Diversity oriented convergent access for collective total synthesis of bioactive multifunctional carbazole alkaloids: Synthesis of carbazomycin A, carbazomycin B, hyellazole, chlorohyellazole, and clausenaline D

Article abstract of DOI:10.1021/ol502721r

Facile syntheses of imperative carbazole alkaloids carbazomycin A, carbazomycin B, hyellazole, chlorohyellazole, and clausenaline D have been demonstrated starting from readily available Boc-protected 3-formylindole and dimethyl maleate. The suitably subs

Full text of DOI:10.1021/ol502721r

Letter
pubs.acs.org/OrgLett
Diversity Oriented Convergent Access for Collective Total Synthesis
of Bioactive Multifunctional Carbazole Alkaloids: Synthesis of
Carbazomycin A, Carbazomycin B, Hyellazole, Chlorohyellazole, and
Clausenaline D
Shivaji B. Markad and Narshinha P. Argade*
Division of Organic Chemistry, National Chemical Laboratory (CSIR), Pune 411 008, India
*
S Supporting Information
ABSTRACT: Facile syntheses of imperative carbazole alkaloids
carbazomycin A, carbazomycin B, hyellazole, chlorohyellazole, and
clausenaline D have been demonstrated starting from readily
available Boc-protected 3-formylindole and dimethyl maleate. The
suitably substituted aromatic rings have been designed comprising
three/four significant C−C bond forming reactions. The competent
Wittig reaction, selective monoalkylations, one-pot regioselective
Weinreb amide formation and Boc-deprotection, well designed
Grignard reactions, dehydrative intramolecular cyclizations, and
Baeyer−Villiger rearrangement of aromatic aldehydes were the main
features.
otal synthesis of bioactive natural products leading to1
transporting electroluminescent materials and are potential
building blocks in functional materials owing to their electrical
and thermal properties. Therefore, carbazoles have been crucial
T
essential medicines is the priority area in science.
Development of new synthetic stratagems for the collective
total synthesis of different classes of natural products is a
6
target compounds and several elegant product specific syntheses
2
3
−5
challenging task of contemporary interest. A large number of
of these have been reported during the past few decades. The
main steps involved in their synthesis were acid/base/metal/
heat/light-catalyzed aryl−carbon/aryl−nitrogen/aryl−aryl cou-
plings of two suitably substituted building blocks and the specific
carbazole alkaloids have been isolated from plant, animal,
3
microbial, and marine genesis (Figure 1). They are an important
3
−5
intramolecular cyclizations.
In the synthesis of carbazoles,
construction of an appropriate fully functionalized aromatic ring
system is the important assignment for steric and/or electronic
7
factors and reactivity reasons. Despite tremendous synthetic
efforts to develop regioselective installation of appropriate
substituents on these heterocyclic structures, general and
3
−6
efficient methods are still limited.
In the continuation o8f
our studies on the total synthesis of bioactive natural products,
we reasoned that the readily available suitably substituted 3-
formylindole derivatives and dimethyl maleate would constitute
a diversity oriented new approach to this important class of
compounds. In this context we herein report the robust route to
essential carbazole alkaloids (Schemes 1−3).
Figure 1. Diversely substituted carbazole alkaloids.
A careful search of major carbazole alkaloid structures revealed
that a pathway encompassing a completely open scope for
introduction of a broad range of substituents at appropriate
positions would provide a general approach to an array of
fascinating bioactive natural and unnatural carbazole and fused
carbazole architectures. A general representation and concise
class of natural products from the point of view of novel
structural topographies and major biological activities.
3
−5
Carbazoles exhibit well proven antitumor, antibiotic, antiviral,
anti-HIV, anti-inflammatory, antimalarial, psychotropic, anti-
histaminic, antioxidative, and significant antituberculosis activ-
ities. Moreover, carbazoles are used in the treatment of
hypertension, ischemic heart disease, and congestive heart
Received: September 17, 2014
Published: October 9, 2014
3
−5
failure.
They have also been used in illustrious hole-
©
2014 American Chemical Society
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dx.doi.org/10.1021/ol502721r | Org. Lett. 2014, 16, 5470−5473

Organic Letters
Letter
Scheme 1. General Representation of Known Major Carbazole Alkaloids and Their Retrosynthetic Analysis
Scheme 2. Synthesis of Carbazole Alkaloids from Boc-Protected 3-Formylindole and Dimethyl Maleate
retrosynthetic analysis of carbazole alkaloids has been depicted in
Scheme 1. Retrosynthetically, the stepwise inter- and intra-
molecular coupling reactions of three carbon units from suitably
substituted 3-formylindole derivatives with three carbon units
from dimethyl maleate would constitute a general pathway to the
majority of diversely substituted carbazole alkaloids. As
represented in Scheme 1, multiple electro- and nucleophilic
reactions would be possible on the potential antecedent dimethyl
indolylmethylenesuccinate. Productively, it bears the correct
number of electro- and nucleophilic C-atoms, accurately located,
to introduce the appropriate substituents and/or functional
groups along with a site for dehydrative intramolecular
cyclization to form the desired aromatic ring structure.
Accordingly, we prepared a synthetic plan to accomplish the
collective synthesis of five carbazole alkaloids: carbazomycin A
tributylphophine was not very efficient, and the required product
was formed only in 33% yield. However, the alternatively
performed reaction of Boc-protected 3-formylindole (1) with the
same Wittig reagent stereoselectively furnished the desired
potential precursor 3 in 90% yield, essentially under neutral
reaction conditions (Scheme 2). The E-geometry of product 3
was confirmed on the basis of peri interaction of the vinylic
8a,16
proton with an ester carbonyl group.
The conjugation of the
3-formyl unit with the lone pair of electrons on the indole N-
atom is responsible for its decline in reactivity, and therefore the
N-Boc protection activates it for the anticipated Wittig reaction.
The NaHMDS induced chemo- and regioselective monomethy-
lation of an activated allylic methyelene carbon in 3 with methyl
iodide provided the necessary product 4 in 76% yield.
Providentially, the formed carbanion did not endure the plausible
intramolecular 1,6-addition course generating a 6−5−5 hetero-
cyclic system. Trimethylaluminum prompted the regioselective
coupling reaction of N,O-dimethylhydroxylamine hydrochloride
(
antibiotic) and carbazomycin B (antibiotic and 5-lipoxygenase
9
inhibitor) isolated from Streptoverticillium ehimense, hyellazole
and chlorohyellazole from Hyella caespitosa, and clausenaline D
10
11
17
from Clausena lansium. Several syntheses of the first four target
with diester 4 to form the desired product 5 in 82% yield.
compounds and their closely related analogues have been known
Conveniently, transformations of the more reactive uncon-
jugated ester unit to a Weinreb amide and N-Boc-deprotection
took place in the presence of trimethylaluminum/N,O-dimethyl-
hydroxylamine hydrochloride. The well-organized chemoselec-
tive reactions of methylmagnesium bromide and phenyl-
magnesium bromide with Weinreb amide 5 cleanly delivered
corresponding ketones 6a,b in 74% and 71% yields, respectively.
1
2−14
in the literature.
related furocarbazoles have also been known in the literature,
Several syntheses of structurally closely
15
while the synthesis of the very recently isolated clausenaline D is
awaited.
The initially studied reaction of 3-formylindole with an in situ
16
generated Wittig reagent from dimethyl maleate (2) and
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Organic Letters
Letter
Scheme 3. First Total Synthesis of Clausenaline D
p-TSA catalyzed dehydrative intramolecular cyclization of
ketones 6a,b, providing the products 7a,b in 90%/93% yields,
efficiently constituting the rightly functionalized aromatic ring C.
LAH-reduction of the ester function in 7a,b followed by the
PCC-oxidation of formed intermediate alcohols 8a,b yielded
corresponding aromatic aldehydes 9a,b in very good overall
yields in two steps. The Baeyer−Villiger oxidation of aromatic
aldehydes 9a,b formed the corresponding unisolable formate
esters 10a,b which, upon in situ hydrolysis, transformed to the
delivered the desired natural product clausenaline D (21) in
94% yield over two steps. The analytical and spectral data
obtained for clausenaline D were in complete agreement with the
11
reported data.
In summary, starting from easily available chemicals and
reagents, we have demonstrated a diversity oriented convergent
access for collective synthesis of five different carbazole alkaloids
with very good overall yields. In the present approach, generation
of a suitably substituted basic carbazole skeleton in just 4/5 steps
utilizing three carbon fragments from each 3-formylindole and
dimethyl maleate is remarkable. Furthermore, the generation of
biaryl systems without the aryl−aryl coupling is also noteworthy.
Syntheses of these alkaloids have been accomplished without
involving any discrete protection−deprotection steps. For an
appropriate functionalization of aromatic ring A, the synthesis
itself can begin with suitably substituted indole derivatives;
otherwise, the functionalization of aromatic ring A toward the
end part of the total synthesis would also be feasible. Further
studies on the total synthesis of a few selected carbazole alkaloids
using different types of electro- and nucleophilic reactions on the
pivotal intermediate dimethyl indolylmethylenesuccinate are in
progress in our laboratory. The present approach is general in
nature and can provide access to a large number of carbazole
alkaloids and their congeners for SAR studies.
1
2p,q
known phenolic compounds 11a,b
in 95%/96% yields. The
present Baeyer−Villiger oxidation reaction was highly temper-
ature and time sensitive. Fortunately, we did not notice any
oxidation of aldehyde to the corresponding carboxylic acid under
the employed set of reaction conditions. The base-catalyzed
chemoselective O-methylation of compound 11a gave the
corresponding known methyl ether intermediate 12a in 98%
yield from which the three-step synthesis of carbazomycin B (13)
12q
is known in the literature. Carbazomycin B (13) on simple O-
12q
methylation forms carbazomycin A (14) in high yield.
Methylation of compound 11b furnished the natural product
hyellazole (12b) in 97% yield from which a two-step synthesis of
yet another natural product, chlorohyellazole, is known in the
1
3c
literature. The analytical and spectral data obtained for both
showed that the advanced intermediate 12a of carbazomycins
and hyellazole (12b) were in complete agreement with the
1
2,13
reported data.
ASSOCIATED CONTENT
In the next phase, the total synthesis of fused carbazole
clausenaline D was planned by assigning the present protocol
■
*
S
Supporting Information
(Scheme 3). The NaHMDS induced regioselective introduction
Experimental procedures for the preparation of all compounds
of an allyl group at an activated allylic methyelene carbon in 3
with allyl bromide provided the required product 15 in 73% yield.
Controlled base-catalyzed regioselective hydrolysis of the more
reactive unconjugated ester unit in 15 was feasible under ambient
reaction conditions and directly formed the Boc-deprotected
1
13
and their tabulated analytical and spectral data. H NMR,
NMR, and DEPT spectra of compounds 3−9, 11, 12, and 15−
1. This material is available free of charge via the Internet at
C
2
http://pubs.acs.org.
monoacid 16 in 92% yield. The Ac O−AcONa stimulated
2
AUTHOR INFORMATION
dehydrative intramolecular cyclization of product 16 directly
delivered the corresponding N- and O-acylated carbazole
derivative 17 in 88% yield. The transformation of the C−C
double bond in compound 17 to the corresponding diol followed
by an in situ oxidative cleavage using OsO and NaIO delivered
■
*
Corresponding Author
E-mail: np.argade@ncl.res.in.
Notes
4
4
the desired aliphatic aldehyde 18 in 93% yield. The use of diacyl
protection in substrate 17 was essential, as the corresponding
The authors declare no competing financial interest.
free phenolic compound on treatment with OsO /NaIO4
4
ACKNOWLEDGMENTS
resulted in a complex reaction mixture. A one-pot p-TSA/
MeOH mediated double deacylation of 18 followed by
dehydrative intramolecular cyclization in refluxing p-TSA/
toluene yielded the furocarbazole 19 in 94% yield. The
DIBAL-H reduction of the aromatic ester unit in 19 to the
corresponding alcohol 20 followed by its PCC-oxidation
■
S.B.M. thanks CSIR, New Delhi, for the award of a research
fellowship. N.P.A. thanks the Department of Science and
Technology, New Delhi, for financial support. We also gratefully
acknowledge the financial support from the CSIR-Network
Project.
5
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Organic Letters
Letter
S.; Lee, M.; Kerns, E.; Krishnan, B.; Matson, J.; Hesler, G. J. Antibiot.
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