Construction of 1,2,5-tricarbonyl compounds using methyl cyanoacetate as a glyoxylate anion synthon combined with copper(I) iodide-catalyzed aerobic oxidation

Article abstract of DOI:10.1002/adsc.201100431

A practical and efficient synthesis of various 1,2,5-tricarbonyl compounds is described. The synthesis has been carried out by a conjugate addition of methyl cyanoacetate to the β-position of α,β-unsaturated carbonyl compounds and a subsequent copper(I) iodide-catalyzed aerobic oxidation. In addition, various α-aryl- and α-alkyl-α-keto esters have been synthesized using a similar approach. Copyright

Full text of DOI:10.1002/adsc.201100431

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DOI: 10.1002/adsc.201100431
Construction of 1,2,5-Tricarbonyl Compounds using Methyl
Cyanoacetate as a Glyoxylate Anion Synthon Combined with
Copper(I) Iodide-Catalyzed Aerobic Oxidation
Se Hee Kim,a Ko Hoon Kim,a and Jae Nyoung Kima,*
a
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Korea
Fax : (+82)-062-530-3389; phone: (+82)-062-530-3381; e-mail: kimjn@chonnam.ac.kr
Received: May 25, 2011; Revised: July 17, 2011; Published online: November 29, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100431.
Abstract: A practical and efficient synthesis of vari-
ous 1,2,5-tricarbonyl compounds is described. The
synthesis has been carried out by a conjugate addi-
tion of methyl cyanoacetate to the b-position of
a,b-unsaturated carbonyl compounds and a subse-
quent copper(I) iodide-catalyzed aerobic oxidation.
In addition, various a-aryl- and a-alkyl-a-keto
esters have been synthesized using a similar ap-
proach.
Keywords: aerobic oxidation; copper(I) iodide;
glyoxylate anion synthon; methyl cyanoacetate;
1,2,5-tricarbonyl compounds; umpolung
Scheme 1. Umpolung strategy for 1,4-dicarbonyl com-
pounds.
way to 2,5-diketo esters theoretically, as shown in
The construction of a 1,4-dicarbonyl functionality is Scheme 1. Very recently, a conjugate addition of a
important in organic synthesis.[1–4] However, such a glyoxylate anion equivalent to nitroalkenes[4a] and a
synthesis through the bond formation either at 1,2- or Stetter reaction of glyoxamide anion synthon to a,b-
2,3-bonds required one part to have an unusual polar- unsaturated carbonyl compounds[4b–d] have been suc-
ity (umpolung synthon). Since Corey and Seebach in- cessfully achieved to synthesize a-keto esters and a-
troduced the concept of umpolung extensive progress keto amides. However, the Stetter reaction between
has been achieved.[2] Although numerous methods ethyl glyoxylate and an a,b-enone was not studied in
have been developed for the preparation of 1,4-dicar- depth except for only one entry in the report of
bonyl compounds,[1] the most widely used approach is Rovis,[4b] to the best of our knowledge. The reason
a conjugate addition reaction of masked acyl anions might be ascribed to a difficult accessibility for pure
and their equivalents to a,b-unsaturated carbonyl ethyl glyoxylate.[5] Ethyl glyoxylate mostly exists in its
compoundss. The construction of 1,4-dicarbonyl com- hydrate[5a,b] or polymeric forms,[5c] and this renders the
pounds by 1,2-bond formation using an acyl anion Stetter reaction difficult in the presence of an N-het-
equivalent is depicted in Scheme 1.
erocyclic carbene catalyst.
However, the reported methods for valuable 2,5-
Thus, we envisioned that a two-step synthesis of
diketo esters (1,2,5-tricarbonyl compounds) are highly 2,5-diketo esters (1,2,5-tricarbonyl compounds) could
limited.[3] Reetz and co-workers have used 1,2-dieth- be carried out from a a,b-enone and ethyl cyanoace-
oxy-1,2-disilyloxyethylene as a glyoxylate anion syn- tate via a conjugate addition and a subsequent aero-
thon,[3a] while Flores-Parra and Khuong-Huu used bic oxidation, as shown in Scheme 2. To our delight,
methyl dimethoxyacetate during their synthesis of 2,5- we found that a conjugate addition product, the a-
diketo esters.[3c] An extension of the acyl anion to a cyano-d-keto ester, can be converted to the 2,5-diketo
glyoxylate anion synthon could provide an efficient ester in high yield by copper-catalyzed aerobic oxida-
Adv. Synth. Catal. 2011, 353, 3335 – 3339
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3335

Se Hee Kim et al.
Table 1. Copper-mediated aerobic oxidation of 1a.[a]
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Scheme 2. Synthetic strategy for 2,5-diketo esters.
Entry
Catalyst[b]
Solvent
Time [h]
2a [%][c]
1
2
3
4
5
6
7
8
CuI
CuI
CuI
CuI
CuI
CuI
CuCl
CuBr
CuCN
CH3CN
CH3CN
DMF
40
3
4
4
4
10
6
40
4
40
5
57[d]
89[e]
75
0
0
tion. Ethyl cyanoacetate was used as an efficient ethyl
glyoxylate anion equivalent in the reaction, and we
wish to report herein the preliminary results.
Copper- or base-mediated oxidations of benzylic[6a,b]
or secondary nitriles[6c,d] to ketones, a-cyano esters to
a-keto esters,[7] a-cyano amides[8a] or a-amino nit-
toluene
CH2Cl2
aq. EtOH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
66
33[f]
39[g]
62[h]
13[i]
0
ACHTUNGTRENNUNG
riles[8b,c] to a-keto amides have been reported. The ox-
idations were effected using molecular oxygen,[6,7] per-
acetic acid,[8a] NaOCl[8b] or Oxone.[8b] Although the
yields were moderate, a variety of precedent reports
implied that our synthetic approach could be realiza-
ble. With this confidence in mind, we decided to use
O2 as an oxidant and examined the optimum reaction
conditions with ethyl 2-cyano-5-oxo-3,5-diphenylpen-
tanoate (1a) which was prepared from ethyl cyanoace-
tate and chalcone (LiClO4/Et3N/room temperature,
10 min, 92% yield).[9a] A base-mediated aerobic oxi-
dation of 1a was not fruitful. The reaction of 1a in
DMF in the presence of Cs2CO3 under an O2 balloon
atmosphere produced 4-oxo-2,4-diphenylbutyronitrile
in moderate yield (45%) instead of the desired prod-
uct, ethyl 2,5-dioxo-3,5-diphenylpentanoate (2a). The
mechanism is not clear at this stage.
9
10
11
Cu
N
CuCl2
[a]
Carried out under O2 balloon atmosphere at 258C.
1.1 equiv. of copper salt were used except for entry 1
(0.2 equiv.).
Isolated yield.
3a (15%).
Selected as conditions A.
1a (15%), 3a (5%) and ethyl 2-chloro-2-cyano-5-oxo-3,5-
diphenylpentanoate (27%).
1a (28%), 3a (5%) and ethyl 2-bromo-2-cyano-5-oxo-3,5-
diphenylpentanoate (8%).
3a (17%).
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
1a (17%) and 3a (25%).
Thus we examined the conversion of 1a to 2a under
various copper-catalyzed oxidation conditions, and
the results are summarized in Table 1. From the re-
sults the use of CuI (1.1 equiv.) in CH3CN at 258C
was selected as the optimum conditions (entry 2). The
use of a catalytic amount of CuI (entry 1) or other
solvents such as DMF, toluene, CH2Cl2 and aqueous
EtOH (entries 3–6) was less efficient. Among the
copper salts, CuI was the reagent of choice. With
other copper catalysts (entries 7–11) a sluggish reac-
tion was observed and/or the amounts of side prod-
ucts increased including cyanohydrin 3a and halogen-
ated compounds (see Table 1).
Then we turned our attention to a catalytic version
in combination with a suitable ligand. As shown in
Table 2, the use of DMEDA (L1), TMEDA (L2), and
ethylenediamine (L3) showed low to moderate yields
of 2a (entries 1–3). The use of 1,10-phenanthroline
(L4) gave a reasonable yield of 2a (entry 4) while 8-
quinolinol (L5) and l-proline (L6) were totally inef-
fective (entries 5 and 6). Reducing the amounts of
CuI and 1,10-phenanthroline lowered the yield of 2a
(entries 7–9). Overall, the conditions using 1,10-phen-
Table 2. CuI-catalyzed aerobic oxidation of 1a in the pres-
ence of a ligand.[a]
Entry CuI (equiv.) Ligand (equiv.)[b] Time [h] 2a [%][c]
1
2
3
4
5
6
7
0.2
0.2
0.2
0.2
0.2
0.2
0.05
0.1
0.05
L1 (0.3)
L2 (0.3)
L3 (0.3)
L4 (0.3)
L5 (0.3)
L6 (0.3)
L4 (0.1)
L4 (0.2)
L4 (0.1)
3
3
3
5
24
24
24
24
5
58
27
53
78[d]
0
0
14
34
53
8
9[e]
[a]
[b]
CH3CN/O2 balloon/258C is common.
Ligands: L1, N,N-dimethylethylenediamine;
N,N,N’,N’-tetramethylethylenediamine; L3, ethylenedi-
AHCTUNGTREGaNNNU mine; L4, 1,10-phenanthroline; L5, 8-quinolinol; L6, l-
L2,
proline.
Isolated yield.
Selected as conditions B.
Carried out at 458C.
[c]
[d]
[e]
A
In order to show the generality, we examined vari-
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Adv. Synth. Catal. 2011, 353, 3335 – 3339

Construction of 1,2,5-Tricarbonyl Compounds using Methyl Cyanoacetate as a Glyoxylate Anion Synthon
Scheme 4. Synthesis of a-aryl-a-keto esters.
vided the corresponding a-keto esters in good yields.
However, the method cannot be applied for arenes
bearing an electron-withdrawing group. Thus, our
two-step protocol can be used efficiently in such
cases.
In addition, some a-alkyl-a-keto esters were also
synthesized via a similar strategy, as summarized in
Scheme 5. Starting materials 1s–w were prepared by
alkylation of the corresponding alkyl bromides or
chlorides (K2CO3, CH3CN, room temperature).[9] The
CuI-mediated aerobic oxidation of these a-cyano
esters provided the corresponding alkyl-substituted a-
keto esters 2s–u in good yields (76–84%). However,
Scheme 3. Synthesis of 1,2,5-tricarbonyl compounds.
B (entry 4 in Table 2). Some representative starting the reaction of 1v produced benzil as the major prod-
materials 1a–l were prepared from various a,b-enones
via a conjugate addition of methyl cyanoacetate, ethyl
cyanoacetate, allyl cyanoacetate, and 2-cyanoacet-
ACHTUNGTRENNUNG
amide according to the reported methods.[9a,b] The re-
sults for the synthesis of 1,2,5-tricarbonyl compounds
2a–l are summarized in Scheme 3. Various cyclic and
acyclic 1,2,5-tricarbonyl compounds were prepared in
moderate to good yields (72–93%). Usually, condi-
tions A showed better results than conditions B; how-
ever, the yield of amide derivative 2f was quite low
under conditions A.
As a next trial, methyl (or ethyl) 2-arylcyanoace-
tates 1m–r were prepared via a base-mediated SNAr
reaction or a palladium-catalyzed a-arylation from
the corresponding aryl halides according to the re-
ported methods.[9c–h] As shown in Scheme 4, CuI-
mediated aerobic oxidation of 1m–r provided the cor-
responding a-keto esters 2m–r in reasonable yields
(56–76%) under the conditions A. Usually, a Friedel–
Crafts acylation of electron-rich arenes with oxalyl
chloride and subsequent treatment with MeOH pro- Scheme 5. Synthesis of a-alkyl-a-keto esters.
Adv. Synth. Catal. 2011, 353, 3335 – 3339
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Se Hee Kim et al.
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Experimental Section
Typical Experimental Procedure for CuI-Mediated
Aerobic Oxidation of Ethyl 2-Cyano-5-oxo-3,5-di-
phenylpentanoate (1a)
A mixture of 1a (321 mg, 1.0 mmol) and CuI (210 mg,
1.1 mmol) in CH3CN (3.0 mL) was stirred at 258C for 3 h
under an O2 balloon atmosphere. Then the mixture was fil-
tered through a pad of Celite and washed with CH2Cl2. The
organic solvents were removed by evaporation, and the resi-
due was purified by flash column chromatography (hexanes/
EtOAc, 10:1) to afford ethyl 2,5-dioxo-3,5-diphenylpent-
AHCTUNGTRENNUNG
;
CDCl3): d=1.29 (t, J=7.2 Hz, 3H), 3.43 (dd, J=18.3 and
3.9 Hz, 1H), 4.03 (dd, J=18.3 and 10.5 Hz, 1H), 4.28 (q, J=
7.2 Hz, 2H), 5.15 (dd, J=10.5 and 3.6 Hz, 1H), 7.26–7.60
(m, 8H), 7.93–7.98 (m, 2H); 13C NMR (75 MHz, CDCl3):
d=13.89, 43.12, 48.42, 62.49, 127.95, 128.16, 128.63, 128.87,
129.13, 133.49, 135.35, 135.93, 160.34, 192.17, 197.37; ESI-
MS: m/z=333 [M+Na]+; anal. calcd. for C19H18O4: C 73.53,
H 5.85; found: C 73.67, H 5.98.
Scheme 6. Plausible reaction mechanism.
uct (69%) instead of the desired product. In addition,
the reaction of 1w also failed and produced apprecia-
ble amounts of benzaldehyde. The results of 1v and
1w might be ascribed to the preferential oxidation at
the benzylic position rather than the a-position of a
cyano group.
Acknowledgements
A plausible reaction mechanism of Cu-mediated
aerobic oxidation of 1 was proposed tentatively, as
shown in Scheme 6. The key radical intermediate I
was generated by hydrogen atom abstraction from 1
with molecular oxygen, meanwhile, Cu(I) was oxi-
dized to Cu(II) and oxygen was transformed to hydro-
peroxide anion.[10] Subsequently, the intermediate I
reacted with the hydroperoxide anion to form the hy-
droperoxide II. In addition, Cu(II) was reduced to
Cu(I) which could enter to the next catalytic cycle
(vide infra). As reported in a similar case, an elimina-
tion of isocyanic acid (HNCO) from hydroperoxide II
via a dioxetane intermediate III afforded the a-keto
ester 2.[11] In one part, the hydroperoxide II was re-
duced to a cyanohydrin 3 and eventually to 2. As pro-
posed in the mechanism, a catalytic amount of CuI
could catalyze the reaction; however, CuI might be
destroyed to a certain unidentified copper salt by the
reaction with liberated HNCO.[12] Thus, an equimolar
amount of CuI was required for the efficient conver-
sion of 1 to 2. However, further studies must be per-
formed for a detailed reaction mechanism in order to
explain the results in a catalytic version (conditions
B).
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technolo-
gy (2010-0015675). Spectroscopic data were obtained from
the Korea Basic Science Institute, Gwangju branch.
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