Copper-Catalyzed Carbonylative Synthesis of β-Homoprolines from N-Fluoro-sulfonamides

Article abstract of DOI:10.1021/acs.orglett.0c00227

A new methodology for the carbonylative transformation of N-fluoro-sulfonamides into N-sulfonyl-β-homoproline esters has been described. In the presence of a catalytic amount of Cu(OTf)₂, a range of β-homoproline derivatives were prepared in moderate to good yield. The reaction proceeds via the intramolecular cyclization and intermolecular carbonylation of a free carbon radical. Notably, this procedure offers the possibility to build potential functionalized bioactive molecules.

Full text of DOI:10.1021/acs.orglett.0c00227

pubs.acs.org/OrgLett
Letter
Copper-Catalyzed Carbonylative Synthesis of β‑Homoprolines from
N‑Fluoro-sulfonamides
Youcan Zhang, Zhiping Yin, and Xiao-Feng Wu*
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ABSTRACT: A new methodology for the carbonylative trans-
formation of N-fluoro-sulfonamides into N-sulfonyl-β-homoproline
esters has been described. In the presence of a catalytic amount of
Cu(OTf)2, a range of β-homoproline derivatives were prepared in
moderate to good yield. The reaction proceeds via the intra-
molecular cyclization and intermolecular carbonylation of a free carbon radical. Notably, this procedure offers the possibility to build
potential functionalized bioactive molecules.
yrrolidines are important and valuable functional hetero-
genation8f,g were applied for constructing β-homoproline
derivatives. However, besides their advantages, limitations
such as limited substrate expansion, stoichiometric additives,
and multistep reactions still restricted their applications.
Recently, Fustero, del Pozo, and coworkers reported a novel
procedure for the synthesis of diastereomeric fluorinated β-
homoproline derivative (Scheme 2, eq a).9 Noda and
Shibasaki’s group developed an interesting Rh-catalyzed C−
P
cycles units in the fields of bioactive molecules and natural
products.1 Prolines with a structural element of pyrrolidine, in
particular, β-homoproline, are of particular interest due to their
special applications in biology and pharmacology.2 For
example, β-homoproline was used in place of proline to
produce biologically active homotetrapeptide I, which has
better affinity toward the μ-opioid receptor and is more
resistant to enzymatic hydrolysis (Scheme 1).3 Usually, β-
Scheme 2. Synthesis of β-Homoproline Derivatives and Our
Design
Scheme 1. Selected Bioactive Homoprolines
homoprolines are also used as intermediates to prepare various
bioactive molecules,4 like tripeptide II. A new type of HCV
NS3 peptidomimetic inhibitor tripeptide III has been
developed as well.5 In recent years, it has been confirmed
that β-amino acids construct more highly stable secondary
structures comparable to their α-amino acid analogs, although
they are not susceptible to protease-type hydrolases.6 The
exploration of β-peptides will play an increasingly important
role in biomedical research and drug development. Even
though many synthetic methods for synthesizing β-amino acid
derivatives have been reported,7 only a few methods are
available for synthesizing β-homoproline derivatives.8 In
general, methods like Michael reactions,8a dipolar cyclo-
additions,8b Arndt−Eistert homologation,8c−e and hydro-
Received: January 16, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00227
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
H bond activation of isoxazolidin-4-ones,10 although the
substrate scope is limited (Scheme 2, eq b). Therefore, a
new method for synthesizing β-homoproline derivatives is still
in demand and will be useful for scientists in related areas.
Carbonylation is an effective and important strategy for
constructing various compounds with carbonyl functionality;
meanwhile, carbon monoxide as a cheap and approachable
carbonyl source has been widely applied in synthetic
chemistry.11 We have been attracted by CO chemistry as
well and been interested in using CO as a carbonyl source to
synthesize various heterocyclic compounds.11d−f We envisaged
that a low-cost metal-catalyzed carbonylation of N-fluoro-
sulfonamides to give β-homoproline derivatives might be
realized by the intramolecular cyclization and intermolecular
carbonylation of free radicals.
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
ligand + base
solvent
MeCN
yield (%)
1
2
3
4
5
6
7
8
Cu(OTf)2
Cu(OAc)2
CuI
L1 + Li2CO3
L1 + Li2CO3
L1 + Li2CO3
L1 + Li2CO3
L1 + LiOH
L1 + K2CO3
L1 + pyridine
L1
L1 + Li2CO3
L1 + Li2CO3
L1 + Li2CO3
L1 + Li2CO3
L2 + Li2CO3
L3 + Li2CO3
L4 + Li2CO3
L5 + Li2CO3
L6 + Li2CO3
Li2CO3
38
32
26
28
trace
35
20
nd
46
44
31
50
42
51
56
52
48
28
71
76
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
Inspired by the previous achievements (Scheme 2, eq c)12
and to achieve our hypothesis, we chose N-fluoro-4-methyl-N-
(pent-4-en-1-yl)benzenesulfonamide 1a as the model substrate
and MeOH as the nucleophile. Combining the recent research
progress of N-fluoro-sulfonamides13 and our previous experi-
ence on cheap metal-catalyzed carbonylation,14 we initiated
our study by using Cu(OTf)2 and bipyridine as the catalytic
system and Li2CO3 as the base under pressure of CO (30 bar)
in MeCN at 80 °C. To our delight, we observed the desired β-
homoproline ester 2a in 38% yield (Table 1, entry 1). After
comparing different copper catalyst effects, Cu(OTf)2 was
found to be the most effective catalyst among these copper
catalyst precursors (Table 1, entries 1−4). Different base
sources were then tested, including LiOH, K2CO3, and
pyridine, and the yield of the desired product was decreased
in those cases (Table 1, entries 5−7). In the case of LiOH, the
decreased yield might be due to the reaction between the
target ester and the hydroxide. In the absence of base, no target
product was detected (Table 1, entry 8). Then, we examined
other various solvents, including THF, PhCF3, and DCE;
however, the starting material 1a could not be better
transformed into 2a (Table 1, entries 9−11). The use of
DMAc or DMF as the solvent also lead to a low yield.
Delightfully, when the reaction was performed in a mixed
solvent of THF/PhCF3 (4:1), a 50% yield of the target product
2a could be obtained (Table 1, entry 12). Afterward, we turned
our attention to explore the ligand effects (Table 1, entries 12−
17). A 42% yield of the desired product can be produced with
1,10-phenanthroline used as the ligand, and the reaction
efficiency can be improved when it is substituted with a methyl
group (Table 1, entries 13 and 14). Improved yields can be
obtained when 2,2′-bipyridine is substituted with an alkyl
group, and a 56% yield was achieved with L4 as the ligand
(Table 1, entries 15 and 16). However, a decreased yield was
obtained when 2,2′-bipyrimidine was applied as the ligand, and
the yield of the desired product was further decreased to 28%
in the absence of ligand (Table 1, entries 17 and 18).
Additionally, we considered adjusting the temperature and the
CO pressure of the reaction as well. Satisfactorily, the yield of
the target product could be improved to 71%, when the CO
pressure was increased to 50 bar (Table 1, entry 19). On the
basis of this reaction condition, the reaction yield could be
improved to 76% when we changed the temperature to 100 °C
(Table 1, entry 20). Subsequently, we continued to increase
the CO pressure to 60 bar, and only a small improvement in
the reaction outcome was observed (Table 1, entry 21).
Finally, we assessed the substrate scope of this reaction with 5
mol % of Cu(OTf)2, 10 mol % of 5,5′-dimethyl-2,2′-bipyridine,
CuF2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
9
10
11
12
13
14
15
16
17
18
19
20
21
PhCF3
DCE
c
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
THF + PhCF3
c
c
c
c
c
cd
,
ce
,
L4 + Li2CO3
L4 + Li2CO3
L4 + Li2CO3
cf
,
cf
,
g
79 (73)
a
Reaction conditions: 1a (0.1 mmol), MeOH (0.1 mL), catalysts (5
mol %), ligands (10 mol %), and base (1 equiv) in solvent (1 mL) at
80 °C for 20 h under CO (30 bar). Yields were determined by GC-
FID analysis using n-hexadecane as the internal standard. THF/
PhCF3 4:1. 80 °C, CO (50 bar). 100 °C, CO (50 bar). 100 °C, CO
(60 bar). Isolated yield. nd = no detection. THF = tetrahydrofuran.
b
c
d
e
f
g
DCE = 1,2-dichloroethane.
and 1 equiv of Li2CO3 under CO pressure (50 bar) in a mixed
solvent of THF/PhCF3 (4:1) at 100 °C.
With the optimized reaction conditions established (Table 1,
entry 20), we then explored the scope of this reaction with a
range of N-fluoro-sulfonamides and alcohols. As shown in
Scheme 3, the starting materials of 1a−d bearing electron-
donating or -neutral groups were smoothly converted into the
corresponding N-sulfonyl-β-homoproline ester products 2a−d
in good yield. The aromatic ring of N-fluoro-sulfonamides with
no functional group or ortho-chloro substitution can be
prepared as well, and the desires products 2b and 2e were
obtained in 70 and 46% yield, respectively. Notably, different
alkyl alcohols as nucleophiles including ethanol, propanol,
isopropanol, and n-butanol, were reacted with 1a, and the
desired products 2f−i were successfully obtained in moderated
yield (51−57%). Furthermore, the alcohol bearing a terminal
alkenyl or trifluoromethyl group was well-tolerated as well,
forming the desired products 2j−l in 32−41%. We scaled up
the reaction to a 1.0 mmol level under the optimal reaction
conditions. To our delight, the desired product 2a could be
prepared in a 60% isolated yield when the reaction of 1a (1.0
B
https://dx.doi.org/10.1021/acs.orglett.0c00227
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a
a
Scheme 3. Scope of N-Fluoro-sulfonamides and Alcohols
Scheme 4. Scope of C2-Substituted N-Fluoro-sulfonamides
a
Reaction conditions: 1 (0.2 mmol), ROH (0.2 mL), Cu(OTf)2 (5
mol %), 5,5′-dimethyl-2,2′-bipyridine (10 mol %), and Li2CO3 (1.0
a
Reaction conditions: 1 (0.2 mmol), ROH (0.2 mL), Cu(OTf)2 (5
mol %), 5,5′-dimethyl-2,2′-bipyridine (10 mol %), and Li2CO3 (1.0
equiv) in THF + PhCF3 (2 mL, 4:1) at 100 °C for 20 h under CO
(50 bar). Isolated yield.
equiv) in THF + PhCF3 (2 mL, 4:1) at 100 °C for 20 h under CO
b
(50 bar). 1a (1.0 mmol), MeOH (1.0 mL), THF + PhCF3 (7 mL,
4:1). Isolated yield.
mmol) and MeOH (1.0 mL) was carried out in 7 mL of a
mixed solvent of THF/PhCF3 (4:1).
5). Moreover, we attempted to remove the tosyl group by SmI2
but failed.14c
Subsequently, a range of other N-fluoro-sulfonamides was
prepared and investigated. These substrates with functional
groups in the alkyl branched chain were all transformed
smoothly, leading to the desired products in 40−73% yield
(Scheme 4, 2m−p). Additionally, N-(2,2-dimethylpent-4-en-1-
yl)-N-fluoro-sulfonamides with different substitutions on the
aromatic ring were tested as well and gave the desired products
2q−t in moderate to good yield. Interestingly, the product of
methyl 2-(4,4-dimethyl-1-(methylsulfonyl)pyrrolidin-2-yl)-
acetate 2u could be formed from the substrate 1n in good
yield. Next, we explored the carbonylation of secondary and
tertiary carbon radicals. As the results show in Scheme 4, a
mixture of products 2v from carbonylation and elimination was
achieved with a secondary radical intermediate. Only
elimination product 2w was formed with the tertiary carbon
radical intermediate. Then, the substrates with different carbon
chain lengths (1q N-(but-3-en-1-yl)-N-fluoro-4-methylbenze-
nesulfonamide, 1r N-(2,2-dimethylhex-5-en-1-yl)-N-fluoro-4-
methylbenzenesulfonamide, and 1s N-fluoro-N-(hex-5-en-1-
yl)-4-methylbenzenesulfonamide) were also tested; however,
only noncarbonylative cyclization products 2x and 2y were
detected. Here the two products were obtained via 1,5-proton
transfer and then the C−N coupling process.14 Additionally,
no target product was formed, and only the defluorination
compound 2z could be detected by GC-MS from the
corresponding N-fluoro-sulfonamide. Finally, under our stand-
ard reaction conditions, substrate 3 from the biologically
important celecoxib can also be successfully reacted and
transformed into the desired product 4 in 46% yield (Scheme
Scheme 5. Carbonylation of N-Fluoro-N-allylcelecoxib
a
Reaction conditions: 3 (0.2 mmol), MeOH (0.2 mL), Cu(OTf)2 (5
mol %), 5,5′-dimethyl-2,2′-bipyridine (10 mol %), and Li2CO3 (1.0
equiv) in THF + PhCF3 (2 mL, 4:1) at 100 °C for 20 h under CO
(50 bar). Isolated yield.
On the basis of the above results and literature,12−15
a
possible reaction mechanism is proposed in Scheme 6. Initially,
amidyl radical A is produced by the CuI-induced single-
electron reduction of 1a while forming the CuII species. After
an intramolecular cyclization of amidyl radical A, a new carbon
radical B is generated. Then, carbon radical B would be
captured by CO and CuII species to give the intermediate C.
Subsequently, a ligand-exchange procedure of intermediate C
with ROH occurs to produce the intermediate D. Finally, the
reductive elimination of intermediate D provides the eventual
product and the active CuI species, which is used for the next
catalytic cycle.
In summary, a new strategy for the copper-catalyzed
intramolecular cyclization and intermolecular carbonylative
transformation of N-fluoro-sulfonamides has been developed.
C
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Scheme 6. Proposed Reaction Mechanism
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ASSOCIATED CONTENT
* Supporting Information
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(5) Nurbo, J.; Peterson, S. D.; Dahl, G.; Helena Danielson, U.;
The Supporting Information is available free of charge at
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AUTHOR INFORMATION
Corresponding Author
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́
Rep. 2014, 31, 1056−1073. (d) Katarzynska, J.; Mazur, A.; Bilska, M.;
Xiao-Feng Wu − Leibniz-Institut fur Katalyse e.V. an der
̈
Adamek, E.; Zimecki, M.; Jankowski, S.; Zabrocki, J. Synthesis and
immunosuppressive activity of new cyclolinopeptide A analogs
modified with β-prolines. J. Pept. Sci. 2008, 14, 1283−1294.
̈
Universitat Rostock 18059 Rostock, Germany; orcid.org/
0000-0001-6622-3328; Email: xiao-feng.wu@catalysis.de
́
(7) (a) Weiner, B.; Szymanski, W.; Janssen, D. B.; Minnaard, A. J.;
Authors
Youcan Zhang − Leibniz-Institut fu
Feringa, B. L. Recent advances in the catalytic asymmetric synthesis of
β-amino acids. Chem. Soc. Rev. 2010, 39, 1656−1691. (b) Noda, H.;
Shibasaki, M., Recent Advances in the Catalytic Asymmetric Synthesis
of β2-and β2,2-Amino Acids. Eur. J. Org. Chem. 2019 DOI: 10.1002/
ejoc.201901596. (c) Lelais, G.; Seebach, D. β2-amino acids-syntheses,
occurrence in natural products, and components of β-peptides1,2.
Biopolymers 2004, 76, 206−243.
̈
r Katalyse e.V. an der
Universitat Rostock 18059 Rostock, Germany
r Katalyse e.V. an der
̈
Zhiping Yin − Leibniz-Institut fu
̈
̈
Universitat Rostock 18059 Rostock, Germany
Complete contact information is available at:
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(8) (a) Chippindale, A. M.; Davies, S. G.; Iwamoto, K.; Parkin, R.
M.; Smethurst, C. A.; Smith, A. D.; Rodriguez-Solla, H. Asymmetric
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A.; Engel, S. L. Bradykinin analogs containing. beta.-homoamino
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Chinese Scholarship Council for financial
support. The analytical support from Dr. W. Baumann, Dr. C.
Fischer, S. Buchholz, and S. Schareina (all in LIKAT) is
gratefully acknowledged.
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