Gold(i)-catalyzed tandem cyclization of cyclopropylidene-tethered propargylic alcohols: An approach to functionalized naphtho[2,3-: C] pyrans

Article abstract of DOI:10.1039/d0cc03285a

An unprecedented gold(i)-catalyzed cyclization of cyclopropylidene-tethered propargylic alcohols via a 6-endo-dig enyne cyclization followed by a ring-opening of cyclopropane and nucleophilic closure reaction to construct naphtho[2,3-c]pyrans was developed. This transformation represents a highly efficient method for the synthesis of polycyclic compounds in one-pot, and two new rings are formed in an atom economic manner.

Full text of DOI:10.1039/d0cc03285a

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DOI: 10.1039/D0CC03285A
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Gold(I)-Catalyzed Tandem Cyclization of Cyclopropylidene-
tethered Propargylic Alcohols: An Approach to Functionalized
Naphtho[2,3-c]pyrans
Received 00th January 20xx,
Accepted 00th January 20xx
Jian Lia,b, Fang Yanga, Weiwei Hua, Bo Rena, Zi-Sheng Chen, a,* and Kegong Jia,*
DOI: 10.1039/x0xx00000x
An
unprecedented
gold(I)-catalyzed
cyclization
of widely used in diversified organic transformations by Shi, and
cyclopropylidene-tethered propargylic alcohols via a 6-endo-dig other groups.7 We envisioned that the cyclopropylidene-
enyne cyclization followed by a ring-open of cyclopropane and tethered propargylic alcohols could be employed in a 6-exo-dig
nucleophilic closure reaction to construct naphtho[2,3-c]pyrans enyne cyclization followed by a concerted ring-open of
was developed. This transformation represents a highly efficient cyclopropane. As part of our ongoing work in the field of gold
method for the synthesis of policyclic compounds in one-pot, and catalysis,8 we reported herein a gold(I)-catalyzed tandem
two new rings are formed in an atom economic manner.
enyne cyclization/cyclopropane opening reactions from
cyclopropylidene-tethered propargylic alcohols (Scheme 1b).
Notably the reaction, if developed, would offer novel and rapid
access to valuable fused naphtho[2,3-c]pyrans.
Natural products
Fused pyrans are important heterocycles,1 of which
naphtho[2,3-c]pyran moieties are important structural motifs
embedded in numerous natural products showing a broad
range of remarkable biological activities containing cytotoxicity,
H3CO
OH
H3CO
OH
OH
O
Me
O
O
O
anti-inflammatory activity, antimalarial activity, antitubercular
H3CO
CH3
CH3
H3CO
CH3
OAc
HO
Me
2
activity and Anti-HSV-1 activity (Figure 1)
.
However, the
R2 R3
Me
OAc
R1
OAc
O
OH
synthetic methods have been reported were limited, and most
of which are only applied to furnish the products with a
narrow scope of structural diversity.3 Therefore, the
development of new strategies to construct naphtho[2,3-
c]pyrans is highly desirable.
HO
H3CO
H3CO
O
O
O
GlcO
Me
H3CO
OH
H3CO
Bioxanthracenes C
OH
Bioxanthracenes A, R1 = OAc
Bioxanthracenes B, R1 = H
Deoxyprotoaphin, R2 = R3 = H
Protoaphin fb, R2 = OH, R3 = H
Protoaphin sl, R2 = H, R3 = OH
Figure 1. Representative natural products.
Gold-catalyzed4 tandem cyclizations of 1, n-enynes are
useful strategies to construct various active heterocyclic
systems,5 and of which 1,n-enynol, bearing a hydroxyl group
on the alkyne, has also been used to participate in different
kinds of regioselective transformations. For examples, Bandini
reported a stereoselective, intramolecular gold(I)-catalyzed
dearomatization of indole derivative via 5-exo-dig cyclization
of enynol to access special polycyclic compounds. Maestri and
co-workers developed a gold(I)-catalyzed cascade 6-endo-dig
bicyclization of 1,5-enynols for the construction of
heterocycles with structural diversity (Scheme 1a).6
Cyclopropylidene, as one kind of particular alkene, has been
Previous works
1a.
R'
R
NH
R''
R
Maestri et al.
Bandini et al.
Z
O
O
Z
OH
Z
6-endo
5-exo
Z = O,TsN
Z = C(CO2Et)2
This work
1b.
R
R
R
6-endo
Concerted
OH
R'
O
OH
Au(I)
Ring-open/
nucleophilic closure
R'
AuI R'
Scheme 1. Cascade reactions of enynols.
To test the feasibility of our hypothesis, an initial scouting
reaction was conducted with cyclopropylidene-ynol 1a in the
presence of various gold catalysts (Table 1). The use of Au(III)
catalyst did not promote the reaction efficiently, only leading
to a collection of inseparable products (Table 1, entry 1). When
Ph3PAuCl/AgNTf2 was used, the expected product did not
obtain but the starting material remained intact (Table 1, entry
2). Moreover, the use of more electron-rich or electron-
a. Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of
Chemistry and Pharmacy, Northwest A&F University, Yangling 712100, Shaanxi,
P. R. China.
b. College of Plant Protection, Northwest A&F University, Yangling, Shaanxi
712100, China.
E-mail: chenzsh@nwsuaf.edu.cn; jikegong@nwsuaf.edu.cn
Electronic Supplementary Information (ESI) available: CCDC 1985419. For ESI and
crystallographic data in CIF or other electronic format. See DOI:
10.1039/x0xx00000x
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deficient or bulky ligands such as BrettPhos, Phosphite, IPr and
XPhos, all led to messy products (Table 1, entries 3 - 6).
Gratifyingly, we were pleased to find that switching to the Au(I)
Table 2 Substrate Scope of Substituted Enynolsa
View Article Online
DOI: 10.1039/D0CC03285A
Ph
Ph
TMetBuXPhosAuNTf2 (5 mol %)
85 oC, PhF, 36 h
OH
R1
O
t
R1
complex Me4 BuXPhosAuCl along with AgNTf2 (halide
scavenger) afforded the desired product 2a in 44% yield (Table
1, entry 7). With this promising result, we proceeded further to
achieve the optimal reaction conditions. The subsequent
solvents screening results showed that the fluorobenzene was
the optimal solvent (Table 1, entries 8 - 11). Increasing the
reaction temperature to 85 oC led to the isolation of
naphthopyran 2a in 83% yield in 24 h. A brief screening of the
Ag salts identified AgNTf2 as the best halide scavenger.
Entry
Substrate
Product
Ph
2
1
Yield (%)
83
Ph
1b
1a
2a
Me
O
OH
OH
OH
Me
Ph
Ph
Ph
Ph
Me
2
3
1b
1c
2b
2c
81
95
O
O
O
Me
Table 1 Optimization of Reaction Conditionsa
Me
tBu
Me
tBu
Me
Ph
NTf2
iPr
Ph
Au
iPr
P
Reaction Conditions
36 h
OH
Me
Ph
Ph
Me
O
Ph
Ph
Ph
iPr
Me
Me
Me
4
5
6
1d
1e
1f
2d
2e
2f
98
90
97
Me4tBuXPhosAuNTf2
1a
2a
OH
OH
OH
entry
1
Catalyst (5 mol %)
conditions
DCE, 75 oC
DCE, 75 oC
DCE, 75 oC
DCE, 75 oC
DCE, 75 oC
DCE, 75 oC
DCE, 75 oC
toluene, 75 oC
PhCF3, 75 oC
PhCl, 75 oC
PhF, 75 oC
PhF, 85 oC
PhF, 85 oC
yield (%)b
Messy
N.R.
Messy
Messy
Messy
Messy
44
PicAuCl2
Ph
O
2
Ph3PAuCl/AgNTf2
(2,4-tBu2PhO)3PAuCl/AgNTf2
BrettPhosAuCl/AgNTf2
IPrAuCl/AgNTf2
Ph
3
Ph
Ph
4
Ph
O
O
5
Ph
6
XPhosAuCl/AgNTf2
t
7
Me4 BuXPhosAuCl/AgNTf2
Ph
Ph
Ph
OBn
t
8
Me4 BuXPhosAuNTf2
60
7
8
9
1g
1h
1i
2g
2h
2i
93
94
95
t
OH
OH
OH
9
Me4 BuXPhosAuNTf2
55
OBn
t
10
11
12c
13
Me4 BuXPhosAuNTf2
77
Ph
Ph
t
Me4 BuXPhosAuNTf2
81
OTBDPS
O
O
t
Me4 BuXPhosAuNTf2
83
OTBDPS
t
Me4 BuXPhosAuCl/ AgX
<24
a Reaction conditions: All reactions were run in vials in the presence of 1a
(0.2 mmol); [1a] = 0.05 M. b Isolated yields. c Reaction time = 24 h. X- = OTs-,
OTf-, SbF6 .
-
With the optimized reaction conditions in hand, we
subsequently investigated the substrate scope of this cascade
cyclization (Table 2). As expected, the ynols comprising propyl,
hexyl and cyclohexyl groups were suitable for this annulation,
rendering 3,4-dihydro-1H-benzo[g]isochromenes 2b-2d in high
to excellent yields. Varying substituents of the ynol moieties to
a benzyl or a phenethyl had no significant effects on the
outcomes of the reaction and gave corresponding products 2e
and 2f in 90% and 97% yields, respectively. Functional groups
such as -OBn and -OTBDPS were also tolerated, delivering
desired compounds 2g-2h in excellent yields. Derivative of the
ynol bearing 3-cyclohexenyl group participated in this tandem
cyclization smoothly, and the desired product 2i was obtained
in 95% yield. The substrates equipped with π system in the α-
position of –OH were also applicable in this reaction and the
desired naphthopyrans 2j-2k were isolated in 68% and 76%
yields, respectively, albeit with higher reaction temperatures.
Moreover, the primary alcohol 1l also underwent the reaction
smoothly, leading to the corresponding product 2l in a good
yield.
Ph
Ph
Ph
Ph
Me
OH
10c
11c
12c
1j
2j
68
76
O
Me
O
Ph
Cl
F
1k
1l
2k
2l
F
OH
Cl
Ph
77
O
(74)d
OH
a All reactions were carried out in 4 mL PhF in the presence of 1 (0.2
b
c
mmol); isolated yields are reported.
Reactions time = 24 h.
o
d
Reactions carried out in 4 mL PhCl at 120 C for 24 h. The reaction
was carried out at 1 gram scale.
Next, substrates bearing different functional groups on the
aromatic rings were also investigated (Table 3). Notably, electron-
donating and electron-withdrawing substituents on the phenyl ring
of the cyclopropylidene moieties were readily accommodated,
delivering the final products 2m-2q in moderate to good yields.
2 | J. Name., 2012, 00, 1-3
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Table 3. Substrate Scope of Aromatic Ethersa
a major role in the reaction outcomes. Electron-defVicieiwenAtrticelne yOnnolinles
(4-F and 3-Cl) underwent the reaction sDmOoIo: 1t0h.l1y0,39p/rDo0vCidCin0g328th5Ae
corresponding products 2s-2t in moderate yields, while the electron
-rich enynols (3-Me and 4-OMe) gave the final products 2u-2v in
higher yields. The structure of 2v was established by X-ray
crystallographic analysis (CCDC 1985419) and other naphthopyran
derivatives were assigned by analogy.9 Notably, aliphatic derived
substrate 1w successfully afforded the desired product 2w in high
yield.
R2
R2
R3
TMetBuXPhosAuNTf2 (5 mol %)
85 oC, PhF, 36 h
OH
O
R3
Me
Me
Entry
Substrate
Cl
Product
2
1
Yield (%)
Cl
13
14
15
1m
2m
2n
2o
66
Me
OH
O
Me
Cl
Cl
1n
60
Me
OH
O
Me
CF3
F3
C
1o
57
Me
OH
O
Me
Me
Me
Figure 2. Solid-state molecular structure of 2v
Interestingly, the 6-endo-dig cyclization product 2 was
obtained as the sole product while the the 5-exo-dig
cyclization product was not observed. This outcome led by the
use of an electron-rich and sterically hindered ligand which
would make the cationic intermediate less electropositive and
then slowed the rate of deauration by proton. Moreover, the
naphthalene would be preferred due to its aromatic and
thermodynamically stable characters.11
16
1p
2p
64
Me
OH
O
Me
OMe
MeO
17
1q
2q
71
Me
OH
O
Me
S
S
On the basis of the above observations and control
experiments (see ESI†), we propose the following plausible
mechanisms for this transformation.7d Gold(I) first coordinates
with the C-C triple bonds, a subsequent 6-endo-dig cyclization
to yield the carbon cationic intermediate B, which stabilized by
aryl or aliphatic substituents is more favorable than
intermediate B’.10 Then the nucleophilic addition of the
intramolecular hydroxyl leads to ring open of cyclopropane to
18
19
1r
2r
53
64
Me
OH
O
Me
Ph
Ph
F
1s
2s
Me
OH
O
F
Me
Ph
Ph
Me
OH
20
21
1t
2t
53
68
form intermediate C in a concerted manner. Finally,
protodeauration gives the desired product 2.
O
Cl
Me
Cl
Ph
Ph
Au
Me
OH
1
1u
2u
O
Me
tBu
NTf2
iPr
Me
Me
tBu
Me
Au
P
iPr
iPr
H
Ph
Ph
Me
MeO
2
22
23
Me
OH
1v
2v
95
76
Me
Me
Me
O
MeO
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
tBu
tBu
tBu
tBu
OH
Me
iPr
P
P
Me
R1
Me
iPr
OH
iPr
iPr
R1
AuL
R
Au
Au
LAu
OH
1w
2w
Me
OH
OH
O
R
A
C
iPr
iPr
Me
R2
R2
a All reaction were carried out in 4 mL PhF in the presence of 1 (0.2 mmol);
isolated yields are reported.
favored
disfavored
B
B'
Ph
Importantly, substrate derived from thiophene was also
compatible with this annulation yielding naphthopyran derivative
2r in a moderate yield. Varying the substituents on the phenyl ring
of the phenylacetylene moieties resulted in considerable changes in
the yields of the desired products, and the electronic factor played
OH
AuL
R
B
Scheme 2. Proposed Reaction Mechanism.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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To demonstrate the synthetic utility of the current method,
the oxidation of product 2l could afford the lactone 3 in a high yield
(Scheme 3).
Krause and C. Winter, Chem. Rev., 2011, 111, 1994;V(ije)wMAr.tiRcluedOonllpinhe
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Ph
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DDQ (1.2 eq)
0 oC~rt, MeOH
O
O
91 %
2l
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O
Scheme 3. Oxidation of 2l
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In conclusion, we have established an unprecedented
gold(I)-catalyzed cascade cyclization of enynols via a concerted
ring-open of cyclopropane and nucleophilic closure to afford
biologically important naphtho[2,3-c]pyran derivatives in a
one-pot reaction. This reaction features high yields, broad
substrate scope and easy operation. We anticipate that this
approach would be valuable in both natural products synthesis
and medicinal chemistry.
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21702170), Natural Science Foundation
of Shaanxi (2018JQ2072, 2019JQ647) and the Youth Training
Program of Northwest A&F University (Nos. 2452016006,
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4 | J. Name., 2012, 00, 1-3
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