Condensed isoquinolines. 21. Condensation of o- bromomethylphenylacetonitrile with substituted anthranilic acids

Article abstract of DOI:10.1007/s10593-007-0066-1

The reaction of substituted anthranilic acids and esters with o-bromomethylphenylacetonitrile give 2,3-R,R¹-7,12-dihydro-5H- isoquino[2,3-a]quinazolin-5-one hydrobromides. It was found that 7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones can exist in the two tautomeric imine and enamine forms. The tautomeric equilibrium position depends on the nature and position of the substituent in the quinazoline fragment. The borohydride reduction, oxidation, and reaction of 2,3-R,R¹-7,12- dihydro-5H-isoquino[2,3-a]quinazolin-5-ones with electrophilic reagents has been studied.

Full text of DOI:10.1007/s10593-007-0066-1

Chemistry of Heterocyclic Compounds, Vol. 43, No. 4, 2007
CONDENSED ISOQUINOLINES. 21*. CONDENSATION
OF o-BROMOMETHYLPHENYLACETONITRILE
WITH SUBSTITUTED ANTHRANILIC ACIDS
L. M. Potikha, V. A. Kovtunenko, and V. M. Kisil
The reaction of substituted anthranilic acids and esters with o-bromomethylphenylacetonitrile give 2,3-R,R1-
7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one hydrobromides. It was found that 7,12-dihydro-
5H-isoquino[2,3-a]quinazolin-5-ones can exist in the two tautomeric imine and enamine forms. The
tautomeric equilibrium position depends on the nature and position of the substituent in the quinazoline
fragment. The borohydride reduction, oxidation, and reaction of 2,3-R,R1-7,12-dihydro-
5H-isoquino[2,3-a]quinazolin-5-ones with electrophilic reagents has been studied.
Keywords: anthranilic acid, o-bromomethylphenylacetonitrile, 7,12-dihydro-5H-isoquino[2,3-a]-
quinazolin-5-one, enamine.
We have previously developed a relatively simple procedure for the synthesis of isoquino[2,3-a]quin-
azoline system derivatives via the reaction of o-bromomethylphenylacetonitrile (o-BMPA) with the ester and
nitrile of anthranilic acid. The properties of 7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one (1) prepared by
this method have been studied in a series of publications [1, 4-10]. Isoquinoquinazoline 1 shows high reactivity
towards electrophilic reagents [4-8] being readily reduced by NaBH4 to 6,6a,7,12-tetrahydro derivatives [4, 9]
and also readily oxidized to give aromatic derivatives [1, 5, 10]. All of these reactions affecting principally the
N(6)–C(6a)–C(7) triad permitted preparation of a series of isoquino[2,3-a]quinazolines substituted at positions 6 and
7. We made attempts to carry out classical aromatic electrophilic substitution reactions. However, because of the
high tendency of compound 1 to oxidize not depending on reaction conditions, these experiments led to tarring
or to formation of a complex mixture of unidentified products. The only solution to this problem was the use of
substituted starting reagents in the synthesis of the isoquinoquinazolines 1. With this in mind we have studied
the interaction of o-BMPA with substituted anthranilic acids and their esters.
The reactions were carried out by the previously reported method [2] of melting an equimolar mixture of
reagents at 130-150ºC or by heating their solutions in 2-propanol. The use of the unsubstituted anthranilic acid
2c gives the 7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one hydrobromide (1c) which was identical in
constants and spectroscopic properties with that obtained before. The reaction in this case needed a longer
heating time or higher melting temperature and the yield of the target product proved a little lower (by 15-20%)
than when using the ester and this is fully consistent with the lowered reactivity when compared with the ester.
_______
* For Communication 20 see [1]
__________________________________________________________________________________________
Taras Shevchenko National University, Kiev 01033, Ukraine; e-mail: potikha_l@mail.ru. Translated
from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 562-573, April, 2007. Original article submitted
December 1, 2005.
460
0009-3122/07/4304-0460©2007 Springer Science+Business Media, Inc.

Reaction of o-BMPA with substituted anthranilic acids and their esters also gave the target isoquino[2,3-a]quin-
azolines 1b-g in good yields (50-60%). When the reaction was carried out in 2-propanol the yield of the salts 1b-
g was a little lower but the reaction products did not generally need further purification.
CH2Br
R1
R
CO2X
NH2
CH2CN
2a–g
R1
R
XO2C
N
R1
R
XO2C
N
H
+
NH2
–
Br
CH2CN
4a–g
3a–g
R
R
R1
R1
–
Br
+
N
N
.
HBr
O
N
O
N
H
H
A
B
1a–g
R
R
R1
R1
N
N
O
N
O
N
H
B
A
5a–g
1−5 a R = R1 = OMe; b R = H, R1 = Me; c R = R1 = H; d R = Cl, R1 = H; e R = H, R1 = Cl;
f R = H, R1 = Br; g R = CO2Me, R1 = H; a, g X = OMe, b−f X = OH
The nature of the substituents in the anthranilic acids and esters proves to have a marked effect on the
yield and structure of the products. It was previously shown that formation of isoquino[2,3-a]quinazolines is a
multistage process including the formation of the alkylation products 3 and 2-aryl-1,4-dihydro-3(2H)-isoquino-
line iminium bromides 4.
In the case of the anthranilic acid ester the separation of the intermediate structure products 3 and 4 was
difficult due to their high reactivity which immediately led to the cyclic product 1c. For the reaction of o-BMPA
with other amines it was found [11] that the determining effect on the yield of the isoquinoline imine 4 was the
basicity of the amino group, the yield of compound 4 being lower with increased basicity. In turn the likelihood
of the cyclization of 4 to 1 is determined by the reactivity of the carboxyl group and this depends on the
electronic effects of the substituent. As a consequence of these factors our reaction led to a mixture of methyl
461

462

463

464

2-{[2-(cyanomethyl)benzyl]amino}-4,5-dimethoxybenzoate (3a) and 2-[4,5-dimethoxy-2-(carbomethoxy)-
phenyl]-1,4-dihydro-3(2H)-isoquinoline iminium bromide (4a) when the 4,5-dimethoxyanthranilate 2a was used
in 2-propanol. The reaction of o-BMPA with ester 2a occurs rapidly. After refluxing for 15 min a precipitate
containing compounds 3a and 4a (1 : 2) is formed and further heating of the reaction mixture does not lead to a
1
marked change in its composition. Formation of products 3a and 4a was established from their H NMR
spectroscopic data in which there are observed proton signals for the imonium group of compound 4a (s,
9.51 and s, 8.27 ppm) and methylene groups C(1)H2 and C(4)H2 as AB spin systems with 2J = 15.2 (d, 4.98 and d,
2
4.72 ppm) and J = 18.4 Hz (d, 4.22 and 4.06 ppm) respectively. The broad signal at 6.16 ppm corresponds to
the resonance of the amino group proton of the benzylaminobenzoate 3a and the two-proton singlets at 4.54 and
4.08 ppm to the resonance of the CH2NH and CH2CN methylene groups. The signals for the aromatic protons at
7.57-7.26 ppm and methoxy groups at 3.92-3.55 ppm are readily assigned in both compounds. The position and
multiplicity of the signals for the aliphatic protons are fully in agreement with the characteristics previously
shown for related structures [11]. The target 2,3-dimethoxyisoquino[2,3-a]quinazoline 1a was obtained when
attempting to separate the mixture 3a, 4a by crystallization from acetic acid or DMF.
The presence of powerful electron-acceptor groups in the anthranilic acid markedly lowers the likely
formation of alkylation product 3 and, correspondingly, the formation of the target isoquinoquinazoline 1. Hence
when the reaction of o-BMPA with 5-nitroanthranilic acid in 2-propanol was carried out even after 30 h
refluxing only traces of the target 3-nitroisoquino[2,3-a]quinazoline were formed in the reaction mixture
(according to TLC and 1H NMR). Further heating led only to an increase in the amount of side products and the
melting at 150ºC led to tarring.
In all of the other examples (2b,d-g) given in the Scheme the formation of intermediate products of type
3 and 4 were not registered and the difference in yields of salts 1 amounted to not more than 10%. Treatment of
the salts 1a,b,d-g with Et3N gave the free bases 5a,b,d-g.
1
The spectroscopic characteristics (IR, H NMR spectra of solutions of the salts in CF3CO2D and the
bases in CDCl3) for the 4-substituted isoquinoquinazolines 1a,b,d-g agreed overall with those of the
unsubstituted isoquinoquinazoline 1c [2]. We have previously reported [5-7] the presence of enamine properties
for compound 1c (ready proton/deuterium exchange at C(6), ability to take part in alkylation and acylation
1
reactions at C(6) etc.) but a clear establishment of the enamine form was unsuccessful. A study of the H NMR
spectra of the salts 1a-g and their free bases 5a-g in DMSO-d6 showed that these compounds can exist in polar
aprotic solvents in the two tautomeric imine and enamine forms as indicated by a double set of signals
corresponding to the predominant imine (A) and enamine (B) forms (full data is given in Table 1). Base 5 has a
clearer tendency to tautomerism under these conditions. The ratio of the two A and B forms depends on the
nature and position of the substituent in the quinazoline fragment, moreover the effect of the latter factor proved
different for salt 1 and base 5. These compounds are placed in order of increasing percentage of enamine form B
content in the mixture in Table 1. The amount of form B correlates well with the general Hammett σ-constant
values for the substituents [12] with respect to the bridging nitrogen atom N(13) for the bases and the C(5)=O
carbonyl group in the salts. Change in temperature also affects the ratio of these two forms. An increase in
temperature leads to a small decrease in the content of form B in the mixture. A similar prototropic effect and
corresponding temperature dependence has been observed earlier [13] for substituted 2-alkylquinazolin-4-ones
but in a less clear form.
1
In the aromatic proton region the H NMR spectra of the protonated salts 1 show several changes from
the spectra of the bases 5 in terms of the relative position of the signals for the H-1 and H-4 protons. In the
spectra of salt 1A the H-1 signal is observed to low field of H-4 but in form B these are reversed. This is
explained well by realization of the structure 6H-isoquino[2,3-a]quinazolin-5-one with protonation 1A while the
3H form is more stable for quinazolin-4-ones [14]. Support for the 6H form of salt 1A also comes from the
observed difference in the chemical shifts of the H-11 protons (∆δ about 0.45 ppm) and of C(12)H2 (about
0.5 ppm) of forms A and B, which for 1A is found to lower field than 1B.
465

It turns out that the nature and position of the substituent in the quinazoline ring markedly affects the
chemical properties in the isoquinoquinazolines obtained 1a-g and 5a-g. We have found that salts 1d-g are
unstable in DMSO solution and are readily oxidized to give mixtures of products, the main component of which
(about 60-80%) is the 2-R- or 3-R-5-oxo-5H,6H-isoquino[2,3-a]quinazolin-13-ium bromide 6a-d. Moreover, the
rate of oxidation (from about 3 h for 1g to about 8 h for 1d) is fully in agreement with the dependence found for
the ratio of the A and B tautomeric forms in solutions of the salts (Table 1). It was found that dehydrogenation of
salt 1c is observed under more rigid conditions (refluxing in benzonitrile) [1]. The formation of a compound of
structure 6 is indicated by the presence in the 1H NMR spectra of the mixture of singlet protons for H-12 (11.59,
6a; 11.91, 6b,c, 11.78 ppm, 6d), H-1 (9.70 ppm), and for H-11 (d, 8.60 ppm, 6a and 9.0-9,12 ppm, 6b-d) and H-
7 (8.3-8.5 ppm) in the region characteristic of the aromatic salts of isoquino[2,3-a]quinazoline [1, 10].
R
R
R1
R1
–
Br
+
N
N
1a,d–g
5a–d
.
O
N
HBr
O
N
H
6a–d
R
NMe2
R1
R
7a–c
R1
N
N
O
O
N
H
O
N
9a–d
H
Me
8a,b
6 a R = CO2Me, R1 = H; b R = H, R1 = Cl; c R = H, R1 = Br; d R = Cl, R1 = H;
7 a R = R1 = OMe; b R = H, R1 = Cl; c R = CO2Me, R1 = H; 8 a R = R1 = OMe; b R = Cl,
R1 = H; 9 a R = R1 =OMe, b R = H, R1 = Me; c R = H, R1 = Br; d R = Cl, R1 = H
The observed dependence for the rate of reaction of salts 1d-g is also observed for their condensation
with p-dimethylaminobenzaldehyde in acetic anhydride. A short heating of the mixture of reagents (3 min for
1b, 10 min for 1e, 30 min for 1a) leads to the formation of the dark-violet colored 7-{[4-dimethyl-
amino]phenyl]methylene}-7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones 7a-c.
We have previously shown [5] that the isoquinoquinazoline 5c readily forms the products of
C(7)-acylation when heated with carboxylic acid chlorides in anhydrous pyridine and that this reaction can come
about under milder conditions. Hence the isoquinoquinazolines 5a,b are readily acylated by acetic anhydride in
the presence of AcONa to give good yields of the 7-acetyl derivatives 8a,b. The reaction is not accompanied by
the formation of side products but difference of the behavior of the bases 5a and 5b involves only the reaction
time (greatest for 5a and least for 5d, see Experimental) and it correlates with the content of enamine form B for
the starting bases 5.
466

TABLE 2. Physicochemical Characteristics of the Compounds Synthesized
Found, %
Calculated, %
——————
Com-
pound
Empirical
formula
Yield, %
(method)
mp, °C*
C
H
Br (Cl)
N
1a
1b
C18H17BrN2O3
C17H15BrN2O
55.46
55.54
59.39
59.49
4.10
4.40
4.36
4.41
20.54
20.53
23.30
23.28
7.30
7.20
8.20
8.16
272-273
50 (A)
> 300 dec.
28 (A),
59 (B)
48 (A),
67 (B)
1d
1e
1f
C16H12BrClN2O
C16H12BrClN2O
C16H12Br2N2O
52.74
52.85
3.28
3.33
22.00
21.97
(9.74)
(9.75)
21.99
21.97
(9.76)
(9.75)
7.80
7.70
> 300 dec.
> 300 dec.
> 300 dec.
52.75
52.85
3.23
3.33
7.81
7.70
50 (А),
70 (B)
46.89
47.09
2.83
2.96
39.17
39.16
6.91
6.86
47 (A),
69 (B)
48 (A)
1g
5a
5b
5d
5e
5f
C18H15BrN2O3
C18H16N2O3
55.75
55.83
70.01
70.12
77.76
77.84
67.89
67.97
67.85
67.97
58.65
58.74
70.50
70.58
62.27
62.31
3.82
3.90
5.19
5.23
5.32
5.38
3.88
3.92
3.87
3.92
3.29
3.39
4.57
4.61
4.99
5.04
20.67
20.63
—
7.30
7.23
9.12
9.09
10.65
10.68
10.00
9.91
9.95
9.91
8.57
8.56
9.20
9.15
8.16
8.07
127-219
263-265
215-217
250-253
235-237
226-229
206-208
249-251
222-225
69
70
85
80
82
79
70
80
C17H14N2O
—
C16H11ClN2O
C16H11ClN2O
C16H11BrN2O
C18H14N2O3
12.56
12.54
12.52
12.54
24.47
24.42
—
5g
7a
7b
C27H26BrN3O3
C25H21BrClN3O
15.39
15.35
16.16
16.15
60.62
60.68
4.19
4.28
8.52
8.49
(7.19)
(7.16)
7c
8a
8b
9a
9b
9c
9d
C27H24BrN3O3
C20H18N2O4
C18H13ClN2O2
C18H18N2O3
C17H16N2O
62.48
62.56
68.49
68.56
66.50
66.57
69.60
69.66
77.19
77.25
58.26
58.38
67.39
67.49
4.60
4.67
5.10
5.18
3.99
4.03
5.79
5.85
6.09
6.10
3.89
3.98
4.56
4.60
15.41
15.41
—
8.12
8.11
8.10
8.00
8.65
8.63
9.10
9.03
10.62
10.60
8.60
8.51
9.89
9.84
255-256
237-240
232-233
190-193
191-193
202-205
238-240
75
65
68
60
49
40
42
10.96
10.92
—
—
C16H13BrN2O
C16H13ClN2O
24.28
24.27
12.47
12.45
_______
* Solvent for recrystallization: DMF (compounds 1a, 5a,d-f, 8a,b,
9a,c,d); AcOH (1b,e,g); AcOH-DMF, 3: 1 (1d,f); 1,4-dioxane (5g,b),
2-propanol (9b); Ac2O (7a); acetone (7b,c).
In our view, a characteristic pointing to the presence of the imine structural fragment in the molecule
(form A) is its borohydride reduction. It was found that the multiple C(6a)=N(6) bond in the Ar-unsubstituted
isoquinoquinazoline 5c is readily reduced by NaBH4 in methanol hence it was logical to propose that the rate of
this reaction in the isoquinoquinazolines 5a-g will be determined by the ratio of the two tautomeric forms A and
467

B. In fact, the time needed for the reaction of 5a,b,d,e to the corresponding 6,6a,7,12-tetrahydro-5H-
isoquino[2,3-a]quinazolin-5-ones 9a-d is decreased in the case of 5a,b (content of form B less or absent than in
5c) but increased in the case of 5d,e (form B content greater than in 5c).
EXPERIMENTAL
1
IR spectra were recorded on a Pye Unicam SP3-300 instrument (KBr tablets). H NMR spectra were
obtained on a Varian Mercury 400 (400 MHz) instrument using DMSO-d6 and with TMS as internal standard.
UV spectra were taken on a Specord M400 instrument. The melting points of the compounds synthesized were
determined on a Boetius type heating block and are not corrected. Monitoring of the reaction course and the
purity of the compounds obtained was carried out by TLC on Silufol UV-254 plates.
2,3-Dimethoxy-7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one Hydrobromide (1a). A solution of
o-BMPA (2.1 g, 10 mmol) and methyl 2-amino-4,5-dimethoxybenzoate 2a (2.11 g, 10 mmol) in 2-propanol
(15 ml) was heated for 1 h. After cooling the precipitate was filtered off and washed with 2-propanol. The solid
material was a mixture of methyl 2-{[2-(cyanomethyl)benzyl]amino}-4,5-dimethoxybenzoate (3a) and 2-[4,5-
dimethoxy-2-(methoxycarbonyl)phenyl]-1,4-dihydro-3(2H)-isoquinoleniminium bromide (4a) in the ratio 1 : 2.
The mixture of compounds 3a and 4a was dissolved with heating in DMF and was refluxed for 3 min. After
cooling the precipitated solid was filtered off and washed with DMF and alcohol.
2,3-R,R1-7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one Hydrobromides (1b-g). A. The
isoquinoquinazoline hydrobromides 1b-f were prepared according to the method in [1] by heating a mixture of
equimolar amounts of o-BMPA and the anthranilic acids 2b-f in 2-propanol. The methyl 5-oxo-7,12-dihydro-
5H-isoquino[2,3-a]quinazoline-2-carboxylate hydrobromide (1g) was prepared using dimethyl 2-amino-
terephthalate (2g).
B. A mixture of o-BMPA (2.1 g, 10 mmol) and the anthranilic acid 2b-f (10 mmol) was heated on an oil
bath at 130-150ºC for 4 h. After cooling, the melt was triturated in acetone (5 ml). The precipitate was filtered
off and washed with acetone to give the isoquinoquinazoline hydrobromides 1b-f.
2,3-R,R1-7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones (5a,b,d-g). Et3N (1.5 ml) was added to a
suspension of the salts 1a,b,d-g in 2-propanol (10 ml) and refluxed for 10 min. Solvent and excess Et3N were
evaporated and water (50 ml) was added to the residue. The precipitate was filtered off and thoroughly washed
with water and alcohol to give the isoquinoquinazolines 5a,b,d-g.
7-{[4-(Dimethylamino)phenyl]methylene}-2,3-R,R1-7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one
Hydrobromides (7a-c). A mixture of the isoquinoquinazoline hydrobromide 1a,e,g (2.57 mmol) and p-dimethyl-
aminobenzaldehyde (0.38 g, 2.57 mmol) in acetic anhydride (10 ml) was refluxed for 30 min (for 1a), 10 min (for
1e), or 3 min (for 1g). After cooling the precipitated solid was filtered off and washed with acetone. UV spectrum
(MeOH), λmax, nm (ε×10-3): compound 7a 204 (276.93), 232 (249.02), 265 (152.41), 328 (74.34), 415 (159.34);
compound 7b 205 (247.41), 242 (157.96), 328 (66.61), 423 (85.64); compound 7c 250 (155.52), 320 (55.73),
423 (102.38).
7-Acetyl-2,3-R,R1-6,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones 8a,b.
A
mixture of the
isoquinoquinazoline 5a,d (0.1 mmol) and anhydrous sodium acetate (0.1 g, 0.12 mmol) in acetic anhydride
(10 ml) was heated for 3 h (5a) or for 1.5 h (5d). The product was cooled and left to stand overnight. The
precipitate was filtered off and washed with acetone.
2,3-R,R1-6,6a,7,12-Tetrahydro-5H-isoquino[2,3-a]quinazolines 9a-d. NaBH4 (0.76 g, 20 mmol) was
added in small portions to a suspension of the isoquinoquinazoline 1a,b,d,f (10 mmol) in methanol (50 ml). At
the end of the vigorous reaction during which the starting salt dissolved the mixture was refluxed for 15 min.
Solvent was distilled off under reduced pressure and the residue was treated with NaOH solution (10%, 20 ml).
The solid material was filtered off and washed with water.
468

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