Synthesis of Spirobidihydropyrazole through Double 1,3-Dipolar Cycloaddition of Nitrilimines with Allenoates

Article abstract of DOI:10.1021/acs.orglett.7b01961

The double 1,3-dipolar cycloaddition of allenoates with nitrilimines has been achieved under mild reaction conditions, affording a variety of spirobidihydropyrazoles in moderate to excellent yields with excellent diastereoselectivities. The reaction diastereoselectively constructs double dihydropyrazole moieties and two chiral centers including a spiro carbon center.

Full text of DOI:10.1021/acs.orglett.7b01961

Letter
pubs.acs.org/OrgLett
Synthesis of Spirobidihydropyrazole through Double 1,3-Dipolar
Cycloaddition of Nitrilimines with Allenoates
Honglei Liu, Hao Jia, Bo Wang, Yumei Xiao, and Hongchao Guo*
Department of Applied Chemistry, China Agricultural University, Beijing 100193, P. R. China
S
* Supporting Information
ABSTRACT: The double 1,3-dipolar cycloaddition of
allenoates with nitrilimines has been achieved under mild
reaction conditions, affording a variety of spirobidihydropyr-
azoles in moderate to excellent yields with excellent
diastereoselectivities. The reaction diastereoselectively con-
structs double dihydropyrazole moieties and two chiral centers
including a spiro carbon center.
pirocyclic compounds having cyclic structures fused at a
central carbon have received considerable attention because
of their ubiquitous presence in natural products and their
interesting conformational features.1 Spiro heterocyclic com-
pounds have been found to exhibit diversified biological
activities and pharmacological and therapeutical properties.2
Common approaches3 (Figure 1) to spirocyclic compounds
carbon double bonds of allenoates in one pot is obviously a
great strategy for synthesis of spirocyclic compounds.
S
Due to the interesting biological activities of various spiro
pyrazolines, their synthesis has attracted great attention. A
number of methods have been developed for the synthesis of
the functionalized spiropyrazolines.14 In general, classical
syntheses of the spiropyrazolines utilized protocols based on
1,3-dipolar cycloaddition15−18 or condensation19 as an essential
step. A few examples of 1,3-dipoles applied in the synthesis of
spiropyrazolines include nitrilimines,15 diazoalkanes,16 related
diazo derivatives,17 and diaziridines,18 and the usual dipolar-
ophile for this process is an alkene or alkyne. Nitrilimines that
are generated in situ from the corresponding hydrazonyl halides
in the presence of a base are important transient 1,3-dipolar
species in organic synthesis and have been utilized as useful
synthons of spiropyrazoline derivatives.20 As shown in Scheme
1a, a typical synthesis of spiropyrazoline was accomplished via a
1,3-dipolar cycloaddition of nitrilimines with heterocyclic α,β-
enone compounds possessing exocyclic double bonds (Scheme
1a).20 Herein, we report a synthesis of spirobidihydropyrazoles
through double 1,3-dipolar cycloaddition of nitrilimines with
allenoates (Scheme 1b).
Figure 1. Methods for synthesis of spirocyclic compounds.
include (a) alkylation methods,4 (b) transition-metal-based
processes,5 (c) radical cyclizations,6 (d) ring closure of
geminally substituted compounds,7 (e) metathesis processes,8
(f) Diels−Alder reactions,9 (g) cycloaddition tactics,10
rearrangement based processes,11 ring-expansion and -contrac-
tion methods, cleavage of bridged ring systems, and so on to
give spirocyclic compounds (Figure 1). Most strategies for
construction of spiro structures are through constructing a new
ring on an existing carbo- or heterocycle. Limited examples on
the formation of two rings through a double-intramolecular 1,3-
dipolar cycloaddition of diene in one pot for a spirocyclic
compound have also been reported.12 Although several
cycloaddition reactions of allenoates with 1,3-dipoles have
been achieved, only a carbon−carbon double bond was
involved in these reactions.13 As shown in method h of Figure
1, the double 1,3-dipolar cycloaddition involving both carbon−
Scheme 1. Synthesis of Spiropyrazolines via 1,3-Dipolar
Cycloaddition of Nitrilimines
Received: June 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01961
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
In our initial investigation, the reaction of nitrilimine
precursor 1a with allenoate 2a was chosen as the model
reaction. With the use of Et3N as the base, the reaction was
performed in dichloromethane at room temperature for 48 h to
give the corresponding product spirobipyrazoline 3aa in 82%
yield (Table 1, entry 1). Then different bases were screened to
Table 2. Scope of Nitrilimine
b,c
a
entry
R1, R2 in 1
Ph, Ph (1a)
3
yield (%)
Table 1. Screening of the Reaction Conditions
1
3aa
3ba
3ca
3da
3ea
3fa
91
63
85
88
72
47
84
62
86
87
73
81
65
79
87
85
67
64
65
80
72
35
2
2-FC6H4, Ph (1b)
3-FC6H4, Ph (1c)
4-FC6H4, Ph (1d)
2-ClC6H4, Ph (1e)
3-ClC6H4, Ph (1f)
4-ClC6H4, Ph (1g)
2-BrC6H4, Ph (1h)
3-BrC6H4, Ph (1i)
4-BrC6H4, Ph (1j)
3-MeC6H4, Ph (1k)
4-MeC6H4, Ph (1l)
3
4
5
6
b,c
entry
base
Et3N
solvent
yield (%)
7
3ga
3ha
3ia
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
82
67
8
2
i-Pr2NEt
t-BuOK
NaOH
9
3
trace
trace
trace
47
10
11
12
13
14
15
16
17
18
19
20
21
22
3ja
4
3ka
3la
5
KOH
6
Cs2CO3
K2CO3
3-OMeC6H4, Ph (1m)
4-OMeC6H4, Ph (1n)
1-naphthyl, Ph (1o)
2-naphthyl, Ph (1p)
2-furanyl, Ph (1q)
n-Pr, Ph (1r)
3ma
3na
3oa
3pa
3qa
3ra
3sa
3ta
7
52
8
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
91
9
81
10
11
12
13
14
15
16
CHCl3
AcOEt
THF
75
43
85
Ph, 3-FC6H4 (1s)
Ph, 3-ClC6H4 (1t)
Ph, 3-BrC6H4 (1u)
Ph, 4-MeC6H4 (1v)
MeCN
MeOH
toluene
DMF
88
trace
trace
trace
3ua
3va
a
Reactions of 1 (0.22 mmol), 2a (0.1 mmol), and Na2CO3 (0.22
a
b
Reactions of 1a (0.22 mmol), 2a (0.1 mmol) and base (0.22 mmol)
mmol) were carried out in 1 mL of CH2Cl2 at rt for 48 h. Isolated
b
c
1
were carried out in 1 mL of solvent at rt for 48 h. Isolated yields.
yields. Unless indicated otherwise, dr is >20:1, determined by H
c
Unless indicated otherwise, dr is >20:1, determined by 1H NMR
NMR analysis of the crude product.
analysis of the crude product.
5, and 8). The fused aromatic and heteroaromatic nitrilimines
such as 1-naphthyl, 2-naphthyl, and 2-furanyl nitrilimine (1o−
q) were also tolerated and yielded products (3oa−qa) in
moderate to high yields (67−87%) with excellent diaster-
eoselectivities (entries 15−17). Satisfactorily, the nitrilimine
scope could be further expanded to the alkyl-substituted
substrate 1r, which delivered the product 3ra in 64% yield
(entry 18). In addition, different R2-substituted nitrilimines also
worked well to give the cycloadducts (entries 19−22).
improve the yield (entries 2−8). Unfortunately, only a trace of
product was observed with the use of t-BuOK, NaOH, or KOH
as the base (entries 3−5). In comparison, the product 3aa was
obtained in moderate yields with the use of i-Pr2NEt, Cs2CO3,
or K2CO3 as the base (entries 2, 6, and 7). To our delight, the
yield of the product 3aa was increased to 91% with the use of
Na2CO3 as the base (entry 8). The impact of the solvent was
also examined. Compared with dichloromethane, 1,2-dichloro-
ethane (DCE), chloroform, ethyl acetate, tetrahydrofuran
(THF), and acetonitrile led to decreased yields (entries 9−
13). Only a trace of product was observed when toluene,
methanol, or N,N′-dimethylformamide (DMF) was used as the
solvent (entries 14−16). The relative configuration of the
cycloaddition product has been determined through X-ray
crystallographic data of the product 3aa.21
With the optimal reaction conditions in hand, we next
explored the scope of the nitrilimine precursors. As shown in
Table 2, the reaction could tolerate a wide scope of nitrilimines
bearing different R1 groups, giving products 3 in moderate to
high yields with excellent diastereoselectivities (entries 1−18).
Regardless of the electronic nature (electron-donating or
electron-withdrawing) of the substituents on the benzene
ring, the corresponding products 3aa−na were obtained in
good yields (62−91%) with excellent diastereoselectivities
(entries 1−14). However, substrates bearing substituents at the
2-position of the aryl showed relative weak reactivities, leading
to the corresponding products in a little lower yields (entries 2,
Next, we carried out an investigation on allenoates 2 (Table
3). A series of α-substituted allenoates 2 bearing the electron-
donating or electron-withdrawing substituents on the benzene
ring could smoothly perform the reaction under mild
conditions, affording the products 3 in good yields (76−94%)
with excellent diastereoselectivities (entries 1−16). Generally,
the position of the substituent on the benzene ring does not
have a remarkable effect on the yields and diastereoselectivities
(entries 1−16). A moderate 67% yield was obtained using the
allenoate-containing dimethoxyl-substituted aryl as the sub-
strate (entry 17). 2-Naphthyl-substituted allenoate is also a
compatible substrate, giving the product 3as in 75% yield
(entry 18). The α-(ethoxycarbonylmethyl)allenoate 2t per-
formed the reaction to give the product 3at in 58% yield.
Varying the ester moiety of allenoates could be tolerated, and
the corresponding products were isolated in moderate to high
yields (entries 20−23).
To further demonstrate the practical utility of the synthetic
method for spirocyclic compounds, the reaction of 2a was
B
DOI: 10.1021/acs.orglett.7b01961
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
As shown in Scheme 3, a plausible mechanism was proposed.
The nitrilimine generated in situ from the corresponding
Table 3. Scope of Allenoate
Scheme 3. Proposed Mechanism
b,c
entry
R3, R4 in 2
3
yield (%)
1
2-FC6H4, Et (2b)
3-FC6H4, Et (2c)
4-FC6H4, Et (2d)
2-ClC6H4, Et (2e)
3-ClC6H4, Et (2f)
4-ClC6H4, Et (2g)
2-BrC6H4, Et (2h)
3-BrC6H4, Et (2i)
4-BrC6H4, Et (2j)
3ab
3ac
3ad
3ae
3af
83
87
92
83
77
86
88
81
84
76
82
78
80
94
86
83
67
75
58
52
85
56
64
2
3
4
hydrazonyl halide 1 in the presence of a base reacts with the
carbon−carbon double bond of allenoate 2 between the α- and
β-carbon to give the intermediate dihydropyrazoline A, which
5
6
3ag
3ah
3ai
7
1
could be observed on the H NMR spectra of the reaction
8
mixture (see the Supporting Information). Then the second
nitrilimine performs the cycloaddition with the terminal olefin
of the intermediate A to produce the product 3.
9
3aj
10
11
12
13
14
15
16
17
18
19
20
21
22
23
3-CF3C6H4, Et (2k)
4-CF3C6H4, Et (2l)
2-MeC6H4, Et (2m)
3-MeC6H4, Et (2n)
4-MeC6H4, Et (2o)
4-t-BuC6H4, Et (2p)
3-OMeC6H4, Et (2q)
3,5-OMe2C6H3, Et (2r)
2-naphthyl, Et (2s)
CO2Et, Et (2t)
3ak
3al
In summary, we have developed the double 1,3-dipolar
cycloaddition of allenoates with nitrilimines, affording a variety
of spirobidihydropyrazoles in up to 94% yield with excellent
diastereoselectivities. A wide range of allenoates and
nitrilimines are compatible with the mild reaction conditions.
This reaction provides an efficient method for synthesis of
spirobidihydropyrazoline. The double 1,3-dipolar cycloaddition
involving both carbon−carbon double bonds of allenoates in
one pot is achieved for the first time.
3am
3an
3ao
3ap
3aq
3ar
3as
3at
Ph, Me (2u)
3au
3av
3aw
3ax
Ph, t-Bu (2v)
ASSOCIATED CONTENT
* Supporting Information
Ph, Cy (2w)
■
Ph, Ph (2x)
S
a
Reactions of 1a (0.22 mmol), 2 (0.1 mmol), and Na2CO3 (0.22
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01961.
b
mmol) were carried out in 1 mL of CH2Cl2 at rt for 48 h. Isolated
yields. Unless indicated otherwise, dr is >20:1, determined by H
NMR analysis of the crude product.
c
1
Experimental procedure, characterization data, and NMR
spectra (PDF)
X-ray data for compound 3aa (CIF)
X-ray data for compound 3wa (CIF)
performed on a gram scale, producing the desired product 3aa
in 78% yield (Scheme 2). Interestingly, when two different
Scheme 2. Gram-Scale Synthesis and Cycloaddition of Two
Different Nitrilimines with Allenoate in One Pot
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hchguo@cau.edu.cn.
ORCID
Hongchao Guo: 0000-0002-7356-4283
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work is supported by the NSFC (Nos. 21372256 and
21572264).
■
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DOI: 10.1021/acs.orglett.7b01961
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DOI: 10.1021/acs.orglett.7b01961
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