Lewis acid-promoted synthesis of unsymmetrical and highly functionalized carbazoles and dibenzofurans from biaryl triazenes: Application for the total synthesis of clausine C, clausine R, and clauraila A

Article abstract of DOI:10.1002/chem.201102802

A natural approach: A Lewis acid-promoted nucleophilic aromatic substitution approach to the regioselective synthesis of highly substituted carbazoles and dibenzofurans has been developed. The annulation process is applied to the total synthesis of carbazole alkaloids clausine C, clausine R, and clauraila A (see scheme).

Full text of DOI:10.1002/chem.201102802

COMMUNICATION
DOI: 10.1002/chem.201102802
Lewis Acid-Promoted Synthesis of Unsymmetrical and Highly Functionalized
Carbazoles and Dibenzofurans from Biaryl Triazenes: Application for the
Total Synthesis of Clausine C, Clausine R, and Clauraila A
Weijun Yang, Jun Zhou, Binjie Wang, and Hongjun Ren*[a]
Carbazole and dibenzofuran motifs are commonly seen in
bioactive natural products, such as dicytodendrin B (1a,
Scheme 1),[1] vanillic acid derivatives (1b and 1c,
Scheme 2. Approaches to dibenzofuran and carbazole motifs. LG=leav-
ing group.
H bond activation by Knolker,[12] amination/intramolecular
direct arylation reaction by Ackermann and co-workers[13]
Scheme 1. Natural products with dibenzofuran and carbazole motifs.
À
and tandem decarboxylation/C H activation reaction by
Glorius and co-workers[14] (path b, Scheme 2). Furthermore,
the approach to carbazole based on organometallic reagents,
such as Grignard reagents has also been reported (path c,
Scheme 2).[15]
Scheme 1)[2] and murrayazoline (1d, Scheme 1).[3] They have
received significant attention as synthetic targets due to
their intriguing structural features and promising biological
activities. A large number of classical and nonclassical meth-
ods have been developed for the synthesis of carbazole and
dibenzofuran frameworks.[4] Since transition-metal-catalyzed
Although considerable efforts have been directed to the
synthesis of carbazole and dibenzofuran skeletons, and
many useful synthetic procedures have been developed, the
synthesis of highly substituted carbazole and dibenzofuran
remains difficult. In view of the particularly interesting phar-
macological properties[16] and wide applications in materials
science,[17] new practical synthetic methods for the construc-
tion of carbarzole and dibenzofuran units are highly desired.
The nucleophilic aromatic substitution of phenyl cation[18] is
a straightforward, efficient, and green way to construct the
carbazole and dibenzofuran cores. However, the research in
this area is still rare because the phenyl cation is unstable,
which limits its application in organic synthesis. Herein,
based on the generation of highly activated phenyl cation in-
termediates, we describe a BF3·OEt2-promoted intramolecu-
lar amination and oxylation strategy for the direct formation
of dibenzofurans and carbazoles from biaryl triazenes[19]
under Lewis acid conditions (path d, Scheme 2).
À
À
intermolecular formation of C N/C O bond through nitro-
gen/oxygen nucleophilic displacement of aryl halides can
promote various annulation reactions and provide efficient
synthetic transformation for the synthesis of heterocyclic
molecules, it was extensively investigated for the formation
of dibenzofuran and carbazole units.[5] Recently, combined
À
À
À
C H bond activation/C N or C O formation route to car-
bazole and dibenzofuran has been scrutinized by the groups
of Buchwald,[6] Gaunt,[7] Shi,[8] Chang,[9] and Liu[10] by using
À
Pd or copper as catalyst (path a, Scheme 2). In addition, C
H-bond activation and intramolecular direct arylation pro-
vide efficient methods for the synthesis of carbazole and di-
benzofuran.[11] Recent elegant examples include double C
À
[a] W. Yang, J. Zhou, B. Wang, Prof. H. Ren
Department of Chemistry, Zhejiang University
38 Zheda Road, Hangzhou 310027 (P.R. China)
Fax : (+86)571-8795-1512
Initially, we treated amino-triazene 2 with BF3·OEt2 in
DMF, leading to the formation of the desired carbazole 4a
in 50% yield (Scheme 3). Interestingly, when we treated di-
triazene 3a (R1 =R2 =Bn) with BF3·OEt2 in EtOH, the
yield of the formation of carbazole 4a increased to 80%.
Subsequently, we tested different substituents on the tri-
E-mail: renhj@zju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102802.
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13665

Next we extended the work
to construct dibenzofuran units.
Initially, we treated phenol tria-
zene 8a, which can be prepared
by using the Suzuki–Miyaura
cross-coupling reaction, with
BF3·OEt2 (4.0 equiv) in EtOH
at room temperature and the
desired product dibenzofuran-
2-carboxylic acid ethyl ester
(9a, Scheme 5) was obtained in
30% yield. However, when
DMF was used as the solvent,
the yield increased to 81%.
Upon exploring the scope of
the intramolecular oxylation, a
number of phenol–triazene sub-
strates (8b–8i) were examined
Scheme 3. Synthesis of functionalized carbozoles from diphenyltriazenes.
azene motif and found that when R1 =R2 =-(CH2)4- (3c),
the yield of the formation of carbazole 4a increased to
88%. Whereas using aromatic amine as the protecting
group (3d, R1 =nBu, R2 =Ph), the yield of the formation of
carbazole 4a decreased to 18%. We then extended the reac-
tion to diiodo substrate 3e, which was obtained from the ho-
mocoupling of monoiodo triazene Grignard reagent in the
presence of FeACHTUNGTRENNUNG
(acac)3,[15] (acac=acetylacetonate) and the
diiodocarbazole 4b was formed in 53% yield.
Following the success of the formation of carbazoles 4a
and 4b, the scope of this transformation was studied using
various functionalized biaryl triazenes. The results of the
tandem cross-coupling/nucleophilic aromatic substitution ap-
proach to functionalized carbazoles are summarized in
Scheme 4. Thus, heating (2-bromo-phenyl)-pyrrolidin-1-yl-
diazene (5a) with 3-borono-4-(1-pyrrolidinylazo)-benzoic
acid ethyl ester (6a) in the presence of PdACHTNUTRGNE(UNG PPh3)4 gave
biaryl triazene in excellent yield, which was subsequently
treated with BF3·OEt2, to give the desired 9H-carbazole-3-
carboxylic acid ethyl ester (7a) in 90% yield. The bromo-
triazenes bearing either electron-withdrawing groups or
electron-donating groups can all be converted to their corre-
sponding carbazoles (7b–h, 7p–r, Scheme 4) after coupling
with boronic triazenes and being treated with BF3·OEt2 in
65–98% yields. Sterically hindered substrates can also be
converted to their corresponding carbazoles (7i–l, 7n and
7s) in 59–93% yields. The boronic triazenes with different
substituted groups, such as CN, OiPr, and dimethyl groups,
can also be converted to their corresponding carbazoles in
59–92% yields (7p–t). In particular, when it came to di-
methyl phenyl triazene boronic acid, carbazoles 7s and 7t
were formed directly without further treatment with
BF3·OEt2. Functional groups, such as carbonyl group of
ketone (7 f and 7r) and aldehyde (7g and 7p), hydroxyl
(7j), alkynyl (7n), alkenyl (7l and 7s), and nitro groups
(7h), can be tolerated in this Lewis acid-promoted process.
Scheme 4. Scope of synthesis functionalized carbazoles by
cross-coupling/nucleophilic aromatic substitution approach. All reactions
were carried out with bromotriazene (0.5 mmol), boronic triazene
a tandem
(0.8 mmol), Cs2CO3 (1.25 mmol) and [PdACTHNUTRGNEU(NG PPh3)4] (0. 025 mmol) in diox-
ane (4 mL) at 1008C for 12 h. Then, the crude cross-coupling product
reacts with BF3·OEt2 (2.0 mmol) in EtOH (4 mL) at room temperature
for 10 min to 1.5 h. Yields shown are of the isolated product after flash
column chromatography. Carbazoles 7s and 7t were formed directly
without further treatment with BF3·OEt2.
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Lewis Acid-Promoted Synthesis
COMMUNICATION
phosphine and provided the coupling product 10a in 68%
yield (Scheme 6). After Mg/I exchange with iPrMgCl·LiCl
and transmetalation with ZnBr2, the iodo-dibenzofuran 9i
underwent a Negishi cross-coupling reaction with 5-bromo-
1,3-difluoro-2-(trifluoromethyl)benzene and provided the F-
substituted compound 10b in 70% yield. The Sonogashira
and Suzuki coupling reactions with the diiodo-carbazole 4b
gave the corresponding coupling products 10c and 10d in 89
and 75% yields, respectively.
The Lewis acid-promoted approach is a powerful method
for the construction of functionalized carbazoles in total
synthesis of carbazole alkaloids. Scheme 7 outlines the total
synthesis of clausine C,[21] clausine R,[22] and clauraila A[23]
using the BF3·OEt2-promoted annulation process as a key
step. The Suzuki-Miyaura cross-coupling between iodo-tri-
azene 11a and triazene–boronic acid 11b can furnish the
formation of biaryl ditriazene. After removing the catalyst
Scheme 5. Scope of synthesis functionalized dibenzofurans from phenol–
triazene substrates. All reactions were carried out with phenol–triazene
(0.5 mmol), BF3·OEt2 (2.0 mmol) in DMF (20 mL) at 50 or 708C for 3–
7.5 h. Yields shown are of the isolated product after flash column chro-
matography.
and the results were summarized in Scheme 5. The electron
properties at both the phenol moiety and the triazene
moiety do not significantly influence the outcome of the re-
action. The substrates bearing either electron-donating (8b,
8c, and 8d) or electron-withdrawing groups on the phenols
(8e–i) all give their corresponding cyclization products in
good to excellent yields (68–86%). Functional groups, such
as carbonyl of ketone (9g) and aldehyde (9h), ester (9d–e
and 9g), cyano (9c), and iodo (9i) can be tolerated in this
Lewis acid-promoted reaction.
The iododibenzofuran and iodocarbazole can be convert-
ed to their corresponding cross-coupling products using
transition-metal-catalyzed reactions.[20] Thus, the Heck reac-
tion took place when we heated iodo-dibenzofuran 9i with
butyl acrylate in the presence of PdACTHNUTRGNENUG(OAc)2 and tri-o-tolyl-
Scheme 7. Total synthesis of clausine C, clausine R, and clauraila A.
by flash column chromatogra-
phy, extended treatment of the
biaryl ditriazene with BF3·OEt2
led to the corresponding prod-
uct clausine C in 71% yield.
Carbazole 11d can be prepared
in a same procedure. Demethy-
lation of 11d with BBr3 afford-
ed carbazole clausine R in 68%
yield. Also in this case, claurai-
la A was formed in 72% yield
after treating 11d with DIBAL-
H and Dess–Martin reagents
(Scheme 7).
Scheme 6. Coupling reactions of iodo-substituted dibenzofuran and carbazole.
Chem. Eur. J. 2011, 17, 13665 – 13669
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In summary, we have developed a Lewis acid-promoted
nucleophilic aromatic substitution approach to the regiose-
lective synthesis of highly substituted carbazoles and diben-
zofurans in good to excellent yields. These reactions are car-
ried out in mild conditions with good tolerance to a variety
of functional groups. Furthermore, this new protocol was
successfully applied to the synthesis of natural carbazole al-
kaloids, such as clausine C, clausine R and clauraila A. Fur-
ther investigations of the reaction mechanism and synthetic
applications are underway.
Experimental Section
In a 50 mL Schlenk tube, aryltriazene 5a (0.5 mmol, 1.0 equiv), phenyl-
boronic acid 6a (0.8 mmol, 1.6 equiv), PdACHTNURTGNE(UNG PPh3)4 (0.025 mmol,
0.05 equiv), and Cs2CO3 (1.25 mmol, 2.5 equiv) were dissolved in dioxane
(4.0 mL). The tube was filled with N2 and immediately sealed with a
Teflon lined cap. The reaction mixture was heated to 1008C in an oil
bath and stirred for 12 h. After the reaction mixture was cooled to room
temperature, water was added. The resulting mixture was extracted with
ethyl acetate. The organic layer was dried over Na2SO4 and concentrated
under reduced pressure. The obtained mixture was dissolved in EtOH
(4.0 mL) and cooled to 08C. Next, BF3·OEt2 (0.26 mL, 4.0 equiv) was
added dropwise. After the mixture was stirred for 10 min at 08C, it was
warmed to room temperature and stirred for another 10 min to 1.5 h.
Then H2O was added and the reaction mixture was extracted with ethyl
acetate. The organic layer was dried over Na2SO4, concentrated under re-
duced pressure. The pure carbazole 7a was obtained in 90% yield after
purification by silica gel chromatography.
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Acknowledgements
We thank the financial support from National Nature Science Foundation
of China (20902081), Zhejiang University as well as the Fundamental Re-
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Keywords: alkaloids
nucleophilic substitution · regioselectivity · triazene
·
carbazole
·
dibenzofuran
·
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Received: September 7, 2011
Published online: November 8, 2011
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