Efficient and selective iron-mediated reductive Claisen rearrangement of propargyloxyanthraquinones to anthrafurandiones in ionic liquids

Article abstract of DOI:10.1139/cjc-2017-0328

An efficient and rapid method is described for the reductive Claisen rearrangement of different propargyloxyanthraquinones to anthra[1,2-b]furan-6,11-diones for first time using iron powder in a mixture of two ionic liquids, namely 1-methylimidazolium tetrafluoroborate [Hmim]BF₄ and 1-benzyl-3-methylimidazolium chloride [Bzmim]Cl. The present method is able to execute single or double Claisen rearrangements of 1,4-or 1,5-bispropargyloxyanthraquinones selectively, so that the desired anthra(mono)furandiones or anthra(bis)furandiones are produced, respectively, as the major product.

Full text of DOI:10.1139/cjc-2017-0328

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ARTICLE
Efficient and selective iron-mediated reductive Claisen
rearrangement of propargyloxyanthraquinones to
anthrafurandiones in ionic liquids
Samaneh Nadali, Ghasem Aghapour, and Zahra Rafieepour
Abstract: An efficient and rapid method is described for the reductive Claisen rearrangement of different propargyloxyanthraquinones
to anthra[1,2-b]furan-6,11-diones for first time using iron powder in a mixture of two ionic liquids, namely 1-methylimidazolium tetra-
fluoroborate [Hmim]BF4 and 1-benzyl-3-methylimidazolium chloride [Bzmim]Cl. The present method is able to execute single or double
Claisen rearrangements of 1,4- or 1,5-bispropargyloxyanthraquinones selectively, so that the desired anthra(mono)furandiones
or anthra(bis)furandiones are produced, respectively, as the major product.
Key words: propargyloxyanthraquinone, Claisen rearrangement, anthrafurandione, iron, ionic liquid.
Résumé : Le présent article est le premier a` décrire une méthode efficace et rapide pour effectuer le réarrangement de Claisen
de différentes propargyloxyanthraquinones en anthra[1,2-b]furan-6,11-diones par voie réductrice en présence de poudre de fer
dans un mélange de liquides ioniques, a` savoir le tétrafluoroborate de 1-méthylimidazolium ([Hmim]BF4) et le chlorure de
1-benzyl-3-méthylimidazolium ([Bzmim]Cl). Cette méthode permet d’exécuter sélectivement un réarrangement de Claisen
unique ou double de 1,4-bispropargyloxyanthraquinones ou de 1,5-bispropargyloxyanthraquinones pour produire respective-
ment les anthra(mono)furandiones ou anthra(bis)furandiones désirées comme produit majoritaire. [Traduit par la Rédaction]
Mots-clés : propargyloxyanthraquinone, réarrangement de Claisen, anthrafurandione, fer, liquide ionique.
propargyloxyanthraquinones producing anthrafurandiones using
iron powder in a mixture of two ionic liquids 1-methylimidazolium
tetrafluoroborate [Hmim]BF4 and 1-benzyl-3-methylimidazolium
chloride [Bzmim]Cl in short reaction times (Scheme 1). As far as we
know, this is the first method for reductive Claisen rearrangement of
propargyloxyanthraquinones.
Introduction
Anthraquinone derivatives are considered an important group
in organic chemistry due to their applications in various fields
such as preparation of photochemical and colorimetric sensor
systems1 and the dye industry (e.g., textile dyestuff),2 as well as
their anti-inflammatory,3 anticancer,4–6 antioxidant,7 antimicro-
bial, and antiviral8 properties. Moreover, some anthraquinone
derivatives have been extracted from slimming tea,9 and some
others are found in some other plants such as lichens and fungi
and even in some insects, forming their colour as natural pigments.
However, a narrow choice of reactions is considered for the function-
alization of anthraquinones due to inertness of their skeleton to a
variety of electrophilic substitution reactions. In this connection, re-
ductive Claisen rearrangement of allyloxyanthraquinones can be used
as a known and progressive way for this purpose and introduction of
a carbon–carbon new bond onto the structure of anthraquinones.10
This [3,3] sigmatropic rearrangement has been carried out using
various reductants10 such as dithionate11 and glucose.12 However,
these reported methods suffer from some disadvantages such as
long reaction times, reduction or isomerization of double bond in
the rearranged allyl group, and low yields. Also, the use of sodium
metabisulfite as a reagent has been reported for performing this
rearrangement.13
16
Experimental
Solvents, reagents, and chemicals were obtained from Merck,
Fluka, or Aldrich chemical companies. Some substrates and prod-
ucts are known compounds10,17,18 and were characterized by com-
parison of their physical or spectral data with authentic samples.
Fourier transform infrared (FTIR) spectra were recorded on
a Perkin-Elmer RXI spectrophotometer. Nuclear magnetic reso-
nance (NMR) spectra were recorded on Brucker Avance 300, 400,
and 500 spectrometers. UV–vis spectra were obtained with an
Analytik Jena SPECORD 205 spectrometer. Also, elemental analy-
ses were performed using an Elementar vario EL III analyzer.
Melting points were determined in open capillary tubes in an
Electrothermal 9100 melting point apparatus. Thin layer chroma-
tography (TLC) was carried out on silica gel 254 analytical sheets
obtained from Fluka.
Typical procedure for the conversion of 1-hydroxyanthraquinone
(3) to 1-(prop-2=-ynyloxy)anthraquinone (1)
In continuation of our previous work on the reductive Claisen
rearrangement of allyloxyanthraquinones with silver/potassium
iodide as a reagent in acetic acid,14 and also with respect to im-
portance of ionic liquids and their applications in organic synthe-
sis as environmentally friendly solvents,15 we report an efficient
and selective method for the reductive Claisen rearrangement of
Dry potassium carbonate (4.5 mmol, 0.62 g) was added to a flask
containing a solution of 3 (2.23 mmol, 0.50 g) in DMF (30 mL). The
reaction mixture was stirred at 90 °C for 15 min. Then, propargyl
bromide (4.5 mmol, 0.48 mL) was added and the stirring was con-
tinued about 30 min. Propargyl bromide (2 mmol, 0.21 mL) was
Received 29 May 2017. Accepted 5 July 2017.
S. Nadali, G. Aghapour, and Z. Rafieepour. School of Chemistry, Damghan University, Damghan, 36715-364, Iran.
Corresponding author: Ghasem Aghapour (email: Gh_Aghapour@du.ac.ir).
Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.
Can. J. Chem. 00: 1–7 (0000) dx.doi.org/10.1139/cjc-2017-0328
Published at www.nrcresearchpress.com/cjc on 8 August 2017.

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Can. J. Chem. Vol. 00, 0000
Scheme 1. Iron-mediated reductive Claisen rearrangement of 1-(prop-2=-ynyloxy)anthraquinone (1) to 2-methyl-6,11-dihydroanthra[1,2-b]furan-
6,11-dione (2) in a mixed ionic liquids [Hmim]BF4 and [Bzmim]Cl.
added again and the stirring was continued until TLC showed the
completion of the reaction after 15 min. The reaction mixture was
filtered and the residue was washed with hot DMF (10 mL). The
product was precipitated after addition of distilled water (30 mL)
to the filtrate and then filtered. 1 was obtained after washing of
precipitate with water (2 × 20 mL) and crystallization from ace-
tone as yellow needles in 95% yield (0.55 g); mp = 218–220 °C (lit.18a
219–220 °C).
1.01 Hz), 7.62–7.68 (m, 2H), 7.82–7.85 (dd, 1H, J = 7.69, 1.02 Hz),
7.89–7.92 (dd, 1H, J = 7.67, 1.08 Hz) ppm; 13C NMR (CDCl3, 75 MHz):
␦ 56.50, 57.27, 77.16, 78.02, 118.02, 118.95, 120.26, 121.09, 123.82,
125.10, 133.55, 134.02, 134.71, 134.82, 157.09, 159.46, 182.56,
183.78 ppm; IR (KBr): 3265 (m), 3015 (w), 2921 (m), 2851 (w), 2132 (w),
1667 (s), 1588 (s), 1465 (w), 1442 (w), 1315 (s), 1282 (s), 1240 (s), 965 (s),
742 (m) cm−1; UV (MeOH), ␭
(log ␧): 288 (3.66), 382 (4.25) nm;
max
Anal. Calcd for C18H12O4: C, 73.97; H, 4.10. Found: C, 72.81; H, 3.74.
Typical procedure for the reductive Claisen rearrangement
of 1 to 2-methyl-6,11-dihydroanthra[1,2-b]furan-6,11-dione (2)
Iron powder (2 mmol, 0.11 g) was added to a flask containing a
solution of two ionic liquids [Hmim]BF4 (0.85 g) and [Bzmim]Cl
(2.50 g) and 1 (1 mmol, 0.26 g) in an oil bath at 160 °C. The reaction
mixture was stirred until TLC showed the completion of the reac-
tion (15 min). The reaction mixture was cooled to room tempera-
ture and then filtered after addition of CH2Cl2 (2 × 20 mL). The
organic layer was washed with water (50 mL), saturated NaHCO3
(40 mL), and brine (40 mL). 2 was obtained after evaporation of
organic solvent and column chromatography of crude mixture on
silica gel 60 using n-hexane:ethyl acetate (30:1) as eluent in 84%
yield (0.22 g) together with 3 in 5% yield (0.011 g).
1-Hydroxy-5-(prop-2=-ynyloxy)anthraquinone (17)
1
Orange needles; mp = 155–157 °C; H NMR (CDCl3, 300 MHz): ␦
2.58–2.60 (t, 1H, J = 2.4 Hz), 4.94–4.95 (d, 2H, J = 2.4 Hz), 7.23–7.79
(m, 5H), 8.03–8.06 (m, 1H), 12.45 (s, 1H) ppm; 13C NMR (CDCl3,
75 MHz): ␦ 57.14, 76.97, 77.54, 115.51, 119.40, 120.63, 121.02, 122.45,
123.10, 134.76, 134.84, 135.43, 136.95, 158.17, 161.96, 181.46,
188.32 ppm; IR (KBr): 3427 (br), 3234 (s), 3006 (w), 2925 (s), 2856 (m),
2116 (w), 1654 (m), 1634 (m), 1602 (m), 1587 (m), 1454 (m), 1284 (s),
1263 (s), 1240 (s), 1072 (m), 705 (m) cm−1; UV (MeOH), ␭
290 (3.87), 402 (4.28) nm; Anal. Calcd for C17H10O4: C, 73.38; H, 3.59.
Found: C, 72.75; H, 3.24.
(log ␧):
max
1,5- Bis(prop-2=-ynyloxy)anthraquinone (19)
Bright yellow needles; mp = 257–259 °C; 1H NMR (CDCl3,
300 MHz): ␦ 2.56–2.58 (t, 2H, J = 2.37 Hz), 4.94 (d, 4H, J = 2.35 Hz),
7.43–7.46 (d, 2H, J = 8.39 Hz), 7.69–7.75 (t, 2H, J = 7.9 Hz), 7.96–7.98
(d, 2H, J = 7.77 Hz) ppm; 13C NMR (DMSO, 75 MHz): ␦ 56.55, 78.65,
79.05, 119.36, 119.56, 120.73, 135.16, 136.84, 157.02, 181.20 ppm; IR
(KBr): 3236 (s), 3014 (w), 2924 (w), 2856 (w), 2115 (w), 1666 (s),
1585 (s), 1458 (m), 1442 (m), 1278 (s), 1261 (s), 1241 (s), 1072 (m),
Data
1-Methoxy-4-(prop-2=-ynyloxy)anthraquinone (6)
Bright yellow needles; mp = 170–172 °C; 1H NMR (CDCl3,
400 MHz): ␦ 2.58 (s, 1H), 4.04 (s, 3H), 4.90 (s, 2H), 7.36–7.38 (d, 1H, J =
9.2 Hz), 7.54–7.57 (d, 1H, J = 9.2 Hz), 7.74 (s, 2H), 8.19 (s, 2H) ppm; 13
C
NMR (CDCl3, 100 MHz): ␦ 56.93, 58.60, 76.53, 78.43, 119.30, 119.50,
122.76, 125.00, 126.46, 126.54, 133.35, 133.50, 134.05, 134.28, 151.52,
155.52, 178.82, 183.35 ppm; IR (KBr): 3260 (s), 3014 (w), 3005 (w),
2950 (w), 2850 (w), 2132 (m), 1667 (s), 1587 (s), 1441 (s), 1314 (s), 1282 (s),
1239 (s), 965 (s), 742 (s) cm−1; UV (MeOH), ␭max (log ␧): 280 (4.07),
415 (4.09) nm; Anal. Calcd for C18H12O4: C, 73.97; H, 4.10. Found:
C, 73.27; H, 3.74.
983 (s), 704 (m) cm−1; UV (MeOH), ␭
(log ␧): 290 (3.43), 376
max
(3.92) nm; Anal. Calcd for C20H12O4: C, 75.94; H, 3.79. Found: C,
75.29; H, 3.50.
2-Methyl-6,11-dihydroanthra[1,2-b]furan-6,11-dione (2)
Yellow needles; mp = 191–193 °C (lit.19a 192–193 °C); H NMR
1
(CDCl3, 300 MHz): ␦ 2.65 (s, 3H), 6.55 (s, 1H), 7.78–7.86 (m, 3H),
1-Hydroxy-8-(prop-2=-ynyloxy)anthraquinone (11)
8.20–8.23 (d, 1H, J = 8.1 Hz), 8.30–8.34 (m, 2H) ppm.
Bright yellow needles; mp = 163–165 °C; 1H NMR (CDCl3,
400 MHz): ␦ 2.64 (s, 1H), 5.00 (s, 2H), 7.32–7.34 (d, 1H, J = 8.00 Hz),
7.54–7.56 (d, 1H, J = 8.00 Hz), 7.64–7.68 (m, 1H), 7.78–7.82 (m, 2H),
8.06–8.08 (d, 1H, J = 7.60 Hz), 12.93 (s, 1H) ppm; 13C NMR (CDCl3,
100 MHz): ␦ 57.22, 77.09, 77.34, 117.02, 118.95, 120.52, 121.28, 124.82,
126.64, 133.38, 134.32, 135.52, 136.01, 158.72, 162.52, 182.55,
188.75 ppm; IR (KBr): 3432 (br), 3265 (m), 3080 (w), 2925 (s),
2856 (m), 2120 (w), 1673 (m), 1641 (s), 1582 (m), 1455 (s), 1264 (s),
5-Methoxy-2-methyl-6,11-dihydroanthra[1,2-b]furan-6,11-dione (7)
1
Yellow needles, mp = 225–228 °C; H NMR (CDCl3, 400 MHz): ␦
2.63 (s, 3H), 4.08 (s, 3H), 6.46 (s, 1H), 7.39 (s, 1H), 7.73–7.80 (m, 2H),
8.24–8.29 (m, 2H) ppm; 13C NMR (CDCl3, 100 MHz): ␦ 14.77, 56.91,
102.97, 109.14, 117.35, 118.93, 126.19, 127.14, 132.77, 133.07, 134.07,
134.99, 137.52, 146.98, 156.80, 163.25, 182.70, 183.35 ppm; IR (KBr):
3071 (w), 2923 (w), 2861 (w), 1667 (s), 1591 (m), 1567 (m), 1467 (s),
1255 (s), 1234 (s), 984 (s), 725 (s) cm−1; UV (MeOH), ␭max (log ␧): 255
(3.76), 280 (3.69), 410 (3.35) nm; Anal. Calcd for C18H12O4: C, 73.97;
H, 4.10. Found: C, 73.81; H, 4.00.
1096 (s), 1020 (s), 803 (s), 710 (m) cm−1; UV (MeOH), ␭
210 (3.78), 225 (3.90), 255 (4.58), 270 (4.51), 390 (3.67) nm; Anal.
Calcd for C17H10O4: C, 73.38; H, 3.59. Found: C, 72.53; H, 3.34.
(log ␧):
max
2,5-Dimethyl-7,12-dihydroanthra[1,2-b: 3,4-b=]difuran-7,12-dione (9)
1-Methoxy-8-(prop-2=-ynyloxy)anthraquinone (13)
1
Yellow needles; mp = 295–297 °C (lit.19c 296–297 °C); H NMR
1
Golden needles; mp = 170–172 °C; H NMR (CDCl3, 300 MHz): ␦
2.53–2.55 (t, 1H, J = 2.39 Hz), 4.01 (s, 3H), 4.93–4.94 (d, 2H, J =
2.39 Hz), 7.29–7.32 (d, 1H, J = 8.38 Hz), 7.47–7.51 (dd, 1H, J = 8.37,
(CDCl3, 300 MHz): ␦ 2.681–2.684 (d, 6H, J = 0.78 Hz), 6.647–6.650 (d,
2H, J = 0.89 Hz), 7.75–7.78 (m, 2H), 8.29–8.32 (m, 2H) ppm.
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Nadali et al.
3
Scheme 2. The propargyloxyanthraquinones used as substrates and the produced anthrafurandiones.
2-Methyl-5-(prop-2=-ynyloxy)-6,11-dihydroanthra[1,2-b]furan-
162.07, 162.42, 182.42, 188.66 ppm; IR (KBr): 3433 (br), 3083 (w),
2922 (m), 2830 (m), 1669 (s), 1638 (s), 1583 (s), 1481 (s), 1456 (s),
1356 (s), 1286 (s), 1240 (s), 1022 (s), 779 (s), 744 (s) cm−1; UV (MeOH),
6,11-dione (10)
1
Yellow needles, mp = 247–248 °C; H NMR (CDCl3, 400 MHz): ␦
2.65 (s, 3H), 2.70 (s, 1H), 4.96 (s, 2H), 6.51 (s, 1H), 7.59 (s, 1H), 7.76–
7.79 (m, 2H), 8.25–8.29 (m, 2H) ppm; 13C NMR (CDCl3, 100 MHz): ␦
14.76, 58.33, 76.53, 78.38, 101.98, 103.08, 113.68, 126.28, 126.72,
127.16, 132.80, 133.23, 133.72, 134.10, 134.81, 137.16, 154.21, 163.30,
182.45, 183.09 ppm; IR (KBr): 3386 (m), 3065 (w), 2925 (s), 2855 (m),
2150 (m), 1667 (m), 1592 (m), 1461 (s), 1351 (s), 1230 (s), 1219 (s), 1014 (s),
784 (m) cm−1; UV (MeOH), ␭max (log ␧): 295 (4.09), 400 (4.00) nm; Anal.
Calcd for C20H12O4: C, 75.94; H, 3.79. Found: C, 75.34; H, 3.39.
␭
max (log ␧): 280 (3.96), 395 (3.39) nm; Anal. Calcd for C17H10O4: C,
73.38; H, 3.59. Found: C, 73.64; H, 3.32.
10-Methoxy-2-methyl-6,11-dihydroanthra[1,2-b]furan-6,11-dione (14)
Brick red needles; mp = 160–162 °C; 1H NMR (CDCl3, 500 MHz): ␦
2.61 (s, 3H), 4.05 (s, 3H), 6.51 (s, 1H), 7.32–7.34 (d, 1H, J = 8.35 Hz),
7.68–7.77 (m, 2H), 7.94–7.96 (d, 1H, J = 7.85 Hz), 8.11–8.12 (d, 1H, J =
8.05 Hz) ppm; 13C NMR (CDCl3, 125 MHz): ␦ 14.62, 56.49, 103.08,
117.89, 119.58, 121.53, 122.17, 124.83, 128.43, 131.02, 134.70, 135.63,
136.99, 151.93, 160.21, 161.87, 182.41, 183.67 ppm; IR (KBr): 3099 (w),
2924 (s), 2853 (s), 1663 (m), 1583 (m), 1450 (m), 1321 (s), 1302 (s),
1279 (s), 1244 (s), 1111 (m), 740 (w) cm−1; UV (MeOH), ␭max (log ␧): 294
(3.58), 380 (3.58) nm; Anal. Calcd for C18H12O4: C, 73.97; H, 4.10.
Found: C, 72.86, H, 3.74.
10-Hydroxy-2-methyl-6,11-dihydroanthra[1,2-b]furan-6,11-dione (12)
Red needles, mp = 211–214 °C; 1H NMR (CDCl3, 400 MHz): ␦ 2.68
(s, 3H), 6.57 (s, 1H), 7.31–7.33 (d, 1H, J = 8.4 Hz), 7.67–7.71 (t, 1H, J =
7.6 Hz), 7.84–7.88 (m, 2H), 8.19–8.21 (d, 1H, J = 8.0 Hz), 12.72 (s,
1H) ppm; 13C NMR (CDCl3, 100 MHz): ␦ 14.71, 103.33, 119.46, 122.52,
124.36, 125.75, 126.44, 126.93, 133.37, 133.95, 136.61, 136.97, 151.88,
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Can. J. Chem. Vol. 00, 0000
Table 1. Reductive Claisen rearrangement of propargyloxyanthraquinones
with iron in a mixture of two ionic liquids [Hmim]BF4 and [Bzmim]Cl
at 160 °C.
2,10-Dimethyl-6,12-dihydroanthra[1,2-b:8,7-b=]difuran-6,12-dione (16)
1
Yellow needles, mp = 290–291 °C; H NMR (CDCl3, 400 MHz): ␦
2.65 (s, 6H), 6.55 (s, 2H), 7.82–7.84 (d, 2H, J = 8.0 Hz), 8.20–8.22 (d,
2H, J = 8.4 Hz) ppm; 13C NMR (CDCl3, 100 MHz): ␦ 14.66, 103.04,
121.26, 122.02, 125.45, 129.05, 136.74, 151.88, 161.91, 182.35, 183.40 ppm;
IR (KBr): 3102 (w), 2996 (w), 2943 (w), 2830 (w), 1697 (m), 1671 (s),
1586 (s), 1439 (s), 1318 (s), 1276 (s), 1234 (s), 1053 (s), 982 (s), 787 (m),
Molar
Entry
no.
ratio
Time Yield
Substrate Product
AQ:Fe (min) (%)
1
1
2
1:2
15
84
5
744 (s) cm−1; UV (MeOH), ␭
(log ␧): 260 (4.00), 290 (4.15), 335
max
Depropargylated
(3.87), 365 (3.96) nm; Anal. Calcd for C20H12O4: C, 75.94; H, 3.79.
Found: C, 75.39; H, 3.62.
2
3
4
6
5
7
5
9
10
5
1:2
1:2
5
10
98
83
12
67
15
8
7-Hydroxy-2-methyl-6,11-dihydroanthra[1,2-b]furan-6,11-dione (18)
Yellow needles; m p = 210–212 °C; 1H NMR(CDCl3, 500 MHz): ␦
2.64 (s, 3H), 6.54 (s, 1H), 7.28–7.30 (dd, 1H, J = 8.4, 1.0 Hz), 7.65–7.68
(m, 1H), 7.83–7.84 (m, 2H), 8.19–8.20 (d, 1H, J = 8.15 Hz), 12.74 (s,
1H) ppm; 13C NMR(CDCl3, 125 MHz): ␦ 14.65, 100.93, 103.26, 119.17,
121.85, 124.14, 125.69, 128.80, 133.83, 133.90, 136.47, 137.42, 151.86,
162.37, 162.49, 181.94, 188.79 ppm; IR (KBr): 3439 (s, br), 3099 (w),
2919 (m), 2856 (m), 1671 (m), 1625 (s), 1598 (s), 1579 (m), 1447 (m),
4
8
1:2
2
5
6
8
8
9
5
9
1:2
1:1
6
3
90
8
5
10
5
72
7
1308 (m), 1278 (s), 1232 (s), 919 (m), 774 (m) cm−1; UV (MeOH), ␭
Doubly depropargylated
5
max
(log ␧): 294 (3.78), 414 (3.68) nm; Anal. Calcd for C17H10O4: C, 73.38;
H, 3.59. Found: C, 72.54; H, 3.28.
7
8
9
11
13
15
12
1:3
1:2
1:4
12
5
73
15
62
8
73
16
96
81
10
7
Depropargylated
14
12
16
2-Methyl-7-(prop-2=-ynyloxy)-6,11-dihydroanthra[1,2-b]furan-
6,11-dione (21)
4
Bright yellow needles, mp = 170–172 °C; 1H NMR (CDCl3,
500 MHz): ␦ 2.58 (s, 1H), 2.63 (s, 3H), 4.96 (s, 2H), 6.52 (s, 1H),
7.49–7.51 (d, 1H, J = 8.25 Hz), 7.72–7.75 (t, 1H, J = 8.55 Hz), 7.81–7.83
(d, 1H, J = 8.05 Hz), 8.05–8.06 (d, 1H, J = 8.3 Hz), 8.15–8.17 (d, 1H, J =
8.1 Hz) ppm; 13C NMR (CDCl3, 125 MHz): ␦ 14.60, 57.33, 76.70, 77.88,
103.07, 117.29, 120.73, 120.88, 122.23, 122.50, 125.91, 130.82, 134.51,
135.89, 136.22, 151.35, 157.90, 161.55, 182.42, 182.58 ppm; IR (KBr):
3233 (m), 3015 (w), 2923 (s), 2852 (m), 2118 (w), 1670 (m), 1595 (m),
1581 (m), 1443 (w), 1269 (s), 1235 (m), 1040 (m), 926 (w) cm−1; UV
Doubly depropargylated
10
11
17
19
18
20
21
18
20
21
18
1:2
1:0.5
7
15
12
19
1:0.5
5
23
52
5
Note: The yields are isolated yields. AQ, anthraquinone.
(MeOH), ␭
(log ␧): 298 (2.74), 378 (3.45) nm; Anal. Calcd for
max
C20H12O4: C, 75.94; H, 3.79. Found: C, 75.16; H, 3.50.
selectivity via the controlling of the molar ratio of the reductant
or reaction time (Table 1, compare entries 5 with 6 and also
11 with 12).
Results and discussion
First, we selected 1 as a model compound and studied its reduc-
tive Claisen rearrangement using iron powder (2 equiv.) in mixed
ionic liquids [Hmim]BF4 and [Bzmim]Cl at 160 °C. Under these
conditions, we found that the desired product 2 was produced in
84% yield together with depropargylated product (5%) after only
15 min. It must be noted that the omission of each of these ionic
liquids caused the yield of the desired product 2 to decrease to
0%–20%. However, thermal operation of this rearrangement in the
absence of iron gave 2 in only 10% yield with concomitant forma-
tion of depropargylated product in 50% yield. In addition, reduc-
tion of the reaction temperature to 130 °C caused the yield of
the product 2 to decrease to 40%. Also, this rearrangement was
unsuccessful when potassium chloride KCl was used instead of
[Bzmim]Cl. In this case, the starting material (1) remained com-
pletely intact after 2 h. Then, we applied these conditions for
performing the reductive Claisen rearrangement of other struc-
turally different propargyloxyanthraquinones to their correspond-
ing anthrafurandiones (Scheme 2). The results are shown in
Table 1. All of these propargyloxyanthraquinones as substrates
mentioned in this table were conveniently synthesized from the
reaction of the corresponding hydroxyanthraquinones with
propargyl bromide in the presence of dry potassium carbonate in
DMF as solvent at 90 °C with reaction times up to 2 h.
In addition, for obtaining deeper insight into the applicability
and other aspects of selectivity of the present method, we exam-
ined the possibility of the reductive Claisen rearrangement of
propargyloxyanthraquinones in the presence of some other func-
tional groups in different binary mixtures. For this purpose, re-
ductive Claisen rearrangement of 1 was studied in the presence of
another compound containing a specified functional group (1:1)
using iron powder (2 equiv.) in mixed ionic liquids [Hmim]BF4 and
[Bzmim]Cl at 160 °C. The results and conversion yields of these
selective reactions are shown in Scheme 3.
As shown in Scheme 3, this rearrangement can also be carried
out in the presence of some other functional groups including
aldehydes, carboxylic amides, phenols, and alcohols with excel-
lent selectivity via the present method.
As mentioned above, the Claisen rearrangement of 1 via the
present method and in the absence of acidic or chloride contain-
ing ionic liquid or Fe powder was unsuccessful; therefore, in these
cases the desired 2 was obtained in only 0%–20% yield. With this
description, although the exact mechanism of this reaction is not
clear, it is suggested that the anthraquinone core is reduced using
iron in the presence of [Bzmim]Cl under an acidic condition created
by [Hmim]BF4 to its electron-rich hydroquinone state A, which is
more active to perform of this rearrangement (Scheme 4).
As shown in Table 1, structurally different anthrafurandiones are
synthesized from their corresponding propargyloxyanthraquinones
via the present method with short reaction times and in good
to excellent yields. Most notable in this table, is that it is pos-
sible to carry out reductive Claisen rearrangement of 1,4- or
1,5-bispropargyloxyanthraquinone so that singly or doubly rear-
ranged product is obtained as major product with remarkable
After this [3,3] sigmatropic rearrangement19 followed by eno-
lization, the intermediate C is formed, which benefits from its
intramolecular hydrogen bonding. Then, internal cyclization of
this intermediate affords anthrafuran D, which is finally con-
verted to the desired product 2 under air oxidation. It seems that
[Bzmim]Cl is necessary for facilitating the electron transfer from
iron to the anthraquinone core and that [Hmim]BF4 is necessary
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Scheme 3. Various selectivities in the Claisen rearrangement of propargyloxyanthraquinones using iron powder (2 equiv.) in a mixed ionic
liquids [Hmim]BF4 and [Bzmim]Cl at 160 °C.
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Can. J. Chem. Vol. 00, 0000
Scheme 4. The suggested mechanism of the reductive Claisen rearrangement of propargyloxyanthraquinones using Fe, [Bzmim]Cl, and
[Hmim]BF4.
due to its acidic property facilitating this rearrangement. Also,
Acknowledgements
this mixture of ionic liquids acts as environment friendly solvent
in the present method. In agreement with this suggested mecha-
nism, we found that this reaction was unsuccessful under N2 at-
mosphere. In this condition, the desired product 2 was formed in
only 25% yield after 30 min with concomitant formation of 3 in
20% yield.
We gratefully acknowledge the support from the Damghan
University Research Council.
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