figure 3).
10 h), studies towards the generality of the reaction were carried
out with different imines 1a–c derived from aromatic alde- We believe that the mechanism of the present furanone syn-
hydes with 4-chloro-acetoacetates 2a,b.
thesis from imines is similar to that reported for activated
Figure 2: Generalisation with aromatic and aliphatic imines (reaction conditions: 1 (1.0 equiv), 2 (1.1 equiv), Pd2dba3·CHCl3 (5 mol %), dppe
(10 mol %), Na2CO3 (2.0 equiv), dioxane (2 mL), 50 °C, 10 h, isolated yield).
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Beilstein J. Org. Chem. 2014, 10, 1462–1470.
Figure 3: Thermal ellipsoid diagrams (50% probability levels) of 4-substituted-3(2H)-furanones 7 (above) and 10 (below).
alkenes [40]. This tandem protocol has two steps, the first being Having established an efficient method for the synthesis of
a Mannich addition of the enolate to the imine. To find out highly functionalized furanones from imines, we were inter-
whether Mannich addition is palladium catalyzed, we carried ested next in extending the range of the electrophiles by intro-
out two reactions of tosylimine 1a with methyl acetoacetate. ducing different functionalities at the 4-position of the 3(2H)-
The first transformation was carried out under basic conditions furanone. We chose to check the reactivity of diisopropylazodi-
(Na2CO3 (1.0 equiv), dioxane, 50 °C, 6 h) and the second under carboxylate (19a) with ethyl 4-chloroacetoacetate (2a) under
optimized conditions. Both reactions afforded the Mannich ad- palladium catalysis. In the initial reaction, 19a was treated with
dition product in similar yields, proving that Mannich addition 2a in the presence of Pd(PPh3)4 as catalyst and base (K2CO3) in
was uncatalyzed.
dioxane at 50 °C. After 10 hours, the expected product
4-hydrazino-3(2H)-furanone 20 was isolated in 58% yield
Based on these results, we propose a plausible mechanism for (Scheme 4).
3(2H)-furanone synthesis from tosylimines and 4-chloroace-
toacetate (Scheme 3). The reaction starts with the Mannich ad- Table 2 describes the screening of different reaction conditions
dition of the enolate 15 to the carbon atom of the imine double to improve the yield of 20. First we tested the efficacy of
bond to form intermediate 16. This step is the same for both the different bases K2CO3, Na2CO3 and KOt-Bu, of which Na2CO3
catalyzed and the uncatalyzed pathway. The second step of the proved to be the best (Table 2, entries 1–3). The highest yield
catalyzed route involves the oxidative addition of Pd(0)Ln to the for 4-hydrazino-3(2H)-furanone was given when Pd(PPh3)4
C–Cl bond of Mannich adduct 16 to form 17. The oxy-π-allyl- alone was used among different catalyst/ligand combinations
palladium intermediate 18 can then be formed from intermedi- tested with Pd(PPh3)4 and Pd2dba3.CHCl3 as catalysts and dppe
ate 17 [4-8]. The final step of the catalyzed mechanism, i.e., the and P(o-furyl)3 as ligands (Table 2, entries 2, and 4–8). The
ring closure towards the formation of 3(2H)-furanone is initi- product was obtained in better yields when dioxane was used as
ated by the abstraction of the acidic proton by the base and the solvent (Table 2, entries 2, 9 and 10). At room temperature
consequent ester enolate attack to the end carbon of the oxy-π- the reaction with optimised catalyst/solvent system furnished
allylpalladium intermediate.
only 38% of the substituted furanone even after 24 hours
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Scheme 3: Mechanism of formation of the 3(2H)-furanone derivative from an imine.
Scheme 4: Pd-catalyzed synthesis of 3(2H)-furanone from diazoester 19a.
(Table 2, entry 11). A control experiment was done without the The possibility for further functionalization of 4-substitued
catalyst/ligand combination but only in the presence of base, furanones was checked by treating 11 with n-heptylamine in
which afforded the furanone in 69% yield (Table 2, entry 12). MeOH at 40 °C. After 10 hours the aza-prostaglandin analogue
Thus by comparing the control experiment and the reaction [26] 27 was isolated in 85% yield (Scheme 5).
depicted in entry 2 of Table 2 we can confirm that palladium
catalyzes the reaction.
Conclusion
We have developed an efficient protocol for the synthesis of
The generality of the reaction was checked with different diazo 4-substituted 3(2H)-furanones by the reaction of imines or diazo
esters 19a–c with 4-chloroacetoacetate esters 2a and 2b under esters with 4-chloroacetoacetates under palladium catalysis. We
the optimized conditions (diazo ester (1.0 equiv), 4-chloroace- could extend the reaction to imines derived from aromatic and
toacetate (1.1 equiv), Pd(PPh3)4 (5 mol %), Na2CO3 (2.0 equiv) aliphatic aldehydes. The reaction proceeded via a catalyzed
in dioxane at 50 °C for 10 h). All reactions afforded the corres- tandem Mannich addition–palladium-catalyzed ring-closing
ponding 4-hydrazino-3(2H)-furanones in excellent yields pathway to afford various 3(2H)-furanones in good to excellent
(Figure 4).
yields. We could further apply this route to the preparation of an
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Beilstein J. Org. Chem. 2014, 10, 1462–1470.
Table 2: Optimisation studiesa.
Entry
Catalyst
Ligand
Base
Yield 20b(%)
1
2
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd2dba3.CHCl3
Pd2dba3.CHCl3
Pd2dba3.CHCl3
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
–
–
K2CO3
Na2CO3
KOt-Bu
58
90
–
3
–
35c
75
4
P(o-furyl)3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
5
dppe
79
6
–
83
7
P(o-furyl)3
80
8
dppe
76
9
–
–
–
–
58d
82e
38f
69
10
11
12
aReaction conditions: 19a (1.0 equiv), 2a (1.1 equiv), catalyst (5 mol %), ligand (10 mol %), base (2.0 equiv), dioxane (2 mL), 50 °C, 10 h. bIsolated
yield. cDimerization product 4 was formed in 23% yield. dCH3CN instead of dioxane. eTHF instead of dioxane. frt, 24 h.
Figure 4: Generalisation with diazo esters (reaction conditions: 19 (1.0 equiv), 2 (1.1 equiv), Pd(PPh3)4 (5 mol %), Na2CO3 (2.0 equiv), dioxane
(2 mL), 50 °C, 10 h, isolated yield).
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Beilstein J. Org. Chem. 2014, 10, 1462–1470.
Scheme 5: Synthesis of aza-prostaglandin analogue.
13.Kupchan, S. M.; Sigel, C. W.; Matz, M. J.; Gilmore, C. J.; Bryan, R. F.
J. Am. Chem. Soc. 1976, 98, 2295–2300. doi:10.1021/ja00424a050
14.Smith, A. B., III; Guaciaro, M. A.; Schow, S. R.; Wovkulich, P. M.;
Toder, B. H.; Hall, T. W. J. Am. Chem. Soc. 1981, 103, 219–222.
doi:10.1021/ja00391a054
aza-prostaglandin analogue. The synthesized molecules are
currently being screened for biological activities. We have also
extended the reaction to triple-bonded electrophiles such as
acetylenes, benzyne and nitriles; the results will be reported in
due course. Studies are in progress to develop a stereoselective
version of the process.
15.Smith, A. B., III; Levenberg, P. A.; Jerris, P. J.; Scarborough, R. M., Jr.;
Wovkulich, P. M. J. Am. Chem. Soc. 1981, 103, 1501–1513.
doi:10.1021/ja00396a034
16.Dreyer, D. L.; Lee, A. Phytochemistry 1972, 11, 763–767.
doi:10.1016/0031-9422(72)80045-1
Supporting Information
17.Jerris, P. J.; Smith, A. B., III. J. Org. Chem. 1981, 46, 577–585.
doi:10.1021/jo00316a018
Supporting Information File 1
18.Haug, T. T.; Kirsch, S. F. In Targets in Heterocyclic Systems;
Attanasi, O. A.; Spinelli, D., Eds.; Royal Society of Chemistry:
Cambridge, 2009; Vol. 13, pp 57–91.
Experimental part and NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-150-S1.pdf]
See for a book chapter on synthesis and chemistry of 3-(2H)-furanone.
19.Mack, R. A.; Zazulak, W. I.; Radov, L. A.; Baer, J. E.; Stewart, J. D.;
Elzer, P. H.; Kinsolving, C. R.; Georgiev, V. S. J. Med. Chem. 1988, 31,
1910–1918. doi:10.1021/jm00118a008
Acknowledgements
JJ thanks the Alexander von Humboldt foundation for a post-
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