From PtCl2- and acid-catalyzed to uncatalyzed cycloisomerization of 2-propargyl anilines: Access to functionalized indoles

Article abstract of DOI:10.1002/anie.200604026

(Chemical Equation Presented) On the move: PtCl₂-catalyzed electrophilic activation of an alkyne moiety triggers a heterocyclization accompanied by migrations of both an allyl group and an acetate group to produce an indole (see scheme). Variat

Full text of DOI:10.1002/anie.200604026

Angewandte
Chemie
DOI: 10.1002/anie.200604026
Synthetic Methods
From PtCl2- and Acid-Catalyzed to Uncatalyzed Cycloisomerization of
2-Propargyl Anilines: Access to Functionalized Indoles**
Kevin Cariou, Baptiste Ronan, Serge Mignani, Louis Fensterbank,* and Max Malacria*
Indoles are ubiquitous motifs in pharmaceuticals as well as in
important natural products. New and straightforward meth-
ods to access these substrates are thus always highly
desirable.[1] In this context, the metal-catalyzed cycloisome-
rization of polyunsaturated precursors is an ideal process to
be explored. One of the main strategies has consisted of a 5-
endo-dig metal-catalyzed[2,3] cyclization of acetylenic deriva-
tives (Scheme 1). Ortho-Halogenoanilines constitute valuable
the best of our knowledge, the alternative 5-exo-dig isomer-
ization approach from precursors 2 has received much less
attention,[5] and we decided to examine this potentially new
route.
Platinum(II)-based catalysis has recently witnessed a
tremendous development which has led to new synthetic
methods[6] as well as versatile applications in the total
synthesis[7] of natural products and asymmetric catalysis.[8]
Recently, we reported on the use of allenyne and enynamide
partners,[9] and showed that the substituent at the propargylic
position had a dramatic influence on the course of the PtCl2-
catalyzed cycloisomerization of various enyne systems.[10] To
examine the scope of the reaction and to generate diverse
platforms we have thus investigated flexible propargylic
precursors of type 3[11,12] (Scheme 1) on which we can easily
vary the oxygen, nitrogen, and alkyne substituents (X, R1, and
R2).
Our initial studies involving substrate 3a (Scheme 2,
Eq. (1) were highly encouraging, and enabled indole 4a to
be isolated in 91% yield. We next examined more challenging
Scheme 1. Transition-metal-catalyzed formation of indoles from o-alky-
nylanilines and o-propargylanilines. M=metal.
starting materials for the synthesis of substrates 1 and can
even be used to generate in situ the akynylaryl species by a
Sonogashira-type coupling reaction prior to cyclization.[2d]
A
recent variant based on imines has also been reported.[4] To
[*] K. Cariou, Prof. Dr. L. Fensterbank, Prof. Dr. M. Malacria
Laboratoire de Chimie Organique, UMR CNRS 7611
Institut de Chimie MolØculaire, FR 2769
UniversitØ Pierre et Marie Curie, Paris 6, case 229
4, place Jussieu, 75005 Paris (France)
Fax: (+33)1-4427-7360
Scheme 2. Indole formation and allyl transfer.
E-mail: fensterb@ccr.jussieu.fr
malacria@ccr.jussieu.fr
Dr. B. Ronan, Dr. S. Mignani
precursors. Gratifyingly, N,N-diallyl precursor 3b also under-
went the transformation [Scheme 2, Eq. (2)]. In this case, an
additional transfer of an allyl group from the nitrogen to the
terminal alkyne carbon atom occurred (Scheme 2, entry 1).
This formally constitutes an aminoallylation of the triple bond
followed by an isomerization of the unsaturated bond. An
analogous allyl transfer has been previously described by
Fürstner et al. in the synthesis of furan derivatives. However,
in this case, stabilization of the vinyl–metal intermediate via
an enolate species seems necessary since only acetylenic
Oncology Dept., Medicinal Chemistry
Centre de Recherche de Paris, Bât. Grignard
Sanofi-Aventis
13, Quai Jules Guesde, 94400 Vitry-sur-Seine (France)
[**] We acknowledge Sanofi-Aventis for a PhD grant to K.C. and for their
financial support. We also thank the IUF, Minist›re de la Recherche
and CNRS for generous financial support and L. M. Chamoreau
(UPMC) for performing X-ray analyses.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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esters and nitriles were reported to undergo the transfer
reaction.[6b]
In our case, such activation is not required. More
potential migrating ability was present.[10,15] This reaction
showed a dramatic temperature effect (Scheme 5, entries 1–
4). The precursor underwent a clean transformation to the
interestingly, this transformation can take place under
metal-free conditions: simple stirring of the reaction mixture
in the presence of silica (500 wt%) provided a similar yield of
4b (entry 2).[13] When the N-methyl,N-allyl substrate 3c was
submitted to both reaction conditions, only the allyl moiety
was transferred (entries 3 and 4). The presence of silica
provided a smooth and better-yielding reaction.
There are several mechanistic issues associated with this
new reaction which depend on the reaction conditions. The
preliminary step in the case of the proton-catalyzed reaction
would correspond to an activation of the triple bond via a
putative vinylic carbocation that is immediately trapped by
the internal nitrogen-based nucleophile (Scheme 3). The
Scheme 5. Reactivity of O-acyl substrates 3e–g. [a] A diene derivative
resulting from a loss of AcOH was also isolated (13%). [b] Complete
conversion was observed during flash chromatography. PTSA=para-
toluenesulfonic acid, Bn=benzyl.
Scheme 3. Proposed mechanism for Brønsted acid catalysis.
expected indole 4e at RT, while performing the reaction
above 808C gave a new indole adduct (4’e) in which an
additional migration of the acetate group had occurred. Thus,
this cycloisomerization process involves the migration of two
groups.
The use of a gold(III) catalyst gave 4e as the major
product, along with 4’e as a minor fraction (Scheme 5,
entries 5 and 6). We also examined the scope of the acid-
catalyzed cycloisomerization of precursor 3e (entries 7–9).
Once again, silica proved to be a better reagent for catalyzing
the 3e to 4e transformation than PTSA, while BF3·Et2Oled
only to degradation. It should be noted that no trace of 4’e was
observed in these acid-catalyzed reactions. Thus, this new
route for achieving a heterocyclic ring closure with concom-
itant reorganization of an enyne system under metal-free
conditions is highly attractive in terms of simplicity and
versatility.
resulting ammonium intermediate is then ready to undergo
a charge-accelerated 3-aza-Cope rearrangement.[14] A final
generation of the aromatic indole nucleus would then steer
the evolution of the reaction. In the case of the metal-
catalyzed process, a similar mechanism based on an initial
p complexation of the alkyne partner would be involved.
These findings led us to explore catalyst-free conditions,
and we carried out the following sequence from 3b. After
deprotonation, methylchloroformate was added to activate
the alkyne function towards the 5-exo attack of the nitrogen
atom, which enabled indole 4d to be obtained in 63% yield
(Scheme 4). Presumably, intramolecular Michael addition to
The N-methyl,N-allyl substrate 3 f reacted in a similar
fashion, with the transfer of the allyl group only. No real
selectivity in the formation of product 4’f could be obtained
when PtCl2 was used as the catalyst (Scheme 5, entries 10–12).
In contrast, the N-benzyl,N-allyl precursor 3g showed total
selectivity, and gave exclusively indole 4’g when exposed to
PtCl2. Regioisomer 4g was smoothly obtained when silica was
used as the catalyst (Scheme 5, entries 13 and 14).
The formation of 4e–g likely occurs through the same
mechanistic pathways as discussed before (Scheme 3). Inter-
estingly, 4e could not be isomerized into 4’e upon prolonged
heating (refluxing toluene) or in the presence of PtCl2. This
finding implies that migration of the acetate moiety takes
place during the cycloisomerization process. Studies on this
Scheme 4. Cyclization by activation of the triple bond.
the triple bond generates an allenolate system that can
undergo [3,3] sigmatropic rearrangement.[14c] Final proton
exchange yields 4d. This new cascade, which includes five
elementary steps, opens up new perspectives for the straight-
forward preparation of functionalized indole scaffolds.
We then examined precursor 3e, which presented an
additional challenge since a propargylic acetate group with
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particular issue are currently underway. It
has to be noted that intractable mixtures
were obtained with internal alkynes (phenyl,
cyclopropyl, and n-butyl groups were tested).
The structures of 4e and 4’e prompted us
to attempt a ring-closing metathesis reaction
between the two pendant unsaturated groups
to access an azepinoindole skeleton.[16]
Indeed, these substrates reacted cleanly to
afford tricyclic indoles 5a and 5b in the
presence of the Grubbs first-generation cat-
alyst (Scheme 6).[17]
The behavior of diprenyl substrates 3h
and 3i followed the same fate (Scheme 7)
and provided additional mechanistic insight,
since no scrambling occurred in the transfer
of the prenyl group. This finding would argue
Scheme 8. Formation of hydroxyindolones. TMS=trimethylsilyl.
cycloisomerizes to give indolone 6 through the previously
discussed 3-aza-Cope rearrangement. Subsequent oxidation
gives rise to the 2-hydroxyindol-3-one 7, which can be
considered as the kinetic product. Indeed, increasing the
reaction time (to 18 h) allowed the selective formation of 8
(Scheme 8). These simple reaction conditions allow the
formation of a fairly complex structure in a single step that
Scheme 6. Ring-closing metathesis to provide azepinoindoles.
involves
a
desilylation/cycloisomerization/oxidation/rear-
rangement sequence and features the formal breaking of
the triple bond (highlighted by the black dots).
In conclusion, we have devised an expedient route to 2,3-
functionalized indoles and notably 3-alkoxyindoles, which
relies on the use of PtCl2 or proton catalysis. The most
intriguing aspect of this process is that the tuning of
substituents on the nitrogen atom, as well as reaction
conditions, notably temperature, allows an easy and versatile
access to a myriad of indole substrates. Complementary to
this approach is the unprecedented skeletal rearrangement
described in the transformation of 3j into 3-hydroxyindolone
8, which can serve as a versatile scaffold for further elabo-
rations, including natural product synthesis.[21]
Received: September 29, 2006
Revised: November 28, 2006
Published online: February 2, 2007
Scheme 7. Behavior of N,N-diprenyl substrates 3h and 3i.
Keywords: cycloisomerization · fused-ring systems · indoles ·
platinum · synthetic methods
against the intervention of a dissociative pathway in this
step[6b] and is consistent with a concerted process, possibly
catalyzed by platinum.
.
Having several valuable protected 3-hydroxyindoles in
our hands, we turned our attention to 3-indolones. Meth-
anolysis of 4e readily led to 2-hydroxyindol-3-one 7, presum-
ably by rapid oxidation of indolone 6 (Scheme 8).[18] By taking
advantage of the structure of 7, we were able to promote an a-
ketol rearrangement under the same reaction conditions,[19]
thus obtaining the 3-hydroxyindol-2-one 8.[20] This interesting
finding prompted us to improve the efficiency by using a less
complex precursor. Desilylation of compound 3j with K2CO3
furnished 2-hydroxyindol-3-one 7 directly in 2 h (Scheme 8).
We postulate that compound 3j is first desilylated and then
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