Silver-Catalyzed Site-Selective Ring-Opening and C-C Bond Functionalization of Cyclic Amines: Access to Distal Aminoalkyl-Substituted Quinones

Article abstract of DOI:10.1021/acs.orglett.9b01496

Distal aminoalkyl-substituted quinones have been efficiently prepared through silver-catalyzed site-selective deconstruction and C-C bond transformation of unstrained N-acylated cyclic amines. This method enjoys mild reaction conditions, high selectivity, a broad scope of substrates, and a low catalytic loading of silver. This strategy can also be applied to the modification of peptides bearing cyclic amine residues.

Full text of DOI:10.1021/acs.orglett.9b01496

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Silver-Catalyzed Site-Selective Ring-Opening and C−C Bond
Functionalization of Cyclic Amines: Access to Distal Aminoalkyl-
Substituted Quinones
Rui-Hua Liu, Yi-Heng He, Wei Yu, Bo Zhou, and Bing Han*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
7
30000, People’s Republic of China
*
S Supporting Information
ABSTRACT: Distal aminoalkyl-substituted quinones have been
efficiently prepared through silver-catalyzed site-selective decon-
struction and C−C bond transformation of unstrained N-acylated
cyclic amines. This method enjoys mild reaction conditions, high
selectivity, a broad scope of substrates, and a low catalytic loading
of silver. This strategy can also be applied to the modification of
peptides bearing cyclic amine residues.
uinone, a vital scaffold, is present in medicines,
functional materials, and natural products. Quinone
Catalytic ring-opening and C−C bond functionalization of
strained cyclic compounds has been well explored in the past
few years for their privileged advantages to modifying and
reorganizing the scaffolds of commonplace compounds to
1
Q
derivatives possess a broad spectrum of biological and
pharmacological activities in living organisms for their unique
2
,3
6
properties of electron and proton metastasis. Among them,
many aminoalkyl-substituted quinones exhibit pharmacological
more valuable molecules. However, the unstrained cyclic
compounds are seldom used in these strategies because of their
3
7
activities (Figure 1). Considering the significance of function-
inherent low ring-strain energy. Saturated unstrained N-
heterocycles such as piperidines and pyrrolidines are plentiful
and useful chemicals. Their frameworks are widely distributed
in many natural products and pharmaceutical molecules
including proline, paroxetine, and pholcodine. Thus, the
transformation of simple cyclic amines to more complex
compounds and the modification of pharmaceutical molecules
bearing cyclic amines groups are both attractive to chemists
7a−e,8
and pharmacologists.
Among them, most strategies focus
on the direct functionalization of C−H bonds on cyclic
9
amines, whereas the deconstruction of cyclic amines involving
the heterocyclic ring-opening and C−C transformation to
7a−e
reorganize their core skeletons have less been achieved.
Higuchi’s group presented the first transformation of N-
acylated cyclic amines to amino acids by ruthenium-catalyzed
7a
oxidative ring-opening of cyclic amines (Scheme 1, a). Very
recently, Sarpong’s group reported an unprecedented stoichio-
metric silver-promoted deconstructive fluorination of cyclic
amines through an alkoxy radical-involved process (Scheme 1,
Figure 1. Drugs and bioactive molecules containing aminoalkyl-
substituted quinones.
7b
b). The same group also developed a protocol for the
deconstructive chlorination and bromination of cyclic amines
through a different process of sequential C−N cleavage and
C−C bond conversion by using stoichiometric amount of
alized quinones, a great deal of tactics for the incorporation of
simple aryl and alkyl groups into quinones have been
4
developed. However, the construction of aminoalkyl-sub-
7
c
5
silver salt (Scheme 1, b). Given the importance of
deconstructive functionalization of cyclic amines, we herein
report the first silver-catalyzed oxidative ring-opening/degra-
stituted quinones has been rarely achieved. In those sparse
3
b
strategies, the multistep transformation or the use of
diversity-limited short chain amino acids as starting materials
5
has to be relied on. The research on the directly preparation
of aminoalkyl-substituted quinones from the multitudinous
and readily available sources still remains challenging.
Received: April 29, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01496
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Methods for the Ring-Opening and
Table 1. Optimization of the Reaction Conditions
Functionalization of N-Acylated Cyclic Amines
f
b
entry
catalyst
AgPF6
AgOAc
AgNO3
AgTFA
organic solvent
oxidant
yield (%)
1
2
3
4
5
6
7
8
9
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
acetone
DCM
PhCH3
Na S O
45
80
81
70
0
2
2
8
8
8
8
8
8
Na S O
2 2
Na S O
2 2
Na S O
2 2
Cu(NO ) ·3H O
Na S O
2 2
3
2
2
Fe(NO ) ·9H O
Na S O
2 2
0
3
3
2
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
(NH ) S O
79
76
0
4
2
2
8
K S O
8
2
2
TBHP
DTBP
10
11
12
13
14
15
16
17
18
0
Na S O
15
76
17
7
0
0
2
2
8
8
8
8
8
Na S O
2
2
Na S O
2
2
CH CN
Na S O
2 2
3
DCE
DCE
DCE
DCE
Na S O
2
2
c
AgNO3
AgNO3
AgNO3
dation/C−C functionalization of cyclic amines by the reaction
of them with simple quinones (Scheme 1, c). From economic
and environmental perspective, the low catalytic loading of
silver salt will be useful to reduce the cost of practical
deconstructive conversion of cyclic amines. This method was
also successfully applied to the transformation of various
peptides bearing cyclic amine residues. Consequently, a variety
of aminoalkyl-substituted quinones were effectively prepared
by this strategy.
d
e
Na S O
61
79
2
2
8
Na S O
2
2
8
a
Reaction conditions: quinone 1a (0.2 mmol, 1 equiv), amine 2a (0.2
mmol, 1 equiv), catalyst (0.02 mmol, 0.1 equiv), solvent (2 mL of
b
H O and 0.4 mL of organic solvent), rt, Ar, dark, 18 h. Isolated yield.
2
c
d
AgNO (0.8 mmol, 4 equiv) was used as the oxidant. AgNO (0.01
3
3
e
mmol, 0.05 equiv) was used. AgNO (0.03 mmol, 0.15 equiv) was
used. DCE = 1,2-dichloroethane, DCM = dichloromethane, TBHP =
tert-butyl hydroperoxide, DTBP = di-tert-butyl peroxide.
3
f
We initiated this research by subjecting a mixture of 1,4-
naphthoquinone 1a and N-pivaloyl piperidine 2a to the dark
conditions of Na S O and AgPF6 in 1,2-dichloroethane
2
2
8
aqueous solution under argon atmosphere for 18 h. The
desired reaction occurred spontaneously at room temperature,
giving aminoalkyl-substituted quinones 3a in 45% yield (Table
generation of product 3c was accompanied by the formation of
a small amount of N-formyl byproduct 3c′. In addition,
piperidines 2d−h bearing a 4-substituent, such as Me, MeO,
1
, entry 1). Encouraged by the result, we subsequently
Ph, CO Me, or F, were all compatible for the reaction,
2
screened a series of silver catalysts such as AgOAc, AgNO3,
providing the corresponding products 3d−h in moderate to
good yields. Notably, 2-methylpiperidine 2i was a good
candidate to react with 1a via the eminently site-selective
ring-opening, giving rise to product 3i in satisfying yield.
Product 3j was obtained in excellent yield when 3,5-
dimethylpiperidine 2j was used for the reaction, promising
the successful conversion of the tertiary carbon in the tactic.
Both spiropiperidine 2k and fused piperidine 2l could be used
as the reaction partners with 1a, furnishing the corresponding
products 3k and 3l in moderate yields. Among them, the
slightly lower yields of 3g and 3k could be attributed to the
easy oxidation of both the benzylic C−H bond of 2g and the
activated C−H bonds of glycol ether moiety of 2k to form
complex mixtures in the reaction. Significantly, morpholine 2m
was a befitting partner for this reaction, providing the product
3m in 65% yield. Besides piperidine, the cyclic amines 2n−q
with different ring sizes transformed smoothly in this reaction,
gaining the quinones bearing aminoalkyl chains of varying
lengths in good yields (3n−q). A small amount of N-formyl
byproduct 3q′ was obtained when 2q was involved. When
cyclic amines bearing a chiral ester group such as methyl (S)-
piperidine-2-carboxylate 2r and L-proline methyl ester 2s were
subjected to this reaction, the chirality retained products 3r
and 3s were formed in moderate to good yields.
and AgTFA (Table 1, entries 2−4). All of those silver salts
promoted this reaction readily, AgNO was selected as the best
3
catalyst, and the yield of 3a was increased to 81% (Table 1,
entry 3). In contrast, other transition-metal catalysts such as
Cu(NO ) ·3H O and Fe(NO ) ·9H O were inoperative in
3
2
2
3 3
2
this reaction (Table 1, entries 5 and 6). Next, a variety of
organic and inorganic oxidants were also investigated. Besides
Na S O , (NH ) S O and K S O were applicable oxidants
2
2
8
4 2
2
8
2
2
8
for this tactic as well, delivering 3a in comparable yields (Table
, entries 7 and 8). Peroxides such as TBHP and DTBP were
1
ineffective for this reaction (Table 1, entries 9 and 10). No
better result was obtained when other organic solvents such as
acetone, DCM, toluene, and acetonitrile were involved (Table
1
, entries 11−14). Moreover, no reaction took place without
either silver salts or peroxysulfates (Table 1, entries 15 and
6). The yield of 3a could not be further improved by
1
screening the dosage of AgNO (Table 1, entries 17 and 18).
3
Having established the optimum conditions (Table 1, entry
3
(
), the scope of N-acylated cyclic amines 2 was first examined
Scheme 2). Piperidines 2a−c with various N-protecting
groups such as Piv (pivaloyl), Bz (benzoyl), and Cyc
cyclohexanecarbonyl) participated readily in this strategy,
affording the desired products 3a−c in nice yields. The
(
B
DOI: 10.1021/acs.orglett.9b01496
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Scope of Cyclic Amines
Scheme 3. Transformations of Di- and Tripeptides
Containing L-Proline Residue
a
b
Isolated yields. Amine 2 (0.5 mmol, 2.5 equiv) was used, and the
reaction time was prolonged to 48 h.
Scheme 4. Scope of Quinones
a
b
Isolated yields. Amine 2 (0.5 mmol, 2.5 equiv) was used, and the
c
reaction time was prolonged to 30 h. Quinone 1a (4.0 mmol) and
amine 2 (4.0 mmol, 1.0 equiv) were used, and the reaction time was
prolonged to 30 h. Amine 2 (0.24 mmol, 1.2 equiv) was used. The
d
e
f
reaction time was prolonged to 48 h. 67% of 1a and 86% of 2r were
recovered.
a
b
Isolated yields. Amine 2a (0.5 mmol, 2.5 equiv) was used, and the
c
To further verify the utility of this approach, peptides
containing cyclic amine residues were also investigated
reaction time was prolonged to 48 h. Amine 2a (0.3 mmol, 1.5
equiv) was used, and the reaction time was prolonged to 30 h. DCM
d
(1 mL) was used.
(Scheme 3). Dipeptides 2t−v derived from the condensation
of L-proline with various amino acid derivatives such as L-
valine, L-aspartic acid, and L-glutamic acid reacted with 1a very
well, affording the chirality retained products 3t−v in moderate
yields. L-Proline-involved tripeptide 2w was also a suitable
reactant for this protocol and generated the corresponding 3w
in 59% yield.
Next, the scope of quinones was examined by their reaction
with 2a, and the results are described in Scheme 4.
Unsubstituted benzoquinone proceeded smoothly and gave
the anticipated product 3x in 66% yield. When asymmetric
monosubstituted benzoquinone participated in the reaction, as
in the case of 2-tert-butylbenzoquinone, a mixture of the
positional isomers 3y and 3y′ was procured in 58% yield. A
variety of disubstituted and trisubstituted benzoquinones were
all applicable for this conversion, providing the desired
products 3z−af in moderated to good yields. In addition,
when 2-Cl-1,4-naphthoquinone and 1,4-anthraquinone were
subjected to the reaction, the desired products 3ag and 3ah
were formed in 65% and 42% yields, respectively. It is worth
10a
noting that thymoquinone,
which possesses anticancer
activity, was also applicable to this strategy and provided the
desired products 3ai and 3ai′ in a combined yield of 65%.
1
0b
Moreover, menadione,
a drug for the treatment of
hypoprothrombinemia, participated very well in the reaction,
producing the desired product 3aj in 86% yield.
C
DOI: 10.1021/acs.orglett.9b01496
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Organic Letters
Letter
The protocol was also suitable for the synthesis of
disubstituted quinones by incorporating symmetrical and
nonsymmetrical aminoalkyl chains in one pot. By increasing
the usage amount of amine 2a and oxidants, symmetrical
disubstituted product 3ak was efficiently synthesized (Scheme
Scheme 7. Proposed Mechanism
5
, eq 1). In addition, nonsymmetrical diaminoalkyl-substituted
Scheme 5. Synthesis of Symmetrical and Nonsymmetrical
Diaminoalkyl-Substituted Naphthoquinones
transfer (HAT) process to yield the C-radical intermediate A
which is subsequently oxidized by Ag(II) to give the iminium
7b,c,11b
ion B.
Compound B is immediately trapped by water to
generate hemiaminal C, which is in equilibrium with the linear
7
a−d
aldehyde D via C−N cleavage of cyclic amine.
The
naphthoquinone 3al could be easily prepared by the sequential
addition of different cyclic amines to the reaction in one pot
intermediate D is subsequently oxidized to N-acylated amino
acid E, which further undergoes the well-known silver-
catalyzed radical decarboxylation to yield the corresponding
(
Scheme 5, eq 2).
To shed light on the mechanism of this reaction, control
12
alkyl radical F. In the capture of F by quinone 1 to form the
carbon radical G, the latter is finally oxidized to product 3. On
the other hand, hemiaminal C can also experience a
competitive reaction through silver-catalyzed oxidation to
produce the alkoxy radical H. The radical ring-opening of H
gives the corresponding alkyl radical I as well. The interception
of I by quinone 1, followed by subsequent oxidation, yields the
byproduct 3c′ or 3q′.
experiments were carried out (Scheme 6). When the sample
Scheme 6. Control Experiments
In summary, a facile and practical silver-catalyzed oxidative
strategy of tandem site-selective ring-opening/carbon degra-
dation/C−C bond functionalization of N-acylated cyclic
amines has been developed by using quinones and cyclic
amines as the commercially available starting materials and
persulfate as the oxidant. By using this method, a variety of
distal aminoalkyl-substituted quinones were effectively pre-
pared. Moreover, this protocol features mild reaction
conditions, high product selectivity, and broad substrate
scope. The site-selective chemical modification of the core
framework of peptides bearing cyclic amine residues by
installing useful quinones might find applications in new
reaction was interrupted after 2 h, 26% of 3a was obtained and
the N-acylated amino acid 4 was detected by the HRMS from
the reaction system (Scheme 6, eq 1). When the N-acylated
amino acid 4 was subjected to the optimal conditions, the
desired product 3a was formed in 63% yield accompanied by
the symmetrical disubstituted byproduct 3ak in 15% yield.
This result clearly indicates that the aminoalkyl-substituted
quinone was produced by silver-catalyzed radical decarbox-
ylation of N-acylated amino acid to generate the degraded
aminoalkyl radical, which then occurred alkylation with
quinone (Scheme 6, eq 2). Notably, the N-formyl product
13
bioactive molecules and pharmaceutical discovery.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01496.
Detailed experimental procedures and spectral data for
all products (PDF)
3q′ could not be transformed to 3q under the optimal
conditions (Scheme 6, eq 3). This result manifests that the
formation of 3q did not experience the deformylation process
7b
of N-formyl product 3q′.
AUTHOR INFORMATION
Corresponding Author
■
*
On the grounds of previous studies on deconstructive
conversion of cyclic amines and the results of control
experiments, a possible mechanism is proposed in Scheme 7.
Initially, Ag(I) reacts with persulfate anion to form the Ag(II),
E-mail: hanb@lzu.edu.cn.
ORCID
11a
sulfate, and sulfate radical anion. N-Acylated cyclic amine 2
reacts first with a sulfate radical anion via a hydrogen atom
Wei Yu: 0000-0002-3131-3080
Bo Zhou: 0000-0002-8713-5906
D
DOI: 10.1021/acs.orglett.9b01496
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
d) Osberger, T. J.; Rogness, D. C.; Kohrt, J. T.; Stepan, A. F.;
Bing Han: 0000-0003-0507-9742
Notes
White, M. C. Nature 2016, 537, 214−219. (e) Yu, C.; Shoaib, M. A.;
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The authors declare no competing financial interest.
5
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Angew. Chem., Int. Ed. 2016, 55, 15319−15322. (h) Ota, E.; Wang,
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
Nos. 21873041, 21632001, and 21422205), the Fundamental
Research Funds for the Central Universities (Nos. lzujbky-
016-ct02 and lzujbky-2016-ct08), Program for the Changjiang
Scholars and Innovative Research Team in University (IRT-
■
H.; Frye, N. L.; Knowles, R. R. J. Am. Chem. Soc. 2019, 141, 1457−
1462. (i) He, Y.; Zheng, Z.; Liu, Y.; Qiao, J.; Zhang, X.; Fan, X. Org.
(
Lett. 2019, 21, 1676−1680. (j) He, K.; Li, P.; Zhang, S.; Chen, Q.; Ji,
H.; Yuan, Y.; Jia, X. Chem. Commun. 2018, 54, 13232−13235. (k) Xu,
Y.; Qi, X.; Zheng, P.; Berti, C. C.; Liu, P.; Dong, G. Nature 2019, 567,
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546−550.
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5R28), and the “111” Project for financial support.
(
8) Galloway, W. R. J. D.; Isidro-Llobet, A.; Spring, D. R. Nat.
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DOI: 10.1021/acs.orglett.9b01496
Org. Lett. XXXX, XXX, XXX−XXX