Gold(I)-catalyzed intramolecular cycloisomerization of propargylic esters with furan rings

Article abstract of DOI:10.1002/chem.201500214

A gold-catalyzed intramolecular cycloisomerization of a-yne-furans 1 is described in this contribution. A variety of cyclic α,β-unsaturated aldehyde or ketone derivatives and nitrogen-containing tricyclic adducts were obtained selectively in moderate to excellent yields under mild conditions by varying the substituents on the standard substrates.

Full text of DOI:10.1002/chem.201500214

DOI: 10.1002/chem.201500214
Communication
&
Gold Catalysis
Gold(I)-Catalyzed Intramolecular Cycloisomerization of Propargylic
Esters with Furan Rings
Jin-Ming Yang,[a] Xiang-Ying Tang,*[b] and Min Shi*[a, b]
Abstract: A gold-catalyzed intramolecular cycloisomeriza-
tion of a-yne-furans 1 is described in this contribution. A
variety of cyclic a,b-unsaturated aldehyde or ketone deriv-
atives and nitrogen-containing tricyclic adducts were ob-
tained selectively in moderate to excellent yields under
mild conditions by varying the substituents on the stan-
dard substrates.
Over the past decade, homogeneous gold catalysis has wit-
nessed rapid growth for CÀC, CÀO or CÀN bond-forming reac-
tions mainly due to the soft carbophilic Lewis acid properties
of gold complexes toward CÀC multiple bonds.[1] In compari-
son to other transition-metal catalysts, gold complexes can ef-
ficiently catalyze reactions under very mild conditions that are
tolerant of air or moisture to give excellent chemoselectivity
and wide functional group compatibility. In particular, gold-cat-
alyzed cycloisomerizations of a-yne-furans have attracted
much attention owing to the efficient synthesis of anellated
carbo- or heterocyclic compounds.[2] In 2000 and 2001, the pio-
neering work on the intramolecular alkyne/furan cyclization re-
actions catalyzed by AuIII[2a] or PtII[2b,c] to afford phenols I and II
was reported by Hashmi, Echavarren and their co-workers
(Scheme 1a), respectively. Afterwards, the group of Hashmi dis-
covered several new reaction patterns by varying the substitu-
ents on a-yne-furans 1: In the case of silyl substituents at the
5-position of furan rings, gold-catalyzed reactions led to the
formation of anellated furans III, presumably through a hydro-
arylation reaction pathway;[2d] When there is an aryloxy sub-
stituent at the terminal position of the alkyne moiety, the
chiral, polycyclic compound IV could be obtained.[2e] As for the
intermolecular reaction of a furan with an alkyne, the synthesis
Scheme 1. Previous and our work on the cyclization of a-yne furan.
of phenols V (Scheme 1b) has been realized by the elegant
work of Hashmi[2f] and Echavarren[2g] as well.
On the other hand, propargylic esters are very important
feedstocks in organic synthesis.[3] In the presence of gold,[4–10]
platinum[11] or rhodium[12] catalysts, propargylic esters have ver-
satile reactivities to undergo 1,2- or 1,3-acyloxy migration,[4–12]
leading to the formation of a metal–carbene or an allenic inter-
mediate that is allowed further transformation to diversified
substances. As a part of our ongoing interest on the transition-
metal-catalyzed or mediated reactions of propargylic esters,
we have already reported several interesting transformations
of propargylic ester derivatives.[13] Very recently, Echavarren dis-
covered intermolecular reactions of propargylic esters with
furans to afford the corresponding cyclized products in good
yields (Scheme 1b).[2h] Based on these findings contributed by
Hashmi and Echavarren, we envisaged that a-yne-furans bear-
ing propargylic esters might undergo very interesting cycloiso-
[a] J.-M. Yang, Prof. Dr. M. Shi
Key Laboratory for Advanced Materials and Institute of Fine Chemicals
East China University of Science and Technology
Meilong Road No. 130
Shanghai, 200237 (China)
[b] Prof. Dr. X.-Y. Tang, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Road
Shanghai 200032 (China)
E-mail: siocxiangying@mail.sioc.ac.cn
mshi@mail.sioc.ac.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500214.
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merizations if catalyzed by a gold complex. To
our great delight, upon treatment of substrate
1 with a gold catalyst, the key spirointermediate
VII was formed from a gold–carbene species,
leading to switchable synthesis of a,b-unsaturat-
ed aldehydes/ketones 2 or [4C+3C] cycloadducts
4, respectively (Scheme 1c).
Table 1. Substrate scope of gold-catalyzed cycloisomerization of 1.[a]
We commenced our study with a-yne-furan 1a
as the standard substrate. After extensive optimi-
zation (see the Supporting Information for de-
tails), we found that treatment of 1a with
5 mol% [(IPr)Au(CH3CN)][SbF6] (IPr=1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) in dry 1,2-
dichloroethane at 808C gave the desired product
a,b-unsaturated aldehyde 2a in 88% yield. The
structure of 2a has been unequivocally confirmed
by X-ray diffraction.[14] With the optimized condi-
tions in hand, we turned our attention to deter-
mine the scope and limitations of the reaction. As
summarized in Table 1, all of the reactions pro-
ceeded smoothly to afford the corresponding
products 2b–g in 89–99% yields when the sulfo-
nyl group was p-bromobenzenesulfonyl (Bs), o-
methylbenzenesulfonyl, m-methylbenzenesulfon-
yl, benzenesulfonyl, mesitylenesulfonyl (Mes) and
2,4,6-triisopropylbenzenesulfonyl, which indicated
that the electronic properties and steric effect of
sulfonyl group did not have significant impact on
the reaction outcome. Next, the scope of this
transformation with respect to the substituent of
the propargylic esters was examined. When both
R2 and R3 were an ethyl group (2h), the chemical
yield of the desired product was only 43%. How-
ever, to our delight, cycloalkyl groups gave much
better results (2i,j). Reactions of starting materials
with a pendant benzoyl (Bz, 1k) or pivaloyl (Piv,
1l) group instead of an acyl (Ac) migrating group
were found to be well-tolerated under the stan-
dard conditions, furnishing the desired products
2k,l in 80 and 99% yields, respectively. We further
tested the effect of substituents at R4. Similarly,
[a] Reaction conditions: 1 (0.2 mmol); [(IPr)Au(CH3CN)][SbF6] (5 mol%); anhydrous DCE
(1.0 mL). Yields are those of the isolated products. [b] Mixture of Z/E isomers.
substrates 1m–o underwent the cycloisomerization smoothly
to give the corresponding products but as Z- and E-isomeric
mixtures. Furthermore, to expand the scope of this reaction,
we also investigated the effect of the substituent at R5. We
were pleased to find that 1p–r gave much better results. Sub-
stituted furan 1s also produced the corresponding a,b-unsatu-
rated methyl ketone[15] 2s in 92% yield as approximately 10:1
(E/Z) isomers. Substrates 1t and 1w bearing different substitu-
ents on the propargyl esters were also suitable for this reac-
tion, giving the corresponding cyclized products as
catalyst loading could be reduced to 2 mol%, and the synthe-
sis of 2a was also possible on a gram scale [Eq. (1)], highlight-
ing the good reproducibility and practicability of this cycloiso-
merization reaction.
To determine more precisely the influence of the substitu-
tion, we prepared the substrate 3a with two carbon atoms in
the tether between the propargylic ester and the furan subunit
and treated it with the gold catalyst. To our surprise, a nitro-
gen-containing tricyclic adduct 4a was obtained in 69% yield
isomeric mixtures in reasonable yields. Moreover, the
oxygen- or carbon-tethered substrates 1u and 1v
were also suitable for this cycloisomerization, provid-
ing the desired aldehydes in 65 and 96% yields, re-
spectively. Thus, this reaction scope was highly gen-
eral with respect to the R1–R6 groups. Notably, the
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instead of the expected product a,b-unsaturated aldehyde.
This might be due to an increase of the steric flexibility with
a longer tether that will be favored for the formal [4C+3C] cy-
cloaddition pathway.[16] The structure of 4a has been unequiv-
ocally confirmed by X-ray diffraction.[14] We then performed the
reaction at room temperature to synthesize tricyclic products 4
and the results are summarized in Table 2. As can be seen
from Table 2, substrates 3b and 3c bearing benzenesulfonyl or
mesitylenesulfonyl (Mes) gave the desired products 4b and 4c
in respective yields of 68 and 54%. Meanwhile, substrates in
which the methyl groups on the acetate carbon center were
replaced with a more sterically demanding cyclobutane (3d,e),
cyclopentane (3 f) or cyclohexane (3g) were also found to un-
dergo the reaction smoothly, giving the corresponding spirote-
tracyclic adducts 4d–g in 38–94% yields. Furthermore, starting
material 3h with a pendant benzoyl (Bz) group instead of an
acyl (Ac) migrating group was also suitable for this reaction,
giving the corresponding tricyclized product in a reasonable
yield. Only when both R2 and R3 were propyl moieties (3i), the
desired product could not be obtained (see Supporting Infor-
mation).
Table 3. Substrate scope of gold-catalyzed cycloisomerization of 5.[a]
It is worth noting that when substrate with a a-methyl
group on furan ring was treated with gold catalyst, the a,b-un-
saturated methyl ketone 6a was obtained in 68% yield in the
same pathway as shown in Table 1. Due to the steric hindrance
of the methyl group, the formal [4C+3C] cycloaddition was
blocked out, which might be able to account for the formation
of 6. We next examined the generality of the reactions with re-
[a] Reaction conditions:
(5 mol%); anhydrous DCE (1.0 mL). Yields are those of the isolated prod-
ucts. [b] Reaction time: 1 h. [c] Reaction time: 2 h.
5
(0.1–0.2 mmol); [(IPr)Au(CH3CN)][SbF6]
Table 2. Substrate scope of gold-catalyzed cycloisomerization of 3.[a]
hexane (5e) instead of the methyl groups were also
found to undergo this reaction very well, affording
the corresponding nitrogen-containing cyclohexene
adducts 6c–e in 67–73% yields. Meanwhile, all of
the reactions proceeded smoothly to afford the cor-
responding products 6 f–i[17] in 40–86% yields when
the sulfonyl group was p-bromobenzenesulfonyl (Bs),
mesitylenesulfonyl (Mes) and 2,4,6-triisopropylbenze-
nesulfonyl. When both R2 and R3 were ethyl moieties
(5j), the 1,3-acyloxy migration allene product were
exclusively formed instead of the desired a,b-unsatu-
rated ketone (see the Supporting Information).
Moreover, the reaction could also be extended to
the alkyl-substituted alkynes[18] and the results are
summarized in Table 4. Substrates bearing methyl,
isopropyl and cyclopropyl groups could produce the
desired a,b-unsaturated aldehydes 8a–c in moderate
yields. When a more sterically large cycloalkane such
as cyclopentane (7d) or cyclohexane (7e) was intro-
duced to the substrate, we found that the reaction
was inefficient under the standard conditions. How-
[a] Reaction conditions: 3 (0.1–0.2 mmol); [(IPr)Au(CH3CN)][SbF6] (5 mol%); anhydrous
DCE (1.0 mL). Yields are those of the isolated products. [b] Reaction performed at
808C.
spect to various substrates 5 under the optimized conditions
and the results are shown in Table 3. The substrate 5b (R4 =
Me), containing another methyl group at the b-position of
furan ring, also underwent the reaction smoothly to give the
corresponding product 6b in 80% yield. Pleasingly, substrates
with a more sterically demanding cyclopentane (5c,d) or cyclo-
ever, changing the gold catalyst into sterically bulky phosphine
coordinated gold catalyst A and increasing the reaction time
gave better results (Table 4).
To verify the reaction pathway of the nitrogen-containing tri-
cyclic adducts 4, the control experiment has been performed
by treating 3a with the sterically bulky phosphine coordinated
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consists of an initial activation of the propargylic ester 3a to
generate the gold carbene intermediate F through a reversible
1,2-acyloxy migration. Notably, the gold–carbene intermediate
F can be also obtained from an allenic ester 9a through a re-
versible 1,2-acyloxy migration, which is initially formed from
3a through a reversible 1,3-acyloxy migration in the presence
of gold catalyst.[7d,f] A nucleophilic attack of the a-position of
furan results in the formation of the spirointermediate G,[21]
which then undergoes a cyclization process to give product
4a and regenerates the gold catalyst.
Table 4. Substrate scope of gold-catalyzed cycloisomerization of 7.[a]
The synthetic utility of these products is shown in Scheme 5
by further transformations into other valuable products. For in-
stance, compound 2a was readily converted into the conjugat-
ed tetraene 12a in 54% yield by Wittig reaction.[22] Further-
more, the tricyclic products 4a and 4c could be smoothly hy-
drolyzed under mild basic conditions with potassium carbon-
ate in methanol, thus yielding the corresponding ketones 10a
and 10c in 70 and 72% yields, respectively. Subsequently
treatment of 10a with 3.0 equivalent of meta-chloroperbenzoic
[a] Reaction conditions: 7 (0.1 mmol); [(IPr)Au(CH3CN)][SbF6] (5 mol%); an-
hydrous DCE (1.0 mL). Yields are those of the isolated products. [b] Reac-
tion performed at 808C. [c] Cat. A was used.
gold catalyst A in dry DCE[13d]
and it was found that the 1,3-
acyloxy migration product 9a
could be obtained in quantita-
tive yield after 15 min. When
using 9a as the starting material
and subjecting it to the stan-
dard conditions, 4a was formed
in 43% yield, thus indicating
that the acyloxy-substituted
allene 9a was the plausible in-
termediate of this reaction to
generate the key intermediate G
Scheme 2. Control experiment of 3a.
for
the
cycloisomerization
(Scheme 2).
A possible mechanism[19] to
account for the above experi-
mental results is shown in
Scheme 3. Gold(I) activation of
the triple bond in 1a promotes
the formation of the gold–car-
bene intermediate C through
a reversible 1,2-acyloxy migra-
tion. A nucleophilic attack of
the a-position of furan results
in the formation of the spiroin-
termediate D, which undergoes
a CÀO cleavage to produce in-
termediate E as Z/E isomeric
mixture. Finally, the thermody-
namically more stable product
2a is formed upon heating.[20]
As for the [4C+3C] cycloaddi-
tion pathway, based on the con-
trol experiments and previous
literature,[16] the mechanistic
working hypothesis (Scheme 4)
Scheme 3. A plausible reaction mechanism for the formation of 2a.
Scheme 4. A plausible reaction mechanism for the formation of 4a.
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Communication
hai Municipal Committee of
Science
and
Technology
(11JC1402600), the National
Natural Science Foundation of
China (20472096, 21372241,
21361140350,
21102166, 21121062, 21302203,
20732008, 21421091, and
20672127,
21372250), and the Fundamen-
tal Research Funds for the Cen-
tral Universities (WJ1014034 and
WK1013004).
Keywords: acyloxy migration ·
cycloisomerization
·
furans
·
gold catalysis
esters
·
propargylic
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In conclusion, we have developed a highly efficient and ac-
cessible gold-catalyzed intramolecular cycloisomerization of
propargylic esters with furan rings. By varying the substituents
on the substrates, we could obtain two different types of cy-
clized products in moderate to excellent yields under mild con-
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the control experiments and previous literature, and some in-
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Communication
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Received: January 20, 2015
Published online on && &&, 0000
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Communication
COMMUNICATION
&
Gold Catalysis
J.-M. Yang, X.-Y. Tang,* M. Shi*
&& – &&
Gold(I)-Catalyzed Intramolecular
Cycloisomerization of Propargylic
Esters with Furan Rings
Going for gold: A variety of cyclic a,b-
unsaturated aldehyde or ketone deriva-
tives and tricyclic adducts were ob-
tained selectively through a gold-cata-
lyzed intramolecular reaction of propar-
gylic esters with furan rings (see
scheme). Several interesting transforma-
tions of the products are also reported.
&
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Chem. Eur. J. 2015, 21, 1 – 8
www.chemeurj.org
8
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!