An improved synthesis of β-carboline and azepino-and azocino[3,4-b]indole derivatives from lactams

Article abstract of DOI:10.1007/s10593-014-1463-x

The Vilsmeier formylation of butyrolactam, N-methylbutyrolactam, N-methylvalerolactam, and N-methylcaprolactam followed by treatment with aryl hydrazines afforded the 2,3,4,9-tetrahydro-1H-β-carbolin-1-ones, 3,4,5,10-tetrahydroazepino[3,4-b]indol-1(2H)-ones, and 2,3,4,5,6,11-hexahydro- 1H-azocino[3,4-b]indol-1-ones. The synthesis was performed as a one-pot process without isolating the intermediate a-formyl lactams.

Full text of DOI:10.1007/s10593-014-1463-x

Chemistry of Heterocyclic Compounds, Vol. 50, No. 2, May, 2014 (Russian Original Vol. 50, No. 2, February, 2014)
AN IMPROVED SYNTHESIS OF β-CARBOLINE AND AZEPINO-
AND AZOCINO[3,4-b]INDOLE DERIVATIVES FROM LACTAMS
1
*
G. P. Tokmakov
The Vilsmeier formylation of butyrolactam, N-methylbutyrolactam, N-methylvalerolactam, and
N-methylcaprolactam followed by treatment with aryl hydrazines afforded the 2,3,4,9-tetrahydro-1Н-
β-carbolin-1-ones, 3,4,5,10-tetrahydroazepino[3,4-b]indol-1(2Н)-ones, and 2,3,4,5,6,11-hexahydro-
1Н-azocino[3,4-b]indol-1-ones. The synthesis was performed as a one-pot process without isolating the
intermediate α-formyl lactams.
Keywords: arylhydrazines, azepino[3,4-b]indoles, azocino[3,4-b]indoles, β-carbolines, lactams, Fischer
reaction, rearrangements, Vilsmeier reaction.
In a series of previous publications [1-3] we described the Fischer reaction involving the α-formyl
lactams 2 or their enamines 3, obtained from the lactams 1, as the carbonyl component. Depending on the ring
size of the starting lactams 1, the reaction gave 2,3,4,9-tetrahydro-1Н-β-carbolin-1-ones 5 (n = 1),
3,4,5,10-tetrahydroazepino[3,4-b]indol-1(2Н)-ones 5 (n = 2), or 2,3,4,5,6,11-hexahydro-1Н-azocino[3,4-b]-
indol-1-ones 5 (n = 3) in up to 80% yields (Scheme 1, route А).
Scheme 1
CHO
O
NMe2
( )n
( )n
( )n
Route А [1-3]
O
+
N
O
1. DMF, POCl3
2. H2O
N
N
R
1a–d
R
R
2a–d
3a–d
R2
R1
N
NH2
Route B
4a–d
(without isolation of
R1
compounds 2 and 3)
( )n
1. DMF, POCl3, 

4a–d, EtOH, 
N
R
N
O
R2
5a–h
1a,b, 5a–c n =1; 1c, 5d–f n = 2; 1d, 5g,h n = 3; 1a, 5a R = H; 1b–d, 5b–h R = Me; 4a,c,d, 5a–d,f–h R1 = H;
4b, 5e R1 = Me; 4a,b, 5b,d,e,g R2 = H; 4c, 5c R2 = Ph; 4d, 5a,f,h R2 = CH2Ph
_______
*To whom correspondence should be addressed, e-mail: tokmakovgp@gmail.com.
1Moscow Agricultural Academy named after K. A. Timiryazev, 49 Timiryazevskaya St., Moscow 127550,
Russia.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 235-240, February, 2014. Original
article submitted December 28, 2013; revision submitted February 21, 2014.
0009-3122/14/5002-0211©2014 Springer Science+Business Media New York
211

The proposed mechanism of this reaction [1] includes a sequence of two intramolecular rearrangements:
the Fischer reaction and a rearrangement with lactam ring expansion. The first stage of the process involves
interaction of the aldehyde 2 or enamine 3 with the arylhydrazine 4, forming the hydrazone 6, which undergoes
Fischer cyclization in acidic medium, leading to the spiroindoline 7. After the protonation of amino group in the
spiroindoline 7, a molecule of ammonia is eliminated, forming the cation 8. The latter undergoes a
rearrangement with lactam ring expansion, forming the cation 9, which is stabilized by deprotonation and thus
gives the tricyclic compound 5 (Scheme 2).
Scheme 2
R
R
( )n
( )n
H
N
N
2 + 4
3 + 4
R1
R1
O
O
NH2
N
H
N N
R2
R2
6
7
R
( )n
+
N
( )n
H+
– H+
+
R1
R1
5
N
O
N
R
– NH3
H
N
H
R2
O
R2
8
9
The heterocycles 5, and especially the β-carbolines attract a significant interest of researchers due to the
broad range of biological activity [4-6] and the utility as starting materials for the further modification of indole
molecules. Thus, the β-carbolines 5 (n = 1) are used for the preparation of tryptamines with various patterns of
substitution [7], as well as the known medication incazane [8].
Quite a lot methods are known in the literature for the synthesis of β-carbolines 5 (n = 1). These
methods are complicated and rely on starting materials that are costly or difficult to find. The better known of
these methods is the Abramovich–Shapiro reaction [9] – the Fischer cyclization of piperidine-2,3-dione 3-aryl-
hydrazones, obtained by the Japp–Klingemann reaction of aryldiazonium salts with 2-oxopiperidine-
3-carboxylates. The azepinoindoles 5 (n = 2) were also obtained by the Fischer reaction from α-oxocaprolactam
enamines [10]. The azocinoindoles 5 (n = 3) were unknown prior to our investigations.
Our goal was to simplify the synthesis of compounds 5 by avoiding the laborious isolation of the
intermediate aldehydes 2 and enamines 3. We applied a Vilsmeyer formylation reaction using DMF and POCl3
to convert the starting lactams 1 to the aldehydes 2 and the respective enamines 3 [2, 11]. Several methods for
the synthesis of α-formyl lactams have been described in the literature, based on Claisen condensation of
N-substituted lactams with ethyl formate in the presence of potassium metal. These approaches are complicated
and provide poor yields of the aldehydes: 3.6% [12], 15% [13] from N-methylbutyrolactam, and 23% [14] from
N-methylvalerolactam. Our Vilsmeier formylation method gave formyl lactams in significantly higher yields
(38-64%) and, in addition, provided the opportunity to obtain N-unsubstituted formyl lactams [2]. However, our
experimental procedure was still quite complicated. The Vilsmeyer reaction with its aqueous work-up was
followed by a laborious isolation of compounds 2 and 3 from water-containing phases. The good aqueous
solubility of these compounds required salting out, multiple extractions, and finally, a vacuum distillation. An
additional drawback of the method was the formation of the aldehydes 2 in mixtures with the enamines 3.
212

We assumed that the isolation of compounds 2 and 3 was not necessary and aqueous solutions of the
aldehydes 2 and/or the enamines 3 obtained after the aqueous work-up of formylation reactions of lactams 1
could be directly introduced into reactions with the arylhydrazines 4 (Scheme 1, route B). It was expected that
the unreacted starting lactams 1 and DMF should not interfere with the formation of arylhydrazones and with the
further rearrangements, and also with the isolation of the products. It was also assumed that the HCl and H3PO4
formed by aqueous work-up would catalyze the Fischer reaction.
The formylation reaction of the lactams 1 was concluded by a careful vacuum evaporation of the solvent
(benzene) in order to obtain a homogeneous aqueous phase for the further transformations. The residue was
worked up by adding cold water or ice. The obtained aqueous solution (or its aliquot) was used in reactions with
the arylhydrazines 4. The reactions successfully produced the heterocycles 5, and the yields of these compounds
from the lactams 1a-d were comparable to the yields from pure andehydes 2 and enamines 3 [1-3]. Thus, the
route B (without isolation of compounds 2 and 3) was even more efficient than the route А.
It was further established that acidic aqueous solutions of the N-substituted α-formyl lactams 2b-d were
relatively stable when stored in a refrigerator and could be used at least for a week without a significant decrease in
the yield of the heterocycles 5. It should be noted that the adducts obtained in the Vilsmeier reaction from lactams,
DMF, and POCl3 were also quite stable. For instance, such an adduct of the lactam 1c (after the removal of benzene)
was stored in a refrigerator in an airtight vessel for more than a year. After dissolving it in water and reacting with
the arylhydrazines 4a,d, we obtained the azepinoindoles 5d,f in nearly the same yields as from a freshly prepared
adduct. Thus we consider these adducts to be convenient synthetic equivalents of the aldehydes 2.
The structures of the obtained compounds 5a-h were confirmed by 1Н and 13С NMR data. The signals
were assigned based on COSY, DEPT, and HMQC experiments. Compounds 5a-h were also shown to be
identical with previously obtained samples [1-3] by the lack of melting point depression of mixed samples,
equivalent chromatographic properties, and matching IR spectra.
As was previously pointed out [3], the 1Н NMR spectrum of the 11-benzyl-substituted azocinoindole 5h
exhibited non-equivalent СН2 group protons, unlike the other spectra. We should emphasize that this was not
observed neither for the 11-unsubstituted azocinoindole 5g, nor for the 10-benzyl-substituted azepinoindole 5f.
This can be explained by a steric hindrance between the benzyl group and the 8-membered lactam ring that
inhibits the free rotation of the benzyl group and/or the conformational flux in the lactam ring. Such an
interpretation was confirmed by the temperature-dependent coalescence of two benzyl proton doublets into a
singlet [3].
Thus, we significantly streamlined the synthesis of β-carboline and azepino- and azocino[3,4-b]indole
derivatives by eliminating the laborious isolation of α-formyl lactam intermediates generated by Vilsmeier
formylation of lactams. Performing this sequence of reactions in one pot enabled us to use commercially
available and affordable lactams, while the yield remained practically the same.
EXPERIMENTAL
IR spectra were recorded on a Perkin Elmer Spectrum 400 FTIR spectrometer in KBr pellets. 1Н and 13С
NMR spectra were acquired on a Bruker Avance III 500 spectrometer (500 and 125 MHz, respectively) in CDCl3,
1
with residual solvent protons as internal standard (7.28 ppm for Н; 76.50 ppm for 13С). Melting points were
determined in glass capillaries on a electroheating block. TLC was performed on Silufol UV-254 plates with 6:1
benzene–acetone as an eluent, visualization under UV light or with iodine vapor.
N-Methylvalerolactam (1с) [15] and N-methylcaprolactam (1d) [16] were obtained according to
literature methods. p-Tolylhydrazine hydrochloride (4b) (Reakhim), 1,1-diphenylhydrazine hydrochloride (4с)
(Aldrich), 1-benzyl-1-phenylhydrazine hydrochloride (4d) (Kharkov chemical reagent plant), butyrolactam (1а)
213

(Merck), and POCl3 (Fluka) were used without additional purification. N-Methylbutyrolactam (1b) (Ferak) was
purified by vacuum distillation. Phenylhydrazine hydrochloride (4а) (Shostka chemical reagent plant) was
purified according to the method [17].
Synthesis of Compounds 5a-h (General Method). A solution of POCl3 (65 mmol) in anhydrous
benzene (10 ml) was stirred and cooled on a water bath, while a solution of the lactam 1a (15 mmol) or freshly
distilled lactam 1b-d (30 mmol) and anhydrous DMF (33 mmol) in anhydrous benzene (8 ml) was added
dropwise over 10-15 min. The reaction mixture turned into a yellowish two-phase system, which was further
stirred and refluxed for 5.5 h on a water bath in a flask equipped with a reflux condenser, while protecting from
air moisture, and left overnight. The flask with the reaction mixture was transferred to a rotary evaporator,
benzene and the excess of POCl3 were removed by distillation over 1 h at 80°С. The residue was cooled and
cautiously treated with cold water (15 ml) (lactams 1b-d) or treated with ice (lactam 1a), while avoiding
overheating, stirred until dissolution and maintained for 1-1.5 h. The obtained solution was mixed together with
a solution or suspension of the arylhydrazine hydrochloride 4а-d (15 mmol for the lactam 1a or 30 mmol for the
lactams 1b-d) in ethanol (50 ml), refluxed for 2 h in a flask with a reflux condenser, and cooled in a refrigerator.
The precipitate of compounds 5a-g was filtered off, washed with 50% ethanol and water, air-dried, and
recrystallized from ethanol. The product 5h was isolated by evaporation of the reaction mixture under vacuum,
treating the residue with benzene and water, agitation of the two-phase system and separation of the benzene
layer, washing with 10% HCl and water, drying over MgSO4, and evaporation of benzene. The residue was
treated with ether (30 ml), the undissolved debenzylated by-product 5g was filtered off and washed with ether.
The ether solution was evaporated, the residue was crystallized from a 1:1 mixture of hexane and cyclohexane.
9-Benzyl-2,3,4,9-tetrahydro-1Н-β-carbolin-1-one (5а). Yield 38%, colorless plates, mp 182-184°С
(yield 82%, mp 183-184°С (2-PrOH) [2]). 1H NMR spectrum, δ, ppm (J, Hz): 3.11 (2Н, t, J = 6.9, 4-СН2); 3.69
(2Н, td, J = 6.9, J = 1.9, 3-СН2); 5.70 (1Н, br. s, NН); 5.93 (2Н, s, PhCH2); 7.16-7.23 (4H, m, Н-6, Н-2,4,6 Ph);
7.27 (2H, t, J = 7.2, H-3,5 Ph); 7.31 (1H, t, J = 7.9, H-7); 7.38 (1H, d, J = 7.9, H-8); 7.64 (1H, d, J = 8.0, H-5).
13C NMR spectrum, δ, ppm: 20.7 (С-4); 41.3 (С-3); 47.3 (PhCH2); 110.5 (C-8); 119.8, 119.9 (C-4a,5,6); 124.0
(C-4b(9а)); 124.6 (С-7); 124.9 (C-9а(4b)); 126.3 (С-2,6 Ph); 126.6 (С-4 Ph); 127.9 (C-3,5 Ph); 137.9, 138.2 (C-8a,
C-1 Ph); 162.6 (C=O).
2-Methyl-2,3,4,9-tetrahydro-1Н-β-carbolin-1-one (5b). Yield 61%, mp 233-235°С (yield 60%;
1
mp 234°С (2-PrOH) [1]). H NMR spectrum, δ, ppm (J, Hz): 3.10 (2Н, t, J = 7.1, 4-СН2); 3.21 (3H, s, CH3);
3.73 (2Н, t, J = 7.1, 3-СН2); 7.15 (1Н, t, J = 7.9, H-6); 7.31 (1Н, t, J = 7.9, H-7); 7.52 (1Н, d, J = 7.9, H-8); 7.59
(1Н, d, J = 7.9, H-5); 10.25 (1Н, br. s, NH). 13C NMR spectrum, δ, ppm: 20.1 (C-4); 33.6 (CH3); 49.7 (C-3);
112.1 (C-8); 117.3 (C-4a); 119.5 (C-5,6); 124.0 (C-7); 124.8, 126.6 (C-4b,9a); 137.1 (C-8a); 161.5 (C=O).
2-Methyl-9-phenyl-2,3,4,9-tetrahydro-1Н-β-carbolin-1-one (5с). Yield 59%, colorless plates,
mp 158-159°С (yield 62%, mp 160°С (MeOH) [1]). 1H NMR spectrum, δ, ppm (J, Hz): 3.10 (3H, s, CH3); 3.17
(2Н, t, J = 7.0, 4-СН2); 3.76 (2Н, t, J = 7.0, 3-СН2); 7.21 (1Н, d, J = 7.9, H-8); 7.23 (1Н, t, J = 7.9, H-6); 7.30
(1Н, t, J = 7.9, H-7); 7.43 (2H, d, J = 7.5, H-2,6 Ph); 7.47 (1H, t, J = 7.5, H-4 Ph); 7.54 (2H, t, J = 7.5, H-3,5
Ph); 7.67 (1Н, d, J = 7.9, H-5). 13C NMR spectrum, δ, ppm: 20.3 (С-4); 33.7 (CH3); 49.1 (С-3); 111.1 (С-8);
119.5 (С-5); 119.6 (С-4а); 120.2 (С-6); 124.0 (C-4b(9а)); 124.6 (C-7); 126.9 (C-9а(4b)); 127.2 (С-4 Ph); 127.4
(С-2,6 Ph); 128.3 (С-3,5 Ph); 138.0, 139.6 (C-8a, C-1 Ph); 160.3 (C=O).
2-Methyl-3,4,5,10-tetrahydroazepino[3,4-b]indol-1(2Н)-one (5d). Yield 56%, mp 237-239°С (yield
1
55%, mp 238°С (2-PrOH) [3]). H NMR spectrum, δ, ppm (J, Hz): 2.22-2.26 (2H, m, 4-СН2); 3.11 (2Н, t,
J = 6.6, 5-СН2); 3.26 (3H, s, CH3); 3.60-3.62 (2Н, m, 3-СН2); 7.14 (1Н, t, J = 7.8, H-7); 7.32 (1Н, t, J = 7.8,
13
H-8); 7.42 (1Н, d, J = 7.8, H-9); 7.61 (1Н, d, J = 7.8, H-6); 9.19 (1Н, br. s, NH). C NMR spectrum, δ, ppm:
24.7 (C-5); 25.9 (C-4); 36.7 (CH3); 50.6 (C-3); 111.1 (C-9); 116.9 (C-5a); 119.1 (C-7); 119.6 (C-6); 124.2
(C-8); 127.0, 127.6 (C-5b,10a); 135.0 (C-9a); 163.0 (C=O).
2,7-Dimethyl-3,4,5,10-tetrahydroazepino[3,4-b]indol-1(2Н)-one (5e). Yield 73%, mp 231-232°С
1
(yield 76%, mp 232°С (2-PrOH) [3]). H NMR spectrum, δ, ppm (J, Hz): 2.19-2.25 (2H, m, 4-СН2); 2.48 (3H,
214

s, 7-CH3); 3.07 (2Н, t, J = 6.6, 5-СН2); 3.25 (3H, s, NCH3); 3.58-3.60 (2H, m, 3-СН2); 7.15 (1Н, d, J = 8.4,
H-8); 7.32 (1Н, d, J = 8.4, H-9); 7.38 (1Н, s, Н-6); 9.25 (1Н, br. s, NH). 13C NMR spectrum, δ, ppm: 21.0
(7-СН3); 24.7 (С-5); 25.9 (С-4); 36.7 (NCH3); 50.6 (C-3); 110.8 (C-9); 116.4 (C-5a); 119.0 (C-6); 126.1 (C-8);
127.0, 127.8 (C-5b,10a); 128.3 (C-7); 133.3 (C-9a); 163.0 (C=O).
10-Benzyl-2-methyl-3,4,5,10-tetrahydroazepino[3,4-b]indol-1(2Н)-one (5f). Yield 60%, mp 137-138°С
1
(yield 57% and 70%; mp 139°С (2-PrOH) [3]). H NMR spectrum, δ, ppm (J, Hz): 2.16-2.22 (2H, m, 4-СН2);
3.06 (2Н, t, J = 7.5, 5-СН2); 3.17 (3H, s, NCH3); 3.39-3.41 (2H, m, 3-СН2); 5.74 (2Н, s, PhCH2); 7.08 (2H, d,
J = 7.3, H-2,6 Ph); 7.16 (1Н, t, J = 7.8, H-7); 7.19 (1Н, t, J = 7.3, H-4 Ph); 7.24 (2Н, t, J = 7.3, H-3,5 Ph); 7.27
(1Н, t, J = 7.8, H-8); 7.33 (1Н, d, J = 7.8, H-9); 7.65 (1Н, d, J = 7.8, H-6). 13C NMR spectrum, δ, ppm: 19.9
(С-5); 27.6 (С-4); 34.2 (СН3); 47.2 (PhCH2); 49.0 (C-3); 110.1 (C-9); 117.4 (C-5a); 119.0, 119.3 (C-6,7); 123.7
(C-8); 125.8 (C-5b(10а)); 126.0 (С-2,6 Ph); 126.4 (C-4 Ph); 127.9 (С-3,5 Ph); 128.8 (C-10а(5b)); 137.6, 138.3
(C-8a, C-1 Ph); 164.7 (C=O).
2-Methyl-2,3,4,5,6,11-hexahydro-1Н-azocino[3,4-b]indol-1-one (5g). The reaction mixture was
diluted to one half concentration with water and left overnight in refrigerator. Yield 11%, mp 208-210°С (yield
1
10%; mp 210°С (benzene) [3]). H NMR spectrum, δ, ppm (J, Hz): 1.87-1.91 (2H, m, 4-СН2); 2.01-2.05 (2H,
m, 5-СН2); 3.03-3.05 (2Н, m, 6-СН2); 3.16 (3H, s, CH3); 3.70-3.72 (2H, m, 3-СН2); 7.14 (1Н, t, J = 7.8, H-8);
7.28 (1Н, t, J = 7.8, H-9); 7.39 (1Н, d, J = 7.8, H-10); 7.58 (1Н, d, J = 7.8, H-7); 9.00 (1Н, br. s, NH). 13C NMR
spectrum, δ, ppm: 21.2 (С-5); 24.2 (С-6); 27.1 (С-4); 32.9 (СН3); 48.6 (С-3); 110.9 (С-10); 117.9 (С-6а); 119.1,
119.2 (С-7,8); 123.7 (С-9); 125.7, 127.6 (С-6b,11a); 134.9 (C-10a); 164.7 (C=O).
11-Benzyl-2-methyl-2,3,4,5,6,11-hexahydro-1Н-azocino[3,4-b]indol-1-one
(5h).
Yield
43%,
1
colorless prisms, mp 78-79°С (yield 45%; mp 78°С (petroleum ether–cyclohexane) [3]). H NMR spectrum, δ,
ppm (J, Hz): 1.61-1.70 (1Н, m, 4-СНAHB); 1.70-1.82 (1Н, m, 5-СНAHB); 1.90-2.09 (2Н, m, 4-СНAHB,
5-СHAHB); 2.92-3.05 (3H, m, 3-СНAHB, 6-СН2); 3.06 (3Н, s, СН3); 3.71-3.82 (1Н, m, 3-СНAHB); 5.47 (1Н, d,
J = 15.9) and 5.65 (1Н, d, J = 15.9, PhCH2); 7.10 (2H, d, J = 7.4, H-2,6 Ph); 7.17 (1Н, t, J = 7.9, H-8);
7.19-7.27 (3Н, m, Н-3,4,5 Ph); 7.28 (1H, t, J = 7.9, Н-9); 7.35 (1Н, d, J = 7.9, Н-10); 7.61 (1Н, d, J = 7.9, Н-7).
13C NMR spectrum, δ, ppm: 21.5 (С-5); 23.7 (С-6); 26.8 (С-4); 32.4 (СН3); 47.0 (PhCH2); 48.9 (C-3); 109.7
(C-10); 117.5 (C-6a); 119.0, 119.1 (C-7,8); 123.2 (C-9); 126.3 (C-6b(11а)); 126.3 (С-2,6 Ph); 126.6 (C-4 Ph);
127.8 (C-3,5 Ph); 127.9 (C-11а(6b)); 137.2, 138.2 (C-10a, C-1 Ph); 164.2 (C=O).
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