A gold-catalyzed 1,2-acyloxy migration/intramolecular cyclopropanation/ring enlargement cascade: Syntheses of medium-sized heterocycles

Article abstract of DOI:10.1039/c5cc05808b

The synthesis of medium-sized heterocycles possessing a trans double bond is still a challenge. Herein, gold(I)-catalyzed 1,2-acyloxy migration/intramolecular cyclopropanation/ring enlargement cascade reaction of furans has been developed, providing highly efficient access to ten- and eleven-membered heterocycles with a broad substrate scope under mild reaction conditions. The reaction outcome features high chemoselectivity at the C5-position of furan. Moreover, a trans-double bond was embodied in the medium ring system.

Full text of DOI:10.1039/c5cc05808b

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ARTICLE TYPE
Gold-Catalyzed
1,2-Acyloxy
Migration/Intramolecular
Cyclopropanation/Ring Enlargement Cascade: Syntheses of Medium-
Sized Heterocycles
5
YinꢀWei Sun,† XiangꢀYing Tang‡* Min Shi†,‡*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Furan rings are valuable fiveꢀmembered aromatic heterocycles
55 due to the unique diversity of their chemical transformations
under gold catalysis. The intramolecular reactions of furan are
particularly attractive due to its versatile reactivities in the
presence of gold(I) complex, providing easy access to
heterocycles.[8] Accordingly, there are several types of
60 intramolecular reactions of furan with other functional groups:
nucleophilic attack from C3ꢀ or C2ꢀposition[9d] of furan (Scheme
1, A and B),[9] and cycloaddition taking place at both C2ꢀ and C5ꢀ
positions (Scheme 1, C) under gold catalysis.[10]
The synthesis of medium-sized heterocycles possessing a trans
double bond is still a challenge. Herein, a gold(I)-catalyzed
10 1,2-acyloxy migration/intramolecular cyclopropanation/ring
enlargement cascade reaction of furans has been developed,
providing a highly efficient access to ten- and eleven-
membered heterocycles with broad substrate scope under
mild reaction conditions. The reaction outcome features high
15 chemoselectivity at the C5-position of furan. Moreover, a
trans-double bond was embodied into the medium ring
system.
Mediumꢀsized heterocycles possessing trans double bonds are
20 prevalent structure in a large number of biologically active
natural products, for example Herbarumin, Abyssomicin and
Madangamine.[1] However, efficient synthetic access to install
such ring systems remains a big challenge.[2] Thus far, very
limited methods have been accomplished. The ringꢀclosing
25 metathesis (RCM) is the most versatile and efficient method to
construct cisꢀCꢀC double bonds, but there are only a few
examples for the synthesis of cyclic transꢀalkenes.[3,4] The
Yamaguchi macrolactonization offers another excellent solution,
however, stoichiometric chloride and base are required.[5] In
30 recent years, gold catalysis has witnessed intensive
advancements.[6] Alternatively, gold catalyzed cycloadditions and
cycloisomerizations have served as a powerful toolꢀbox to build
medium ring systems.[7] For example, She’s group has developed
a facile protocol to construct mediumꢀsized ring catalyzed by
35 gold(I) catalysis.[7e] Nevertheless, formation of cisꢀalkenes is still
unavoidable. Thus, the development of a new strategy with atom
efficiency and practical simplicity to construct medium sized
heterocycles, especially those with trans double bonds, is still
highly demanded.
40
†
65
Key Laboratory for Advanced Materials and Institute of Fine
Scheme 1. Different reaction patterns on furan ring.
Chemicals, School of Chemistry & Molecular Engineering, East China
University of Science and Technology, and 130 MeiLong Road, Shanghai
45 200237, People’s Republic of China.
Surprisingly, reactions specifically occur at furan’s C5ꢀposition
are extremely rare. As reported by Doyle, the tandem
70 cyclopropanation and ringꢀopening process gave a 14ꢀmembered
ring compound as a Z/E isomeric mixture in only 40% yield
(Scheme 1, D).[11] Herein, we reported an intramolecular reaction
of furanꢀring at C5ꢀposition under gold catalysis, affording ten or
elevenꢀmembered heterocycles with a transꢀdouble bond in high
75 chemoselectivity and regioselectivity in up to 98% yield.
‡
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
Shanghai 200032, P. R. China. E-mail: siocxiangying@mail.sioc.ac.cn;
mshi@mail.sioc.ac.cn.
50 † Electronic Supplementary Information (ESI) available: Experimental
procedures, characterization data of new compounds, and CCDC 973301,
996019, 966978, 972417, 1401380, 1406576 and 997457. See DOI:
10.1039/b000000x/
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DOI: 10.1039/C5CC05808B
Goldꢀcatalyzed 1,2ꢀacyloxy migration of propargylic esters to
form a gold vinyl carbenoid species has been widely reported.[12ꢀ
14]
It is well known that such electrophilc carbenes can readily
5
accept nucleophilic attack. We thus envisage that if the chain
between furan and carbene center is long enough, the reaction of
gold carbene with furan would take place at C5ꢀposition, and the
only challenge may rely on the competitive reaction which takes
place at the alkene part of vinly gold carbene.
10
Initially, the ester 1a was selected as substrate to screen the
reaction conditions (Table 1). Pleasingly, using IPrAuCl/AgSbF6
as catalyst, 2a[15] was isolated in 30% yield, along 3ꢀphenylꢀ1ꢀ
tosylꢀ1Hꢀpyrrole 3[15,16] in 31% yield (entry 1). Among various
t
phosphine ligands, Me4 BuXphos turned out to be the most
15 effective with 70% isolated yield of 2a (Table 1, entries 2 and 3).
t
Me4 BuXphosAu(MeCN)OTf was chosen as the best catalyst,
affording 2a in 91% yield (Table 1, entry 4). We were pleased to
find that if cut the loading of catalyst to 2.5 mol% and 1 mol%,
2a was isolated in 95% and 96% yields, respectively (entries 5
20 and 6) (see Table SIꢀ1 for more information).
Table 1. Screening of the conditions of this reaction.
55
Table 3. Goldꢀcatalyzed reaction for construction of elevenꢀ
memberedꢀring compounds.
25
With the optimized reaction conditions in hand, we set out to
define the scope of this cascade reaction (Table 2). Other
propargyl esters (OPiv and OBz) were suitable for this
cyclization, with the formation of 2b and 2c in 72% and 81%
yields, respectively (Table 2, entries 1 and 2). It was found that
n
X
Me4tBuXPhosAu(MeCN)OTf
DCE, rt
OAc
O
n X
m
m
OAc
O
R2
R2
products
n = 1-3, m = 1-2
1
X = NTs, O
2
yield (%)b
entry
substrates
30 R2 could be various aryl rings. When either electronꢀwithdrawing
or electronꢀdonor groups were introduced on the benzene ring of
R2, the reactions proceeded smoothly to give the corresponding
products 2dꢀ2h in moderate to excellent yields (Table 2, entries
3ꢀ7). When R2 were 1ꢀnaphthalenyl and 2ꢀchlorophenyl groups,
35 the reactions produced the desired products 2i and 2j in 20% and
45% yields, and the relatively low yields were probably due to
the increased steric hindrance (Table 2, entries 8 and 9). Other
sulfonyl groups (R3), such as PhSO2, Ms, 2ꢀClC6H4SO2, Bs and
2ꢀMeC6H4SO2 were well tolerated under the standard reaction
40 conditions, furnishing 2k-2o in 81ꢀ94% yields (Table 2, entries
10ꢀ14). However, treatment of Nꢀphenyl group (R3) protected
substrate under gold catalysis, the reaction became complex,
presumably due to this phenyl ring could also take part in the
reaction as that of furan ring (Table 2, entry 15). When substrate
45 1q with Me group on its C3ꢀposition of furan ring was employed,
the reaction gave product 2q in 96% yield successfully (Table 2,
entry 16). Notably, when all carbon chain tethered substrate, such
as 1r, was treated under the standard reaction conditions, the
reaction worked very well to give tenꢀmembered rings 2r in 82%
50 yield (Table 2, entry 17).
n = 1, m = 2
OAc
Ar
Ts
N
O
N
OAc
O
Ts
Ar
1
2
3
1s: Ar = Ph
2s
98
98
80
1t: Ar = 4-MeC6H4
2t
1u: Ar = 4-BrC6H4
2u
n = 2, m = 1
OAc
X
O
X
OAc
Ph
O
Ph
4
5
1v: X = NTs
1w: X = O
2v
97
78
2w
OAc
nX
m
O
Ph
1x: n = 2, m = 2
1y: n = 3, m = 1
6c
-
complex
a The reaction was carried out on a 0.2 mmol scale with 2.5 mol% catalyst in
solvent (2.0 mL). b Isolated yield. c Both reaction became complexed.
60
Moreover, the reaction could also be extended to synthesize
elevenꢀmemberedꢀring system. As shown in Table 3, for
substrates 1sꢀ1u, in which n = 1, m = 2, the reactions gave the
corresponding elevenꢀmembered heterocycles 2sꢀ2u in 80ꢀ98%
65 yields (Table 3, entries 1ꢀ3). [15] While, when n = 2, m = 1, the
reactions of 1v and 1w went on smoothly to furnish the desired
products 2v and 2w in 78% and 97% yields, respectively (Table 3,
entries 4 and 5). The nitrogen tether could also be replaced by
oxygen; as for substrate 1u, the corresponding elevenꢀmembered
70 cyclic ether was obtained in 78% yield (Table 3, entry 5). The
Table 2. Goldꢀcatalyzed reaction for formation of tenꢀmembered
rings.
2
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reaction failed in the syntheses of twelveꢀmembered cyclic
amines; as for substrates 1x and 1y with longer chain, the
reactions gave complex product mixtures (Table 3, entry 6).
regenerates the catalyst.[11] The steric hindrance of aryl groups in
intermediate C may cause the cyclopropanation exclusively
taking place at C4ꢀ and C5ꢀpositions. Finally, the ring opening
To further define the substrate scope of this reaction, some 45 process of syn-D provides the desired product 2a.
different types of substrates were synthesized and new reaction
5
patterns were observed (Scheme 3).[17] As for nonꢀterminal
propargyl ester 4, a formal [4 + 2] cycloaddition reaction took
place to form oxoꢀbridged bicyclic compound 5[17] in 99% yield
(Scheme 3, eq. 1). When furan 1z with its C5ꢀposition being
10 blocked by Br atom was employed as substrate, the reaction
became complex (Scheme 3, eq. 2). When R2 was replaced by
alkyl (Me) groups, fragmentation was observed and 3ꢀmethyl
pyrrole was obtained in 76% yield (Scheme 3, eq. 3 and more
information is included in Table SIꢀ2). Interestingly, if R2 was an
15 allyl group, the reaction of 8 delivered 9 in 50% yield via
cyclopropanation (Scheme 3, eq. 4).[13c,13h] The structure of 9 was
assigned by 1H NMR, 13C NMR and 2D NMR spectroscopic
experiments (see SI for the details). These results suggested that
the terminal alkyne and aryl substituent of R2 were two critical
20 factors for the desired ring synthesis.
Scheme 5. Proposed Mechanisms.
50
In conclusion, we have developed a highly efficient procedure
for the formation of ten and elevenꢀmembered carboꢀ and
heterocycles by goldꢀcatalyzed cascade reaction. The appropriate
distance from furan ring and the carbene center may be the key to
form these mediumꢀsized ring systems. We believe that this
55 transformation will provide new insights into gold catalyzed
intramolecular cyclization reactions and new future of furan’s
reaction.
Acknowledgement: We are grateful for the financial support
60 from the National Basic Research Program of China (973)ꢀ
2015CB856603, and the National Natural Science Foundation of
China (20472096, 21372241, 21361140350, 20672127,
21421091, 21372250, 21121062, 21302203 and 20732008).
65 Notes and references
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Scheme 3. Other products were observed using different
substrates.
25
Then we tried to use other heterocyclic substrates. When using
thiophene derivative 10 as substrate, the reaction became
complex (Scheme 4, eq. 1). As for NꢀMe pyrrole 11a, a Friedelꢀ
Crafts reaction gave product 12[15] in 97% yield (Scheme 4, eq. 2).
30 If using NꢀTs pyrrole 11b as substrate, 13[15] was obtained in
65% yield (Scheme 4, eq. 3).
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A mechanistic proposal is proposed in Scheme 5. Gold(I)
catalyst activation of triple bond in 1a gives complex A, which
undergoes a 5ꢀexoꢀdig attack to the activated triple bond results in
intermediate B. Subsequent 1,2ꢀacyloxy migration delivers gold
40 carbene intermediate C.[12] Following a cyclopropanation at C4ꢀ
and C5ꢀpositions of furan delivers intermediate syn-D and
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ChemComm
View Article Online
DOI: 10.1039/C5CC05808B
Gold catalyzed 1,2ꢀacyloxy migration/intramolecular
cyclopropanation/ring enlargement cascade of furan
substrates provides a new highly efficient procedure for
the formation of ten and elevenꢀmembered ring
Gold-Catalyzed
1,2-Acyloxy
Migration/Intramolecular
Cyclopropanation/Ring Enlargement Cascade: Syntheses of
Medium-Sized Heterocycles
compounds.
A
new transformation type of
intermolecular furan compounds catalyzed by gold
catalyst was reported.
‡
YinꢀWei Sun,† XiangꢀYing Tang, ‡* Min Shi†,
*
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5