Copper-Catalyzed Cascade Cyclization for the Synthesis of Trifluoromethyl-Substituted Spiro-2H-azirines from 1,6-Enynes

Article abstract of DOI:10.1002/adsc.201500510

A method for the synthesis of trifluoromethyl CF₃-substituted spirocyclic compounds containing with a unique quaternary carbon center from readily available starting materials has been developed. The reaction provides a facile access to 2H-azir

Full text of DOI:10.1002/adsc.201500510

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DOI: 10.1002/adsc.201500510
Very Important Publication
Copper-Catalyzed Cascade Cyclization for the Synthesis of
Trifluoromethyl-Substituted Spiro-2H-azirines from 1,6-Enynes
Yu-Tao He,a Qiang Wang,a Jiahui Zhao,a Xiao-Zhen Wang,a Yi-Feng Qiu,a
Yu-Chen Yang,a Jing-Yuan Hu,a Xue-Yuan Liu,a and Yong-Min Lianga,b,*
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science,
b
Lanzhou 730000, People’s Republic of China
Fax: (+86)-931-891-2582; phone: (+86)-931-891-2596; e-mail: liangym@lzu.edu.cn
Received: May 27, 2015; Revised: July 12, 2015; Published online: September 11, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500510.
Abstract: A method for the synthesis of trifluoro-
methyl CF3-substituted spirocyclic compounds con-
taining with a unique quaternary carbon center
from readily available starting materials has been
developed. The reaction provides a facile access to
2H-azirines via cascade cyclization. These com-
pounds constitute a new class of functionalized syn-
thetic intermediates, which can be used for the syn-
thesis of various nitrogen-containing heterocycles
and biologically active CF3-containing compounds.
methods to construct new functionalized 2H-azirines
is of great interest. With these demands in mind, Park
and co-workers developed a method for the synthesis
of quaternary carbon centered 2H-azirine-2-carboxyl-
ic esters by the rearrangement of a-diazo oxime
ethers (Scheme 1b).[10] Although this process enables
an efficient synthesis of 2H-azirines, the hardly acces-
sible substrates and the use of noble-metal catalysts
restrict its further application in synthetic chemistry.
Highly substituted pyrrolidines are ubiquitous sub-
structures in a large number of natural alkaloids ex-
hibiting important biological activities.[11] Various syn-
thetic methods have been developed reported towards
substituted pyrrolidine derivatives synthesis, such as
the reaction via cyclization of N-linked 1,6-enynes.[12]
However, the construction of quaternary carbon cen-
tered spirocyclic pyrrolidines is still a challenge. With
Keywords: 2H-azirines; cascade cyclization; copper;
enynes; spirocyclic skeleton
The unusual synthetic motif of the 2H-azirines makes recent development of radical chemistry, the cascade
them important reactive intermediates with ever-in- radical cyclization of 1,6-enynes provides a new way
creasing applications in organic synthesis.[1] They are for the synthesis of valuable pyrrolidines.
stable towards manipulation of the unique structure
Thus, we speculated whether both azirine and pyr-
featuring a C=N bond embedded in a highly strained rolidine could be simultaneously achieved in one pro-
three-membered cycle, and they have been exploited cess. On the other hand, the introduction of important
as useful precursors for constructing different azacy- functionalized groups, such as CF3 group,[13–16] into
clic compounds by using transition metal catalysis or building blocks is also highly desirable. In this con-
UV light irradiation.[2] In addition, as unique building text, we are interested in the synthesis of trifluoro-
blocks, 2H-azirines have been used for the synthesis methyl-substituted azirines with structural complexity,
of various nitrogen-containing heterocycles, such as which could show great potential of subsequent trans-
indoles,[3] pyrroles,[4] pyridines,[5] isoxazoles[6] and formation in organic synthesis. The synthesis of such
others.[7] Both the cleavage of C N and C C bonds a unique structure was previously studied by the
were classified to be general strategies for the ring- Rçschenthaler group who used imine substrates with
opening reaction of 2H-azirines. Despite broad utility, a pre-introduced CF3 group (Scheme 1c).[17] To the
the synthesis of this unique structure is rarely stud- best of our knowledge, the introduction of a new CF3
ied.[8] The traditional synthetic method relied on the group to construct trifluoromethyl-substituted 2H-
use of the corresponding ketones (Scheme 1a).[9] The azirine from readily available starting materials has
limited approaches to 2H-azirines has seriously ham- not been reported. Herein, we disclose a one-pot
pered the further exploitation of their synthetic po- route to trifluoromethylated spiro-2H-azirines via
tential. Thus, the development of practical synthetic a copper-catalyzed cascade cyclization of 1,6-enynes
À
À
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Table 1. Optimization of the reaction conditions.[a]
Entry Catalyst
Solvent
t [h] Yield [%][b]
1
2
3
4
5
6
7
8
Cu(OAc)2
CuSO4
Cu(OTf)2
DMF
DMF
DMF
5.0
5.0
5.0
5.0
5.0
5.0
5.0
69
trace
62
70
76
70
59
56
Cu(MeCN)4PF6 DMF
Cu
Cu
Cu
Cu
Cu
Cu
Cu
–
DMF
CH3CN
DCE
1,4-dioxane 5.0
9
NMP
DMF
DMF
DMF
DMF
5.0
6.0
6.0
6.0
6.0
35
10
11[c]
12[d]
13[e]
82 (2.0:1 dr)
72
0
Cu
trace
[a]
Reaction conditions: 1a (0.2 mmol), Togniꢁs reagent 2a
(0.5 mmol), TMSN3 (0.4 mmol), Cu powder (10 mol%),
solvent (1.5 mL), 908C, under argon.
Isolated yield (3a+3a’).
Under air conditions.
Without a copper catalyst.
[b]
[c]
[d]
[e]
NaN3 was used instead of TMSN3.
Cu(OAc)2 in DMF at 908C under argon. To our de-
light, the desired 2H-azirine product was isolated in
69% yield after 5 h (Table 1, entry 1). The molecular
structure of 3a was unambiguously confirmed by X-
ray crystallography (see Supporting Information).[20]
The study of various Cu pre-catalysts revealed that
the Cu powder gave the best result (Table 1, entry 5).
A brief survey on the solvent revealed that DMF is
still the best choice for this reaction (Table 1, en-
Scheme 1. Previous and proposed methodologies for the
synthesis of 2H-azirines.
(Scheme 1d). The significance of the present reaction tries 5–9). An increased yield of the desired 2H-azir-
is three-fold: (i) The produced 2H-azirines represent ine (82%) was obtained when the reaction time was
not only a highly valuable class of compounds found prolonged to 6 h (Table 1, entry 10). The reaction
in natural products,[18] but also an important synthetic worked well under an air atmosphere and gave 3a+
motif in the synthesis of various heterocycles. (ii) The 3a’ in 72% yield (Table 1, entry 11). Other attempts
introduction of CF3 is of great importance for the to promote this process proved to be less effective
modification of this novel fragments. (iii) We have (see the Supporting Information). The control experi-
synthesized a new pyrrolidine bearing a spirocyclic ment suggested that the copper catalyst was essential
skeleton motif via cascade cyclization.
to the transformation (Table 1, entry 12). In contrast,
On the basis of the above scenario, the commercial- only a trace amount of 2H-azirine was observed with
ly available azidotrimethylsilane (TMSN3) was chosen NaN3 as nitrogen source (Table 1, entry 13). Finally, it
as the nitrogen source to explore the reaction with was confirmed that the optimal reaction conditions
1,6-enyne 1a and Togniꢁs reagent 2a.[19] Initially, the were Cu powder (10 mol%) in DMF at 908C under
reaction was carried out in the present of 10 mol% argon for 6 h.[21]
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Copper-Catalyzed Cascade Cyclization
Table 2. Scope of the copper-catalyzed cascade cyclization of 1,6-enynes.[a,b]
[a]
Reaction conditions: 1 (0.2 mmol), Togniꢁs reagent 2a (0.5 mmol), TMSN3 (0.4 mmol), Cu powder (10 mol%), DMF
(1.5 mL), 908C, under argon, isolated yield.
The ratio of diastereomers was determined by crude H NMR. Structures of the major diastereomers are shown.
[b]
1
The optimal catalytic conditions were then applied internal position of the alkene had a dramatic effect
to a range of 1,6-enynes to examine the substrate on the diastereoselectivities, and the selectivity to-
scope of this reaction. As summarized in Table 2, the wards product 3’ was increased with increased steric
reaction was not significantly affected by the substitu- hindrance of substituents from H, Me to Ph (3n, 3a
ents on the phenyl ring of the 1,6-enynes. Both elec- and 3o). The reaction also proceeded smoothly with
tron-donating and electron-withdrawing groups per- a sterically-hindered naphthalene and gave the corre-
formed well under the optimal reaction conditions sponding product 3p. The reactivities of several
(3b–3l). Meanwhile, this transformation was less af- carbon-tethered 1,6-enynes (1q–1s) were subsequently
fected by the substituent with a 2-thiophenyl group investigated, and the corresponding 2H-azirines (3q–
(3m) attached to the triple bond, which was obtained 3s) were obtained in good yields. The use of oxygen-
in 79% yield. It is worth noting that the unsubstituted tethered 1,6-enynes gave an excellent diastereoselec-
allylic substrate (1n) afforded a decreased yield of tivity for spiro amide 3t (>20:1 dr). Finally, the mo-
25% with a slightly improved diastereoselectivity lecular structures of 3b’ and 3i’ were unambiguously
(4.0:1 d.r.). This diastereomeric ratio is higher than confirmed through X-ray crystallography (see the
those of all other substrates (except 3t). This case sug- Supporting Information)[20]
gests that the favored diastereomer may be a kinetic The butyrolactone skeleton is present in a wide va-
.
product and the other a thermodynamic product. As riety of natural products with significant biological ac-
the yield increases, the thermodynamic diastereomer tivities, such as antibiotic and anti-tumor properties.[22]
may be more prevalent, decreasing the diastereomeric It is also a versatile building block in organic synthe-
ratio. In addition, enyne (1o) with a phenyl substitu- sis. Encouraged by those versatile results and the
ent efficiently participated in the reaction and provid- unique role of butyrolactone, we wished to further in-
ed the desired product 3o in 75% yield with the dia- vestigate the scope of the reaction by using allylic al-
stereomeric ratio changed to 1:2. Substituents on the kynoates 4. As shown in Table 3, allylic alkynoates
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Table 3. Substrate scope of the allylic akynoates.[a,b]
[a]
Reaction conditions: 1 (0.2 mmol), Togniꢁs reagent 2a
(0.5 mmol), TMSN3 (0.4 mmol), Cu powder (10 mol%),
DMF (1.5 mL), 908C, under argon, isolated yield.
The ratio of diastereomers was determined by crude
1H NMR. Structures of the major diastereomers are
shown.
Scheme 2. Further synthetic transformations.
[b]
4a–4g were smoothly transformed into the corre-
sponding spiro-2H-azirines in moderate yields. The
tolerance of the process for the halogen substituent in
4f was remarkable, giving the desired product in 69%
yield. These butyrolactone skeletons might prove to
have some potential value in medicinal chemistry in
the future.
Scheme 3. Gram-scale reaction.
This copper-catalyzed synthesis of spiro-2H-azirines
proved to be synthetically useful to construct complex
heterocycles in organic chemistry. For instance, spiro out to detect the corresponding CF3 addition prod-
azirine product 3u was reacted with ynamide 6 in the ucts. Both oxytrifluoromethylation product 8 (38%)[24]
presence of 3 mol% of gold complex M in DCM at and hydrotrifluoromethylation product 9 (15%) were
708C for 12 h,[23] the cycloaddition occurred smoothly obtained under the optimized conditions (Scheme 4a).
and gave the spiro product 7[20] in 43% yield An increased yield of hydrotrifluoromethylation prod-
(Scheme 2).
uct (46%) was observed when 2.0 equiv. of 9-borabi-
Furthermore, the reaction could be easily scaled up cyclo[3.3.1]nonane (9-BBN) were added as hydride
to 1.7 gram and gave the desired product in a good source (Scheme 4b). Moreover, when the radical scav-
yield (Scheme 3).
enger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
The possible mechanism for the transformation of was added to the reaction system, no product was de-
1,6-enynes into 2H-azirines was also studied tected with 89% of the starting material being recov-
(Scheme 4). The reaction without TMSN3 was carried ered. With another radical inhibitor 2,6-di-tert-butyl-
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Scheme 4. Trapping experiments.
Scheme 5. Proposed mechanism.
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References
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obtained followed by intermediate G, which is just
a resonance structure of the alkenyl nitrene F.
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An oven-dried tube was charged with 1,6-enyne (0.2 mmol),
Togniꢁs reagent 2a (0.5 mmol) and Cu powder (0.02 mmol).
The tube was evacuated and backfilled with argon. Then,
TMSN3 (0.4 mmol) dissolved in DMF (1.5 mL) was added.
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the “111” Project, J1103307, Program for Changjiang Schol-
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(CYCXJJ-2015003) for support of this work.
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