Q-Tube assisted MCRs for the synthesis of 2,3-dihydroquinazolin-4(1H)-ones

Article abstract of DOI:10.24820/ark.5550190.p010.506

A Q-tube assisted, efficient method for the preparation of 2-phenylquinazolin-4-ones is here presented. The target structures were prepared through the one-pot, multicomponent reaction between isatoic anhydride, an aromatic aldehyde and a primary amine following a catalyst-free approach. The use of the commercially available Q-tube apparatus allowed to perform reactions at external temperature higher to the solvent boiling point generating medium pressure conditions, shortening the reaction times and affording good yields in all the example herein reported. Formula parented.

Full text of DOI:10.24820/ark.5550190.p010.506

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Arkivoc 2018, part iii, 270‐278
Q‐Tube © assisted MCRs for the synthesis of 2,3‐dihydroquinazolin‐4(1H)‐ones
Luca Sancineto,a Bonifacio Monti,b Orsola Merlino,b Ornelio Rosati,b and Claudio Santib*
aCentre of Molecular and Macromolecular Studies, Polish Academy of Science,
Sienkiewicza 112, 90‐363 Łódź Poland
bDepartment of Pharmaceutical Sciences, University of Perugia, via del Liceo 1, 06100 Perugia Italy
Email: claudio.santi@unipg.it
Received 01‐29‐2018
Accepted 03‐15‐2018
Published on line 04‐29‐2018
Abstract
A Q‐tube assisted, efficient method for the preparation of 2‐phenylquinazolin‐4‐ones is here presented. The
target structures were prepared through the one‐pot, multicomponent reaction between isatoic anhydride, an
aromatic aldehyde and a primary amine following a catalyst‐free approach. The use of the commercially
available Q‐tube© apparatus allowed to perform reactions at external temperature higher to the solvent
boiling point generating medium pressure conditions, shortening the reaction times and affording good yields
in all the example herein reported.
Keywords: 2,3‐Dihydroquinazolin‐4(1H)‐ones, 2‐phenylquinazolin‐4‐ones, Q‐tube, isatoic anhydride,
chemoselectivity
DOI: https://doi.org/10.24820/ark.5550190.p010.506
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Introduction
Quinazolinone scaffolds, either in the reduced (compound I, 2,3‐dihydroquinazolin‐4(1H)‐one) or in the
oxidized (compound II, quinazolin‐4(3H)‐one) form, are widely distributed in biologically relevant molecules
(Figure 1).
Figure 1. Drugs and pharmacologically relevant molecules characterized by the quinazoline scaffold.
The quinazolinone scaffold characterizes the marketed diuretic drugs Metazolone, Quinetazone and
Fenquizone that are currently used to treat hypertension and related diseases or congestive heart failure.1 In
the antimicrobial research field, some anti leishmanial2 and antiviral derivatives3 have been developed very
recently, the latters through a fragment based drug discovery approach.4 Methaqualone, marketed using the
brand name of Quaalude is a sedative and hypnotic medication widely used in US also, regrettably, as
substance of abuse.5 The synthesis of the first in class, anti‐hypertensive drug Prazosin and its congeners
entails, as key step, the preparation of the quinazoline scaffold which is then converted into the 4‐amino
quinazoline core by a tree‐step procedure.6
As a result of the pharmacological importance, a plethora of synthetic methodologies for the preparation
of these privileged scaffolds have been reported and are summarized in Scheme 1.7‐9
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Scheme 1. General procedure for the preparation of Quinazolinones.
All the reported procedures first entailed the preparation of the reduced quinazolinone I which is then
converted into the corresponding oxidized form II. In many cases it is impossible to prevent the oxidation
reaction thus the compounds having the C2 with a sp3 hybridization represent per se a synthetic target. The
literature survey revealed that the synthetic methodologies could be clustered into 4 groups. In the first one 2‐
aminobenzamide is cyclo‐condensed with aromatic and aliphatic aldehydes,10 or rarely with substituted benzyl
alcohols11 or gem‐dibromomethylarenes.12 Recently, also 2‐aminobenzonitrile was used as a starting point in a
green procedure for the preparation of quinazolinones. The reactions took place in presence of aromatic
aldehydes in water with K3PO4 as inorganic base leading to the preparation of various products.13
Among the MCRs, in 2014 Guo and co‐workers reported the synthesis of the target compounds I starting
from 2‐bromobenzamide, which reacted with aldehydes and aqueous ammonia in DMSO for 24 h. In principle,
this approach is suited for the modification of the N‐3 position by replacing ammonium hydroxide with a
different primary amine, nevertheless the authors reported just the use of the inorganic amine to selectively
obtain compounds II.14 Isatoic anhydride is the elective starting material for a multicomponent approach
because it gives the possibility to introduce 3 points of diversity in the desired final structures (R1, R2 and R3 in
Scheme 2).
Scheme 2. Synthesis of quinazolinone starting from isatoic anhydride.
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The commonly used methodology entails the treatment of isatoic anhydride with an aldehyde, a primary
amine or an ammonium salt under basic conditions. In one case the Lewis acid gallium triflate was used to
catalyse the reaction.15 As previously stated, the selective synthesis of type I compounds is not easy and often
the reaction mixtures contain, beside the desired products also appreciable amounts of the oxidized
derivatives II. For this reason, in most of the reported procedures a mild oxidant, such as iodine or DMSO, is
used to force the reaction directly to the oxidated product II without even attempting to isolate the reduced
intermediate I.
Here we report the Q‐Tube mediated, one pot, multicomponent procedure for the selective preparation of
compounds I, under mild conditions. The protocol is straightforward and affords the quantitative conversion
of the starting material into the target products. The use of the Q‐Tube technology permits to exceed the
solvent boiling point thus shortening the reaction times.16
Result and Discussion
We started the investigations exploring the best combination solvent/temperature for the reaction between
isatoic anhydride 1, benzaldehyde 3a and ammonium acetate 2 in a ratio 1:1:1.5 in the presence of a catalytic
amount of the Lewis acid niobium oxide, as prosecution of our interest in exploring the reactivity of this
scarcely studied catalyst.
Table 1
Conv.
Entry
Solvent
T (°C)
1:2:3a
Cat (mol %)
4a : 5a Time (h)
(Yield) %a
‐‐
1
2
3
4
5
6
7
8
9
EtOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
H2O
H2O
toluene
toluene
reflux
25 ))))
50
50 ))))
100
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.2:1
Nb2O5 (10%)
Nb2O5 (10%)
Nb2O5 (10%)
Nb2O5 (10%)
Nb2O5 (10%)
Nb2O5 (10%)
‐‐
Nb2O5 (10%)
‐‐
Nb2O5 (10%)
‐‐
PFPAT (10%)d
2.5
2.5
2.5
‐‐
traces
100 (79)
100 (89)
100 (91)
100 (86)
‐‐
1 : 99
1 : 99
23 : 77
30 : 70
2.5
2.5
1.5
1.5
1.5
1.5
1.5
1.5
3
170
170
170 (Q‐tube) c
170 (Q‐tube)c
170 (Q‐tube)c
170 (Q‐tube)c
reflux
‐‐
‐‐
10
11
12 b
100 (90)
90
8 : 92
aConversions were determined by 1H‐NMR based on the consumption of the aldehyde. Yields are referred to
the amount of 4b+5b recovered as pure mixture after workup without further purification b see reference 15. c
Temperature of the oil bath.d PFPAT= Pentafluorophenylammonium Triflate.
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The reaction in refluxing EtOH (entry 1) as well as in DMSO at room temperature under sonication (entry
2) did not produce the desired product. Nevertheless, the reactivity in DMSO resulted nicely controlled by
temperature since at 50°C, 100°C, and 170°C the formation of 4a:5a was obtained in traces, 89% and 91%,
respectively (entries 3, 5 and 6) allowing, in the latter case, to reduce the reaction time at 1.5 h. The
chemoselectivity resulted to be dependent on the temperature, as well. The almost complete formation of 5a
was observed at 100°C whereas at 170° the oxidized form 4a was detected in a 4a/5a ratio of 23:77. We also
investigated the possibility to use ultrasounds for the activation of the reaction at lower temperature without
appreciable improvements (entry 4). We proved that at 170°C the catalysis of niobium oxide is minimal,
discouraging its use for a better atom economy of the entire process (see entry 6 vs entry 7). Several different
solvents were also tested and, at the same time, a Q‐Tube© device was used to perform reactions at external
temperatures higher than those of the solvents boiling point and under medium pressure conditions. DMF and
water did not afford desired product whereas using toluene in Q‐Tube© in an oil bath at 170°C 2‐
phenylquinazolin‐4‐one 5a was obtained in 90% yield (entry 11). In toluene, Q‐tube© allowed to avoid the use
of catalysts, reducing reaction time, yielding the target compound in 1.5 hours rather than 3 hours, as
reported in literature for refluxing conditions (entry 11 and 12).17 In addition the solvent can be recovered by
1
distillation in its pure form (confirmed by H‐NMR analysis) and reused for subsequent reactions, reducing
waste production and the issues connected with the toxicity and environmental risks.
Scheme 3. General reaction setup.
A similar comparison of the different reaction conditions has been performed starting from p‐
chlorobenzaldehyde 3b and the results are summarized in Table 2.
In this case, the catalytic effect of Nb2O5 is slightly evident in DMSO at 170 °C (entry 1 vs entry 2). The use
of the Q‐tube © continues to be superior affording in 1.5 hours 5b in 98 % yield without catalyst (Table 2,
entry 4) affording better results in comparison with the refluxing conditions in the presence of
pentafluorophenylammonium triflate as catalyst.
Having elected as the best conditions those depicted in Table 1 entry 12, we explored the scope of the
reaction. As reported in Table 3 entries 1‐6, aldehydes bearing both electron donating and withdrawing groups
were tested indicating that the nature of substituent did not influence the yield that, in all the cases, are very
good. On the contrary, the chemoselectivity appear to be influenced by the electronic properties of
substituents: the electron‐withdrawing effect of halogens facilitates the exclusive formation of compound 5b,
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5e while the methoxy group favours the oxidation reaction leading to a mixture of 4c:5c in both toluene and
DMSO (Table 3, entry 5 and 6).
Table 2
4b:5b
Entry
Solvent
T(°C)
1:2:3b
Cat (mol %)
Nb2O5 (10%)
Yield (%)a
Time (h)
1
2
3
4
5b
DMSO
DMSO
THF
Toluene
Toluene
170
170
170 (Q‐tube) e
170 (Q‐Tube)e
reflux
1:1.5:1
1:1.5:1
1:1.5:1
1:1.5:1
1:1.2:1
91
78
73
98
95
25 : 75
23 :77
1 : 99
1 : 99
n.r.d
1.5
1.5
1.5
1.5
3
‐‐
‐‐
‐‐
PFPAT (10%)c
1
aConversions, determined by H‐NMR based on the consumption of the aldehyde, were in all the cases
quantitative. Yields are referred to the amount of 4b+5b recovered as pure mixture after workup without
further purification purification bReference 15 c PFPAT: Pentafluorophenylammonium Triflate. d Not reported. e
Temperature of the oil bath.
Table 3
Entry
R
R’
yield (%)a
4 : 5
1
2
H
Cl
H
H
90
98
8 : 92
1 : 99
3
4
5
6c
7
F
H
H
H
H
96
1 : 99
NO2
OMe
OMe
Cl
96b
82
1 : 99b
30 : 70
20 : 80
0 : 100d
88
52e
nBu
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1
aConversions, determined by H‐NMR based on the consumption of the aldehyde, were in all the cases
quantitative. Yields are referred to the amount of 4b+5b recovered as pure mixture after workup without
b
further purification excepted where otherwise indicated Some other impurities (<10%) are present and
c
cannot be removed due to the instability of 5d. Reaction was performed in DMSO at 170°C and 10 mol%
Nb2O5 was used. d 40 % of the amide 9 was recovered. e Isolated yield by chromatography
In the case of the nitro aldehyde 3d the corresponding 5d was obtained almost quantitatively as proved by
1
the H‐NMR analysis of the crude, but it was impossible to isolate in pure form due to its instability. In one
example, ammonium acetate was replaced with n‐butylamine 6, giving the target compound 8 in moderate
yield but in mixture with the butyl‐substituted anthranilamide 9. The isolation of this intermediate gives some
information about a plausible reaction mechanism and can be supposed that, in this case, after isatoic
anhydride decomposition and the formation of 9, the butyl substituent exerts a steric hindrance that
disfavours the successive condensation reaction with the aldehyde.
Conclusions
In conclusion we described the possibility to use the Q‐tube © apparatus to promote the chemoselective
synthesis of 2,3‐dihydroquinazolin‐4(1H)‐ones skeletons starting from isatoic anhydride without any catalyst
or additive. The organic solvent used for the synthesis can be recovered and reused reducing the
environmental issues correlated to the use of toluene.
Experimental Section
General. Reactions were conducted in Q‐tube® and were stirred with Teflon‐coated magnetic stirring bar.
Solvents and reagents were used as received unless otherwise noted. Analytical thin layer chromatography
(TLC) was performed on silica gel 60 F254 pre‐coated aluminium foil sheets and visualized by UV irradiation or
by use of a KMnO4 stain. Silica gel Kieselgel 60 (70–230 mesh) was used for column chromatography. NMR
experiments were obtained at 25 °C on a Bruker DPX 200 spectrometer operating at 200 MHz for 1H and 50.31
MHz for 13C experiments or in a Bruker DRX spectrometer operating at 400 MHz for 1H and 50.31 MHz for 13C
experiments. 1H and 13C chemical shifts (δ) are reported in parts per million (ppm) and they are relative to TMS
0.0 ppm and the residual solvent peak of DMSO‐d6 at δ 2.54 and δ 40.45 in 1H and 13C NMR, respectively. Data
are reported as follows: chemical shift (multiplicity, number of hydrogens, coupling constants where
applicable, and assignment where possible). Abbreviations are as follows: s (singlet), d (doublet), t (triplet), q
(quartet), dd (doublet of doublet), dt (double of triplet), tt (triplet of triplet), m (multiplet), bs (broad signal).
Coupling constant (J) quoted in Hertz (Hz) to the nearest 0.1 Hz. All the starting material are commercially
available.
General procedure for synthesis of 2H‐quinazolin‐4‐one 5a‐e and 8. Q‐tube is charged with isatoic anhydride
1 (150 mg, 0.919 mmol), ammonium acetate 2(106 mg, 1.378 mmol) to obtain compounds 5a‐e, or n‐
butylamine 6 (137 µL, 1.378 mmol) to obtain compound 8, aldehyde 3a, 3b, 3c, 3d, 3e (0.919 mmol) with 0.8
mL of toluene. The reaction mixture is than vigorously stirred at 170 °C for 1.5 hour. After cooling at room
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temperature, the solvent was recovered by distillation and the crude residue was diluted with water (15 ml)
and extracted three times with EtOAc (3 X 15 ml). The organic layers were dried using Na2SO4 and then
evaporated under reduced pressure. Crude products starting from 3c were purified by chromatographic
column affording 5c and 8, in all the other cases the target compounds were obtained after workup as a pure
mixture of 4 and 5 in the ratios reported in the Tables. Spectral data for all the compounds agree with those
reported in literature and for the major products, 1H NMR and selected examples of 13C and 19F NMR spectra,
extrapolated from the mixture, are reported below.
2‐Phenyl‐2,3‐Dihydroquinazolin‐4(1H)one (5a). 1H‐NMR (DMSO‐d6, 400 MHZ): δ 8.31 (s, 1 H), 7.63 (d, 1 H, J 7.2 Hz),
7.50 (d, 2 H, J 6.9 Hz), 7.38‐7.42 (m, 3 H), 7.24‐7.26 (m, 1 H), 7.13 (s, 1 H), 6.75 (d, 1 H, J 8.0 Hz), 6.68 (t, 1 H, J 7.3 Hz),
5.76 (s, 1 H) ppm. Physical data agree with those reported in literature.13
2‐(4‐chlorophenyl)‐dihydroquinazolin‐4(1H)one (5b). 1H‐NMR (DMSO‐d6, 200 MHZ): δ 8.39 (s, 1 H), 7.63 (dd, 2 H, J 1.6,
7.7 Hz), 7.54 (m, 3 H), 7.29 (t, 1H), 7.19 (s, 1 H), 6‐68‐6.79 (m, 2 H), 5.80 (s, 1 H) ppm. Physical data agree with those
reported in literature.13
2‐(4‐Methoxyphenyl)‐dihydroquinazolin‐4(1H)one (5c). 30% yield after chromatographic purification using as eluent
Ethyl acetate/petroleum ether (8:2). 1H‐NMR (DMSO‐d6, 400 MHZ): δ 8.15 (s, 1 H), 7.59 (d, 1 H, J 7.7 Hz), 7.40 (d, 2 H, J
8.6 Hz), 7.21 (t, 1 H, J 13.9 Hz), 7.00 (s, 1 H), 6.93 (d, 2 H, J 8.6 Hz), 6.72 (d, 1 H, J 8.0 Hz), 6.66 (t, 1 H, J 14.8), 5.69 (s, 1H)
ppm. Physical data agree with those reported in literature.13
2‐(4‐Nitrorophenyl)‐dihydroquinazolin‐4(1H)one (5d). 1H‐NMR (DMSO‐d6, 400 MHz): δ 8.62 (s, 1H) ; 8.30‐8.25 (m, 2H) ;
7.80‐7.75 (m,2H) ; 7.65 (dd, 1H, J 1.5 and 9.9 Hz) ; 7.30 (s, 1H) ; 7.25 dt, 1H, J 1.7 and 8.2Hz) ; 6.61‐6.39 (m, 2H ; 6.05 (t,
1H, J 2.2Hz) Physical data agree with those reported in literature.15
1
2‐(4‐Fluorophenyl)‐dihydroquinazolin‐4(1H)one (5e). H‐NMR (DMSO‐d6, 400 MHz): δ 8.29 (s, 1 H), 7.58 (d, 1 H, J 7.3
Hz), 7.50‐7.54 (m, 2 H, J 8.4 Hz), 7.21‐7.25 (m, 3 H), 7.09 (s, 1 H), 6.72 (d, 1 H, J 7.8 Hz), 6.67 (t, 1 H, J 14.9 Hz), 5.75 (s, 1
H) ppm.13C‐NMR (DMSO‐d6 50.31 MHz) : 65.93, 114.68 (d, J 22.64 Hz), 114.91, 115.34, 117.28, 127.38, 129.07 (d, J 8.55
Hz), 133.39, 137.83, 147.86, 162.17 (d, J 244.00 Hz), 163.58 19F‐NMR (DMSO‐d6, 376 MHz): 114.29 ppm. Physical data
agree with those reported in literature.18
3‐Butyl‐2‐(4‐chlorophenyl)‐2,3‐dihydroquinazolin‐4(1H)one(8). Purified eluting with petroleum ether/ethyl acetate
(7:3). 1H‐NMR (DMSO‐d6, 400 MHz): δ 7.64 (d, 1 H, J 7.8 Hz), 7.33‐7.43 (m, 5 H), 7.19‐7.22 (m, 1 H), 6.62‐6.68 (m, 2 H),
5.87 (s, 1 H), 3.87‐3.94 (m, 1 H), 2.71‐2.77 (m, 1 H), 1.43‐1.57 (m, 2 H), 1.24‐1.31 (m, 3 H), 0.84‐0.98 (m, 3H) ppm.
Physical data agree with those reported in literature.14
Acknowledgements
University of Perugia and the “Fondo Ricerca di Base 2017” are warmly acknowledged. Authors also thanks
CBMM (Companhia Brasileira de Metalurgia e Mineração) that provided Niobium oxide. L.S. acknowledges for
the financial support from the Polish National Science Centre (Polonez 2 (Reg. No: 2016/21/P/ST5/ 03512)).
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