3+3 -cycloaddition reactions of α-acidic isocyanides with 1,3-dipolar azomethine imines

Article abstract of DOI:10.1021/ol501829k

α-Acidic isocyanides are versatile reagents in organic synthesis, especially for the synthesis of five-membered heterocycles via [3 + 2]-cycloaddition reactions with activated multiple bonds. In this communication, the first [3 + 3]-cross-cycloaddition of α-acidic isocyanides with 1,3-dipolar azomethine imines to generate a series of 1,2,4-triazine derivatives with significant regiochemical control under mild catalytic reaction conditions is described. This new strategy shows that α-acidic isocyanides can also be taken as potent reagents for the synthesis of six-membered heterocycles through [3 + 3]-cross-cycloaddition reactions with 1,3-dipoles.

Full text of DOI:10.1021/ol501829k

Letter
pubs.acs.org/OrgLett
[3 + 3]-Cycloaddition Reactions of α‑Acidic Isocyanides with
1,3‑Dipolar Azomethine Imines
Juan Du, Xianxiu Xu,* Yifei Li, Ling Pan, and Qun Liu*
Department of Chemistry, Northeast Normal University, Changchun 130024, China
S
* Supporting Information
ABSTRACT: α-Acidic isocyanides are versatile reagents in
organic synthesis, especially for the synthesis of five-membered
heterocycles via [3 + 2]-cycloaddition reactions with activated
multiple bonds. In this communication, the first [3 + 3]-cross-
cycloaddition of α-acidic isocyanides with 1,3-dipolar
azomethine imines to generate a series of 1,2,4-triazine
derivatives with significant regiochemical control under mild
catalytic reaction conditions is described. This new strategy
shows that α-acidic isocyanides can also be taken as potent
reagents for the synthesis of six-membered heterocycles
through [3 + 3]-cross-cycloaddition reactions with 1,3-dipoles.
imidazoline,8 and thiazole derivatives, etc.9 For comparison,
ynthesis of heterocycles based on α-acidic isocyanides,
1
Sdiscovered by Schollkopf in 1968,
however, there has been no evidence for the [3 + 3]-
cycloaddition reaction on the basis of α-metalated isocya-
nides,1−11 although a formal [3 + 3]-cycloaddition intermedi-
ate, 3,4-dihydro-2H-1,2,5-oxadiazine, has been proposed for the
reaction of α-metalated isocyanides with nitrones to produce
five-membered 2-imidazolidinones (Scheme 1, eq 2).2b
has become very
̈
popular due to the diverse array of α-metalated isocyanides (a
nucleophilic α-carbanion and a divalent isocyanide carbon).2
Among numerous methods developed, the particularly power-
ful transformation is the [3 + 2]-cycloaddition reaction of α-
metalated isocyanides with activated multiple bonds to produce
a wide range of five-membered heterocycles (Scheme 1, eq 1),2
such as pyrrole,3 pyrroline,4 oxazole,5 oxazoline,6 imidazole,7
In principle, α-metalated isocyanides are ambivalent reagents
because both contain nucleophilic and electrophilic centers.1−11
Consequently, we envisioned that the [3 + 3]-cycloaddition
reaction of α-acidic isocyanides with certain 1,3-dipoles should
happen to form six-membered heterocycles. As part of our
ongoing interest in developing cycloaddition reactions based on
α-acidic isocyanides,12 we present herein the first example of [3
+ 3]-cycloaddition reactions based on α-acidic isocyanides,
including CuI-catalyzed [3 + 3]-cross-cycloaddition of α-acidic
isocyanides with azomethine imines and the AgOTf-catalyzed
one-pot [3 + 3]-cycloaddition of ethyl isocyanoacetate with N-
iminoisoquinolinium ylides, generated in situ from (E)-N′-(2-
alkynylbenzylidene)-4-methylbenzenesulfonohydrazides, to de-
liver di- and tricyclic 1,2,4-triazine compounds (Scheme 1, eq
3).13,14 Triazine derivatives have been identified as pharmaco-
logically important heterocycles with diverse biological
activities,13a−d found in natural products,13e,f and applied in
organic synthesis.13g−j
Scheme 1. Cycloaddition Reactions of α-Acidic Isocyanides
In the present research, the [3 + 3]-cycloaddition of (Z)-2-
benzylidene-5-oxopyrazolidin-2-ium-1-ide 1a14 and ethyl iso-
cyanoacetate 2a was employed to screen the reaction
conditions. It was found that, catalyzed by CuI (20 mol %)
and in the presence of DBU (DBU = 1,8-diazabicyclo[5.4.0]-
undec-7-ene, 20 mol %) in ethanol at room temperature for 0.5
Received: June 24, 2014
Published: July 23, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/ol501829k | Org. Lett. 2014, 16, 4004−4007

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Letter
h, only a pyrrole product, diethyl 3-phenyl-1H-pyrrole-2,4-
dicarboxylate 3a, instead of the desired [3 + 3]-cycloaddition
product, was obtained in 42% yield from the reaction of the
azomethine imine 1a (0.3 mmol) and ethyl isocyanoacetate 2a
(1.2 equiv) (Scheme 2 and Table 1, entry 1). This result
(entry 7), CuBr (entry 8), Cu(OAc)2 (entry 9), CuBr2 (entry
10), Ag2CO3 (entry 11), AgOAc (entry 12), and AgOTf (entry
13) were less effective catalysts than CuI. For the bases tested,
DBU was the best choice (entry 6 versus entries 14−17)
applied in the present research. In addition, it was found that
lower product yields of 1,2,4-triazine 4a were obtained by
lowing CuI or DBU loadings (entries 18 and 19). In the
absence of CuI (entry 20), no 4a was detected, showing the
importance of CuI for the formation of 1,2,4-triazines 4.
Under optimal conditions (Table 1, entry 6), a series of
experiments were performed, and the results are summarized in
Scheme 3. According to the experimental results, a series of
Scheme 2. Reaction of 1a with 2a in Ethanol
Scheme 3. Synthesis of Pyrazolo[1,2-a][1,2,4]triazin-8(4H)-
ab
,
one 4
indicates that, under basic conditions in protic solvents,
solvolysis of 1a may occur to give benzaldehyde, which
condensed with 2a followed by stepwise [3 + 2]-cycloaddition
with 2a to give 3a.15
Under identical conditions as above but using aprotic
reaction media, the yields of pyrrole 3a were substantially
reduced. In these cases, the desired [3 + 3]-cross-cycloaddition
product, pyrazolo[1,2-a][1,2,4]triazine-3-carboxylate 4a, could
be obtained in low to fair yields (Table 1, entries 2−5).
Optimization of the reaction conditions could lead to 4a in 72%
isolated yield at room temperature within 1 h when chloroform
was selected as the solvent (entry 6). For comparison, CuCl
a
Table 1. Screening of Reaction Conditions
b
yield (%)
entry
catalyst
CuI
base
solvent
4a
3a
1
2
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
Et3N
EtOH
42
12
14
10
35
11
15
15
7
CuI
CH2Cl2
DCE
47
48
23
45
72
41
64
68
38
47
49
67
3
CuI
a
Reactions were carried out with 1a (0.3 mmol), 2a (0.36 mmol), CuI
4
CuI
CH3CN
THF
(20 mol %), and DBU (20 mol %) in CHCl3 (3 mL) at room
5
CuI
b
temperature. Isolated yields.
6
CuI
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
7
CuCl
CuBr
Cu(OAc)2
CuBr2
Ag2CO3
AgOAc
AgOTf
CuI
pyrazolo[1,2-a][1,2,4]triazin-8(4H)-ones 4a−o were obtained
in good to high yields under extremely mild conditions within 1
h by the reactions of ethyl isocyanoacetate 2a with a wide range
of azomethine imines 1 bearing various R1 groups, such as para-
(4a−f), meta- (4g and 4h), and ortho-substituted aryl groups
(4i), 2-naphthalenyl (4j), and heteroaryl groups (4k and 4l). In
addition, 1,2,4-triazine derivatives (4m−o) were also prepared
in high yields from the reactions of 2a with azomethine imines
1 bearing varied R2 and R3 substituents. Furthermore, it was
proved that tosylmethylisocyanide 2b could also react well with
1 having an aryl or an alkyl group to give the desired [3 + 3]
products (4p and 4q) under identical conditions. For
comparison, 1-(isocyanomethyl)-4-nitrobenzene 2c was not
reactive for the [3 + 3]-cycloaddition due to, probably, the
steric hindrance of the bulky 4-nitrophenyl group.
8
9
10
11
12
13
14
15
16
17
5
14
24
21
12
11
11
12
13
5
CuI
Cs2CO3
NaOH
t-BuOK
DBU
DBU
DBU
57
21
42
65
64
CuI
CuI
c
18
CuI
d
19
CuI
20
a
Reactions were carried out with 1a (0.3 mmol) and 2a (0.36 mmol)
b
c
The above reactions are stereospecific with two substituents
at 3- and 4-positions of 4 being trans to each other according to
in solvent (3 mL) at room temperature. Isolated yields. CuI (10 mol
%) was used. DBU (10 mol %) was used.
d
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Letter
1
the H and 13C NMR spectra of 4a−4q. The structure was
Scheme 5. One-Pot Synthesis of [1,2,4]Triazino[6,1-
a]isoquinolines 7
further confirmed by the X-ray single-crystal analysis of 4q,16
and the relative configurations of 4m and 4n were determined
by NOE experiments (see Supporting Information).
a b
,
On the basis of the above experimental results (Table 1 and
Scheme 3) and related reports,2b,3d,e,4,12,13,17 a possible
mechanism for the [3 + 3]-cycloaddition of α-metalated
isocyanides 2 with azomethine imines 1 is proposed (Scheme 4,
Scheme 4. Proposed Mechanism for Formation of 4
a
aReactions were carried out with 5 (0.3 mmol) and AgOTf (10 mol
%) in DCE (3 mL) at 60 °C for 2 h, then 2a (0.36 mmol) and DBU
b
(20 mol %) were added. Isolated yields.
catalyzed conditions. As a result, the desired products,
[1,2,4]triazino[6,1-a]isoquinolines 7, bearing electron-rich (7b
and 7c) and electron-poor aryl groups (7d and 7e) and an alkyl
group (7f), were prepared, respectively, in good to high yields.
The reaction of ethyl isocyanoacetate 2a with N′-(2-alkynyl-
benzylidene)hydrazides 5 proceeds in a highly chemo- and
stereoselective manner. In the above reactions, no other
diastereomers could be detected by NMR spectra. The
structure of product 7a was confirmed by the X-ray single-
crystal analysis.16
In summary, we have developed a new transformation of α-
acidic isocyanides. In the presence of a catalytic amount of
copper salt, the [3 + 3]-cross-cycloaddition reaction of α-acidic
isocyanides with 1,3-dipolar azomethine imines proceeded
smoothly at ambient conditions within 1 h. This new
transformation was further expanded by a silver-catalyzed
one-pot N-iminoisoquinolinium ylide formation and subse-
quent [3 + 3]-cycloaddition cascade. The reaction features
rapidity, mild reaction conditions, broad substrate scope, and
high regio- and diastereoselectivity and provides a straight-
forward and general method for the synthesis of a variety of
1,2,4-triazine derivatives from readily available starting materi-
als. We expect this isocyanide [3 + 3]-cross-cycloaddition to
complement the existing [3 + 2] protocol and to gain broad
applications in the preparation of six-membered heterocycles.
with the reaction of isocyanoacetate 2a with imine 1a as an
example). The reaction starts with the formation of α-
cuprioisocyanide A or its tautomer A′ from isocyanoacetate
2a in the presence of CuI and DBU.3d,e,4 Then, the nucleophilic
addition of α-cuprioisocyanide A on the imine 1a takes place to
form intermediate B. Insertion of isonitrile into the N−Cu
bond generates the imidoyl−copper intermediate C3e,18
followed by protonation to give 1,2,4-triazine 4a with
regeneration of the catalysts, CuI and DBU, for the next
catalytic cycle (Scheme 4).
The [3 + 3]-cycloaddition of α-acidic isocyanides with 1,3-
dipoles described above represents a simple and efficient
protocol for the direct synthesis of the six-membered
heterocycles, [1,2,4]triazine derivatives 4.13,14 Encouraged by
the successful application of α-acidic isocyanides 2 in the [3 +
3]-cross-cycloaddition with azomethine imines 1, next, we
examined the one-pot reaction of ethyl isocyanoacetate 2a with
(E)-N′-(2-alkynylbenzylidene)-4-methylbenzenesulfono-
hydrazides 5 as the precursors of N-iminoisoquinolinium ylides
6 (Scheme 5).19 Fortunately, [1,2,4]triazino[6,1-a]isoquinoline
7a was obtained in 94% isolated yield when N′-(2-alkynyl-
benzylidene)hydrazide 5a was treated with AgOTf (10 mol %)
in DCE (DCE = 1,2-dichloroethane) at 60 °C for 2 h followed
by the addition of 2a and DBU (20 mol %). It was found that
product 7a could not be detected for the above reaction if
AgOTf was replaced by CuI probably because CuI was not a
suitable catalyst for the generation of N-iminoisoquinolinium
ylides 6 via the 6-endo cyclization of 5.19c
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data for all
compounds and X-ray data of 4q and 7a. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
The scope of the one-pot intramolecular cyclization/[3 + 3]-
cycloaddition reaction was then investigated using selected N′-
(2-alkynylbenzylidene)hydrazides 5 under the above AgOTf-
*E-mail: xuxx677@nenu.edu.cn.
*E-mail: liuqun@nenu.edu.cn.
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Letter
12228. (e) Wu, T.; Pan, L.; Xu, X.; Liu, Q. Chem. Commun. 2014, 50,
1797.
Notes
The authors declare no competing financial interest.
(13) (a) Badran, M. M.; Ismail, M. A. H.; Abdu, N.; Abdel-Hakeem,
M. Alexandria J. Pharm. Sci. 1999, 13, 101. (b) Thiele, K.; Gordon, C.
J. S.; Fischer, J.; Jahn, U. Eur. Pat. Appl. EP 28660, 1981. (c) Zhu, Z.;
Greenlee, W. J.; Cole, D. J.; Pissarnitski, D. A.; Gallo, G. V.; Li, H.;
Josien, H. B.; Qin, J.; Knutson, C. E.; Mandal, M.; Vicarel, M. L.;
Rajagopalan, M.; Dhondi, P. K.; Xu, R.; Sun, Z.-Y.; Bara, T. A.; Huang,
X.; Zhu, X.; Zhao, Z.; Clader, J. W.; Palani, A.; Asberom, T.;
McCracken,T.; Bennett, C. E. PCT Int. Appl. WO 2010054067, 2010.
Biological activity for [1,2,4]triazino[6,1-a]isoquinolines: (d) Messmer,
ACKNOWLEDGMENTS
■
Financial support of this research provided by the NNSFC
(21172030, 21272034, and 21202015) is greatly acknowledged.
REFERENCES
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