New heteroaromatic derivatives of 6-amino-4-methylangelicin

Article abstract of DOI:10.1023/A:1017695719880

New heteroaromatic derivatives of 6-amino-4-methylangelicin, the pyrrolofurocoumarins and 6-pyrazolyl-4-methylangelicin, have been obtained.

Full text of DOI:10.1023/A:1017695719880

Chemistry of Heterocyclic Compounds, Vol. 37, No. 4, 2001
NEW HETEROAROMATIC DERIVATIVES
OF 6-AMINO-4-METHYLANGELICIN
I. I. Sakharuk, I. G. Makarov, and V. F. Traven
New heteroaromatic derivatives of 6-amino-4-methylangelicin, the pyrrolofurocoumarins and
6-pyrazolyl-4-methylangelicin, have been obtained.
Keywords: 6-azido-4-methylangelicin, aliphatic hydrazones, 6-amino-4-methylangelicin, 4-methyl-6-
(3,5-dimethyl-1-pyrazolyl)angelicin,
angelicin, Fischer method.
pyrrolofurocoumarins,
4-methyl-6-(p-toluenesulfonamido)-
Amongst the furocoumarins the amino derivatives of angelicin are the most promising derivatives for
studying biological activity. The high water-solubility provides active transport of a furocoumarin salt in
biological substrates and a correspondingly more marked pharmacological effect. The number of studies on the
synthesis of aminoangelicins is however small. 4-Aminoangelicins are obtained by condensing
5-aminomethyleno-6,7-dihydrobenzofuran-4-one [1] with dichloroacetic acid chloride in the presence of a
tertiary amine into 3-chloro-3,4,5,6-tetrahydroangelicin with subsequent dehydrochlorination and aromatization.
Aminomethyl derivatives of angelicins are obtained by substituting the halogen of a halomethyl group of the
appropriate angelicin [2].
Investigation of the biological activity of a series of aminoangelicins has confirmed their promise as
photochemotherapeutic agents [2, 4, 7]. Aminoangelicins studied in most detail contain an amino group on the
lactone or furan ring.
We have reported previously the synthesis of 6-amino-4-methylangelicin in which the amino group is
found on the benzene ring [8-13]. In the present work reactions of 6-amino-4-methylangelicin (1) are considered
which enable new heteroaromatic derivatives of angelicin to be synthesized. 6-Aminoangelicin (1) possesses
properties typical of aromatic amines. It is readily acylated at the amino group leading to 6-acetamido-,
6-formamido-, and 6-(p-toluenesulfonamido)-4-methylangelicins (compounds 2a,b and 3 respectively), and
forms Schiff's bases with aldehydes. For example 6-(p-hydroxyphenylaldimino)-4-methylangelicin (4) was
obtained in good yield.
Me
Me
Me
N
O
RHN
H2N
HO
CHO
O
O
O
O
O
O
O
O
HO
2a R = COMe
2b R = COH
3 R = Ts
1
4
__________________________________________________________________________________________
Russian D. I. Mendeleev University of Chemical Technology, Moscow 125047; e-mail:
traven@main-gw.muctr.edu.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 501-506,
April, 2001. Original article submitted April 8, 1999.
0009-3122/01/3704-0453$25.00©2001 Plenum Publishing Corporation
453

The amino group in 6-aminoangelicin 1 is subject to diazotization. The diazonium salt 5 obtained in this way
is reduced with stannous chloride in hydrochloric acid at low temperature (-15°C) to the corresponding hydrazine 6.
_
_
Me
Me
O
Cl
Cl
+
+
H
N
H3N
N
N
SnCl2
HCl
NaNO2
HCl
1
O
O
O
O
O
6
5
At higher temperatures (0°C) the reduction leads to obtaining of the initial amine 1. Possibly the
reaction passes through the intermediate formation of the hydrazine salt 6, since the hydrochloride of hydrazine
6 obtained previously is reduced smoothly under the same conditions to amine 1. The diazo group in compound
5 is readily substituted by other functional groups (for example azide). On reaction with sodium azide 6-azido-4-
methylangelicin (7) is obtained.
Reaction of hydrazine 6 with certain substituted benzaldehydes and ketones gives the corresponding
hydrazones 8,9.
Me
H
O
R1R2C
N N
R1
R2
6
O
O
O
8, 9
8 R1 = 4-O2N–C6H4, R2 = H; 9 R1 = 4-HO–C6H4, R2 = H
Aliphatic hydrazones from compound 6, in spite of their instability, are capable of cyclization by the
Fischer method to the previously unknown pyrrolofurocoumarins. Due to the presence of donor groups in the
benzene ring of angelicin position 5 is sufficiently reactive. Carbonyl components are most frequently used for
Fischer cyclization [15], and the cyclizing reagents used, according to [14], are mixtures of sulfuric and acetic
acids, hydrochloric and acetic acids, thionyl chloride and ethanol, and also polyphosphoric acid. When cyclizing
hydrazone 10 the best yield was achieved using polyphosphoric acid, but for hydrazones 12 and 14 a mixture of
hydrochloric and acetic acids was best.
O
Me
O
EtO
H
N
8
O
9
Me
7
Me
N
O
10 HN
Me
OEt
PPA
6
5
6
O
O
EtO
O
O
O
1O
2
4
10
3
11
Me
Me
HN
Me
Me
O
O
H
Et
N
N
HCl
Et
Me
Me
6
AcOH
O
O
O
O
O
12
13
454

Me
O
H
Me
O
N N
HN
O
O
HCl
6
O
O
AcOH
O
15
14
We also established that hydrazine 6 condensed with acetylacetone forming the corresponding
6-(3,5-dimethyl-1-pyrazolyl)-4-methylangelicin (16) in good yield.
Me
Me
O
O
N
N
Me
Me
Me
6
O
O
O
16
Probably the reaction occurs with the intermediate formation of the corresponding hydrazone and its
subsequent cyclization. To carry out such a type of cyclization using acetoacetic ester was unsuccessful. In this
case the reaction stopped at the stage of forming the corresponding hydrazone.
EXPERIMENTAL
The 1H NMR spectra were recorded on a Bruker WP 200 instrument with DMSO-d6 or CDCl3 as solvent
and TMS as internal standard. Mass spectra were recorded on a Finnigan MAT SSQ 710 instrument at an
ionizing voltage of 70 eV.
6-Acetamido-4-methylangelicin (2a). A mixture of compound 1 (0.1 g, 0.4 mmol) and acetic
anhydride (5 ml) was heated for 10 min then cooled to precipitate a solid. The product obtained was
1
recrystallized from acetic acid. Yield 0.1 g (90%); mp 279-281°C. H NMR spectrum (DMSO-d6), , ppm,
δ
J (Hz): 2.16 (3H, s, NAc); 2.45 (3H, d, 4-Me, JMe,3 = 1.1); 6.38 (1H, q, 3-H, J3,Me = 1.1); 7.27 (1H, d,
9-H, J9,8 = 2.1); 8.09 (1H, s, 5-H); 8.17 (1H, d, 8-H, J8,9 = 2.1); 10.14 (1H, s, NH). Mass spectrum: 257 (47), 215
(-CH2C=O, 100), 187 (-CH2C=O, -CO, 70), 159 (-CH2C=O, -2CO, 11). Found, %: C 65.20; H 4.55; N 5.39.
C14H11NO4. Calculated, %: C 65.37; H 4.31; N 5.44.
6-Formamido-4-methylangelicin (2b). Compound 1 (0.5 g, 2.3 mmol) was dissolved with heating in
formic acid (1 ml). The precipitate obtained was filtered off, and recrystallized from DMF. Yield 0.3 g (65%);
mp 238-240°C. 1H NMR spectrum (DMSO-d6), , ppm, J (Hz): 2.50 (3H, d, 4-Me, JMe,3 = 1.1); 6.32 (1H, q, 3-H,
δ
J
3,Me = 1.1); 7.23 (1H, d, 9-H, J9,8 = 2.1); 8.11 (1H, s, 5-H); 8.35 (1H, s, CH); 8.41 (1H, d, 8-H, J8,9 = 2.1); 10.51
(1H, s, NH). Mass spectrum: 243 (100), 215 (-CO, 21), 187 (-2CO,12), 159 (-3CO). Found, %: C 64.60; H 3.55;
N 5.69. C13H9NO4. Calculated, %: C 64.20; H 3.73; N 5.76.
4-Methyl-6-(4-toluenesulfonamido)angelicin (3). A mixture of compound 1 (0.2 g, 0.93 mmol), tosyl
chloride (0.2 g, 1 mmol), pyridine (0.5 ml), and chloroform (3 ml) was heated under reflux for 1 h. The
chloroform was distilled off and the residual reaction mixture was poured into water. The solid obtained was
1
filtered off, dried, and recrystallized from chloroform. Yield 0.27 g (70%); mp 215-218°C. H NMR spectrum
(DMSO-d6), , ppm, J (Hz): 2.33 (3H, d, 4-Me, JMe,3 = 1.1); 6.36 (1H, q, 3-H, J3,Me = 1.1); 7.21 (1H, d, 9-H,
δ
455

J
9,8 = 2.1); 7.25 (1H, s, 5-H); 7.32 (2H, d, 3'-H, 5'-H, J3',2' = J5',6' = 7.8); 7.63 (2H, d, 2'-H, 6'-H, J2',3' = J6',5' = 7.8);
8.04 (1H, d, 8-H, J8,9 = 2.1); 10.47 (1H, s, NH). Mass spectrum: 369 (16), 214 (– 4-MePhSO2, 100). Found, %:
C 61.63; H 4.11; N 3.93. C19H15NO5S. Calculated, %: C 61.78; H 3.71; N 3.79.
4-Methyl-6-angelicindiazonium Chloride (5). Compound 1 (0.22 g, 1 mmol) was dissolved with
heating in conc. hydrochloric acid (2 ml), the solution cooled to 0 to 2°C, and sodium nitrite (0.075 g, 1.1 mmol)
as a 30% solution was added with stirring. The mixture was stirred a further 30 min, the solution filtered, and
then used for the further reactions.
6-Azido-4-methylangelicin (7). Sodium azide (0.7 g, 1.1 mmol) was added as a 30% aqueous solution
to a solution of diazonium chloride 5 obtained by the method given above. The precipitated solid was filtered
off, washed several times with water, dried, and recrystallized from DMF. Yield 1.45 g (60%); mp 204°C
1
(decomp.). H NMR spectrum (DMSO-d6), , ppm, J (Hz): 2.48 (3H, d, 4-Me, JMe,3 = 1.1); 6.41 (1H, q, 3-H,
δ
J
3,Me = 1.1); 7.32 (1H, d, 9-H, J9,8 = 2.1); 7.37 (1H, s, 5-H); 8.23 (1H, d, 8-H, J8,9 = 2.1). Mass spectrum: 241 (3),
213 (-CO, 100). Found, %: C 59.50; H 2.85; N 17.27. C12H7N3O3. Calculated, %: C 59.76; H 2.93; N 17.42.
6-(4-Hydroxybenzylideneimino)-4-methylangelicin (4). p-Hydroxybenzaldehyde (0.11 g, 0.87 mmol)
was added to a solution of compound 1 (0.2 g, 0.87 mmol) in dioxane and the mixture was heated at 80°C for
15 min. The solid precipitated on cooling was filtered off, and recrystallized from dioxane. Yield 0.2 g (60%);
mp 250°C (decomp.). 1H NMR spectrum (DMSO-d6), , ppm, J (Hz): 2.42 (3H, d, 4-Me, JMe,3 = 1.1); 6.30 (1H,
δ
q, 3-H, J3,Me = 1.1); 6.93 (2H, d, 2'-H, 6'-H, J2',3' = J6',5' = 8.6); 6.96 (1H, s, 5-H); 7.20 (1H, d, 9-H, J9,8 = 2.1); 7.75
(2H, d, 3'-H, 5'-H, J3',2' = J5',6' = 8.6); 8.11 (1H, d, 8-H, J8,9 = 2.1); 9.79 (1H, s, CH). Mass spectrum: 319 (100),
291 (-CO, 31), 171 (– 4-OHPhCH=N, – CO, 28), 144 (– CO, – HCO, – 4-OHPhCH=N, 40). Found, %: C 71.50;
H 4.41; N 4.49. C19H13NO4. Calculated, %: C 71.47; H 4.10; N 4.39.
6-Hydrazino-4-methylangelicin (6). Stannous chloride (0.57 g, 3 mmol) in conc. hydrochloric acid
(1 ml) was added dropwise to a cooled solution of 4-methyl-6-angelicindiazonium chloride (1 mmol) so that the
temperature did not exceed 10°C. The precipitated white solid hydrazine salt was filtered off and dried in the
air. Yield 0.72 g (90%).
Hydrazones from 6-Hydrazino-4-methylangelicin (General Procedure). The appropriate ketone was
added to a solution of 6-hydrazino-4-methylangelicin 6 (0.8 g, 1 mmol) in ethanol at room temperature. The
precipitated solid was filtered off and recrystallized.
4-Methyl-6-(4-nitrobenzylidenehydrazino)angelicin (8). Yield 78%; mp 252-255°C (dioxane).
1H NMR spectrum (DMSO-d6), , ppm, J (Hz): 2.58 (3H, d, 4-Me, JMe,3 = 1.1); 6.32 (1H, q, 3-H, J3,Me = 1.1);
δ
7.19 (1H, d, 9-H, J9,8 = 2.1); 7.56 (1H, s, 5-H); 7.07 (1H, d, 8-H, J8,9 = 2.1); 7.85 (2H, d, 2'-H, 6'-H,
J
2',3' = J6',5' = 9.0); 7.94 (1H, s, CH); 8.28 (2H, d, 3'-H, 5'-H, J3',2' = J5',6' = 9.0). Mass spectrum: 363 (73), 214
(– 4-NO2PhCH=N, 96), 186 (– CO, – 4-NO2PhCH=N, 100). Found, %: C 62.50; H 4.41; N 11.49. C19H15N3O5.
Calculated, %: C 62.46; H 4.14; N 11.50.
6-(4-Hydroxybenzylidenehydrazino)-4-methylangelicin (9). Yield 82%; mp 247-250°C (dioxane).
1H NMR spectrum (DMSO-d6), , ppm, J (Hz): 2.49 (3H, d, 4-Me, JMe,3 = 1.1); 6.63 (1H, q, 3-H, J3,Me = 1.1);
δ
6.82 (2H, d, 2'-H, 6'-H, J2',3' = J6',5' = 8.6); 7.24 (1H, d, 9-H, J9,8 = 2.1); 7.42 (1H, s, 5-H); 7.54 (2H, d, 3'-H, 5'-H,
J3',2' = J5',6' = 8.6); 8.14 (1H, s, CH); 8.16 (1H, d, 8-H, J8,9 = 2.3); 9.64 (1H, d, OH); 10.37 (1H, d, NH). Mass
spectrum: 334 (40), 214 (– 4-OHPhCH=N, 98), 186 (– CO, – 4-OHPhCH=N, 100). Found, %: C 68.50; H 4.41;
N 8.49. C19H14N2O4. Calculated, %: C 68.26; H 4.22; N 8.38.
6-(1-Ethoxycarbonylethylidenehydrazino)-4-methylangelicin (10). Yield 80%; mp 198-200°C
1
(dioxane). H NMR spectrum (DMSO-d6), , ppm, J (Hz): 1.30 (3H, t, COOCH2CH3); 2.18 (3H, s, CH3C=N);
δ
2.48 (3H, d, 4-Me, JMe,3 = 1.1); 4.21 (2H, q, COOCH2CH3); 6.39 (1H, q, 3-H, J3,Me = 1.1); 7.29 (1H, d, 9-H,
J
9,8 = 2.1); 7.54 (1H, s, 5-H); 8.20 (1H, d, 8-H, J8,9 = 2.1); 9.88 (1H, s, NH). Mass spectrum: 328 (67), 214
(-CH3CNCOOC2H5, 100), 186 (-CH3CNCOOC2H5, -CO, 92). Found, %: C 62.24; H 4.72; N 8.46. C17H16N2O5.
Calculated, %: C 62.19; H 4.91; N 8.53.
456

1
6-(2-Butylidenehydrazino)-4-methylangelicin (12). Yield 82%; mp 144-146°C (dioxane). H NMR
spectrum (acetone-d6), , ppm, J (Hz): 1.20 (3H, t, CCH2CH3); 2.05 (3H, s, CH3C=N); 2.38 (2H, q, CCH2CH3);
δ
2.51 (3H, d, 4-Me, JMe,3 = 1.1); 6.25 (1H, q, 3-H, J3,Me = 1.1); 7.18 (1H, d, 9-H, J9,8 = 2.1); 7.53 (1H, s, 5-H);
7.97 (1H, d, 8-H, J8,9 = 2.1); 8.00 (1H, s, NH). Mass spectrum: 284 (48), 214 (-CH3CNC2H5, 94), 188
(-CH3CNC2H5, -CO, 100). Found, %: C 67.59; H 5.67; N 9.85. C16H16N2O3. Calculated, %: C 67.39; H 5.81;
N 9.43.
1
6-Cyclohexylidenehydrazino-4-methylangelicin (14). Yield 77%; mp 198-200°C (dioxane). H NMR
spectrum (CDCl3), , ppm, J (Hz): 1.63 (6H, s, 3'-CH2, 4'-CH2, 5'-CH2); 2.48 (3H, d, 4-Me, JMe,3 = 1.1); 2.50
δ
(4H, m, 2'-CH2, 6'-CH2); 6.32 (1H, q, 3-H, J3,Me = 1.1); 7.23 (1H, d, 9-H, J9,8 = 2.1); 7.37 (1H, s, 5-H); 8.13 (1H,
d, 8-H, J8,9 = 2.1); 8.87 (1H, s, NH). Mass spectrum: 310 (48), 214 (-(CH2)5=N, 100), 186 (-(CH2)5=N, -CO, 94).
Found, %: C 69.59; H 5.67; N 9.25. C18H18N2O3. Calculated, %: C 69.66; H 5.85; N 9.03.
e
g
Hydrazone
9-Ethoxycarbonyl-7-methyl-5-oxopyrano[3,2- ]furo[3,2- ]indole (11).
10
(0.33 g,
1 mmol) was added to polyphosphoric acid (6 ml) and the mixture stirred at 75°C for 15 min, then poured into
ice water. The precipitated solid was filtered off, washed with water, and recrystallized from acetic acid.
1
Yield 0.093 g (30%); mp 280°C (decomp.). H NMR spectrum (DMSO-d6), , ppm, J (Hz): 1.38 (3H, t,
δ
CH2CH3); 2.77 (3H, d, 7-Me, JMe,7 = 1.1); 4.39 (2H, q, CH2CH3); 6.37 (1H, d, 6-H, J6,Me = 1.1); 7.32 (1H, d, 3-H,
J
3,2 = 2.1); 7.82 (1H, s, 8-H); 8.19 (1H, d, 2-H, J2,3 = 2.1); 13.17 (1H, s, NH). Mass spectrum: 311 (35), 265
(-C2H5OH, 80), 237 (-C2H5OH, -CO, 100). Found, %: C 65.39; H 4.33; N 4.25. C17H13NO5. Calculated, %:
C 65.59; H 4.21; N 4.50.
e
g
7,8,9-Trimethyl-5-oxopyrano[3,2- ]furo[3,2- ]indole (13).
Several drops of hydrochloric acid were
added with stirring to a mixture of hydrazone 12 (0.28 g, 1 mmol) and acetic acid (5 ml). The mixture was
stirred at room temperature for 3 h, the precipitated solid was filtered off, dried, and recrystallized from DMF.
Yield 0.23 g (85%); mp 247°C (decomp.). 1H NMR spectrum (CDCl3), , ppm, J (Hz): 2.41 (3H, s, 8-Me); 2.47
δ
(3H, s, 9-CH3); 2.73 (3H, d, 7-Me, JMe,6 = 1.1); 6.21 (1H, d, 6-H, J6,Me = 1.1); 7.17 (1H, d, 3-H, J3,2 = 2.1); 7.55
(1H, d, 2-H, J2,3 = 2.1); 8.53 (1H, s, NH). Mass spectrum: 267 (100), 239 (-CO, 76), 225 (-CO, -CH2, 33).
Found, %: C 71.86; H 4.73; N 5.25. C16H13NO3. Calculated, %: C 71.90; H 4.90; N 5.24.
c
a
7-Methyl-5-oxo-8,9,10,11-tetrahydropyrano[2,3- ]furo[2,3- ]carbazole (15).
Several drops of
hydrochloric acid were added to a solution of hydrazone 14 (0.33 g, 1 mmol) in acetic acid (5 ml). The mixture
was heated under reflux for 1 h, then cooled, and poured onto ice. The precipitate was filtered off, dried, and
1
recrystallized from toluene. Yield 0.26 g (90%); mp 253°C (decomp.). H NMR spectrum (CDCl3), , ppm,
δ
J (Hz): 2.87-2.93 (8H, m, 8,9,10,11-CH2); 2.74 (3H, d, 7-Me, JMe,6 = 1.1); 6.21 (1H, d, 6-H, J6,Me = 1.1); 7.19
(1H, d, 3-H, J3,2 = 2.1); 8.47 (1H, d, 2-H, J2,3 = 2.1); 8.47 (1H, s, NH). Mass spectrum: 293 (71), 265 (-CO, 37),
237 (-2CO, 100). Found, %: C 73.86; H 5.23; N 4.65. C18H15NO3. Calculated, %: C 73.71; H 5.15; N 4.78.
6-(3,5-Dimethyl-1-pyrazolyl)-4-methylangelicin (16). Acetylacetone (0.86 g, 1.0 mmol) was added to
a solution of hydrazine 6 (0.8 g, 1.0 mmol) in alcohol at room temperature. The mixture was stirred until the
formation of a white solid, which was filtered off, and recrystallized from chloroform. Yield 2.1 g (70%);
1
mp 208-210°C. H NMR spectrum (DMSO-d6), , ppm, J (Hz): 2.21 (3H, s, 3'-Me); 2.34 (3H, s, 5'-Me); 2.51
δ
(3H, d, 4-Me, JMe,3 = 1.1); 6.09 (1H, s, 4'-H); 6.33 (1H, q, 3-H, J3,Me = 1.1); 7.22 (1H, d, 9-H, J9,8 = 2.1); 7.60
(1H, s, 5-H); 7.70 (1H, d, 8-H, J8,9 = 2.1). Mass spectrum: 294 (100), 199 (– 3'-CH3-pyrazolyl, – 5'-CH3-pyrazolyl,
49). Found, %: C 69.30; H 4.61; N 9.28. C17H14N2O3. Calculated, %: C 69.38; H 4.79; N 9.52.
REFERENCES
1.
2.
L. Mosti, P. Schenone, and G. Menozzi, J. Heterocycl. Chem., 25, 407 (1988).
F. Dall'Acqua, D. Vedaldi, S. Caffieri, A. Guiotto, P. Rodighiero, F. Baccichetti, F. Carlassare, and
F. Bordin, J. Med. Chem., 24, 178 (1981).
3.
J. Mazur, M. Grabowska, and T. Zawadowski, Acta Pol. Pharm., 44, 155 (1987).
457

4.
5.
A. Guiotto, P. Rodighiero, G. Pastorini, F. Bordin, F. Baccichetti, F. Carlassare, D. Vedaldi,
F. Dall'Acqua, and E. J. Land, Farmaco, Ed. Sci., 36, 536 (1981).
G. Bastian, L. Rene, J. P. Buisson, R. Royer, D. Averbeck, and S. Averbeck, Eur. J. Med. Chem.-Chim.
Ther, 16, 563 (1981).
6.
7.
8.
9.
Z. Kubaj, Roczn. Chem., 42, 1029 (1968).
P. Valenti, P. Montanari, G. Scapini, P. Giusti, and L. Cima, Arch. Pharm., 313, 449 (1980).
V. F. Traven, D. V. Kravtchenko, and T. A. Chibisova, Mendeleev Commun., 21 (1995).
V. F. Traven, D. V. Kravtchenko, T. A. Chibisova, S. V. Shorshnev, R. Eliason, and D. H. Wakefield,
Heterocycl. Commun., 2, 345 (1996).
10.
11.
12.
V. F. Traven, D. V. Kravtchenko, and T. A. Chibisova, Mendeleev Commun., 249 (1997).
D. V. Kravtchenko, V. F. Traven, and T. A. Chibisova, Heterocycl. Commun., 3, 331 (1997).
V. F. Traven, R. V. Rozhkov, A. Y. Tolmachev, N. F. Kuznecova, N. Y. Podhaluzina, and
E. A. Carberry, Heterocycl. Commun., 3, 4 (1997).
V. F. Traven, I. I. Saharuk, and D. V. Kravtchenko, Heterocycl. Commun., 4, 429 (1998).
W. J. Houligan (editor), The Chemistry of Heterocyclic Compounds. Indoles. Part II, New York (1972).
S. L. Vorob'eva, V. N. Buyanov, I. I. Levina, and N. N. Suvorov, Khim. Geterotsikl. Soedin., 783
(1989).
13.
14.
15.
458