Tandem Spirocyclopropanation/Rearrangement Reaction of Vinyl p-Quinone Methides with Sulfonium Salts: Synthesis of Spirocyclopentenyl p-Dienones

Article abstract of DOI:10.1021/acs.orglett.7b00516

A novel base-mediated tandem spirocyclopropanation/rearrangement reaction of vinyl p-quinone methides (p-VQMs) with sulfonium salts is described. The unprecedented reactivity of p-VQMs was explored for the first time in the spiroannulation cascade, providing a stereoselective approach to the construction of synthetically interesting, densely functionalized spirocyclopentenyl p-dienones.

Full text of DOI:10.1021/acs.orglett.7b00516

Letter
pubs.acs.org/OrgLett
Tandem Spirocyclopropanation/Rearrangement Reaction of Vinyl
p‑Quinone Methides with Sulfonium Salts: Synthesis of
Spirocyclopentenyl p‑Dienones
†
†,§
†
†
†
†
Xiang-Zhi Zhang, Yu-Hua Deng, Kang-Ji Gan, Xu Yan, Ke-Yin Yu, Fang-Xin Wang,
,†,‡
and Chun-An Fan*
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222
Tianshui Nanlu, Lanzhou 730000, China
‡
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China
§
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, China
*
S Supporting Information
ABSTRACT: A novel base-mediated tandem spirocyclopropanation/
rearrangement reaction of vinyl p-quinone methides (p-VQMs) with
sulfonium salts is described. The unprecedented reactivity of p-VQMs
was explored for the first time in the spiroannulation cascade, providing a
stereoselective approach to the construction of synthetically interesting,
densely functionalized spirocyclopentenyl p-dienones.
he spirocyclopentyl p-dienone unit A (Figure 1) featuring
the quaternary carbon center and cyclohexadiene moiety is
Scheme 1. Approaches to Spirocyclopent(en)yl p-Dienones
T
Figure 1. Representative natural products containing spirocyclopentyl
p-dienones.
topic, recently, two palladium-catalyzed [3 + 2] annulations of p-
widely distributed in many biologically interesting natural
products. Chemically, its inherent characteristic with the
1
quinone methides with vinylcyclopropanes (route c) have been
7a
7b
elegantly developed by Yao and Zhao, respectively. Despite
this great progress, it still remains highly desirable to explore a
novel methodology to access this class of synthetically interesting
spirocycles characterized by the unique p-dienone unit.
enone group and the spirocyclic skeleton renders such
functionalized carbocycles to be a class of synthetically useful
2
building blocks in organic chemistry. Because of its unique
molecular architecture as well as synthetic potential, the
development of methods to construct the spirocyclopentyl p-
dienones has received considerable attention in the synthetic
community. Among them, as shown in Scheme 1, dearomative
8
,9
p-Quinone methides (p-QMs) have attracted increasing
attention in the synthetic community due to their unique
structural architecture featuring a bisvinylogous enone system.
Several reaction modes of p-QMs have been discovered, mainly
3
4
spirocyclizations (route a) involving nucleophilic, electrophilic,
10,11
12
5
6
including 1,6-conjugate additions,
[2 + 1]-annulations, [3
radical, or carbene entities have been generally developed on
the basis of various available phenol precursors. Meanwhile, 1,6-
enyne cyclization (route b) has also been used in the
Received: February 20, 2017
2
construction of spirocyclopentyl p-dienones. Focusing on this
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00516
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
3,7
14
+
2]-annulations, and [4 + 2]-annulations. Compared with
16). In terms of yield and stereoselectivity, EtOAc as the reaction
solvent gave the best result (96% yield, 12:1 dr, entry 14).
With the optimized conditions in hand, we then preliminarily
explored the scope of sulfonium salts (2b−e). As shown in
many reports on the application of p-QMs in organic synthesis,
surprisingly, there is little known on the methodology design
based on the chemistry of vinyl p-quinone methides (p-VQMs),
despite the fact that p-VQMs have been described in organic
chemistry for decades. With our continuing interest in the
chemistry of p-QMs,
a
Scheme 2, sulfonium salts containing ester groups (R = t-BuO,
15
11a,j,k
a,b
together with our previous exploration
Scheme 2. Scope of Substrates
12c
in the spirocyclopropanation of p-QMs with sulfonium salts,
recently we have developed a novel tandem spiroannulation
reaction of p-VQMs with sulfonium salts (Scheme 1) wherein an
unprecedented cascade process including a spirocyclopropana-
tion and subsequent spontaneous vinylcyclopropane (VCP)
rearrangement is involved. This tandem reaction provides an
effective method for the synthesis of various functionalized
spirocyclopentenyl p-dienones. Herein, we report our prelimi-
nary results on this aspect.
We commenced our investigation by using p-VQM 1a and
dimethylsulfonium acetate bromide 2a as the model substrates.
As tabulated in Table 1, several inorganic bases (e.g., hydroxides,
a
Table 1. Optimization of Reaction Conditions
b
c
entry
base
solvent
CH Cl
time (h) yield (%)
dr
ND
11:1
1
2
3
4
5
6
7
8
9
Na CO3
48
48
48
12
24
20
15
17
48
24
48
36
11
12
11
1
0
76
39
81
86
90
72
76
<5
84
60
89
49
96
83
0
2
2
2
2
2
2
2
NaOH
CH Cl
2
K CO3
CH Cl
12:1
11:1
12:1
11:1
4:1
2
2
KOH
CH Cl
2
Cs CO3
CH Cl
2
2
K PO ·3H O
CH Cl
3
4
2
2
2
t-BuOK
DBU
CH Cl
2
2
2
2
2
CH Cl
3:1
2
DABCO
TMG
CH Cl
ND
2
a
Performed with p-VQMs 1a−n (0.2 mmol) and sulfonium salts 2a−e
0.3 mmol) in the presence of K PO ·3H O (0.4 mmol) in EtOAc (4
10
11
12
13
14
15
16
CH Cl
7:1
12:1
10:1
6:1
2
(
K PO ·3H O
PhCH3
CHCl3
3
4
2
3
4
2
mL) at 25 °C. The yields refer to the isolated products, the dr’s were
K PO ·3H O
3
4
2
determined by the crude NMR, and major isomers were indicated
K PO ·3H O
CH CN
b
3
4
2
3
graphically for all cases. The relative configuration of 3aa was
K PO ·3H O
EtOAc
THF
12:1
11:1
3
4
2
established by X-ray crystallographic analysis, and accordingly, the
reaction stereoselectivity in other cases was assigned by analogy.
K PO ·3H O
3
4
2
c
K PO ·3H O
DMSO
ND
3
4
2
Performed at 40 °C.
a
Performed with p-VQM 1a (0.1 mmol) and sulfonium salt 2a (0.15
mmol) in the presence of base (0.2 mmol) in solvent (2 mL) at 25 °C.
BnO) could smoothly afford the corresponding annulation
products 3ab and 3ac with high yields (96% yield) and good
diastereoselectivities (12:1 dr). However, compared with the
ester sulfonium salts having a less acidic α-hydrogen, the ketone
b
c
Yield of isolated product. Determined by the crude NMR. ND = not
determined, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, DABCO =
,4-diazabicyclo[2.2.2]octane, TMG = 1,1,3,3-tetramethylguanidine.
1
a
a
ones 2d (R = Ph) and 2e (R = 4-methoxyphenyl) led to a
decrease both in reaction efficiency and stereoselectivities, giving
3ad (74% yield, 2.8:1 dr) and 3ae (79% yield, 3.7:1 dr),
respectively. After the above investigations on the one-carbon
component in this tandem annulation reaction, a series of
structurally diverse vinyl p-quinone methides (p-VQMs) as a
four-carbon component were further probed. For p-VQMs (1b−
j) bearing electronically different aryl substituents, generally the
expected spirocyclic products 3ba−ja could be afforded in good
to excellent yields (85−96% yield) with satisfactory stereo-
selectivities (9:1−14:1 dr), but prolonging the reaction time was
necessary for the reaction of p-VQMs (1g−i) with an electron-
donating C4-aryl group. In addition, two p-VQMs (1k and 1l)
having the heteroaryl groups (Ar = 2-furanyl, 2-thienyl) were also
carbonates, the phosphate) were first tested for this reaction in
CH Cl at room temperature (entries 1−6). Among them,
2
2
K PO ·3H O delivered the desired product 3aa with 11:1 dr in a
3
4
2
higher yield of 90% yield (entry 6), wherein the relative cis-
configuration of spirocyclopentenyl p-dienone 3aa was assigned
16
by X-ray crystallographic analysis. It is noteworthy that some
organic bases (e.g., t-BuOK, DBU, DABCO, and TMG) were
also examined in the current model reaction (entries 7−10), but
there was no positive improvement observed in the reaction
reactivity (up to 84% yield) and diastereoselectivity (3:1 to 7:1).
Encouraged by the above promising results, the influence of
solvents in this model reaction was then examined (entries 11−
B
DOI: 10.1021/acs.orglett.7b00516
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
examined, giving the corresponding products 3ka (96% yield,
Scheme 4. Plausible Mechanism
1
0:1 dr) and 3la (96% yield, 12:1 dr). Moreover, one example
b
c
using p-VQM 1m (R = t-Bu, R = Me, Ar = Ph, E/Z 1.5:1) with a
mixture of geometrical isomers was also conducted, and the
product 3ma was obtained with poor diastereoselectivity (1.3:1b
dr) despite a high yield (96% yield). Meanwhile, p-VQM 1n (R
c
=
Me, R = H, Ar = Ph) containing two methyl groups at the α,α′-
position gave the desired product 3na only in 47% yield and 2:1
dr, showing a negative influence of less bulky α,α′-substituents in
p-VQM on the reactivity and selectivity. To explore the synthetic
utility of this protocol, a gram-scale reaction of 1a and 2a was
additionally conducted (Scheme 3), delivering the model
product 3aa with analogous reactivity (98% yield) and selectivity
(12:1 dr).
Scheme 3. Gram-Scale Reaction
process undergoes an intramolecular S 2-type nucleophilic
N
substitution in a favorable steric zwitterionic TS-A, generating
the spirocyclopropanyl p-dienone intermediate 4 with a trans-
12c
configuration. Due to the driving force of ring strain of the
vinylcyclopropane framework in 4, a vinylcyclopropane(VCP)−
18,19
cyclopentene rearrangement
involving a C1−C2 cleavage
and a concomitant C1−C4 connection through an orbital
symmetry allowed TS-B might account for the stereoselective
formation of major products 3aa−af and 3ba−na with cis-
configuration.
Notably, when the sulfonium salt 2f bearing an amide group
was subjected to the optimized conditions, as shown in eq 1, the
In conclusion, an unprecedented and highly efficient approach
for the synthesis of spirocyclopentenyl p-dienones was
developed via a base-mediated tandem spirocyclopropanation/
rearrangement reaction of p-VQMs with sulfonium salts. A series
of highly functionalized spirocycles were achieved in good yields
(
up to 96% yield) and diastereoselectivities (up to 14:1 dr) under
mild conditions. This reaction not only provides a new pathway
to the spirocyclopentenyl p-dienone building blocks but also
manifests the new reactivity of p-VQMs in methodology design.
Further exploration of the chemistry of p-VQMs in organic
synthesis is underway in our laboratory.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00516.
X-ray data for compound 3aa (CIF)
1
desired spirocyclopentenyl p-dienone 3af was procured only as
the minor product (26% yield, 4.5:1 dr), wherein the unexpected
spirocyclopropane 4af (>20:1 dr) was isolated as the major
Experimental procedure, characterization data, and H and
13
C NMR spectra (PDF)
17
product in 65% yield. Interestingly, the spirocyclopropane 4af
>20:1 dr) could be smoothly transformed into the spirocyclo-
pentene 3af (4:1 dr) in 85% yield in the presence of K PO ·
AUTHOR INFORMATION
Corresponding Author
■
*
(
3
4
E-mail: fanchunan@lzu.edu.cn.
3
H O at 40 °C (eq 2). Compared with the standard conditions
2
described in eq 1, a controlled experiment using 1a and 2f at 40
C was then performed, furnishing the desired spirocyclopentene
af in 96% yield with 4:1 dr (eq 3). Importantly, the present
results obtained from the above experiments mechanistically
imply the possibility that the spirocyclopropane intermediate
resulting from the spirocyclopropanation process could be
involved in this tandem annulation reaction.
ORCID
°
3
Chun-An Fan: 0000-0003-4837-3394
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
According to the above-mentioned results, as shown in
Scheme 4, a plausible reaction mechanism was then proposed.
First, a nucleophilic 1,6-addition of sulfonium salts 2a−f to p-
VQMs 1a−n takes place in the presence of K PO ·3H O as base,
We are grateful for financial support from NSFC (21572083,
21322201, 21290180), FRFCU (lzujbky-2015-48, lzujbky-2016-
ct02, lzujbky-2016-ct07), PCSIRT (IRT_15R28), the 111
Project of MOE (111-2-17), and the Chang Jiang Scholars
Program (C.-A.F.).
3
4
2
and subsequently, a diastereoselective spirocyclopropanation
C
DOI: 10.1021/acs.orglett.7b00516
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
9, 958. (n) Zhuge, R.; Wu, L.; Quan, M.; Butt, N.; Yang, G.; Zhang, W.
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(
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1
4
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