Pentannulation Reaction by Tandem Gold(I)-Catalyzed Propargyl Claisen Rearrangement/Nazarov Cyclization of Enynyl Vinyl Ethers

Article abstract of DOI:10.1021/acs.orglett.8b02141

The tandem gold(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization of propargyl vinyl ether derivatives, followed by in situ reduction of the resulting carbonyl group, provides functionalized cyclopentadienes fused with various N-hetero- and

Full text of DOI:10.1021/acs.orglett.8b02141

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pentannulation Reaction by Tandem Gold(I)-Catalyzed Propargyl
Claisen Rearrangement/Nazarov Cyclization of Enynyl Vinyl Ethers
†
́
Antonia Rinaldi, Martina Petrovic, Stefano Magnolfi, Dina Scarpi,* and Ernesto G. Occhiato*
Department of Chemistry “U. Schiff”, University of Florence, Via della Lastruccia 13, 50019, Sesto Fiorentino (FI), Florence, Italy
S
* Supporting Information
ABSTRACT: The tandem gold(I)-catalyzed propargyl Claisen rearrange-
ment/Nazarov cyclization of propargyl vinyl ether derivatives, followed by
in situ reduction of the resulting carbonyl group, provides functionalized
cyclopentadienes fused with various N-hetero- and carbacycles, including
indoles, in good to excellent yields. The reaction occurs with high
regioselectivity, with the position of the double bonds in the five-
membered ring depending on the type of (hetero)cycle bearing the
propargylic moiety and the side chain on the latter.
he Nazarov reaction, in its classical version, is the Lewis
or Brønsted acid catalyzed 4π-electrocyclization of 1,4-
However, allenes can be prepared not only from propargylic
acetates but also by the gold-catalyzed [3,3]-rearrangement of
propargyl vinyl ethers (propargyl Claisen rearrangement, eq
2).16 We envisaged that this reaction could be included in an
unprecedented tandem process entailing a final Nazarov
cyclization for the generation of cyclopentadienes (Scheme
1).17 Thus, a suitably built enynyl vinyl ether (1), upon
T
dien-3-ones to form cyclopentenones.1,2 The Nazarov reaction
has now become one of the most important and versatile tools
for the construction of five-membered rings,3 as in the past two
decades a number of innovative approaches have been
developed for the generation of the requisite pentadienyl
cation undergoing the cyclization process.4 Suitably function-
alized allenes in particular have proven to be very useful as
substrates for the Nazarov reaction under a variety of
conditions, including treatment with oxidants, Brønsted
acids, and transition-metal complexes.4a,5
Scheme 1. Proposed Tandem Process
Gold-catalyzed cycloisomerization of aryl allenes6 and vinyl
allenes (eq 1)7,8 were first reported in 2006 and 2007,
treatment with a gold catalyst, rearranges to give the
corresponding allene−gold(I) complex (2). This, in turn,
should generate the requisite pentadienyl cation intermediate
(3) which cyclizes to ultimately form the target pentannulated
compound (4). While with the rearrangement of enynyl
acetates the final products are cyclopentenones, this process
provides cyclopentadienes bearing, on the side chain, an
aldehyde group which could be subjected to further
elaboration, even in situ, for chain elongations or other
transformations. For this study, we decided to use substrates
respectively, and were later exploited for the synthesis of
pentannulated carba- and heterocycles.9 Gold(I)-catalyzed
cascade processes in which allene intermediates are generated
by formal [3,3]-rearrangement of 5-acyloxy-1,3-enynes10 have
also been reported to efficiently provide five-membered rings
through a final Nazarov cyclization step.9a,b,11,12
Due to our interest in the synthesis of cyclopenta-fused
heterocycles by the Nazarov reaction,13 we recently extended
the latter approach to the pentannulation of N-heterocycles14
and the synthesis of natural compounds.14a,15
Received: July 9, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02141
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
embodying the enynyl double bond in various N-heterocycles,
including indoles, and carbacycles (compounds 7, Scheme 2),
so that the corresponding pentannulated products could
eventually be obtained.
Table 1. Optimization of the Reaction Conditions
Scheme 2. Preparation of the Substrates
d
time
(h)
7a
8a
8a′
9a
b
c
c
c
c
entry
1
catalyst
(%)
(%)
(%)
9
(%)
[Ph3PAuCl]/
AgSbF6
1
−
74
29
96
17
29
−
2
3
[Ph3PAuCl]/
AgBF4
[Ph3PAuCl]/
AgOTf
5
37
−
5
4
0.5
4
5
[LAuCl]/AgSbF6
4
84
−
46
−
−
1
3
5
12
−
−
−
−
−
−
−
21
−
e
tBu3PAuNTf2
1.5
3.5
2.5
0.25
1.5
3.5
0.5
1
95
49
92
94
78
54
22
−
f
6
7
8
9
10
11
12
13
[IPrAuCl]/ AgSbF6
[IPrAuCl]/AgSbF6
[IPrAuCl]/AgOTf
5
8
6
22
15
3
−
−
e
IPrAuNTf2
−
[IPrAuCl]/AgBF4
31
g
h
AuCl3/AgSbF6
75
h
AgSbF6
79
i
h
TfOH
0.25
100
−
a
Conditions: Reactions carried out on 0.2−0.3 mmol of 7a in CH2Cl2
(0.05 M) at 25 °C under N2 atmosphere; L = [(2,4-di-t-
BuC6H3)O]3P; IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene.
b
Prepared by mixing the silver salt (3 mol %) and the gold chloride (3
c
mol %) in CH2Cl2 before addition of the substrate. Relative amount
d
determined by 1H NMR of the crude reaction mixture. Formed
e
f
g
The required enynyl vinyl ethers 7 were prepared by the
Sonogashira reaction of either vinyl triflates or phosphates 5
(which were obtained from the corresponding lactams and
cycloalkanones) with differently substituted propargyl alcohols,
followed by O-vinylation18 of the coupling products 6 (Scheme
2).19 In order to find the optimal reaction conditions for the
Au(I)-catalyzed tandem process, we subjected N-Ts-protected
compound 7a to a variety of conditions (Table 1), starting
with those reported for the cycloisomerization of vinyl allenes7
and the propargyl Claisen rearrangement.16a Using 3 mol %
during the workup. Commercially available. Carried out a 0 °C. 9
mol %. Alcohol 6a. 1.5 mol %.
h
i
54% conversion, entry 6), or when the reaction was slow
because of the anion used (entry 10),23 which suggests a very
fast cyclization of the gold−allene complex once formed under
these conditions. The use of AuCl3 (entry 11), AgSbF6 (entry
12), and TfOH (entry 13) caused mainly devinylation of the
starting material.
In assessing the scope of the reaction, because of the
instability of the aldehydes during the chromatographic
purification,22 these were reduced in situ to the corresponding
alcohols by NaBH4 (Scheme 3) once the starting material 7
had disappeared (TLC monitoring). Interestingly, the alcohols
deriving from the reduction of isomers 8′ were not observed
when adopting this two-step procedure. Thus, in the presence
of t-Bu3PAuNTf2 as the catalyst, 7a was quantitatively
converted to alcohol 10a, which was obtained in 72% yield
after chromatography. With compound 7b, bearing a n-Bu
chain, the reaction carried out with the same catalyst was
sluggish, providing after 16 h a mixture of alcohol 10b (63%)
and the corresponding allene (29%), together with traces of
the unreacted starting material (8%). Instead, with the
[IPrAu]+TfO− catalyst the reaction was faster (reaction
complete in 4 h), although still slower than with substrate
7a, providing alcohol 10b only, which was isolated in 71%
yield. With gem-dimethyl substituted 7c the reaction quickly (1
h) led to Nazarov product 10c (81% yield) only when the
reaction was carried out in the presence of the [IPrAu]+TfO−
catalyst, with one double bond obviously forced at the ring
junction. Interestingly, with substrate 7h bearing the same gem-
dimethyl moiety, and in which the N atom is not conjugated
with the endocyclic double bond, the reaction carried out with
t-Bu3PAuNTf2 provided alcohol 10h in 81% yield but as a
[Ph3PAu]+SbF6 in CH2Cl2 (Table 1, entry 1) the reaction
−
was complete in 1 h and provided target compound 8a (and its
minor isomer 8a′, formed during the aqueous workup) as the
major product (74%), but in a mixture with allene 9a
(17%).20,21 With BF4 as the counterion, the reaction was
−
similarly sluggish, being still incomplete after 5 h and providing
a mixture of starting material, product 8, and allene 9a (entry
2). Using TfO− as the anion (entry 3), the reaction was instead
much faster (0.5 h) and yielded 8 only. With an electron-poor
ligand such as the phosphite [(2,4-di-t-BuC6H3)O]3P (entry 4)
the reaction was much slower, with conversion of 7a into
products (mainly allene 9a) being only 16%. With t-
Bu3PAuNTf2 as the catalyst (entry 5) the reaction was
complete in 1.5 h, providing aldehyde 8 only.
We also carried out a series of experiments with the NHC
ligand IPr (IPr = 1,3-bis(diisopropylphenyl)imidazol-2-yli-
dene) and various anions, and in all cases, the reaction
provided the 8a/8a′ mixture only (entries 6−10). Again, the
reaction was faster with TfO− as the noncoordinating anion
(entry 8), being complete in 15 min. With this NHC ligand,
we never observed allene 9a when monitoring the reactions by
1H NMR, even when carrying out the reaction at 0 °C (for
example, only the signals of the starting material and the
1
Nazarov product were present in the H NMR spectrum at a
B
DOI: 10.1021/acs.orglett.8b02141
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
To demonstrate that this cascade process can be extended to
carbacyclic systems, substrates 7i−k, were subjected to various
reaction conditions. While in the case of N-heterocycle alcohol
10a (as well as 10b and 10d) we never observed double bond
isomerization in the five-membered ring during the chromato-
graphic purification on silica gel, likely because of the fully
conjugated π system from the N atom to C5; this was not the
case of the products deriving from carbacycle substrates, for
the purification of which eluents containing Et3N had to be
used. So, with both 7i and 7j, the reaction, best carried out in
Scheme 3. Scope of the Reaction
−
the presence of 3 mol % [IPrAu]+SbF6 , was very fast
(complete in 10−20 min) and provided, after reduction,
isomerically pure alcohols 10i−j in excellent yields after
chromatography (81−96%). With cycloheptene derivative 7k,
the reaction was even faster at 25 °C utilizing 1.5 mol % of
[IPrAu]+SbF6 , quantitatively providing 10k as a 21:1 mixture
−
with a minor isomer having one double bond at the ring
junction. However, this compound proved isomerically less
stable as the relative amount of the major isomer gradually
decreased in solution (both in CDCl3 and in CD3OD) during
NMR analysis.
As the cyclopenta[b]indole motif is present in several natural
and synthetic biologically active compounds,25,26 the scope of
the process was evaluated with some indole derivatives as well
(compounds 7l−o). Thus, the cascade process provided
substituted cyclopenta[b]indoles 10l−o (58−65% yields) in
which isomerization of the double bond occurred to restore
the aromaticity of the indole ring. With substrate 7l, the
−
reaction was complete in 2 h with [IPrAu]+SbF6 (3 mol %),
whereas, with the gem-dimethyl-substituted propargylic de-
rivative 7n, 5 h were necessary for a complete conversion with
the same catalyst. Both n-butyl-substituted derivative 7m and
Br-substituted indole 7o were instead best reacted in the
presence of [IPrAu]+TfO− (3 mol %) as the catalyst. With
indole substrates 7l−o, both the yields and reaction rates
decreased if compared to those obtained with carbacycles 7i−
k. Despite the possible accelerating effect by the electron
donor N-Me on the vinyl moiety,3e,13 disruption of the indole
aromaticity could slow down the cyclization,14a causing some
degradation of the starting material.
The proposed catalytic cycle for this tandem process is
reported in Scheme 4. The initial coordination of gold to the
triple bond triggers the [3,3]-rearrangement to form allene−
gold complex IV. Gold(I)−complex IV then evolves toward
allene V or rearranges to pentadienyl cation VI which cyclizes
to give intermediate VII. The position of the double bonds in
the final product IX is consistent with the mechanism
proposed by Toste for the cycloisomerization of vinyl allenes,7
i.e. a [1,2]-H shift from position 5 to 6 in VII to generate
cation VIII and eventual LAu+ elimination.27 This is further
supported by an experiment we carried out with deuterated 7a
([D]-7a, Scheme 5) in the presence of 3 mol % t-Bu3PAuNTf2
which resulted in most of the deuterium (77%) being
incorporated at position 6 of the final product (see Supporting
Information). The incomplete deuterium incorporation could
be explained by a minor pathway involving deprotonation at
C5 followed by protodeauration.11a,b,14b Clearly, with gem-
dimethyl-substituted substrates like 7c, the [1,2]-H shift
cannot be effective and a base-assisted deprotonation at 4a
position followed by protodeauration must instead take place.
In this case, the fast cyclization observed when TfO− is present
in the reaction medium could be accounted for by its greater
basicity compared to the other anions used in this study.28
a
b
c
Prepared by mixing [LAuCl] and AgX. Carried out at 0 °C. 21:1
d
isomeric mixture at 100% conversion. Carried out with 1.5 mol % of
catalyst.
1.2:1 mixture of isomers. With a phenyl-substituted propargylic
moiety (substrate 7d) the reaction was much cleaner when
carried out at 0 °C and it provided, after 3.5 h and in situ
reduction, alcohol 10d in 60% yield.
With the seven-membered ring containing substrates, the
presence of the n-Bu chain in 7f also slowed down the reaction
rate (compared to 7e), when catalyzed by the t-Bu3PAuNTf2
complex, as it was complete in 2 h and provided 10f in mixture
with about 10% of the corresponding allene. With the
[IPrAu]+TfO− catalyst, the reaction was faster (0.5 h) and
provided 10f only (74% yield after chromatography). Quite
interestingly, with these seven-membered ring derivatives,
isomerization of the double bonds occurred to form
thermodynamically more stable compounds 10e−f.24 Finally,
as in the case of 7c, also with substrate 7g the corresponding
product (10g) with the double bond at the ring junction was
obtained (74% yield) after 6 h in the presence of the
[IPrAu]+TfO− catalyst.
C
DOI: 10.1021/acs.orglett.8b02141
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
provides, in general, isomerically stable products with high
regioselectivity and in good to excellent yields. The position of
the double bonds in the five-membered ring is dictated by the
type of (hetero)cycle bearing the propargylic moiety and the
side chain on the latter. The presence of an aldehyde
appendage on the cyclopentadiene allows for further in situ
elaboration to increase the complexity of the products.
Extension of the methodology to other systems and application
to the synthesis of natural compounds is ongoing in our
laboratory.
Scheme 4. Plausible Reaction Mechanism
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.8b02141.
Experimental details and full spectroscopic data for all
new compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: dina.scarpi@unifi.it.
*E-mail: ernesto.occhiato@unifi.it.
ORCID
Dina Scarpi: 0000-0001-7211-4881
Ernesto G. Occhiato: 0000-0003-2187-2409
Present Address
†
́
Fidelta Ltd., Prilaz baruna Filipovica 29, 10000 Zagreb,
Scheme 5. Control Experiment
Croatia.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.P. thanks the European Commission for a Marie Curie
fellowship (FP7-PEOPLE-2012-ITN, Project ECHONET, No.
316379).
Finally, the fact we never observe the formation of allene V
in the presence of the IPr ligand (e.g., entries 6−10, Table 1,
for 7a), even when the reaction is slow, suggests that the
cyclization of allene−gold complex IV, to give the Nazarov
product, must be fast once IV is formed.29
REFERENCES
■
(1) Nazarov, I. N.; Zaretskaya, I. I. Izv. Akad. Nauk. SSSR, Ser. Khim.
1941, 211.
(2) Habermas, K. L.; Denmark, S. E.; Jones, T. K. Org. React. 1994,
45, 1 and references cited therein..
To demonstrate that chain elongation can be carried out in
situ, PhMgBr (1 M in THF) was added at −78 °C to the DCM
solution containing aldehyde 8c just after completion of the
gold-catalyzed process (Scheme 6). The reaction provided
alcohol 11 in 55% yield after chromatography.
In conclusion, we have developed a gold(I)-catalyzed
propargyl Claisen rearrangement/Nazarov cyclization cascade
process for the synthesis of functionalized cyclopentadienes
fused with various N-hetero- and carbacycles, including
indoles. The reaction occurs under mild conditions and
(3) Most recent review on the Nazarov reaction: (a) Vinogradov, M.
G.; Turova, O. V.; Zlotin, S. G. Org. Biomol. Chem. 2017, 15, 8245.
See also: (b) West, F. G.; Scadeng, O.; Wu, Y.-K.; Fradette, R. J.; Joy,
S. The Nazarov cyclization. In Comprehensive Organic Synthesis, 2nd
ed.; Knochel, P., Molander, G. A., Eds.; Elsevier: Amsterdam, 2014;
pp 827−866. For older reviews on the Nazarov reaction covering the
past decade, see: (c) Frontier, A. J.; Collison, C. Tetrahedron 2005,
61, 7577. (d) Tius, M. A. Eur. J. Org. Chem. 2005, 2005, 2193. For
the effect of heteroatoms on the Nazarov reaction, see: (e) He, W.;
Sun, X.; Frontier, A. J. J. Am. Chem. Soc. 2003, 125, 14278.
(4) For a review on unconventional Nazarov reactions, see:
(a) Spencer, W. T., III; Vaidya, T.; Frontier, A. J. Eur. J. Org.
Chem. 2013, 2013, 3621. Selected recent papers on alternative
Nazarov reactions: (b) Fradette, R. J.; Kang, M.; West, F. G. Angew.
Chem., Int. Ed. 2017, 56, 6335. (c) Komatsuki, K.; Sadamitsu, Y.;
Sekine, K.; Saito, K.; Yamada, T. Angew. Chem., Int. Ed. 2017, 56,
11594. (d) Sudhakar, G.; Mahesh, S. K.; Phani Babu Vemulapalli, S.;
Babu Nanubolu, J. Org. Lett. 2017, 19, 4500. (e) Carmichael, R. A.;
Sophanpanichkul, P.; Chalifoux, W. A. Org. Lett. 2017, 19, 2592.
(f) Huang, Y.-W.; Frontier, A. J. Org. Lett. 2016, 18, 4896. (g) Shu,
Scheme 6. One-Pot Cascade Process and Chain Elongation
D
DOI: 10.1021/acs.orglett.8b02141
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
C.; Wang, Y.-H.; Shen, C.-H.; Ruan, P.-P.; Lu, X.; Ye, L.-W. Org. Lett.
2016, 18, 3254.
(5) For a review on allene ether Nazarov cyclization, see: Tius, M. A.
132, 4720. (b) For a review on the chemistry of propargyl vinyl
ethers, see: Zhu, Z.-B.; Kirsch, S. F. Chem. Commun. 2013, 49, 2272.
(18) We used a variant of the Hg(OAc)2-catalyzed vinylation
reported in ref 17a. See also: Grissom, J.; Klingberg, D.; Huang, D.;
Slattery, B. J. J. Org. Chem. 1997, 62, 603.
Chem. Soc. Rev. 2014, 43, 2979.
́
́
(6) Marion, N.; Díez-Gonzalez, S.; de Fremont, P.; Noble, A. R.;
Nolan, S. P. Angew. Chem., Int. Ed. 2006, 45, 3647.
(19) Vinyl ethers 7 were not stable when neat and were better stored
in the chromatographic eluent with 1% Et3N until their use.
(20) The structure of 8a was easily determined by 1H NMR studies.
The CHO proton resonates at 9.76 ppm as a singlet, and the α
hydrogens to the carbonyl group form an AB system at 3.73 and 3.41
ppm. 6-H is a singlet at 6.01 ppm, and the methyl group, a singlet at
1.86 ppm. The bridgehead 4a-H resonates at 2.00 ppm and couples
with protons at C4. Its isomer 8a′ has the singlet of 6-H at about 6.15
ppm, and the aldehyde H resonates at 9.77 ppm. Bridgehead 7a-H is a
singlet at about 3.65 ppm.
(7) Lee, J. H.; Toste, F. D. Angew. Chem., Int. Ed. 2007, 46, 912.
(8) PtCl2 also has been reported to catalyze cycloisomerization of
vinyl allenes: Funami, H.; Kusama, H.; Iwasawa, N. Angew. Chem., Int.
Ed. 2007, 46, 909.
̀
(9) (a) Lemiere, G.; Gandon, V.; Cariou, K.; Fukuyama, T.;
Dhimane, A.-L.; Fensterbank, L.; Malacria, M. Org. Lett. 2007, 9,
̀
2207. (b) Lemiere, G.; Gandon, V.; Cariou, K.; Hours, A.; Fukuyama,
T.; Dhimane, A.-L.; Fensterbank, L.; Malacria, M. J. Am. Chem. Soc.
2009, 131, 2993. (c) Chen, B.; Fan, W.; Chai, G.; Ma, S. Org. Lett.
2012, 14, 3616. (d) Imino-Nazarov variant: Ma, Z.-X.; He, S.; Song,
W.; Hsung, R. P. Org. Lett. 2012, 14, 5736. For an example of related
PtCl2-catalyzed process, see: (e) Bhunia, S.; Liu, R. S. J. Am. Chem.
Soc. 2008, 130, 16488.
1
(21) Diagnostic H NMR signals of allene 9a are the quartet at 4.6
ppm and the doublet at 1.8 ppm (CH and Me group of the allene
moiety, respectively) as well as the triplet at 5.48 ppm of 3-H
(heterocycle). The aldehyde proton resonates as a triplet at 9.7 ppm,
and the α hydrogens resonate as a multiplet at 3.32 ppm.
(22) Aldehydes 8 tend to form various unidentified byproducts
during chromatography on silica gel, and migration of the double
bond to the exocyclic position also occurs to generate the
corresponding α,β-unsaturated aldehydes (iso-8) (see Supporting
Information).
(10) For reviews on propargylic esters in gold catalysis, see:
(a) Shiroodi, R. K.; Gevorgyan, V. Chem. Soc. Rev. 2013, 42, 4991.
(b) Marion, N.; Nolan, S. P. Angew. Chem., Int. Ed. 2007, 46, 2750.
(c) Marco-Contelles, J.; Soriano, E. Chem. - Eur. J. 2007, 13, 1350.
For a discussion on the mechanism, see: (d) Correa, A.; Marion, N.;
Fensterbank, L.; Malacria, M.; Nolan, S. P.; Cavallo, L. Angew. Chem.,
Int. Ed. 2008, 47, 718.
(11) (a) Zhang, L.; Wang, S. J. Am. Chem. Soc. 2006, 128, 1442.
(b) Shi, F. Q.; Li, X.; Xia, Y.; Zhang, L.; Yu, Z. X. J. Am. Chem. Soc.
2007, 129, 15503. (c) Congmon, J.; Tius, M. A. Eur. J. Org. Chem.
2018, 2018, 2926.
(12) The metal-catalyzed rearrangement of 3-acyloxy-1,4-enynes,
called the Rautenstrauch rearrangement, occurs instead via initial 1,2-
shift of the carboxylate and is another way to prepare five-membered
rings; see: (a) Shi, X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc.
2005, 127, 5802. For recent examples, see: (b) Zi, W.; Wu, H.; Toste,
(23) We monitored this reaction by 1H NMR at 10, 30, 60, and 120
min but never observed even traces of allene 9a.
(24) RHF/3-21G(*) calculations (PC Spartan Pro software, Wave
function, Inc.) resulted in compound 10e being 8.9 kcal/mol lower in
energy than its isomer with double bonds at C6−C7 and C8−C8a.
(25) For a review on the biological significance of, and the metal-
catalyzed synthetic approaches to, cyclopenta[b]indoles, see:
Vivekanand, T.; Satpathi, B.; Bankar, S. K.; Ramasastry, S. S. V RSC
Adv. 2018, 8, 18576.
(26) For other approaches to pentannulated indoles which exploit
gold(I) catalysis, see: (a) Mei, L.-Y.; Wei, Y.; Tang, X.-Y.; Shi, M. J.
Am. Chem. Soc. 2015, 137, 8131. (b) Alcaide, B.; Almendros, P.;
F. D. J. Am. Chem. Soc. 2015, 137, 3225. (c) Burki, C.; Whyte, A.;
̈
Arndt, S.; Hashmi, A. S. K.; Lautens, M. Org. Lett. 2016, 18, 5058.
For other metal-catalyzed processes involving either 1,2- or 1,3-
acyloxy migration in substrates embodying a propargyl alcohol
moiety, ultimately leading to five-membered rings: (d) Prasad, B. A.
B.; Yoshimoto, F. K.; Sarpong, R. J. Am. Chem. Soc. 2005, 127, 12468.
(e) Hoffmann, M.; Weibel, J. M.; de Fremont, P.; Pale, P.; Blanc, A.
Org. Lett. 2014, 16, 908. (f) Shiroodi, R. K.; Sugawara, M.; Ratushnyy,
M.; Yarbrough, D. C.; Wink, D. J.; Gevorgyan, V. Org. Lett. 2015, 17,
4062. See also refs 6 and 10.
(13) (a) Occhiato, E. G.; Prandi, C.; Ferrali, A.; Guarna, A.;
Venturello, P. J. Org. Chem. 2003, 68, 9728. (b) Prandi, C.; Ferrali, A.;
Guarna, A.; Venturello, P.; Occhiato, E. G. J. Org. Chem. 2004, 69,
7705. (c) Prandi, C.; Deagostino, A.; Venturello, P.; Occhiato, E. G.
Org. Lett. 2005, 7, 4345. (d) Cavalli, A.; Masetti, M.; Recanatini, M.;
Prandi, C.; Guarna, A.; Occhiato, E. G. Chem. - Eur. J. 2006, 12, 2836.
(e) Larini, P.; Guarna, A.; Occhiato, E. G. Org. Lett. 2006, 8, 781.
(f) Cavalli, A.; Pacetti, A.; Recanatini, M.; Prandi, C.; Scarpi, D.;
Occhiato, E. G. Chem. - Eur. J. 2008, 14, 9292.
́
Alonso, J. M.; Fernandez, I. J. Org. Chem. 2013, 78, 6688. (c) Ma, Z.-
X.; He, S.; Song, W.; Hsung, R. P. Org. Lett. 2012, 14, 5736.
(d) Dhiman, S.; Ramasastry, S. S. V. Org. Lett. 2015, 17, 5116.
(e) Dhiman, S.; Ramasastry, S. S. V. Chem. Commun. 2015, 51, 557.
(f) Kothandaraman, P.; Rao, W.; Foo, S. J.; Chan, P. W. H. Angew.
Chem., Int. Ed. 2010, 49, 4619. See also refs 9c and 12b.
(27) The transformation of allylic cation VII to a localized cation
VIII is feasible likely because of the large stabilizing hyperconjugative
β-Au effect. See: (a) Alabugin, I. V.; Gilmore, K.; Peterson, P. W.
WIREs Comput. Mol. Sci. 2011, 1, 109 See also:. (b) Peterson, P. W.;
Mohamed, R. K.; Alabugin, I. V. Eur. J. Org. Chem. 2013, 2013, 2505
and references therein.
(28) (a) Lu, Z.; Han, J.; Okoromoba, O. E.; Shimizu, N.; Amii, H.;
Tormena, C. F.; Hammond, G. B.; Xu, B. Org. Lett. 2017, 19, 5848.
For the important role of additive AgOTf, see also: (b) Alcaide, B.;
Almendros, P.; Fernandez, I.; Martínez del Campo, T.; Naranjo, T.
Adv. Synth. Catal. 2013, 355, 2681. Reviews on counterion effects in
homogeneous gold catalysis: (c) Jia, M.; Bandini, M. ACS Catal.
2015, 5, 1638. (d) Schießl, J.; Schulmeister, J.; Doppiu, A.; Wcŗ ner,
E.; Rudolph, M.; Karch, R.; Hashmi, A. S. K. Adv. Synth. Catal. 2018,
360, 2493.
́
́
́
(14) (a) Scarpi, D.; Petrovic, M.; Fiser, B.; Gomez-Bengoa, E.;
́
Occhiato, E. G. Org. Lett. 2016, 18, 3922. (b) Petrovic, M.; Scarpi, D.;
́
Fiser, B.; Gomez-Bengoa, E.; Occhiato, E. G. Eur. J. Org. Chem. 2015,
(29) When product mixtures containing allene 9a were treated with
the [IPrAu]+TfO− catalyst under the optimized reaction conditions,
conversion of the allene into 8a occurred but it was much slower than
the conversion of 7a, reasonably for the minor “allenophilicity” of the
LAu+ species compared to its “alkynophilicity”.
2015, 3943.
(15) Scarpi, D.; Faggi, C.; Occhiato, E. G. J. Nat. Prod. 2017, 80,
2384.
(16) (a) Sherry, B. D.; Toste, F. D. J. Am. Chem. Soc. 2004, 126,
15978. (b) Wang, D.; Gautam, L. N. S.; Bollinger, C.; Harris, A.; Li,
M.; Shi, X. Org. Lett. 2011, 13, 2618. (c) Vidhani, D. V.; Cran, J. W.;
Krafft, M. E.; Alabugin, I. V. Org. Biomol. Chem. 2013, 11, 1624.
(17) (a) The Au-catalyzed propargyl Claisen rearrangement of 3-
cyclopropyl propargyl vinyl ethers, leading to cyclopentenes via
́
cyclopropane ring opening, has been reported: Garayalde, D.; Gomez-
Bengoa, E.; Huang, X.; Goeke, A.; Nevado, C. J. Am. Chem. Soc. 2010,
E
DOI: 10.1021/acs.orglett.8b02141
Org. Lett. XXXX, XXX, XXX−XXX