Metal-Free Direct Amidation of Naphthoquinones Using Hydroxamic Acids as an Amide Source: Application in the Synthesis of an HDAC6 Inhibitor

Article abstract of DOI:10.1021/acs.orglett.6b02740

A novel synthetic approach to amidoquinones by the reaction of naphthoquinones with hydroxamic acids under basic conditions was developed. The reaction is mild and operationally simple, and it affords high yields of amidoquinones. With this new method, a novel, very strong HDAC6 inhibitor, which showed high toxicity to AML cells, was successfully synthesized.

Full text of DOI:10.1021/acs.orglett.6b02740

Letter
pubs.acs.org/OrgLett
Metal-Free Direct Amidation of Naphthoquinones Using Hydroxamic
Acids as an Amide Source: Application in the Synthesis of an HDAC6
Inhibitor
Cheng Zhang*,† and C. James Chou*,‡
†Department of Applied Chemistry, China Agricultural University, 2 Yuanmingyuan West Road, Beijing 100193, China
‡Department of Drug Discovery and Biomedical Sciences, Medical University of South Carolina, 280 Calhoun Street, MSC140
QF307, Charleston, South Carolina 29425, United States
S
* Supporting Information
ABSTRACT: A novel synthetic approach to amidoquinones
by the reaction of naphthoquinones with hydroxamic acids
under basic conditions was developed. The reaction is mild
and operationally simple, and it affords high yields of
amidoquinones. With this new method, a novel, very strong
HDAC6 inhibitor, which showed high toxicity to AML cells,
was successfully synthesized.
inhibitor, NQN-1, that showed selective toxicity toward acute
myeloid leukemia (AML) cells at micromolar concentration was
developed.2 To date, mainly two methods are available to access
such amidoquinone structures (Scheme 1b): (1) An amino-
quinone is obtained by directly warming a mixture of a quinone
and NaN3 in AcOH, and the aminoquinone is then reacted with
an acyl chloride in the presence of strong base to form the final
amidoquinone product. The strong base, such as NaH, is
indispensable in the acylation reaction because the amino group
is highly deactivated by the quinone ring, and the generation of a
nitrogen anion with the strong base is necessary to make the
nucleophilic substitution reaction proceed. As a result, the need
to work with this corrosive and explosive NaN3/AcOH reaction
system and to handle the susceptible acylation reaction make this
method considerably labor-intensive and time-consuming.3 (2)
The amidoquinone is directly formed by oxidation of the
corresponding dimethoxy-N-phenylamide with a strong oxidant
such as a hypervalent iodine compound. This method can afford
various amidoquinones with different substituents.4 However,
substrates that are susceptible to strong oxidants cannot tolerate
these reaction conditions. In this context, the development of
more concise and environmentally benign synthetic methods to
access these amidoquinone structures is highly desirable and of
great significance.
midoquinones are core structures of many biologically
Aactive natural products and synthetic molecules. Some of
these compounds, such as hygrocin A and salinisporamycin
(Scheme 1a), exhibit very important anticancer and antibiotic
activities.1a−e Besides their biological applications, amidoqui-
nones have also been used as metal chelators in catalysis.1f
Recently, a novel amidoquinone-based HDAC6-selective
Scheme 1. Amidoquinone Structures and Synthetic Methods
Many methods to access aryl C−N bonds have been reported,
and the most reliable ones are Buchwald−Hartwig amination, the
Ullman coupling reaction, and the Chan−Lam coupling
reaction.5 The advent of direct amination using hydroxylamine
or its derivatives as amination reagents by N−O bond cleavage
provides chemists with new powerful tools to create such C−N
bonds, but these N−O bond cleavages usually need the
Received: September 12, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02740
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Organic Letters
Letter
assistance of transition metals.6 Recently, metal-free synthetic
methods to access arylamines have attracted more and more
attention.7 To our knowledge, March and Engenito8 reported
the first noncatalyzed direct amidation of aromatic compounds
via N−O bond cleavage with hydroxamic acid using poly-
(phosphoric acid) (PPA) as a solvent, but this method cannot be
practically useful because of its extremely limited substrate scope,
low yields, and harsh conditions. In 2015, Chen and Wang9
reported a direct amidation method to access aminophenol
compounds with N-hydroxyindolinones, which involved a
regioselective [1,3] arrangement. Very recently, Cheng, Li, and
co-workers reported direct aminations of benzofuran-2(3H)-
ones with hydroxylamine derivatives via a single electron transfer
event in which the two types of reactants act as electron donors
and electron receptors, repectively.10 Despite these achieve-
ments, direct amidation of quinones has never been reported. We
envisioned that the amidoquinone structure could be achieved by
direct amidation of quinones with hydroxamic acid or its
derivatives under specific reaction conditions. With our
continuing interest in quinone-based organic molecules,2,3c,11
we report here a metal-free direct amidation of naphthoquinones
with hydroxamic acids (Scheme 1c) and its application in the
synthesis of a highly potent HDAC6 inhibitor.
CH2Cl2, MeOH, and DMF offered the desired product 3ad in
60%, 21%, 0%, and 72% yield, respectively (entries 3−6). Besides
K2CO3, other types of bases, including both organic and
inorganic bases, were carefully tested for the reaction. Finally,
iPr2NEt was proved to be the most efficient base for the coupling
reaction, affording 3ad in 87% yield (entry 8), while others, such
as Et3N, pyridine, etc., were also effective bases but gave lower
yields (entries 10 and 11). However, the reaction was messy
when the stronger organic base DBU was employed (entry 12). It
was also found that 2 equiv of iPr2NEt (based on 2d) is required
for the best performance of the reaction, and the yield of 3ad
decreased when the loading of iPr2NEt was lowered (entry 9).
The reaction was slightly less productive when performed at
room temperature, giving an 80% yield of the final product (entry
13). There was no difference when the reaction was performed
under argon atmosphere (entry 14), which indicated that oxygen
and moisture do not affect the reaction.
Subsequently, the substrate scope of hydroxamic acids was
explored with the most effective protocol. As shown in Scheme 2,
a
Scheme 2. Substrate Scope of Hydroxamic Acids
Initially, we employed 1,4-naphthoquinone (1a) and N-
hydroxybenzamide (2d) as starting materials to interrogate
different reaction conditions (Table 1). To our great delight, the
a
Table 1. Optimization of the Reaction Conditions
b
entry
solvent
base (equiv)
t (°C)
yield (%)
1
2
1,4-dioxane
1,4-dioxane
THF
K2CO3 (2.0)
−
70
70
70
70
70
70
70
70
70
70
70
70
25
70
68
none
60
3
K2CO3 (2.0)
K2CO3 (2.0)
K2CO3 (2.0)
K2CO3 (2.0)
K2CO3 (2.0)
iPr2NEt (2.0)
iPr2NEt (1.0)
Et3N (2.0)
4
CH2Cl2
MeOH
21
5
none
72
6
DMF
7
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
75
8
87
9
53
a
10
11
12
76
Reactions were performed by warming a mixture of 1a (0.6 mmol), 2
pyridine (2.0)
DBU (2.0)
iPr2NEt (2.0)
iPr2NEt (2.0)
69
(0.5 mmol), and i-Pr2NEt (1.0 mmol) in CH3CN (4 mL) for 12 h.
b
messy
80
The reaction was performed in DMF.
c
13
d
14
86
a
substitution of the hydroxamic acid at different positions of the
phenyl ring did not affect the yield of the final product (3aa−ac).
Electron-withdrawing or electron-donating substituents at the
para position of the phenyl ring of the hydroxamic acid seemed
to have little influence on the reaction, and the desired products
were obtained in good yields (3ae−ah). N-hydroxynaphtha-
mides have very poor solubility in CH3CN, but reactions went
smoothly in DMF and gave slightly lower isolated yields of the
final products (3ai and 3aj). Hydroxamic acids with hetero-
aromatic rings are also suitable substrates, giving the
corresponding products in good yields (3ak−an). Alkyl
hydroxamic acids are more challenging substrates, affording the
desired substrates in moderate yields (3ao−aq). Notably, Boc-
protected hydroxylamine can also be applied under the optimal
reaction conditions, furnishing the product in high yield (3ar).
Reactions were performed by warming a mixture of 1a (0.6 mmol),
2d (0.5 mmol), and the base (1.0 mmol) in the indicated solvent (4
b
c
mL) for 12 h. Isolated yields based on 2d. The reaction was
performed at room temperature. The reaction was performed under
argon.
d
desired product (3ad) was obtained in 68% yield when the
starting materials were treated with K2CO3 at 70 °C in 1,4-
dioxane (entry 1). However, no desired product was found when
1a and 2d were stirred under the same reaction conditions but in
the absence of the base (entry 2). This indicated that the base is
indispensable for the coupling reaction between 1a and 2d.
Following the initial attempt, several other solvents were then
screened for the reaction. CH3CN was found to be the most
suitable solvent, affording 3ad in 75% yield (entry 7), while THF,
B
DOI: 10.1021/acs.orglett.6b02740
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Organic Letters
Letter
To further test the limitations of this reaction, the scope of
various quinones was also investigated (Scheme 3). The results
Scheme 4. Control Experiments
a
Scheme 3. Substrate Scope of Quinones
mechanism based on the results obtained (Scheme 5). The
reaction takes place via initial abstraction of H+ from compound
Scheme 5. Proposed Mechanism
a
Reactions were performed by warming a mixture of 1 (0.6 mmol), 2d
or 2l (0.5 mmol), and i-Pr2NEt (1.0 mmol) in CH3CN (4 mL) for 12
b
h. Pyridine was used as the base.
indicated that naphthoquinones with an electron-deficient group
at the 2-position afford higher yields of the desired products (3cd
and 3dd), while naphthoquinones with an electron-donating
group at the 2-position afford lower yields of the desired products
(3ed−fd and 3il). However, a 1,4-naphthoquinone with a 2-
chlorophenyl substituent at the 2-position gave an extremely low
yield of the corresponding product (3bd), probably because the
steric hindrance hindered the formation of 3bd. A 1,4-
naphthoquinone with an amido substituent at the 2-position
gave a high yield of 3gd. Quinoline-5,8-dione is also a favorable
substrate for this reaction that afforded a 79% yield of the desired
product 3hd. 6-Methyl-1,4-naphthoquinone gave a high yield of
amido-substituted products, but it was a 1:1 mixture of 2- and 3-
substituted products, which is difficult to purify (3jd). However,
when 2-methylnaphthoquinone (1k), 1,4-benzoquinone (1l), or
2,5-dimethyl-1,4-benzoquinone (1m) was subjected to the
standard conditions, no desired amidoquinone product was
detected.13
In order to probe the mechanism of the reaction, we carried
out several control experiments as shown in Scheme 4. The
model reaction proceeded well in the presence of excess 2,2,6,6-
tetramethylpiperidin-1-oxyl (TEMPO), which indicated that the
reaction does not go through a radical pathway (Scheme 4a).
There was no reaction when 1,4- naphthoquinone was treated
with N-methylhydroxamic acid (Scheme 4b). However, the
desired product was obtained in 78% yield when 1,4-
naphthoquinone was treated with O-methylhydroxamic acid
(Scheme 4c). The results imply that the generation of amide
anion is essential for the reaction. We also evaluated the kinetic
isotope effect (KIE) for the reaction, and a small KIE value was
observed (Scheme 4d). This indicated that the reaction does not
involve direct C−H activation. We then proposed a plausible
2d by the base to form intermediate A,12 which then attacks 1a
via nucleophilic addition, affording intermediate B. A molecule of
water is abstracted from B to form intermediate C, and the final
product 3ad is then generated after electron rearrangement.
With great interest in amidoquinone-based HDAC6 inhib-
itors, we had been endeavoring to synthesize compound 9 but
were unsuccessful.13 With our newly developed method of direct
amidation of 1,4-naphthoquinones, however, compound 9 was
obtained in 31% overall yield (Scheme 6). Compounds 1a and 7
were subjected to the optimal reaction conditions to yield
compound 8, and deprotection of 8 in MeOH with a catalytic
Scheme 6. Synthesis of Novel HDAC6 Inhibitor 9 by the
Newly Developed Direct Amidation Reaction of Quinones
C
DOI: 10.1021/acs.orglett.6b02740
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Organic Letters
Letter
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amount of p-TsOH·H2O afforded compound 9. Biological
studies of 9 showed that it has extremely high inhibition potency
against human recombinant HDAC6 enzyme with an IC50 of 6
nM; it is stronger than SAHA (vorinostat), the first FDA-
approved HDAC inhibitor in the clinic, which has an IC50 of 484
nM against HDAC6. Besides, Western immunoblot showed that
9 induces hyperacetylated tubulin because of HDAC6 inhibition.
Upregulation of Hsp70 suggests HDAC6-associated Hsp90
inhibition followed by degradation of the oncogenic protein
FLT-3 and STAT5 in AML cells. In the end, compound 9
exhibited high toxicity toward AML cells with an EC50 of 367 nM,
compared with approximately 1 mM for SAHA, which indicates
promising biological prospects.13
In conclusion, we have developed a new metal-free method to
construct biologically valuable amidoquinone structures by the
direct reaction of quinones with hydroxamic acids under mild
basic conditions, and it unveiled a new N−O bond cleavage
reaction of hydroxamic acids. The newly developed C−N bond
formation reaction is convenient and operationally simple, has a
broad substrate scope, and affords the final products in high yield
(up to 91%). In particular, with this newly developed method, we
successfully synthesized a highly potent HDAC6 inhibitor (IC50
= 6 nM) that is hard to access by usual methods, and the inhibitor
exhibits promising biological activities, such as high toxicity
toward AML cells (EC50 = 367 nM). Further biological studies
are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02740.
Synthesis of quinones 1 and hydroxamic acids 2,
1
experimental procedures for products 3, and H and 13C
NMR spectra for 3 (PDF)
(7) (a) Kim, H. J.; Kim, J.; Cho, S. H.; Chang, S. J. Am. Chem. Soc. 2011,
133, 16382. (b) Kantak, A. A.; Potavathri, S.; Barham, R. A.; Romano, K.
M.; DeBoef, B. J. Am. Chem. Soc. 2011, 133, 19960. (c) Antonchick, A.
P.; Samanta, R.; Kulikov, K.; Lategahn, J. Angew. Chem., Int. Ed. 2011, 50,
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Org. Lett. 2012, 14, 5518. (e) Manna, S.; Serebrennikova, P. O.;
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4588. (f) Xiao, Q.; Tian, L.; Tan, R.; Xia, Y.; Qiu, D.; Zhang, Y.; Wang, J.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zhangc9711@cau.edu.cn, cheunggilbert@gmail.com.
*E-mail: chouc@musc.edu.
Author Contributions
Both authors have given approval to the final version of the
manuscript.
Notes
J. R.; Kurti, L. J. Am. Chem. Soc. 2012, 134, 18253. For a review, please
̈
see: (i) Coeffard, V.; Moreau, X.; Thomassigny, C.; Greck, C. Angew.
Chem., Int. Ed. 2013, 52, 5684.
The authors declare no competing financial interest.
(8) March, J.; Engenito, J. S. J. Org. Chem. 1981, 46, 4304.
(9) Chen, Z.; Wang, Q. Org. Lett. 2015, 17, 6130.
(10) Yang, C.; Liu, Y.; Yang, J.-D.; Li, Y.-H.; Li, X.; Cheng, J.-P. Org.
Lett. 2016, 18, 1036.
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(12) Ventura, O. N.; Rama, J. B.; Turi, L.; Dannenberg, J. J. J. Am.
Chem. Soc. 1993, 115, 5754.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health/
National Cancer Institute (CA163452 to C.J.C.) and the
Department of Drug Discovery and Biomedical Science,
SCCP, Medical University of South Carolina. Support from
the College of Science of China Agricultural University is also
gratefully acknowledged.
(13) For more information, please see the Supporting Information.
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