Gold-Catalyzed Formal [4 + 2] Cycloaddition of 5-(Ethynylamino)pent-2-yn-1-yl Esters to 1,2,3,5-Tetrahydrobenzo [g] quinolines

Article abstract of DOI:10.1021/acs.orglett.8b00267

A synthetic method to prepare 1,2,3,5-tetrahydrobenzo[g]quinolines efficiently that relies on gold(I)-catalyzed cycloisomerization of 5-(ethynylamino)pent-2-yn-1-yl esters at room temperature under atmospheric conditions is described. The proposed reaction mechanism presents a unique instance of an in situ formed allenic ester and gold keteniminium species to undergo a formal [4 + 2] cycloaddition pathway.

Full text of DOI:10.1021/acs.orglett.8b00267

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Gold-Catalyzed Formal [4 + 2] Cycloaddition of
5‑(Ethynylamino)pent-2-yn-1-yl Esters to 1,2,3,5-
Tetrahydrobenzo[g]quinolines
Xiaoyu Chen,† James Theodore Merrett,† and Philip Wai Hong Chan*,†,‡
†School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
‡Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom
S
* Supporting Information
ABSTRACT: A synthetic method to prepare 1,2,3,5-
tetrahydrobenzo[g]quinolines efficiently that relies on gold-
(I)-catalyzed cycloisomerization of 5-(ethynylamino)pent-2-
yn-1-yl esters at room temperature under atmospheric
conditions is described. The proposed reaction mechanism
presents a unique instance of an in situ formed allenic ester
and gold keteniminium species to undergo a formal [4 + 2] cycloaddition pathway.
old-catalyzed cycloisomerization of propargyl esters and
Gynamides provides one of the most powerful and versatile
strategies for carbocyclic and heterocyclic synthesis (Schemes
1a and 1b).1−10 Mechanistically, the reactions of propargyl
esters have often relied on their propensity to undergo either
1,2- or 1,3-acyloxy migration to give the corresponding gold
carbenoid or allene intermediates I and II.2−4 For the nitrogen-
substituted alkynes, it is thought the transformations proceed
via a gold keteniminium species III.5−7 In both cases, further
functionalization by a remaining pendant group or a second
starting material was then posited to give the product. A recent
study also described the trapping of the organogold species of
one by the CC bond of the other in reactions employing
both substrates that is followed by a 1,2-acyloxy shift to give 1-
amino-1,3-cyclopentadienes (Scheme 1c).7 However, the
analogous cascade reactions in which a gold carbenoid or
allene intermediate subsequently engages with a gold
keteniminium species to effect carbon−carbon bond formation
has so far remained unrealized. With this in mind, we were
drawn to the potential gold-catalyzed cycloisomerization
chemistry of 5-(ethynylamino)pent-2-yn-1-yl esters 4 (Scheme
1d). In doing so, the putative (2-iminiovinyl)gold adduct IV
furnished in this manner was discovered to undergo a formal [4
+ 2] cycloaddition to give the 1,2,3,5-terahydrobenzo[g]-
quinoline ring system.8,9,11 Herein, we disclose the details of
this Au(I)-catalyzed method for the efficient and selective
synthesis of a potentially useful new member of the N-
heterocyclic compound family in good to excellent yields from
a ynamide-tethered propargyl ester. Accomplished at room
temperature under mild conditions that did not require the
exclusion of moisture or air, the transformations represent the
first example of the two in situ formed cycloisomeric motifs
participating in a formal [4 + 2] cycloaddition pathway. It also
provides a rare instance of a gold-activated ynamide taking on
the role of the 4π cycloaddition partner in gold catalysis.9
To test the feasibility of our hypothesis, the ynamide-
tethered propargyl acetate 4a was chosen as the model
substrate to establish the optimum reaction conditions (Table
1). The ensuing study revealed that treatment of the starting
material with 5 mol % of gold(I) phosphine complex A in
nondistilled 1,2-dichloroethane at room temperature under
atmospheric conditions for 12 h gave the best result (entry 1).
Scheme 1. Gold-Catalyzed Reactivities of Propargyl Esters
and Ynamides
Received: January 25, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00267
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
outcome was found in a final set of control experiments
mediated by AgOTf or AgSbF6, which were observed to give an
inseparable mixture of two compounds that could not be
Table 1. Optimization of the Reaction Conditions
1
identified by H NMR analysis (entries 12 and 14).
With the optimum reaction conditions in hand, we next
turned our attention to evaluating the generality of the present
procedure by surveying a variety of 5-(ethynylamino)pent-2-yn-
1-yl esters (Figure 1). These reactions demonstrated the
b
c
entry
catalyst
solvent
yield (%)
1
A
A
A
A
A
A
B
(CH2Cl)2
CH2Cl2
90
67
36
d
2
3
MeCN
4
THF
5
toluene
46
f
e
6
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
toluene
7
75
50
f
8
Ph3PAuNTf2
Ph3PAuNTf2
C
e
e
9
10
11
12
54
f
C
AgOTf
AgOTf
AgSbF6
g
h
13
d
14
(CH2Cl)2
g
a
All reactions were performed with 0.1 mmol of 4a and 5 mol % of
gold(I) catalyst in 1 mL of solvent at room temperature under
b
atmospheric conditions for 12 h. Reaction performed in nondistilled
c
d
solvent. Isolated yield. Mixture of unidentifiable decomposition
products were obtained based on TLC analysis and 1H NMR
measurements. Reaction performed in distilled (CH2Cl)2 and with 4
e
Å MS (50 mg) under a nitrogen atmosphere at room temperature for
f
3 h and then at reflux temperature for 4 h. Substrate recovered in 70−
g
96% yield. Mixture of two compounds were obtained that could not
h
1
be identified by H NMR analysis. Reaction conducted with 10 mol
% of AgOTf at reflux temperature for 5 h.
Under these reaction conditions, the anticipated formal [4 + 2]
cycloaddition product 5a was obtained in 90% yield. The
structure of the nitrogen-containing cyclic adduct was
determined by comparison with the NMR measurements and
X-ray crystallography of a closely related analogue vide infra.
Lower product yields of 36−67% were furnished on repeating
the reaction with dichloromethane, acetonitrile, or toluene in
place of 1,2-dichloroethane as the solvent (entries 2, 3, and 5).
Likewise, the analogous control reactions mediated by the
gold(I) phosphine complex B, Ph3PAuNTf2 or NHC−gold(I)
(NHC = N-heterocyclic carbene) complex C were found to
give product yields of 50−75% (entries 7, 8, and 10). In
contrast, the introduction of 4 Å molecular sieves (MS) to the
gold(I) phosphine complex A- or C- or Ph3PAuNTf2-catalyzed
experiments in distilled 1,2-dichloroethane was found to result
in no reaction being detected by TLC analysis after 3 h (entries
6, 9, and 11). Continuing the experiments at reflux temperature
for 4 h was also observed to have no influence on the course of
the reaction and led to the recovery of only the substrate in
70−96% yield. The analogous reactions catalyzed by gold(I)
phosphine complex A with THF as the solvent or AgOTf (10
mol %) in toluene at reflux temperature for 5 h, on the other
hand, were found to give a mixture of unidentifiable
decomposition products (entries 4 and 13).12 A similar
Figure 1. Formal [4 + 2] cycloaddition of 4b−v catalyzed by Au(I)
complex A. All reactions were performed at the 0.1−1 mmol scale with
5 mol % of Au(I) complex A in 1,2-dichloroethane (0.1 M) at room
temperature under atmospheric conditions for 1−18 h. Values in
parentheses denote isolated product yields. (a) Reaction conducted at
60 °C.
gold(I) complex A-catalyzed reaction conditions to be broad,
providing a series of 1,2,3,5-tetrahydrobenzo[g]quinolines in
32−89% yield from the corresponding substrates 4b−v. The
rearrangement of substrates with a OPNB (4b) or OPMB (4c)
instead of OAc migrating group were found to proceed to
afford the corresponding N-heterocycles 5b and 5c in yields of
64 and 62%, respectively. The structure of the former partially
hydrogenated quinoline was also confirmed by X-ray single-
crystal analysis. Reactions with starting materials containing
other aryl (4d−i), 3-furanyl (4j), alkyl (4k,l), or cycloalkyl
(4m,n) groups at the ester carbon center were also found to be
well tolerated and gave the anticipated nitrogen-containing
heterocycles 5d−n in 32−89% yield. Similarly, the presence of
an electron-withdrawing (4p−r,v) or -donating (4s−u)
substituent at the ortho- or para-position of the phenyl group
B
DOI: 10.1021/acs.orglett.8b00267
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
at the distal ynamide carbon center of the substrate was found
to have little influence on the course of the reaction. In these
reactions, the corresponding tricyclic adducts 5p−v were
furnished in 46−82% yield. The cycloisomerization of the
tertiary ester 4o was the only instance that required a slight
modification of the reaction conditions, with a reaction
temperature of 60 °C needed to furnish the desired N-
heterocyclic product 5o in 65% yield. Under the gold(I)-
catalyzed standard conditions at room temperature, the allene
6o was obtained in 71% yield.
Scheme 3. Control Experiment with 6o
of substrate in control reactions with 4 Å MS and a nitrogen
atmosphere detailed in Table 1, entries 6, 9, and 11.10
In summary, we have elucidated an efficient gold(I)-catalyzed
formal [4 + 2] cycloaddition process to assemble 1,2,3,5-
tetrahydrobenzo[g]quinolines from 5-(ethynylamino)-pent-2-
yn-1-yl esters under mild conditions at room temperature. The
cycloisomerization was shown to be robust as it did not require
the exclusion of air or moisture and demonstrated a new mode
of reactivity between in situ formed allenic esters and gold
keteniminium species. Efforts to realize an asymmetric version
and explore the potential synthetic applications of the present
reaction are currently in progress.
A tentative mechanism for the present Au(I)-catalyzed
formal [4 + 2] cycloaddition is outlined in Scheme 2. With
Scheme 2. Proposed Mechanism for the Au(I)-Catalyzed
Formal [4 + 2] Cycloaddition of 5-(Ethynylamino)pent-2-
yn-1-yl Esters Represented by 4a
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00267.
Detailed experimental procedures, characterization data
1
and H and 13C NMR spectra for all starting materials
and products (PDF)
Accession Codes
CCDC 1817966 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
4a as representative example, this could involve the activation
of the propargyl ester moiety in the substrate by the metal
complex to give the gold-coordinated species Va. As a result,
this may trigger [3,3]-sigmatropic rearrangement of the acetate
group and formation of the allenic ester VIIa via the 1,3-dioxin-
1-ium species VIa. This is followed by gold-activated ynamide−
gold keteniminium tautomerization to provide the putative
organogold intermediate IVa. This is the putative active species
that undergoes the formal [4 + 2] cycloaddition process
involving nucleophilic attack of the allenic ester group to the
gold keteniminium motif in IVa.8,9 Subsequent Friedel−Crafts-
type alkylation of the phenyl group by the vinyl oxonium unit
in the ensuing vinyl gold species VIIIa would deliver the
Wheland-type intermediate IXa. Deprotonation and proto-
deauration of this newly formed tricyclic carbocation species
might then produce the product 5a.10 The proposed sequential
1,3-acyloxy migration followed by a gold-activated ynamide−
gold keteniminium tautomerization pathway would be
consistent with our findings showing 5o and 6o being afforded
in the respective reactions at 60 °C and room temperature as
described in Figure 1. It also supports the posited involvement
of the putative (2-iminiovinyl)gold adduct IV put forward in
Scheme 1. This argument was further corroborated by the
observation that when 6o was resubjected to the Au(I)-
catalyzed standard conditions at 60 °C, the expected
benzofused quinoline product 5o was obtained in 71% yield
(Scheme 3). The role of an advantageous trace amount of water
in facilitating the rearomatization and protodeauration of the
vinyl gold species IXa would be in agreement with the recovery
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: phil.chan@monash.edu, p.w.h.chan@warwick.ac.uk.
ORCID
Philip Wai Hong Chan: 0000-0002-8786-6143
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a Discovery Project Grant
(DP160101682) from the Australian Research Council.
■
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DOI: 10.1021/acs.orglett.8b00267
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