Synthesis of ω-and (ω - 1)-acetylenic acids from five-, six-, or seven-membered cycloalkanones

Article abstract of DOI:10.1023/A:1011303024847

A convenient method for the synthesis of ω-and (ω - 1)-acetylenic acids involves free-radical oxidative scission of cycloalkanones containing five-, six, or seven-membered cycles to give the corresponding ω-olefinic acids followed by bromination of the latter and subsequent dehydrobromination under the action of alkalis.

Full text of DOI:10.1023/A:1011303024847

Russian Chemical Bulletin, International Edition, Vol. 50, No. 5, pp. 833—837, May, 2001
833
Organic Chemistry
Synthesis of ω- and (ω – 1)-acetylenic acids from five-, six-,
or seven-membered cycloalkanones
E. K. Starostin,« A. V. Ignatenko, M. A. Lapitskaya, K. K. Pivnitsky,
and G. I. Nikishin
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. E-mail: li@ioc.ac.ru
A convenient method for the synthesis of ω- and (ω – 1)-acetylenic acids involves free-
radical oxidative scission of cycloalkanones containing five-, six, or seven-membered cycles to
give the corresponding ω-olefinic acids followed by bromination of the latter and subsequent
dehydrobromination under the action of alkalis.
Key words: 4-pentynoic acid, synthesis from cyclopentanone; 4-hexynoic acid, synthesis
from cyclohexanone; 5-hexynoic acid, synthesis from cyclohexanone; 5-heptynoic acid,
synthesis from cycloheptanone; 6-heptynoic acid, synthesis from cycloheptanone; 4,5-di-
bromopentanoic acid, dehydrobromination; 5,6-dibromohexanoic acid, dehydrobromination;
6,7-dibromoheptanoic acid, dehydrobromination.
Recently,1 we have found that the results of
dehydrobromination of 5,6-dibromohexanoic acid (1b)
with alkali metal hydroxides depend substantially on the
nature of alkali metal. Thus, the reactions with LiOH or
NaOH smoothly afforded 5-hexynoic acid (2b), whereas
the reaction with KOH was accompanied by intense
isomerization of this acid into 4-hexynoic acid. The
aim of the present study was to synthesize ω- and
(ω – 1)-acetylenic acids from the corresponding five-,
six-, and seven-membered cycloalkanones taking into
account the above-mentioned effect.
urated acids,2 eicosanoids, oxylipins, and enzyme inhibi-
tors of their biosynthesis3 as well as other natural com-
pounds.4 Although the syntheses of the above-mentioned
acetylenic acids have been described in many stud-
ies,5—13 these procedures are either multistage or incon-
venient. Hence, the development of a simple preparative
procedure for their synthesis is still a topical problem.
The syntheses of ω- and (ω – 1)-acetylenic acids
from the corresponding five-, six-, and seven-mem-
bered cycloalkanones were performed according to
Scheme 1. ω-Alkenoic acids (1a—c) are rather readily
accessible. 5-Hexenoic acid (1b) and 6-heptenoic acid
(1c) were prepared according to a known procedure14
by the reactions of C6- and C7-cycloalkanones, respec-
tively, with hydrogen peroxide (0.5 mol-equiv.) and FeII
and CuII sulfates (the yields were 22—24%).*
Results and Discussion
ω-Acetylenic carboxylic acids are the useful starting
compounds in the syntheses of compounds containing
triple and Z-double bonds or other functional fragments
separated from the carboxyl group by several (1—4)
methylene groups. Among these are many natural unsat-
* Hereinafter, unless otherwise stated, the yields are given with
respect to the ketone introduced into the reaction without
regard for its excess and partial recovery from the reaction.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 797—801, May, 2001.
1066-5285/01/5005-833 $25.00 © 2001 Plenum Publishing Corporation

834
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
Starostin et al.
Scheme 1
considered above) so that the yields of acids 1a—c
reached 65% with respect to the consumed ketone.14,15
Acid 1a, which was most difficult to prepare by the
method under consideration, can also be synthesized
according to an alternative two-stage procedure involv-
ing C-allylation of diethyl malonate with allyl acetate
catalyzed by palladium complexes16 followed by decom-
position of the malonate fragment.17
O
a
COOH
n
(CH2)n
1a—c
n = 2—4
b
Bromination of alkenoic acids 1a—c proceeded
smoothly due to which dibromoacids 2a—c obtained in
virtually quantitative yields can be used in the subse-
quent stage without additional purification. We studied
double dehydrobromination of dibromoacids 2a—c un-
der the action of NaOH and KOH (Table 1). In all
experiments with hydroxides, aqueous solutions con-
taining poly(ethylene glycol) with the molecular weight
of 2000 (PEG-2000) were used. Previously, it has been
demonstrated1 that the addition of PEG-2000 is favor-
able for dehydrobromination. For comparison, sodium
amide in liquid ammonia was used as the standard
reagent for the synthesis of acetylenes from 1,2-di-
bromides.13
As in the case of dibromoacid 2b reported previ-
ously,1 dehydrobromination of dibromoacids 2a,c with
NaOH proceeded under rather mild conditions (80 °C).
Complete elimination of the second HBr molecule oc-
curred only in the case of formation of two (out of
three) intermediate isomeric bromoolefinic acids, viz.,
ω-bromo-(Z)- and (ω – 1)-bromo-(ω – 1)-alkenoic
acids (see Table 1, runs 1, 4, and 8). Under the condi-
tions used, both dehydrobromination of ω-bromo-(E)-
(ω – 1)-alkenoic acids (5a—c), which cannot undergo
trans-elimination, and subsequent conversions of the
resulting ω-acetylenic acids 3a—c into (ω – 1)-isomers
4a—c proceeded to only a small extent. Dehydro-
bromination of the highest homolog 2c (see Table 1,
runs 8 and 9), which unexpectedly required the larger
Br
COOH
c
Br
COOH
n
n
3a—c
2a—c
d
e
COOH
Br
COOH
n
n–1
5a—c
4b,c
n = 2 (a), 3 (b), 4 (c)
Reagents and conditions: a) H2O2, CuSO4, FeSO4; b) Br2,
CH2Cl2, –40 °C; c) NaNH2, NH3, –33 °C; d) NaOH,
PEG-2000, 80 °C; e) KOH, PEG-2000, 120 °C.
4-Pentenoic acid (1a) was obtained from cyclo-
pentanone only in 8% yield due to the low conversion
of the latter. Besides, acid 1a thus obtained contained
up to 12% of n-pentanoic acid due to competitive
detachment of the H atom from cyclopentanone under
the action of carboxybutyl radicals. The use of 2 moles
of hydrogen peroxide per mole of cyclopentanone re-
sulted in an increase in the yield of acid 1a to 12% with
a simultaneous decrease in the amount of n-pentanoic
acid to 8%.
Oxidative decyclization of the ketones used was not
accompanied by essential side processes (except for that
Table 1. Reaction conditions and results of dehydrobromination of dibromoacids 2a—c
Run
Com-
pound
n
Reaction conditions
Composition of the products (%)a
Total
yield (%)
Base
[PEG-2000] (%) T/°C
τ/h
3
4
5
1
2
2a
2a
2a
2b
2b
2b
2b
2c
2c
2c
2c
2
2
2
3
3
3
3
4
4
4
4
NaOH
KOH
NaNH2
NaOH
KOH
5
8
—
8
13
—
—
8
80
120
–33
82
120
120
–33
80
8
8
1.5
8
8
8
1.5
8
16
8
62
—
100
57
—
—
100
24
61
—
100
3
—
—
1
100
100
—
<0.5
4
35b
—
—
39
—
—
—
48d
35
—
88
86c
69
91
92
87
65
3
4
5
6
KOH
7
8
NaNH2
NaOH
NaOH
KOH
e
—
9
8
17
—
80
120
–33
89
85
63
10
11
100
—
NaNH2
1.5
—
a Data from GLC analysis.
b An additional amount (0.6%) of the corresponding (Z) isomer was found by GLC.
c A mixture of oligomeric unsaturated acids.
d An additional amount (28%) of the corresponding (Z) isomer was found by GLC.
e The yield was not determined.

Synthesis of ω- and (ω – 1)-acetylenic acids
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
835
Table 2. Chemical shifts in the 1H NMR spectra (200.13 MHz, CDCl3) of the compounds synthesized
Com-
pound
δ
H(1)
H(2)
H(3)
H(4)
H(5)
H(6)
—
H(7)
—
2a
11.35 (br.s)
1.94—2.16 (m, 1 H);
2.47—2.77 (m, 3 H)
2.44 (t)
4.26
(br.t)
1.70—2.36 (m)
3.64 (t, A);
3.90 (dd, B)
4.17
2b*
2c
11.5 (br.s)
10.7 (br.s)
3.63 (t, A);
3.86 (dd, B)
4.17 (m)
—
(dddd)
2.44 (t)
1.40—2.30 (m)
3.63 (t, A);
3.87 (dd, B)
3a
11.44 (br.s)
11.62 (br.s)
11.8 (br.s)
11.45 (br.s)
11.8 (br.s)
10.83 (br.s)
11.72 (br.s)
11.2 (br.s)
2.55 (t)
2.48 (t)
2.35 (t)
2.55 (m)
2.48 (t)
2.31—2.57 (m)
2.36 (t)
2.38 (t)
2.46 (t)
2.01 (m)
1.55 (quint)
2.45 (m)
—
2.29 (m)
1.70 (quint)
—
2.19 (m)
6.20 (m)
2.13 (q)
1.98 (t)
—
2.17 (br.t)
—
1.98 (t)
—
1.76 (t)
—
—
—
1.96 (t)
—
2.19 (m)
—
3b*
3c
4b
4c
—
—
1.74 (m)
5a
5b*
5c
—
1.76 (quint)
1.47 (quint)
5.97—6.24 (m)
2.07 (q)
—
1.65 (quint)
5.98—6.27 (m)
* The data published in the literature1 are given for comparison.
(3 m½3 mm) with the 6% SE-30 phase on Chromosorb W
(60—80 mesh) using nitrogen as the carrier gas (30 mL min–1).
The melting points were determined on a Kofler instrument.
We used PEG-2000 (98% purity) purchased from Austro-
waren Wien Laboratoriums Reagentien. The starting alkenoic
acids 1a, b.p. 86—87 °C (14 Torr), 1b, b.p. 93—95 °C (12 Torr),
and 1c, b.p. 116—118 °C (12 Torr), were prepared according to
a procedure reported previously14* (the results were discussed
above).
Dibromoacids 2a—c (general procedure). An equimolar
amount of a solution of Br2 (80 g, 0.5 mol) in CH2Cl2
(100 mL) was added with intense stirring to a solution of
alkenoic acid 1a—c (0.5 mol) in CH2Cl2 (200 mL) at –40 °C
for 1 h. The reaction mixture was stirred at this temperature for
0.5 h and then concentrated in vacuo. The residue was recrys-
tallized from hexane. The spectral data for these products are
given in Tables 2—4.
4,5-Dibromopentanoic acid (2a) was prepared from acid 1a
containing 11% of n-pentanoic acid in 82% yield (with respect
to acid 1a), m.p. 55—57 °C (see Ref. 5). Found (%): C, 23.61;
H, 3.14; Br, 61.89. C5H8Br2O2. Calculated (%): C, 23.10;
H, 3.10; Br, 61.49.
5,6-Dibromohexanoic acid (2b). The yield and physico-
chemical characteristics have been reported previously.1
6,7-Dibromoheptanoic acid (2c). The yield was 89%,
m.p. 29 °C. Found (%): C, 28.98; H, 4.03; Br, 55.93.
C7H12Br2O2. Calculated (%): C, 29.19; H, 4.20; Br, 55.49.
Dehydrobromination of dibromoacids 2a—c with aqueous
solutions of sodium and potassium hydroxides (general proce-
dure). A solution of dibromoacid 2 in a 10 M aqueous solution
of NaOH or KOH containing 10 mol of alkali per mole of acid
2 with the addition of PEG-2000 (up to 5—15% by weight) was
vigorously stirred (the conditions are listed in Table 1). The
reaction mixture was diluted with an equal volume of water and
reaction time, is characterized by the same regularities.
The resulting mixtures of acetylenic (3a—c) and
bromoolefinic acids (5a—c) can be readily separated by
fractionation in vacuo giving rise to ω-acetylenic acids
3a—c in 49—61% yields. The conventional procedure
for dehydrobromination involving sodium amide, which
is experimentally less convenient, afforded the same
acids 3a—c in slightly higher yields (63—69%, see
Table 1, runs 3, 7, and 11).
Dehydrobromination of dibromoacids 2a—c with
KOH was carried out under more severe conditions
(120 °C) so as to complete the conversion of bromo-
alkenoic acids 5a—c, which were difficult to dehydro-
brominate. In this case, intermediate ω-acetylenic acids
3a—c were completely isomerized to the corresponding
(ω – 1)-isomers 4b,c. As a result, dibromides 2b,c were
converted into the corresponding (ω – 1)-acetylenic
acids 4b,c in high yields (see Table 1, runs 5, 6, and 10).
Apparently, this is the best procedure for their prepara-
tion. However, we failed to synthesize acid 4a by this
method (see Table 1, run 2) because isomerization of
acid 3a under the reaction conditions proceeded by a
different mechanism specific to this acid.8
To summarize, a divergent and experimentally
simple procedure for the synthesis of ω- and
(ω – 1)-(C5—C7)-acetylenic acids starting from five-,
six-, and seven-membered cycloalkanones was devel-
oped based on the difference in the behavior of NaOH
and KOH as reagents for dehydrobromination of ω- and
(ω – 1)-dibromoacids.
Experimental
* Caution: Note that regeneration of the excess ketones accord-
ing to a procedure described previously14 should be carried out
only after thorough removal of all peroxide compounds, includ-
ing rather stable dialkyl peroxides. When these rules were not
met, an explosion occurred at the beginning of distillation of
ketone.
The 1H and 13C NMR spectra were recorded on a Bruker
AC-200 instrument (200.13 and 50.32 MHz, respectively). The
GLC analysis was carried out on an LKhM-80MD instrument
equipped with a flame ionization detector and a column

836
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
Starostin et al.
Table 3. Spin-spin coupling constants of the compounds syn-
thesized
(1 Torr), m.p. 52—53 °C. Redistillation (1 Torr) of the distilla-
tion residue in a flask afforded a higher-boiling distillate (3% by
weight) consisting (according to the 13C NMR spectrum) of
bromoacid 5a and its (Z) isomer in a ratio of 4 : 1.
5-Hexynoic (3b) and 6-bromohex-5(E)-enoic (5b) acids
(see Table 1, run 4). The yield and physicochemical character-
istics have been reported previously.1
4-Hexynoic acid (4b). The crystalline substance obtained in
runs 5 and 6 (see Table 1) was in fact chromatographically pure
acid 4b; the yields were 92 and 87%, respectively, m.p.
102—103 °C (from aqueous MeOH) (Refs. 8 and 11).
Com-
pound
J/Hz
H(2)—H(3) H(3)—H(4) H(4)—H(5)
Other
2a
—
9.6
9.6 (4—5A), 9.6 (6A—6B)
5.1 (4—5B)
2b*
6.7
—
2.9 (4A—5), 10.2 (6A—6B)
8.7 (4B—5)
6-Heptynoic (3c)6 and 7-bromohept-6(E)-enoic (5c) acids.
According to the data from GLC analysis (the column tem-
perature was 135 °C, the evaporator temperature was 250 °C;
the retention times of methyl esters of 3c and 5c were 4.05 and
18.0 min, respectively), the mixture obtained in run 8 (see
Table 1) contained a substantial amount of methyl ester of
(6Z)-5c (the retention time was 13.2 min). The mixture was
heated until ester of 5c completely disappeared (run 9). Frac-
tionation of the resulting mixture afforded acid 3c with 95%
purity in 61% yield, b.p. 130—132 °C (14 Torr) (lit data:6
m.p. 18—20 °C), and bromoacid 5c, b.p. 128—131 °C (1 Torr),
m.p. 40—42 °C. Found (%): C, 40.94; H, 6.01; Br, 38.01.
C7H11BrO2. Calculated (%): C, 40.60; H, 5.35; Br, 38.59.
5-Heptynoic acid (4c). The crystalline substance obtained
in run 10 (see Table 1) was in fact chromatographically pure
acid 4c, the yield was 85%, m.p. 37—39 °C (from aqueous
MeOH; Ref. 8).
2c
6.6
6.6
7.5
7.2
—
7.1
7.7
7.5
—
—
—
7.2
—
—
—
—
10.2 (6A—6B)
3a
3b*
3c
2.3 (3—5)
—
2.0 (4—6)
7.2
—
—
4b
4c
5b*
5c
1.8 (5—7)
—
7.7
7.5
—
—
—
7.7
7.5
Note. The H(5)—H(6) spin-spin coupling constants for com-
pounds 2b and 5c are 10.2 (5—6A), 4.4 (5—6B), and 7.5 Hz,
respectively. The H(6)—H(7) spin-spin coupling constants for
compound 2c are 10.2 (5—6A) and 4.8 (5—6B).
* The data published in the literature1 are given for com-
parison.
acidified with concentrated HCl to pH 2. The solution was
extracted with ether. The extract was dried with anhydrous
MgSO4 and concentrated. The resulting oily or crystalline
residues (2—3 mg) were methylated with an ethereal solution of
CH2N2 and analyzed by GLC and NMR spectroscopy (see
Tables 2—4). The procedures used for the preparative isolation
are included in Table 1 along with the characteristics of the
final products.
4-Pentynoic (3a) and 5-bromopent-4(E)-enoic (5a) acids.
Acid 3a and bromoacid 5a were isolated from the reaction
mixture obtained in run 1 (see Table 1) by fractionation in
vacuo. The yield of 3a was 51%, b.p. 94—95 °C (10 Torr), m.p.
54—56 °C (see Ref. 5). The yield of 5a was 18%, b.p. 65—68 °C
Dehydrobromination of dibromoacids 2a—c with sodium
amide (see Table 1, runs 3, 7, and 11). A solution of dibromoacid
2 in ether (1.33 M) was added with vigorous stirring to a 2 M
suspension of NaNH2 in liquid NH3, which was prepared in
situ from metallic Na, at –33 °C for 30 min. The viscous
suspension was stirred at this temperature for 15 min. Ammonia
was evaporated and one-half of the volume of water was added
with caution to the solid residue heated to 20 °C. The resulting
solution was acidified with concentrated HCl to pH 2 at 0 °C
and extracted with ether. The extract was washed with water,
dried with MgSO4, and concentrated. Vacuum distillation of
the residue afforded acid 3; the yields are given in Table 1; the
physicochemical characteristics are listed above.
Table 4. 13C NMR spectra (50.32 MHz, CDCl3, δ) of the compounds synthesized
Compound
Ñ(1)
Ñ(2)
Ñ(3)
Ñ(4)
Ñ(5)
Ñ(6)
Ñ(7)
—
2a
178.86
179.66
180.04
178.25
179.80
180.24
178.82
180.03
178.79
178.79
179.97
173.26
180.07
31.56
33.08
33.79
33.08
32.73
33.51
33.82
32.77
32.88
32.34
33.01
33.07
33.70
31.04
21.94
23.79
13.92
23.38
23.63
14.41
23.75
27.76
24.79
23.35
23.21
23.85
51.16
35.14b
26.25
82.10
17.84
27.66
76.65b
17.99
135.43
132.36
32.04
28.85
27.86
35.75
51.90
35.61
69.28
83.07
—
2ba
2c
35.85b
52.38
—
36.10
—
—
68.76
—
3.24
—
—
—
—
104.64
3a
3ba
3c
69.40
83.76
3.45
77.62b
—
18.06
4b
4c
5a
(4Z)-5ac
5ba
(5Z)-5ba,d
5c
76.91b
76.52b
106.19
109.44
136.63
133.51
32.45
—
105.38
108.58
137.34
a The data published in the literature1 are given for comparison.
b These data in the rows can be interchanged.
c The data were obtained from the spectrum of a mixture of (4Z)-5a and 5a.
d The corresponding methyl ester.

Synthesis of ω- and (ω – 1)-acetylenic acids
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 5, May, 2001
837
(c) D. Bouyssi, J. Gore, and G. Balme, Tetrahedron Lett.,
1992, 33, 2811.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99-03-
33053) and by the Russian Federation Government
Program "Leading Scientific Schools" (Project No. 00-
15-97328).
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Received October 9, 2000;
in revised form December 22, 2000