Free radical reaction between 2-benzoyl-1,4-benzoquinones and 1,3-dicarbonyl compounds

Article abstract of DOI:10.1039/b907369h

A manganese(III)-mediated reaction between 2-benzoyl-1,4-benzoquinones and 1,3-dicarbonyl compounds that produces benzo[c]furan-4,7-diones and anthracene-1,4-diones with high chemoselectivity is described. With ethyl butyrylacetate, by changing the solven

Full text of DOI:10.1039/b907369h

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Free radical reaction between 2-benzoyl-1,4-benzoquinones and 1,3-dicarbonyl
compounds†
Kuang-Po Chen, Hen-Qun Lee, Yu-Chih Cheng and Che-Ping Chuang*
Received 14th April 2009, Accepted 6th July 2009
First published as an Advance Article on the web 5th August 2009
DOI: 10.1039/b907369h
A manganese(III)-mediated reaction between 2-benzoyl-1,4-benzoquinones and 1,3-dicarbonyl
compounds that produces benzo[c]furan-4,7-diones and anthracene-1,4-diones with high
chemoselectivity is described. With ethyl butyrylacetate, by changing the solvent,
benzo[c]furan-4,7-diones and anthracene-1,4-diones can be generated in high chemoselectivities. With
ethyl benzoylacetate, N,N-dimethyl acetoacetamide and 1,3-diones, benzo[c]furan-4,7-diones were
produced effectively with high selectivity. With 2-alkyl-5-benzoyl-1,4-benzoquinones, the
regioselectivity of this reaction was also studied and the corresponding benzo[c]furan-4,7-dione and
anthracene-1,4-dione derivatives were obtained in high regioselectivity.
addition to the expected 6-hydroxy-naphthacene-5,12-diones 6b,
the novel naphtho[2,3-c]furan-4,9-diones 5 were also produced
as the major products (Scheme 1) This is presumably due to the
electron deficiency of the benzoyl group on 4b which disfavours
Introduction
Free radical reactions have received considerable attention during
the last two decades and there is now a wealth of new radical
reactions designed for organic synthesis.1,2 The oxidative addition
the intramolecular cyclization of electrophilic radical intermediate
onto the C–C double bond of the benzoyl group.5g These
naphthacene-5,12-diones and naphtho[2,3-c]furan-4,9-diones can
also be generated directly from the intermolecular oxidative free
radical reaction of 2-benzyl-1,4-naphthoquinones and 2-benzoyl-
1,4-naphthoquinones with 1,3-dicarbonyl compounds.5d,h This
report describes our results on manganese(III) acetate mediated re-
actions between 2-benzoyl-1,4-benzoquinones and 1,3-dicarbonyl
compounds.
of carbon-centered radicals to alkenes mediated by metal salts has
received considerable attention in organic synthesis for the con-
struction of carbon–carbon bonds. Among these, manganese(III)
acetate and cerium(IV) ammonium nitrate have been used most
efficiently,2–5 and the free radical reaction of 1,4-quinones has been
reported.5,6
Compounds containing the quinone group represent an im-
portant class of biologically active molecules that are widespread
in nature7 such as viocristin (1),8a XR651 (2)8b and bhi-
mamycin B (3)8c (Fig. 1). Previously, we found that the man-
ganese(III) acetate mediated oxidative free radical reaction of
2-benzyl-(3-ethoxycarbonylmethyl)-1,4-naphthoquinones 4a pro-
duced naphthacene-5,12-diones 6a effectively. In contrast, with
2-benzoyl-(3-ethoxycarbonylmethyl)-1,4-naphthoquinones 4b, in
Scheme 1 Intramolecular free radical reaction of 1,4-naphthoquinones.
Fig. 1
Results and discussion
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan,
70101, Republic of China. E-mail: cpchuang@mail.ncku.edu.tw; Fax: +886-
6-2740552
† Electronic supplementary information (ESI) available: Experimental
procedures, compound characterization and copies of NMR spectra. See
DOI: 10.1039/b907369h
We began our studies of this manganese(III)-mediated re-
action with 2-benzoyl-5,6-dimethyl-1,4-benzoquinones 7 and
ethyl butyrylacetate (8a, R3
=
nPrCO) (Table 1). When 5-(4-
methylbenzoyl)-2,3-dimethyl-1,4-benzoquinone (7a, R1 = H, R2 =
Me) was treated with 8a and manganese(III) acetate in acetic
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Table 1 Reaction with ethyl butyroylacetate (8a)a
radical 13 onto the C–C double bond of benzoyl group (path
a). Interestingly, reaction of 7 bearing an additional electron-
withdrawing halogen group gave the corresponding products 9
and 10 in excellent 9/10 ratio (entries 3 and 4). These results can be
rationalized by considering that the electron deficiency of radical
intermediate 7 makes the rate of six-membered-ring cyclization to
the benzene ring bearing an electron-withdrawing halogen group
(path a) much slower, meaning that the competitive oxidation of 7
(path b) becomes the major route.
Solvent effects play an important role in the manganese(III) ac-
etate mediated oxidative free radical reaction.11 Reaction between
7a and 8a was next conducted in neutral solvents. The change of
solvent to acetonitrile, chloroform and benzene gave 9a and 10a
with a much higher 9a/10a ratio than that performed in acetic
acid (entries 7, 12 and 16), and gave best results when acetonitrile
and chloroform were used. Analogous results were obtained with
other 5-benzoyl-2,3-dimethyl-1,4-benzoquinones 7, and are also
summarized in Table 1 (entries 7–16). In all cases, benzo[c]furan-
4,7-diones 9 were dominant. These results can be accounted for by
the steric effect of the benzoyl group; 13-B is the minor conformer
in neutral solvents, and therefore the cyclization of 13 (path a) is
slower and the oxidation of 13 (path b) becomes the major route.
This reaction was next performed in formic acid. Treatment of
7a and 8a with manganese(III) acetate in formic acid at 70 ◦C
for 30 min resulted in the formation of 10a exclusively in 25%
yield (entry 17). This selective formation of 10a, in contrast to
those performed in neutral solvent, is presumably due to the
coordination of proton with the carbonyl groups, leading to 14a-B
being the major conformer. The cyclization of radical 13a (path
a) via 14a-B becomes the major route. It may also result from
the coordination of proton with R3 and R4, which decreases the
electron density and retards the oxidation of radical 13a (path b).
Considering the electron-withdrawing effect of the benzoyl
group, it is likely that 12a is produced via the nucleophilic
addition of 8a to the C–C double bond of 7a followed by
manganese(III) acetate oxidation. To test this hypothesis, we
next performed the reaction between 5-(4-methylbenzoyl)-2,3-
dimethyl-1,4-benzoquinone (7a) and ethyl butyrylacetate (8a) in
the absence of manganese(III) acetate (Scheme 3). Treatment
of 7a and 8a with sodium acetate in acetonitrile at 70 ◦C
gave 18a, which was converted to 19a during purification as a
stereoisomeric mixture in 82% yield. This nucleophilic adduct 19a
was then oxidized by manganese(III) acetate in acetonitrile at
room temperature to give 12a in 72% yield.
Reaction
Entry Quinone
Solvent time (h) Product (yield (%))
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
7a: R1 = H, R2 = Me HOAc
0.5
0.5
0.5
0.5
0.5
0.5
3
3
3
3
3
9a (36), 10a (29)
9b (35), 10b (29)
9c (41), 10c (4)
9d (45), 10d (5)
9e (56), 10e (10)
9f (53), 10f (15)
9a (50), 10a (5)
9b (41), 10b (6)
9c (28), 10c (0)
9e (54), 10e (8)
9f (59), 10f (5)
9a (50), 10a (0)
9b (45), 10b (0)
9e (62), 10e (trace)
9f (64), 10f (0)
9a (26), 10a (0)
9a (0), 10a (25)
9a (0), 10a (22)
7b: R1 = H, R2 = H
7c: R1 = H, R2 = Cl
7d: R1 = H, R2 = Br
HOAc
HOAc
HOAc
7e: R1 = Me, R2 = H HOAc
7f: R1 = Me, R2 = Me HOAc
7a: R1 = H, R2 = Me CH3CN
7b: R1 = H, R2 = H
7c: R1 = H, R2 = Cl
CH3CN
CH3CN
7e: R1 = Me, R2 = H CH3CN
7f: R1 = Me, R2 = Me CH3CN
7a: R1 = H, R2 = Me CHCl3
4
4
4
4
7b: R1 = H, R2 = H
CHCl3
7e R1 = Me, R2 = H: CHCl3
7f: R1 = Me, R2 = Me CHCl3
7a: R1 = H, R2 = Me C6H6
4
7a: R1 = H, R2 = Me HCO2H 0.5
7b: R1 = H, R2 = H
HCO2H 0.5
a All reactions were carried with 7 (0.60 mmol), 8 (2.47 mmol) and
manganese(III) acetate (2.50 mmol) at 70 ◦C.
acid at 70 ◦C, benzo[c]furan-4,7-dione 9a and anthracene-1,4-
dione 10a were obtained in 36% and 29% yields, respectively
(Table 1, entry 1). Although the mechanistic details of this reaction
are unclear, 9a and 10a may be formed via the reaction route
presented in Scheme 2. Manganese(III) acetate oxidation of 8a
produces radical 11a. This radical intermediate 11a undergoes
intermolecular addition to the quinone ring followed by oxidation
to give 12a,9 which is then oxidized by manganese(III) acetate
to generate 13a. Radical 13a undergoes either (path a) a six-
membered-ring free radical cyclization via 14a-B and subsequent
aromatization to give 15a, which undergoes a further retro-Claisen
condensation to produce 10a,10 or (path b) oxidation to give 16a.
This cationic intermediate 16a undergoes a five-membered-ring
cyclization followed by retro-Claisen condensation to generate 9a.
The generalities of this reaction were examined with other
5-benzoyl-2,3-dimethyl-1,4-benzoquinones 7 (see Table 1, entries
1–6). In all cases, benzo[c]furan-4,7-diones 9 and anthracene-1,4-
diones 10 were obtained in fair yields. In contrast to the reactions
of 2-benzyl-1,4-naphthoquinones with b-ketoesters, 9 is the major
product.5d This indicates that the electron deficiency of the benzoyl
group disfavours the intramolecular cyclization of electrophilic
With 12a and 19a in hand, the radical reaction of 12a and 19a
was next examined. When ◦12a was treated with manganese(III)
acetate in acetic acid at 70 C, 9a and 10a were obtained in 39%
and 30% yields, respectively (Table 2, entry 1). Reaction of 19a
with manganese(III) acetate in acetic acid at 70 ◦C also gave 9a and
10a in 43% and 32% yields, respectively (Table 2, entry 2). Other
examples with acetonitrile and formic acid as solvents are also
shown in Table 2. The results are similar to those in Table 1, except
that 10a was produced in a much better reaction yield in formic
acid, and in all cases, 9a and 10a were obtained with high selectivity
depending on the solvent used (entries 3–6). Based on these results,
we believe that 12a is formed mainly via the nucleophilic addition
of 8a to 7a followed by manganese(III) oxidation.
The formation of benzo[2,3-c]furan-4,7-diones 9 is interest-
ing. The corresponding isofuran derivatives have served as
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Scheme 2 Probable mechanism.
Scheme 3 Nucleophilic reaction between 7a and 8a.
quinodimethane synthetic analogues in Diels–Alder reaction and
are widely used in the preparation of complex molecules.12 To im-
prove the chemoselectivity for the formation of benzo[2,3-c]furan-
4,7-diones 9, this manganese(III)-mediated reaction of 5-benzoyl-
2,3-dimethyl-1,4-benzoquinones 7 with other b-ketoesters 8b-c
was next investigated. Reaction of 5-benzoyl-2,3-dimethyl-1,4-
reaction increases as the size of substituents (R3) increases. This
can be attributed to the steric effect exerted by R3 group – the
cyclization rate of 13 (path a) is retarded by the larger R3 group
and the oxidation of 13 (path b) becomes the major route. On
the basis of this finding, by choosing ethyl benzoylacetate (8c)
as the nucleophilic precursor, the generalities of this reaction
were also examined with a variety of 5-benzoyl-2,3-dimethyl-1,4-
benzoquinones 7. The results are summarized in Table 3 (entries
3–8). In all cases, 5-benzoyl-2,3-dimethyl-1,4-benzoquinone 7
was converted to the corresponding benzo[c]furan-4,7-dione 9
with high chemoselectivity. This reaction was also conducted
i
benzoquinone (7b) with ethyl isobutyrylacetate (8b, R3 = PrCO)
and manganese(III) acetate in acetic acid afforded 9b and 10b in
42% and 12% yields, respectively (Table 3, entry 2). The 9b/10b
ratio rose to 42/6 when ethyl benzoylacetate (8c, R3 = PhCO)
was employed (Table 3, entry 4). The chemoselectivity of this
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Table 2 Radical reaction of nucleophilic adduct (see right-hand side of
Scheme 3)
Table 4 Reaction with 1,3-diketonesa
Entry
Quinone
1,3-Diketone
Product (yield (%))
Nucleophilic
adduct
Reaction
time (h)
7a: R1 = H, R2 = Me
7b: R1 = H, R2 = H
7c: R1 = H, R2 = Cl
7d: R1 = H, R2 = Br
7e: R1 = Me, R2 = H
7f: R1 = Me, R2 = Me
7a: R1 = H, R2 = Me
7b: R1 = H, R2 = H
7c: R1 = H, R2 = Cl
7d: R1 = H, R2 = Br
7e: R1 = Me, R2 = H
7f: R1 = Me, R2 = Me
8e
8e
8e
8e
8e
8e
8f
8f
8f
8f
8f
8f
9k (36), 10k (7)
9l (31), 10l (5)
9m (35), 10m (0)
9n (39), 10n (0)
9o (42), 10o (0)
9p (43), 10p (0)
9q (81), 10q (0)
9r (81), 10r (0)
9s (54), 10s (0)
9t (55), 10t (0)
9u (74), 10u (0)
9v (79), 10v (0)
Entry
Solvent
Product (yield (%))
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
12a
19a
12a
19a
12a
19a
HOAc
HOAc
CH3CN
CH3CN
HCO2H
HCO2H
0.5
0.5
4
4
0.5
0.5
9a (39), 10a (30)a
9a (43), 10a (32)b
9a (45), 10a (0)a
9a (48), 10a (0)b
9a (6), 10a (67)a
9a (3), 10a (73)b
a These reactions were carried with 12a (0.43 mmol) and manganese(III)
acetate (1.08 mmol) at 70 ◦C. b These reactions were carried with 19a
(0.37 mmol) and manganese(III) acetate (1.81 mmol) at 70 ◦C.
10
11
12
a All reactions were carried with 7 (0.60◦ mmol), 8 (2.47 mmol) and
manganese(III) acetate (2.50 mmol) at 70 C in acetic acid (10 cm3) for
30 min.
Table 3 Reaction with b-ketoester and b-ketoamidea
b-Ketoester or
Results Quinone
b-ketoamide
Product (yield (%))
Table 5 Chemoselective formation of anthracene-1,4-dione 10
1
2
3
4
5
6
7
8
9
7a: R1 = H, R2 = Me
8b
8b
8c
8c
8c
8c
8c
9a (45), 10a (2)
9b (42), 10b (12)
9a (44), 10a (7)
9b (42), 10b (6)
9c (27), 10c (0)
9d (31), 10d (0)
9e (66), 10e (0)
9f (69), 10f (4)
9g (69), 10g (0)
9h (62), 10h (0)
9i (48), 10i (0)
9j (75), 10j (0)
7b: R1 = H, R2 = H
7a: R1 = H, R2 = Me
7b: R1 = H, R2 = H
7c: R1 = H, R2 = Cl
7d: R1 = H, R2 = Br
7e: R1 = Me, R2 = H
7f: R1 = Me, R2 = Me 8c
7a: R1 = H, R2 = Me
7b: R1 = H, R2 = H
7c: R1 = H, R2 = Cl
8d
8d
8d
10
11
12
7f: R1 = Me, R2 = Me 8d
a All reactions were carried with 7 (0.60◦ mmol), 8 (2.47 mmol) and
manganese(III) acetate (2.50 mmol) at 70 C in acetic acid (10 cm3) for
30 min.
with N,N-dimethyl acetoacetamide (8d). Reaction of 7a and 8d
with manganese(III) acetate under above conditions led to the
formation of 9g in 69% yield, and no 10g was found (Table 3,
entry 9). The results of the reaction between other 5-benzoyl-
2,3-dimethyl-1,4-benzoquinones 7 and 8d are also summarized
in Table 3 (entries 10–12). In all cases, 9 is the only product.
Again, this high chemoselectivity can be ascribed to the steric effect
exerted by the larger R4 ( = CONMe2) group of 8d (compared to
that of 8a, R4 = CO2Et), resulting in the oxidation of 13 being the
major route (path b).
Next, we investigated this manganese(III)-mediated reaction
with 1,3-diketones 8e and 8f. When 7a was treated with pen-
tandione (8e) and manganese(III) acetate in acetic acid, 9k and
10k were obtained in 36% and 7% yields, respectively (Table 4,
entry 1). The scope of this reaction is shown in Table 4 (entries
1–6). In all cases, 5-benzoyl-2,3-dimethyl-1,4-benzoquinone 7
was converted to the corresponding isofuran product 9 with
high chemoselectivity. It can be rationalized that the radical
intermediate 13e bearing a stronger electron-withdrawing R4 ( =
COMe) group is more electron-deficient and this makes the
intramolecular cyclization of 13e to the benzoyl group (path a)
slower than that of 13a (R4 = CO2Et). With dibenzoylmethane
(8f), bearing a larger benzoyl group, isofuran products 9 were
obtained exclusively (Table 4, entries 7–12).
Nucleophilic adduct
(yield (%), ratio)
Entry Quinone
Product (yield (%))
1
2
3
4
5
6
7a: R1 = H, R2 = Me 19a (82, 11:1)a,b
7b: R1 = H, R2 = H 19b (82, 11:1)a,b
7f: R1 = H, R2 = Me 19c (82, 11:1)a,b
9a (3), 10a (73)c
9b (5), 10b (75)c
9f (trace), 10f (75)c
9a (0), 10a (58)d
9b (trace), 10b (52)d
9f (trace), 10f (55)d
7a: R1 = H, R2 = Me
7b: R1 = H, R2 = H
7f: R1 = H, R2 = Me
—
—
—
a These reactions were carried with 7 (0.69 mmol), 8a (1.40 mmol) and
sodium acetate (2.09 mmol) in acetonitrile (10 cm3) at 70 ◦C for 1 h.
b The stereoisomeric ratio was determined by NMR integration. c These
reactions were carried with 19 (0.37 mmol) and manganese(III) acetate
(1.81 mmol) in formic acid (10 cm3) at 70 ◦C for 30 min. d These one-
pot reactions were carried by heating 7 (0.59 mmol), 8 (1.18 mmol) and
sodium acetate (1.17 mmol) in acetonitrile (5 cm3) at 70 ◦C for 1 h, and
then manganese(III) acetate (2.95 mmol) and formic acid (15 cm3) were
added. This reaction mixture was then heated at 70 ◦C for 30 min.
tivity (entry 6). Compounds containing the anthracene-1,4-dione
subunit are an important class of biologically active molecules that
are widespread in nature.7,8a,b We therefore continued to study this
manganese(III)-mediated reaction with other nucleophilic adducts
19 in formic acid, and the results are listed in Table 5 (entries 1–3).
In all cases, anthracene-1,4-diones 10 were obtained selectively
from nucleophilic adducts 19. In attempt to enhance the efficiency
of this reaction, we investigated the development of a one-pot
process, in which 19 was generated in situ from 7 and 8. Reaction
As shown in Table 2, with formic acid as solvent, anthracene-
1,4-dione 10a was produced from the radical reaction between 19a
and manganese(III) acetate in 73% yield with high chemoselec-
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Table 6 Regioselectivity of 2-alkyl-5-benzoyl-1,4-benzoquinones 20a
Entry
Quinone
b-Ketoester or b-ketoamide
Product (yield(%))
1
2
3
4
5
6
7
8
9
20a: R = Me, R1 = H, R2 = Me
20b: R = Me, R1 = H, R2 = H
20c: R = Me, R1 = H, R2 = Cl
20d: R = Et, R1 = H, R2 = Me
8a
8a
8a
8a
8a
8a
8d
8d
8d
8d
8d
8d
21a (34), 22a (20)
21b (36), 22b (22)
21c (56), 22c (4)
21d (37), 22d (28)
21e (37), 22e (24)
21f (37), 22f (26)
21g (54), 22g (0)
21h (47), 22h (0)
21i (43), 22i (0)
21j (64), 22j (0)
21k (62), 22k (0)
21l (66), 22l (0)
t-
20e: R = Bu, R1 = H, R2 = Me
20f: R = Bn, R1 = H, R2 = Me
20a: R = Me, R1 = H, R2 = Me
20b: R = Me, R1 = H, R2 = H
20c: R = Me, R1 = H, R2 = Cl
20d: R = Et, R1 = H, R2 = Me
10
11
12
t-
20e: R = Bu, R1 = H, R2 = Me
20f: R = Bn, R1 = H, R2 = Me
a All reactions were carried with 20 (0.64 mmol), 8 (2.61 mmol) and manganese(III) acetate (2.66 mmol) in acetic acid (10 cm3) at 70 ◦C for 30 min.
◦
of 7a and 8a with sodium acetate in acetonitrile at 70 C for 1 h
was followed by the addition of manganese(III) acetate and formic
acid without the isolation of 19a. This reaction mixture was heated
further at 70 ◦C for 30 min to give 10a in 58% yield (entry 4). Other
examples are also shown in Table 5 (entries 5 and 6). In all cases,
5-benzoyl-2,3-dimethyl-1,4-benzoquinone 7 was converted to the
corresponding anthracene-1,4-dione 10 effectively by this one-pot
process.
cyclization reactions. This reaction provides a method for the
synthesis of benzo[c]furan-4,7-diones and anthracene-1,4-diones
from readily available 2-benzoyl-1,4-benzoquinones and 1,3-
dicarbonyl compounds. With ethyl butyrylacetate, by changing
the solvent, benzo[c]furan-4,7-diones 9 and anthracene-1,4-diones
10 can be generated in high chemoselectivities. With ethyl
benzoylacetate, N,N-dimethyl acetoacetamide and 1,3-diones,
benzo[c]furan-4,7-dione 9 was produced effectively in high se-
lectivity. We also studied this reaction with 2-alkyl-5-benzoyl-
1,4-benzoquinones 20, and found that benzo[c]furan-4,7-diones
21 and anthracene-1,4-diones 22 derived from the nucleophilic
addition of 8 to C6 of 20 were obtained in high regioselectivity.
We then continued to examine the regioselectivity of
this oxidative free radical reaction with 2-alkyl-5-benzoyl-1,4-
benzoquinones (20). We first tried the reaction between 2-methyl-
5-(4-methylbenzoyl)-1,4-benzoquinone (20a, R = Me) and ethyl
butyrylacetate (8a, R3 = PrCO) (Table 6). When 20a was treated
n
with 8a and manganese(III) acetate in acetic acid at 70 ◦C,
benzo[c]furan-4,7-dione 21a (34%) and anthracene-1,4-dione 22a
(20%) were obtained (Table 6, entry 1). No product derived
from the addition of 8a to the C3 of 20a was found. This is
presumably due to the rate of nucleophilic addition of 8a to
the C–C double bond of the quinone ring (bearing an electron-
withdrawing benzoyl group) being much faster. Analogous results
wereobtainedwith other 2-alkyl-5-benzoyl-1,4-benzoquinones 20,
and are listed in Table 6 (entries 1–6). In all cases, 21 and 22
were obtained regioselectively. N,N-Dimethyl acetoacetamide (8d)
behaved similarly, giving only 21 derived from the addition of 8d
to the C6 of 20 (entries 7–12).
Experimental
General considerations
Melting points are uncorrected. Infrared spectra were taken with
1
a Hitachi 260-30 spectrometer. H and 13C NMR spectra were
recorded on a Bruker AVANCE-300 or AMX-400 spectrometer.
Chemical shifts are reported in ppm relative to TMS as internal
reference. Elemental analyses were performed with Heraeus CHN-
Rapid Analyzer. Mass spectra were recorded on a Jeol JMS-
SX 102A mass spectrometer. Analytical thin-layer chromatog-
raphy was performed with precoated silica gel 60 F-254 plates
(0.25 mm thick) from EM Laboratories and visualized by UV.
The reaction mixture was purified by column chromatography
over EM Laboratories silica gel (70–230 mesh). The starting
2-benzoyl-1,4-benzoquinones 7 and 20 were synthesized by the
CAN oxidative demethylation13 of corresponding 2-benzoyl-1,4-
dimethoxybenzenes.14
Conclusions
1,4-Benzoquinone 12, generated via the nucleophilic addition
of 1,3-dicarbonyl compounds 8 to the C–C double bond of
the quinone ring, undergoes efficient manganese(III)-mediated
4078 | Org. Biomol. Chem., 2009, 7, 4074–4081
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Typical experimental procedure for the manganese(III)-mediated
reaction of 2-benzoyl-1,4-benzoquinones with b-ketoesters,
b-ketoamide and 1,3-diketones
d, J 7.6, ArH), 7.49 (2 H, d, J 7.6, ArH) and 10.42 (1 H, s, OH);
dC(75.4 MHz; CDCl3; Me4Si) 11.7 (q), 13.1 (q), 13.6 (q), 14.1 (q),
16.6 (t), 21.7 (q), 40.9 (t), 57.2 (d), 61.5 (t), 109.4 (s), 115.6 (s),
118.0 (s), 127.1 (s), 127.8 (s), 129.12 (2 ¥ d), 129.20 (2 ¥ d), 136.5
(s), 143.3 (s), 149.8 (s), 153.8 (s), 169.8 (s) and 199.6 (s); m/z (EI)
412.1896 (M+. C24H28O6 requires 412.1885), 394 (13%), 348 (19),
324 (35), 296 (100) and 283 (39).
A
mixture of 5-benzoyl-2,3-dimethyl-1,4-benzoquinone (7b)
(143 mg, 0.60 mmol), ethylbutyrylacetate (8a)(391mg, 2.47mmol)
and manganese(III) acetate (671 mg, 2.50 mmol) in acetic acid
(10 cm3) was heated at 70 ◦C for 30 min. The reaction mixture
was diluted with ethyl acetate (100 cm3), washed with saturated
aqueous sodium bisulfite (50 cm3), water (3 ¥ 50 cm3), aqueous
saturated sodium bicarbonate (3 ¥ 50 cm3), dried (Na2SO4),
and concentrated in vacuo. The crude product was purified by
column chromatography over silica gel (20 g) (eluting with 2:1
dichloromethane–hexane) followed by recrystallization (CHCl3–
hexane) to give 9b (68 mg, 35%) and 10b (56 mg, 29%).
Manganese(III)-mediated oxidation reaction of nucleophilic
adduct 19a
A mixture of 19a (150 mg, 0.36 mmol) and manganese(III) acetate
(195 mg, 0.73 mmol) in acetonitrile (10 cm3) was stirred at RT for
3 h. The reaction mixture was diluted with ethyl acetate (100 cm3),
washed with saturated aqueous sodium bisulfite (50 cm3), water
(3 ¥ 50 cm3), dried (Na2SO4), and concentrated in vacuo. The
crude product was purified by column chromatography over silica
gel (20 g) (eluting with 1:15 ethyl acetate–hexane) to give 12a
(108 mg, 72%).
4,7-Dihydro-5,6-dimethyl-4,7-dioxo-3-phenybenzo[c]furan-1-
carboxylic acid ethyl ester 9b. Yellow crystals; mp 161–162 ◦C
(from CHCl3–hexane); (Found: C, 70.27; H, 4.93. C19H16O5
requires C, 70.36; H, 4.97%); nmax(CHCl3)/cm-1 3015, 1730, 1665,
1400, 1375 and 1280; dH(400 MHz; CDCl3; Me4Si) 1.48 (3 H, t, J
7.1, CH3), 2.17 (6 H, s, 2 ¥ CH3), 4.51 (2 H, q, J 7.1, OCH2), 7.49–
7.56 (3 H, m, ArH) and 8.39–8.46 (2 H, m, ArH); dC(75.4 MHz;
CDCl3; Me4Si) 13.18 (q), 13.25 (q), 14.2 (q), 62.3 (t), 117.3 (s),
124.9 (s), 127.6 (s), 128.52 (2 ¥ d), 128.55 (2 ¥ d), 131.7 (d), 141.8
(s), 145.3 (s), 145.5 (s), 157.4 (s), 157.5 (s), 179.4 (s) and 180.3 (s).
2-[1,4-Dihydro-3-(4-methylbenzoyl)-1,4-dioxo-naphthalen-2-yl]-
3-oxo-hexanoic acid ethyl ester 12a. Yellow oil; nmax(KBr)/cm-1
3270, 1650, 1605, 1455, 1285, 825; dH(300 MHz; CDCl3; Me4Si)
0.86 (3 H, t, J 7.3, CH3), 1.19 (3 H, t, J 7.1, CH3), 1.42–1.57 (2
H, m, CH2), 2.00–2.21 (2 H, m, CH2), 2.09 (3 H, s, CH3), 2.13 (3
H, s, CH3), 2.39 (3 H, s, CH3), 3.93–4.22 (2 H, m, OCH2), 7.19 (2
H, d, J 8.0, ArH), 7.70 (2 H, d, J 8.0, ArH), 12.96 (1 H, s, OH);
dC(75.4 MHz; CDCl3; Me4Si) 12.2 (q), 12.7 (q), 13.7 (q), 14.1 (q),
19.1 (t), 21.7 (q), 35.4 (t), 60.8 (t), 94.1 (s), 129.1 (2 ¥ d), 129.2 (2 ¥
d), 133.0 (s), 137.6 (s), 140.9 (s), 141.7 (s), 143.4 (s), 145.3 (s), 170.3
(s), 178.9 (s), 185.8 (s), 185.9 (s), 191.6 (s); m/z (EI) 410.1734 (M+.
C24H26O6 requires 410.1739), 393 (58%), 347 (66) and 321 (100).
1,4-Dihydro-9-hydroxy-2,3-dimethyl-1,4-dioxoanthracene-10-
carboxylic acid ethyl ester 10b. Red crystals; mp 169–170 ◦C
(from CHCl3–hexane); (Found: C, 70.26; H, 4.96. C19H16O5
requires C, 70.36; H, 4.97%); nmax(CHCl3)/cm-1 2990, 1660, 1605,
1300 and 1270; dH(300 MHz; CDCl3; Me4Si) 1.47 (3 H, t, J 7.2,
CH3), 2.20 (3 H, s, CH3), 2.22 (3 H, s, CH3), 4.63 (2 H, q, J 7.2,
OCH2), 7.66–7.80 (2 H, m, ArH), 7.83 (1 H, d, J 8.0, ArH), 8.52
(1 H, d, J 8.0, ArH) and 14.35 (1 H, s, OH); dC(75.4 MHz; CDCl3;
Me4Si) 12.5 (q), 13.3 (q), 14.1 (q), 62.1 (t), 107.6 (s), 123.9 (s),
124.9 (d), 126.9 (s), 127.0 (d), 127.3 (s), 129.1 (d), 131.7 (d), 132.8
(s), 144.4 (s), 146.1 (s), 162.6 (s), 169.1 (s), 183.1 (s) and 189.1 (s).
Typical experimental procedure for the manganese(III)-mediated
radical reaction of 12a
A mixture of 12a (178 mg, 0.43 mmol) and manganese(III)
acetate (290 mg, 1.08 mmol) in acetic acid (10 cm3) was heated
at 70 ◦C for 30 min. The reaction mixture was diluted with ethyl
acetate (100 cm3), washed with saturated aqueous sodium bisulfite
(50 cm3), water (3 ¥ 50 cm3), aqueous saturated sodium bicarbon-
ate (3 ¥ 50 cm3), dried (Na2SO4), and concentrated in vacuo. The
crude product was purified by column chromatography over silica
gel (20 g) (eluting with 2:1 dichloromethane–hexane) followed by
recrystallization (CHCl3–hexane) to give 9a (57 mg, 39%) and 10a
(44 mg, 30%).
Typical experimental procedure for the nucleophilic reaction
between 2-benzoyl-1,4-benzoquinones and ethyl butyrylacetate
A
solution of 5-(4-methylbenzoyl)-2,3-dimethyl-1,4-benzo-
quinone (7a) (174 mg, 0.69 mmol), ethyl butyrylacetate (8a)
(221 mg, 1.40 mmol) and sodium acetate (171 mg, 2.09 mmol) in
acetonitrile (10 cm3) was heated at 70 ◦C for 1 h. The reaction
mixture was diluted with ethyl acetate (100 cm3), washed with
water (3 ¥ 50 cm3), dried (Na2SO4), and concentrated in vacuo.
The crude product was purified by column chromatography over
silica gel (20 g) (eluting with 1:15 ethyl acetate–hexane) to give 19a
(225 mg, 82%) as a mixture of isomeric products in 11:1 ratio. The
major isomer can be obtained in pure form by recrystallization
(ethyl acetate–hexane)
4,7-Dihydro-5,6-dimethyl-3-(4-methylphenyl)-4,7-dioxobenzo-
[c]furan-1-carboxylic acid ethyl ester 9a. Yellow crystals; mp 161–
162 ◦C (from CHCl3–hexane); (Found: C, 70.89; H, 5.42. C20H18O5
requires C, 70.99; H, 5.36%); nmax(CHCl3)/cm-1 3010, 1730, 1665,
1610 and 1280; dH(300 MHz; CDCl3; Me4Si) 1.47 (3 H, t, J 7.1,
CH3), 2.14 (6 H, s, 2 ¥ CH3), 2.42 (3 H, s, CH3), 4.50 (2 H, q, J 7.1,
OCH2), 7.30 (2 H, d, J 8.2, ArH) and 8.32 (2 H, d, J 8.2, ArH);
dC(75.4 MHz; CDCl3; Me4Si) 13.0 (q), 13.2 (q), 14.1 (q), 21.6 (q),
62.1 (t), 116.7 (s), 124.75 (s), 124.81 (s), 128.4 (2 ¥ d), 129.2 (2 ¥ d),
141.4 (s), 142.3 (s), 145.0 (s), 145.4 (s), 157.4 (s), 157.6 (s), 179.3
(s) and 180.1 (s).
(2S*,3R*)-2,3-Dihydro-2,5-dihydroxy-6,7-dimethyl-4-(4-me-
thylbenzoyl)-2-propylbenzo[b]furan-3-carboxylic acid ethyl ester
19a. Yellow crystals; mp 88–89 ◦C; nmax(KBr)/cm-1 3445, 2965,
1730, 1615 and 1335; dH(400 MHz; CDCl3; Me4Si) 1.00 (3 H, t,
J 7.5, CH3), 1.08 (3 H, t, J 7.1, CH3), 1.53 (2 H, sextet, J 7.5,
CH2), 1.85–1.95 (2 H, m, CH2), 2.22 (6 H, s, 2 ¥ CH3), 2.44 (3 H, s,
CH3), 3.73–3.97 (2 H, m, OCH2), 3.88 (1 H, s, CH), 7.25 (2 H,
1,4-Dihydro-9-hydroxy-2,3,6-trimethyl-1,4-dioxoanthracene-10-
carboxylic acid ethyl ester 10a. Red needles; mp 168–169 ◦C
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(from CHCl3–hexane); (Found: C, 70.91; H, 5.36. C20H18O5
requires C, 70.99; H, 5.36%); nmax(CHCl3)/cm-1 2925, 1720, 1655,
1500 and 1265; dH(300 MHz; CDCl3; Me4Si) 1.47 (3 H, t, J 7.1,
CH3), 2.17 (3 H, s, CH3), 2.19 (3 H, s, CH3), 2.54 (3 H, s, CH3), 4.63
(2 H, q, J 7.1, OCH2), 7.50 (1 H, d, J 8.4, ArH), 7.57 (1 H, s, ArH),
8.37 (1 H, d, J 8.4, ArH) and 14.31 (1 H, s, OH); dC(75.4 MHz;
CDCl3; Me4Si) 12.4 (q), 13.2 (q), 14.1 (q), 22.1 (q), 62.0 (t), 107.1
(s), 124.1 (s), 124.7 (d), 125.2 (s), 126.2 (d), 126.4 (s), 131.1 (d),
133.0 (s), 142.5 (s), 144.3 (s), 145.9 (s), 162.6 (s), 169.2 (s), 183.1
(s) and 188.8 (s).
d, J 1.4, CH3), 4.51 (2 H, q, J 7.1, OCH2), 6.78 (1 H, q, J 1.4,
CH), 7.32–7.59 (3 H, m, ArH) and 8.38–8.49 (2 H, m, ArH);
dC(75.4 MHz; CDCl3; Me4Si) 14.1 (q), 16.2 (q), 62.3 (t), 117.6 (s),
124.7 (s), 127.3 (s), 128.4 (2 ¥ d), 128.6 (2 ¥ d), 131.9 (d), 138.0 (d),
142.3 (s), 149.6 (s), 157.2 (s), 157.3 (s), 179.9 (s) and 180.2 (s).
1,4-Dihydro-9-hydroxy-3-methyl-1,4-dioxoanthracene-10-carbo-
xylic acid ethyl ester 22b. Red crystals; mp 143–144 ◦C (from
CHCl3–hexane); (Found: C, 69.65; H, 4.56. C18H14O5 requires C,
69.67; H, 4.55%); nmax(KBr)/cm-1 2990, 1730, 1645, 1600 and 1225;
dH(400 MHz; CDCl3; Me4Si) 1.47 (3 H, t, J 7.2, CH3), 2.21 (3 H,
d, J 1.4, CH3), 4.63 (2 H, q, J 7.2, OCH2), 6.91 (1 H, q, J 1.4, CH),
7.66–7.81 (2 H, m, ArH), 7.84 (1 H, d, J 7.8, ArH), 8.52 (1 H, d, J
7.8, ArH) and 14.13 (1 H, s, OH); dC(100.6 MHz; CDCl3; Me4Si)
14.1 (q), 16.8 (q), 62.2 (t), 107.9 (s), 123.6 (s), 124.9 (d), 127.1 (d),
127.3 (s), 127.4 (s), 129.3 (d), 131.8 (d), 132.7 (s), 136.4 (d), 150.6
(s), 162.5 (s), 168.8 (s), 183.8 (s) and 189.1 (s).
Typical experimental procedure for the manganese(III)-mediated
radical reaction of nucleophilic adduct 19
A mixture of 19a (151 mg, 0.37 mmol) and manganese(III) a◦cetate
(486 mg, 1.81 mmol) in acetonitrile (10 cm3) was heated at 70 C for
4 h. The reaction mixture was diluted with ethyl acetate (100 cm3),
washed with saturated aqueous sodium bisulfite (50 cm3), water
(3 ¥ 50 cm3), dried (Na2SO4), and concentrated in vacuo. The
crude product was purified by column chromatography over silica
gel (20 g) (eluting with 2:1 dichloromethane–hexane) followed by
recrystallization (CHCl3–hexane) to give 9a (60 mg, 48%).
Acknowledgements
We are grateful to the National Science Council of the ROC for
financial support (Grant No. NSC-96-2113-M-006-010-MY3).
Typical experimental procedure for the one-pot reaction
References
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A
solution of 5-(4-methylbenzoyl)-2,3-dimethyl-1,4-benzo-
quinone (7a) (150 mg, 0.59 mmol), ethyl butyrylacetate (8a)
(186 mg, 1.18 mmol) and sodium acetate (145 mg, 1.77 mmol)
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◦
70 C for another 30 min, the reaction mixture was diluted with
ethyl acetate (100 cm3), washed with saturated aqueous sodium
bisulfite (50 cm3), water (3 ¥ 50 cm3), aqueous saturated sodium
bicarbonate (3 ¥ 50 cm3), dried (Na2SO4), and concentrated in
vacuo. The crude product was purified by column chromatography
over silica gel (20 g) (eluting with 2:1 dichloromethane–hexane)
followed by recrystallization (CHCl3–hexane) to give 10a (115 mg,
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Typical experimental procedure for the manganese(III)-mediated
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A mixture of 2-benzoyl-5-methyl-1,4-benzoquinone (20b)(144 mg,
0.64 mmol), ethyl butyrylacetate (8a) (412 mg, 2.61 mmol)
and manganese(III) acetate (712 mg, 2.66 mmol) in acetic acid
(10 cm3) was heated at 70 ◦C for 30 min. The reaction mixture
was diluted with ethyl acetate (100 cm3), washed with saturated
aqueous sodium bisulfite (50 cm3), water (3 ¥ 50 cm3), aqueous
saturated sodium bicarbonate (3 ¥ 50 cm3), dried (Na2SO4),
and concentrated in vacuo. The crude product was purified by
column chromatography over silica gel (20 g) (eluting with 2:1
dichloromethane–hexane) followed by recrystallization (CHCl3–
hexane) to give 21b (70 mg, 36%) and 22b (44 mg, 22%).
4,7-Dihydro-6-methyl-4,7-dioxo-3-phenylbenzo[c]furan-1-carbo-
xylic acid ethyl ester 21b. Yellow crystals; mp 115–116 ◦C (from
CHCl3–hexane); (Found: C, 69.62; H, 4.56. C18H14O5 requires C,
69.67; H, 4.55%); nmax(KBr)/cm-1 2985, 1715, 1675, 1535 and 1215;
dH(400 MHz; CDCl3; Me4Si) 1.48 (3 H, t, J 7.1, CH3), 2.17 (3 H,
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4080 | Org. Biomol. Chem., 2009, 7, 4074–4081
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9 Considering the electron-withdrawing effect of benzoyl group, 12a may
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oxidation.
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5320.
This journal is
The Royal Society of Chemistry 2009
Org. Biomol. Chem., 2009, 7, 4074–4081 | 4081
©