Gold(I)-catalyzed 1,2-acyloxy migration/[3+2] cycloaddition of 1,6-diynes with an ynamide propargyl ester moiety: Highly efficient synthesis of functionalized cyclopenta[b]indoles

Article abstract of DOI:10.1002/chem.201405965

A gold-catalyzed cycloisomerization of 1,6-diynes containing an ynamide propargyl ester or carbonate moiety has been developed that provides an attractive route to a diverse-substituted 3-acyloxy-1,4-dihydrocyclopenta[b]indoles. Mechanistic studies indicate that the reaction likely proceeds through a competitive 1,2-OAc migration followed by [3+2] cycloaddition of the vinyl gold-carbenoid intermediate with the pendant triple bond. The synthetic utility of the obtained cyclopenta[b]indole products was demonstrated by their efficient transformations by deprotection or double-bond isomerization reactions.

Full text of DOI:10.1002/chem.201405965

DOI: 10.1002/chem.201405965
Communication
&
Synthetic Methods
Gold(I)-Catalyzed 1,2-Acyloxy Migration/[3+2] Cycloaddition of
1,6-Diynes with an Ynamide Propargyl Ester Moiety: Highly
Efficient Synthesis of Functionalized Cyclopenta[b]indoles
Jun Liu,[a] Ming Chen,[a] Liangwei Zhang,[a, b] and Yuanhong Liu*[a]
catalysts, ligands, reaction conditions, or additives, etc. For ex-
Abstract:
A gold-catalyzed cycloisomerization of 1,6-
ample, it was reported that 1,7-diyne containing a propargyl
benzoate and a terminal alkyne moiety underwent a concerted
double cyclization initiated by nucleophilic attack of the aryl
ring at the propargylic position.[3c] Recently, ynamides[5] have
attracted considerable interest in p-acid transition-metal-cata-
lyzed reactions[6] due to the enhanced electrophilicity arising
from the polarized triple bond compared with the normal al-
kynes. During our ongoing project on gold-catalyzed cycloiso-
merizations of 1,n-diynes bearing a propargyl carbonate moie-
ty,[3a,d] we envisioned that the presence of a heteroatom-teth-
ered propargyl carboxylate such as an ynamide functionality in
1,n-diynes[7] may have an important impact on the reaction
patterns, and may also allow efficient access to valuable nitro-
gen heterocycles. However, to the best of our knowledge, the
effect of such heteroatom substituents has not been investi-
diynes containing an ynamide propargyl ester or carbon-
ate moiety has been developed that provides an attractive
route to a diverse-substituted 3-acyloxy-1,4-dihydrocyclo-
penta[b]indoles. Mechanistic studies indicate that the reac-
tion likely proceeds through a competitive 1,2-OAc migra-
tion followed by [3+2] cycloaddition of the vinyl gold–car-
benoid intermediate with the pendant triple bond. The
synthetic utility of the obtained cyclopenta[b]indole prod-
ucts was demonstrated by their efficient transformations
by deprotection or double-bond isomerization reactions.
In recent years, gold-catalyzed cycloisomerizations of
1,n-diynes have emerged as an important methodol-
ogy in the construction of diversely functionalized
polycyclic molecules.[1,2] A particularly attractive strat-
egy is based on the cycloisomerizations of 1,n-diynes
bearing a propargyl ester or carbonate moiety.[3] Gen-
erally, these reactions are initiated through the selec-
tive activation of the propargyl moiety by gold cata-
lyst[4] to trigger an efficient generation of a gold-co-
ordinated carboxyallene A through 3,3-rearrange-
ment[3a–j] or vinyl gold–carbenoid B through 1,2-acyl-
oxy migration,[3k–l] depending on the electronic or
steric nature on either end of the propargyl moiety
(Scheme 1). Both A and B show unique reactivities
for further functionalizations by reactions with the
pendant alkyne. Usually internal propargyl carboxy-
lates undergo 3,3-rearrangement, whereas terminal
propargyl carboxylates prefer 1,2-acyloxy migration.
Scheme 1. Gold-catalyzed cycloisomerizations of diynes bearing a propargyl ester.
However, the reaction pathway may be altered by ef-
fects such as the structural factors of the substrates,
gated.[8] Herein, we report the first example of gold-catalyzed
cycloisomerization of 1,6-diyne 1 with an ynamide propargyl
ester moiety, which provides a straightforward route to oxy-
genated 1,4-dihydrocyclopenta[b]indole 2, a commonly occur-
ring structural motif in pharmaceuticals and natural products,[9]
with high efficiency (Scheme 1). Notably, the reaction likely
proceeds by a competitive 1,2-acyloxy migration of an internal
propargyl moiety to afford vinyl gold–carbenoid C, which
serves as a three-atom building block for formal [3+2] cycload-
dition with the pendant alkyne according to mechanistic stud-
ies.[10] Prior to this study, there are only few reports dealing
[a] Dr. J. Liu, M. Chen, L. Zhang, Prof. Y. Liu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences,
345 Lingling Lu, Shanghai 200032 (P. R. China)
E-mail: yhliu@mail.sioc.ac.cn
[b] L. Zhang
Center of Drug Discovery, College of Pharmacy
China Pharmaceutical University, 24 Tongjia Xiang Road
Nanjing 210009 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405965.
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with the reactivity of the in-situ formed gold–carbenoid inter-
mediates from 1,n-diynes, including cyclopropenation/Nazarov
cyclization,[3l] gold–carbene transfer,[2a] and [2+2+1] cycloaddi-
tion with aldehydes.[3k]
drolysis with a trace amount of water (entry 6). The reaction
could also proceed in 1,2-dichloroethane, with a slight lower
yield of 2a (76%, entry 7). However, switching the solvent to
toluene did not give satisfactory results (entry 8). The frequent-
ly used gold(I) complexes such as [Au(IPr)]SbF6, [Au(PPh3)]SbF6,
or [Au(PPh3)]NTf2 were not effective for this transformation,
and the diyne 1a remained in 24–73% (entries 9–11). The re-
sults indicated that the nature of the ligand on gold catalyst
played an important role in this cascade reaction. Interestingly,
when a gold(III) complex, such
Based on the superior catalytic activity of [Au(Johnphos)
(MeCN)]SbF6 (catalyst A) in various gold-catalyzed reactions,
we first investigated the cycloisomerization of benzene-
bridged 1,6-diynyl acetate 1a in the presence of catalyst A.
The results are shown in Table 1. We were pleased to see that
as [AuCl2(Pic)], was used as the
Table 1. Optimization studies for the formation of 1,4-dihydrocyclopenta[b]indole 2a.
catalyst, a-acyl-a,b-unsaturated
4a was obtained exclusively in
88% yield with a E geometry of
the double bond[11] (entry 12).
The formation of 4a can be ra-
tionalized by a gold-catalyzed
tandem 3,3-rearrangement/oxo-
carbenium ion formation/acyl-
migration process similar to that
previously reported for gold(III)-
catalyzed acyl migration of prop-
argylic esters by Zhang et al.[12]
Entry
Cat. (5 mol%)
[Substrate] [m]
Solvent
t [h]
Yield [%][a]
2a
1a
3a[b]
1
2
3
4
A
A
A
A
A
A
A
A
0.1
0.03
0.02
0.015
0.01
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
ClCH2CH2Cl
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
3
3
3
3
3
3
1
3
4
4
5
1
4
–
–
–
–
–
–
–
9
24
38
73
–
68
75
80
83
83
56
76
–
–
–
–
–
5
–
6[c]
7
25 (only E)
–
58 (52:6)
35 (only E)
22 (19:3)
–
Control
experiments
with
AgSbF6 alone could not afford
the desired 2a (entry 13). The
structure of 1,4-dihydrocyclopen-
ta[b]indole product 2 was con-
firmed by X-ray crystallographic
analyses of 2a, 2i, and 2m[13]
(vide infra). Apparently, a formal
1,2-OAc migration took place
during the reaction according to
the structure of product 2.
8[d]
9[d,e]
10[d,f]
11
12[g]
13[d]
[AuCl(IPr)]/AgSbF6
[AuCl(PPh3)]/AgSbF6
[Au(PPh3)]NTf2
[AuCl2(Pic)]
AgSbF6
–
–
–
–
–
76
–
7 (3:4)
[a] Isolated yields. All the reactions were carried out on 0.3 mmol scale in the presence of 75 mg 4ꢁ MS.
[b] The ratio of E/Z isomers is shown in parentheses. [c] Without MS. [d] 1H NMR spectroscopic yields using
CH2Br2 as the internal standard. [e] 25% 4a was also formed. [f] 4% 4a was also formed. [g] 4a was isolated in
88% yield. IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; Pic=2-picolinate.
We chose the reaction condi-
tions shown in Table 1, entry 4
to examine the scope of this
novel cascade reaction. The re-
sults are shown in Table 2. The
effect of the protection groups
was examined first. Ac and Piv,
treatment of 1a with 5 mol% of catalyst A and 4 ꢁ molecular
sieves in dichloromethane at room temperature for 3 h ena-
bled complete consumption of 1a to afford 3-acyloxy-1,4-dihy-
drocyclopenta[b]indole 2a in 68% yield (Table 1, entry 1). It
was noticed that some amounts of colored byproducts were
also formed during the process according to TLC analysis. We
reasoned that the concentration of the substrates may have an
effect on the reaction course. To our delight, decreasing the
substrate concentration from 0.1 to 0.03m improved the yield
of 2a to 75% (entry 2). Further decreasing the substrate con-
centration to 0.015 or 0.01m resulted in the significant im-
provement of the product yield, as the same 83% yields of 2a
were obtained (entries 4–5). Performing the reaction in the ab-
sence of molecular sieves led to 2a in 56% yield, along with
25% of undesired a,b-unsaturated imide 3a formed by 3,3-re-
arrangement of the propargyl acetate moiety followed by hy-
as well as Bz groups, could be well-accommodated in this reac-
tion, leading to 2a–c in 50–83% yields. The presence of a steri-
cally demanding Piv group in 1b gave 2b in moderate yield of
50%, which indicated that the reaction is sensitive to the bulki-
ness of the migrating group. Propargyl carbonate 1d also un-
derwent the reaction smoothly to furnish 2d in 79% yield.
Next, we examined the effect of R3 group on the alkyne termi-
nus. The reaction applied to a wide variety of aryl-substituted
alkynes, and the functionalities of Cl-, F-, Me-, MeO-, and 3,4,5-
(MeO)3 groups on aromatic rings were tolerated well during
the reaction, affording the corresponding products 2e–i and
2k in good to high yields. Especially, sterically encumbered o-
MeO-substituted substrate 1i was smoothly converted into the
corresponding 2i in a good yield of 76%. A thienyl group
could also be incorporated successfully into the sequence, pro-
viding 2j in 86% yield. Substrate 1l bearing a cyclohexenyl
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73–81% yields. However, when
R2 is a phenyl group, only alkene
product 3r was obtained as
a mixture of Z/E isomers. The
electronic nature of the substitu-
ent on the parent phenyl ring
also has a strong influence on
the reaction course, for example,
while electron-donating methyl-
substituted 1s worked efficiently
to afford 2s in a good yield of
72%, the electron-withdrawing
F- or Cl-substituted 1t or 1u af-
forded the desired 2t and 2u
only in low yields of 33 and
29%, respectively. The results
may be due to the decreased
nucleophilicity of the alkyne
moiety, which cannot trap the
gold–carbenoid intermediate in
an efficient way.
Table 2. Gold-catalyzed cyclizations of 1,6-diynyl esters to cyclopenta[b]indoles.[a]
The utility of the cyclopenta-
[b]indole products 2 was dem-
onstrated by transformation re-
actions of 2a (Scheme 2). Depro-
tection of 2a under acidic condi-
tions afforded cyclopentenone
5a with high stereoselectivity. To
our surprise, stirring a CH2Cl2 so-
lution of 2a in the presence of
basic Al2O3 afforded double-
bond-isomerized product 6a in
90% yield. The structure of 6a
was unambiguously confirmed
by X-ray crystallography of its
analogue 6k.[13,14]
We also prepared the deuteri-
um-labeled diynyl ester to probe
the reaction mechanism. Full in-
corporation of deuterium at the
alkyl carbon atom of the cyclo-
pentadiene ring was observed
upon cyclization of [D]1a. In ad-
dition, cyclization of 1a in the
presence of 2.0 equivalents of
D2O afforded 2a in 35% yield
without deuterium incorpora-
[a] Isolated yields. [b] Z/E=1.1:1. [c] 50% of 3t (E/Z=1.3:1) was also isolated. [d] 43% of 3u (E/Z=1.4:1) was
also isolated.
group converted into 2l with a high isolated yield (90%). A cy-
clopropyl-substituted alkyne 1m was efficiently transformed to
2m in 84% yield. However, the use of a butyl-substituted
alkyne 1n did not deliver the desired cyclopenta[b]indole
product; instead, alkene 3n was formed in 95% yield with ex-
cellent E stereoselectivity. This result is likely due to the fast hy-
drolysis of the allenic intermediate formed by a competing 3,3-
rearrangement. The substituent effect on propargylic position
(R2) was also investigated. Alkyl groups, such as Me, Et or more
bulky iPr groups as R2 were all compatible, leading to 2o–q in
tion, along with 43% of [D]3a with moderate deuterium incor-
poration. These results implied that H-migration process might
occur in a concerted way. To have a better understanding of
the ÀOAc migration process and the subsequent transforma-
tions into heterocycles, we examined the cycloisomerization of
carbonyl-labeled 18O-1a under gold-catalyzed conditions. It
was found that 18O-2a was produced in 78% yield with the 18
O
label located at the acyloxy position[14,15] (Scheme 3).
Based on the above observations, we propose the following
mechanism for this transformation (Scheme 4). First, selective
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a 6-endo-dig cyclization could be favored in this step since the
alkynyl carbon atom adjacent to nitrogen atom is more elec-
trophilic due to the electronic polarization of the triple bond,
this reaction pathway might be suppressed by fast transforma-
tion through a 1,2-migration-initiated cascade sequence. 3,3-
Rearrangement might also be reversible with 1,2-acyloxy mi-
gration.[16] After that, two reaction pathways may be possible
for the formation of cyclopenta[b]indole product 2. In path a,
attack of the remaining alkyne moiety to the gold carbenoid
affords vinyl cationic species 10. Subsequent nucleophilic
attack of the alkene moiety to the vinyl cation followed by
elimination of the gold catalyst leads to intermediate
11, which undergoes 1,5-H shift to deliver the final
product 2. Trapping of the gold carbenoid by an
alkyne followed by attack of the aryl ring to the vinyl
cation has been recently reported by Davies et al.[17]
Alternatively, transfer of the gold carbenoid moiety
across the triple bond, possibly by ring opening of
the cyclopropene intermediate[3l,10] 12, affords the
gold–carbenoid 13. Cyclization of 13 results in the
formation of cation 14. Compound 14 might under-
go direct deauration to give 11, which isomerizes to
2. It may also undergo deprotonation and proto-
deauration to provide the product 2. However, this
process is not supported by a deuterium-label experi-
ment result, since no deuterium incorporation was
found when the reaction was carried out in the pres-
ence of D2O.
Scheme 2. Transformation of OAc-substituted cyclopenta[b]indole 2a.
In summary, we have disclosed a new reaction pat-
tern for gold-catalyzed cycloisomerizations of 1,6-
diynes containing an ynamide propargyl ester moiety
that provides an attractive route to a diverse-substi-
Scheme 3. Isotopic-labeling experiments.
tuted
3-acyloxy-1,4-dihydrocyclopenta[b]indoles.
Mechanistic studies indicate that the reaction likely
proceeds through a competitive 1,2-OAc migration
followed by [3+2] cycloaddition of the vinyl gold–
carbenoid intermediate with the pendant triple
bond. It is also noted that the reactions initiated
through a 1,2-acyloxy migration of internal alkynes
are quite rare in gold catalysis.[16a,18] Our results also
indicated that the 1,2- versus 1,3-acyloxy migration
can be affected by the nature of the catalysts and
the alkyne substituents. Further exploration and ap-
plications of this chemistry are underway in our
group.
Acknowledgements
We thank the National Natural Science Foundation of
China (Grant Nos. 21125210, 21372244, 21121062),
Chinese Academy of Science, and the Major State
Basic Research Development Program (Grant No.
2011CB808700) for financial support.
Scheme 4. Proposed reaction mechanism.
activation of the propargyl acetate moiety by gold results in
the competitive formation of a-vinyl gold–carbenoid 9 by 1,2-
acyloxy migration. Compound 9 might be stabilized by the ad-
jacent nitrogen atom. Although 3,3-rearrangement through
Keywords: cycloaddition · gold · homogeneous catalysis ·
propargyl ester · ynamides
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Received: November 5, 2014
Published online on December 2, 2014
Chem. Eur. J. 2015, 21, 1009 – 1013
www.chemeurj.org
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