Condensed isoquinolines 23. Reaction of o-bromomethylphenyl-acetonitrile with anthranilic acids: Synthesis of 6H, 12H,17H-dibenzo[3,4:6,7][1,8]- naphthyridino[1,8-ab]quinazoline-6,17-diones

Article abstract of DOI:10.1007/s10593-007-0141-7

The direction of the reaction of anthranilic acids with o-bromomethylphenylacetonitrile upon fusion depends on the temperature and nature of the substituent in the anthranilic acid. The reaction may lead to three types of products: Derivatives of 7,12-dihydro-5H-isoquino[2,3-a] quinazolin-5-ones below 150°C and to 6,11-dihydro-13H-isoquino[3,2-b] quinazolin-13-one or derivatives of 6H,12H,17H-dibenzo[3,4:6,7][1,8] naphthyridino[1,8-ab]quinazoline-6,17-diones above 150°C depending on the nature of the substituent in the anthranilic acid. A study was carried out on the mechanism for the formation of 6H,12H,17H-dibenzo[3,4:6,7][1,8] naphthyridino[1,8-ab]quinazoline-6,17-diones, which permitted the preparation of 6-(4-methylphenyl)-6,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one.

Full text of DOI:10.1007/s10593-007-0141-7

Chemistry of Heterocyclic Compounds, Vol. 43, No. 7, 2007
CONDENSED ISOQUINOLINES
23*. REACTION OF o-BROMOMETHYLPHENYL-
ACETONITRILE WITH ANTHRANILIC ACIDS:
SYNTHESIS OF 6H,12H,17H-DIBENZO[3,4:6,7][1,8]-
NAPHTHYRIDINO[1,8-ab]QUINAZOLINE-6,17-DIONES
L. M. Potikha, V. A. Kovtunenko, and A. V. Turov
The direction of the reaction of anthranilic acids with o-bromomethylphenylacetonitrile upon fusion
depends on the temperature and nature of the substituent in the anthranilic acid. The reaction may lead
to three types of products: Derivatives of 7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones below
150°C and to 6,11-dihydro-13H-isoquino[3,2-b]quinazolin-13-one or derivatives of 6H,12H,17H-
dibenzo[3,4:6,7][1,8]naphthyridino[1,8-ab]quinazoline-6,17-diones above 150°C depending on the
nature of the substituent in the anthranilic acid. A study was carried out on the mechanism for the
formation of 6H,12H,17H-dibenzo[3,4:6,7][1,8]naphthyridino[1,8-ab]quinazoline-6,17-diones, which
permitted the preparation of 6-(4-methylphenyl)-6,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one.
Keywords: anthranilic acid, o-bromomethylphenylacetonitrile, 6H,12H,17H-dibenzo[3,4:6,7][1,8]-
naphthyridino[1,8-ab]quinazoline-6,17-dione, 6-(4-methylphenyl)-6,12-dihydro-5H-isoquino[2,3-a]quinazolin-
5-one.
In previous work [1, 2], we showed that the reaction of o-bromomethylphenylacetonitrile (o-BMPA)
with anthranilic acids 1 leads to derivatives of 7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones 2 or products
of their rearrangements, namely, derivatives of 6,11-dihydro-13H-isoquino[3,2-b]quinazolin-13-one (3) (when
there are substituents at the positions ortho to the amino group of the acid). The reaction was carried out by
fusion at a temperature not exceeding 145°C or in a 2-propanol solution. In this case, 2- (3b) and
3-halo-6,11-dihydro-13H-isoquino[3,2-b]quinazolin-13-ones (3c) were obtained by rearrangement of their
angular isomers 2b or 2c in N-methyl-2-pyrrolidone [1]. In order to improve the synthesis of linear
isoquinoquinazolines, we studied the reaction of o-BMPA with anthranilic acids above 150°C, i.e., under
conditions permitting high-temperature rearrangement [3].
As in the case of fusion of o-BMPA with the methyl ester of anthranilic acid [3], fusion with the acid at
>150°C gives already the product of the rearrangement of isoquino[2,3-a]quinazoline 2a, namely,
isoquino[3,2-b]quinazoline 3a. The best yield of this product was obtained at 165-170°C, which is a temperature
higher than when using the ester since the rate of formation of 2a is slower due to the lower carbonyl activity of
_______
* For Communication 22 see [1].
__________________________________________________________________________________________
Taras Shevchenko National University, Kiev 01033, Ukraine; e-mail: vkovtunenko@univ.kiev.ua.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1059-1067, July, 2007. Original article
submitted June 10, 2006.
0009-3122/07/4307-0893©2007 Springer Science+Business Media, Inc.
893

the acid in comparison with the ester. We expected a similar result in the case of halo-substituted anthranilic
acids 1b-d. However, fusion at only 150-160°C is already sufficient for the facile formation of a product, whose
spectral data were not in accord with the expected characteristics of isoquino[3,2-b]quinazolines 3b-d. The
1H NMR spectra of the reaction products show only one signal in the region for the aliphatic methylene group
protons, namely, a singlet at 5.23-5.26 ppm, while the total integral intensity of the aromatic protons proved
greater than for the expected reaction products. The mass spectrometric data of one of the products (the product
of the reaction with acid 1b) suggest that this product is formed from one molecule of o-BMPA and two
molecules of anthranilic acid. The finding of only one signal for the aliphatic methylene group protons in the
1H NMR spectrum indicated the formation of the product of substitution both at the carbon and nitrogen atoms.
This led us to propose the structure 6H,12H,17H-dibenzo[3,4:6,7][1,8]naphthyridino[1,8-a]quinazoline-
6,17-dione (4), which was in complete agreement with the mass spectral and elemental analysis data. Further
evidence for this assignment was found in the 13C NMR spectra, which showed signals for 23 carbon atoms,
including two corresponding to C=O group carbon atoms in regions characteristic for amides (159-160 ppm,
C(10), Table 1) and ketones (~173 ppm, C(5)).
O
R2
R1
N
N
3a-c
R1
R2
CO2H
NH2
R2
_
Br
R1
+
N
CH2Br
1a-d
N
H
O
CH2CN
2a-d
2 (1a–d)
R1
R2
O
1
11
1
4
11
N
N
6
N
7
N
O
H
N
O
H
1'
10'
9
O
11'
6''
6
R1
N
R2
4a-d
6
1–4 a, c, d R1 = H, b R1 = Cl, a, b R2 = H, c R2 = Br, d R2 = Cl
Comparison with the 13C NMR spectra of structurally similar 6-methyl-5,6-dihydro-5H-isoquino[2,3-a]-
quinazolin-5-one and 7-acyl-7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones [4] permitted us to assign the
signal at 102 ppm to C(4b) in 4 since the corresponding signal for C(7) in 6-methyl- and 7-acyl-
isoquino[2,3-a]quinazolines is seen at 82-94 ppm.
The fusion of o-BMPA with anthranilic acid 1a at 180-190°C also led to a compound with the structure
of naphthyridino[1,8-ab]quinazoline (4a). Support for the structure of the unknown reaction products as
6H,12H,17H-dibenzo[3,4:6,7]naphthyridino[1,8-ab]quinazoline-6,17-diones 4a-d was obtained by a NOE
experiment. Saturation of the frequency of the methylene group protons in 4a led to an increase in the intensity
of the signals of the two aromatic protons (7.88 and 7.38 ppm), which was found to be a criterion for assigning
an angular isoquinoquinazoline structure [3]. The observed pattern for the aromatic protons corresponds to
894

R
R
HO2C
N
CH2Br
NH2
Me
CH2CN
N
1a-d
_
180-190°C
+
N
O
B
NH2
5a–d
7
< 150°C
> 150°C
2a-d
1a–d
Me
R2
R1
R2
R1
R2
R1
HO2C
N
HO2C
HO2C
N
N
H
N
_
+
O
H
N
N
H
Br
O
HN
. HBr
H2N
_
+
H3N
O
Br
10
8
9
R1
R2
N
4a-d
CO2H
N
H
. HBr
O
structure of 4 and the signals were assigned on the basis of COSY HH experiments. A characteristic feature of
the spectra of 4a-d (Table 1) is the presence of signals for four protons at low field (δ > 8.0 ppm). Two of these
signals are for the protons in the peri position to the carbonyl groups (8.14-8.33, H-6 and 8.05-8.20 ppm, H-11)
and two signals are for the protons in the deshielding region of the carbonyl groups (8.63-8.78, H-4 and
8.50-8.57 ppm, H-9).
We attempted to elucidate several features of the mechanism of formation of naphthyridino[1,8-ab]-
quinazolines 4a-d, which, as suggested by their structure, result from the nucleophilic substitution of the imino
group nitrogen atom and acylation of the methylene group. In previous work [2, 5, 6], we have already noted that
the formation of 7,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-ones 2 is a multistep process involving
cyclization of 2-(2-carboxyphenyl)-1,4-dihydro-3(2H)-isoquinoliniminium bromides 5. The structures of
isoquinolinimines 5 and isoquinoquinazolines 2 both have electrophilic sites (the carbonyl group and imino
group carbon atoms), at which nucleophilic substitution may proceed. In order to determine the step in which
this reaction occurs, we fused isoquinoquinazoline hydrobromide 2a with anilines and their protic salts. In all
cases, irrespective of the reactivity of the nucleophilic reagent, the thermal rearrangement of salt 2a took place to
give isoquino[3,2-b]quinazoline 3a. However, the fusion with anthranilic acid 1a gave an unexpectedly high
yield (50%) of the product of oxidative dimerization of isoquino[3,2-b]quinazoline 3a, namely,
6,11,6',11'-tetrahydro-11'H-[6,11']bi[isoquino[3,2-b]quinazolinyl]-13,13'-dione (6).
895

896

In previous work [7], in a study of the rearrangement of protic salts of isoquino[2,3-a]quinazoline 2a to
give isoquino[3,2-b]quinazoline 3a, we noted the strong tendency of isoquino[3,2-b]quinazoline 3a to undergo
oxidation by atmospheric oxygen at high temperatures, leading to a mixture of oxidation products and dimeric
products. The structures of some of these dimers have been reliably established. The spectral data of the mixtures
obtained suggest the formation unsymmetrical dimer 6 but this product could not be isolated as a pure
compound.
1
The H NMR spectrum of dimer 6, similar to the spectrum of 6,11-dihydro-11'H-[6,11']bi[isoquino-
[3,2-b]quinazolinyl]-13,6',13'-trione [7] with analogous structure, showed an extremely complex pattern in the
aromatic region with total integral intensity 17H. Four signals were found in the aliphatic proton region at 20°C:
two single-proton doublets with different coupling constants at 5.73 and 4.10 ppm and two broad multiplets at
4.64 (2H) and 4.30 ppm (1H). However, at 80°C, we observed five single-proton doublets (see Experimental).
The NOE and COSY HH experiments indicated that these signals belong to methylene group protons C(11)H2
2
2
(5.27 ppm, dd, ∆δ = 1.0 ppm, J = 16.8 Hz) and C(6')H2 (4.30 ppm, dd, ∆δ = 0.3 ppm, J = 19.2 Hz) and methine
3
proton H-6 (4.62 ppm, J = 9.8 Hz). The signal of the second methine proton H-11' was found in the aromatic
proton region at 6.37 ppm. The temperature dependence of the pattern of the signals of HB-11 and HA-6' is
attributed to conformational changes in the pyridine rings of the isoquinoline fragments in 6. Analysis of the
coupling constants of the methine protons (H-11' and H-6) and the spatial models, it appears most likely that the
structure for dimer 6 has transoid arrangement of the hydrogen atoms in the C(6)–C(11') fragment. We used the
modified Karplus equation [8] and the coupling constant to find that the H–C(6)–C(11')–H dihedral angle is 143°.
Indirect evidence justifying this hypothesis is found in the position of the two doublets of aromatic protons H-10'
and H-7 upfield (6.50 and 6.46 ppm) relative to the other Ar-H protons and the corresponding signals of
isoquinazoline 3a [7]. These protons fall in the shielding region of double bonds N(5)=C(5a) and C(13')=O only in
the trans isomer with virtually parallel orientation of the planes of the isoquino[3,2-b]quinazoline fragments. The
linear structure of the isoquinoquinazoline fragments in dimer 6 is supported by comparative analysis of the
electronic spectra of isoquinoquinazoline 3a and product 6, which display considerable similarity.
Since isoquinoliniminium bromide 5a could not be isolated as a pure compound due to its instability
[2, 5], we attempted to carry out nucleophilic substitution in imine 5a by the consecutive addition of the reagents
in the fusion (180-190°C) of o-BMPA with acid 1a and anilines. In the experiment with p-toluidine, which was
added to the reaction mixture 1.5 h after the onset of heating of a mixture of o-BMPA with acid 1a, we obtained
6-(4-methylphenyl)-6,12-dihydro-5H-isoquino[2,3-a]quinazolin-7-one (7) in low yield. This result is indicated
1
by the H NMR spectrum of this product, which showed a single-proton signal of methine proton H-7 at
4.21 ppm along with the signals of the introduced p-tolyl group. 6-Arylisoquinoquinazoline 7 is formed as the
result of two consecutive nucleophilic substitution reactions. In order to determine whether the initial attack of
the nucleophilic reagent occurs at the C=O or C=N electrophilic site in isoquinolinimine 5a, we fused o-BMPA
with the N-benzylamide of anthranilic acid. We assumed that the anthranilic acids with this reagent at high
temperatures (>150°C) may form diamides [9], which are likely intermediates in the synthesis of
naphthyridino[1,8-ab]quinazolines 4a-d. However, using N-alkylamide, which is a more reactive nucleophilic
reagent than N-arylamide, we assumed that the reaction would proceed under milder conditions and give a better
yield of the 6-substituted isoquinoquinazoline product. Indeed, fusing with the N-benzylamide led to the reaction
product at a lower temperature (140°C), which proved, however, to be isoquino[2,3-a]quinazoline 2a.
The formation of halo-substituted naphthyridino[1,8-ab]quinazolines 4b-d at relatively lower
temperatures than for unsubstituted 4a is probably a consequence of the difference in the carbonyl activity of the
anthranilic acids. This finding along with the considerable difficulty in obtaining the nucleophilic substitution
products in the reaction with other anilines, including more reactive compounds, suggested that acylation is the
likely first step in the formation of 4a-d, namely, intermolecular acylation at the imine group in isoquinoline 5,
leading to intermediate 8. The imino group in 8 is additionally activated by the electrophilic substituent, which
facilitates the nucleophilic addition of the amino group of the anthranilic acid fragment. This
897

intramolecular reaction may give spiro compound 9. Since triaminomethane derivatives are extremely unstable,
opening of the resultant pyrimidine ring occurs readily. The intramolecular acylation of enamine 10 by the amide
completes formation of the new isoquinoline system. The reaction is accompanied by loss of ammonium
1
bromide, which was detected in the reaction products using H NMR spectroscopy. We should note that
acylation by amides at the β-position of enamines was observed in our previous work for the intramolecular
reaction leading to the formation of spiro[benzimidazo[1,2-b]isoquinoline-6(11H),2'-indane]-1',11-dione from
the amide of 2-[(11-oxo-5,11-dihydrobenzimidazo[1,2-b]isoquinoline-6-yl]benzoic acid. Cyclization in the final
step leads to naphthyridino[1,8-ab]quinazolines 4a-d.
We made additional attempts to obtain further proof of the proposed mechanism for the formation of
4a-d. In particular, we attempted to obtain products of the reaction of already reported 2-aryl-1,4-dihydro-3(2H)-
isoquinilinium bromides [6] with anilines, benzoic acids, and anthranilic acids under conditions for the formation
of naphthyridino[1,8-ab]quinazolines 4a-d. However, these attempts did not lead to the expected results,
probably due both to the insufficient activity of the imino group for nucleophilic substitution and the instability
of the products of acylation at the imino group.
EXPERIMENTAL
The melting points were determined on a Boetius block and not corrected. The IR spectra were taken for
1
KBr pellets on a Pye Unicam SP3-300 spectrometer. The H and 13C NMR spectra were taken on a Varian
Mercury 400 spectrometer at 400 and 100 MHz, respectively in DMSO-d6 with TMS as the internal standard.
The UV spectra were taken on a Specord M-400 spectrophotometer. The mass spectrum of 4b was taken on a
Waters Integrity System with a Thermabeam detector (acetonitrile as the mobile phase), while the mass spectrum
of 6 was obtained using high-pressure liquid chromatography on a AGILENT/100-Series instrument (CI,
acetonitrile, 0.05% formic acid as the mobile phase). The course of the reactions and purity of the products were
monitored by thin-layer chromatography on Silufol UV-254 plates.
6,11-Dihydro-13H-isoquino[3,2-b]quinazolin-13-one (3a). A mixture of o-bromomethylphenyl-
acetonitrile (2.1 g, 10 mmol) and anthranilic acid 1a (1.37 g, 10 mmol) was heated on an oil bath at 165-170°C
for 5 h. The melt gradually hardened during the reaction. After cooling, the melt was triturated with acetone
(5 ml). The precipitate was filtered off and washed with acetone. The solid was dissolved in morpholine (3 ml)
and water (20 ml) was added. The precipitate was filtered off and washed with acetone to give 3a (1.24 g,
50%), mp 161-163°C (DMF, mp 162°C [3]).
6H,12H,17H-Dibenzo[3,4:6,7][1,8]naphthyridino[1,8-ab]quinazoline-6,17-diones 4a-d. A mixture of
o-bromomethylphenylacetonitrile (2.1 g, 10 mmol) and anthranilic acid 1b-d (20 mmol) was heated on an oil
bath at 150-160°C for 4 h. After cooling, the melt was triturated with acetone (5 ml). The precipitate was
filtered off and washed with acetone. The solid was dissolved in morpholine (3 ml) upon heating. The mixture
was cooled and water (15 ml) was added. The precipitate was filtered off and washed with water and acetone.
Acid 1a was used to obtain naphthyridinoquinazoline 4a (the bath temperature in this case was 185-190°C).
Dione 4a. Yield 1.4 g (40%); mp 250-252°C (DMF). Found, %: C 78.76; H 3.98; N 8.01. C23H14N2O2.
Calculated %: C 78.84; H 4.03; N 8.00.
Dione 4b. Yield 2.73 g (65%); mp 240-242°C (DMF) (2.73 g). Mass spectrum, m/z (Irel, %): 418* [M]+
(100), 389 (20), 353 (8). Found, %: C 65.81; H 2.79; Cl 16.93; N 6.70. C23H12Cl2N2O2. Calculated, %: C 65.89;
H 2.88; Cl 16.91; N 6.68.
Dione 4c. Yield 3.05 g (60%); mp 236-238°C (DMF). Found, %: C 54.29; H 2.30; Br 31.46; N 5.52.
C23H12Br2N2O2. Calculated, %: C 54.36; H 2.38; Br 31.45; N 5.51.
_______
* Calculated for isotope 35Cl.
898

Dione 4d. Yield 2.64 g (63%); mp 256-256°C (DMF). Found, %: C 65.79; H 2.80; Cl 16.92; N 6.70.
C23H12Cl2N2O2. Calculated, %: C 65.89; H 2.88; Cl 16.91; N 6.68.
6,11,6',11'-Tetrahydro-11'H-[6,11']bi[isoquino[3,2-b]quinazolinyl]-13,13'-dione (6). A mixture of
hydrobromide salt of isoquino[2,3-a]quinazoline 2a (1 g, 3.04 mmol) and anthranilic acid 1a (0.42 g,
3.04 mmol) was heated on an oil bath at 175-180°C for 2 h. The reaction proceeded vigorously in the first
30 min and the melt then gradually hardened. After cooling, the melt was triturated with acetone (5 ml). The
precipitate was filtered off and washed with acetone. The solid was dissolved in morpholine (2 ml) upon
heating. After cooling, water (15 ml) was added. The precipitate was filtered off and washed with water and
acetone to give 2.48 g (50%) 6, mp >300°C (dec.) (DMF). IR spectrum (neat), ν, cm-1: 1670 (C=O), 1598, 1472,
1
1398, 774, 757, 694. UV spectrum (MeOH), λmax, nm, (qualitative): 270, 306, 317. H NMR spectrum at 80°C,
δ, ppm (J, Hz): 8.12 (1H, d, oJ = 7.2, H-1'); 7.73-7.02 (13H, m, Ar-H); 6.50 (1H, d, oJ = 7.6, H-10'); 6.46 (1H, d,
3
2
2
oJ = 7.6, H-7); 6.37 (1H, d, J = 9.8, H-11'); 5.75 (1H, d, J = 16.8, C(11)HAHB); 4.78 (1H, d, J = 16.8,
C(11)HAHB); 4.62 (1H, d, 3J = 9.8, H-6); 4.45 (1H, d, 2J = 19.2, C(6')HAHB); 4.15 (1H, d, 2J = 19.2, C(6')HAHB). 13C
NMR spectrum at 20°C, δ, ppm: 161.02 (C5a)); 160.84 (C(5a')); 153.71 (C(13)); 152.93 (C(13')); 147.29 (C(4a));
147.19 (C(4a')); 134.80, 134.28, 131.37 (br.); 129.64, 129.07, 128.43, 128.34, 127.64 (br.); 126.85 (br.); 126.67,
126.43 (br.); 126.34, 126.25 (br.); 126.19, 119.76, 119.70, 56.43 (C(11')); 50.94 (C(6)), 44.15 (C(11)); 36.57 (C(6')).
Mass spectrum (CI), m/z (Irel, %): 495 [M+1]+ (50), 247 [1/2 M]+ (100). Found, %: C 77.66; H 4.40; N 11.34.
C32H22N4O2. Calculated, %: C 77.72; H 4.48; N 11.33.
6-(4-Methylphenyl)-6,12-dihydro-5H-isoquino[2,3-a]quinazolin-5-one
(7).
A
mixture
of
o-bromomethylphenylacetonitrile (2.1 g, 10 mmol) and anthranilic acid 1a (1.37 g, 10 mmol) was heated on an
oil bath at 125-130°C for 1.5 h. Then, p-toluidine (1.07 g, 10 mmol) was added to the melt and heating was
continued at 175-180°C for an additional 2 h. After cooling, the melt was triturated with acetone (5 ml). The
precipitate was filtered off and washed with acetone to give 0.85 g (25%) 7, mp 138-140°C (DMF). IR spectrum
1
o
(neat), ν, cm-1: 1673 (C=O), 1619, 1570, 1481, 755. H NMR spectrum, δ, ppm (J, Hz): 7.89 (1H, d, J = 7.2,
H-4); 7.61 (1H, t, oJ = 8.0, H-2); 7.37 (2H, d, oJ = 8.4, H-2',6'); 7.33 (1H, d, J = 8.8, H-1); 7.16 (2H, d, oJ = 8.4,
H-3',5'); 7.12 (1H, d, J = 7.6, H-8); 7.00 (2H, m, H-3,10); 6.90 (1H, t, J = 7.2, H-9); 6.57 (1H, d, J = 7.2,
H-11); 5.15 (2H, s, C(12)H2); 4.21 (1H, s, H-7); 2.48 (3H, s, CH3). Found,%: C 81.55; H 5.30; N 8.29.
C23H18N2O. Calculated, %: C 81.63; H 5.36; N 8.28.
o
o
o
REFERENCES
1.
2.
L. M. Potikha, Khim. Geterotsikl. Soedin., 899 (2007). [Chem. Heterocycl. Comp., 43, 759 (2007)].
L. M. Potikha, V. A. Kovtunenko, and V. M. Kisil, Khim. Geterotsikl. Soedin., 562 (2007). [Chem.
Heterocycl. Comp., 43, 460 (2007)].
3.
4.
V. M. Kisil, V. A. Kovtunenko, A. V. Turov, A. K. Tyltin, and F. S. Babichev, Dokl. Akad. Nauk, 306,
628 (1989).
V. M. Kisil, L. M. Potikha, R. M. Gutsul, V. A. Kovtunenko, and A. V. Turov, Khim. Geterotsikl.
Soedin., 113 (2006). [Chem. Heterocycl. Comp., 42, 100 (2006)].
5.
6.
V. M. Kisil, V. A. Kovtunenko, A. K. Tyltin, and F. S. Babichev, Ukr. Khim. Zh., 56, 749 (1990).
F. S. Babichev, V. K. Patratii, V. A. Kovtunenko, N. G. Prodanchuk, V. G. Zinchenko, and V. M. Kisil,
Khim.-farm. Zh., 24, No. 5, 32 (1990).
7.
L. M. Potikha, V. A. Kovtunenko, A. V. Tarasevich, J. G. Wolf, and Ch. André, Khim. Geterotsikl.
Soedin., 430 (2007). [Chem. Heterocycl. Comp., 43, 347 (2007)].
8.
9.
C. A. G. Haasnoot, F. A. A. M. de Leeuw, and C. Altona, Tetrahedron, 36, 2783 (1980).
Kh. M. Shakhidoyatov, A. Irisbaev, L. M. Yun, E. Oripov, and Ch. Sh. Kadyrov, Khim. Geterotsikl.
Soedin., 1564 (1976). [Chem. Heterocycl. Comp., 12, 1286 (1976)].
10.
L. M. Potikha, N. V. Danileiko, V. M. Kisil, and V. A. Kovtunenko, Khim. Geterotsikl. Soedin., 1214
(2004). [Chem. Heterocycl. Comp., 40, 1052 (2004)].
899