A catalytic allylic cation-induced intermolecular allylation-semipinacol rearrangement

Article abstract of DOI:10.1039/C8CC04285C

A catalytic intermolecular semipinacol rearrangement induced by allylic carbocations has been realized. This tandem reaction is highly efficient under the catalysis of ZnBr₂, generating a wide range of α-homoallyl substituted ketones which contain all-carbon quaternary centres in good to excellent yields (up to 98%) with moderate to high diastereoselectivities (up to >20?:?1). Synthetic application of this novel methodology in the construction of core structures of natural products is also reported.

Full text of DOI:10.1039/C8CC04285C

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Catalytic Allylic Cation-induced Intermolecular Allylation-
Semipinacol Rearrangement
Ming-Hui Xu, a Kun-Long Dai, a Yong-Qiang Tu,*ab Xiao-Ming Zhang,*a Fu-Min Zhang, a Shao-Hua
Wanga
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A catalytic intermolecular semipinacol rearrangement induced by
In the past decade, our group has engaged in the
of novel intermolecular semipinacol
allylic-carbocations has been realized. This tandem reaction is development
highly efficient under the catalysis of ZnBr2, generating a wide rearrangement reactions induced by carbocations.7 A number
range of ɑ-homoallyl substituted ketones which contain an all- of activated carbon electrophiles, such as dimethyl acetal,7b
carbon quaternary centre in good to excellent yields (up to 98%) propargyl,7d and β,γ-unsaturated α-ketoester7e (Scheme 1a)
with moderate to high diastereoselectivities (up to > 20:1). have been found to be suitable in promoting the
Synthetic applications of this novel methodology on the rearrangement, and the products obtained could be
construction of core structures of nature products are also transformed into more complex core structures of nature
reported.
products,7b,8 further demonstrating the versatile utility of
these methodologies. In connection with our continued
interest in this research topic, we hypothesized that the allylic
cations generated from suitable allylic alcohols9 would be
viable electrophiles to induce the rearrangement reactions
All-carbon quaternary centres are widely found in a number of
bioactive natural products and represent
a challenging
structural unit in organic synthesis due to its congested
nature.1 Over the past decades, various synthetic methods,2
especially the asymmetric ones,3 have been established for the
construction of such moieties. Although these novel
approaches have enriched the arsenal of synthetic chemists in
assembling these crowded carbon centres, more general and
efficient methodologies capable of introducing useful
functionalities for further elaboration are still rather limited.4
Among the numerous strategies documented, cation-induced
rearrangement is considered to be a versatile reaction,5 as a
wide range of electrophiles, such as proton, halonium ion,
iminium, oxonium, or even carbocations, could be introduced
to the β position of the quaternary carbon atom after its
formation.6 Compared with the heteroatom electrophiles, the
carbon electrophile induced tandem reactions would generate
molecular complexity more expeditiously in that two C-C
bonds could be forged in a single transformation with the
introduction of other functionalities initially attached to the
electrophile. However, despite the potential synthetic utility,
development of such rearrangements has met with difficulties
because of the low electrophilicity of carbocations.
(Scheme 1b). Such an allyl functionality would be
a
synthetically useful scaffold for late-stage derivation.10 Herein,
preliminary results of our investigation on this tandem
reaction are reported.
or
E
E
CO2Et
OAc
H
R
E+
OH
E:
X
X
Lewis acid
O
or
(OC)3Co
Co(CO)3
O
rk:
ZnBr2(OH)
R2
R3
R1
n
OH
R3
ZnBr2
O
R2
+
OH
R2
R3
R1
3
n
Scheme 1. Design of the allylic cation-induced intermolecular
semipinacol Rearrangement
Initially, allylic alcohols 1a and 2a were selected as model
substrates to investigate the allylic cation-induced semipinacol
rearrangement under the catalysis of ZnBr2. To our delight, the
desired ɑ-homoallyl substituted ketone 3a could be obtained,
albeit with only marginal yield. The low conversion of this
reaction was presumably attributed to the coordination of the
a.State Key Laboratory of Applied Organic Chemistry
Lanzhou University, Lanzhou 730000, P. R. China. Email: tuyq@lzu.edu.cn
b.School of Chemistry
& School of Chemistry,
&
Chemical Engineering, Shanghai Jiao Tong University, diethyl ether with ZnBr2, which reduced the Lewis acidity of
Shanghai 200240, P. R. China. Email: tuyq@sjtu.edu.cn
Electronic Supplementary Information (ESI) available. CCDC 1836270 (3q); 1836259
(3u). For ESI and crystallographic date in CIF or other electronic formate see DOI:
10.1039/x0xx00000x
the catalyst. Therefore, various other solvents were next
screened (Table 1, entries 1-10). Consistent with our
speculation, solvents that had a strong coordination with the
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catalyst (entries 2-7) would completely prohibit the reaction, the reactivity of several β-alkyl substituted allylic alcohols was
and no conversion of 1a was observed in these cases. The investigated (3k 3q). Despite the generation of the desired
-
reaction generally performed better in solvents like DCM, ketones in moderate yields, the rearrangement reactions only
CHCl3, and DCE (entries 7-10), with the best result obtained in proceeded with diminished diastereoselectivities probably due
DCM (98% yield, d.r. 5.5 : 1). Next, the influence of Lewis acids to a less steric interaction of the alkyl group with the allylic
on the reaction outcome was investigated in order to further cation of 2a. The same low diastereoselectivity was also
improve the diastereoselectivity. Although the rate of the observed in the reaction of allylic alcohol 1o with a vinyl ether
reaction could be enhanced under the catalysis of some strong motif (3o). In this case, only low yield of 3o was obtained.
Lewis acids such as Sc(OTf)3, AlCl3 and AlBr3, the yield or Finally, the 1,2-migration could be extended to involve the
diastereoselectivity was lower in each case (entries 11-13). unstrained cyclopentanol and cyclohexanol, whose expansions
Additionally, for other Lewis acids with similar acidity with were unprecedented in previously reported semipinacol
ZnBr2 (entries 14-16), decreased product yields were obtained. rearrangement induced by carbon electrophiles (3p-3q).
Table 1. Optimization of the allylic cation-induced semipinacol
rearrangement reactiona
Table 2. Scope of tertiary allylic alcohols as rearrangement
substratesa
Entry
Catalyst
Solvent
Yieldb(%)
d.r.c
1
2
3
4
5
6
7
8
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
AlCl3
Et2O
THF
MeOH
EA
MeCN
toluene
DCM
CHCl3
CCl4
2%
NR
NR
NR
trace
29
98
57
9
96
67
80
95
76
71
87
—
—
—
—
—
3.7:1
5.5:1
3.9:1
2.7:1
5.1:1
5.3:1
2.0:1
5.0:1
5.0:1
5.0:1
5.5:1
9
10
11
12
13
14
15
16
DCE
DCM
DCM
DCM
DCM
DCM
DCM
AlBr3
Sc(OTf)3
Cu(OTf)2
Zn(OTf)2
ZnCl2
aUnless otherwise noted, the reaction was conducted at room temperature
with allylic alcohols 1a (0.15 mmol, 1.0 equiv.), 2a (1.0 equiv.), and catalyst
(20 mol%) in solvent (1.0 mL); bYield of isolated product. The diastereomers
of the product were inseparable. cd.r. determined by 1H NMR. THF =
tetrahydrofuran, EA = ethyl acetate, DCM = dichloromethane, DCE = 1,2-
dichloroethane.
a1 (0.15 moml, 1.0 equiv.), allylic alcohols 2a (1.0 equiv.), and ZnBr2 (20
mol%) in DCM (1.0 mL) were stirred at room temperature. The
diastereomers of the products were inseparable. d.r. determined by 1H NMR.
With the optimal conditions in hand, we began to probe the
substrate scope of this tandem reaction. As summarized in
Table 2, a wide range of tertiary allylic alcohols 1a-1q were
shown to be suitable rearrangement substrates. Generally,
their reactions with 2a could afford the ɑ-homoallyl
substituted ketones in moderate to good yields with moderate
diastereoselectivities. For β-aryl substituted allylic alcohols,
different substitutions on arene had a strong influence on the
diastereoselectivities of the reactions, although the product
Subsequently, various carbon electrophiles
2
were
examined under the standard reaction conditions. 1,3-diaryl
substituted allylic alcohols 2b and 2c with para-electron-
withdrawing groups on the phenyl rings would furnish the
rearrangement products in relatively higher yields and
diastereoselectivities compared with the reaction of alcohol 2d
yield of each reaction varied slightly (3a
alcohols with an electron-rich aryl group would give better
diastereoselectivities than those with an electron-deficient aryl
group (3b 3e 3h 3j). The position of the electron-donating
group also played an important role in the diastereoselection,
and decrease of selectivity was observed when the
-3j). Specifically,
with para-methyl phenyl groups
substituted alcohols 2e 2g were also amenable to the
rearrangement reaction, affording 3u 3w in low to moderate
(3r-3t). The 1-methyl
1
-
-
,
,
-
yields with good diastereoselectivities. Although these allylic
electrophiles would probably generate two allylic carbocations
a
in the reaction sequence,
a regioselective electrophilic
substituent was placed from para- to ortho-site (3b-3g). Next,
addition of the non-benzyl cation to the alkene was generally
2 | J. Name., 2012, 00, 1-3
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favoured and produced only one isomer (3u
,
3v, or 3w) with produce aldehyde
4, which, under the catalysis of PTS, would
complete regioselection. Despite a low yield of 3v obtained take part in an intramolecular aldol reaction to deliver [3.2.1]-
due to a low conversion of 2f, the formation of the other cyclic ketone12
in 60% total yield (Scheme 1, entry 1). This
5
regioisomer was not observed in this reaction. These results bridged cyclic ketone motif is widely found in a number of
suggested that a steric repulsion between the substituent of bioactive natural products as represented by ialibinone A,
the allylic cation and the β-phenyl group of 1a should exist ialibinone B and enaimeone A.13 From intermediate
4, a
during the electrophilic addition step, and the minimal Pinnick oxidation of the aldehyde followed by intermolecular
interaction between the methyl substituent and the phenyl Friedel-Crafts acylation would give tricyclic diketone
7
, a core
group could only exist when the non-benzyl cation was added structure of naturally occurring merochlorins A and B.14
Me
to the alkene. In similar ways, allylic alcohol 2h would generate
F
O
PTS
Ph
O3
O
OH
O
3x as the sole product. The relative configurations of the major
diastereoisomers of 3q and 3u have unambiguously been
confirmed by X-ray diffraction (Scheme 2). From these results,
(1)
Me
DCM
DCE
O
-78 oC - 0 o
C
Me
H
78%, d.r. 8.1:1
60%, d.r. 2.9:1
3u
5
4
it is deduced that the major diastereoisomers of other
disubstituted ketones would have the same relative
configuration as 3q and 3u 11
ɑ-
Me
NaClO2
NaH2PO4
2-Me-2-butene
O
.
O
(COCl)2/DMF
AlCl3
O
OH
Me
(2)
DCM , 0 oC
Me
Table 3. Scope of allylic alcohols as carbon electrophilesa
t-BuOH/H2
O
O
O
O
75%, d.r. 7.0:1
60%, d.r. 1.5:1
OH
R3
OH
4
6
7
ZnBr2 (20 mol%)
DCM, r.t.
O
R2
+
Scheme 3. Transformations of cyclopentanone 3u
.
R2
R3
Conclusions
1a
2b-2h
Ph
3r-3x
F
Cl
O
Me
In conclusion, we have developed a ZnBr2-catalyzed allylic
cation-induced intermolecular allylation-semipinacol
Ph
Ph
O
O
rearrangement. This tandem reaction has a broad substrate
scope in terms of both the tertiary allylic alcohols and the
carbon electrophiles. Generally, high product yields and
diastereoselectivities are obtained for these reactions, and the
allyl group introduced to the ketone products could be further
functionalized and used in the synthesis of the core structures
of nature products. Further applications of this reaction on the
total synthesis of natural products are currently underway in
our lab.
F
Cl
Me
52%, d.r. 2.4:1
3r
3s
3t
94%, d.r. 5.0:1
90%, d.r. 5.8:1
F
Me
Ph
Me
Me
Me
Ph
Ph
Ph
O
O
O
O
Conflicts of interest
3u 88%, d.r. 11.4:1
3v 54%, d.r. 10.2:1
3w 71%, d.r. 12:1
3x 27%
There are no conflicts to declare.
a1a (0.15 mmol, 1.0 equiv.), allylic alcohols 2 (1.0 equiv.), and ZnBr2 (20
mol%) in DCM (1.0 mL) were stirred at room temperature. The
diastereomers of the products were inseparable. d.r. determined by 1H NMR.
Acknowledgement
We would like to thank NSFC (No. 21290180, 21372104,
21472077, 21772076, 21772071, and 21502080), the “111”
Program of MOE, and the fundamental research funds for the
central universities (lzujbky-2017-116, lzujbky-2018-134) for
their financial support.
Notes and references
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Catalytic Allylic Cation-induced Intermolecular
Allylation-Semipinacol Rearrangement
A catalytic intermolecular semipinacol rearrangement induced by allylic carbocations have been
realized and used in the synthesis of ɑ-homoallyl ketones.