Comparison of reactivities of 1- and 4-oxotetrahydrocarbazoles in reactions with nucleophilic and electrophilic reagents

Article abstract of DOI:10.1007/s11172-006-0054-0

The half-wave potentials of polarographic reduction of the carbonyl group in unsubstituted and N-methyl- and N-phenylsulfonyl-substituted 1- and 4-oxotetrahydrocarbazoles and their reactivities in reactions with nucleophilic (NaBH₄, malonodinitrile, and cyanoacetamide) and electrophilic (DMF dimethyl acetal) reagents were compared. 4-Oxotetrahydrocarbazoles are much less reactive than 1-oxotetrahydrocarbazoles.

Full text of DOI:10.1007/s11172-006-0054-0

Russian Chemical Bulletin, International Edition, Vol. 54, No. 8, pp. 1887—1891, August, 2005
1887
Comparison of reactivities of 1ꢀ and 4ꢀoxotetrahydrocarbazoles
in reactions with nucleophilic and electrophilic reagents
S. Yu. Kukushkin,a P. Yu. Ivanov,a L. M. Alekseeva,a V. I. Levina,a K. I. Kobrakov,b
N. B. Grigor´ev,a and V. G. Granika
ꢀ
aState Research Center of Antibiotics,
3a ul. Nagatinskaya, 117003 Moscow, Russian Federation.
Fax: +7 (495) 231 4284. Eꢀmail: vggranik@mail.ru
bA. N. Kosygin Moscow State Textile Academy,
1 ul. Malaya Kaluzhskaya, 119991 Moscow, Russian Federation.
Fax: +7 (495) 952 1440
The halfꢀwave potentials of polarographic reduction of the carbonyl group in unsubstituted
and Nꢀmethylꢀ and Nꢀphenylsulfonylꢀsubstituted 1ꢀ and 4ꢀoxotetrahydrocarbazoles and their
reactivities in reactions with nucleophilic (NaBH4, malonodinitrile, and cyanoacetamide) and
electrophilic (DMF dimethyl acetal) reagents were compared. 4ꢀOxotetrahydrocarbazoles are
much less reactive than 1ꢀoxotetrahydrocarbazoles.
Key words: 1,2,3,9ꢀtetrahydroꢀ4Hꢀcarbazolꢀ4ꢀones, 2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbazolꢀ1ꢀ
ones, reduction, voltammetry, dimethylformamide dimethyl acetal.
Fused carbazoles, for example, the antidepresꢀ
sants pyrazidole and tetrindole, belonging to the pyraziꢀ
no[3,2,1ꢀj,k]carbazole series are among important pharꢀ
maceuticals that are widely used in medical practice.1—4
Recently, we have demonstrated that 6ꢀmethylꢀ2,3,4,9ꢀ
tetrahydroꢀ1Hꢀcarbazolꢀ1ꢀone (1a) is a promising startꢀ
ing compound. In addition to the synthesis of pyrazinoꢀ
carbazole derivatives based on 1a, the latter can be used
in two new approaches. Thus, compound 1a can be inꢀ
volved in the synthesis of hydrogenated carbazoles conꢀ
taining the urea or thiourea substituents at position 1.5
These compounds exhibited anticonvulsant activity in
pharmacological experiments. Besides, compound 1a can
be used for the construction of tetracycles containing the
fused pyridine or pyrimidine rings
at positions 1 and 2 (Scheme 1).6
The aim of the present study
was to compare the reactivities of
carbazolone 1a and its analog, viz.,
1,2,3,9ꢀtetrahydroꢀ4Hꢀcarbazolꢀ4ꢀ
one (2a)7 containing the oxo group
at position 4. We expected that the
Scheme 1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1832—1836, August, 2005.
1066ꢀ5285/05/5408ꢀ1887 © 2005 Springer Science+Business Media, Inc.

1888
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 8, August, 2005
Kukushkin et al.
electronꢀwithdrawing effect of the keto group on the adꢀ
jacent CH2 group in compound 2a would be lower than
that in 1ꢀoxo derivative 1a.
We also studied reduction of the oxo group in molꢀ
ecules 2a,b and Nꢀphenylsulfonyl derivative 2c (which
was prepared according to a procedure described earꢀ
lier8), because it is the corresponding carbinols (reducꢀ
tion products) in the series of 1ꢀsubstituted carbazoles
that are the key compounds in the synthesis of the aboveꢀ
mentioned anticonvulsants.
Earlier,5 it has been found that reduction of ketone 1a
with sodium borohydride in ethanol at 50—55 °C was
completed within 40 min. Under the same conditions,
reduction of ketone 2a (see Scheme 2) required 17 h for
completion (TLC control, the disappearance of the startꢀ
ing ketone in the reaction mixture). However, the latter
reaction did not produce the target alcohol 3. Apparently,
the reaction proceeds through the formation of carboꢀ
cation 5, and tetrahydrocarbazole 6 was the only product
isolated in high yield. The latter was also prepared by
reduction of substrate 2a with sodium cyanoborohydride.
It should be noted that sodium borohydride reduces
the carbonyl group of ketone 2a to the hydroxy group
under milder conditions (methanol—water, 20 °C, 14 h)
with satisfactory selectivity, which allowed us to isolate
and characterize alcohol 3.9
Actually, in some cases, compounds 1a and 2a differ
substantially in the reactivity. For example, the reaction
of 1a with DMF dimethyl acetal afforded the 2ꢀdimethylꢀ
aminomethylene derivative, which was then transformed
into pyridocarbazoles,6 whereas condensation of 2a with
DMF dimethyl acetal under the same conditions involves
only methylation of the indole nitrogen atom to form
derivative 2b. The latter was also prepared independently
by methylation of 2a with dimethyl sulfate (Scheme 2).
Scheme 2
We also performed reduction of Nꢀsubstituted keꢀ
tones 2b,c. It appeared that reduction of Nꢀmethylꢀsubꢀ
stituted compound 2b occurred smoothly at 80 °C for
1.5 h to give the target alcohol 4 in 67% yield. The reacꢀ
tion with Nꢀphenylsulfonylꢀsubstituted compound 2c as
the starting reagent (Scheme 3) afforded the correspondꢀ
ing alcohol 8 in 54% yield. It should be noted that the
conditions of the reaction 2b → 4 are more rigid that
those used for reduction of ketone 1a, whereas the reacꢀ
tion 2c → 8 proceeds analogously to that with 1a, under
much milder conditions.
Scheme 3
Attempts to perform the reaction of ketone 2b with
DMF acetal under the same or even more drastic condiꢀ
tions (in an excess of acetal, 125 °C, 3 h or refluxing in an
excess of acetal in DMF for 7 h) failed. In all cases, only
the starting compound 2b was isolated in high yield. An
analogous situation was observed when attempting to perꢀ
form condensation of ketone 2a at the carbonyl group
with such strong CHꢀacids as malonodinitrile and cyanoꢀ
acetamide. These reactions with 1ꢀoxo derivative 1a ocꢀ
cur rather easily6 (ammonium acetate, AcOH in toluene,
60 °C, 5 h), whereas ketone 2a does not react with these
compounds under the same or even more drastic condiꢀ
tions (in the presence of MeONa in methanol or DMF on
refluxing).

1ꢀ and 4ꢀOxotetrahydrocarbazoles
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 8, August, 2005
1889
Scheme 4
Reduction of substrate 2c with the complex BH3•Py
instead of NaBH4 afforded tetrahydrocarbazole 7 in 32%
yield. The latter was also prepared by reduction of alcoꢀ
hol 8 with this complex. Apparently, the alcohols are
intermediates in the transformation of ketones 2a and 2c
into products 6 and 7, respectively.
Therefore, ketone 2a differs substantially in the reacꢀ
tivity from ketone 1a, and this difference can be associꢀ
ated with the fact that the carbonyl group in molecule 2a
is attached to electronꢀrich position 3 of the indole
fragment.
The qualitative characterization of the reactivities
agrees well with the halfꢀwave potentials (E1/2) of polaroꢀ
graphic reduction of ketones 1 and 2.
Compound
1a
1b
1c
2a
2b
2c
E1/2/V
–2.17 –2.16 –1.78 –2.73 –2.67 –1.83
type 1). This difference is apparently attributed to a subꢀ
stantial increase in the electron density at position 4 comꢀ
pared to position 1.
The halfꢀwave potentials E1/2 characterizing polaroꢀ
graphic reduction of ketones are known10 to correlate
with the ability of the substituents to change the electron
density on the ketone carbonyl group. As can be seen
from the polarographic data, our main assumption that
the carbonyl group at position 4 of the carbazole ring
possesses higher electron density than that at position 1
was completely confirmed. A comparison of the halfꢀ
Experimental
The mass spectra were recorded on a JSQꢀ900 spectrometer
using a direct inlet system. The 1H NMR spectra were measured
on a Bruker ACꢀ200 spectrometer in DMSOꢀd6.
wave potentials of parent ketones 1a and 2a (∆E1/2
=
0.56 eV) shows that the latter is more difficult to reduce.
This is evidence that the carbonyl group in compound 2a
provides lower CHꢀacidity of the adjacent methylene
group than the carbonyl group in compound 1a, which is
responsible for the fact that DMF acetal cannot undergo
condensation at this position under the conditions used.
At the same time, the low electrophilicity of the C=O
group in compound 2a prevents condensation at the carꢀ
bon atom of compounds containing the active methylene
group. The carbonyl group at position 4 accepts the elecꢀ
tron upon polarographic reduction or the hydride ion
upon reduction with sodium borohydride less readily
(compared to position 1), which accounts for the aboveꢀ
described characteristic features of reduction (electroꢀ
chemical reduction of ketone 2a occurs with difficulty,
whereas removal of the hydroxy group from reduced prodꢀ
uct 3 in the reaction with borohydride occurs easily). It
should be noted that the introduction of a strong elecꢀ
tronꢀwithdrawing substituent at position 1 of the indole
fragment gives rise to compounds 1c and 2c characterized
by similar E1/2, and reduction of these compounds with
sodium borohydride affords the corresponding alcohols 8
(see Scheme 3) and 9 (Scheme 4) under the same condiꢀ
tions (see the Experimental section).
Polarographic measurements were carried out on a PUꢀ1
polarograph (Belarus). The halfꢀwave potentials E1/2 are given
for the first reduction wave at a dropping mercury electrode
relative to a saturated calomel electrode. The concentration of
the compound under study was 2.3•10–4 mol L–1 in a 0.01 M
tributylammonium iodide solution in DMF.
The course of the reactions and the purity of the compounds
were monitored by TLC on Silufol UVꢀ254 plates using a 1 : 1
ethyl acetate—benzene system as the eluent. The microanalysis
data and mass spectra of compounds 1c, 2b, 4, and 7—9 are
given in Table 1. The 1H NMR spectroscopic data are presented
in Table 2. 6,9ꢀDimethylꢀ2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbazolꢀ1ꢀone
(1b) has been prepared earlier.11
6ꢀMethylꢀ9ꢀphenylsulfonylꢀ2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbazolꢀ
1ꢀone (1c). Sodium hydride (80% suspension, 0.39 g, 13 mmol)
was added to a solution of ketone 1a (2 g, 10 mmol) in THF
(30 mL) under argon, and the mixture was stirred at room temꢀ
perature for 1 h. Benzene sulfochloride (1.8 mL, 14 mmol) was
added to the resulting yellow suspension for 10 min in such a
way that the temperature was maintained below 25 °C. The
reaction solution was stirred at room temperature for 3.5 h. The
solvent was distilled off, the residue was diluted with water
(70 mL), and the resulting precipitate was filtered off, washed
with water, and dried in vacuo. After recrystallization from DMF,
compound 1c was obtained in a yield of 1 g (31%), m.p.
185—187 °C.
1,2,3,9ꢀTetrahydroꢀ4Hꢀcarbazolꢀ4ꢀone (2a) was synthesized
according to a known procedure7 from cyclohexaneꢀ1,3ꢀdione
monophenylhydrazone12 in 41% yield, m.p. 218—220 °C.
9ꢀMethylꢀ1,2,3,9ꢀtetrahydroꢀ4Hꢀcarbazolꢀ4ꢀone (2b).
A. A suspension of compound 2a (0.46 g, 2.5 mmol) in DMF
dimethyl acetal (3.3 mL) was refluxed with stirring for 3 h. The
resulting solution was cooled to 15 °C. The precipitate that
The comparative study of the properties of ketones of
the tetrahydrocarbazole series showed that ketones conꢀ
taining the carbonyl group at position 4 (compounds 2a,b)
are less reactive in reactions with electrophilic and nuꢀ
cleophilic reagents than the corresponding isomers conꢀ
taining the carbonyl group at position 1 (compounds of

1890
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 8, August, 2005
Kukushkin et al.
Table 1. Elemental analysis and mass spectrometric data for substituted tetrahydrocarbazoles 1c, 2b, 4, and 7—9
Comꢀ
pound
Found
Calculated
Molecular
formula
MS,
m/z (Irel (%))
(%)
C
H
N
S
1c
2b
4
67.34 5.09 4.07 9.45
67.24 5.05 4.13 9.45
C19H17NО3S
C13H13NО
339 [M]+ (21), 198 [M – SO2Ph]+ (18),
156 [M – CH2CO – SO2Ph]+ (100)
199 [M]+ (23)
78.01 6.78 7.05
78.36 6.58 7.03
77.63 7.60 6.84
77.58 7.51 6.96
—
—
C13H15NO
201 [М]+ (17)
7
69.81 5.48 4.51 10.15
69.43 5.50 4.50 10.30
66.09 5.01 4.12 10.02
66.03 5.23 4.28 9.79
66.52 5.45 4.06 9.70
66.84 5.61 4.10 9.39
C18H17NО2S
C18H17NO3S
C19H19NО3S
311 [М]+ (58),
170 [M – SO2Ph]+ (100)
327 [М]+ (51), 310 [М – ОН]+ (10),
186 [М – SO2Ph]+ (61), 168 [M – OH – SO2Ph]+ (100)
341 [M]+ (36), 323 [M – H2O]+ (54),
182 [M – H2O – SO2Ph]+ (100)
8
9
1
Table 2. H NMR spectra of substituted tetrahydrocarbazoles 1c, 2b, 4, and 7—9
Comꢀ
pound
δ (J/Hz)
H(1)—Н(4)
Н(5)—Н(8)
Substituent at the N(9) atom
1ca
2b
4
2.08 (q, 2 Н, 2 H(3)); 2.54, 2.92 (both t,
2 H each, 2 H(2), 2 H(4), J = 6.0)
2.19 (q, 2 Н, 2 Н(2)); 2.43, 3.00 (both t,
2 H each, 2 H(1), 2 H(3), J = 6.0)
1.94, 2.37, 2.70 (all m, 3 H each, 1 Н, 2 Н,
2 H(1), 2 H(2), 2 H(3));
7.43 (dd, 1 Н, Н(7), Jo = 8.8,
Jm = 1.7); 8.10 (d, 1 Н, Н(8), Jo = 8.8)
7.18 (m, 2 Н);
7.45, 8.01 (both m, 1 H each)
7.03 (m, 2 Н);
7.52—7.76 (m, 4 Н)b;
8.01 (m, 2 Н)
3.74 (s, 3 H, Me)
3.58 (s, 3 H, Me)
7.32, 7.54 (both m, 1 H each)
5.01 (t, 1 Н, J = 3.4)с
7
1.79 (m, 4 Н, 2 H(2), 2 H(3)); 2.55, 3.00
(both m, 2 H each, 2 H(1), 2 H(4))
1.81, 2.94 ( both m, 4 H each, 2 Н, 2 H(1),
2 H(2), 2 H(3)); 4.78 (m, 1 Н, Н(4))с
1.62—2.12 (m, 4 Н)d;
7.26 (m, 2 Н); 7.40 (m, 1 Н); 7.60 (m, 3 Н); 7.80 (m, 2 Н); 8.02 (m, 1 Н)
7.26 (m, 2 Н); 7.63 (m, 4 Н); 7.86 (m, 2 Н); 8.01 (m, 1 Н)
8
9a
7.13 (dd, 1 Н, Н(7), Jo = 8.8, Jm = 1.7);
7.24 (br.s, 1 Н, H(5));
7.42—7.70 (m, 3 Н);
7.98 (m, 2 Н)
4.99 (m, 1 Н, Н(1))с
7.85 (d, 1 Н, Н(8), Jo = 8.8)
a The spectra of 1c and 9 show signals of C(6)Me at δ 2.43 (s, 3 H) and 2.34 (s, 3 H), respectively.
b The signal for H(5) falls in this region.
c In the spectrum of 4, the signal for OH is absent due to exchange with water of the solvent; the spectra of 8 and 9 have the signal at
δ 5.05 (d, 1 H, J = 6.6 Hz ) and 5.25 (br.s, 1 H), respectively.
d Some of the 2 H(2)—2 H(4) signals are observed at δ 2.38—2.79 and overlap with the signal of the solvent (δ 2.50).
formed was filtered off, washed with MeOH (2 mL), dried
in vacuo, and recrystallized from MeOH to obtain ketone 2b in a
yield of 0.32 g (65%), m.p. 196—197 °C (cf. lit. data13: m.p.
198—200 °C).
9ꢀPhenylsulfonylꢀ2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbazolꢀ4ꢀone (2c)
was synthesized from ketone 2a and benzene sulfochloride acꢀ
cording to a procedure described earlier,8 the yield was 60%,
m.p. 169—170 °C (cf. lit data8: m.p. 170 °C).
B. A solution of NaOH (12.8 g) in water (25 mL) was added
to a solution of ketone 2a (11.56 g, 62 mmol) in acetone (75 mL).
Then dimethyl sulfate (13.3 mL) was added with vigorous stirꢀ
ring for 10 min. The reaction mixture warmed up to 45 °C and
the alkylation product precipitated. The resulting suspension
was stirred at room temperature for 1 h and diluted with water
(100 mL). The precipitate was filtered off, washed with water,
dried in vacuo, and recrystallized from benzene to give ketone 2b
in a yield of 10.35 g (83%), m.p. 196—197 °C.
4ꢀHydroxyꢀ9ꢀmethylꢀ2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbazole (4).
A mixture of ketone 2b (1 g, 5 mmol), NaBH4 (0.9 g, 24 mmol),
and EtOH (20 mL) was refluxed with stirring for 1.5 h. The
ethanol was distilled off and the reaction mixture was diluted
with water (50 mL). The precipitate was filtered off, dried, and
recrystallized from benzene. The yield of alcohol 4 was 0.67 g
(67%), m.p. 159—162 °C (with decomp.).
2,3,4,9ꢀTetrahydroꢀ1Hꢀcarbazole (6). A. Sodium boroꢀ
hydride (0.9 g, 24 mmol) was added to a suspension of ketone 2a

1ꢀ and 4ꢀOxotetrahydrocarbazoles
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 8, August, 2005
1891
(1 g, 5.4 mmol) in EtOH (20 mL). The reaction mixture was
stirred at 45—50 °C for 17 h. The ethanol was distilled off and
the pasteꢀlike residue was mixed with water (50 mL). The preꢀ
cipitate was filtered off, washed with water, dried in vacuo, and
recrystallized from hexane to obtain compound 6 in a yield of
0.56 g (61%), m.p. 118—120 °C. (cf. lit data14: m.p. 114—116 °C).
B. A 11% HCl solution in MeOH was added to a suspension
of ketone 2a (0.5 g, 2.7 mmol) and NaBH3CN (0.18 g, 2.8 mmol)
in MeOH (5 mL) to pH 3. The reaction mixture was stirred at
room temperature for 20 h, filtered, neutralized with Na2CO3,
and evaporated on a rotary evaporator. The residue was worked
up as described above (method A). Compound 6 thus preꢀ
pared was identical to that described above, the yield was
0.27 g (58%).
9ꢀPhenylsulfonylꢀ2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbazole (7).
A. A 95% BH3•Py (0.8 mL, 7.5 mmol) was added portionwise to
a suspension of ketone 2c (1.0 g, 3 mmol) in MeOH (20 mL)
cooled to –5 °C under argon, the temperature being maintained
in the range from –5 to –2 °C. Then the reaction mixture was
cooled to –10 °C, and a 11% HCl solution in MeOH (6.2 mL)
was added in such a way that the temperature was maintained
below –5 °C, and kept at room temperature for 3.5 h. The
reaction mixture was neutralized with a solution of NaOH (0.6 g)
in water (10 mL). The precipitate was filtered off, washed with
water, dried in vacuo, and recrystallized from MeCN to obtain
compound 7 in a yield of 0.3 g (32%), m.p. 106—108 °C (cf. lit.
data15: m.p. 107—109 °C).
B. Compound 7 was synthesized according to the aboveꢀ
described procedure (method A) from carbinol 8 (0.5 g,
1.5 mmol) and 95% BH3•Py (0.34 mL, 3.7 mmol) in a yield of
0.32 g (69%), m.p. 106—108 °C.
4ꢀHydroxyꢀ9ꢀphenylsulfonylꢀ2,3,4,9ꢀtetrahydroꢀ1Hꢀcarbꢀ
azole (8). Sodium borohydride (0.8 g, 21 mmol) was added to a
suspension of ketone 2c (cf. lit data8) (1.5 g, 4.6 mmol) in EtOH
(20 mL) and the reaction mixture was stirred at room temperaꢀ
ture for 7 h. The ethanol was distilled off on a rotary evaporator.
Water (70 mL) was added to the pasteꢀlike residue. The precipiꢀ
tate was filtered off, washed with water, dried in vacuo over
CaCl2 at 50 °C, and recrystallized from PriOH to obtain alcohol
8 in a yield of 0.81 g (54%), m.p. 146—150 °C.
This study was financially supported by the Federal
Agency for Science and Innovations of the Russian Fedꢀ
eration (Contract No. 1/05).
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1ꢀHydroxyꢀ6ꢀmethylꢀ9ꢀphenylsulfonylꢀ2,3,4,9ꢀtetrahydroꢀ
1Hꢀcarbazole (9). Sodium borohydride (0.25 g, 6.6 mmol) was
added to a suspension of ketone 1c (0.50 g, 1.5 mmol) in EtOH
(5 mL) and the reaction mixture was stirred at room temperaꢀ
ture for 6 h. The ethanol was distilled off and the residue was
treated with water (20 mL). The precipitate was filtered off,
washed with water, dried in vacuo, and recrystallized (0.37 g
from 3 mL of MeCN) to obtain alcohol 9 in a yield of 0.30 g
(59%), m.p. 147—149 °C.
Received May 20, 2005;
in revised form September 5, 2005