Synthesis of pyrrolophenanthridone alkaloid kalbretorine from indolecarboxylic acids via hypervalent iodine(III) mediated halodecarboxylation and reduction

Article abstract of DOI:10.1016/j.tetlet.2012.01.132

The treatment of 8,9,10-trimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-4,5- dicarboxylic acid with phenyliodine diacetate and potassium iodide in tetrahydrofuran gave the corresponding 4,5-diiodo derivative, which was converted to kalbretorine via reductio

Full text of DOI:10.1016/j.tetlet.2012.01.132

Tetrahedron Letters 53 (2012) 1924–1927
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Tetrahedron Letters
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Synthesis of pyrrolophenanthridone alkaloid kalbretorine from
indolecarboxylic acids via hypervalent iodine(III) mediated
halodecarboxylation and reduction
⇑
Yasuyoshi Miki , Hideaki Umemoto, Masashi Dohshita, Hiromi Hamamoto
School of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae, Higashi-Osaka 577-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The treatment of 8,9,10-trimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-4,5-dicarboxylic acid with
phenyliodine diacetate and potassium iodide in tetrahydrofuran gave the corresponding 4,5-diiodo deriv-
ative, which was converted to kalbretorine via reduction, demethylation, followed by alkylation.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 7 January 2012
Revised 26 January 2012
Accepted 31 January 2012
Available online 9 February 2012
Keywords:
Decarboxylation
Halogenation
Hypervalent iodine
Indolecarboxylic acids
Pyrolophenanthridone alkaloid
Carboxylic acids are an attractive substrate class in chemical
synthesis, because they are available at low cost from natural
and synthetic sources, and are easy to store and handle.1 In the last
decade, a great deal of effort has been invested to provide the syn-
thetic strategies to form carbon–carbon or carbon–heteroatom
bonds starting from carboxylic acids via decarboxylative transfor-
mations.1–3 Alternatively, simple decarboxylation, that is, protode-
carboxylation, to form carbon–hydrogen bond is also of synthetic
interest for the removal of carboxylate groups left behind as a re-
sult of the chosen synthetic route. However, protodecarboxylation
is difficult except activated acid and the methods generally require
harsh, forcing conditions.4
Recently, we developed an efficient halodecarboxylation meth-
od of arenecarboxylic acids to give halogenated compounds utiliz-
ing phenyliodine diacetate (PhI(OAc)2) and lithium halides
combination (PhI(OAc)2/LiX).5,6 This method seems to be suitable
for the synthesis of natural products or bioactive compounds be-
cause the reaction can be carried out under mild and safe condi-
tions relative to other halodecarboxylation methods such as the
Hunsdiecker-type reaction which requires highly toxic heavy me-
tal salts.7 Moreover, the resulting halogenated compounds could
easily be converted to other substituent or removed as an alterna-
tive to give the corresponding protodecarboxylation products
from carboxylic acids. An efficient use of this method in the
halodecarboxylation of indolecarboxylic acids have been achieved
by us and several haloindole derivatives were successfully ob-
tained in high yields.8 In this Letter, we report the use of the halod-
ecarboxylation utilizing PhI(OAc)2/LiX system and the following
reduction as the key step for the effective synthesis of pyrrolophe-
nanthridone alkaloid kalbretorine and its derivatives.
Pratosine9 and hippadine10 are 7H-pyrrolo[3,2,1-de]phenanthri-
done alkaloids, isolated from various species of Amaryllidaceae and
have no significant biological activities.11 However, the synthesis
of pratosine and hippadine was reported by many groups including
us12 and they were mainly synthesized by three methods from the
2-protected indole,12,13 indoline derivatives,14 or 7-haloindoles,
and related compounds.15,16 On the contrary, kalbretorine is also
a member of the 7H-pyrrolo[3,2,1-de]phenanthridone alkaloids
and shows antitumor activity. It contains three oxygen groups as
one hydroxy group and one methylenedioxy group compared to
N
N
N
O
O
O
OH
MeO
O
O
O
O
OMe
Hippadine
Kalbretorine
Pratosine
⇑
Corresponding author.
E-mail address: y_miki@phar.kindai.ac.jp (Y. Miki).
Figure 1. Some 7H-pyrrolo[3,2,1-de]phenanthridone alkaloids.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.132

Y. Miki et al. / Tetrahedron Letters 53 (2012) 1924–1927
1925
pratosine and hippadine, which have two methoxy groups or one
methylenedioxy group, respectively, but its mechanism of antitu-
mor activity is still not clarified17 (Fig. 1).
The first in low yield synthesis of kalbretorine was reported by
Ghosal.17 Iwao prepared kalbretorine by introducing the hydroxyl
group to 4,5-dihydrohippadine in a 33% yield using tert-BuLi and
B(OMe)3.15g Meyers also showed an elegant synthesis of kalbreto-
rine from N-benzyl-7-bromoindoline as the starting material, the
yield is still insufficient.15e
In the course of our synthetic studies on the use of indolecarb-
oxylic acid as building blocks for bioactive compounds, we re-
ported the synthesis of pratosine and hippadine from the
palladium-catalyzed intramolecular cyclization of dimethyl 1-(2-
bromobenzyl)indole-2,3-dicarboxylate, followed by decarboxyl-
ation. But the difficulty in the decarboxylation of the correspond-
ing dicarboxylic acids brought significant drawbacks on this
approach.12 Herein, we investigated the efficacy of the halodecarb-
oxylation of 7H-pyrrolo[3,2,1-de]phenanthridine-4,5-dicarboxylic
acids utilizing PhI(OAc)2/LiX system to achieve the simple and use-
ful synthesis of kalbretorine and its derivatives.
presence of 10% Pd–C in hot methanol produced pratosine in an
83% yield (Scheme 1).
We also examined the synthesis of hippadine from the corre-
sponding 9,10-methylenedioxy-7H-pyrrolo[3,2,1-de]phenanthri-
dine-4,5-dicarboxylic acid 4. In a similar manner as described for
1, the treatment of 4 with PhI(OAc)2 and a metal bromide (Li, Na,
and K) in THF, acetonitrile, 1,2-dichloroethane, or TFE as the sol-
vent resulted in a complex mixture (Scheme 2). These results
clearly indicated that the methylenedioxy group may be unstable
compared to the methoxy group under these reaction conditions.
The
8,9,10-trimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-
4,5-dicarboxylic acid 7 was synthesized as follows in a preparation
similar to 1. The treatment of the benzyl derivative 5 in the pres-
ence of catalytic Pd(PPh3)4 and potassium acetate in hot dioxane,
which was prepared from dimethyl indole-2,3-dicarboxylate and
6-bromo-2,3,4-trimethoxybenzyl bromide in the presence of
potassium carbonate in hot acetonitrile (87%), gave the dimethyl
8,9,10-trimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-4,5-dicar-
boxylate 6 (85%). Hydrolysis of the dimethyl ester 6 (LiOH, diox-
ane-H2O) gave the 4,5-dicarboxylic acid 7 in quantitative yield.
However, 8 could not be isolated from 7 under similar reaction
conditions and a complex mixture was obtained (Scheme 3).
Next, we examined the iododecarboxylation8 of the dicarbox-
ylic acid 7. When 7 was treated with 6 equiv of PhI(OAc)2 and
the same equivalent of lithium iodide, the 4-iodo-5-carboxylic acid
Initially, we tried to apply PhI(OAc)2/LiX system8 for the
synthesis of pratosine as a model approach to attain kalbretorine
synthesis. The treatment of 9,10-dimethoxy-7H-pyrrolo[3,2,1-de
]phenanthridine-4,5-dicarboxylic acid 112 with PhI(OAc)2 (6 equiv)
and lithium bromide (6 equiv) in THF gave a mixture of 4-bromo-
5-carboxylic acid 2 and 4,5-dibromo compound 3 in 15% and 64%
yields, respectively (Table 1, entry 1). The reaction of 1 with
8 equiv of PhI(OAc)2 gave 3 in a slightly higher yield (68%), but
when 1 was treated with 6 equiv of PhI(OAc)2 and an equimolecu-
lar of sodium bromide or potassium bromide, 2 was formed in 72–
75% yields and 3 could not be found (entries 2–4).8 In acetonitrile,
1,2-dichloroethane or 2,2,2-trifluoroethanol (TFE) as the solvent
instead of THF, 3 could not be isolated and a complex mixture
was obtained (entries 5–7). The 4-bromo-5-carboxylic acid 2 could
be converted to the corresponding 4,5-dibromo compound 3 by the
treatment of PhI(OAc)2 (4 equiv) and lithium bromide (4 equiv) in
TFE (3 h, rt) in a 69% yield. The reduction of 3 with NaBH4 in the
Br
10% Pd/C
NaBH4
N
Br
N
MeOH
O
O
MeO
MeO
OMe
OMe
3
Pratosine (87%)
Scheme 1. Reduction of dibromoindole derivative 3 affords pratosine.
Table 1
Hypervalent iodine(III) mediated halodecarboxylation of 9,10-dimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-4,5-dicarboxylic acid 1
Br
N
CO2H
O
CO2H
CO2H
MeO
PhI(OAc)2
MBr
N
2
OMe
+
0°C 3 h
then rt 3 h
Br
Br
MeO
OMe
N
1
O
MeO
OMe
3
Entry
PhI(OAc)2 (equiv)
MBr (equiv)
Solvent
Yield of 2a (%)
Yield of 3a (%)
1
2
3
4
5
6
7
6
8
6
6
6
6
6
LiBr (6)
LiBr (8)
NaBr (6)
KBr (6)
LiBr (6)
LiBr (6)
LiBr (6)
THF
THF
THF
THF
(CH2Cl)2
CH3CN
TFE
15
10
72
75
64
68
—
—
—
—
b
b
b
—
a
Yield of isolated product.
3 could not be isolated and a complex mixture was obtained.
b

1926
Y. Miki et al. / Tetrahedron Letters 53 (2012) 1924–1927
Br
I
I
CO2H
CO2H
PhI(OAc)2
MBr
n-Bu4NBH4
SnCl2
N
N
Br
reduction
N
N
N
O
O
O
O
O
MeO
BBr3
O
OMe
OMe
MeO
O
OMe
OMe
4
O
O
O
Hippadine
(86%)
10
11
Scheme 2. Synthetic approach to hippadine using hypervalent iodine(III) mediated
halodecarboxylation.
BrCH2Cl
Na2CO3
N
N
CO2Me
CO2Me
CO2Me
Na2SO3
O
O
Pd(PPh3)4
CO2Me
N
N
Br
HO
OH
OH
O
KOAc
O
OH
OMe
OMe
MeO
OMe
OMe
6 (85%)
(76%)
MeO
12
Kalbretorine (63%)
Scheme 4. Synthesis of kalbretorine from 10.
5
Br
Br
CO2H
CO2H
PhI(OAc)2
sium iodide, 10 was obtained in an 82% yield (entries 2,3). When the
reaction of 7 with 4 equiv of PhI(OAc)2 was examined, the desired
product 10 was not isolated and the monoiodo compound 9 was
obtained in high yield (83%) (entry 4). However, when the reaction
of 7 with PhI(OAc)2 was carried out in 1,2-dichloroethane or
acetonitrile instead of in THF, the yields of 10 were relatively low
(25–36%) and 9 was also isolated (42–50%) (entries 5, 6). In addi-
tion, the reaction of 7 in TFE instead of in THF afforded a complex
mixture (entry 7), but 9 was converted to 10 at room temperature
in TFE in the presence of 4 equivalents of PhI (OAc)2 and the same
equivalent of potassium iodide in an 84% yield.
LiOH
N
N
O
LiBr
MeO
OMe
OMe
MeO
OMe
OMe
(quant)
8
7
Scheme 3. Preparation of 8,9,10-trimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-
4,5-dicarboxylic acid 7 and its hypervalent iodine(III) mediated halodecarboxylation.
The diiodo compound 10 could be converted into kalbretorine
as follows (Scheme 4). The reduction of 10 with tetrabutylammo-
nium borohydride and tin(II) chloride in THF gave the dihydro
derivative 1118 in an 86% yield. 11 was reacted with boron tribro-
mide (À20 °C, 32 h in dichloromethane) to produce the triol 12 in a
9 was produced in low yield, but the 4,5-diiodo compound 10 was
not isolated (Table 2, entry 1). The treatment of 7 with PhI(OAc)2
and sodium iodide instead of lithium iodide provided a mixture of
9 and 10 in 63% and 21% yields, respectively, and when using potas-
Table 2
Hypervalent iodine(III) mediated halodecarboxylation of 8,9,10-trimethoxy-7H-pyrrolo[3,2,1-de]phenanthridine-4,5-dicarboxylic acid 7
I
N
CO2H
O
CO2H
CO2H
OMe
OMe
MeO
9
N
PhI(OAc)2
PhI(OAc)2
MI
MI
+
60°C, 20 h
OMe
MeO
I
I
OMe
N
7
O
OMe
MeO
OMe
10
a
a
Entry
PhI(OAc)2 (equiv)
MI (equiv)
Solvent
Yield of 9 (%)
Yield of 10 (%)
1
2
3
4
5
6
7
6
6
6
4
6
6
6
LiI (6)
NaI (6)
KI (6)
KI (4)
KI (6)
KI (6)
KI (6)
THF
THF
THF
THF
(CH2Cl)2
CH3CN
TFE
13
63
—
83
50
42
—
21
82
—
36
25
b
—
a
Yield of isolated product.
3 could not be isolated and a complex mixture was obtained.
b

Y. Miki et al. / Tetrahedron Letters 53 (2012) 1924–1927
1927
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76% yield, followed by treatment with bromochloromethane in the
presence of sodium carbonate and sodium sulfite in DMF (80 °C,
2 h) to afford kalbretorine19 in a 63% yield.
In conclusion, we have achieved a novel synthesis of kalbreto-
rine from indolecarboxylic acids based on hypervalent iodine(III)
mediated halodecarboxylation and reduction as key steps. Such
efficient alternative access to protodecarboxylation tool under
mild conditions may find wide application in the synthesis of nat-
ural products or bioactive compounds. Further studies along this
line are now in progress.
Acknowledgments
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This work was partially supported by Grant-in-Aid for Scien-
tific Research (C) from the Japan Society for the Promotion of
Science (JSPS), Young Scientists (B) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT), Kinki Uni-
versity Research Grant Project, and also in part ‘High-Tech
Research Center Project’ for Private Universities and matching
fund subsidy.
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18. 7H-8,9,10-Trimethoxypyrrolo[3,2,1-de]phenanthridin-7-one (11): mp 133–
134 °C (from THF). 1H NMR (CDCl3) d: 3.99 (3H, s, OMe), 4.06 (3H, s, OMe),
4.11 (3H, s, OMe), 6.86 (1H, d, J = 4.0 Hz, H-4), 7.45 (1H, dd, J = 8.0, 7.2 Hz, H-2),
7.56 (1H, s, H-11), 7.74 (1H, d, J = 8.0 Hz, H-3), 7.95 (1H, d, J = 7.2 Hz, H-1), 8.06
(1H, d, J = 4.0 Hz, H-5). 13C NMR (CDCl3) d: 157.34, 156.77, 156.37, 143.41,
132.79, 131.04, 128.11, 123.52, 123.36, 122.44, 117.88, 115.97, 114.81, 109.77,
100.54, 61.88, 61.28, 56.01. HRMS m/z (M+) calcd for C18H16ON4; 31.1031
Found; 310.1079.
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