1,2-Gold Carbene Transfer Empowers Regioselective Synthesis of Polysubstituted Furans

Article abstract of DOI:10.1021/acs.orglett.8b01152

A gold-catalyzed cascade cycloisomerization/ring-opening reaction of allenyl ketones bearing a cyclopropyl moiety with a wide variety of alcohols or ketones is developed. This reaction involves an unprecedented 1,2-gold carbene transfer to provide regioselective and modular access to tri- or tetrasubstituted furans in moderate to high yields and with broad substrate tolerability.

Full text of DOI:10.1021/acs.orglett.8b01152

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
1,2-Gold Carbene Transfer Empowers Regioselective Synthesis of
Polysubstituted Furans
Maozhong Miao,* Huaping Xu, Mengchao Jin, Zhengkai Chen, Jianfeng Xu, and Hongjun Ren*
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, P. R. China
S
* Supporting Information
ABSTRACT: A gold-catalyzed cascade cycloisomerization/ring-open-
ing reaction of allenyl ketones bearing a cyclopropyl moiety with a
wide variety of alcohols or ketones is developed. This reaction involves
an unprecedented 1,2-gold carbene transfer to provide regioselective
and modular access to tri- or tetrasubstituted furans in moderate to
high yields and with broad substrate tolerability.
olysubstituted furans are frequently found as key structure
motifs in many bioactive natural products,1 pharmaceuti-
Homogeneous gold catalysis has grown explosively during
the past decade to become one of the most exciting research
fields for rapidly building molecular complexity.6 In particular,
considerable attention has been paid to gold-catalyzed
generation of carbene species7 from some typical substrate
precursors,8 together with exploration of their new reactivities.
However, developing new reactions involving formation of gold
carbene complexes based on these strategies, thereby
permitting carbene species transfer from one carbon atom to
another remains challenging to implement. In 2007, Toste’s
group9 has demonstrated that a gold carbene can be transferred
across an alkyne from a diazo substrate. Chan10 and co-workers
have disclosed a tandem process involving carbenoid trans-
formation through an in situ formed cyclopropene intermedi-
ate. Notably, Hashmi11 has recently reported an oxidative
cyclization of diynes via 1,6-carbene transfer for the synthesis of
indenones (Scheme 1b). While these types of gold carbene
transfer are limited to the 1,6- or 1,7-positions, the short
distance transfer such as 1,2-neighboring transfer, to the best of
our knowledge, has not been reported to date.
P
cally important substances,2 and functional materials,3 and they
also serve as versatile building blocks in organic synthesis
(Scheme 1a).4 Although much progress has been made on the
Scheme 1. Background and Working Hypothesis
On the other hand, allenyl ketones have been served as cyclic
metal carbene precursors to afford a range of furans via 1,2-
functional groups migration12 (involving intermediates I and II,
Scheme 1c). For example, Marshall13 has illustrated the RhI- or
AgI-catalyzed method for the synthesis of furans through a
formal 1,2-hydrogen shift of allenyl ketones. A related strategy
has also been explored by Hashmi14 using PdII and AuCl3 as
efficient catalysts. In this context, Gevorgyan15 and co-workers
have presented fruitful results in transition-metals catalyzed
cycloisomerization of allenyl ketones with various 1,2-FG (FG
= alkyl, aryl, halide, acyloxy, silyl, boryl, etc.) migration as a key
step in the assembly of highly substituted furans.
In addition, Zhang16 has reported two interesting gold-
catalyzed reactions using 1-(1-alkynyl)cyclopropyl ketones as
an alternative substrate. The oxocarbenium species III was
synthesis of multiple substituted furans via transition-metal-
catalyzed reactions,5 development of mild and practical
synthetic methods to access this privileged heterocyclic core
by utilizing new catalytic properties of transition metals is still
of great significance in organic chemistry.
Received: April 11, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01152
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Letter
a,b
generated when the carbonyl group attacked the Au-activated
C−C triple bond via 5-endo-dig cyclization. Further trans-
formation of III would lead to conceptually novel Au-
containing all-carbon 1,4-dipole or Au carbenoid-containing
carbonyl ylides (Scheme 1d). In this area, we17 have disclosed a
PdCl2-catalyzed cycloisomerization of cyclopropyl-tethered
allenyl ketones to form furan-fused cyclobutenes with high
potential via C−C bond cleavage of the strained ring. Inspired
by these achievements and diverse gold catalysis, we conceived
that allenyl ketones bearing a cyclopropyl moiety will be
converted to cyclic gold carbene intermediate IV via 5-endo-trig
cyclization in the presence of cationic gold(I) catalyst, which
may be able to shift to the neighboring carbon to form cyclic
gold carbene species V via adjacent cyclopropyl moiety
consecutive scission (Scheme 1e). Here, we report the
realization of 1,2-gold carbene transfer and the development
of a gold(I)-catalyzed cycloisomerization/ring-opening reaction
of allenyl ketones containing tethered cyclopropyl functionality
with alcohols or ketones that delivers tri- or tetrasubstituted
furans efficiently and regioselectively.
Scheme 3. Varying Alcohols in Formation of 2
With this idea in mind, we initially investigated the
transformation of cyclopropyl-tethered allenyl ketone 1a with
ethanol in DCM at room temperature in the presence of 5 mol
% of Au(PPh3)OTf. Gratifyingly, a mixture of 2,3,4-
trisubstituted furan 2b and 2,3,5-trisubstituted furan 2b′ (2b/
2b′ = 49:51) was afforded in 92% total yield (Scheme 2). We
a
The reaction was carried out using 1a (0.2 mmol), alcohols (2
mmol), Au2(rac-BINAP)Cl2 (2.5 mol %), and AgSbF6 (10 mol %) in 2
b
mL of DCM under N2 atmosphere. Isolated yield.
Scheme 2. Initial Attempt
appropriate yields, rendering this late-stage modification
method broadly useful. Finally, when n-octyl mercaptan was
employed, the reaction afforded the Michael addition product
in 98% yield (1.3:1 E/Z mixtures) instead of the desired furan.
Subsequently, we turned our attention to defining the
capacity of different cyclopropyl-tethered allenyl ketones 1 with
iPrOH (Scheme 4). With respect to the substituents at the R1
position, diverse aryl, heteroaryl, and alkyl groups could be
employed for this transformation, and the corresponding
presumed that the reaction involving 1,2-gold carbene transfer
between cyclic carbene IV and V would yield the furan 2b.
Considering that the selective formation of 2b is abnormal and
particularly interesting, we then focused on optimizing the
reaction with respect to 2b including yield and selectivity. After
an extensive survey of the reaction parameters (Table S1)
including metals, anions, ligands, silver effect, and solvents, the
optimal conditions were identified to be a mixture of Au2(rac-
BINAP)Cl2 (2.5 mol %) and AgSbF6 (10 mol %) as the
catalyst, DCM as the solvent at room temperature, wherein the
desired product 2b was afforded in 73% yield, rr > 99:1.
With the above optimal conditions in hand, we examined the
scope of alcohols using 1a as the model substrate (Scheme 3).
Simple aliphatic alcohols such as MeOH, EtOH, iPrOH,
tBuOH, BnOH, and cycloheptanol (2a−f) afforded the desired
functionalized furans in moderate to high yields. A moderate
regioselectivity was observed when MeOH was employed.
Sterically hindered 1-adamantanol (2g) was suitable for this
reaction, yielding the corresponding product 2g with good yield
and selectivity. A variety of alcohols containing a fluorenyl (2h),
polyether (2i and 2j), hydroxyl (2k), cyclopropyl (2l), alkenyl
(2m), silyl (2n), and ester (2o) functionalities were all
smoothly and effectively accommodated into this process.
Notably, such a gold-catalyzed carbene-transfer reaction was
applied to the synthesis of furans containing bioactive moieties.
Natural occurring alcohols and their derivatives such as N-Boc-
L-prolinol (2p), α-D-glucose derivative (2q), L-menthol (2r),
and cholesterol (2s) could be introduced to the furan linker in
a,b
Scheme 4. Varying Substrate 1 in Formation of 2
a
The reaction was carried out using 1 (0.2 mmol), iPrOH (2 mmol),
Au2(rac-BINAP)Cl2 (2.5 mol %), and AgSbF6 (10 mol %) in 2 mL of
b
DCM under N2 atmosphere. Isolated yield.
B
DOI: 10.1021/acs.orglett.8b01152
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Letter
trisubstituted furans 2ca−ck were achieved in modest to good
yields (33−88%). The nature of the substituents had a
demonstrable effect on this reaction. As for the electron-
donating 4-methyl and 4-methoxy groups, 2ca and 2cb were
afforded in modest yields. Moreover, the product 2ce
containing the sterically hindered 2-Br phenyl substituent was
provided in 85% yield, however, with lower regioselectivity (rr
= 12:1). The R2 substituents with an array of substituted
aromatic ring could also be successfully altered, leading to
furans 2cl−cr in moderate to high yields, and the electronic
nature of the R2 substituents had little effect on the reaction.
Notably, cyclohexane-fused substrate could also be employed to
reach the target furan scaffold (2cs) in high yield with excellent
trans-stereoselectivity.
analysis. Thus, the scope of the reaction was briefly examined.
The acetone-d6 (7b), cyclopentanone (7c), cyclohexanone
(7d), and acetylacetone (7e) could be tolerated to afford furans
in moderate to high yield with excellent selectivity. Unfortu-
nately, the reaction of isobutylaldehyde with 1a turned to
generate a complex mixture. With respect to cyclopropyl-
tethered allenyl ketones 1, typical substrates bearing sub-
stitutions at the 1- or 2-position were examined, which all
delivered the products 7f−i in acceptable yields.
Mechanistic studies were conducted to gain insight into the
aforementioned transformations (Scheme 7). First, subjecting
Scheme 7. Mechanism Study
To demonstrate the synthetic value of the method, we
investigated one-pot transformations using 2,3,4-trisubstituted
furans 2 as the intermediates (Scheme 5). The cascade
Scheme 5. Synthetic Application of Trisubstituted Furans
diarylmethylcarbocation electrophilic substitution, Vilsmeier−
Haack formylation, and Friedel−Crafts acylation of 2 were
achieved, respectively, affording tetrasubstituted furans 3−5 in
65−71% yields. Moreover, the highly functionalized isocroto-
nolactone 6 was obtained in 56% yield via NBS-mediated
oxidative dearomatization of the furan skeleton.
With such a ring-opening reaction in hand, we then
considered the possibility of extending the above protocol to
synthesize cyclic tetrasubstituted furans 7 by trapping the
migrated gold−carbene intermediates by ketones. After some
trials, the product 7a was obtained in 65% yield with high
regioselectivity by using a mixture of Au2(rac-BINAP)Cl2 (2.5
mol %) and AgSbF6 (5 mol %) as the catalyst, acetone as the
reagent, and solvent (Scheme 6). The structure of 7a was
unambiguously confirmed by single-crystal X-ray diffraction
the cyclopentyl-substituted allenyl ketone 1v with iPrOH to the
reaction conditions led to the six-membered ring fused furan 8
instead of the desired ring-opening product (eq A). This
observation rules out that the cyclopropyl moiety in substrate 1
plays a crucial role in the cascade cycloisomerization/carbene-
transfer/ring-opening reaction. Subsequently, to determine
whether this cascade reaction involves a 1,2-carbene transfer
process, several parallel experiments were performed. For
example, the reaction of substrate 1i without addition of any
trappers resulted in unknown polymers. To our delight, when
the diphenyl sulfoxide,18 a typical gold carbene scavenger, was
added under the same conditions, the 2(3H)-furanone 9 and
diphenyl sulfide were observed (eq B). This experiment clearly
indicates the formation of gold−carbene intermediate V during
the reaction. In addition, when the furan-fused cyclobutene
1017b was subjected to reaction with iPrOH and diphenyl
sulfoxide,the desired products 2ch and 9, respectively, were
successfully afforded. These results indicated that furan-fused
cyclobutene 10 is an important intermediate in the reactions.
Moreover, the Brønsted acid showed different reactivities, and
ring-contraction product 11 was obtained when TsOH (5 mol
%) was employed instead of gold catalyst (eq C).
a,b
Scheme 6. Varying Substrate 1 in Formation of 2
,
Based on the above experimental results and previous
reports, a tentative mechanism is outlined in Scheme 8. First,
the C−C bond of allene moiety is activated by a cationic
gold(I) complex, which is then attacked by the carbonyl oxygen
to form spirocyclic oxonium19 Int-A or its resonance structure,
cyclic gold carbene complex IV (note: oxidation compound of
carbene intermediate IV was not observed yet in trapping
experiments probably due to the relatively fast ring-expansion).
Next, the competitive reaction occurs. The intermediate Int-A
a
The reaction was carried out using 1 (0.2 mmol), Au2(rac-
BINAP)Cl2 (2.5 mol %), and AgSbF6 (5 mol %) in ketones as
solvent under N2 atmosphere. Isolated yield. 40 °C. 70 °C.
b
c
d
C
DOI: 10.1021/acs.orglett.8b01152
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Organic Letters
Letter
Scheme 8. Tentative Mechanism for the Selective
Production of 2
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: mmzok@zstu.edu.cn.
*E-mail: renhj@zstu.edu.cn.
ORCID
Maozhong Miao: 0000-0001-6777-7189
Zhengkai Chen: 0000-0002-5556-6461
Jianfeng Xu: 0000-0003-2111-2944
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21302169, 21602202, 21602203), the Program for
Innovative Research Team of Zhejiang Sci-Tech University
(Grant Nos. 13060052-Y), and the Zhejiang Provincial Top
Key Academic Discipline of Chemical Engineering and
Technology of Zhejiang Sci-Tech University for financial
support.
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Experimental procedures and H and 13C NMR spectra
for all new compounds (PDF)
Accession Codes
CCDC 1587707 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_
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