Ortho -Naphthoquinone-catalyzed aerobic oxidation of amines to fused pyrimidin-4(3 H)-ones: A convergent synthetic route to bouchardatine and sildenafil

Article abstract of DOI:10.1039/d0ra06820a

A facile access to fused pyrimidin-4(3H)-one derivatives has been established by using the metal-free ortho-naphthoquinone-catalyzed aerobic cross-coupling reactions of amines. The utilization of two readily available amines allowed a direct coupling strategy to quinazolinone natural product, bouchardatine, as well as sildenafil (Viagra) in a highly convergent manner. This journal is

Full text of DOI:10.1039/d0ra06820a

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ortho-Naphthoquinone-catalyzed aerobic
oxidation of amines to fused pyrimidin-4(3H)-ones:
a convergent synthetic route to bouchardatine and
sildenafil†
Cite this: RSC Adv., 2020, 10, 31101
*
Kyeongha Kim, Hun Young Kim
and Kyungsoo Oh
Received 7th August 2020
Accepted 14th August 2020
A facile access to fused pyrimidin-4(3H)-one derivatives has been established by using the metal-free
ortho-naphthoquinone-catalyzed aerobic cross-coupling reactions of amines. The utilization of two
DOI: 10.1039/d0ra06820a
readily available amines allowed
a direct coupling strategy to quinazolinone natural product,
rsc.li/rsc-advances
bouchardatine, as well as sildenafil (Viagra™) in a highly convergent manner.
N-Heterocyclic compounds with a pyrimidin-4(3H)-one core yields. The Nguyen group also developed the FeCl3$6H2O-
constitute a large number of natural products and biologically catalyzed condensation of 2-nitroanilines and benzylamines
active molecules. For example, quinazolinone alkaloids possess in the presence of 20 mol% of S8, where six quinazolinones were
a phenyl-fused pyrimidin-4(3H)-one structure and display obtained in 68–75% yields.34 While the cross condensation of
a wide spectrum of pharmacological activities (Scheme 1a).1 anthranilamides and benzylamines was accomplished, there
Sildenal (Viagra™), a potent and selective inhibitor of type 5 exists a signicant knowledge gap due to the limited substrate
phosphodiesterases on smooth muscle cell, is based on the scope combined with less optimal reaction conditions (i.e. high
pyrazole-fused pyrimidin-4(3H)-one structure and marketed for
erectile dysfunction.2 The synthetic approaches to the phenyl-
fused pyrimidin-4(3H)-ones, quinazolinones, typically involve
the condensation between anthranilamides and aldehydes to
give aminal intermediates that in turn oxidized to quinazoli-
nones under oxidation conditions (Scheme 1b). The oxidation
catalysts include Cu,3 Fe,4 Ga,5 Ir,6 Mn,7 iodine,8 peroxide,9
however the aerobic oxidation of aminal intermediates is also
known at 150 C.10 The utilization of alcohols also effects the
ꢀ
one-pot synthesis of quinazolinones through in situ oxidation to
aldehydes in the presence of Fe,11 Ir,12 Mn,13 Ni,14 Pd,15 Ru,16 V,17
Zn,18 and iodine catalysts.19 Other precursors to aldehydes have
been also identied using alkynes,20 benzoic acids,21 indoles,22
a-keto acid salts,23 b-keto esters,24 styrenes,25 sulfoxides,26 and
toluenes.27 Non-aldehyde approaches to quinazolinones have
been also demonstrated in the cross coupling of amidines,28
amines,29 benzamides,30 isocyanides,31 and nitriles.32 In 2013,
the Nguyen group disclosed the synthesis of four quinazoli-
nones, utilizing the autooxidation of benzylamines to imines
that subsequently condensed with anthranilamides.33 While
ꢀ
a closed system at 150 C was necessary, the use of 40 mol%
AcOH without solvent provided the quinazolinones in 46–75%
Center for Metareceptome Research, Graduate School of Pharmaceutical Sciences,
Chung-Ang University, 84 Heukseok-ro, Dongjak, Seoul 06974, Republic of Korea.
E-mail: kyungsoooh@cau.ac.kr
Scheme 1 Biologically active fused pyrimidin-4(3H)-one derivatives
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06820a
and their synthetic methods.
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reaction temperature, closed system in neat conditions and the corresponding homocoupled imine 2a0 under aerobic
excess use of amines). In addition, it is not entirely clear if the conditions. However, the subsequent in situ condensation
cross condensation of amide-containing amines and benzyl- reaction of 2a0 with anthranilamide 1a only provided the aminal
amines would work for other fused pyrimidin-4(3H)-one deriv- product 3a in 11% yield (entry 1). To facilitate the cross coupling
atives.35 To address such shortcomings in the cross between the amine 2a and anthranilamide 1a, a catalytic
condensation of amines, the ortho-naphthoquinone (o-NQ)- amount of TFA was utilized where a signicant improvement in
catalyzed aerobic cross amination strategy was investigated yield was observed for 3a (entry 2). Faced with the inability to
(Scheme 1c).36 Herein, we report a highly general approach to oxidize the aminal 3a to the corresponding product, quinazo-
fused pyrimidin-4(3H)-one derivatives in the presence of o-NQ linone 4a, other ortho-naphthoquinone catalysts were screened
catalyst, culminating to the direct aerobic coupling of two without much success (entries 3–5). To our delight, the exami-
amines to bouchardatine and sildenal.
nation of solvents revealed that the reaction temperature of
Given that the o-NQ-catalyzed aerobic cross coupling of 100 ꢀC was needed for the formation of 4a (entries 6–11).38 The
benzylamines and aniline derivatives such as o-phenylenedi- reaction in DMSO lowered down the ratio of 1a and 2a from
amines provided a facile approach to heterocyclic compounds 1.0 : 1.5 to 1.0 : 1.2 (entry 12) and the catalyst loading to 5 mol%
including benzoimidazoles,37 the use of anthranilamide 1a and without affecting the overall reaction efficiency (entry 13). The
benzylamine 2a was examined as a model study (Table 1). The control experiments conrmed the critical roles of both o-NQ1
catalytic use of o-NQ1 smoothly converted benzylamine 2a to and TFA (entries 15–18), and the reaction utilized molecular
oxygen as a terminal oxidant (entries 19 and 20). Piecing
together the experimental data, the employment of 5 mol% o-
NQ1 and 20 mol% TFA in DMSO at 100 ꢀC was selected for
further studies.
Table 1 Optimization of o-NQ-catalyzed aerobic cross-coupling of
amines to quinazolinonea
The optimized aerobic cross-coupling condition was applied
to a variety of benzylamine derivatives (Scheme 2). In general,
the electronic and steric characters of benzylamines did not
signicantly affect the formation of quinazolinones (4a–4m).
However, the use of halogen-substituted and dimethoxy-
substituted benzylamines led to the slightly lower yields of
quinazolinones (4e, 4f and 4m) in 58–75% yields. In addition,
the current aerobic cross-coupling reaction tolerated the furanyl
and thiophenyl moieties, where the corresponding quinazoli-
nones (4o and 4p) were obtained in 54% and 75% yields,
respectively.
Entry
Cat. (mol%)
Solvent
T (ꢀC)
Yieldb (%)
1
2
3
4
5
6
7
8
o-NQ1 (10)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
MeOH
EtOH
80
80
80
80
80
65
78
150
150
100
80
100
100
100
100
100
100
100
100
100
3a, 11
3a, 83
3a, 13
3a, 66
3a, 32
3a, 5
3a, 31
4a, >95
4a, >95
4a, >95
3a, 30
4a, >95
4a, >95 (93)
3a/4a, 45/50
3a, 10
3a/4a, 34/51
NR
3a, 25
3a, 33
NR
o-NQ1 (10)/TFA (20)
o-NQ2 (10)/TFA (20)
o-NQ3 (10)/TFA (20)
o-NQ4 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (10)/TFA (20)
o-NQ1 (5)/TFA (20)
o-NQ1 (5)/TFA (10)
—
DMF
9c
10
11
12d
13d
14
15
16
17
18
19e
20f
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
o-NQ1 (10)
TFA (20)
o-NQ1 (5)/AcOH (20)
o-NQ1 (5)/TFA (20)
o-NQ1 (5)/TFA (20)
a
Reaction using 1a (0.20 mmol), 2a (0.30 mmol), and o-NQ in solvent
b
(0.2 M) under O2 balloon for 24 h. Yields based on internal standard
and isolated yield in parentheses. Reaction for 6 h. Use of 2a
(0.24 mmol, 1.2 equiv.). Reaction under air. Reaction under argon.
c
d
e
f
Scheme 2 Substrate scope of aerobic oxidation to quinazolinones
NR ¼ no reaction.
(areaction for 36 h, breaction for 12 h).
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Further extension of the current aerobic cross-coupling
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reactions of amines is illustrated in Scheme 3. Thus, an array
of substituted anthranilamides was readily employed to give the
fused pyrimidin-4(3H)-one derivatives (4q–4x) in 61–84% yields.
In particular, the N-substituted anthranilamides also partici-
pated in the current aerobic cross-coupling reaction in excellent
yields (4y–4za). While the use of 3-amino-2-naphthamide led to
the corresponding quinazolinone 4zb in 46% yield, the
synthetic advantage of the current method was well demon-
strated in the preparation of heteroaryl fused pyrimidin-4(3H)-
one derivatives (4zc–4zh), where a variety of heterocyclic amines
were successfully utilized in a tandem sequence of aerobic
oxidation processes.
The mechanistic rationale of the aerobic cross-coupling
reactions of amines is depicted in Scheme 4. Thus, the benzyl-
amine 2a is condensed with the o-NQ1 catalyst to give the
naphthol-imine species A.37a While the nucleophilic attack of 2a
to the naphthol-imine A is favored due to the low nucleophi-
licity of the anthranilamide 1a, the presence of TFA promotes
the cross-coupling between naphthol-imine A and anthranila-
mide 1a to give the naphthol-aminal B1. This process releases
the hetero-coupled imine 3a0 and naphthol-amine C. The use of
TFA promotes the intramolecular Mannich cyclization of imine
3a0, leading to the aminal 3a that in turn converts to the desired
fused pyrimidin-4(3H)-one 4a with the help of o-NQ1 catalyst
and molecular oxygen. Alternatively, the naphthol-imine A can
produce the homocoupled imine 2a0 and the naphthol-amine C
via the naphthol-aminal B2 through the nucleophilic attack of
benzylamine 2a. The conversion of the naphthol-amine C to o-
NQ1 catalyst is effected upon exposure to oxygen atmosphere.38
Scheme 4 Mechanistic rationale for aerobic cross coupling reaction
of amines.
reaction supports the involvement of 2a0. However, the major
pathway to the fused pyrimidin-4(3H)-one 4a appears to involve
the naphthol-aminal B1 since the use of benzaldehyde instead
of benzylamine 2a under the optimized conditions only led to
the 80% conversion. The o-NQ1 catalyst without added TFA
provided a mixture of aminal 3a and quinazolinone 4a in 34%
and 51% yields, respectively (Table 1, entry 16). Thus, it is likely
that the role of TFA is the catalyst for the cross-coupling of two
amines to give the naphthol-aminal B1 and the cyclization of
the imine 3a0 to aminal 3a. Our control experiments also
revealed that TFA alone slowly oxidize 3a to 4a, but rapidly
oxidized by the action of o-NQ1 within 10 h.39
ꢀ
The homocoupled imine 2a0 undergoes hydrolysis at >80 C to
the benzaldehyde and benzylamine 2a that in turn re-enters the
catalytic cycle.37b Our experimental observation of the homo-
coupled imine 2a0 by the 1H NMR and TLC analysis during the
The synthetic utility of the aerobic cross-coupling strategy is
demonstrated in the synthesis of quinazolinone alkaloids and
sildenal (Scheme 5). The direct cross coupling of a commer-
cially available pyrazole amine 1o and benzylamine 2r afforded
Scheme 3 Further substrate scope for fused pyrimidin-4(3H)-one Scheme
5
Synthetic utilization to quinazolinone alkaloids and
b
derivatives (areaction at 120 ꢀC, reaction at 140 ꢀC).
sildenafil.
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a highly convergent synthetic approach to sildenal.40 Likewise,
the employment of anthranilamide 1a and 2-(aminomethyl)
indole 2s provided the desired quinazolinone 5 in 70% yield,
and the subsequent formylation under the Zeng's conditions41
paved a way to the total synthesis of bouchardatine. In addition,
while the basicity of quinolin-2-ylmethanamine 2t required an
excess of TFA, the corresponding quinazolinone 6 was obtained
in 60% yield under the optimized conditions. The conversion of
6 to the luotonin natural products has been reported by the
Argade group and others.42
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Conflicts of interest
There are no conicts to declare.
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