Gold-Catalyzed Oxidative Cyclization of Tryptamine Derived Enynamides: A Stereoselective Approach to Tetracyclic Spiroindolines

Article abstract of DOI:10.1002/adsc.201800001

In this work, we describe a new gold-catalyzed oxidative cascade cyclization of conjugated enynamides. The reaction could be carried out under relatively mild condition. In presence of a cationic gold catalyst bearing an N-heterocyclic carbene ligand, enynamides derived from tryptamine react with pyridine N-oxide, leading to the formation of a variety of tetracyclic spiroindolines in a stereoselective manner. (Figure presented.).

Full text of DOI:10.1002/adsc.201800001

Title: Gold-Catalyzed Oxidative Cyclization of Tryptamine Derived
Enynamides: A Stereoselective Approach to Tetracyclic
Spiroindolines
Authors: Meijun Lin, Lei Zhu, Jiajin Xia, Yinghua Yu, Jianxin Chen,
Zhifeng Mao, and Xueliang Huang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800001
Link to VoR: http://dx.doi.org/10.1002/adsc.201800001

10.1002/adsc.201800001
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Gold-Catalyzed Oxidative Cyclization of Tryptamine Derived
Enynamides: A Stereoselective Approach to Tetracyclic
Spiroindolines
Meijun Lina,b,Lei Zhua,c, Jiajin Xiaa,c, Yinghua Yua, Jianxin Chen,*b Zhifeng Mao*a and
Xueliang Huang*a,c
a
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Center for Excellence in Molecular Synthesis,
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
E-mail: maozf@fjirsm.ac.cn; huangxl@fjirsm.ac.cn.
College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China.
b
E-mail: jxchen@fjnu.edu.cn
University of Chinese Academy of Sciences, Beijing 100049, China.
c
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. In this work, we describe a new gold-catalyzed
oxidative cascade cyclization of conjugated enynamides.
The reaction could be carried out under relatively mild
condition. In presence of a cationic gold catalyst bearing an
N-heterocyclic carbene ligand, enynamides derived from
tryptamine react with pyridine N-oxide, leading to the
formation of a variety of tetracyclic spiroindolines in a
stereoselective manner.
Keywords: gold catalyst; enynamide; tetracyclic
Figure 1. Naturally occurring polycyclic spiroindolines.
spiroindolines; gold carbene; stereoselective
Transition metal carbene is a reactive intermediate
that has found great potential in the synthetic
community.[1] Diazo compounds are easily accessible,
possessing tuneable reactivity. Thus they are
frequently used as precursors for the generation of
metal carbene species.[2] However, in view of their
hazardous and potentially explosive nature, the
development of an alternative carbene precursor has
been a longstanding goal for the organic chemists.
Pioneered by Toste[3] and Zhang,[4] and later was
further exploited by others,[5] the strategy involving
oxidative addition of alkynes in presence of a proper
gold catalyst to produce α-oxo gold carbene has
received considerable attention.[6,7]
Polycyclic spiroindolines are prevalent structural
motifs that embedded in a number of alkaloid natural
products and pharmaceutical molecules with potent
biological activities (Figure 1).[8] Among all the
synthetic approaches to construct these frameworks,
the methods based on catalytic dearomatization of the
existing indole ring are of particular interests,
regarding the availability of the starting materials and
the high efficiency on the fused ring formation.[9]
Scheme 1. Trapping metal-carbene intermediates by indole
derivatives.
Recently, Qin and co-workers have reported
transition-metal catalyzed cyclopropanation of diazo
compounds with indole derivatives (Scheme 1a),[10]
which was further applied to the total synthesis of
(±)-Communesin F,[10d] (±)-Vincorine[10e] and Kopsia
indole alkaloids.[10f] More recently, Ye and co-
1
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10.1002/adsc.201800001
Advanced Synthesis & Catalysis
workers reported an intermolecular trapping of α-
oxo-gold carbene intermediate by indoles and
anilines (Scheme 1b).[11] Inspired by these advances
and our recent research interests in transition metal
carbene chemistry,[12,13] we envisioned that a gold-
catalyzed oxidative intramolecular annulation of
tryptamine derived ynamide in presence of pyridine
N-oxide might offer a direct approach to tetracyclic
spiroindolines (Scheme 1c). Given the well-known
procedure for the preparation of ynamides,[14,15] and
the nondiazo feature, we believe that the successful
implementation of our hypothesis might have the
potential for the rapid and large scale synthesis
complex molecules.
Remarkably, tetracyclic spiroindoline 3u containing
two contiguous tetrasubstituted carbon stereocenters
was isolated in excellent yield as
a single
diastereoisomer. Aryl/heteroaryl goups at the
terminal position of the double bond with different
Table 1. Reaction scope for the synthesis of spiroindolines
3.a)
At the outset, we chose enynamide 1a as the model
substrate to react with pyridine N-oxide 2a to test our
hypothesis. Pleasingly, after brief examination of the
reaction conditions,[16] we found that the reaction of
1a and 2a catalyzed by a cationic gold catalyst
IPrAuNTf2
(IPr
=
1,3-bis(diisopropylphe
nyl)imidazol-2-ylidene) in 1,4-dioxane could proceed
smoothly at 80oC, and the target tetracyclic
spiroindoline 3a was isolated in nearly quantitative
yield. Control experiments further highlight the
critical role of the cationic nature for the gold catalyst,
[5j]
and Brønsted acid HNTf2 was not efficient as well
(Scheme 2).
Scheme 2. Gold-catalyzed oxidative annulation of 1a in
presence of pyridine N-oxide 2a.
After establishing a set of effective reaction
conditions for 3a. The reaction scope and limitation
were further investigated and the results are compiled
in Table 1. As depicted, the effects of protecting
groups in amide moiety were firstly evaluated. The
enynamides bearing Ms-, Ts-, Bs-, Ns-groups are all
compatible with current transformation, giving the
products in good to excellent yields (cf. 3a-3d, 70-
99%). Replacement of the methyl group with a
benzyl group on the nitrogen atom of the indole
moiety led to a slightly decrease of the reaction yield
(cf. 3e). When protecting group on the indole
nitrogen moiety was switch to a carbamate group (e.g.
Boc), a major product α-ketoamide resulting from
over oxidation was obtained (results not shown).
Introducing a variety of substituents on to the indole-
ring has no strong influence on the reaction outcomes,
regardless of the electronic or steric nature (cf. 3f-3l).
a)
The reaction was carried out under an atmosphere of
argon, in 0.3 mmol scale. Yields reported are for pure,
isolated compounds.
2
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10.1002/adsc.201800001
Advanced Synthesis & Catalysis
electronic or steric characters were tolerated under
current conditions, giving the corresponding products
in good to excellent yields (cf. 3m-3s, 3u). Because
of the over oxidation, the reactions of ynamides
expansion of the fused cyclopropane ring would
eventually furnish 3a. Alternatively, the reaction may
proceed via
a
stepwise cyclization pathway.
Nucleophilic addition of 3-position of the indole-ring
to the gold carbene carbon center would build up the
spiro framework III. Intramolecular trapping of the
iminium ion intermediate by tethered stryrene moiety
would also furnish 3a.
derived
from
phenylacetylene
or
2-
ethynylnaphthalene gave the spiroindoline 3v or 3w
in 27% and 44% yields, respectively. It is worthwhile
to mention that replacing the aryl group (R4) with an
alkyl group led to negligible formation of
corresponding spiroindoline (results not shown).
Interestingly, the reaction of Yang’s indole-ynamide
1x[17] under standard conditions gave a tosyl protected
tryptamide SI-2 in 72% yield. The structural
configuration of the products and diastereoselectivity
of current reaction in presence of 2a were further
confirmed by X-ray crystal analysis of 3a (Figure
2).[18]
Under standard conditions, a gram-scale reaction
was also carried out, and 3a was obtained in 91%
yield. Subsequently, preliminary manipulation of the
tetracyclic spiroindoline 3a was sought (Scheme 4).
The methylsulfonyl group could be removed
following a radical pathway. The resulting crude
product was used without purification for the next
step. Spiroindoline 4 was obtained in 81% overall
yield.
Scheme 4. Synthetic manipulation on tetracyclic
spiroindoline 3a.
In conclusion, we have developed an efficient
gold-catalyzed oxidative annulation of tryptamine
derived enynamides. Based on our own results and
previous reports, catalytic cycle involving α-oxo-gold
carbene intermediate was proposed. In this event, a
number of tetracyclic spiroindolines bearing three
continuous carbon centers were obtained with high
stereoselectivity. Furthermore, considering the well-
established procedure for the preparation of ynamides,
the method on modular synthesis of spiroindolines
reported here may be helpful to build up a library of
molecules with different structural complexities.
Studies to extend the substrate scope and further
applications on the synthesis of pharmaceutical
relevant compounds are on-going in our laboratory.
Figure 2. Crystal structure of spiroindoline 3a. The
thermal ellipsoids are drawn at 50% probability; hydrogen
atoms are omitted for clarity.
Experimental Section
General procedure:
Scheme 3. Plausible mechanism for gold-catalyzed
oxidative annulation of enynamide 1a.
An oven-dried 10 mL schlenk tube equipped with a
magnetic stirring bar was charged with enynamide 1a (0.30
mmol, 113.5 mg), pyridine N-oxide 2a (0.45 mmol, 42.8
mg), IPrAuNTf2 (3 mol%, 7.8 mg) and 3 mL anhydrous
1,4-dioxane. The tube was then capped and the mixture
According to the results obtained and previous
reports on gold-catalyzed oxidative functionalization
of alkynes, plausible reaction pathways leading to
tetracyclic spiroindolines 3 were proposed in Scheme
3. The reaction was initiated by the coordination of
the cationic gold catalyst with C-C triple bond. The
oxidation of 1a by pyridine N-oxide 2a probably
generates α-oxo-gold carbene intermediate I.[6] At
this stage, an intramolecular cyclopropanation of the
carbene intermediate takes place giving II.[10,11] Ring
o
stirred at 80 C under an atmosphere of argon until the
reaction was complete, then the mixture was cooled to
room temperature and the solvent was removed under
reduced pressure. The resulting residual was purified by
column chromatography (petroleum ether/ethyl acetate) on
silica gel to afford tetracyclic spiroindolines 3a.
Acknowledgements
3
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10.1002/adsc.201800001
Advanced Synthesis & Catalysis
We are grateful for the financial support by NSFC (Grant No.
21402197, 21502190), NSF of Fujian (Grant No. 2015J05043,
2017J01031), Hundred-Talent Program and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(Grant No. XDB20000000).
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5
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Advanced Synthesis & Catalysis
COMMUNICATION
Gold-Catalyzed Oxidative Cyclization of
Tryptamine Derived Enynamides: A
Stereoselective Approach to Tetracyclic
Spiroindolines
Adv. Synth. Catal. Year, Volume, Page – Page
Meijun Lina,b,Lei Zhua,c, Jiajin Xiaa,c, Yinghua Yua,
Jianxin Chen,*b Zhifeng Mao*a and Xueliang
Huang*a,c
6
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