Synthesis of 3,4-dihydroisoquinolines using nitroalkanes in polyphosphoric acid

Article abstract of DOI:10.1007/s11172-019-2518-z

A new method for the synthesis of 6,7-dimethoxy-3,4-dihydroisoquinoline based on the reaction of 2-(3,4-dimethoxyphenyl)ethan-1-amine (homoveratrylamine) with aliphatic nitro compounds in polyphosphoric acid (PPA) was developed.

Full text of DOI:10.1007/s11172-019-2518-z

Russian Chemical Bulletin, International Edition, Vol. 68, No. 5, pp. 1047—1051, May, 2019
1047
Synthesis of 3,4-dihydroisoquinolines using nitroalkanes in polyphosphoric acid
N. A. Aksenov,a V. V. Malyuga,a G. M. Abakarov,b D. A. Aksenov,a L. G. Voskressensky,с and A. V. Aksenova
aNorth Caucasus Federal University,
1A ul. Pushkina, 355009 Stavropol, Russian Federation.
E-mail: radioanimation@rambler.ru
bDagestan State Technical University,
70 ul. Imama Shamilya, 367010 Makhachkala, Russian Federation
сPeoples´ Friendship University of Russia,
6 ul. Miklukho-Maklaya, 117198 Moscow, Russian Federation
A new method for the synthesis of 6,7-dimethoxy-3,4-dihydroisoquinoline based on the
reaction of 2-(3,4-dimethoxyphenyl)ethan-1-amine (homoveratrylamine) with aliphatic nitro
compounds in polyphosphoric acid (PPA) was developed.
Key words: phenylethylamines, 6,7-dimethoxy-3,4-dihydroisoquinoline, nitro compounds,
polyphosphoric acid, alkaloids.
Modern organic chemistry is mainly governed by the
interest of the pharmaceutical industry to generation of
new synthetic drugs, as well as modification of already
existing ones in order to increase their activity and reduce
their toxicity. The solution of these problems is impossible
without using new effective synthetic methods. Such
techniques should not involve usage of heavy metals and
other toxic compounds that complicate the following
purification of the target products. Therefore, the search
for methods of assembling new structures that enables one
to effectively synthesize large libraries of compounds for
screening biological activity is still an urgent task. The
isoquinoline fragment as well as partially hydrogenated
isoquinoline derivatives are often found in natural com-
pounds, for example, in the structure of alkaloids of the
isoquinoline series, such as papaverine 1, barberine 2,
scoulerine 3, cochlaurine 4, No-spa 5, etc. Many of these
alkaloids demonstrate different physiological activities and
therefore various derivatives of this heterocyclic system,
including partially hydrogenated, are widely used for the
search of new drugs.1—7
The synthesis of 3,4-dihydroisoquinolines usually
involves the assembly of the pyridine core by the Bischler—
Napieralski,8—10 Ritter reactions,11—13 and by other
methods (see, for example, Ref. 14). Carboxylic acid de-
rivatives, which are not always readily available, are often
utilized in these methods. In addition, the possibility of
functionalization of isoquinolines and their derivatives
with partially hydrogenated heterocycles should be men-
tioned.15—17
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1047—1051, May, 2019.
1066-5285/19/6805-1047 © 2019 Springer Science+Business Media, Inc.

1048 Russ. Chem. Bull., Int. Ed., Vol. 68, No. 5, May, 2019
Aksenov et al.
In the present work, we report on a highly effective
modification of the Bischler—Napiralski reaction, which
serves to produce 3,4-dihydroisoquinolines from activated
nitroalkanes but not from carboxylic acid derivatives.
The binary phosphoric anhydride—water system has
great potential from the viewpoint of the design of "smart"
reaction media for conducting organic reactions catalyzed
by Brønsted acids. Thus, when the content of phosphoric
anhydride is over 75%, polyphosphoric acid (PPA) is pres-
ent in the system, which allows one to tune such physical
and chemical parameters of the medium as polarity, ef-
fective acidity, and anhydride activity. The behavior of
nitroalkanes in PPA is of particular interest. Nitro com-
pounds are one of the most convenient and widely used
in organic synthesis of synthons for building the carbon—
carbon and carbon—heteroatom bonds.18,19 The avail-
ability of aliphatic nitro compounds, as well as their high
reactivity, make them attractive building blocks in the
synthesis of a wide variety of natural and biologically active
nitrous compounds.20 Nevertheless, utilization of nitro com-
pounds in organic synthesis is limited mainly by the intro-
duction of electrophilic reagents in the -position. This fact,
evidently, limits the scope of their application to basic and
weakly acidic media. In PPA, nitroalkanes 6 form a stable,
double phosphorylated aci-form 7, which can act as an
electrophilic component in many selective processes that
proceed similarly to the Nef reaction with C-nucleo-
philes21—27 and N-nucleophiles28—30 participating. Some
examples of such reactions in which C- and N-nucleophiles
participate simultaneously (Scheme 1) are reported.31
The reaction of the aci-form 7 with electron rich aro-
matic hydrocarbons usually results in phosphorylated
N-hydroxyhydroxylamines 8, which further eliminate
orthophosphoric acid to form phosphorylated oximes 9.
Under the same conditions, oximes 9 can participate in
either the Beckmann rearrangement or the Wagner—
Meerwein rearrangement providing an additional oppor-
tunity for the introduction of functional groups into the
aromatic ring (see Scheme 1).21—30 The reaction of the
aci-form 7 with N-nucleophiles yields a stabilized ami-
dinium ion 10. In the presence of a second nucleophilic
functional group or an aromatic ring, intramolecular cyclo-
condensation is possible, which allowed us to develop
effective synthetic approaches to imidazoles 11, oxazoles
12,28 tetraazopyrenes 13,29 and 11H-indolo[ 3,2-c] quin-
olines 1431 (see Scheme 1).
Application of nitro compounds instead of carboxylic
acids has several advantages. First, nitro compounds, in
particular 2-aryl-1-nitroethanes, synthesized by the reduc-
tion of the corresponding nitroalkenes, are readily available
compounds. Secondly, utilization of nitro compounds
made it possible to decrease the reaction temperature by
100 C compared to a similar method in which carboxylic
acids were used.32
We suggested that the same strategy can be applied in
the synthesis of 3,4-dihydroisoquinolines 15 by the reac-
tion of nitro alkanes, as activated electrophiles, with
2-(3,4-dimethoxyphenyl)ethane-1-amine (16). The usage
of aliphatic amines in such transformations has not been
reported earlier. To verify this suggestion, the mixture of
Scheme 1

6,7-Dimethoxy-3,4-dihydroisoquinolines
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 5, May, 2019
1049
standard. Electrospray ionization mass spectra were obtained
using a Bruker maXis impact time-of-flight mass spectro-
meter equipped with a direct injection system. A mixture of
HCO2Na—HCO2H was used to calibrate the instrument. IR
spectra were measured on a Shimadzu IR Affinity-1S FT-IR
spectrometer equipped with an attenuated total reflectance attach-
ment. Purity of isolated compounds were controlled by TLC on
Silufol UV-254 plates, eluting with ethanol—ethyl acetate (2 : 3);
UV radiation was used for the visualization of the spots of com-
pounds. Melting points were measured on a Stuart SMP30 ap-
paratus.
11H-Indolo[3,2-c]isoquinolines 15a—f (general protocol).
A mixture containing 90 mg (0.5 mmol) of homoveratrylamine
16, 1.5 mmol of nitroalkane 6a—f, and polyphosphoric acid
(1.0 g, P2O5 content was 86%) was placed in an Erlenmeyer flask
and stirred at 100—120 C (100 C in the case of nitromethane
(6a) and nitroethane (6b)) until complete consumption of the
reactants (ca. 1.5 h, TLC monitoring). Then the reaction mixture
was colled down to room temperature, diluted with water (40 mL),
and neutralized with 20% aqueous ammonia (ca. 7 mL) to obtain
alkaline medium. The resulted mixture was extracted with ethyl
acetate (4×15 mL), solvents were evaporated in vacuo, and the
product was isolated by column chromatography (ethanol—
ethyl acetate, 1 : 101 : 1.5).
amine 16 with nitroethane 6b (R = Me) was stirred in 86%
polyphosphoric acid at 100 C for 2 h. Under these condi-
tions, the corresponding 6,7-dimethoxy-3,4-dihydroiso-
quinoline (15b) was obtained with a yield of 63% (Scheme 2).
Similar reactions were carried out with nitroalkanes 6a,
c—f, however, in these cases, the reaction temperature was
elevated in accordance with a decrease in the volatility of
the nitroalkane used, which allowed us to reduce the reac-
tion time. Reactions involving all the used nitroalkanes
6a—f proceeded smoothly to give 6,7-dimethoxy-3,4-
dihydroisoquinolines 15a—f in the yields of 36–63% (see
Scheme 2).
Scheme 2
6,7-Dimethoxy-3,4-dihydroisoquinoline (15a). Yield 50%,
light yellow liquid. Rf 0.52 (ethyl acetate—EtOH, 3 : 2). IR,
ν/cm–1: 2949, 2835, 1630, 1611, 1577, 1519, 1458, 1275, 1119.
1Н NMR, : 2.73—2.63 (m, 2 H, C(4)); 3.72 (t, 2 H, C(3),
J = 7.2 Hz); 3.89 (s, 3 H, C(6)OMe); 3.91 (s, 3 H, C(7)OMe);
6.66 (s, 1 H, C(5)); 6.80 (s, 1 H, C(8)); 8.22 (s, 1 H, C(1)).
13C NMR, : 24.9(C(4)), 47.4 (C(3)), 56.1 (C(6)OMe), 56.2
(C(7)OMe), 110.4 (C(7)), 110.5 (C(8)), 121.5 (C(4a)), 130.0
(C(8a)), 147.9 (C(6)), 151.3 (C(7)), 159.8 (C(1)). MS, found:
m/z 192.1024 [M + H]+. Calculated for C11H14NO2: 192.1019.
6,7-Dimethoxy-1-methyl-3,4-dihydroisoquinoline (15b). Yield
63%, light brown solid. M.p. 87.2—89.5 С. Rf 0.21 (ethyl
acetate—EtOH, 3 : 2). IR, ν/cm–1: 2945, 2839, 2362, 1668, 1626,
1
1607, 1577, 1515, 1378, 1325, 1275, 1218, 1161, 1060. Н NMR,
: 2.26 (s, 3 H, C(1)Me); 2.52 (t, 2 H, C(4), J = 7.6 Hz); 3.51
(t, 2 H, C(3), J = 7.6 Hz); 3.79 (s, 3 H, C(6)OMe); 3.80 (s, 3 H,
C(7)OMe); 6.58 (s, 1 H, C(5)); 6.88 (s, 1 H, C(8)). 13C NMR,
: 23.1 (C(1)Me), 25.5 (C(4)), 46.6 (C(3)), 55.8 (C(6)OMe),
56.0 (C(7)OMe), 108.8 (C(8)), 110.1 (C(5)), 122.2 (C(4a)), 130.9
(C(8a)), 147.3 (C(6)), 150.7 (C(7)), 163.7 (C(1)). MS, found:
m/z 206.1171 [M + H]+. Calculated for C12H16NO2: 206.1176.
1-Ethyl-6,7-dimethoxy-3,4-dihydroisoquinoline (15c). Yield
36%, brown liquid. Rf 0.29 (ethyl acetate—EtOH, 3 : 2). IR,
ν/cm–1: 2938, 2835, 2366, 1664, 1607, 1573, 1515, 1458, 1325,
Compounds 6 and 15
R
H
Me
Et
C7H15
Ph
Bn
Yield of 15 (%)
a
b
c
d
e
f
50
63
36
52
48
39
1
1271, 1210, 1149, 1073, 1023. Н NMR, : 1.23 (t, 3 H, C(1)Et,
In summary, in the present work, a convenient method
for the synthesis of 6,7-dimethoxy-3,4-dihydroisoquino-
lines was developed. The reaction of nitroalkanes activated
by PPA with aliphatic amines was carried out for the
first time.
J = 7.5 Hz); 2.65 (t, 2 H, C(4), J = 7.5 Hz); 2.77 (q, 2 H, C(1)Et,
J = 7.4 Hz); 3.65 (t, 2 H, C(3), J = 7.5 Hz); 3.90 (s, 3 H,
C(6)OMe); 3.91 (s, 3 H, C(7)OMe); 6.70 (s, 1 H, C(5)), 7.01
(s, 1 H, C(8)). 13C NMR, : 11.7 (C(1)Et), 25.9 (C(4)), 28.7
(C(1)Et), 46.2 (C(3)), 56.1 (C(6)OMe), 56.4 (C(7)OMe), 109.0
(C(8)), 110.5 (C(5)), 121.4 (C(4a)), 131.9 (C(8a)), 147.7 (C(6)),
151.4 (C(7)), 168.9 (C(1)). MS, found: m/z 220.1333 [M + H]+.
Calculated for C13H18NO2: 220.1332.
Experimental
1-Heptyl-6,7-dimethoxy-3,4-dihydroisoquinoline (15d). Yield
52%, light yellow liquid. Rf 0.58 (ethyl acetate—EtOH, 3 : 2).
IR, ν/cm–1: 2938, 2366, 1680, 1577, 1520, 1275, 1153, 1031.
1Н NMR, : 0.86 (t, 3 H, C(1)C7H15, J = 6.7 Hz); 1.46—1.21
1Н and 13С NMR spectra were recorded on a Bruker
AVANCE III HD (400.40 (1Н) and 100.61 (13С) MHz) instru-
ment in СDCl3, using residual signals of CHCl3 as an internal

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Russ. Chem. Bull., Int. Ed., Vol. 68, No. 5, May, 2019
Aksenov et al.
(m, 8 H, C(1)C7H15); 1.69—1.59 (m, 2 H, C(1)C7H15); 2.63
(t, 2 H, C(4), J = 7.5 Hz); 2.70 (t, 2 H, C(1)C7H15, J = 7.8 Hz);
3.63 (t, 2 H, C(3), J = 7.5 Hz); 3.90 (s, 3 H, C(6)OMe); 3.92
(s, 3 HC(7)OMe); 6.70 (s, 1 H, C(5)); 7.00 (s, 1 H, C(8)). 13C NMR,
: 14.2 (C(1)C7H15), 26.0 (C(1)C7H15), 22.8 (C(4)), 27.5
(C(1)C7H15), 29.2 (C(1)C7H15), 29.6 (C(1)C7H15), 31.9
(C(1)C7H15), 35.9 (C(1)C7H15), 46.4 (C(3)), 56.1 (C(6)OMe),
56.4 (C(7)OMe), 109.1 (C(8)), 110.5 (C(5)), 121.7 (C(4a)), 131.9
(C(8a)), 147.6 (C(6)), 151.2 (C(7)), 167.9 (C(1)). MS, found:
m/z 290.2118 [M + H]+. Calculated for C18H28NO2: 290.2115.
6,7-Dimethoxy-1-phenyl-3,4-dihydroisoquinoline (15e). Yield
48%, light yellow liquid. Rf 0.19 (ethyl acetate—EtOH, 3 : 2). IR,
ν/cm–1: 2938, 2838, 2351, 1611, 1561, 1515, 1462, 1363, 1279,
1218, 1119, 1031. 1Н NMR, : 2.74 (t, 2 H, C(3), J = 7.4 Hz);
3.73 (s, 3 H, C(4)); 3.82 (t, 2 H, C(6)OMe, J = 7.4 Hz); 3.95
(s, 3 H, C(7)OMe); 6.79 (s, 1 H, C(5)); 6.79 (s, 1 H, C(8));
7.48—7.40 (m, 3 H, C(3)C(1)Ph, C(4)C(1)Ph, C(5)C(1)Ph); 7.61
(dd, 2 H, C(2)C(1)Ph, C(6)C(1)Ph, J = 6.8 Hz, J = 2.4 Hz).
13C NMR, : 26.1 (C(4)), 47.7 (C(3)), 56.2 (C(6)OMe), 56.3
(C(7)OMe), 110.3 (C(8)), 111.7 (C(5)), 121.6 (C(4a)), 128.3
(2 C, C(3)C(1)Ph, C(5)C(1)Ph), 128.9 (2 C, C(2)C(1)Ph, C(6)C(1)Ph),
129.5 (C(4)C(1)Ph), 132.8 (C(8a)), 139.1 (C(1)C(1)Ph), 147.2
(C(6)), 151.1 (C(7)), 167.0 (C(1)). MS, found: m/z 268.1337
[M + H]+. Calculated for C17H18NO2: 268.1332.
1-Benzyl-6,7-dimethoxy-3,4-dihydroisoquinoline (15g). Yield
39%, light yellow liquid. Rf 0.55 (ethyl acetate —EtOH, 3 : 2).
IR, ν/cm–1: 2949, 2839, 1672, 1611, 1573, 1515, 1458, 1367, 1321,
1275, 1153, 1046. 1Н NMR, : 2.65 (t, 2 H, C(4), J = 7.7 Hz);
3.71 (s, 3 H, C(7)OMe); 3.71 (t, 2 H, C(3), J = 7.7 Hz); 3.86
(s, 3 H, C(6)OMe); 4.04 (s, 2 H, CH2Ph); 6.65 (s, 1 H, C(5)); 6.94
(s, 1 H, C(8)); 7.31—7.23 (m, 5 H, CH2Ph). 13C NMR, : 25.8
(C(4)), 43.4 (CH2Ph), 47.1 (C(3)), 55.9 (C(6)OMe), 56.0
(C(7)OMe), 109.6 (C(8)), 110.3 (C(5)), 121.5 (C(4a)), 126.5
(C(8a)), 128.6 (2 C, C(3)Bn, C(5)Bn), 128.7 (2 C, C(2)Bn, C(6)Bn),
131.8 (C(8a)), 138.1 (C(1)Bn), 147.2 (C(6)), 150.7 (C(7)), 165.6
(C(1)). MS, found: m/z 282.1482 [M + H]+. Calculated for
C18H20NO2: 282.1489.
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Received December 20, 2018;
in revised form February 1, 2019;
accepted March 25, 2019