Regioselective Synthesis of Chirally Enriched Tetrahydrocarbazolones and Tetrahydrocarbazoles

Article abstract of DOI:10.1021/acs.orglett.8b01656

A new one-step, reagent-directed regioselective synthesis of chirally enriched tetrahydrocarbazolones and tetrahydrocarbazoles from a common type of substrate has been developed. The salient features of this method include inherited stereodiversity, a bro

Full text of DOI:10.1021/acs.orglett.8b01656

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Regioselective Synthesis of Chirally Enriched
Tetrahydrocarbazolones and Tetrahydrocarbazoles
Chintam Narayana,† Priti Kumari,† and Ram Sagar*,†,‡
†Department of Chemistry, School of Natural Sciences, Shiv Nadar University (SNU), NH91, Tehsil-Dadri, Gautam Buddha Nagar,
Uttar Pradesh 201314, India
‡Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India
S
* Supporting Information
ABSTRACT: A new one-step, reagent-directed regioselective synthesis of chirally enriched tetrahydrocarbazolones and
tetrahydrocarbazoles from a common type of substrate has been developed. The salient features of this method include
inherited stereodiversity, a broad substrate scope, a quick reaction time, and a benign catalyst. The method is applicable to the
synthesis of a bioactive cryptosanguinolentine precursor.
arbazoles and their derivatives are important nitrogen-
Sporangium cellulosum strain Soce375, which exhibits cytotox-
Ccontaining heterocyclic compounds having a tricyclic icity against a mouse fibroblast cell line.6 Similarly, there are
structure that are widespread in nature.1 Tetrahydrocarbazo-
chiral tetrahydrocarbazole (CTHC) based bioactive molecules
currently used as drug candidates (Figure 1).7
lone is a subclass of carbazoles that consists of an indole ring
Considering the biopotential of tetrahydrocarbazolones and
tetrahydrocarbazoles, several groups have developed various
methods for the synthesis of these derivatives. The basic
method is the Fisher indole synthesis;8a however, the
drawbacks of this method are the highly acidic conditions
and poor regioselectivity it displays.8b Later transition metal
(Pd and Cu)-catalyzed intramolecular cyclizations of o-halo
aryl enaminones were also reported.9 Wang and co-workers
have also reported metal-free hypervalent (III) iodine
promoted oxidative annulation of 2-aryl enaminones for
carbazolone synthesis.10
fused at the C-2 and C-3 position with a cyclohexanone.
Carbazolones are key intermediates in the synthesis of natural
product alkaloids such as murryaquinone A2 as well as
pyrayaquinone A and B.3 Compounds with a carbazolone
nucleus exhibit diverse biological activities including serving as
HIV-integrase inhibitors,4a potassium channel blockers,4b and
antipsychotics.4c Furthermore, chiral tetrahydrocarbazolones
are one of the most important structural motifs present in drug
molecules. (R)-Ondansetron is a 5-HT3 receptor antagonist,
used for treatment of nausea and vomiting, caused during
chemotherapy in cancer patients (Figure 1). Its S-isomer and
racemic mixture are not suitable for medical use.5 Sorazolone A
is a chirally functionalized tetrahydrocarbazolone isolated from
Reductive cyclization is another important method for
̈
carbazolone synthesis. Soderberg and co-workers have further
reported a Pd-catalyzed reductive N-heteroannulation for the
synthesis of carbazolones, but it requires CO.11a Yao and co-
workers have developed an Fe/AcOH mediated intramolecular
reductive cyclization of 3-hydroxy-2-(2-nitrophenyl) enones
for carbazolone synthesis.11b Recently, Zhu and co-workers
have accomplished an efficient synthesis of substituted
carbazolones by reductive cyclization of chirally substituted
(2-nitro phenyl) enones in the presence of aqueous TiCl3 and
NH4OAc.11c However, there are only a few synthetic reports
available for the synthesis of chirally enriched tetrahydro-
carbazolones.12 The development of an efficient method for
Figure 1. Some drugs and natural products with chirally enriched
tetrahydrocarbazolones (a−c) and tetrahydrocarbazoles (d−f).
Received: May 25, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01656
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the synthesis of chiral tetrahydrocarbazolones thus remains an
attractive challenge.
usually complete within 20 min and resulted in the Ullmann
product being formed in very good yield. After successful
synthesis of the Ullmann products, the next key step attempted
was reductive cyclization. The reaction optimization was
carried out as summarized in Table 1. We attempted several
On the other hand, there are quite a few synthetic methods
that have been reported for the synthesis of chiral
tetrahydrocarbazoles (CTHCs). Chen and co-workers re-
ported an asymmetric [4 + 2] cycloaddition reaction between
α,β-unsaturated aldehydes and in situ generated active indole-
2,3-quinodimethanes under mild acidic conditions for the
synthesis of CTHCs.13a Kerr and co-workers developed a
Zn(NTf)2 catalyzed [3 + 3] annulative strategy based upon the
reaction of donor−acceptor (DA)-cyclopropanes with 2-
alkynylindoles for the synthesis of substituted tetrahydrocarba-
zoles.13b Ghorai and co-workers reported a diastereoselective
synthesis of functionalized chirally enriched tetrahydrocarba-
zoles via Lewis acid mediated domino-ring-opening cyclization
of donor−acceptor cyclopropanes with 2-vinylindoles.13c
However, the stereoselective synthesis of polysubstituted chiral
tetrahydrocarbazoles remains a highly challenging task.
Herein, we disclose a new regioselective synthesis of
polysubstituted chirally enriched tetrahydrocarbazolones and
carbazoles in one step starting from Ullmann products. The
Ullmann products were prepared from commercially available
o-bromo nitroarenes and readily preparable stereodiverse chiral
synthones, α-iodo cyclohexanones 1−3 (Scheme 1).
Table 1. Reaction Optimization for Synthesis of
Tetrahydrocarbazolone 29
a
b
entry
reductant
temp (°C)
time
yield (%)
1
2
3
4
5
6
7
8
9
none
none
none
PPh3
PPh3
PPh3
pinacol
pinacol
pinacol
pinacol
70
90
130
70
90
130
130
130
130
130
15 min
15 min
15 min
15 min
15 min
15 min
15 min
15 min
15 min
12 h
NR
NR
NR
traces
10
60
50
70
65
c
d
e
10
60
a
Reactions were conducted using 5 mol % MoO2Cl2(dmf)2 and 4
b
c
Scheme 1. Synthesis of Ullmann Products 15−28
equiv of reductant. μW heating. 2 equiv of reductant used in this
case. Dimethylacetamide (DMA) used as solvent. Thermal heating.
d
e
NR: No reaction.
reductive cyclization conditions (Supporting Information (SI))
on substrate 15, but these always resulted in a statistical
mixture of products, which were difficult to purify and
characterize. We therefore looked for new reagents and
reaction conditions for arriving at our designed tetrahydro-
carbazolone product 29.
Recent reports have revealed that metal-oxo complexes can
be useful reagents in organic synthesis.16 A plethora of reports
are available on MoO2Cl2 complexes and their importance in
several organic reactions such as oxidation, reduction, catalytic
hydrogen transfer, etc.16a Among these, catalytic oxo-transfer
reactions are a very important process. Fernandes and co-
workers reported reduction of pyridine N-oxides to pyridine
and sulfoxide to sulfide using MoO2Cl2 as a catalyst and
triethylsilane as a reductant.16b Santz and co-workers reported
MoO2Cl2 catalyzed reductive cyclization of nitro aromatics
using PPh3 toward carbazole preparation.16c Later, Santz and
co-workers reported several organic transformations, viz nitro
to amine, sulfoxide to Sulphone, and deoxygenation of
heteroaromatic N-oxides using pinacol as a reducing agent.16d
Here, we have developed a new method for the synthesis of
chirally enriched tetrahydrocarbazolones using
MoO2Cl2(dmf)2 as a catalyst and pinacol as a green reductant.
As shown in Table 1, the catalyst MoO2Cl2(dmf)2 alone was
not sufficient to transform substrate 15 into product 29, even
at high temperature (Table 1, entries 1−3), whereas use of 4
equiv of PPh3 as a reductant in the same reaction at 130 °C in
toluene resulted regioselectively in tetrahydrocarbazolone 29
(60%) within 15 min (Table 1, entry 6). The other possible
We planned to transfer natural chirality into our designed
tetrahydrocarbazolones. Therefore, we used carbohydrates as
chiral synthons. The required α-iodo cyclohexenones 1−3
were synthesized from D-glucose, D-galactose, and D-mannose
respectively as per the literature.14 Palladium-catalyzed
Ullmann coupling15 between α-iodo cyclohexenones 1−3
and commercially available o-bromo nitroarenes 4−14
furnished the α-substituted cyclohexenones 15−28 in good
yield (Scheme 1). We also performed selected Ullmann
reactions under microwave conditions, where the reaction was
1
regioisomer was not seen even by crude H NMR spectros-
copy. Use of less equivalents of reductant or a lower
temperature decreases the product yield and increases the
reaction time, inferring that 4 equiv of reductant and 130 °C
temperature are the optimal conditions. We were interested in
B
DOI: 10.1021/acs.orglett.8b01656
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Organic Letters
Letter
replacing the reductant PPh3, as it is converted to the
corresponding oxide, which was difficult to separate from
product 29. We planned to use pinacol as a reductant, as it
gives acetone and water as byproducts, which can be
evaporated. After careful screening of the reaction conditions,
as shown in Table 1, use of 4 equiv of pinacol and 5 mol %
MoO2Cl2(dmf)2 catalyst afforded clean regioselective tetrahy-
drocarbazolone 29 in 70% isolated yield (entry 8) within 15
min under microwave heating at 130 °C in toluene. When we
carried out the same transformation in polar solvents such as
acetonitrile and DMSO, it did not result in any product,
whereas dimethylacetamide furnished product 29 in 65%
isolated yield (Table 1, entry 9). This transformation was also
feasible at 130 °C but required 12 h (Table 1, entry 10).
After optimization of the reaction conditions for reductive
cyclization, we next examined substrate scope for this
transformation. We screened several functional groups bearing
the stereodivergent substrates 15−28, e.g. fluoro, chloro,
bromo, keto, ester, etc., on the aromatic ring, and they were
well tolerated and furnished the corresponding tetrahydro-
carbazolones 29−42 regioselectively in acceptable to very good
yields (Scheme 2). Substrates with a nitro group (22) or a
Scheme 3. Synthesis of Compound 43
Figure 2. Plausible Mechanism for Tetrahydrocarbazolone For-
mation.
Scheme 2. Synthesis of Tetrahydrocarbazolone 29−42
MoO2Cl2(dmf)2 reacts with pinacol to form pinacolate
complex A and water. Pinacolate complex A releases acetone
to give the molybdenum complex B. Oxidative cleavage of the
pinacolate by Mo(VI) converts it to Mo(IV) complex B and
acetone. Then the nitro group in substrate 15 is converted to
the nitroso intermediate C through oxo group transfer and the
oxidation state VI of the molybdenum catalyst is restored. The
nitroso intermediate C then undergoes 6π electrocyclization
resulting in the N-oxide intermediate D. The catalyst
MoO2Cl2(dmf)2 and reductant undergo a similar catalytic
cycle on intermediate D to furnish the tetrahydrocarbazolones
29.
After successfully obtaining these results, we turned our
attention toward synthesis of chiral tetrahydrocarbazoles using
the common substrate 15. Initially, we treated 15 with Zn dust
in aqueous NH4Cl followed by NaCNBH3 in methanol, which
furnished tetrahydrocarbazole 44 in 20% yield along with a
mixture of products.17 Fe-catalyzed reductive cyclization in
HCl18 resulted in a complex mixture along with carbazole. In
2003, Banwell and co-workers reported a Pd/C/H2 gas
mediated reductive cyclization.19a Later Soderberg and co-
̈
workers reported a similar protocol for tetrahydrocarbazole
synthesis.19b However, Pd/C/H2 gas mediated reductive
cyclization failed on our substrate 15. Later, we attempted
SnCl2 and Pt/C/H2 conditions, which were also unsuccessful
(Table 2, entries 5 and 6). Recently, Banwell and co-workers
reported a Raney Ni and H2 gas mediated reductive annulation
reaction for tetrahydrocarbazoles.19c When we carried out a
Raney Ni mediated reductive annulation on substrate 15 in
methanol under H2 gas, it furnished chirally enriched
tetrahydrocarbazoles 44 in 60% isolated yield. This is the
first report on preparation of chirally divergent and substituted
tetrahydrocarabazoles through a reductive cyclization process.
After having identified and optimized our reaction
conditions, we next examined the scope of this reaction for
synthesis of substituted tetrahydrocarbazoles. Therefore, we
screened differently substituted substrates 15−20 on the
ketone group (25) gave product 36 or 39 respectively in lesser
yields. We did not observe dehalogenation on halogenated
substrates, nor reduction of keto or ester containing substrates.
All tetrahydrocarbazolones 29−42 were well characterized (SI)
including by single crystal X-ray analysis for 31 (Scheme 2).
We next investigated if there is any electronic substituent
effect on the chiral carbacycle for this identified trans-
formation. Therefore, electron-withdrawing groups (OAc)
were introduced after debenzylation and acetylation on
substrate 16 (SI). As shown in Scheme 3, when the resulting
tri-O-acetyl substrate was treated under the optimized reaction
conditions, this furnished tetrahydrocarbazolone 43 in good
isolated yield (63%).
The mechanism of this regioselective transformation can be
postulated to occur as shown in Figure 2. First the catalyst
C
DOI: 10.1021/acs.orglett.8b01656
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Organic Letters
Letter
Table 2. Reaction Optimization for Synthesis of
Tetrahydrocarbazole 44
Scheme 6. Debenzylation of 35
a
yield
Finally, we have shown the synthesis of the methyl derivative
of an indoloquinoline alkaloid, cryptosanguinilentine precursor
56, using this methodology. It possesses antimicrobial and
cytotoxic activity. The synthesis started with a palladium
catalyzed Suzuki−Miyaura cross-coupling reaction (SI)
followed by molybdenum catalyzed reductive cyclization on
55 under optimized reaction conditions (Scheme 7) to afford
56 in 68% isolated yield.
entry
reaction condition
(%)
1
2
3
4
5
6
Zn dust (8 equiv), NH4Cl, NaCNBH3, MeOH, rt, 2 h
Fe powder (5 equiv), 12 N HCl, EtOH/H2O, 80 °C, 12 h
Pd/C, H2 gas, MeOH, rt, 3 h
SnCl2 (5 equiv), EtOH, 80 °C, 2 h
Pt/C, MeOH, H2 gas, rt, 2 h
20
mix
NR
mix
NR
60
Raney Ni, MeOH, H2 gas, rt, 3 h
a
Mix: Complex mixture was observed. NR: No reaction.
Scheme 7. Synthesis of Natural Product Precursor 56
benzene ring and stereodivergent substrates 26−28. As shown
in Scheme 4 they were successfully transformed into products
Scheme 4. Synthesis of Compounds 44−52
In summary, we have developed an efficient one-step
protocol for the regioselective synthesis of highly function-
alized stereodiverse chirally enriched tetrahydrocarabazolones
and tetrahydrocarabazoles from a common substrate. The
process is catalytic for the synthesis of chirally enriched
tetrahydrocarabazolones and requires only 15 min. Mild
reaction conditions, operational simplicity, and a green
reducing agent are the advantages of this method. The
protocol has a wide substrate scope, and electron-donating and
-withdrawing groups were well tolerated. We have shown that
chirally enriched tetrahydrocarbazole can be transformed into
the corresponding spiro product. Furthermore, we have
applied this methodology for the synthesis of a crypto-
sanguinolentine precursor.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01656.
■
44−52 in acceptable to good yields. In the case of substrate 19,
debromination was observed and the desired product 48 was
obtained in 45% isolated yield.
In further explorations, we have transformed chiral
tetrahydrocarabazole 45 into the chirally substituted spiro
compound 53 using N-bromosuccinimide (NBS) in AcOH/
H2O/THF (1:1:1) with a 57% isolated yield (Scheme 5). It is
worth mentioning that the spirooxoindoles are privileged
scaffolds with ubiquity in numerous natural products and
pharmacologically important drugs.20
Debenzylation was carried on the substrate 35 using BCl3 in
CH2Cl2 at −40 °C, which furnished polyhydroxy substituted
chirally enriched tetrahyrocarbazolones 54 in 60% yield
(Scheme 6).
S
Experimental procedures, reaction optimization con-
1
ditions, H and 13C NMR spectra, crystallographic data
and other supplementary data (PDF)
Accession Codes
CCDC 1586085 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 5. Synthesis of Chiral Spirooxoindole 53
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: ram.sagar@snu.edu.in; ram.sagar@bhu.ac.in.
ORCID
Ram Sagar: 0000-0003-2472-6247
D
DOI: 10.1021/acs.orglett.8b01656
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are thankful to the Shiv Nadar University (SNU)
for providing required laboratory facilities. C.N. and P.K. are
thankful to the Council of Scientific and Industrial Research
(CSIR), India for senior research fellowships. The authors are
also thankful to the Department of Science and Technology
(DST) India for financial support (DST-EMR/2014/000320).
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G.; Soderberg, B. C. Tetrahedron 2007, 63, 1183. (c) Yan, Q.; Gin, E.;
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Org. Lett. XXXX, XXX, XXX−XXX