Synthesis of new benzo[c]phenanthridine derivatives

Article abstract of DOI:10.1134/S1070428010120134

Three-component condensation of naphthalen-1-amine with triethyl orthoformate and dimedone or cyclopentane-1,3-dione, as well as the reaction of naphthalen-1-amine with 2-acetyl-5,5-dimethylcyclohexane-1,3-dione, gave the corresponding 2-[1-(α-naphthylamino)alkylidene]cycloalkane-1,3-diones which underwent intramolecular cyclization to 7,8,9,10-tetrahydrobenzo[c] phenanthridin-7-one derivatives on heating in polyphosphoric acid. 9,9-Dimethyl-7,8,9,10-tetrahydrobenzo[c]phenanthridin-7-one was reduced to 9,9-dimethyl-7,8,9,10-tetrahydrobenzo[c]phenanthridin-7-ol with sodium tetrahydridoborate. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070428010120134

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 12, pp. 1843–1847. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.N. Pyrko, 2010, published in Zhurnal Organicheskoi Khimii, 2010, Vol. 46, No. 12, pp. 1831–1834.
Synthesis of New Benzo[c]phenanthridine Derivatives
A. N. Pyrko
Sakharov International State Ecological University, ul. Dolgobrodskaya 23, Minsk, 220070 Belarus
e-mail: pyrko@yandex.ru
Received December 2, 2009
Abstract—Three-component condensation of naphthalen-1-amine with triethyl orthoformate and dimedone or
cyclopentane-1,3-dione, as well as the reaction of naphthalen-1-amine with 2-acetyl-5,5-dimethylcyclohexane-
1,3-dione, gave the corresponding 2-[1-(α-naphthylamino)alkylidene]cycloalkane-1,3-diones which underwent
intramolecular cyclization to 7,8,9,10-tetrahydrobenzo[c]phenanthridin-7-one derivatives on heating in poly-
phosphoric acid. 9,9-Dimethyl-7,8,9,10-tetrahydrobenzo[c]phenanthridin-7-one was reduced to 9,9-dimethyl-
7,8,9,10-tetrahydrobenzo[c]phenanthridin-7-ol with sodium tetrahydridoborate.
DOI: 10.1134/S1070428010120134
Synthesis of new benzo[c]phenanthridine deriva-
tives like V and VI attracts interest, taking into account
that such compounds exhibit a broad spectrum of bio-
logical activity [1, 2]. Among naturally occurring
analogs, benzo[c]phenanthridine alkaloids are known
[3, 4]. The synthesis of tetrahydrobenzo[c]phenanthri-
dine derivatives Vd and Ve via reaction of naphthalen-
1-amine (I) with 2-acylcyclohexane-1,3-diones IVd
and IVe and subsequent cyclization of enamino di-
ketones IIId and IIIe in polyphosphoric acid was
described previously [5] (Scheme 1). Martinez et al.
[6] synthesized compound Va in three steps. The re-
action of naphthalen-1-amine (I) with triethyl orthofor-
mate gave the corresponding formamidine VII which
reacted with dimedone (IIa) to produce imino ketoenol
IIIa, and the latter was subjected to cyclization on
heating in polyphosphoric acid.
The structure of the isolated compounds was con-
firmed by their elemental analyses and NMR, IR, and
mass spectra. All compounds displayed the molecular
ion peaks in the mass spectra. The structure of the
enamino diketone fragment in IIIa followed from the
presence of signals from two carbonyl carbon atoms
in the 13C NMR spectrum (δC 196.10, 200.18 ppm).
cis–trans Isomerism with respect to the exocyclic
double bond is not possible, for the substituents on the
C2 atom are similar. The s-cis orientation of the car-
bonyl group relative to the double bond is fixed by
1
the cyclohexane ring. The H NMR spectrum of IIIa
contained two downfield doublets at δ 13.66 and
8.68 ppm. The first of these corresponds to the NH
proton, and the second, to the olefinic proton coupled
3
with the NH proton. The coupling constant J is equal
to 12.5 Hz, indicating anti orientation of these protons
about the C–N bond. On the whole, the 1H NMR spec-
trum of IIIa corresponds to the cis-s-cis structure of
the enamino carbonyl system [7, 8]. Obviously, the
formation of intramolecular hydrogen bond is the main
factor stabilizing the antiplanar conformation which
appears due to rotation about the C–N bond in mole-
cules IIIa–IIIe. In keeping with published data, cis-s-
cis conformation of enamino carbonyl system is more
stable than imino ketone or imino enol [7, 9]. The
compound obtained in [6] was assigned imino enol
structure A with Z configuration [10] of substituents
about the double C=N bond. Obviously, this structure
excludes formation of intramolecular hydrogen bond
between the enol proton and nitrogen atom, whereas
the chemical shift of the hydroxy proton (δ 11.3 ppm)
In the present work tetracyclic compound Va was
synthesized in two steps. Three-component condensa-
tion of naphthalen-1-amine (I) with triethyl ortho-
formate and dimedone (IIa) in the presence of acetic
acid afforded enamino diketone IIIa which was then
heated in polyphosphoric acid to obtain the target
product, benzo[c]phenanthridine derivative Va
(Scheme 1). Likewise, three-component reaction of
naphthalen-1-amine (I) with triethyl orthoformate and
cyclopentane-1,3-dione (IIb) gave enamino diketone
IIIb. However, compound IIIb failed to undergo intra-
molecular cyclization on heating in polyphosphoric
acid at 170°C, whereas a complex mixture of products
was formed at a higher temperature.
1843

PYRKO
1844
Scheme 1.
H
·
·
·
O
O
O
R
X
I
+
X
X
O
O
R
O
IVc, IVd, IVe
H
NH2
O
N
R
N
X
+
CH(OEt)3 +
O
I
IIa, IIb
IIIa–IIIe
Va, Vc–Ve
NaBH4
OH
N
N
CH(OEt)3
IIa
H
I
N
Me
Me
VII
VI
X = CMe2 (a, c, e), bond (b), CH2 (d); R = H (a, b), Me (c), Et (d), Ph (e).
s-trans imino ketoenol structure also excludes forma-
suggests that such hydrogen bond does exist. In terms
of s-cis–s-trans isomerism for a system of two con-
jugated double bonds [7], structure A is characterized
by cis-s-cis configuration. In addition, cis-s-trans con-
figuration is possible, but it also excludes intramolec-
ular hydrogen bonding. The formation of intramolec-
ular hydrogen bond could be expected for the E
isomer; however, in this case trans-s-cis imino enol
structure B and the enamino ketone structure of IIIa
shown in Scheme 1 are tautomeric, and they cannot be
distinguished in the NMR spectra. The transformation
from one tautomer to another involves proton transfer
from the nitrogen atom in the enamino tautomer to the
ketone carbonyl oxygen atom and vice versa, from the
enol oxygen atom to the imino nitrogen atom. The
energy barrier to such transition is low. Possible trans-
tion of intramolecular hydrogen bond. Although there
1
are some differences in the H NMR spectra of
samples of IIIa isolated in the present work and in [6],
they correspond most probably to the same compound,
for transformation of any of the above isomeric struc-
ture into another form could occur via intra- or inter-
molecular proton exchange [7]. Thus compound IIIa
may be presumed to exist in solution an equilibrium
mixture of tautomers, the enamino ketone structure
prevailing.
1
The H NMR spectrum of enamino diketone IIIb
contained a downfield broadened singlet at
δ 12.66 ppm, which was assigned to the NH proton
involved in intramolecular hydrogen bond with the
carbonyl oxygen atom.
Naphthalen-1-amine (I) reacted with triketone IVc
to give enamino diketone IIIc which showed in the
1H NMR spectrum a signal at δ 15.16 ppm due to the
NH proton (H-bonded). The presence in the 13C NMR
spectra of enamino diketones IIId and IIIe of signals
from two carbonyl carbon atoms (δC 196.54 and
201.36 ppm for IIId and δC 195.15 and 200.55 ppm for
IIIe) indicated that these compounds exist as enamino
diketone tautomers.
Me
Me
O
O
O
Me
Me
H
·
·
·
N
N
OH
1
In the H NMR spectra of compounds Va and Vc–
A
B
Ve a one-proton singlet was observed in a weak field
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SYNTHESIS OF NEW BENZO[c]PHENANTHRIDINE DERIVATIVES
1845
(δ 9.35–9.45 ppm, 6-H). The reduction of ketone Va
with sodium tetrahydridoborate gave alcohol VI whose
spectral parameters and elemental composition were
consistent with the assumed structure. The coupling
constants for the 7-H proton and protons in the neigh-
boring methylene group (J = 5.0, 12.0 Hz) indicated
pseudoaxial orientation of 7-H and hence pseudoequa-
torial orientation of the hydroxy group in the cyclo-
hexene ring occurring in a half-chair conformation.
H2O]. Found, %: C 77.61; H 6.29; N 4.83. C19H19NO2.
Calculated, %: C 77.68; H 6.20; N 4.77.
2-[(Naphthalen-1-ylamino)methylidene]cyclo-
pentane-1,3-dione (IIIb). A mixture of 1.43 g
(10 mmol) of naphthalen-1-amine (I), 0.98 g
(10 mmol) of cyclopentane-1,3-dione (IIb), 3 ml of
triethyl orthoformate, and 2 ml of glacial acetic acid
was heated for 1.5 h under reflux. Volatile compounds
were removed under reduced pressure, and the residue
was purified by column chromatography on silica gel
(Silicagel L, 100–160 μm) using ethyl acetate–hexane
(1:3, 1:2, 1:1) as eluent. Yield 1.53 g (61%), mp 164–
166°C (from EtOAc). IR spectrum, ν, cm–1: 3440
(NH); 1700, 1635 (C=O); 1625, 1615, 1598 (C=C);
1
It should also be noted that the H NMR and mass
spectra given in [6] for the reaction product of ketone
Va with ethylene glycol coincided with the corre-
sponding spectral data for the initial ketone rather than
for the assumed cyclic acetal.
1
1580, 1520, 1490. H NMR spectrum, δ, ppm: 2.32 m
(4H, CH2), 7.30 m (2H, Harom), 7.48 m (2H, Harom),
7.62 d (1H, J = 8 Hz), 7.76 d (1H, J = 8 Hz), 7.86 d
(1H, J = 8 Hz), 8.16 s (1H), 12.66 br.s (NH). Mass
spectrum: m/z 256 [M]+. Found, %: C 76.41; H 5.30;
N 5.63. C16H13NO2. Calculated, %: C 76.48; H 5.21;
N 5.57.
EXPERIMENTAL
The progress of reactions and the purity of products
were monitored by TLC on Silufol UV-254 plates
using ethyl acetate as eluent; spots were visualized
under UV light or by treatment with iodine vapor. The
melting points were determined on a Boetius melting
point apparatus. The IR spectra were recorded in KBr
on a UR-20 instrument. The mass spectra (electron im-
pact, 70 eV) were obtained on a Varian MAT-311 mass
spectrometer with direct sample admission into the ion
5,5-Dimethyl-2-[1-(naphthalen-1-ylamino)ethyli-
dene]cyclohexane-1,3-dione (IIIc). A mixture of
1.82 g (10 mmol) of 2-acetyl-5,5-dimethylcyclohex-
ane-1,3-dione (IVa) and 1.43 g (10 mmol) of naphtha-
len-1-amine in 60 ml of toluene was heated for 2 h
under reflux in a flask equipped with a Dean–Stark
trap. The solvent was distilled off, and the residue was
crystallized from 20 ml of ethyl acetate. Yield 2.70 g
(88%), mp 149–151°C (from EtOAc). IR spectrum, ν,
cm–1: 3440 (N–H); 1645, 1630, 1580 (C=C, C=O).
1H NMR spectrum, δ, ppm: 1.12 s (6H, CH3), 2.46 s
(5H, CH2, CH3), 2.58 s (CH2), 7.30 m (1H), 7.56 m
(3H), 7.88 m (3H), 15.16 br.s (NH). Mass spectrum,
m/z: 307 [M]+, 279 [M – CO]. Found, %: C 78.21;
H 6.99; N 4.64. C20H21NO2. Calculated, %: C 78.13;
H 6.88; N 4.56.
source. The H and 13C NMR spectra were measured
on a Bruker AC-200 instrument from solutions in
CDCl3; the 13C NMR spectra were recorded with
decoupling from protons.
1
5,5-Dimethyl-2-[(naphthalen-1-ylamino)methyli-
dene]cyclohexane-1,3-dione (IIIa). A mixture of
1.43 g (10 mmol) of napthalen-1-amine (I), 1.4 g
(10 mmol) of 5,5-dimethylcyclohexane-1,3-dione (IIa,
dimedone), 3 ml of of triethyl orthoformate, and 2 ml
of acetic acid was heated for 1.5 h under reflux. Vola-
tile compounds were removed under reduced pressure,
and the residue was crystallized from ethyl acetate.
Yield 2.50 g (85%), mp 148–150°C (from EtOAc).
IR spectrum, ν, cm–1: 3450 (N–H); 1677, 1605, 1580
Compounds IIId and IIIe were synthesized in
a similar way.
2-[1-(Naphthalen-1-ylamino)propylidene]cyclo-
hexane-1,3-dione (IIId) was synthesized from 1.68 g
(10 mmol) of 2-(1-oxopropyl)cyclohexane-1,3-dione
(IVd) and 1.43 g (10 mmol) of naphthalen-1-amine.
Yield 2.67 g (91%), mp 172–174°C (from EtOAc). IR
spectrum, ν, cm–1: 3440 (N–H); 1650, 1570, 1550
1
(C=O, C=C). H NMR spectrum, δ, ppm: 1.08 s (6H,
CH3), 2.39 s (2H, CH2), 2.46 s (2H, CH2), 7.30–7.70 m
(5H, Harom), 7.79 d (1H, Harom, J = 7 Hz), 8.00 d (1H,
Harom, J = 7 Hz), 8.68 d (1H, CH=, J = 12 Hz), 13.66 d
(1H, NH, J = 12 Hz). 13C NMR spectrum, δC, ppm:
28.57 (CH3), 31.16 (C3), 51.26 and 51.53 (C2, C4),
109.53 (C6), 114.07 (CH), 120.44 (CH), 125.23 (C),
125.60 (CH), 126.88 (CH), 127.29 (CH), 128.57 (CH),
134.02 (C), 134.60 (C), 151.75 (CH), 196.10 (CO),
200.18 (CO). Mass spectrum, m/z: 293 [M]+, 275 [M –
1
(C=C, C=O). H NMR spectrum, δ, ppm: 1.04 t (3H,
CH3, J = 7 Hz), 2.00 q (2H, CH2, J = 7 Hz), 2.62 m
(4H, CH2), 2.86 q (2H, J = 7 Hz), 7.30–7.90 m (7H,
H
arom), 15.29 br.s (NH). 13C NMR spectrum, δC, ppm:
13.32 (CH3), 19.35 (CH2), 24.74 (CH2), 38.88 (CH2),
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PYRKO
1846
40.10 (CH2), 108.38 (C6), 122.47 (CH), 124.09 (CH),
125.13 (CH), 126.92 (CH), 127.41 (CH), 128.35 (CH),
128.59 (CH), 129.78 (C), 132.85 (C), 134.16 (C),
179.72 (C), 196.54 (CO), 201.36 (CO). Mass spec-
trum, m/z: 293 [M]+, 265 [M – CO]. Found, %:
C 77.63; H 6.24; N 4.85. C19H19NO2. Calculated, %:
C 77.68; H 6.20; N 4.77.
1.54 g (5 mmol) of enamino diketone IIIc. Yield
1.28 g (88%), mp 145–147°C (from EtOH). IR spec-
trum, ν, cm–1: 1678 (C=O); 1628, 1570 (C=C, C=N).
1H NMR spectrum, δ, ppm: 1.15 s (6H, CH3), 2.54 s
(CH2), 3.07 s (CH3), 3.18 s (CH2), 7.64–7.92 m (5H),
9.35 m (1H, 12-H). Mass spectrum, m/z: 289 [M]+,
274 [M – CH3], 261 [M – CO]. Found, %: C 83.09;
H 6.71; N 4.91. C20H19NO. Calculated, %: C 83.01;
H 6.62; N 4.84.
5,5-Dimethyl-2-[α-(naphthalen-1-ylamino)ben-
zylidene]cyclohexane-1,3-dione (IIIe) was synthe-
sized from 2.44 g (10 mmol) of 2-benzoylcyclohexane-
1,3-dione (IVe) and 1.43 g (10 mmol) of naphthalen-1-
amine. Yield 3.33 g (90%), mp 201–202°C (from ethyl
acetate). IR spectrum, ν, cm–1: 3440 (N–H); 1658,
6-Ethyl-7,8,9,10-tetrahydrobenzo[c]phenanthri-
din-7-one (Vd) was synthesized from 1.46 g (5 mmol)
of enamino diketone IIId. Yield 1.22 g (89%),
mp 170–172°C (from EtOAc). IR spectrum, ν cm–1:
1
1
1580, 1536 (C=C, C=O). H NMR spectrum, δ, ppm:
1680 (C=O); 1562, 1509 (C=C, C=N). H NMR spec-
1.16 s (6H, CH3), 2.38 s (2H, CH2), 2.64 s (2H, CH2),
6.74 d (1H, Harom, J = 7 Hz), 6.90–7.70 m (9H, Harom),
8.10 d (1H, Harom, J = 7 Hz), 15.05 br.s (NH).
13C NMR spectrum, δC, ppm: 28.47 (CH3), 30.30 (C3),
52.37 (CH2), 53.24 (CH2), 108.83 (C6), 122.14 (CH),
123.60 (C), 124.61 (CH), 126.54 CH), 127.17 (CH),
127.31 (C), 127.80 (CH), 128.27 (CH), 128.74 (CH),
129.41 (CH), 133.14 (CH), 133.65 (CH), 134.06 (C),
171.06 (C), 195.15 (CO), 200.55 (CO). Mass spec-
trum, m/z: 369 [M]+, 341 [M – CO]. Found, %:
C 81.21; H 6.20; N 3.86. C25H23NO2. Calculated, %:
C 81.27; H 6.27; N 3.79.
trum, δ, ppm: 1.46 t (3H, CH3, J = 7 Hz), 2.22 q (2H,
CH2, J = 6 Hz), 2.72 t (2H, CH2, J = 6 Hz), 3.34 t (2H,
CH2, J = 6 Hz), 3.44 q (2H, CH2, J = 7 Hz), 7.76 m
(5H, Harom), 9.39 m (1H, Harom). 13C NMR spectrum,
δC, ppm: 13.03 (CH3), 21.97 (CH2), 26.19 (CH2), 31.89
(CH2), 40.16 (CH2), 120.74 (CH), 122.03 (C), 124.65
(C), 125.60 (CH), 127.01 (CH), 127.15 (CH), 127.45
(CH), 128.80 (CH), 131.33 (C), 134.16 (C), 146.12
(C), 151.31 (C), 162.47 (C), 199.13 (CO). Mass spec-
trum: m/z 275 [M]+. Found, %: C 82.80; H 6.31; N 5.17.
C19H17NO. Calculated, %: C 82.87; H 6.23; N 5.09.
9,9-Dimethyl-6-phenyl-7,8,9,10-tetrahydrobenzo-
[c]phenanthridin-7-one (Ve) was synthesized from
1.80 g (5 mmol) of enamino diketone IIIe. Yield
1.54 g (88%), mp 246– 247°C (from EtOAc). IR spec-
trum, ν, cm–1: 1678 (C=O); 1570, 1552, 1500 (C=C,
9,9-Dimethyl-7,8,9,10-tetrahydrobenzo[c]phen-
anthridin-7-one (Va). A mixture of 1.46 g (5 mmol) of
enamino diketone IIIa and 40 g of polyphosphoric
acid (prepared from 134 g of P2O5 and 65 ml of
H3PO4) was stirred for 1.5 h at 170°C. The mixture
was cooled, carefully diluted with 50 ml of ice water,
and neutralized with solid potassium hydroxide to
pH 6.0. The precipitate was filtered off, washed with
water, and dried in air. Yield 1.24 g (90%), mp 196–
197°C (from ethyl acetate). IR spectrum, ν, cm–1: 1620
(C=N), 1572, 1513. 1H NMR spectrum, δ, ppm: 1.22 s
(6H, CH3), 2.50 s (CH2), 3.14 s (CH2), 7.70–7.96 m
(5H), 9.34 m (1H), 9.45 s (1H, 6-H). 13C NMR spec-
trum, δC, ppm: 28.56 (CH3), 33.13 (C9), 38.95 (C10),
51.59 (C8), 120.72 (CH), 123.94 (C), 124.23 (C),
125.60 (CH), 127.32 (CH), 127.37 (CH), 127.65 (CH),
129.11 (CH), 131.52 (C), 134.08 (C), 146.90 (CH),
148.32 (C), 197.70 (CO). Mass spectrum, m/z: 275
[M]+, 260 [M – CH3], 247 [M – CO]. Found, %:
C 82.79; H 6.29; N 5.15. C19H17NO. Calculated, %:
C 82.87; H 6.23; N 5.09.
1
C=N). H NMR spectrum, δ, ppm: 1.22 s (6H, CH3),
2.62 s (2H, CH2), 3.34 s (2H, CH2), 7.40–8.00 m (10H,
Harom), 9.36 m (1H, Harom). 13C NMR spectrum, δC,
ppm: 28.64 (CH3), 33.36 (C9), 40.12 (C10), 53.11 (C8),
120.64 (CH), 122.99 (C), 124.72 (C), 125.80 (CH),
127.21 (CH), 127.48 (CH), 127.77 (CH), 127.98 (CH),
128.09 (CH), 128.58 (C), 129.10 (CH), 131.46 (C),
134.22 (CH), 142.11 (C), 146.46 (C), 149.98 (C),
157.76 (C), 198.23 (CO). Mass spectrum: m/z 351
[M]+. Found, %: C 85.39; H 6.11; N 4.05. C25H21NO.
Calculated, %: C 85.44; H 6.02; N 3.99.
9,9-Dimethyl-7,8,9,10-tetrahydrobenzo[c]phen-
anthridin-7-ol (VI). A mixture of 0.3 g of ketone Va,
50 ml of propan-2-ol, 5 ml of water, and 0.5 g of
NaBH4 was stirred for 3 h and was then kept for 20 h
at room temperature. The solvent was distilled off, and
the residue was treated with 50 ml of ethyl acetate and
50 ml of water. The organic phase was separated and
dried over magnesium sulfate, the solvent was distilled
off, and the residue was crystallized from ethyl acetate.
Compounds Vc–Ve were synthesized in a similar
way.
6,9,9-Trimethyl-7,8,9,10-tetrahydrobenzo[c]-
phenanthridin-7-one (Vc) was synthesized from
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SYNTHESIS OF NEW BENZO[c]PHENANTHRIDINE DERIVATIVES
1847
3. Dostal, J. and Potacek, M., Collect. Czech. Chem.
Commun., 1990, vol. 55, p. 2840.
4. Olugbade, T.A., Waigh, R.D., and Mackay, S.P.,
Yield 0.22 g (73%), mp 176–178°C (from EtOAc). IR
1
spectrum, ν, cm–1: 1578, 1515 (C=C, C=N). H NMR
spectrum, δ, ppm: 0.90 s (3H, CH3), 1.04 s (3H, CH3),
1.46 t (1H, 8-Hax, J = 12 Hz), 2.02 d.d (1H, 8-Heq, J =
5, 12 Hz), 2.47 and 2.81 (1H each, 10-H, AB system,
J. Chem. Soc., Perkin Trans. 1, 1990, p. 2657.
5. Pyrko, A.N., Khim. Geterotsikl. Soedin., 1990, p. 1693.
J
AB = 17 Hz), 4.98 d.d (1H, 7-Hax, J = 5, 12 Hz), 7.60–
6. Martinez, R., Toscano, R.A., Linzaga, I., and San-
chez, H., J. Heterocycl. Chem., 1992, vol. 29, p. 1385.
7.98 m (5H, Harom), 9.15 s (1H, 6-H), 9.24 d (12-H,
J = 8 Hz). Mass spectrum, m/z: 277 [M]+, 276
[M – H], 260 [M – OH], 244, 221. Found, %: C 82.20;
H 7.00; N 5.11. C19H19NO. Calculated, %: C 82.28;
H 6.90; N 5.05.
7. Freimanis, Ya.F., Khimiya enaminoketonov, enamino-
iminov, enaminotionov (Chemistry of Enamino Ketones,
Enamino Imines, and Enamino Thiones), Riga: Zinatne,
1974, p. 274.
8. Pyrko, A.N., Khim. Geterotsikl. Soedin., 1999, p. 774.
9. Yakimovich, S.I. and Zerova, I.V., Enaminy v organi-
cheskom sinteze (Enamines in Organic Synthesis),
Aleksandrov, B.B., Ed., Sverdlovsk: Ural. Otd. Akad.
Nauk SSSR, 1990, p. 90.
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