Benzannulation of 2-Alkenylindoles using Aldehydes by Sequential Triple-Relay Catalysis: A Route to Carbazoles and Carbazole Alkaloids

Article abstract of DOI:10.1002/adsc.201700092

Benzannulation of 2-alkenylindoles with readily available aldehydes, under one-pot sequential triple-relay-catalysis, provides an easy access to several structurally unique carbazoles including 2- and 3-alkenylcarbazoles. This protecting group-free method enabled one-pot synthesis of alkaloids such as hyellazole and 6-chlorohyellazole, and the formal syntheses of seven other alkaloids. Construction of the core structure, present in murastifoline A, murrafoline E, and related alkaloids was also demonstrated. Even conjugated 3,3′-biscarbazoles can also be synthesized by one-pot, two-fold sequential triple-relay catalysis. (Figure presented.).

Full text of DOI:10.1002/adsc.201700092

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DOI: 10.1002/adsc.201700092
Benzannulation of 2-Alkenylindoles using Aldehydes by Sequen-
tial Triple-Relay Catalysis: A Route to Carbazoles and Carbazole
Alkaloids
Ankush Banerjee,a Samrat Sahu,a and Modhu Sudan Majia,*
a
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur – 721302, India
E-mail: msm@chem.iitkgp.ernet.in
Received: January 23, 2017; Revised: March 29, 2017; Published online: && &&, 0000
Supporting information for this article can be found under http://dx.doi.org/10.1002/adsc.201700092.
Abstract: Benzannulation of 2-alkenylindoles with
readily available aldehydes, under one-pot sequen-
tial triple-relay-catalysis, provides an easy access to
several structurally unique carbazoles including 2-
and 3-alkenylcarbazoles. This protecting group-free
method enabled one-pot synthesis of alkaloids such
as hyellazole and 6-chlorohyellazole, and the formal
syntheses of seven other alkaloids. Construction of
the core structure, present in murastifoline A, mur-
rafoline E, and related alkaloids was also demon-
strated. Even conjugated 3,3’-biscarbazoles can also
be synthesized by one-pot, two-fold sequential
triple-relay catalysis.
comparatively well explored, it requires functional-
ized aniline derivatives, prepared by tedious synthetic
efforts. On the other hand, benzannulation methods
recently have gained particular interest due to the
easy accessibility of the desired indole precursors. For
instance, Itami et al. developed the formation of car-
bazoles from N-protected indoles using a Pd-Cu-Ag
trimetallic system.[11b] Huang and Wang reported
a tandem Cp*Rh(III) and Brønsted acid-catalyzed
Keywords: alkaloids; annulation; carbazoles; conju-
gated materials; sequential catalysis
The carbazole framework is a privileged structural
motif (Figure 1) present in a large number of natural
products, drugs, and biologically active compounds.[1–3]
In addition, numerous compounds, containing the
carbzole core, show antiviral, antimalarial, and antitu-
mor activities indicating their increasing potential in
medicinal chemistry.[4] Beside this, cabazoles have
broad range of importance in materials chemistry due
to their wide band gap, thermal, electrical, and optical
properties.[5] The carbazole-based oligomers are very
important scaffolds due to their significant applica-
tions in photo-conductors,[6] electroluminescent mate-
rials,[7] charge transporting, and emitting materials in
organic light-emitting diodes.[8]
A literature survey on the construction of carba-
zoles reveals that the synthetic strategies can be cate-
gorized into two major classes: (i) formation of the
middle pyrrole ring via either C–N or C–C coupling
of aniline derivatives,[9,10] and (ii) benzannulation of
functionalized indoles.[11,12] Although the first one is Figure 1. Some representative carbazole alkaloids.
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benzannulation of indoles for the synthesis of substi- taldehyde 2a were chosen as model substrates. Tetra-
tuted carbazoles.[11c] Reddy et al. also demonstrated hydrofuran (THF) was found to be the suitable sol-
a [4+2] benzannulation method to obtain carbazoles vent for the C-3 alkenylation of indole. Reaction of
from N-protected 2-alkenylindoles.[11g]
1a with 2a by using 5 mol% PTSA·H2O, 5 mol%
Despite their own advantages, most of these strat- DBU, and 5 mol% Pd/C in THF-xylene solvent
egies are based on N-protected indoles. Apart from system furnished 3a in 44% yield (Table 1, entry 1).
this, the synthesis of carbazoles by the reported syn- Increasing the Pd/C catalyst loading to 10 mol% and
thetic strategies faces serious limitations such as func- temperature for the step 3, improved the yields signif-
tional group intolerance, limited substrate scope, com- icantly (entries 2–4). Executing the first step at 608C
plicated starting materials, regioselectivity of C–C or while maintaining room temperature for the second
C–N bond formation reactions, and use of expensive step provided better results (entries 5 and 6). In the
catalysts. With the expectation that the benzannula- absence of NaH, the yield decreased considerably
tion strategy will provide ample opportunity to com- (entry 7). Changing the solvent for the step 3 to deca-
plement the above-mentioned limitations, we have lin, accelerated the reaction rate (entry 8). Other high
undertaken the challenge to develop a general, step- boiling solvents, such as mesitylene and 1,2-dicholoro-
and atom-economic method to synthesize several benzene were not suitable (entries 9 and 10). The
functionalized carbazoles and carbazole alkaloids by highest yield was obtained by conducting the last step
protecting group-free, one-pot sequential triple-relay at 1808C (84%, entry 11). While the absence of
catalysis.[13,14] Our recently developed indole C-3 alke- PTSA·H2O led to a complex reaction mixture, omis-
nylation method enabled us to achieve this benz- sion of DBU only gave carbazole 3a in 35% yield,
ACHTUNGTRENNUNG
annulation reaction using simple aldehydes.[14] To the proving their necessity for the reaction (entries 12 and
best of our knowledge, simple aldehydes have not 13). Finally, omitting the Pd/C catalyst, and conduct-
been used directly as annulating agents to construct ing whole sequence under air provided product 3a in
the carbazole ring. We anticipated that this strategy 38% yield (entry 14). We also investigated the com-
may provide a direct approach to install various alkyl, patibility of other oxidants for the final aromatization
alkenyl, aryl, heteroaryl, and alkoxy functional groups step. Here, after conducting the 6p-electrocyclization
at the C-3 position of carbazole, which is still a major reaction at 1608C for 24 h in decalin (for all oxidant
challenge when using the existing methods. Beside screenings), the reaction mixture was treated with
this, it may open a new route to synthesize various 0.25 equiv. of iodine in dimethyl sulfoxide (1.0 mL,
structurally unique bis-carbazoles and carbazole alka- 24 h, at 1008C) to furnish 3a in 42% yield. Further-
loids in a step-economic way.
more, one-pot treatment of the electrocyclized prod-
To achieve the goal, four completely different reac- uct with either tert-butyl hydroperoxide or benzoqui-
tions need to be executed one after another without none as oxidants (2 equiv., 24 h) at room temperature
isolating a single intermediate (Scheme 1). The four failed to provide 3a. However, the same reaction with
reactions include: (i) a first Brønsted acid-catalyzed 2.0 equiv. of Dess–Martin periodinane (24 h, at room
generation of sulfonylindole A using PhSO2H;[14] (ii) temperature) provided 3a in 49% yield.
next, a Brønsted base-catalyzed in situ generation of
With the optimized conditions in hand, the scope of
2,3-dialkenylated indole B;[14] (iii) then, a 6p-electro- this method was studied with various aldehydes
cyclization reaction to obtain dihydrocarbazole C; having different functional groups. 4-Chlorophenyl-
and (iv) finally the dehydrogenation of C to furnish and 1-naphthylacetaldehydes successfully took part in
the desired carbazole 3.
the reaction and carbazoles 3b and 3c were isolated in
To optimize the reaction conditions, (E)-2-styryl- good yields (70–79%, Scheme 2). Heteroarylacetalde-
1H-indole 1a and commercially available phenylace- hydes were also found to be suitable reaction partners
Scheme 1. Strategy for the synthesis of carbazole.
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Table 1. Optimization of the reaction conditions.[a]
Entry
DBU [mol%]
Solvent (step 3)
Tempertaure [oC] (steps 1/2/3)
Time [h] (steps 1/2/3)
Yield[b] [%]
1[c,d]
2[d]
3[d]
4
5
5
5
xylene
xylene
xylene
xylene
xylene
xylene
xylene
decalin
mesityline
o-DCB
decalin
decalin
decalin
decalin
r.t./r.t./140
r.t./r.t./140
r.t./r.t./160
r.t./r.t./160
60/60/160
60/r.t./160
60/–/160
60/r.t./160
60/r.t./160
60/r.t./160
60/r.t./180
60/–/–
3/0.5/72
3/0.5/72
3/0.5/60
2.5/0.3/60
1.2/0.3/60
1.2/0.3/60
1.2/48
1.2/0.3/48
1.2/0.3/60
1.2/0.3/60
1.2/0.3/44
–
44
54
72
76
69
78
51
78
<2
<2
84
–
10
10
10
30
10
10
10
10
10
–
5
6
7[e]
8
9
10
11
12[f]
13
14[g]
60/r.t./180
60/r.t./180
1.2/3/48
1.2/0.3/44
35
38
10
[a]
Reactions were carried out in a sealed pressure tube using 1.5 equiv. of 2a, 1.2 equiv. of PhSO2H, 2.5 equiv. of NaH.
Isolated yields.
[b]
[c]
[d]
[e]
[f]
5.0 mol% of Pd/C were used.
5.0 mol% of PTSA·H2O were used.
Without NaH, and omitting step 2.
Without PTSA·H2O, step 1 was complex.
Without Pd/C, in the presence of air.
[g]
(3d and 3e, 40–77% yields). To synthesize compara-
The scope of this method was further explored by
tively less explored 3-alkylated carbazoles, various ali- varying the 2-alkenylindole partner. Several 2-arylat-
phatic aldehydes were subjected to the standard con- ed carbazoles 3t–v were prepared by using the corre-
ditions. Aliphatic aldehydes having normal as well as sponding 1d and 1e (62–82%, Scheme 3). Although
branched chains were suitable substrates (3f–j, 45– installation of two sterically hindered aryl substituents
67%). Aldehydes bearing ester and amide functional at the vicinal position of an aromatic ring is rarely re-
groups also furnished the desired carbazoles 3k–m in ported in the literature, 2,3-dinaphthylcarbazole 3w
42–68% yields. Benzannulation using acetaldehyde was readily synthesized in 88% yield. Installation of
was also accomplished to provide 2-phenylcarbazole three phenyl substituents at the 1,2, and 3 positions of
3n in 51% yield. This opens up a route to synthesize carbazole 3x was also accomplished in 66% yield. Fur-
several 2-arylcarbazoles which are difficult to synthe- thermore, 2,3-dialkylated carbazole 3y was obtained
size by traditional cross-coupling reactions due to the in moderate yield (40%). An electron-deficient 2-al-
difficulties associated with the synthesis of 2-halocar- kenylindole was also found to be a suitable substrate
bazole electrophiles.[15] Acetone was also a suitable (3z, 52%).
substrate for this annulation reaction albeit furnishing
3-Hydroxy-
and
3-methoxycarbazoles
are
a lower yield of the carbazole 3o. Alkenylated carba- a common structural motif present in many biologi-
zoles are important building blocks for the synthesis cally active carbazole natural products (Figure 1).[1,3]
of OLED materials, and the installation of an alkenyl Installation of a hydroxy functionality at the C-3 posi-
unit on carbazole required significant efforts.[16] Pleas- tion of carbazole will eventually lead to many carba-
ingly, 3-alkenylated carbazoles 3p and 3q were direct- zole alkaloids or their advanced intermediates directly
ly synthesized in 45–55% yields using appropriate al- from the respective 2-alkenylindoles 1. The success of
kenylacetaldehydes. Gratifyingly, readily synthesized our strategy depends on the generation of highly elec-
2-indolyl-1,3-dienes 1b ꢁand 1c were compatible under tron-rich enol ether intermediate B (Scheme 1, R4 =
the reaction conditions to provide the challenging 2- OR). To confirm the sustainability of the alkoxy
alkenylated carbazoles 3r and 3s in 50% and 46% group present in C under the reaction conditions, first
yields (Scheme 3).
aldehyde 2r was reacted with 1a. Pleasingly, carbazole
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Scheme 2. Scope of aldehydes. Reaction conditions: 1a (1.0 equiv.), aldehyde 2 (1.5 equiv.), PhSO2H (1.2 equiv.), PTSA·H2O
(10 mol%), THF, 608C; NaH (2.5 equiv.), DBU (10 mol%), THF, room temperature; Pd/C (10 mol%), decalin, 1808C; iso-
lated yields.
Scheme 3. Scope of 2-alkenylindoles. Reaction conditions: 2-alkenylindole 1 (1.0 equiv.), aldehyde 2 (1.5 equiv.), PhSO2H
(1.2 equiv.), PTSA·H2O (10 mol%), THF, 608C; NaH (2.5 equiv.), DBU (10 mol%), THF, room temperature; Pd/C
(10 mol%), decalin, 1808C; isolated yields.
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4a was isolated in 38% yield along with 26% of its de- aldehyde 2s with 1j furnished the desired hyellazole
benzylated analogue 4b (Scheme 4). Conducting the natural product in 31% yield in one-pot.[1a,3a,c,g] In
whole sequence under air as discussed earlier a similar fashion, synthesis of 6-chlorohyellazole was
(Table 1, entry 14), provided product 4a in 17% yield, also accomplished from 1k in 38% yield. Finally, o-
and no 4b was detected. To avoid debenzylation and methylcarazostatin 4d was synthesized in one-pot
to install the required methoxy functional group, alde- from 1l. o-Methylcarazostatin 4d was previously used
hyde 2s was chosen, and product 4c was isolated in for synthesis of antiostatin A4, antiostatin B4, and car-
43% yield. To our delight, despite the presence of bazoquinocin C.[3c,f,k,l] Two more advanced intermedi-
benzyloxy or methoxy leaving groups, no dealkoxylat- ates 4e and 4f were also prepared in 26% and 21%
ed product 3n was detected. Treatment of the crude yields, respectively. Synthesis of alkaloid antiostatin
B2 was reported from 4e,[3f] and carbazomycin A and
carbazomycin B from 4f.[3b,h–j]
Among the four possible core structures of biscar-
bazoles accessible via C–N coupling, 3,9’- and 2,9’-bis-
carbazoles are of particular interest due to their well-
studied pharmacological properties.[1a,c,17] Beside
these, 3,9’-linked carbazole oligomers exhibit photo-
emission properties with high quantum yield.[8b] The
present methods to prepare them by transition metal-
catalyzed C–N cross-coupling reactions require a pro-
tecting group for the carbazole electrophile.[18] A pro-
tecting group-free synthesis of 3,9’-biscarbazole 4g
was realized by using aldehyde 2t (63%, Scheme 5).
The core structure present in biscarbazole 4g is also
found in many carbazole alkaloids, such as murastifo-
line A.[17a] In a similar way methylene-linked biscarba-
zole 4h was also prepared in 40% yield using alde-
hyde 2u. The core structure of 4h is found in the bis-
carbazole-based natural products murrafoline E, bis-
murrayafoline A, and bismurrayafolinol.[17c,d] Acetate-
protected cholic acid-derived aldehyde 2v also reacted
with 1a to provide the highly functionalized carbazole
5 in 32% yield.
Considering the importance of 3,3’-biscarbazole for
the preparation of conjugated poly(3,6-carbazole)s,[19]
a method was developed to obtain 6 directly from 1a.
Here biscarbazole 6 was synthesized in 43% yield
using succinaldehyde by the one-pot, 2-fold sequential
triple-relay catalysis associated with eight distinct
steps having an average yield of 90% for each
(Scheme 6). Similarly, a methylene-linked biscarba-
zole 7 was also synthesized in 45% yield using gluta-
raldehyde (Scheme 6).
In summary, benzannulation of 2-alkenylindoles
using readily available aldehydes was achieved to con-
struct structurally diverse carbazoles. The usefulness
of the method was demonstrated by synthesizing car-
bazole alkaloids or their advanced intermediates. Al-
kenylated carbazoles, an important class of monomers
to obtain conjugated-organic materials, were also pre-
pared in one step. A protecting group-free synthesis
of 3,9’-biscarbazole was also realized by this method.
This method was also further extended to furnish 3,3’-
biscarbazole by one-pot, 2-fold sequential triple-relay
catalysis. We believe that this method has high poten-
tial to provide a facile route to carbazole-based conju-
Scheme 4. Synthesis of carbazole alkaloids.
gated materials in a step- and atom-economic way.
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Experimental Section
Representative Method for the Synthesis of 3a
THF (0.4 mL) was added to
a mixture of PhSO2H
(0.24 mmol) and PTSA·H2O (0.02 mmol) in a 15-mL sealed
pressure tube. To the resulting reaction mixture 1a
(0.20 mmol) and a solution of 2a (0.30 mmol) in THF
(0.6 mL) were added and the mixture heated at 608C with
continuous stirring. Upon complete consumption of the
starting material, the reaction mixture was cooled down to
room temperature, NaH (0.50 mmol) and DBU (0.02 mmol)
were added successively and stirring was continued at room
temperature. Upon complete consumption of sulfonylindole
A, Pd/C (0.02 mmol) was added to the reaction mixture
along with 2.0 mL decalin, the sealed tube was filled with ni-
trogen gas and heated to 1808C. After completion of the re-
action, the crude residue was directly purified by silica gel
column chromatography to obtain 3a; yield: 84%; see the
Supporting Information for details.
Acknowledgements
M.S.M. gratefully acknowledges DST/INSPIRE Faculty
Award/2013/DST/INSPIRE/04/2013/000681 and SRIC, IIT
Kharagpur (ISIRD grant) for funding. S.S. thanks CSIR
India, and A.B. thanks UGC, India for fellowships.
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8 Benzannulation of 2-Alkenylindoles using Aldehydes by
Sequential Triple-Relay Catalysis: A Route to Carbazoles
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Ankush Banerjee, Samrat Sahu, Modhu Sudan Maji*
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