Synthesis of carboxyalkylindolenines from 6-methyl-7-oxooctanoic acid

Article abstract of DOI:10.1007/s11172-008-0095-7

Novel 3-(4-carboxybutyl)indolenines were obtained. A method for the synthesis of 6-methyl-7-oxooctanoic acid, the precursor for these indolenines, was developed.

Full text of DOI:10.1007/s11172-008-0095-7

Russian Chemical Bulletin, International Edition, Vol. 57, No. 3, pp. 608—611, March, 2008
608
Synthesis of carboxyalkylindolenines from
6ꢀmethylꢀ7ꢀoxooctanoic acid

V. E. Kuznetsova and A. V. Chudinov
V. A. Engelhardt Institute of Molecular Biology, Russian Academy of Sciences,
32 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 1405. Eꢀmail: chud@eimb.ru
Novel 3ꢀ(4ꢀcarboxybutyl)indolenines were obtained. A method for the synthesis of 6ꢀmethylꢀ7ꢀ
oxooctanoic acid, the precursor for these indolenines, was developed.
Key words: indolenines, 2ꢀacetylcyclohexanone, oxo acids, heterocyclization.
2ꢀMethylindolenines are important intermediates in
sponding 1ꢀ(ωꢀcarboxyalkyl)indoleninium salts.6 Comꢀ
the synthesis of indocyanine dyes used as fluorescent
labels for biological analysis.1—5 The properties of fluoꢀ
rescent labels largely depend on the type and location of
various substituents. The position of the reactive carboxy
group in the indolenine fragment affects the spatial orienꢀ
tation of the label relative to a labeled biomolecule (proꢀ
tein or oligonucleotide).
For the synthesis of dyes with a carboxyl function,
carboxylꢀcontaining indoleninium bases are used. They
can be prepared in several ways. NꢀAlkylation of indoꢀ
lenines with ωꢀbromo carboxylic acids gives the correꢀ
pounds with the carboxy group in position 5 are syntheꢀ
sized by the Fischer method from carboxylꢀcontaining
phenylhydrazines and isopropyl methyl ketone.7
3ꢀCarboxyalkylindolenines are of particular interest.
A hydrocarbon chain between the carboxyl function and
the heterocycle diminishes their reciprocal influence.8,9
The present study was devoted to the synthesis of indoꢀ
lenines from 6ꢀmethylꢀ7ꢀoxooctanoic acid.
The Fischer synthesis is the most accessible route to
indolenines (Scheme 1). Condensation of various pheꢀ
nylhydrazines 1 with oxo acids 2 in acidic media involves
Scheme 1
R = Alk, SO3–, Ar.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 595—598, March, 2008.
1066ꢀ5285/08/5703ꢀ608 © 2008 Springer Science+Business Media, Inc.

Carboxyalkylindolenines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008
609
two sequential reactions: the formation of hydrazones 3
or 5 and their cyclization. Arylhydrazones of ketones
with an αꢀbranched alkyl chain are known to underꢀ
go cyclization only at the tertiary C atom to give
2ꢀmethylindolenines 6.10 This cyclization pathway is preꢀ
ferred because ene hydrazine 4 is more stable (conꢀ
tains more substituents at the double bond) than ene
hydrazine 5.
2 of the ring depends on the alkylating agent and
the reaction conditions. We methylated 2ꢀacetylcycloꢀ
hexanone16,17 (10) with MeI in the presence of aqueous
NaOH 15 to give 2ꢀacetylꢀ2ꢀmethylcyclohexanone (11)
in 75% yield. This method is very simple. The presence
of unreacted ketone 10 in the product was controlled by
a color reaction with FeCl3 in EtOH (a qualitative reacꢀ
tion for enols).
The heterocyclization pathway is strongly influenced
by the acidity of the medium. For instance, ZnCl2, AcOH,
the bisulfate ion, and aqueous H3PO4 always yield
cyclization products in the more branched chain of
the ketone (compounds 6). Polyphosphoric acid and
conc. H2SO4 and HCl give mixtures of products,
the yields of compounds 7 increasing with the acidity
of the medium.
We found that diketone 11 reacts with bases to give
three major products rather than two ones reported earliꢀ
er13,18,19 (Scheme 3).
Scheme 3
For the synthesis of 3ꢀcarboxyalkylindolenines, an
appropriate oxo carboxylic acid (of the type 2) with
the terminal carboxy group is required. Such acids can be
prepared in several ways. 7ꢀMethylꢀ8ꢀoxononanoic acid
has been synthesized from ethyl 2ꢀmethylacetoacetate
and ethyl 6ꢀbromohexanoate.11 Pritzkow et al. have proꢀ
posed to obtain methyl 6ꢀmethylꢀ7ꢀoxooctanoate from
Nꢀnitrosocaprolactam.12 The multistep scheme includꢀ
ing the Claisen retrocondensation of various 2ꢀacetylcyꢀ
cloalkanones affords oxo acids 2.13 It should be noted
that the retrocondensation outcome is sensitive to the ring
size and the reaction conditions. We developed a conveꢀ
nient method for the synthesis of oxo acid 8 from cycloꢀ
hexanone (9) (Scheme 2).
The reaction in aqueous 10% NaOH mainly resulted
in deacetylation of compound 11: the yield of 2ꢀmethylꢀ
cyclohexanone (13) was 60%, while the yield of acid 8
was less than 10%. The reaction in the presence of MeONa
afforded compounds 12, 13, and 14 in 35, 15, and 30%
yields, respectively. The best results were obtained with
MeOLi. The total yield of byꢀproducts 13 and 14 was
lowered to 30%, and oxo ester 12 was isolated by vacuum
distillation in 52% yield.
Scheme 2
We found optimum conditions for the acid hydrolysis
of ester 12. The highest yield (75%) of oxo acid 8 was
achieved with 30% HCl.20
Further reactions of oxo acid 8 with phenylhydrazines
15a—c were carried out in boiling AcOH in one step,
without isolating intermediate phenylhydrazones 3
(see Refs 6—8 and Schemes 1, 4).
Additional recrystallization from Et2O—hexane (16a)
or AcOEt—hexane (16c) gave the indolenines in 78
and 84% yields, respectively. Waterꢀsoluble indoleꢀ
nine 16b was converted into its potassium salt under
the action of KOH in MeOH (see Ref. 6) and isolated in
83% yield.
To sum up, we obtained novel carboxylꢀcontaining
indolenines 16a—c, which can serve as intermediates for
the synthesis of cyanine dyes. The presence of a carboxyꢀ
alkyl group in position 3 and various substituents in posiꢀ
tion 5 of indolenine provides great scope for the preparaꢀ
tion of new efficient fluorescent labels.
2ꢀAcetylꢀ2ꢀalkylcyclohexanones can be prepared in
several ways.13—15 The degree of methylation at position

610
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008
Kuznetsova and Chudinov
Scheme 4
colorless. The organic extracts were combined, washed with brine
(100 mL), and dried with MgSO4. The solvent was removed and
the residue was fractionally distilled in vacuo to give three products
12, 13, and 14.
2ꢀMethylcyclohexanone (13). The yield was 0.8 g (11%), b.p.
1
51—52 °C (22 Torr) (cf. Ref. 13: b.p. 165—166 °C). H NMR
(CDCl3), δ: 1.02 (d, 3 H, C(2)CH3, J = 6.5 Hz); 1.37, 1.81 (m, 2 H,
H2C(4)); 1.65 (m, 2 H, H2C(3)); 2.05 (m, 2 H, H2C(5)); 2.36
(m, 2 H, H2C (6)).
Methyl 2,3ꢀdimethylcyclohexꢀ1ꢀenecarboxylate (14). The yield
was 2.1 g (19%), b.p. 125—130 °C (20 Torr). MS (EI, 70 eV), m/z
(Irel (%)): 168.0 [M+] (100%). 1H NMR (CDCl3), δ: 0.97 (d, 3 H,
C(3)CH3, J = 7 Hz); 1.31 (s, 3 H, C(2)CH3); 1.49—1.78 (m, 4 H,
H2C(4), H2C(5)); 1.87 (m, 3 H, H(3), H2C(6)); 3.69 (s, 3 H,
COOCH3). 13C NMR (CDCl3), δ: 14.34 (3ꢀCH3); 19.57 (2ꢀCH3);
21.23 (C(5)); 26.78 (C(6)); 36.06 (C(4)); 46.52 (C(3)); 49.42
(COOCH3); 86.73 (C(1)); 121.20 (C(2)); 178.78 (COOCH3).
Methyl 6ꢀmethylꢀ7ꢀoxooctanoate (12). The yield was 6.2 g
(52%), b.p. 139—143 °C (20 Torr) (cf. Ref. 18: b.p. 120—124 °C
(12 Torr)). 1H NMR (CDCl3), δ: 1.06 (d, 3 H, C(6)CH3, J = 7 Hz);
1.23—1.35 (m, 3 H, H2C(5), H(4)); 1.56—1.69 (m, 3 H, H2C(3),
H(4)); 2.11 (s, 3 H, H3C(8)); 2.28 (t, 2 H, H2C(2), J = 7.5 Hz);
2.49 (m, 1 H, H(6)); 3.63 (s, 3 H, COOCH3).
6ꢀMethylꢀ7ꢀoxooctanoic acid (8). A mixture of ~12% HCl
(37 mL) and methyl 6ꢀmethylꢀ7ꢀoxooctanoate (12) (9.1 g, 0.049
mol) was refluxed with stirring for 6 h. The product was extracted
with ether (50 mL), the solvent was removed, and the residue was
fractionally distilled in vacuo. The yield of acid 8 was 5.9 g (75%),
b.p. 122 °C (0.1—0.01 Torr) (cf. Ref. 13: b.p. 128—129 °C
(0.7 Torr)). 1H NMR (CDCl3), δ: 1.06 (d, 3 H, C(6)CH3, J = 7 Hz);
1.25—1.39 (m, 3 H, H2C(5), H(4)); 1.58—1.69 (m, 3 H, H2C(3),
H(4)); 2.12 (s, 3 H, H3C(8)); 2.33 (t, 2 H, H2C(2), J = 7.5 Hz);
2.50 (m, 1 H, H(6)). 13C NMR (CDCl3), δ: 16.29 (6ꢀCH3); 19.57
(2ꢀCH3); 24.73 (C(3)); 26.72 (C(4)); 28.08 (C(8)); 32.45 (C(5));
33.92 (C(2)); 47.02 (C(6)); 179.55 (COOH); 213.09 (C(7)).
3ꢀ(4ꢀCarboxybutyl)ꢀ2,3,5ꢀtrimethylindolenine (16a). A mixꢀ
ture of 4ꢀtolylhydrazine hydrochloride (15a) (825 mg, 5.2 mmol),
glacial AcOH (8 mL), 6ꢀmethylꢀ7ꢀoxooctanoic acid (8) (913 mg,
5.3 mmol), and anhydrous AcOK (980 mg, 10 mmol) was refluxed
with stirring for 6 h. The solvent was removed and the residue was
dissolved in chloroform (10 mL). The resulting solution was washed
with water (20 mL) and brine (20 mL) and dried with MgSO4.
The solvent was removed and the residue was recrystallized
from Et2O—hexane (1 : 10). The yield of indolenine 16a was
1.13 g (84%), dark orange crystals. UV (MeOH), λmax/nm:
254; m.p. 138—140 °C. Found: m/z 260.7 [M]+. C16H21NO2.
Calculated: M = 259.34. 1H NMR (CDCl3), δ: 0.60—0.83
(m, 2 H, CH2CH2CH2CH2COOH); 1.25 (s, 3 H, H3C(3));
R = Me (15a, 16a), SO3H (15b), SO3K (16b), H (15c, 16c)
Experimental
1
H and 13C NMR spectra were recorded on a Bruker AMXꢀ400
pulse Fourier spectrometer (Germany) in CDCl3 (with Me4Si as
the external standard) and D2O. MALDIꢀTOF mass spectra were
recorded on a Kompact MALDI 4 instrument (Kratos Analytical,
US) with 2,5ꢀdihydroxybenzoic acid as a matrix for lowꢀmolecularꢀ
weight organic compounds. Mass spectra of lowꢀmolecularꢀweight
compounds were recorded on a Kratos MSꢀ30 instrument (230 °C,
70 eV). Melting points were determined on a Kofler Boetius hot
stage (Germany). UV spectra were recorded on a Jasco Vꢀ550
spectrophotometer (Japan).
4ꢀTolylhydrazine hydrochloride,21 4ꢀhydrazinobenzenesulfoꢀ
nic acid,22 and 2ꢀacetylcyclohexanone16,17 were prepared accordꢀ
ing to known procedures.
2ꢀAcetylꢀ2ꢀmethylcyclohexanone (11). A threeꢀneck roundꢀ
bottom flask fitted with a reflux condenser, a stirrer, and a thermoꢀ
meter was charged with MeOH (83 mL), 2ꢀacetylcyclohexanone (10)
(29.0 g, 26.9 mL, 0.21 mol), and MeI (44.0 g, 19.3 mL, 0.31 mol).
A solution of NaOH (8.3 g, 0.21 mol) in water (41.4 mL) was added
dropwise at 0 °C to the stirred reaction mixture. The mixture was
stirred at ~20 °C for 20 h and then refluxed for 2 h. On cooling,
water (85 mL) was added and the product was extracted with
chloroform (2×85 mL). The organic extracts were combined, washed
with 2 M NaOH (200 mL) and brine (100 mL), and dried with
MgSO4. The solvent was removed and the residue was fractionally
distilled under nitrogen in vacuo. The yield of compound 11
was 23.8 g (75%), b.p. 127—128 °C (32 Torr) (cf. Ref. 15:
b.p. 74—76 °C (3 Torr)).
1H NMR (CDCl3), δ: 1.23 (s, 3 H, C(2)CH3); 1.41—1.48 (m, 1 H,
H(4)); 1.61—1.69 (m, 3 H, H(4), H2C(5)); 1.93—1.99 (m, 1 H,
H(3)); 2.09 (s, 3 H, COCH3); 2.25—2.34 (m, 1 H, H(3)); 2.42—2.48
(m, 2 H, H2C(6)). 13C NMR (CDCl3), δ: 20.76 (2ꢀCH3); 22.31
(C(4)); 25.61 (COCH3); 27.26 (C(5)); 36.80 (C(3)); 41.15 (C(6));
63.76 (C(2)); 207.56 (COCH3); 210.26 (C(1)).
Methyl 6ꢀmethylꢀ7ꢀoxooctanoate (12). A mixture of anhyꢀ
drous MeOH (52.4 mL), metallic Li (0.5 g, 0.071 mol), and
2ꢀacetylꢀ2ꢀmethylcyclohexanone (11) (10.0 g, 0.065 mol) was reꢀ
fluxed with stirring for 1 h and then kept at room temperature for
12 h. Then the reaction mixture was acidified with conc. H2SO4 to
pH 4—5. Water (55 mL) was added to the resulting suspension and
the product was extracted with ether until the aqueous phase became
1.46 (m,
2
H, CH2CH2CH2CH2COOH); 1.72—1.88
(m, 2 H, CH2CH2CH2CH2COOH); 2.09—2.18 (m, 2 H,
CH2CH2CH2CH2COOH); 2.28 (s, 3 H, H3C(2)); 2.37
(s, 3 H, H3C(5)); 7.01—7.40 (m, 3 H, Ar); 10.11 (s, 1 H,
CH2CH2CH2CH2COOH).
Potassium 3ꢀ(4ꢀcarboxybutyl)ꢀ2,3ꢀdimethylindolenineꢀ5ꢀsulꢀ
fonate (16b). A mixture of 4ꢀhydrazinobenzenesulfonic acid (15b)
(1.8 g, 9.7 mmol), 6ꢀmethylꢀ7ꢀoxooctanoic acid (8) (2.0 g,
11.6 mmol), and glacial AcOH (15 mL) was refluxed with stirring for
8 h. Then another portion of AcOH (15 mL) was added and
the mixture was treated with activated charcoal. The solvent was
removed and the residue was dissolved in MeOH (10 mL).
A solution of KOH (600 mg, 10.6 mmol) in MeOH (7 mL), PriOH

Carboxyalkylindolenines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008
611
(15 mL), and Et2O (2 mL) was added dropwise. The precipitate that
formed was filtered off and dried in a vacuum desiccator over P2O5.
The yield of potassium salt 16b was 3.5 g (83%), light beige crystals.
UV (MeOH), λmax/nm: 258; m.p. 198—200 °C. Found: m/z 327.2
[M – K]+. C15H19NO5S. Calculated: M = 325.38. 1H NMR
(D2O), δ: 0.45—0.70 (m, 2 H, CH2CH2CH2CH2COOH); 1.31
(s, 3 H, H3C(3)); 1.36 (m, 2 H, CH2CH2CH2CH2COOH);
1.88—2.14 (m, 4 H, CH2CH2CH2CH2COOH); 7.53 (d, 1 H, Ar,
J = 9 Hz); 7.80 (m, 2 H, Ar).
3ꢀ(4ꢀCarboxybutyl)ꢀ2,3ꢀdimethylindolenine (16c). A mixture
of 6ꢀmethylꢀ7ꢀoxooctanoic acid (8) (723 mg, 4.2 mmol), phenylꢀ
hydrazine (15c) (454 mg, 414 µL, 4.2 mmol), and glacial AcOH
(4.2 mL) was heated in an inert atmosphere at 118 °C for 3.5 h.
The solvent was removed and the oily residue was dissolved in
chloroform (10 mL). The resulting solution was washed with water
(20 mL) and brine (20 mL), dried with MgSO4, and concentrated.
The residue was recrystallized from AcOEt—hexane (0.6 : 2).
The yield of indolenine 16c was 0.8 g (78%), yellowish orange
crystals. UV (MeOH), λmax/nm: 256; m.p. 112—113 °C. Found:
m/z 246.3 [M]+. C15H19NO2. Calculated: M = 245.32. 1H NMR
(CDCl3), δ: 0.56—0.81 (m, 2 H, CH2CH2CH2CH2COOH); 1.28
(s, 3 H, H3C(3)); 1.43—1.51 (m, 2 H, CH2CH2CH2CH2COOH);
1.76—1.92 (m, 2 H, CH2CH2CH2CH2COOH); 2.13—2.21 (m, 2 H,
CH2CH2CH2CH2COOH); 2.24 (s, 3 H, H3C(2)); 7.18—7.54
(m, 4 H, Ar).
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This work was financially supported by the Internaꢀ
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