Self-Relay Gold(I)-Catalyzed Pictet-Spengler/Cyclization Cascade Reaction for the Rapid Elaboration of Pentacyclic Indole Derivatives

Article abstract of DOI:10.1002/chem.201503941

Gold-catalyzed cascade reactions allow the rapid elaboration of pentacyclic indolo[2,3-a]quinolizidines from N-allyl tryptamines and ortho-alkynylarylaldehydes. The tandem process combines a gold-catalyzed Pictet-Spengler reaction and a cyclization occurring concomitantly with an allyl transfer from the nitrogen atom to the stilbene function. Various substituted allyls were successfully transferred, furnishing the products in yields typically ranging from 60-98 % in high diastereoselectivity. Tryptamines bearing a butenol chain undergo an additional cyclization to chiral hemiaminals in high diastereoselectivities. Going for gold! Gold-catalyzed cascade reactions allow the rapid elaboration of pentacyclic indolo[2,3-a]quinolizidines from N-allyl tryptamines and ortho-alkynylarylaldehydes (see scheme). The tandem process combines a gold-catalyzed Pictet-Spengler reaction and a cyclization occurring concomitantly with an allyl transfer from the nitrogen atom to the stilbene function.

Full text of DOI:10.1002/chem.201503941

DOI: 10.1002/chem.201503941
Communication
&
Homogenous Catalysis
Self-Relay Gold(I)-Catalyzed Pictet–Spengler/Cyclization Cascade
Reaction for the Rapid Elaboration of Pentacyclic Indole
Derivatives
ValØrian GobØ, Pascal Retailleau, and Xavier Guinchard*[a]
Abstract: Gold-catalyzed cascade reactions allow the
rapid elaboration of pentacyclic indolo[2,3-a]quinolizidines
from N-allyl tryptamines and ortho-alkynylarylaldehydes.
The tandem process combines a gold-catalyzed Pictet-
Spengler reaction and a cyclization occurring concomi-
tantly with an allyl transfer from the nitrogen atom to the
stilbene function. Various substituted allyls were success-
fully transferred, furnishing the products in yields typically
ranging from 60–98% in high diastereoselectivity. Trypta-
mines bearing a butenol chain undergo an additional cyc-
lization to chiral hemiaminals in high diastereoselectivities.
The development of tandem catalysis strategies fastens the
discovery for new reactions and the elaboration of molecular
complexity, by performing several steps in one single flask.[1]
Such processes can be the result of several steps concurrently
catalyzed by the same catalyst by self-relay catalysis (also
known as autocatalysis) or, alternatively, by the combination of
Scheme 1. Previous approaches and proposed work.
two catalysts, in an orthogonal manner, each of which being
responsible for the catalysis of one single step.[2] Tandem catal-
ysis is, however, a challenging approach since catalysts and
lyzed Pictet–Spengler reaction[9] followed by a self-relay palla-
substrates should be compatible to ensure efficient conversion
in the desired product and avoiding side reactions. Gold com-
plexes have emerged over the last decade as powerful tools
for the synthesis of functionalized molecules.[3] In addition, the
gold tolerance to oxygen, moisture, and many functional
groups is very high, and make this metal an ideal candidate for
the development of tandem catalytic processes, either in or-
thogonal catalysis with Brønsted acid,[4] or self-relay catalysis.[5]
Hence numerous gold complexes have been used in elegant
cascade reactions developed for the synthesis of complex com-
pounds by both approaches.[6,7]
dium-catalyzed cyclization, featuring both the deallylation and
the cyclization steps, leading to polycyclic compounds. Very re-
cently, Waldmann and Kumar reported a two-step approach
consisting in an Yb(OTf)3-catalyzed Pictet–Spengler reaction
followed by a AuI-catalyzed hydroamination leading to penta-
cyclic derivatives in moderate yields (Scheme 1, Eq. (2)).[10]
For our part, we hypothesized that a gold-catalyzed cascade
combining a Pictet–Spengler reaction between N-allyltrypta-
mines 1 and o-alkynylarylaldehydes 2 and a cyclization with
a concomitant allyl transfer from intermediate 3 would lead to
analogous polycyclic derivatives 4 in a very rapid one-pot pro-
cess (Scheme 1, Eq. (3)). The allyl group, which is in general
considered a protecting group, would therefore be converted
into a three-carbon synthon, potentially useful for further syn-
thetic transformations. Such carboaminations of N-allyl nucleo-
philes on alkynes proceeding with an allyl transfer are known
to be very efficient and have led to a variety of heterocy-
cles.[11,12]
Recently, we used allenals as bifunctional key building
blocks[8] for the synthesis of polycyclic indolic chiral architec-
tures (Scheme 1, Eq. (1)).[8b,c] The strategy relied on a two-steps
sequence consisting of an asymmetric phosphoric acid cata-
[a] V. GobØ, Dr. P. Retailleau, Dr. X. Guinchard
Institut de Chimie des Substances Naturelles, CNRS UPR 2301
UniversitØ Paris-Sud, UniversitØ Paris-Saclay, 1 avenue de la Terrasse
91198 Gif-sur-Yvette (France)
In this paper we thus report our efforts in the development
of this cascade reaction leading to pentacyclic indolo[2,3-a]qui-
nolizidines[13] 4, in a self-relay AuI-catalyzed sequence.
E-mail: xavier.guinchard@cnrs.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503941.
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Table 1. Optimization of reaction conditions.[a]
Table 2. Scope of the tandem gold(I)-catalyzed process.[a]
Entry
[Au] (mol%)
Ratio 1/2 [mol] Yield 3a [%] Yield 4a [%]
1
2
3
4
5
6
7
8
5a (5)/DPP (5)
5a (5)
5b (5)
5c (5)
5d (5)
5e (5)
AuCl3 (5)
AuCl3/AgPF6 (5) 1:2
5c (5)/DPP (5)
5c (5)
1:2
1:2
1:2
1:2
1:2
1:2
1:2
0
4
35
8
34
64
36
92
0
80
78
15
84
57
24
11
8
Entry
R1
R2
R3
R4
R5
Yield [%][b]
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
nBu
SiMe3
3-pyridine
Ph
pMeO2CC6H4
mOMeC6H4
Ph
Ph
Ph
Ph
4a, 97
4b, 72
4c, 86
4d, 91[c]
4e, 82
4 f, 98
4g, 62
4h, 72
4i, 94[c]
4j, 75
5-OMe
5-F
6-OMe
H
H
H
H
H
H
H
H
H
H
H
H
H
H
9
10
11
1:2
1:2
1:2
94
93[b]
20
H (X=N)
H
H
5-NO2
5-F
6-F
5-OMe
5-OMe
5-OMe
H
0
75
9
5c (2)
10
11
12
13
14
15
16
17
18
[a] Yields measured by 1H NMR spectroscopy with 1,3,5-trimethoxyben-
zene as the internal standard. [b] The yield dropped to 60% when the re-
action was performed in the absence of molecular sieves.
4k, 69
4l, 76
4m, 89
4n, 77
4o, 97
4p, 33
4q, 60
4r, 88[d]
pMeO2CC6H4
mOMeC6H4
Ph
H
Ph
[a] Reaction conditions: 1a (0.12 mmol), 2a (0.1 mmol), 5c (5 mol%), mo-
lecular sieves (4 Š, 100 mg), solvent (1.5 mL). [b] Isolated yields. [c] Reac-
tion performed using the conditions of Table 1, entry 9. [d] d.r.>95:5.
We initiated our study by establishing the suitable catalytic
system for the Pictet–Spengler gold-catalyzed cyclization one-
pot process (Table 1).
To this aim, N-allyl tryptamine 1a and aldehyde 2a were re-
acted in the presence of catalytic amounts of diphenyl phos-
phate (DPP, 5 mol%),[14] to ensure catalysis of the Pictet–Spen-
gler reaction and Echavarren catalyst 5a (5 mol%), in dichloro-
ethane at 508C. Product 4a, stemming as expected from
a one-pot Pictet–Spengler reaction and gold-catalyzed cycliza-
tion was obtained in 80% yield (Table 1, entry 1). Pictet–Spen-
gler reactions can be catalyzed by AuI or AuIII complexes,
though their utilization in this reaction is scarce.[15] Indeed, the
reaction performed with 5a as the sole catalyst furnished
product 4a in 78% yield (entry 2), showing the ability for AuI
to catalyze the Pictet–Spengler step. The carbene complex
[(IPr)Au(MeCN)]·BF4 (5b) led to a mixture mainly composed of
the intermediate tetrahydro-b-carboline 3a (entry 3), while the
stable cationic catalyst [(Ph3P)Au(NTf2)][16] (5c) enhanced the
yield of 4a to 84% (entry 4). The use of catalysts 5d,e and AuIII
catalysts dropped the yield of 4a (entries 5–8). The catalyst 5c
was then selected for the following of the study and it was
checked whether the addition of DPP (5 mol%) would be ben-
eficial to the reaction. Though the yield increased to 94%
(entry 9), the effect of DPP was considered moderate and fur-
ther optimization was performed with AuI as the sole catalyst.
The ratio between 1a/2a was then optimized to 1.2:1 which
allowed an excellent yield and facilitated the purification pro-
cess (entry 10). A decrease of the catalyst loading to 2 mol% of
[(Ph3P)Au(NTf2)] dropped the yield to 20% (entry 11).
stituents introduced on the aryls on the reactivity in this
tandem process (Table 2). Numerous o-alkynylarylaldehydes 2
were reacted with tryptamines
1 in the presence of
[(Ph3P)Au(NTf2)] (5c). We first studied the influence of the sub-
stituent present on the indole ring and found that both elec-
tron-withdrawing (EWG) and -donating groups (EDG) R1 groups
have little influence on the reactivity, furnishing the corre-
sponding products 4a–d in good to excellent yields (Table 2,
entries 1–4). The reaction was next successfully performed
using alkynes substituted by either alkyl or silyl groups, fur-
nishing the corresponding products 4e and 4 f in 82 and 98%
yields, respectively (entries 5 and 6). The sequence was also
compatible with the use of basic heterocyles. Reactions per-
formed with pyridyl groups on either R5 or with X=N fur-
nished the corresponding heterocycles in 62 and 72% yields,
respectively (Table 2, entries 7 and 8).[17] Different functions
were next introduced on the aryl moieties at the R4- and R5-po-
sitions. The reaction proceeded well with EWG or EDG groups
at R5 (entries 9 and 10). Substitution on the arylaldehyde using
a nitro functionality led to compound 4k in a good 69% yield
(entry 11). Fluoro derivatives similarly afforded the correspond-
ing compounds 4l and 4m (entries 12 and 13). Electron-rich
aryl groups were next used successfully in combination with
a Ph- or a p-CO2Me-substituted R5 group (entries 14 and 15). A
good 77% yield was obtained with R5 =Ph (entry 14), increas-
ing to 97% in compound 4o with an electron-poor R5 group
(entry 15). However, the yield in 4p dropped to 33% with R5 =
The scope and the limitations of the reaction were next
studied, to determine in particular the influence of the sub-
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m-OMeC6H4 (entry 16), showing that combined electron-donat-
ing effects are detrimental to the reaction. Overall it is clear
that the reaction is strongly favored with electron-poor sub-
stituents, likely by impoverishing the triple bond, thereby in-
creasing its electrophilicity. The migratory ability of various allyl
groups was next studied. Reaction of tryptamines 1e–f func-
tionalized by a methallyl or a cinnamyl group were reacted
with aldehyde 2a in the presence of catalyst 5c. The corre-
sponding compounds 4q and 4r were obtained in good 60
and 88% yields (entries 17 and 18). The relative stereochemis-
try of 4r, obtained as a sole diastereomer, was assessed by X-
ray crystallography (see the Supporting Information).
tion reactions. A point to note is that a stereogenic quaternary
center is created within the process and controlled with full
diastereoselectivity. The same reaction was performed using al-
dehydes 2d and 2 f, furnishing the corresponding compounds
6b and 6c in 56 and 54% yields. This complex process allows
the formation of four stereogenic centers, three of them being
contiguous, in one single synthetic step and in high diastereo-
control.
The mechanism of this gold-catalyzed transformation and in
particular its exquisite stereocontrol is worth considering. The
migration of an allyl group from a heteroatom to an alkyne
function in the course of a gold-[11,12] or platinum-catalyzed[18]
process has been previously reported. A dissociative mecha-
nism is proposed in the case of an allyl transfer from an
oxygen atom,[12a–d] but it is usually recognized that a concerted
pathway is operating from N-allyl functions,[11] by an aza-Clais-
en reaction. The mechanistic pathway of our transformation
can be postulated as follows (Scheme 3). After the Pictet–Spen-
We finally evaluated the migratory ability of the (E)-4-hy-
droxybut-2-en-1-yl group of tryptamine (1g), possessing a free
hydroxyl function (Scheme 2). The reaction of 1g with alde-
Scheme 2. Orthogonal catalysis to hemiaminals 6.
hyde 2a in the presence of 5c (5 mol%) furnished the expect-
ed cyclized product 8a which, upon chromatography on silica
gel, evolved partially to the chiral hemiaminal hexacyclic com-
pound 6a. The use of DPP (5 mol%) as an additional acidic cat-
alyst furnished directly compound 6a in 80% yield, as a single
isomer for which relative stereochemistry was attributed both
by nOe and X-ray crystallography studies (Figure 1).
Scheme 3. Mechanistic hypothesis (counterions have been omitted for clari-
ty).
gler step catalyzed by AuI, complexation of the intermediate
tetrahydro-b-carboline 3 with the gold catalyst triggers the
cyclization by addition of the amine function to the activated
triple bond, leading to a quaternary ammonium (i). It then un-
dergoes a [3,3]-sigmatropic suprafacial aza-Claisen reaction
leading to intermediate (ii), with the generation of an addition-
al stereogenic center in high diastereoselectivity when R¼ H.
The deauration step releases the product 4. Such a level of dia-
stereoselectivity in the allyl transfer is characteristic of a con-
certed intramolecular migration. A dissociative migration that
would proceed by an allyl cation would on the contrary result
in low diastereoselectivity and potentially low regioselectivity
(mixture of branched and linear compounds).
This compound is the result of a cascade process, catalyzed
both by the AuI complex and the phosphoric acid, including
the Pictet–Spengler/cyclization/allyl migration/hemiaminaliza-
In conclusion, we have developed a rapid access to pentacy-
clic indolic derivatives 4 from N-allyltryptamines 1 and o-alky-
nylaryladehydes 2. The route features a tandem gold-catalyzed
Pictet–Spengler reaction and a cyclization proceeding with
concomitant allyl migration. The scope of this multicatalytic
pathway is wide and allows the fast elaboration of molecular
complexity. Tryptamines bearing substituted allyl groups led to
the corresponding cyclized derivatives with a high level of dia-
Figure 1. Ortep drawing for compound 6a.
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V.G. thanks the ICSN for financial support. The authors thank
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and V. Servajean (ICSN) for technical assistance.
Keywords: cascade reaction · gold catalysis · homogenous
catalysis · indoles · nitrogen heterocycles
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Received: October 1, 2015
Published online on October 20, 2015
Chem. Eur. J. 2015, 21, 17587 – 17590
www.chemeurj.org
17590
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