Domino reactions of donor-acceptor-substituted cyclopropanes for the synthesis of 3,3'-linked oligopyrroles and pyrrolo[3,2-e]indoles

Article abstract of DOI:10.1002/anie.201205880

Multiple displacement of oxygen: Electron-rich oligopyrroles and pyrrolo[3,2-e]indoles are generated by a domino process induced by donor-acceptor-substituted cyclopropanes. Up to seven molecules of water are eliminated, thus allowing the introduction of nitrogen and aromaticity. Copyright

Full text of DOI:10.1002/anie.201205880

Angewandte
Chemie
DOI: 10.1002/anie.201205880
Synthetic Methods
Domino Reactions of Donor–Acceptor-Substituted Cyclopropanes for
the Synthesis of 3,3’-Linked Oligopyrroles and Pyrrolo[3,2-e]indoles**
Johannes Kaschel, Tobias F. Schneider, Daniel Kratzert, Dietmar Stalke, and Daniel B. Werz*
Pyrroles, oligopyrroles, and their derivatives are widespread
in nature.[1] Furthermore, they play an important role in
material science[2] and medicinal chemistry.[3] Among oligo-
pyrroles, differentiation is made between the 2,2’-linked
derivatives and those which are connected at the 3-position.
Methods for the synthesis of 2,2’-linked oligopyrroles are
multifarious, including Paal–Knorr cyclizations,[4] Vilsmeier
condensations,[5] dipolar cycloadditions,[6] Ullmann cou-
plings,[7] and other metal-mediated coupling reactions.[8] In
contrast, the number of reports concerning the 3,3’-linkage of
pyrrole moieties is much lower.[9,10] 3,3’-Linked dimers have
been accessed, for instance, by Pd-catalyzed rearrangements
of N-bridged diynes,[11] by Barton–Zard synthesis on pyrrole
derivatives,[12] or by oxidative couplings.[13] Potential cross-
coupling reactions suffer from the heavy accessibility of
3-substituted pyrroles. To the best of our knowledge, the only
approach to 3,3’-linked oligopyrroles with more than two
pyrrole units was performed by Magnus et al.[14] They applied
a repetitive route using olefins and (p-toluenesulfonyl)methyl
Scheme 1. Previously reported oligoacetal synthesis (top) as well as
envisioned bispyrrole synthesis (bottom) starting from furan (1).
isocyanide as key reagent to build up the pyrrole moieties in
a stepwise fashion, giving rise to electron-poor N-tosyl
substituted oligomers.[14]
rearrange immediately to 3 (Scheme 1, top).[19] On the basis of
computational investigations regarding the ring enlargement
of a variety of D-A cyclopropanes, we realized that imine
acceptors as in 4 have a similar activation barrier for the
cyclopropane–heterocyclopentene rearrangement compared
to the respective aldehydes.[20] Thus, we thought of the
formation of imines (from the corresponding stable ketones)
leading to immediate ring enlargement (Scheme 1,
bottom).[21] We anticipated that the annulated 2,3-dihydro-
pyrrole moieties in 5 obtained in this transformation should
eliminate water, affording bispyrroles of type 6, as the
tendency towards aromatization is much higher than in the
case of the analogous 2,3-dihydrofuran derivative 3.
Herein we report the preparation of 3,3’-oligopyrroles,
and even hitherto unknown electron-rich congeners, by
making use of a domino sequence[15] initiated by the ring-
enlargement of donor–acceptor-substituted (D-A) cyclopro-
panes.[16–18] This method allows a very fast and versatile access
to bispyrroles, but also to ter- and quaterpyrroles.
Recently, we developed a method, starting from furan (1),
to oligocyclic oligoacetals such as 3. Key intermediates are D-
A cyclopropanes with aldehyde acceptors of type 2 that
[*] Dipl.-Chem. J. Kaschel, Dipl.-Chem. T. F. Schneider,
Priv.-Doz. Dr. D. B. Werz
To test our hypothesis, we prepared aliphatic and aromatic
diketones 8 as starting materials (Table 1). Therefore, the
diesters of type 7, which are available in one step from
furan,[22] were converted via the Weinreb amides into the
respective tricyclic diketones 8. Diketone 8a (R1 = H; R2 =
Me) was selected as model substrate to explore optimized
reaction conditions for the anticipated cascade consisting of
imine formation, ring enlargement, and elimination of water.
Aniline was chosen as the reaction partner. The best results
were obtained using p-toluenesulfonic acid in benzene at
808C after a reaction time of 2 h (for details of the
optimization, see the Supporting Information).[23]
Institut fꢀr Organische und Biomolekulare Chemie
Georg-August-Universitꢁt Gçttingen
Tammannstrasse 2, 37077 Gçttingen (Germany)
E-mail: dwerz@gwdg.de
Homepage: http://werz.chemie.uni-goettingen.de
Dipl.-Chem. D. Kratzert, Prof. Dr. D. Stalke
Institut fꢀr Anorganische Chemie
Georg-August-Universitꢁt Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
[**] We thank the German Research Foundation (DFG), the German–
Israeli Foundation (G.I.F.), the Danish National Research Founda-
tion (DNRF), and the Fonds der Chemischen Industrie (Emmy
Noether Fellowship and Dozentenstipendium to D.B.W.). We thank
Prof. Dr. Rolf Gleiter and Prof. Dr. Lutz F. Tietze for helpful
discussions.
The scope of this reaction was evaluated under these
optimized conditions by changing both the diketone 8 as well
as the amine. Six different diketones 8a–8 f were employed.
As substituent R1 on the three-membered ring, H, Me, and Ph
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205880.
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Table 1: Synthesis of bispyrroles 6.
Table 2: Synthesis of terpyrroles 11.
Entry
8
R1
R2
R3
Yield [%]
6
9
1[a]
8a
8a
8a
8a
8a
8b
8b
8b
8b
8b
8c
8d
8d
8e
8e
8e
8e
8e
8 f
H
H
H
H
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
nPr
nPr
nPr
nPr
nPr
iPr
Ph
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
81
89
90
58
42
77
19
70
72
47
45
54
–
67
75
72
69
39
64
54
–
–
–
–
–
17
74
13
20
37
34
32
89
–
–
–
–
–
–
–
2[b]
4-MeOC6H4
4-HOC6H4
4-BrC6H4
Me
Entry
10
R1
R2
Yield [%]
3[b]
4[a]
1
2
3
4
5
6
10a
10a
10a
10a
10b
10b
Me
Me
Me
Me
Ph
Ph
11a
11b
11c
11d
11e
11 f
64
38
58
53
32
34
5[a,c]
6[b]
4-MeOC6H4
4-BrC6H4
4-MeC6H4
Ph
4-MeOC6H4
4-NCC6H4
4-MeC6H4
3-MeC6H4
2-MeC6H4
4-MeOC6H4
4-MeOC6H4
4-MeC6H4SO2
4-MeOC6H4
4-MeC6H4SO2
Ph
7[a]
8[a]
9[a]
Ph
4-NO2C6H4
10[a]
11[b]
12[b]
13[a]
14[b]
15[a]
16[a]
17[a]
18[a]
19[a]
20[a]
Ph
Ph
H
with aniline derivatives under the catalytic influence of p-
TsOH furnished the desired trimers 11 in only one single step
(Table 2). A plausible reaction mechanism to afford the
domino product 11 is depicted in Table 2. The twofold imine
formation to 12 is followed by the ring enlargement of both
cyclopropane moieties. The resulting dihydropyrrole units in
13 are prone to aromatization. Twofold ring cleavage takes
place, providing intermediate 14 with two aromatic pyrrole
units and two hemiacetal moieties located at the central ring.
As the final step, the inner pyrrole unit is formed in a Paal–
Knorr-like course of action leading to the final compound 11.
To demonstrate the potential of this domino sequence, we
targeted the construction of quaterpyrrole 16 in a single
reaction starting with the oligocyclic bisketone 15.[23,25] After
eight steps (similar to those presented in Table 2), quaterpyr-
role 16 is formed in 25% yield (Scheme 2). To the best of our
Me
Me
Me
Me
Me
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
4-F3CC6H4
Boc
Ph
s
t
8 f
4-F3CC6H4
[a] p-TsOH was used as catalyst. [b] AcOH was used as catalyst. [c] The
product is very air-sensitive.
were chosen, whereas Me, nPr, iPr, and Ph served as residues
at the ketone (see Table 1). Substituents of the amines were
varied in anticipation of modulating the nucleophilicity from
electron-donating to electron-withdrawing groups. Electron-
rich anilines proved to be most suited for this transformation;
therefore, the best results were obtained with 4-hydroxyani-
line and 4-methoxyaniline. To assure solubility of these
amines, acetic acid was used instead of p-TsOH. N-Alkyl-
substituted reaction products proved to be very unstable; only
the methyl-substituted congener 6e could be isolated in
moderate yield. As seen in Table 1, with anilines carrying an
electron-withdrawing group (such as CF3, CN) or with
p-toluenesulfonamide as nitrogen source, a second product,
a monopyrrole derivative 9, was formed.[24] Interestingly, this
product was only found in the case of R1 = H. Carbamates
such as BocNH2 also delivered the respective bispyrrole 6r,
but only in fair yield.
Scheme 2. Synthesis of quaterpyrrole 16.
knowledge, electron-rich 3,3’-linked ter- and quaterpyrroles
have never been reported to date. Their intrinsic instability is
a major problem and might also explain the only moderate to
low yields obtained in these transformations.
After the successful construction of 3,3’-linked bispyr-
roles, we tried to extend this sequence to access correspond-
ing ter- and quarterpyrroles. Extended oligoacetalic diketones
of type 10 served as starting materials.[23,25] Their treatment
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It has not escaped our notice that 3,3’-linked bispyrroles 6
would act as excellent nucleophiles that are comparable with
enamines and ketene acetals.[26] This assumption was vali-
dated in the transformation of bispyrroles with maleic
anhydride, maleimide, and p-benzoquinone into pyrrolo[3,2-
e]indoles 17–19. We observed that for bispyrrole formation,
the intermolecular Michael addition of the in situ generated
pyrrole nucleophile to a,b-unsaturated carbonyl compounds
and the condensation can be combined in an one-pot process
starting from 8 (Scheme 3). As anticipated, a strongly elec-
In conclusion, we have developed a novel strategy to build
up 3,3’-linked oligopyrroles by using a domino approach.
Ketone-substituted cyclopropanes were converted into the
corresponding imines setting the stage for the domino
cascade. Key for the success is the high tendency of donor–
acceptor-substituted cyclopropanes to undergo ring enlarge-
ment combined with the bias for aromatization to pyrroles. In
up to eight steps, bis-, ter-, and quaterpyrroles were obtained.
The cascade can be further expanded by the addition of
Michael acceptors reacting with the highly nucleophilic
bispyrroles formed in situ to afford pyrrolo[3,2-e]indoles.
These structural constituents are relevant subunits of duo-
carmycins, which are potent antitumor agents. Related studies
directed to analogous reactions with thiocarbonyl acceptors
are underway in our laboratory and will be reported in due
course.
Received: July 20, 2012
Published online: September 28, 2012
Keywords: cyclopropanes · domino reactions · donor–
.
acceptor systems · pyrroles · ring enlargement
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