Stereoselective Synthesis of β-Lactam-triflones under Catalyst-Free Conditions

Article abstract of DOI:10.1021/acs.orglett.5b02827

The first example of the synthesis of β-lactam-triflones is described. Treatment of 2-diazo-1-aryl-2-(trifluoromethylsulfonyl)ethanones 1c-f with imines 2 under catalyst-free heating conditions provides pharmaceutically attractive multisubstituted β-lacta

Full text of DOI:10.1021/acs.orglett.5b02827

Letter
pubs.acs.org/OrgLett
Stereoselective Synthesis of β‑Lactam-triflones under Catalyst-Free
Conditions
Zhongyan Huang, Chen Wang, Etsuko Tokunaga, Yuji Sumii, and Norio Shibata*
Department of Nanopharmaceutical Sciences, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan
S
* Supporting Information
ABSTRACT: The first example of the synthesis of β-lactam-
triflones is described. Treatment of 2-diazo-1-aryl-2-(trifluoro-
methylsulfonyl)ethanones 1cf with imines 2 under catalyst-free
heating conditions provides pharmaceutically attractive multi-
substituted β-lactam-triflones 3 in good to high yields with regio-
and diastereoselectivities. A successive Wolff rearrangement and
Staudinger [2 + 2] cycloaddition reaction are key elements for the success of this transformation.
Scheme 1. Synthesis of Heterocyclic Triflones from Ar−C−
C−SO2CF3 Building Blocks 1
he development of novel and efficient methods for the
Tsynthesis of organofluorine compounds is of great
importance in the fields of material sciences, agrochemicals,
and pharmaceuticals.1 Trifluoromethylated (CF3) compounds
are frequently encountered in these areas of study due to the
unique properties induced by the high lipophilicity and strong
electron-withdrawing effect of the CF3 group.2 Recently, interest
in CF3-heteroatom variants has expanded, exemplified by the X−
CF3 group, such as O−CF3 (trifluoromethoxy) and S−CF3
(trifluoromethylthio), etc.3 Among the potential X−CF3 groups,
we have become interested in triflones. Triflones are compounds
with a trifluoromethanesulfonyl (trifly, SO2CF3) group in their
structures.4 The triflyl group is the strongest electron-with-
drawing group (σm = 0.79, σp = 0.93), but its lipophilicity is
somewhat milder (π = 0.55) than that of the trifluoromethyl
group (CF3, π = 0.88, σm = 0.43, σp = 0.54).5 The uniqueness of
the triflyl group has prompted chemists to develop novel
methodologies for the synthesis of triflones and to use this
functional group for the structural design of bioactive
compounds,6 catalysts,7 and functional materials.8 In the past
few decades, numerous methodologies have been developed for
the introduction of a triflyl group into molecules,3e including
oxidation of corresponding trifluoromethylthio compounds or
trifluoromethylsulfoxides,9 direct triflylation reagents,10 trans-
formation from SO2CF3-containing building blocks,11 and
intramolecular rearrangement.12 Although tremendous progress
has been made in the synthesis of triflones, the vast majority of
these methodologies has focused on the synthesis of aryl
triflones. Thus, the synthesis of triflones with heterocyclic
frameworks still remains challenging. In this context, our group
reported an efficient synthesis of heteroaryl triflones using
SO2CF3-containing aryl-2-carbon building blocks, Ar−C−C−
SO2CF3 1. Thus, a synthesis of 4-triflyl isoxazoles from α-triflyl
aryl ketones 1a was achieved with imidoyl chlorides (Scheme
1a),13 and a synthesis of pyrazole triflones and pyrazolo[5,1-
a]isoquinoline triflones via cycloaddition of aryl triflyl alkynes 1b
with the corresponding hydrazonoyl chlorides or C,N-cyclic
azomethine imines (Scheme 1b).14
As an extension to our research on heterocyclic triflones, we
became focused on β-lactam triflones. β-Lactams are a
biologically important class of heterocyclic compound,15 and a
large number of derivatives have been registered in the SciFinder
database.16 However, there is no report of β-lactams having a
triflyl group at their carbon(s) in the ring.17 We herein report a
highly stereoselective synthesis of 3,3-aryltriflyl multisubstituted
β-lactams 3 via a Staudinger [2 + 2] cycloaddition of imines 2
with aryl trifly ketenes A generated in situ from 2-diazo-1-aryl-2-
(trifluoromethylsulfonyl)ethanones 1cf18 by a Wolff rear-
rangement in satisfactory to good yields and with a broad scope
(Scheme 1c). The method shows wide substrate generality, and a
variety of β-lactam triflones 3 can be obtained under catalyst-free
conditions. The reaction mechanism and stereochemistry of the
successive Wolff rearrangement/Staudinger cycloaddition are
also discussed.
To generate a carbenoid intermediate,19 we initiated our
investigation with the reaction of 2-diazo-1-phenyl-2-(trifluoro-
Received: September 30, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02827
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
methylsulfonyl)ethanones 1c and imine 2a under catalysis of
CuF2 in DMF (Table 1, entry 1). However, no desired product
attempt the reaction optimization. After screening several
catalysts, we found that no catalyst afforded the best result
(i.e., 38%) while metal catalysts gave lower yields (entries 57).
A higher reaction temperature did not improve yield, and toluene
was found to be the best solvent for this transformation (entries
711). The yield was increased to 50% when the reaction was
conducted in two steps in one pot. First, diazo-triflone 1c was
heated in toluene at 100 °C for 1 h, and then imine 2a in toluene
was added dropwise (entry 12). The yield increased as diazo-
triflone 1c was increased to 1.52.0 equiv, whereupon the
desired β-lactam 3a was detected in 85% yield (entries 13 and
14). Heating was crucial for this transformation, and the reaction
did not proceed under UV light (entry 15).
With these optimized conditions in hand, the scope of this
cyclization reaction was then investigated with 1c and a variety of
imines 2 (Scheme 2). All the imines 2aq, with R1 bearing
either electron-donating or -withdrawing substituents and
halogen in different positions on the benzene ring, were
smoothly converted to the corresponding multisubstituted β-
lactam-triflones 3aq in good to high yields independent of the
electronic character and the steric hindrance of the aryl moieties
in the imines. The substituted position in the phenyl group of R1
might influence the cyclization to the β-lactam ring due to an
electronic effect and steric hindrance (3bd). Interestingly,
imine 2m, which was transformed into the corresponding β-
lactam-triflone 3m in 21% yield, contains a double bond for
further transformation. All N-alkyl and N-aryl substituted aryl
imines 2nr could be transformed to the corresponding β-
lactams 3nr in good yields except for 2r. To our surprise, bis-
imine 2s afforded bis-β-lactam-triflone 3s in 56% yield. The
diastereoselectivity is generally high in all cases, except the case of
3e, 3q, and 3s. To our knowledge, this is the first example of the
synthesis of β-lactam-triflones.15−17
Table 1. Optimization of Reaction Conditions for Successive
a
Wolff Rearrangement/Staudinger Cycloaddition to 3a
b
entry
solvent
cat. (mol %)
temp (°C) time (h) yield (%)
1
2
3
4
5
6
DMF
CuF2 (20)
CuF2 (20)
Ag2O (20)
CuF2 (20)
Ag2O (20)
Rh2(AcO)4 (5)
50
70
12
12
12
5
0
0
THF
THF
70
0
toluene
toluene
toluene
toluene
toluene
p-xylene
DCE
100
100
100
100
120
100
100
110
100
100
100
25
2
2
35
3
2
c
7
2
38
35
35
30
32
50
66
85
0
8
2
9
2
10
11
8
dioxane
toluene
toluene
toluene
THF
2
d
12
3
de
,
13
14
15
3
df
,
3
g
10
a
Reaction conditions: Imine 2a (0.1 mmol), diazo-triflone 1c (0.1
b
mmol), cat., solvent (1.0 mL), heat. Yields were determined by 19F
NMR spectroscopy with trifluoromethylbenzene as an internal
c
standard. A small amount of minor isomer was detected. Isolated
d
yield. Diazo-triflone 1c (0.1 mmol), solvent (3.0 mL), 100 °C,
heating for 1 h, then imine 2a (0.1 mmol) in 0.5 mL of toluene was
e
added and stirred at 100 °C for additional 2 h. 0.15 mmol diazo-
f
g
triflone 1c was used. 0.2 mmol diazo-triflone 1c was used. Under UV.
After the success of different imines 2 with diazo-triflone 1c,
we further attempted the reaction using different diazo-triflones
1df with imine 2o. The results in Scheme 3 show that the
electron-rich aryl group in diazo-triflone 1d (X = OMe, σp =
−0.28)5c allowed β-lactam-triflones 3do to be obtained in 62%
yield, while the electron-deficient diazo-triflone 1e (X = Cl, σp =
0.22)5c afforded 3eo in low yield (19%). The trifluoromethyl
3a was obtained. We next attempted the reaction of THF in the
presence of a catalytic amount of CuF2 or Ag2O under reflux for
12 h. No product 3a or trace amount was detected (entries 24,
6). Gratifyingly, the desired β-lactam 3a was detected in 35%
yield in toluene (entry 5). This result encouraged us to further
a
Scheme 2. Scope and Limitations of the Reaction of Diazo-triflone 1c with Imines 2
a
Reaction conditions: diazo-triflone 1c (0.4 mmol), toluene (6.0 mL), 100 °C for 1 h, then imines 2 (0.2 mmol) in 1.0 mL toluene was added and
stirred at 100 °C for another 2 h. All yields refer to isolated yields of major isomer, except for 3q and 3s as a mixture of isomers. 19F NMR yields (a
b
mixture of isomers) are shown in parentheses. The dr indicates a ratio of isomers determined by crude 19F NMR. 3s was conducted with 1c (0.6
mmol), toluene (6.0 mL), 100 °C for 1 h, then imine 2s (0.15 mmol) in 1.5 mL of toluene was added slowly in 5 min, and stirred at 100 °C for
1
another 5 h; the dr ratio was determined by H NMR.
B
DOI: 10.1021/acs.orglett.5b02827
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
zwitterionic intermediates. The ketene-triflone A21 generated
from 1c via carbene B by a Wolff rearrangement intermediate
reacts with imine 2a to give a zwitterionic intermediate C where
the donor, a phenyl group, faces outward, while the acceptor, a
triflyl group, prefers to reside inside because of a torquoelectronic
effect.22 The strong electron-withdrawing effect of the triflyl
group decreases the nucleophilicity of the enolate in C, which
significantly slows the direct ring closure (decrease in k1) to
(3S*,4S*)-3a. On the other hand, the imine moiety in C is
isomerized to provide another zwitterionic intermediate D for
steric reasons. An electrocyclic conrotatory ring closure in the
Staudinger [2 + 2] cycloaddition reaction would give (3S*,4R*)-
β-lactam-triflone 3a.20b The possibility of an intramolecular
nucleophilic addition of the enolate to the imine moiety in D to
form (3S*,4R*)-3a, instead of the electrocyclic conrotatory
mechanism, cannot be denied.20b
It should be noted that a nonfluorinated, CH3SO2 analogue of
1c, i.e., 2-diazo-1-phenyl-2-(methanesulfonyl)ethanone,23 did
not give a corresponding β-lactam under the same reaction
conditions. Besides, to our knowledge, there is no example of
direct formation of 3-methanesulfonyl-β-lactams in the [2 + 2]
ring closure in the Staudinger ketene−imine cycloaddition
reaction.24 Therefore; the trifluoromethyl group should play an
important role in the activation of this [2 + 2] ring-closing
reaction.
a
Scheme 3. Reaction of Diazo-triflones 1d−f with Imine 2o
a
Reaction conditions: diazo-triflones 1 (0.4 mmol), toluene (6.0 mL),
100 °C for 1 h, then imine 2o (0.2 mmol) in 1.0 mL of toluene was
added and stirred at 100 °C for another 5 h. All yields refer to isolated
yields of major isomer; values in the parentheses refer to 19F NMR
yield; the dr ratio was determined by crude 19F NMR.
analogue 1f was unreactive under the same reaction conditions
(X = CF3, σp = 0.54).5c The reactivity of 1df decreased as the
electron-withdrawing effects of the aryl functional groups
increased (σp: CF3 > Cl > OMe) due to the lower migration
abilities of the electron-deficient aryl groups providing ketene
intermediates under the Wolff aryl rearrangement. Information
gleaned from H NMR, 13C NMR, 19F NMR, IR, and mass
1
spectra led to the formulation of β-lactam-triflones 3. Finally, the
stereochemistry of 3 was confirmed unambiguously by the single
crystal X-ray structure analysis of β-lactam-triflone 3g (Figure 1,
In summary, we disclose here a highly stereoselective synthesis
of 3,3-aryltriflyl multisubstituted β-lactams from the reaction of
diazo-triflones with imines. The protocol is the first report about
the introduction of a triflyl group into the 3-position of a β-
lactam. The electronic effect and steric hindrance of R2 in the
imines, and the electronic effect of the aryl group in the diazo-
triflones, influence the final [2 + 2] ring closure in the Staudinger
ketene−imine cycloaddition reaction. The triflyl- and aryl-
substituted groups in ketenes direct the stereoselectivity of the
final β-lactams. Further applications and demonstration of the
utility of diazo-triflones reagent 1c are under investigation.
Figure 1. X-ray crystallographic structure of β-lactam 3g shows
(3S*,4R*) conformation (CCDC 1412034).
CCDC1412034). While the isolation of minor diastereomers
was not easy due to the high selectivities, a minor isomer of 3q
was isolated to confirm its structure. The existences of other
minor isomers were tentatively assigned by analogy based on the
19F NMR.
ASSOCIATED CONTENT
■
S
* Supporting Information
The proposed transition state for the highly stereoselective
construction of a β-lactam ring is shown in Figure 2. According to
a report by Xu and co-workers,20 the cis/trans stereoselectivity is
generated as a result of the competition between direct ring
closure (k1, k3) and isomerization of the imine moiety (k2) in
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02827.
General procedures; NMR spectra (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nozshiba@nitech.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research is partially supported by the Platform Project for
Supporting in Drug Discovery and Life Science Research from
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), the Japan Agency for Medical Research
and Development (AMED), the Advanced Catalytic Trans-
formation (ACT-C) from the Japan Science and Technology
(JST) Agency, and Kobayashi International Foundation.
Figure 2. Proposed transition-state models for the stereoselective [2 +
2] cycloaddition of ketene-triflone A with imine 2a.
C
DOI: 10.1021/acs.orglett.5b02827
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
A.; Tokunaga, E.; Shibata, N. Org. Lett. 2011, 13, 4854. (f) Zhang, K.;
Xu, X.-H.; Qing, F.-L. J. Org. Chem. 2015, 80, 7658.
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−162.200 ppm) after heating. See Supporting Information for details.
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DOI: 10.1021/acs.orglett.5b02827
Org. Lett. XXXX, XXX, XXX−XXX