Ring transformations of heterocyclic compounds. XXII [1]. Pyrido[1,2-a]indolium salts from 2-methyl-3H-indoles by pyrylium mediated three carbon annelation

Article abstract of DOI:10.1002/jhet.5570390203

The synthesis of pyrido[1,2-a]indolium perchlorates 8,11 from 2,4,6-triarylpyrylium perchlorates 1 and 2-methyl-3H-indoles 6,9 in the presence of a basic condensing agent (anhydrous sodium acetate, piperidine acetate, triethylamine/acetic acid, triethylamine) in ethanol by a 2,4-[C₃+C₂N] pyrylium ring transformation is reported. Spectroscopic data of the transformation products and their mode of formation are discussed.

Full text of DOI:10.1002/jhet.5570390203

Mar-Apr 2002
Ring Transformations of Heterocyclic Compounds. XXII [1].
Pyrido[1,2-a]indolium Salts from 2-Methyl-3H-indoles by
Pyrylium Mediated Three Carbon Annelation
263
Thomas Zimmermann* and Lothar Hennig
Institut für Organische Chemie der Universität Leipzig, Permoserstraße 15, D-04303 Leipzig, Germany
Received February 16, 2001
The synthesis of pyrido[1,2-a]indolium perchlorates 8,11 from 2,4,6-triarylpyrylium perchlorates 1 and 2-
methyl-3H-indoles 6,9 in the presence of a basic condensing agent (anhydrous sodium acetate, piperidine
acetate, triethylamine/acetic acid, triethylamine) in ethanol by a 2,4-[C +C N] pyrylium ring transformation
3
2
is reported. Spectroscopic data of the transformation products and their mode of formation are discussed.
J. Heterocyclic Chem., 39, 263 (2002).
Some years ago it was found that 2,4,6-triarylpyrylium
When the 2,4,6-triarylpyrylium perchlorates 1a-h and
the 3,3-dimethyl substituted 3H-indoles 6a-h were
refluxed in ethanol in the presence of anhydrous sodium
acetate, instead of the expected spiro compounds of the
type 4, the pyrido[1,2-a]indolium perchlorates 8a-o were
obtained. In the same manner the 3H-indoles 6i-k with dif-
ferent substituents or a spiro-fused carbocycle at C-3 gave
rise to the indolium perchlorates 8p-r. The example 1i +
6a → 8s shows that the transformation can be extended to
3,5-disubstituted 2,4,6-triarylpyrylium salts. In the case of
benzo-fused 3H-indoles the success of the transformations
strongly depends on the position of the additional benzene
ring. Whereas the benzo[e]indole 9 reacted smoothly with
the pyrylium salt 1a to give the benzo[e]pyrido[1,2-
a]indolium perchlorate 11, no pyridinium salt 12 could be
obtained starting with the benzo[g]isomer 10. Obviously, a
strong steric interaction between the α-bonded phenyl sub-
stituent and the fused benzene ring in 12, which is not pre-
sent in 11, prevents its formation. In all the transformations
the sodium acetate, used as basic condensing agent, can be
successfully substituted by piperidine acetate or triethy-
lamine/acetic acid, whereas triethylamine itself diminishes
the yield and sodium ethanolate is totally ineffective (cf.
experimental part).
salts 1 [2] react with 2-methyleneindolines 2 (R = Me, Ph;
R'= R''= Me) [3] by a diastereoselective ring transforma-
tion to give the aroylspiro[cyclohexadiene-indolines] 4
[4,5], which represent first examples of a new class of pho-
tochromic compounds, the aroylspiro[cyclohexadiene-aza-
heterocycles] [6]. The transformation was later extended to
2-methyleneindolines 2 with two different substituents or a
spiro-fused carbocycle/heterocycle at C-3 [7,1] as well as
to their benzo-fused analogues [8].
Acridine derivatives were found to be further candidates
for the synthesis of spiro compounds [9]. The transforma-
tion of the salts 1 with 9-methylenedihydroacridines 3 led
to aroylspiro[cyclohexadiene-dihydroacridines] 5 [4] also
showing interesting photochromic properties [10]. Since
9-methylacridine (7) reacted to give the N-unsubstituted
spirodihydroacridines 5 (R = H), the idea arose to study
transformations of the pyrylium salts 1 with the struc-
turally related 3H-indoles 6 with the aim to synthesize
spiroindolines 4 of the secondary amine type (R = H),
which would offer possibilities for further manipulations
at the nitrogen atom. Surprisingly, the reactions proceeded
quite differently. In this paper we wish to report on these
investigations.

264
T. Zimmermann and L. Hennig
Vol. 39
Pyrido[1,2-a]indolium salts of the type 8 have been
indolium perchlorates 8 are obtained. As shown for the
transformation 1a + 6a → 8a, the ketone by-product can
be isolated from the reaction mixture as a 2,4-dinitro-
phenylhydrazone derivative. Since in the course of this
transformation a pyridinium moiety is built up from three
reported in the patent literature to be optical whiteners
[11]. In comparison to the only other known method for
their preparation, the reaction of 3H-indoles with α,β-
unsaturated ketones in the presence of strong acids [12],
their synthesis via pyrylium ring transformation proceeds
under milder conditions, with shorter reaction times and
with higher yields.
carbon atoms of the pyrylium cation and a C N-fragment
2
of the 3H-indole, which connects the former positions 2
and 4 of 1, the reaction can be classified as a 2,4-[C +C N]
3
2
Concerning the mechanism of the transformations one
may assume that the 3H-indole 6 attacks the pyrylium
cation of 1 in the primary step at the preferred position 2
ring transformation [14].
If the 3H-indole 6 acts as an nitrogen nucleophile via the
intermediates 13', 14' and 15' the isomeric pyrido[1,2-a]-
indolium perchlorates 8', as the result of a 2,4-[C +NC ]
[2] as a carbon nucleophile with the C-atom of the 2-posi-
tioned methyl group or as a nitrogen nucleophile with its
N-atom. In the first case the 2H-pyran intermediate 13 [13]
would be formed, from which after electrocyclic ring
opening to 14, recyclization to 15 and perchloric acid
assisted alkylarylketone elimination the pyrido[1,2-a]-
3
2
ring transformation [14], would be formed. Because the
reaction of the pyrylium salts 1a and 1i leads to the same
products according to both mechanisms, a decision
between the mechanistic alternatives cannot be made. The
transformations of the salts 1b-h give only indolium salts of

Mar-Apr 2002
Ring Transformations of Heterocyclic Compounds. XXII
265
the type 8, clearly indicating that the 3H-indole 6 attacks
the pyrylium cation with carbon in the primary step.
by the almost perpendicular orientation of the aryl ring to
the molecular plane due to its steric interaction with the
indolium system. This signal can be located at 5.78-6.42
ppm, split into a doublet by coupling with the ortho-bonded
proton with a characteristic coupling constant of 5.7-9.4 Hz
[15]. NOe experiments have shown that in the case of the
pyrido[1,2-a]indolium perchlorates 8b-h the aryl substituent
Ar' and not Ar is found adjacent to the proton at C-4, thus
ruling out the alternative structures 8'b-h. By the same type
of experiment, it was found that the doublet at 8.91-9.11
ppm corresponds to a proton in the neighbourhood of a
methyl group at C-10 of 8 and C-12 of 11. Hence, this signal
has to be attributed to the H-atom in position 9 of 8 (position
11 of 11) and the doublet at 8.32-8.43 ppm to the remaining
proton of the pyridinium ring. The coupling constant of 1.1-
2.0 Hz determined for both doublets is comparable to those
observed for protons in 3,5-position of other pyridinium
derivatives [16].
A characteristic feature of the uv-spectra is a strong
absorption band at 329-356 nm [17]. FAB mass spectra,
recorded for 8a, 8b and 8f, show exactly the mass peaks of
the corresponding cations.
Further support for the structure of the pyrido[1,2-a]-
indolium perchlorates 8,11 was obtained by an indepen-
dent synthesis of 8a and 8c. According to a modified liter-
ature procedure, 1,3-diphenylpropen-1-one and 1-(4-
methoxyphenyl)-3-phenylpropen-1-one, respectively,
were treated with 2,3,3-trimethyl-3H-indolium perchlo-
rate. The pyrido[1,2-a]indolium perchlorates 8a and 8c
obtained were identical in all respects with those synthe-
sized by ring transformation of the pyrylium salts 1a,1c
with the 3H-indole 6a.
EXPERIMENTAL
The melting points were measured on a Boëtius hot stage appara-
1
13
tus. The H nmr and C nmr spectra were recorded on a Varian
1
13
Gemini 200 spectrometer ( H: 199.975 MHz, C: 50.289 MHz)
and on a Varian Gemini 2000 spectrometer ( H: 200.041 MHz, C:
1
13
50.305 MHz) in deuteriochloroform or dimethyl-d sulfoxide at 25°
6
with hexamethyl disiloxane as internal standard and uv spectra on a
Zeiss M 40 instrument (acetonitrile, 25°). Mass spectra were deter-
mined on a VG ZAB HSQ Analytical Instruments spectrometer
(FAB, matrix: 3-nitrobenzyl alcohol). The pyrylium perchlorates 1a
[18], 1b [19], 1c [20], 1d [21], 1e [22], 1f-h [23], 1i [24], 1-naph-
thylhydrazine [25], 3-methyl-4-phenylbutan-2-one [26] and 1-(4-
methoxyphenyl)-3-phenylpropen-1-one [27] were synthesized
according to literature procedures. The 2,3,3-trimethyl-3H-indol,
phenylhydrazine, 4-methylphenylhydrazine hydrochloride, 4-fluo-
rophenylhydrazine hydrochloride, 4-chlorophenylhydrazine
hydrochloride, 4-bromophenylhydrazine hydrochloride, 3-
methylbutan-2-one, 3-methylpentan-2-one and 1,3-diphenylpropen-
1-one were purchased from Aldrich, 4-methoxyphenylhydrazine
hydrochloride and 1,1,2-trimethyl-1H-benzo[e]-indole from Acros,
4-tert-butylphenylhydrazine hydrochloride as well as 4-iodophenyl-
hydrazine from Lancaster and 1-cyclhexylethanone from Fluka.
The elemental analyses and the spectroscopic data (cf.
Tables 1 and 2) strongly support the structure proposed for
1
the pyrido[1,2-a]indolium perchlorates 8,11. In the H nmr
spectra the methyl groups at C-10 of 8 and C-12 of 11 show
the expected singlet at 1.72-2.06 ppm. The multiplet at 6.49-
8.50 ppm can be attributed to protons of the benzene ring(s)
of the indolium moiety and the two aryl substituents. The
signal of the proton at C-4 of 8 and C-6 of 11 is remarkably
shifted to higher field by the anisotropy effect of the aryl
ring at C-6 of 8 and C-8 of 11, respectively. This is caused

266
T. Zimmermann and L. Hennig
Vol. 39
Preparation of the 3H-Indoles 6 and 10.
This compound was obtained from 4-methoxyphenylhydrazine
hydrochloride and 3-methylbutan-2-one in 95% yield; H nmr
1
The 3H-indoles 6b-k and 10 were synthesized by Fischer
indolization [28] from N-arylhydrazines or their hydrochlorides
and α-branched ketones in the presence of perchloric acid in boil-
ing ethanol as previously reported [1]. The resulting oily crude
products were used without further purification. Their identity
was checked by nmr spectroscopy and in the case of known com-
pounds the data were compared with reported values.
(deuteriochloroform): δ 1.21 (s, 6H, 3,3-(CH ) ), 2.17 (s, 3H,
3 2
2-CH ), 3.75 (s, 3H, 5-OCH ), 6.73-7.38 (m, 3H, arom-H).
3
3
5-tert-Butyl-2,3,3-trimethyl-3H-indole (6d).
This compound was obtained from 4-tert-butylphenylhy-
drazine hydrochloride and 3-methylbutan-2-one in 96% yield;
1
H nmr (deuteriochloroform): δ 1.21 (s, 6H, 3,3-(CH ) ), 1.27
3 2
2,3,3,5-Tetramethyl-3H-indole (6b) [29].
(s, 9H, 5-C(CH ) ), 2.18 (s, 3H, 2-CH ), 7.20-7.41 (m, 3H,
3 3
3
This compound was obtained from 4-methylphenylhydrazine
hydrochloride and 3-methylbutan-2-one in 89% yield; H nmr
(deuteriochloroform): δ 1.20 (s, 6H, 3,3-(CH ) ), 2.17 (s, 3H, 5-
arom-H).
1
5-Fluoro-2,3,3-trimethyl-3H-indole (6e) [31].
3 2
CH ), 2.31 (s, 3H, 2-CH ), 7.00-7.30 (m, 3H, arom-H).
3
3
This compound was obtained from 4-fluorophenylhydrazine
hydrochloride and 3-methylbutan-2-one in 88% yield; H nmr
1
5-Methoxy-2,3,3-trimethyl-3H-indol (6c) [30].

Mar-Apr 2002
Ring Transformations of Heterocyclic Compounds. XXII
267
(deuteriochloroform): δ 1.21 (s, 6H, 3,3-(CH ) ), 2.19 (s, 3H, 2-
5-Iodo-2,3,3-trimethyl-3H-indole (6h) [31].
3 2
CH ), 6.85-7.42 (m, 3H, arom-H).
3
This compound was obtained from 4-iodophenylhydrazine and
3-methylbutan-2-one in 94% yield; H nmr (deuteriochloroform)
5-Chloro-2,3,3-trimethyl-3H-indole (6f) [32].
1
δ 1.22 (s, 6H, 3,3-(CH ) ), 2.19 (s, 3H, 2-CH ), 7.21-7.56 (m,
3H, arom-H).
This compound was obtained from 4-chlorophenylhydrazine
hydrochloride and 3-methylbutan-2-one in 88% yield; H nmr
3 2
3
1
(deuteriochloroform): δ 1.22 (s, 6H, 3,3-(CH ) ), 2.20 (s, 3H, 2-
3 2
3-Ethyl-2,3-dimethyl-3H-indole (6i) [33].
CH ), 7.17-7.39 (m, 3H, arom-H).
3
5-Bromo-2,3,3-trimethyl-3H-indole (6g) [31].
This compound was obtained from phenylhydrazine and
3-methyl-4-phenylbutan-2-one in 84% yield; H nmr (deuteri-
1
This compound was obtained from 4-bromophenylhydrazine
hydrochloride and 3-methylbutan-2-one in 98% yield; H nmr
1
ochloroform): δ 0.33 (t, 3H, 3-CH CH ), 1.21 (s, 3H, 3-CH ),
2
3
3
1.78 (m, 2H, 3-CH CH ), 2.17 (s, 3H, 2-CH ), 7.11-7.48 (m, 4H,
(deuteriochloroform): δ 1.18 (s, 6H, 3,3-(CH ) ), 2.16 (s, 3H, 2-
2
3
3
3 2
arom-H).
CH ), 7.30 (s, 3H, arom-H).
3

268
T. Zimmermann and L. Hennig
Vol. 39
3-Benzyl-2,3-dimethyl-3H-indole (6j) [34].
(5 mmoles) 2,3,3-trimethyl-3H-indolium perchlorate, prepared
by dropping an equimolar amount of perchloric acid (70% in
water) to an etheral solution of 2,3,3-trimethyl-3H-indol and fil-
tration of the crystalline product. After magnetically stirring the
reaction mixture for 5 hours at 120° (oil bath), addition of 10 ml
of absolute ethanol, refluxing for 10 minutes and cooling to room
temperature, the precipitate was filtered by suction and washed
with ethanol and ether to give 0.45 g (20%) of 10,10-dimethyl-
6,8-diphenyl-10H-pyrido[1,2-a]indolium perchlorate (8a), mp
331-332° (lit yield 30%, mp >310°) and 0.62 g (26%) of 6-(4-
methoxyphenyl)-10,10-dimethyl-8-phenyl-10H-pyrido[1,2-
a]indolium perchlorate (8c), mp 259-260° (lit yield 43%, mp
245°), respectively. Both compounds were identical in all
respects with those obtained from the pyrylium salts 1a,1c and
the 3H-indole 6a.
This compound was obtained from phenylhydrazine and
3-methyl-4-phenylbutan-2-one in 82% yield; H nmr (deuteri-
1
ochloroform): δ 1.30 (s, 3H, 3-CH ), 2.26 (s, 3H, 2-CH ), 2.76
3
3
(d, J = 13.5 Hz, 1H, 3-CH Ph), 3.10 (d, J = 13.5 Hz, 1H,
2
3-CH Ph), 6.71-7.38 (m, 9H, arom-H).
2
2'-Methylspiro[cyclohexane-1,3'-indol] (6k) [35].
This compound was obtained from phenylhydrazine and
1
1-cyclohexylethanone in 98% yield; H nmr (deuteriochloro-
form): δ 1.18-1.91 (m, 10H, 3',3'-(CH ) ), 2.23 (s, 3H, 2'-CH ),
2 5
3
6.95-7.66 (m, 4H, arom-H).
2,3,3-Trimethyl-3H-benzo[g]indole (10) [32].
This compound was obtained from 1-naphthylhydrazine and
1
3-methylbutan-2-one in 98% yield; H nmr (deuteriochloro-
Acknowledgement.
form): δ 1.25 (s, 6H, 3-CH ), 2.30 (s, 3H, 2-CH ), 7.32-8.52 (m,
3
3
The financial support by the Deutsche Forschungs-
gemeinschaft is gratefully appreciated.
6H, arom-H).
Synthesis of Pyrido[1,2-a]indolium Perchlorates 8,11 from 2,4,6-
Triarylpyrylium Perchlorates 1 and 2-Methyl-3H-indoles 6,10.
REFERENCES AND NOTES
General Procedure (cf. Tables 1 and 2).
To absolute ethanol (30 ml) 5 mmoles pyrylium perchlorate 1,
5 mmoles 3H-indole 6,10 and anhydrous sodium acetate (0.82 g,
10 mmoles) were added. The reaction mixture was then refluxed,
if not otherwise stated, for two hours. The pyrido[1,2-a]indolium
perchlorates 6,11 crystallized from the hot reaction mixture or
crystallization was initiated by cooling. The salts were filtered by
suction, washed with ethanol, water (to remove sodium salts),
again with ethanol, finally with ether and recrystallized from
ethanol/acetonitrile.
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1
2
[4] The formula scheme shows only one enantiomer of the
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Determination of the Influence of the Condensing Agent Used on
the Product Yield.
To study how various condensing agents influence the product
yield, the transformation 1a + 6a → 8a was performed instead in
the presence of sodium acetate with equimolar amounts of the
following agents (yield of 8a in parenthesis): piperidine acetate
(33%), triethylamine/acetic acid (48%), triethylamine (14%),
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Isolation of the Alkylarylketone Formed as By-product.
The transformation 1a + 6a → 8a was carried out according to
the general procedure. After isolation of 8a by filtration the
mother liquor obtained was poured into a solution of 2,4-dinitro-
phenylhydrazine (0.99 g, 5 mmole) in 5 ml of concentrated sulfu-
ric acid, 7.5 ml of water and 25 ml of ethanol under magnetic stir-
ring. The acetophenone-2,4-dinitrophenylhydrazone precipitated
was filtered by suction, washed with water and ethanol and dried
in vacuo over phosphorous pentoxide, yield 65%, mp 247-248°
(ethylacetate), lit mp 248° [36].
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Independent Synthesis of the Pyrido[1,2-a]indolium Perchlorates
8a and 8c.
The pyrido[1,2-a]indolium perchlorates 8a and 8c were syn-
thesized applying a modified literature procedure [12]. To 1,3-
diphenylpropen-1-one (1.04 g, 5 mmoles) and 1-(4-methoxy-
phenyl)-3-phenylpropen-1-one, respectively, were added 1.30 g
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