The regioselective coupling of 2-arylquinazolinone C-H with aldehydes and benzyl alcohols under oxidative conditions

Article abstract of DOI:10.1039/d0nj03721d

The direct and regioselective palladium-catalyzed dehydrogenative coupling of aldehydes and benzyl alcohols with 2-aryl-quinazolinone C-H endowed with a quinazolinone nucleus, which is one of the most fascinating and vital core units, as an inherent directing group under oxidative conditions was developed. This atom/step economic aroylation offers an attractive handle and a very green approach for highly convergent and diversity-oriented synthetic modifications and the construction of vital molecules of therapeutic interest. This journal is

Full text of DOI:10.1039/d0nj03721d

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Regioselective Coupling of 2-Arylquinazolinone C-H with Aldehyde
and Benzyl Alcohol under Oxidative Condition
Received 00th January 20xx,
Accepted 00th January 20xx
Umesh A. Kshirsagar,*a,b Deepali S. Waghmare,b Shrikant D. Tambeb
DOI: 10.1039/x0xx00000x
A direct and regioselective palladium catalyzed dehydrogenative coupling of aldehydes and benzyl alcohols with 2-aryl-
quinazolinones C-H endowed with quinazolinone nucleus, which is one of the most fascinating and vital core units as an
inherent directing group under oxidative condition was developed. This atom/step economic aroylation offers an
attractive handle and very green approach for highly convergent and diversity-oriented synthetic modifications and
construction of vital molecules of therapeutic interest.
The Synthetic manipulations that create new carbon-carbon
bonds are among the most important transformations in
organic chemistry, as they enable the design of artificial
chemical structures essential to modern life. Cross coupling
reactions catalyzed by transition metals have been developed
and improved to a level that almost any structure can be
prepared.1-3 However, requirement of pre-functionalization of
substrates, tedious purification stages and ultimately reduced
overall material/atom and step economy in synthesis and
chemical waste are associated with it. Current challenge is not
only to develop new synthetic tools for performing a given
transformation, but rather to perform it in a practical,
environmental friendly, atom- and step-economical way. In
past two decades, direct transformation of carbon-hydrogen
(C–H) bond i.e. C–H activation have attracted much attention4
and excellent achievements, and widespread applications have
been made in the synthesis of pharmaceuticals, natural
products, and agrochemicals. However, C–H bond activation
still requires at least one functionalized partner to generate
desired C–C bond. In this context, direct coupling of two C-H
bonds under oxidative condition such as dehydrogenative
coupling5 pioneered by Murahashi5p and Li5o to form a new
carbon-carbon bond is highly desirable as it address most of
the above challenges.6,7 Quinazolinone, one of the most
fascinating and vital nucleus found in biologically active and
medicinally important natural products and drug candidates,
consequently received an intensive focus by synthetic and
medicinal chemists (Figure 1).8,9 Several marketed drugs and
intermediates with range of biological and pharmacological
activities containing quinazolinone as privileged core are
known and making their synthetic modifications tempting. In
addition, quinazolinone substrates are endowed with inherent
Figure 1. Selected bioactive quinazolinone drug and natural products.
directing group which can stimulate site selective C–H
activation. Mhaske et. al disclosed quinazolinone directed C-H
activation for monoarylation using diaryliodonium salts in
presence of Pd-catalyst and Na2CO3 (Scheme 1, eq. a).10 Back
to back this, Hong and Park co-workers reported Pd-catalyzed
and ligand assisted annulation of unmasked 2-arylquinazolin-
4(3H)-ones with diaryliodonium salts via C-arylation followed
by intramolecular N-arylation in presence of silver salt and
base.11 Later, Ru-catalyzed alkenylation of 2-aryl-quinazolin-
a. Discipline of Chemistry, Indian Institute of Technology Indore, Indore, INDIA.
b. Department of Chemistry, Savitribai Phule Pune University, Pune, INDIA.
Electronic Supplementary Information (ESI) available: Characterization data, NMR
spectra and X-ray Crystallographic data (CIF file). See DOI: 10.1039/x0xx00000x
Scheme 1. Previous work and this work.
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4(3H)-ones by coupling terminal alkynes as alkene partner column chromatography (table 1, entry 2). WhenVrieewacArttiioclenOwnlianse
(Scheme 1, eq. b)12 and Ir-catalyzed alkynylation of 2-aryl- performed using PdCl2 as catalyst,DOrI:e1s0u.1lt0s39/Dw0eNrJe037n21oDt
quinazolin-4(3H)-ones with TIPS-EBX as alkyne source (Scheme encouraging (table 1, entry 3). In order to optimize the
1, eq. b)13 have been reported. On the other hand, heck type reaction condition for better yield, various solvent, oxidizing
coupling of quinazolinone, halogenation, amination, agent were pursued. It was decided to perform the reaction in
annulations and recently, oxidative annulations with aryl other solvent using Pd(OAc)2 catalyst. Various solvents were
aldehyde and C-H sulfonylation as well as Rh-catalyzed screened (table 1, entries 4-8) but only ACN, and
annulation of quinazolinones with sulfoxonium ylidesare also chlorobenzene found to give desired product in moderate
reported.14 Owing to the privileged biological and yield (table 1, entry 6 & 8). Next, it was planned to screen
pharmacological importance of quinazolinone derivatives, and combination of these solvents (table 1, entries 9-11) and found
demand of direct coupling of two C-H bonds from atom/step- that acetonitrile:chlorobenzene (1:4) combination produced
economy, convergent and diversity oriented synthesis point of the product in good yield (entry 11). Replacement of oxidant
view, herein, we have disclosed the straightforward and regio with DTBP, K2S2O8, (NH4)2S2O8 (entries 12-14) as well as
selective coupling of 2-aryl-quinazolinones C-H having performing the reaction in presence of some additives such as
quinazolinone as intrinsic directing group with aldehydes and PPh3, NaOAc, Na2CO3 did not turned on any expectations
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benzyl alcohols under oxidative condition (Scheme 1, eq. c).
(entries 15-17). Decrease in the yield was observed by
Our investigation commenced with the reaction of 2-phenyl- lowering the equivalents of oxidant (entry 18 & 19) and no
quinazolinone (1a) with benzaldehyde (2a) in the presence of substantial increase in yield was received after increasing
o
Pd(OAc)2 (5 mol%) and TBHP (3.0 equiv.) in DCE at 80 C. After oxidant equivalent (entry 20).
36 h, we observed no recognizable change in the reaction Having the optimized reaction condition, we investigated the
mixture (table 1, entry 1). Hence, reaction was performed at reaction of several 2-aryl-quinazolinones (1b-k) with
120 oC in sealed tube. After heating the reaction in sealed tube
for 36 h, product 3a was isolated in 25% yield using silica-gel
Table 1. Optimization of the reaction conditions for Pd-catalyzed CDC reaction.a
entry
1
2
catalyst
Pd(OAC)2
Pd(OAC)2
PdCl2
solvent
DCEc
DCE
Additive
yieldb
trace
25
-
-
3
DCE
15
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18g
19h
20i
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
Pd(OAC)2
THF
dioxane
ACN
DMSO
-
-
-
-
-
-
-
-
16
29
43
nr
45
60
41
70
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31
36
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PhCl
ACN:PhCl (1:1)
ACN:PhCl (4:1)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
ACN:PhCl (1:4)
DTBPd
K2S2O8
(NH4)2S2O8
PPh3
NaOAcf
Na2CO3
d
d
e
f
-
-
-
72
aConditions: 1a (1.0 mmol, 1.0 equiv.), 2a (2.0 equiv.), catalyst (10 mol%), oxidant
c
(3 equiv.), solvent (3.0 mL), 120 oC, 36 h. bisolated yields. refluxed at 80 oC. das
oxidant (3.0 equiv.) instead of TBHP. eas additive (20 mol%). fas additive (3.0
equiv.). g1.0 equiv. of TBHP. h2.0 equiv. of TBHP. i4.0 equiv. of TBHP.
Scheme 2. Scope of 2-aryl-quinazolinones.
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substitution on 2-aryl ring as well as with N-methyl, N-ethyl
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DOI: 10.1039/D0NJ03721D
and N-methoxy quinazolinones in order to assess the scope of
the optimized cross-dehydrogenative coupling reaction
(Scheme 2). Consequently, 2-aryl-quinazolinones bearing
electron attracting groups such as -Cl, -Br, -F, -CF3 (1b-f) with
N-methyl and N-ethyl group afforded the corresponding
desired products in moderate yields (46-57%). N-Methoxy-2-
Aryl-quinazolinone also produced desired product (3k) in 59%
yield. Under similar conditions, 2-aryl-quinazolinones holding
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electron releasing groups such as alkyl and alkoxy groups (1g-j
)
reacted effectively with benzaldehyde to providing
corresponding desired acylated products (3g-j) in good yields
(68-81%). Interestingly, with unsymmetrical aryl ring, acylation
occurs selectively at less hindered side only (3f and 3j). Next,
the effect of substitution on aryl ring of quinazolinone core
structure was demonstrated (Scheme 3). Quinazolinone having
-fluoro and -chloro group as well as methyl group produced
the desired products (3l, 3m, 3n and 3o) in good to moderate
yield (51, 56, 59 and 61% respectively), whereas
quinazolinones having methoxy group delivered the product
Scheme 4. Oxidative coupling of benzyl alcohols with quinazolinones.
(
3v and 3w) in moderate yield (50 and 54% respectively).
Other than substituted benzaldehydes, 2-furylaldehyde and 2-
thienylaldehyde also reacted to affording corresponding
product (3z and 3aa) in moderate yield (41 and 60%
respectively).
(
3p and 3q) in 63 and 68% yield respectively.
Further, the scope of aryl aldehyde was investigated (Scheme
3). Treatment of aryl-aldehydes having electron attracting and
releasing groups proceeded to afford desired product (3r-y) in
good to moderate yields (50-65%). Arylaldehydes having
hydroxy group (-OH) at 2 or 4 position gave expected product
Considering that the C-H coupling of benzyl alcohol under
oxidative condition,15 the behaviour of our substrate under
similar condition with benzyl alcohol was planned (Scheme 4).
When 2-phenyl-quinazolinone (1a) was allowed to react with
benzyl alcohol (4a) under our optimized condition, desired
acylated product 3a was observed in low yield (35%). Reaction
was also performed in other solvents such as DCE, THF, DMSO,
and toluene but no significant improvement in yield was
observed. Next, we conducted reaction in trifluorotoluene as
solvent and 4.0 equivalent of TBHP. To our delight reaction
went smoothly to give desired product 3a in 61% yield. Some
more benzyl alcohols were treated under similar condition
with 2-phenyl-quinazolinone
(
1a
) to give corresponding
desired product (3u 3s 3ab
,
,
, 3c) in good to moderate yields
(Scheme 4). When our optimized condition was tested on NH-
quinazolinone (2-phenylquinazolin-4(3H)-one), we observed
complex reaction mixture (multiple spots).
In order to trace the mechanistic path of our developed
reactions, some control experiments were planned (Scheme
5). We performed the radical trapping experiment during the
reaction of 2-aryl-quinazolinone 1a with benzaldehyde 2a in
the presence of varied equivalents of classical radical
scavengers 3,5-di-tert-butyl-4-hydroxytoluene (BHT) and
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). Observations
of these experiments revealed the rapid decrease in yield of
product 3a in the presence of 2−4 equivalents of the radical
scavengers individually to the reaction under standard
condition. Expected radical trapped adduct product
(5)
generated by interaction of acyl radical with TEMPO was
1
isolated and characterized using H, 13C and HRMS data. When
reaction was performed with benzyl alcohol in presence of
TEMPO (4.0 equiv.), TEMPO trapped adduct was formed and
no desired product formation was observed. When reaction
was performed in absence of TBHP, with 10 mol% as well as
1.0 equivalent of palladium catalyst, no product formation was
Scheme 3. Scope of aryl-aldehydes and quinazolinones.
This journal is © The Royal Society of Chemistry 20xx
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developed. Looking the assurance of functionalization of
ketone group and the medicinal and pharmacological
significance of the quinazolinone motif, atom and step
efficacy, attractive substrate scope and good yields makes this
direct aroylation protocol more enticing and prominent
alternative for synthetic modifications and drug design.
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DOI: 10.1039/D0NJ03721D
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Conflicts of interest
“There are no conflicts to declare”.
Acknowledgements
We thank Savitribai Phule Pune University for providing the
infrastructure and research facility. D.S.W. thanks CSIR-New
Delhi for research fellowship.
Scheme 5. Studies for tracing mechanistic path.
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Conclusions
In
conclusion,
palladium
catalyzed
regioselective
dehydrogenative coupling of 2-arylquinazoline-4-one C–H with
aldehyde and oxidative coupling with benzyl alcohol has been
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