Chemoselective double annulation of two different isocyanides: Rapid access to trifluoromethylated indole-fused heterocycles

Article abstract of DOI:10.1021/acs.orglett.7b02582

An unprecedented chemoselective double annulation of α-trifluoromethylated isocyanides with o-acylaryl isocyanides has been developed. This new reaction provides a rapid, efficient, and complete atom-economic strategy for the synthesis of trifluoromethylated oxadiazino[3,2-α]indoles in a single operation from readily available starting materials. Isocyanide insertion into C=O double bonds is disclosed for the first time as indicated by the results of ¹⁸O-labeling experiment. A mechanism for this domino reaction is proposed involving chemoselective heterodimerization of two different isocyanides, followed by indole-2,3-epoxide formation and rearrangement.

Full text of DOI:10.1021/acs.orglett.7b02582

Letter
pubs.acs.org/OrgLett
Chemoselective Double Annulation of Two Different Isocyanides:
Rapid Access to Trifluoromethylated Indole-Fused Heterocycles
Yuelei Gao,†,§ Zhongyan Hu,†,§ Jinhuan Dong,‡ Jun Liu,† and Xianxiu Xu*,†,‡
†Department of Chemistry, Jilin Province Key Laboratory of Organic Functional Molecular Design and Synthesis, Northeast Normal
University, Changchun 130024, China
‡College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for
Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Institute of
Molecular and Nano Science, Shandong Normal University, Jinan 250014, China
S
* Supporting Information
ABSTRACT: An unprecedented chemoselective double
annulation of α-trifluoromethylated isocyanides with o-acylaryl
isocyanides has been developed. This new reaction provides a
rapid, efficient, and complete atom-economic strategy for the
synthesis of trifluoromethylated oxadiazino[3,2-a]indoles in a
single operation from readily available starting materials.
Isocyanide insertion into CO double bonds is disclosed
for the first time as indicated by the results of 18O-labeling
experiment. A mechanism for this domino reaction is proposed involving chemoselective heterodimerization of two different
isocyanides, followed by indole-2,3-epoxide formation and rearrangement.
Scheme 1. Annulation of Two Different Isocyanides
ue to their unique structures and reactivities, isocyanides
1
Dare versatile building blocks in chemical synthesis. Beside
the well-known multicomponent Ugi1a,2 and Passerini3 reaction,
isocyanide-based [3 + n]4 and [1 + m]5 annulations became the
most powerful tools for the preparation of heterocycles in recent
years. Generally, activated methylene isocyanides act as three-
atom components and react with polar multiple bonds,4a,b 1,3-
dipole,4c or 1,6-dipole4d to participate in the [3 + n] reaction, and
isocyanidescanalsoactasaone-atomcomponenttoparticipatein
the [1 + m] reactions.5 Among these transformations, the
chemoselective annulation of two different isocyanides has
seldom been explored,6−10 probably because of the readiness of
homodimerization6 or polymerization1a of isocyanides. In 2005,
Yamamoto’s group first reported the cross-cycloaddition of
activated methylene isocyanides with aryl isocyanides for the
synthesis of imidazoles (Scheme 1, eq 1).7 Recently, Hong’s8 and
Bi’s9 groups successfully extended the scope of this reaction for
the preparation of 1,4-diaryl- and 1,4,5-trisubstituted imidazoles,
respectively. We recently found that a [4 + 2] rather than [3 + 2]
annulation took place to access fused pyridones when α-
substituted isocyanoacetamides were employed instead of other
activated methylene isocyanides (Scheme 1, eq2).10a Moreover, a
tandem heterocoupling and [3 + 2] cycloaddition of 2-
isocyanochalcones and isocyanides was also developed by our
research groups recently (Scheme 1, eq 3).10b Despite these
research efforts, exploiting new annulation of different
isocyanides associated with isocyanide new reactivity profiles
remains highly desirable.
isocyanide insertion into heteroatom−H,11a,12 C−H,11a,13 C−
heteroatom,11a,14 C−metal,11a−c,15 and C−C bonds16 have been
investigated. Recently, isocyanide insertion reactions with
radicals have emerged as an efficient and useful protocol for the
synthesisofheterocycles.11d,17 Althoughisocyanideinsertioninto
On the other hand, isocyanide insertion reactions have
attracted extensive attention for the construction of nitrogen-
containing heterocycles in the past few decades.11 Accordingly,
Received: August 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02582
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
single bonds,11−14,16 including C−O bonds of epoxides18 and
ketals or acetals,19 was reported, to our knowledge, the insertion
into the CO double bond has not yet been established.
To develop a practical and efficient protocol to access valuable
trifluoromethylated heterocycles, we recently designed α-
trifluoromethylated methyl isocyanides as new synthons.20
Preliminary results revealed that a wide range of CF3-containing
oxazolines, imidazolines, and pyrrolines were conveniently
constructed by formal [3 + 2] cycloaddition of α-trifluoromethy-
lated methyl isocyanides with polar multiple bonds.20 In
continuation of our studies on isocyanide-based annulation
reactions,21 we report herein a unique and unprecedented
chemoselective double annulation of α-trifluoromethylated
methyl isocyanides with o-acylaryl isocyanides for the efficient
synthesis of trifluoromethylated oxadiazino[3,2-a]indoles with
complete atom-economy (Scheme 1, eq 4). In this domino
reaction, new reactivity profiles are disclosed, including
isocyanide formal insertion into the CO bond and chemo-
selective heterodimerization of two different isocyanides. More-
over, indoles22 and oxadiazines23 are widespread skeletons in
numerous biologically active natural products and pharmaceut-
icals. It was also reported that fused oxadiazines were ideal drug
motifs for treating Alzheimer’s disease.24 This double annulation
provides an efficient protocol for the rapid assembly of new
oxadiazine-fused indole scaffolds.
was produced in 80% yield (Table 1, entry 9), whereas at 120 °C,
70% yield of 3aa was obtained (Table 1, entry 10). The isolated
yield of 3aa was improved to 91% when 2.0 equiv of 2a was added
to the reaction mixture (Table 1, entry 11).
With the optimal conditions in hand (Table 1, entry 11), the
substrate scope toward this double annulation was investigated.
The reaction of viable o-acylphenyl isocyanides 1 with isocyanide
2a was examined (Scheme 2). First, wide variation of the R1 group
a b
,
Scheme 2. Scope of Isocyanides 1
Initially, the reaction of o-acylphenyl isocyanide 1a and 1-
methyl-4-(2,2,2-trifluoro-1-isocyanoethyl)benzene 2a was em-
ployed to optimize the reaction conditions. It was found that,
catalyzed by Ag2CO3 (30 mol %) in acetonitrile at 100 °C for 6 h,
the trifluoromethylated oxadiazino[3,2-a]indole 3aa25 was
obtained in 57% yield (Table 1, entry 1). When the amount of
a
Table 1. Optimization of Reaction Conditions
a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), Ag2CO3 (50 mol
b
c
b
%), CH3CN (2 mL) for 6 h. Isolated yields. Yield of 1.0 mmol scale.
entry
x
solvent
temp (°C)
3aa (%)
1
2
30
50
60
50
50
50
50
50
50
50
50
CH3CN
CH3CN
CH3CN
1,4-dioxane
THF
100
100
100
100
100
100
100
100
110
120
110
57
74
70
53
39
on isocyanides 1 including electron-neutral (e.g., Ph), -poor (e.g.,
4-ClC6H4), and -rich (e.g., 4-MeC6H4) aryl and alkyl groups (e.g.,
Me, Et, n-Pr, and i-Pr) allowed the formation of diverse
trifluoromethylated oxadiazino[3,2-a]indoles (3aa−ga) in 48−
91% yields. Furthermore, this annulation tolerated isocyanides 1
bearing both electron-withdrawing (e.g., F, Cl, and Br) and
electron-donating (e.g., Me) R2 groups on either 4 or 5 positions
and afforded the corresponding trifluoromethylated oxadiazino-
[3,2-a]indoles (3ha−oa) in good to high yields. Notably, the
chloro and bromo groups are compatible with these reaction
conditions and can be used as handles for further diversification.
Subsequently, the scope of this double annulation was
evaluated with respect to some selected trifluoromethylated
isocyanides 2 (Scheme 3). Trifluoromethylated isocyanides 2
bearing various R groups including electron-neutral phenyl, -poor
aryl (e.g., 4-FC6H4, 4-ClC6H4, and 4-BrC6H4), and -rich aryl (e.g.,
4-MeOC6H4, 3-MeC6H4) groups afforded the oxadiazino[3,2-
a]indoles (3ab−ag) in high yields. Furthermore, heteroaryl-
substituted isocyanide 2h produced the oxadiazino[3,2-a]indole
3ah in moderate yield. When benzyl-substituted trifluoromethyl
isocyanide 2i was employed in this double annulation, the
corresponding oxadiazino[3,2-a]indole 3ai was not detected. In
the case of α-phenylethyl isocyanide 2j, replacing the
3
4
5
6
CH3OH
CF3Ph
7
41
27
80
70
8
DME
9
CH3CN
CH3CN
CH3CN
10
11
c
87 (91)
a
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), Ag2CO3 (30
b
mol %), in solvent (2 mL) for 6 h. Determined by 1H NMR
spectroscopy using 1,2-dibromoethane as an internal standard; isolated
c
yields are given in parentheses. 2a (2.0 equiv).
Ag2CO3 wasincreased to50mol%inthereaction, theyieldof 3aa
was improved to 74% (Table 1, entry 2). Further increasing the
amount of Ag2CO3 to 60 mol % led to a comparable yield of 3aa
(Table 1, entry 3). A survey of solvents such as 1,4-dioxane, THF,
methanol, benzotrifluoride, and dimethoxyethane identified
acetonitrile as the most suitable medium (Table 1, entry 2 vs
entries 4−6). When the reaction was performed at 110 °C, 3aa
B
DOI: 10.1021/acs.orglett.7b02582
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 3. Scope of Trifluoromethylated Isocyanides 2
Scheme 5. Proposed Mechanism
a
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), Ag2CO3 (50 mol
trifluoromethylated isocyanide 2a, followed by the abstraction of
a proton, forms a silver complex I.6,7,10 Meanwhile, Ag2CO3
coordinates to isocyanide 1a to form complex II. Nucleophilic
attack of the intermediate I on complex II produces the imidoyl
silver intermediate III.9,10 Bicyclization of III generates the
zwitterionic intermediate IV, followed by intramolecular
cylization to form the indole epoxide intermediate V,26 which is
protonated to regenerate the catalyst Ag2CO3 along with the
production of intermediate VI. Finally, ring opening of the
epoxide26 and imidazoline forms the zwitterionic intermediate
VII, which generates the oxadiazino[3,2-a]indoles 3aa by ring
closing.27
b
c
%), CH3CN (2 mL) for 6 h. Isolated yields. ND = not detected.
trifluoromethyl with a methyl group, the corresponding
oxadiazino[3,2-a]indole 3aj was not detected either. This result
indicated that the trifluoromethyl group is essential for this
transformation.
To shed light on the mechanism of this double annulation,
several control experiments were performed. When 2.0 equiv of
TEMPO or 2,6-di-tert-butyl-4-methylphenol was added to the
standard reaction, 3aa was obtained in 61 and 62% yields,
respectively (Scheme 4, eqs 1 and 2). These results indicated that
In summary, an unprecedented silver-promoted chemo-
selective double annulation of α-trifluoromethylated isocyanides
with o-acylaryl isocyanides has beendeveloped. Thisnewreaction
provides a rapid, efficient, and complete atom-economic strategy
for the synthesis of biologically potential trifluoromethylated
oxadiazino[3,2-a]indoles in a single operation from readily
available starting materials. In this domino transformation, new
reactivity profiles such as isocyanide insertion into CO bonds,
chemoselective heterodimerization of two different isocyanides,
and an indole-2,3-epoxide formation and rearrangement were
exhibited. Investigation of the reaction mechanism and the
synthetic potential of this double annulation is ongoing.
Scheme 4. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b02582.
this double annulation probably does not proceed via a radical
pathway. Moreover, to determine the source of the oxygen atom
in product 3, we carried out an isotope-labeling experiment.
When 18O- labeled isocyanide 1a-18O was used as substrate under
the standard conditions, 18O-labeled product 3aa-18O was
obtained in 81% yield, as evidenced by mass spectrometry
(Scheme 4, eq 3). This result clearly illustrated that the oxygen
atom in oxadiazino[3,2-a]indoles 3 comes from the carbonyl
group of isocyanides 1, and that adventitious H2O or O2 are not
involved in the double annulation process.
Experimental procedures and characterization data for all
compounds (PDF)
X-ray data of 3aa (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xuxx677@sdnu.edu.cn.
ORCID
On the basis of above experimental results and literature
precedent,6,7,9,10,25,26 a possible reaction pathway leading to
oxadiazino[3,2-a]indoles 3 is depicted in Scheme 5 (exemplified
by the generation of 3aa). The coordination of Ag2CO3 to
Xianxiu Xu: 0000-0001-7435-7449
Author Contributions
§Y.G. and Z.H. contributed equally to this work.
C
DOI: 10.1021/acs.orglett.7b02582
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
S.; Munoz-Torrero, D.; Per
́
ez, B.; Luque, F. J.; Gam
Angew. Chem., Int. Ed. 2016, 55, 8994.
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́
ez-Montano, R.
Notes
̃
̃
The authors declare no competing financial interest.
́
ACKNOWLEDGMENTS
■
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Financial support provided by the NSFC (21672034), the
Natural SciencesFoundation of Jilin Province (20160101330JC),
and Shandong Normal University (108-100801) is gratefully
acknowledged.
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Org. Lett. XXXX, XXX, XXX−XXX